Fluid‐rich damage zone of an ancient out‐of‐sequence thrust, Kodiak Islands, Alaska

Abstract

The Uganik Thrust is a fossil out‐of‐sequence thrust fault which was active over a period of ∼3 Ma during the early Tertiary until activity ceased with the subduction of the Kula‐Farallon spreading ridge at ∼57 Ma. During this period the fault experienced at least ∼1 km of throw and developed a strongly asymmetric damage zone. The brittle damage zone in the footwall of the fault acted as a conduit for fluid advection during the active faulting. A similar asymmetrical footwall damage zone has been interpreted as a fluid conduit at the Nobeoka Thrust, Shimanto Belt, SW Japan. Thermal indicators in the uppermost footwall give similar maximum paleotemperatures to those in the hanging wall (∼280°C), while previous work elsewhere in the footwall formation suggests maximum burial temperatures of ∼240°C. In this case, similar to the Irish Canyon thrust in the Franciscan accretionary complex, the location of the thermal anomaly is spatially offset from the structural fault which caused it owing to thermal overprinting in the vicinity of the fault.