Geological Society of Malaysia Annual Geological Conference 2001 June 2-3 2001, Pangkor Island, Perak Darul Ridzuan, Malaysia

Wrench tectonics in Sundaland - Subsurface and offshore evidence

H.D. TnA

PETRONAS Research & Scientific Services Sdn Bhd Lots 3288 & 3289 Off Jalan Ayer ltam, Bangi Institutional Area, 43000 Kajang

Abstract: Wrenching is widespread in Sundaland. Convincing evidence from onshore mapping is now combined with lesser known information from the subsurface and from the offshore. Maximum principal stress (SH) directions were determined from wrench patterns, well-bore breakouts, and first-motion of major earthquakes occurring in the last century. Most of Sundaland is currently subjected to north-south Sw Towards its margins the stress trajectories deviate from the meridian, probably as result of interference with SH of the convergence of adjoining megaplates and the southeast extrusion of Indosinia. From at least the late Oligocene onward Sundaland has been the focus of converging plates and subplates. Fracture zones that are suitably orientated with respect to the convergence direction in various parts of the region responded by wrenching. Until approximately the onset of Mid-Miocene most wrenching was transtensional forming pullapart depressions and modifying the structuration of the large depocentres: the backarc basins of Sumatra­Java, the aulacogens Malay-Penyu-West Natuna, and the forearc/marginal basins Soikang-Sarawak- NW Sabah-East Kalimantan. Cessation of spreading in the Philippine Sea and Caroline basins by Mid-Miocene changed the wrenching into transpressional structures that was accompanied by slip-sense reversals and substantial structural inversion.

Abstrak: Sesaran mendatar adalah umum di Tanah Sunda. Buktinya yang diperoleh menerusi pemetaan di darat telah dipadukan dengan maklumat daripada bawah-tanah dan lepas pesisir. Tegasan utama maksimum (SH) dapat ditentukan berdasarkan corak sesaran mendatar, bujuran lubang gerudi serta gegaran perdana gempabumi gadang yang berlaku di kurun lepas. Semasa ini hampir seluruh Tanah Sunda mengalami SH berjurus utara-selatan. Penyimpangan arah SH berlaku berhampiran tepian Tanah Sunda di mana pengaruh penumpuan keping-keping raksasa dan ekstrusi Indosinia ke tenggara dipercayai sedang berlaku. Sejak sekurang-kurangnya penghujung Oligosen Tanah Sunda telah menjadi tumpuan pergerakan keping-keping litosferajiran. Jalur-jalur retakan yang berjurus sesuai terhadap arah penumpuan di masing-masing sektor Tanah Sunda telah bertindak sebagai sesar mendatar. Pada permulaan sesaran mendatar berupa transtensi yang menerbitkan lekukan"pullapart" dan meminda penstrukturan deposenter besar lainnya seperti lembangan­buri-arka Sumatra-Jawa, aulakogen Melayu-Penyu-Natuna Barat, lembangan-hadapan/tepian Soikang-Sarawk-Sabah Baratlaut-Kalimantan Timur. Selepas tamatnya pemuaian lembangan Filipina and Carolin sekitar Miosen Tengah, sesaran mendatar di rantau ini bertukar sifat kepada transpresi yang disertai pembalikan arah sesaran dan penyongsangan struktur yang jelas.

INTRODUCTION

Sundaland is the geological region compnsmg the large islands Borneo/Kalimantan, Sumatra and Java, further the Malay Peninsula, the Sunda Shelf and its smaller islands and island groups (Figure 1). To biologists, Sundaland is the Southeast Asian region west of Strait Makassar. On almost all sides Sundaland extends to the 200-m isobath of the Indian Ocean, Strait Makassar, South China Sea, and the Andaman Sea. The northern boundary is artificial and uncertain but it may be considered to partly coincide with the Thailand-Malaysia international boundary. The Sunda Platform is a smaller entity that comprises the pre-Tertiary outcropping areas within Sundaland. The structural grain of Sundaland is semicircular and approximately centred about the South China Sea Basin. This structural pattern has been highlighted in van Bemrnelen's undation theory (1977, update of his original1933 theory) and updated in Katili's (1973) plate tectonic outline of the Indonesian Archipelago. The western and southern half of Sundaland is dominated by regional, north-south fracture zones. North­south fractures in Peninsular Malaysia have been considered

the oldest set of faults (Geological Survey of Malaysia, summaries in its annual reports, e.g. 1973). Perhaps theN­S faults existed since early Jurassic and some have subsequently experienced reactivation (Tjia, 1978). Wood (1985) extended land-based regional faults into the offshore using generally proprietary material acquired by offshore hydrocarbon exploration. The theory of plate tectonics requires lithospheric plates to move laterally. At convergent and transform plate boundaries the movements create stress regimes where the maximum principal stress is horizontal. The convergence angle mainly determines if thrusting (including subduction) or transcurrent faulting occurs. Transcurrent faulting takes place at oblique convergence, and wrenching is possibly most easily achieved where the convergence angle is in the region of 30 degrees, as shear failure laboratory experiments by Donath (1961) seems to indicate. At local scale, decoupling of stress systems probably occurs and the pattern of stress tensors across the plate boundary may seem inconsistent. On a regional scale the motion-sense of the neighbouring plates has been shown to extend far into the Southeast Asian region (Tjia, 1973).

This paper presents less known evidence of the

72 H.D. TJIA

PLATE BOUNDARIES: TRENCHES (1) SUNDA (12) LUZON (13) MINDANAO SUTURES (4) BENTONG-BENGKALIS (7) RED RIVER (8) VIETNAM SHEAR &

LUPAR TRANSCURRENT FAULTS (2) SUMATRA (3) SAGAING (5) THREE PAGODAS (Sa) AXIAL MALAY (6) MAE PING (9) WEST BARAM (1 0).BALABAC (11) PHILIPPINE (14) PALU-KORO (15) MATANO (16) MADURA STRAIT (17) BANYUMAS (18) RATU (19} MENTAWAI (20) KHLONG MARUI-RANONG

Figure 1. Tectonic framework of Sundaland and adjacent regions.

significant role of wrench faulting since the Jurassic in shaping and modifying Sundaland. Most of the present illustrations originate from hydrocarbon exploration but for obvious reasons are restricted to nonproprietary material.

REGIONAL STRIKE-SLIP FAULTS

Sumatra

Before the acceptance of plate tectonics, least as a working hypothesis and by most geologists as full-fledged theory, part of the Sumatra Fault zone, its so-called "Oeloe Aer" (Ulu Air) segment in the northern half of the island, was recognised by Durham (1940) to possess significant lateral displacements. By publication Katili and Hehuwat (1967) and Katili (1970) placed the entire 1600-km long Sumatra fault zone on the world geology map. Dextral displacement ranges from about 42 km (Hahn andWeber, 1981) for Middle Miocene(?) granitoids to several metres during earthquake events. Hamilton (1979, p. 82) classifies the Sumatra fault zone (SFZ) as a dextral transform that extends northward and across the Andaman spreading ridge continues as the Sagaing dextral transform into Myanmar. Evidence of lateral slips on both transforms were referred to by Harding (1974) while expounding on the seismic characteristics of wrench faults. In the south, in the vicinity of Strait Sunda, the SFZ possibly ends into the Sunda subduction trench. Offshore the western shoreline of

ANDAMANSEA REGIME

COMPRESSION

MAJOR WRENCH FAULTS NORTH SUMATRA BASIN

,..,..,

NORTH

Figure 2. In the North Sumatra Basin, northerly striking regional faults have dextral slip sense, except the Pe.usangan fault which is sinistral. Motion sense on the latter is probably controlled by the spreading direction (solid arrow) of the Andaman Basin.

Sumatra, geophysical evidence for a so-called Mentawai dextral wrench fault was reported by Diament et al. ( 1992). This regional fault is strikes parallel to the SFZ and occupie the forearc basin.

Regional mapping in the late 1970s and early 1980s by cooperative efforts of the Indonesian Geological Survey and British Overseas Geology Department recognised the existence of a good number of strike-slip faults in the northern half of Sumatra (see Cameron et al., 1980). Hydrocarbon exploration contributed a fair share of the growing understanding on the role played by lateral fault displacements (Situmorang and Barlian Yulihanto, 1985; Sosrornihardjo, 1988; Yarmanto and Aulia, 1988; Wajzer et al., 1991; Ryacudu et al., 1992; Barlian Yulihanto, 1997). Wrench motion on the Sumatran faults is most often dextral (Figure 4b) and this takes place along northerly and northwesterly strikes. The NW-striking Sumatra fault zone is a prime example. The Bengkalis depression in Central Sumatra strikes north-south and has notable dextral strike­slip segments (see Eubank and Makki, 1981; Villaroel, 1985). An exception is the northerly striking Peusangan fault that shows left-lateral displacements in the order of 20 km (Figure 3). East of the Peusangan fault zone in North Sumatra, the other northerly striking faults Arun, 98 , Tanjungpura and Tanjung Morawa have notable dextral displacements (Figure 2). Examples of wrenching in the three Sumatran petroleum basins follow. The Rantau field of the North Sumatra basin is represented by the depth structure map of the upper Miocene Keutapang Formation (Figure 4a). It consists of a faulted anticline striking ESE­WSW. The en echelon pattern of the normal faults suggest sinistral wrenching parallel to the anticlinal trend. Figure 4b shows the two major structural trends in the Central Sumatra basin, NW (Lirik trend) and N (Merbau trend). Both have reportedly dextral displacements. The Benakat field of the South Sumatra basin at the Miocene Talangakar horizon shows two zones of en echelon fault patterns (Figure

Geological Society of Malaysia Annual Geological Conference 2001

WRENCH TECTONICS IN SUNDALAND - SUBSURFACE AND OFFSHORE EVIDENCE 73

.. / I 12'

' / .·

\ I

\ I

\

FAULT ZONE

Figure 3. The Peusangan fault and its indicators in North Sumatra.

4c) . The zones strike NNE and easterly and correspond with dextral and sinistral wrenching, respectively. The regional maximum principal stress that produced the wrench patterns in the Sumatran oil basins is interpreted to have operated in NNE-SSW direction.

Strait Melaka

Exploration for hydrocarbon on the Malaysian side of the Strait of Melaka discovered more than ten faulted depressions, each in the order of 50 km x 25 km. The faulted depressions are grabens and half-grabens (Figure Sa). The structures seem associated with N-S fault zones in the pre-Tertiary basement (Liew, 1994, 1997). En echelon arrangements of many of these depressions suggest formation in an early Palaeogene transtensional regime where the regional compression acted in N30°E (Tjia 2000). Structural inversion in early to middle Miocene deformed the graben-filling sediments and indicate a corresponding transpressional stress regime, that was still dominated by N30°E regional compression.

Peninsular Malaysia

The large fracture zones onshore Peninsular Malaysia have established lateral displacements. A recent study and review was by Zaiton Harun (1992). In descending order of total length and frequency the strike large fracture zones cluster about north, north-northwest, northwest and west­northwest. The Ma'Okil fault striking is the sole known ,

June 2-3 2001, Pangkor Island, Malaysia

I

'•· ~

\ ... " \

RANTAU FIElD, NORTl-1 SUIIIATRA FAULTED ANTICLINE OF UPPER MIOCENE

KEUTAPANG FORMATION

\ \

Figure 4a. En echelon faults across the Rantau anticline, North Sumatra Basin. Simplified after Courteney et al. (1989).

STRUCTURES, SOURCE KITCHEN IN THE SOUTHERN CENTRAL SUMATRA BASIN S.P. Todd et al. (1997) Geol. Soc. Spec. Publ. 126

0 SOURCE KITCHEN AREA

• OIL FIELD Figure 4b

Figure4b. N-S and NW -SE structural trends in the Central Sumatra Basin. Both possess dextral slip. After Todd et al. (1997).

strike-slip representative with dextral displacement (Burton, 1965). The northerly striking Bentong Suture has sli ckensided surfaces corresponding with dextral slip. Its interpreted extension in Central Sumatra, the north-striking Bengkalis Trough, is also marked by dextral displacement of lower Neogene geological contacts. However, the fracture zones representing the Bentong Suture and other regional fau lts onshore the Peninsula were active only up to the Middle Eocene. This assumption is based on probably reset K/Ar ages of fault breccia and mylonite (Zaiton Harun,

74 H.D. TJIA

103 .ca

BENAKAT FIELD, SOUTH SlJMATRA Structure Top Talang Akar- c.i. 50 m

Figure4c. Benakat field, South Sumatra Basin. Dextral and sinistral slip sense as ind icated by en echelon fault patterns. The generating maximum principal stress direction is approximately NE-SW. Simplified after Courteney et al. ( 1990).

1992). The latest strike-slip motions on the Peninsular regional faults together with other directions of geological transport cones pond to tectonic compression that operated about 070° - 250°.

Malay Basin

Lateral slips on the major fa ult zones in the Malay Basin are common. The youngest sed imentary units affected are of the Middle Miocene. Time sli ces down to near pre­Tertiary basement of the Sotong fie ld show a rhombic pullapart, actual offset of a geological structure (Mastura Abdul Malik and Tjia, 1998) beside the more common en echelon patterns indicative of transcurrent motion. In se ismic sections, flower structures can be clearly demonstrated. The Malay Basin probably began as one of three rift arms (the other two being the Penyu and West Natuna depressions) in the Late Cretaceous when a hot spot developed in central Sundaland. The triple junction of the ensuing Malay Dome marked the area where the three rifts come together. A SE extension of the Three Pagodas fault is beheved to control the NW grain of the Malay basin. This basement fault is named as Axial Malay fault (AMF). The extrusion of Indos ini a according to Tapponnier's et al. (1982) tectonic hypothesis caused the AMP to slip left-laterally during the Late Oligocene. The regional stress field was then transtensional and E-W oriented half-grabens associated with the AMF developed

in the pre-Tertiary basement. These depressions acted a satellite depocentres of the large Malay basin. The regional stress field became transpressional, mainly in Middle Miocene, as result of changing plate dynamics. The AMF then moved dextrally and the sedimentary half-graben fill were squeezed into fo lds whose orientations were essentially controlled by the original trend of the depressions , that is, east-west. Inversion tecton ics are also demonstrated by younger reverse motion on large growth fa ults, and by rhomboid pullaparts that are incongruent with the occunence of anticlines built of their sedimentary fil ings (Figure 6). The plans of the Berantai-C pullaparts along the Western Hinge Line fau lt zone and the associated en echelon faults indicate dextral wrenching in a transtensional stress regime. However, their sedimentary fills were deformed in to folds in a subsequent transpressional stress f ield and indicate reversal of sense in strike-s lip motion. The en echelon faults of the Besar f ie ld a lso reflects this.

Java Sea West

In the SW corner of Sundaland are the Asri and Sunda Cenozoic basins. Wrench fau lting is shown by discrete zones of en echelon fau lts (Figure 7). Left and right-lateral wrench zones appear to have responded to a regional compression in abo ut N020°E direction. This is also the approximate trend of the Thousand Islands fault zone, and two other fau lt zones to the west of Kartini and Karmila fie lds, whose fau lt patterns are anastomosing. These relatively wide zones comprising curv ing fault plans are consistent with extens ion parallel to the regional maximum principal stress direction (SH). This stress is interpreted to have operated in NNE - SSW direction.

Northwestern Borneo

The northwestern side of Borneo has structural trends that are disturbed by regional fracture zones (Figure 8). Wrenching is shown by a few of these fracture zones; the various slip senses are indicated on the figure. The sinistral displacement on the Tubau fau lt amounts to about 25 km (Tj ia et al., 1998) The northerly trending Belait belt and Jerudong-Morris fault are regional fractures. Abrupt change of the structural grain occurs across these regional fau lts; left lateral drag is indicated. The West Baram Line figured prominently in the offshore palaeobathymetry and separated deep marine conditions to the east fro m shallow-marine environments to its west.

Eastern Borneo

The Attaka fie ld of the Kutei basin in East Kalimantan shows the structure of the upper M iocene deltaic clastics as in Figure 5b. It is a faulted anticl ine striking Nl0°W transected by NW -SE normal faults. These faults down throw between 150 m and 300 m to the NE and are en echelon.. Sinistral wrenching is indicated for the northerly trending zone occupied by the anticline. The interpreted maximum principal stress direction operated in the NW quadrant.

Geological Society of Malays ia Annual Geological Conference 2001

WRENCH TECTONICS IN SUNDALAND - SUBSURFACE AND OFFSHORE EVIDENCE 75

r· ) A.s I I

c.i. 0.2sTWT

Figure 5a. Central and Southern grabens are Cenozoic depressions on the Malaysian side of Strait Melaka. The en echelon fault patterns suggestNE-SW maximum principal stress direction. After Liew (1997).

CONCLUSIONS

The earlier mentioned examples for major wrench faulting in Sundaland comprise but a small part of the known indications of strike-slip deformation that has affected Sundaland during the Cenozoic. The available information combined with movements of adjacent megaplates, well-bore breakout directions (Tjia and Mohd Idrus Ismail, 1994) and earthquake SH directions (Tjia, 1983) has resulted in Figure 9 which represents the current stress field of Southeast Asia. For the region proper, Strait Makassar appears as a major geotectonic boundary. Stress trajectories to its east are clearly influenced by the WNW­ward convergence of the Pacific Plate. The core of Sundaland experiences northerly SH that deviates significantly towards its margins. In the northwest the deviation can be attributed to the opening direction of the Andaman Basin; towards the geotectonic line in the Makassar Strait, the influence of the Pacific Plate convergent direction increases in strength. The stress trajectories in the SW and southern margins of Sundaland probably bend in accordance with the convergent direction of the Indian Ocean-Australian plate.

June 2-3 2001, Pangkor Island, Malaysia

ATTAKA STRUCTURE

KUTEI BASIN, KALTIM ~~ JO'f.

~k~

Depth in 1000 feet, Late Miocene 44-7 Sand Central anticlinal area is approx. 25 km2

Figure 5b. The Attaka structure in the Kutei Basin, East Kalimantan. Simplified after Schwartz et al. (1973).

Figure 6. Pullapart depressions and en echelon fault patterns are products of transcurrent motion along the Western Hinge Line fault. The wrenching had opposite sense during different deformation episodes (see text). Adapted from Tjia and Liew (1996).

76 H.O. TJIA

106"30' E.

10km

REGIONAL COMPRESSION

DIRECTION

Figure 7. Asri and Sunda basins, Java Sea-West. The maximum principal stress in NNE-SSW direction was responsible for the wrench zones (en echelon fault pattern). Simplified after Fainstein and Prarnono (1985) and Wight et al. (1997).

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-··

TERnARY STRUCTURAL TRENDS

SARAWAK · SABAH

"' Sea

I WEST BAUNGIAN LINE - · 2 WEST BARAM LINE 3 BALABAC FAULT 4 MULU SHEAR 5 JERUOONG·MORRIS F. 6 BElAIT BELT 7TINJARUNE 8 OENGAN FAULT

K KINASAI..U SUTURE 9 TUSAU FAULT L WPAR SUTURE 10 MERSING UNE

I

Figure 8. Major wrench zones in the Sarawa.k Basin region.

Figure 9. The current stress field of Southeast Asia.

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