Structure and evolution of the Ban Pong Basin, Chiang Mai Province, Thailand

Abstract

The Ban Pong Basin (BPB) is a small intermontane basin of Neogene age on the western margin of the Chiang Mai Basin in northern Thailand. The BPB was discovered in 2012 because of new quarrying activity. The basin complex formed in the hanging wall of a major Cenozoic low-angle normal fault (LANF) called the Inthanon Detachment, and provides a unique record of the Neogene deformation related to the detachment. The BPB’s preservation is probably due to its location in a synform within the rising Western Ranges metamorphic complex, which forms the Suthep-Inthanon mountain range. Initially, extensional faults in the BPB were oriented ∼N-S but then rotated to NNW-SSE, NW-SE, and ENE-WSW to NE-SW directions, which we interpret as arising due to local stress rotation related to the adjacent rising metamorphic complex. Folds, thrusts, and inverted normal faults both within Neogene sediments of the Mae Rim Formation and adjacent Palaeozoic metasedimentary rocks indicate a phase of compression that affected the basin late in its history (probably latest Miocene-Pliocene based on the regional timing of inversion). Complete Bouguer anomaly interpretations of 235 reduced gravity stations reveal two en echelon sub-basins, bounded by gravity highs related to shallow-lying Palaeozoic rocks and underlying high-grade metamorphic basement rocks. Two gravity-density cross-section models indicate the basin fill comprises up to 250 m of weakly lithified syn-rift sediments. The absence of a classic half graben morphology is probably related to inversion and erosion that significantly modified the extensional basin geometry and original thickness.