DFE04-PO-023

INDONESIAN PETROLEUM ASSOCIATION Proceedings, Deepwater And Frontier Exploration In Asia & Australasia Symposium, December 2004

GEOLOGICAL CONDITION OF THE CONVERGENT MARGIN SYSTEM OFF WEST JAVA AND SOUTHERN SUMATRA

Afiat Anugrahadi* H.S. Koesnadi**

Yusuf Surachman*** Djunaedi Muljawan***

ABSTRACT The Sunda arc (Sunda Trench) represents a large convergent margin system that extends of about 5,000 km from the Bengal Fan to the south of Sumba Island in eastern Indonesia. The arc displays morphology of trench, slope basin, outer arc ridge, forearc basin and volcanic chain. From the trench to the outer arc ridge, the structure has been filled in by sediment. This structure was deformed by tectonic activity that forms the accretionary prism. The combination of remote data and insitu data such as refraction and reflection seismic methods together with magnetic, gravimetric, and bathymetric measurements has led to a significantly improved understanding of the composition, structure, and tectonic evolution of the convergent margin system off West Java and South Sumatra. This data was acquired during a cruise conducted using Research Vessel SONNE belonging to the government of Germany. Analyses of sediments, rocks, and benthic fauna, recovered during the offshore work of the project, provided new information on the sedimentary processes in the convergent margin system. These results show the difference of lithologic condition and geological process of convergent margin system between Java and Sumatra. The seafloor depth in the fore-arc basin is 3000 m off West Java and around 2000 m off the Sunda Strait and southern Sumatra. The Sunda trench marks the deepest part of the survey area. Water depths are more than 6500 m off Java and more than 6000 m off Sumatra

* Trisakti University ** Marine Geological Institute *** Agency for the Assessment and Application of Technology

( BPPT)

INTRODUCTION The arc-trench system changes along strike from continental crust along the Sumatra Island through to the transitional crust of Java, to oceanic crust in the eastern Sunda Arc (Hamilton, 1988). The Sunda arc represents a large convergent margin system that extends about 5,000 km from the Bengal Fan to the south of Sumba Island in eastern Indonesia. The convergent margin system displays a morphology of trench, slope basin, outer arc ridge, forearc basin and volcanic chain in cross section (Figure 1). From the trench to the outer arc ridge, the structure has been filled in by the sediment. This structure was deformed by tectonic activity forming an accretionary prism. Some of these structures represent the outer arc ridge that has emerged above the sea level such as Simeulue, Banyak, Nias, Batu, Siberut, and Enggano islands. The subduction zone at the Sunda arc is divided into two kinds of subduction system: oblique and perpendicular (frontal) subduction. Oblique subduction is taking place to the south of Sumatra, while frontal subduction is occurring in southern Java, Bali, Lombok and Sumbawa islands. Subduction rates range from 60 mm/yr. in Sumatra to 78 mm/yr. at the easternmost part of the arc. The depth of the Benioff zone reaches only 300 km under Sumatra, but 700 km under Java (Ghose et al., 1990). The oblique subduction has resulted in a huge strike-slip fault system in Sumatra Island, called the Sumatra fault zone. This fault zone extends from the Andaman Sea to the southeastern end of Sumatra Island and bends to the Sunda Strait. Four principle structural domains were distinguished in the fore-arc province: the oceanic, the accretionary, the outer-arc high complexes, and the fore-arc basin (Beiersdorf et al. 2001).

This paper will describe in sequence of a new strike-slip fault system within forearc basin south of the Sumatra Island that runs parallel with the Sumatra fault zone called the Mentawai Fault zone. The very similar tectonic style of the older part throughout the extension of the outer-arc high indicates that the subduction direction in the Paleogene was uniform off South Sumatra and West Java, other than the condition of lithologic and geological process.

DATA ACQUISITION The cruise was conducted using Research Vessel SONNE belonging to the government of Germany. It was carried out in the area south of Cilacap waters, southern Sunda Strait and southern Bengkulu waters. This project is named GINCO, which is an acronym for "Geoscientific Investigation along the Convergence Zone between the Eastern Eurasian and Indo-Australian Plate of Indonesia"; a joint cooperation on geoscientific investigation between the government of Indonesia (BPPT) and the government of Germany (BGR) The goal of GINCO project was focused on the subduction zone of southern Sumatra and West Java described by Beiersdorf in the cruise report (1999). The objectives from this project were: 1). Studying the structure and composition of a reference area on the Indo-Australian Plate just a passing the Sunda Trench; 2). Studying the structure and composition of the subducting complex and of the overriding plate include the accretionary complex; 3). Searching for splinters of oceanic crust which have been thrusted into the accretionary prism; 4). Deciphering the tectonic mechanisms which have led to the structural inventory of the convergence zone, in particular placing emphasis on the structural difference between areas of oblique versus areas with frontal subduction; 5). Studying the heat flow and fluid flow regimes at the deformation front and within the accretionary complex; 6). Searching for bottom simulating reflectors as indicators for the occurrences of gas hydrates; and 7). Searching for active venting of methane in fault settings and studying biota associated with the venting. The cruise was divided into three legs. Leg I (SO-137), from 21 November 1998 to 28 December 1998, was to assess the structural inventory of the subduction complexes of western Java and southern Sumatra (Figure 2). Leg II (SO-138), from 29 December 1998 to 21 January 1999, was to carry out seismic refraction work using GEOMAR ocean

bottom hydrophones and ocean bottom seismometer. Leg III (SO-139), was a geological leg carried out from 30 January to 27 February 1999, to sample the sediment, rock and seafloor biota. The processing of geophysical and geological data, emphasized on cruise SO 137 and SO 139, is part of the agreement between Indonesia-German cooperation in which Indonesian scientists took part in data processing in BGR, Hannover, Germany. The processing was carried out between 22 June 2000 and 18 August 2000. GEOPHYSICAL MEASUREMENTS AND SATELLITE OBSERVATION Remote data was acquired from geophysical measurements and satellite altimeter observation. Three thousand kilometers of sediment-echographic and swath-bathymetric profiling was achieved (Beiersdorf et al, 2001). Three consecutive cruises were carried out: 1) SO-137 GINCO 1 aimed at the assessment of

the structural inventory of the subduction complexes off western Java and southern Sumatra. The cruise used the BGR multi-channel reflection seismic system, the HYDROSWEEP swath mapping system and the high resolution sub bottom profiling system PARASOUND of SONNE, as well as the BGR gradient magnetometer and the BGR gravitymeter (Reichert et al., 1999). During Cruise SO 137 a total 5500 kilometers of geophysical profiles were achieved. Magnetic, gravimetric, sediment-echographic, and swath-bathymetric measurements were almost continuously carried out over this length. On 4100 kilometers, multi-channel seismic profiling was undertaken. The very similar tectonic style of the older part throughout the extension of the outer-arc high indicates that the subduction direction in the Paleogene was uniform off South Sumatra and West Java.

2) SO-138 GINCO 2 main goal was to carry out

refraction seismic work using GEOMAR ocean bottom hydrophones and ocean bottom seismometers in order to delineate deep structures of the continental margin and to assess their seismic velocities. Additional bathymetric, reflection seismic, magnetic and gravimetric profiling was performed in order to

expand the geophysical data base of the previous cruise (Flueh et al., 1999).

Cruise SO 138 was dedicated to obtain

information on p-wave-velocities via refraction seismic measurement. Along 9 profiles, with a total length of 1860 kilometres, 111 locations were occupied by ocean-bottom hydrophones and ocean-bottom seismometers. Magnetic, gravimetric, bathymetric, and single-channel reflection seismic profiling also took place along these 9 profiles.

Gravity measurements carried out during the

SONNE cruises SO-137 and SO-138 measured along 16210 kilometers. This data covers the structural units within the curved active subduction zone of the Indian Ocean off west Java and southern Sumatra from about 100.5°E to 108°E with profile spacing of around 50 km. Furthermore, dense measurements with profile spacing of less than 10 km were carried out in the Sunda Strait. Processing of the gravity data consisted essentially of the following steps: time shift of 175 seconds due to the overcritical damping of the sensor, conversion from reading units (r.u.) to mGal by applying the conversion factor |of 0.94542 mGal/ru., correction of the observed gravity data to the world gravity net IGSN 71, correction for the Eotvos effect using the navigation data, correction for the instrumental drift, and subbtraction of the normal gravity (WGS67). The result shows the free-air anomaly obtained. The accuracy of the final data is about 1 mGal seen from discrepancies at crossovers of gravity profiles. In this study, we tried to combine the ship-borne observations with the information from Krakatau National Geophysical Data Center in 1988 (Reichert, et al. 1999).

However, the gravimeter had a critical problem

during the cruise, causing insufficient data quality in parts. Therefore, this was compared to the gravity anomalies derived from satellite altimetry (Figure 3), which is measured at a distance of about 100 km from the coast. Possible explanations for discrepancies are an asymmetry of the ocean waves in this area or the presence of strong ocean currents. Such effects result in a systematical error of the calculation of free-air gravity data from altimetry data, as the calculation is based on the assumption that the mean sea level is a function of the mass distribution only. Nevertheless,

free-air gravity anomalies derived from satellite altimetry are of great importance to get an overview of the gravity field in an oceanic area. For detailed investigations, however, shipboard gravity measurements are indispensable.

3) SO-139 GINCO 3 aimed at sampling of

sediments, rocks and seafloor biota, making TV seafloor observations as well as heat flow measurements in order to verify the interpretation of geophysical data and to study heat flow and fluid migration systems of various forearc realms. Particular emphasis was placed on search for methane vent systems and associated biota. During Cruise SO 139, 101 stations were occupied for hydrographic and geothermal measurements as well as for water and geological sampling.

GEOLOGICAL SAMPLING Insitu data acquired from geological sampling during cruise SO139 performed several purposes, which are described by Wiedicke et al. (1999). These are: 1) define nature and age of acousto-stratigraphic sequences where accessible for sampling devices; 2) provide sediment cores for gas analyses of near surface sediments and for pore water analyses; 3) acquire sediment cores from different parts of the accretionary complex for stratigraphic and sedimentological analyses; 4) test seafloor properties in view of a subsequent deployment of the heat flow probe; and 5) assist the search for communities of life forms associated with fluid seeps. This approach allowed sampling and data acquisition from the deep-sea trench to the fore arc basin and was based on coherent structural information observed from the previous cruises. Our work at the BGR focused on sample analysis from core numbers: 11 KL, 14 KL, 29 KL, 31 SL, 37 KL, 50 KL, and 88 SL. Three cores were analyzed together with colleagues from BGR. The cores were stored in the cool storage at the temperatures of about 7°C, to protect from secondary changes and fungi. Lithology from 1m sections of core were described in detail, before sample analysis. The work started with preparing the descriptions of the sediment cores. It followed by subsequent laboratory work including a suite of sedimentological investigations; color scanning of the cores, several detailed sampling sessions to provide suitable material for different

investigations, freeze drying, wet sieving and weighted samples, and analysis of carbonate content, as well as the investigation of micropaleontological and foram samples by extraction of selected foraminiferal species and electron-microscope photography. Carbonate content was measured using a special machine at the BGR from weighted 2 gr of dry sample filled in the tube. The output from this process is a table of water content in % and carbonate content in %, as depicted in Figure 4.

RESULT AND DISCUSSION The combination of remote data and insitu data such as refraction and reflection seismic methods together with magnetic, gravimetric, and bathymetric measurements led to a significantly improved understanding of the composition, structure, and tectonic evolution of the fore-arc complex off West Java and South Sumatra. Geothermal, hydro-acoustic, and hydrographic measurements, as well as analyses of seawater, sediments, rocks, and benthic fauna, recovered during the offshore work of the project, provided new information on the geothermal regime, seismostratigraphy, sedimentary processes, hydrocarbon generation, fluid migration, and fauna associated with fluid seeps in the fore-arc complex. The results show different lithologic conditions and geological processes between Java and Sumatra. The seafloor in the fore-arc basin is 3000 m depth off West Java, and around 2000 m off the Sunda Strait and southern Sumatra. Free-air gravity anomalies are different between Java and Sumatra. Moreover, the southern coast of West Java contains much clay and some volcanic ash layers as well as a carbonate content 10 – 15 % less than that off South Sumatra. We found a new strike-slip fault system that parallels the Sumatra fault called the Mentawai Fault zone in the area within the forearc basin south of Sumatra Island. The very similar tectonic style of the older part throughout the extension of the outer-arc high indicates that the subduction direction in the Paleogene was uniform off South Sumatra and West Java. The subducting slab of the Indo-Australian lithosphere was traced for 135 km from the Sunda Trench landward, and to a depth of 30 km under the outer-arc high. The subduction angle increases

landward from 3.5° to 7°. The MOHO in the upper slab is clearly marked by an abrupt change of p-wave velocities from 7.2 to 8 km/s. Under the Sunda Strait this change occurs at 28 km depth. The style of deformation divides the accretionary complex into a Neogene to Recent part between the trench and the outer-arc high, and a Paleogene part under the outer-arc high. The younger part is dominated by landward dipping thrust sheets, while in the older part tectonic flakes, associated with backstop-backthrust faults, dominate. Moderate p-wave velocities of up to 5.5 km/s in this part of the accretionary complex suggest that it has a mainly sedimentary nature. The very similar tectonic style of the older part throughout the extension of the outer-arc high, indicates that the subduction direction in the Paleogene was uniform off South Sumatra and West Java. A velocity decrease in the younger part of the accretionary complex towards the trench hints at active accretion in its frontal section. The Sunda trench marks the deepest part of the survey area where water depths are more than 6500 m of Java and more than 6000 m of Sumatra. Faults were formed due to oblique subduction in southern part of Sumatra Offshore. The almost north-south direction of subduction is normal in front of Java, and oblique in front of west Java Offshore as well as in the Forearc Basin that shows the same structure such as Mentawai Fault Zone. The free-air gravity anomalies derived from satellite altimetry are of great importance to get an overview of the gravity field in an oceanic area. Based on the interpretation of satellite altimetry it was observed the different condition of free-air gravity anomalies exist between Java and Sumatra. Piston cores from the infra-slope basins and the fore-arc basin showed that their youngest fill consists of Pliocene-Quaternary olive-gray hemipelagic muds with intercalated dacitic to rhyolitic ash layers, as well as turbidites. Results from the laboratory investigations were used to establish a first stratigraphic correlation between several sediment cores and for initial geological interpretations. The general trend found in cores 11 KL, 14KL, 31 KL, 88SL located at the southern of West Java contain much clay and some volcanic ash layers. Carbonate content is 10 – 15 % less than the western part of the area (south of Sumatra). Oxygen isotope from 74 KL showed more than 9 glacial cycles in a core length of about 20 m (personal consultation, Wiedicke, 2000). Volcanic ash layers were found in most cores. In

general, cores south of Sumatra appear to contain more ash layers than cores taken further east. In summary the subduction zone around Sunda Strait as well as different environment conditions were observed in the Indian Ocean off West Java and the Southern Sumatra as a result of this project. ACKNOWLEDGMENT We gratefully thank to Prof.Dr. Thoby Mutis, Rector of Trisakti University, Ir.H. Moch.Thamrin, Dean of Faculty of Mineral Technology, and Dr.Ir. Indroyono Soesilo, MSc, the deputy chairman of BPPT. We would like to give special thanks to Prof.Dr. B.Buttkus from Geophysik Meeres und Polarforschung, Prof.Dr. Chriatian Reichert from Geophysikalische Forschung, and Prof.Dr. Helmut Beiersdorf and M. Wiedicke from Geologicshe Forschung which is provided us for funding and the opportunity for training and data processing on the data of GINCO in Germany, and especially appreciate of using the available geological and geophysical data that acquired R/V Sonne through the GINCO project. We thank to the colleagues of Marine Geology group and Geophysic group, who are help us patiently during the training and data processing in BGR Hannover, Germany. REFERENCES CITED Beirsdorf, H., and all participants, 2001. GINCO, Geoscientific investigations along the active convergence zone between the eastern Eurasian and Indo Australian plates, Summary and Synthesis RV Sonne Cruise so-137 – 139, and Final Report Cruise SO-139, BGR, Annex I-VII, Hannover, Germany, p. 1-18. Beirsdorf, H. 1999. Introduction, Implementation of Cruise SO-139, in Cruise Report, Sonne Cruise SO-139, Geoscientific investigations along the active convergence zone between the eastern Eurasian and Indo Australian plates off Indonesia (GINCO 3), BGR, Hannover, Germany, p. 1-5

Flueh, E., Schreckenberger, B., Bialas, J. and Shipboard scientific party, 1999. FS SONNE Cruise Report SO138- GINCO-2 GEOMAR Report, 81, ISSN0936-5788, 333p. Ghose, R., Yoshioka, S. and Oike, K, 1990. Three-dimensional numerical simulation of the subduction dynamic in the Sunda arc region, Southeast Asia - Tectonophysics, v. 181, p. 223-255. Hamilton, W., 1988. Plate tectonics and island arcs. Geol. Soc. A. Bull, v. 100, p. 1503-1527. Kieckhefer, R.M., Shor, G.G. Jr, Curray, J. R., Sugiarta, W., and Hehuwat, F., 1980. Seismic refraction studies of the Sunda trench and forearc basin, Journal of Geophysical Research, v. 85, p. 863-889. Wiedicke, M., Sahling, H., Delisle, G., Faber E., Neben, S., Beiersdorf, H., Marchig, V., Weiss, W., Von Mirbach, N., and Afiat, A., 2002. Characteristics of an active vent in the fore-arc basin of the Sunda Arc, Indonesia, Marine Geology, v. 184, p. 121-141. Wiedicke, M., Weiss, W., Udrekh, Premana Haryadi, and Steinmann, 1999. Geological Sampling and Core Logging, in Cruise Report, Sonne Cruise SO-139, Geoscientific investigations along the active convergence zone between the eastern Eurasian and Indo Australian plates off Indonesia (GINCO 3), BGR, Hannover, Germany, p. 63-73. Reichert, C., and GINCO Scientific party, 1999. Geoscientific investigations on the active convergence zone between the east Eurasian and Indo-Australian plates along Indonesia (GINCO 1), Cruise Report, SONNE Cruise SO 137. -BGR, unpublished report, archives no. 118844, 141 p.

Figure 1 - Plate tectonic situation and the morphology of Subduction zone in Indian Ocean off West Java and the Southern Sumatra (Kieckhefer, et.al, 1980 and Wiedickie, 2002)

Figure 2 - Satellite image of Location of Research area including bathymetry (in m).

Figure 3 - Comparison of the measured free-air gravity anomalies (ship-based) and derived from satellite altimetry along line S0137-12/12A (Reichert et al., 1999).

Figure 4 - Diagram of carbonate content from S-139 – 88 SL (West Java). and SO-139 – 74 KL (South Sumatra).