RECEIVER FUNCTION INVESTIGATION OF THE UPPER MANTLE TRANSITION ZONE BENEATH NORTHERN CALIFORNIA

This study is to determine if there is a correlation between lithospheric tectonic features and the thermal state of the upper mantle beneath Northern California. This region is characterized by every type of tectonic boundary; transform plate boundary to the south, the Juan de Fuca ridge to the west, subduction and the associated Cascadia range to the north and the extensional tectonics of the Basin and Range to the east. The upper mantle transition zone (TZ) is usually defined as the interval from the 410 km discontinuity to the 660 km discontinuity which is generally considered to be the results of thermally regulated phase changes in the olivine mineral system. To investigate the thermal state of the upper mantle (which can be correlated to depth anomalies to upper mantle discontinuities) beneath Northern California, we produce 3-D images of the upper mantle by stacking Ps phases from the upper mantle discontinuities using 8 years of data from the Berkley Digital Seismic Network (BDSN) and all available data from the EarthScope Transportable Array (TA, 3 months to date but is ongoing). Our current images find that the 410 km discontinuity beneath this region is structured south of the Mendocino Triple Junction (MTJ) but with no obvious correlation to surface tectonics. North of the MTJ, the TZ is thickest to the northwest indicating low temperatures beneath the slab. While low temperatures are expected in association with a slab, this particular slab is very near the ridge and would be warm and the expected flat geometry would keep it from reaching the TZ in this region. A cool TZ in the northwestern part of our region would not necessarily be expected unless localized convection beneath the slab chills the TZ. The TZ is thin to the northeast which may be expected for warm mantle temperatures that may be associated with Basin and Range tectonics. South of the MTJ we observe the 720 km discontinuity (hypothesized to be associated with phase change in garnet composition system) as well as a 660, but the 720 does not appear to be present beneath northernmost California. The 220 and 520 km discontinuities may also be present in our preliminary images.