PUBLICATIONS. Tectonics. Forearc structure and morphology along the Sumatra- Andaman subduction zone RESEARCH ARTICLE 10.

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1 PUBLICATIONS Tectonics RESEARCH ARTICLE Key Points: Sunda accretionary prism and forearc structure vary significantly along strike Sediment thickness and basement topography dominate control of prism structure Landward vergence of thrust faults is common in the Sunda prism toe Correspondence to: L. C. McNeill, Citation: McNeill, L. C., and T. J. Henstock (2014), Forearc structure and morphology along the Sumatra-Andaman subduction zone, Tectonics, 33, , doi:. Received 2 NOV 2012 Accepted 16 DEC 2013 Accepted article online 27 DEC 2013 Published online 25 FEB 2014 Forearc structure and morphology along the Sumatra- Andaman subduction zone Lisa C. McNeill 1 and Timothy J. Henstock 1 1 Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Southampton, UK Abstract The Sunda subduction margin, the location of recent magnitude 8 9 megathrust earthquakes, is characterized by major changes in prism and forearc morphology and structure along its 5000 km length. On the Sumatra-Andaman section, measurements of prism width and surface slope (α) indicate along-strike segments, inclu1ding abrupt changes, of prism morphology: (1) a narrow and steep prism between Burma and the Andamans; (2) broad with an averaged gentle slope in the Andamans, Nicobars, and North Sumatra; (3) steep and narrow in Central Sumatra; and (4) wider and less steep offshore South Sumatra, decreasing in width to West Java. Prism width varies from ~90 to 180 km and average surface slope from ~1 to 3 with a strong inverse correlation between width and slope, also observed globally. The prism deviates from typical taper geometry in parts of the margin, notably offshore North Sumatra where it is characterized by a steep toe and broad plateau. Along-strike changes in morphology are strongly linked to input sediment thickness. Sections of the prism toe represent key global examples of unusual landward vergent thrusting. These sections correspond to a thick sediment input and to a wide prism with shallow surface slope. A low basal shear stress or backstop mechanism may drive this style of faulting. Prism morphology and structure appear to be driven predominantly by input sediment thickness linked to oceanic basement topography, with sediment properties, plate smoothness, and orthogonal subduction rate and obliquity also contributing, and no clear role of plate age or dip. 1. Introduction The Sunda subduction zone (Figures 1 and 2) extends over 5000 km from Burma in the northwest to Sumba Island in the southeast. The structure and morphology of the subduction forearc vary considerably along its length and therefore provides opportunities to examine what processes and parameters drive these changes. The margin has recently experienced a number of very large magnitude plate boundary earthquakes including two of the largest ever recorded (although no similar magnitude earthquakes have affected the margin offshore Java and further east). This allows comparison between the earthquake rupture process and the forearc structure. The 26 December 2004 (Mw ) earthquake ruptured ~ km of the northwestern segment of this margin, followed by a ~400 km rupture to the south on 28 March 2005 (Mw 8.7) [e.g., Ammon et al., 2005; Briggs et al., 2006] and a series of smaller plate boundary ruptures in 2007 and 2010 [e.g., Konca et al., 2008; Lay et al., 2011]. Following the 2004 earthquake, several marine geophysical expeditions have expanded our knowledge of the margin structure and morphology. These include multibeam bathymetry and seismic reflection data collection that, for example, in North Sumatra confirm details of an unusual margin morphology (a broad plateau with steep slopes at the seaward and landward edges) and reveal reasons for earthquake rupture unusually far seaward beneath the accretionary prism [e.g., Henstock et al., 2006; Dean et al., 2010; Gulick et al., 2011; Geersen et al., 2013]. The new data also confirm that along parts of the margin, the prism frontal thrusts generate landward vergentfolds(drivenbyseawarddippingfaultsincontrast to the more usual landward dipping thrusts of foldthrust belts; see Figure 3c), as postulated from earlier seismic surveys [Moore et al., 1980; Karig et al., 1980]. This style of prism faulting can have implications for the physical properties of the prism sediments and basal boundary, which in turn affect fault slip behavior particularly along the plate boundary megathrust. Overall, the Sunda margin, from the Andaman Islands to West Java, is a typical accretionary margin subducting variable age oceanic lithosphere ( Ma), with relatively low convergence rates and oblique convergence along much of its length and with high sediment thicknesses being accreted due to the Bengal-Nicobar Fan sediment source with increasing dominance of exposed oceanic plate basement topography to the south away from the sediment source. In this study, we examine the structure and morphology of the Sunda margin from North of the Andaman Islands to westernmost Java in the South (the Sumatra-Andaman section of the margin, MCNEILL AND HENSTOCK American Geophysical Union. All Rights Reserved. 112

2 Figure 1. The Sunda convergent margin and Indian-Australian oceanic plate illustrated using satellite altimetry gravity data [Sandwell and Smith, 1997] highlighting basement topography on the oceanic plate and primary forearc features. Seafloor profiles and seismic profiles used in this study indicated by black lines and grouped according to region (refer to Tables 1 and 2). Sources of data in text, Table 1, and Figure 4 caption, lettered profiles are shown in Figure 4. Stars indicate epicenters of 2004 and 2005 earthquakes. FZ = fracture zone; WSR = Wharton Spreading Ridge; SFS = Sumatran Fault System; BF = Batee Fault; IFZ = Investigator Fracture Zone. Outer-arc faults: WAF = West Andaman Fault; MF = Mentawai Fault; EMF = Eastern Margin Fault. Forearc islands: And = Andaman; Nic = Nicobar; S = Simeulue; N = Nias; Sib = Siberut; Eng = Enggano. Note: Gravity lows in the forearc coincide with forearc basins and can be used alongside bathymetric data (Figure 2) to estimate prism widths. Convergence vectors (white arrows and black numbers) and subduction velocities accounting for forearc motion (red arrows and numbers) are derived from Simons et al. [2007], Delescluse and Chamot-Rooke [2007], Chlieh et al. [2008], and Kamesh Raju et al. [2004]. Figures 1 and 2). For simplicity, we continue to refer to the Sunda margin, but note that the Central Java to Sumba Island eastern margin, characterized by thin sediment, basement topography, erosion, and normal convergence, is not included in this study. The morphology of an accretionary prism (Figure 3) has been linked to (a) the subducting plate dip and the physical properties including pore fluid pressures of the accreted sediments and basal boundary, as outlined in the Coulomb taper theory of Davis et al. [1983] and by Saffer and Bekins [2002], (b) the thickness of incoming MCNEILL AND HENSTOCK American Geophysical Union. All Rights Reserved. 113

3 Tectonics Figure 2. The Sunda convergent margin and Indian-Australian plate illustrated using bathymetric data (derived from General Bathymetric Chart of the Oceans grid of primarily satellite-derived bathymetry [Sandwell and Smith, 1997]). Primary properties of forearc morphology (prism width and surface slope) and structure (prism toe vergence) are given for each region along strike (see also Table 2). Refer to Figure 1 for explanation of abbreviations. sediments and degree of accretion versus subduction or underplating over time [e.g., von Huene and Scholl, 1991; Lallemand et al., 1994; Clift and Vannucchi, 2004], and (c) the dynamics of plate motions, e.g., convergence rate [Clift and Vannucchi, 2004]. Across-strike changes in sediment properties from the deformation front into the older prism are expected due to consolidation and dewatering, diagenesis, low-temperature metamorphism, and lithification as temperature and pressure increase [e.g., Moore and Saffer, 2001]. These transitions and the boundary between accreted material and the forearc basement material are often referred to as backstops, and they can play a key role in the evolution of the forearc (and vice versa) and in forearc morphology. The relative activity and style of faulting within the prism also affect the local and regional morphology of the forearc. The predominant fault geometry within accretionary prisms is that of landward dipping thrusts, generating seaward vergent folds, normally forming an imbricate fold-thrust belt building seaward. Deviations from this MCNEILL AND HENSTOCK American Geophysical Union. All Rights Reserved. 114

4 a) deformation front Prism Width (PW) seaward edge of forearc basin 50 km outer prism accretionary prism outer arc crest x forearc basin input sediment moho outer arc marginal fault b) 50 km outer prism Prism Width (PW) α50 km α rear elevation for α measurement, avoiding bias of underfilled forearc basin α + β = taper angle β c) landward -vergent thrust folds seaward-vergent thrust folds Figure 3. Cartoons illustrating forearc features, defined measurements, and fault geometry and terminology. (a) Key elements of the Sunda forearc and measurement of the prism width (PW) from the deformation front to either the seaward edge of the forearc basin (basin-prism boundary) or outer-arc marginal fault. Bathymetric and seismic profiles and bathymetry and gravity data (Figures 1 and 2) used to derive PW. (b) Measurement of the prism surface slope (α) and 50 km outer prism slope (α50km). The rear prism elevation is taken at the basin edge or outer-arc marginal fault or at the high point adjacent to these features to avoid bias where the basin is underfilled (as illustrated). The taper angle is the sum of the prism surface slope (α) and the basal slope (β). (c) Geometry of prism thrusts and folds defined by fold vergence: landward vergent folds with seaward dipping faults and seaward vergent folds with landward dipping thrusts. simple geometry include out-of-sequence thrusts which are active or activating within the older prism and seaward dipping thrusts ( landward vergent folds) developing in all or part of the prism (Figure 3). The latter style of faulting is relatively rare in active accretionary prisms, with the primary cited example being the Cascadia margin [e.g., MacKay et al., 1992; Flueh et al., 1998; Adam et al., 2004]. Several models have been proposed for landward vergence, summarized by Gutscher et al. [2001], including (a) low basal shear stress [e.g., Seely, 1977; Mackay et al., 1992], often linked to overpressured incoming sedimentary sections; and (b) a seaward dipping backstop [e.g., Byrne and Hibbard, 1987]. We have coupled published and unpublished single and multichannel seismic profiles and bathymetric data along the Sunda margin from Burma/Andaman Islands to West Java (Figure 1) to assess along-strike variability of forearc morphology and structure and likely driving mechanisms. By comparing our analysis of along-strike variability at one margin with other margins and with global compilations of the same parameters, we are building on the studies of, e.g., Davis et al. [1983], von Huene and Scholl [1991], Lallemand et al. [1994], and MCNEILL AND HENSTOCK American Geophysical Union. All Rights Reserved. 115

5 Clift and Vannucchi [2004]. A key difference in this study is an increase in detail of along-strike variations at one particular margin. The Sunda margin has been acknowledged as a margin with particularly poor data coverage for one of such global significance in terms of its total margin length [Clift and Vannucchi, 2004].In addition, this study provides an examination of variations in structural vergence along one margin and hence possible driving mechanisms. 2. Sunda Margin Morphology and Structure The Sunda margin is an ideal location to study the impact of various parameters on the shape and structure of the forearc due to its along-strike extent, varying sediment input, varying orthogonal convergence rate, andgooddatacoveragefollowingrecentearthquakes.compiledpublishedresultsandunpublisheddata allow us to compare and contrast prism morphology (e.g., width and surface slope) and fault geometry (e.g., dip direction/vergence) along the margin, building on the previous work of Moore et al. [1980], Curray [2005], and Cochran [2010] in the northern margin. Data incorporated into the study include unpublished seismic profiles in the northern part of the margin, between North Sumatra and Burma (courtesy of J. Curray, examples published in Moore et al. [1980]), and in North and Central Sumatra from recent UK surveys, extending the length and detail of the margin that can be examined. 3. Methodology The results are reported in Tables 1 and 2 and discussed further below. For consistency along the margin, prism widths have been defined using seismic profiles or bathymetric data (Figure 3a) as the distance from the deformation front to either the seaward edge of the forearc basin or the outer-arc, margin-parallel fault system just landward of the outer arc islands (normally close to the boundary of the forearc basin and the prism). We measure to this position because it is more easily definable than the conventional outer-arc high crest, because, on this margin, the outer arc high is locally extremely broad. In much of the data, a crystalline backstop and its intersection point with the subducting plate is not clearly resolved (as is common elsewhere); therefore, again, we choose not to select this as a control point. We do not define only the steep section of the outer prism as the deforming active prism for our measurements, because our data and published analysis [Fisher et al., 2007] show that deformation is not restricted to this part of the prism. We believe our defined prism width is broadly equivalent to the distance from the trench to backstop used by von Huene and Scholl [1991] and Clift and Vannucchi [2004], because the toe of the backstop is expected to underlie the high located at the rear of the tapered wedge. Prism widths are given as margin-perpendicular values where profiles are oblique. Prism surface slopes are given as (a) an average over the defined prism width and (b) for the outer 50 km of the prism (Figure 3b). The latter is broadly equivalent to the average prism surface slopes used by Clift and Vannucchi [2004] and Saffer and Bekins [2002]. For surface slope (α), we measure to the high point adjacent to the seaward edge of the forearc basin/outer-arc fault, in order to avoid bias due to a relatively underfilled forearc basin (see Figure 3b). For study of prism thrust vergence, we have focused on the prism toe (outermost thrusts) where landward vergence is globally more common and where data and fault resolution are greatest. Dominant vergence state is given (seaward or landward, Figure 3c) or designated mixed if vergence changes rapidly along and/or across strike. Sediment thicknesses in the trench are reported in both time and depth (Tables 1 and 2). For conversion from time to depth, the following sources of velocity analysis or depth conversion (both local and regional) are used for specificornearbyprofiles: Kieckhefer et al. [1980], Moore et al. [1982], Moore and Curray [1980], Curray et al. [1982], Dean et al. [2010], Singh et al. [2010], and Kopp and Kukowski [2003]. These studies suggest typical average sediment velocities of ~2.5 km/s, but with some notable exceptions increasing average velocities to ~3 km/s, e.g., due to high sediment velocities at depth where sediment thicknesses are very high, such as in the North Sumatra trench [Dean et al., 2010; Singh et al., 2010]. 4. Results 4.1. North Region ( N) Seismicity, volcanism, and a tomographically imaged subucting slab suggest active convergence continues offshore Burma, but convergence is highly oblique [e.g., Rao and Kalpna, 2005; Cummins, 2007; Stork et al., 2008]. North of 15 N, the prism is narrow and steep with a steep toe rising to a shelf plateau [Cochran, 2010; Nielsen et al., 2004]. North of the Andaman Islands (~14 15 N) the prism is ~90 km wide with a steep surface MCNEILL AND HENSTOCK American Geophysical Union. All Rights Reserved. 116

6 Table 1. Properties of the Prism and Forearc of the Sunda Margin and Sources of Data a Region Seismic or Bathymetric Profile (Numbers and Letters of Original Profile in Data Source) Bathy Profile in Figure 4 Latitude (deg) Average Prism Width PW Prism Surface (km) b Slope, α (deg) b Prism Surface Slope Over Outer 50 km (deg) b Trench Sediment Thickness (Two Way Time) (TWT s) c Trench Sediment Thickness (km) c Data Sources, Notes North Bathymetry profile from Cochran [2010, Figure 13]. PW to Eastern Margin fault/ seaward edge of forearc basin (see gravity and bathymetry data of Figures 1 and 2). T57-58 or 6 (Figure 5 insert) T55-56 or 7 (Figure 5) A Bathymetry and seismic profile from Curray [2005, Figure 8] and Cochran [2010, Figure 12]. PW calculated from map view to Eastern Margin fault Bathymetry and seismic profile from this study and Moore et al. [1980, Figure 6], Curray [2005, Figure 8], Cochran [2010, Figure 12], and Curray et al. [1982, Figure 5]. PW calculated from map view to Eastern Margin fault. Andaman J Bathymetry profile from Cochran [2010, Figure 7]. K Bathymetry profile from Cochran [2010, Figure 7]. T5-7 or L B Bathymetry and seismic profile from this (Figure 6) study, Moore et al. [1980, Figure 5], Curray [2005, Figure 8], Cochran [2010, Figure 6], and Curray et al. [1982, Figure 4]. PW calculated from Cochran [2010] bathy profile, measured to Eastern Margin Fault. Nicobar N Bathymetry profile from Cochran [2010, Figure 6]. P Bathymetry profile from Cochran [2010, Figure 6]. R Bathymetry profile from Cochran [2010, Figure 5]. North Sumatra M8-9 (Figure 7) C >2.4 d >2.8 3 d Bathymetry and seismic profile from this study, Curray [2005, Figure 8]. See also Cochran [2010, profiles S, T, Figure 5]. PW calculated to West Andaman Fault location, just landward of forearc island trend and crest of outer arc ridge. T Bathymetry profile from Cochran [2010, Figure 5]. Scott D Bathymetry profile from data of Henstock et al. [2006]. MCNEILL AND HENSTOCK American Geophysical Union. All Rights Reserved. 117

7 Table 1. (continued) Region Seismic or Bathymetric Profile (Numbers and Letters of Original Profile in Data Source) Bathy Profile in Figure 4 Latitude (deg) Average Prism Width PW Prism Surface (km) b Slope, α (deg) b Prism Surface Slope Over Outer 50 km (deg) b Trench Sediment Thickness (Two Way Time) (TWT s) c Trench Sediment Thickness (km) c Data Sources, Notes Central Sumatra I Central Sumatra II South Sumatra E42-43 E Bathymetry and seismic profile from Curray [2005, Figure 8], Henstock et al. [2006]. and Moore et al. [1980]. Similar profiles shown in Gulick et al. [2011]. Sediment thicknesses from nearby unpublished profiles. SUME15 (Figure 8) Bathymetry from Henstock et al. [2006] and seismic profile from this study. Similar profile shown in Dean et al. [2010, Figure 2] SUME03 F Bathymetry and seismic profile from this study. Similar profile shown in Dean et al. [2010, Figure 4]. SUMD01 and SUMD07 (Figure 9) G Bathymetry and seismic profiles from this study. SO H Bathymetry and seismic profile from Kopp et al. [2001] and Kopp and Kukowski [2003]. PW calculated to seaward edge of forearc basin. West Java SO I Bathymetry and seismic profile from Kopp et al. [2002] and Kopp and Kukowski [2003]. PW calculated to seaward edge of forearc basin. a See Figures 1 and 2 for region and profile locations. b See Figure 3 for parameter definitions. c Sediment thicknesses are measured at the deformation front and are derived from Moore et al. [1980, 1982], Moore and Curray [1980], Curray et al. [1982], Kopp et al. [2002, 2009], Kopp and Kukowski [2003], Franke et al. [2008], Klingelhoefer et al. [2010], Dean et al. [2010], Gulick et al. [2011], Curray [2005], Singh et al. [2008, 2011], and this study (see Figures 5 9, for examples). See section 3 for details of velocity information used (where direct velocity data are not available, sediment thicknesses converted to depth using an average km/s). d Base of sediments not imaged; therefore, value given here is a minimum sediment thicknesses. MCNEILL AND HENSTOCK American Geophysical Union. All Rights Reserved. 118

8 Table 2. Summary of Sunda Forearc Properties and Subduction Parameters Along the Margin Region Latitude Prism Width (km) a Average Prism Surface Slope (deg) a Prism Slope (Outer 50 (km) (deg) a Trench Sediment Thickness (km) b Percent of Total Sediment Thickness Accreted at Prism Toe (km) b North N ~ ? Average Plate Dip Beneath Prism (Equivalent to β) (deg) e Plate Dip Range, 0 50 km From Trench (deg) j Andaman N ~ ? Nicobar 6 10 N ~ >3 c Unknown c ~3 12 North Sumatra Central Sumatra I N ~ ~ ef ~ N ~ d ~ eg ~1 7 Central Sumatra II 3 0 S ~ d ~ ~0 8 South Sumatra 5 7 S ~ ~ eh ~2 7 West Java S ~ ~ ei ~1 6 slope of 2 (Table 1 and Figure 4a). Moving south toward the Andaman Islands, the prism widens with reduced average surface slope (e.g., width ~130 km, α ~ 1.2 ) [see also Mooreetal., 1980]. Seismic profiles indicate seaward vergence of the frontal thrust fold and of folds further landward, with some examples of unclear or possible mixed vergence (e.g., Figure 5) [Moore et al., 1980; Curray, 2005] Andaman Region ( N) Seismic profiles across the prism here are limited. The prism widens south of the Andaman Islands, reaching ~155 km, and surface slopes are on average reduced (α ~1 1.6 ) relative to the North Region. Most profiles show a steep prism toe with a broad outer arc plateau (Figure 4b), but some have a normal wedge taper morphology [see Curray, 2005; Cochran, 2010]. Seismic profiles indicate either landward vergence at the prism toe reverting to seaward vergence into the prism (Figure 6), or seaward vergence: therefore, this region is described as mixed vergence [see Moore et al., 1980; Curray, 2005] Nicobar Region (6 10 N) The prism morphology gradually changes from a steep toe-plateau morphology in the north to a normal taper morphology further south (Figure 4c). The prism is extremely wide (~ km) and surface slopes relatively low (α ~1 1.7 ) with evidence of an inverse correlation between surface slope and width from north to south (see Table 1). The seismic profile in Figure 7 [see Curray, 2005] images landward vergence of the frontal two folds North Sumatra Region (2.4 6 N) The forearc and prism are extremely wide (prism width ~ km) with a km broad plateau and steep toe (Figures 4d and 4e) [Moore et al., 1980; Henstock et al., 2006; Fisher et al., 2007; Singh et al., 2008; Gulick et al., 2011], similar to parts of the Andaman and Nicobar Region margin (e.g., Figure 4b). The average surface slope is ~ , but the outer 50 km slope is ~ , reflecting the steep toe (the toe itself has an average surface slope of ~5 6 and locally > 20 ). Landward vergence of thrust folds dominates the prism toe with the frontal fold landward vergent and folds two to three transitioning from landward to seaward vergence with conjugate thrusts and in some cases out-of-sequence thrusts developing (Figure 8) [Henstock et al., 2006; Fisher et al., 2007; Mosher et al., 2008; Dean et al., 2010; Gulick et al., 2011]. This section is coincident with the very steep prism toe and the first clear seaward vergent fold often marks the seaward edge of the prism plateau. The unusual plateau structure of the North Sumatran prism is argued to contain a strong wedge interior and to deform as a duplex [Ladage et al., 2006; Gaedicke et al., 2006; Fisher et al., 2007]. The geometry of thrust folds MCNEILL AND HENSTOCK American Geophysical Union. All Rights Reserved. 119

9 Table 2. Summary of Sunda Forearc Properties and Subduction Parameters Along the Margin Plate Dip (Average for Slab Depth: 0 50 km) (deg) k Convergence Velocity (mm/yr) l Estimated Subduction Velocity (mm/yr) l Approx. Plate Age (Ma) m Prism Toe Vergence Prism Morphology Mechanisms Driving Morphology and Vergence Seaward Narrow, steep Highly oblique convergence. Trench sediment thickness restricted by Ninety East Ridge Mixed Widening, steep toe Moderate trench sediment thickness and plateau Landward Wide, steep toe and plateau, transitioning to normal taper (but restricted by Ninety East Ridge) Likely thick trench sediment, with specific sediment properties Landward Wide, steep toe and plateau Thick trench sediment, with specific sediment properties Mixed Narrow, steep Thinning and variable trench sediment thickness, increased seafloor roughness, variable input properties Mixed Narrow, steep Thin trench sediment, increased seafloor roughness, variable input properties Mixed Moderate width and slope Seaward Slightly narrowed, moderate slope Thin trench sediment, reduced seafloor roughness Thin trench sediment, increased seafloor roughness a See Figure 3 for parameter definitions. b See Table 1 caption for sources of sediment thickness. c Data imaging base of sediments not available; therefore, value given here is a minimum sediment thicknesses and percentage of sediment accreted unknown. d Other profiles from this study and from Moore and Curray [1980] used to supplement data in Table 2 and derive sediment thickness ranges. e Plate dip values derived from seismic refraction and reflection data. f Singh et al. [2008], Chauhan et al. [2009], and Klingelhoefer et al. [2010]. g Tang et al. [2013] and Franke et al. [2006, 2008]. h Kopp et al. [2001] and Singh et al. [2011]. i Kopp et al. [2002]. j Plate dip values from Hayes et al. [2012]. k Plate dip values from Cruciani et al. [2005]. l Convergence velocity (motion between India-Australia and Sunda plates) and subduction velocities (accounting for estimates of forearc motion) derived from Simons et al. [2007], Delescluse and Chamot-Rooke [2007], Kamesh Raju et al. [2004], and Chlieh et al. [2008] (see text for details). m Plate age information from Mueller et al. [1997], Cande and Kent [1995], and Cruciani et al. [2005]. further into the prism is apparently dominantly seaward vergent with occasional landward vergent structures (back thrusts) [e.g., Fisher et al., 2007] Central Sumatra Region (3 S 2.0 N) In the area offshore Simeulue Island (~2 2.5 N), the prism abruptly narrows with increased average surface slope (Figures 2 and 4) [Moore and Curray, 1980; Franke et al., 2008; Kopp et al., 2008; Dean et al., 2010]. This represents a transitional zone between the North Sumatra and Central Sumatra regions (as described and defined in Tables 1 and 2 and Figure 2). At this location, a broad basement high related to the N-S trending 96 fracture zone is subducting, generating variations in sediment thickness across the oceanic plate [see Dean et al., 2010]. The forearc structure and morphology changes at ~2.4 N but sediment thicknesses remain large until the basement high is reached (at ~2 N) defining the transitional zone. The prism width decreases from ~150 km to ~100 km and average surface slope increases from ~1 to 3 over <100 km along strike (Table 1 and Figure 4). The Central Sumatra Region south of Simeulue is characterized by variable oceanic plate basement topography, sediment cover, and seafloor roughness associated with ridge and fracture zone basement structure and decreasing sediment thickness away from the Bengal Fan source (Figures 1 and 2). Although it could be argued that this interpretation may be a function of increased data density in this region, the gravity anomaly data of Figure 1 highlight the variability of basement topography and the bathymetry data in Figure 2 highlight the seafloor roughness of the Central Sumatra oceanic plate relative to other parts of the margin, as also demonstrated by Kopp et al. [2008]. The prism has a more normal wedge morphology (Figure 4f), but with a steep toe-shelf plateau still developed (e.g., Figure 4g). Kopp et al. [2008] define a morphological segment boundary at 1.5 S (between their Siberut and Enggano segments); however, changes in prism morphology southward may be more gradual or an alternative MCNEILL AND HENSTOCK American Geophysical Union. All Rights Reserved. 120

10 (a) (b) (c) (d) (e) (f) (g) (h) (i) Figure 4. Bathymetric profiles across the prism/forearc from north to south along the Sunda margin, scaled to margin perpendicular where profiles are oblique. Prism width indicated by thick black line. Prism widths (PWs) and average prism surface slope (α) across the full prism width (see Figure 3 and text for definition and measurement method). All depths are in kilometers. Sources of profiles: (a, b) Curray [2005] and Cochran [2010]; (c, e) Curray [2005]; (d) Henstock et al. [2006]; (f, g) this study; (h, i) Kopp et al. [2001, 2002] and Kopp and Kukowski [2003]. See also Moore et al. [1980] and Cochran [2010]. MCNEILL AND HENSTOCK American Geophysical Union. All Rights Reserved. 121

11 (b) (a) Figure 5. (a) Seismic profile T55-56 across the accretionary prism offshore Northern Andaman Islands at ~14 N [after Curray, 2005; Moore et al., 1980]. An example of dominant seaward vergence (interpreted seaward vergence of at least the first two thrust folds). (b) Line drawing of adjacent parallel profile T57 T58 to the north [Curray, 2005], Figure 4a also indicating seaward vergence. Other nearby profiles indicate possible landward or mixed vergence at the prism toe but with seaward vergence dominating overall. Probable top of eastern basement flank of the Ninety East Ridge at deformation front yields ~ s or ~2 2.3 km sediment thickness (using velocities of Curray et al. [1982] and Dean et al. [2010] average km/s). Vertical exaggeration of ~1:9 at seafloor. boundary can be picked at ~4 5 S, where oceanic plate roughness is diminished and the deformation front steps seaward. Published data [e.g., Moore and Curray, 1980; Karig et al., 1980; Neben et al., 2006] and data analyzed in this study (e.g., Figure 9) indicate that vergence of prism toe structures varies significantly along and across strike. We therefore interpret mixed vergence for this region Southern Sumatra Region (5 7 S) The prism broadens slightly here to ~ km, and the prism surface slope decreases to ~2 [Kopp et al., 2001, 2008; Kopp and Kukowski, 2003]. This transition may be gradual from north to south with the decreasing effects of oceanic basement topography, but data are rather limited in this region. South of Enggano Island (Figures 1 and 2), the prism includes three sections across strike, each becoming shallower in slope landward (Figure 4h) [Kopp et al., 2001; Kopp and Kukowski, 2003]. Kopp et al. [2001], Schlüter et al. [2002], and Kopp and Kukowski [2003] correlate the slope break at ~30 km with the outer edge of a landward dipping dynamic backstop. A similar prism morphology forms north of Enggano Island [Singh et al., 2011]. The plateaux developed on this part of the margin aremoresubtlerelativetothoseofthenorthsumatraregion (compare Figure 4h with Figures 4d and 4e). Seismic and bathymetric data [Kopp et al., 2001; Kopp and Kukowski, 2003] suggest mixed vergence of prism toe thrust folds. Profile S suggests a landward vergent frontal fold also interpreted by Kopp and Kukowski [2003]. MCNEILL AND HENSTOCK American Geophysical Union. All Rights Reserved. 122

12 km W 100 km E (b) T5-7 (a) 2 3?? 4 5 Two Way Time (s) 20 km 4 W (c) 20 km T7d-8 Seaward vergent frontal fold E 4 6 Top of basement TWT (s) Top of basement Figure 6. (a) Seismic profile T5-7 across accretionary prism offshore Southern Andaman Islands at ~10 N [after Curray, 2005; Moore et al., 1980; Curray et al., 1982]. An example of mixed vergence (landward vergence of frontal thrust reverting to seaward vergence). Other profiles in the area show mixed or seaward vergence (e.g., inset c). (b) Bathymetric profile across forearc [Curray, 2005] (Figure 4b); box indicates main seismic profile location. Probable top of eastern basement flank of the Ninety East Ridge at deformation front yields ~2 2.2 s or ~ km sediment thickness (using velocities of Curray et al. [1982] and Dean et al. [2010] average 2.5 km/s). (c) Line drawing of adjacent profile T7e-8 extends further onto the oceanic plate showing basement shallowing to the west, i.e., topography of the Ninety East Ridge flank. Vertical exaggeration of main profile ~1:12.5 at seafloor West Java Region ( S) A change in margin orientation here generates convergence close to orthogonal and the fault systems accommodating margin-parallel motion offshore Sumatra (Sumatran Fault and Mentawai-West Andaman-Eastern Margin Faults) become inactive (Figures 1 and 2). The prism width is ~110 km with an average surface slope of 50 km M8-9 E 2 W Depth (km) km Two way time (secs) Figure 7. Seismic profile M8-9 across accretionary prism west of Nicobar Islands [after Curray, 2005]. An example of landward vergence of the prism toe folds. Zoom illustrates landward vergence at the frontal thrust. Top of basement is not imaged in the full seismic profile depth (recorded to 8 s two-way time). Therefore, a sediment thickness at the deformation front of 2.4 s or km is estimated (using an average upper sediment velocity of ~ km/s). Bottom right inset shows bathymetric profile of entire line (Figure 4c), with box showing section illustrated here. Vertical exaggeration of main profile ~1:14 at seafloor. MCNEILL AND HENSTOCK American Geophysical Union. All Rights Reserved. 123

13 2 3 SW NE SUME15 4 Landward-vergent thrust fold Time (s) Landward-vergent frontal thrust fold 8 9 Top of oceanic basement Distance (km) Figure 8. Seismic profile SUME15 across prism toe offshore North Sumatra. An example of landward vergence of the prism toe folds. See similar structures and profiles in Dean et al. [2010] and Mosher et al. [2008]. Top of basement imaged at deformation front and depth converted sediment thickness of ~4.7 km. Vertical exaggeration ~1:4 at seafloor. ~2.3 and a relatively normal wedge morphology (Figure 4i) [see also Kopp et al., 2002; Kopp and Kukowski, 2003], slightly reduced in width and increased in slope relative to the South Sumatra Region. Seismic data of Kopp and Kukowski [2003] and Kopp et al. [2009] suggest frontal folds are predominantly seaward vergent. Data and contrasting interpretations of fold vergence in the Sunda Strait (between West Java and South Sumatra; see Figure 1) indicate ambiguous frontal fold vergence [Kopp and Kukowski, 2003; Schlüter et al., 2002]; therefore, an interpretation of mixed prism toe vergence is inferred for the Sunda Strait area. For discussion of the Sunda margin further east, we refer to Kopp et al. [2006], Lueschen et al. [2011], and Planert et al. [2010]. SW 5 NE SUMD07 6 Seaward-vergent Seaward-vergent thrust fold frontal thrust fold Landward-vergent, potentially switching to seaward-vergent Time (s) 7 8 Top of oceanic basement Distance (km) Figure 9. Seismic profile SUMD07 across the outer accretionary prism offshore Batu Islands at ~1 S, Central Sumatra. An example of mixed vergence of prism toe thrust folds (seaward vergence of the frontal and second thrust folds but landward vergence at the third thrust fold, potentially switching to seaward vergence). Top of basement imaged at deformation front and depth converted sediment thickness of ~1.8 km. Vertical exaggeration ~1:4 at seafloor. MCNEILL AND HENSTOCK American Geophysical Union. All Rights Reserved. 124

14 a) Prism slope ( ) b) Prism width (km) Prism width (km) SIMEULUE ISLAND c) 70 AND SUMATRA JAVA Latitude along Sunda margin Prism width (km) Trench sediment thickness (km) Figure 10. Prism and subduction parameters for the Sunda margin (derived from data in Table 1). (a) Prism slope against width along the margin, (b) prism width plotted against latitude (positive = north and negative = south) showing variation along strike (dashed line indicates major change at Simuelue Island segment boundary, AND = Andaman Islands), (c) prism width against input sediment thickness along the margin. Unfilled data point and arrow indicates minimum sediment thickness. Note off-trend data point at 100 km prism width: This is likely a function of sediment thickness changing over time in a region of significant oceanic basement topography (sediment thickness is anomalously thick relative to long-term average). 5. Summary of Sunda Margin Structure 5.1. Prism Geometry and Morphology Earlier studies, particularly Moore et al. [1980], suggested consistent gradual changes in prism width and morphology along the Sunda margin with prism width gradually increasing from Java to North Sumatra then deviating from this trend further north. They attributed this structural change to changes in both incoming sediment thickness and the orthogonal component of subduction (a function of obliquity and forearc motion). Our integration and reexamination of the old and new data show that while these trends are generally correct, there is evidence for (a) abrupt changes along strike and (b) distinct prism morphologies characterizing specific sections of the margin (Tables 1 and 2 and Figures 1, 2, 4 and 10b). We observe distinct along-strike changes in prism morphology producing morphological segmentation of the margin, as also delineated by Kopp et al. [2008] for part of the margin. The prism is, from north to south, (1) narrow and steep between Burma and the Andaman Islands (North Region), (2) broad, with a relatively shallow MCNEILL AND HENSTOCK American Geophysical Union. All Rights Reserved. 125

15 average surface slope between the Andaman and Simeulue Islands (Andaman, Nicobar, and North Sumatra Regions), (3) steep and narrow from Simeulue to the Mentawai Islands (Central Sumatra Regions), and (4) wider and less steep offshore South Sumatra, decreasing in width to West Java (Tables 1 and 2 and Figures 1, 2, 4, and 10). The profile of the accretionary prism is quite unusual in parts of the margin (Figure 4). For example, offshore North Sumatra and parts of the Andaman and Nicobar Regions, the prism morphology is a very steep toe and flat plateau (Figures 4b, 4d, and 4e) [Moore et al., 1980; Curray, 2005; Henstock et al., 2006; Singh et al., 2008], deviating from a classic taper. The most distinct along-strike change in prism morphology occurs abruptly at Simeulue Island (2.5 N) where there is a transition from a broad, on average shallow-dipping prism with an unusual plateau morphology in the north to a steep narrow prism with a normal taper in the south over a short distance of ~ km. Figure 10b, displaying prism width along the margin, illustrates this abrupt change. Values of prism surface slope (α) are in the range 1 2.7, and, where available, values of plate dip at the base of the prism (equivalent to β) are in the range 5 8 (Tables 1 and 2), generating taper angles (α + β)of6 10. These detailed values of α and β for the Sunda margin sit generally close to the previously proposed values for the Sunda margin and other accretionary margins within global compilations [e.g., Davis et al., 1983; Lallemand et al., 1994; Clift and Vannucchi, 2004]. However, our results suggest that in some cases, α values have been overestimated and β values underestimated in previous studies. There is a good general correlation between width and average surface slope of the prism, where increasing surface slope is correlated with decreasing width (Figure 10a). A similar trend is observed at other margins worldwide [e.g., Davis et al., 1983], and this is partly a function of a shallower surface slope naturally increasing the prism width if the accreted volume is unchanged (and in some cases is a function of definition, i.e., increasing the prism width further landward at any one location will naturally decrease the averaged slope) Prism Thrust Fold Vergence Asignificant length of the Sunda margin prism toe (North Sumatra Region between 2.2 and 5 N, for the youngest two to three thrust folds) is dominated by landward vergence (Figures 3c and 8) and a similar structure is observed in the Nicobar Region (7 N, Figure 7). This represents a margin section ~600 km in length with prism toe vergence predominantly landward, making the Sunda margin an important global example of landward vergence. The most northerly (North Region) and southerly (West Java Region) parts of the study area are dominated by seaward vergence. Between these predominantly seaward and landward vergent provinces, prism toe structure is characterized by mixed vergence with evidence of both landward and seaward vergent structures that change rapidly along and/or across strike (Andaman, Central Sumatra, and South Sumatra Regions). We acknowledge however the limitations of sparse data availability in some of these regions. These domains of structural vergence are illustrated in Figure 2. Comparing prism toe fault geometry and prism morphology, there is a general correlation of a broad and shallow averaged-surface-slope prism with landward vergence, e.g., Nicobar and North Sumatra Regions, and of a narrow and steep prism with seaward or mixed vergence, e.g., North and Central Sumatra Regions (Figures 1, 2, and 4 and Table 2). In many cases, the landward vergent prism toe correlates with the very steep outer part of the broad prisms (e.g., Figures 4d and 4e profiles) [Henstock et al., 2006; Dean et al., 2010], but there are exceptions where landward vergence occurs, and the prism lacks this distinct steep toe and has a more normal surface profile (e.g., Figure 4c profile and Figure 7 seismic line). Further linkages between landward vergence and sediment input (sediment thickness and sediment properties) are discussed below. 6. Discussion 6.1. Controls on Sunda Prism Morphology and Structure A number of studies, including von Huene and Scholl [1991], Clift and Vannucchi [2004], and Lallemand et al. [1994], have reviewed and compared forearc character of different convergent margins, including potential correlations between forearc structure and accretion versus erosion processes and subduction parameters. Clift and Vannucchi [2004] argue that normal convergence rate and trench sediment thickness have a significant control on prism geometry as well as rates of accretion. Saffer and Bekins [2002] focus on hydrological (pore pressures) and strain rate (convergence rate) control of the morphology of the prism, building on the work of Davis et al. [1983]. Here we investigate the changing forearc characteristics and subduction parameters along the Sunda margin and compare with other margins to assess likely cause and effect. MCNEILL AND HENSTOCK American Geophysical Union. All Rights Reserved. 126

16 Input and Accreted Sediment Thickness Trench sediment thickness on this margin is a function of (a) distance from the Bengal Fan source, (b) oceanic plate basement topography, and (c) oceanic plate basement barriers reducing southward transport of sediment. The generally thick sediment cover on the oceanic plate is the result of the Himalayan-sourced Bengal Fan and the Nicobar Fan located between the Ninety East Ridge and Investigator Fracture Zone (Figure 2). Water depth at the trench generally decreases from south to north, reflecting distance from this sediment source [Moore et al., 1980] rather than oceanic plate age. Sediment thickness varies locally due to oceanic basement topography created by fossil ridge segments and fracture zones (Figures 1 and 2), leading to thin and variable sediment cover in places and sometimes acting as a barrier to sediment transport along the trench. Sediment thickness (as a function of basement topography) is particularly variable between Simeulue and Siberut Islands in the Central Sumatra Region (2 N to 3 S, indirectly shown in Figures 1 and 2). Sediment thicknesses discussed here are given in Tables 1 and 2 and are derived from Moore et al. [1980, 1982], Moore and Curray [1980], Curray [2005], Curray et al. [1982], Kopp et al. [2002, 2009], Kopp and Kukowski [2003], Franke et al. [2008], Singh et al. [2008, 2011], Klingelhoefer et al. [2010], Dean et al. [2010], Gulick et al. [2011], and this study. Average incoming sediment thickness is a maximum in the Nicobar Fan in the North Sumatra and Nicobar Regions (up to 5 km or more between 2.5 and 7 N (Table 1)), and north of our study area offshore Burma [Moore et al., 1980; Curray et al., 1982]. Sediment thickness is minimum offshore South Sumatra and West Java (1 2 km) and is locally thin and highly variable as a result of basement topography offshore Central Sumatra. In the North and Andaman Regions, the topography of the eastern flank of the Ninety East Ridge reduces trench sediment thicknesses (Figures 5 and 6) which would otherwise be very high due to proximity to the Bengal Fan source. The large sediment thicknesses from North Sumatra-Nicobar and Burma are comparable with the thickest global trench sections (Makran: up to 7.5 km [Smith et al., 2012] and Southern Lesser Antilles: up to 7 km [Westbrook et al., 1984]). For most of the margin, the full sediment section can be imaged and the position of the décollement identified or its position approximately inferred based on the deepest extent of prism compressional deformation therefore providing an indication of the proportion of incoming sediment accreted at the prism toe. Where the décollement position can be inferred, the majority of sediment appears to be accreted (Table 2, in most cases >70 80% accreted). Therefore, the variability of input sediment thickness along the margin is a good approximation for variability of accreted sediment thickness. There is a general trend of increasing prism width (and hence decreasing surface slope) with increasing input sediment thickness (and hence accreted sediment thickness) along the margin (Table 1 and Figure 10c). The anomalous value on this plot (a low prism width relative to sediment thickness) is from the central Sumatra region and is likely to result from accretion of highly variable sediment thickness sections over time due to basement topography on the oceanic plate, i.e., the current sediment thickness is larger than the long-term average. Alternatively, subducting basement features may enhance subduction erosion processes contributing to narrower prism widths, climatic change will influence input sediment thickness through time, and prism age will also influence width. In the Southern Andaman Islands, although the present trench sediment thickness is <3 km, a very broad, shallow dip prism has developed, and this may also be a function of changing input sediment thickness over time or longevity of the prism on this part of the margin. These detailed results from a single margin give important support to the similar correlation of prism width and slope from the global compilation of accretionary margins in Clift and Vannucchi [2004, Table 1]. There also appears to be a general correlation between structural vergence and sediment thickness, with the primary region of prism toe landward vergence coincident with the thickest incoming sediment section (North Sumatra and Nicobar Regions). On the Central Sumatra margin, the narrowing and steepening of the prism and mixed vergence of prism toe structures correlate with variable and reduced sediment thickness driven by distance from the source and oceanic plate basement topography. The results lead to the following interpretation: where sediment thickness is ~3 4 km, we see pervasive landward vergence, continuous over long distances, at the prism toe (first 2 3 thrust folds); where sediment thickness is < ~1 1.5 km, we observe predominantly seaward vergence; between ~1 and 3 km we typically see vergence varying rapidly along and across strike (mixed). Central Sumatra is an example of mixed and rapidly changing vergence (sediment thickness ~1 3 km in most places), and here the increased variation in oceanic basement topography (see Figures 1 and 2) [Kopp et al., 2008] may be a more important factor. Below we discuss the potential role of sediment properties and links to basement in this region. MCNEILL AND HENSTOCK American Geophysical Union. All Rights Reserved. 127

17 Input and Prism Sediment Properties Many of the apparent links between prism structure/morphology and sediment thickness could also be a function of changing sediment properties, which are often related to sediment thickness by change in sedimentation rate or source, by thermally driven diagenetic and metamorphic reactions at depth related to thickness of the section or by the significance of oceanic basement processes including fluid generation. Abrupt along-strike changes in prism structure/morphology, such as margin sections to the North and South of Simuelue Island [Dean et al., 2010; Gulick et al., 2011; Geersen et al., 2013] also support the role of sediment properties. To the north, trench sediment thickness is large and the prism is wide, on average shallow in dip, and characterized by landward vergence in the toe. To the south, oceanic basement topography and subsequent sediment thickness is highly variable and coincides with the region where the prism is consistently narrow and steep and thrust vergence varies considerably. Geersen et al. [2013] suggest that the thermal profile of the thick input section off North Sumatra can generate sufficiently high temperatures to dehydrate clays thus strengthening the section. The contrasting input sediment densities on either side of the oceanic basement high at Simeulue Island appear to influence how the prism and basal décollement develops on either side of this boundary [Dean et al., 2010] showing a strong link between input sediment properties and prism structure. On the Central Sumatran margin, oceanic basement topographic variability is apparently greater than other parts of the margin and the thin sediment cover in places means that basement more directly interacts with deformation within the prism [e.g., Kopp et al., 2008]. Here the variation of sediment properties around basement highs and the nature of the basement-sediment interface itself are likely to play a role in driving structure and morphology, in addition to primary changes in basement topography and sediment thickness. The primary source of the subducting sedimentary section (distal Bengal-Nicobar Fan) suggests potential for relatively high sedimentation rates and overpressured sediments, and the on average low taper angles would suggest that the prism is poorly drained and pore fluid pressures are elevated [Saffer and Bekins, 2002]. Assessments of sediment physical properties from geophysical data, although rather limited, suggest however that bulk overpressure is limited [e.g., Dean et al., 2010; Gulick et al., 2011]. The absence of a prominent and widespread bottom-simulating reflector [e.g., Kopp, 2002] also suggests a dewatered sediment section and potentially limited overpressure. However, discrete overpressured layers may still play a role, such as the overpressured layer that is thought to develop into the décollement on the North Sumatran margin [Dean et al., 2010]. If this overpressured layer is present throughout the thickened trench sediment section of the North Sumatra and Nicobar Regions, this would generate low basal shear stress and may drive the observed landward vergence in the prism toe [e.g., Seely, 1977]. Importantly, the seismic properties suggest the overpressure of this layer reduces landward, which could drive the change to seaward vergence (through increasing slip on conjugate landward dipping thrusts and the development of out-of-sequence thrusts), coinciding with the prism slope break. Kopp and Kukowski [2003] also infer a weak décollement and low basal friction beneath the South Sumatra prism (but not resulting from overpressure) generating the observed low taper angle (Tables 1 and 2). Offshore North Sumatra, Dean et al. [2010], Gulick et al. [2011], and Klingelhoefer et al. [2010] demonstrate high sediment velocities at the base of the input sediments, with continued high seismic velocities, densities, and opacity within the accreted wedge interior [Singh et al., 2008; Klingelhoefer et al., 2010; McNeill et al., 2006] (Figure 1 for high gravity anomaly coincident with plateau). The abrupt change in prism surface slope (steep toe to flat plateau) at the seaward edge of the North Sumatran plateau may relate to a significant change in properties of the internal and basal prism materials from weaker materials in the outermost prism to the dense materials of the prism interior identified from the geophysical data [e.g., Fisher et al., 2007; Gulick et al., 2011]. This change in slope may also coincide with the inner to outer wedge transition of Wang and Hu [2006] and thus give an estimated seismogenic updip limit position [Gulick et al., 2011]. The increase in strength of prism material properties may also generate a backstop in the outer prism that could drive the observed landward vergence [e.g., Byrne and Hibbard, 1987; Byrne et al., 1993]. However, other studies show that the role of a backstop in generating back thrusting/landward vergence is strongly dependent on the backstop geometry and surface frictional properties [e.g., Graveleau et al., 2012], both difficult parameters to constrain, and that landward vergence will be restricted to the initial thrust(s) [e.g., Gutscher et al., 2001]. In the South Sumatra/Sunda Straits Region, a change in material properties across strike, interpreted as a landward dipping dynamic backstop, separates the active seaward prism from the older prism and coincides with a change in surface slope and seafloor roughness [Kopp and Kukowski, 2003]. We can therefore conclude that MCNEILL AND HENSTOCK American Geophysical Union. All Rights Reserved. 128