LATE CENOZOIC INVERSION IN THE MAE MOH BASIN, NORTHERN THAILAND

The Mae Moh Basin is a NNE-SSW trending Tertiary extensional basin about 100 km southeast of Chiang Mai, northern Thailand. It is part of a N-S trending rift zone that extends from northern Thailand southward into the Gulf of Thailand. The basin fills comprise up to 1000 m of Miocene-Recent fluvial and lacustrine strata, including coal that is currently mined for power generation. Normal fault systems in the basin constitute Western and Eastern sub-basins, separated by a Central Ridge. Reflection seismic interpretation of the Western Sub-basin revealed a half-graben geometry with a west-dipping boundary fault and a depocenter at the eastern margin. The depocenter was folded against and uplifted along the boundary fault, resulting in an anticline next to the fault plane and a syncline further into the basin. Folding and uplift are consistent with erosion of the upper part of the basin fills at the surface. Such structural and stratigraphic configurations are characteristic of compressive inversion of normal faults that has been documented in other extensional basins in northern Thailand. In the Eastern Sub-basin, inversion has also been interpreted based on folding and uplift of the syn-rift succession. The resultant erosion was so severe that the depocenter was completely removed exposing the Pre-Tertiary basement at the surface. At some location, the basal unconformity of the basin fills forms the Present-day geographic limit of the Tertiary strata that is about one kilometer away from the boundary fault. Inversion in the Mae Moh Basin was restricted to larger basin-bounding faults with reactivation-prone orientations and mechanical properties, including less friction that could result from frequent movements. The timing of inversion in the Mae Moh Basin is probably Pliocene to Pleistocene.