Evidence for middle Triassic to Miocene dual subduction zones beneath the Shan-Thai terrane, western Thailand from magnetotelluric data

摘要

Kanchanaburi province, western Thailand, is in the middle of Shan-Thai terrane. Paleomagnetic and geological data support the hypothesis of dual subduction zones beneath the Shan-Thai terrane. During the late Triassic there was a westward subduction in the east where the Lampang-Chiang Rai block subducted beneath the Shan-Thai terrane. In addition, in the early Tertiary, the western Burma terrane subducted underneath the Shan-Thai in the west resulting in an eastward subduction. A pioneer survey of this region was therefore conducted using magnetotelluric (MT) techniques with the aim of finding deep structures associated with these ancient subduction zones. 39 MT stations were deployed to cover most of the province. Resistivity structures are obtained from the 3-D inversion. The near surface resistivity structures correspond well with the surface geology. The mid and lower crusts are conductive and this is interpreted as the crusts being composed of mafic granulites of 3% porosity in which the remnant dehydrated fluid was accumulated during the subduction. In addition, the westward and eastward subduction zones generated mafic/ultramafic intrusions which appear as two conductive zones on the east and west, respectively. These two conductors and the conductive lower crust support the hypotheses of the middle Triassic to Miocene dual subduction zones.