The Geological Framework Asia

This chapter outlines the principal geological features of the region, extending from Myanmar and Taiwan in the north, southwards to include all the ASEAN countries,and extending as far as northern Australia (Figure 1.1). The present-day lithospheric plates and plate margins are described, and the Cenozoic evolution of the region discussed. Within a general framework of convergent plate tectonics, Southeast Asia is also characterized by important extensional tectonics, resulting in the world’s greatest concentration of deep-water marginal basins and Cenozoic sedimentary basins, which have become the focus of the petroleum industry.

The pre-Cenozoic geology is too complex for an adequate analysis in this chapter and the reader is referred to Hutchison (1989) for further details. A chronological
account summarizing the major geological changes in Southeast Asia is given in Figure 1.2. The main geographical features of the region were established in the Triassic, when the large lithospheric plate of Sinoburmalaya (also known as Sibumasu), which had earlier rifted from the Australian part of Gondwanaland, and
collided with and became sutured onto South China and Indochina, together named Cathaysia. The result was a gr eat mountain-building event known as the Indosinian
orogeny. Major granites were emplaced during this orogeny, with which the tin and tungsten mineral deposits were genetically related. The orogeny resulted in general uplift and the formation of major new landmasses, which have predominantly persisted as the present-day regional physical geography of Southeast Asia.
General Plate Tectonics The Indo-Australian Plate is converging at an average rate of 70 mm a −1 in a 003° direction, pushed from the active South Indian Ocean spreading axis. For the most part it is composed of the Indian Ocean, formed of oceanic sea-floor basalt overlain by deep water. It
forms a convergent plate margin with the continental Eurasian Plate, beneath which it subducts at the Sunda
or Java Trench.
The Eurasian continental plate protrudes as a peninsular extension (Sundaland) southwards as far
as Singapore, continuing beneath the shallow Straits of Malacca and the Sunda Shelf as the island of Sumatra and the northwestern part of Borneo (Figure 1.1). A Cenozoic arc of volcanic and non-volcanic islands, related to the Sunda subduction system, links Sumatra with Papua, which geologically is an extension of the continental lithosphere of Australia, continuous with it beneath the shallow Arafura Sea. A complicated volcanic archipelago, subduction, and fault system links the
Bird’s Head of northwestern Papua, through Sulawesi and the Philippines, with Taiwan. The Philippine Sea Plate, pushed westwards by the Pacific Plate, converges on the Eurasian Plate at Taiwan in a 307° direction at 86 mm a −1 (McCaffrey 1996). The present-day physical geography of Southeast Asia is governed by these basic characteristics.

Indian Ocean
Based on magnetic anomaly identification, calibrated by drill site data, three distinct episodes of sea-floor spreading can be discerned (Curray et al. 1982).

1. Anomalies M10 to M25 (Neocomian–Oxfordian) have been identified (Heirtzler et al. 1978) in the Argo Abyssal Plain, between the Sunda (Java) Trench and
western Australia (Figure 1.3). They trend 60° and increase in age towards Australia, dating the original Late Jurassic–Early Cretaceous rifting of India from
Australia. South of the Abyssal Plain, the anomalies trend 30° in the Wallaby Plateau. Thus, while India
separated towards the northwest, some other continental fragment(s) may have moved northerly. Micro-continents containing tin-bearing granites,

2. The spreading pattern was completely reorganized between magnetic anomalies M0 and 34, which is the Cretaceous magnetic quiet period (110–80 Ma ago).From anomaly 34 to 19 (84–44 Ma ago) India made its spectacular rapid northwards flight with rates of 15 to 17 cm a −1 ; the anomalies are aligned east–west, offset by major north–south transform faults. One of the faults is the Investigator Ridge; others lie close to and parallel to the Ninety-East Ridge. The prominent Ninety-East Ridge is the trace of a single mantle hotspot, which now lies under the Kerguelen Plateau in the south Indian Ocean. The furthest end of it is the Rajmahal Traps, 200 km north-northwest of Calcutta, where the basalt has an age of 105 Ma. The  average rate of relative motion of the hotspot trace was
about 11 cm a −1 (Curray et al. 1982).

3. Around magnetic anomaly 19 time (44 Ma ago), spreading completely ceased at the Wharton Ridge in the northern Indian Ocean (Curray and Munasinghe 1989). This striking event in Southeast Asia coincides with the prominent widespread unconformity within the Bengal Fan and with the unconformable continental beginning of many Southeast Asian Cenozoic basins (e.g. Central Sumatra). The cause for this event may be sought in the Eocene collision of India with Eurasia. As India came into full collision, its northwards motion was spectacularly slowed, and spreading became impossible at the Wharton Ridge. A spreading axis between Australia and Antarctica had been in existence since 95 Ma ago. It then propagated westwards to begin the Southeast Indian Ocean Ridge at 44 Ma ago as spreading closed down southeast of India (Veevers 1984). From then to the present, the Indian Ocean of Figure 1.1, India, and Australia, all three belong to a single plate, pushed northwards from the South Indian Ocean spreading axis.

indicating continental crustal origin, have been identified in the Borneo–Sula region (Hutchison 1989), and the Jurassic–Lower Cretaceous ophiolitic basement of northeast Borneo represents the uplifted sea floor, once continuous with the Argo Abyssal Plain, now separated from it by the younger Sunda Trench.

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