Evidence for Small-Scale Mantle Convection from Seasat Altimeter Data

摘要

Evidence for the early development of convective instability in the thermal boundary layer associated with cooling plates has been found from gravity anomalies and residual sea surface heights derived from Seasat altimeter data. Subtle lineated patterns trending in the direction of plate munition in the hot spot reference frame are observed over the younger portions of the fast-moving pacific and Indo-Australian plates. For the east central Pacific Ocean, the Seasat-derived data sets reveal limenations with (1) wavelength in the range 150-500 km (perhaps increasing with plate age), and (2) Peak- to trough amplitudes of 5-20 mGa1 for gravity anomalies. The lineated pattern over the Pacific plate, which first becomes discernable over seafloor 5- to 10-m.y.-old west of the East Pacific Rise, is clearly oblique to the trends of the prominent Pacific-Farallon fracture zones. Onset of convective instability at such early ages can be understood if young oceanic lithospere is underlain by a layer having a viscosity of about to the 18th power pa s 10 to the 19th power), which is about 3 ordes of magnitude less than mantle viscosity inferred form postglacial rebound studies. Recent numerical studies of convection in fluids with a temperature- and pressure- dependent viscosity support an early onset time for convective instability and low-viscosity zones in the upper mantle beneath young lithosphere.