3-DIMENSIONAL THERMAL MODELING FOR THE MENDOCINO TRIPLE JUNCTION AREA

Complex interaction between the pacific, North American, and Juan de F uca plates at the northward migrating Mendocino Triple Junction (MTJ) has had a profound effect on the geological evolution of western North America. This paper presents a three-dimensional thermal model for th e area around the MTJ that is based on its kinematic evolution, incorp orating the effects of an asthenospheric slab window, changes in relat ive plate motions and the trenchward migration of the Juan de Fuca-Pac ific spreading ridge, The thermal equation, including conductive and a dvective heat transport, is solved numerically using finite difference s, Surface heat flow data and the trend in the maximum depth of seismi city south of the MTJ can be quite well explained by the thermal model . A finite lithospheric thickness above the slab window is required to fit heat flow measurements; however, the lack of data west of the San Andreas Fault prevents discriminating between underthrusting and accr etionary mechanisms of lithospheric thickening. A comparison between t he thermal and recent seismic velocity models reveals that P-wave anom alies in the uppermost mantle have smaller wavelengths and larger ampl itudes than predicted if they were purely thermal.