Seismo Lab Seminar

Several aspects of the oceanic lithosphere confound simple models of plate growth through conductive cooling. These include off-ridge non-plume volcanism, systematic departures from expected depth-age relationships, and intraplate gravity lineations. Small-scale convection (SSC), driven by density instability at the base of a cool, dense plate, might explain these phenomena. However, it has not previously been observed tomographically, and computational models of this process are highly contingent on poorly-constrained rheological parameters. To address this, we conducted an ocean-bottom seismic experiment, deploying instruments in the central Pacific in two regions notable for elongated gravity anomalies. Our goal was to image 3-D seismic wavespeeds beneath the plates. At ~40My plate age, we discovered alternating velocity anomalies in the upper mantle in elongated bands parallel to gravity lineations and local plate motion. These features require large (300-500 K) lateral temperature contrasts and/or partial melt. Together with new gravity and bathymetry analysis, this is the first direct evidence for small-scale convective rolls beneath the oceanic plates. At ~90My plage age, we find several upper mantle structures that confound classic plate evolution models, but putative convection cells are less clearly organized. We use these seismic observations as a reference for a new suite of high-resolution dynamical models of maturing oceanic plates that include complex rheology and dynamically evolving grain size. By attempting to match the observed wavelength, depth, and amplitude of SSC, we determine the range of parameters that reproduce this behavior. These models shed new light on the fundamental dynamics which govern 70% of our planet's (sub)surface.