Current Research in EEPS Seminar – Miles Bodmer – Department of Earth Sciences, University of Oregon
Title: Seismic Imaging of Upper Mantle Heterogeneity in the Cascadia Subduction Zone: How asthenospheric anomalies beneath the incoming plate influence subduction dynamics.
Abstract: Subduction zones are key locations in Earths tectonic system where oceanic plate is recycled, arc volcanism is generated, and active deformation and orogenesis occurs. However, investigation of these systems has been limited by the dearth of dense offshore data, obscuring our understanding of upper mantle heterogeneity beneath the incoming plate and its potential influence on subduction zone processes. Recent large amphibious community seismic experiments have begun to amend this, exemplified by the Cascadia Initiative which deployed broadband ocean bottom seismometers spanning the entire incoming Juan de Fuca (JdF) plate. Here, I present passive source seismic imaging work that highlights the heterogeneous nature of Cascadia’s isotropic and anisotropic upper mantle structure. These observations are synthesized with previous inferences from geodynamics, geomorphology, and seismology to understand the subslab mantle’s role in segmentation of both megathrust behavior and landscape evolution of the forearc. First, I use SK(K)S shear wave splitting observations to characterize mantle anisotropy and infer patterns of mantle flow. Beneath the JdF plate, anisotropy is subparallel to APM and is largely consistent with plate driven flow. In contrast, beneath the Gorda microplate splitting orientations parallel JdF-Pacific relative plate motion, which we attribute to a broad zone of shear driven by the motion of the neighboring plates. Contrary to previous hypothesis informed only by onshore anisotropy observations, we find no evidence for toroidal flow induced by slab rollback. Next, I conducted analysis of onshore/offshore teleseismic P-wave tomography, uniquely constraining offshore and subslab mantle structures. Two localized low-velocity anomalies are observed beneath the subducting slab, which are interpreted as independent upwellings (informed by splitting observations) and regions of partial melt. These are inferred to be regions of excess buoyancy and are observed to correlate with patterns of plate locking and tremor density. These observations led to an entirely new hypothesis for subduction zone segmentation in which subslab asthenospheric anomalies modulate forces along the plate interface. Lastly, through collaboration with geodynamics and geomorphology colleagues, I investigate whether subslab buoyancy could influence variations in Cascadia’s forearc topography. Regions of inferred subslab buoyancy are observed to correlate with higher topography, increased short and long-term uplift rates, increases cosesmic subsidence, increased erosion rates, shallower slab dip angles, and greater plate locking. I propose that subslab buoyancy increases the total shear force along the megathrust by decreasing the slab dip and/or increasing plate coupling, thus indirectly influencing where forearc topography develops and helping to provide support. This work provides novel insight into how the subslab upper mantle can influence along-strike variations in subduction zones and the importance of characterizing the asthenosphere structure of the incoming plate. It is still unclear whether these processes are unique to Cascadia or if similar processes can be observed elsewhere, for example at the Alaskan subduction zone, which has well documented along-strike variation in plate coupling, incoming seafloor fabric, and hotspot activity