The Nature of Intra-plate Deformation in the Wharton Basin and Its Consequence on the Subduction Processes

Abstract:

Intra-plate deformation and associated earthquakes are enigmatic features on the Earth. The Wharton Basin is one of the most active intra-plate deformation zones, recently confirmed by the occurrence of the 2012 twin great intra-plate strike-slip earthquakes (Mw≥8.2) in the Wharton Basin. These earthquakes seem to have ruptured the whole lithosphere, but how this deformation is distributed at depth remains unknown. In this paper, we present a seismic reflection image from the Wharton Basin that shows faults down to 45 km depth, the deepest faults ever imaged in the oceanic setting. The amplitude of these reflections in the mantle first decreases linearly with depth down to 25 km and then remains constant down to 45 km. The number of faults imaged along the 233-km long profile show similar behavior. We also find that the number of earthquakes and the cumulative moment released as a function of depth show a similar pattern, suggesting that the lithospheric mantle deformation in this region can be divided into two layers: a highly fractured fluid-filled serpentinized upper layer and a pristine brittle lithospheric mantle where great earthquakes initiate and large stress drop occurs. Moreover, the upper serpentinized layer can transport a significant amount of water into the Wadati-Benioff zone, its base defining the lower boundary of the double Benioff zones, the dehydration boundary.