Thermal and Seismic Signature of the Trailing Fragments of the Farallon Slab

After more than 100 m.y. of continuous subduction, only small parts of the Farallon plate are still subducting below the northwestern U.S. and central America today. Under the southwestern U.S. a window in the subducting plate has been forming since the Pacific-Farallon ridge reached the trench about 30 Ma. Due to the young age of the last parts of the Farallon plate and its high rates of subduction it has been proposed that much of the slab in the upper mantle would have thermally equilibrated. However, seismic tomography has imaged high velocity anomalies in the transition zone below the southwestern U.S. not far from the previous location of the trench. Our objective is to determine the strength and possible location of the seismic anomalies associated with Farallon subduction based on thermal modeling and the plate tectonic history. The location of the slab in the upper mantle is not easy to predict as its path through the mantle is unknown and it may have deformed and/or broken after subduction of the ridge. However, a composite image of the velocity models of Van der Lee and Nolet (1997) and Grand (1997) shows anomalies that are continuous from the upper to lower mantle. Assuming a continuous and not internally deformed slab, the material at the top of the lower mantle would have been subducted around 40 Ma. A three-dimensional thermo-kinematic model for such a slab shows that in spite of the young age, most of the subducted Farallon plate should still have a thermal signature. At 200 km depth, thermal anomalies range from about -800^oC in the currently still subducting part of the plate to 0^o in the slab window. At deeper levels, including the transition zone, thermal anomalies of -200^o to -500^oC are preserved which is sufficient for a resolvable seismic velocity anomaly. A comparison of the volume of upper mantle slab material imaged seismically and the volume thermally modeled will allow us to determine whether the material subducted since 40 Ma is sufficient to account for the large-scale anomalies in the transition zone, or whether significant internal deformation of the slab is required.