Inherited extensional faults and bed-parallel slip in Mae Moh Basin, Thailand with implication for basement fabrics

Abstract

The 3D seismic analysis with structural reconstruction and outcrop study in Mae Moh mine, located in the Central Sub-basin of the Mae Moh Basin, unveil multiple extensional phases and the controls of pre-existing basement fabrics on extensional faults, bed-parallel slip (BPS) and post-rift folding. The coexistence of orthogonal and oblique extensional fault systems characterizes the mine area as a result of regional E-W extension in the Middle Miocene. The dominant NNE-striking normal faults comprise easterly dipping domino-style faults with minor western dipping antithetic faults. Left-stepping en-echelon arrays of NNE-to NNW-striking normal faults at coal levels in the 3D survey follow the deeper early-rift fault that bends along strike from NNE to NW direction. Locations of intra-basin fault bends and offset of basin-bounding faults reflect E-W to ENE-WSW striking basement fabrics that potentially activate during rifting as a transfer zone across the Mae Moh Basin. The focus of rifting migrates eastward with up to 56% extension in the mine. The initial depocenter of early rift phase and the deepest economical coal seams at the center of the post-rift syncline align above the transfer zone. The presence of BPS is observed at outcrop scale, characterized by the offsetting of normal faults or fault zones with antithetic movement. BPS develops during the late rift phase under a NW-SE extension preceding the Late Miocene-Pliocene inversion.