The Moresby Seamount detachment in the Woodlark Basin (east of Papua New Guinea) is arguably the best exposed active detachment fault in the world. We present the results of a high-resolution autonomous underwater vehicle survey of bathymetry, bottom water temperature, and turbidity. In combination with dredging and existing drillhole data, a synthesis of the tectonic geomorphology, kinematics, and mechanics of the detachment is provided. The detachment surface, which has a 30° northward dip and ∼8 km post-Pliocene displacement, is well preserved. Two major smooth areas are tectonically created, and megascopic (kilometer scale) slickensides indicate downdip direction of movement. The detachment is transected by a major sinistral strike-slip fault, suggesting deformation partitioning in the detachment zone in response to the 500 k.y. change in plate kinematics. The mainly gabbroic protoliths and cataclasites from the fault show pervasive syntectonic alteration, leading to large increases in abundance of quartz and, more important, calcite. Resulting quartz-rich and calcite-rich mylonites play a crucial role, as weak fault rocks and ductile microstructures point to detachment operation at low differential stress. A kilometer-sized anomaly in bottom water temperature and turbidity is found at the downdip end of the detachment zone, indicating that it hosts an active hydrothermal system, probably fed by overpressured fluids from a deep crustal source.

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