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Presented By: Earth and Environmental Sciences

Smith Lecture - Dr. Jonathan Delph, Purdue University

Linking heterogeneous expressions of subduction along the Cascadia margin

Spatial correlations between lateral heterogeneity in geophysical, seismogenic, and tectonic features is observed along the Cascadia margin. Both the overriding and downgoing plate have been invoked to play the dominant role in controlling along-strike correlations between seismogenic behavior, potential field measurements, morphological/tectonic characteristics, and seismic structure; however, significant feedbacks likely exist between the two. Our recent seismic images suggest that zones of basal accretion of material from the downgoing plate to the overriding plate may link the seismogenic, geological, and morphological expression of subduction in Cascadia. In the northern and southern portions of the forearc, this “subcreted”
material is also characterized by thick (~10 km) anomalously low shear-wave velocity zones. The thickness, high internal reflectivity, and low Bouguer gravity signatures likely indicate that this subcreted material is composed of dominantly (meta)sedimentary material that has been emplaced through successive subcretion
events over geologic timescales. Furthermore, the anomalously low velocities and spatial correlation with high non-volcanic tremor (NVT) density and short slow slip recurrence intervals indicate that these regions are fluid-rich. While 1 st order variations in the fluids that control NVT and slow slip likely result from differences in the permeability of the downgoing slab as inferred from its stress state and the distribution of intraslab seismicity, these subcreted packages likely represent thick, vertically-impermeable regions in the lower crust that further accentuate this correlation. Variability in the amount of subcretion explains patterns of exhumation and uplift along the Cascadia margin and the resulting forearc topography over geologic timescales, and is likely controlled by some combination plate interface geometry/rheology and overriding plate architecture.

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