Evidence for Deep Uneclogitized Subducted Crust Under Alaska

Abstract:

The dynamics of subduction zones are largely controled by mineralogic reactions and density variations, with one of the most important such reactions being eclogitization. When sinking into the mantle and undergoing eclogitization, the density of the crust increases dramatically. This in turn greatly reduces the seismic contrast between the subducting crust and the surrounding mantle. Therefore, looking for the deepest evidences of seismic scattering using Receiver Functions (RF) can help put constraints on the eclogitization reaction dynamics in subduction zones in order to improve our understanding of their evolution. In this study, we use data from a dense composite array combining stations from TA/USArray and the Alaska Volcano Observatory with stations from the BEAAR, MOOS and SALMON temporary deployments. The region is imaged with a fully 3D Prestack Kirchhoff RF migration scheme which uses the FM3D fast Eikonal solver to compute travel times, accounts for 3D elastic point scattering and includes the first order free surface multiples. We observe scattering signal from the subducted Moho down to about 180km into the mantle, which suggests the presence of deep uneclogitized oceanic crust. We also observe large variations in dip angle and slab thickness below the Denali Volcanic Gap, which confirm the presence and 3D extent of the subducted Yakutat terrane under continental Alaska.