The Pacific Megagash: A future plate boundary?

Seismic anisotropy is an efficient way to investigate the deformation field within the upper mantle. In the framework of rigid tectonic plates, we make use of recent tomographic models of azimuthal anisotropy for deriving the best rotation pole of the Pacific plate in the uppermost 200km of the mantle. It is found to be in good agreement with current plate motion (NUVEL1, HS3 and NNR). However, when dividing the Pacific plate into two subplates, separated by the megagash, an E-W low velocity and anisotropy band extending across the Pacific plate from Samoa-Tonga to the EasterJuan Fernandez Islands, the rotation pole of northern Pacific is still in agreement with current plate motion but not the rotation pole of the southern part of the Pacific, far away from the ”classical” rotation pole of the Pacific plate. This result suggests a differential motion between