Strong seismic anisotropy in the deep upper mantle beneath the Cascadia backarc: Constraints from probabilistic finite-frequency SKS splitting intensity tomography

Abstract

The volcanically active High Lava Plains (HLP) region is a striking tectonomagmatic feature of eastern Oregon, in the backarc of the Cascadia subduction zone. It features young (<12 Ma), bimodal volcanic activity; the rhyolitic volcanism has a spatiotemporal trend that is oblique to that expected from the absolute motion of the North American plate. Several models have been proposed to explain the tectonic evolution of Cascadia backarc and the relationships between upper mantle processes and volcanic activity; however, consensus remains elusive. Because seismic anisotropy in the upper mantle reflects processes such as mantle flow and partial melting, constraints on anisotropic structure can shed light on the connections between mantle dynamics and tectonomagmatic activity in the Cascadia backarc. Anisotropy is often constrained via SKS splitting measurements; however, their interpretation is typically ambiguous because they lack depth resolution. Here we present new constraints on upper mantle anisotropy beneath the HLP region from probabilistic finite-frequency SKS splitting intensity tomography, which provides both lateral and depth constraints on anisotropic structure. Our technique is based on a Markov chain Monte Carlo approach to searching parameter space, and we use finite-frequency sensitivity kernels to relate model perturbations to splitting intensity observations. We use data from broadband stations of the dense High Lava Plains experiment, which provide good resolution of upper mantle anisotropic structure, as demonstrated via resolution tests. We find evidence for particularly strong seismic anisotropy in the deep upper mantle (200-400 km depth) beneath the Cascadia backarc, suggesting that flow in the deep upper mantle, rather than alignment of partial melt in the shallow mantle, provides the first-order control on shear wave splitting delay times. Our model provides additional support for the idea that mantle flow beneath the Cascadia backarc is controlled by rollback subduction. Our results suggest that anisotropy in the deep upper mantle may be more important to the interpretation of SKS splitting measurements in some settings than commonly appreciated, and provides an avenue for reconciling apparently contradictory constraints on anisotropic structure from surface waves and SKS splitting.