The Geological Framework
The Geological Framework
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. 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 great 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. 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.
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