Origin of synchronous extension and inversion in a rift basin: the Phitsanulok Basin, central Thailand

Abstract The Phitsanulok Basin is a Cenozoic rift basin in central Thailand. The basin opened in the Oligocene to Early Miocene with the development of N-S and NNW-SSE trending normal faults. In the Middle Miocene the northern part of the basin continued to experience extension, while the southern part was affected by compressive inversion. Extension resumed in the Late Miocene to Pliocene, followed by post-rift subsidence in the Quaternary. The Middle Miocene anomalous deformation is attributed to activities of the NE-SW Uttaradit Fault and NW-SE Mae Ping Fault, respectively at the northern and southern basin margins. Both faults had limited activities during the Oligocene to Early Miocene rifting, where E-W extension was probably accompanied by vertical maximum principal stress. In the Middle Miocene, the maximum principal stress rotated to a N-S direction and produced sinistral motion on the Uttaradit Fault and dextral motion on the Mae Ping Fault. The strike-slip motions contributed to a counter-clockwise rotation of the continental block west of the Phitsanulok Basin, creating the synchronous extension and contraction. The kinematic model introduces the possibility of local inversion in a rift basin being originated from short-term switch between the maximum and intermediate principal stress axes and reactivation of long-lived strike-slip faults, as opposed to major change in regional stress related to plate movements. The model implies that the segment of the Mae Ping Fault south of the Phitsanulok Basin was inactive during the Oligocene to Early Miocene and was reactivated as a dextral fault in the Middle Miocene.