CONVECTIVE THERMAL INSTABILITIES IN THE WAKE OF THE MIGRATING MENDOCINO TRIPLE JUNCTION, CALIFORNIA

Whether the high seismic-velocity anomalies in the upper mantle under California represent cold downwelling mantle flow or fragments of the subducted Farallon plate has important implications for mantle dynamic s related to the tectonic history of western North America. We investi gate the former possibility by simulating the time-dependent thermal e volution in the asthenosphere beneath California, in the wake of the m igrating Mendocino triple junction (MTJ), using a two-dimensional mode l that represents a transect parallel to the direction of the MTJ migr ation. For viscosity within the typical range for the asthenosphere be neath active tectonic regions (1x10(18) - 2x10(19) Pa s), our model pr edicts vigorous small-scale convection characterized by periodic forma tion of cold downwelling flows near the southern edge of the migrating Gorda plate. Within the relevant time frame, the predicted convective thermal structures are comparable to the major mantle features shown by seismic imaging.