Tropical Southeast Asia

by

Avijit Gupta

• LAST REVIEWED: 25 November 2014
• LAST MODIFIED: 25 November 2014
• DOI: 10.1093/obo/9780199363445-0014

Introduction

The world region of Southeast Asia is separated from the rest of the continent by a vast area of high mountains near the border of India and China, but it is difficult to delineate its other physical boundaries. It, however, can be identified politically as the assemblage of the sovereign area of the countries Myanmar, Thailand, Malaysia, Singapore, Lao People’s Democratic Republic, Cambodia, Vietnam, Indonesia, Timor Leste, Brunei Darussalam, and the Philippines. Southeast Asia is a complex mosaic of continental and maritime environments that include ancient stable landmasses, alluvial plains and deltas of large rivers, inland seas, and volcanic islands. The outer margin of Southeast Asia is marked by a number of islands and deep oceanic trenches with frequent earthquakes, volcanic eruptions, and tsunamis. A long human occupation has modified the landscape, converting alluvial valleys and deltas to agriculture, mostly rice. Closer to the present time, widespread deforestation, spread of plantation agriculture, agricultural expansion, and urbanization have further modified the physical landscape. The related degradation has been particularly severe on rainforests, peat swamps, mangroves, and coral reefs. The original widespread forest cover of Southeast Asia has been reduced to about half its original size. Both plate tectonics and people are thus important contributors to the state of the physical environment in Southeast Asia.

Geology

The early geology of Southeast Asia, which explains much of the distribution of igneous, sedimentary, and metamorphic rocks in the region, is complex. The main characteristics of its physiography started to appear in the Triassic when a lithospheric plate, called Sibumasu, collided with and amalgamated on to Cathaysia, the term for the combined landmass of Indochina and South China (Hutchison 1989, Hutchison 2005). Sibumasu had been detached earlier from the Australian part of Gondwana. The collision gave rise to a mountain-building episode, the Indosinian Orogeny, leading to uplift and formation of mountainous landmasses, granite emplacement, and mineralization of tin and tungsten. The main granitic belt of Southeast Asia extends from northern Thailand and eastern Myanmar through the Malay Peninsula, to as far south as Belitung Island (Oliver and Prave 2013; Oliver, et al. 2014). The continental part of the early Southeast Asia is known as Sundaland. Sundaland went through rifting and graben formation throughout the Cretaceous, Late Eocene, and early Oligocene, leading to the formation of the major oilfields of Southeast Asia. The collision of the Indian Plate with the Eurasian Plate about fifty million years ago formed the Himalaya Mountains and, as Tapponnier, et al. 1982 describes, gave rise to large strike-slip faulting by extrusion tectonics. This opened the South China Sea and formed the alternate pattern of highlands and valleys of mainland Southeast Asia. The continental part of Southeast Asia is bordered by a Cenozoic arc of volcanic and nonvolcanic islands extending from Sumatra to Papua, which is flanked toward the south by a subduction trench known as the Sunda or Java Trench. The subduction is due to the convergence of the Indo-Australian Plate with the continental Eurasian Plate. A subduction trench and a complicated arrangement of volcanic islands and fault systems occur also in the east from northwest Papua to Taiwan through Sulawesi and the Philippines. This subduction zone is due to the westward convergence of the Pacific Plate that is colliding with several smaller plates such as the Philippines Plate. During the Pleistocene, repeated sea level changes due to global ice advance and retreat alternately exposed and inundated a wide extent of shallow continental shelves, known off Asia as the Sunda Shelf and off Australia as the Sahul Shelf. During the last glaciation, the sea dropped about 120 m and rose 6 m above the present level toward the beginning of the Holocene. This led to repeated drainage adjustments; changes in pattern of sedimentation; and migration of vegetation, animals, and people across the exposed land to the present islands (Biswas 1973; Emmel and Curray 1982; Gupta, et al. 1987). Such changes were probably witnessed by Hominins who had been in Southeast Asia for more than one million years.

• Biswas, Buddhadeb. 1973. Quaternary changes in sea-level in the South China Sea. Bulletin of the Geological Society of Malaysia 6:229–256.

  A scholarly account of the sea level changes in the Quaternary, based on sea floor cores from the South China Sea.

• Emmel, Frans J., and Joseph R. Curray. 1982. A submerged late Pleistocene delta and other features related to sea level changes in the Malacca Strait. Marine Geology 47:197–216.

  DOI: 10.1016/0025-3227(82)90069-X

  An account of the drainage development on the exposed sea floor during the Pleistocene ice advance, with special reference to the strait between Sumatra and Malay Peninsula and the surrounding uplands.

• Gupta, Avijit, Ausafur Rahman, Poh Poh Wong, and John Pitts. 1987. The Old Alluvium of Singapore and the extinct drainage system to the South China Sea. Earth Surface Processes and Landforms 12:259–275.

  DOI: 10.1002/esp.3290120305

  Reconstruction of the physical environment in the Pleistocene and associated development of the regional drainage around the South China Sea. Detailed discussion of the Quaternary alluvium in Singapore and neighboring areas.

• Hutchison, Charles D. 1989. Geological evolution of South-East Asia. Oxford Monographs on Geology and Geophysics 13. Oxford: Clarendon.

  The basic monograph on the geology of Southeast Asia. Requires knowledge of principles of geology.

• Hutchison, Charles D. 2005. The geological framework. In The physical geography of Southeast Asia. Edited by Avijit Gupta, 1–23. Oxford: Oxford Univ. Press.

  An introduction to the geology of Southeast Asia. A good place to start.

• Oliver, Grahame, and Anthony Prave. 2013. Palaeogeography of Late Triassic red-beds in Singapore and the Indonisian orogeny. Journal of Asian Earth Science 76:2214–2224.

  Discussion of a logged Triassic red-bed facies including its sedimentology, formation, dates, and linkage with the Indonisian Orogeny. Reports finding of a dinosaur footprint and tracks.

• Oliver, Grahame, Khin Zaw, Mark Hotson, Sebastien Meffre, and Takayuki Manka. 2014. U-Pb zircon geochronology of Early Permian to Late Triassic rocks from Singapore and Johor: A plate tectonic reinterpretation. Gondwana Research 26:132–143.

  DOI: 10.1016/j.gr.2013.03.019

  A revision of the plate tectonic evolution of the Malay Peninsula region, based on U-Pb zircon geochronology of granitic rocks.

• Tapponnier, Paul, Gilles Peltzer, Anne-Yvonne le Dain, Rolando Armijo, and Peter Cobbold. 1982. Propagating extrusion tectonics in Asia: New insights from single experiments with plasticine. Geology 10.12: 611–616.

  DOI: 10.1130/0091-7613(1982)10%3C611:PETIAN%3E2.0.CO;2

  A pioneering discussion on the effect of Himalayan mountain building on the geology and landforms of Southeast Asia.