THE UNIQUE DYNAMICS OF THE PACIFIC HEMISPHERE MANTLE AND ITS SIGNATURE ON SEISMIC ANISOTROPY

Abstract

The portion of Earth’s mantle lying below the hemispherical region occupied by the Pacific Ocean basin is characterized by a unique style of convection. The unique dynamics below the ‘Pacific Hemisphere’ are revealed by analyzing the mantle flow predicted on the basis of high-resolution seismic images of mantle structure. In the Pacific Hemisphere we find a dome-like upwelling, originating in the lower mantle, which actively transports material into the upper mantle. A unique aspect of this flow, found only in the Pacific Hemisphere, is a ‘demi-tour’ or U-turn of the circulation which occurs in the shallow mantle, west of the East Pacific Ridge. This ‘demi-tour’ connects the westward movement of the Pacific plate to the dome-like upwelling below the central Pacific Ocean and it also provides an explanation for the asymmetric distribution of seismic anisotropy on either side of the ridge axis. In a global analysis of the correlation between mantle flow and seismic anisotropy, we find the strongest agreement occurs in the Pacific Hemisphere mantle.