Interactions between Subduction-related Processes and the Upper Plate Lithosphere in Alaska

Abstract:

To resolve the signatures of volatiles and melt in the subduction zone mantle wedge, and how they have interacted with the upper plate, we determined seismic velocity gradients beneath Alaska with Sp receiver function common-conversion point (CCP) stacking. For the CCP stack, waveform components were calculated using a free-surface transform with improved estimates of free-surface velocities determined from P and SV particle motions, and receiver functions were calculated with time-domain deconvolution. In the stack, Sp receiver functions were mapped to space using weighting functions that accurately incorporate the properties of Sp scattering kernels. We applied this method to data from hundreds of stations of the NSF EarthScope Transportable Array, as well as other portable arrays and permanent networks. For data filtered with a 4-100 s bandpass, Sp phases indicate a positive velocity gradient with depth at depths of 150 km. This velocity gradient appears both in the mantle wedge, and in the mantle beneath the subducting lithosphere, and marks the base of low-velocity asthenosphere. A similar velocity gradient is observed near the base of the mantle wedge in the Nicaraguan subduction zone, where Sp CCP stacking has also recently been carried out. In Nicaragua, the depth of the positive velocity gradient matches the lower boundary of a zone of high Vp/Vs from regional body-wave tomography that has been interpreted as particularly high fractions of partial melt. In both regions, the positive velocity gradient likely represents the onset of partial melting. Sp phases from the negative velocity gradient at the base of the upper plate in Alaska are strongest in central Alaska, where lithosphere-asthenosphere (LAB) depths lie at ~60 km, and joint inversions with Rayleigh data show that asthenospheric velocities are particularly low. This zone includes magmatically active regions and likely represents thinning of the upper plate by mantle wedge volatiles, melt, and flow. The LAB phase deepens to the north, reaching depths of ~120 km beneath the northern Arctic Alaska terrane. An increase in the depth of the LAB phase from the arc to the far back-arc is also observed in Nicaragua, providing another example of sculpting of the upper plate by subduction-related processes.