STRUCTURE

OF THE

SUMATRA

SUNDA

TRENCH

LOWER

FROM MULTICHANNEL REFLECTION DATA*

SLOPE

OFF

SEISMIC

GREGORY F. MOORE and J O S E P H R. CURRAY

Geological Research Division, A-015, Scripps Institution of Oceanography, La Jolla, California 92093, U.S.A. (Accepted 4 April, 1980) Abstract. Multichannel seismic reflection profiles across the Sunda Trench slope off central Sumatra reveal details of subduction zone structure. Normal faults formed on the outer ridge of the trench offset deep strata and the oceanic crust, but die out upsection under the trench sediments. At the base of the inner trench slope, shallow reflectors are tilted seaward, while deeper reflectors dip landward parallel to the underlying oceanic crustal reflector. Intermediate depth reflectors can be traced landward through a seaward-dipping monocline. We interpret this fold as the shallow expression of a landward-dipping thrust fault at depth. Landward of this flexure, relatively undeformed strata have been stripped off the oceanic plate, uplifted 700 meters, and accreted to the base of the slope. The oceanic crust is not involved in the deformation at the toe of the slope, and it can be observed dipping landward about 25 km under the toe of the accretionary prism. The middle portion of the trench slope is underlain by deformed accreted strata. Shallow reflectors define anticlinal structures, but coherent deep reflectors are lacking. Reflectors 45 to 55 km landward of the base of the slope dip 40-5~ landward beneath a steep slope, suggesting structural imbrication. A significant sediment apron is absent from the trench slope. Instead, slope basins are developed in 375-1500 m water depths, with an especially large one at about 1500 m water depth that is filled with more than 1.1 seconds of relatively undeformed sediments. The seaward flank of the basin has recently been uplifted, as indicated by shallow landward-dipping reflectors. Earlier periods of uplift also appear to have coincided with sedimentation in this basin, as indicated by numerous angular unconformities in the basin strata.

1. Introduction It h a s b e e n p r o p o s e d t h a t s e d i / n e n t s a r e s t r i p p e d off t h e d e s c e n d i n g o c e a n i c p l a t e a n d a c c r e t e d to t h e b a s e o f t h e i n n e r t r e n c h s l o p e in a c t i v e a r c s y s t e m s w h e r e a t h i c k s e d i m e n t a r y c o v e r is b e i n g s w e p t into t h e s u b d u c t i o n z o n e (e.g., S i l v e r , 1969; S e e l y et al., 1974). A c c r e t i o n in this t y p e o f t r e n c h h a s b e e n inferred primarily from marine seismic reflection surveys, but interpretation of t h e c o m p l e x s t r u c t u r e o f t r e n c h s l o p e s is v e r y difficult w i t h o u t m i g r a t e d m u l t i c h a n n e l s e i s m i c r e f l e c t i o n d a t a , a n d t h e d e t a i l s o f this a c c r e t i o n p r o c e s s r e m a i n o b s c u r e . A l t h o u g h D e e p S e a Drilling h a s c o r r o b o r a t e d t h e a c c r e t i o n a r y m o d e l in t h e S h i k o k u (J. C. M o o r e a n d K a r i g , 1976) a n d n o r t h e r n M i d d l e A m e r i c a T r e n c h e s (J. C. M o o r e et al., 1979), drilling r e s u l t s h a v e c a s e d o u b t s as to t h e a p p l i c a b i l i t y o f t h e m o d e l to t h e J a p a n T r e n c h ( D S D P Staff, 1978) a n d t h e M i d d l e A m e r i c a T r e n c h off G u a t e m a l a ( v o n H u e n e et al., in p r e s s ) . S e i s m i c r e f l e c t i o n profiles a c r o s s t h e O r e g o n - W a s h i n g t o n m a r g i n ( S i l v e r , 1972; C a r s o n et al., 1973), t h e n o r t h e r n S u n d a T r e n c h ( C u r r a y a n d D. G. M o o r e , 1974), t h e \ *Contribution of the Scripps In~stitution of Oceanography, new series.

Marine Geophysical Researches 4 (1980) 319-340. 0025-3235/80/0043-0319 $03.30. Copyright 9 1980 by D. Reidel Publishing Company.

320

G R E G O R Y F. M O O R E A N D J O S E P H R. C U R R A Y

Shikoku Trench (J. C. Moore and Karig, 1976), the Aleutian Trench (Seely, 1977; von Huene, 1979), the Middle America Trench (Seely, 1979; Seely et al., 1974), and the Makran Trench (White and Klitgord, 1976) suggest that trench and/or lower plate sediments are first folded into anticlines at the base of the lower trench slope and may be subsequently incorporated into the subduction complex along thrust faults. Profiles across the Java portion of the Sunda Trench (Beck and Lehner, 1974) suggest that very little folding occurs in that area, with most of the deformation presumably occuring by thrusting. A ridge and trough morphology occurs on trench slopes, with slope sediments accumulating in the troughs. Single-channel analog seismic reflection profiles show that the slope basin sediments are tilted landward (Curray and D. G. Moore, 1974; G. F. Moore and Karig, 1976; White and Klitgord, 1976), but the high vertical exaggeration and shallow penetration of analog profiles have hampered interpretations of slope basin stratigraphy and structure. The forearc region of the Sunda Arc (Figure 1) has often been cited as one of the classic examples of an accretionary margin (see, for example, W. Hamilton, 1977, 1979; Dickinson and Seely, 1979). The region has a sediment-filled forearc basin, an emergent outer-arc ridge (trench slope break), and a thick sediment section on the underthrusting Indian plate (McDonald, 1977; Karig et al., 1979). As part of the International Decade of Ocean Exploration/Studies of East Asian Tectonics and Resources (IDOE/SEATAR) program (CCOP-IOC, 1974), we conducted a multidisciplinary marine study of the central Sumatra portion of the Sunda Arc in cooperation with the Indonesian National Institute of Geology and Mining. During March 1977, we collected 830km of multichannel seismic reflection data across the inner trench slope west of central Sumatra on the R/V Thomas Washington of the Scripps Institution of Oceanography (Figure 1). The multichannel seismic lines were oriented approximately perpendicular to the strike of the trench slope and were run from the trench across the lower trench slope to the outer-arc ridge in order to delineate the structure of the lower slope and to locate a small area for later detailed study using analog seismic reflection and bathymetric methods. A seismic refraction program was also carried out on the inner trench slope. Much of our shallow structural interpretation was made on the basis of the localized detailed survey and has been presented by Karig et al. (in press). The multichannel seismic data presented here confirm and extend the inferences drawn from those single-channel data. The seismic refraction d~ita, interpreted by Kieckhefer et al. (1980), have also been employed in the interpretation of our multichannel seismic profiles.

2. Data Acquisition and Processing

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intervals (150 feet or 45.72 m) provided the seismic sound source. The signal was received by a 24-channel hydrophone array, 6300 feet (1.92 km) long, digitized by a GeoSpace 1590 Digital Field System and redorded on high-density tape drives. Twelve seconds of 1 millisecond data were recorded, but only 6 seconds (subbottom) of 4 msec data were processed. The hydrophone streamer consisted of six sections of 150-foot (45.72 m) group spacing, followed by eighteen sections of 300-foot (91.44 m) groups. As the first six sections are spaced at half the distance of the other 18 sections, data from three sections were deleted during the initial stages of data processing. In essence, we had twenty-one 300-foot (91.44 m) groups to produce twenty-onefold CDP data. The following processing sequence was applied to the data. Steps 1 and 2 were performed at SIO, and the remainder were performed by Digicon Geophysical Corporation of Houston: 1. Reformat to 1600 BPI 9-track tape 2. Demultiplex, edit, binary gain recovery, resample at 4 msec 3. Common Depth Point (CDP) gather 4. Spiking deconvolution (160 msec operator length) 5. Velocity analysis (average of one every two kilometers) 6. Normal Moveout (NMO) correction 7. 21-fold CDP stack 8. Time-variant digital band-pass filter (8-55 Hz to 8-30 Hz) 9. Automatic gain control (0.5 sec operator length) 10. Wave equation migration (Line 7 only). The single-channel analog monitor record shows good resolution of layers in the trench and penetration of greater than 2 seconds is achieved. Resolution and penetration of the landward trench slope, however, is poor. In the multichannel profiles processed through normal moveout and stack (Figures 2, 3, and 4) resolution of structures on the inner trench slope is increased because of CDP stacking and the lower vertical exaggeration. The migrated time section (Figure 5) shows further improvement in resolution, because most diffraction hyperbolae that tend to obscure primary reflectors have been coUasped. We note that this profile is approximately 20~ from perpendicular to the local strike of the trench slope. For this reason, and because of variations in deep structure along the strike of the slope, the assumptions used in the 2-D migration of the profile are probably not completely valid. Several spurious reflections are identified near the base of the slope which cut across more reliable reflectors. We consider the spurious events to be either reflections from Out of the plane of the profile, or artifacts of the migration process and have left them out of our line-drawing interpretation (Figure 6A). We believe, however, that the migration of the reflectors has not been greatly affected by th6 breakdown of 2-D assumptions. Many of the most prominent reflectors on our interpretive section were digitized and converted to depth (Figure 6B) using velocities from sonobuoy and explosive refraction measurements in this area (Hamilton et al., 1977; Kieckhefer et al., 1980; and other unpublished SIO data.

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STRUCTURE OF THE SUNDA TRENCH

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3. Structure and Seismic Stratigraphy 3.1. TRENCH Strata seaward of the inner trench slope are underlain by a strong landwarddipping acoustic basement that is interpreted as the top of oceanic layer 2, based on acoustic character (strong diffraction hyperbolae and low-frequency reflectors) and seismic velocities of 5.8 km/sec, determined by explosive refraction (Kieckhefer et al., 1980). The basement reflector is irregullar, with relief of up to 0.5 seconds. This relief is interpreted as due to offsets along normal faults. Several well-developed normal faults offset the basement on line 3 (Figure 2). At shot Point Number (SPN) 2000 of Line 7 (Figures 3, 5), a major basement fault displaces the landward side up approximately 0.3 seconds relative to the seaward side. This fault may be either a seaward-dipping normal fault or a landwarddipping thrust fault similar to those described in the Peru-Chile Trench (Prince and Kulm, 1975) and in the Sunda Trench off Java (Curray et al., 1977). Overlying the acoustic basement in the trench is a landward-thickening wedge of sediment greater than 2 seconds thick at the base of the trench slope. The deepest reflectors dip landward, approximately parallel to acoustic basement. This lower unit exhibits subparallel to divergent reflection co~ffigurations (Mitchum et al., 1977), and shows characteristic non-systematic lateral reflection terminations. These non-systematic terminations are probably ,caused by lateral thinning of turbidite units below the resolution of our seismic system (Sangree and Widmier, 1978). Lateral frequency and amplitude variations are also common. The lower unit is interpreted as pelagic sediments and turbidites that were deposited both in the Nicobar Fan Complex on the Indian plate, seaward of the trench outer rise, and by lateral transport down the trench axis, as suggested by McDonald (1977) and Bowles et al. (1978). Overlying the Nicobar Fan Complex is a younger trench wedge which has a slight seaward surface slope. Younger strata generally progressively downlap the older sediments farther seaward. Bowles et al. (1978) attributed the seaward surface slope to sediment transport from the landward trench slope. The delineation of small morphologic fans emanating from the base of the landward slope that correspond to this seaward-dipping unit (Karig et al., in press) strengthens this inference. The Nicobar Fan Complex sediments show deformation by faulting, folding, and tilting. Normal faults, down toward Sumatra and the trench, are common in the Indian plate over the trench outer rise seaward of this area (McDonald, 1977). Displacement of this sense along normal faults generally decreases upsection, probably primarily due to the rapid increase in sedimentation rate as the section approaches the inner trench slope by convergence, and secondarily due to a decrease in the rate of faulting. An excellent example of upsection decrease in displacement is at SPN 2000 of Line 3 (Figure 2). The basement is

334

GREGORY

F. MOORE

AND

JOSEPH

R. CURRAY

offset 0.4 seconds (approximately 400 m) and the offset decreases to 0.145 seconds (approximately 110 m) at the surface (as shown by 3.5 kHz data). A horst and a graben are developed on Line 3 between SPN 1825 and 2050. A syncline occurs on Line 7 at SPN 2090 at 1.5 seconds below the seafloor, but dies out upsection.

3.2. LOWERMOST TRENCH SLOPE The oceanic crust reflector continues beneath the toe of the inner trench slope and up to 25 km landward, where it is lost in the water-bottom multiple. The seaward dip of the reflector on the time sections (Figures 2-5) is due to a velocity pullup caused by the overlying wedge of sediment. When corrected for velocity effects (Figure 6B), the reflector dips 6-7 ~ landward. A seafloor ramp with a seaward dip of approximately 12~ is developed at the base of the inner trench slope. On line 7 (Figure 6B), the top of this ramp is a bathymetric peak that is approximately 900 meters above the trench floor (near SPN 1800). Seaward of SPN 1900 (on Line 7), the surface dip decreases, and the surface and near-surface reflectors dip seaward at approximately 5~, while deeper reflectors dip landward nearly parallel to the underlying oceanic crustal reflector. Numerous diffractions obscure the structure below the bathymetric high on the stacked section (Figure 3); however, the structure is clarified on the migrated depth section (Figures 6B and 7), where seaward-dipping reflectors at depths between 5.7 and 7.3 km can be resolved between SPN 1750 and 1850. Reflectors are tilted into a seaward-dipping monocline just landward of the bathymetric high. S.P.N. 2000

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STRUCTURE OF THE SUNDA TRENCH

335

Between SPN 1670 and 1700 of Line 7 is a basin 5 km wide. This basin is underlain by more than 2.3 seconds of nearly fiat-lying sediments. The shallowest sediments ( < 0.1 sec) are probably slope deposits. Similarities in thickness and acoustic character to the trench sediment fill, however, and differences compared to the slope basin strata upslope led Karig et al. (in press) to interpret the deeper sediments as relatively undeformed, uplifted trench sediments. These sediments have been uplifted approximately 700 meters abow~' the level of the trench floor. A similar interpretation is suggested for the secliments between SPN 900 and 1030 of Line 3 (Figure 2) and between SPN 850 and 900 of Line 9 (Figure 4). Beneath the uplifted trench sediments on Line 7 is a one-second thick zone of faint landward-dipping reflectors above the descending oceanic crust reflector. Theses reflectors that are not resolved on the single-channel analog records discussed by Karig et al. (in press) may represent a zone of thrustthickened strata (see below).

3.3.

MIDDLE

TRENCH SLOPE

Several more morphologic ridges are observed landward of the lowermost ridge. Our multichannel lines are too widely spaced to correlate topographic features; however, our detailed bathymetric survey straddles Lines 5 and 7 (Karig et al., in press) and demonstrates that the ridges are nearly parallel to the trend of the arc. There is some indication of slope sediments ponded behind these structural highs, but thicknesses cannot be resolved on our profiles. Landward of the uplifted sediments on Line 9 (Figure 4) is a well-defined anticline at least 8 kilometers wide (SPN 1050 to 1200) and 1.5 ,seconds thick. An anticlinal structure may also be present between SPN 1380 and 1610 on Line 7 (Figures 3, 5), but it is less clear. These coherent sediments are also interpreted as uplifted and accreted trench strata. Beneath the anticline on Line 9, a group of landward-dipping reflectors at 8.5 seconds has a refraction velocity indicative of oceanic crust (Kieckhefer et al., 1980). The rest of the middle trench slope is almost devoid of coherent reflectors (Line 3, SPN 500-800; Line 7, SPN 90001350; Line 9, SPN 1250-1750). Seismic refraction measurements indicate that this part of the slope,, is underlain by material with velocities of 4.9km/sec (Kieckhefer et al., 1980). This wedge probably consists of strongly deformed slope deposits and accreted trench strata that are now acoustically chaotic. Between SPN 690 and 900 on Line 7 (Figures 3, 5, 6), is a landward-dipping zone of irregular reflectors at approximately 4 seconds. The reflectors extend landward about 10 km beneath the slope and dip at about 4-5 ~ (Figure 6B), and deeper reflectors dip more steeply than shallow reflectors. These reflectors are probably relatively undeformed accreted strata.

336

3.4.

GREGORY F. MOORE A N D J O S E P H R. CURRAY

UPPERMOST

TRENCH

SLOPE

On the uppermost trench slope, sediment-filled slope basins approximately 7-12 km wide are developed. The surface of the basin crossed by Line 3 (Figure 2) has a slight seaward slope and is broken by minor faults at SPN 380 and 435. The basin is filled by approximately 0.6 seconds of sediments. The sedimentary reflectors are discontinuous and exhibit large amplitude variations. The acoustic basement of the basin displays overlapping diffraction hyperbolae. Another small basin is developed between SPN 110 and 150. It has a seaward surface slope, a flat reflector at approximately 0.4 seconds subbottom, and a landwarddipping reflector at 0.5 to 0.8 seconds subbottom. The first reflector would dip landward, and the second would dip more steeply with application of a correction for the velocity effect of the surface slope. The surface of the basin in Line 7 is flat from SPN 365 to 560, but has a landward dip seaward of SPN 560. The basin is bounded seaward by a structural high floored by acoustic basement which is overlain by 0.35-0.5 seconds of sediments. T h e sedimentary fill in the flat part of the basin is greater than 1.1 seconds. A strong reflector at 0.2 seconds subsurface dips gently landward. Seaward of SPN 500, reflectors all dip landward, and young reflectors lap onto consistently older reflectors, defining many unconformities. Landward of SPN 500, reflectors do not dip as steeply, but the loss of acoustic coherence suggests that they are structurally disturbed at levels below 0.5 seconds. We believe that these landward dips are caused by differential uplift of the seaward margin of the basin, causing landward tilting of the sedimentary fill. An acoustic basement dips seaward under the landward flank of the basin. The landward slope into the basin is formed of irregular hummocky reflectors characteristic of slumping. Underlying the slope into the basin is a wedge of slope sediments which thins out seaward and overlies a strong reflector. Sediments lap onto the high amplitude reflector on this landward slope. On Line 9 (Figure 4), a small basin is developed between SPN 1970 and 2050. The sediment fill is thickest at the landward margin of the basin (approximately 0.75 sec). A larger basin was crossed between SPN 2150 and 2500. This basin has a seaward surface slope and only a small ( > 0 . 2 sec) section of stratified sediments. Acoustic basement dips landward and is formed mainly of numerous diffractions. The sediment fill is probably too highly deformed to return coherent reflections. Seismic refraction measurements (Kieckhefer et al., 1980) indicate that lowvelocity sediments (less than 2.8 km/sec) in the slope basins are approximately 1 km thick. Underlying these strata is a landward-thickening wedge of material approximately 16 km thick, with seismic velocity of approximately 4.9 km/sec. This wedge is interpreted to be composed of dewatered and deformed slope and accreted trench and Nicobar Fan sediments.

STRUCTURE OF THE SUNDA TRENCH

337

4. Discussion

We believe that the structure at the base of the lower trench slope can best be explained in terms of an imbricate thrust model (e.g., Seely et al., 1974). The structure is most clear on the migrated depth section (Figure 6B). The zone between SPN 1850 and 1950 on Line 7, where shallow reflectors are tilted seaward, may be a zone of incipient thrusting (Figure 7; Karig et al., in press). At this zone, movement along landward-dipping thrusts with minor displacement have probably caused the seaward tilting of shallow strata. The seaward-dipping monocline at a depth of 6 to 7 km between SPN 1750 and 1800 is probably caused by a major landward-dipping thrust fault, along which trench and Nicobar Fan strata have been stripped from the oceanic crust, uplifted approximately 700 m, and accreted to the base of the trench slope. Karig et al. (in press) believe that the dip of this basal thrust fault is approximately 20-30 ~ near the seafloor and flattens with depth to become nearly tangent to the descending plate. A similar speculative interpretation is shown in Figure 7. Beneath the uplifted trench and fan sequence is a 1.5 km thick zone above the oceanic crust. This zone is probably formed of thrust-thickened fan sediments. Our data do not allow us to confidently locate a basal d6collement, but we suspect that it occurs near the base of the thrust-thickened sequence (Figure 7). It thus appears that a large percentage of the sedimentary section that enters this subduction zone is scraped off the oceanic crust and accreted to the lower trench slope. Some sediments undoubtedly are subducted beyond the base of the trench slope, but they are probably added to the accretionary prism before they are carried beyond the outer-arc ridge. Calculations made by Karig et al. (in press) using seismic reflection and refraction data along with structural and age data from Nias indicate that the volume of the accretionary prism from Nias to the trench is consistent with this interpretation. The oceanic crust does not appear to be involved in the offscraping deformation at the toe of the slope, although it is faulted at the outer trench slope. The landward-dipping reflectors on the upper slope (between SPN 700 and 900 on Line 7; Figure 6B) probably represent some form of imbrication on the middle trench slope. They are similar to the landward-dipping midslope reflectors observed on seismic reflection profiles across the Middle America Trench slope that have been interpreted as evidence for structural imbrication (Seely et al., 1974; Ibrahim et al., 1979; Shipley et al., 1980). The reflectors may be analogous to the landward-dipping bedding surfaces of uplifted trench strata drilled on the northern Middle America Trench slope (DSDP Leg 66, J. C. Moore et al., 1979). An alternative, but less likely, interpretation is that the landwarddipping reflectors represent slope basin deposits that have been overthrust by a basin higher on the slope. We do not believe that this imbricate structure, nor the detailed submarine morphology of the inner trench slope (see Karig et al., in press), is consistent with a gravity slide origin (e.g., von Huene, 1972).

338

GREGORY F. MOORE AND JOSEPH R. CURRAY

Unlike the Middle America and eastern Aleutian Trenches (Shipley et al., 1980; von Huene, 1979), there is no clear slope sediment apron on the trench slope west of Nias. Most of the slope sediments appear to be ponded in well-developed slope basins on the upper slope. The numerous angular unconformities in the large slope basin on Line 7 (Figure 5) provide further evidence for the hypothesis that sedimentation and deformation in trench-slope basins is nearly synchronous (Seely et al., 1974; G. F. Moore and Karig, 1976). The existence of a landward-tilted reflector only a few hundred meters below the surface of the basin and the strongly uplifted seaward flank of the basin attest to the youthfulness of deformation of this slope basin, and indicate that deformation on the lower trench slope is continuous almost to the trench slope break.

Acknowledgements We wish to acknowledge the contribution made by P. J. Crampton and the SIO Shipboard Geophysical Group, and J. L. Abbot and the Shipboard Computer Group in establishing a working multichannel seismic system at SIO, and in collecting the data at sea. Initial data processing was made possible by the efforts of P. C. Henkart. We are grateful to Cecil and Ida Green and members of the Scripps Industrial Associates, who made financial contributions for obtaining data acquisition and processing equipment, and to Exxon Production Research Company, who provided the basic field acquisition system. This research was supported by NSF/IDOE Grant OCE76-24101. Dr Fred Hehuwat, Director of the Indonesian National Institute of Geology and Mining and Dr G. G. Shor, Jr., of SIO were instrumental in organizing and planning the data acquisition program. The manuscript was improved by critical reviews by M. A. Arthur, J. C. Moore, C. Mrozowski, D. R. Seely, T. H. Shipley, and R. von Huene.

References Beck, R. H., and Lehner, P.: 1974, 'Oceans, New Frontier in Exploration', Amer. Assoc. Petrol. Geol. Bull. 58, 376-395. Bowles, F. A., Ruddiman, W. F., and Jahn, W. H.: 1978, 'Acoustic Stratigraphy, Structure, and Depositional History of the Nicobar Fan, Eastern Indian Ocean', Marine Geology 26, 269-288. Carson, B., Yuan, J., Myers, P. B., and Barnard, W. D.: 1974, 'Initial Deep-Sea Sediment Deformation at the Base of the Washington Continental Slope: A Response to Subduction', Geology 2, 561-564. CCOP-IOC: 1974, 'Metallogenesis, Hydrocarbons and Tectonic Patterns in Eastern Asia', U. N. Development Program (CCOP), Bangkok, 158 pp. Curray, J. R. and Moore, D. G.: 1974, 'Sedimentary and Tectonic Processes in the Bengal Deep-Sea Fan and Geosyncline'. In Burk, C. A. and Drake, C. L. (eds.), The Geology of Continental Margins, Springer-Verlag, New York, 617--627.

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