Lithospheric and Asthenospheric Structure Beneath Alaska From Bayesian Inversion of Sp and Rayleigh Wave Data

Session: Advances in Seismic Imaging of Earth’s Mantle and Core and Implications for Convective Processes
Type: Oral
Date: 4/29/2020
Time: 09:00 AM
Room: 120 + 130
Description: 

Deformation in Alaska manifests dramatic variations, from convergence and uplift in southern Alaska associated with subduction to more enigmatic deformation in interior Alaska that includes southward surface motions. Modification of upper plate thickness and viscosity by subduction-related processes likely plays a key role in this variation, but the northern extent of this modification at mantle depths is unclear. To measure the properties of the upper plate lithosphere and the asthenosphere beneath Alaska, we are jointly inverting Sp receiver functions and Rayleigh wave phase velocities (periods of 5-140 s) using a Bayesian approach with a Monte Carlo Markov Chain search. Data are from the Alaska Regional Network, other permanent stations and the EarthScope Transportable Array. Tests with synthetic data show that these data types have complementary sensitivities, with Sp receiver functions enhancing resolution of mantle velocity gradients and the Moho and Rayleigh waves providing key control on absolute shear velocities. Models from synthetic tests with Sp receiver functions versus those using stacks of non-deconvolved Sp waveforms are equivalent. Inversions with real data show that the thinnest lithosphere (50-75 km thick) and the lowest velocity asthenosphere typically lie above the subducting plate as illuminated by seismicity. The low velocity mantle wedge is laterally continuous, but has slightly higher minimum shear velocities beneath the break in magmatism termed the Denali Volcanic Gap, potentially indicating a reduction in partial melt content. Upper plate thickness increases to the northwest, reaching values of 120-130 km beneath the Brooks Range. Tractions from the asthenosphere acting on this gradient in lithospheric thickness have potential to explain the southward surface motions in the Alaskan interior. Data from the southernmost stations constrain the thickness of the subducting plate. The subducting plate carrying the Yakutat terrane is 100-120 km thick, versus the ~90 km thick Pacific plate that subducts further west.

Presenting Author: Karen M. Fischer

Authors