Constraints on Cascadia Forearc Structure Off Washington From New Shipboard Gravity Data

The Cascadia Subduction Zone is recognized as a source of great megathrust earthquakes and has recently been the focus of significant geophysical investigation. Previous studies of the plate boundary indicate that the margin comprises two accretionary complexes, an older (Miocene) region that makes up the continental shelf and a younger (Pleistocene) prism extending ~50-100 km beyond the shelf. However, details of the plate boundary, including the structure and mechanics of the accretionary prism, the amount (if any) of sediment being subducted, and the role of fluids at the margin, remain poorly understood. To address these questions, the Cascadia Open-Access Seismic Transects (COAST) cruise recorded 2-D multi-channel seismic (MCS) data, gravity, magnetics, swath bathymetry and acoustic backscatter over the Cascadia margin. Data were collected by the R/V Marcus G. Langseth along nine dip-parallel lines and two strike-parallel lines covering approximately 10,000 km². Here we report preliminary results from the gravity data collected on the COAST cruise. Free air gravity anomalies (FAA) across the region were found to be in the range of -10 to -50 mGals, consistent with values reported by Sandwell and Smith (2009). As the Juan de Fuca plate approaches the plate boundary, gravity values gradually decrease, reflecting the slowly deepening ocean crust at the trench. MCS data indicate that sediments in this area range from 1-3 km in thickness. At the deformation front gravity increases as sediment is compacted and uplifted via folding and thrust faulting. The eastern end of the Pleistocene accretionary complex comprises a broad basin filled with sediments presumed to come from the continental slope. This region is less compacted resulting in lower gravity anomalies. The eastern portion of the accretionary wedge is Miocene in age and thicker than the younger wedge to the west. Here gravity rises again as the bathymetry shallows. Finally, FAA values across the Miocene accretionary complex fall as the line approaches the coast, reflecting the increasing depth to the subducting slab. Large listric normal faults within the continental shelf coincide with gravity lows, indicative of thick sediment packages. The gravity and MCS data collected on the COAST cruise can be used to evaluate the thickness of sedimentary packages present in the Cascadia margin. The presence of a deep reflector, believed to be the subducting oceanic crust, allows us to examine the thickness of the sediments in both the Miocene and Pleistocene accretionary complexes. Densities derived from the gravity data allow us to evaluate the amount of fluid present in the accretionary wedge.