deformation of the northern sumatra accretionary prism from high-resolution seismic reflection...
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Marine Geology 252 (2008) 89–99
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Marine Geology
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Deformation of the northern Sumatra accretionary prism from high-resolutionseismic reflection profiles and ROV observations
D.C. Mosher a,⁎, J.A. Austin Jr. b, D. Fisher c, S.P.S. Gulick b
a Geological Survey of Canada-Atlantic, Natural Resources Canada, 1 Challenger Dr., Bedford Institute of Oceanography, Dartmouth, NS, Canada B2Y 4A2b Institute for Geophysics, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, 10100 Burnet Road (R2200), Austin, TX, 78758-4445 USAc Department of Geosciences, Penn State University, University Park, PA, 16892 USA
⁎ Corresponding author. Tel.: +1 902 426 3149; fax: +E-mail address: [email protected] (D.C. Mosher)
0025-3227/$ – see front matter. Crown Copyright © 20doi:10.1016/j.margeo.2008.03.014
A B S T R A C T
A R T I C L E I N F OArticle history:
Following the 2004 Great Received 13 June 2007Received in revised form 7 March 2008Accepted 17 March 2008Keywords:forearc basinfrontal thruststrike-slip faulttsunamiSumatradeformationsubduction
Sumatran–Andaman Mw≈9.2 earthquake, high-resolution seismic reflection,multibeam bathymetric and remotely operated vehicle data were acquired to investigate the tectonicframework of the rupture zone and search for evidence of seafloor and near-surface displacement. Threedistinct regions off northern Sumatra were investigated; 1) a portion of the Sunda Trench, 2) the adjacentfrontal deformation zone, and 3) the seaward flank of the Aceh (forearc) Basin. A thick (N1.5 km) sedimentsection within the Sunda Trench shows evidence of shallow normal faulting, possibly representing earlystages of assimilation into the accretionary wedge. The frontal deformation zone consists of ridges ofpredominantly landward-verging thrust folds. Seaward-verging backthrust faults at or near the base of thesteep slope commonly reach the seafloor. We do not observe a single, laterally extensive structural offset atthe deformation front that might be interpreted as contributing to the 2004 tsunami. Rather, a series ofsmall-offset (tens of metres) faults were noted across this broad zone of the frontal accretionary wedge. Thewestern boundary of the Aceh (forearc) Basin is the West Andaman strike-slip fault, juxtaposing theaccretionary complex's forearc high with basin fill sediments. Neither the seismic nor ROV data showevidence of recent seafloor displacement along the fault trace. Basin infill demonstrates consistent along-strike patterns of tilting and seaward subsidence during sedimentation, while modern fill is flat-lying andcoherent across the entire basin. Intercalated chaotic layers interpreted as mass transport deposits mayrecord a history of seismicity, but recent examples of such deposits were not observed on the modernseafloor, either seaward of the deformation front or in the Aceh Basin. Lack of any evidence of faulting, offsetor disruption of sediments within Aceh Basin suggests that there was little impact of the 2004 earthquake inthis area. Distributed faults throughout the frontal deformation zone, combined with observations oflandward-verging folds at the deformation front, folding within piggy-back basin sediments, and lack ofevidence of disruption along the West Andaman Fault zone and within the forearc basin all support strainpartitioning across the margin. A proposed strong wedge interior may act as a backstop during major thrustevents, constraining deformation to the frontal deformation zone and the slope apron. Tsunami generation inresponse to the 2004 event did not result from surficial displacements along a single fault or narrow faultzone at the toe of the deformation front, but was more likely a result of vertical displacement across theentire outer forearc.
Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.
1. Introduction
At 00:58:47Z on December 26, 2004, an earthquake of Mw≈9.2(see Bilek et al., 2007) occurred along the Sumatra subduction zone,rupturing ∼1300 km of crust (Ammon et al., 2005; Lay et al., 2005).The resulting tsunami devastated coastal areas surrounding the IndianOcean and appeared on tidal records around the world. Focal
1 902 426 4104..
08 Published by Elsevier B.V. All rig
mechanisms of this earthquake, derived from teleseismic arrivals,indicate that the earthquake was a reverse thrust event at ∼30 kmdepth. It occurred along a shallowly dipping (∼8°) fault striking 329°(Harvard CMT catalog, http://www.seismology.harvard.edu). In themonths following the earthquake, a number of expeditions weredeployed to understand the geologic causes and consequences of theearthquake, including seafloor displacements that resulted in tsunamigeneration (e.g., Arai et al., 2005; Henstock et al., 2005; Sibuet et al.,2005, 2007; Ladage et al., 2006; Carton et al., 2006; Neben et al., 2006;Soh et al., 2006; Singh et al., 2006; Zillmer et al., 2006; Klingelhoeferet al., 2007). Nonetheless, determination of the location, distribution
hts reserved.
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and amount of coseismic displacement accompanying the 2004mega-earthquake remains problematic. Displacement estimates derivedfrom earthquake seismic waves (Ammon et al., 2005; Lay et al., 2005)are constrained by uncertainties in the degree of translation ofsubsurface rupture to seafloor displacement. Geodetic measurementsprovide large-scale results and agree well with results fromseismological (T-wave) studies (Chlieh et al., 2007), but are limitedin resolving seafloor rupture and slip near the Sunda Trench axis.Measurements of seafloor bathymetric change are not feasible, as fewproperly constrained baseline bathymetric data from this marginexisted prior to the earthquake. This study presents evidence of nearsurface and surface deformation of the accretionary wedge offshore ofSumatra that hint at a geologic architecture for the region of the 2004earthquake which may explain the margin's strong tsunamigeniccapability.
Over 40,000 km2 of seafloor multibeam bathymetric data wereacquired above the southern segment of the proposed rupture zoneoffshore Sumatra in early 2005 (Henstock et al., 2005, 2006). Thesedata showed the complex morphology of the seafloor in this region,but did not resolve seafloor features that resulted from the earth-quake. In May 2005, the SEATOS expedition used these multibeambathymetric data as a guide to explore a number of potentiallytsunamigenic seafloor and shallow sub-seafloor features along boththe accretionary front and the forearc (Aceh) basin using high-resolution single-channel seismic reflection profiling, remotely oper-ated vehicle (ROV) observations and ROV-based sediment sampling
Fig. 1. Location map showing SEATOS single-channel seismic profiles (with line numbers) acqand the Aceh (forearc) basin are identified. The 4500 m contour represents the approximate pframework (see also Fig. 8) is after Malod and Kemal (1996) and Sieh and Natawidjaja (2000).also shown.
(Fig. 1; Moran et al., 2005a,b). The longest of the acquired seismicprofiles (Line 1) crossed the entire margin from the Sunda Trench tothe northern Aceh Basin (Fisher et al., 2007). In this paper,interpretations of the SEATOS single-channel seismic data (Figs. 1and 2) and select ROV images, in concert with available multibeamseafloor bathymetric data, are presented to document styles ofdeformation along the front of the accretionary prism and providegeophysical evidence for complicated strike-slip deformation alongthe western flank of the forearc basin. Using these interpretations, wehypothesize that strain partitioning within the prism results in acomplex geologic response, from predominantly landward-vergingthrust faulting along the frontal deformation zone, to symmetricfolding across the upper slope, to right-lateral strike-slip faulting inthe vicinity of the western flank of the Aceh Basin. SEATOS seismicreflection profiles are also used to understand patterns of sedimentaryfill lying within the Aceh Basin; we suggest that associated deforma-tion reflects prominent uplift and subsidence episodes, strike-slipfaulting and gravitationalmasswasting related to recurring seismicity.
2. Geologic setting
The Sunda arc–forearc-trench system extends ∼4000 km fromTimor to the Nicobar Islands. Off Sumatra, the Sunda Trench forms byoblique subduction of the Indian Ocean Plate beneath the Burmamicroplate and the Sunda Plate (Hamilton, 1979). Convergence to thenortheast at a rate of ∼4.7 cm/yr was established in the late Eocene to
uired over parts of the Sumatra accretionary prism in May 2005; the accretionary frontosition of the Sunda Trench axis and the frontal deformation zone. Generalized tectonicThe Hypocenter ( ) of the December 26th, 2004 ∼Mw 9.2 Great Andaman Earthquake is
Fig. 2.Multibeam bathymetry of the Sumatra frontal deformation zone from ∼3°–4.5°N(from Henstock et al., 2006), on which are superimposed locations of the subset ofSEATOS single-channel seismic profiles intended to image the frontal fold and thrustbelt of the leading edge of the accretionary prism. The inset shows details over part ofthe frontal thrust ridge, along the seaward edge of which lies the “ditch”. The “ditch”was examined during SEATOS both seismically and with an ROV (see also Fig. 6) in anattempt to determine its tsunamigenic potential and possible link to the December2004 mega-earthquake.
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early Oligocene (Karig et al., 1979, 1980; Liu et al., 1983; Daly et al.,1991; Hall, 1996, 1997, 1998; Socquet et al., 2006; Delescluse andChamot-Rooke, 2007). Strain associated with this ongoing obliquesubduction of the Indian Plate beneath Sumatra is partitioned intoorthogonal (i.e., trench-perpendicular) and right-lateral (i.e., trench-parallel) components (Malod and Kemal, 1996; Baroux et al., 1998).The trench-perpendicular component includes thrust faulting andfolding within the accretionary wedge and resulted in basin develop-ment in the forearc (e.g., Aceh and Simelue basins, Fig. 1). The trench-parallel component manifests as strike-slip fault systems (i.e., theSumatra, Mentawai and West Andaman strike-slip fault systems),both onshore Sumatra and in the adjacent forearc region (Fig. 1; Katili,1975; Hamilton, 1979; Moore et al., 1980; McCaffrey, 1991; Diament
Fig. 3. Seismic Line 2 showing an example of thick (N1.5 s TWT) Sunda Trench sedimentary fi
disruptions caused by buried channels. Vertical exaggeration ∼10× and relative angles are sh
et al., 1992; Malod and Kemal, 1996; McCaffrey et al., 2000; Sieh andNatawidjaja, 2000; Susilohadi et al., 2005). Late Oligocene–earlyMiocene and ongoing collision of India and Eurasia caused massiveterrigenous sediment input to the eastern Indian Ocean. Resultantinfilling of the Sunda Trench and subsequent accretion of thesesediments during subduction created an extensive accretionary prismoffshore Sumatra (Fig. 1; Matson and Moore, 1992). Today, incomingoceanic crust of Eocene age includes a 2–3 km-thick overlyingsequence of Bengal-Nicobar Fan strata (Moore et al., 1982), overlainin turn by a∼500m-thick trench sedimentwedge derived locally fromthe adjacent frontal deformation zone (Karig et al., 1979).
Observations from bothmultibeam bathymetry and SEATOS Line 1,which crosses the entire margin, suggest that the forearc between 3°and 4.5° N exhibits three morphologic components (Fig. 2): 1) a steep(N8°), southwest-facing outer slope adjacent to the Sunda Trench(Henstock et al., 2006), 2) a broad, more gentle upper slope forming atopographic low between inner and outer highs (Fisher et al., 2007),and 3) another steep (up to 9°), northeast-facing slope flanking theseaward side of the Aceh Basin that borders the volcanic arc (Fig. 1;Henstock et al., 2006). Largely based on such morphologic evidence,Fisher et al. (2007) suggest that active shortening occurs across theentire northern Sumatra accretionary prism, not just at the frontaldeformation zone. They hypothesize that such shortening is probablyrestricted to an upper slope apron and conclude that a mega-earthquake produces: 1) seaward advance of a strong inner wedge,2) movement of the outer forearc high seaward, deformation at theprism toe, and peeling up of weaker, shallower trench fill, and 3)shortening and uplift of the upper slope. This combination of effectswould result in surficial and near-surface geologic structures andassociated stratigraphic disruptions, but would not produce a laterallyextensive, single structure that is in itself tsunamigenic.
3. Methods
3.1. Seismic data acquisition
High-resolution single-channel seismic reflection profiles wereacquiredusing a dual Sercel®Generator Injector (GI) airgun array (7.2 Lvolume) and hydrophone streamer (Moran et al., 2005a). Shot spacingwas 12–14 m. The hydrophone streamer was a 61 m-long analogTeledyne model 28420®. Signals from all active groups were summedand digitally recorded at 24-bit resolution using a sample interval of0.25 ms. Datawere recorded with integrated navigation as SEG-Y files.Data were subsequently processed with a bandpass filter of 10/35 to325/350 Hz, exponential gain (time⁎e1.5), static correction, F-K time-migration with a static velocity of 1520 m/s, and water bottom mute.Processed SEG-Y data were imported to SMT Kingdom Suite® digitalinterpretation software for profile and bathymetry integration,horizon and fault interpretation and final image production.
ll. See Figs. 1 and 2 for line location. Line 2 shows faulting to the seafloor and occasionalown assuming a sediment velocity of 2000 m/s for the travel time to depth conversion.
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Frequency analyses of processed seismic data show a recordedbandwidth of 25–200 Hz with a central peak of ∼110 Hz. Assuming awater velocity of 1500m/s, vertical seismic resolution at the seafloor is∼3–4 m (assuming a Rayleigh criteria of λ /4). The short streamer andshort source-receiver offset led to prevalent out-of-plane reflectionson unmigrated data. The datawere consequently migrated at constantvelocity to collapse these out-of-plane reflections. Correlation ofmigrated seismic datawith seafloor features recognized onmultibeambathymetry data suggests that in-plane horizontal resolution is betterthan 200 m in 4.5 km water depths.
Fig. 4. Examples of seismic profiles crossing the frontal deformation zone, from north to soutlocations of these lines. Seismic data are displayed as envelope of the amplitude (positively rthe text. MTD in F represents mass transport deposit. Scales are consistent for all plots; the vvelocity of 2000 m/s for the travel time conversion.
3.2. Remotely operated vehicle (ROV) operations
Oceaneering's Magellan 825® remotely operated vehicle (ROV)was employed for seafloor imaging and sediment and biologicalsample acquisition. This ROV was capable of diving to 7000 m waterdepth, and was equipped with a CCD video camera and digital stillcamera on a pan-and-tilt mount. Manipulation arms permittedsampling of both seafloor sediment and biology specimens. In fiveseparate deployments, the ROV was lowered 1) within the frontaldeformation zone (at the inner trench wall), 2) on the accretionary
h: A, Line 3, B, Line 17, C, Line 12, D, Line 6, E, Line 8, and F, Line 19. Figs. 1 and 2 show theectified) to highlight faults. Structural and stratigraphic interpretations are discussed inertical exaggeration is 5X and relative slope angles are displayed, assuming a sediment
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prism, and 3) on the flank of the forearc basin, to examine seafloorgeology and biology and to search for seafloor expressions of the 2004earthquake (see Moran et al., 2005a).
4. Results
SEATOS expedition high-resolution single-channel seismic reflec-tion profiles cross the Sumatran accretionary complex and Aceh Basin(forearc) from ∼3°N to 4.5°N (Figs. 1 and 2). Over 3880 line-km ofseismic profile data were collected, concentrating within threemorphologic regions: 1) the Sunda Trench immediately seaward ofthe frontal deformation zone, 2) the frontal fold-and-thrust beltforming the frontal deformation zone, and 3) the seaward flank of theAceh Basin.
4.1. Sunda Trench
Seaward of the frontal deformation zone, in water depths N4.5 km,the Sunda Trench is filled by over 1.5 s two-way travel time (TWT) ofsediment. The top of the oceanic crust was not observed. Reflectionswithin trench sediment fill are largely flat-lying and laterally coherent,except for lenticular-shaped bodies of incoherent reflections repre-senting buried channels (Fig. 3). Similar channels appear at theseafloor at a variety of angles to the Sunda Trench axis (Fig. 2, inset;Henstock et al., 2006). On some profiles, sediment thickening occurstoward the frontal deformation zone (Fig. 4). Incipient folding beneaththe seafloor increases toward the frontal deformation zone on allfrontal thrust profiles (Fig. 4), and some sub-seafloor wipeouts suggestfluid/gas escape. Steep normal faults characterized by tens of metresof offset are also evident (e.g. Figs. 3 and 4B).
4.2. The frontal deformation zone
Eleven single-channel profiles were acquired across the frontaldeformation zone of the northern Sumatran accretionary prism(Figs. 1 and 2). A subset of these profiles is shown in Fig. 4). Prominentfrontal folds and thrust ridges are observed. Reflections within theseridges suggest that they are predominantly landward-verging anticli-
Fig. 5. ROV photo mosaic of part of the wall, ∼12-m high, forming part of the seaward flankangular talus blocks at the base. The inset is a close-up of a portion of the face that shows fresto wall collapse. This may explain the overall fresh appearance of this wall. The inset is app
nes, with their seaward flanks dipping between 7 and 9 degrees(assuming a shallow sediment velocity of 2000 m/s for the time todepth conversion) (Fig. 4). These folds are associated with landward-verging thrust faults and seaward-verging backthrusts (Fig. 4).Conjugate faulting interpreted along the northernmost crossing(Line 3, Fig. 4A) forms a triangular zone at the synclinal hinge line ofthe outermost fold. Slope angles on these conjugate faults are about17°, while most other faults are in excess of 45°. The seafloor isoccasionally offset between 10 to 20 m by these faults (Figs. 2 and 4B,E). Small-scale seafloor roughness (10 s to 100 s of metres) alsosuggests sediment mass-failure near the frontal deformation zone(Figs. 2, 4C, D and E). There is generally little sediment ponding withinthe piggy-back basins near the frontal deformation zone (Fig. 4).
SEATOS acquired six tightly spaced seismic profiles across onesmall region of the frontal deformation zone (Fig. 2, inset) to studywhat appeared to be an extensional seafloor feature known as the“ditch”, identified from multibeam bathymetric data (Henstock et al.,2006). This “ditch” is a 200 m wide and 15–20 m deep gully thatparallels the seaward flank of a frontal fold at the deformation front(Figs. 2 and 4C). Seismic data show surfacing thrust faults at the toe ofthe deformation front proximal to the ditch (Fig. 4C, E). An ROV diveconfirmed that the “ditch” is bounded on its southwest (seaward) sideby a landward-facing vertical wall ∼12-m high that is clearly fresh,based on absence of biological colonization, preservation of delicatesurface striations, sharp edges of the sides and top of this wall, and thepresence of ∼1 m-scale angular blocks of talus at its base (Fig. 5). Thewall morphology strongly resembles fracture/joint surfaces; the facenearest the camera is characterized by clear plumose structures (cf.Pollard and Aydin, 1988) that indicate lateral and downwardpropagation of a joint surface that allows block collapse (see Fig. 5,inset). Fresh talus at the wall's base must be the result of continuingexfoliation.
4.3. Aceh (forearc) Basin
Seven SEATOS reflection profiles cross the seaward flank of theAceh Basin (Fig. 1), acquired to investigate whether the evolution ofbasin sediment infill and the observed flanking structures show
of the “ditch”. Note the freshness and sharpness of wall surfaces, and the presence ofh plumose facture patterns, indicative of recent joint formation and propagation leadingroximate 1 m2 in area; the view is facing west.
Fig. 6. Seismic reflection profiles from Aceh (forearc) Basin. A, Line 20, B, Line 22, C, Line 23–24, D, Line 25, and E, Line 26. See Fig. 1 for locations of these lines. MTD represents masstransport deposit. Data are plotted as amplitude envelops. Colour lines represent contacts between different seismic units. Vertical exaggeration is ∼15× and, relative slope angles areshown in C and E and apply to all sections. Travel time to depth conversions were made assuming 2000 m/s for the shallow sediment column. (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.)
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evidence of active tectonism. We recognize a number of discreteseismic stratigraphic units separated by unconformities
1) The top of Unit 1 is an undulatory surface on some profiles (Fig. 6B,C), and it may crop out in a ridge on Line 20 (Fig. 6A). Acousticenergy is insufficient to image beneath the top of this unit.
2) Unit 2 lies unconformably on top of Unit 1 and is relatively trans-parent, although this echo-character is likely a result of insufficientacoustic energy rather than reduced acoustic impedance (Fig. 6B, C).Generally, Unit 2 dips to the west.
3) Unit 3, ∼0.5 s thick, displays dipping and gently folded reflections.On Line 23 (Fig. 6C), this unit extends along interpreted listric faultssoling into layer-parallel structures. Away from folds, reflectionswithin this unit are generally parallel-continuous, but faulting andevidence of slope failure introduce complexities. This unit also dipsto the west.
4) Unit 4 is the uppermost sequence observed in the seismic strati-graphic succession on all but one profile (Fig. 6A). This unitthickens to the southwest to N1.2 s (Fig. 6B and C), apparently aresult of basin infilling. This unit forms a partial onlap surface ontounderlying Unit 3 (Fig. 6B). Unit 4 consists of generally flat-lying,high amplitude, coherent reflections. Occasional thin (b50 ms),chaotic beds are interspersed within these high-amplitude reflec-tions (Figs. 6B, C and 7). Six such beds are recognizable. Some ofthese, within the upper part of Unit 4, clearly thin and pinch out tothe NNE, implying a source towards Sumatra, while others deeperin the section are thicker and persist across the basin (Fig. 7). Ingeneral, these beds appear to decrease both in thickness andcontinuity in the shallower part of the section.
5) On Line 20 (Fig. 6A), chaotic reflections (Unit 5) lie at or just belowthe seafloor. They exist in two sub-basins, pinching out againstadjacent high ridges.
Line 20 (Fig. 6A) traverses the southeastern-most extension of theAceh Basin, and also crosses a bathymetric feature known as the TubaRidge (Malod and Kemal, 1996). The Tuba Ridge at this locationincludes two bathymetric highs that are dissected by multiple near-vertical fault strands. One sub-basin lying between the forearc highand the Tuba Ridge holds N1 s TWT of sediment.
On all profiles crossing the Aceh Basin north of Tuba Ridge, unit 3and 4 strata on the seaward side of the basin abut abruptly againstinterpreted acoustic basement of the forearc high. This sharpjuxtaposition of seismic units forms a near-vertical, undulatorycontact, with progressive upward warping of basin reflections
Fig. 7. Portion of SEATOS Line 22 showing Aceh Basin sediment infill above a prominent onlafacies suggest mass transport deposits (MTD), separated by more uniformly layered matersediment failure results from major seismic events. Vertical exaggeration is about 7×, assum
(Fig. 6D, E). We interpret this near-vertical contact as a strike-slipfault zone which exhibits an increasing basinward salient along lines23/24 (Fig. 6C), Line 25 (Fig. 6D), and Line 26 (Fig. 6E). These profilescross the purported trend of the West Andaman Fault Zone (Fig. 1;Malod and Kemal, 1996; Sieh and Natawidjaja, 2001).
5. Discussion
5.1. Sunda Trench
Trench fill sediments are over 1.5 km thick, assuming a 2000 m/ssediment velocity (Moore and Curray, 1980; Kieckhefer et al., 1980).Moore and Curray (1980) and Gaedicke et al. (2006), with deeperpenetrating seismic data, observed up to 3.5 s TWT of sedimentseaward of the Trench. Age control is non-existent, but the sequencemust be post-Paleocene given reported seafloor ages in this area(Müller et al., 1997). Therefore, sedimentation rates must be N30 m/Ma. Trench-fill sediments are largely composed of distal Nicobarand Bengal Fan material, supplemented by sediments eroded locallyfrom the adjacent slopes of the accretionary prism (Moore et al.,1982; Henstock et al., 2006). Little Neogene material derives fromthe Sumatran arc terrain, as island-derived sediment tends to betrapped within the forearc basins (Moore et al., 1982). Observed flat-lying, laterally coherent and high-amplitude reflections suggest thepredominance of deep-sea turbidites, interrupted by intercalatedlens-shaped bodies that are likely buried channels. Similar channelsare apparent on the seafloor today in this environment (Fig. 2;Henstock et al., 2006), sourced from the adjacent deformation front,and are also noted as common features from other accretionaryprism fronts (e.g. Goldfinger et al., 2003; Huyghe et al., 2004). Thesechannels are continuous from the frontal slope of the accretionaryprism to the trench floor, suggesting substantive mass-wastingerosion and transportation of recently accreted sediments to supplyupper trench fill. Landslides along the seaward flank of the frontalthrust fold also appear to have transported material to the trench(Fig. 4B, F), perhaps in response to recurring seismic activity (Tappinet al., 2007). Slope apron debris, including coherent blocks, havealso been transported downslope at some locations along thesubduction front (Fig. 4B, F; Moran et al., 2005a,b; Mosher et al.,2005; Henstock et al., 2006; Tappin et al., 2007). Shallow (top100 m), near-vertical faults within subducting trench fill may resultin response to local stresses of collision/subduction and wedgeconstruction nearby.
p surface (see also Fig. 7B). Layers of varying thickness characterized by chaotic seismicial that are likely turbidites and/or hemipelagic beds. The MTDs suggest that periodicing 2000 m/s for the sediment velocity.
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5.2. The frontal deformation zone
Where we observed it seismically, the frontal deformation zonemarks the trace of a syncline with an axial plane that strikes NW(Fig. 2). This part of the front is composed of a series of asymmetric,landward-verging, open, upright anticlinal folds bounded by pre-dominantly landward-verging thrusts (Figs. 2, 4B–E). Folding andthrusting in combination cause the seafloor to shoal to the NE from awater depth of ∼4.5 km (∼6 s) to ∼2.6 km (∼3.5 s) over a horizontaldistance of ∼14 km, representing an average seafloor slope of almost8° (Henstock et al., 2006).
Henstock et al. (2006) noted differences in the bathymetriccharacter of the margin and suggested that zones without clearasymmetric frontal ridges may be seaward-vergent. However, all ofthe seismic profiles acquired for this study show the seaward flank ofthe frontal deformation zone to be composed of steeply-dipping,landward-verging, thrust-bounded anticlinal folds (Fig. 4). Imagedthrust faults, typically in excess of 45°, show tens-of-metres ofdisplacement (∼0.1 s) near or at the seafloor. Some anticlines exhibitdivergent structures (Fig. 4B, C, E), but more generally, fault-relatedfolds are asymmetric with the dominant fault being the landward-vergent one (Fig. 4B, E, F). Therefore, we conclude that landwardverging structures predominate along the frontal deformation zonewithin our survey area.
Landward vergence is generally attributed to low basal shear stresscaused by the rapid accretion of water-rich sediments to the frontaldeformation zone (e.g., Seely, 1977; Carson and Bergland, 1986;MacKay et al., 1992), or backstop geometry and composition (McNeillet al., 2006). The former explanation appears justified for the landwardvergence observed offshore Oregon and Washington (MacKay et al.,1992; Fisher et al., 1996; Flueh et al., 2000; Goldfinger and McNeill,2006). Low basal shear stress is consistent with low surface slopes onthe accretionary prism, but landward-vergent structures that weobserve off Sumatra (Fig. 4) have higher angle slopes and aremore akinto the steep, landward-vergent structures observed in northernCalifornia (Gulick et al., 1998). Fisher et al. (2007) concluded thatlandward vergence off Sumatra is controlled by a more coherent,deeper layer within the prism, peeling up the incoming section inlandward-vergent thrust sheets. McNeill et al. (2006) suggested thathigh sediment densities within the outer prism, revealed by gravitydata, produce this backstop, contributing to landward vergence. Theysuggest that, in turn, this backstop may control the up-dip limit of theseismogenic zone. Gulick et al. (1998) similarly suggested for northernCalifornia that a local backstop rather than basal shear stress generatesthe landward vergence on that margin. Steep flanks of the landward-vergent ridges, along with shallow thrust structures and the argu-ments put forward byMcNeill et al. (2006) and Fisher et al. (2007) for astronger deep layer, lead us to suggest that a local backstop drives thelandward vergence off Sumatra (as illustrated schematically in Fig. 8).Under these circumstances, the structure of the deformation front isanalogous to the “triangle zones” observed along many mountainfronts with thick foreland basin sequences (e.g., Vann et al., 1986).
Seismic profiles over the “ditch” and adjacent frontal deformationzone consistently show that the “ditch” is spatially associated withone or more antithetic faults (seaward dipping). However, the specificsurfacing position of these faults in relation to the ditch varies (Figs. 2,4C and D). For example, on Line 12 (Fig. 4C), a thrust fault reaches theseafloor just seaward of the ditch, on Line 6 (∼4 km SE; Fig. 4D), onelies centrally beneath the “ditch” axis, and on Line 8 (∼8 km SE), a faultsurfaces landward of the “ditch” (Fig. 4E). Henstock et al. (2006)interpreted the “ditch” to be a seafloor displacement feature that mayhave formed as a result of the 2004 mega-earthquake or over asuccession of seismic events. They suggested that this feature was aconsequence of either a backthrust rupture or shortening on an innerarc flexural bend. ROV mosaics of the seaward wall of the “ditch”confirm that parts of it are indeed fresh (Fig. 5). The varying spatial
configuration of the “ditch” with respect to surfacing faults in itsvicinity, however, suggests that it is not a tectonic feature, but insteada product of surficial erosion and collapse, albeit seismically actuatedby underlying fault movement(s). We believe that the freshness of theditch wall in the ROV photomosaic (Fig. 5) is the result of gravitationalslope failure(s) along an erosional furrow. Bedforms developed in softsediments covering the base of the ditch, observed in SEATOS ROVvideos, provide further evidence of recent seafloor erosion along thefrontal deformation zone (Moran et al., 2005a,b).
5.3. Aceh (forearc) Basin
The Aceh Basin is the northernmost of a chain of westernIndonesian forearc basins that extend more than 1800 km tosouthwest Java (Fig. 1). These basins form a subsiding belt betweenthe elevated Sumatra Paleozoic–Mesozoic arc massif of Sumatra andJava and the outer forearc high of the subduction margin accretionarywedge (Karig et al., 1980; Schlüter et al., 2002). Generally, sedimentsfilling the Aceh Basin are N2.3 s TWT thick where they abut the forearchigh and overlie acoustic basement (Fig. 6C–E). Infilling occurred inmultiple phases, although we do not recognize the four unitsdescribed by Susilohadi et al. (2005) in basins to the southeast. Thefive units that we define are based on acoustic character and contactrelationships, the likely result of differential basin subsidence andtilting rather than eustatic sea level change, as suggested by Susilohadiet al. (2005). The most notable acoustic discontinuity lies betweenseismic Units 3 and 4 (Fig. 6B–E). On Line 22 (Fig. 6B), this contact is awell-developed, high-angle onlap surface. On Lines 22 and 23 (Fig. 6B,C), the contact exists along the east (landward) side of the Aceh Basin.Further offshore, towards the outer arc high, reflections within thesediment fill become more conformable (Fig. 6D, E), validating thesuggestion of syndepositional subsidence of the Aceh Basin.
The dominant sedimentation process for forearc basin infillingalong this margin appears to be turbidity current deposition (cf. Kariget al., 1980), leading to the coherent parallel, laterally extensivereflections observed (Fig. 6). Offlap apparent on our reflection profilessuggests that the dominant source region is the hinterland to the NE(Fig.1). Interpretedmass-transport deposits (MTD) intercalatedwithinthe turbidites generally thicken to theNE, particularly in the upper partof the section (Fig. 6C, 7). Unit 5 (Fig. 6A), which we interpret to be asurficial MTD, thickens eastward aswell.We suggest that each of theseMTDs represents a sedimentary response to a significant earthquakethat triggered seafloor instability and mass-failure (c.f., Goldfingeret al., 2003; Mosher et al., 2005). If so, then our observation that thesebeds generally decrease in frequency and thickness up-section impliesthat: 1) earthquake frequency and magnitudes have generallydecreased with time, as suggested by Curray (2006); 2) the forearcarea has become more removed (physically or tectonically) from theearthquake source region with time; and/or 3) sedimentary sourcematerial has become increasingly limited.
Western Indonesian forearc basins are bounded by complex faultsystems (Izart et al., 1994; Malod and Kemal, 1996; Sieh andNatawidjaja, 2000; Susilohadi et al., 2005). Malod and Kemal (1996)interpret the West Andaman Fault to be a landward-verging thrustthat lies along the boundary between the Aceh Basin (within the AcehPlate) and the forearc high of the Burma Plate (Fig. 8). Seismic profilescrossing this contact show near-vertical, undulatory contacts betweenAceh Basin infill and the forearc high that we interpret as fault traces(Fig. 6D, E). Because of these contact characteristics and the nature ofoblique subduction along this part of the margin, we interpret thisboundary as being dominated by active right-lateral strike-slipfaulting, likely within a strand of the West Andaman Fault system(Fig. 8). Upturned reflections of basin sediments at the fault mayindicate a vertical (thrust) component to recent motions, concurrentwith basin deposition. However, such upturning could equally well beexplained by differential sediment compaction.
Fig. 8. Summary schematic interpretation of the tectonic framework of this part of the Sumatra convergent margin, as discerned from previous work and the SEATOS single-channelseismic profiles. The tectonic framework is adapted from Malod and Kemal (1996); average convergence direction is after Delescluse and Chamot-Rooke (2007) and Socquet et al.(2006). Oblique convergence leads to strain partitioning predominantly in the form of landward-vergent deformation within the upper part of the accretionary prism from ∼3°–4.5°N, with right-lateral strike-slip faulting localized along theWest Andaman and Mentawai fault zones which together form the seaward flank of the forearc basins. Inset: Note thelocation of the hypocenter of the 2004 great earthquake; rupture propagation, as modeled by Ammon et al. (2005) and subsequent references, stayed seaward of the seawardboundary of the Aceh Plate, entirely within the Burma Plate.
97D.C. Mosher et al. / Marine Geology 252 (2008) 89–99
At the southern Aceh Basin, Line 20 crosses the Tuba Ridge (Fig. 1,6A). Our seismic data show this ridge is dissected by multiple, near-vertical fault strands. Based on fault characteristics and juxtapositionof observed seismic units, some of these strands appear to be thrustfaults; othersmay represent strike-slip deformation. These interpreta-tions are consistent with transpressional deformation (Figs. 1, 8). TheWest Andaman Fault south of Tuba Ridge was interpreted to be astrike-slip boundary between the Simulue Basin of the Aceh Plate andthe Burma Plate (Malod and Kemal,1996; Sieh andNatawidjaja, 2000);however, we have no data to support or refute this interpretation. Inthis case, the Tuba Ridge appears to be a complex restraining bendbetween the two strike-slip fault systems; together, they form thewestern boundary of the Aceh Plate, a sliver plate similar to theMentawai sliver plate interpreted to the SE (Fig. 8 (inset); Malod andKemal, 1996).
6. Conclusions
High-resolution single-channel seismic reflection data from theSEATOS expedition, in concert with multibeam bathymetry data andremotely operated vehicle observations, provide the bases for severalbroad inferences with respect to tectonics and tsunami generation in
the northern Sumatra subduction zone: 1) submarinemass-wasting inthe frontal thrust region of the accretionary wedge contributessediment to the subducting trench floor, but likely played nosignificant role in tsunami generation in the 2004 event, 2) in thefrontal deformation zone, orientations of abundant near-surface faultswith dips on the order of 45°, landward-verging fold structures andsedimentary-fill deformation in piggy-back basins within fold syn-clines are all indicative of the shallow geologic response to the currenttectonic regime. We interpret them to suggest a local tectonicbackstop causing shallow peel-up of the accretionary wedge material,rather than low basal shear stress in the over-riding wedge. 3) TheWest Andaman Fault along this segment of the margin is principally aright-lateral strike-slip feature. This complex fault system must marka boundary partitioning the normal component of oblique strain to thewest, while also partially accommodating the margin parallel-strainthrough strike-slip tectonics. We hypothesize that as rupture accom-panying the 2004 seismic event occurred in the accretionary wedge ofthe Burma Plate; the West Andaman Fault could have influenced itsdown-dip limit, as we illustrate schematically in Fig. 8.
Fisher et al. (2007) suggest that tectonic shortening during amegathrust subduction event on this margin is probably restricted tothe upper slope apron, with accompanying seaward advance of a
98 D.C. Mosher et al. / Marine Geology 252 (2008) 89–99
strong inner wedge, seaward movement of the forearc high,deformation at the prism toe, peeling up of weaker, shallower trenchfill, and shortening and uplift of the upper slope. Surficial and near-surface features distributed across the frontal deformation zone asdocumented in this study support such an interpretation. Therefore,we suggest that tsunami generation likely did not result fromdisplacement along a single fault or narrow fault zone at the toe ofthe deformation front, but instead resulted from displacement alongthe entire frontal deformation zone with the degree of slip decreasingup-dip, as suggested by Fisher et al. (2007).
Comparison of the Sumatran subduction margin to other, betterstudied convergent margins may provide further insight intotsunamigenesis. Cascadia, for example, is known to have generatedsignificant tsunamis in the past (Satake et al., 1996). Deformation frontarchitecture in the rupture zone of the 2004 event off Sumatra appearssimilar to the deformation front off northern California (Gulick et al.,1998), at the southern end of the Cascadia subduction zone. Thisarchitecture; however, is dissimilar to the landward vergence offnorthern Oregon and Washington (MacKay et al., 1992). Therefore,examination of tsunamigenic hazards on any convergent margin mustconsider the potential importance of local margin architecture,specifically the relationships between surficial features and deep-rooted structures. To understand these relationships off Sumatra willrequire a combination of high-resolution results, such as thosepresented in this paper, and deep-penetration seismic data offSumatra, which are now being collected.
Acknowledgements
The authors would like to express their appreciation to the SEATOSscientific party and officers and crew of the Oceaneering vessel M/VThe Performer. Our greatest thanks go to Drs. K. Moran and S. Grilli fororganizing the SEATOS expedition and inviting us to participate. B.Chapmanwas of critical importance to the mission, for mobilizing andmaintaining the seismic systems. Funding was provided by the BritishBroadcasting Corporation, the Discovery Channel, University of RhodeIsland, Natural Resources Canada, Science Applications InternationalCorporation, BP Marine Limited. Science support and funding was alsoprovided by British Geological Survey, Alfred P. Sloan Foundation'sCensus of Marine Life, University of New Hampshire Center for Coastaland Ocean Mapping, Institute for Geophysics at The University ofTexas at Austin, Pennsylvania State University, Institut de Recherchepour le Dévelopement–“Géosciences Azur”–Observatoire Océanolo-gique, and L'ecole Normale Superieure de Cachan. OceaneeringInternational Corporation provided the vessel M/V The Performer atreduced rates. We thank Mr. JulianWare and Mr. Ed Wardle of DarlowSmithson Production and Mr. David Mearns of Bluewater Recoveries.We also thank the United Kingdom Ministry of Defense andHydrographic Office, the British Embassy Jakarta, and the Republicof Indonesia. Drs. K. Wang, M. Deptuck and J. Wells and the twoanonymous reviewers are thanked for providing insightful criticalreviews of the manuscript. This publication is Natural ResourcesCanada, Earth Sciences Sector contribution number 20070360 andUTIG Contribution #1983.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.margeo.2008.03.014.
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