Stress evolution at the horsetail end of a major strike-slip fault: Tectonic controls on a large porphyry, South Cotabato, Philippines

Tensor solutions calculated from a microtectonic study of the Tampakan area in south Cotabato, Philippines, indicate rotating stress orientations in the past few million years. The area, believed to be straddling the SW flank of an eroded stratovolcano, is traversed by a fault system characterized by an evolving left-lateral shear pair controlled by southern horsetail branches of the Cotabato Fault Zone. This stress regime evolution provides insights to help explain seemingly anomalous fault orientations which are theoretically inconsistent with classical wrench fault tectonics, as well as in understanding the spatial and temporal relationships between mineral concentrations and geologic structures.