Some possible mechanisms for gravity and thrust faults under oceanic trenches

Abstract

Seismic reflection profiles taken across several trenches of the Pacific suggest that gravity faulting is the dominant type of tectonic deformation within the trenches. Studies made of the directions of oceanic crustal movement suggest, however, that the oceanic crust is absorbed into the mantle beneath island arcs by thrust faulting. First-motion studies of earthquakes occurring beneath the trenches suggest horizontal extension of the crust over the sea-floor level and crustal compression at about 30 km below and beneath the trench axes and the island arcs. To accommodate the concept of underthrusting of the oceanic crust under island arcs with gravity-fault deformation in the trenches, a transformed thrust mechanism is suggested as one of a possible number of mechanisms whereby a gravity fault located at the trench surface would become a thrust fault at depths of 30 to 40 km beneath the island arc.