TECTONICS, VOL. 21, NO. 5, 1047, doi:10.1029/2001TC901048, 2002

Tectonic features of the southern Sumatra-western Java forearc of Indonesia

H. U. Schlu¨ter, C. Gaedicke, H. A. Roeser, B. Schreckenberger, H. Meyer, and C. Reichert Bundesanstalt fu¨r Geowissenschaften und Rohstoffe (BGR), Hannover, Germany

Y. Djajadihardja Agency for the Assessment and Application of Technology (BPPT), Jakarta, Indonesia

A. Prexl Veritas DGC Ltd., Crawley, UK Received 23 November 2001; revised 19 April 2002; accepted 28 May 2002; published 12 October 2002.

[1] Multichannel reflection seismic profiles along the Earth: Plate boundary—general (3040); 8158 Tectonophysics: active Sunda Arc, where the Indo-Australian plate Evolution of the Earth: Plate motions—present and recent (3040); subducts under the overriding Eurasian margin 9320 Information Related to Geographic Region: Asia; KEYWORDS: revealed two accretionary wedges: The inner wedge I Indonesia, Sunda forearc, subduction, sediments, seismics, geo- is of assumed Paleogene age, and the outer wedge II is dynamics. Citation: Schlu¨ter H. U., C. Gaedicke, H. A. Roeser, of Neogene to Recent age. The inner wedge I is B. Schreckenberger, H. Meyer, C. Reichert, Y. Djajadihardja, and composed of tectonic flakes stretching from southeast A. Prexl, Tectonic features of the southern Sumatra-western Java forearc of Indonesia, Tectonics, 21(5), 1047, doi:10.1029/ Sumatra across the Sunda Strait to northwest Java, 2001TC901048, 2002. implying a similar plate tectonic regime in these areas at the time of flake development during upper Oligocene. 1. Introduction Today, wedge I forms the outer arc high and the backstop for the younger outer wedge II. The missing [2] New geophysical and bathymetric data were collected outer arc high of the southern Sunda Strait is explained in 1998 with the German R/V Sonne during cruise SO 137 by a combination of Neogene transtension due to a in the frame of the German-Indonesian cooperative project clockwise rotation of Sumatra with respect to Java and GINCO I (Geoscientific Investigations on the Active Con- by arc-parallel strike-slip movements. The rotation vergence zone between the East-Eurasian and Indo-Austral- ian plates off Indonesia). The investigations were carried created transtensional pull-apart basins along the out in the Indian Ocean offshore southeast Sumatra, the western Sunda Strait (Semangka Graben) as opposed Sunda Strait and off northwest Java (Figure 1). Field to transpression and inversion on the eastern Sunda operations were conducted in close cooperation with the Strait, within the new detected Krakatau Basin. The arc- Agency for the Assessment and Application of Technology parallel transpressional Mentawai strike-slip fault zone (BPPT), Jakarta. There was also a close cooperation with (MFZ) was correlated from the Sumatra forearc basin to the German Research Centre for Marine Geosciences the northwest Java forearc basin. Off the Sunda Strait, (GEOMAR), Kiel which applied seismic refraction and northward bending branches of the MFZ are connected wide-angle methods in the same area during the subsequent with the right-lateral Sumatra fault zone (SFZ) along cruise SO-138 [Kopp et al., 2001, 2002]. the volcanic arc segment on Sumatra. It is speculated 1.1. Scientific Objectives that the SFZ was attached to the Cimandiri-Pelabuhan Ratu strike-slip fault of Java prior to the presumed [3] The investigated area covers the active plate boun- rotation of Sumatra, and that since the late lower dary where the Indo-Australian plate subducts below the Miocene the main slip movement shifted from the eastern Eurasian plate. Their zone of interference is asso- volcanic arc position to the forearc basin area due to ciated with strong volcanism (Krakatau eruption 1883, Toba explosion 73,000 years B. P.) and earthquake activity, increasingly oblique plate convergence. INDEX causing occasional tsunamis in the densely populated low- TERMS: 3025 Marine Geology and Geophysics: Marine seismics lands of the Sunda Islands. Therefore, it is important to (0935); 3040 Marine Geology and Geophysics: Plate tectonics know more about the tectonic processes and of the geologic (8150, 8155, 8157, 8158); 8150 Tectonophysics: Evolution of the evolution of this area, in order to assess future hazards and to approach prediction possibilities for the most endangered Copyright 2002 by the American Geophysical Union. regions. On the other hand, the investigated area represents 0278-7407/02/2001TC901048$12.00 probable regions for hydrocarbon exploration in Indonesia.

11 - 1 11 - 2 SCHLU¨ TER ET AL.: TECTONICS OF SOUTHERN SUMATRA

Figure 1. Compilation of generalized structures of the Sunda Arc with locations of seismic profiles.

[4] This implies two scientific aspects: to study the Mantawai forearc basins off Sumatra and the Java forearc geologic-tectonic development of the plate margins and basin in front of the volcanic arc. the geometry and structure of the subduction zone with [6] In recent years the detailed structure of the lithosphere particular emphasis upon the neotectonic processes of the and mantle of the studied area was subject to seismic wide Sunda Strait. The second aspect was to study the facies, angle reflection and tomographic studies [Spakman and stratigraphy, and thickness distribution of the accumu- Bijwaard Vening Meinesz, 1998; Rangin et al., 1999; Lelge- lated sediments and their possible potential of conven- mann et al., 2000]. Since the trench parallel slip component tional hydrocarbons. In this paper we present structural increases from Java to Sumatra, the present Indo-Australian features and a geologic-tectonic model for the Sumatra- plate motion leads to northward drag off Sumatra with Java forearc. geometrical distortion and detachment of the slab below Sumatra [Widiyantoro and van der Hilst, 1996]. However, changes in slab geometry can also be the result of the 2. Plate Tectonic and Geologic Setting subduction of oceanic crust of different ages and with [5] The Sumatra-Java area is part of the Sunda Arc that different rates. The subducting oceanic crust of to day off stretches from the Andaman Sea in the northwest to the Sumatra is 46–60 m.y. old and has a present convergence Banda Sea in the east. Along the Sunda Arc the Indo- rate of 6.81 cm/yr, while the crust off Java has ages of 70– Australian plate subducts under the Eurasian plate. The 100 m.y. [Hamilton, 1979; Ghose et al., 1990] and converges studied area (Figure 1) is a classical example of a sub- with a rate of 7.23 cm/yr (NUVEL 1A [Demets et al., 1994]). duction system, composed of the downgoing Indo-Austral- The geometry of the dipping oceanic slab can be deduced ian slab along the Sumatra-Java trench, an accretionary from earthquake distribution linked to subduction. Seismic wedge, the outer arc ridge forming the backstop [Pubellier activity reaches not below 200 km depth off Sumatra but et al., 1992; Samuel and Harbury, 1996], the Bengkulu- goes down to 650 km off Java [Ghose et al., 1990]. SCHLU¨ TER ET AL.: TECTONICS OF SOUTHERN SUMATRA 11 - 3

2.1. Sumatra Area [Beaudry and Moore, 1981, 1985; Izart et al., 1994; Malod et al., 1995]. According to these data a widespread uplift [7] Previous investigations show that convergence between the Australian and Eurasian plates is nearly orthog- and erosion occurred during the Paleogene followed by onal along the Java Trench. West of the Sunda Strait forearc subsidence since the latest Oligocene-earliest Mio- [Demets et al., 1990; McCaffrey, 1991] slip vectors for cene as evidenced by two transgressive-regressive sequen- earthquakes of the underthrusting Indo-Australian plate ces of limestone and shale due to eustatic changes and rotate into a northeast direction, suggesting that conver- tectonism. During the Plio-Pleistocene, two more sequences gence is oblique and a large part of the plate motion is taken of deltaic clastic and clay material were shed from the up by right-lateral shear within the overriding plate in the Sumatra margin into the subsiding basin which is seg- order of 3.6–4.9 cm/yr [McCaffrey et al., 2000]. The trench mented by transverse ridges into several subbasins [Nata- parallel shear is absorbed by transpressive deformation of widjaja and Sieh, 1994; Genrich et al, 2000]. According to the Eurasian plate leading edge and with an assumed Dickinson [1995] these basins were formed in the Oligo- northward displacement of continental slivers [Baroux cene. However, older basin sediment (early Eocene) are et al, 1998; Simandjuntak and Barber, 1996] of not more found on some islands [Pubellier et al., 1992]. than 100 km [Sieh and Natawidjaja, 2000] to 150 km [11] The forearc ridge (outer arc high) is characterized by [McCarthy and Elders, 1997], indicating the partitioning islands, such as Enggano, Pagai, Siberut, and Nias, which of oblique plate convergence into thrust and strike-slip provide considerable geologic information [Samuel et al., motions [Genrich et al., 2000]. 1997; Moore and Karig, 1980; Harbury and Kallagher, 1991; Simandjuntak and Barber, 1996; Pubellier et al., [8] The most pronounced shear zone of the overriding Eurasian plate is the Sumatra fault zone (SFZ) within the 1992; Samuel and Harbury, 1996]. Fieldwork revealed that volcanic arc (Figure 1). The SFZ accommodates most of during Eocene and Oligocene an increase in the subduction the right-lateral stress of the relative plate motion between rate led to the formation of a me´lange [Karig et al., 1980], the Indo-Australian and Eurasian plates and is seismically containing ultrabasic oceanic components, and to basin active. According to Katili and Hehuwat [1967] and inversion and uplift of the outer arc high. This occurrence Natawidjaja and Sieh [1994] the SFZ is composed of of oceanic crust components induced Hamilton [1979] to several segments with differing slip rates, ranging from propose obduction processes with the formation of oceanic 1.1 cm/yr to 2.8 cm/yr [Baroux et al., 1998]. Slip rates as crustal splinters. Early Miocene sediments were initially determined by SPOT images along stream offsets on deposited in deeper water, while since the middle Miocene Sumatra infer movements of 0.6 cm/yr in the south to 2.3 shallow water clastic and carbonate sequences dominate. On cm/yr in the north [Bellier and Se´brier, 1995]. Slip rates of Enggano Island late Paleogene to Pliocene successions are 2.3–2.4 cm/yr as determined from global positioning folded and thrusted [Geological Research Development measurements are reported by Prawirodirdjo et al. Centre, 1993]. [2000]. According to Malod and Kemal [1996] the right- 2.2. Sunda Strait lateral slip is not only taken up by the SFZ with a rate of 2 cm/yr, but also by the Mentawai fault zone (MFZ) [Dia- [12] The Sunda Strait represents a particular tectonic ment et al., 1992] along the forearc basin with a rate of up element where the strongest bending of the Sunda Arc to 1.1 cm/yr off Nias Island. The northern part of the occurs and forms the transition from frontal to oblique Mentawai fault seems to be connected to the SFZ by the subduction [Malod and Kemal, 1996] or even represent the Batee fault and terminates within the accretionary wedge, boundary of two different geodynamic settings [Ghose et al., indicating two slivers (Mentawai and Aceh slivers) on top 1990]. The Sunda Strait is interpreted either as related to a of which the forearc basin has developed. If this is correct, rotation of Sumatra relative to Java and with the rotation axis the accretionary wedge and the outer arc high with the close to the Sunda Strait during the late Cenozoic [Zen, Islands of Enggano and Nias must be a separated northward 1983], or as an extensional feature [Huchon and Le Pichon, moving feature along the Mentawai fault [Malod et al., 1984] resulting from the northwestward displacement of the 1995; van der Werff, 1996], which is assumed to be a long- southern Sumatra block along the Sumatra fault. The amount lived basin bounding fault with throws of about 5 km since of extension of the Sunda Strait is estimated to 50–70 km the Oligo-Miocene [Samuel and Harbury, 1996]. [Malod and Kemal, 1996] and presumably occurred during [9] Off southeast Sumatra (Figure 1) the SFZ (Semangka- the Pliocene [Diament et al., 1992]. South of Sumatra the segment) bends to the south where it merges the exten- forearc ridge morphology shoals to generally <500 m, and sional, south striking fault systems of the Sunda Strait the forearc basin changes to a complex ridge and trough [Sukmono et al., 1996]. These fault scarps form submarine structure [Moore et al., 1980a] which trends parallel to the pull-apart grabens [Malod et al., 1995] widening to the trench [Malod et al., 1995]. south where they lose their bathymetric signature. Hence, it is an open question whether the SFZ terminates or if the 3. Data Acquisition and Processing SFZ bends to the southeast and is attached to the west-east running dextral Ujung Kulon fault zone off northwest Java. [13] Surveying was done along 45 traverses with a digital [10] A grid of multichannel seismic profiles and well multichannel reflection seismic system (MCS), with gra- information from the Sumatra forearc basin revealed a dient magnetometer and gravimeter systems and with swath stratigraphic framework for the northern and central basins bathymetric (Hydrosweep) and sediment echographic sys- 11 - 4 SCHLU¨ TER ET AL.: TECTONICS OF SOUTHERN SUMATRA tems. Seismic data with a total length of >4100 km and segments are correlatable from offshore Sumatra to the plate magnetic, gravity, and bathymetric data with a length of margin off Java, even across the area off the Sunda Strait. >5500 km were acquired. [20] The inner part of the accretionary prism and adjoin- [14] After loading of the seismic field data corrections for ing to the forearc basin (Figure 3) forms a NW-SE running the true shotpoint-hydrophone geometry were done and outer arc ridge off Sumatra with the Island of Enggano shot-gathers created for quality control and for first inter- being the highest part of this segment. For this outer arc active velocity analyses. Determination of the pre-stack ridge with widths of 30–60 km we use the term accre- parameters were performed by the FOCUS software mod- tionary wedge I. It is made up of 5 to 6 imbricated flakes ules bad trace editing, band-pass filtering, deconvolution, which are thrust to the southwest (Figure 4). Due to velocity analyses. individual characters of the imbricated flakes in morphol- [15] The seismic processing was optimized in order to ogy, size, squeezed synclines, turn-overs and marked thrust image deep crustal reflectors and to receive a sufficient planes it is possible to recognize the flakes on most of the resolution in the sedimentary cover. Hence, the multiple profiles. Of particular importance is a marked strip-thrust suppression was one of the main topics during the process- fold on the western rim of the accretionary wedge I which ing. One or more of the following steps for multiple occurs on all profiles south of Enggano Island. Almost all suppression were applied: (1) FK-filtering after NMO over- subsurface tectonic structures are buried below a thin (0.3– correction to attenuate the multiple-far-trace energy, (2) 0.8 s TWT), but deformed sedimentary cover. Along its mute of the inner traces (near trace) to attenuate the multiple northeastern flank accretionary wedge I is characterized by near-trace energy, and (3) stacking to attenuate residual small-scaled reverse faults to the northeast indicating back multiple energy. thrust toward the forearc basin. Off the Sunda Strait area the [16] After stacking extra filtering and fk-migration was outer arc ridge of accretionary wedge I has a subdued done on all profiles. Instead of the fk-migration an Omega-x seafloor morphology. There, the observed imbricated struc- migration was applied on selected profiles in order to tures are covered by thick (0.5–1.55 TWT) sediments. enhance the results in cases of lateral velocity changes. These sediments are deformed and show onlap, downlap Then the processed data were converted into a SEG-Y and infill seismic pattern in small troughs between thrust format and loaded to the Schlumberger GeoQuest Geo- flakes. The missing outer arc ridge off the Sunda Strait is frame/IESX system for interactive interpretation. explained by extensional tectonics (pull-apart) of the Sunda [17] A main difficulty of interpretation was the low Strait and by partitioning of compression along the Ciman- density of seismic survey lines in the prospect area as well diri-Pelabuhan Ratu and Ujon Kulon wrench zones. as the lack of tie lines for tracing seismic layers from one [21] The top of the imbricated flakes of the forearc ridge line to another. Especially due to the geologic situation with is characterized by the pronounced reflection horizon strong deformation and separated sub-basins tracking of ‘‘blue’’ (Figure 4). Horizon ‘‘blue’’ is characterized below seismic horizons was complicate. To overcome this diffi- the outer arc ridge by a set of high-amplitude low-frequency culty different tools of the IESXTM system were applied and a reflectors which are highly deformed by thrusts and folds. significant improvement of interpretation results was There are southwestward imbricated thrust slices as well as achieved. overturned structures, i.e., strip-thrust folds and overturned [18] In addition we loaded swath bathymetric data of the folds with squeezed synclines. This style of strip-thrust cruises SO137, SO138, and SO139. The combination of folds is explained as superficial deformation resulting from bathymetry and seismic is a very useful method for inter- the gliding of strata over a lubricating sequence and pretation. By comparison of the seabed morphology with compression into folds. In order to test this idea reflection seismic imaging it is possible to cross-check the CDP- pattern from the interpreted lower limb of an overturned locations and to determine the strike and dip of structures fold of line 137-34 were analyzed in respect of their seismic and faults if they are visible at the seafloor. Due to the fact attributes. Instantaneous frequency analysis of the reflec- that the hydrosweep data have some extension to both sides tions from the near surface upper limb and the interpreted of the cruise lines the strike of identified faults were deeper lying lower limb revealed similarity in character recognized and afterwards extrapolated to adjacent lines. indicating the same lithology. Hence, it is reasonable to assume overturned strip-thrust folds below the outer arc ridge. This seems even more reliable because westward 4. Results thrusted and folded calcareous sand-, silt-, and claystones of Paleogene to middle Miocene age occur on Enggano Island 4.1. Tectonic Structure of the Forearc Ridges [Geological Research Development Centre, 1993]. off Sumatra, Java and the Sunda Strait [22] Horizon ‘‘blue’’ could be traced throughout the area [19] Outstanding features of the convergent plate boun- particularly along the outer arc ridges off Java and Sumatra dary along the Sunda Arc are a well structured, 120–140 km and could even be correlated across the Mentawai fault zone broad accretionary prism and a deep forearc basin. The into the forearc basin, as on lines 137-06, 137-36, and 137- accretionary prism could be divided into two major morpho- 42B. Reflector ‘‘blue’’ is interpreted to represent the top of a logical segments (accretionary wedge I and accretionary sequence which is rich in calcareous material. This is wedge II; see Figures 2–5) which differ also in their tectonic supported by geological sampling during Sonne cruise style of deformation. Despite changes in morphology the 139 along the outer arc ridge in the north and south of SCHLU¨ TER ET AL.: TECTONICS OF SOUTHERN SUMATRA 11 - 5

Figure 2. Offshore tectonic features of the sedimentary cover and onshore geology.

Enggano Island, which exhibited calcareous sands and tionary wedge I and stretches to the trench-slope deforma- arenitic limestones of Neogene age. The existence of wide- tion front (Figure 2). For this segment we use the term spread calcareous sediments is also supported by offshore accretionary wedge II. In general accretionary wedge II is drilling on the outer shelf and slope of the forearc basins separated from accretionary wedge I by a pronounced [Beaudry and Moore, 1985; Izart et al., 1994; Rahmat and thrust, probably connected to the seafloor (Figure 6). There- Oemar, 1998]. fore, the top of accretionary wedge II lies about 750– [23] The outer arc ridge of accretionary wedge I off Java 1500 m deeper than accretionary wedge I. The internal is very similar in its structure as to the outer arc ridge off architecture of this segment is more complicated than Sumatra: pronounced imbricated thrust slices which are accretionary wedge I because it is impossible to identify active until sub-recent. The thrust slices could be correlated undeformed sediments in the seismic recordings on top of unequivocally throughout the area. The thrust slices of the this outer part of the prism (Figure 5). But several ridges outer arc ridge act as a backstop for the incoming sediments with intercalated depressions at the seabed in combination of the downgoing plate. The ‘‘blue’’ horizon could be traced with northeast dipping and downward flattening reflections from the outer arc ridge into the forearc basin, which presumably indicate imbricated structures. This seems rea- represent a sedimentary trap for the sediments run off Java. sonable because in the area off Sumatra these features strike This is the reason for thin sedimentary cover above horizon subparallel to the imbricated flakes of accretionary wedge I ‘‘blue’’ across the outer arc ridge. There is also clear and have the same size. Comparable features occur off Java indication for backthrust/reverse faulting off Java, initiating where they have been mapped in detail by Malod et al. a successive northward thrust over the southern forearc [1995] (Figure 2). basin by the accretionary wedge I. [25] Accretionary wedges I and II are underlain by the [24] The outer part of the Sunda Arc accretionary prism is subsiding oceanic crust of the Indo-Australian plate 60–100 km wide, attached to and just west of the accre- (Figure 6) which is characterized in the seismic records 11 - 6 SCHLU¨ TER ET AL.: TECTONICS OF SOUTHERN SUMATRA

Figure 3. Geoseismic section of profile 137-19 off Sumatra (Mentawai Basin, Paleogene deltaic sequences, Neogene basin infill). by two types of top reflections. The first type occurs seismic records within a few kilometers, as it is the case on around Enggano Island, mainly below the accretionary profiles 137-34 and 137-42B. On profile 137-42B there are wedge II off the Sunda Strait and off northwest Java. This indications for strong internal deformation of the second type of top reflection of the oceanic crust is clearly defined type of crust. Nonetheless, on the stacked seismic profiles by a set of continuous reflectors containing high ampli- this second type of crustal top reflection could be correlated tudes and low frequencies which are traceable from the unequivocally from the outer arc high to close to the NW-SE striking deformation front over a distance of Mentawai fault zone (MFZ). Applying velocity information 135 km to below the outer arc high (Figure 7). In general, from seismic processing, depth estimations were made for the top of this oceanic crust shows a smooth irregular, the top of the downgoing slab at the northeasternmost northeastward increasing relief. occurrence, just west of the MFZ on profile 137-19, and [26] The second reflection type from the oceanic crust revealed a depth of about 25 km. This agrees quite well with occurs below the outer arc ridge to the forearc basin and seismic refraction and wide-angle measurements from lines stretches from the southernmost tip of Sumatra along strike P01 and P02 of Sonne cruise 138 [Kopp et al., 2002] where across the Sunda Strait to the northwesternmost tip off Java depths of 21–23 km were determined for the downgoing (Figure 7). It is characterized by discontinuous, weak slab off Sumatra. The reason for these differences in reflecting elements, and with, in general, a strong relief. reflectivity and surface relief of the oceanic crust are not The transition between both types of the crust occurs in the related to thickness variations of the overlying accretionary SCHLU¨ TER ET AL.: TECTONICS OF SOUTHERN SUMATRA 11 - 7

Figure 4. Geoseismic section of profile 137-19 off Sumatra (wedge I with Paleogene tectonic flakes).

wedges I and II and not attached to changes in recording/ tivated older suture zone elsewhere along the paleo-Sunda processing parameters. We speculate that the difference in margin [Pubellier et al., 1992]. the seismic image of the downgoing slab is related to the extensional regime and volcanism along the Sunda Strait 4.2. Tectonic Structure of the Forearc Basin and/or to the strong bending of the Sunda Arc in that area in combination with slab distortion of the subducting Indo- [28] The igneous basement below the central part of the Australian crust. But it cannot be excluded, that the different Mentawai forearc basin is masked by strong multiple seismic images of the oceanic crust are of prime origin, reflections from the seafloor. Only below the northeastern representing changes of the crust generation during the basin margin, there are few weak reflection elements Cretaceous magnetic quiet period. indicating downfaulted and rotated crustal blocks of con- [27] In our geophysical profiles there is no indication for tinental origin (Figure 3). Character of basinward thinning a coherent oceanic crustal splinter along the investigated continental crust is constrained by refraction seismic results forearc ridge off Sumatra and Java neither from seismic and P wave velocities of 6.0–6.7 km/s underneath the reflections or structures nor from magnetic or gravity Sumatra slope and below the northern forearc basin margin anomalies. The peridotites reworked in conglomerates of [Kopp et al., 2002]. The main normal faults have a general the polygenic Nias me´lange are not conclusive for an NW-SE strike and were tentatively correlated along the oceanic splinter, because they may originate from a reac- Sumatra margin. In the area off southeast Sumatra and in 11 - 8 SCHLU¨ TER ET AL.: TECTONICS OF SOUTHERN SUMATRA

Figure 5. Geoseismic section of profile 137-42A off the Sunda Strait (wedge II with Neogene accretion). SCHLU¨ TER ET AL.: TECTONICS OF SOUTHERN SUMATRA 11 - 9

Figure 6. Geoseismic section of profile 137-12A (wedges I + II, downgoing oceanic slab and the forearc basin underlain by thinned Eurasian crust). the Sunda Strait the faults are crosscut by N-S to NNE-SSW sediment thicknesses of up to 5 s (TWT) in the northeast. trending normal fault systems thus forming a grain of According to weak reflections with low frequencies and complex horst structures with pull-apart basins in between. high amplitudes beneath the basin fill the continental base- [29] Off Sumatra, this type of continental basement is ment continues below the entire forearc basin. It is of overlain by seaward prograding wedges composed of particular importance that these reflectors occur without a diverging reflector sequences with a high-amplitude low- large vertical displacement even underneath the MFZ and frequency content (Figure 3). Hitherto these wedges were extend to below the outer arc ridge. This supports the interpreted as large basalt flows (seaward dipping reflector assumption of a Sumatra forearc sliver plate with a uniform sequences, SDRS) [Hinz, 1981], erupted along the former basement [Moore et al., 1980b; Huchon and Le Pichon, passive continental margin of Sumatra and masking the real 1984] and the MFZ being a back-thrust feature [Sieh and basement beneath. According to the whole internal fabric, Natawidjaja, 2000] with a strong transpressional compo- the regional occurrence and extent of these wedges we now nent. Intensity of transpressional forces has changed favor the idea that the wedges rather represent basinward throughout evolution as it is obvious by differences in the prograding sedimentary sequences of presumed Paleogene strength of deformation of the Neogene seismic sequences. age. This interpretation seems more reliable, because indus- [31] Seismic profiles and swath bathymetric results of the try wells from the Bengkulu Basin off Manna, from the GINCO cruises clearly show that the MFZ extends from the Banyak, Simeulue and Aceh Basins off central and north Sumatra forearc basin across the Sunda Strait to the Java Sumatra encountered non-marine to nearshore clastics and forearc basin (Figure 7). Off the southern Sunda Strait there marginal marine mudstones of Eocene to Oligocene ages are several NW to N running branches of the MFZ which [Beaudry and Moore, 1985; Izart et al., 1994; Rahmat and join the Semangka segment of the Sumatra fault zone. Oemar, 1998] below a late Oligocene/early Miocene ero- Branching of the MFZ is explained as due to the strong sional surface. On Nias Island, Pubellier et al. [1992] bending of the Sunda Arc between Java and Sumatra and recognized limestones of apparent in situ early Eocene due to the Sunda Strait extension since the Miocene. age. However, according to Samuel and Harbury [1996] [32] Structure and tectonic deformation of the Sunda these rocks are of upper Oligocene to lower Miocene age Strait differ considerably from the offshore areas of south- containing reworked Eocene fauna. east Sumatra and northwest Java. Outstanding features of [30] The forearc basin off southern Sumatra is highly the Sunda Strait are north to northwest trending horst and affected by the Mentawai fault zone and anticlinal structure, graben systems (Figure 7). The westernmost horst repre- causing a pronounced doming of the seafloor (Figures 3 and sents the seaward continuation of the Semangka Peninsula. 6). The Mentawai fault zone (MFZ) separates a smaller sub- This westward tilted Semangka Horst has a triangular shape basin in the southwest from the main forearc basin with and is bounded in the east by a major, steep normal fault 11 - 10 SCHLU¨ TER ET AL.: TECTONICS OF SOUTHERN SUMATRA

Figure 7. Offshore crustal types with major tectonic elements. down to the adjoining Semangka Graben (Figure 8) and in subsidence of the eastern Semangka Graben is constrained the southwest by normal downfaulted blocks of the Sumatra by up to 2 km thick sediments of early Miocene to margin. Sampling along the eastern flank of the Semangka Pleistocene age with velocities of Vp =1.6À1.9 km/s which Horst revealed various igneous rocks (metamorphites, ande- are underlain by about 4 km of pre-Eocene to Oligocene sites) indicating continental and volcanic arc basement. This sediments with seismic velocities of Vp =2.2À3.8 km/s is also supported by high seismic velocities of Vp > 4.3 km/s [Lelgemann et al., 2000]. The change in structure is also at shallow depths below the horst structures of the Sunda attached to southward increasing but buried positive flower Strait [Lelgemann et al., 2000]. structures along the horst-graben transition, indicative for [33] The steep N-S running major fault along the eastern strike-slip transpressional movements in the pre-Pleisto- flank of the Semangka Horst has displacements of about 2 s cene. Hence, we consider a preceding extensional pull-apart (TWT) in the north which is successively replaced by a set basin morphology followed by a transpressional tectonic of less pronounced smaller scaled faults in the south. phase. Simultaneously, the adjoining Semangka Graben changes [34] Correlation of these doming flower structures hints character from a v-shaped small half-graben structure in the at a south strike which bends into a northwest-southeast north (Semangka Bay) (Figure 8) to a broader basin feature direction in the southern Sunda Strait. There, this deforma- in the south of the Sunda Strait originating from a transten- tional zone merges with the Mentawai fault zone and seems sional (pull-apart) regime which we assume to be of early to be younger because sediments close to the seafloor are Miocene age. The age assessment of transtension is based also domed. According to these observations, transpression on our seismostratigraphic interpretation presented else- and strike-slip movements presumably are time dependent. where. This is in contradiction to Lassal et al. [1989], It is further suspected that the strike-slip component of the who assumed the onset of opening and subsidence of the Sunda Arc is taken up sometimes by the Semangka Graben Sunda Strait to be of latest Miocene age. Transtension and and sometimes by the MFZ creating deformation either SCHLU¨ TER ET AL.: TECTONICS OF SOUTHERN SUMATRA 11 - 11

Figure 8. Geoseismic section of profile 137-25 (Semangka Graben and dyke of Krakatau Basin). along the Semangka Graben or along the MFZ. If this section, particularly on its southern end on seismic line SO assumption is correct then the Sumatra fault zone is coeval 137-27, which led to inversion structures and large-scale to the Mentawai fault zone, which is of Oligo-Miocene age monoclines. The acoustic basement of assumed late Oligo- [Samuel and Harbury, 1996]. At its northern end the trans- cene-early Miocene age is represented by a strong reflection pressional strike-slip zone of the Semangka half-graben is with low frequencies and high amplitudes and shows connected with the Sumatra fault zone (Figure 7). upthrust structures. Similar inverted basin structures and [35] The Semangka Horst and the northward narrowing in prolongation of the Krakatau Basin occur on seismic Semangka Graben terminate off Semangka Bay along a profiles SO 137-30 and SO 137-34 off the south coast of transversal northwest striking horst structure. This horst is west Java. It seems reasonable to interpret the inversional offset to the south by about 14 km and can be correlated features as a common zone of deformation due to east-west with the structural highs of the Panaitan Island and Panaitan compression. This compression may be connected with the Peninsula of northwest Java which are made up of Quater- Cimandiri and Pelabuhan Ratu strike-slip zones. Since the nary volcanics and Tertiary sediments. major inversion occurred predominantly within the lower, [36] In the northeast of the Panaitan Peninsula occur two early to late Miocene sedimentary section we assume the more pull-apart basins: one southward narrowing basin east main deformation to be of that age. of the Semangka Bay and the second basin south of the [37] In contrast to this, the upper and younger sediments Krakatau (Figure 2). The latter, called the Krakatau Basin, is of the Krakatau Basin rather show extensional normal buried below >3 s (TWT) thick sediments and has no faulting. This is particularly valid for the northern Krakatau topographic expression on the seafloor. This basin is char- Basin where the faults even reach the seafloor, indicating acterized by reverse normal faults in the lower sedimentary young extension. 11 - 12 SCHLU¨ TER ET AL.: TECTONICS OF SOUTHERN SUMATRA

Figure 9. Cartoon with four geodynamic stages of evolution.

[38] Along the eastern margin of the Krakatau Basin there Australian oceanic lithosphere occurred beneath Sunda-Java is a 2.5 km broad feature with very steep slopes at depths although the direction and rate of convergence may have below 1 s (TWT) which is characterized by a non-coherent changed [Hall, 1998]. This was associated with back arc internal reflection pattern. The sediments along the flanks rifting and the development of grabens on Sumatra and Java are dragged up and the overlying sediments are domed. This [Longley, 1997]. feature strikes north-south, occurs on profiles SO 137-40, [41] During the early Paleogene the convergence between SO 137-37, and on SO 137-25 and is interpreted as a the Indian and Eurasian plates created a first accretionary volcanic dyke which is related to the Krakatau volcanism wedge along the paleo-Sunda Arc and the subsiding paleo- (Figure 8). Since the dragged sediments belong to the upper forearc basin was fed by seaward prograding slope sedi- sequence injection of the dyke is believed to happened in ments from the adjacent continental margin. Today, parts of the upper Miocene or Pliocene. this accretionary wedge I are exposed on Nias Island. Due to continuous convergence and tectonic uplift and due to 5. Geodynamic Model for the Evolution eustatic sea level lowering in the order of 100–150 m during the lower Oligocene [Haq et al., 1987], the accretionary of the Sunda Forearc prism I as well as marginal parts of the forearc basin were [39] We draw the following scenario for the evolution of eroded. This event is documented by the erosional Paleo- the accretionary wedges I and II which presumably is valid gene-Neogene unconformity of reflection horizon ‘‘blue.’’ for the whole Sunda forearc (Figure 9). 5.2. Stage 2 5.1. Stage 1 [42] Subsequent subsidence of the forearc area presum- [40] During the early Cenozoic, India and Australia ably may be related to changes of the Indian Plate movement became one plate, and northward subduction of Indo- and direction since the time of chron 11 [Glebovsky et al., SCHLU¨ TER ET AL.: TECTONICS OF SOUTHERN SUMATRA 11 - 13

1995], about 32 m.y. ago, which led to a slow down or even ited, followed by prograding clastic and shale-prone sedi- cessation of convergence along the Sunda Arc throughout ments shed from the volcanic arc terrane of the upper plate. the upper Oligocene to earliest Miocene. A simultaneous onset of sea level rise started during the upper Oligocene. 5.4. Stage 4 The transgression is visible in the seismic profiles along the [45] Due to steady subduction of Indo-Australian litho- entire Sunda forearc basin by onlap terminations on the sphere the uplift of the outer arc ridge containing rocks of Oligocene erosional surface and is proven by offshore drill- accretionary wedge I continued during the Plio-Pleistocene. ing results in the Aceh, Meulaboh, Banyak, Bengkulu(- The outer arc ridge reacted as a buttress against the incom- Manna), and Java Basins. This event seems to be time ing sediments of accretionary wedge II which became transgressive with the largest erosional gap on the Sumatra extremely deformed and chaotic already at its front, along and Java shelves and with only a small hiatus on former the trench. This is not common along convergent plate slope areas, below the present Mentawai and Java Basins. boundaries because off Makran, North Sulawesi, North On the Sumatra-Java margins sedimentation above the Palawan and even off Barbados the internal stratification Oligocene unconformity started with shallow water lime- of the first outer thrust slices of the respective wedges stones merging downslope into deepwater carbonates and remained intact. Therefore the high deformation of accre- mudstones and with upward increasing amounts of clastic tionary wedge II off Sumatra and Java are explained by material. Renewed convergence in combination with extreme strain-slip cleavage caused by high compression increased seismicity and steepening of the paleo-slope may against the backstop of accretionary wedge I as well as by have caused submarine disruption and sliding of these beds the obliquity of convergence. These processes influenced above a lubricating layer, causing synsedimentary over- also the mass of accretionary wedge I creating backthrusts turned and strip-thrust folds. to the northeast along the border from the forearc ridge to the forearc basin. It seems even reasonable to assume this 5.3. Stage 3 transpression as the main force for the development of the Mentawai fault zone (MFZ) along the forearc basin. Suc- [43] At the early-middle Miocene the Burma Plate was partly coupled with the north moving Indian Plate creating cessive transpressional phases along the MFZ are indicated stretching of the Sunda continental margin north of Sumatra by differences in the strength of deformation, i.e., by the and with the subsequent formation of oceanic crust in the grade of updoming and steepening of the sedimentary Andaman Sea since the Pliocene. Although convergence sequences across the MFZ. The well-constrained large along the Sunda Arc and the opening of the Andaman Sea subsidence of the forearc basin since the Plio-Pleistocene led to the inversion of older rift basins [McCarthy and cannot be explained by the deformation and loading effect Elders, 1997], marine sediments were deposited due to of the sedimentary cover. Speculative subcrustal erosion or eustatic rise of sea level [Longley, 1997]. The clockwise slivering processes of the leading Eurasian plate below the rotation of Sumatra against Java is suggested by paleomag- forearc region might be the reasons. netic studies [Nishimura et al., 1986] and volcano-tectonic analyses [Zen, 1983], and led to a change of the orientation 6. Conclusions of the Sumatra margin with respect to the Indian plate motion as well as to an increase in obliquity and rate of [46] Two main stages of evolution of the forearc area with convergence, compensated by increased subduction [Hall, the formation of two accretionary wedges during the Pale- 1998]. ogene and since the Neogene are revealed. A first accre- [44] This increasing obliquity and advanced convergence tionary wedge developed during Paleogene along the and subduction of the Indo-Australian plate below the convergence zone between the Eurasian (Sundaland) and Eurasian margin since the middle Miocene we assume as Indo-Australian plates. Remnants of that wedge are repre- the cause for the onset of the second accretionary wedge II sented by me´lange rocks on the Sunda forearc islands. formation in front of the first Paleogene accretion. Today, Although ophiolite components occur within this me´lange the boundary between both wedges is represented by the on Nias Island, the GINCO geophysical data do not hint at pronounced thrust-related step of the seafloor and by differ- an obducted oceanic crustal splinter as assumed previously ent water depths. The Paleogene accretionary wedge I is [Hamilton, 1979]. interpreted as the backstop for the younger trench fill [47] We explain subsequent uplift and erosion of parts of sediments which to some extent descent into the subduction the accretionary wedge I during late Paleogene by changes zone. It cannot be excluded, that there are detachments in plate geometries and direction of movement and by between the younger downgoing sediments and the older eustatic lowering during the lower Oligocene. This main Paleogene wedge I, giving rise to initial uplift, northeast erosional unconformity was correlated on all GINCO seis- tilting and rotation of accretionary wedge I and duplexing at mic profiles off the Sunda Arc and is proven by wells depth. The accreted and consolidated mass of wedge I offshore Sumatra and Java. Below that erosional uncon- started to form the present outer arc ridge. Simultaneous formity there are sequences of seaward divergent reflection subsidence of the forearc basin started in the northeast of the seismic patterns along the Sumatra margin which are evolving outer arc ridge and on top of seaward thinned interpreted as equivalent sequences to the Paleogene pro- continental crust of the developing Mentawai sliver. Since grading deltaic sediments drilled in the Mentawai, Nias, and the lower Miocene sediments rich in carbonate were depos- Simeulue offshore basins. 11 - 14 SCHLU¨ TER ET AL.: TECTONICS OF SOUTHERN SUMATRA

[48] Due to a slow down or even cessation of subduction a clockwise rotation of Sumatra with respect to Java since the accretionary wedge I subsided and with the onset of the lower Miocene attached to compression at the pole of eustatic rise in the upper Oligocene marine calcareous series rotation along the Krakatau Basin and to a bending of the were deposited on the paleo-slope merging toward the Sumatra strike-slip fault zone (Semangka segment) along paleo-shore of Sumatra and Java into shallow water carbo- the Sunda Strait. It cannot be ruled out that the Cimandiri nates. This is supported by our seismostratigraphic inter- and the Pelabuhan Ratu faults of NW Java are the original pretation, by wells, and by geological sampling near correlatives of a former continuous Sumatra-Java fault Enggano Island. Widespread carbonate sedimentation of system. It is further speculated that due to continuous upper Oligocene to lower Miocene age with subsequent rotation of Sumatra and increasing obliquity of convergence subsidence to bathyal depth is common throughout SE Asia, since the late lower Miocene the arc-parallel strike-slip as it is proven for the South China Sea off Palawan and the jumped from the volcanic arc position to the southwest into Dangerous Grounds as well as for the Kutai and Tarakan the forearc basin area, thus creating the Mentawai fault Basins off east Kalimantan. It is suspected that due to zone. Reflection seismic and bathymetric data from GINCO renewed convergence submarine disruption and sliding of cruises clearly show the continuation of the Mentawai fault the slope sediments along the steepening margin of the zone from the Sumatra forearc basin to the NW Java forearc Sunda Arc occurred and synsedimentary overturning and basin. Across the Sunda Strait branches of the assumed strip-thrust folding above the Paleogene wedge happened. younger Mentawai strike-slip system are connected with the This is supported by seismic attribute analysis and by assumed older Sumatra strike-slip zone. Recent seismicity mapping results on Enggano. and structural deformation along both fault zones hint at [49] Increasing convergence rates between the Indo-Aus- strain partitioning within which strike-slip components are tralian and Eurasian plates since the Neogene created the taken up sometimes by the Sumatra fault zone via the second accretionary wedge seaward of wedge I which Semangka segment and sometimes by the Mentawai fault formed the backstop and turned into the rising outer arc zone. high. The outer arc high is made up of 5–6 tectonic flakes which could be distinguished unequivocally throughout the entire area, indicating the same style of deformation of the [51] Acknowledgments. We are indebted to the Government of the sedimentary cover off SE Sumatra (oblique convergence) Republic of Indonesia providing the permission for the investigations in its and off NW Java (frontal convergence). This implies a very territorial waters. The assistance of Dr. S. Indroyono and Dipl.Ing. Basri similar plate tectonic regime at the time of the tectonic flake Gani from BPPT and of Dr. H. Keune from the German Embassy during the development during the upper Oligocene to lower Miocene cruise preparation is highly acknowledged. The authors wish to express their gratitude to the various colleagues who have supported the work along the Sumatra and Java paleo-margins and hence, the during the cruise and carried out the data processing. Particular thanks are transition from frontal to oblique convergence may have directed to the ship’s master H. Papenhagen and the crew of FS Sonne.We been not yet existed. Simultaneously the subsiding forearc are highly indebted to M. Pubellier for his critical review which improved basin became more pronounced and downlap and onlap the manuscript. We highly acknowledge Mrs. G. Bulla for typing and reformatting the manuscript. The research project was carried out with grant seismic pattern of Miocene sediments developed. 03G0137A of the Bundesministerium fu¨r Bildung und Forschung (BMBF), [50] We speculate that the initial transtension and the Germany. This support is highly acknowledged as well as the burden of formation of pull-apart basins of the Sunda Strait are due to administration done by the Projekttra¨ger Ju¨lich GmbH.

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