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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.

The intricate coastline of Southeast Asia, and its many islands and island groups—Indonesia alone has over 17 500 islands—contains 32 per cent (91 700 km2) of the world’s shallow coral reefs (Spalding, Ravilious, and Green 2001). While sedimentary regimes appear to restrict reef development in the East China Sea, the Gulf of Thailand, the South China Sea, and around the island of Borneo, reefs are well developed elsewhere. Fringing reefs characterize island coastlines, and there are also barrier reefs and, in the deeper waters of the South China Sea and to the east, atoll-like reef structures. Although the region has a distinguished history of reef studies—in which the pioneering work of R. B. Seymour Sewell, J. H. F. Umbgrove, and Ph. H. Kuenen on the Snellius expedition (1929–30) come particularly to mind—the lack of detailed information about many areas remains considerable. The coral reefs, and their associated shallow-water ecosystems, within this region are the product of both historical and contemporary processes. A wide range of hypotheses to explain coral distributions have been proposed. These include the importance of the widespread availability of suitable shallow substrates for coral growth with submergence histories determined by regional tectonic and sea-level dynamics (e.g. Hall and Holloway 1998), the variety of habitats present (e.g. Wallace and Wolstenholme 1998), and the more contemporary roles of high sea-surface temperatures and ocean current circulation patterns, including the dynamics of western Pacific Ocean–eastern Indian Ocean connectivity (Tomascik et al. 1997a). Both sets of controls show wide variation across the region. Thus, for example, geological settings range from tectonically stable platforms to rapidly uplifting plate collision zones of considerable seismic and volcanic activity. Present-day environments vary from equable, tranquil interior seas to cycloneand swell wave-dominated environments on the region’s margins. Added to these controls are the perturbations introduced by, for example, periodic coral bleaching and biological catastrophes (e.g. Crown of Thorns starfish infestations; Lane 1996). Taken as a whole, therefore, the coral reefs of Southeast Asia demonstrate enormous complexity and considerable dynamism. These reef resources are, however, under considerable pressure from large, and growing, populations and economic development.

Southeast Asia is a corner of the continent of Asia which ends in an assemblage of peninsulas, archipelagos, and partially enclosed seas. Towards the northwest, the physical contact of this region with the rest of Asia is via a mountainous region that includes the eastern Tibetan Plateau, the eastern Himalaya Mountains, the hills and plateaux of Assam (India) and of Yunnan (China). From this high region a number of large, elongated river basins run north–south or northwest–southeast. These are the basins of rivers such as the Irrawaddy, Salween, Chao Phraya, Mekong, and Sông Hóng (Red). An east–west traverse across the mainland part of Southeast Asia, therefore, is a repetition of alluvium-filled valleys of large rivers separated by mountain chains or plateaux. To the south and to the east are coastal plains, rocky peninsulas, and a number of deltas. Beyond lies the outer margin of Southeast Asia, the arcuate islands of Indonesia, and the Philippines with steep volcanic slopes, intermontane basins, and flat coastal plains of varying size. This assemblage of landforms has resulted from a combination of plate tectonics, Pleistocene history, Holocene geomorphic processes, and anthropogenic modifications of the landscape. Most of the world has been shaped by such a combination, but unlike the rest of the world, in Southeast Asia all four are important. The conventional wisdom of a primarily climate-driven tropical geomorphology is untenable here. The first two factors, plate tectonics and the Pleistocene history, have been discussed in Chapters 1 and 2 respectively. In the Holocene, Southeast Asia has been affected by the following phenomena: • The sea rose to its present level several thousand years ago. • The present natural vegetation, a major part of which includes a set of rainforest formations, achieved its distribution. • A hot and humid climate became the norm, except in the high altitudes and the extreme northern parts. • The dual monsoon systems blowing from the northeast in the northern hemispheric winter and from the southwest in the summer (and in general producing a large volume of precipitation) became strongly developed.