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Tectonic erosion along the Japan and Peru convergent margins R. VON HUENE U.S. Geological Survey, M.S. 999, 345 Middlefield Road, Menlo Park, California 94025, and GEOMAR, Kiel, West Germany S. LALLEM AND Université Pierre et Marie Curie, Paris, France ABSTRACT INTRODUCTION The volume of material removed by subduction erosion can be estimated quantitatively if the position of the volcanic arc, the position of the paleotrench axis, and a paieodepth reference surface are known. Estimates based on these parameters along the Japan and Peru Trenches indicate rates of erosion comparable to well-known rates of accretion. Proposed erosional mechanisms along the plate boundary, where horsts on the lower plate abrade the upper one, appear insufficient to handle the minimum volumes of eroded material. Some mechanisms of tectonic erosion at the base of the trench slope can be observed at colliding seamounts and ridges where structures are large enough to be seismically imaged. Local tectonic erosion of the lower slope of the Japan Trench resulted when seamounts entered the subduction zone, uplifted the slope, and oversteepened it. The oversteepened slope failed, debris slumped into the trench axis, and much of it was then subducted. Where a seamount was subducted, a large re-entrant was left in the slope, which filled rapidly by local accretion of abundant sediment. Subduction of the oblique-trending Nazca Ridge off Peru produced many similar structures. Erosion is dominated by uplift and breakup of the lower slope, with subduction of the debris rather than abrasion under high-stress conditions. Modern convergent margins are commonly associated with the accretion of sediment from the subducting lower plate to the upper plate, whereas erosion of the upper plate has received much less attention. The difference in attention may result from the difficulty in resolving erosional structures in seismic records. Tectonic foreshortening of the lower slope of a trench has been reported and is explained by lateral translation of terranes or by erosion of the upper plate through piecemeal subduction. The latter has been called "subduction erosion" (Scholl and others, 1980). In this paper, we examine subduction erosion but begin with a review of evidence for the subsidence of the Japan and Peru margins and discuss why it probably originates from the tectonic erosion of the upper plate. We then estimate quantities of materials removed along these margins and find rates of erosion that are similar to rates of accretion; these rates exceed those that can be accommodated by the horst and graben chain-saw model (Hilde, 1983). To explain these high rates of erosion, we reexamine some mechanisms of subduction erosion and illustrate morphologies and structures indicative of erosional processes. Another form of tectonic erosion occurs along the base of the upper plate. Its magnitude is indicated by massive subsidence along the margin; however, because of deep burial, the structure resulting from basal erosion is rarely imaged in seismic records. The volume of material eroded along the base of the upper plate exceeds that eroded from the front of the lower slope. Tectonic erosion along modern convergent margins was first demonstrated convincingly when Deep Sea Drilling Project (DSDP) drilling penetrated ancient rocks near the trench axis. The study of drill cores from the Japan (Legs 56 and 57), Middle America (Legs 66,67, and 84), and Peru (Leg 112) Trenches showed nonaccretion or Neogene accretionary complexes from 10 to 15 km wide stacked against a buttress of Mesozoic and Paleozoic consolidated or metamorphosed rocks. The older rocks observed in DSDP cores required tectonic erosion of the missing rock that once covered or extended seaward from them. Geological Society of America Bulletin, v. 102, p. 704-720, 11 figs., 1 table, June 1990. 704 Both tectonic accretion and tectonic erosion occur along the lower slope of convergent margins and along the underside of the upper plate. Erosion along the lower slope of a convergent margin as indicated by landward retreat of the trench slope is herein called "frontal erosion." Erosion farther landward along the base of the upper plate inferred from general margin subsidence during convergence is herein called "basal erosion." EVIDENCE OF REGIONAL SUBSIDENCE, JAPAN AND PERU CONVERGENT MARGINS Regional subsidence during the latest period of plate convergence was established during the DSDP program along the Japan Trench margin (von Huene, Nasu, and others, 1978) by drilling through a subaerial erosion surface many kilometers below sea level. That erosion surface corresponds to an angular unconformity that cuts across tilted beds and is buried beneath subhorizontal strata of the outer shelf and slope (Fig. 1). The unconformity extends throughout a 150-km-long area (Nasu and others, 1980; von Huene and others, 1982) and shows no signs of ending beyond, the published seismic coverage. A similar unconformity is recorded from the adjacent Joban Basin area to the south (Mitsui, 1971; Kato, 1980), from the southern end of the Japan Trench (Lallemand and others, 1989), and from north of the Japan Trench along the Kuril Trench (S. Lallemand and R. von Huene, unpub. data). Across the unconformity, seismic velocities increase abruptly from —1.9 to 4.2 km/s (Murauchi and Ludwig, 1980), consistent with the contact between unconsolidated Oligocene to Quaternary strata and well-consolidated Cretaceous rock as drilled at DSDP Site 439. The sedimentary strata above the unconformity consist of a 48-m-thick breccia and conglomer- 705 TECTONIC EROSION ALONG JAPAN AND PERU CONVERGENT MARGINS ate of dacite and rhyolite boulders, covered by 50 m of medium-grained sand containing abundant little-transported macrofossils, which was in turn buried by silt and sand turbidites (Scientific Party, 1980) with a probable seaward source (von Huene and others, 1982). The upper 800 m of the section consists of Miocene diatomaceous mud. The regional extent of rock types and erosion was explained by subsidence of a landmass during the past 22 m.y. (Scientific Party, 1980; von Huene and others, 1982). Benthic microfossils from the sediments indicate a succession of water depths (Fig. 2) con- sistent with such a history (Arthur and others, 1980; Keller, 1980). Despite questioning the regional extent of such subsidence from drilling results on DSDP Leg 87 (Karig, Kagami, and others, 1983), the depth-versus-age relations observed in samples from DSDP Site 584 (Lagoe, 1986) are consistent with those from DSDP Sites 438/439, located about 50 km landward (G. Keller, 1988, personal commun.). Geophysical and drilling studies along the Peruvian margin have recorded a similar geology. A continuous unconformity from the shelf across the upper and middle slopes is revealed in o- 2- -320 unconformity multichannel seismic reflection records covering an -600-km-long stretch of the margin (Hussong and Wipperman, 1981; Ballesteros and others, 1988; von Huene and Miller, 1988; Moore and Taylor, 1988). At the edge of the shelf, drilling and sampling have produced crystalline basement below the unconformity and sandy Eocene strata containing shallow-water megafossils above it (Kulm and others, 1988). Beneath the continental slope, the Eocene strata above the unconformity also contain shallowwater microfauna (Sites 682, 683, and 688; Suess, von Huene, and others, 1988a; Resig, m/my P r a f a r ú n n c accreted slump debris nr 4- 6 top of oceanic sediment /—» E 8> I I H 10 CL UJ D 14- decollement truncated beds 16 top of igneous ocean crust 18- 20 JAPAN MARGIN inherited normal faults 10 i subducted trench fill and slump debris 20 km i VE = 2 sediment Figure 1. Line drawings of seismic sections across the Japan Trench (ORI78-4) after von Huene and Culotta, 1989) and Peru Trench (CDP-1 after von Huene, Suess and others, 1988), illustrating major tectonic features. Japan Trench stratigraphy was followed from DSDP sites landward of the seismic image shown here. The average subsidence over 22 m.y. is shown by arrow, and the dashed line represents the minimum seaward extent of the 22-m.y. continental slope. The Peru Trench stratigraphy is known from ODP sites (numbered), and the rates of subsidence and paleotopography of the margin at 5 Ma are indicated. Note three unconformities. 706 VON HUENE AND LALLEMAND I » S u b a e r i a l exposure Figure 2. Subsidence history at Sites 438/439, based on benthic foraminiferal stratigraphy (after Keller, 1980, and Arthur and others, 1980). Vertical bars indicate the depth ranges of benthic foraminiferal assemblages from Site S84; arrows indicate depths greater than 2,000 m (Lagoe, 1986). Considerable normal faulting is observed in seismic records, SeaBeam bathymetry, and cores across both the Japan and Peruvian margins.1 Vertical displacement rarely exceeds 100 m, and the structural pattern is commonly like the stacked domino model where blocks about 0.5 to 1.5 km wide are separated by faults that offset the entire sediment section and the underlying unconformity (Fig. 3). DSDP Sites 438/439 on the Japan margin are in a faulted area, and below the 400 m level, the cores contain many microfaults; however, thinning of individual beds by pervasive microfaulting is not noticeable in seismic records when the faulted segments are compared with adjacent unfaulted segments. LATE 0LIG. EARLY MIOCENE MIDDLE MIOCENE 1989). Rocks below the unconformity have a velocity of 5 km/s, indicating the same crystalline basement sampled at the edge of the shelf (Hussong and Wipperman, 1981), and above are stratified rocks with velocities around 2 km/s. Conventional sampling of the middle slope yelded samples containing late Miocene microfauna from shallow environments indicating subsidence (Kulm and others, 1984). At Ocean Drilling Program (ODP) drill Sites 683 and 688 on the lower slope, an upper Miocene upwelling facies was recovered (Suess, von Huene, and others, 1988b). This depth-sensitive facies presently accumulates in water shallower than 500 m. The litho- and biostratigraphy of the Peru margin indicate subaerial erosion of the crystalline basement, followed in Eocene time by deposition in shelf and upper-slope environments. The Eocene sedimentary sequence was again eroded in Oligocene and early Miocene time, followed by regional subsidence since early and middle Miocene time. Part of that regional subsided surface was uplifted before and during subduction of the Nazca Ridge, and beneath the Lima Basin, it has subsided since late Miocene time (von Huene, Suess, and others, 1988). A subaerial rather than submarine origin for these unconformities is supported by differences LATE MIOCENE PUÒ. laUAT. _ l I from disconformities commonly mapped in seismic stratigraphic studies. Along the Japan and Peru Trench margins, the erosional unconformity is a regional feature extending thousands of kilometers beneath the shelf and lower slope. The rock truncated along the unconformity is hard and resistant to erosion; truncated beds have an apparent thickness of at least 1 km, and the metamorphosed and crystalline rock sampled requires the removal of a thick overburden. The older rock below the unconformity is separated from the younger rock above it by a hiatus spanning many cycles of change in sea level. Post-erosional sediment on the unconformity is conglomerate and sandstone of a near-shore shallow-water lithofacies, consistent with the enclosed biofacies upon which mudstone from successively deeper-water environments was deposited. These observations are most easily explained by subaerial and surf-zone erosion followed by regional subsidence to present levels. On the slope, where the sediment section is flexed downward toward the trench, the faults become more numerous. Normal faults between rotated blocks 300-500 m wide also occur in the stacked domino configuration (Leggett and others, 1987). At DSDP Site 584, the stratigraphy in 3 holes drilled 0.5 to 0.75 km apart corresponds with general seismic structure only if many normal faults between the holes are inferred (Karig, Kagami, and others, 1983). The structure imaged with further processing of the seismic record since DSDP drilling supports that interpretation (R. von Huene and J. Miller, unpub. data). The number of small faults increases toward the mid-slope until coherent reflections are no longer resolved with the seismic techniques used. Much of the normal faulting on the slopes is thought to represent slope failure driven by gravity tectonics (von Huene and Culotta, 1989). The structural pattern seen in seismic records off Japan is consistent with structures at the smaller scale of drill cores from the Peruvian margin. Extensional structures on the Peruvian shelf are associated with abundant dewatering features, subsidence, and local extension, whereas on the slope, struc- CAUSES OF SUBSIDENCE Subsidence of a continental margin could result from a change in the configuration of the Benioff zone or from crustal thinning. The mechanisms commonly invoked as causes for crustal thinning are (1) listric normal faulting and (2) subcrustal erosion. 'For Japan, compare Nasu and others, 1980; von Huene and others; 1982; Kagami, Karig, and others, 1986; Leggett and others, 1987; Cadet and others, 1987; von Huene and Culotta, 1989; for Peru, compare Hussong and Wipperman, 1981; von Huene and others, 1985; Thornburg, 1985; Bourgois and others, 1986, Ballesteros and others, 1988; Moore and Taylor, 1988; von Huene and Miller, 1988. TECTONIC EROSION ALONG JAPAN AND PERU CONVERGENT MARGINS 707 Figure 3. Seismic record Shell P-849 (Lehner and others, 1983), reprocessed at the U.S. Geological Survey, showing the unconformities cut across landward-dipping Cretaceous strata and the overlying upper Oligocene to Quaternary section cut by normal faults. Vertical exaggeration at sea floor, 2.5x. ture is commonly related to gravity sliding (Kemp and Lindsley-Griffin, 1989). Crust thinned by normal faults is exemplified in the Basin and Range province of western North America and along some passive margins of the Atlantic (compare Wernicke and Burchfiel, 1982; Beach and others, 1987; Sibuet and others, 1987). Boillot and others (1987) have advanced a simple shear interpretation of the seismic reflection data and results from deep ocean drilling along the Galicia margin where half-graben blocks are 13 to 18 km wide in the upper slope and 9 to 13 km beneath the lower slope. Faults displace the ocean crust from 4 to 7 km, and the possible plane of horizontal detachment is 2 km above the Moho (Sibuet and others, 1987). By comparison, the greatest normal fault displacement imaged along the Peruvian margin offsets an 11-km-wide block 880 m along a steep fault that is not imaged through the upper plate (Bourgois and others, 1988; von Huene and others, 1989). Some faults not so clearly imaged on the Japan margin could involve displacements of several hundred meters (von Huene and Culotta, 1989). Extensional structures along the Japanese and Peruvian margins are much smaller than the extensional structures associated with crustal thinning in the Basin and Range or on passive margins. The Japanese and Peruvian margins have 150-km-wide areas without major extensional structure that have subsided 3 and 4 km (Fig. 3). In such areas, crustal thinning by listric faulting in a simple shear system is not possible. In areas of steeper topography on the landward slope of the trench, the abundant small normal faults may thin the sediment section; however, they are not sufficient to explain the 4 to 6 km of subsidence observed there. Changes in the configuration of a subduction zone may be caused by loading as the accretionary prism grows (Karig and others, 1976) or as the subducted oceanic crust changes density (Langseth and others, 1981). Because a relative negative buoyancy of the descending cold dense oceanic lithosphere is a main force configuring the subduction zone, changes in the age (temperature), the rate and direction of convergence, and the thickness of the subducting lithosphere are the most likely causes of change in subduction zone configuration. The plate-tectonic history of the Japanese margin contains little change in age of the subducting crust, the relative rate of plate convergence, or the subduction of major features on the oceanic lithosphere during the past 22 m.y., as discussed below.2 On northern Honshu, the position of the volcanic front has re- 2 The Japan triple junction remained south of the area of observed subsidence (Jolivet and others, 1989). mained within a belt the width of which is 12% of the arc-trench distance since 22 Ma except for a single extrusion at the southern end of the trench near the triple junction (Tsunakawa, 1986). Furthermore, the retreat of the Japan Trench margin would locally unload the lower plate, causing it to rise rather than subside. A major plate-tectonic change observed along the Peruvian margin is the subduction of the Nazca Ridge. Subduction of the ridge was concurrent with the change from erosional to accretionary tectonics along the northern transect of ODP Leg 112 studies (Suess, von Huene, and others, 1988a). The uplift and subsidence of the Lima Basin correlates approximately with the subduction of the leading and trailing ridge flanks, respectively, based on plate reconstruction (Cande, 1985) and drilling results (Suess, von Huene, and others, 1988a). The adjacent Trujillo and Salaverry Basins show little stratigraphy in seismic records (Thornburg, 1985) or cores to indicate a period of uplift and subsidence during ridge subduction. Only Lima Basin shows the subsidence. Morphology in the present area of ridge subduction shows topographic expression of uplift or subsidence only along the lower continental slope (Prince and others, 1980). The depth of the Benioff zone just landward of Lima Basin decreased after subduction of the ridge (Boyd and others, 1984) in a vertical sense opposite to subsidence of the basin. 708 VON HUENE AND LALLEMAND Although the 8 Ma to Quaternary subsidence of Lima Basin was local, subsidence of the 21- to 22-m.y. surface (Suess, von Huene, and others, 1988a) cut on Eocene and Oligocene strata extends through all forearc basins off Peru. From the latitude of the Nazca Ridge to the Ecuadorian border, plutonic activity from middle Miocene to early Pliocene time appears to have stayed in a single belt (Sillitoe, 1988). The lack of major plate changes, shifts in the volcanic-arc position, and decrease rather than an increase in depth of the Benioff zone suggest little direct influence on subsidence from changes in the configuration of the descending slab. SEISMIC IMAGES OF TECTONIC EROSION Frontal Erosion Frontal erosion is illustrated in a seismic record across the Japan Trench (Fig. 1). A pervasive 1-km-high scarp in the lower slope (Cadet and others, 1987) produces debris that has accumulated at its base. This debris is accreted like the sediment transported into the trench (von Huene and Culotta, 1989), but most of the de- bris and trench sediment must be subducted because accretion has not piled the sediment sufficiently high to stabilize the slope. An indication of the entrainment of debris from mass wasting into the subducted sediment is the 300to 400-m thickening of the underthrust sediment layer as it passes landward of the trench axis (Fig- 1). Frontal erosion is also required to explain the structure seaward of the Lima Basin. The lithoand biostratigraphy established during ODP Leg 112 along the Peruvian margin (Suess, von Huene, and others, 1988a) indicate a subaerial erosion surface beneath the Eocene and Oligocene shelves, and because the Eocene shelf is now near the trench axis, erosion of the missing Eocene trench slope is required. That erosion occurred during the period of plate convergence when the present Andes were formed. The end of frontal erosion along the lower slope north of Lima Basin was dated during Leg 112 at Site 685, and it corresponds in time to the subduction of the Nazca Ridge. Basal Erosion An erosional origin is proposed for truncated bedding seen 15 to 30 km down the Japan Trench subduction zone in a reprocessed seismic record (von Huene and Culotta, 1989). Above the layer of subducting sediment are landwarddipping beds truncated at the décollement surface (Fig. 1). These beds crop out at the sea floor, where dredging produced samples of rocks lithologically equivalent to the Cretaceous silicified mudstones drilled at DSDP Site 439. Positive age equivalence was not established because the dredge samples lacked age-diagnostic fossils, but radiometric analysis of micas indicates an age in excess of 28 m.y. (Takigami and Fujioka, 1989). Because the small accreted wedge along the Japan Trench is 20 m.y. younger than this minimum age where drilled (Scientific Party, 1980) and is lithologically different from the dredged rock, the best correlation is with the Cretaceous or perhaps an unsampled Paleogene basement rock of this margin. Another indication of basal erosion is seen between the Peru Trench and the Lima Basin (Fig. 1). Eocene strata of shallow-water bio- and lithofacies drilled at Site 688 cover rock that has seismic velocities of the crystalline basement. These Eocene rocks can be followed down a seaward-dipping incline and are now truncated at the décollement along the plate boundary. Such a structure requires the removal of considerable underlying crystalline basement (Fig. 1). ESTIMATION OF MATERIAL FLUX ALONG THE JAPAN AND PERU CONVERGENT MARGINS volcanic arc Method 1 100 km The amount of material removed during erosion of the Japan and Peru margins was estimated by reconstructing their former configuration and comparing it with their present one. The effects of large thermal changes were circumvented by selecting a former configuration when the volcanic arc was already in its present position. The parameters involved in the reconstruction are shown in Figure 4, where dashed lines indicate the past, and solid lines the present, configurations of the margin. lokin-l Figure 4. Diagram illustrating parameters from which a quantitative estimate of subduction erosion and retreat of the slope was made. S indicates the depth of subsidence across a margin as indicated by biostratigraphy and other measures of past water depth. Pb is the paleobathymetric profile reconstructed from stratigraphic geometry, such as the downlap on an unconformity observed in seismic records. Rt is the distance that the trench slope retreated landward during erosion. The paleoslope is reconstructed from the seawardmost point (point A) at which a paleodepth can be established. A paleoslope is projected to the paleotrench depth from point A, using the present slope angle under the assumption that the ancient critical-wedge angle was similar to that along the modern trench landward slope. Ps is the paleoslab, and its profile is approximated by joining the paleotrench axis to a 100-km-deep point beneath the paleoarc. The Magnitude of Frontal Erosion along the Japan Trench Margin We estimated erosion of the Japan Trench margin in the period since opening of the Sea of Japan. DSDP drill cores indicate that the beginning of explosive arc volcanism was in early Miocene time (20 Ma) (Cadet and Fujioka, 1980). Togashi (1983) reported a shift to islandarc composition of the volcanic rocks on northern Honshu after 20 Ma. A sudden change in TECTONIC EROSION ALONG JAPAN AND PERU CONVERGENT MARGINS 709 Figure 5. Cretaceous and Jurassic magnetic lineations of the northwest Pacific west and north of Shatsky Rise. For simplicity, only each fifth isochron is shown. Sources of data are Sager and others (1989), dark numbers and solid lines; Hilde and others (1976), light numbers and dotted lines. C.Q.M.Z., Cretaceous quiet magnetic zone; K.K.T., Kuril-Kamtchatka Trench; J.T., Japan Trench; I.B.T., Izu-Bonin Trench; N.T., Nankai Trough; PT, paleotrench; M.T., Mariana Trench; E. smt, Erimo Seamount; D.K. smt, Daiichi Kashima Seamount. Thin-shafted arrow indicates the amount and direction of convergence since 20 Ma. rate of rotation of the Oga Peninsula after 20 Ma is observed in studies of paleomagnetism (Tosha and Hamano, 1988). From these tectonic and volcanic events, we assume that the present arc-trench system began about 20 Ma. Oceanic magnetic lineations along the Japan Trench (Fig. 5) trend N65°E; the youngest is M-8 (129 Ma; Kent and Gradstein, 1985), and the oldest is probably anomaly M-15 (140 Ma) (Sager and others, 1989). Thus, the average age of the present subducting lithosphère is 135 ± 5 m.y. From 0 to 5 Ma, the convergence of the Pacific and Eurasian plates off Japan was 114 km/m.y. at 288°, and from 5 to 28 Ma, it was 710 VON HUENE AND LALLEMAND Figure 6. The present and the reconstructed landward slope of the Japan Trench, based on parameters developed from observations to show the trench retreat for different values of paleotrench depths. From point A, a paleoslope (Rt) is projected to depth Pt (see Fig. 4). The present slope angle was used for the projection, assuming that the missing materials and wedge angle were similar to those along the present trench landward slope. VE = 2x. 94 km/m.y. at 281°-282° (Engebretson and others, 1985). We inferred a paleotrench 100 km oceanward of the present trench axis (developed below). To establish the depth of the paleotrench, it is necessary to backtrack the subducted lithosphere and reconstruct its age 2,400 km west of the 135 m.y. isochron. As the left-lateral transform offset along the southeast-trending fracture zones is subducted, the magnetic lineations west of the Japan Trench axis become younger (Fig. 5). The lack of the history of transform motion on the subducted oceanic crust leaves few constraints on the age of the oceanic crust much beyond the 83 m.y. isochron. Thus, this isochron yields a minimum backtracked crustal age of 61 m.y. at 20 Ma. Hilde and Uyeda (1982) showed an empirical relation between the age of the subducting lithosphere and trench depth. The present Japan Trench is 3.5 km shallower than predicted, despite the conformity of the adjacent ocean lithosphere to the subsidence curve of the Pacific basin off Japan (Parsons and Sclater, 1977; Heestand and Crough, 1981). We assume that at 20 Ma, the trench depth was consistent with a range of 8 ± 1 km in the depths where 61-m.y. or older crust is being subducted. Thus, the paleo-Japan Trench depth minimum is 7 km. With this information, we estimated a retreat of the Japan Trench slope since 20 Ma. The subaerially eroded Paleogene unconformity was followed in seismic records to point A (Fig. 6). The present depth of point A along seismic line ORI 87-4 and Shell P-849 is approximately 6,400 and 5,625 m below sea level, respectively (von Huene and others, 1982; von Huene and Culotta, 1989) and is located only 15 km from the present trench axis in both lines. Along seismic line JNOC-2, point A is imaged 30 km from the trench axis at a depth of 5 km (Nasu and others, 1980) and was subaerial until about 16 Ma3 (Nasu and others, 1980). If an average continental slope is assumed, dipping 5° seaward to the trench axis from point A on seismic line ORI 78-4, the result is a retreat (Rt minimum) of 75 km in 20 m.y. or about 3 km/m.y. (Fig. 6). The same calculation using the parameters from JNOC-2 gives a minimum retreat of 50 km in 16 m.y. or also about 3 km/m.y. This rate of retreat is nearly twice as great as the conservative estimate made previously (von Huene and others, 1982). The Peru Trench Margin The same approach to estimate retreat was applied to the Peru Trench margin in the area of Lima Basin for two time periods. The paleoconfiguration of the Peru margin was first constructed from the regional unconformity that subsided since 20 Ma as shown by ODP Leg 112 drill cores. Seismic data indicate that the unconformity on top of the Eocene continues to its termination near the trench axis (Figs. 1 and 7). Sediment overlying that unconformity was again eroded during the subduction of the Nazca Ridge. The younger unconformity was dated at site 679 by a hiatus between 8 and 11 Ma. The subduction of the Nazca Ridge at this latitude is 3 The age of final inundation of the paleolandmass was estimated at 10 m.y. (von Huene and others, 1982), but this age was revised to 16 m.y. based on improved seismic imaging across the normal faults in the west part of the area shown in Figure 3. seen in the subsidence at Sites 682 and 688 after 5 Ma, which is consistent with the plate reconstruction of Cande (1985). From these two dates, the reaction of the margin to subduction of the Nazca Ridge appears more complex than simple uplift and subsidence of a passive prism over the subducting ridge, and we consider the estimate of an erosional rate from the Sites 679, 682, and 688 paleobathymetry a possible maximum in the Lima Basin area. Oceanic magnetic lineations along the Peru Trench were backtracked to 20 Ma by Cande (1985). Anomalies 30 and those older than 33 were in the trench axis opposite Lima Basin at 8 and 20 Ma, respectively (Fig. 8), resulting in a trench depth of ~8 km (Table 1), in accord with the trench depth/age relationship (Hilde and Uyeda, 1982). In the Lima Basin area, point A' was established from the seawardmost seismic image of the Oligocene-Miocene unconformity (Fig. 1). Point A was established from cores from ODP Sites 683 and 688, located about 30 km landward of the present trench axis. At Sites 683 and 688, cores containing primary coastal upwelling facies with 5-m.y.-old shallow-water microfauna were recovered (Suess, von Huene, and others, 1988b). Similar coastal upwelling facies are presently deposited off Peru in water no deeper than 350 m. Rates derived by inferring subsidence of the unconformity from sea level during the past 5 m.y. are similar to those required by the microfaunal assemblages in dredged samples from Lima Basin (Kulm and others, 1984). Point A was placed along the seawardmost recovery location of primary upwelling deposits that were drilled at Site 688 (Fig. 7). The paleoslope was projected at the average angle of the present lower slope to a PERU TRENCH Lima Basin area Neogene 50km w VE = 5.4 Figure 7. Cross sections of the Japan and Peru Trench subduction zones, showing configurations used to calculate amounts of material eroded. The volume of ocean crust that compensates for removal of surface material is calculated using the method of Karig and others (1976). The observations that constrain subsidence are from DSDP/ODP drilling results and reprocessed seismic records showing extent of subaerial erosion. 712 paleotrench depth of ~ 8 km (Table 1). An estimated 28-km retreat of the lower slope of the trench in 8 m.y. gives an average rate of retreat of about 3.5 km/m.y.; the 50-km retreat in 20 m.y. gives a rate of about 2.5 km/m.y. The latter rate is a minimum, because it was estimated without regard to the recent episode of accretion and advance of the trench slope (Fig. 1) that began about 3 Ma (von Huene, Suess, and others, 1988). THE MAGNITUDE OF BASAL EROSION Basal erosion is a principal explanation for the kilometers of Neogene subsidence along the Japan and Peru margins. Subsurface erosion can be computed from the amount of subsidence; however, it requires a knowledge of isostatic VON HUENE AND LALLEMAND compensation, which has clear maximum and minimum limits. If the lower plate in a subduction zone behaves rigidly, then the mass removed along the base of the upper plate is not much more than subsidence of the sea floor. If, on the other hand, the lower plate is compliant, then high-density lower-plate material will replace and compensate for the lower-density eroded mass. Therefore, at the one extreme, the subsidence at the surface approximates the volume of material eroded, and at the other extreme, it represents only a fraction of the eroded material because the lower plate has isostatically risen to fill some of the space left by subsurface erosion. Isostatic compensation of lithosphère within an oceanic plate has been modeled as a thin elastic layer over a fluid (Watts and Steckler, 1979; Steckler and Watts, 1982; Karner and Watts, 1982). Compensation is largely a function of lithospheric rigidity, which depends on its age; the increased flexural rigidity of the oceanic plate with increasing age is known (Watts and Cochran, 1974; Watts, 1978; Watts and others, 1980). The load of the island of Oahu was compensated in less than 0.5 m.y., despite the Mesozoic oceanic crust on which it was built (ten Brink and Watts, 1985). Thus, isostatic compensation of oceanic lithosphere appears to be rapid relative to the millions of years considered here. Along a convergent margin, the rate of isostatic adjustment appears to be similar. The subducted part of Daiichi Kashima Seamount is compensated by about 1,200 m of crustal depression in 0.2 m.y. of subduction (Lallemand and others, 1989). The rate of compensation is similar to compensation of continental litho- Figure 8. Nazca-South America plate interactions since 20 Ma, after Cande (1985). Arrow shows the backtracked path of a point in Lima Basin along the seismic line used for Figure 7. The locations of the ridge relative to South America at 5 and 10 Ma are plotted, and subducted magnetic lineations are outlined, on the basis of the mirror image of the oceanic crust near Tuamotu Ridge. TABLE 1. SUMMARY OF MODEL PARAMETERS AND RESULTS Japan Time of reconstruction: T Trench depth Subducting lithosphere age Landward trench retreat: Rt Distance trench-point A Depth of point A Mean dip, landward slope Cross-sectional area of subsidence = minimum eroded Peru (Lima Basin) 20 m.y. 8 ± 1 km >61 m.y. 7.5 km 135 ± 5 m.y. 50 km 15 km 6.4 km 4° <0km «5° 8 m.y. 8 km 66 m.y. 6 km 45 m.y. 8 km 35 km 4 km 6° 0-0.35 m 6° 20 m.y. >8 km >60 m.y. 6 km 45 m.y. 50 km 18 km 6.3 km 6° 0 km 6° 800 km 2 290 km 2 480 km 2 Assuming complete isoslatic compensation 1,100 km 2 370 km 2 610 km 2 Estimated erosion rate (40)-55 km 2 /m.y. (361-46 km 2 /m.y. (24)-31 km 2 /m.y. Note: the rates of erosion in parentheses are derived from the area of subsidence alone without any compensation by the lower plate; thus, they are limiting minima. normal faults partially filled re-entrant KATORI S M T . 20km VE=1,5 Figure 9. Perspective diagram of Daiichi Kashima and Katori Seamounts at a vertical exaggeration of 1.5*. Katori Seamount illustrates the first stage of subduction, where the trench axis becomes constricted and normal faults begin to break up the seamount. Note filled re-entrant from collision with a seamount at a prior time just opposite Katori Seamount. Daiichi Kashima was at the stage equivalent to that of Katori 0.1 to 0.2 Ma, and about half of the leading flank has been subducted. Above the subducted leading flank, the upper plate has been pushed up as much as 1,000 m, and beneath the seamount, the ocean crust has been depressed about 1,000 to 1,400 m. 714 sphere during déglaciation; however, calculations of erosion that assume complete compensation yield high volumes of eroded material. We use the model of a one-dimensional bending of a thin elastic plate subjected to a hydrostatic restoring force, developed by Karig and others (1976), to estimate the deflection of the lower plate. Basal Erosion along the Japan Trench Margin The time stratigraphy of the DSDP drill holes was extended into seismic records that show the transgression of Neogene slope deposits over the subaerial erosion surface. At Sites 438/439, the subaerial erosion ceased and marine deposition began in latest Oligocene time (Nasu and others, 1980). The shoreline transgressed eastward of Sites 438/439 across a gentle slope (Fig. 3, right) until about 16 Ma, when the last insular topographic highs at the edge of the shelf were overwhelmed (Fig. 3, left). The history of inundation, clearly observed across the landward flank of the insular topography, is obscured in seismic records on the seaward flank by tectonism, seafloor erosion, and great water depth. Therefore, point A was placed on the landward flank and is a minimum extent of the erosion surface. From 16 Ma to the present, only the average rate of subsidence can be determined except at Sites 438, 439, and 548, where benthic foraminiferal assemblages were recovered. Erosion along the Japan margin is estimated from a generalized crustal cross section (Fig. 7). Due to isostatic adjustment, the configuration of the descending slab adjacent to the trench axis probably changed as the slope subsided and retreated, but the generally stationary volcanic arc on northern Honshu during the past 20 m.y. (Tsunakawa, 1986) suggests general stability of the plate boundary. Subsidence of the Japan margin during the past 20 m.y. has substituted water for a cross-sectional area of 800 km 2 of rock (Table 1). Isostatic compensation (Fig. 7) increases the cross-sectional area of eroded material to 1,110 km 2 . The average rate of erosion per million years is 55 km 2 (Table 1). Transport of this volume of eroded material down the subduction zone at the 100 km/m.y. average rate of convergence requires that on average, a 550-m-thick layer of eroded material be incorporated into the subducting sediment. Basal Erosion along the Peru Trench Regional subsidence of the Peru margin since late Oligocene and early Miocene time is shown by shelf and upper-slope biostratigraphic indicators (Suess, von Huene, and others, 1988a). Su- VON HUENE AND LALLEMAND perposed on the regional subsidence is the local uplift and subsidence of Lima Basin over the past 8 m.y., which is known with greater precision than is the regional subsidence. Subsidence of the middle of Lima Basin occurred at a rate of more than 0.5 km/m.y. over 8 m.y.; however, subsidence began after 5 Ma at Site 688. We calculated the rate of erosion local to Lima Basin over the past 8 m.y. and also erosion over 20 m.y. The estimate of local erosion across Lima Basin was made in the same manner as for the Japan Trench margin. Restoration of the margin at 8 Ma (Fig. 7 and Table 1) was controlled by microfossil assemblages in drill holes (Suess, von Huene, and others, 1988a) and by conventional sampling (Kulm and others, 1984). The eroded cross-sectional area is 370 km 2 , assuming compensation (Karig and others, 1976) for the eroded mass. The estimated erosion rate is 46 km 2 /m.y., which requires subduction of a layer of continental material 460 m thick. The estimate of erosion in the past 20 m.y. is calculated using the regional unconformity as a sea-level reference surface. Point A' is the seaward end of the eroded middle Eocene strata (Fig. 7). The eroded cross-sectional area is 610 km 2 , assuming compensation (Karig and others, 1976) for the eroded mass (Table 1). The 31 km 2 /m.y. erosional rates estimated for the 20m.y. period are about 65% of those estimated for the local erosion of Lima Basin since 8 Ma, but the latter are closer to those estimated along the Japan Trench. DISCUSSION Our estimates of the amount of eroded material are surprisingly large, even those indicated by the limiting minima (Table 1). These minima are equivalent to the area of subsidence and unrealistic, because they fail to account for any isostatic compensation. Retreat of the Japan and Peru Trench margins occurred at similar or greater rates than the advance from tectonic accretion at some other margins.4 Such rates of subduction erosion have not been previously reported, raising the issue of an adequate tectonic mechanism. The most commonly inferred mech- 4 The 3-km/m.y. retreat along the Japan margin exceeds the 2-km/m.y. rate of lower-slope advance from accretion along the Middle America Trench off southern Mexico (20-km-wide accretionary complex that is about 10 m.y. old; Watkins, Moore, and others, 1982). It is about equivalent to the rate of slope advance along the Nankai Trough (Karig and Angevine, 1986) and Barbados lower slope (Mascle, Moore, and others, 1987). Thus, the rate of erosion can be as rapid as that of accretion. anism of subduction erosion is abrasion by the horst and graben on the subducting ocean crust where the horsts scrape away part of the upper plate, and the graben transport this material down the subduction zone (Schweller and Kulm, 1978; Hilde and Sharman, 1978). Such a mechanism is inadequate here, because (1) the observed graben have but half of the volume of the eroded material5 and (2) the subducted graben are 500 to 800 m beneath the décollement (von Huene and Culotta, 1989, and Fig. 1), preventing any contact of the abrasional teeth (horsts) with the upper plate. Furthermore, the overall coherent appearance of the subducted sediment, even - 3 0 km landward of the trench axis and as much as 15 km deep (von Huene and Culotta, 1989), observed overpressure (Carson and others, 1982; Cadet and others, 1987), and the lack of larger subductionzone earthquakes beneath the lower and middle slope (Yoshii, 1979) indicate conditions of reduced, rather than high, friction along the plate boundary. Mechanisms of Frontal Erosion The erosional action of ocean floor topography in the Japan and Peru subduction zones appears to involve wedging of positive topographic features into the subduction zone, with uplift and breakup of the upper plate. The subduction of seamounts and ridges produces structural and morphological effects sufficiently large to detect by swathmapping and seismic techniques. Two of four reported observations (von Huene, 1986; Lallemand and Le Pichon, 1987; Collot and Fisher, 1988, 1989; Lallemand and others, 1989; Yamazaki and Okamura, 1989; Ballance and others, 1989) are illustrated here with perspective diagrams (Figs. 8 and 9) to form the basis for a model of frontal erosion. Two subducting seamounts along the Japan Trench are used to show stages of frontal erosion. Daiichi Kashima Seamount (Kobayashi and others, 1986) at the west end of a chain of seamounts (Fig. 9) is the latest to subduct beneath the Japan Trench. Daiichi Kashima is on the seaward slope of the trench where horst and graben on the flexed oceanic crust continue across the seamount. The subducted leading flank of the seamount, imaged in seismic records (Lallemand and others, 1989), dips less than 15°, and underthrusting of the flank has elevated the upper plate by wedging it upward. In front of and over the subducted flank of the seamount, 5 The present average graben depth ( - 4 0 0 m) and spacing (~ 10 km) with a constant rate of 100 km/m.y. of convergence are assumed as past conditions also. TECTONIC EROSION ALONG JAPAN AND PERU CONVERGENT MARGINS 715 |aubPUCTgp ERIMO SMT. 20km VE=3 Figure 10. Two perspective diagrams of the Erimo Seamount area. Upper: looking northeast toward Hokkaido Island. Smoothed bathymetry is from conventional soundings; area inside light dashed line is from SeaBeam data (Cadet and others, 1987), at 3x vertical exaggeration. Lower: looking north-northwest toward Hokkaido Island, same data as above, with adjacent SeaBeam coverage to the south but only 1.5x vertical exaggeration. contractile strain increases, so that uplift also involves thickening by thrust faulting. Where the landward slope of the trench has been observed and sampled from a submersible, it consists of highly fractured mudstone and includes clasts of seamount material (Cadet and others, 1987). Some of the rubble from the seamount appears to be transferred to the landward slope, but the bulk of the seamount remains with the subducting oceanic plate. At the northern end of the Japan Trench, at its juncture with the Kuril Trench, is Erimo Seamount (Fig. 10). Erimo Seamount is just entering the axis of the Japan and Kuril Trenches, and the normal faults paralleling the Kuril 716 VON HUENE AND LALLEMAND Trench cut the seamount. Adjacent to the seamount on the landward slope of the trench is a large re-entrant where another seamount has just been subducted (Lallemand and Chamot-Rooke, 1986; Yamazaki and Okamura, 1989). Evidence for the buried seamount is a strong magnetic anomaly from a source beneath the bathymetric high reflecting the subducted seamount. Modeling of the anomaly indicates the presence of a buried seamount about 2 km high and about 30 km in diameter. Lallemand and Le Pichon (1987) modeled a subducting seamount like Daiichi Kashima from its present stage of subduction backward in time to its pre-collision configuration. The subducted leading flank of the seamount increased the critical taper of the margin front and steepened its slope, causing collapse. The critical taper is a function of the internal friction in the accreted sediment (Davis and others, 1983), which becomes overcritical as the seamount begins to subduct, thereby causing collapse, and undercritical after the seamount has subducted, causing accretion. The lower slope re-entrants along the Japan Trench from previous seamount subduction are largely filled and difficult to detect even with SeaBeam bathymetry (Fig. 9). This filling appears to be a rapid response to a change in the critical taper as the slope adjusts to the absence of a subducting seamount. The early stages of such filling have recently been described in the New Hebrides subduction zone, where an accretionary ridge along the trench axis has closed a large re-entrant like the one near Erimo Seamount (Collot and Fisher, 1989). trench axis lower slope at critical angle of failure normal fault seamount 5° uplift above leading flank of seamount overcritical, slope in failure From these snapshots, we show in four diagrams a general model of seamount subduction (Figs. 11A-11D). In the initial diagram (Fig. Figure 11. Four stages in a general model of the subduction of a seamount. Part A shows initial stage where only trench sediment has been wedged up. B is a stage comparable to Daiichi Kashima Seamount and is patterned after it. Along trenches without strong faulting on the seaward slope, seamounts are not so severely broken by faults. C illustrates collapse of the lower slope as the seamount crest has subducted and the thickened welt that pushed up in front of the seamount slides down its trailing flank. D illustrates healing of the re-entrant by accretion along the deformation front. This stage occurs after that shown with the re-entrant in Figure 9. undercritical area of sediment ponding B undercritical i fracturing B overcritical <= TECTONIC EROSION ALONG JAPAN AND PERU CONVERGENT MARGINS Figure 11. (Continued). thickened welt of fractured rock slope failure uplift undercritical overcritical sediment ponding accretion D critical slope 5° accretion 717 11 A), the landward flank of a seamount has wedged beneath the sediment filling the trench axis. The second diagram (Fig. 1 IB) shows a stage comparable to that of Daiichi Kashima Seamount (Fig. 9). The front of the upper plate has ridden up the incline of the seamount's leading flank. Horizontal shortening and greater critical taper cause imbrication and thickening of the upper plate. The front of the margin has retreated a small amount due to contractile deformation and failure at the base of the slope. In the third diagram, the trailing flank of the seamount is subducting (Fig. 11C). The thickened welt of faulted and fractured rock that developed above the leading flank is now above the descending slope of the trailing flank. The thickened rock mass, already weakened from fracturing, is further oversteepened by the subduction of a trailing flank and is increasingly prone to gravity failure. This results in accelerated mass wasting and a retreat of the slope. The re-entrant at the southern end of the Kuril Trench (Fig. 9) illustrates a later interval than in Figure 11C, when the slope has further retreated as the crest of the seamount subducted. Mass wasting produces debris avalanches that create a sediment apron in the axis of the trench. Loading of sediment in the trench axis from repeated debris avalanches should cause elevation of pore-fluid pressure in the underlying sediment section. Sediment subduction proceeds efficiently here to remove the debris, otherwise it would accrete against the slope and stabilize it. The fourth diagram (Fig. 11D) shows how the re-entrant forms a backstop for accreting sediment. In our example, the margin is in an accretionary configuration except where the seamount is subducted. Therefore, once the seamount is subducted, the front of the margin will return to its initial state. The debris from mass wasting is left in the wake of the subducting seamount, and along with sediment deposited in the trench axis, it accretes along the deformation front and builds a low ridge. That ridge becomes the seaward flank of a lowerslope basin in which sediment ponds. This process is observed along the New Hebrides Trench where accretion is closing a re-entrant from a subducted seamount (Collot and Fisher, 1988, 1989). Filling of the re-entrant by accretion in the area where the Kashima seamounts are being 718 subducted appears to be completed between the subduction of one seamount and the arrival of the next. The seamounts are spaced about 60 km apart, and the rate of convergence is 100 km/m.y. Thus, the margin is healed in approximately 0.6 m.y. or less. The model indicates the importance of wedge configuration as a factor to determine the accretionary or nonaccretionary condition of the Japan and Peru margins despite the probable less than true Coulomb behavior of the material involved. Our proposed mechanism for frontal erosion of the Japan and Peru margins underscores the role of wedging and breakup of the upper plate to produce debris that is transported away by sediment subduction. The difference with the chain-saw model is that erosion results from subduction of a step or disturbance in the basement topography without requiring sharp hard protrusions to abrade and tear up the lower slope. The Response of Continental Margins to the Subduction of Ridges and Scarps Along the Peru Trench, the last phase of erosion was simultaneous with subduction of the Nazca Ridge; once the ridge was subducted, the margin became accretionary (von Huene, Suess, and others, 1988). The southward-traveling trailing flank of the Nazca Ridge appears to have left collapse structure and debris from mass wasting, as indicated by Leg 112 drilling at Site 685 (Suess, von Huene, and others, 1988a), and buried jumbled material that is difficult to image seismically (Kulm and others, 1986). The scarp from subduction erosion formed a backstop of crystalline and Eocene to middle Miocene sedimentary rock against which upper Miocene and Quaternary sediment accreted. Thus, many processes illustrated in our seamount model appear to have operated during the subduction of Nazca Ridge. Similar tectonic mechanisms may operate during subduction of the 250- to 500-m-high scarps of horst and graben on the ocean floor. Along the Japan Trench, some scarps continue locally as a low ridge in the accreted sediment (Lallemand and others, 1986). These could form low waves of tectonism traveling obliquely along the foot of the slope. Fault scarps are subducted frequently, because the average spacing between horsts off South America and Japan commonly averages less than 10 km (Aubouin and others, 1981; Bourgois and others, 1986; Cadet and others, 1987). Off northern Japan, where the rate of convergence is 100 km/m.y., VON HUENE AND LALLEMAND more than 20 scarps along horsts and graben are subducted every million years. Although it was shown before that the graben off Japan are insufficient to transport all the eroded debris, they may constantly erode at a rate that is less than the rate of accretion. Once the configuration of the margin becomes less accretionary, erosion may dominate. Mechanisms of Basal Erosion The truncation at the base of the upper plate of beds that predate present accretion along the Japan Trench (von Huene and Culotta, 1989, and Fig. 1) suggests erosion. The time of the truncation is unknown, but because the sea floor above the truncated beds subsided in Pliocene time, it was inferred that the basal erosion occurred at the same time. The material detached from the upper plate may be in smaller pieces than resolved in the seismic images. Plucking of many small clasts by traction along the base of the upper plate could occur in an environment of overpressured, circulating pore fluids. Upward migration of the water released during subduction has been suggested as a mechanism to "soften" the base of the upper plate (Murauchi and Ludwig, 1980). Fragmentation by hydrofracturing may produce a slurry of clasts that becomes part of the subducting sediment mass (von Huene and Lee, 1983; Piatt, 1989). Rock at the base of the upper plate may disaggregate as pressured pore fluid invades and permeates through fractures. Highly fractured, consolidated sediment that is broken into pebble- or gravelsized clasts is well known from cores recovered on the Japan margin (Carson and others, 1982; vori Huene and others, 1982; Leggett and others, 1987) and from submersible observations (Cadet and others, 1987). Although the previously discussed estimates of eroded material from the Japan and Peru margins are huge, about 100 km of ocean crust is subducted beneath these margins every million years. A layer of gravelly breccia formed of clasts from the base of the upper plate needs to average from —300 to 550 m thick in the sequence of subducted sediment to transport subcrustally eroded material of the volume estimated (Table 1). Such thicknesses are consistent with the beds paralleling the lower plate (that is, subducted sediment) imaged in seismic records of the Japan and Peru margins. It is difficult, however, to differentiate clearly between underplated and subducting strata, using the seismic data alone. The significance of the huge quantities of material eroded from the overriding plate in a con- vergent margin is the ultimate repository of the material and how this affects lithospheric processes. Where does the subducted erosional material go, to magmatic products of the associated arc, to the base of the crust where it could be magmatically underplated, or to deeper parts of the subduction zone that enter the mantle? The underplating beneath the parts of the upper plate and beneath the volcanic arc should be reflected at the surface as a topographic uplift; however, crustal thickening by overthrusting has been proposed in recent studies of Andean uplift because of the rapid vertical tectonics (Isacks, 1988). Inclusion in the volcanic and plutonic rocks of the arc should be detected by isotopic signatures. Many geochemists, however, find that their data are better explained by a model that employs the mixing of subcrustal and deepcrustal magmas, rather than contamination by continental materials in the source region. The products of the tectonic erosion beneath Lima Basin are presently not reaching the surface because of the lack of volcanic activity in this part of the Peruvian Andes. Subduction beyond the zone of partial melting seems difficult because of the low density and melting temperatures of continental material. Thus, the fate of subducted materials remains a significant problem. SUMMARY Subduction erosion, like accretion, can occur at the base of the slope or along the underside of the upper plate. Erosion along the base of the slope, or frontal erosion, has caused the landward slope of the Japan and Peru Trenches to retreat at average rates of 2 to 3 km/m.y. One probable agent of such erosion is the subduction of many topographic features, from 200-m fault scarps to 3,000-m seamounts and ridges. Subducting seamounts along the Japan Trench illustrate some stages in this process. As these seamounts subduct, the leading flank of the seamount wedges up the overriding plate, and the trailing flank lets it fall; the slope material is pushed up and weakened by fracturing, and the wedge angle becomes overcritical, causing slope failure, which sends debris avalanching into the trench axis. Gravity failure and efficient subduction of the increased volume of trench sediment local to the seamount leave a large re-entrant in the slope after the seamount has subducted. The re-entrant later fills by accretion of sediment from the trench axis when the dip of the subducting plate returns to its former condition. This sequence of processes also appears to operate where ridges are subducted. TECTONIC EROSION ALONG JAPAN AND PERU CONVERGENT MARGINS The subduction of seamounts, ridges, and horst and graben probably accelerates frontal erosion for a short time. Sustained frontal erosion is more likely caused by the fundamental configuration of the margin, but this is not yet well understood. One indication of this sensitivity to configuration is seen in the sequence of changes during seamount and ridge subduction along Japan and Peru. The Japan and the Peruvian margins are presently accretionary although not strongly so. After a seamount or ridge subducts and causes temporary local erosion, the margins return to the accretionary state. The rate of plate convergence has remained constant, and the wedge angle and sediment load have varied, indicating the control of tectonic processes by these parameters. We speculate that the constant subduction of small topographic features (including horst and graben not buried by sediment in the trench axis) is erosive but is here superposed on a stronger accretionary tectonics governed by plate configuration. Erosion along the underside of the upper plate, or basal erosion, removes more material than does frontal erosion from the Japan and Peru margins. It affects these margins by thinning the crust, thereby causing subsidence beneath the continental terrace and the slope. The mechanisms of subsurface erosion are inferential because they have not been observed. One inferred mechanism involves the invasion of overpressured water into fractures, which disaggregates the rock along the underside of the upper plate and makes a profusion of small fragments vulnerable to plucking by traction along the plate boundary. The estimated rates of erosion through a cross section of the northern Japan Trench and the Peru Trench along Lima Basin in the past 20 m.y. is from 31-55 km 2 /m.y. Such rates require subduction of a layer of material about 0.5 km thick in addition to the subducted ocean material. This amount is possible, given the thickness of reflections from subducted material in seismic records. ACKNOWLEDGMENTS We are grateful for the extensive efforts of Mark Brandon, Dan Karig, and David Scholl, who through their thoughtful and thorough reviews have helped us greatly improve earlier versions of manuscripts describing our study. The seismic data of Figure 3 were provided by Shell International Petroleum Mij. 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