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CONTENTS

N-S (N 0⁰ – 10⁰E), WNW-ESE (ca.N 300⁰E) and NW-SE (ca.N 330⁰E) trending major faults constitute the basic basinal framework of the (Tertiary) Palembang Basin in South Sumatra (Fig. 1). The N-S and WNW – ESE faults are of Pre-Tertiary age and principally by way of renewed tectonic activities along these faults, controlled subsequent Tertiary basin history (Pulunggono, 1983). Investigations in the Garba Mts, in the Jambi basinal area and SAR analysis of South Sumatra also reveal striking N-S and WNW – ESE oriented sets of parallel aligned faults of considerable lengths, intersecting and offsetting one another and dissecting Pre-Tertiary as well as Tertiary rock formations. SW-NE (N 35⁰ – 40⁰E) directed faults are also conspicuous in the Jambi area (Trisakti Univ.,1993; Pertamina 1996). N-S and WNW – ESE (N 600W) trending major faults have been recognized also in the Central Sumatra Basin and linked to a Mesozoic deformation period. These faults were important elements during subsequent Tertiary basinal development. (Heidrick & Aulia, 1993). Recent investigations in the Ombilin Basin and onshore parts of the Sibolga forearc basin reveal partly outcropping N-S, WNW – ESE and SW – NE major faults planes, with the Ombilin Basin basically made up of several north – south trending half grabens, bounded by N-S faults which to the east is bounded by a major WNW – ESE (N 3300E) trending faultzone . The western limit is the NW-SE oriented (Barisan) faultzone while a large SW-NE (N 400E) fault act as a principal border fault in the northwest(Fig. 2). Paleostress reconstructions show tensional and compressional systems and recognize paleostress directions representing phases of tectonic activities. Sedimentary cycles have also been determined which related to the phases of tectonism, establish the tectonic and depositional history with the N-S, WNW – ESE and SW-NE major faults clearly controlling basinal development. Since Late Eocene time, successive tectonic movements of tensional and compressional character along those SW-NE, N-S and WNW-ESE oriented major faults were responsible for consequent depositional and structural features i.e. fan deposits, half-grabens and strike-slip faults. Subsequent tectonic movements of mainly compressive character since Mid-Miocene time were (also) focused along the established WNW – ESE, N-S and SW-NE fault configuration resulting a.o. in thrusting, reverse faults and inversions along the WNW – ESE Takung fault (Fig.3; ITB, 1995-1998). Abovementioned structural and depositional phenomena have also been recognized or inferred within the Central Sumatra, Jambi and Palembang basinal areas with the ever crucial role of the WNW – ESE, SW-NE and N-S trending faults (Fig. 3). The distributional pattern of especially the WNW-ESE and N-S trending faults throughout Sumatra and the obvious similarity in character and times of inception of structural and depositional features in South-,Central- and West Sumatra (Fig. 4) which are persistently linked to the N-S and the WNW-ESE plus their N 300 – 400E faults, point to the existence of a regional system of N-S and WNW-ESE trending megashears, incepted within the Mesozoic, apparently closely linked and subsequently also instrumental in shaping the present pre-Tertiary (basement) lateral configuration of Sumatra which is a mosaic of continental and oceanic microplates (Fig 5; Pulunggono and Cameron, 1984). The Eo – Oligocene plate tectonic setting of southwestern Sundaland i.e. proto-Sumatra, was dominated by the northward directed impingement of India into Eurasia (45-50Ma). The marked reduction of convergence rate of the northward moving Indian oceanic plate initiated a rift system along North-South major transforms, whereby Eocene spreading south and southwest of Sumatra was predominantly oriented E-W with major transforms oriented parallel to the N 0⁰ – 15⁰E trending Ninety-east ridge (Fig. 6). The major Tertiary (N-S) rift system of onland Sumatra were initiated during the Eocene, probably also under roll-back conditions of the obliquely subducting Indian oceanic plate under Sundaland, and rifting phase was well under way during the Late Eocene. Eo-Oligocene rifting took place along the old N-S set of basement “weak zones” or megashears of Mesozoic age and along the N 30⁰-40⁰E faults. In conjunction, movements along the existing pre-Tertiary WNW-ESE megashears (reactivation) took place and were expressed as Late Eocene strike-slip movements. In a regional Southeast Asian context, this implies that during most of the Paleogene and Neogene, Sundaland did not behave as a “rigid” block as believed by some authors. Impact between India and Southeast Asia had also affected southwestern Sundaland (proto Sumatra) and should have induced dextral strike-slip faults but expressed as (reactivated) movements along the existing pre-Tertiary WNW-ESE trending shears. The possibility of “extrusion” of segments of Sundaland to the southeast (or ESE) along those WNW-ESE series of shears is not out of the question. The Neogene compressive regime i.e. the aforementioned inverted structures along the WNW-ESE Takung Faultzone in West Sumatra and also seen in South Sumatra Basin (the WNW-ESE Lematang Faultzone and the Saka Faultzone) and in Central Sumatra Basin (Rokan-Kempas-Beruk and Daludalu-Kampar-Tigapuluh foldbelts) can not only be attributed to the increase of convergence rate of the north-northeastward moving Indian oceanic plate, but may as well justify the existence during Mid-Late Miocene of a southwestward directed stress towards Sumatra consequent to Borneo’s anticlockwise rotation as proposed by Hall in 1995. Furthermore, Sumatra’s Eocene (Oligocene ?) magmatic arc was clearly trending WNW-ESE (N 300⁰E) compared with the current NW-SE (N 330⁰E) orientation of the Barisan Mountain Range and Barisan Faultzone of Mid-Miocene origin (Fig. 7). This explains the presence of Eocene (Oligocene ?) volcanics in the present South Sumatra backarc basin and their obvious absence in present-day Central and North Sumatra backarc basins with those volcanics being anticipated within presently known forearc environments of Central and North Sumatra, west of the Barisan Mountain Range. In this context, it is worthy to note the presence of calc-alkaline intrusions dated in the 52-57 Ma range in the southwestern edge part of the Ombilin Basin and igneous rocks of 56 Ma-49 Ma age at Sibagindar, west of the Barisan Mts while igneous rocks of Oligocene age were reported in the Padang area, also west of the present Barisan Mts (ITB, 1995-1998). Also, the distribution of (Jurassic and Cretaceous) intrusive granitoids is obviously WNW-ESE (N 300⁰E) oriented. Segments of the WNW – ESE oriented proto-Sumatra were progressively offsetted by the ever northward moving Indian Ocean plate with its SSW-NNE transforms (or the onland S-N megashears) notably since Miocene time, hence shaping the present NW-SE (N 3300E) orientation of the isle of Sumatra, in contrary to the idea of a clockwise (or even anti clockwise) rotation of Sumatra. The N 3000E structural trend had been described by Karig in 1975 on Musala island in the present-day forearc region west of North Sumatra and later on in 1990 a joint French/LIPI team reported N 3000E trending faults crossing the Mentawai Faultzone west of Sumatra. Furthermore, Daines’s pre-collision reconstruction of Eurasia based upon Tapponier’s extrusion tectonics model (Tapponier, 1984) suggests an extensive “Jurassic Suture”, separating Indochina and Sunda, which propagated the paralelly aligned WNW-ESE (N 3000E) trending Malay-Natuna-Lupar shear zone along which subsequent Late Eocene (40 Ma) graben development took place (West Natuna) with rifting along SW-NE faults (Daines, 1985). This is in congruence with the initiation of the Tertiary basins in Sumatra further south with the understanding that the Sumatran WNW-ESE shear zone projected along Natal-Takung-Duabelas Mts-Lematang is also a suture between the continental (Perm-Carbon) Mergui-and accretionary (Juro-Cret.) Woyla terranes. Obvious N-S trending Tertiary basins within the Gulf of Thailand (Shaw, 1997) and the N-S running pre-Tertiary Raub-Bentong suture in Malacca Peninsula may have their southward continuations within the present-day Sumatra backarc basins. Within a likely a set-up of conjugate WNW-ESE strike-slip faults in the region, a similar mode of basin development, namely N-S pull-apart basins, could be inferred for the Tertiary Sumatra basins (in particular the North Sumatra Basin) like the N-S trending Tertiary basins in the Gulf of Thailand as well as in Central Thailand. The N-S and WNW-ESE Sumatran megashears are part of a fault system of Mesozoic (or older?) age which appear over large portions of Southeast Asia in a rhombic geometry. This fundamental pattern, representing zones of weakness, is well-developed throughout the pre-Tertiary “basement” and is crucial for subsequent Tertiary sedimentary basin development.