Structure of the Lesser Antilles subduction zone backstop and its role in a large accretionary system

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B7, 2358, doi: /2002jb002040, 2003 Structure of the Lesser Antilles subduction zone backstop and its role in a large accretionary system Nathan L. Bangs, Gail L. Christeson, and Thomas H. Shipley University of Texas Institute for Geophysics, Austin, Texas, USA Received 19 June 2002; revised 31 January 2003; accepted 4 March 2003; published 31 July [1] The role of a backstop in subduction zones has been the subject of numerous laboratory and numerical modeling studies; however, few field observations exist revealing how backstops control deformation in subduction zones and accretionary wedge construction. A seismic reflection and refraction survey acquired in 1998 with the R/V Maurice Ewing reveals the geometry of the forearc igneous crust, accretionary wedge, and forearc basin structure of the northern Guadeloupe area of the Lesser Antilles forearc. An accreted block of buoyant crust, accreted in the late Miocene, forms the toe of the overriding arc crust and forms the backstop. We imaged the top of this surface, beneath the forearc basin, to its seaward edge where it meets the subducting oceanic crust. The toe of the backstop was thrust upward and forms a steep buttress in contact with the lower half of the accretionary wedge. The steep buttress produces a narrow inner deformation zone with minimal backthrusting of the accretionary complex landward over the backstop, and a narrow <10 km transition between accreted and forearc basin sediment. Seismic reflections from the subducting crust and the decollement appear beneath the entire accretionary wedge and below the backstop toe. Separating the decollement and the subducting crust is an interval, usually between 500 and 750 m, of underthrust sediment carried underneath the accretionary wedge and subducted 15 km landward and beneath the toe of the backstop. We speculate that the upturned geometry of the toe of the backstop and a weak fluid-rich decollement may facilitate sediment subduction beneath the backstop and potentially into the mantle. INDEX TERMS: 3025 Marine Geology and Geophysics: Marine seismics (0935); 3040 Marine Geology and Geophysics: Plate tectonics (8150, 8155, 8157, 8158); 8015 Structural Geology: Local crustal structure; 8020 Structural Geology: Mechanics; 8150 Tectonophysics: Plate boundary general (3040); KEYWORDS: subduction zone, backstop, Barbados Ridge, forearc basin, accretionary prism, Lesser Antilles Citation: Bangs, N. L., G. L. Christeson, and T. H. Shipley, Structure of the Lesser Antilles subduction zone backstop and its role in a large accretionary system, J. Geophys. Res., 108(B7), 2358, doi: /2002jb002040, Introduction [2] The distinctive kinematic boundary, or backstop, at the front of the overriding plate at a convergent margin plays two roles in the behavior of active subduction zones. In one role, the backstop partitions subducting material directing it either into the accretionary wedge, or diverting material deep into the subduction zone to be recycled into the mantle. Despite the important ramifications for mantle evolution and margin development, few studies have made observations of sediment fluxes through subduction zones by examining the interaction of the relatively stable, rigid backstop structure of the overriding plate with the rear of the accretionary wedge and the subducting plate [e.g., Karig and Kay, 1981; von Huene and Scholl, 1993; Plank and Langmuir, 1993]. [3] In a second role, that of forearc construction, the backstop controls wedge deformation above and in front of the backstop [Byrne et al., 1988, 1993]. Numerical [Byrne et Copyright 2003 by the American Geophysical Union /03/2002JB002040$09.00 al., 1993] and laboratory (sandbox) models [Wang and Davis, 1996] show the geometry of the rigid boundary provided by the backstop significantly modifies the propagation of stresses into the wedge. A seaward dipping backstop is conducive to wedge thickening and shortening through development of backthrusts [Silver and Reed, 1988] that transport the accretionary wedge landward over the toe of the backstop to produce an outer arc structural high and inner deformation zone along the landward side of the accretionary wedge [Byrne et al., 1993; Wang and Davis, 1996]. Despite numerous modeling studies of the effects of the backstop geometry on subduction zone processes [e.g., Byrne et al., 1988, 1993; Calassou et al., 1993; Mulugeta and Koyi, 1992; Larroque et al., 1995; Gutscher et al., 1998; Wang and Davis, 1996], relatively few field studies have been conducted on these tectonic settings to observe the role of the backstop in forearc construction. Good crustal models of a backstop and accretionary wedge were obtained with comprehensive multichannel and large-aperture (ocean bottom seismometer (OBS) and onshore/offshore) seismic experiments along the Cascadia margin [e.g., Hyndman et EPM 6-1

2 EPM 6-2 BANGS ET AL.: LESSER ANTILLES SUBDUCTION ZONE BACKSTOP Figure 1. Location map showing the track lines for the seismic acquisition. The dip lines (D1 D4) are spaced 10 km apart. D5 is centered between D2 and D3. These lines were acquired at 50 m shot spacing. The strike lines (S1 S8) are also 10 km apart but were shot at 100 m shot spacing. OBSs (circles) are located at the intersections of the lines. The outer arc high forms above the contact between the subducting and overriding lithosphere. Black lines are sections of the data shown in Figures 2, 4, 5, 6, 7, and 8. See color version of this figure at back of this issue. al., 1990; Spence et al., 1991; Tréhuet al., 1994] to examine its landward dipping backstop. Few similar observations for seaward dipping backstops exist because as the seaward dipping backstop dips down beneath the forearc basin to the accretionary wedge, it becomes a difficult target for seismic imaging. Most settings are too deep to penetrate seismically and returns are usually obscured by seafloor multiples. [4] The Lesser Antilles is an ideal location to examine the interaction between the subducting crust and the overriding arc crust, which serves as the backstop. Seismic profiles and gravity modeling [Westbrook, 1975; Westbrook et al., 1988] have identified a seaward dipping forearc basement that extends from the exposed arc massif downward to near the top of the subducting crust. The seaward dipping geometry of this backstop is a type example of commonly observed seaward dipping versus landward dipping, geometry [Silver and Reed, 1988; Byrne et al., 1988]. The general structure and morphology of the rear of the Lesser Antilles wedge are consistent with observations made from laboratory and numerical models of stress and deformation in front of rigid seaward dipping backstops [Byrne et al., 1988, 1993]. Of particular importance for studies of backstops, the Guadeloupe transect of the Lesser Antilles forearc that we have selected (Figure 1) is the only sediment-rich setting where the backstop lies in deep water and most, if not all, of it can be imaged without interference from water column multiples. The purpose of this paper is to examine the structures of the deep part of the Lesser Antilles subduction zone with multichannel reflection and OBS refraction data to reveal the geometry of the backstop, and interpret its role in determining material partitioning and construction of a sediment-rich convergent margin. 2. Background and Data Acquisition 2.1. Tectonic Setting [5] The Barbados Ridge accretionary complex (Figure 1) has formed by accretion of sediment from the North American plate as it subducts at a modest rate of km/m.y. beneath the Caribbean plate [Mann et al., 2002]. Subduction magmatism along the current arc began in the Late Cretaceous, and has constructed the Lesser Antilles island arc on oceanic crust of the Caribbean plate. Subduction accretion has produced an accretionary wedge (Figure 1) that began accumulating in the Eocene and has grown to more than 300 km in width and 20 km in thickness at its widest point along southern Barbados Ridge, south of the island of Barbados. [6] Our survey area lies along the northern Lesser Antilles arc system, east of the islands of Guadeloupe and La

3 BANGS ET AL.: LESSER ANTILLES SUBDUCTION ZONE BACKSTOP EPM 6-3 Desirade (Figure 1). The arc has evolved in episodes of magmatic and tectonic activity [Bouysse and Westercamp, 1990]. A westward shift in the axis of magmatism occurred in the late Oligocene, primarily north of our study area, but extended southward to the island of Guadeloupe. This event coincided with a cessation of volcanism for 10 m.y. during late Oligocene and early Miocene [Bouysse and Westercamp, 1990]. This segment of the arc is also noted for its rapid uplift of the eastern edge of the island arc. The island of La Desirade was uplifted 2000 m relative to its surroundings also during the late Oligocene to early Miocene reorganization of arc magmatism. Major NE-SW and NW-SE fault systems intersect the arc in the vicinity of Guadeloupe, which is the southernmost extent of major tectonism. Bouysse and Westercamp [1990] argue that this major tectonic and magmatic episode was the result of a collision with, and obduction onto the arc, of a buoyant magmatic ridge on the subducting Atlantic Ocean crust. However, other tectonic mechanisms have been proposed including changes in the subduction angle due to subduction of buoyant pieces of crust [McCann and Sykes, 1984], and tectonic erosion of the forearc [Westbrook and McCann, 1994]. The Antigua-Dominica group of islands forms a continuous N-S ridge that ends just to the north of our study area (Figure 1). [7] The morphology of the subducting North American plate includes several prominent aseismic, uncompensated crustal ridges that have had significant, but an order of magnitude less impact on the forearc crust than the hypothesized early Miocene collision and obduction event. South of the Antigua-Dominica area, the Tiburon Ridge lies seaward of the deformation front. The ridge trends NW- SE, is km wide, and rises 1850 m above the surrounding abyssal plain. On the basis of a convergence rate of 35 km/m.y. [McCann and Sykes, 1984], Westbrook and McCann [1986] place the initial collision of the Tiburon Ridge with the forearc crust at 10.5 Ma in the Greater Antilles arc to the north (Figure 1) and subsequent southward migration of the collision zone through our study area began 3.5 Ma. Currently, the Tiburon Ridge is subducting beneath the toe of the arc crust south of our study area. [8] East of Guadeloupe (16 N), the accretionary complex is a massive accumulation of sediment. Accretion at the toe scrapes off 250 m of the 750 m of incoming sediment on the subducting basement. The Guadeloupe segment of the wedge lies north of the current position of the underthrusting Tiburon Ridge and at the distal edge of the hemipelagic sediments sourced from South America that are part of the Oronoco fan, so it is intermediate in size between the accretionary complex to the north where it is <40 km wide and 7 km maximum thickness to the south where it is 300 km wide and up to 20 km thickness [Westbrook, 1984]. Bordering the Lesser Antilles arc, accretion began in Eocene and has constructed one of the largest accretionary complexes on Earth Seismic Experiment [9] We acquired multichannel and OBS seismic data during a 3-week seismic reflection and refraction experiment on board the R/V Ewing, EW9803, from 15 March to 6 April This cruise included 7 days of transit, leaving only 14 days for data acquisition. During the short cruise, 1850 km of 180-channel seismic reflection data were acquired in a grid pattern that spanned the intersection between the forearc crust and the subducting oceanic lithosphere. Four dip lines and eight strike lines were spaced 10 km apart on a regular grid (Figure 1). One additional dip line runs through the center of the survey. The seismic experiment was designed to use the R/V Ewing s highenergy, tuned 20-air gun (8400 cubic inch) seismic array to penetrate 10 km subseafloor and image beneath the entire accretionary complex and underlying oceanic plate. Dip lines were shot on distance at 50-m spacing and strike lines at 100-m spacing. The survey region overlaps with a previous experiment that also imaged the entire accretionary complex with a much smaller (2500 cubic inch) air gun array [Westbrook et al., 1988]. [10] In addition to the seismic reflection survey, 36 OBS deployments using 20 instruments provided large offset seismic reflection and refraction returns (Figure 1). These data were gathered to reveal the basement of the forearc and subducting crust particularly where it is difficult to detect in seismic reflection data. The combination of both seismic reflection and refraction information provided a means to map the geometry of the underthrusting crust and accretionary wedge. The OBS results are presented in a related publication by Christeson et al. [2003]. [11] Significant problems arose with the data acquisition that degraded the quality of seismic images. Rough sea states, tail buoy drag and the large stress it imposed on the streamer, and the lack of working tow leader and stretch sections all contributed to noisy recording by the multichannel seismic streamer. The poor condition of the streamer required painstaking examination of the data traces and hand editing of shots to eliminate the worst of the noise. The shots, decimated by 10 25% after editing, were used for processing. [12] Reflection data processing included both conventional and prestack processing. Conventional processing consisted of velocity analysis, predictive deconvolution to remove a small bubble pulse, normal moveout and stack, and poststack F-K migration. Prestack processing was also performed on the five dip lines (Figure 1) using Geodepth 2 software. These lines were migrated with prestack depth migration, which required detailed velocity analysis as part of the iterative migration processes. The depth-migrated lines were converted back to time because of the difficulty in recognizing correlative events after stretching to produce depth sections and to correlate with crossing strike lines that were processed only as time sections. 3. Results [13] Despite the difficulties with signal-to-noise ratio, and the challenges of imaging the very thick and structurally complicated setting of the Lesser Antilles wedge, new, fundamental observations arose that improve our understanding of the interaction between the forearc and subducting crust and their role in controlling the evolution of this convergent margin Forearc Basement Structure [14] One of the striking observations of the profiles is that they reveal the geometry of the top of the island arc basement and the subducting oceanic crust beneath the entire forearc basin and accretionary wedge (Figure 2). This

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5 BANGS ET AL.: LESSER ANTILLES SUBDUCTION ZONE BACKSTOP EPM 6-5 surface can be interpreted on dip line images with aid from the crossing strike lines and the OBS data. The OBS velocity models reveal a significant velocity change between basement rocks and overlying sediment that is distinctive in the OBS data, km/s versus 3.0 km/s, respectively (Figure 2; for more details on the OBS data, see Christeson et al. [2003]). (Note that the velocity data presented in Figure 2 are from the inversion models of the data presented by Christeson et al. [2003] and differ somewhat from the 3-D tomography models also presented by Christeson et al. [2003]. The 3-D tomography results tend to be smoother than the 2-D inversion models.) The velocity contrast at the top of the arc basement generates a discontinuous basal seismic reflection that is identifiable beneath the forearc basin in the western half of the profile and appears discontinuously, usually recognizable in the anticlines, as a weak reflection beneath the eastern half of the forearc basin sequence. Typically, this basal reflection separates undulating, reflection horizons of the forearc basin sedimentary sequences above it from incoherent crustal reflections and noise beneath it, thus distinguishing this basal horizon as the top of the arc basement underlying the forearc basin. The dashed line in Figure 2 marks the top of the island arc basement interpreted from OBS velocities and from the observable basement reflections on the crossing strike and dip lines in between the OBS stations. We placed the top of the basement at the base of the forearc basin sequence and assumed that it conforms with the overlying forearc basin sequence except near ridge B, where it is clearly unconformable. [15] The top of the subducting oceanic crust can be traced from the eastern edge of the profile to the junction with the top of arc basement and possibly beneath the arc basement. The reflection from the top of the ocean crust typically has high amplitude along 10- to 15-km-long segments and is weak in between. Some of the difficulty in imaging this surface continually is due to the nature of this rough surface, which scatters reflections and does not produce laterally coherent reflections. The junction of the oceanic and arc basement surfaces lies generally at the transition from intensively folded, sheared and disrupted sediment of the accretionary wedge to broadly folded, strata of the forearc basin. This relation is generally consistent with models for a seaward dipping backstop [Byrne et al., 1993]. The arc basement we identified in profiles serves as the backstop to the accretionary wedge Backstop Geometry [16] A surprising and consistent observation in each of the five dip lines is the high relief of the top of the island arc basement beneath the forearc basin. Figure 2 shows two dip lines with the interpretation of the top of the island arc basement (dashed line). Both of these lines (D3 and D4) and the other dip lines show 2 3 km of relief. There is also considerable relief in the strike direction as depicted in Figure 3. There are no apparent thrusts cutting through the forearc basin sequences that would indicate a detachment between the basement and the overlying sequences. Therefore the forearc basin sequence generally mimics and emphasizes the current backstop geometry. [17] The sharp downward deflection in the top of the arc basement at the toe of the backstop forms the principal contact of the backstop with the accretionary wedge. The toe of the backstop rises steeply 2 3 km from the top of the subducting crust and bends into a narrow ridge (ridge A in Figure 3). The buttress formed at the toe of the backstop is evident from seismic profiles and the OBS velocities. On line D3, for example, basement velocities are observed at 9.5 s where crossed by line S7, but 10 km seaward at S8, velocities typical of accretionary wedge sediment are observed down to the top of the subducting slab at 12 s. Island arc basement is not detected at all at S8. Figure 3 shows a 3-D perspective view and map of the surface of the arc crust underlying a 250 km 2 area of the forearc surveyed by dip and strike lines. Ridge A, which formed at the toe of the backstop, has local peaks and is steepest on lines D3 and D4, but regionally it is part of the toe of the backstop, which is a steep buttress oriented NW-SE along the survey area. An enlargement of D3 in the vicinity of ridge A (Figure 4) shows a series of tilted stratigraphic layers below horizon b draped over and bent conformably with ridge A. Although this is not a depth section, these stratigraphic horizons appear conformable and were probably deposited horizontally in an undisturbed forearc basin on top of the arc crust. This sequence therefore reveals a significant change in backstop geometry at the toe of the backstop. On the basis of layer pinch outs onto horizon b (Figure 4), horizon b lies along an unconformity produced by a significant change in the geometry of the basin around the time horizon b was deposited. [18] A comparison of two strike lines across ridge A, S6 and S7 (Figures 5 and 6), reveals a significant change from tight to broad folding in the forearc basin sequence coincident with ridge A. The top of the forearc basement on line S7 is defined by OBS models as the boundary between sediment velocities of km/s and basement velocities of 4.5 km/s. Although most of the sediment sequence is folded and disrupted beyond what can be imaged seismically, the strata above the basement at crossing lines D1 and D2 (Figure 5) are recognizable and dip roughly south. These strata indicate that 5-km sections are tilted and disrupted as partially coherent units. In contrast, the basin strata overlying the backstop in line S6 is broadly folded (e.g., beneath D3 on line S6). A marked transition in folding and disruption of the forearc basin sequence occurs between S7 and S6, which coincides with ridge A, the seaward edge of the Figure 2. (opposite) Seismic reflection lines D4 and D3 between crossing lines S1 and S8 (see Figure 1 for location). Dashed line shows the interpretation of the top of the igneous forearc crust interpreted from the OBS inversion models and basement reflections in both dip and crossing strike lines. Horizontal bars mark the top of the arc s igneous crust indicated by the OBS refraction data. Arrows mark the top of the subducting oceanic crust. The band of reflections above the arrows is interpreted as underthrusting sediment. Note that the 3-D tomography inversion is presented by Christeson et al. [2003, e.g., Figure 10], and it is not the same inversion result as the 2-D inversion models displayed here. The 3-D inversion produces a similar, but smoother result than the 2-D inversions.

6 EPM 6-6 BANGS ET AL.: LESSER ANTILLES SUBDUCTION ZONE BACKSTOP Figure 3. Map view and 3-D perspective view of the island arc crust that forms the backstop to the accretionary wedge. Seismic lines are shown on the map and correspond to lines shown in Figure 1. The backstop crust is buckled into two ridges, ridge A at the toe of the backstop and ridge B. These structural highs were probably formed by collisions of the arc with aseismic ridges. See color version of this figure at back of this issue.

7 BANGS ET AL.: LESSER ANTILLES SUBDUCTION ZONE BACKSTOP EPM 6-7 Figure 4. Enlarged view of line D3 at the toe of the backstop. Thin lines show the geometry of the forearc basin sediment overlying the backstop. The toe of the backstop is defined by the OBS models from lines S5 to S8. It extends seaward to somewhere between S7 and S8. Note the parallel orientation of the forearc basin strata b e and the onlapping sequence above b. This is interpreted as recent deformation of the backstop between deposition of horizons a and b. The arrows mark the top of the subducting oceanic crust. forearc basement. In the dip line direction, the forearc basin sequence can be traced continuously across the forearc basin over ridge A to <5km of the toe of the backstop where it abruptly loses coherency, (e.g., line D3, Figure 4). Ridge A is a significant abutment that fronts the accretionary wedge and coincides with a transition from an intensively to a moderately disrupted forearc basin sequence. [19] In addition to ridge A at the toe of the backstop, a series of two to three smaller ridges lie between ridges A and B. These ridges are observed on each of the dip lines; however, they are not continuous from line to line (Figure 2). The basin stratigraphy above these ridges resembles that above ridge A with an unconformity at horizon b. [20] A second major ridge (ridge B in Figure 3) forms in the arc crust on the western edge of the forearc basin. Ridge B has a distinctively different overlying stratigraphy relative to that of the more seaward basement ridges. Ridge B appears on each of the dip lines but is largest and most apparent on lines D3 D4 between crossing lines S3 S4 (Figure 2). Unlike the more seaward basement ridges, the forearc basin sequence deposited immediately above ridge B onlaps against it. Truncations of the deep basin stratigraphy are seen on lines D4 and D3 on both sides of ridge B (Figure 2). Sequences are thinner over the top of the ridge than they are on either side, which is consistent with deposition against a preexisting ridge. Second, sequences deposited on top of horizon d on D3 (Figure 2) onlap against horizon d over the crest of ridge B. Horizons c and b onlap against horizon d, so the unconformity lies deeper stratigraphically than the unconformity associated with ridge A at horizon b. It is also notable that ridge B is oriented nearly due north, close to perpendicular to the convergence direction, and oblique to the steep buttress at the toe of the backstop (ridge A in Figure 3). The seismic velocity of the basement derived from the OBS data decreases from 6.0 km/s at ridge B to km/s east to ridge A. These observations suggest distinct differences in the compositions and tectonic histories for ridges A and B, spatial and temporal changes to the geometry of the backstop, and changes in the interaction between the backstop and the accretionary wedge as discussed further below Sediment Subduction [21] Beneath the entire accretionary complex, and then deeper beneath the backstop, there appear to be indications

8 EPM 6-8 BANGS ET AL.: LESSER ANTILLES SUBDUCTION ZONE BACKSTOP Figure 5. A portion of line S7 shows S dipping forearc basin strata above the backstop, consistent with deformation following the collision of the NW oriented Tiburon Ridge. Forearc basin sequences are intensively deformed indicating a weak/deformable backstop. Compare with S6 in Figure 6. The numbers are seismic velocities in km/s derived from the OBS models and show the top and base of the backstop between D1 and D3 and its absence on D4. of several hundred meters of sediment attached to the subducting crust. While our profiles do not extend seaward to the toe of the accretionary wedge, they do overlap with profiles that show that the subducting basement and decollement can be traced into our survey area [Westbrook et al., 1988; Ladd et al., 1990]. Figure 7 shows the top of the subducting crust as a rough reflection at 10 s on the eastern edge of the profile. The high-amplitude reflections are discontinuous, which is typical of the rugged m of ocean crust relief commonly observed at the wedge toe. Approximately s above the basement reflection lies a subparallel reflection that is somewhat smoother than the underlying basement surface. We interpret this as the decollement surface, as indicated by its position overlying the basement, by its high-amplitude reflection [Bangs et al., 1999], and by the suggestion of the soling of out-ofsequence thrusts into this surface (Figure 7). We cannot unambiguously trace this reflection to the backstop because of the large variation in amplitude of the reflections and the poor signal strength beneath the deepest parts of the accretionary wedge, but we can see a series of discontinuous reflections that dip beneath the accretionary wedge that are significantly higher amplitude than the rest of the section (arrows in Figure 7). These reflections are typically paired with separation of 0.5 s and are interpreted as the subducting basement and overlying decollement. In the strike line S8, which lies just seaward of the backstop, there is also a pair of high-amplitude reflections. OBS velocity models (Figure 8) determine that basement velocities lie beneath the lower reflection, and the upper reflection is probably the decollement rather than the top of basement. If this interpretation is correct, there is m of underthrust sediment has been subducted 110 km beneath the wedge to the toe of the backstop. [22] Evidence also exists that the sediments are subducted beneath the backstop. The backstop structure as identified above, extends seaward from the arc to somewhere in between lines S7 and S8. On dip lines D3 and D4 (Figure 2), reflections lie beneath the backstop at S6 and S7. However, these are located just above the seafloor multiple, and are partially obscured by it. On strike line S6 (Figure 6), a pair of reflections is observed with similar characteristics to those traced beneath the accretionary wedge on D4 (Figure 7). Two subparallel reflections appear on S6, especially at crossing line D3, each with distinctively different characteristics indicating that they are not ringing or source effects, but are interpreted as two separate events separated by s. These lie beneath the backstop as defined by the refraction data. A detachment also separates the gently folded sediment above from the pair of reflections below, as the lower reflections do not conform to the fold pattern of the overlying strata. We interpret this reflection pair as the top of the underthrust sediment sequence and the top of the subducting oceanic crust. On the basis of this interpretation, and using a velocity of km/s for sediment above oceanic crust (Figure 8), m of

9 BANGS ET AL.: LESSER ANTILLES SUBDUCTION ZONE BACKSTOP EPM 6-9 Figure 6. A part of line S6 showing the top of the backstop as indicated by OBS models (in km/s). The top of the subducting basement is shown by the up arrows. The down arrows are interpreted to be the base of the backstop. This implies m of sediment subducted beneath the backstop. Note the mild deformation of the forearc basin sediment with respect to that revealed along the more seaward section S7, Figure 5. sediment is subducted at least 15 km beneath the toe of the backstop. 4. Discussion [23] Despite the challenging setting for seismic imaging the data presented here offer a rare examination of an island arc backstop, which we can compare to numerical and laboratory models to show the role of the backstop in accretionary wedge construction. [24] We speculate that the arc basement in this segment of the Lesser Antilles forearc may be a composite of arc igneous rocks and obducted or accreted crustal components that have evolved during subduction related tectonic events. While the arc basement is distinctive from the old accretionary wedge material on the basis of seismic velocity (typically km/s for basement versus km/s for sediment in the forearc basin and the oldest parts of the accreted wedge) velocities in most of the basement are not typical of arc crystalline crust. Usually arc igneous has a velocity of >6.0 km/s [e.g., Holbrook et al., 1999]. On the basis of basement velocity, ridge B may be the seaward extent of the arc crystalline crust and the basement seaward of ridge B probably has a different composition and origin than ridge B. We speculate that the basement seaward of ridge B could be related to the hypothesized late Oligocene early Miocene aseismic ridge collision that resulted in the obduction or accretion of oceanic crust to the arc and cessation of volcanism as described by Bouysse and Westercamp [1990]. If this interpretation is correct, then the obducted/accreted block would have to be highly fractured, primarily extrusive rocks, or altered to have the low velocities of km/s. Given the basement velocities, the composition of the backstop is probably not the oldest accreted sediment as it is in other margins [e.g., von Huene et al., 1998]. Here crystalline crust may abut the accretionary wedge, although it is probably highly fractured or altered, making it a weak, deformable backstop to the accretionary wedge. [25] Interestingly the extent of the backstop buckling and folding of overlying forearc basin sediment does not gradually decrease toward the arc in rough proportion to the thickening of the arc backstop, but it is focused at ridges A and B. The focused backstop deformation suggests two independent tectonic events may be involved. First, on the basis of some of the forearc basin stratal relationships, uplift of ridge B initiated well before the collision with the Tiburon Ridge. Uplift of ridge B initiated after some of the earliest forearc basin deposition. Collision deformed the oldest sediment sequences to produce an unconformity at horizon d (Figure 2) with 2 4 km of overlying forearc basin deposits. The sediment accumulation rates in the forearc basin near Barbados, which are believed to be substantially higher due to the closer proximity to sediment sources, places the age as no younger than Miocene [Speed et al., 1989], well before the collision with the Tiburon Ridge. Sediment deposited directly on ridge B is also some of the oldest forearc basin sediment suggesting that they may have

10 EPM 6-10 BANGS ET AL.: LESSER ANTILLES SUBDUCTION ZONE BACKSTOP Figure 7. The seaward portion of line D4. The up arrows are interpreted as the top of the subducting crust. The down arrows are interpreted as the decollement. OOST is the out-of-sequence thrust, which runs through the accretionary wedge and intersects preexisting thrusts formed at the toe of the wedge during initial deformation. The dashed line on left is the inferred position of the backstop. Note that m of underthrust sediment is carried beneath the entire accretionary wedge to the backstop.

11 BANGS ET AL.: LESSER ANTILLES SUBDUCTION ZONE BACKSTOP EPM 6-11 Figure 8. A portion of line S8 that shows the top of subducting crust as defined by OBS velocity models (numbers in km/s). Note the reflection overlying the basement, which is interpreted as the decollement and the top of the underthrust sediment section. been deposited soon after the formation of the basin. We speculate that ridge B formed as the result of collision unrelated and much earlier than the collision with the Tiburon Ridge. The earlier collision involved arc crust and an unnamed buoyant ridge that accreted an aseismic ridge onto the arc crust in early Miocene as proposed by Bouysse and Westercamp [1990] (Figure 9). We speculate that this block may have been accreted rather than obducted based on our interpretation of the uplift of ridge B (see Figure 9). If this interpretation is correct, then the backstop is a composite of arc crust and accreted, highly fractured crust. A preexisting accretionary complex may have been tectonically eroded from the margin during crustal accretion. On the basis of the styles and the timing of forearc basin deformation, a second collision, a collision between the toe of the backstop and the Tiburon Ridge, may have caused a substantial amount of the seaward most deformation. The parallel layering below horizon b in Figure 2 and the onlap above this horizon are roughly consistent with the timing of Tiburon Ridge collision, which began in this segment of the arc 3.5 Ma [Westbrook and McCann, 1986]. Both of these events have produced significant changes to the backstop structure and geometry. [26] One of the surprising observations from these data is the lack of an inner deformation zone and a prominent backthrust that is a distinguishing feature of forearcs with a seaward dipping backstop [Silver and Reed, 1988]. The inner deformation zone and backthrust are observed farther south at the forearc basin and accretionary wedge transition near the island of Barbados [Torrini and Speed, 1989], and is predicted from laboratory and numerical models of a seaward dipping backstop [Wang and Davis, 1996; Byrne et al., 1988, 1993]. Although the backstop offshore Guadeloupe has undergone substantial modification as described above, the overall backstop geometry is seaward dipping, which numerical models predict will result in thrusting of the accretionary wedge over the backstop along a backthrust to form a significant inner deformation zone (see Wang and Davis, [1996] for description) of accreted forearc basin sedimentary masses. Byrne et al. [1993] argue that the formation of the backthrust and inner deformation zone is not very sensitive to the geometry of a seaward dipping backstop and that a blunt backstop geometry, intermediate between seaward dipping and landward dipping, will produce similar structures as the seaward dipping backstop [cf. Byrne et al., 1993, Figures 6 and 9]. [27] Our observations preclude the possibility that the accretionary wedge is thrust significantly over the backstop. As described above, we observe the forearc basin sequence draped over ridge A to the toe of the backstop, with less than 10 km of over thrusting of the accretionary wedge (Figures 4 and 9). This leaves a narrow zone for an inner deformation zone and backthrust to exist. The equivalent structure farther south is on the order of 50 km wide [Torrini and Speed, 1989]. We speculate that the lack of a inner deformation zone is because it was incorporated into the rear of the accretionary wedge during modification to the backstop discussed above, or the inner deformation zone and backthrust are not developed with the backstop geometry that has evolved along this segment of the margin. The backthrust and retrowedge may not form here because the toe of the backstop is too blunt, >25 dip, and abuts a substantial part of the accretionary wedge rather than forming a gently tapered, seaward descending surface to the top of the subducting ocean crust. Even before the uplift of the toe of the backstop, the backstop probably formed a significant vertical boundary (Figure 9). If this interpretation is correct, then the geometry of the backstop has had a significant role in the development of the accretionary wedge, causing a narrow transition from forearc basin to accretionary wedge material.

12 EPM 6-12 BANGS ET AL.: LESSER ANTILLES SUBDUCTION ZONE BACKSTOP Figure 9. Schematic interpretation of the history of aseismic ridge collision and deformation history of the backstop. The interpretation of these data implies that the backstop has evolved from ridge collisions, which produced a backstop with a steep buttress. The result is a narrow transition between the accretionary wedge (yellow) and forearc basin (orange), and a backstop geometry that is conducive to sediment subduction beneath the backstop. See color version of this figure at back of this issue.

13 BANGS ET AL.: LESSER ANTILLES SUBDUCTION ZONE BACKSTOP EPM 6-13 [28] The geometry of the backstop may also promote sediment subduction. This could occur from the stress state that develops as the backstop concentrates stress high in the accretionary wedge rather than forming a deeply rooted backthrust. The lack of well-developed fluid conduits from backthrusts or out-of-sequence thrusts may inhibit dewatering leading to underconsolidation of the accretionary wedge and the underthrust sediment. The thick sequence of sediment underthrust beneath the entire accretionary wedge suggests a substantial volume of fluid may be carried to the toe of the backstop with the subducting sediment, a circumstance that could maintain low shear stresses and prevent the decollement from stepping down and underplating sediment to the base of the accretionary wedge. The large reflection amplitudes are consistent with large physical property contrasts. Large amplitude decollement reflections near the accretionary wedge toe are caused by underconsolidated water-rich sediments that maintain high porosity [Bangs and Westbrook, 1991]. The large reflection amplitudes beneath the wedge suggest underconsolidated sediments may extend beneath the entire accretionary wedge down to at least the toe of the backstop. [29] Whatever the mechanism involved, substantial evidence exists that sediment is also subducted beneath the toe of the backstop. The underthrust section appears to be at least m thick 15 km seaward of the backstop toe, where the underthrusting section is imaged at its deepest point (Figure 7). The coincidental relationship between the blunt, thick geometry of the toe of the backstop and the thick underthrust section suggests the backstop geometry may produce a thick subduction channel that subducts a thick section beneath the backstop. Transport of sediment beneath the rigid backstop of the arc crust also implies that subducting ocean floor sediment is not likely to be exhumed and returned to the surface. The fate of subducted sediment is either underplating to the base of the arc crust or recycling into the mantle. 5. Conclusions [30] From this study we infer several conclusions concerning the behavior of the seaward dipping backstop in the development of the Lesser Antilles subduction zone. [31] 1. Refraction velocities and deformation of the forearc basin sediment reveals the geometry of a deformed backstop. The backstop is probably a composite of island arc crust and pieces of an accreted aseismic ridge that forms the toe of the backstop. The backstop is a broad, 50-km-wide zone extending from the arc massif across a fractured and deformed block of accreted crustal material. It is this material that pushes against the rear of the accretionary wedge and causes the transition from highly deformed sediment of the accretionary wedge to the mildly deformed forearc basin sequence. [32] 2. At the junction between the accreted block and the island arc crust, an uplifted ridge has formed within the backstop. This ridge, ridge B, initially buckled upward soon after crustal accretion deforming some of the oldest forearc basin sediments. Ridge B has also reactivated following recent subduction of the Tiburon Ridge causing further uplift and deformation of the forearc basin sediment. Ridge B is the landward most section of the backstop that has evidence of significant deformation. [33] 3. Collision with the Tiburon Ridge (Figure 9) has caused the toe of the backstop geometry to become extremely blunt. The toe of the backstop has been deformed upward to form a steep, 25 35, buttress that impinges directly on the lower half of the accretionary wedge. The deformation is believe to have occurred coincidentally with subduction of the aseismic Tiburon Ridge within the past 3.5 my. The steep buttress at the seaward edge of the backstop has resulted in an accretionary complex that has a poorly developed backthrust and inner deformation zone system with minimal thrusting of the accretionary complex over the backstop, and a transition between accreted and forearc basin sediment that forms within a narrow 5 10 km region. [34] 4. A 500- to 750-m-thick sequence of underthrust sediment can be traced from beneath the accretionary wedge to the backstop. This sequence is imaged 15 km beneath the backstop, indicating that sediment is being carried beneath the igneous basement. Sediment is transported well beyond the uplifted frontal ridge (ridge A) of the backstop, beyond that explained as a consequence of deformation of the toe of the backstop. As much as half of the sediment underthrust at the toe of the accretionary wedge appears to be subducted to the toe of the backstop. Uplift of the toe of the backstop and the blunt geometry of the toe of the backstop may open the subduction channel to allow a significant thickness of sediment to be subducted beneath the arc crust and potentially deeper into the subduction zone or even all the way into the mantle. [35] Acknowledgments. We thank the crew and captain of the R/V Maurice Ewing for their efforts during EW9803. This work was funded by NSF with grant These data were processed with Paradigm s Geodepth 2 software. We thank the Journal of Geophysical Research reviewers David Scholl and Marc-Andre Gutscher for their thorough and thoughtful reviews, which contributed significantly to this manuscript. References Bangs, N., and G. K. Westbrook, Seismic modeling of the décollement zone at the base of the Barbados Ridge accretionary complex, J. Geophys. Res., 96, , Bangs, N. L., T. H. Shipley, J. C. Moore, and G. F. Moore, Fluid accumulation and channeling along the northern Barbados Ridge décollement thrust, J. Geophys. Res., 104, 20,399 20,414, Bouysse, P., and D. Westercamp, Subduction of Atlantic aseismic ridges and Late Cenozoic evolution of the Lesser Antilles island arc, Tectonophysics, 175, , Byrne, D. E., D. M. Davis, and L. R. Sykes, Loci and maximum size of thrust earthquakes and the mechanics of the shadow region of subduction zones:, Tectonics, 7, , Byrne, D. E., W. Wang, and D. M. Davis, Mechanical role of backstops in the growth of forearcs:, Tectonics, 12, , Calassou, S., C. Larroque, and J. Malavielle, Transfer zones of deformation in thrust wedges: An experimental study, Tectonophysics, 221, , Christeson, G. L., N. L. Bangs, and T. H. Shipley, Deep structure of an island arc backstop, Lesser Antilles subduction zone, J. Geophys. Res, 108, doi: /2002jb002243, in press, Gutscher, M.-A., N. Kukowski, J. Malavieille, and S. Lallemand, Episodic imbricate thrusting and underplating: Analog experiments and mechanical analysis applied to the Alaskan Accretionary Wedge, J. Geophys. Res., 103, 10,161 10,176, Holbrook, W. S., D. Lizarralde, S. McGeary, N. Bangs, and J. Diebold, Structure and composition of the Aleutian island arc and implications for continental crustal growth, Geology, 27, 31 34, Hyndman, R. D., C. Y. Yorath, R. M. Clowes, and E. E. Davis, The northern Cascadia subduction zone at Vancouver Island: Seismic structure and tectonic history:, Can. J. Earth Sci., 27, , Karig, D. E., and R. W. Kay, Fate of sediments on the descending plate at convergent margins, Philos. Trans. R. Soc. London, 301, , Ladd, J. W., G. K. Westbrook, P. Buhl, and N. Bangs, Wide-aperture seismic profiles across the Barbados Ridge Complex, Proc, Ocean Drill. Program Initial Rep., Part B, 110, 3 6, 1990.

14 EPM 6-14 BANGS ET AL.: LESSER ANTILLES SUBDUCTION ZONE BACKSTOP Larroque, C., S. Calassou, J. Malavieille, and F. Chanier, Experimental modelling of forearc basin development during accretionary wedge growth, Basin Res., 7, , Mann, P., E. Calais, J. C. Ruegg, C. DeMets, P. E. Jansma, and G. S. Mattioli, Oblique collision in the northeastern Caribbean from GPS measurements and geological observations, Tectonics, 21(6), 1057, doi: /2001tc001304, McCann, W. R., and L. R. Sykes, Subduction of aseismic ridges beneath the Caribbean Plate: Implications for the tectonics and seismic potential of the northeastern Caribbean, J. Geophys. Res., 89, , Mulugeta, G., and H. Koyi, Episodic accretion and strain partitioning in a model sand wedge, Tectonophysics, 202, , Plank, T., and C. H. Langmuir, Tracing trace elements from sediment input to volcanic output at subduction zones, Nature, 362, , Silver, E. A., and D. L. Reed, Backthrusting in accretionary wedges, J. Geophys. Res., 93, , Speed, R., R. Torrini, and P. L. Smith, Tectonic evolution of the Tobago Trough forearc basin, J. Geophys. Res., 94, , Spence, G. D., R. D. Hyndman, E. E. Davis, and C. Y. Yorath, Seismic structure of the northern Cascadian accretionary prism: Evidence from new multichannel seismic reflection data, in Continental Lithosphere: Deep Seismic Reflections, Geodyn. Ser., vol. 22, edited by R. Meissner et al., pp , AGU, Washington, D.C., Torrini, R., and R. C. Speed, Tectonic wedging in the forearc basin-accretionary prism transition, Lesser Antilles forearc, J. Geophys. Res., 94, 10,549 10,584, Tréhu, A. M., I. Asudeh, T. M. Brocher, J. Luetgert, W. D. Mooney, J. L. Nabelek, and Y. Nakamura, Crustal architecture of the Cascadia forearc, Science, 265, , von Huene, R., and D. W. Scholl, The return of sialic material to the mantle indicated by terrigenous material subducted at convergent margins, Tectonophysics, 219, , von Huene, R., D. Klaeschen, M. Gutscher, and J. Fruehn, Mass and fluid flux during accretion at the Alaska margin, Geol. Soc. Am. Bull., 110, , Wang, W. H., and D. M. Davis, Sandbox model simulation of forearc evolution and noncritical wedges, J. Geophys. Res., 101, 11,329 11,339, Westbrook, G. K., The structure of the crust and upper mantle in the region of Barbados and the Lesser Antilles, Geophys. J.R. Astron. Soc., 43, , Westbrook, G. K., Magnetic lineations and fracture zones, in Lesser Antilles Arc and Adjacent Terranes, Atlas 10, Ocean Drilling Project, Reg. Atlas Ser., vol. 10, edited by R. C. Speed and G. K. Westbrook, sheet 5, Mar. Sci. Int., Woods Hole, Mass., Westbrook, G. K., and W. R. McCann, Subduction of the Atlantic lithosphere beneath the Caribbean, in The Geology of North America, vol. M, TheWesternNorthAtlanticRegion,editedbyP.R.VogtandB.E. Tucholke, pp , Geol. Soc. of Am., Boulder, Colo., Westbrook, G. K., J. W. Ladd, P. Buhl, N. Bangs, and G. Tiley, Cross section of and accretionary wedge: Barbados Ridge Complex, Geology, 16, , N. L. Bangs, G. L. Christeson, and T. H. Shipley, University of Texas Institute for Geophysics, 4412 Spicewood Springs Road, Bldg. 600, Austin TX , USA. (nathan@utig.ig.utexas.edu; gail@utig.ig.utexas. edu; tom@utig.ig.utexas.edu)

15 BANGS ET AL.: LESSER ANTILLES SUBDUCTION ZONE BACKSTOP Figure 1. Location map showing the track lines for the seismic acquisition. The dip lines (D1 D4) are spaced 10 km apart. D5 is centered between D2 and D3. These lines were acquired at 50 m shot spacing. The strike lines (S1 S8) are also 10 km apart but were shot at 100 m shot spacing. OBSs (circles) are located at the intersections of the lines. The outer arc high forms above the contact between the subducting and overriding lithosphere. Black lines are sections of the data shown in Figures 2, 4, 5, 6, 7, and 8. EPM 6-2

16 BANGS ET AL.: LESSER ANTILLES SUBDUCTION ZONE BACKSTOP Figure 3. Map view and 3-D perspective view of the island arc crust that forms the backstop to the accretionary wedge. Seismic lines are shown on the map and correspond to lines shown in Figure 1. The backstop crust is buckled into two ridges, ridge A at the toe of the backstop and ridge B. These structural highs were probably formed by collisions of the arc with aseismic ridges. EPM 6-6