Friday Seminar (21 October 2022) James Gaherty (Northern Arizona Univ.)

Title: An Ocean-Bottom View of Mantle Convection Beneath the Pacific Basin



The Pacific basin provides an outstanding natural laboratory for studying a wide range of tectonic and upper-mantle dynamic processes: seafloor spreading, hotspots and other midplate melting and volcanism, multiscale thermal convection, and subduction dynamics, to name just a few.  Historically, direct geophysical constraints on these processes were limited to low-resolution, long-wavelength imaging due to the restriction of most geophysical instrumentation on the surrounding continents and isolated islands.  Over the last decade, advances in seafloor instrumentation (both seismic and electromagnetic) has enabled innovative high-resolution, localized investigation of many of these processes under the umbrella of the grassroots international collaboration PacificArray.  Here I introduce new emerging results from the US ORCA (OBS Research on Convecting Asthenosphere) project, consisting of two year-long seismic arrays in the central and south Pacific.  ORCA was designed to image upper-mantle seismic wavespeeds in two regions where satellite-derived gravity variations display long, linear structures suggestive of small-scale convection in the upper mantle.  At the first site, we find linear blobs of fast and slow material in the mantle beneath the oceanic plate, parallel to the gravity features. These represent cold sinking and warmer rising material, revealing a highly dynamic convective system underneath the plate.  By combining these results with constraints on absolute shear velocity and seismic anisotropy in the two ORCA regions and previous analyses from across the Pacific, we are gaining an improved understanding of the processes controlling the formation, modification, and evolution of the ocean plates and the underlying convection system.