東構造節的伸展和折返：泰國北部裂谷的重力信號

東構造節的伸展和折返：泰國北部裂谷的重力信號

E-mail: gladassoc@btinternet.com.

Although many of its details have been criticised,the indenter model of India-Asia collision (Fig 1)is still widely accepted as providing a workable first-order explanation for the structural development of the eastern syntaxis and the initiation of the major NW-SE faults that segment the mainland of Southeast Asia. Arguments have largely concerned the extent to which the arrival of the continental crust of greater India has been accommodated by thickening beneath the Tibetan Plateau or by extrusion of blocks into Southeast Asia. Second-order structures relevant to such questions include elevated plateaus,metamorphic core complexes and the networks of rift basins with broadly N-S trends. In northern Thailand,one such rift network has been responsible for the development of a characteristic basin-and-range morphology.

Gravity patterns in northern Thailand are scale-dependent. At the broadest scale, there is a northwards decrease in gravity produced by crustal thickening that is considerably greater than would be expected from the average topographic level of the extremely rugged terrain (Fig. 2). At the other extreme,of scales of a few tens of kilometers or less, gravity lows define rift depocentres. At intermediate levels there is a very direct association between rifted regions and gravity highs. More than simple isostasy is at work in these areas, because isostatic balance is being achieved at neither the regional nor the local level.

One difficulty in studying this area lies in the lack of uniformity in the distribution of geological and geophysical data. Subsurface data, in the form of seismic surveys, drill cores and mine workings, are abundant for many of the intermontane basins as consequences of exploration for and exploitation of oil,gas and coal resources. Information from the topographic highs is generally limited to the results of surface mapping but, as a consequence of unroofing by low-angle detachments, the surface mappers are often studying much deeper crustal layers than the workers in the valleys.

Fig. 1 The indenter physical model(a) and the indenter effect in SE Asia(b)(Illustrations based on figures downloaded from http://perso-sdt.univ-brest.fr/～jacdev/ens/the3_modeles.pdf)

Fig. 2 Regional Bouguer gravity in northern Thailand. CR = Chiang Rai Basins; CM = Chiang Mai Basin; F = Fang Basin;MS – Mae Sot Basin. UFZ = UttararditFault Zone; MPF =Mae Ping Fault; TPF = Three Pagodas Fault

The same lack of uniformity in coverage affects gravity data, and regional Bouguer maps of Thailand based on widely-separated points give very different pictures from maps provided by the relatively detailed surveys in the valleys. Special techniques, based in part upon manual rather than machine processing, have been needed to make progress in interpretation, given the present status of survey coverage. The preliminarygravity models now being produced are providing constraints on models involving lower crust and mantle flow to accommodate the indenter, but are representative only of work in progress.

Extension and Exhumation in the Eastern Syntaxis: Gravity Signatures of Rift Valleys in Northern Thailand

John MILSOM
Department of Geological Sciences, University of Hong Kong

Gravity; indenter model; Northern Thailand

10.3975/cagsb.2012.s1.22