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Geol Rundsch (1994) 83: 822-831 © Springer-Verlag 1994

N. Kukowski. R. von Huene • J. Matavieille S. E. Lallemand

Sediment accretion against a buttress beneath the Peruvian continental margin at 12 ° S as simulated with sandbox modeling

Received: 21 January 1994/ Accepted : 16 June 1994

Abstract Reflection seismic data from the Peruvian continental margin at 12 ° S clearly reveal an accretionary Introduction wedge and buttress. Sandbox experiments applying the physical concept of the Coulomb theory allow the The concept of a buttress or backstop has been firmly systematic investigation of the growth and deformation implanted in published work on convergent margins of such an accretionary structure. The style of deforma- (Byrne etal., 1988; 1993). It is considered to be the tion of the buttress and the internal structure of the wedge seaward edge of the or continental margin older is observed in the sandbox models. The possibility of rock framework against which an accretionary prism underplating material beneath the buttress and the forms (von Huene and Scholl, 1993). Conceptually, this amount of tectonic erosion depend on the physical body is the anvil against which offscraped trench sedi- properties of the materials, mainly internal friction, ment is accreted and tectonically deformed. cohesion and basal friction. Boundary conditions such as A good example of an actual buttress is imaged in the height of the gate and the thickness of seismic records across the Peruvian continental margin. incoming sand also constrain the style of growth of the Beneath the slope, a wedge-shaped body of crystalline model accretionary structure. rock with probable continental affinity has been imaged The configurations of two experiments were closely through careful processing of seismic records (Moore and scaled to reflection seismic depth sections across the Taylor 1988; von Huene and Miller 1988; yon Huene, in Peruvian margin. A deformable buttress constructed of press). Against this wedge-shaped buttress a prism of compacted rock powder is introduced to replicate the trench sediment about 10 km wide has been accreted. rock which allows deformation similar to that Deformation visible in the overlying sediments may give in the seismic data. With the sandbox models it is possible a hint that deformation also occurs in the narrow tapered to verify a proposed accretionary history derived from end of the buttress. seismic and borehole data. The models also help in The evolution and deformation of the accretionary understanding the mechanisms which control the amount body in front of a buttress can be forward modeled of accretion, subduction and underplating as a function applying a sandbox technique. However, in most former of physical properties, boundary conditions and the models the buttress was modeled as a hard, non-defor- duration of convergence. mable member and sand was accreted against a landward Key words Accretionary prisms • Convergent margins- or seaward dipping rigid stationary buttress (Byrne et al., Peru. Analogue modeling. Tectonics • Accretion - Tecto- 1988; Mulugeta 1988; Lallemand et al., 1992). To achieve nic erosion. Marine geophysics more realistic conditions, Byrne et al. (1993) used a defor- mable backstop made of wet sand. Considering the tectonic deformation in the buttress off Peru, we intro- duced a deformable buttress (compacted rock powder) in the sandbox modeling. Furthermore, we scaled the mo- N. Kukowski (~:~); R. von Huene dels after observations in seismic records across the GEOMAR, Forschungszentrum ffir Marine Geowissenschaften, Peruvian margin to replicate features observed in nature Wischhofstrasse 1--3, D-24148 Kiel, Germany, Fax: (49)-431- 72 02-293, e-mail: [email protected] and we allowed material to leave the system through a 'subduction window' to simulate that in nature a certain J. Malavieille - S. E. Lallemand Equipe de G6ophysique et Tectonique, Universit6 Montpellier II, amount of material is transported to greater depths. Case postale 060, Place E. Bataillon, F-34095 Montpellier Cedex In this paper we report the results of our sandbox 05, France modeling. A main objective of our models was to test 823

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Fig. 1. Map of the Peruvian continentalmargin showing available lines were pre-stack depth migrated at GEOMAR (von reflection seismic lines near 9°S and 12°S latitude and ODP drillholes Huene and Pecher, unpublished data). From early wide-angle seismic data (Hussong and Wipperman, 1981) it was known that seismic velocities of a kinematic restoration proposed from geophysical and 5 km s -1 and greater characterize upper plate rocks geological observations off Peru that include information landward of the first 10 km of the margin. In seismic from ODP drilling (yon Huene, Suess et al., 1988). By reflection records, coherent reflections were visible along observing the intermediate stages during convergence in boundaries above and below a high velocity wedge, but the sandbox, we gained many ideas about the possible incoherent events characterized its interior. Rocks reco- tectonic history, the material paths and the parameters vered from outcrops at the edge of the shelf and along the controling the evolution of the tectonic structure of lower slope indicated that the area of incoherent reflec- convergent margins. Observing the processes taking place tions could represent equivalents of the schists and in the sandbox, we saw that the ratio of material supplied gneisses that are exposed in the coastal mountains. After by the moving plate to material removed from the system drilling leg 112, these rocks were observed from a submer- through the subduction window may be one of the most sible in outcrops at the foot of the margin in Chiclayo critical parameters controlling the accretionary evolu- Canyon (Sosson et al., 1992). During leg 112 a shallow tion. water Eocene sandstone which is known to rest unconfor- mably on the metamorphic rock cropping out at the edge of the shelf was recovered from two sites on the lower slope near 12°S. The unconformity is seen in all multi- channel seismic records across the continental shelf and is Tectonic history of the Peruvian margin inferred to extend onshore. This regional unconformity, now up to 5 km below sea level, was a subaerial erosion Four seismic lines (1017, 1018, CDP-1, HIG-14) covering surface in pre-Middle Eocene time (von Huene, Suess about 30 km along the Peru margin near 12 ° S show the et al., 1988) because the shallow water continental origin and the buttress (Fig. 1). Line CDP-1 of the sandstone on the unconformity makes it unlikely from the Nazca Plate Project and Shell 1017 were that the underlying basement is an Eocene deep water processed in the time domain to a pre-stack migration in accretionary wedge. Minor deformation affects the thin preparation for ODP drilling on leg 112 (yon Huene and tapered part and only extensional deformation of the Miller, 1988). HIG-14, which was also a part of the basement is observed to cut the basement from the middle drilling transect (Moore and Taylor, 1988), was post- slope landward (Fig. 2). These observations indicate that stack time migrated. The Shell line 1018 is also very close the wedge-shaped buttress is a relatively rigid body to the main drilling sites. Because of this proximity and probably composed of metamorphic rock similar to that because the Shell data are of fairly high quality, both Shell in the coastal massif (Kulm et al., 1988). 824

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10- -10 KM b Fig, 2. a Time-migratedstack ofthenear-trenchpart of CDP-1 and One aim of our study was to test this kinematic b interpreted drawing showing the accretinary prism and buttress interpretation physically with sandbox experiments.

In the northern lines (CDP-1, Shell 1018, HIG 14), the Sandbox experiments buttress forms a wedge tapering about 20 °. The upper surface is usually bounded by a continuous reflection with Model concept strong amplitude (Fig. 2), which correlates with the Modeling the evolution of an accretionary wedge with the Eocene shallow water sandstone at ODP site 688. Its sandbox technique is based on the assumption that the lower boundary is less distinct because of the greater frontal part of a subduction zone, i.e. the accretionary depth and complex ray path; however, underplated wedge, the underplated sediment and the frontal part of sediment is consistently imaged there. Subducting sedi- the buttress, behaves as a Coulomb material (Davis et al., ment beneath the inferred metamorphic rocks is observed 1983; Dahlen, 1984). Thus the scale-independent Cou- many tens of kilometers landward along the plate bound- lomb theory can be applied. Dahlen (1984) derives an ary. exact solution for the critical taper a uniform wedge for The evolution of the accretionary wedge began when the case of a non-cohesive material. The mechanical state accretion was initiated after the Nazca Ridge trailing of the accretionary wedge is, for a given rock density, flank had been subducted (von Huene and Miller, 1988) a function of only four parameters, the internal and basal (Fig. 3). Several studies show that the area, where the pore pressures and friction angles, but not of time or Nazca Ridge subducts, migrated southward with time temperature. However, natural rocks are cohesive mate- (Cande 1985; yon Huene and Lallemand, 1990). From rials. Dahlen etal. (1984) showed that the Coulomb plate tectonic reconstruction (Nur and Ben Avraham, theory can be extended to cohesive materials with cohe- 1981) the time when ridge trailing flank subducted at the sion, Co, as the fifth of the parameters which characterize trench axis near the position of 1018 is estimated at 3 Ma. the state of stress in a critically tapered wedge. 825

Quaternary Lallemand et al. (1994) compiled the mechanical para- meters for many examples of convergent margins. How- ever, there are no measurements available for the Peru- "k. vian subduction zone. In our first two experiments, which will be described in detail in the following section, we used different basal frictions. Thereby we tested which para- meter set better matches observations to give a first constraint for the mechanical properties of the Peruvian continental margin. In both experiments the buttress was constructed of darkly coloured rock powder (rock sawdust) from a Mio- cene sandstone which was carefully compacted during construction. Thin, light marker horizons of the same material were repeatedly inserted to make any deforma- .:#~/i~iiii tion occurring during the experiment visible. The oceanic and slope sediments were modeled using dry, fine-grained (< 300 gm), differently coloured eolian quartz sand. The mechanical properties of both materials were measured Miocene giving ~b, = 2300 kgm -3, Cobu = 130 Pa, #bu = 0.6 for Pliocene the sawdust, ~s = 1900 kg m -3, Cos = 20 Pa, /~s = 0.57 erod for the sand. ,u is scale-independent and ~ 0.6, a reason- able value (Lallemand et al., 1994), cohesion scales to 13 MPa, 2 MPa, respectively which are again reason- able values (Hoshino et al., 1972). ~3 Ma As indicated in the seismic lines, the buttress in the first experiment had an initial taper of 20 ° and the slope had NAZCA RIDGE a taper of 9 ° (Fig. 4a). Sedimentation in the trench axis was simulated by adding material during the course of the Fig. 3. Unscaled sketches showing (in a sequence) a reconstruction experiment on top of an originally 0.5 cm thick section of of the evolution of the accretionary wedge as proposed from seismic material representing pelagic and hemipelagic sediments and borehole data (after von Huene and Miller, 1988) and initial trench-fill. In this experiment, double-sided adhesive tape was attached to the moving plate giving Construction of the model experiments a basal friction /~b of 0.5. During the first 30 cm of convergence, an increasing amount of sand was added in The experiments were carried out using a glass-sided the trench axis to simulate material supply from the rectangular box (Malavieille, 1984). The is continental plate. As a result of this, the ratio R of represented by a plastic plate which can be bent to model incoming to outgoing material changed from R = 2 to the geometry of a subduction zone. On its surface, R = 3. No more material was added to the trench axis, materials such as sticky paper or a Mylar sheet are fixed to and R decreased to about R = 1 after 80 cm of convergen- simulate different basal frictions. The main difference ce and then remained constant. between our experiments and previous work is the use of The construction of the second model was designed to a deformable backstop made of compacted rock powder answer questions raised by the interpretation of the first (rock sawdust), which means that the frontal part of the experiment and especially to investigate the role of the buttress is also assumed to behave as a Coulomb material. subduction channel. During this experiment no material In the first experiment, a curved plate was used. was added to the trench as it became clear from the first We constructed'the model geometry matching the experiment that the material input could be simulated angular relations observed in the seismic records. Scaling without constant sedimentation in the trench axis. A criti- was such that 1 km in nature was equivalent to about cal parameter is the height of the subduction channel in 1 cm in a model (scale factor ~ 105). Subduction was seismic records, which is uncertain because of its great simulated by dragging a lower plate at a specified angle depth (12-15 km). An error of 100-200 m might have beneath the buttress. The thickness of incoming sedi- a significant effect on the results in a sandbox model. To ments was scaled according to the thickness of sediments test this, the thickness of incoming material was the same near the trench observed in the seismic records. Material as that of the subducted material during the whole course balance was controlled by constructing a fixed window of the experiment (R = 1 = const.). ('subduction window') at the 'landward edge' of the model box through which subducted sand is allowed to exit with the moving plate from the model. The thickness of coherent reflections was assumed to represent the height of the subduction window. 826

O. O

e

I C I f

Fig. 4a--f. Interpretative line drawings of a initial configuration buttress and caused the formation of conjugate thrusts in and several steps during the run of the first experiment: b after the sandy cover (Fig. 4 c). 10cm; e after 30cm; d after 40cm; e after 50cm; and f after 85 cm of dragging of the lower plate. The dotted pattern refers During the first 30 cm of convergence, material was to the region where material is underplated accreted only beneath the buttress, which resulted in uplift and a decrease in the thickness of the trenchward part of the buttress. After more than 35 cm of convergen- Results of the sandbox experiments ce, the formation of the frontal accretionary wedge and duplexes began. First experiment To accommodate the shortening, several nearly parallel backthrusts dipping 43 o_ 45 ° towards the toe developed in The first experiment was started with a slope angle the buttress. As the input of material continued, sediment = 10.5 ° and a dip angle]? = 8 ° (Fig. 4a), which should was partly accreted in front of the buttress, partly below it. give a stable initial taper according to Coulomb theory. Material leaving the experimental box through the subduc- During the run of the experiment, the rigid lower plate tion window originated mostly from the subducted sand, was dragged a total of 105 cm, taking a photograph for but also to a small amount from the buttress. This resulted in documentation every 5 cm. Several times during the first the formation of faults nearly perpendicular to the stages of this experiment, sand was added to the trench to backthrust in the lower part of the buttress near the rigid match a possible effect of an increase in sediment volume backstop. The deforming buttress was tilted arcward and just after the subduction of the Nazca Ridge. uplifted by the growing wedge beneath it (Fig. 5). During the first 20 cm, the slope angle decreased to During the course of the experiment, an out of about 8 ° in the 'arcward' part of the model wedge and sequence thrust (top d6collement) developed just beneath then remained constant. Consequently, the taper also the buttress (Fig. 4). Basal shear was partitioned between changed, matching the critical taper for a basal friction of this 'top d6collement' and the 'basal d6collement' be- 0.5 (Lallemand et al., 1994). In this stage, the buttress, tween the sand and the rigid lower plate. Between the top especially at its toe, underwent additional compaction. d~collement and the buttress material originating from Thus the bending of the toe of the buttress due to frontal the moving plate was underplated. Above the top d6colle- accretion induced shortening in the upper part of the ment, no movement occurred (Fig. 4 d-f). 827

top d~collement marks the boundary between accreted and underplated sediment. 7&;-----z z-. The buttress undergoes shortening and tilting at its trenchward edge. Despite the contraction of the buttress the accretionary wedge grows at its seaward edge such ~m~al_____---Z7.c_ -conditio~ ..... a) that the accretionary advance is about twice the contrac- tion of the buttress after a dragging distance of 105 cm (Fig. 6).

...... arcward tilting ...... '22Z'-'--~ ...... of the buttress Second experiment

b) The initial geometry of this experiment was scaled to a more refined model from an early MIGPACK pre-stack depth migration version of line 1018 (Klingelh6fer, 1992) with c~ = 11.5 ° and fl = 4.5 ° (Fig. 7a). The seismic record shows sediments 800 m thick enter- ing at the trench axis and sediments of various thickness being subducted to a greater depth, but we do not know . uplift of the the history of subduction. Therefore, one major objective of this experiment was to investigate the effect of trench sediment supply versus its disposal by subduction. For c) this reason, the incoming material had the same height (8 ram) as the subduction window. Both heights were held convergence constant during the experiment. The coefficient of basal friction was lower (JZb = 0.43). This was achieved by Fig. fa--e. Sketches showing the geometry of an early, inter- attaching a Mylar sheet on top of the moving plate. mediate and late stage of the first experiment indicating the main regional deformation and growth of the wedge In the early stages of this experiment, the trenchward edge of the buttress was compacted, shortened and thickened due to compression. Its leading edge was also The first duplex was fully developed after a convergen- tilted and a small amount of sand was accreted below it ce of 50 cm. During the formation of a new duplex, the (Fig. 7b), concurrent with accretion in front of the slope angle changed, reaching a minimum when a new buttress. Seaward verging faults in the sandy cover fault originated, resulting in a periodicity of the specific developed above the buttress' leading edge due to the steps of duplex formation which is documented in several shortening. The buttress was involved in the underthrust distinct steps of different steepness along the slope in the sequence such that a major thrust ate its landward part. three-dimensional view of the frontal part of the accretio- Schlieren-like accumulations of the light coloured sand in nary structure (Fig. 6 a). Subsequent duplexes were pro- the dark sand indicate internal deformation and faulting gressively more closely spaced. Not all faults were active within the accretionary wedge (Fig. 8). at the same time or during the whole course of the During the experiment, the wedge steadily grew at experiment, so that movement occurred at one time only a nearly constant taper. The material removed through in connection with the two or three latest duplexes. After the subduction channel was almost all from the buttress. the maximum convergence of 105 cm at the end of the Thus an upper part of the continental slope with a slope experiment, seven duplexes had been built (Fig. 6 b). The angle of about 12 °- 13 ° above the landward part of the movement along the out of sequence thrust stopped after buttress can be distinguished and a more gentle lower about 60 cm of convergence and the shape and amount of continental slope above the critically tapered trenchward underplated material remained constant. This might be part of the buttress and the accretionary wedge. However, a consequence of the change in the amount of material the volume of the whole model accretionary complex remained constant. This might be a consequence of the remained almost unchanged during the run of the experi- change in the amount of input, which was no longer in ment, reflecting the balance between the input and excess of the output at the subduction window. After the removal of material. The wedge itself grew steadily while development of a duplex was completed, no more move- the slope angles remained constant. Accretion began ment occurred along the bounding faults. immediately at the trench axis and was compensated by Summarizing the main features observed during this erosion from the base of the buttress. This experiment experiment, we point out that the backward tilting of the indicates that accretion and tectonic erosion can occur buttress occurred as soon as enough underthrust sand simultaneously but at different locations if there is was built beneath it, which is a function of greater input a certain amount of material being subducted and if than output (Fig. 5). The thickness of the material distributed shear occurs along the plate boundary. How- accreted beneath the buttress increased continuously. The ever, this material flux has to be tested carefully by 828

Fig. 6 a, b. Photographs showing in detaii the accretionary wedge in might be due to the fact that the top d~collement runs a an early stage and b a late stage of the first experiment through the buttress, which was eroded. The absolute position of the uppermost d~collement is a function of the comparison with data and observations in nature because basal friction (Lallemand, 1992); the relative position in these processes are forced to a certain amount in sandbox relation to the buttress also depends on the thickness of experiments. material accreted beneath the buttress, the different Underplating did not occur, a result which is funda- position of the top d6collement, which again is a function mentally different to that of the first experiment. This of basal friction. If we open the subduction window, the 829

a C

b d

Fig. 7a--d. Interpretative line drawings of a initial configuration obtained in the sandbox models shows a great similarity and several steps during the run of the second experiment: b after to what can be imaged in the seismic records as well as 35 cm; c after 75 cm; and d after 110 cm of dragging of the lower plate some structures such as back-thrusts. This indicates that applying the Coulomb theology for the frontal part of a convergent margin may be realistic and may give a first thickness of the removed layer increases as the basal constraint for the mechanical parameters of convergent friction decreases. In other words, with a high basal margins, Therefore, from physical experiments we can friction as in the first experiment, the thickness of the learn something about the fundamental principles of removed layer is more or less equivalent to the height of accretion and erosion, and then try to constrain a kinema- the window, whereas in the second experiment with a low tic evolution for a specific case. basal friction the thickness of the removed layer is about Hussong and Wipperman (1981) proposed that the twice the height of the window. This is supported by the vertical movement of the trenchward part of the continen- observation that the removed layer constituted almost tal crystalline rocks (= buttress) in the Peruvian conti- only sand in the first experiment, but sand and rock nental margin is a consequence of the thickening of powder in the second. This means that if there is low basal material from the oceanic plate in the early stages of friction, there is a great potential for tectonic erosion so convergence. Comparing the observed structure of the that material removal is dominantly constrained by the Peruvian continental margin with the final stages of the height of the subduction window. The processes control- sandbox models, the first experiment seems to match the ling the height of the subduction window and, therefore, observations more closely than the second. A value of the amount of material which can be transported to basal friction of > 0.45 simulates the natural processes greater depths are only very poorly understood in nature. better. Here, physical modeling may be helpful in detecting the The simultaneity of accretion and erosion as observed controlling processes and parameters. in the second sandbox model poses the question of From this experiment it can be shown that the basal whether this is also reasonable and possible in nature. friction coefficient and the ratio of incoming to removed There is some evidence that both mechanisms were active material are the parameters dominantly controlling the simultaneously along the Peruvian continental margin. shape and growth of the accretionary wedge. Here, the basal The Lima Basin, located on the middle part of the slope, friction determines the deformation behaviour of the subsided from about 5 Ma ago to at least 2 Ma ago, as buttress, whereas the input to removal ratio controls the interpreted from foraminiferal data (von Huene, in growth rate of the wedge and the total change in volume. press). The subsidence is inferred to be caused by the removal of material at the plate boundary. Therefore, although still hypothetical, accretion in front of the Discussion buttress and tectonic erosion at the back are not in conflict with observations and inferences about the The sandbox experiments scaled geometrically from processes at the plate boundary. seismic images of the subduction zone off Peru at 12 ° S The experiments have revealed the ratio of incoming to showed that the tectonic reconstruction proposed from removed material as one of the most crucial parameters seismic and geological data (Fig. 3) is consistent with the controlling the mechanics of a subduction zone. Basal development of an accretionary wedge as modeled in friction is another dominant parameter controlling distri- a physical approach. The shape of the stable buttress buted shear in a layer along the plate boundary. However, 830

Fig. 8 a, b. Photographs showing a an initial and b the final stage of modeling. However, with numerical modeling it is not the second experiment possible to model the dynamic evolution of an accretiona- ry wedge. Here, sandbox modeling is a good tool to derive constraints for mechanical parameters that can be entered the thickness of the subduction channel in nature and its into numerical models for fluid transport. Thus a combi- role with regard to mass balance are poorly known. An nation of different modeling techniques constrained by associated question is the effect of changing the ratio of good geophysical data could be a key to understanding incoming material to material removed by subduction to the dynamic transport processes taking place in subduc- greater depths. Similarly, the conditions conductive to tion zones. underplating need quantification. We suggest that under- Acknowledgements We kindly thank the Deutsche Akademische plating can only take place if the ratio R is significantly Austauschdienst (DAAD) and the Minist~re des Affaires Etrang~res larger than unity and if basal friction is sufficiently high, Fran~ais for financial travel support to conduct the experiments in so that the out of sequence thrust will be located beneath the Laboratoire de G6ologie Structurale of the Universit~ de the buttress. Before being able to derive a complete model Montpellier II within the French German PROCOPE program. Fundings of experiments came from the French Program 'Dynami- of the growth of an accretionary wedge in response to que et Bilan de Ia Terre'. CNRS-INSU-DBT contribution constraints from material supply, further systematic No. 80713. We also thank Marc Sosson and an anonymous reviewer experiments are needed. for their valuable suggestions. Seismic imaging and numerical modeling (Shiet al., 1990) have shown the importance of fluid transport within an accretionary wedge. Fluid pressure is a function References of the hydraulics of the rocks present and controls the Byrne D E, Davis DM, Sykes LR (1988) Loci and maximum size of shear strength. This parameter is absent from sandbox thrust earthquakes and the mechanics of the shallow region of experiments and must be considered through numerical subduction zones. Tectonics 7: 833--857 831

Byrne DE, Wang W-H, Davis DM (1993) Mechanical role of diagenetic, and metamorphic lithologies of a subsiding continen- backstops in the growth of forearcs. Tectonics 12: 123-144 tal block: central Peru forearc. In: Suess E, von Huene R et al. Cande S (1985) Nazca-South America plate interactions since 50 (eds) Proc ODR Init Rep 112: 91-107 My B. R to present, in Peru- Chile trench off Peru. In: Hussong Lallemand SE (1992) Transfer de mati&e en zone de subduction. DM et al. (eds) Ocean Drillion Program, Regional Atlas S. Habilitation fi Diriger des Recherches. M~m Sci Terre Univ Marine Sciences Inernational, Woods Hole. P M Curie Paris 92-27, vol 1:189 pp Dahlen FA (1984) Noncohesive critical Coulomb wedges: an exact Lallemand SE, Malavieille J, Calassou S (1992) Effects of oceanic solution. J Geophys Res 89:10125-10133 ridge subduction on accretionary wedges: experimental modeling Dahlen FA, Suppe J, Davis D (1984) Mechanics of fold-and-thrust and marine observations. Tectonics 11:1301-1313 belts and accretionary wedges : cohesive Coulomb theory. J Geo- Lallemand SE, Schnurle R Malavieille J (1994) Coulomb theory phys Res 89:10087-10101 applied to accretionary and non-accretionary wedges - possible Davis D, Suppe J, Dahlen FA (1983) Mechanics of fold-and-thrust causes for tectonic erosion and/or frontal accretion. J Geophys belts and accretionary wedges. J Geophys Res 88:1153-1172 Res, 99 : 12033-12055 Hoshino K, Koide H, Inami K, Iwamura S, Mitsui S (1972) Malavieille J (1984) Mod~lisation exp~rimentale des chevauche- Mechanical properties of Japanese Tertiary sedimentary rocks ments imbriqu~s: application aux cha~nes des montagnes. Bull under high confining pressures. Rep 244, Geol Surv Jpn: 1 -200 Soc G~ol Fr 7:129-138 Huene R von. A sedimentary and tectonic history of Lima Basin, an Moore GF, Taylor B (1988) Structure of the Peru forearc from offshore contemporary of Pisco Basin. J Geol Soc London, in multichannel seismic-reflection data. In: Suess E, Huene R von press et al. (eds) Proc ODP, Init Rep 112:71-76 Huene R yon, Lallemand SE (1990) Tectonic erosion along the Mulugeta G (1988) Modeling the geometry of Coulomb thrust Japan and Peru convergent margins. Geol Soc Am Bull 29: wedges. J Struct Geol 10:847-859 704 - 720 Nur A, Ben-Avraham Z (1981) Volcanic gaps and the consumption Huene R yon, Miller J (1988) Migrated multichannel seismic- of aseismic ridges in South America. In: Kulm LD, Dymond J, reflection records accross the Peru continental margin. In: Suess Dasch EJ, Hussong DM (eds) Nazca Plate: Crustal Formation E, von Huene R et al. (eds) Proc ODE Init Rep 112:109-124 and Andean Convergence. Geol Soc Am Mem 154:729--740 Huene R von, Scholi DW (1993) The return of sialic material to the Shi Y, Wang C-Y, yon Huene R (1990) Modeling of pore pressure, mantle indicated by terrigenous material subducted at convergent Central Peru Margin. in: Suess, E, von Huene R et al. (eds) Proc margins. Tectonophysics 219: 163-175 ODP, Sci Res 112:663--670 Huene R yon, Suess E et al. (1988) Ocean Drilling Program Leg 112, Sosson M Mercier de Lepinay B, Bourgois J, de Wever P, Michand F, Peru continental margin: part 1: tectonic history. 16: Barron J, Fourtanier E (1992) Deep sea dives in the Chiclayo 934-938 Canyon Northern Peru: tectonic regime of the Andean conver- Hussong DM, Wipperman LK (1981) Vertical movement and gent margin. EOS 73 (suppl): 152 tectonic erosion of the continental wall of the Peru- Chile trench near 11°30'S latitude. In: Kulm LD, Dymond J, Dasch EJ, Hussong DM (eds) Nazca Plate: Crustal Formation and Andean "This is a short but fascinating paper. This sort of medeling is not easy Convergence. Geol Soc Am Mem 154: 509-524 to do well, but the authors have succeeded admirably. The importance Klingelh6fer F (1992) Prozessing seismischer Daten der Subduk- of the relative size of the subduction window has never before been tionszone vor Peru. Unpublished Diplom Thesis, Univ Kiel addressed as well as in this paper, the simultaneous accretion and Kulm L D, Thornburg TM, Suess E, Resing J, Fryer P (1988) Clastic, erosion is another very important result." Anonymus reviewer