Tectonic Erosion Along the Japan and Peru Convergent Margins

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 Both tectonic accretion and tectonic erosion occur along the lower slope of convergent mar- The volume of material removed by sub- Modern convergent margins are commonly gins and along the underside of the upper plate. duction erosion can be estimated quantita- associated with the accretion of sediment from Erosion along the lower slope of a convergent tively if the position of the volcanic arc, the the subducting lower plate to the upper plate, margin as indicated by landward retreat of the position of the paleotrench axis, and a paieo- whereas erosion of the upper plate has received trench slope is herein called "frontal erosion." depth reference surface are known. Estimates much less attention. The difference in attention Erosion farther landward along the base of the based on these parameters along the Japan may result from the difficulty in resolving ero- upper plate inferred from general margin subsi- and Peru Trenches indicate rates of erosion sional structures in seismic records. Tectonic dence during convergence is herein called "basal comparable to well-known rates of accretion. foreshortening of the lower slope of a trench has erosion." Proposed erosional mechanisms along the been reported and is explained by lateral transla- plate boundary, where horsts on the lower tion of terranes or by erosion of the upper plate EVIDENCE OF REGIONAL plate abrade the upper one, appear insuffi- through piecemeal subduction. The latter has SUBSIDENCE, JAPAN AND PERU cient to handle the minimum volumes of been called "subduction erosion" (Scholl and CONVERGENT MARGINS eroded material. Some mechanisms of tec- others, 1980). In this paper, we examine subduc- tonic erosion at the base of the trench slope tion erosion but begin with a review of evidence Regional subsidence during the latest period can be observed at colliding seamounts and for the subsidence of the Japan and Peru mar- of plate convergence was established during the ridges where structures are large enough to gins and discuss why it probably originates from DSDP program along the Japan Trench margin be seismically imaged. Local tectonic erosion the tectonic erosion of the upper plate. We then (von Huene, Nasu, and others, 1978) by drilling of the lower slope of the Japan Trench re- estimate quantities of materials removed along through a subaerial erosion surface many kilo- sulted when seamounts entered the subduc- these margins and find rates of erosion that are meters below sea level. That erosion surface tion zone, uplifted the slope, and oversteep- similar to rates of accretion; these rates exceed corresponds to an angular unconformity that ened it. The oversteepened slope failed, debris those that can be accommodated by the horst cuts across tilted beds and is buried beneath slumped into the trench axis, and much of it and graben chain-saw model (Hilde, 1983). To subhorizontal strata of the outer shelf and slope was then subducted. Where a seamount was explain these high rates of erosion, we re- (Fig. 1). The unconformity extends throughout a subducted, a large re-entrant was left in the examine some mechanisms of subduction ero- 150-km-long area (Nasu and others, 1980; von slope, which filled rapidly by local accretion sion and illustrate morphologies and structures Huene and others, 1982) and shows no signs of of abundant sediment. Subduction of the indicative of erosional processes. ending beyond, the published seismic coverage. oblique-trending Nazca Ridge off Peru pro- Tectonic erosion along modern convergent A similar unconformity is recorded from the ad- duced many similar structures. Erosion is margins was first demonstrated convincingly jacent Joban Basin area to the south (Mitsui, dominated by uplift and breakup of the lower when Deep Sea Drilling Project (DSDP) drill- 1971; Kato, 1980), from the southern end of the slope, with subduction of the debris rather ing penetrated ancient rocks near the trench axis. Japan Trench (Lallemand and others, 1989), than abrasion under high-stress conditions. The study of drill cores from the Japan (Legs 56 and from north of the Japan Trench along the Another form of tectonic erosion occurs and 57), Middle America (Legs 66,67, and 84), Kuril Trench (S. Lallemand and R. von Huene, along the base of the upper plate. Its magni- and Peru (Leg 112) Trenches showed nonaccre- unpub. data). Across the unconformity, seismic tude is indicated by massive subsidence along tion or Neogene accretionary complexes from velocities increase abruptly from —1.9 to 4.2 the margin; however, because of deep burial, 10 to 15 km wide stacked against a buttress of km/s (Murauchi and Ludwig, 1980), consistent the structure resulting from basal erosion is Mesozoic and Paleozoic consolidated or meta- with the contact between unconsolidated Oligo- rarely imaged in seismic records. The volume morphosed rocks. The older rocks observed in cene to Quaternary strata and well-consolidated of material eroded along the base of the upper DSDP cores required tectonic erosion of the Cretaceous rock as drilled at DSDP Site 439. plate exceeds that eroded from the front of missing rock that once covered or extended The sedimentary strata above the unconformity the lower slope. seaward from them. consist of a 48-m-thick breccia and conglomer-

Geological Society of America Bulletin, v. 102, p. 704-720, 11 figs., 1 table, June 1990.

704 TECTONIC EROSION ALONG JAPAN AND PERU CONVERGENT MARGINS 705 ate of dacite and rhyolite boulders, covered by sistent with such a history (Arthur and others, multichannel seismic reflection records covering 50 m of medium-grained sand containing 1980; Keller, 1980). Despite questioning the re- an -600-km-long stretch of the margin (Hus- abundant little-transported macrofossils, which gional extent of such subsidence from drilling song and Wipperman, 1981; Ballesteros and was in turn buried by silt and sand turbidites results on DSDP Leg 87 (Karig, Kagami, and others, 1988; von Huene and Miller, 1988; (Scientific Party, 1980) with a probable seaward others, 1983), the depth-versus-age relations ob- Moore and Taylor, 1988). At the edge of the source (von Huene and others, 1982). The upper served in samples from DSDP Site 584 (Lagoe, shelf, drilling and sampling have produced crys- 800 m of the section consists of Miocene diato- 1986) are consistent with those from DSDP talline basement below the unconformity and maceous mud. The regional extent of rock types Sites 438/439, located about 50 km landward sandy Eocene strata containing shallow-water and erosion was explained by subsidence of a (G. Keller, 1988, personal commun.). megafossils above it (Kulm and others, 1988). landmass during the past 22 m.y. (Scientific Geophysical and drilling studies along the Beneath the continental slope, the Eocene strata Party, 1980; von Huene and others, 1982). Peruvian margin have recorded a similar geol- above the unconformity also contain shallow- Benthic microfossils from the sediments indi- ogy. A continuous unconformity from the shelf water microfauna (Sites 682, 683, and 688; cate a succession of water depths (Fig. 2) con- across the upper and middle slopes is revealed in Suess, von Huene, and others, 1988a; Resig,

o- -320 m/my

2- accreted unconformity Prafarúnnc nr slump debris 4- top of oceanic sediment 6

/—» E 8- > I I 10 H subducted CL UJ trench fill and D inherited slump debris normal faults 14- decollement truncated beds 16 top of igneous 10 20 km 18- ocean crust i i JAPAN MARGIN VE = 2 20

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» Subaerial 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. On the slope, where the sediment section is flexed downward toward the trench, the faults become more numerous. Normal faults be- LATE EARLY MIDDLE LATE PUÒ. laUAT. tween rotated blocks 300-500 m wide also 0LIG. MIOCENE MIOCENE MIOCENE _l I occur in the stacked domino configuration (Leggett and others, 1987). At DSDP Site 584, 1989). Rocks below the unconformity have from disconformities commonly mapped in the stratigraphy in 3 holes drilled 0.5 to 0.75 a velocity of 5 km/s, indicating the same seismic stratigraphic studies. Along the Japan km apart corresponds with general seismic crystalline basement sampled at the edge of the and Peru Trench margins, the erosional uncon- structure only if many normal faults between shelf (Hussong and Wipperman, 1981), and formity is a regional feature extending thousands the holes are inferred (Karig, Kagami, and above are stratified rocks with velocities around of kilometers beneath the shelf and lower slope. others, 1983). The structure imaged with 2 km/s. The rock truncated along the unconformity is further processing of the seismic record since Conventional sampling of the middle slope hard and resistant to erosion; truncated beds DSDP drilling supports that interpretation yelded samples containing late Miocene micro- have an apparent thickness of at least 1 km, and (R. von Huene and J. Miller, unpub. data). fauna from shallow environments indicating the metamorphosed and crystalline rock sam- The number of small faults increases toward subsidence (Kulm and others, 1984). At Ocean pled requires the removal of a thick overburden. the mid-slope until coherent reflections are no Drilling Program (ODP) drill Sites 683 and The older rock below the unconformity is sepa- longer resolved with the seismic techniques 688 on the lower slope, an upper Miocene up- rated from the younger rock above it by a hiatus used. Much of the normal faulting on the welling facies was recovered (Suess, von Huene, spanning many cycles of change in sea level. slopes is thought to represent slope failure and others, 1988b). This depth-sensitive facies Post-erosional sediment on the unconformity is driven by gravity tectonics (von Huene and presently accumulates in water shallower than conglomerate and sandstone of a near-shore Culotta, 1989). The structural pattern seen in 500 m. shallow-water lithofacies, consistent with the seismic records off Japan is consistent with The litho- and biostratigraphy of the Peru enclosed biofacies upon which mudstone from structures at the smaller scale of drill cores margin indicate subaerial erosion of the crystal- successively deeper-water environments was from the Peruvian margin. Extensional struc- line basement, followed in Eocene time by dep- deposited. These observations are most easily tures on the Peruvian shelf are associated with osition in shelf and upper-slope environments. explained by subaerial and surf-zone erosion fol- abundant dewatering features, subsidence, and The Eocene sedimentary sequence was again lowed by regional subsidence to present levels. local extension, whereas on the slope, struc- eroded in Oligocene and early Miocene time, followed by regional subsidence since early and CAUSES OF SUBSIDENCE middle Miocene time. Part of that regional sub- 'For Japan, compare Nasu and others, 1980; von sided surface was uplifted before and during Subsidence of a continental margin could Huene and others; 1982; Kagami, Karig, and others, subduction of the Nazca Ridge, and beneath the result from a change in the configuration of the 1986; Leggett and others, 1987; Cadet and others, Lima Basin, it has subsided since late Miocene Benioff zone or from crustal thinning. The 1987; von Huene and Culotta, 1989; for Peru, compare Hussong and Wipperman, 1981; von Huene and time (von Huene, Suess, and others, 1988). mechanisms commonly invoked as causes for others, 1985; Thornburg, 1985; Bourgois and others, A subaerial rather than submarine origin for crustal thinning are (1) listric normal faulting 1986, Ballesteros and others, 1988; Moore and Taylor, these unconformities is supported by differences and (2) subcrustal erosion. 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 3 and 4 km (Fig. 3). In such areas, crustal mained within a belt the width of which is (Kemp and Lindsley-Griffin, 1989). thinning by listric faulting in a simple shear 12% of the arc-trench distance since 22 Ma Crust thinned by normal faults is exempli- system is not possible. In areas of steeper to- except for a single extrusion at the southern fied in the Basin and Range province of west- pography on the landward slope of the trench, end of the trench near the triple junction ern North America and along some passive the abundant small normal faults may thin the (Tsunakawa, 1986). Furthermore, the retreat margins of the Atlantic (compare Wernicke sediment section; however, they are not suffi- of the Japan Trench margin would locally un- and Burchfiel, 1982; Beach and others, 1987; cient to explain the 4 to 6 km of subsidence load the lower plate, causing it to rise rather Sibuet and others, 1987). Boillot and others observed there. than subside. (1987) have advanced a simple shear interpre- Changes in the configuration of a subduc- A major plate-tectonic change observed along tation of the seismic reflection data and results tion zone may be caused by loading as the the Peruvian margin is the subduction of the from deep ocean drilling along the Galicia accretionary prism grows (Karig and others, Nazca Ridge. Subduction of the ridge was con- margin where half-graben blocks are 13 to 18 1976) or as the subducted oceanic crust current with the change from erosional to accre- km wide in the upper slope and 9 to 13 km changes density (Langseth and others, 1981). tionary tectonics along the northern transect of beneath the lower slope. Faults displace the Because a relative negative buoyancy of the ODP Leg 112 studies (Suess, von Huene, and ocean crust from 4 to 7 km, and the possible descending cold dense oceanic lithosphere is others, 1988a). The uplift and subsidence of the plane of horizontal detachment is 2 km above a main force configuring the subduction zone, Lima Basin correlates approximately with the the Moho (Sibuet and others, 1987). By com- changes in the age (temperature), the rate subduction of the leading and trailing ridge parison, the greatest normal fault displace- and direction of convergence, and the thick- flanks, respectively, based on plate reconstruc- ment imaged along the Peruvian margin ness of the subducting lithosphere are the tion (Cande, 1985) and drilling results (Suess, offsets an 11-km-wide block 880 m along a most likely causes of change in subduction von Huene, and others, 1988a). The adjacent steep fault that is not imaged through the zone configuration. Trujillo and Salaverry Basins show little stratig- upper plate (Bourgois and others, 1988; von The plate-tectonic history of the Japanese raphy in seismic records (Thornburg, 1985) or Huene and others, 1989). Some faults not so margin contains little change in age of the cores to indicate a period of uplift and subsi- clearly imaged on the Japan margin could in- subducting crust, the relative rate of plate con- dence during ridge subduction. Only Lima Basin volve displacements of several hundred meters vergence, or the subduction of major features shows the subsidence. Morphology in the pres- (von Huene and Culotta, 1989). on the oceanic lithosphere during the past 22 ent area of ridge subduction shows topographic Extensional structures along the Japanese m.y., as discussed below.2 On northern Hon- expression of uplift or subsidence only along and Peruvian margins are much smaller than shu, the position of the volcanic front has re- the lower continental slope (Prince and others, the extensional structures associated with 1980). The depth of the Benioff zone just crustal thinning in the Basin and Range or on landward of Lima Basin decreased after subduction of the ridge (Boyd and others, 1984) in passive margins. The Japanese and Peruvian 2The Japan triple junction remained south of the margins have 150-km-wide areas without area of observed subsidence (Jolivet and others, a vertical sense opposite to subsidence of major extensional structure that have subsided 1989). the basin. 708 VON HUENE AND LALLEMAND

Although the 8 Ma to Quaternary subsidence bris and trench sediment must be subducted be- Trench subduction zone in a reprocessed seismic of Lima Basin was local, subsidence of the 21- to cause accretion has not piled the sediment record (von Huene and Culotta, 1989). Above 22-m.y. surface (Suess, von Huene, and others, sufficiently high to stabilize the slope. An indica- the layer of subducting sediment are landward- 1988a) cut on Eocene and Oligocene strata ex- tion of the entrainment of debris from mass dipping beds truncated at the décollement sur- tends through all forearc basins off Peru. From wasting into the subducted sediment is the 300- face (Fig. 1). These beds crop out at the sea the latitude of the Nazca Ridge to the Ecuado- to 400-m thickening of the underthrust sediment floor, where dredging produced samples of rocks rian border, plutonic activity from middle Mio- layer as it passes landward of the trench axis lithologically equivalent to the Cretaceous silici- cene to early Pliocene time appears to have (Fig- 1). fied mudstones drilled at DSDP Site 439. Posi- stayed in a single belt (Sillitoe, 1988). The lack Frontal erosion is also required to explain the tive age equivalence was not established because of major plate changes, shifts in the volcanic-arc structure seaward of the Lima Basin. The litho- the dredge samples lacked age-diagnostic fossils, position, and decrease rather than an increase in and biostratigraphy established during ODP Leg but radiometric analysis of micas indicates an depth of the Benioff zone suggest little direct 112 along the Peruvian margin (Suess, von age in excess of 28 m.y. (Takigami and Fujioka, influence on subsidence from changes in the Huene, and others, 1988a) indicate a subaerial 1989). Because the small accreted wedge along configuration of the descending slab. erosion surface beneath the Eocene and Oligo- the Japan Trench is 20 m.y. younger than this cene shelves, and because the Eocene shelf is minimum age where drilled (Scientific Party, SEISMIC IMAGES OF TECTONIC now near the trench axis, erosion of the missing 1980) and is lithologically different from the EROSION Eocene trench slope is required. That erosion dredged rock, the best correlation is with the occurred during the period of plate convergence Cretaceous or perhaps an unsampled Paleogene when the present Andes were formed. The end basement rock of this margin. Frontal Erosion of frontal erosion along the lower slope north of Another indication of basal erosion is seen Lima Basin was dated during Leg 112 at Site between the Peru Trench and the Lima Basin Frontal erosion is illustrated in a seismic rec- 685, and it corresponds in time to the subduction (Fig. 1). Eocene strata of shallow-water bio- and ord across the Japan Trench (Fig. 1). A perva- of the Nazca Ridge. lithofacies drilled at Site 688 cover rock that has sive 1-km-high scarp in the lower slope (Cadet seismic velocities of the crystalline basement. and others, 1987) produces debris that has ac- Basal Erosion These Eocene rocks can be followed down a cumulated at its base. This debris is accreted like seaward-dipping incline and are now truncated the sediment transported into the trench (von An erosional origin is proposed for truncated at the décollement along the plate boundary. Huene and Culotta, 1989), but most of the de- bedding seen 15 to 30 km down the Japan Such a structure requires the removal of considerable underlying crystalline basement (Fig. 1).

ESTIMATION OF MATERIAL FLUX ALONG THE JAPAN AND PERU volcanic arc CONVERGENT MARGINS

Method

1 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 cir- cumvented by selecting a former configuration when the volcanic arc was already in its present position. The parameters involved in the recon- 100 km struction are shown in Figure 4, where dashed lines indicate the past, and solid lines the present, configurations of the margin. lokin-l The Magnitude of Frontal Erosion along the Figure 4. Diagram illustrating parameters from which a quantitative estimate of subduction Japan Trench Margin 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 paleobathy- We estimated erosion of the Japan Trench metric profile reconstructed from stratigraphic geometry, such as the downlap on an uncon- margin in the period since opening of the Sea of formity observed in seismic records. Rt is the distance that the trench slope retreated landward Japan. DSDP drill cores indicate that the begin- during erosion. The paleoslope is reconstructed from the seawardmost point (point A) at which ning of explosive arc volcanism was in early a paleodepth can be established. A paleoslope is projected to the paleotrench depth from point Miocene time (20 Ma) (Cadet and Fujioka, A, using the present slope angle under the assumption that the ancient critical-wedge angle was 1980). Togashi (1983) reported a shift to island- similar to that along the modern trench landward slope. Ps is the paleoslab, and its profile is arc composition of the volcanic rocks on north- approximated by joining the paleotrench axis to a 100-km-deep point beneath the paleoarc. ern 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 Oceanic magnetic lineations along the Japan average age of the present subducting litho- Ma is observed in studies of paleomagnetism Trench (Fig. 5) trend N65°E; the youngest sphère is 135 ± 5 m.y. (Tosha and Hamano, 1988). From these tec- is M-8 (129 Ma; Kent and Gradstein, 1985), From 0 to 5 Ma, the convergence of the Pa- tonic and volcanic events, we assume that the and the oldest is probably anomaly M-15 cific and Eurasian plates off Japan was 114 present arc-trench system began about 20 Ma. (140 Ma) (Sager and others, 1989). Thus, the 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 the present trench axis in both lines. Along seis- seen in the subsidence at Sites 682 and 688 after others, 1985). We inferred a paleotrench 100 mic line JNOC-2, point A is imaged 30 km 5 Ma, which is consistent with the plate recon- km oceanward of the present trench axis (devel- from the trench axis at a depth of 5 km (Nasu struction of Cande (1985). From these two oped below). and others, 1980) and was subaerial until about dates, the reaction of the margin to subduction To establish the depth of the paleotrench, it is 16 Ma3 (Nasu and others, 1980). If an average of the Nazca Ridge appears more complex than necessary to backtrack the subducted litho- continental slope is assumed, dipping 5° sea- simple uplift and subsidence of a passive prism sphere and reconstruct its age 2,400 km west of ward to the trench axis from point A on seismic over the subducting ridge, and we consider the the 135 m.y. isochron. As the left-lateral trans- line ORI 78-4, the result is a retreat (Rt min- estimate of an erosional rate from the Sites 679, form offset along the southeast-trending fracture imum) of 75 km in 20 m.y. or about 3 km/m.y. 682, and 688 paleobathymetry a possible max- zones is subducted, the magnetic lineations west (Fig. 6). The same calculation using the param- imum in the Lima Basin area. of the Japan Trench axis become younger (Fig. eters from JNOC-2 gives a minimum retreat Oceanic magnetic lineations along the Peru 5). The lack of the history of transform motion of 50 km in 16 m.y. or also about 3 km/m.y. Trench were backtracked to 20 Ma by Cande on the subducted oceanic crust leaves few con- This rate of retreat is nearly twice as great as the (1985). Anomalies 30 and those older than 33 straints on the age of the oceanic crust much conservative estimate made previously (von were in the trench axis opposite Lima Basin at 8 beyond the 83 m.y. isochron. Thus, this isochron Huene and others, 1982). and 20 Ma, respectively (Fig. 8), resulting in a yields a minimum backtracked crustal age of 61 trench depth of ~8 km (Table 1), in accord with m.y. at 20 Ma. The Peru Trench Margin the trench depth/age relationship (Hilde and Hilde and Uyeda (1982) showed an empirical Uyeda, 1982). In the Lima Basin area, point A' relation between the age of the subducting lith- The same approach to estimate retreat was was established from the seawardmost seismic osphere and trench depth. The present Japan applied to the Peru Trench margin in the area of image of the Oligocene-Miocene unconformity Trench is 3.5 km shallower than predicted, de- Lima Basin for two time periods. The paleocon- (Fig. 1). Point A was established from cores spite the conformity of the adjacent ocean figuration of the Peru margin was first con- from ODP Sites 683 and 688, located about 30 lithosphere to the subsidence curve of the Pacific structed from the regional unconformity that km landward of the present trench axis. At Sites basin off Japan (Parsons and Sclater, 1977; subsided since 20 Ma as shown by ODP Leg 112 683 and 688, cores containing primary coastal Heestand and Crough, 1981). We assume that at drill cores. Seismic data indicate that the uncon- upwelling facies with 5-m.y.-old shallow-water 20 Ma, the trench depth was consistent with a formity on top of the Eocene continues to its microfauna were recovered (Suess, von Huene, range of 8 ± 1 km in the depths where 61-m.y. termination near the trench axis (Figs. 1 and 7). and others, 1988b). Similar coastal upwelling or older crust is being subducted. Thus, the pa- Sediment overlying that unconformity was facies are presently deposited off Peru in water leo-Japan Trench depth minimum is 7 km. again eroded during the subduction of the Nazca no deeper than 350 m. Rates derived by in- With this information, we estimated a retreat Ridge. The younger unconformity was dated at ferring subsidence of the unconformity from sea of the Japan Trench slope since 20 Ma. The site 679 by a hiatus between 8 and 11 Ma. The level during the past 5 m.y. are similar to those subaerially eroded Paleogene unconformity was subduction of the Nazca Ridge at this latitude is required by the microfaunal assemblages in followed in seismic records to point A (Fig. 6). dredged samples from Lima Basin (Kulm and The present depth of point A along seismic line others, 1984). Point A was placed along the 3 ORI 87-4 and Shell P-849 is approximately The age of final inundation of the paleolandmass seawardmost recovery location of primary up- 6,400 and 5,625 m below sea level, respectively was estimated at 10 m.y. (von Huene and others, welling deposits that were drilled at Site 688 1982), but this age was revised to 16 m.y. based on (Fig. 7). The paleoslope was projected at the (von Huene and others, 1982; von Huene and improved seismic imaging across the normal faults in Culotta, 1989) and is located only 15 km from the west part of the area shown in Figure 3. average angle of the present lower slope to a PERU TRENCH Neogene Lima Basin area

50km VE = 5.4 w

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 VON HUENE AND LALLEMAND paleotrench depth of ~ 8 km (Table 1). An esti- compensation, which has clear maximum and Watts, 1982). Compensation is largely a func- mated 28-km retreat of the lower slope of the minimum limits. If the lower plate in a subduction of lithospheric rigidity, which depends on trench in 8 m.y. gives an average rate of retreat tion zone behaves rigidly, then the mass re- its age; the increased flexural rigidity of the of about 3.5 km/m.y.; the 50-km retreat in 20 moved along the base of the upper plate is not oceanic plate with increasing age is known m.y. gives a rate of about 2.5 km/m.y. The latter much more than subsidence of the sea floor. If, (Watts and Cochran, 1974; Watts, 1978; Watts rate is a minimum, because it was estimated on the other hand, the lower plate is compliant, and others, 1980). The load of the island of without regard to the recent episode of accretion then high-density lower-plate material will re- Oahu was compensated in less than 0.5 m.y., and advance of the trench slope (Fig. 1) that place and compensate for the lower-density despite the Mesozoic oceanic crust on which it began about 3 Ma (von Huene, Suess, and oth- eroded mass. Therefore, at the one extreme, the was built (ten Brink and Watts, 1985). Thus, ers, 1988). subsidence at the surface approximates the vol- isostatic compensation of oceanic lithosphere ume of material eroded, and at the other appears to be rapid relative to the millions of THE MAGNITUDE OF extreme, it represents only a fraction of the years considered here. BASAL EROSION eroded material because the lower plate has iso- Along a convergent margin, the rate of iso- statically risen to fill some of the space left by static adjustment appears to be similar. The Basal erosion is a principal explanation for the subsurface erosion. subducted part of Daiichi Kashima Seamount is kilometers of Neogene subsidence along the Isostatic compensation of lithosphère within compensated by about 1,200 m of crustal de- Japan and Peru margins. Subsurface erosion can an oceanic plate has been modeled as a thin pression in 0.2 m.y. of subduction (Lallemand be computed from the amount of subsidence; elastic layer over a fluid (Watts and Steckler, and others, 1989). The rate of compensation is however, it requires a knowledge of isostatic 1979; Steckler and Watts, 1982; Karner and 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 Peru (Lima Basin)

Time of reconstruction: T 20 m.y. 8 m.y. 20 m.y. Trench depth 7.5 km 8 ± 1 km 6 km 8 km 6 km >8 km Subducting lithosphere age 135 ± 5 m.y. >61 m.y. 45 m.y. 66 m.y. 45 m.y. >60 m.y. Landward trench retreat: Rt 50 km 8 km 50 km Distance trench-point A 15 km 35 km 18 km Depth of point A 6.4 km <0km 4 km 0-0.35 m 6.3 km 0 km Mean dip, landward slope 4° «5° 6° 6° 6° 6°

Cross-sectional area of subsidence = minimum 800 km2 290 km2 480 km2 eroded

Assuming complete 1,100 km2 370 km2 610 km2 isoslatic compensation

Estimated erosion rate (40)-55 km2/m.y. (361-46 km2/m.y. (24)-31 km2/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 SMT.

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 VON HUENE AND LALLEMAND sphere during déglaciation; however, calcula- perposed on the regional subsidence is the local anism of subduction erosion is abrasion by the tions of erosion that assume complete compen- uplift and subsidence of Lima Basin over the horst and graben on the subducting ocean crust sation yield high volumes of eroded material. past 8 m.y., which is known with greater preci- where the horsts scrape away part of the upper We use the model of a one-dimensional bending sion than is the regional subsidence. Subsidence plate, and the graben transport this material of a thin elastic plate subjected to a hydrostatic of the middle of Lima Basin occurred at a rate down the subduction zone (Schweller and restoring force, developed by Karig and others of more than 0.5 km/m.y. over 8 m.y.; how- Kulm, 1978; Hilde and Sharman, 1978). Such a (1976), to estimate the deflection of the lower ever, subsidence began after 5 Ma at Site 688. mechanism is inadequate here, because (1) the plate. We calculated the rate of erosion local to Lima observed graben have but half of the volume of Basin over the past 8 m.y. and also erosion over the eroded material5 and (2) the subducted Basal Erosion along the Japan 20 m.y. graben are 500 to 800 m beneath the décolle- Trench Margin The estimate of local erosion across Lima ment (von Huene and Culotta, 1989, and Fig. Basin was made in the same manner as for the 1), preventing any contact of the abrasional The time stratigraphy of the DSDP drill holes Japan Trench margin. Restoration of the margin teeth (horsts) with the upper plate. Furthermore, was extended into seismic records that show the at 8 Ma (Fig. 7 and Table 1) was controlled by the overall coherent appearance of the sub- transgression of Neogene slope deposits over the microfossil assemblages in drill holes (Suess, von ducted sediment, even -30 km landward of the subaerial erosion surface. At Sites 438/439, the Huene, and others, 1988a) and by conventional trench axis and as much as 15 km deep (von subaerial erosion ceased and marine deposition sampling (Kulm and others, 1984). The eroded Huene and Culotta, 1989), observed overpres- began in latest Oligocene time (Nasu and others, cross-sectional area is 370 km2, assuming com- sure (Carson and others, 1982; Cadet and oth- 1980). The shoreline transgressed eastward of pensation (Karig and others, 1976) for the ers, 1987), and the lack of larger subduction- Sites 438/439 across a gentle slope (Fig. 3, right) eroded mass. The estimated erosion rate is 46 zone earthquakes beneath the lower and middle until about 16 Ma, when the last insular topo- km2/m.y., which requires subduction of a layer slope (Yoshii, 1979) indicate conditions of re- graphic highs at the edge of the shelf were over- of continental material 460 m thick. duced, rather than high, friction along the plate whelmed (Fig. 3, left). The history of inunda- The estimate of erosion in the past 20 m.y. is boundary. tion, clearly observed across the landward flank calculated using the regional unconformity as a of the insular topography, is obscured in seismic sea-level reference surface. Point A' is the sea- Mechanisms of Frontal Erosion records on the seaward flank by tectonism, seaward end of the eroded middle Eocene strata floor erosion, and great water depth. Therefore, (Fig. 7). The eroded cross-sectional area is 610 The erosional action of ocean floor topog- point A was placed on the landward flank and is km2, assuming compensation (Karig and others, raphy in the Japan and Peru subduction zones a minimum extent of the erosion surface. From 1976) for the eroded mass (Table 1). The 31 appears to involve wedging of positive topo- 16 Ma to the present, only the average rate of km2/m.y. erosional rates estimated for the 20- graphic features into the subduction zone, with subsidence can be determined except at Sites m.y. period are about 65% of those estimated for uplift and breakup of the upper plate. The sub- 438, 439, and 548, where benthic foraminiferal the local erosion of Lima Basin since 8 Ma, but duction of seamounts and ridges produces struc- assemblages were recovered. the latter are closer to those estimated along the tural and morphological effects sufficiently large Erosion along the Japan margin is estimated Japan Trench. to detect by swathmapping and seismic tech- from a generalized crustal cross section (Fig. 7). niques. Two of four reported observations (von Due to isostatic adjustment, the configuration of DISCUSSION Huene, 1986; Lallemand and Le Pichon, 1987; the descending slab adjacent to the trench axis Collot and Fisher, 1988, 1989; Lallemand and probably changed as the slope subsided and re- Our estimates of the amount of eroded mate- others, 1989; Yamazaki and Okamura, 1989; treated, but the generally stationary volcanic arc rial are surprisingly large, even those indicated Ballance and others, 1989) are illustrated here on northern Honshu during the past 20 m.y. by the limiting minima (Table 1). These minima with perspective diagrams (Figs. 8 and 9) to (Tsunakawa, 1986) suggests general stability of are equivalent to the area of subsidence and un- form the basis for a model of frontal erosion. the plate boundary. Subsidence of the Japan realistic, because they fail to account for any Two subducting seamounts along the Japan margin during the past 20 m.y. has substituted isostatic compensation. Retreat of the Japan and Trench are used to show stages of frontal ero- water for a cross-sectional area of 800 km2 of Peru Trench margins occurred at similar or sion. Daiichi Kashima Seamount (Kobayashi rock (Table 1). Isostatic compensation (Fig. 7) greater rates than the advance from tectonic ac- and others, 1986) at the west end of a chain of increases the cross-sectional area of eroded cretion at some other margins.4 Such rates of seamounts (Fig. 9) is the latest to subduct be- material to 1,110 km2. The average rate of ero- subduction erosion have not been previously re- neath the Japan Trench. Daiichi Kashima is on sion per million years is 55 km2 (Table 1). ported, raising the issue of an adequate tectonic the seaward slope of the trench where horst and Transport of this volume of eroded material mechanism. The most commonly inferred mech- graben on the flexed oceanic crust continue down the subduction zone at the 100 km/m.y. across the seamount. The subducted leading average rate of convergence requires that on av- flank of the seamount, imaged in seismic records erage, a 550-m-thick layer of eroded material be (Lallemand and others, 1989), dips less than 4 incorporated into the subducting sediment. The 3-km/m.y. retreat along the Japan margin 15°, and underthrusting of the flank has elevated exceeds the 2-km/m.y. rate of lower-slope advance the upper plate by wedging it upward. In front from accretion along the Middle America Trench off Basal Erosion along the Peru Trench southern Mexico (20-km-wide accretionary complex of and over the subducted flank of the seamount, that is about 10 m.y. old; Watkins, Moore, and others, Regional subsidence of the Peru margin since 1982). It is about equivalent to the rate of slope advance along the Nankai Trough (Karig and Angevine, late Oligocene and early Miocene time is shown 5 1986) and Barbados lower slope (Mascle, Moore, and The present average graben depth (-400 m) and by shelf and upper-slope biostratigraphic indica- others, 1987). Thus, the rate of erosion can be as rapid spacing (~ 10 km) with a constant rate of 100 km/m.y. tors (Suess, von Huene, and others, 1988a). Su- as that of accretion. 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 clasts of seamount material (Cadet and others, At the northern end of the Japan Trench, at involves thickening by thrust faulting. Where 1987). Some of the rubble from the seamount its juncture with the Kuril Trench, is Erimo the landward slope of the trench has been ob- appears to be transferred to the landward slope, Seamount (Fig. 10). Erimo Seamount is just en- served and sampled from a submersible, it con- but the bulk of the seamount remains with the tering the axis of the Japan and Kuril Trenches, sists of highly fractured mudstone and includes subducting oceanic plate. and the normal faults paralleling the Kuril 716 VON HUENE AND LALLEMAND

Trench cut the seamount. Adjacent to the sea- trench axis lower slope at critical mount on the landward slope of the trench is a angle of failure large re-entrant where another seamount has just been subducted (Lallemand and Chamot-Rooke, 1986; Yamazaki and Okamura, 1989). Evi- dence 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 normal fault function of the internal friction in the accreted sediment (Davis and others, 1983), which be- seamount comes overcritical as the seamount begins to subduct, thereby causing collapse, and undercrit- 5° ical 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 uplift above leading New Hebrides subduction zone, where an accre- flank of seamount overcritical, slope in failure tionary ridge along the trench axis has closed a large re-entrant like the one near Erimo Sea- mount (Collot and Fisher, 1989). 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 com- undercritical area of sediment ponding parable 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. B 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 undercritical B overcritical 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. i fracturing <= TECTONIC EROSION ALONG JAPAN AND PERU CONVERGENT MARGINS 717

thickened welt Figure 11. (Continued). of fractured rock

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 Sea- mount (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 slope developed above the leading flank is now above uplift failure the descending slope of the trailing flank. The thickened rock mass, already weakened from undercritical overcritical fracturing, is further oversteepened by the subduction of a trailing flank and is increasingly prone to gravity failure. This results in acceler- ated 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 ponding sediment apron in the axis of the trench. Load- accretion ing 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 effi- ciently 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 depos- D ited in the trench axis, it accretes along the deformation front and builds a low ridge. That ridge becomes the seaward flank of a lower- critical slope slope basin in which sediment ponds. This proc- 5° accretion ess 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 VON HUENE AND LALLEMAND subducted appears to be completed between the more than 20 scarps along horsts and graben are vergent margin is the ultimate repository of the subduction of one seamount and the arrival of subducted every million years. Although it was material and how this affects lithospheric proc- the next. The seamounts are spaced about 60 km shown before that the graben off Japan are in- esses. Where does the subducted erosional apart, and the rate of convergence is 100 sufficient to transport all the eroded debris, they material go, to magmatic products of the asso- km/m.y. Thus, the margin is healed in approxi- may constantly erode at a rate that is less than ciated arc, to the base of the crust where it could mately 0.6 m.y. or less. The model indicates the the rate of accretion. Once the configuration of be magmatically underplated, or to deeper parts importance of wedge configuration as a factor to the margin becomes less accretionary, erosion of the subduction zone that enter the mantle? determine the accretionary or nonaccretionary may dominate. The underplating beneath the parts of the upper condition of the Japan and Peru margins despite plate and beneath the volcanic arc should be the probable less than true Coulomb behavior of Mechanisms of Basal Erosion reflected at the surface as a topographic uplift; the material involved. however, crustal thickening by overthrusting has Our proposed mechanism for frontal erosion The truncation at the base of the upper plate been proposed in recent studies of Andean uplift of the Japan and Peru margins underscores the of beds that predate present accretion along the because of the rapid vertical tectonics (Isacks, role of wedging and breakup of the upper plate Japan Trench (von Huene and Culotta, 1989, 1988). Inclusion in the volcanic and plutonic to produce debris that is transported away by and Fig. 1) suggests erosion. The time of the rocks of the arc should be detected by isotopic sediment subduction. The difference with the truncation is unknown, but because the sea floor signatures. Many geochemists, however, find chain-saw model is that erosion results from above the truncated beds subsided in Pliocene that their data are better explained by a model subduction of a step or disturbance in the base- time, it was inferred that the basal erosion oc- that employs the mixing of subcrustal and deep- ment topography without requiring sharp hard curred at the same time. The material detached crustal magmas, rather than contamination by protrusions to abrade and tear up the lower from the upper plate may be in smaller pieces continental materials in the source region. The slope. than resolved in the seismic images. Plucking of products of the tectonic erosion beneath Lima many small clasts by traction along the base of Basin are presently not reaching the surface be- The Response of Continental Margins to the the upper plate could occur in an environment cause of the lack of volcanic activity in this part Subduction of Ridges and Scarps of overpressured, circulating pore fluids. Up- of the Peruvian Andes. Subduction beyond the ward migration of the water released during zone of partial melting seems difficult because of the low density and melting temperatures of Along the Peru Trench, the last phase of ero- subduction has been suggested as a mechanism continental material. Thus, the fate of subducted sion was simultaneous with subduction of the to "soften" the base of the upper plate (Murau- materials remains a significant problem. Nazca Ridge; once the ridge was subducted, the chi and Ludwig, 1980). Fragmentation by hy- margin became accretionary (von Huene, Suess, drofracturing may produce a slurry of clasts that and others, 1988). The southward-traveling becomes part of the subducting sediment mass SUMMARY trailing flank of the Nazca Ridge appears to have (von Huene and Lee, 1983; Piatt, 1989). Rock left collapse structure and debris from mass at the base of the upper plate may disaggregate Subduction erosion, like accretion, can occur wasting, as indicated by Leg 112 drilling at Site as pressured pore fluid invades and permeates at the base of the slope or along the underside of 685 (Suess, von Huene, and others, 1988a), and through fractures. Highly fractured, consolidated the upper plate. Erosion along the base of the buried jumbled material that is difficult to image sediment that is broken into pebble- or gravel- slope, or frontal erosion, has caused the land- seismically (Kulm and others, 1986). The scarp sized clasts is well known from cores recovered ward slope of the Japan and Peru Trenches to from subduction erosion formed a backstop of on the Japan margin (Carson and others, 1982; retreat at average rates of 2 to 3 km/m.y. One crystalline and Eocene to middle Miocene sedi- vori Huene and others, 1982; Leggett and others, probable agent of such erosion is the subduction mentary rock against which upper Miocene and 1987) and from submersible observations of many topographic features, from 200-m fault Quaternary sediment accreted. Thus, many (Cadet and others, 1987). Although the pre- scarps to 3,000-m seamounts and ridges. processes illustrated in our seamount model viously discussed estimates of eroded material Subducting seamounts along the Japan appear to have operated during the subduction from the Japan and Peru margins are huge, Trench illustrate some stages in this process. As of Nazca Ridge. about 100 km of ocean crust is subducted be- these seamounts subduct, the leading flank of the neath these margins every million years. A layer Similar tectonic mechanisms may operate seamount wedges up the overriding plate, and of gravelly breccia formed of clasts from the during subduction of the 250- to 500-m-high the trailing flank lets it fall; the slope material is base of the upper plate needs to average from scarps of horst and graben on the ocean floor. pushed up and weakened by fracturing, and the —300 to 550 m thick in the sequence of sub- Along the Japan Trench, some scarps continue wedge angle becomes overcritical, causing slope ducted sediment to transport subcrustally eroded locally as a low ridge in the accreted sediment failure, which sends debris avalanching into the material of the volume estimated (Table 1). (Lallemand and others, 1986). These could form trench axis. Gravity failure and efficient subduc- Such thicknesses are consistent with the beds low waves of tectonism traveling obliquely tion of the increased volume of trench sediment paralleling the lower plate (that is, subducted along the foot of the slope. Fault scarps are sub- local to the seamount leave a large re-entrant in sediment) imaged in seismic records of the ducted frequently, because the average spacing the slope after the seamount has subducted. The Japan and Peru margins. It is difficult, however, between horsts off South America and Japan re-entrant later fills by accretion of sediment to differentiate clearly between underplated and commonly averages less than 10 km (Aubouin from the trench axis when the dip of the sub- subducting strata, using the seismic data alone. and others, 1981; Bourgois and others, 1986; ducting plate returns to its former condition. Cadet and others, 1987). Off northern Japan, The significance of the huge quantities of mate- This sequence of processes also appears to oper- where the rate of convergence is 100 km/m.y., rial eroded from the overriding plate in a con- ate where ridges are subducted. TECTONIC EROSION ALONG JAPAN AND PERU CONVERGENT MARGINS 719

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Trench area, Leg 57, Deep Sea Drilling Project, in Scientific Party, Initial Boillot, G.. Winterer. E. L., Meyer, A. W., Applegate, J., Baltuck, M., Bergen, reports of the Deep Sea Drilling Project, 56, 57 (Part 2): Washington, indicating the control of tectonic processes by J. A., Comas, M. S., Davies, T. A., Dunham, K., Evans, C. A., D.C., U.S, Government Printing Office, p. 835-866. these parameters. We speculate that the constant Gérardany, J., Goldberg, G., Haggarty, J., Jansa, L. F., Johnson, J. A., Kemp, A.E.S., and Lindsley-Griffin, N., 1989, Variations in structural style Kasahara, J., Lareau, J. 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P., Aubouin, J., Dubois, J., von Huene, R., Jolivet, L., posed on a stronger accretionary tectonics gov- de Lepinay, B„ Monge, F., Monlaii, J., Pelletier, B., Sosson, M., and von Kanazawa, T., Kasahara, J., Koizumi, K., Lallemand, S., Nakamura, Y., Huene. R„ 1986, Tectonic regime of the Andean convergent margin off Pautot, G., Suyehiro, K., Tani, S., Tokuyama, H., and Yamazaki, T., erned by plate configuration. Peru (SEAPERC cruise of the R/V Jean Charcot, July 1986): Comptes 1986, Normal faulting of the Daiichi Kashima Seamount in the Japan Rendus de l'Académie des Sciences, Paris, France, p. 1599-1604. Trench revealed by the Kaiko I cruise, Leg 3, in Project Kaiko, Erosion along the underside of the upper 1988, SeaBeam and seismic-reflection imaging of the tectonic regime of p. 257-266. plate, or basal erosion, removes more material the Andean continental margin off Peru (4°S to I0°S): Earth and Plane- Kulm, L. D., Suess, E., and Thornburg, T. M., 1984, Dolomites in organic-rich tary Science Letters, v. 87, p. 111-126. muds of the Peru forearc basins: Analogue to the Monterey Formation, than does frontal erosion from the Japan and Boyd. T. M„ Snoke. J. A.. Sacks, I. S., and Rodriguez, B. A., 1984, High- in Garrison, R. E., Kastner, M., and Zenger, D. H., eds.. Dolomites of resolution determination of the Benioff zone geometry beneath southern the Monterey Formation and other organic-rich units: Society of Eco- Peru margins. It affects these margins by thin- Peru: Seismological Society of America Bulletin, v. 74, no. 2, nomic Paleontologists and Mineralogists, Pacific Section, Special Publi- ning the crust, thereby causing subsidence be- p. 559-568. cation 41, p. 29-47. Cadet. J. P.. and Fujioka, K., 1980, Neogene volcanic ashes and explosive Kuln, L. D., Miller, J., and von Huene, R., 1986, The Peru continental margin, neath the continental terrace and the slope. The volcanism; Japan Trench transect. Leg 57, Deep Sea Drilling Project, in record section 2, in von Huene, R., ed., Seismic images of modern Scientific Party, Initial reports of the Deep Sea Drilling Project, Legs 56 convergent margin tectonic structure: American Association of Petro- mechanisms of subsurface erosion are inferential and 57 (Part 2): Washington, D.C., U.S. Government Printing Office, leum Geologists Studies in Geology 26, p. 37-40. because they have not been observed. One in- p. 1027-1042. Kulm, L. D., Thornburg, T. M., Suess, E., Resig, J., and Fryer, P., 1988, Clastic, Cadet. J. P., Kobayashi, K., Aubouin, J., Boulégue, J., Déplus, C., Dubois, J., diagnetic, and metamorphic lithologies of a subsiding continental block: ferred mechanism involves the invasion of over- von Huene, R., Jolivet, L, Kanazawa, T., Kasahara, J., Koizumi, K., Central Peru forearc, in Proceedings of the Ocean Drilling Program, Lallemand, S., Nakamura, Y., Pautot, G., Suyehiro, K., Tani, S., initial reports (Volume 112): College Station, Texas Ocean Drilling pressured water into fractures, which disaggre- Hidekazu, T., and Yamazaki. T., 1987, The Japan Trench and its junc- Program, p. 91-107. gates the rock along the underside of the upper ture with the Kuril Trench, cruise results of the Kaiko Project, Leg 3: Lagoe, M. B., 1986, Foraminifers from the Nankai Trough and the Japan Earth and Planetary Science Letters, v. 83, p. 267-284. Trench, Leg 87, in Initial reports of the Deep Sea Drilling Project plate and makes a profusion of small fragments Cande, S. C.. 1985, Nazca-South America plate, interactions since 50 m.y. B.P. (Volume 87): Washington, D.C., U.S. Government Printing Office, to present in Hussong, D. M., Dang, St. P., Kulm, L. D., Couch, R., and p. 587-604. vulnerable to plucking by traction along the Hilde, T.W.C., eds.. Peru-Chile Trench off Peru: Ocean Margin Drilling Lallemand. S., and Chamot-Rooke, N., 1986, Sur la cause du décrochement plate boundary. Program, Regional Atlas Series: Woods Hole, Massachusetts, Marine senestre entre les fosses du Japon et des Kouriles: Subduction-collision Sciences International. d'un ancien volcan sous-marin: Comptes Rendus de l'Académie des The estimated rates of erosion through a cross Carson, B., von Huene, R., and Arthur, M., 1982, Small-scale deformation Sciences, Paris, v. 303, Série II, n. 16, p. 1443-1448. structures and physical properties related to convergence in Japan Lallemand, S., and Le Pichon, X., 1987, Coulomb wedge model applied to section of the northern Japan Trench and the Trench slope sediments: Tectonics, v. 1, no. 3, p. 277-302. subduction of seamounts in the Japan Trench: Geology, v. 15, Collot, J -Y., and Fisher, M. A., 1988, Tectonic effects of the collision of the p. 1065-1069. Peru Trench along Lima Basin in the past 20 Bougainville Guyot and another seamount on the New Hebrides accre- Lallemand, S., Cadet, J. P., and Jolivet, L., 1986, Méchanisme de tectonenèse à m.y. is from 31-55 km2/m.y. Such rates require tionary complex: EOS (American Geophysical Union Transactions), la base du mur inter ne de la fosse du Japon (au large de Sanriku, Japon v. 69, no. 44, p. 1407. NE): Rejeu des failles océaniques sous la marge: Comptes Rendus de subduction of a layer of material about 0.5 km 1989, Formation of forearc basins by collision between seamounts and l'Académie des Sciences, Paris, France, v. 302, Série II, n. 16, accretionary wedges: An example from the New Hebrides subduction p. 319 324. thick in addition to the subducted ocean mate- zone: Geology, v. 17, p. 930-933. Lallemand, S., Culotta, R., and von Huene, R., 1989, Subduction of the Daiichi rial. This amount is possible, given the thickness Davis, D„ Suppe, J., and Dahlen, F. A., 1983, Mechanics of fold-and-thrust Kashima Seamount in the Japan Trench: Tectonophysics, v. 160, belts and accretionary wedges: Journal of Geophysical Research, v. 88, p. 231-247. of reflections from subducted material in seismic p. 1153-1172. Langseth, M. G., von Huene, R., Nasu, N., and Okada, H., 1981, Subsidence of Engebretson, D. C., Cox, A., and Gordon, R. G., 1985, Relative motions the Japan Trench forearc region off northern Honshu: Oceanologica records. between oceanic and continental plates in the Pacific basin: Geological Acta, Supplement to Volume 4, p. 173-179. Society of America Special Paper 206, 59 p. Leggett, J. K., Lundberg, N., Bray, C. J., Cadet, J. P., Karig, D. E., Knipe, R. J., Heestand. R. L., and Crough, S, T., 1981, The effects of hot spots on the and von Huene, R., 1987, Extensional tectonics in the Honshu forearc, ACKNOWLEDGMENTS oceanic age-depth relation: Journal of Geophysical Research, v. 86, Japan: Integrated results of DSDP Legs 57,87, and reprocessed multi- p. 6107-6114. channel seismic reflection profiles, in Coward, M. P., Dewey, J. F., and Hilde, T. W.C., 1983, Sediment subduction versus accretion around the Pacific: Hancock, P. L., eds.. Continental extensional tectonics: Geological Tectonophysics, v. 99, p. 381-397. Society of London Special Publication 28, p. 593-609. We are grateful for the extensive efforts of Hilde, T.W.C., and Sharman, G. F„ 1978, Fault structure of the descending Lehner, P., Daust, H., Bakker, G., AHenbach, P., and Gueneau, Y., 1983, Japan Mark Brandon, Dan Karig, and David Scholl, plate and its influence on the subduction process: EOS (American Geo- Trench, line P-849: American Association of Petroleum Geologists physical Union Transactions), v. 59, p. 1182. Studies in Geology Series 15, v. 3, p. 3.4.2-1-3.4.2-20. who through their thoughtful and thorough re- Hilde, T.W.C., and Uyeda, S., 1982, Trench depth: Variation and significance, Mascle, A., Moore, C., and others, 1987, Accretionary complex penetrated, in Hilde, T.W.C., and Uyeda, S., eds., Geodynamics of western Pacific- defined: Geotimes, v. 32, no. 1, p. 13-15. views have helped us greatly improve earlier Indonesian region: American Geophysical Union Geodynamics Series, Mitsui, S-, 1971, Studies on the mechanism of deformation of sedimentary versions of manuscripts describing our study. v. 11, p. 75-89. rocks in the Iwaki area of the Joban Coal Field, Fukushima Prefecture: Hilde, T.W.C., Isezaki, N„ and Wageman, J. M-, 1976, Mesozoic sea-floor Science Reports of the Tohoku University Series 2, v. 42, p. 199-272. The seismic data of Figure 3 were provided by spreading in the North Pacific: American Geophysical Union Geophysi- Moore, G. F., and Taylor, B., 1988, Structure of the Peru forearc from multi- cal Monograph 19, p. 205-226. channel seismic reflection data, in Proceedings of the Ocean Drilling Shell International Petroleum Mij. B.V. in the Hussong, D. M., and Wipperman, L. K., 1981, Vertical movement and tectonic Program, initial reports (Volume 112): College Station, Texas, Ocean Hague and were reprocessed by Rutt and John erosion of the continental wall of the Peru-Chile Trench near 11°30'S Drilling Program, p. 71-76. latitude, in Nasca plate: Crustal formation and Andean convergence: Murauchi, S., and Ludwig, W. J., 1980, Crustal structure of the Japan Trench: Miller at the U.S. Geological Survey, Denver. Geological Society of America Memoir 154, p. 509-524. The effect of subduction of ocean crust, in Scientific Party, Initial reports 720 VON HUENE AND LALLEMAND

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Cenozoic tectonic history along the I POD Japan Trench transect: Geo- REVISED MANUSCRIPT RECEIVED AUGUST 21, 1989 Suess, E., von Huene, R., and others, 1988a, Proceedings of the Ocean Drilling logical Society of America Bulletin, v. 93, p. 829-846. MANUSCRIPT ACCEPTED AUGUST 25, 1989

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