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ELSEVIER Tectonophysics 295 (1998) 259–306 Accelerating late Quaternary uplift of the New Georgia Island Group (Solomon island arc) in response to subduction of the recently active Woodlark spreading center and Coleman seamount Paul Mann a,Ł , Frederick W. Taylor a , Martin B. Lagoe b,1 , Andrew Quarles b,2 , G. Burr c a Institute for Geophysics, University of Texas at Austin, 4412 Spicewood Springs Rd. Bldg. 600, Austin, Texas 78759-8500, USA b Department of Geological Sciences and Institute for Geophysics, University of Texas at Austin, Austin, Texas 78712, USA c National Science Foundation Accelerator Facility, PAS Building #81, University of Arizona, Tucson, Arizona 85721, USA Received 18 June 1996; accepted 28 May 1998 Abstract The New Georgia Island Group of the Solomon Islands is one of four places where an active or recently active spreading ridge has subducted beneath an island arc. We have used coral reef terraces, paleobathymetry of Neogene sedimentary rocks, and existing marine geophysical data to constrain patterns of regional Quaternary deformation related to subduction of the recently active Woodlark spreading center and its overlying Coleman seamount. These combined data indicate the following vertical tectonic history for the central part of the New Georgia Island Group: (1) subsidence of the forearc region (Tetepare and Rendova Islands) to water depths of ¾1500 m and deposition of marine turbidites until after 270 ka; (2) late Quaternary uplift of the forearc to sea level and erosion of an unconformity; (3) subsidence of the forearc to ¾500 m BSL and deposition of bathyal sediments; and (4) uplift of the forearc above sea level with Holocene uplift rates up to at least 7.5 mm=yr on Tetepare and 5 mm=yr on Rendova. In the northeastern part of the New Georgia Island Group, our combined data indicate a slightly different tectonic history characterized by lower-amplitude vertical motions and a more recent change from subsidence to uplift. Barrier reefs formed around New Georgia and Vangunu Islands as they subsided >300 m. By 50–100 ka, subsidence was replaced by uplift that accelerated to Holocene rates of ¾1 mm=yr on the volcanic arc compared with rates up to ¾7.5 mm=yr in the forearc area of Tetepare and Rendova. Uplift mechanisms, such as thermal effects due to subduction of spreading ridges, tectonic erosion, or underplating of deeply subducted bathymetric features, are not likely to function on the 270-ka period that these uplift events have occurred in the New Georgia Island Group. A more likely uplift mechanism for the post-270-ka accelerating uplift of the forearc and volcanic arc of the New Georgia Island Group is progressive impingement of the Coleman seamount or other topographically prominent features on the subducting plate. Regional effects we relate to this ongoing subduction-related process include: (1) late Quaternary (post-270 ka), accelerating uplift of the Rendova–Tetepare forearc area in response to initial impingement of the Coleman seamount followed by exponentially increasing collisional contact between the forearc and seamount; (2) later Quaternary propagation of uplift arcward to include the volcanic arc as the area of collisional contact between the forearc and seamount increased; and (3) large-wavelength folding that has produced regional variations in late Holocene uplift rates observed in both forearc (southern Rendova, Tetepare) and volcanic arc (New Georgia Island) Ł Corresponding author. E-mail: [email protected] Deceased December 26, 1995. 2 Present address: ARCO International, P.O. Box 260888, Plano, Texas 75026-0888, USA. 1 0040-1951/98/$19.00 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 9 8 ) 0 0 1 2 9 - 2 260 P. Mann et al. / Tectonophysics 295 (1998) 259–306 areas. We propose that the dominant tectonic effect of Coleman seamount impingement is horizontal shortening of the forearc and arc crust that is produced by strong coupling between the subducting seamount and the unsedimented crystalline forearc of the New Georgia Island Group. The horizontal forces due to mechanical resistance to subducting rugged ridge and seamount topography may have terminated spreading of the Woodlark spreading center entering the trench (Ghizo ridge) and converted it to a presently active strike-slip fault zone. 1998 Elsevier Science B.V. All rights reserved. Keywords: Solomon island arc; New Georgia Island Group; Woodlark spreading center; late Quaternary uplift rates; seamount subduction 1. Introduction 1.1. Active ridge subduction There are four places where a recently active spreading ridge is subducting. These include: (1) the Chile ridge beneath central Chile (Cande et al., 1987); (2) the Juan de Fuca ridge at British Columbia (Rohr and Furlong, 1995); (3) the Woodlark spreading system at the Solomon Islands (Crook and Taylor, 1994); and (4) the Ayu trough at the western New Guinea trench (Milsom et al., 1992) (Fig. 1). The seafloor subducting at all three ridges ranges in age from about 4 to 0 Ma and ranges in topographic relief from 0.3 to 3 km because of rough, seafloor topography and off-axis seamounts. Of these four examples, only the Chile ridge exhibits active and unequivocal spreading in the trench. Active spreading has ceased on the Woodlark ridge segment adjacent to the San Cristobal trench about 500 ka ago, but oblique subduction of the ridge, continues at ¾97 mm=yr (Crook and Taylor, 1994) (Fig. 1). The mechanical difficulties of subducting rough, ridge-generated topography may explain the cessation of spreading at the Woodlark and Juan de Fuca ridges and the occurrence of active strike-slip faulting along both former ridges. 1.2. Objectives of this paper Eighteen geological effects theoretically accompany active ridge subduction (cf. review in Taylor and Exon, 1987), but have not been directly observed by marine and onshore studies of the triple junction area. This paper seeks to test the validity of some of these proposed geologic effects of ridge subduction by examining the record of vertical tectonism in the New Georgia Island Group where the recently active Woodlark ridge is subducting at the San Cristobal trench (Fig. 1). Key questions to answer include the following. (1) What is the late Cenozoic setting of the tectonically complex area of ridge subduction beneath the Solomon island arc? (2) How has vertical tectonism changed in the New Georgia Island Group from the Early Pleistocene (time recorded by the youngest marine sedimentary rocks in the outer forearc area) to the Late Pleistocene and Holocene (time recorded by raised coral reefs fringing the islands)? (3) What tectonic mechanism best explains the sequence and pattern of vertical tectonism in the New Georgia Island Group? 2. Tectonic setting of the Solomon Islands, Ontong Java Plateau, and adjacent oceanic basins and spreading ridges 2.1. Tectonic setting The Solomon Island Group is a 900-km-long linear, double chain of islands composed of arc crust of the northern Melanesian arc system now juxtaposed against oceanic plateau material accreted during late Neogene shallow subduction of the Cretaceous Ontong Java Plateau (Hughes and Turner, 1977) (Fig. 2). The area of accreted and folded Ontong Java crust, the Malaita accretionary prism of Mann et al. (1996), is exposed in the D-shaped area bounded by the North Solomon trench and the Kia-Kaipito-Korigole fault zone. This fault has been mapped on Santa Isabel as high-angle reverse or strike-slip fault (Petterson et al., 1997) and extends as a southeast-dipping and active thrust between Malaita and Guadalcanal. Late Cenozoic collision between the thick crust of the Ontong Java Plateau P. Mann et al. / Tectonophysics 295 (1998) 259–306 261 Fig. 1. (A) Geography of the Solomon Islands area (boxed area represents map shown in Fig. 2). The Solomon Islands, New Britain, and New Ireland constitute the northern Melanesian island arc system. The Ontong Java Plateau is an Early Cretaceous submarine oceanic plateau. (B) Tectonic setting of the Solomon Islands showing convergence of Ontong Java Plateau (OJP) and Pacific plate on the northern Melanesian arc system and the Australian plate and continent. Oblique convergence at a rate of about 10 cm=yr in a WSW–ENE direction between the OJP and Australian continent over the past 4 m.y. (DeMets et al., 1994) may have reoriented arc systems and convergent margins previously moving in the direction indicated by 1 into the direction marked by 2. This radical change in microplate motions towards the oceanic ‘free face’ to the southeast may have activated extension in the New Guinea, Manus and Woodlark basins. Black dots are 1185 earthquake epicenters from the ISC database with depths from 0 to 20 km and magnitudes >4.5. 262 P. Mann et al. / Tectonophysics 295 (1998) 259–306 Fig. 2. (A) Free-air gravity anomalies of the Solomon Islands region derived from Geosat and ERS-1 satellite altimetry data (Sandwell and Smith, 1997). Land areas are shown in black and data are illuminated from an azimuth of 128º. (B) Tectonic interpretation of the area shown in (A). Land is dark gray; late Neogene oceanic crust of the Woodlark basin is light gray; areas of thick crust of the Ontong Java Plateau and Louisiade Plateau are shown with horizontally ruled pattern; heavy lines with barbs represen t subduction boundaries; double lines indicate active spreading ridges; circles represent Plio–Pleistocene volcanic centers; triangles represent volcanic centers with histori c activity; large black arrow gives direction and rate of convergence of the Pacific plate relative to the Australia plate from DeMets et al. (1994). Earthquakes with magnitudes >4.5 are shown as black dots and were compiled by the International Seismological Centre for the period 1963–1992. Key to geographic abbreviations: PNG D Papuan Peninsula (Papua New Guinea); NB D New Britain; B D Bougainville (PNG); C D Choiseul (Solomon Islands, SI); SH D Shortland Islands (SI); NG D New Georgia Island Group (SI); RI D Russell Islands; SI D Santa Isabel (SI); M D Malaita (SI); FI D Florida Islands; G D Guadalcanal (SI); MA D Makira (SI). Key to abbreviations of tectonic features: TT D Trobriand trench; NBT D New Britain trench; KT D Kilinailau trench; WSR D Woodlark spreading center; SR D Simbo ridge; GR D Ghizo ridge; MAP D Malaita accretionary prism; KKKF D Kia-Kaipito-Korigole fault zone; SCT D San Cristobal trench; NST D North Solomon trench; CJT D Cape Johnson trench. Bars marked A, B, C and D indicate transect areas of earthquakes plotted as sections A, B, C and D in Fig. 3. Two dashed lines through the northern and southern Solomon island arc represent gravity lineaments seen in (A) that may correspond to transverse faults. 263 Fig. 2 (continued). P. Mann et al. / Tectonophysics 295 (1998) 259–306 264 P. Mann et al. / Tectonophysics 295 (1998) 259–306 and the northern Melanesian arc may explain the waning volcanic activity of this arc and the reorientation of the much more active New Britain arc in a southeasterly direction in which it can subduct normal oceanic crust (Fig. 1B). Island outcrops of arc and accreted rocks of the Ontong Java Plateau constitute topographic uplifts ranging from about 200 m to over 2 km with associated gravity highs >50 mGal (Fig. 2). High topography to the west in the New Georgia group (1768 m) and Bougainville (2743 m) is related to the construction of large, Pliocene to recent stratovolcanoes while high topography on Guadalcanal (2447 m), Makira (1250 m), and Malaita (1251 m) is related to the uplift and erosion of the deeper, crustal levels of the northern Melanesian arc and Malaita accretionary prism (Fig. 2). The deeper level of erosion and greater crustal seismicity of the Guadalcanal– Makira–Malaita area may reflect the position of these islands in the convergent zone between the Ontong Java oceanic plateau on the Pacific plate and oceanic plateau or continental crust of the Louisiade Plateau on the Australia plate (Mann, 1997) (Fig. 1). Global plate motion models predict about 100 mm=yr of oblique convergence between continental crust of the Australian plate and oceanic plateau crust of the Ontong Java Plateau, a largely submerged feature of the Pacific Ocean (Neal et al., 1998) (Fig. 1). The plateau would behave as a continent upon entering a subduction zone because its crustal thickness is roughly four times that of normal oceanic crust and is similar to that of many continental areas. In the area of initial oceanic plateau–arc contact east of the Solomon Islands, the Vitiaz subduction zone was unable to consume the thick oceanic plateau crust and underwent a subduction polarity reversal that is now in an advanced stage (Fig. 1). Steady west-southwest motion of the Ontong Java Plateau relative to the Melanesian arc progressively continued the polarity reversal process in Late Miocene–Early Pliocene time along the Solomon Islands segment of the arc system. This event initiated subduction of oceanic crust of the Woodlark basin (Ghizo ridge) at the San Cristobal trench (Fig. 2). Subduction of the Woodlark ridge may have begun at the San Cristobal trench adjacent to Guadalcanal and progressed to the northwest to the area of the Ghizo ridge adjacent to the New Georgia Island Group. To a first approximation, one may assume that emerged islands in the Solomons chain area uplifting and offshore areas are subsiding (Fig. 2). Previous studies of numerous uplifted Quaternary reefs found throughout the Solomon Islands confirm that most of the islands have been uplifting in Quaternary time (Grover, 1965; Stoddart, 1969a,b; Neef, 1978; Ramsay, 1982; Taylor and Tajima, 1987; Ridgway, 1987). The Florida Island Group and most of Santa Isabel and Choiseul appear to be subsiding based on drowned river valleys and coastlines and their location adjacent to the central Solomons intra-arc basin, a major zone of late Quaternary synclinal downwarping and sedimentation (Bruns et al., 1986) (Fig. 2). 2.2. Morphology of the North Solomon and San Cristobal trenches 2.2.1. North Solomon trench The 3500- to 6000-m-deep North Solomon trench marks a southwest-dipping subduction zone separating the Malaita accretionary prism from the Ontong Java Plateau (Fig. 2). The Kilinailau trench extends to the northwest of the North Solomon trench and juxtaposes the Ontong Java Plateau with arc rocks in Bougainville (Bruns et al., 1989) (Fig. 2). 2.2.2. San Cristobal trench The New Britain–San Cristobal trench marks a northeast-dipping subduction zone separating the Solomon–Bougainville section of the northern Melanesian arc system from the Australia and Solomon Sea plates (Fig. 2). The New Britain–San Cristobal trench can be divided into four main segments based on water depth. The well-defined New Britain trench northeast of the Woodlark rise ranges from 6000 to 8000 m deep. The New Britain trench shallows abruptly from a depth of ¾6000 m to the northwest of the Woodlark rise to a depth of ¾5000 m to the southeast of the rise where the trench becomes known as the San Cristobal trench. Young oceanic crust of the Woodlark basin is subducted beneath the Solomon arc at the San Cristobal trench from the Woodlark rise to the Pocklington rise near Guadalcanal (Fig. 2). In most places the San Cristobal ‘trench’ is not a well developed physiographic trench because of P. Mann et al. / Tectonophysics 295 (1998) 259–306 limited flexure of the subducting Woodlark oceanic crust (Taylor, 1987). The depth to the base of the steep trench slope composed of arc-related rocks averages about 3500 m and the small trench formed at the base of the inner slope is commonly devoid of sediment. Near the trenchward projection of the Pocklington rise, the third segment of the San Cristobal trench exhibits an average water depth of 4000–5000 m and juxtaposes older oceanic plateau or continental crust of the Louisiade Plateau with the arc (Fig. 2). The fourth and final segment of the San Cristobal trench is defined by a deepening to 7500 m. The lack of a prominent physiographic trench, the shoaling of the trench area and the less frequent earthquakes along the San Cristobal trench are consistent with subduction of young and buoyant oceanic crust of the Woodlark basin into the central part of the trench (Cooper and Taylor, 1985). 2.2.3. Woodlark basin The V-shaped, 3- to 5-km-deep Woodlark basin formed by westward propagation of the Woodlark spreading ridge between continental crust of the Woodlark and Pocklington rises (Taylor et al., 1995) (Fig. 2). Active spreading is occurring along 3.5- to 4.0-km-deep short ridge segments characterized by 6- to 7-km-wide axial valleys clearly visible on the regional gravity map shown in Fig. 2A. Mann (1997) proposed that the formation of the Pliocene to recent Woodlark spreading system may be related to the rifting of the continental part of the Solomon Sea plate as its Oligocene ocean floor undergoes simultaneous slab pull from coeval subduction occurring at the New Britain and Solomon trenches (Fig. 1). The Ghizo ridge, the short spreading segment adjacent to the San Cristobal trench and Solomon island arc, became extinct at about 500 ka when motion between the Australia and Solomon Sea plate shifted eastwards to a right-lateral transpressional fault parallel to the Simbo ridge (Crook and Taylor, 1994) (Fig. 2). This shift in the plate boundary transferred the small triangular area of Woodlark oceanic crust north of the Ghizo ridge from the Solomon Sea plate to the Australia plate. 2.2.4. Subducted slabs beneath the Solomon Islands Fig. 3 summarizes seismic evidence for the presence of subducted slabs along four earthquake 265 hypocenter transects through the Solomon Islands at the Shortland Islands, the New Georgia Island Group, the Russell Islands and Guadalcanal. Two slabs of opposing dip and extending to the San Cristobal and North Solomon trenches are present on all transects in Fig. 3. The 75- to 100-km-long, northeast-dipping slab that projects to the San Cristobal trench corresponds to subducted late Neogene oceanic crust of the Woodlark basin or subducted Oligocene oceanic crust of the Solomon Sea to the northeast of the Woodlark rise (Fig. 2). The southwestward-dipping slab at the North Solomon trench is much less seismogenic than the northeastwarddipping slab (Fig. 3). Previous workers like Weissel et al. (1982), Cooper and Taylor (1985) and Taylor and Exon (1987) have noted that the subducted slab of Woodlark basin crust beneath the New Georgia Islands is not well defined. They have related the slab’s poor definition to the buckling or underthrusting of its warm lithosphere and=or the rapid thermal equilibration of its young (3 Ma) lithosphere in the surrounding mantle material. However, more recent earthquake data, compiled for the New Georgia area in Fig. 3C, show that the subducted slab of Woodlark oceanic crust containing the subducted spreading center is a coherent slab to a depth of 75 km at an average dip of 45º. The shorter, ¾75 km length of the Woodlark slab relative to the slab subducted at the North Solomon trench probably reflects the recent onset of subduction at the San Cristobal trench over the past several million years. The average 45º dip of the young, 3 Ma Woodlark oceanic crust over the length of the San Cristobal trench is similar to the average dip of Cretaceous oceanic and oceanic plateau crust along the North Solomon trench. Despite the existence of two subducted slabs beneath much of the Solomons and Bougainville, historically active arc volcanism is restricted to only three localities plotted in Fig. 2B. In the Solomon Islands, this volcanism may be related to the melting of the Ontong Java Plateau or its adjacent oceanic crust that has been subducted to depths greater than 100 km. Localized crustal convergence between the Ontong Java Plateau and oceanic plateau or continental crust of the Louisiade Plateau south of the Solomon Islands may result in the larger and more intense seismic ac- 266 P. Mann et al. / Tectonophysics 295 (1998) 259–306 tivity in the southeastern Solomon Islands along with the narrow width, greater topography, deeper erosional level, and more rapid Quaternary uplift of the southwestern Solomon island arc (Fig. 2). 3. Geologic setting of the New Georgia Island Group 3.1. Main tectonic features The New Georgia Group with a total land area of 5060 km2 trends northwest for 235 km and forms the part of the Solomon island arc that is closest to the Woodlark spreading ridge (Fig. 2). The seafloor subducting in an east-northeast direction beneath the New Georgia Group was formed by north–south seafloor spreading in the Woodlark basin over the past 4 m.y. (Taylor et al., 1995) (Fig. 4A) and is dominated by two major ridges impinging on the New Georgia Island Group: (1) The north-trending Simbo ridge rises more than 1500 m above the general level of the seafloor and parallels the Simbo right-lateral transform fault (Fig. 4A). The Simbo fault zone is the main active plate boundary proposed by Crook and Taylor (1994) to separate the Solomon Sea and Australia plates following the cessation of spreading at the Ghizo ridge 500 ka ago. (2) The Ghizo ridge is the former eastward continuation of the Woodlark spreading center (Taylor, 1987) (Fig. 4A). The eastward end of the Ghizo ridge passes into an elevated area that includes the twin, sub-circular seamounts, Kana Keoki and Coleman (Fig. 4A). The Coleman seamount rises up 2.8 km above the surrounding seafloor while the twin- peaked Kana Keoki seamount rises up 2.3 km. The Coleman seamount is undissected and very reflective materials are confined to the summit region (Crook et al., 1991; Crook and Taylor, 1994). These authors concluded that the Coleman edifice may still be in a phase of vigorous volcanic growth. We estimate that ¾20 km of the Coleman seamount has subducted based on the unsubducted, sub-circular shape of the seamount based on high-resolution bathymetric surveys by Crook et al. (1991) and Crook and Taylor (1994) (Fig. 4A). With the notable exception of the atypical, active near-trench volcanism in New Georgia (Kavachi volcano, Fig. 2), there is no apparent thermal signature in the New Georgia forearc and intra-arc basins above the subducted spreading ridge (Hobart and Weissel, 1987). This observation is consistent with the result of Crook and Taylor (1994) that spreading on the subducting Ghizo ridge segment ceased by about 500 ka. Uplift and strike-slip faulting of the Ghizo ridge and non-parallelism of its adjacent spreading fabric with other areas of the Woodlark basin may indicate shortening effects on the subducting plate related to the resistance to underthrusting of the Ghizo ridge beneath the New Georgia arc. Based on extensive geophysical surveys, Crook and Taylor (1994) suggested that spreading ceased on the Woodlark spreading center when the Ghizo ridge was about 70 km in front of the trench. The Kana Keoki and Coleman submarine volcanoes were constructed at the east end of the Ghizo ridge over a segment of the former Woodlark spreading center which had become extinct progressively from east to west between 640 and 500 ka. The cessation of spreading along the E–W-striking ridge has transferred the area north of the ridge from the Solomon Fig. 3. Profiles of ISC earthquake hypocenters from the period 1962–1992 beneath bathymetric profiles of the northern Melanesian arc system (see Fig. 2B for transect locations). Hypocenters are projected onto the centerlines of the four 150-km-wide transect areas shown in Fig. 2A. The vertical exaggeration is greater for the bathymetric profile in the upper panel than for the hypocenters in the lower panel in order to accentuate tectonic features. Solid lines within hypocenters are the inferred top of the downgoing plate. (A) Bougainville transect. Subducted slabs of Eocene age oceanic crust of the Solomon Sea plate and Cretaceous age Ontong Java oceanic plateau or its adjacent Mesozoic oceanic crust are present at depth but do not appear to be in contact. (B) New Georgia transect. Subducted slabs of late Neogene (4.5–0 Ma) oceanic crust of the Woodlark basin and Ontong Java and=or its adjacent Mesozoic oceanic crust are present at depth but do not appear to be in contact. (C) Russell Islands. Subducted slabs of late Neogene (4.5–0 Ma) oceanic crust of the Woodlark basin and Ontong Java and=or its adjacent Mesozoic oceanic crust are present at depth but do not appear to be in contact. (D) Guadalcanal transect. Subducted slabs of late Neogene oceanic crust of the Woodlark basin and Ontong Java and=or its adjacent Mesozoic oceanic crust are present at depth and may be in contact. P. Mann et al. / Tectonophysics 295 (1998) 259–306 Sea plate to the Australia plate and activated rightlateral transform motion along the former Simbo ridge fracture zone. The model, therefore, predicts the progressive transfer, uplift, and rotation of aban- 267 doned spreading ridges from the Solomon Sea to the Australia plate. It is possible that a similar tectonic process may have accompanied the approach of other previously spreading segments of the Wood- 268 P. Mann et al. / Tectonophysics 295 (1998) 259–306 Fig. 4. (A) Bathymetry of the New Georgia Islands and adjacent Woodlark basin. Note the presence of the outer barrier reef at several places around islands in the northeast. Circles represent volcanic centers. (B) Two stages in the subsidence of a volcanic feature on the seafloor and the formation of barrier reefs and atolls modified from Darwin (1842). Reversal of subsidence by tectonic uplift has exposed barrier reefs around the New Georgia Islands. P. Mann et al. / Tectonophysics 295 (1998) 259–306 lark ridge that may have subducted to the east of the New Georgia Island Group (Fig. 2). 3.2. Sedimentary rocks of the forearc basin in the New Georgia Islands Islands in the forearc area (Tetepare, southern Rendova, and Ranongga) of the New Georgia Island Group are overlain by forearc marine sedimentary rocks of Plio–Pleistocene age (Dunkley, 1986) (Fig. 4A). Lithologies include turbiditic sandstone and shale, mudstone, pelagic limestone, and interbedded breccia and tuff. Much of the sediment was derived from the main volcanic chain to the north and northeast and was emplaced into the deepwater forearc basin by turbidity currents and debris flows. Micropaleontological studies done by Hughes et al. (1986) and by M.B. Lagoe for this study indicate an initial period of Early Pleistocene forearc subsidence followed by very rapid and very Late Pleistocene uplift of the forearc region. The structure, age and paleobathymetry of these rocks constrain a longer-term uplift history than provided by the Quaternary coral record alone. Like corals, the elevation of these rocks records the net uplift since their deposition on the seafloor. By dating the rocks using planktonic foraminifera and calcareous nannofossils and establishing the depth of deposition using benthic foraminifera, one can determine the average uplift rate since the sediments were deposited on the seafloor. Benthic foraminifera of known depth ranges and the presence or absence of pteropods record changes in water depth through continuous sections and across unconformities. Highly stratified marine sedimentary rocks provide numerous datum planes for determining deformational history. Major and minor faults and folds and angular contacts at unconformities can provide information on the mechanism for uplift and the directions of convergence and divergence active at the time of uplift. 3.3. Quaternary reef limestone in the New Georgia Island Group The coastlines of the New Georgia Island Group exhibits outer barrier reef islands indicative of subsidence of their volcanic island foundation (Darwin, 269 1842; Stoddart, 1969a,b) (Fig. 4B). These barrier reefs are particularly well developed at the Marovo and Nggerasi lagoons north of New Georgia Island and at Roviana and Nono lagoons south of New Georgia Island (Fig. 4A). Stoddart (1969a,b) noted that emerged barrier reefs around New Georgia Island reached up to 25 m above sea level and displayed emerged notches and corals. He concluded that the barrier reef-to-atoll subsidence sequence illustrated in Fig. 4B had been interrupted by subsequent tectonic uplift. Stoddart did not carry out sufficient radiometric dating to verify the timing of this uplift hypothesis. 3.4. Common shoreline features used to determine tectonic uplift 3.4.1. Limestone distribution Fig. 5A shows a generalized cross-section of a New Georgia Group shoreline where a late Quaternary barrier reef formed on a substrate of late Cenozoic volcanic rocks. The regional dip of the bedrock substrate averages about 10º and reflects the seaward slopes of the large stratovolcanoes making up the islands (e.g., Stoddart, 1969a,b; Dunkley, 1986). On several of the islands, a shallow (<5 m deep) lagoon separates an outer barrier reef island from the main island. Holocene reefs, indicating ongoing uplift, are best developed on the seaward edges of the barrier islands, although reefs also occur on the landward sides of the barrier islands. Reefs or mangrove swamps occur on the landward edge of the lagoon, where reef growth is inhibited by turbidity and fresh-water runoff from the islands. Pre-Quaternary recrystallized limestone deposits occur on eastern New Georgia Island up to hundreds of meters above sea level. These outcrops are erosional remnants of much older limestone that once covered larger areas and is not related to coastal coral reef limestones. 3.4.2. Shoreline features The typical features observed on late Quaternary barrier islands are summarized in Fig. 5B and include the following. (1) A living, fringing coral reef up to tens of meters wide is generally present at sea level. (2) A well preserved emerged coral reef is com- 270 P. Mann et al. / Tectonophysics 295 (1998) 259–306 Fig. 5. (A) Generalized distribution of late Quaternary reef limestone in the New Georgia Island Group based on observations made in this study. Reef limestone formed during the subsidence of underlying arc volcanoes. (B) Shoreline features observed in boxed area in (A) and used to determine tectonic uplift of coastlines in the New Georgia Island Group. See text for discussion. monly present on a gently sloping surface at elevations ranging from several meters to ¾35 m above sea level. This reef commonly onlaps older reefs deposited during pre-Holocene high stands of sea level or pre-Quaternary bedrock. Although 14 C ages for corals (determined using a 5568 yr 14 C half-life uncorrected for the reservoir effect) from this terrace range from 470 to 7430 yr B.P., those coral samples taken from near the reef crest tend to cluster at ¾5500–6500 yr B.P. (Table 1), approximately P. Mann et al. / Tectonophysics 295 (1998) 259–306 271 Table 1 Radiocarbon ages of Holocene corals, New Georgia Island Group and Russell Islands (numbered locations shown in Fig. 7B) Island and sampled locality a Lat. (S), Long. (E) Field sample number Lab number Percent aragonite Height (above Age b of sample living coral, in m) (yr B.P.) A. Rendova Island 6. Asovo Point Peninsula 6. Asovo Point Peninsula 6. Asovo Point Peninsula 10. South Rava Point 7. Rano Village 8. South Mbarora Bay 8º350 , 157º200 8º350 , 157º200 8º350 , 157º200 8º410 , 157º190 8º380 , 157º170 8º400 , 157º180 RDV-E-2 RDV-E-6 RDV-E-8 RDV-F-1 AQ-1 AQ-REND-4 TX7584 TX7592 A10962 TX7608 TX7605 A10965 100% 100% 100% 100% 100% 100% 6.5 18 18.1 1.6 28 11.9 8. South Mbarora Bay 8º400 , 157º180 AQ-REND-5 A10967 100% 1.7 8. South Mbarora Bay 8. South Mbarora Bay 8º400 , 157º180 8º400 , 157º180 AQ-REND-9 A10966 AQ-REND-10 TX7603 100% 100% 15.7 22.1 2630 š 60 4910 š 80 6690 š 65 5170 š 70 5430 š 80 5340 š 55 5370 š 65 600 š 50 730 š 55 4845 š 60 5520 š 80 B. Tetepare Island 12. West of Kioroso River 8º430 , 157º370 E-TET-3 TX7599 100% 46.75 7430 š 70 8º010 , 156º360 RAN-B-2 RAN-C-2 RAN-D-1 RAN-G-3 RAN-G-1a TX7587 A10962 A10964 TX7585 TX7586 100% 99% 97% 100% 100% 11.9 32 14.0 1.43 1.63 3620 š 60 7025 š 60 4355 š 55 470 š 50 590 š 50 8º220 , 157º170 8º220 , 157º320 8º030 , 157º170 8º030 , 157º170 RER-A HOT-A-1 VIR-A-3 VIR-A-1 NJA-A-3 RAM-A-1 ROV-A AR-A NG-A-1 NG-A-2 TX7593 TX7590 TX7597 TX 7602 TX7609 A10963 TX7596 TX7595 TX7588 A10959 100% 100% 100% 100% 100% 100% 100% 100% 100% 99% 0.42 0.75 6.5 4.1 2.0 2.61 3 3.6 1.63 4.9 180 š 40 6430 š 70 5500 š 70 5570 š 80 4120 š 70 6100 š 60 6220 š 80 6430 š 70 720 š 60 6810 š 60 E. Kolombangara Island 63. Ruvi Bay 8º040 , 156º570 61. Unamed islet South of Vila Point 8º070 , 157º30 KOL-A-2 VV-B TX7589 A10958 100% 100% 1.0 3.26 2250 š 60 4330 š 50 F. Ghizo Island 67. Logha Island 8º020 , 156º470 GZ-A-1 TX7591 100% 1.0 5340 š 60 G. Simbo Island 70. Nusasimbo Island 8º170 , 156º320 SIM-B-1 TX7606 100% 1.0 5490 š 60 H. Vella Lavella Island 72. Maravari Village 77. Paramata 73. Northeast Lambulambu 7º500 , 156º410 7º440 , 156º320 7º410 , 156º470 VL-A VLA-C VL-B A10961 TX7601 TX 7604 100% 100% 100% 1.0 1.0 1.0 6180 š 85 870 š 60 6490 š 90 C. Ranongga Island 14. Kukuri Point 15. N of Konggu village 16. Pienuna Village 18. South of Kolomali Village 18. South of Kolomali Village D. New Georgia and Parara Islands 26. Rereghana Island 47. Hotoanivena Island 57. Viru Harbor 57. Viru Harbor, 39. West Maitu Island 33. Ramata Island 25. Roviana Island 27. Ararosa Pass 29. Vululi Point North 29. Vululi Point North 8º010 , 156º350 8º010 , 156º350 8º180 , 157º230 8º440 , 158º040 8º290 , 157º350 8º290 , 157º350 8º340 , 158º080 a All locality names from 1 : 100,000-scale published geologic maps, Geological Survey Division, Ministry of Energy, Water, and Mineral Reservoir, Honiara, Solomon Islands. b Standard radiocarbon age calculated with the 5568 yr half life and no corrections for initial 14 C=12 C or for the reservoir effect; all errors are 2¦ . 272 P. Mann et al. / Tectonophysics 295 (1998) 259–306 Fig. 6. Interplay of Holocene sea level and tectonic uplift. See text for discussion. the time of the mid-Holocene high stand shown in Fig. 6. (3) In areas of rapid uplift, the coral limestone forms a thin coral mantle on a steep slope or cliff, rather than a more typical, flat-lying coral terrace. The terrace width can vary from nil to 100 m and is usually bounded on its landward edge by a gradual to steep rise in slope onto either older Quaternary limestone or pre-Quaternary bedrock (Fig. 5B). Most of the ages reported in Table 2 are from samples collected on the seaward edge of this coral terrace. (4) An erosional sea-level notch recording the mid-Holocene sea-level high stand is commonly present at an elevation of 1–10 m ALC (above living coral). Holocene notches are associated with Holocene coral reefs and appear fresh compared with older higher notches. These freshly cut Holocene notches are commonly encrusted at their bases by insitu coral and oysters that grew during or soon after the time when the notch formed. We report one age in Table 2 (sample RER-A, 180 š 40 yr B.P.) from Rereghana Island for oysters encrusting the upper part of the modern notch 0.42 m higher than living intertidal oysters. (5) Coral limestone above this notched slope is older, recrystallized limestone that is commonly densely vegetated with subdued karst topography on the terrace surfaces (Fig. 5B). We found no coral P. Mann et al. / Tectonophysics 295 (1998) 259–306 samples from the pre-Holocene reefs suitable for isotopic dating. 4. Field and laboratory methods for using coral reefs to determine uplift rates Coral reefs and shorelines provide the primary evidence for vertical deformation used in this paper. We summarize the methodology for interpreting tectonic uplift from coral reefs. A more detailed explanation of these methods is given by Taylor et al. (1990) and Ota et al. (1993). 4.1. Measurement of the elevations of coral limestone paleoshorelines and samples Coral elevations are measured from the shallowest presently living corals and given as heights in meters above living coral (ALC) (Fig. 5B). We determine the height of bioerosion notches and other paleosealevel indicators relative to the presently forming notch or relative to living corals if the modern notch is not developed (Fig. 5B). Handlevels and tape measures allow us to produce results accurate to at least 0.5 m for low sites and about 1 m for sites higher than 20 m. Healthy coral reefs can track the movements of sea level on tropical coasts. Generally, the highest living corals occur near mean low tide level, although the exact level depends on coral species and a variety of local factors (Fig. 5B). There is usually no evidence that a fossil coral reef grew up to its maximum potential level unless individual coral heads exhibit clear ‘microatoll’ growth forms (Taylor et al., 1990). Because most corals lived well below sea level rather than at the shallowest possible depths, an uplift rate calculated from the height and age of a coral is a minimum estimate (e.g. >2.5 mm=yr). In addition, emerged fossil reef flats are often lowered by sea spray erosion, especially on unprotected coastlines exposed to large waves. Solution notches that occur just above a Holocene reef flat are paleosea-level indicators, but the notches themselves are erosional and cannot be directly dated (Fig. 5B). If a notch is very closely related to a Holocene reef, then we may interpret it as a Holocene paleosea-level indicator and use it to 273 infer uplift rates. Notches commonly indicate that Holocene sea level reached one or two meters higher than the associated reef flat and fossil corals. Erosional notches are not lowered by erosion, but they, too, record only the minimum height attained by Holocene sea level relative to the coast. In the New Georgia Island Group, we date coral samples from many points to establish that the reef and associated notch are indeed Holocene (Table 1). The Holocene reef and associated notches are usually such continuous coastal features that dating of samples from widely spaced intervals is sufficient to confirm their age and continuity. Once we established these features to be Holocene in age, sea-level history was used to estimate uplift at many other undated sites (Table 2). 4.2. Interplay of Holocene sea level and tectonic uplift Both sea-level changes and vertical tectonics have contributed to the present vertical position of paleoshorelines and reefs in the New Georgia Island Group. Established Quaternary sea-level history allows more precise calculation of uplift rates and limits the possible age range of an emerged paleosea-level indicator (Fig. 6). 4.2.1. Holocene sea-level history Contribution to ocean volume by melting ice largely ceased by ¾5500 to 6000 yr B.P. (uncorrected 14 C years). Sea level subsequently fell relative to southwest Pacific coasts due to postglacial hydro-isostatic adjustments (Nakada and Lambeck, 1989; Tushingham and Peltier, 1991) (Fig. 6). This proposed fall of sea level since ¾5500 yr B.P. is confirmed by data from Fiji (Miyata et al., 1990), New Caledonia (Cabioch et al., 1989) and many other sites in Australia and the western Pacific and Indian Ocean (e.g. Nakada and Lambeck, 1989; Eisenhauer et al., 1993). Comparison of widely distributed sealevel data cited above reveals that sea level reached ¾1–2 m higher than at present at ¾5500–6000 yr B.P. We choose C1 m for this study in the New Georgia Island Group, but whether the actual amount was C0.5 m or C2 m is negligible for our interpretations. For any Holocene reef in the New Georgia Island Group study area, the maximum paleosea level had 274 P. Mann et al. / Tectonophysics 295 (1998) 259–306 Table 2 Uplift rates based on highest recognized Holocene paleosea levels (numbered locations shown in Fig. 7B) Studied locality A. Rendova Island 1. Lubaria Island 2. Kenelo Point 3. 2 km south of Kenelo Pt. 4. Asovo Point East 5. Asovo Point West 6. Rano Village South 7. South Mbarora Bay 14 C sample (Table 1) Uplift rate in mm=yr Highest Holocene Total Holocene Uplift rate from Table 1 sea level indicator emergence (m) (mm=yr) (sample age=height) (m) Reef Flat Reef Flat Reef Flat Reef Flat Reef Flat Reef Flat 1.5 6 10 20 20 28 0.1 0.9 1.6 3.5 3.5 4.9 22 8 3.5 3.8 1.3 0.5 RDV-E-2=6=8 AQ-1 AQ-REND-4= 5=9=10 2.5=3.7=2.7 5.2 8. Rava Point North 9. Rava Point South RDV-17-1 0.3 Reef Flat Reef Flat Reef Flat B. Tetepare Island 10. Waugh Bay 11. East Tetepare E-TET-3 6.3 Reef Flat Coral (No Flat) 5.5 47 0.8 8.4 RAN-B-2 RAN-C-2 RAN-D-1 3.3 4.6 3.2 RAN-G-1A=3 2.8=3.0 Reef Flat Reef Flat Reef Flat Reef Flat Reef Flat Reef Flat Reef Flat Reef Flat Reef Flat Reef Flat Reef Flat 8 21 33 24 22 16.5 16.5 16.5 11.5 0 0 1.3 3.6 5.7 4.2 3.8 2.8 2.8 2.8 1.9 <0 <0 4.5 6 5 5 2 5.5 5.5 2 3.5 5 4 4 2 2.5 2 1.5 1.5 4 5 5 5 4 2 0.6 0.9 0.7 0.7 0.2 0.8 0.8 0.2 0.5 0.7 0.5 0.5 0.2 0.3 0.2 0.1 0.1 0.5 0.7 0.7 0.7 0.5 >0.2 C. Ranongga Island 12. Lungga Point West 13. Kukuri Point West 14. Konggu Village North 15. Pienuna 16. 3 km north of Pienuna 17. Kolomali 18. 1 km north of Kolomali 19. New Mala 20. Emu Harbor 21. Bolu Island 22. Islet in Wilson Strait D. New Georgia, Vangunu, and Parara Islands 23. Munda 24. Roviana Island ROV-A 25. Rereghana Island RER-A 26. Ararosa Passage AR-A 27. Noro 28. Valuli Pt. North NG-A-UNG-A-2 29. Visu Visu Point 30. Kolumbaghea 31. Singgo Island 32. Ramata Island RAM-A-1 33. Uipi Island 34. Lumalihe Passage 35. Matiu Island West 36. Matiu Island (Stoddart, 1969a) 37. Matiu Island East 38. Porepore Island West 39. Porepore Island East 40. Njajapuchanjorno Island (Stoddart, 1969a) 41. Njapuana Island West 42. Njapuana Island East NJA-A-3 43. Karunjou Island 44. Mboli=Minjanga Island (Stoddart, 1969a) 45. Chalu Island (Stoddart, 1969a) 2.2=2.6=3.2=4 0.5 2.3 0.7 2.2=0.7 0.4 0.5 Reef Flat Reef Flat Notch Notch Reef Flat Reef Flat Notch Notch Notch Reef Flat=Notch Reef Flat=Notch Notch Notch Notch Notch Notch Notch Notch Notch Notch=Reef Flat Notch Notch Eroded Reef Flat P. Mann et al. / Tectonophysics 295 (1998) 259–306 275 Table 2 (continued) Studied locality 14 C sample (Table 1) Uplift rate in mm=yr from Table 1 (sample age=height) Highest Holocene sea level indicator (m) 46. Hotoanivena Island West 47. Ulukoro Island (Stoddart, 1969a) 48. Wickham Anchorage (Stoddart, 1969a) 49. Mbopo Bay 50. Votana Point 51. Ankara Island North 52. Reynolds Bay Peninsula 53. Rapichana Point 54. Choki Point 55. Viru Harbor Mouth 56. Viru Harbor West 2 km 57. Tambaka Point 58. Mbelombelo Island East 59. Mbelombelo Island West 60. Parara West 61. Patutoka Islet HOT-A-1 0.1 VIR-A-113 0.7=1.2 VV-B 0.8 Eroded Reef Flat Eroded Reef Flat Reef Flat Eroded Reef Flat Eroded Reef Flat Reef Flat=Notch Notch Notch Notch Notch Notch Notch=Reef Flat Notch=Reef Flat Notch Reef Flat Reef Flat 2 2 4 1.5 1.5 3 3 5 5.5 4.5 5 8.5 6.5 5 3.5 3.5 >0.2 >0.2 0.5 >0.2 >0.2 0.4 0.4 0.7 0.8 0.6 0.7 1.4 1.0 0.7 0.5 0.5 KOL-A-2 0.4 Reef Flat Reef Flat=old cliff Notch=Reef Flat Reef Flat 1.5 1.5 1 2 0.1 0.1 0 0.2 GZ-A-1 0.2 Notch Reef Flat Reef Flat 1 2 1.5 0 0.2 0.1 SIM-B-1 0.6 Eroded Reef Flat Reef Flat 1.4 4.5 A. 1 0.6 VL-A VI-B 1.0 0.2 7.5 8.5 3.5 0 1 0 1 -1 1.2 1.4 0.5 <0 0 <0 0 <0 E. Kolombangara 62. Ruvi Bay 63. Vanga 64. Rei 65. Ringgi Cove F. Ghizo Island 66. Mbambanga Island 67. Logha Island South 68. Nusatupe Island G. Simbo Island 69. Nusa Islet 70. Nusasimbo Island H. Vella Lavella 71. Vonunu 72. Maravari 73. Lambulambu 74. Supato 75. Mbava Island 76. Tirovilu 77. Paramata 78. Vorambare Bay VLA-C 0.6 to occur near ¾5500 yr B.P. unless: (1) the uplift rate exceeds the pre-¾5500 yr B.P. sea-level rise rate and the reef flat began to emerge even before ice ceased melting; or (2) the site is subsiding faster than sea level has fallen since ¾5500 yr B.P. and a younger reef has overgrown the 5500 yr B.P. surface. The uplift rate exceeds 5 mm=yr only in a few isolated areas of the New Georgia Island Group. For Notch=Reef Notch=Reef Notch=Reef Reef Flat Notch=Reef Reef Flat Reef Flat Reef Flat Flat Flat Flat Flat Total Holocene emergence (m) Uplift rate (mm=yr) example, at site RAN-C on Ranongga, the Holocene reef flat is at about 33 m ALC and the reef-crest coral RAN-C-2 gave an age of 7025 š 60 yr B.P. (Table 1) when sea level was still several meters below its peak at ¾5500 yr B.P. These data indicate an uplift rate of >5 mm=yr and earlier emergence than on slowly uplifting or static coasts. 276 P. Mann et al. / Tectonophysics 295 (1998) 259–306 4.2.2. Notches and sea-level history Fig. 6 illustrates the relationship between the high stand of late Holocene sea level at ¾5500 yr B.P. (Nakada and Lambeck, 1989) to the morphology of the erosional notches. If the rate of Holocene tectonic uplift is fast enough (>0.5 mm=yr), less-wellpreserved, pre-Holocene notches commonly occur at higher elevations on the slope (Fig. 6). These higher, older, weathered notches commonly exhibit weathering features like stalagtites and generally contrast with the fresh and sharply defined Holocene notches. The older pre-Holocene notches tend to be discontinuous features because of mass wasting on the steep slopes, whereas Holocene notches are nearly continuous features along sea cliffs. If the Holocene uplift rate is slow enough (<0.2 mm=yr), the Holocene notch and reef are superimposed on the older notches or bury them beneath Holocene reef. In a few areas of the New Georgia Islands, there is a particularly large, overhanging notch where the Holocene notch was cut directly onto an older notch. 4.2.3. Uplift rate calculations in the New Georgia Island Group A hypothetical Holocene reef and associated notch that formed at ¾1 m ALC at the ¾5500 yr B.P. high stand (shown in Fig. 6) that is now found at elevation of 7.8 m ALC has been uplifted at a rate of .7:8 1:0/ D 6:8 m=5500 yr, or 1.25 mm=yr. A younger, hypothetical coral shown in Fig. 6 and dated at 2630 yr B.P. is now found at an elevation of 3.6 m ALC. This coral was uplifted at a rate of .3:6 0:5/ D 3:1 m=2630 yr or >1.18 mm=yr because about half of the ¾5500 high stand of sea level had receded by 2630 yr B.P. We also assume this is a minimum uplift rate because the coral is unlikely to have grown up to its shallowest possible level. 4.3. Reefs and shorelines related to pre-Holocene sea levels Holocene coral reefs provide the best indication of ongoing uplift rates for each field site. However, we will show that the uplift of pre-Holocene notches and reefs is slower than the Holocene notches and reefs because uplift in the New Georgia Island Group accelerated in late Quaternary time. Because of this process of accelerated uplift, much older and weath- ered corals and reefs are present only a few meters above the Holocene ones. The occurrence of three sea-level high-stand events are useful constraints on the tectonic history of the New Georgia Island Group. These high-stand events are referred to by their correlative deep-sea core oxygen-isotope substage numbers 5a, 5c, and 5e. Isotope substage 5e correlates with the most recent full interglacial high-stand event before present at about 120–130 ka (commonly called the ‘last interglacial’). Substages 5c and 5a correlate to interstadial glacial retreats and lower-than-present high stands of sea level for which the approximate ages are 105 ka and 80 ka. The relative sea levels of 5e, 5c, and 5a are generally accepted as having been about C6, 15, and 15 m relative to present (e.g. Chappell et al., 1996). However, neither these exact sea levels nor their ages are essential to our interpretation, as long as one accepts that these sea levels occurred at these approximate times and that the sea levels of 5a and 5c were lower than present sea levels in the western Pacific. Other workers, including Stoddart (1969a,b), Morton and Challis (1969) and Morton (1974) have measured coral limestone elevations ranging from 0 to 60 m in the New Georgia Island Group. The distribution and approximate elevations of Quaternary limestone in the New Georgia Island Group have been compiled on geologic maps and accompanying reports by Dunkley (1986). However, these previous studies were not accompanied by regionally extensive radiometric dating and therefore were not adequate for detailed tectonic interpretations. 4.4. Treatment of samples prior to dating Treatment of coral samples prior to 14 C dating included: (1) slicing coral into 5-mm-thick slabs and trimming discolored or abnormal-looking material; (2) soaking about 72 h in 2% sodium hypochlorite solution; (3) cleaning in distilled water with an ultrasonic probe; and (4) examining for excessive aragonite cement. Powder X-ray diffraction scans on each sample prior to dating ensured that calcite was not detectable in the dated samples. The standard radiocarbon ages in Table 1 are calculated with the 5568 yr half life with no corrections for shifts in 14 C=12 C in the atmosphere, or the reservoir effect. P. Mann et al. / Tectonophysics 295 (1998) 259–306 5. Late Quaternary uplift of the New Georgia Island Group 5.1. Distribution of samples and coastline observations The pattern of pre-Holocene and Holocene uplift was determined by detailed observations from 78 coastal sites around the New Georgia Island Group (Fig. 7A; Table 2) and 34 14 C dates on samples of measured elevation (Table 1). All localities are shown on the location map of Fig. 7B along with the locations of the outcrop photographs used in this paper. Each locality is numbered (1–78) in Fig. 7B and keyed to the 14 C ages listed in Table 1 and the uplift rates based on highest Holocene paleosea-level indicators listed in Table 2. Geographic names of the main features and towns of the New Georgia Island Group are shown in Fig. 7A. Geographic names for the localities that correspond to the names of small islands or villages, are keyed in Table 2 to localities 1–78 in Fig. 7B. All geographic names and their spellings in Fig. 7A and in Table 2 are taken from the seven 1 : 100,000 geologic maps of the New Georgia Island Group compiled in 1987 by the Geological Survey Division, Ministry of Energy, Water, and Mineral Resources (MEWMR), Solomon Islands (Geological Survey Division, 1987). A location key for these geologic maps that are identified as maps NG 1 through NG 7 is given in Fig. 7A. These maps are available for sale to the public through the Geological Survey Division in Honiara. Between the 78 landings where direct observations were made, we made continuous observations of the rise and fall in terrace elevations while traveling along the coast in small boats. The heights of these features constrain uplift rates between sites with 14 C ages (Fig. 8B; Table 2). 5.2. Pattern of pre-Holocene uplift 5.2.1. Outcrops of older limestone at higher elevation on eastern New Georgia Island A recrystallized, reefal limestone plateau reaching 50–80 m in elevation is found in coastal and interior areas of eastern New Georgia Island (Dunkley, 1986; Map NG 3, Geological Survey Division, 1987) (Fig. 8A). The outcrop does not follow topographic 277 contours and there are many adjacent areas in the 30- to 80-m-altitude range on eastern New Georgia and nearby islands where this limestone is absent. In outcrop, the limestone is massive, heavily recrystallized, and contains recognizable coral and shell fragments. The edge of the limestone plateau forms a steep escarpment and the plateau surface exhibits subdued karst landforms as seen north of Munda (site 23, Fig. 7B). On Roviana Island 8 km east of Munda (site 24, Fig. 8A), an erosional outlier of the platform is present with a 50-m-high summit that is roughly concordant with the average elevation of the plateau north of Munda. It is unlikely that the higher, isolated outcrops of this extensive limestone formed at sea level during the last interglacial about 125,000 years ago (isotope substage 5e) when sea level was about 6 m higher than at present (e.g., Chappell and Shackleton, 1986). If the sea covered the New Georgia Islands Group to an elevation of C80 m during the past few hundred thousand years, then continuous coral limestone and younger, lower terraces should cover the islands rather than only occurring on the plateau of eastern New Georgia Island. 5.2.2. Age of older limestone It is likely that this older, plateau-forming limestone is early Quaternary or Pliocene in age as suggested by the recrystallized state and as indicated on survey map NG 4 (Geological Survey Division, 1987) (Fig. 7A). There is no doubt that substantial reefs developed in this region during the last interglacial at ¾125 ka. The last interglacial reef most likely forms the continuous, emerged barrier reef around New Georgia Island and the low coastal reefs ranging up to ¾30 m on New Georgia and ¾80 m on Nggatokae Island (Fig. 8A). The last interglacial high stand may have modified the slopes of the 50-m-high escarpment near Munda and on Roviana Island (Fig. 8A), but sea level did not reach the 50m-high crest of the plateau in this area during the transgression preceding 125 ka. 5.2.3. Outcrops of limestone at higher elevation on southern Rendova In contrast to the higher limestones on New Georgia island, we include the undated and poorly studied limestone on southern Rendova Island in the pre- 278 P. Mann et al. / Tectonophysics 295 (1998) 259–306 Fig. 7. (A) New Georgia Island Group geographic locality map and key to geologic survey map. (B) Locality map for late Quaternary reef sites (small squares) examined during this study. The numbers of these sites are keyed to the numbers in Table 1 showing radiocarbon dates and Table 2 listing all visited localities. The locations of photographs showing reef outcrops are also indicated along with the locations of three rivers where unconformities and paleobathymetry of Plio–Pleistocene sedimentary rocks were studied (Fig. 13). P. Mann et al. / Tectonophysics 295 (1998) 259–306 279 Fig. 8. (A) Contours of maximum elevation in meters of late Quaternary limestone, major late Quaternary tectonic blocks and plate boundaries of the New Georgia Island Group. The 400-m-high elevation of a poorly described reef limestone on Rendova Island that we did not examine during this study is consistent with the late Quaternary uplift history proposed here. The submarine outcrop, faults and late Neogene volcanic cones shown as white circles on the Ghizo and Simbo ridges are from Crook and Taylor (1994). (B) Holocene uplift rate in mm=yr based on 14 C age dating and elevations of coral samples of known elevation. Ages of all samples are listed in Table 1. 280 P. Mann et al. / Tectonophysics 295 (1998) 259–306 Holocene uplift map shown in Fig. 8A. This area of probable late Quaternary limestone has been uplifted to an elevation range of 400 to 800 m with a strong northeastward tilt consistent with the tilt and trends of fold axes and thrusts in underlying forearc sedimentary rocks of Plio–Pleistocene age (Takeda, 1973; Map NG 6, Geological Survey Division, 1987) (Fig. 7A). We include the Rendova limestone on the preHolocene uplift map for several reasons: (1) extremely high Holocene uplift rates up to 7.7 mm=yr based on coastal reefs (Table 1) are consistent with the 400–800 m elevation and late Quaternary age for these limestones; (2) these limestones are contiguous with known Holocene limestone along the eastern and western coasts of Rendova (Fig. 8B); and (3) Dunkley (1986; NG Map 6, Geological Survey Division, 1987) describes these deposits as raised reefs although they are recrystallized and their exact age has yet to be determined (Fig. 8A). The 200-m-contour of pre-Holocene uplift is extended around the coasts of southern Rendova and Tetepare to account for the present-day topographic elevations of Plio– Pleistocene sedimentary rocks occurring in these areas and underlying the outcrop of the 400- to 800m-high limestones of southern Rendova (Fig. 8A). 5.2.4. Forearc and volcanic arc blocks indicated by elevations of pre-Holocene reefs The elevations of late Quaternary limestone shown in Fig. 8A and paleobathymetric data indicate that total uplift ranges from hundreds to thousands of meters in the forearc area of Tetepare and Rendova, but total late Quaternary uplift is only 0–80 m in the volcanic arc area of New Georgia Island to the northeast. Variations in reef elevations over the 60-km distance between Tetepare and the northeast coast of New Georgia Island indicate the presence of distinct structural blocks underlying the forearc and volcanic arc areas (Fig. 8A). The forearc blocks contain the islands of Tetepare, Rendova, Ghizo and Ranongga and the volcanic arc blocks the islands of Nggatokae, New Georgia, Kohinggo, Parara, Kolombangara and Vella Lavella (Fig. 7A). Differences in uplift history within the volcanic arc are the basis for subdividing the volcanic arc into smaller blocks separated by inferred, northeast-striking faults (Fig. 8A). 5.2.5. Fault boundaries between forearc and volcanic arc blocks The areas of highest uplift rates of the volcanic arc are adjacent to an unnamed submarine thrust fault inferred between the forearc and arc area along the northeastern edge of the Blanche Channel near the southwest coast of New Georgia Island (Fig. 8A). We infer another submarine thrust fault on the steep trench slope southwest of Tetepare and Rendova (Crook and Taylor, 1994) (Fig. 8A). Uplift along the northeastern side of the trench slope may contribute to the uplift pattern elongate in the direction parallel to the fault and the long axes of the land areas of southern Rendova and Tetepare (Fig. 8A). The axial trends of large-scale folds on Tetepare and Rendova (Dunkley, 1986; Map NG 6, Geological Survey Division, 1987), the submarine Blanche Channel syncline (Taylor, 1987) and the Ghizo anticline (Dunkley, 1986; Map NG2, Geological Survey Division, 1987) are all roughly parallel to our two proposed, submarine faults shown as queried features in Fig. 8A. 5.3. Pre-Holocene block structure of the volcanic arc area 5.3.1. Block structure The tops of the New Georgia barrier reefs mark the level of maximum transgression of the most recent phase of subsidence and provide a datum from which to measure subsequent net emergence as shown graphically in Fig. 6. The elevations of the barrier reefs show that maximum total uplift of the ongoing uplift cycle is only a few tens of meters (Fig. 8A). Variations in preHolocene reefs define several blocks separated by inferred northeast-striking, high-angle faults (Fig. 8A). 5.3.2. Nggatokae block Nggatokae has uplifted about 80 m with strong tilting toward the north-northwest and is clearly a distinct tectonic block from the rest of the volcanic arc area of New Georgia Island to the west which is characterized by a total late Quaternary uplift within a range of 0–30 m. The limited amount of total late Quaternary uplift of New Georgia Island in turn dramatically contrasts with the hundreds to thousands of meters of forearc uplift of Tetepare and Rendova 30–60 km to the southwest and Ranongga 70–150 km to the west (Fig. 8A). P. Mann et al. / Tectonophysics 295 (1998) 259–306 5.3.3. Vangunu block Late Quaternary uplift of this block is intermediate between the fast-uplifting Nggatokae block to the east and the New Georgia block to the west (Fig. 8A). The Vangunu block exhibits a southwestward tilt but limestone only up to an elevation of 20 m. 5.3.4. New Georgia block Late Quaternary limestone reaches a maximum elevation of only ¾30 m (Fig. 8A). The 10-mcontour of uplift of this late Quaternary limestone roughly outlines the coasts of New Georgia, Kohinggo, and Parara Islands (Fig. 7A). The eastern part of the block and New Georgia Island exhibits tilting to the northeast similar to that seen to the south on the forearc block of Tetepare and Rendova (Fig. 8A). The western part of the block (western New Georgia, Kohinggo and Parara Islands) exhibits southwestward tilting. 5.3.5. Kolombangara–Vella Lavella block This block exhibits no uplift or tilting. A fault boundary is inferred beneath the marine straits (Blackett Strait and Kula Gulf) separating the stable and untilted area of Kolombangara from the area of faster uplift and tilting in western New Georgia, Kohinggo, and Parara Islands) (Fig. 7A). 5.3.6. Paraso rift on Vella Lavella This rift appears active based on its topographic expression cross-cutting the island: solfataric and geothermal activity in the basin block (Dunkley, 1986); prominent fault scarps along its southeastern margin (Grover, 1965); and evidence for coastal drowning along the southeastern rift shoulder during the 1959 earthquake (Grover, 1965) (Fig. 8A). However, late Quaternary limestone exhibits no differential uplift or tilting and it appears that the rift area was part of a stable Kolombangara–Vella Lavella block (Fig. 8A). 5.4. Pattern of Holocene uplift in the New Georgia Island Group 5.4.1. Method and outcrop A total of 34 Holocene samples from strategic coastal areas (Table 1) verify the Holocene age of 281 the nearly continuous reefs and notches in the intervening areas (Table 2). The uplift rate map of Fig. 8B represents rates derived directly from the 78 sites listed in Table 2 along with these inferred rates. Minimum uplift rates are given in Table 1 based on the height of the Holocene reef flat or emerged solution notch as discussed previously and shown in Fig. 6. 5.4.2. Pattern of Holocene uplift of the forearc Holocene uplift of the forearc block in Tetepare and southern Rendova exhibits a northwest-trending axis of maximum uplift parallel to the coasts of the two islands: the Blanche Channel syncline; the two inferred submarine thrust faults; and the San Cristobal trench (Fig. 8A). The elongate uplifting area of Tetepare and Rendova lies where the subducting Coleman seamount (Australia plate) at the eastern end of the Ghizo ridge impinges on the forearc (Fig. 8A). In contrast, the Holocene uplift of the forearc block in Ranongga exhibits a north– south uplift parallel to its coasts and the strike of the Simbo transform fault on the subducting Solomon Sea plate (Crook and Taylor, 1994) (Fig. 8A). The long axis of the Ranongga uplift is at about 45º to the convergence vector of the Solomon Sea plate and Simbo ridge. A more northeast-trending extension of the Ranongga uplift includes the east coast of Vella Lavella of the volcanic arc block as seen from rates determined at localities 72, 73, and 74 (Fig. 8B; Table 2). The maximum uplift rate on Tetepare and in the entire Solomon Islands to date is 7.7 mm=yr based on the height=age calculation for a fossil coral sample, E-TET-3, collected at locality 11 on the northern coast of the island (Table 2). This sample gave an age of 7430 yr B.P. when sea level was ¾10 m below its present level (Fig. 6). Therefore, the fossil coral has been uplifted ¾57 m to its present height. Holocene uplift rates decrease northward as seen from the elevation of reefs at localities 1, 2 and 3 along the west coast of Rendova (Table 2). The maximum uplift rate estimated on Ranongga to date is ¾5.7 mm=yr at locality 14 on the east coast of Ranongga (Fig. 8B) where sample RAN-C-2 gave an age of 7025 š 60 for a coral from a terrace at 33 m ALC. The age and rapid uplift of this terrace suggest that it began to emerge when sea level was still ¾7 m 282 P. Mann et al. / Tectonophysics 295 (1998) 259–306 lower than at present (Fig. 6) so that the total uplift is ¾40 m. 5.5. Coastal geomorphology of the forearc area of Tetepare and Rendova 5.5.1. Tetepare and southern Rendova Coastal areas of rapid to moderate uplift in the forearc area exhibit uplifted Holocene reefs with steeper topographic profiles, less weathered surfaces, and more slumped features than those found in slower uplifting areas of the volcanic arc. The rapidly uplifting coastline of the north-central coastline of Tetepare at locality 11 in Fig. 7A is characterized by a Holocene terrace forming a thin mantle above a 50-m-high, heavily vegetated coastal bluff (Fig. 9A). Slopes are steep to vertical with large trees growing out from the steep slope at the coast and overhanging the sea. Large slump blocks and boulders of older recrystallized and Holocene corals are present along the narrow shoreface and in the surf zone. The western end of Tetepare at locality 10 in Fig. 7B is experiencing a much slower uplift rate of 0.8 mm=yr and is characterized by an exposed, roughly planar reef flat on a gentle slope. Farther to the west on southern Rendova at localities 4, 5, 6, and 7 (Fig. 7B), steep, heavily vegetated slopes similar to those of north-central Tetepare at locality 11 are associated with high uplift rates in the range of 3.5–4.9 mm=yr (Fig. 8B). Low values of uplift in western Tetepare and southeastern Rendova may reflect recent subsidence or less uplift of the area of the Balfour Channel separating the two islands (Fig. 7A). The topography and underlying geologic structure of both areas suggest northeastward tilting of the islands associated with the formation of a large anticline with a slightly overturned southwestern limb (Dunkley, 1986; Map NG 6, Geological Survey Division, 1987). For this reason we suspect that late Quaternary reefs shown on map NG 6 of the Geological Survey Division (1987) might exhibit even higher uplift rates than those along the more accessible northern edges of the islands. We did not investigate the southern coasts of Tetepare and Rendova because they are exposed to the open sea and are difficult to access in a small boat. 5.5.2. West coast of Rendova The west coast of Rendova exhibits a gradual southwest to northeast transition in coastal geomorphology from steep slopes mantled with uplifted Holocene reef in the higher uplift areas to the southwest (localities 2, 3, Fig. 7; Fig. 9B is a photograph of locality 3) to low relief, Holocene reef flats associated with a barrier reef in the northeast (locality 1) (Fig. 8B). The presence of a raised barrier reef provides a general indicator of uplift rate in the forearc volcanic arc area of the New Georgia Island Group. There is usually no barrier reef present on forearc and volcanic arc coastlines with uplift rates greater than about 0.8 mm=yr (Fig. 8B). On Rendova the barrier reef is confined to the northern part of the island where the uplift rate at locality 1 is 0.1 mm=yr (Fig. 8B). 5.6. Coastal geomorphology of the forearc area of Ranongga Terraces along the east coast of Ranongga that are uplifting at rates of about 1–5 mm=yr form more planar surfaces than those seen on Tetepare, but nonetheless reach elevations up to 33 m ASL (compared to 47 m ASL in the forearc area of Tetepare) (Fig. 8B). Raised Holocene terraces along the east-central coast form planar reef flats about 8 m above sea level. These raised reef flats decrease in elevation and disappear towards the northern end Fig. 9. Comparative morphology of coral terraces uplifting at extremely fast to moderate rates in the forearc area of Tetepare and Rendova. (A) Rapidly uplifting coastline at a rate of ¾7.7 mm=yr along the north coast of Tetepare (locality 11 in Fig. 7A). In this photograph, Holocene reef limestone mantles the steep slope up to an elevation of 47 m but is obscured by vegetation. (B) Moderately uplifting coastline at a rate of ¾0.9 mm=yr near Kenelo Point (locality 3 in Fig. 7A) on the west coast of Rendova Island. The south to north decrease in uplift across Rendova is reflected in the coastal geomorphology of the Holocene reefs. In the south at Kenelo Point, the faster-uplifting Holocene reef mantles a steeper coast than locality 1 in the north where the Holocene reef is uplifting at a slower rate (¾0.1 mm=yr) in a barrier island setting. (C) Slowly uplifting coastline at a rate of ¾0.7 mm=yr near Arasosa Passage (locality 26 in Fig. 7) on New Georgia Island. In this slowly uplifting setting, weathering effects of the coral terrace are more pronounced. P. Mann et al. / Tectonophysics 295 (1998) 259–306 283 284 P. Mann et al. / Tectonophysics 295 (1998) 259–306 of the island characterized by slow uplift or possibly subsidence (Fig. 8B). Higher terraces are present along a steeply dipping substrate of Plio–Pleistocene forearc basin sedimentary rocks and older crystalline basement rocks that crop out in the interior highlands of the island. The areas which are not covered by coral reefs on Ranongga Island and other rapidly uplifting areas like Tetepare, are probably due to the steep coastal zones composed of easily erodible forearc basin sedimentary rocks. These rocks would form a poor substrate for reef growth during rapid uplift of the coast. The topography and underlying geologic structure of Ranongga suggest eastward to east-northeastward tilting of the island (Dunkley, 1986; Map NG 2, Geological Survey Division, 1987). As in the case of Tetepare and southern Rendova, we suspect that uplift rates along the western coast of the island may be faster than those measured from Holocene reefs on the east coast and shown in Fig. 8A. Unfortunately, there is no late Quaternary coral limestone present on the steep western coast according to Map NG 2 of the Geological Survey Division (1987) (Fig. 7A). We did not investigate the west coast of Ranongga because it forms steep cliffs that are exposed to the open sea and difficult to access in a small boat. 5.7. Coastal geomorphology of the forearc area of Ghizo A Holocene terrace at Mbambanga Island (locality 67, Fig. 7A) indicates that this and similar islets east of Ghizo are not uplifting. At Mbambanga Island, the Holocene reef forms a 50- to 100-m-wide reef flat near sea level. The reef flat is eroded in the typical appearance of a non-uplifting coastline. Outcrops of recrystallized and tectonically fractured, pre-Holocene limestone in the interior of the island indicate that the Holocene reef forms a thin mantle on much older limestone. On Nusatupe (locality 69 in Fig. 7A) and Logha Islands (locality 68), closer to the main island of Ghizo, uplift rates are ¾0.1 mm=yr and ¾0.2 mm=yr, respectively. Again, this slowly uplifting area exhibits wide, eroded reef flats. Pre-Holocene notches were not observed at any of the three localities near Ghizo probably because the notches have been overgrown by Holocene highstand coral deposits. The topography and underlying geologic structure of Ghizo indicates that the island is a large doubly plunging anticline whose axial trace trends to the northwest and parallels convergent structures in the forearc area of Tetepare and southern Rendova (Dunkley, 1986; Map NG 3, Geological Survey Division, 1987) (Fig. 8A). The lack of Holocene uplift on the plunging nose of this anticline on Mbambanga Island east of the main island of Ghizo (locality 67) and higher rates of uplift more to the west near the main island at localities 68 and 69 suggest that tectonic uplift of the land surface of Ghizo above the forearc anticline may continue into the Holocene. 5.8. Coastal geomorphology of Simbo on the subducting Australia plate Simbo is the only emergent area of the Simbo ridge and the only island in the New Georgia Island Group that occupies the subducting Australia plate (Fig. 4A). The San Cristobal trench crosses the bathymetric saddle beneath the 10-km-wide marine strait that separates Simbo and Ranongga. Holocene reefs on Simbo indicate slow uplift ranging from a rate of >0.1 on Nusa Islet on the western side of the main island (locality 70 in Fig. 7A) to >0.6 mm=yr on Nusasimbo Islet on the southeastern side of the main island (locality 71). In both areas, 100- to 300-m-wide, eroded reef flats characteristic of nonor slowly uplifting coasts are present. 5.9. Pattern of Holocene uplift on the volcanic area of New Georgia Island The northwest-trending axis of the volcanic arc area, that includes the large islands of New Georgia, Kolombangara and Vella Lavella, is characterized by moderate to slow uplift of 0.5 to 1.4 mm=yr in comparison to the faster areas of more geographically concentrated forearc uplift on Tetepare, southern Rendova and Ranongga (Fig. 8B). The area of maximum Holocene uplift in the volcanic arc area occurs along the southeast coast of New Georgia Island adjacent to the Blanche Channel at localities 57 (1.4 mm=yr) and 58 (1.0 mm=yr) (Figs. 7 and 8A; Table 1). Uplift decreases to extremely low values on Nggatokae Island and the southern part of Vangunu Island and near westernmost New Georgia Island, P. Mann et al. / Tectonophysics 295 (1998) 259–306 Hatherton Sound and Kohinggo Island (Fig. 7A). A single 10- to 15-km-wide band of slow Holocene uplift in the range of 0.5 to 1.4 mm=yr affects the eastern coast of Vella Lavella and is perhaps related to activity along large fault scarps bounding the southeastern edge of the Paraso rift (Grover, 1965). The western and northern coasts of Vella Lavella and the adjacent island of Kolombangara to the east exhibit little or no evidence of Holocene uplift. 5.10. Coastal geomorphology of the volcanic arc area of New Georgia Island Relatively recent, Late Pleistocene cessation of subsidence and uplift of barrier reefs surrounding New Georgia Island, that is shown graphically in Fig. 4B, is well illustrated by the large-scale coastal geomorphology of the Roviana, Nggerasi, Marovo and Nono lagoons (Fig. 7A). 5.10.1. Barrier island of the Roviana Lagoon The uplifted barrier reef isolating the Roviana Lagoon can be traced 40 km from near Munda on eastern New Georgia Island to the central part of New Georgia Island in the east (Fig. 7A). As on Rendova, the barrier reef merges with the coast of New Georgia Island when the uplift rate exceeds about 0.8 mm=yr near localities 58 and 59 at the eastern edge of the lagoon (Fig. 8A). Uplift caused the former lagoon to emerge and become dry land where the uplift was greatest. 5.10.2. Holocene reef terraces of the Roviana Lagoon Variations in uplift rates account for differences in coastal geomorphology around New Georgia Island as shown graphically in Fig. 5. For example, the low, intensely eroded Holocene terrace near Araroso Passage (locality 26) at the eastern end of the Roviana Lagoon is being uplifted at a relatively slow rate of about 0.7 mm=yr (Fig. 9C). Coral sample AR-A from an elevation of 3.7 m on the notched cliff shown in Fig. 15 yielded an age of 6430 š 70 yr B.P. (Table 1). Based on the Holocene notch at 5 m and the assumption of 1 m higher sea level at ¾5500 yr B.P., we estimate an uplift rate of 0.7 m for the notch and reef terrace at locality 26. Erosion lowered the terrace level seen in the 285 photograph of Fig. 9C as it uplifted, so that its present height is only ¾1 m ASL. With slower uplift, erosion may lower a terrace surface as fast as it emerges. The associated notch 1 shown in Fig. 15A suggests several meters of erosion-related lowering of the top of the terrace shown in Fig. 14 or that it did not grow up to the sea surface at ¾5500 yr B.P. 5.10.3. Holocene notches of the Roviana Lagoon Sea cliffs of barrier islands forming the Roviana and Marovo lagoons contain many examples of Holocene and pre-Holocene notches that are not directly contiguous with a Holocene reef as in the case of locality 26 at Arasosa Passage (Fig. 10). Some of these sea cliff notches were previously described by Stoddart (1969a,b) and many other examples were found and described by us during this study. The notches are cut into the pre-Holocene coral limestone forming the raised barrier reef systems. At locality 25 on Rereghana Island on the barrier island of the Roviana Lagoon, the lowest notch is encrusted with oysters giving an age of 180 š 40 yr B.P. (sample RER-A in Table 1) and suggestive of recent relative sea-level change (Fig. 7B). An uplift rate of 0.7 mm=yr is inferred from the elevation of these notches and the assumption of C1 m sea level at ¾5500 yr B.P. (Fig. 8B). A higher notch at ¾6.5 m may also be Holocene. 5.10.4. Barrier islands of the Nggerasi and Marovo lagoons Barrier islands along the northeast coast of New Georgia Island isolate a 100-km-long lagoon that extends from the northern tip of New Georgia Island to Nggatokae Island (Fig. 7A). The widest, 5- to 20-km parts of the lagoon formed by the barrier reef are called the Nggerasi Lagoon to the northwest and the Marovo Lagoon to the southeast. The intervening area between the two lagoons is marked by an area where the barrier reef is as close as 1 km to the main island of New Georgia. Although data points are sparse, we suggest that this intervening area with a narrow lagoon may be undergoing slightly higher uplift (>0.7 mm=yr) than the wider adjacent areas of the Nggerasi and Marovo lagoons on either side. This area of slightly greater uplift is opposite the area of highest uplift on the southwest coast of New Georgia between localities 53 and 58 (Fig. 8B). 286 P. Mann et al. / Tectonophysics 295 (1998) 259–306 Fig. 10. Morphology of notches in late Quaternary limestone at Ararosa Passage between Petani and New Georgia (see Fig. 7 for location; locality 26 in Tables 1 and 2). Sample ARA-A collected at the base of the higher person’s feet yielded a 14 C age of 6430 š 70 years and confirms the presence of late Holocene shoreline features in this area uplifted at an average rate of 0.7 mm=yr. This uplift rate predicts that the notches are Holocene in age up to about 5 m or the level of the higher person’s head. Similarly on the southwest coast, the Marovo and Nono lagoons merge with the coastline in the area of higher uplift. The Nggerasi Lagoon and the adjacent barrier island of Ramata provide excellent examples of coastal geomorphology produced by the uplifting barrier reefs and islands that enclose the Nggerasi Lagoon in its widest part (Fig. 16). A dated sample (RAM-A-1) from Ramata Island and Holocene paleosea-level indicators confirm that late Holocene uplift rate of ¾0.7 mm=yr on the barrier islands (Fig. 8B). Uplift has raised Ramata Island to a total elevation ¾17 m ASL. P. Mann et al. / Tectonophysics 295 (1998) 259–306 287 Fig. 11. Morphology of notches at south end of Porepore Island on the Marovo Lagoon (see Fig. 7 for location; locality 38 in Table 2). The inferred uplift rate at this locality of 0.1 mm=yr (based on samples RAM-A-1 and NJA-A-3) from the Marovo Lagoon area) predicts Holocene features elevated to a height of 1.5 m at the base of the cliff. The higher, older notches above the Holocene notches at the base of the cliff exhibit solution effects expected from features pre-Holocene in age. Because of this process of accelerated uplift in the New Georgia Island Group, much older and weathered Quaternary corals and reefs are present only a few meters above Holocene ones. 5.10.5. Holocene notches of the Nggerasi and Marovo lagoons As in the case of the Roviana Lagoon, notches not directly contiguous to reef terraces are abundant on sea cliffs at several localities in the Marovo Lagoon and were used to estimate uplift rates. On the seaward edge of Porepore Island in the Marovo Lagoon where uplift is slower at about 0.1 mm=yr, one to three lower Holocene notches occur beneath two older and much more weathered pre-Holocene notches (Fig. 11). In some places, the Holocene notches are superimposed on older notches creating deep overhangs in the cliff face that are up to 6 m in depth. 288 P. Mann et al. / Tectonophysics 295 (1998) 259–306 5.11. Coastal geomorphology of the volcanic arc area of Kolombangara, Vella Lavella, and Ghizo The islands of Kolombangara, northwestern Vella Lavella, and Ghizo exhibit little or no Holocene uplift or subsidence (Fig. 8B). However, southeastern Vella Lavella exhibits an area of moderate uplift at rates of 1–2 mm=yr on the southeast flank of the Paraso rift. This uplift may reflect Holocene uplift of the southeastern footwall of that rift along the major scarps mapped by Grover (1965) and map NG 1 (Geological Survey Division, 1987) (Fig. 7A). 5.11.1. Coastal effects of the 1959 earthquake During the magnitude 7 earthquake of August 18, 1959, fissures and subsidence affected the area indicated in Fig. 8A on southern Vella Lavella and northernmost Ranongga in a direction parallel to the eastern boundary fault of the Paraso rift on Vella Lavella (Grover, 1965). We observed submerged trees, eroding beaches and other drowned coastline features along the southwestern projection of the Paraso rift faults at and near the village of Supato (locality 75 in Fig. 7B), and local witnesses confirmed to us coseismic subsidence in 1959. Similar features indicative of drowned coastlines were seen on the small islets (localities 21, 22) in Wilson Strait between Vella Lavella and Ranongga and along the coast of northwestern Vella Lavella (localities 77, 78, 79). According to Grover (1965), the main shock of the 1959 event originated west of Ranongga because small tsunamis approached Ranongga from the west. tween the stable northern edge of the outer forearc block and the uplifted area of New Georgia Island is abrupt: uplift rates on Rendova decrease to 0 at its northern coast rate while the barrier islands along the southern coast of New Georgia Island are uplifting at ¾1 mm=yr. We infer a submarine thrust fault at the northern margin of the syncline (Fig. 8A). This proposed thrust may account for the highest uplift rates of the volcanic arc block along the southern coastline of New Georgia Island and account for the merging of the Roviana, Nggerasi and Marovo barrier reefs with the mainland in this region (Fig. 8A). 6. Deformation and uplift of Plio–Pleistocene forearc basin sedimentary rocks 6.1. Relation between pre-Holocene uplift pattern and distribution of forearc basin sedimentary rocks A 100-km-long and 50-km-wide, discontinuous belt of deformed, marine sedimentary rocks crops out on the outer forearc islands of Tetepare, Rendova, and Ranongga (Fig. 12). We conducted parallel studies of the paleobathymetry and structure of these rocks in order to better understand the long-term cycles of subsidence and uplift inferred from the Holocene coral reef data. Holocene coral reef data in the outer forearc zone indicate that these marine sedimentary rocks are presently being uplifted at rates as high as 7.5 mm=yr. 5.12. Boundary between forearc and volcanic arc blocks in the New Georgia Island Group 6.2. Structure and stratigraphy of forearc basin rocks An intervening basinal low between the forearc and volcanic arc areas corresponds to the Blanche Channel syncline (Fig. 8A). This syncline separates the outer arc area characterized by hundreds to thousands of meters of uplift from the volcanic arc uplift of only a few tens of meters. The discontinuity be- 6.2.1. Geologic map data Previous structural information is reported by Dunkley (1986) and on the accompanying seven 1 : 100,000-scale, colored geologic map sheets (Geological Survey Division, 1987) (Fig. 7A). This report and maps represent a synthesis of a large and sys- Fig. 12. Relation between pre-Holocene uplift and distribution of Plio–Pleistocene forearc basin rocks examined in this study for their paleobathymetric information. Forearc basin rocks comprise two formations described in detail by Dunkley (1986) and Hughes et al. (1986) and crop out discontinuously on islands in the outer forearc area. The orientations of bedding and fold axial planes in these rocks compiled from Dunkley (1986) and this study are shown as plotted as poles to planes in lower hemisphere, equal area projections. P. Mann et al. / Tectonophysics 295 (1998) 259–306 289 290 P. Mann et al. / Tectonophysics 295 (1998) 259–306 tematic mapping effort in the New Georgia Islands that occupied Dunkley and four other MEWMR field geologists (A. Smith, D. Abraham, G.W. Hughes, and S. Booth) from 1979 to 1983. Using the pace and compass method, this group made geologic traverse and sample maps for almost all major stream and river valleys in the New Georgia Islands. These data are on file at the Geological Survey Division offices at the MEWMR in Honiara. Our own structural mapping effort in pre-Quaternary rocks was confined to much smaller areas: (1) northern coastal sections on Ghizo; (2) river sections on Ranongga (Lea, Bangu, Randoi, Kolomali), southern Rendova (Findovanga, Rano, Ozivudo) and Kiorosi River on Tetepare (Fig. 12). To obtain better structural control on events affecting the forearc sedimentary rocks, we have combined our own structural measurements in these rocks with those taken from the traverse maps of Ranongga, Ghizo, Rendova, and Tetepare which Mr. Donn Tolia, director of the Geological Survey Division, MEWMR, kindly allowed us to copy and use as part of this study. The maps were also useful in locating our biostratigraphic samples in the Kiorosi River of Tetepare with the samples described by Hughes et al. (1986). Combined measurements of 407 total bedding planes from the three islands and 64 fold axial planes from Tetepare Island are plotted as lower hemisphere, equal-area pole projections in Fig. 12 and are discussed below. All stratigraphic terminology used below is described in detail by Dunkley (1986). 6.2.2. Structure of Tetepare The island of Tetepare is composed almost entirely of Plio–Pleistocene, marine sedimentary rocks of the Tetepare Formation (Fig. 12). Mapping by Dunkley (1986) and lower hemisphere plots of poles to bedding shown in Fig. 12 indicate that these rocks are folded to a higher degree than the sedimentary sections on Ranongga and Rendova. The geologic map and cross-section of the entire island by Dunkley (1986) and the lower hemisphere plot of poles to Plio–Pleistocene bedding indicate Late Pleistocene folding of the entire island into a large anticline with a slightly steeper-dipping, southwestern limb. The steeper limb of the large anticline is not well represented on the plot of poles to bedding shown in Fig. 12 because the steeper limb crops out mainly along the rugged southern coast of the island. This coast is generally inaccessible for small boats and can only be reached by hiking across the island from the north coast. Smaller-scale folds mapped by ministry geologists are parasitic to this larger anticline and consist of a series of asymmetrical folds with short, sometimes vertical southwestern limbs and long gently dipping northeastern limbs. Minor thrust and reverse faults were observed by ministry geologists in most river sections and were mapped by us in three localities in the Kiorosi River section. 6.2.3. Stratigraphy of Tetepare Basement is limited to one exposure of a brecciated basalt in the central part of the island. Based on the presence of this outcrop and large basaltic boulders in many of the rivers, Dunkley (1986) speculates that an extensive basaltic basement similar to that on Rendova underlies most of Tetepare. The contact between the isolated basement outcrop and the overlying Tetepare Formation is faulted. A relatively undeformed, 800-m-thick section of the Tetepare Formation is well exposed in the Kiorosi River and has been measured and described by Hughes et al. (1986) and mapped and sampled by us (Fig. 13A). The section consists of thinly bedded, turbiditic sandstone and siltstone with subordinate mudstone, calcisiltite, and calcilutite. Relatively pure limestone is confined to the upper half of the formation with some beds up to 9 m in thickness. Chaotic slump breccias of debris-flow origin and Fig. 13. (A) Summary stratigraphic section of the Kiorosi River section on Tetepare Island from Hughes et al. (1986) showing the locations of their samples and samples analyzed by M.B. Lagoe for this study. On Tetepare, Rendova and Ranongga we focussed our biostratigraphic sampling on the major unconformity separating the deep marine sequence from an overlying shallow marine sequence. Samples collected immediately above and below this angular unconformity revealed a major shallowing event that was the harbinger of the present pattern of uplift of late Holocene reefs. (B) Summary stratigraphic section showing angular unconformity exposed in the Findovanga River on southern Renodva Island. (C) Summary stratigraphic section showing angular unconformity exposed in the Lea River on Ranongga Island. Locations of all three rivers are shown in Fig. 7B. P. Mann et al. / Tectonophysics 295 (1998) 259–306 291 292 P. Mann et al. / Tectonophysics 295 (1998) 259–306 south-verging slump folds in stratified, finer-grained rocks occur through the section and range in thickness from 1 to 126 m. Clasts are composed of the enclosing lithologies, exhibit fluidization structures formed in soft sediment, and are commonly a few tens of centimeters in diameter. Slump breccias in the lower part of the formation contain volcanic cobbles and pebbles and angular blocks of coral. Clasts of other lower crust and mantle rock types including pyroxenite and peridotite are common in the river bed and have not been seen by Dunkley (1986) in any other part of the New Georgia Islands. The section is rich in microfauna that is described in detail by Hughes et al. (1986) and described by M.B. Lagoe using samples collected by us (Fig. 13A). The lower two thirds of the section is Early Pleistocene in age and the upper third of the section is Late Pleistocene in age and containing zones commencing at 0.44 Ma and 0.27 Ma. Our sampling in the Kiorosi River on Tetepare as well as rivers on southern Rendova and Ranongga emphasized a major angular unconformity separating the deep marine rocks of the folded or tilted Plio– Pleistocene rocks from an overlying gently tilted to horizontal shallow marine section (Fig. 13A). 6.2.4. Structure of southern Rendova The southern half of Rendova is a plateau rising to 800 m with a lower elongate peninsula that is collinear with Tetepare (Fig. 12). The plateau is composed of an uplifted, unnamed forearc basement of volcanic rocks overlain by marine sedimentary rocks of the Tetepare Formation. The occurrence of basement rocks along the southern part of the island and the steady northward and northeastward dip of the overlying Plio–Pleistocene forearc basin section by 20–30º suggest that uplift of southern Rendova has been controlled by vertical movements along thrust or reverse faults in the coastal area or on the trench slope (Crook and Taylor, 1994). Lower hemisphere plots of poles to bedding of Plio– Pleistocene age show this tilting to the north and northeast (Fig. 12). 6.2.5. Stratigraphy of southern Rendova The basement rocks consist of pillow basalts with intercalations of marine, calcareous sandstone and siltstone in the upper part of the pile. The Tetepare Formation which is at least 170 m thick on Rendova consists of mainly interbedded sandstone and siltstone with intercalations of tuffs and volcanic sandstone and breccia. The base of the formation is taken as the highest pillow lava. Planktic foraminifera identifications by Dunkley (1986) placed an Early Pleistocene or latest Pliocene age on the formation. The rocks are uplifted as high as 800 m around the central part of the island where they are unconformably overlain by coral limestone not sampled in this study (Takeda, 1973). 6.2.6. Higher coral limestone of central Rendova The coral limestone on central Rendova is continuous from sea level up to about 600 m ASL (Geological Survey Division, 1987). We document Holocene uplift rates in this area are >5 mm=yr (Fig. 8B). At Asovo Point on the east coast of Rendova (localities 4 and 5, Fig. 7B), the Holocene reef reaches to at least 20 m with an uplift rate of ½3.5 mm=yr. A series of older terraces rises inland to an elevation of nearly 300 m. Although we did not date these older reefs, their continuity with the Holocene reef and their terraced morphology contrast with the isolated remnants of ancient coral limestone found at elevations of 40–80 m on western New Georgia Island. We infer that the higher coral limestone of central Rendova was uplifted hundreds of meters during the past 100–200 ka as recorded by the paleobathymetry of the underlying Plio–Pleistocene marine sedimentary rocks. The absence of coral limestone in some areas of Rendova is probably related to the steep coastal slopes and poor reef substrate provided by the easily erodible forearc basin sedimentary rocks combined with the extremely rapid passage of the substrate through the zone of coral reef growth. 6.2.7. Structure of Ranongga Plio–Pleistocene bedding of the Tuara Formation of Ranongga is tilted to the east-northeast (Fig. 12). This pattern of tilting exposes basement rocks along the southwest and south coasts of the island. The occurrence of basement rocks along the western part of the island and the steady eastward and northeastward dip of the overlying Plio–Pleistocene forearc basin section by 20–30º suggest that uplift of Ranongga has been controlled by vertical movements along strike-slip faults of the Simbo ridge (Crook P. Mann et al. / Tectonophysics 295 (1998) 259–306 and Taylor, 1994). Oblique subduction of the Simbo ridge in a northeasterly direction beneath Ranongga may also contribute to the island’s uplift. 6.2.8. Stratigraphy of Ranongga Lavas, breccias, tuffs, and hyaloclastites of the Kela Formation of Mio–Pliocene age form the basement of the forearc sedimentary succession on Ranongga. These more resistant basement rocks form high, rugged ground in the southern and central parts of the island and are named after the highest point on the island, Mount Kela, which rises to 860 m (Dunkley, 1986). The volcanic basement of Ranongga has undergone low-grade metamorphism, and is the only basement in the New Georgia group that show signs of regional metamorphism. Forearc sedimentary rocks of the Tuara Formation occupy the northern third of the island and also occur in a narrow strip down the eastern coast of the island (Fig. 12). The Tuara Formation is subdivided into two conformable marine sedimentary members, the Vori Member and Lea Member, which have maximum thicknesses of 1400 and 500 m, respectively (Dunkley, 1986). Contacts between these rocks and the underlying Kela Formation are frequently faulted, although, where undeformed, the contact between the two formations is gradational with the Vori Member being marked by the appearance of sandstones within the volcanic rocks. Volcanic rocks decrease upward to the top of the Vori Member which is marked by the last volcanic bed. The overlying Lea Member of thinly bedded, calcareous sandstone and siltstone of turbiditic character is devoid of volcanic intercalations. Microfauna is common within both the sandy and silty beds. Planktic foraminifera identifications by Dunkley (1986) placed a Late Pliocene (N21) age for the base of the Vori Member and a definite Pleistocene age for the upper part of the Lea Member cropping out near the coast. 293 Paleobathymetric estimates are based on three different lines of evidence: (1) benthic foraminiferal biofacies; (2) planktic foraminiferal abundance; and (3) carbonate sediment distribution. Relationships among these three types of information form the paleobathymetric model used in this study (Fig. 14). 6.3.2. Benthic foraminiferal biofacies Benthic foraminiferal biofacies have long been used to estimate paleobathymetry and have proved extremely useful in tectonic studies. Because benthic foraminiferal distributions are largely correlated to water mass changes in the modern ocean, and these water masses vary both geographically and temporally, it is important to calibrate the paleobathymetric model to regional oceanographic conditions. For the western equatorial Pacific Ocean, data were compiled from Frerichs (1970), Biswas (1976), Burke (1981), Hughes et al. (1986), Resig (1986, 1989), Van Marle et al. (1987), Hughes (1989), and De Smet et al. (1990) to establish a series of paleo-bathymetrically significant, benthic foraminiferal biofacies. 6.3. Paleobathymetry of forearc basin rocks 6.3.3. Planktic foraminifera The abundance of planktic foraminifera in marine sediments can give information about paleobathymetry as well (Grimsdale and van Markhoven, 1955; Wright, 1977; Gibson, 1989). Planktic foraminifera are rare or absent in very shallow marine coastal waters, increase rapidly with depth across marine shelves and dominate (>90% of the total marine fauna) in deeper slope and deep-sea environments (above the calcite compensation depth). These types of relationships have been used to estimate paleobathymetry in the western equatorial Pacific Ocean by Van Marle et al. (1987). A typical planktic foraminiferal abundance vs. depth curve for this region (from Van Marle et al., 1987) serves as a valuable adjunct to the benthic foraminiferal biofacies for estimating paleobathymetry. 6.3.1. Methods Several samples from the forearc section were collected to determine paleobathymetric history and the results of micropaleontological analysis by M.B. Lagoe on these samples is reported below and shown in Fig. 13. 6.3.4. Carbonate distribution The distribution of carbonate sediments is the final relationship used in the paleobathymetric model. Two main aspects are used: (1) carbonate dissolution relationships and (2) the distribution of specific types of carbonate allochems. Calcite and aragonite have 294 P. Mann et al. / Tectonophysics 295 (1998) 259–306 Fig. 14. Paleobathymetric histories for islands in the forearc area of the New Georgia Island Group (cf. Fig. 12 for locations). (A) Tetepare Island. (B) Southern Rendova Island. (C) Ranongga Island. Paleobathymetry is based on the oceanographic model discussed in the text and the results of analysis of samples by M.B. Lagoe in the immediate area of the unconformity at the top of the deep marine section. The longer-term Plio–Pleistocene depth history of Tetepare Island is from Hughes et al. (1986). The Holocene uplift history is based on coral reef studies summarized in Fig. 8B. P. Mann et al. / Tectonophysics 295 (1998) 259–306 295 variable dissolution vs. depth relationships in various parts of the ocean (e.g. Milliman, 1974; Berger et al., 1976). In the western equatorial Pacific Ocean, the aragonite compensation depth (ACD) is approximately 500 m, while the calcite compensation depth (CCD) is 4500 m. The distribution of aragonite in the Solomon Islands samples is particularly important. The distribution of carbonate allochems (sand-sized biogenic particles) is most useful in determining shallow marine (shelf, lagoon) from deep marine (slope and deeper environments) sediments. Deep marine carbonate sediments are composed predominantly of foraminiferal tests and calcareous nannoplankton ooze. Shallow marine carbonates in this area contain a diverse mix of allochems including coral fragments, bivalves, gastropods, alcyonarian spicules, echinoid spines, holothurian sclerites, bryozoans, and ostracodes. inantly of foraminifera, with the finer matrix containing abundant calcareous nannoplankton. Above the unconformity the sediments are significantly different (Fig. 13A). The benthic foraminiferal biofacies are upper bathyal with admixed neritic species. Planktic foraminifera are less common than below (68–78% of the total assemblage) and the planktic assemblage also contains common pteropods. The presence of pteropods is significant because pteropods are aragonitic and must be deposited above the aragonite compensation depth (<ca. 500 m). Also present is a diverse assemblage of carbonate allochems including benthic gastropods, bivalves, ostracodes and occasional sponge spicules, alcyonarian spicules and echinoid spines. All of these relationships are consistent with an upper slope environment near a shallow marine shelf or lagoon and a water depth of 150–400 m. 6.3.5. Previous study of paleobathymetry of Plio–Pleistocene sedimentary rocks on Tetepare Island Hughes et al. (1986) provide an overview of the paleobathymetry of the sediments on Tetepare Island, based primarily on benthic foraminiferal biofacies. They also give biostratigraphic ages based on planktic foraminifera and calcareous nannoplankton. They document a largely deep water, but shoaling upwards history for these sediments. Our sampling on Tetepare supplements that of Hughes et al. (1986) and concentrates on a prominent unconformity near the top of the section (Fig. 13A). This unconformity was present and sampled on Tetepare, Rendova and Ranongga Islands. 6.4.2. Rendova Island On Rendova Island the stratigraphy bracketing the unconformity is similar to that of Tetepare (Fig. 13B). Sediments below the unconformity are characterized by lower bathyal benthic foraminiferal biofacies, high abundance of planktic foraminifera (94% of the total assemblage) and carbonate allochems dominated by foraminifera. Above the unconformity sediments contain outer neritic benthic foraminiferal biofacies, only moderately abundant planktic foraminifera (20% of the total assemblage), pteropods and diverse carbonate allochems. The latter include benthic gastropods, bivalves alcyonarian spicules, echinoid spines, sponge spicules, holothurian sclerites, crab remains, ostracodes, bryozoans and coral fragments. An outer shelf to shelfedge environment (50–150 m water depth) is indicated. 6.4.1. Tetepare Island This island has the longest stratigraphic record studied here (Fig. 13A). Based on a reanalysis of the Hughes et al. (1986) and our own data, the sediments below the unconformity gradually shallow from approximately 2500 m to 1500 m (Fig. 13A). These sediments contain deep-water benthic foraminifera (upper abyssal to lower bathyal biofacies), very abundant planktic foraminifera (96–98% of the total assemblage), and allochems consisting predom- 6.4.3. Ranongga Island The stratigraphic relations across the unconformity on Ranongga Island appear to be different, although this relationship is only based on one sample above and one sample just below the unconformity (Fig. 13C). Sediments below the unconformity are similar to those at Rendova and Tetepare Island and represent a lower bathyal environment. The sediments above the unconformity are similar to those below, instead of indicating a shallower environment 6.4. Paleobathymetric results of this study 296 P. Mann et al. / Tectonophysics 295 (1998) 259–306 as at the other two islands. They too represent a lower bathyal environment with appropriate benthic foraminiferal biofacies, high planktic foraminiferal abundance (99% of the total assemblage) and low carbonate allochem diversity, mostly foraminifera. 6.4.4. Paleobathymetric comparison between islands A paleobathymetric history of the three islands based on integration of micropaleontologic (Fig. 13) and coral reef age data (Fig. 8B) is presented in Fig. 14. On the islands Tetepare and Rendova, micropaleontologic evidence indicate significant uplift across the unconformity with estimates ranging from 1000 m on Tetepare and 1350 m on Rendova. Both uplift amounts are minima, because the values are based on present sea level rather than the topographic elevations of the individual samples that range from sea level to about 250 m. At Ranongga Island there is no evidence of uplift except the unconformity itself. Some uplift probably occurred to form the unconformity but it was not enough to change the benthic foraminiferal biofacies or other paleobathymetric indicators used here. In the following discussion, we integrate this earlier subsidence and uplift history with the uplift history known from our study of Holocene reefs overlying these older sedimentary rocks. 7. Discussion 7.1. Uplift models to explain Holocene uplift rates and coastal geomorphology of the New Georgia Islands Inferences based on established knowledge of global sea-level history are needed to reconstruct the late Quaternary subsidence and uplift history of the New Georgia Islands. Two uplift models are graphically shown in Fig. 15 and their implications are contrasted and compared below. 7.1.1. Constant uplift model Fig. 15A graphically illustrates a constant uplift model for the New Georgia Islands by assuming an average uplift rate of 0.5 mm=yr throughout the late Quaternary (Fig. 15A). There are two major problems with this constant uplift model: (1) the large-scale coastal geomorphology of the New Georgia Group with its outer barrier islands suggests a pre-Holocene history dominated by subsidence, not uplift (Fig. 8B); and (2) constantly uplifted reefs formed during sea-level high stands at 200 ka, 125 ka, 105 ka, and 80 ka would occur at elevations of approximately 100 m, 70 m, 38 m, and 25 m, respectively (Fig. 15A). We did not observe such a sequence of terraces anywhere along the New Georgia volcanic arc. Instead, we observe that the highest coral limestone in inland areas is less than 20–30 m ALC even in areas where the adjacent Holocene reef is uplifting at a rate of ½1 mm=yr (Fig. 8A). Therefore, we do not favor a constant uplift model because of the presence of the outer barrier reefs and because there is little late Quaternary limestone present above elevations of 30 m. Two exceptional areas are the higher coral limestone mapped of the Nggatoke block at the eastern end of New Georgia Island and the limestone plateau on southern Rendova Island (Fig. 8A). 7.1.2. Subsidence–uplift model This alternative model recognizes that (1) the outer barrier islands formed on a subsiding volcanic substrate (Fig. 4B), and (2) that this phase of subsidence ceased about 100,000 years ago and was replaced by uplift (Fig. 8B). In this model, shown graphically in Fig. 15B, Quaternary subsidence lowers the 200 ka and earlier reefs to allow the 125 ka and younger reefs to grow on top of them. The 5c and 5a paleosea levels are represented only by solution notches on the nearly vertical reef wall constructed by the older, aggrading transgressive reef sequence. Subsequent late Quaternary uplift required by Holocene uplift data shown in Fig. 8B would raise the shoreline features to elevations above sea level, but to much lower elevations than predicted by the constant uplift model (Fig. 15A). An important point is that ¾25% of the total uplift of the New Georgia barrier reef system is recorded by the midHolocene reef alone since about 5.5 ka. We prefer the subsidence–uplift model because it explains two important characteristics of the New Georgia Islands: (1) the formation of the outer barrier islands (Fig. 4); and (2) the lack of Late Pleistocene coral limestone higher than about 30 m in most areas of the New Georgia Island Group (Fig. 8A). P. Mann et al. / Tectonophysics 295 (1998) 259–306 297 Fig. 15. (A) Constant uplift model for late Quaternary coastal features of the New Georgia Group. Sea level curve in left box is from Chappell and Shackleton (1986). (B) Subsidence–uplift model favored by us to explain the vertical tectonism in this region. See text for discussion. Another possible scenario for the vertical tectonic history of the New Georgia Islands not illustrated in Fig. 15 is cessation of subsidence around 200 ka so that reefs of this age would be predicted to form the New Georgia barrier reef. This model would also predict that the pre-Holocene emerged notches 298 P. Mann et al. / Tectonophysics 295 (1998) 259–306 could have formed only during the 5e sea level. This model does not explain our observations because, if Holocene uplift since ¾5.5 ka is subtracted from the heights of the notches, the hypothetical 5e notches would occur at or below present sea level. We know that the 5e sea level was ¾6 m higher than the present sea level so that a 5e notch would occur at C6 m even without uplift. In summary, the subsidence–uplift model illustrated in Fig. 15B is the model we favor because it provides the simplest possible explanation that is consistent with our observations and established sea-level history. Unfortunately, we can not precisely specify the time when uplift of the volcanic arc began except that it was before 5.5 ka, but probably near the time or after cutting of the presumed 5a and 5c notches at 80 ka and 105 ka. 7.2. Integration of Plio–Pleistocene sedimentary and late Quaternary coral reef to constrain the cycle of subsidence and uplift in the New Georgia Group In the forearc area, combination of both data sets indicates that four distinct vertical tectonic phases affected the area as seen in Fig. 16B: (1) subsidence of the outer forearc region (Tetepare and Rendova islands) to depths of ¾1500 m and deposition of marine turbidites until after 270 ka; (2) uplift to sea level and erosion of an unconformity (Fig. 13); this uplift phase produced a major shallowing of 1000 to 1350 m on Ranongga and Tetepare and an insignificant shallowing on Ranongga (Fig. 14); (3) subsidence to ¾500 m BSL and deposition of bathyal sediments; and (4) uplift above sea level with Holocene uplift rates up to at least 7.5 mm=yr on Tetepare and 5 mm=yr on Rendova (Fig. 8B). In the volcanic arc area, two distinct phases can be seen: (1) subsidence >300 m of the volcanic arc (New Georgia and Vangunu Islands) occurred until about 100 ka and led to the development of barrier reefs by the model shown in Fig. 4B; (2) uplift of 0–30 m replaced subsidence about 50–100 ka and accelerated to Holocene rates of ¾1 mm=yr with ¾25% of total uplift occurring since 5.5 ka. These rates are much slower than the maximum rates of ¾8 mm=yr observed in the outer forearc area of Rendova and Tetepare (Fig. 8B). Outer arc subsidence occurred after subsidence ceased at 270 ka, but <5% of total late Quaternary uplift is Holocene. Older reefs on Rendova are continuous with the Holocene reef and the paleobathymetry indicates hundreds of meters of net uplift compared with only tens of meters of net uplift of New Georgia Island and Vangunu. About 25% of the uplift of the volcanic arc area of New Georgia Island and Vangunu has occurred just since 5.5 ka. The high terraces up to hundreds of meters on Rendova (Fig. 8A) indicate that the uplift of the outer forearc area began much earlier than that on the volcanic arc. This period of uplift probably began by ¾125 ka. 7.3. Requirements of regional, tectonic mechanisms to explain observed subsidence and uplift pattern in the New Georgia Island Group We seek a regional uplift mechanism that (1) allows regional subsidence prior to regional uplift, (2) initiates uplift over a large part of the forearc area by ¾100–200 ka at rates up to nearly 10 mm=yr, (3) causes nearly simultaneous cessation of subsidence of volcanic arc area over an area of 130 ð 50 km, (4) explains nearly simultaneous uplift of the entire volcanic arc at Holocene rates as great as 1–2 mm=yr (the present pattern of uplift shows three large areas of uplift with rates greater than 0.5 mm=yr; two areas are elongate in a direction parallel to the trench in New Georgia Island and Rendova and Tetepare and one are is elongate in a direction at a high angle to the trench on Ranongga; Fig. 16B), and (5) explains the pattern of subsidence followed by uplift in an obliquely convergent tectonic setting involving the subduction of a recently active spreading ridge segment capped by the high relief (>2 km) Coleman seamount flanked by young oceanic crust ranging in age from 0 to 3 Ma (Fig. 16A). 7.4. Unlikely tectonic mechanisms at the observed time scales Uplift mechanisms such as thermal effects due to subduction of spreading ridges, tectonic erosion or underplating, and isostatic adjustments to subducted bathymetric features are not likely to function in the 270-ka period that these uplift events have occurred in the New Georgia Island Group. Total convergence of the Woodlark basin oceanic crust with the P. Mann et al. / Tectonophysics 295 (1998) 259–306 299 Fig. 16. (A) Horizontal projection of subducted oceanic crust of the Woodlark spreading center assuming the same plate motions as today and that the subducted crust is a mirror image of the unsubducted crust. Magnetic anomalies are from Taylor (1987). For simplicity, we assume that the dip of the slab beneath the forearc area is horizontal. We know from the depth of hypocenters shown in Fig. 3B and from earthquake studies by Cooper and Taylor (1985) that the Benioff zone dips at an average angle of 45º to the northwest. (B) Projection of subducted oceanic crust beneath the New Georgia Islands assuming an average Benioff zone dip of 45º beneath the volcanic arc area and the New Georgia Sound. The 50-isobath of the Benioff zone is from Cooper and Taylor (1985) and the areas of Holocene uplift in gray are from this study. See text for discussion. 300 P. Mann et al. / Tectonophysics 295 (1998) 259–306 Solomon island arc was only about 20 km at the New Georgia Island Group since 200 ka (Fig. 16B). Given the oblique direction of convergence (Fig. 16A), there has been only ¾7 km of subduction in the direction normal to the trend of the arc. Isostatic response of the volcanic arc area of New Georgia Island and the forearc area of Rendova and Tetepare to a subducted bathymetric feature is not a satisfactory explanation for our observations because there has not been sufficient time to subduct a bathymetric feature like the Coleman seamount beneath the arc to produce the observed reversal from subsidence to accelerating uplift that has affected both the forearc and volcanic arc areas (Fig. 16B). The 60 by 30 km area of the New Georgia forearc began uplifting about 100 to 200 ka ago, yet during this time only about 19 km of the Woodlark basin crust has been subducted beneath the forearc in the direction of convergence. Moreover, the direction of subduction was oblique to the forearc and the zone of maximum Holocene uplift (Fig. 8B). Another possibility is that the subducted Woodlark plate was subjected to a ‘slab pull’ force that broke the plate along a line of weakness such as those provided by either subducted spreading ridge or fracture zone boundaries (Fig. 16A). Slab pull forces are probably minimal because the oldest part of the subducted slab is only a few million years old and extends only to depths of about 70 km (Fig. 3B). The Woodlark spreading system is likely being forcibly thrust beneath the arc by convergent relative motion between the Australia and Pacific plates, which is driven on a regional scale by slab pull (Fig. 1). 7.5. Proposed tectonic mechanism for regional deformation Far-field deformation of the arc crust by horizontal forces imparted by the initial subduction of the Coleman seamount is our preferred mechanism for the widespread, rapid, and oscillatory nature of vertical movements in the New Georgia Island Group. Such a mechanism implies that the arc crust should display large-wavelength deformation as suggested by regional variations in reef terrace elevations (Fig. 16B). This model of regional crustal shortening related to the impingement of the Coleman seamount is shown in Fig. 17, a 12-times, vertically exaggerated, regional cross-section through the Coleman seamount and the New Georgia forearc and volcanic arc area (line of section shown in Fig. 16B). The main tectonic features on the section in Fig. 17 include the following. (1) The Coleman seamount above the Ghizo ridge with 2.5 km of bathymetric relief relative to the surrounding 0 to 1.6 Ma oceanic seafloor of the Woodlark basin (Fig. 17). The seamount is obliquely subducting beneath the forearc along the San Cristobal trench at a rate of ¾97 mm=yr (Fig. 8A). The base of the seamount beneath the overriding forearc is projected on the cross-section using the shape of the unsubducted seamount mapped in detail using high-resolution sidescan mapping techniques (Crook and Taylor, 1994). Normal faults on the north flank of the seamount indicate that it is flexing and dismembering along trench-parallel normal faults as it enters the subduction zone. (2) The subducted slab of the Woodlark basin is poorly known at shallow (<25 km) depths and is not shown on the cross-section of Fig. 17. We assume a dip of about 20º in the upper 25 km of the subduction zone as inferred from the sparse seismic data plotted in Fig. 3B. We have shown the horizontal projection of a slab with a 20º dip on a map of the New Georgia Island Group in the map shown in Fig. 16B. On this projection and on the regional map of Fig. 16A, we assume that the subducted part of the plate exhibits a symmetry across its spreading ridge with the unsubducted part of the plate mapped by Taylor (1987) in the Woodlark basin (Fig. 16A). The horizontal projection of a subducted slab dipping 20º in Fig. 16B indicates the possible presence of fracture zones and subducted and extinct spreading ridges beneath the eastern part of the New Georgia Island Group. (3) The steep trench slope of the New Georgia Island Group separating the San Cristobal trench from the rapidly uplifting islands of Tetepare and southern Rendova in the forearc area (Fig. 17). The bathymetric slope of the trench slope is roughly the same as that of the Coleman seamount and the summit of the Coleman seamount extends more than half way up the trench slope. We infer two thrust faults on the line of section in Fig. 17. A lower thrust fault is inferred at the top of a small P. Mann et al. / Tectonophysics 295 (1998) 259–306 301 Fig. 17. Holocene uplift rates compared with the topographic profile across the downgoing Coleman seamount on the Ghizo ridge and the overriding New Georgia island arc (line of section shown in Fig. 16B). Subduction of the Coleman seamount at the San Cristobal trench accounts for the observed pattern of late Holocene uplift and the convergent structures affecting forearc rocks on Rendova and Tetepare. The folded structure of the arc at depth has not been observed on the seismic reflection profile but is inferred from the pattern of deformation in the Holocene reef and the underlying forearc basin section (cf. Fig. 12). high at the base of the slope that is possibly a small accretionary prism that would be expected in this sediment-starved environment. A higher thrust fault is inferred near the top of the trench slope to explain the elongate uplift pattern of the forearc area (Fig. 17). Single-channel seismic profiling by Taylor (1987) and sidescan mapping by Crook and Taylor (1994) have shown that the trench slope is largely devoid of sedimentary cover and probably represents the same type of crystalline igneous rocks exposed on Tetepare and southern Rendova (Fig. 8B). (4) Forearc high and islands at the crest of the forearc high. The line of section in Fig. 17 passes through two highs in the forearc area. The southern high is a zone of folded Plio–Pleistocene sedimentary rocks of the Tetepare Formation which exhibit small-scale fold axes dipping to the northeast. To the east of the area of the line of section on the eastern continuation of this high on Tetepare, a larger-scale and more strongly overturned fold structure is present in Plio–Pleistocene sedimentary rocks (Fig. 12). Based on this surficial pattern of folding, we infer folds present in the upper crust to a depth of several kilometers and exhibiting a slight southwest vergence over the impinging Coleman seamount. The sedimentary rocks exposed on this high were deposited between about 1.9 Ma and 270 ka, uplifted and eroded and finally subsided to bathyal depths. On Tetepare and Rendova there is micropaleontologic evidence for significant (1000–1350 m) uplift across the unconformity (Fig. 14). These folded sedimentary rocks are now unconformably overlain by Holocene coral reefs that have themselves been uplifted at rates up to 7.5 mm=yr (Fig. 8B). The northern high of the forearc area on the cross-section in Fig. 17 is the Rendova stratovolcano which formed in late Neogene time and reaches a present topographic elevation of 2600 m. (5) Forearc low (Blanche Channel) between the forearc high and the volcanic arc. This low is the topographic expression of the Blanche Channel syncline defined by northerly dips of Plio–Pleistocene rocks on Tetepare and Rendova and imaged on single-channel seismic lines by Taylor (1987). We infer 302 P. Mann et al. / Tectonophysics 295 (1998) 259–306 a northeast-dipping thrust fault on the northern edge of the syncline based on the rapid uplift of the southwest coast of New Georgia Island (Fig. 8B). (6) Volcanic arc high (New Georgia Island). This high is the topographic expression of several coalesced stratovolcanoes that make up New Georgia Island. We infer the presence of a large anticline beneath the island based on the regional dips inferred in the forearc area and the Blanche Channel syncline. No stratal dip data are available from this area of the New Georgia Islands because sedimentary rocks are covered by younger, massive volcanic flows. Based on the cross-sectional distribution of these features, we propose that impingement of the Coleman seamount and the additional horizontal force applied to the overriding forearc are responsible for large-scale folding of the forearc and volcanic arc crust and that this folding is in turn responsible for the observed pattern of Holocene uplift (Fig. 16B). The area of highest Holocene uplift on Tetepare is aligned along the convergence vector of the impinging seamount (Fig. 16B). We propose that the subduction decollement is reshaping itself around the 2.5 km high asperity of the Coleman seamount with an accompanying increase in the elevation of the outer forearc region. It is this ‘reshaping’ process and accompanying shortening of the overriding plate that is concentrating uplift in the Tetepare and Rendova areas (Fig. 8B). In the absence of a sedimentary accretionary wedge, hard rock contacts lacking sediment lubrication form between the downgoing seafloor topography and volcanic and plutonic rocks of the forearc. Cloos (1993) predicts that in such a situation only small earthquakes would be generated. In contrast, Scholz and Small (1997) observe large aseismically, locked zones in western Pacific arcs undergoing seamount subduction. The present relative quiescence in large earthquakes along this segment of the trench could support either of these viewpoints, although we favor the Scholz and Small (1997) view that the area may be a locked zone characterized by large and infrequent events. The differently oriented Holocene uplift of the Ranongga area may involve a similar fold mechanism related to the impingement of the Simbo ridge on the trench slope of the arc (Fig. 24B). The northern prolongation of this Holocene uplift on the island of Vella Lavella may reflect rift flank uplift asso- ciated with the northeast-trending rift crossing that island. We expect that this folding=uplift process will continue to affect an ever widening area in the Tetepare and Rendova area and probably accelerate through time as more and more surface area of the Coleman seamount is coupled with the overriding trench slope area (Fig. 17). As the seafloor topography penetrates more deeply into the thicker parts of the upper plate wedge, a zone of subhorizontal contraction would project arcward from the high and apply greater and more extensive forces that would result in upper plate flexure and more distant uplift events such as the type seen on the volcanic arc part of the New Georgia Island Group today. Large-scale folding is probably accompanied by localized thrust faulting on the trench slope, in the Blanche Channel syncline and possibly in the ‘back-arc’ area to the northeast of New Georgia Island in the New Georgia Sound (Fig. 8B). These thrust faults may account for rapid changes in uplift rate that are commonly manifested as shorelines, such as the shoreline between southern Rendova–Tetepare and the trench slope and the shoreline between New Georgia Island and the Blanche Channel syncline. We are less confident about proposing previous cycles of uplift and subsidence related to other seamounts probably subducted with the downdip extension of the Ghizo ridge (Fig. 16A). It is possible that the previous subsidence cycle from about 1.9 Ma to 2.7 ka in the New Georgia Island Group (Fig. 14) is in part related to the collapse of the forearc following the passage of a seamount similar to the Coleman seamount and possibly overlying the Woodlark spreading center (Fig. 16A). Certainly, other seamounts may have formed along the Woodlark ridge segments now subducted (Fig. 16B) and this uplift–collapse cycle has been well documented from other active (e.g., von Huene and Lallemand, 1990) and ancient (Wagreich, 1995) subduction margins. Seafloor-related topographic highs like the Coleman seamount may eventually become ‘decapitated’ with accompanying large earthquakes and subsequently accrete to the overlying forearc plate (Cloos, 1993). Decapitation of the subducted high would produce a subplanar thrust surface that offers less resistance to subduction and regional subhorizontal contraction might cease. P. Mann et al. / Tectonophysics 295 (1998) 259–306 8. Conclusions The New Georgia Island Group of the Solomon Islands is one of four places where previous workers have proposed that a spreading ridge is actively subducting beneath an island arc. We have dated coral reef terraces of known elevation, dated and determined paleobathymetry of late Cenozoic sedimentary rocks. These results are integrated with existing marine geophysical data to constrain patterns of regional Quaternary deformation related to subduction of the Ghizo ridge, a formerly active segment of the Woodlark spreading center and its overlying Coleman seamount. Our main conclusions include the following. (1) Highest Holocene uplift rates based on radiocarbon coral ages range from 3 to 7.5 mm=yr and coincide with two areas: an outer forearc area of arcward tilting adjacent to the point where the Ghizo ridge is being subducted (Tetepare and southern Rendova) and an outer forearc area, eastward- and northeastward-tilting away from the Simbo ridge, an active, right-lateral transform fault (Ranongga Island) (Fig. 8B). (2) The uplift pattern of the outer forearc area is centered on the Coleman seamount on the subducting plate that has bathymetric relief up to 2000 m and is currently impinging on the trench slope adjacent to Tetepare and southern Rendova (Fig. 8B). Previously subducted parts of the Ghizo ridge or other ridge-related bathymetric features may also be strongly pressing against the arc crust of the overriding plate (Fig. 16). Uplift in this and other ridge subduction settings may relate more to the coupling between high-standing ridge topography and ridgerelated seamounts than to thermal, compositional, or other properties of the subducting spreading ridge itself. Our data do not bear directly on the question of whether the Ghizo Ridge on the downgoing plate is actively spreading, has become extinct, and=or has converted to a zone of strike-slip faulting (Crook and Taylor, 1994). (3) The volcanic arc of the New Georgia Islands adjacent to the rapidly uplifting forearc area is characterized by an unusual Quaternary coral reef morphostratigraphy consisting of emergent barrier reefs enclosing broad lagoons and barrier reefs formed during subsidence of the volcanic arc (Fig. 4). Ra- 303 diocarbon ages document a modest late Quaternary to present pulse of uplift of the volcanic arc probably related to the latest phase of subduction of seamounts beneath the forearc area (Fig. 8A). The presence of early Holocene and pre-Holocene notches at 4–6 m ASL within this area is consistent with a late Quaternary pulse of uplift (Figs. 10 and 11). The lack of Quaternary coral limestone above an elevation of about 80 m ASL suggests that the present period of late Quaternary uplift was preceded by a period of earlier subsidence that drowned shoreline features (Fig. 14). (4) Paleobathymetric data from the outer forearc high of the New Georgia Island Group document a series of vertical movements: subsidence; uplift to sea level after 270 ka; subsidence to ¾500 m; and then hundreds of meters of uplift that continues to the present (Fig. 14). Present-day uplift is demonstrated by the continuous series of uplifted coral reef terraces on Rendova (Fig. 8A) and the high level of mid-Holocene coral reefs on southern Rendova and Tetepare (Fig. 8B). These reefs demonstrate that uplift began earlier on the outer forearc (perhaps by 200 ka or at least by 100 ka) than on the volcanic arc and that the amplitude of uplift is hundreds of meters compared to a maximum of a few meters to tens of meters on the volcanic arc. (5) A basin-forming period of subsidence of many hundreds of meters affected the volcanic arc in the New Georgia Island Group. This subsidence was in part coeval across the arc. Subsidence changed to uplift progressively from the outer forearc to the volcanic arc (Fig. 17). We relate the intiation and continuation of uplift to the approach of the Coleman seamount and=or a recently subducted bathymetric feature similar to the Coleman seamount (Fig. 8B). (6) Accelerating uplift of the New Georgia Island Group is attributed to a predicted exponential increase in physical contact and coupling between the subducting or subducted topographic high and the thinly-sedimented trench slope of the forearc. We propose that the outer forearc area deforms and shortens by sub-horizontal compression in the manner shown in Fig. 17. With continued collision, horizontal shortening effects propagate arcward in map view in a bullseye-shaped pattern centered on the point of initial contact between the seamount and trench slope (Fig. 8B and Fig. 12). 304 P. Mann et al. / Tectonophysics 295 (1998) 259–306 (7) The earlier subsidence of hundreds to thousands of meters that affected the New Georgia Island Group may have been the aftermath of a previous subduction cycle involving a large bathymetric high that was perhaps similar in relief to the now-subducting Coleman seamount (Fig. 17). This feature may have ceased to couple strongly to the forearc as a result of its “beheading” and its subsequent transfer from the subducting plate to the overriding forearc. The seamount “beheading” process may have resulted in a relaxation of compressive forces acting on the forearc: and its subsequent collapse and subsidence by the observed amount of about 1500 m (Fig. 14). (8) The cycle of vertical uplift and subsidence described above is characterized by a present-day phase of strong coupling and uplift related to the Coleman seamount and=or a similar recently subducted feature (Fig. 17). We propose that this present-day period of strong coupling=uplift will eventually terminate by seamount “beheading”, relaxation of compressive stresses acting on the forearc, subsidence, and forearc basin sedimentation. Forearc subsidence and basin sedimentation may persist until the next difficult-to-subduct part of the incoming plate arrives at the trench. (9) The horizontal forces due to mechanical resistance to subducting rugged ridge and seamount topography may have terminated spreading of the Woodlark spreading center entering the trench (Ghizo ridge) and converted it to a presently active strike-slip zone. Acknowledgements We dedicate this work to our friend and colleague, Martin B. Lagoe (1950–1995). We greatly appreciate the logistic and scientific support of the Ministry of Energy, Water, and Mineral Resources (MEWMR) of the Solomon Islands. 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