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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, B06302, doi:10.1029/2003JB002790, 2004 Continental crust under compression: A seismic refraction study of South Island Geophysical Transect I, South Island, New Zealand Harm J. A. Van Avendonk,1,2 W. Steven Holbrook,1 David Okaya,3 Jeffrey K. Austin,1,4 Fred Davey,5 and Tim Stern6 Received 9 September 2003; revised 27 February 2004; accepted 13 April 2004; published 15 June 2004. [1] The 1996 South Island Geophysical Transect (SIGHT) active source seismic survey was designed to show the style of lithospheric thickening due to late Cenozoic oblique convergence across the Australian-Pacific plate boundary in New Zealand. As part of this study, two seismic refraction lines were shot across central South Island and offshore extensions of the continental crust in the Tasman Sea and Pacific Ocean. We present the data and a 603 km long seismic velocity profile of the crust and uppermost mantle along one of these seismic transects. A tomographic inversion of 62,563 travel times from crustal and upper mantle refractions and wide-angle reflections resulted in a model with a two-layer crust. Upper crustal velocities were between 5.9 and 6.3 km/s, and lower crustal velocities were between 6.5 and 7.0 km/s. Continental compression has locally reduced the seismic velocities in the Pacific plate crust by 0.2–0.3 km/s, a possible effect of high strain and fluids in the crust. The thickening of the crust from 28 km at the east coast of South Island to 37 km beneath the Southern Alps can account for about 25% of the 80–110 km shortening of Pacific plate crust, while the rest must be accounted for by rapid erosion of Mesozoic sedimentary rocks on the west side of the orogen. In our model the lower crust forms a continuous 2–6 km thick layer beneath central South Island. The asymmetric topography of the Southern Alps is reflected in the crustal root which has a steeper flank at the west coast. This observation is consistent with westward underthrusting of Pacific lower lithosphere INDEX TERMS: 3025 Marine beneath South Island that has been suggested in earlier studies. Geology and Geophysics: Marine seismics (0935); 8102 Tectonophysics: Continental contractional orogenic belts; 8105 Tectonophysics: Continental margins and sedimentary basins (1212); 8110 Tectonophysics: Continental tectonics—general (0905); KEYWORDS: crustal structure, continental collision, refraction seismology Citation: Van Avendonk, H. J. A., W. S. Holbrook, D. Okaya, J. K. Austin, F. Davey, and T. Stern (2004), Continental crust under compression: A seismic refraction study of South Island Geophysical Transect I, South Island, New Zealand, J. Geophys. Res., 109, B06302, doi:10.1029/2003JB002790. 1. Introduction [2] Owing to its buoyancy, the continental crust has been deformed but rarely subducted during various collision events in the Earth’s history. Orogens from Proterozoic to recent times are well preserved [e.g., Lucas et al., 1993; Meissner and Tanner, 1993], providing ample opportunities to investigate the role of deep-seated geological processes 1 Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming, USA. 2 Now at University of Texas Institute for Geophysics, Austin, Texas, USA. 3 Department of Geological Sciences, University of Southern California, Los Angeles, California, USA. 4 Now at ChevronTexaco, Houston, Texas, USA. 5 Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand. 6 School of Earth Sciences, Victoria University of Wellington, Wellington, New Zealand. Copyright 2004 by the American Geophysical Union. 0148-0227/04/2003JB002790$09.00 during the creation of these mountain belts [Arndt and Goldstein, 1989; Nelson, 1992]. Yet it is difficult to infer the tectonic stresses and lithospheric rheology at the onset of continental compression. The yield strength of continental lithosphere, the distribution of strain in the deep crust, and the mechanical coupling with the lower lithosphere may be obscured as the orogen matures and erodes. To gain more insight in the behavior of continental crust under compression we must look at young convergent zones, such as the Southern Alps of New Zealand. [3] The Southern Alps of South Island, New Zealand, are the surface expression of a relatively small orogen that formed over the last 5 –10 Myr in this submerged continent by transpression across the Pacific-Australian plate boundary. The relative plate motion was strike slip in the early Miocene [Kamp, 1987; Sutherland, 1995], but a change in the Euler rotation pole of the Pacific plate resulted in 80– 100 km of shortening [Stock and Molnar, 1982; Cande and Kent, 1995; Sutherland, 1995; Walcott, 1998]. Most of the shearing and compressional deformation has taken place near the Alpine Fault [e.g., Norris et al., 1990], a major B06302 1 of 16 B06302 VAN AVENDONK ET AL.: CONTINENTAL COMPRESSION SOUTH ISLAND Figure 1. Two SIGHT transects crossed the island where it is 150 km wide, and two transects investigated the structure that ran parallel to the plate boundary on the Australian and Pacific side. The R/V Maurice Ewing air gun shots (in black) were recorded by ocean bottom instruments (circles) and dense arrays of land seismometers on the two transects. On-land explosions (stars) were also recorded by the land arrays. B06302 debate [e.g., Pysklywec et al., 2002] because the seismic constraints on the deep crustal structure were insufficient prior to the 1996 South Island Geophysical Transect (SIGHT) study. [5] During the SIGHT experiment, seismic refraction and reflection data were collected along four seismic transects by a consortium of U.S. and New Zealand scientists. The width of South Island, 150 km from the Tasman Sea to the Pacific Ocean, made it a suitable target for a controlledsource, two-sided onshore-offshore seismic experiment [Okaya et al., 2002]. Two of these transects are parallel to the east and west coast of South Island. The two other transects extend from the Tasman Sea across South Island into the Pacific Ocean (Figure 1). Linear arrays of land seismometers across the island recorded air gun shots that were fired offshore from both the east and west coast, such that both flanks of the crustal root could be imaged [Davey et al., 1998]. This data set, combined with land explosion and ocean bottom refraction data, provides an unprecedented opportunity to image the deeper structure of an orogen and undeformed portions of the flanking continental plates [Okaya et al., 2002]. In this paper we present a twodimensional (2-D) seismic velocity model obtained by a tomographic inversion of seismic refraction data that were recorded along the northern of the two transects across the waist of South Island, referred to here and elsewhere as Transect 1 (Figure 1). Our seismic velocity model consists of layers with progressively increasing seismic velocity bounded by continuous, reflecting interfaces. 2. Geologic Setting lithological discontinuity that runs straight along the west flank of the Southern Alps. Significant strain has also been accommodated in Pacific plate crust to the east [Walcott, 1984; Langdale and Stern, 1998]. The geologic observations are generally consistent with the current strain distribution known from Global Positioning System (GPS) surveys [Beavan et al., 1999; Beavan and Haines, 2001] across South Island. These detailed kinematic constraints make the Southern Alps an attractive place to study the deep structure of a continental collision. [4] Over the years a number of studies have shown that the Alpine Fault dips 40– 50 southeast from its surface trace, forming an oblique thrust along which sedimentary rocks of the Pacific plate are transported from lower crustal levels to the surface [Wellman, 1979; Sibson et al., 1979; Rattenbury, 1986; Cooper and Norris, 1994; Kleffmann et al., 1998]. The localization of convergence and uplift in the Southern Alps is reflected in the thermal history of rocks exposed in South Island [Adams, 1980; Tippett and Kamp, 1993; Batt et al., 2000], by shoaling of the brittle-ductile transition [Leitner et al., 2001; Wannamaker et al., 2002], and in low seismic velocities in the Pacific plate crust [Kleffmann et al., 1998; Stern et al., 2001; EberhartPhillips and Bannister, 2002]. In contrast with the shortening in the upper crust, the strain in the mantle lithosphere is inferred to be distributed over a few hundred kilometers [Klosko et al., 1999; Molnar et al., 1999; Scherwath et al., 2002; Kohler and Eberhart-Phillips, 2002]. The coupling between these two modes of deformation is an issue of [6] We describe some of the New Zealand geology with a focus on the area where the South Island Geophysical Transect (SIGHT) active seismic data were collected. The distribution in age and origin of basement rocks of South Island reveals two very distinct metamorphic belts, the Western and Eastern Province, which are joined by a suture called the Median Tectonic Zone (MTZ, Figure 2) [Landis and Coombs, 1967; Bradshaw, 1993; Kimbrough et al., 1993]. The 450 km dextral slip of the Alpine Fault has offset the MTZ over much of South Island in the Cenozoic, such that in central South Island the Western and Eastern Provinces lie on the Australian and Pacific plates, respectively (Figure 2). [ 7 ] The Western Province, comprising the NelsonWestland block in the north and Fiordland block in the south, is considered an autochthonous part of Gondwana [e.g., Spörli, 1987]. It consists of lower Paleozoic metasediments that were cratonized during the middle Paleozoic and intruded by large volumes of granitoids [Cooper, 1979; Tulloch, 1983; Mattinson et al., 1986]. During the early Cretaceous, accretion of the Eastern Province onto the Gondwana margin was accompanied by continental arc volcanism. Some granulites are found in the Nelson block [Tulloch, 1983], and these rocks may constitute a significant portion of the lower crust as in much of the aged continental crust worldwide [Fountain and Salisbury, 1981; Rudnick and Fountain, 1995]. SIGHT Transect 1 intersects only a 25 km wide subaerial strip of the Westland, but Paleozoic basement has also been dredged offshore on the Challenger Plateau [Tulloch et al., 1991]. Therefore the western portion 2 of 16 B06302 VAN AVENDONK ET AL.: CONTINENTAL COMPRESSION SOUTH ISLAND B06302 side of the Southern Alps. The divide between these watersheds, 50 km from the west coast locally reaches 1700 m elevation. Transect 2 lies just 60– 70 km southward along the strike of the Alpine Fault [Scherwath et al., 2003], and crosses a higher section of the divide just south of the Mount Cook area, where the band of Haast Schist narrows to 12 km, and the Bouguer gravity anomaly deepens toward the south [Woodward, 1979]. The variation in thickness of the Haast Schist terrane may be the result of more pervasive compression at Transect 2 compared with Transect 1. 3. Wide-Angle Data [10] An overview of the SIGHT active source seismic data was given by Davey et al. [1998] and Okaya et al. [2002]. We summarize the source and receiver geometry along Transect 1 and give an inventory of the travel time data that were used in the tomographic inversion. Before we can use the data in a tomographic inversion we must interpret and label travel time arrivals. We present a travel time interpretation based on a two-layer crust that applies to all the subbasement seismic refraction data collected along SIGHT Transect 1. Figure 2. Map of South Island shows the location of terranes from the Eastern and West Provinces relative to the Alpine Fault in central South Island. Pearson et al. [1995] measured 38 mm/yr convergence in central South Island. MTZ, Median Tectonic Zone. of Transect 1 covers a large portion of old Gondwana foreland that is characterized mostly by metagraywackes and granitic rocks in the upper crust. [8] The Eastern Province includes several terranes that appear to have been derived from an island arc system [e.g., Coombs et al., 1976], but most of the eastern and central South Island is covered by the Torlesse graywackes. These quartzofeldspathic sediments, mainly Permian to Jurassic in age in central South Island, may have been deposited on Pacific oceanic crust [Smith et al., 1995] near a trench system [Adams and Kelley, 1998]. In the Early Cetaceous, sedimentary rocks of the Eastern Province were exhumed from mid to lower crustal depths. Because of the more recent deeper exhumation near the Alpine Fault, the metamorphic grade of basement ranges from zeolite and prehnite-pumpellyite facies in the Torlesse terrane in the east [MacKinnon, 1983] to greenschist in most of the Haast Schist terrane [Wood, 1978]. The dextral shear between the Australian and Pacific plates in the Cenozoic has bent the Eastern Province in a reversed ‘S’ shape [Molnar et al., 1975; Walcott, 1978], and the concurrent offset across the Alpine Fault has placed the Paleozoic crust of the Western Province against the Haast Schists where SIGHT Transect 1 crosses central South Island. The Torlesse and Haast terranes probably extend offshore east of South Island on Transect 1, since Mesozoic sandstones and schists are found on Chatham Rise and on the Chatham Island [Cullen, 1965; Andrews et al., 1978; MacKinnon, 1983]. [9] The seismic Transects 1 and 2 both make use of river valleys that provide access into the Southern Alps. Transect 1 follows the Whataroa river through Westland, and then it follows the Rangitata river system at the other 3.1. Marine Seismic Data [11] Marine seismic data were acquired using an array of 20 air guns on board the R/V Maurice Ewing with a total capacity of 139 liter. The air guns were fired every 50 m on the offshore portions of the seismic transects (Figure 1). The two-way travel time to the acoustic basement is constrained using stacked multichannel seismic (MCS) data on both the east and west coast of South Island. Eight ocean bottom instruments were deployed in both the Tasman Sea and the Pacific Ocean in order to provide data coverage at large distances of the plate boundary. All instruments recorded with a 10 ms sample interval. After suppression of previous shot noise (PSN) [Nakamura et al., 1987], predictive deconvolution and band-pass filtering, the ocean bottom records show coherent arrivals to 80 km source-receiver offset. Crustal refractions (labeled Pg) have apparent velocities somewhat less than 6 km/s in receiver gathers, and midcrustal and Moho reflections are visible at offsets larger than 40 km with apparent velocities larger than 6 km/s (Figure 3). A midcrustal reflection, referred to as PiP in this paper, was observed and correlated between receiver gathers on both the east and west coast of South Island. Mantle refractions (Pn) are observed intermittently in the ocean bottom data. 3.2. Onshore-Offshore Data [12] The R/V Maurice Ewing air gun shots were also recorded by an array of 126 land seismometers along Transect 1. The instruments were spread from the east to the west coast with an average spacing of 1.5 km, although the gaps between instruments in the Southern Alps were larger. The sample interval of all instruments was 10 ms. Noise levels varied strongly among instrument sites, but some of these receiver gathers are of very high quality (Figure 4). In comparison with the ocean bottom data the onshore-offshore data have a better signal/noise ratio and the signal does not degrade as fast with distance, presumably because the recording is not affected by PSN. 3 of 16 B06302 VAN AVENDONK ET AL.: CONTINENTAL COMPRESSION SOUTH ISLAND B06302 Ocean onshore-offshore records. Offshore to the west, the PmP arrivals are weaker than PiP, but in the Pacific Ocean records PmP are usually the strongest phase. Weak mantle refractions (Pn) are observed in most onshoreoffshore records followed by relatively strong reflections that appear to arrive from the uppermost mantle. Figure 3. An example ocean bottom seismometer (OBS) gather shows seismic refractions and reflections from instrument C1 at 145 km on the Australian plate west of South Island. The horizontal distances are measured along the 603 km long Transect 1 from northwest to southeast. The vertical time axis is reduced by 6 km/s. Inset shows the true shot spacing of 50 m. [13] A crustal seismic refraction (Pg) with apparent velocity of approximately 6 km/s is observed in the offshore data east and west of South Island to sourcereceiver offsets of 150 km. In most onshore-offshore receiver gathers the lower crust is characterized by a few wide-angle reflections. Some of these lower crustal reflections are spatially incoherent, but the first arriving of these reflections is always a clear waveform that correlates well with the midcrustal PiP observed in the ocean bottom seismograph data. Offshore to the west, PiP is sometimes observed to source-receiver distances larger than 200 km, but at the east coast most PiP are recorded within 150 km. The Moho reflection (PmP) is also a prominent arrival on both the Tasman Sea and Pacific 3.3. Explosion Data [14] The onshore-offshore seismic data provide excellent coverage of both flanks of the Southern Alps, but instruments positioned near the east and west coast did not record many useful offshore data from across the 150 km wide land section of Transect 1. Seismic coverage of the crust beneath central South Island was provided mostly by 16 large chemical explosions along Transect 1 in South Island, recorded by 417 instruments spaced at 400 m (Figure 1). Turning waves and postcritical reflections with a dominant frequency of 10 Hz constitute most of the observed seismic phases in the ocean bottom and onshore-offshore air gun data. In contrast the explosion data, recorded with 4 ms sample interval, contain seismic energy to at least 35 Hz. High-frequency precritical seismic reflections from the deep crust are commonly observed. The turning wave in the crust (Pg) is generally the first and strongest arrival in the explosion data, and it is often coherent to offsets over 100 km (Figure 5). On some explosion shot records, distinct PiP and PmP arrivals are consistent with the onshoreoffshore data recorded at larger source-receiver offsets. Other explosion records show a train of seismic energy that appears to arrive from the deep crust. We interpret the onset of this package as PiP and the tail as PmP. 3.4. Travel Time Picks [15] The four seismic travel time phases Pg, PiP, PmP, and Pn can be used to constrain a two-layer crustal model provided that these arrivals are consistent among the different seismic gathers. Reversed coverage on ocean bottom and explosion records allowed us to verify source-receiver Figure 4. Onshore-offshore data shot from the Tasman Sea recorded by an instrument located at shot point 7 (SP07) located at 301 km. The horizontal and vertical axes are as in Figure 2. An upper crustal refraction (Pg) is observed to 130 km source-receiver offset. Very strong and continuous reflections PiP and PmP come in with higher apparent velocity and are recorded to 180 km source-receiver offset. Another strong mantle reflection is visible after PmP. A weak mantle arrival (Pn) appears at 150 km offset. Inset shows the coherence of wide-angle reflection PiP. 4 of 16 B06302 VAN AVENDONK ET AL.: CONTINENTAL COMPRESSION SOUTH ISLAND Figure 5. Explosion refraction data, from shot point SP07 at 301 km, recorded across South Island show a strong firstarrival Pg. The time and distance axes are as in Figure 2. The instruments were spaced every 400 m along much of the 150 km wide traverse across South Island, except for the high portions of the Southern Alps between 253 and 265 km where helicopters were used to deploy instruments. reciprocity. The onshore-offshore data are unreversed, but the dense receiver spacing results in consistent seismic arrival times at constant offset between neighboring land instruments. [16] In section 4 we describe a least squares inversion of all wide-angle seismic refraction and reflection travel times. Appropriate weighting of the travel time data is necessary to ensure that the modeling procedure finds a good balance between fitting explosion and air gun data, and to account for different noise levels. Although the shot spacing in the air gun data is 50 m, we picked the arrivals in these receiver gathers with intervals of at least 400 m, or 8 seismic traces, to achieve a data density comparable to the explosion data. Data errors were assigned visually to all travel time picks. They varied between 70 and 150 ms in the ocean bottom data, between 40 and 120 ms in the onshore-offshore data, and between 40 and 160 ms in the explosion data. The 62,563 travel time picks are separated by data type and seismic phase in Table 1. 4. Tomography [17] Tomographic inversions aim to invert seismic refraction data for a minimum-structure seismic velocity model [Zelt, 1999; Zelt et al., 2003]. The smooth seismic velocity models that result from this approach usually give a conservative but robust representation of the true earth structure [Zelt et al., 2003]. However, the multichannel seismic data collected during SIGHT show reflections from the basement offshore New Zealand [e.g., Davey et al., 1998]. These reflections are caused by a contrast in seismic properties at this boundary where probably Mesozoic rocks are overlain by postcollision Tertiary rocks. Sharp seismic boundaries can also be inferred from the consistent occurrence of wide-angle reflections PiP and PmP in the wideangle data. We use the method of Van Avendonk et al. [2001a] to construct a model with smooth spatial variations in seismic velocity that is intersected by four velocity discontinuities: The seafloor, acoustic basement, lower crust and Moho. 4.1. Ray Tracing [18] The calculation of ray paths and travel times in a reference seismic velocity model is an essential step in a B06302 tomographic inversion. When applied to seismic refraction data, a ray-tracing scheme must handle dense source or receiver gathers efficiently. Korenaga et al. [2000] and Van Avendonk et al. [2001b] showed that a combination of a grid-based global travel time solver such as the shortest path method [Moser, 1991] or eikonal equation solver [Vidale, 1988] with a local optimization scheme [i.e., Um and Thurber, 1987; Moser et al., 1992] is computationally efficient. Using a shortest path grid spacing of 500 by 150 m in the method of Moser [1991] and the ray-bending method of Moser et al. [1992] we calculate travel times with an accuracy of approximately 10 ms. [19] An apparent problem with travel time solvers is their inability to find secondary travel time arrivals, with the exception of reflections from first-order seismic velocity discontinuities [Moser, 1991]. We model the seismic reflections PiP and PmP as reflections from a midcrustal and crust-mantle seismic velocity step. Seismic reflections and refractions originating from a velocity discontinuity in the model can be found by first calculating the travel times from a source to the interface, and by propagating the travel times back to the Earth’s surface using the interface as a timevarying source. 4.2. Inversion [20] The travel time data carry information on both the seismic velocities and reflecting boundaries. We therefore seek to solve for these parameters simultaneously. However, the structure of the acoustic basement appears to be more complicated than the deeper structure. We therefore carry out a ‘‘layer-stripping’’ step to determine the seismic structure of the shallow sediments and basement before we image the deep crustal structure. By forward modeling of shallow seismic refractions from within the sediments and basement reflections from multichannel seismic data [Davey et al., 1998] we obtained a basement profile along Transect 1. The PiP and PmP arrival times of the explosion data recorded in the Southern Alps are consistent with a crustal root that extends deeper than the crust outside the collision zone [e.g., Smith et al., 1995]. We generated a starting model with the lower crust and Moho deepened over a wide region beneath South Island (Figure 6). The seismic velocity of the model layers and the geometry of the lower crust and Moho tomographic inversion [21] The tomographic inversion solves for a model vector m that represents both seismic slowness, which is the reciprocal of seismic velocity, and the depth of the velocity discontinuities [Van Avendonk et al., 2001a]. The inversion procedure seeks a model perturbation dm that minimizes Table 1. Number of Picks for Each Data Type and Phase Phase Pg PiP PmP Pn Total 5 of 16 Ocean Bottom Onshore-Offshore Land Explosion 2362 796 1838 69 5065 11,043 16,709 15,541 4,707 48,000 3712 3231 2564 0 9507 VAN AVENDONK ET AL.: CONTINENTAL COMPRESSION SOUTH ISLAND B06302 B06302 Figure 6. Starting model for tomographic inversion of deep crustal arrivals contains detailed basement topography and a first guess of the geometry of the crustal root. both the data misfit and the roughness of the new velocity model using least squares: F ðdmÞ ¼ ðd A dmÞT C1 d ðA dm dÞ þ lðm þ dmÞT DT Dðm þ dmÞ: ð1Þ The matrix A is the Fréchet derivative matrix that maps dm into the data residual vector d. Assuming that travel time pick errors are uncorrelated, the a priori data covariance matrix Cd reduces to a diagonal matrix with elements Cii = s2i , where s are the errors assigned to the travel time data. The second term of equation (1) minimizes both horizontal and vertical gradients in seismic velocity and the slopes of layer boundaries in the model. The step in seismic velocity across these boundaries is not restricted in the inversion [Van Avendonk et al., 2001a]. The first-derivative roughness matrix D applies to the newly constructed model m + dm. This strategy has been named the ‘‘jumping’’ approach [Shaw and Orcutt, 1985] because it attempts to jump from the starting solution to the final solution without restraining the size of dm. The relative strength of this smoothing term is varied by the scalar l. The trade-off between data misfit and model roughness is tuned to achieve a normalized data misfit that approximately averages unity: c2 ¼ 2 1 12 Cd ðA dm dÞ 1: N ð2Þ [22] We carried out regularized inversions on a rectangular grid with horizontal node spacing of 1 km and the vertical cell spacing varied between 200 m near the surface and 500 m at 50 km depth. The resultant model vector m of 3.3 104 parameters was determined by 1.5 105 equations. The fraction of nonzero elements in the inversion matrix was 1.3 103, sufficiently sparse to invert the system of equations using LSQR [Paige and Saunders, 1982]. 4.3. Data Fit [23] We carried out 22 iterations of ray tracing and inversion of all the wide-angle travel time picks. The procedure yielded a stable solution after eight steps. Although a c2 data misfit of 1.0 is preferred, the trade-off between model roughness and data misfit is subjective, and we found that a c2 misfit of 1.3 produced a more conservative model that is reproduced between different tomography iterations (Figure 7). The inversion procedure guarantees that the summed data misfit compares well with the assigned errors, but the spatial distribution of the data misfit may still be important where details of the seismic velocity model are concerned. [24] In Figure 8 we show the ray paths and travel time fits for three ocean bottom seismometers (OBSs) on Transect 1. The data density on the distal portions of Transect 1 was not high because the instruments were widely spaced and observed deep refractions and reflections were intermittent. However, the good data fit of these ocean bottom records validates the two-layer model of the extended continental crust on both the east and west side of the collision zone. The ray paths of the onshore-offshore data (Figure 9) sample the east and west flank of the orogen, and many deep reflections in the explosion data return from the keel of Figure 7. Seismic velocity structure along Transect 1 stretches from submerged and undeformed Australian continental crust to the northwest across the west coast of South Island at 213 km, the Alpine Fault at 228 km, the east coast at 271 km, and into the submerged Pacific plate. Red triangles show the location of ocean bottom and land instruments that recorded the wide-angle seismic data. See color version of this figure at back of this issue. 6 of 16 B06302 VAN AVENDONK ET AL.: CONTINENTAL COMPRESSION SOUTH ISLAND B06302 Figure 8. Ocean bottom data from the (a and b) Tasman Sea and (c) Pacific Ocean show a good data fit of seismic refractions that turn in the upper crust and wide-angle reflections from the top of the lower crust (PiP) and Moho (PmP). Picked travel times are shown as solid lines, and travel times calculated in our new seismic velocity model are shown by dashed curves. the crustal root. The Moho and lower crust beneath the plate boundary (Figure 7) apparently dip too steeply to the east to be illuminated by wide-angle reflections. 4.4. Seismic Velocity Model [ 25 ] The 603 km long seismic velocity model of Transect 1 covers both undeformed and compressed portions of continental crust (Figure 7). At large distance from the collision zone, both the Australian and Pacific plate crust is characterized by a 15 km thick upper crust with seismic velocities of 5.7– 6.2 km/s overlying a thinner lower crust with seismic velocities ranging from 6.5 to 7.1 km/s (Figure 7). This distinct bimodal velocity distribution fits the travel time data, and it explains the prominence of midcrustal reflection PiP on both sides of the plate boundary. However, the contrast between these two crustal layers may be accentuated by the additional constraints in the inversion. The roughness operator D was applied to seismic velocity gradients within layers such that locally the flattest possible seismic velocity structure was found. The increase in seismic velocity between the upper and lower crust was not penalized. The Australian lower crust is substantially thicker and laterally more heterogeneous than the Pacific lower crust. At 200 km northwest of the plate boundary the lower crustal velocity averages 7.1 km/s, but it decreases to 6.4 km/s near the Alpine Fault. [26] The thick crustal root beneath the Southern Alps lies entirely on the Pacific side of the plate boundary. The maximum Moho depth of 37 km below sea level is reached at 275 km in the profile, 45 km southeast of the surface expression of the Alpine Fault. The upper crust makes up most of the crustal root beneath the Southern Alps and its seismic velocity is only 5.8 km/s. The lower crust is only 3 – 5 km thick beneath the crustal root and its seismic velocity of 6.5 km/s is also less than the 7.0 km/s in the lower crust eastward in the Pacific 7 of 16 B06302 VAN AVENDONK ET AL.: CONTINENTAL COMPRESSION SOUTH ISLAND B06302 Figure 9. Seismic records of six explosion records are shown along with onshore-offshore refraction data that were recorded from the same location as these large explosions. The Figures 9a to 9f are records from shot point 14 (233.1 km), shot point 11 (265.7 km), shot point 10 (272.7 km), shot point 7 (301.3 km), shot point 4 (333.6 km) and shot point 3 (347.8 km), respectively. The shallow waters offshore the east and west coast resulted in gaps in these data records. plate. The upper mantle beneath South Island also has reduced seismic velocities. The minimum seismic velocity in the upper mantle (7.8 km/s) is found near the west coast and the Alpine Fault. The crustal thickness decreases rapidly from the plate boundary, but the east flank of the crustal root beneath South Island is more gradual, extending over at least 100 km. The seismic velocity beneath the Canterbury plains is significantly higher than the velocity further offshore to the east: The upper crustal seismic velocities of 6.2– 6.3 km/s there are higher than elsewhere in the Mesozoic upper crust. The lower crust is variable in thickness (2 – 6 km), and the mantle seismic velocity beneath the east coast is very high at 8.6 km/s. 4.5. Model Resolution [27] Before we discuss implications of the new seismic velocity model for Transect 1, we must assess the non- 8 of 16 B06302 VAN AVENDONK ET AL.: CONTINENTAL COMPRESSION SOUTH ISLAND B06302 Figure 9. (continued) uniqueness in the model. The tomographic inversion was designed to find a simple seismic velocity model that fits the travel time data. A first impression of reliability of our solution in Figure 7 is given by the derivative weight sum (DWS) [Scales, 1989; Toomey and Foulger, 1989]. By summing up the elements for each column of derivate matrix A and scaling by data errors we obtain a measure of ray density (Figure 10). The SIGHT experiment succeeded in covering the crust beneath the Southern Alps with ray paths without leaving large gaps. However, model resolution also depends on the rank of A. Crossing ray paths are essential to independently resolving model parameters. The solution dm that minimizes the penalty function (1) can be expressed in the generalized inverse of d: 1 T T ^ ¼ AT C1 dm A d: d A þ lD D ð3Þ The resolution matrix can be found by substituting A dm for d in equation (3) [e.g., Tarantola, 1987]. The resolution matrix R, a blurring filter between the ‘‘true’’ model dm and ^ can be expressed as the minimum norm solution dm, 1 T T R ¼ AT C1 A A: d A þ lD D ð4Þ We calculate R using an eigenvalue decomposition of T ATC1 d A + l D D by Lanczos iteration [Vasco et al., 1999]. 9 of 16 B06302 VAN AVENDONK ET AL.: CONTINENTAL COMPRESSION SOUTH ISLAND Figure 9. (continued) 10 of 16 B06302 VAN AVENDONK ET AL.: CONTINENTAL COMPRESSION SOUTH ISLAND B06302 B06302 Figure 10. Derivative weight sum (DWS) measures the ray density scaled by the errors in the travel time data. Thickness of the reflecting boundaries indicates the density of coverage of these model boundaries. The diagonal of the resolution matrix gives a direct indication of the quality of the solution (Figure 11). The resolution is very good at depths less than 10 km in the collision zone. Adequate resolution is also obtained in the lower crust of the Australian and Pacific plates where paths of PiP and PmP cross. Deeper in the crustal root and farther offshore South Island the resolution becomes poor. Compared to the ray density (Figure 10) the resolution decreases faster with depth, presumably because the long ray paths of PiP, PmP, and Pn arrivals cover many model parameters in the deep crust that cannot be determined independently. 5. Discussion [28] We have obtained a detailed crustal seismic velocity profile across the Southern Alps of New Zealand in central South Island. A similar seismic profile was obtained across the Mount Cook area [Scherwath et al., 2003], with similar results. These two transects currently provide the most detailed information on the deep crustal structure of the collision zone where 20– 25 km thick Pacific and Australian plates are juxtaposed. The 3-D tomographic inversion of first-arrival times in South Island of Eberhart-Phillips and Bannister [2002] shows that the crustal structure does not vary much along strike in central South Island. 5.1. Structure of Continental Crust Offshore Central South Island [29] The distinct two-layer crust in the Australian and Pacific plates is consistent with the geological history of the Western and Eastern Provinces of New Zealand. In the Australian plate, the Westland crust has experienced both convergent margin and extensional processes [e.g., Bradshaw et al., 1981; Bishop, 1992] that have resulted in high-grade metamorphic rocks in the lower crust. A thin lower crust with high seismic velocity beneath the Pacific plate was previously observed by Smith et al. [1995]. They interpreted this layer as the old oceanic crust on which the Caples and Torlesse sedimentary rocks were deposited [e.g., Wood, 1978]. The thickness of 3 – 6 km and the seismic velocity of 7.0 km/s in the lower crust in our model of Transect I are consistent with this interpretation. [30] The seismic velocity structure along Transect 1 exhibits large variations in crustal thickness. The crustal thickness of the Australian plate averages 23 km in our profile. The Pacific plate crust in the Bounty Trough is just 18 km thick (Figure 12). These values are low in comparison with global compilations of continental crustal thickness [e.g., Mooney et al., 1998], but they are not unusual for extended and submerged continental lithosphere. The continental lithosphere of New Zealand was stretched mostly during and after the separation from eastern Gondwana in the late Cretaceous [Carter et al., 1974; Kamp, 1986], and smaller rift basins can also be seen at 75 km and 475 km in our new crustal profile (Figure 7). The gradual westward thickening of Pacific crust from 18 km in the head of the Bounty Trough to 28 km beneath the east coast (Figure 12) is unlikely caused by the Cenozoic convergence in New Zealand because GPS and geochronology studies indicate that the recent and current Figure 11. Diagonal of the resolution matrix shows very good resolution near the surface in South Island. Resolution is much poorer at larger depth in the model. Trade-offs between various model parameters at large depth result in low resolution. See color version of this figure at back of this issue. 11 of 16 B06302 VAN AVENDONK ET AL.: CONTINENTAL COMPRESSION SOUTH ISLAND B06302 parallel to the plane of foliation [Godfrey et al., 2002]. The seismic velocities of 6.3 km/s imaged in the middle crust between 350 and 400 km (Figure 7) may be due to anisotropy, but we do not have sufficient ray coverage at depths below 10 km to distinguish anisotropic effects from spatial variations in isotropic seismic velocity structure. Figure 12. Seismic velocity as a function of depth in our model at 65 km, 275 km, 370 km, and 500 km shows that seismic velocities in the upper crust are very consistent but the depth of the top of lower crust and Moho varies laterally. deformation is largely confined to South Island [e.g., Kamp and Tippett, 1993; Beavan and Haines, 2001]. Besides the mentioned Cretaceous thinning in the Bounty Trough, the South Island crust may have been thicker toward the plate boundary. The Torlesse terrane that largely makes up the Pacific upper crust in central South Island may have been laterally compressed when it docked in the late Triassic or early Jurassic [e.g., Landis and Bishop, 1972; MacKinnon, 1983]. [31] The seismic velocity of 6.0– 6.2 km/s in the upper crust in the Australian and Pacific plates is not unusual for rocks of granitic and quartzofeldspathic composition [Holbrook et al., 1992; Christensen and Mooney, 1995]. Although the seismic velocity in the Australian and Pacific plates falls well within the range of global observations, the vertical gradient is remarkably low (Figure 12). This modest increase in seismic velocity with depth may indicate that these upper crustal layers are homogeneous in composition [Christensen and Mooney, 1995]. [32] Petrophysical studies of Torlesse greywacke and Haast schists of South Island show an average compressional velocity of just over 6.0 km/s [Okaya et al., 1995] which is in agreement with our velocity model. However, Haast schist samples exhibit up to 20% anisotropy with fast velocities in the plane of foliation [Okaya et al., 1995; Godfrey et al., 2000]. A small seismic refraction study offshore Otago showed compressional seismic velocities parallel and perpendicular to the inferred foliation differed only 6% on a macroscopic scale [Smith, 1999]. The presence of Haast schist could therefore result in higher seismic velocities if seismic refraction paths run predominantly 5.2. Shortening of South Island [33] Walcott [1998] reviewed conceptual models for deformation of the South Island crust based on data collected prior to the SIGHT experiment. The role of two basic modes of crustal shortening, exhumation and thickening of the crust, cannot be quantified without knowledge of the geometry of the deeper crust. The crustal root beneath South Island, with a crustal thickness of up to 37 km along Transect 1, and the Cenozoic sediments offshore provide new constraints on the mass transfers through this young orogen. The crustal structure along SIGHT Transect 2 is mostly similar to our seismic velocity model, although Scherwath et al. [2003] found a maximum crustal thickness in the root of 44 km. This difference can be explained as an along-strike variation in the amount of compression along the plate boundary, which has also raised the Mount Cook area, where Transect 2 crosses the Main Divide, to the highest elevations of South Island. It should also be noted, however, that our tomographic inversion was designed to find the smoothest model that fits the data. The keel of the crustal root in our image may be biased toward smaller depths. The minimum-parameter model [Zelt and Smith, 1992] of Scherwath et al. [2003] is a model with minimum features, but does not apply smoothing of the model layer boundaries. [34] To explain the westward increase in uplift rate and metamorphic grade in the Mesozoic basement, Wellman [1979] suggested that the Pacific crust is sheared from the subducting upper mantle in South Island and upturned along the Alpine Fault toward the plate boundary. If the rock volume eroded at the plate boundary equals the amount of crustal shortening, the orogeny may be in a steady state. However, the band of highly metamorphosed schists that are currently exhumed in the Southern Alps only represent 15 km thickness of incoming Pacific crust [Grapes and Watanabe, 1992]. Consequently, these schists are probably bounded at the base by décollement within the Pacific crust [Norris et al., 1990]. The section of Pacific crust beneath the décollement is transferred into a crustal root, and thus causes the root to grow over time (Figure 13). [35] Until recently the contributions of thickening and erosion to crustal shortening have been difficult to evaluate because of limited constraints on deep crustal structure in South Island. If the mean elevation was near sea level at the onset of convergence [Kamp and Tippett, 1993], crustal thickening must have dominated at least until the Southern Alps rose high enough to increase precipitation. Comparing river-suspended sediment load with rock uplift, Adams [1980] concluded that shortening and erosion are now approximately balanced in South Island, so that further growth of the crustal root beneath the Southern Alps must be small. Using older constraints on the deep structure of South Island [e.g., Stern, 1995], Walcott [1998] estimated that about one third of the total crustal rocks that entered the orogen from the east now reside in the crustal root. The 12 of 16 B06302 VAN AVENDONK ET AL.: CONTINENTAL COMPRESSION SOUTH ISLAND B06302 Figure 13. Schematic picture shows our seismic velocity model without vertical exaggeration. Location of the Alpine Fault is projected on the profile. other two thirds of the mass budget are then dispersed as sediments, perhaps as far as the Taranaki basin and Tasman Sea. Our estimate of 28 km for undeformed Pacific crust is slightly thicker than Walcott’s assumed 25 km thickness. Our thickness of the crustal root is a little less than Walcott’s assumed 10 to 15 km thickening of the original crust, but it is in better agreement with the results of Transect 2 [Scherwath et al., 2003]. [36] It remains difficult to refine the 3-D crustal mass balance with 2-D constraints on the crustal structure because of the along-strike variation in structure [Eberhart-Phillips and Bannister, 2002; Scherwath et al., 2003] and the large component of strike-slip motion (38 mm/yr) in the orogen [Walcott, 1998]. However, the new seismic constraints confirm that 75% of the incoming crust must have eroded off the Southern Alps. The first-order comparison shows that surface erosion has been the dominant factor in the mountain building process and may lead to a flux steady state where all crustal rocks are exhumed [Willet and Brandon, 2002]. Alternatively, if erosion cannot keep pace with convergence, the crustal root of this young mountain belt could over time reach depths where phase changes make it dense enough to allow delamination, as probably happened in the European Alps [e.g., Chopin, 1987; Laubscher, 1988; Kissling, 1993; Marchant and Stampfi, 1997]. [37] The Pacific lower crust forms a wide synform beneath the crustal root of the Southern Alps, apparently without substantial thickening. Both its thickness and seismic velocity vary along Transect 1, but its unique reflective character is evident in almost every seismic refraction record in the explosion refraction and east coast onshoreoffshore data (Figure 8). The downward deflection of the lower crustal layer (Figure 7) can account for half of its 80– 110 km convergence. Given the low model resolution at large depth, we cannot rule out that pieces of the missing lower crustal material have been mixed into the crustal root. In our modeling we have parameterized the lower crust as a layer that is distinct from the upper crust. The two-layer model seems justified because of the widespread observation of the PiP reflection. However, there is no evidence that the lower crust remains intact in the west flank of the root where it may have been intensely deformed during the oblique convergence. The image of the lower crustal keel along transect 2 [Scherwath et al., 2003] shows that the Pacific lower crust is deformed in the Mount Cook area. [38] Seismic studies of the mantle beneath South Island [Stern et al., 2000; Kohler and Eberhart-Phillips, 2002] show that the crustal root of the Southern Alps overlies a 2 – 4% high-velocity and high-density mantle root. The area experiences earthquakes between 30 and 70 km depth [Kohler and Eberhart-Phillips, 2003]. The anomaly indicates lower upper mantle temperatures that may be caused by a cold downwelling beneath the collision zone. 5.3. Crustal Deformation [39] The oblique compression in South Island evolved from a strike-slip setting but crustal deformation now occurs almost entirely on the Pacific side, leaving the Australian plate undisturbed. Numerical modeling shows that greater initial crustal thickness and weaker rheology of the Pacific plate may have caused it to deform more easily, effectively making the Australian plate an indentor [e.g., Beaumont et al., 1996]. The maximum depth of upper mantle earthquake hypocenters beneath South Island increases from 40 km near the east coast of South Island to 70 km beneath the Alpine Fault [Reyners, 1987; Kohler and Eberhart-Phillips, 2003]. Reyners [1987] interpreted this trend as due to westward underthrusting of Pacific mantle lithosphere. However, the large shear strain beneath the plate boundary may also contribute to the occurrence of these deep earthquakes [Kohler and Eberhart-Phillips, 2003]. [40] The new seismic constraints from the SIGHT experiment indicate that the crustal structure of the Pacific and Australian role may also have played a role in the asymmetric development of the Southern Alps. If we can assume that the seismic velocity structure at the east coast is the best approximation of the Pacific crustal structure before convergence started, quartzofeldspathic rocks of the Torlesse or Haast terranes may have been juxtaposed against more competent rocks of the Westland lower crust at 20– 25 km depth at the start of convergence. The upper crust of the cratonic blocks of the western Province may also be stronger than the Torlesse graywacke (Figure 13). Indentation of the more competent Australian plate subsequently resulted in thickening, deformation, and strain softening in the Pacific plate crust [e.g., Koons, 1987; Gerbault et al., 2002, 2003], probably greatly aided by the expulsion of fluids from the crustal root [Koons et al., 1998; Wannamaker et al., 2002]. The low-resistivity anomaly imaged by Wannamaker et al. [2002] roughly coincides with a low seismic velocity anomaly of 0.2– 0.3 km/s beneath the Southern Alps in our study of SIGHT Transect 1. In other seismic studies of central South Island [Kleffmann et al., 1998; Stern et al., 2001; Eberhart-Phillips and Bannister, 2002] this anomaly is found to be larger in amplitude and confined to a smaller region in the Pacific crust. The wider low-velocity anomaly in our study (Figure 7) may result from our use smoothness con- 13 of 16 B06302 VAN AVENDONK ET AL.: CONTINENTAL COMPRESSION SOUTH ISLAND straints (equation (1)) instead of damping. The damped inversion of Eberhart-Phillips and Bannister [2002] did not attempt to minimize the seismic velocity gradients in the model. Low seismic velocities in the crust of the Alpine Fault zone may be evidence for elevated pore pressures [Christensen, 1984; Stern et al., 2001] which further facilitates strain localization in the Pacific plate crust. [41] Two other positive feedback mechanisms that may have localized deformation are mechanical unloading of the crust by erosion and the advection of heat. The interaction between rock uplift and surface erosion has been studied extensively in New Zealand and other mountain belts [Koons, 1989; Beaumont and Quinlan, 1994; Batt and Braun, 1999]. The prevailing west wind in South Island may have accelerated erosion near the west coast, and it may be largely responsible for the 40 km lateral offset between the Alpine Fault and the root of the Southern Alps [Willett, 1999]. The upward advection of relatively hot lower crust locally elevates the depth of the brittle-ductile transition and thus influenced the mechanical strength and strain distribution near the plate boundary [Adams, 1980; Shi et al., 1996; Batt and Braun, 1999; Leitner et al., 2001]. [42] In our interpretation of the continental compression we have ignored the large amount of strike slip at the plate boundary. Three-dimensional models [e.g., Koons, 1994; Braun and Beaumont, 1995] can better predict the failure and deformation of crustal rocks in an obliquely convergent setting. Earthquake studies have provided evidence for partitioning of strike-slip and convergence along different fault systems in northern South Island [Anderson et al., 1993; Doser et al., 1999; Abercrombie et al., 2000; Gledhill et al., 2000; Leitner et al., 2001]. In this area the crustal stresses are influenced by the Hikurangi subduction zone, which lies just north of the continent-continent collision [Anderson et al., 1993; Reyners and Cowan, 1993; Leitner et al., 2001]. In contrast, in the crust of central South Island the Alpine Fault appears to accommodate both strike slip and convergence [Sibson et al., 1979; Norris et al., 1990; Cooper and Norris, 1994]. The lack of strain partitioning in the crust may be characteristic for continental collision zones, depending on the degree obliquity of convergence [Braun and Beaumont, 1995]. While convergence deeper in the crust is thought to be accommodated by a listric detachment further east [e.g., Norris et al., 1990], the strike-slip deformation probably extends more vertically down into the zone of reduced upper mantle seismic velocity between 200 and 260 km in our profile (Figure 7). This low-velocity anomaly, with seismic velocities of 7.8 km/s, can be interpreted as a zone of distributed strain in the uppermost mantle that gives rise to azimuthal anisotropy with the fast velocities parallel to the plate boundary [Klosko et al., 1999; Scherwath et al., 2002]. Seismic velocities measured in the same location on Transect 1 in the direction parallel to the Alpine Fault are as high as 8.2 km/s (A. Melhuish et al., Crustal and upper mantle seismic structure of the Australian plate, South Island, New Zealand, submitted to Tectonophysics, 2004). 6. Conclusions [43] Tomographic inversion of seismic refraction travel times of the SIGHT data has provided unprecedented B06302 constraints on the deep structure of South Island, New Zealand, that give us more insight in the style of crustal shortening in this young continent-continent collision. [44] Our seismic velocity model with a two-layer crust covers crust from the Pacific and Australian plates both inside and outside the orogen. The 80– 110 km of convergence has locally increased the crustal thickness from 28 km to 37 km beneath the Southern Alps. The deepest part of the crustal root lies 40 km southeast of the surface expression of the Alpine Fault. Extensive crustal thickening has taken place over a 80 km wide zone in the Pacific plate crust, and it can account for about half the crustal shortening in central South Island. In contrast, the Australian plate crust is largely undeformed. We attribute the onset of deformation in the Pacific plate crust to the larger thickness of weaker upper crust at the start of transpression. The upper crust of the Pacific plate shows seismic velocities that are 0.2– 0.3 km/s lower than elsewhere in the quartzofeldspathic rocks of the Pacific plate crust. Overpressured fluids released from the crustal root may be responsible for lowering the seismic velocity in the continental collision zone. The exhumation of hot lower crustal rocks and coupling between uplift and erosion in the Southern Alps has also contributed to the localization of strain in a narrow zone southeast of the Alpine Fault. The results from the SIGHT experiment can provide much improved constraints for geodynamic models of this plate boundary. [45] Acknowledgments. We thank the land crew in South Island and officers and crew of the R/V Maurice Ewing that made the SIGHT study a success. Discussions with M. 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([email protected]) F. Davey, Institute of Geological and Nuclear Sciences, P.O. Box 30368, Lower Hutt, New Zealand. ([email protected]) W. S. Holbrook, Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071-3006, USA. ([email protected]) D. Okaya, Department of Geological Sciences, University of Southern California, University Park, Los Angeles, CA 90089-0740, USA. ([email protected]) T. Stern, School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand. ([email protected]) H. J. A. Van Avendonk, University of Texas Institute for Geophysics, 4412 Spicewood Springs Road, Austin, TX 78759, USA. ([email protected]) 16 of 16 B06302 VAN AVENDONK ET AL.: CONTINENTAL COMPRESSION SOUTH ISLAND Figure 7. Seismic velocity structure along Transect 1 stretches from submerged and undeformed Australian continental crust to the northwest across the west coast of South Island at 213 km, the Alpine Fault at 228 km, the east coast at 271 km, and into the submerged Pacific plate. Red triangles show the location of ocean bottom and land instruments that recorded the wide-angle seismic data. Figure 11. Diagonal of the resolution matrix shows very good resolution near the surface in South Island. Resolution is much poorer at larger depth in the model. Trade-offs between various model parameters at large depth result in low resolution. 6 of 16 and 11 of 16 B06302