1.6 INTRODUCING THE GEOLOGICAL HISTORY
OF THE MEKONG BASIN

From the perspective of global tectonics, Southeast Asia is a fascinating jigsaw puzzle. International research during the last decades has led to the recognition that the geological backbone of Southeast Asia consists of a series of continental fragments. Geoscientists often refer to them as "terranes". The terranes of Southeast Asia are all derived from the ancient continent of Gondwanaland.

Global tectonic activity map The global tectonic activity map reveals all major plate boundaries and faults that were active over the past one million years (see legend on map). The most important structure in the vicinity of the Mekong river basin is the Red River Fault , named after the Red River in Viet Nam which runs as a straight line in a NW-SE direction. Source: [core2.gsfc.nasa.gov/dtam]

Today, these pieces make up an intricate pattern of blocks that include China, Southeast Asia, parts of the China Sea and Burma. Recent plate tectonic theory has postulated, convincingly, that the geological history of this area is closely linked to a sequence of earth moving events that culminated with the collision of a considerable chunk of Gondwana, a tectonic plate now known as India, into Eurasian continent. This process moved ahead at a record velocity of some 6 cm per year, causing the mountain-building process of the Himalayas and the uplifting of a huge chunk of continental plate, now known as the Tibetan Plateau.

Interestingly, this model of deformation of the Eurasian landmass had been described by Argand as early as 1921. His model illustrates the thickening of the Tibetan crust by under-thrusting of the leading portions of the Indian continent. This remarkable section predates formal plate-tectonic concepts by over 45 years!

Distribution of principal tectonic blocks Distribution of principal tectonic blocks ('terranes') and geologic welding lines ('sutures') in SE Asia. (After Metcalfe, 1996). 'Terranes' that make up the mosaic of present day Asia. On the schematic map, the outline of the modern coastline is shown in blue for reference. Note the straight suture and fault line of the Red River (1). The names of other sutures and blocks are given below.

The relatively recent event of the collision of India and Eurasia has also had a pronounced effect upon the whole region east of the impact. Simply put, the plate tectonic movements can be replicated with a small experiment by pressing one's hand on a tablecloth and by moving the hand slowly forward for about 30 cm. The resulting fan-shaped folding pattern of the cloth, especially to the right of the hand is not unlike the tectonic pattern that emerged in Southeast Asia. It is marked by a series of the geologic faults, for example the Red River Fault of Northern Viet Nam, and the outward arching valleys of the principal rivers, including the Brahmaputra, Salween, Mekong, Red River and Yangtze. In a more scientific approach, geoscientists have experimented with plasticine models and achieved remarkable results that closely resemble the modern tectonic map of Asia.

The previous maps illustrate that compression and faulting is the main pattern of deformation, but it accounts for perhaps half of India's penetration into Eurasia. The other half is made up of China and Southeast Asia being pushed eastward and out of the way of India like toothpaste is squeezed out of a tube. Those movements took place along the enormously long strike-slip faults (see Map 4 and 5). The mountains, plateaus, faults and earthquakes within the Mekong basin of Southeast Asia, many hundreds and even thousands of km from the suture, are thus influenced by the continuing collision of India and Eurasia.

In summary, the Mekong basin as a whole spans a wide range of altitude, latitude, climate and vegetation zones along the 4,200 km length of the river. The upper Mekong basin in China resembles a long and relatively narrow corridor. The lower Mekong basin begins downstream of where the river enters Lao PDR, Myanmar, Cambodia and Viet Nam. It covers 77% of the entire basin area and account for more than four-fifths of the water that drains the basin each year. The entire area is under the influence of monsoon climate and thus marked by great seasonal variation in rainfall. Typical low (May) and high (September) flow volumes of the Mekong show a difference of the order of 15 times. This fluctuation is a defining characteristic of the environmental conditions and has a profound impact on resource management strategies within the basin.