Upper Plate Deformation Associated with Seamount Subduction

ELSEVIER Tectonophysics 293 (1998) 207±224

Upper plate deformation associated with seamount subduction

S. Dominguez a,Ł, S.E. Lallemand a, J. Malavieille a,R.vonHueneb a Laboratoire de GeÂophysique et Tectonique, UMR 5573, UniversiteÂ de Montpellier II, 34095 Montpellier, France b GEOMAR, Christian-Albrechts-UniversitaÈt, 24148 Kiel, Germany Received 4 March 1997; accepted 24 March 1998

Abstract

In many active margins, severe deformation is observed at the front of the overriding plate where seamounts or aseismic ridges subduct. Such deformation appears to be a main tectonic feature of these areas which in¯uences the morphology and the seismicity of the margin. To better understand the different stages of seamount subduction, we have performed sandbox experiments to study in detail the evolution of deformation both in space and time and thus complement seismic images and bathymetry interpretation. We focus, in this paper, on the surface deformation directly comparable with sea¯oor morphology. Two types of subducting seamounts were modelled: relatively small conical seamounts, and larger ¯at-topped seamounts. The indentation of the margin by the seamount inhibits frontal accretion and produces a re-entrant. The margin uplift includes displacement along backthrusts which propagate from the base of the seamount, and out-of-sequence forethrusts which de®ne a shadow zone located on the landward ¯ank of the seamount. When the seamount is totally buried beneath the margin, this landward shielded zone disappears and a larger one is created in the wake of the asperity due to the elevated position of the deÂcollement. As a consequence, a section of the margin front follows behind the seamount to greater depth. A `slip-line' network develops concurrently above the subducting seamount ¯anks from the transtension along the boundaries of the shadow zone. In a ®nal stage, normal faults, controlled by the shape of the seamount, develop in the subsiding wake of the asperity. Swath-bathymetric data from the Costa Rica margin reveal detailed surface deformation of the margin above three subducting seamounts. Shaded perspective views highlight the detailed structure of the sea¯oor and compare well with surface deformation in the sandbox experiments. The good correlation between the marine data and experimental results strengthen a structural interpretation of the Costa Rican seamount subduction.  1998 Elsevier Science B.V. All rights reserved.

Keywords: seamount subduction; sandbox experiment; slip-line fracture network; backthrust

1. Introduction rameters such as the nature of the asperity, the structure and the dip of the oceanic plate, and also the ge- The subduction of oceanic highs, seamounts, vol- ology and the tectonic regime of the overriding plate. canic chains or oceanic plateaus, involves a vigorous Many authors have studied these problems, es- deformation of the upper plate (Cloos, 1992; Scholtz pecially in the western Paci®c Ocean. These stud- and Small, 1997). Numerous bathymetric and seismic ies of large seamount subduction, like the Daiichi± records show that deformation depends on several pa- Kashima in the Japan Trench (Mogi and Nishizawa, 1980; Tomoda and Fujimoto, 1980; Oshima et al., Ł Corresponding author. E-mail: [email protected] 1985; Kobayashi et al., 1987; Lallemand et al., 1989;

Fig. 1. Morphological analysis of seamounts located close to active margins (vertical exaggeration is 5).

Dominguez et al., 1995a), the Erimo at the Japan± deeply. To improve detection and interpretation, ana- Kuril Trench junction (Cadet et al., 1985; Lallemand logue modelling is used for additional information and Chamot-Rooke, 1986; Lallemand and Le Pichon, on morphology and structure associated with sub- 1987; Dubois and Deplus, 1989), the Bougainville in ducted seamounts. the New Hebrides Trench (Collot and Fisher, 1989; With this objective, we will present in the ®rst Collot et al., 1992) and several seamounts in the Mid- section of this paper, the results of experimental dle America Trench (Von Huene and Flueh, 1993; data based on sandbox experiments performed using Von Huene et al., 1995), have played a major role granular materials and realistic boundary conditions in the evaluation of deformation from a subducting patterned after marine data. To better simulate ®eld asperity. Lallemand et al. (1990, 1992, 1994) have data, we have analyzed the morphology of volcanic proposed a ridge and seamount subduction sequence seamounts at various stages of subduction. It appears based on marine data and sandbox modelling. Scholtz from this work that we can divide them into two and Small (1997) have studied the in¯uence of sub- groups according to their size and shape. The ®rst ducting seamount on seismic coupling and earthquake group corresponds to seamounts less than 3 km high activity across the subduction interface, mainly in with a conical shape and the second corresponds to the Izu±Bonin and Tonga±Kermadec Trench. Cloos ¯at-topped seamounts higher than 3 km (Fig. 1). (1992) has also studied seismic slip and rupture area In the second section, the results of these general of large earthquakes in the region of seamount sub- experimental studies are compared with subducting duction. He suggests that these earthquakes are re- conical seamounts beneath the Costa Rica margin lated with shearing off of the seamounts. (Von Huene et al., 1995) and ¯at-topped seamounts Generally, the ®rst indication of a subducting subducting into the Japan Trench. These areas have seamount is given by an anomaly in the morphology been chosen because several seamounts are subduct- of the margin like a re-entrant, a scarp or an up- ing and swath-mapping, with high resolution and lifted bulge. In the Mediterranean ridge (Von Huene, 100% coverage, is available. Furthermore, we have 1997), it is even possible to observe directly the top been able to compare our models with high resolu- of the Bannock seamount due to the high dissolution tion 2-D and 3-D shaded views of the Costa Rica and rate, around the subducting seamount, of the evapor- Japan margin bathymetry. itic sediments forming the accretionary wedge. Anal- yses of magnetic anomalies (Dubois and Deplus, 1989) or seismic pro®les (Hinz et al., 1996; Ye et al., 2. Experimental apparatus and procedure 1996) may help to localize the subducted asperity. However, morphological and geophysical evi- Each sandbox experiment was performed under dence is less clear when the seamount is subducted normal gravity using an apparatus (Fig. 2) which S. Dominguez et al. / Tectonophysics 293 (1998) 207±224 209

Fig. 2. Experimental set-up used to perform the sandbox experiments of seamount subduction. simulates the basic geometry and the main mecha- buttress. Then, a low cohesive sand wedge was built nisms of a subduction zone. This experimental set-up to simulate an accretionary wedge in front of the is similar to the one used by Malavieille (1984) and buttress. On the mylar sheet, successive coloured Malavieille et al. (1991). A 1.2 m wide mylar sheet sand layers simulated the oceanic sediments (Fig. 3). (analogue of the oceanic plate), lying on a horizon- The seamount consisted of a rigid core attached to tal plate, is pulled beneath a rigid backstop. The the mylar sheet and was covered with low cohesive use of an inclined basal plate does not change the sand. The aeolian sand used in this study is rounded mechanisms of the deformation. Indeed, we have with a grain size of less than 300 µm and a density already observed in previous experiments that the ² D 1600 kg=m3. The internal coef®cient of friction only signi®cant effect is a slight modi®cation of is ¼ D 0:6 and the cohesion C0 D 20 Pa. topographical slopes. The granular materials used to build the margin This device allows more than 1.5 m of conver- and the oceanic sediments have frictional properties gence for each experiment. In front of the backstop, satisfying the Coulomb theory (Dahlen et al., 1984; a wedge was constructed that represents the structure Dahlen, 1984) and thus, they are fair analogues of commonly observed in seismic records. This wedge the rocks of the upper crust (Krantz, 1991). Scaling was constructed of a cohesive material (compacted is discussed in Lallemand et al. (1992), and 1 cm in rock powder or mortar) to simulate a deformable our experiment is equivalent to 1 km in nature. Two

Fig. 3. Schematic cross-section of the experimental set-up showing the internal structure at initial stage. 210 S. Dominguez et al. / Tectonophysics 293 (1998) 207±224 cameras and one video recorder recorded each stage Contraction in front of the seamount is largely of deformation. Structural interpretations are based accommodated by backthrusts. The backthrusts are on planar views recorded at the same time as the curved landward and have a small lateral extension. morphological perspective views presented. Offsets are about 1 to 2 mm which can be compared with 100 m in nature. Associated with the differential shortening, a 3. First experiment: Subduction of a conical transtensive fracture network develops (Dominguez seamount et al., 1995b, 1996). These fractures are directly in- duced by the indentation of the margin (Fig. 4b and During the ®rst stage of subduction, the seamount Fig. 5b). The fracture network consists of subvertical indents the base of the inner trench slope (Fig. 4a strike-slip faults that follow the slip-line orientation. and Fig. 5a). The front of the margin is uplifted and They grow above the top of the subducting seamount slides on the subducted ¯ank of the seamount. At this and diverge slightly on both sides from the direction stage, some thrusts reactivation can be observed due of the convergence (Fig. 4c and Fig. 5c). The geome- to the increase of the dip angle of the deÂcollement, try of the fractured zone can be compared with a fan which is now located on the landward ¯ank of the structure, developing intermittently as the seamount seamount. subducts. The length of these faults is a few centime- As subduction progresses, a shadow zone forms tres in our experiment (several kilometres in nature) in the wake of the seamount (Fig. 4b and Fig. 5b) and the offset is about 1 to 2 mm (100±200 m in because the deÂcollement is deviated upwards above nature). The root zone of the slip-lines is intensively the seamount landward ¯ank. The detachment rises fractured by subvertical faults and forms a weak zone to a higher level in the front of the margin and where mass-sliding (and canyon erosion in nature) the sediments located underneath subduct, then, with is active. When the seamount is further subducted little deformation. (Fig. 4c and Fig. 5c), frontal accretion resumes. The In the wake of the seamount, frontal accretion trench ®ll, in the former shadow zone located in the stops and the oceanic sedimentary sequence subducts wake of the seamount, accretes because the deÂcolle- without any deformation. On both sides of the re- ment moves from the top of the subducting sediment entrant, new imbricates are added to the accretionary sequence to its base (Dominguez et al., 1994). wedge. Close to each side of the re-entrant, these Above the seamount, local uplift of the margin new imbricates are curved in the direction of the forms a circular knoll whose geometry and volume convergence and connect with the landward ¯ank of are controlled by the shape and size of the asperity. the subducted seamount. The point of maximum uplift is located landward of The scarp, associated with the re-entrant, is af- the top of the subducting seamount. In this region, fected by intense mass-sliding. Slumped masses pro- small horsts and grabens (few millimetres offsets) ceeding from the uplifted part of the margin, deposit trending parallel to the direction of the convergence, on the subducting sequence located in the shadow develop above the subducting asperity. The bending zone (Fig. 4b and Fig. 5b). They are subducted in of the margin above the subducted seamount pro- the wake of the seamount as long as the deÂcollement duces a normal fault network. These faults are super- crops out in the upper part of the frontal margin. imposed on the `slip-line' fracture network (Fig. 4d Such processes allow material from the margin to be and Fig. 5d). subducted to greater depth. As the whole seamount subducts beneath the co- After a certain amount of seamount subduction hesive part of the margin, the backthrust spacing (15 cm), the deÂcollement switches to a deeper level, increases (Fig. 4d and Fig. 5d). These are cut by located at the top of the subducting sequence. Then landward-dipping normal faults that develop sub- ®nally, the deÂcollement returns to its basal level sequently in the wake of the subducted seamount. (at the base of the subducting sequence) causing Displacements on these normal faults are only a few underplating of the unit previously dragged behind millimetres. The subsided area, observed behind the the seamount. crest of the seamount, is elongated in the direction S. Dominguez et al. / Tectonophysics 293 (1998) 207±224 211 Fig. 4. (a through d) First experiment. Subduction of a conical seamount. 212 S. Dominguez et al. / Tectonophysics 293 (1998) 207±224

Fig. 5. (a through d) Tectonic interpretation of the ®rst experiment. The circle outlines the location of the subducted seamount. of the convergence. It could be ®lled by sediments seamount subducts, extension, expressed by normal and then form a basin. It develops when frontal ac- faulting, occurs in the wake. cretion resumed and corresponds to the underplating After 50 cm of convergence, deformation re- of the sediments situated in the shadow zone. As the lated to the seamount subduction ends because the S. Dominguez et al. / Tectonophysics 293 (1998) 207±224 213 seamount is deeply subducted and located farther uplifted zone cut by numerous backthrusts devel- from the front of the accretionary wedge. Only a ops over the seamount leading ¯ank and over-critical gentle uplift without any fracture network can be slopes with gravity slides develop on the trailing ¯ank. detected. On both sides of the re-entrant, the wedge is To better study the scarps associated with the frac- dragged upslope and two lateral conjugated shear ture network, we performed the same experiment us- zones, composed of several strike-slip faults with ver- ing a very ®ne granular material to favour fracturing tical displacement, develop (Fig. 7c,d). These faults and a larger seamount to increase fault displacement bound a triangular-shielded zone in the wake of the (Fig. 6a,b). The `slip-line fracture network' is clearly guyot which allows a great part of the margin front to expressed in surface morphology and we observe the be dragged landward behind the subducting guyot. right- and the left-lateral displacement of conjugated When the seamount is totally subducted beneath strike-slip faults accommodating the indentation of the front of the margin, the large uplifted area pro- the margin by the seamount. The normal faults are duced by the guyot becomes subcircular and the also clearly observable in this experiment above the backthrust spacing increases rapidly. At the same trailing ¯ank of the seamount (Fig. 6a,b). time, the height of the bulge decreases. Frontal ac- We have also performed sandbox experiments to cretion resumes in the wake of the seamount and study the in¯uence of seamounts slope angles. With the scarp associated with guyot subduction becomes this objective, we have used three seamounts with inactive. The two lateral shear-bands continue prop- the same diameters but different heights (Fig. 6c). agating landward and become subparallel (Fig. 7d). It appears from these experiments that the shape of Superimposed on the backthrusts is a dense net- the subducting seamount directly controls the devel- work of small slip-lines, subparallel to the direction opment of the backthrust and the slip-line network. of the convergence. This network is not strongly di- Seamounts with ¯at slopes (less than 5ë) do not vergent as in the case of a conical seamount which is generate slip-line faults and the backthrust spacing probably related to the ¯at top of the seamount and is very short. Seamounts with steeper slopes (more the low angle of its slopes. The root zone of these than 5ë) produce a well developed slip-line network faults is intensely fractured, displacement is small and widely spaced backthrusts (Fig. 6d). but they extend laterally over more than 20 cm (20 to 30 km in nature). In the very large subsided area (Fig. 7d), trans- 4. Second experiment: Subduction of a verse normal faults and late-stage strike-slip faults on ¯at-topped seamount both sides of the re-entrant cut early-stage structures. These faults are similar to those observed in the The experiment was performed using the same conical seamount experiment. They produce a large materials and parameters as previously. The main subcircular basin bounded seaward by the uplifted differences are the shape of the asperity (¯at-topped, accretionary wedge dragged upslope in the wake of higher summit and lower slope angles) and its larger the guyot (Fig. 7c). size compared with the accretionary wedge and the At the end of the experiment, the main morpho- margin (Fig. 7a). tectonic features were: a re-entrant in the front of The main processes, obtained during guyot sub- the margin about three times the diameter of the duction, are comparable to those produced during the guyot, an elongated trough in its wake and an area main stages of a conical seamount subduction. How- highly deformed by backthrusts and transverse verti- ever, due to the size and the shape of the asperity, the cal faults above the subducting guyot. deformation of the margin differs slightly. The indentation of the margin by the guyot rapidly affects the accretionary wedge. Shortening is accom- 5. Swath-bathymetry reprocessing modated by reactivation of landward-dipping out-of- sequence thrusts. A shielded zone develops behind the The experimental results have been compared to trailing ¯ank of the seamount (Fig. 7b). A very large reprocessed swath-mapping (Hydrosweep) morphol- 214 S. Dominguez et al. / Tectonophysics 293 (1998) 207±224 re well observable. (b) f a sandbox experiment Fig. 6. (a) Perspective views and tectonic interpretation of an experiment of conical seamount subduction. The fractures following the slip-lines a Tectonic interpretation of a conical seamount subduction showing the relations between the different fracture networks. (c, d) Perspective views o showing the relations between slip-line, backthrust development and subducting seamount shapes. S. Dominguez et al. / Tectonophysics 293 (1998) 207±224 215 ogy in areas of seamount subduction. It is common The studied area (Fig. 8) corresponds to a very practice to smooth bathymetric data to eliminate bad deformed part of the Costa Rica margin. In this soundings and other artefacts. Reprocessing of raw area, three well-developed seamount tracks illustrate data and un®ltered soundings are presented to re- three stages of seamount subduction. Fig. 9 shows solve the very ®ne morphotectonic features which different angles of illumination of the third re-entrant would normally have been discarded. Perspective to reveal the very ®ne fracture network above the images with arti®cial illumination generated with the supposed position of the subducted seamount. GMT software (Wessel and Smith, 1991), allowed us The ®rst re-entrant (9ë000N, 84ë400E) in the to signi®cantly improve resolution. Examples from southeast shows morphotectonic features typical of the Costa Rica margin (Fig. 8) and the Japan Trench the preliminary stages of subduction observed in the (Fig. 12) are compared with the sandbox experiments sandbox experiments (Fig. 9). The subcircular scarp previously described to allow a detailed structural in- associated to this re-entrant is nearly 1.5 km high and terpretation. shows recent gravity collapse. The wedge is indented and shows a re-entrant more than 8 km long. Frontal accretion is inhibited in a shielded zone and occurs 6. Effect of conical seamount subduction as only on both sides of the re-entrant. The deÂcollement illustrated in Costa Rica comes to the surface at the top of the oceanic sediment sequence. The circular bulge associated with Along this portion of Middle America, the Co- the re-entrant (dashed line on Fig. 10) is 13 km long, cos plate, of Late Miocene age subducts beneath the 15 km wide and 1 km high. All these features sug- Caribbean plate at a rate of about 9 cm=year (Demets gest a conical shape for the subducted volcanic relief et al., 1990). On the basis of seismic data, Bour- as seen from other seamounts exposed on the oceanic gois et al. (1984), Crowe and Buf¯er (1985), have plate (Fig. 8). The fractures due to the indentation proposed that the Costa Rica margin is mainly con- of the margin by the seamount are not very well ex- stituted by the seaward prolongation of the Nicoya pressed at this stage, but some evidence of strike-slip ultrama®c rocks complex. They have interpreted the faults (slip-lines oriented fractures network) can be geomorphology of the margin offshore as the result observed at the toe of the scarp. The vertical dis- of an extensional tectonic regime affecting the whole placement on these faults could be about 20±50 m overriding plate. which is near the limit of resolution in the bathymet- Most recent works have shown that this inter- ric data. Furthermore, since sediment deposition is pretation cannot explain the margin deformations rapid on the Costa Rican margin, there is little time observed. The ®rst description of seamount subduc- for fracture development. Upslope from the bulge, tion beneath the Costa Rica margin (Von Huene and some backthrusts and a faint divergent pattern oc- Flueh, 1993; Lallemand et al., 1994) identi®ed three curs. We can also observe two diverging canyons seamount tracks across the margin. The high resolu- which shape is controlled by the landward propa- tion morphology indicates that the tectonics of the gation of the bulge associated with the subducted Costa Rica margin is signi®cantly affected by the seamount. subduction of seamounts on the Cocos plate (Fig. 8). The second re-entrant (9ë050N, 84ë500E) in the Several local uplifted zones, associated seaward with middle of the area corresponds to a more advanced sedimentary slides, contrast with the smoothed mor- stage of subduction. This re-entrant has two lateral phology of the rest of the margin. scarps, about 1 km high, trending parallel to the To improve the resolution, different angles of direction of convergence. Its landward headwall is illumination were viewed to derive a high resolution formed by active landslides located more than 25 tectonic map (resolution of about 50 m to 100 m, km from the trench. The geometry of this re-entrant depending on depth). After processing, the tectonics suggests subduction of a seamount comparable in of subducting seamounts was at a scale similar to size with the previous one. the sandbox model and the structural map compared The present front of the margin at the seamount better with the sandbox experiments. trail is not indented because the seamount has mi- 216 S. Dominguez et al. / Tectonophysics 293 (1998) 207±224 Fig. 7. (a, b, c) Second experiment. Subduction of a ¯at-topped seamount (guyot). S. Dominguez et al. / Tectonophysics 293 (1998) 207±224 217

Above the position of the subducted asperity outlined by the magnetic anomalies (Von Huene et al., 1995) is a circular bulge, 15 km in diameter and 500 m high. This uplifted zone is cut by many small faults. High resolution imagery shows several curved backthrusts dipping seaward with little displacement (<30 m) close to the landward boundary of the bulge. A dense network of faults oriented along slip-lines trends is now observable in the reprocessed data. The fan-shape distribution of the faults is comparable to the experiment in which the conical seamount was subducted (Fig. 6c). Their lengths are about 5 to 10 km and the maximum offset associated to these faults is about 100 m. The third seamount trail observed in the area is probably a large surface scar resulting from seamount subduction even earlier than the previous one. The long trail between the trench and the eroded scarp indicates a seamount deeply buried beneath the margin. There is no associated bulge on the margin but the contours show a wide elongated remnant basin, suggesting that the asperity which Fig. 7 (continued). (d) Structural interpretation of the second experiment corresponding to the ®nal stage viewed from above. caused this trail was probably larger than those that caused the two previous ones. grated suf®ciently far to allow the frontal accretion 7. Case of a ¯at-topped seamount as illustrated to resume. The initial V-shape indentation remains by the Daiichi±Kashima seamount in the Japan observable and is comparable to the one produced by Trench the ®rst seamount. Immediately seaward of the steepest part of the Very few ¯at-topped seamounts have been de- scarp is a small ¯at area that may correspond to the scribed in detail (with swath-bathymetry available) beginning of the subsidence observed in the sandbox in subduction zones. The two most famous are experiments as the seamount was deeply subducted. the Daiichi±Kashima in the Japan Trench and the Subsidence along previous strike-slip faults which Bougainville in the New Hebrides Trench. Unfortu- developed behind the seamount forms the seamount nately these guyots are in the ®rst stages of sub- trail across the accretionary wedge and backstop. duction and there is no clear evidence in any active There are no normal faults perpendicular to the margin that a guyot is presently subducting. How- convergence. Perhaps the cohesive part of the margin ever, the subduction of the Daiichi±Kashima, despite (the relative deformable backstop) was just recently the fact that a huge normal fault affects the guyot affected by the asperity. The sediments in the wake (Lallemand et al., 1989; Dominguez et al., 1995a), of the subducted seamount probably belong to the deforms signi®cantly the Japan margin. pre-existing accretionary wedge and slope deposits First, its western ¯ank is completely subducted and are not cohesive enough to produce these faults. beneath the front of the Japan margin. A bulge with As shown in the sandbox experiments, the normal a curved shape can be clearly observed, as well as faults develop when the seamount starts subducting fractures, using 3-D shaded views (Fig. 11). These beneath the cohesive part of the margin. fractures probably correspond to the incipient de- 218 S. Dominguez et al. / Tectonophysics 293 (1998) 207±224

Fig. 8. General 2-D illuminated view of the Costa Rica margin showing several re-entrants caused by conical seamount subductions. velopment of a slip-line fracture network associated 8. Discussion with the beginning of the margin indentation. Second, the Daiichi±Kashima belongs to a vol- The good ®t between sandbox experiments and canic chain of big seamounts and, as noticed by natural examples (Costa Rica and Japan sea¯oor Lallemand et al. (1989), there is a possibility that morphology), suggests a fairly good replication of some of them have been previously subducted. In seamount subduction. Based on this evidence and the upper part of the margin, we can observe an reprocessed morphology, we have studied the me- uplifted area. The presence of two diverging canyons chanics of the deformation in the sandbox models (like in the Costa Rica margin), curved ridges (back- to better understand mechanisms in active margins. thrusts) and an eroded area (remnant scarp of the The subducting seamounts have shaped the segment re-entrant), suggests that a big seamount is deeply of the Costa Rica margin, at least during Plio± buried beneath the Japan margin (Fig. 11). Quaternary times if we assume that the third imprint Nevertheless, the average quality of swath- on Fig. 10 was caused by a huge subducting asperity. bathymetry (Seabeam), the incomplete survey of this They were responsible for: (1) short interrup- region and the complex morphology of the margin tions of frontal accretion and re-entrants into the make a more detailed interpretation dif®cult. accretionary wedge; (2) net removal of accretionary S. Dominguez et al. / Tectonophysics 293 (1998) 207±224 219

Fig. 9. Zooms of the second re-entrant, with different angles of illumination, showing the fan-shape fault network following the slip-lines. 220 S. Dominguez et al. / Tectonophysics 293 (1998) 207±224

Fig. 10. Tectonic map of the studied area in the Costa Rica margin. wedge material from the front of the margin; (3) The deÂcollement moulds the subducted seamount transverse sedimentary traps in the wake of subduct- (and laterally, it tends to close again in its wake ing seamounts; (4) weak zones along the seamount (Fig. 12a). trails. These weak zones are characterized by the A continuing progression of backthrusts and frac- dense network of subvertical faults as the top of the tures develops across the margin. First, backthrusts seamount migrates beneath the wedge that are later nucleate as the compression increases in front of reactivated as normal faults. the indenter. Second, a ®rst generation of fan-shaped S. Dominguez et al. / Tectonophysics 293 (1998) 207±224 221 Fig. 11. 3-D shaded view of the Japan margin in the area of the Daiichi±Kashima guyot. Vertical exaggeration is about 5. 222 S. Dominguez et al. / Tectonophysics 293 (1998) 207±224

Fig. 12. (a) Sketch showing the shape of the deÂcollement in the area of a subducted seamount. Gridding represents the maximum and minimum stress directions. (b) Sketch showing the geometry of the fault planes on the landward part of the subducting seamount at the time of their nucleation. Only one set of strike-slip faults and one backthrust is represented on this cartoon. During seamount subduction, other sets of faults superimpose on the ®rst one. subvertical fractures develops (Fig. 12b) whose ge- vated during the next stages of seamount subduction ometry is controlled by the asperity shape. Then, until compression reaches again the yield limit of the both backthrusts and subvertical fractures are reacti- material and another generation of backthrusts and S. Dominguez et al. / Tectonophysics 293 (1998) 207±224 223 then subvertical fractures develops. The ®nal stage Acknowledgements involves the interaction between pre-existing faults and newly activated ones. The authors wish to thank D. Davis, H. Kerr, J. Bourgois and C. Scholz for their reviews of the manuscript. We also thank B. Sanche for his tech- 9. Conclusion nical support in performing sandbox experiments, Niita is acknowledged for the grammatical correc- The impact of the subducting seamounts or vol- tions of the manuscript. canic ridges on the tectonics of active margins is generally well documented in the literature. Natural examples have been described but the kinematics and References the fault networks associated with the subduction of such oceanic relief is poorly understood. As we have Bourgois, J., Azema, J., Baumgartner, P.O., Tournon, J., Desmet, seen in a comparison between experimental results A., Aubouin, J., 1984. The geologic history of the Caribbean± and the Costa Rica margin, the sandbox experiments Cocos plate boundary with special reference to the Nicoya of seamount subduction are a useful aid to improve ophiolite complex (Costa Rica) and D.S.D.P. results (Legs 67 and 84 off Guatemala): a synthesis. Tectonophysics 108, 1±32. interpretation of the geomorphology associated with Cadet, J.P., Kobayashi, K., Aubouin, J., Boulegue, J., Dubois, this process. We describe the weak zone above and J., von Huene, R., Jolivet, L., Kanazawa, T., Kasahara, J., in the wake of the asperity. The indentation of the Koizumi, K., Lallemand, S.E., Nakamura, Y., Pautot, G., margin by the seamount causes a fan-like fault net- Suyehiro, K., Tani, S., Tokuyama, H., Yamazaki, T., 1985. work associated with backthrusts. The margin re- De la fosse des Kouriles: Premiers reÂsultats de la campagne covers its initial slope as the seamount is deeply oceÂanographique franco±japonaise Kaiko (Leg 3). C. R. Acad. Sci. Paris 301, 287±296. subducted. These experiments allow us to observe Cloos, M., 1992. Thrust-type subduction-zone earthquake and the kinematics and the timing of deformation and seamount asperities: A physical model for seismic rupture. the interactions between concurrent compressional Geology 20, 601±604. and extensional deformations above the subducting Collot, J.Y., Fisher, M.A., 1989. Formation of forearc basins seamount. by collision between seamounts and accretionary wedges: an Results of the analogue modelling and example from the New-Hebrides subduction zone. Geology 17, 930±933. swath-mapping, show that the subduction of vol- Collot, J.Y., Lallemand, S., Pelletier, B., Essein, J.P., Glacon, G., canic asperities involves very strong deformation Fisher, M.A., Greene, H.G., Boulin, J., Daniel, J., Monzier, and intense fracturing of the margin. They affect M., 1992. 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Mechanics of fold-and- seamounts increase locally the normal stress across thrust belts and accretionary wedges: cohesive Coulomb the- the subduction interface. This could explain the large ory. J. Geophys. Res. 89, 10087±10101. interplate earthquakes events recorded in areas of Demets, C., Gordon, R.G., Argus, D.F., Stein, S., 1990. Current seamount subduction. The other explanation, as sug- plate motions. Geophys. J. R. Astron. Soc. 101, 425±478. gested by Cloos (1992), is that the seamounts are Dominguez, S., Lallemand, S.E., Malavieille, J., 1994. New re- ®nally shearing off when they are deeply subducted sults from sandbox modeling of seamount subduction and possible applications (abstr.). AGU Fall Meeting, San Francisco, beneath the inner part of the overriding plate. We EOS 75 (44), p. 671. believe that sandbox experiments could clarify this Dominguez, S., Lallemand, S.E., Malavieille, J., Pouclet, A., point. 1995a. Nouvelle interpreÂtation structurale du mont sous-marin 224 S. 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