Active-Source Wide-Angle Seismic Observations of a Shallow Mantle Discontinuity Within the 8-9-Ma Juan de Fuca Oceanic Plate Off Cascadia Subduction Zone

In June-July 2012 we conducted an active-source seismic study of the Juan de Fuca (JdF) oceanic plate. This two-ship experiment (cruises RV Langseth MGL1211 and RV Oceanus OC1206A) acquired coincident two-dimensional multichannel seismic reflection data and long-offset wide-angle reflection/refraction data using ocean bottom seismometers (OBSs) to study the evolution and state of hydration of this young oceanic plate prior to subduction beneath the Cascadia margin. Data were collected along two spreading-parallel profiles extending from zero-age crust at the JdF Ridge to the Cascadia deformation front, and along a margin-parallel profile a few km seaward off the deformation front over 8-9 Ma crust. OBS record sections from the margin-parallel profile show the expected crustal (Pg) and shallow mantle (PmP and Pn) P-wave arrivals, with the Pg-PmP-Pn triplication typically observed at 40-50 km source-receiver offsets. For larger offsets, data from OBSs deployed along the southern portion of the profile show clear sub-Moho reflections characterized by a high-amplitude triplication at 140-160 km source-receiver offsets. We will use a combination of travel-time tomography and waveform forward modeling to constrain some of the properties of the upper mantle seismic discontinuity that produces the observed wide-angle reflections: its depth, thickness, and P-wave velocity structure (i.e., velocity inversion or positive impedance contrast). We expect that our observations and models will provide new constraints on the nature of upper mantle discontinuities, contributing to our understanding of the development of the lithosphere-asthenosphere boundary and/or other intra-lithospheric structures beneath young oceanic plates.