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].