1994-structure and morphotectonics of the accretionary prism along the e sunda-w banda arc_van der...

Pergamon Journal of Southeast Asian Earth Sciences, Vol. I I. No. 4, pp. 309-322, 1995

~ 1995 Elsevier Science Ltd 0743-9547(94)00038-7 Printed in Great Britain. All fights reserved

0743-9547/95 $9.50 + 0.00

Structure and morphotectonics of the accretionary prism along the Eastern Sunda-Western Banda Arc

W. van der Werff Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands

(Received 30 May 1994; accepted for publication 13 October 1994)

Abstract--Seismic reflection profiles across the accretionary prism along the Eastern Sunda-Western Banda Arc reveal variation in structure that relates to the incipient collision with Australia. The morphology of the arc-trench system changes from ridged, south of Bali, Lombok and Sumbawa, to sloped south of Sumba. East of Sumba, the accretionary wedge is backthrust over the forearc basin, incorporating forearc sediments and basement. Frontal accretion in the Java Trench is characterized by imbricate thrusting of a "thin" pelagic sediment cover at the toe of the accretionary wedge. The morphology near the toe of the wedge appears to be controlled by faults in the subducting oceanic plate that strike parallel to the deformation front and may have originated in the outer-trench swell. The prism incorporates oceanic basement by trenchward verging thrusts, which cut across the accretionary prism and extend into the subducting slab. A comparison between the volume of the accretionary prism and the amount of sediment delivered to the Java Trench in the past 30 Ma shows that probably little of the sediment has been subducted. The decrease in width of the prism from Bali to Sumbawa corresponds to an eastward younging trend of the arc-trench system from late Oligocene to early Miocene. South of Sumba the width of the accretionary prism increases considerably, due to the accretion of thick continental margin carbonates which deform by thrust-bounded folds. Buoyancy of the partially subducted marginal Scott plateau increases basal shear stresses, adding to the growth of a large accretionary wedge. Further east, the subduction of thick continental crust results in even higher basal shear stresses that are distributed throughout the accretionary wedge. They cause the progressive development of backthrusts and internal deformation, leading to shortening and thickening of the wedge.

Introduction

The Eastern Sunda-Western Banda Arc (Fig. 1) represents an active margin that is affected by an oblique collision involving the north-western margin of Aus- tralia. It has been widely recognized as an actualistic model for older arc-continent collision zones (Hamilton, 1979; Von der Borch, 1979; Audley-Charles, 1986; Price and Audley-Charles, 1987). Major controversial issues originally existed regarding the location of the surface trace of the plate boundary and the provenance and emplacement mechanisms of the major tectonic units on Timor (Audley-Charles, 1968; Barber et al., 1977; Chamalaun and Grady, 1978; Hamilton, 1979). One group of workers suggested that the surface trace is situated to the north of Timor (Audley-Charles, 1986; Price and Audley-Charles, 1987). They considered the Timor Trough as a foreland basin, developed entirely within the Australian craton. Recent studies indicate, however, that the zone of plate contact and major compressional deformation that lies along the Java Trench continues directly eastward into the Timor Trough (Masson et al., 1991). The Banda collision zone west of Timor is, in its present stage of evolution, still very similar to a normally subducting arc-trench system (Karig et al., 1987). An integration of both geological and geophysical data from the Banda orogen reveals variation in structural style with time (Johnston and Bowin, 1981). The collision complex represents a westward younging system where progressive deformation towards the east is increasingly absorbed away from the toe of the orogenic wedge (Harris, 1991).

Shortening of the Australian margin becomes parti- tioned between frontal accretion, subcretion and backthrusting.

Most recent studies focused on the style of sediment accretion along the Java-Timor trench and the across- arc extent of deformation on a local scale using both side scan sonar and seismic data (Reed, 1985; Breen et al., 1986; Karig et al., 1987; Masson et al., 1991). This study represents a regional synthesis. It discusses the variation in morphology and structure of the accretionary prism in relation to the time of convergent margin initiation, the nature of the subducting plate, and the type and thickness of sediment on the subducting plate. In addition, the effects of the introduction of continental crust into the subduction zone are discussed. A first attempt is made to quantify processes such as wedge growth and sediment subduction vs sediment accretion.

To approach these problems, the accretionary prism in the Eastern Sunda-Western Banda Arc has been mapped between 114 ° and 122°30'E (Fig. 2) using single- channel seismic profiles recorded during the Indone- sian-Dutch Snellius-II expedition (Jongsma 1986; Van Weering 1986; Van Weering et al., 1989; Jongsma et al., 1989a,b). These data were complemented by single- channel data of the Rama 12 expedition (1982), supplied by the Scripps Institute of Oceanography, and multi- channel profiles, supplied by Shell and the Geological Survey of Japan (Fig. 2A). In general, multi-channel data reveal the deeper structures of the accretionary prism, while single-channel data only show the near-surface character. There are no well data of the accretionary prism available to us. DSDP Site 261, however, gives

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information about the nature of the sediments and crust of the subducting slab.

Regional Setting

The Sunda-Banda Arc, which forms the southern margin of the Indonesian Archipelago from Sumatra to western Irian Jaya, is the zone where the Indian-Australian plate is subducted underneath the south-east Asian plate (Fig. 1).

The Western Sunda Arc is characterized by thick ocean floor and trench-fill sediments that are dominated by detritus from the Himalayan mountains and trans- ported southward by turbidity currents over a distance of 3000km to the coast of S.E. Java (Ingersoll and Suzcek, 1979; Moore et al., 1982). Accretion of these sediments led to the construction of a wide accretionary prism and a high outer-arc ridge that is backthrust over the Sumatra forearc region (Curray and Moore, 1974; Hamilton, 1979; Karig et al., 1980; Moore and Curray, 1980; Silver and Reed, 1988). South of Java, a major accretionary prism developed, which deformed the outer forearc basin strata by tilting and folding (Bolliger and De Ruiter, 1975; unpublished data, MGI).

The Eastern Sunda arc occupies the western extension of the collision zone between Australia and eastern Indonesia that started to form 3-5 Ma ago (Johnston and Bowin, 1981; Harris, 1991). East of Sumba, this collision resulted in backthrusting of the accretionary prism over the forearc basin (Reed, 1985; Reed et al., 1986). An earlier collision may have occurred in the late Miocene (10 Ma), when a marginal plateau collided with the Western Banda Arc (Reed, 1985). South of central Sumba, the present transition from subduction to collision is marked by the oblique intersection of the continent-ocean boundary of Australia with the Java Trench at NI20°I0'E (Breen et al., 1986). This boundary

trends at a high angle to the trench axis and separates the rifted continental crust of the Scott Plateau from the late Jurassic oceanic crust of the Argo Abyssal Plain (Stagg, 1978; Ludden and Gradstein, 1990). Where continental crust has been subducted beneath the inner- trench slope, the outer-arc ridge has been lifted up above sea level. This resulted in the formation of the islands of Savu, Roti and Timor.

Subduction along Sumatra, Java and Bali has been taking place since the late Oligocene (Hamilton, 1979). The Eastern Sunda Arc from Sumbawa to Central Flores was initiated in the early Miocene (19-21 Ma), and is younging towards Atauro and Wetar (Van Bemmelen, 1949; Abbott and Chamalaun, 1981; Nishimura et al., 1981). The rate of plate convergence has been estimated at 5 cm/a between 30 Ma and 10 Ma. After 10Ma, the convergence increased to 7cm/a (Karig et al., 1980; Liu, 1983; Beaudry, 1983; Curray, 1989).

The trench bottom is flat in the Western Sunda arc and V-shaped in the central and Eastern Sunda arc region (Ganie et al., 1987), suggesting an absence of trench-fill deposits in the east (Van Weering et al., 1989).

Side-scan images of the outer-trench slope of the eastern Java Trench show a pattern of normal faults (Masson et al., 1990). Collision of seamounts with the accretionary prism resulted in local steepening of the inner-trench slope and slumping of sediments into the trench (Masson et al., 1990). In the Java Trench, sidescan sonar images indicate that sediment accretion occurs by uniform, small-scale folding. South of Sumba, deformation on the inner-trench slope is concentrated within 15-25 km from the thrust front (Breen et al., 1986). In this area, thicker sedimentary sequences deform into thrust-bounded folds. Mud diapirs which rise in front of the deformation front indicate high pore fluid pressures (Reed, 1985; Masson et al., 1991). South of West Timor, high angle reverse faults offset trench

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turbidites and thick continental margin strata at the base of the inner-trench slope (Karig et al., 1987).

Results

The western segment of the Eastern Sunda-Western Banda Arc between 114 ° and 118°30'E is formed by an intra-oceanic-volcanic arc system that can be classified as a broad ridged forearc (Dickinson and Seely, 1979). Towards the east between 118°30 ' and 120°30'E, the arc-trench system is affected by the collision of the Scott Plateau and can be described as a sloped forearc.

Between 120°30 , and 122°30'E, the forearc collides with the partially subducting Australian continental slope and margin. The accretionary prism has been thrust over the forearc basement (Fig. 2B).

1. Intra oceanic-volcanic arc (114-118°30'E)

South of the Java Trench, an outer-trench swell rises from the abyssal plain, and bends steeply into the subduction zone. It displays typical horst and graben structures that indicate extension of the upper part of the oceanic lithosphere. This is caused by flexural down- bending of the subducting slab into the trench (Fig. 3;

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Section 1). The oceanic basement locally has a rugged topography (Fig. 4). It is covered by 0.4 sec TWT of normally faulted sediments, which display two major seismic units, a lower unit (1), mostly characterized by high amplitude reflections, and an upper unit (2), with a transparent facies. The oceanic basement is of late Jurassic age and is covered by Cretaceous claystones, Upper Miocene and Pliocene nannofossil oozes and Quaternary radiolarian clays as drilled in DSDP Site 261 (Heirtzler et al., 1974; Hinz et al., 1978).

The Java Trench here has a depth of 6800 m and is largely devoid of sediments. Locally, sediments that were originally deposited on the outer-trench slope have slumped into the trench as a consequence of slope steepening and normal faulting as the outer-trench slope moves towards the subduction zone (Fig. 3; Section 1). A thin layer of trench-fill sediments is present in the eastern Java Trench (Fig. 5; 4).

The accretionary wedge has a smoothly tapered geometry of which the inner-trench slope increases slightly towards the toe (Fig. 6; Section 1). The wedge is 100 km wide south of Bali and Lombok and decreases to about 70 km south of Sumbawa. The accretionary prism at- tains a minimum thickness of 8 km close to the trench- slope break (defined at the location of major change in trench-slope inclination).

The deformation front is located at the position where oceanic basement is underthrust beneath the accretionary prism. The top of the subducting oceanic plate can be traced for 70 km underneath the wedge dipping towards the north at an angle of 4 ° (Fig. 6; Section 1). The inner-trench slope covers a relief of 4 km and has a

relatively low slope angle of 3 ° . Fracturing of the oceanic basement below the toe of the accretionary wedge resulted in small changes in slope angle (Fig. 6; Section 1). The rupture of the oceanic crust is indicated by a "broken" high amplitude reflector which forms the top of the oceanic basement (Fig. 6; Section 1). The inner- trench slope is characterized by a seismic amorphous or hyperbolic character (Figs 3-5). Where imbricated thrust segments crop out, slope basins have developed which contain little or no sediments. In the eastern Lombok Ridge, many trench slope basins are present at the lower inner-trench slope (Fig. 5; Section 1).

The trench-slope break is flanked in the north by a major slope basin (Fig. 3; Section 2). This basin forms a regional structural feature that can be traced along the top of the Lombok Ridge. The slope sediments on the prism have a thickness of 0-0.5 sec TWT and up to 1 sec TWT in the slope basin. In the slope basin, the sediments have a seismic facies characterized by parallel to subpar- allel reflectors with lateral variation in reflectivity. These reflectors dip towards the south and reflect differential uplift of the northern "wedge high" (defined as that part of the outer-arc high located to the north of the slope basin). The seismic facies in the deeper part of the basin is blurred by acoustic voids. The contact between the slope basin and the wedge high is along an E-W trending north dipping reverse fault. In the eastern part of the slope basin, the seismic character of the sediments has been completely disturbed (Fig. 5; 6), probably the consequence of an increased eastward compression along the accretionary wedge, due to the nearby collision of the Scott Plateau south of Sumba.


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illustrating the compressive character between the accretionary prism and the forearc basin.

The transition from the accretionary wedge to the forearc basin is marked by the uplift and tilt of the southern forearc basement and basin fill deposits, and the presence of a mud-volcano located north of the eastern Lombok Ridge (Fig. 5; 7).

2. Transitional segment (118030 '- 120°30'E)

The subducting plate is covered by sediments with a thickness that increases from 0.5 sec TWT in the west, to 2 sec TWT in the east. These sediments can be divided into two units which are both characterized by a seismic facies of even-bedded, low amplitude reflectors and separated by a band of high amplitude reflectors (Fig. 7; Section 2). The location of the Scott Plateau roughly coincides with the 4000 m depth contour (Fig. 2B). Along the outer-trench slope, the basement of the Scott Plateau is downfaulted into the Timor Trough by a steep north-dipping fault with an offset of more than 0.8 sec TWT (Fig. 7; Section 2).

The accretionary prism has a ridged taper with a convex morphology and is 70 km wide in the west and l l 0km in the east. At 118°30'E, a north-trending strike-slip fault is interpreted which offsets the eastern part of the accretionary wedge to the north (Fig. 2B).

The deformation front is here the location where the sediments of the subducting plate actually are deformed, uplifted and incorporated at the base of the accretionary prism. Frontal thrusts advance irregularly across the

Timor Trough by a mechanism of thrust-bounded folding. On profile Pac 104, the decollement is formed by the top of the subducting oceanic basement (Fig. 8). It can be traced for a distance of about 10km, extending almost horizontally below the accretionary prism. The basement is broken by a number of steep north-dipping reverse faults. At the base of the inner-trench slope, several accreted units rotate towards steeper inclinations as they move up the inner-trench slope (Fig. 8).

The inner-trench slope ranges from about 5 km depth in the Timor Trough to 2 km at the trench-slope break and has a slope angle of less than 2 °. A mid slope terrace at a depth of 3700 m (Fig. 7; Section 1) is covered by slope sediments with a minimum thickness of 0.5 sec TWT. The seismic character of these sediments suggests increasing deformation with depth. Due to this deformation, the sediments have largely lost their original seismic signature. North of the mid slope high, the inner-trench slope continues as deformed and folded slope sediments similar to that of the mid slope terrace.

The northern part of the outer-arc high is formed by a block, 25 km wide, which has been thrust underneath the "Sumba" forearc basement.

3. Arc-continent collision (120°30'-122°30'E)

The subducting plate is covered by sediments with a minimum thickness of 2 sec TWT. The top of the basement correlates with a band of discontinuous high

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amplitude reflectors and consists of Late Triassic flit blocks (Stagg and Exon, 1981; Reed, 1985) (Fig. 9A). Two seismic facies units characterize the sedimentary sequence. The lower unit (1) has a thickness of 1 sec TWT and is composed of a transparent facies. The upper unit (2), which has the same thickness, is composed of continuous north-dipping high amplitude reflectors.

The accretionary wedge south-east of Sumba, south of Savu, and west of Roti forms a southward bulge (Fig. 2B). The wedge is asymmetric and has a maximum width of 140 km. South of Roti, the prism thins and is 60-80 km wide. Its taper is ridged at the lower inner- trench slope and smooth at the upper part (Fig. 9A). The wedge has a relief of 4 km, which steepens towards the toe. In the north, four backthrusts are directed over the "Sumba" basement (Fig. 9B).

A frontal thrust is poorly defined at the toe of the wedge and may not extend to the seafloor (Fig. 9A). The decollement level has been observed at the facies transition between units (1) and (2). It occurs near the transition from upper Cretaceous shallow marine clastics to overlying deep marine fine grained carbonates (Stagg and Exon, 1981). Apparently, the impermeable cap of fine-grained carbonates allows for high pore fluid pressures in the clastic strata controlling the position of the decollement. The decollement can be traced for a distance of 25 km underneath the accretionary prism and is converging at a low angle with the basement. Thinning of the subducted unit may be due to water loss caused by the pressure of the weight of the accretionary prism. Post-breakup strata are incorporated at the front of the accretionary prism. A zone of broken reflectors with a thickness of 0.4 sec TWT and a width of 8.5 km in front of the deformation front may have resulted from exces- sive pore fluid pressures. This zone possibly evolves into a future thrust fault and corresponds to a region where mud volcanoes and mud ridges developed (Breen et aL,

1986; Masson et al., 1991). The lack of deformation of the deeper reflectors suggests that mud diapirism does not extend beneath the level of decollement. Frontal sediment accretion occurs by a mechanism of thrust bounded folding (Fig. 9A).

The inner-trench slope is 80 km wide and has a surface slope that increases from 2 ° in the west to 4 ° in the east. It is composed of two mid-slope highs separated by a major slope basin which are barely covered by slope sediments (Fig. 9A).

The outer-arc high has a width of 60 km. The trench- slope break is formed by a north dipping thrust fault. North of this fault, four backthrusts are observed of which the most northern one corresponds to the Savu Thrust (Fig. 9B; backthrust 1). These thrusts are covered by deformed slope sediments with a maximum thickness of 900 m, characterized by high amplitude/low frequency reflectors. Tilting of the reflectors on top and in front of the backthrusts indicates differential movements along the thrusts. North of the Savu Thrust, the Sumba Ridge is downfaulted along steep, south-dipping normal faults. South of Sumba, the forearc basement itself has been incorporated into the thrusting (Van der Werff et al., 1994a their Fig. 9). Between the Savu Thrust and the Sumba Ridge, a topographic relief of 750 m extends over a distance of 30 km. To the east, the Savu Thrust has been thrust over the Sumba Ridge and is positioned at the same level. A horst structure located to the south of the thrust represents the offshore westward extension of the island of Rajuna (Fig. 10). To the east, the backthrusts converge and disappear, except for the most southern one.

Discussion

Accretionary prisms are sites of incipient mountain building where oceanic sediments and rocks are uplifted,


deformed and ultimately transformed to continental crust (Moore and Silver, 1987). They form by frontal tectonic accretion of sediments or rocks at the toe of the inner-trench slope (Seely et al., 1974; Karig and Shar- man, 1975), while basal accretion may occur by under- plating (Moore and Silver, 1987). Accretionary prisms may be accreted in a snowplow fashion against backstops of forearc basement origin, which represent the initial site of convergent margin initiation (Le Pichon et al., 1982; Hamilton, 1988). Backstops either dip towards the arc or towards the trench in various convergent margin systems (Silver and Reed, 1988; Hamilton, 1988; Fig. 11).

1. In tra ocean ic -vo lcan ic arc (114°00'-118°30'E)

Important differences between sediment starved intra- oceanic forearcs and clastic dominated continental margin systems include non-or episodic accretion and subduction erosion of the inner-trench slope, ac- companied by extension and subsidence of the outer-

forearc region (Lundberg, 1983; Hawkins et al., 1984). The late Jurassic age of the subducting Indian Ocean Plate (Heirtzler et al., 1974), the steeply dipping Benioff zone (McCaffrey, 1989), and the moderate convergence rate of 7 cm/a of the Indian Plate relative to the SE Asian Continent (De Mets et al., 1990), suggest that the inter-plate coupling between the two plates is relatively weak as compared to other active plate boundaries. The Argo Abyssal Plain has a pelagic sediment cover of only 600 m, and generally is considered sediment starved. In addition, the Java Trench is almost devoid of sediments (Van Weering et al., 1989; Masson et al., 1990). These arguments would support the assumption that south of Bali, Lombok and Sumbawa, little or no sediment has been accreted to the leading edge of the Asian Plate as suggested by Reed (1985). An example of other non-accretionary intra-oceanic arcs with a comparable tectonic configuration are the Mariana and Izu-Bonin Arcs (Fryer et al., 1990). These arc-trench systems are characterized by an inner-trench slope composed of volcanic rocks.

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the inner-trench slope.

The presence of a well developed accretionary prism located to the south of Bali, Lombok and Sumbawa illustrates, however, that sediment accretion can still be an important mechanism of accretionary prism formation along trenches that lack thick volcaniclastic turbidite deposits.

A comparison between the volume of the prism located to the south of Bali and the amount of material fed to the trench may provide more insights into the contri- bution of sediment accretion to accretionary prism development. Such a calculation is inherently dangerous because of the greatly extrapolated sedimentation and subduction rates• Van der Werff et al (1994b) concluded that forearc basement composed of thinned continental crust should extend for some distance into the accretionary prism. The minimum area supposed to be underlain by this basement is indicated in Fig. 6 by a stippled pattern.

We assume that the arc-trench system was initiated during the late Oligocene at approximately 30 Ma ago (Hamilton, 1988). If the oceanic plate subducted at a rate of 5 cm/a between 30 and 10 Ma and at 7 cm/a between 10 Ma and present (Liu, 1983), then about 1700 km of the lithosphere has been consumed at the convergent margin system. The Argo Abyssal Plain is covered by sediments that range in thickness from 300 m in the west to 1 km in the east (Heezen et al., 1977).

If we assume an average sediment thickness of 600 m on top of the subducting plate, a total volume of 220,000 km 3 of sediments has been delivered to the subduction zone since the late Oligocene.

A simple volume calculation for the accretionary prism between 114 and 116°E, using the wedge geometry in profile P-7 (Fig, 6) has been made. The wedge has a maximum thickness of 8 km below the trench-slope break, a length of 62.5 km between the trench-slope

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SEISMIC PROFILE PAC-109 (SECTION I)

N

DEFORMATION FRONT

I

~ " ~ " ~ ...--"~-. -.--A~-..--~.. ~ - - " - o 10KM V E 2.3 .... " " " "" "'"" '" "" '" " ""'"""r-l':'*~ .... ":"

• " - . . . . AUSTRALIAN CONTINENTAL CRUST .' . . ' " ". "."

T w r SEC

2

4

6

8

SEISMIC PROFILE PAC-109 (PART 2)

TRENCH-SLOPE BREAK SLOPE SEDIMENTS _ . ~ , ~ _ . _ . . . . . . ~ , ~ -, _ _

" - :~ ,BACKTHRUST4 ~ : ~ " ~ - ' - " " - - / ~ ~ - ,. ~,~I ~...~.~-.~..--Z--~-~_ ~ , - ~ ..~-.-- ~ - - -~ x, j.

BACKTHRUST 2 ..~ . .1.~v~.-'-/'\ ~..~'~ ~ : ' . . , ' / . . . - . " BACKTHRUST I .~ I / " SUMBA RIDGE

0 10 KM V.E. 2.3 I [

/

Fig. 9. (A) Line drawing and interpretation of part of seismic profile Pac 109 (Southeast of Sumba). The location is shown in Fig. 2. For legend see Fig. 3. The decollement coincides with the boundary between seismic facies units (l) and (2). Note that the decollement converges with the basement as it is progressively situated further underneath the accretionary wedge. (B) Line drawing and interpretation of part of seismic profile Pac 109 (East of Sumba). The location is shown in Fig. 2. For legend see Fig. 3. The forearc basement

(Sumba Ridge) appears to be downflexed by the load of the backthrusts.


S E I S M I C P R O F I L E SN-521 S N

1 7 0 0 2 8 0 0

T W T . . . . . . . 'ARC BASI~ SEC ~ FORE

F ~ ('~' F I

6 I H O R S T BACKTHRUST I i

V.E. 2.6 0 10 KM I j

Fig. 10. Line drawing and interpretation of seismic profile N-521 (West of Savu). The location is shown in Fig. 2. For legend see Fig. 3. The horst structure possibly represents the westward extension of the island of

Rajuna.

break and the deformation front, and a width of 222.36 km (120 NM) between 114 and 116°E. The outer part of the wedge between the deformation front, the trench-slope break and the subducting oceanic slab can be approximated as a triangle. Its volume = 0.5 x max. thickness x length x width = 55,600 km 3. The volume of the remaining part of the wedge can be approximated by assuming that the oceanic basement remains parallel to the surface of the wedge, from the trench-slope break to the forearc basement. This results in an additional volume of: thickness (8 km) x length (64 km) x width (222.36 km)= 113,800 km 3. The prism has a total volume of 169,400 km 3.

Sediment that enters a subduction zone generally contains about 50% water, while uplifted exposed sequences have porosities of less than 10% (Bray and Karig, 1985). DSDP Site 261 recovered a core with a total thickness of 532m. The cored sediments are characterized by average porosities of 50% up to a depth of 370 m and decrease to 25% at deeper levels (Heirtzler et al., 1974). DSDP Site 765, located in the south of the Argo Abyssal Plain contains sediments with a porosity ranging from 75% at the shallow depth to 50% between 400 and 850 m (Ludden et al., 1990).

Studies on the density and porosity of accretionary

prisms indicate a gradual increase in density and decrease in porosity away from the deformation front (Bray and Karig, 1985; Fowler et al., 1985; Moore et al., 1988). We can approach the change in volume caused by the densification and dewatering assuming the following boundary conditions: the accreted sediments did not experience any reduction in porosity, and most of the sediments experienced a maximum reduction in porosity to 10% shortly after accretion.

Without porosity reduction, an excess volume of approximately 50,000 km 3 has been delivered to the trench and must have been subducted. With porosity reduction, the volume of solid grains is preserved. The initial proportion of solid grains is 50%, while the final proportion is 90%. So the final volume = 5/9 the initial volume. The initial volume of accreted sediments, incorporated into the accretionary prism then has been about 305,000 km 3. A comparison between the calculated values shows that the volume of the accretionary prism is 1.4 times the volume of all the sediments delivered to the Java Trench. The original volume of the accreted sediments thus will be in between the 169,400 and 305,000 km 3 and probably accommo- dates most of the sediments (220,000 km 3) delivered to the trench.

A

0

.zlO-

m~2 0-

3 0 -

4 0 -

MELANGE ARC-WARD DIPPING BACKSTOP

A m ,

---," ~ " ~ . . . . . . ~ . . . . . : .7

B

0

Z I 0 " ~

~. 2 0 -

830-

0 50 KM I [

M E L A N G E T R E N C H - W A R D DIPPING BACKSTOP

- - - . . -._ ~ .....'.'.--:'. " . . . ' ; "'.......'.....

0 50 KM I I

Fig. 11. (A) Profile across an accretionary wedge with an arc-ward dipping backstop (adopted from Hamilton, 1979). (B) Profile across an accretionary wedge with a trench-ward dipping backstop (adopted from Silver

and Reed, 1988). S E A E S H 4- E


Processes other than sediment accretion at the toe of the wedge also have added to the construction of the accretionary prism. Seismic profile P-7 indicates that segments of the subducting oceanic plate are incorporated at the base of the accretionary prism by imbricate thrusting (Fig. 6). Imbricate thrusting of oceanic basement may have occurred along pre-existing fault zones that originated on the outer-trench slope by flexural down bending. Near the crest of the outer-arc high, magnetic anomalies indicate shallow depths to basement and suggest that portions of the subducting oceanic crust also are incorporated further north underneath the accretionary prism (Beck and Lehner, 1974).

The decrease in width of the prism, from 100 km south of Bali to 70 km south of Sumbawa, may reflect an eastward younging trend of the arc-trench system from late Oligocene (30 Ma) to early Miocene (19-21 Ma). In that case, the average rate of wedge growth remains fairly constant along the prism at a rate of 3.0-3.5 km/Ma. This rate is relatively low in comparison with other active margins such as the Mediterranean (5-20km/Ma), Makran (7.5km/ma), and Barbados Ridges (5 km/Ma), but conforms to those of Peru, the Middle America Trench and the Nankai Trough (Kas- tens, 1991). Along the Western Sunda Arc off Nias, the wedge growth is significantly higher (5 km/Ma) and compatible with the amount of material fed to the trench-slope by plate convergence (Karig et al., 1980; Kastens, 1991).

The regional geometry of the unique slope basin on top of the ridge suggest that its formation is related to a fundamental tectonic mechanism. The basin may represent the initial site of trench formation. If so, the wedge high may represent a backstop composed of forearc basement. The wedge high, however, is not characterized by a seismic facies distinct from the inner-trench slope or trench-slope break, and therefore it is likely that it has a similar composition (Figs 3 and 5). The opaque seismic facies of the wedge high probably results from progressive deformation and dewatering of accreted sediments and metamorphic processes which alter the composition and density of the rocks.

On the basis of the presently available geophysical data, it is not possible to clarify whether the backstop underlying the accreted sediments is dipping arc- or trenchward. The uplift and northward tilt of the forearc basin strata located at the inner side of the accretionary prism (Van der Werff et al., 1994b), and the inferred presence of oceanic basement near the crest of the outer-arc high (Beck and Lehner, 1974) may be sugges- tive for an arcward dipping backstop. The absence of arcward verging thrusts along the inner side of the wedge, commonly observed along forearcs with trenchward dipping backstops, may add to this suggestion although it is not diagnostic (Westbrook, 1982; Silver and Reed, 1988).

Seismic refraction data, on the other hand, suggest that most of the ridge is underlain by accreted sedimentary rocks and basement with velocities that range from 1.6 to 6.2km/s (Curray et al., 1977). If part of the wedge is underlain by rifted continental crust (Van der Werff et al., 1994b), it appears more likely that the trench originally was initiated along the continent-ocean boundary and that the accretionary prism is underlain by a trenchward dipping backstop.

2. Transi t ional segmen t (118°30'-120°30'E)

The outer-trench swell morphology is determined by the geometry of the Scott Plateau. The steep outer- trench slope is formed by pre-existing basement faults which are reactivated as the plateau moves towards the subduction zone (Fig. 7; Section 2).

The transition in frontal accretion from small-scale folding and imbricate thrusting to large-scale thrust- bounded folding reflects a change in lithology and physical properties of the subducting sediments from thin weak pelagic oozes and clays to thicker more competent continental margin carbonates (Heirtzler et al., 1974; Hinz et al., 1978; Von Rad and Exon, 1982; Reed, 1985; Breen et al., 1986).

The increase in width of the accretionary prism by 40km between 119°20 ' and 120°30'E relates to an eastward increase in sediment thickness on top of the subducting plate. The subduction of buoyant continental crust belonging to the Scott marginal plateau resulted in an increased shear stress at the base of the prism, adding to the process of wedge growth. Assuming that the growth of the southward bulge of the accretionary prism is related to the arc--continent collision which started 5 Ma ago, the wedge growth is estimated at 8 km/Ma. This growth contrasts with the relatively slow average of 3.0-3.5 km/Ma of the accretionary prism in the intra- oceanic-volcanic arc segment.

A similar transition in the structure of the accretionary prism has been described along the southern part of the Lesser Antilles Arc (Westbrook, 1982; Biju-Duval et al., 1982; Speed, 1985). In this region, thin distal turbidites of South American origin have been deposited in the northern trench floor and are incorporated into a slightly tapered wedge characterized by abundant mud volcanoes (Westbrook and Smith, 1983). In the south, thick deposits of proximal turbidites derived from the Orinoco river delta are deformed into a thick accretionary prism characterized by gently asymmetric folds riding on westward dipping thrusts.

3. A r c - c o n t i n e n t collision (120°30'-122°30'E)

The maximum width of the accretionary prism coincides with the area where backthrusts develop. The increase in width of the prism, as it grows, increases the force exerted to the overriding plate because of an increase in surface area over which shear stress at its base is applied. Also, as the wedge thickens, the shear stress increases because of the increase in normal stress. In addition, the subduction of progressively thicker buoyant continental crust into the Timor Trough results in increased basal shear stress. These three factors increase the total force imparted at the arcward site of the wedge and are responsible for the initiation of the backthrusts.

The subsequent stacking of the backthrusts from west to east as they advance over the Sumba Ridge suggests an increase in eastward deformation, and coincides with the subduction of the Australian continental slope and margin. The eastward decrease in width of the inner- trench slope and increase in surface slope from less than 2-4 ° suggests that part of the plate convergence is taken up by distributed internal deformation of the prism. This results in thickening and uplift of the accretionary wedge.


Conclusions

The Lombok Ridge has been growing wider since the late Oligocene at an average rate of 3.0-3.5 km/Ma. The total volume of sediments that subducted into the Java Trench has probably been incorporated into the accre- t ionary prism. The eastward th inn ing of the prism corresponds to the decrease in age of the a rc- t rench system from 30 Ma south of Bali to 21 Ma south of Sumbawa.

South of Sumba, the presence of thick cont inenta l margin deposits and the collision of the a rc- t rench system with the Scott Margina l Plateau resulted in a rapid southward growth of the accret ionary prism by 8 km/Ma. The style of sediment accretion changed from small-scale folding and imbricate thrust ing to larger scale th rus t -bound folding and reflects the t ransi t ion from pelagic sediments towards more competent continental margin carbonates.

East of Sumba, the subduct ion of cont inenta l crust of the Aust ra l ian slope and margin resulted in an increased interplate coupling and basal shear stress between the subduct ing plate and the accret ionary prism. This is taken up by backthrus t ing and distr ibuted internal de- format ion, leading to a progressive eastward thickening of the accret ionary prism.

Acknowledgements - - I would like to thank T. C. E. van Weering (Netherlands Institute for Sea Research), J. E. van Hinte, A. R. Fortuin (Free University of Amsterdam) for critically reviewing this manuscript and providing suggestions for improvement. Bert Aggenbach is acknowledged for his assistance in the production of the figures. I thank the Geologi- cal Survey of Japan, the Shell Petroleum Corporation and the Marine Geology Institute of Bandung (Indonesia) for providing multi-channel data. Scripps Institute of Oceanography is thanked for the use of single-channel Rama-12 profiles. This study was supported by the Netherlands Marine Science Foundation (SOZ) in the Hague. This paper benefited from the additional comments of two anonymous reviewers.

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1994-structure and morphotectonics of the accretionary prism along the e sunda-w banda arc_van der...

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