EVIDENCE FOR LITHOSPHERIC DRIP FEATURES BENEATH SOUTHERN CALIFORNIA FROM COMMON CONVERSION POINT STACKING OF RECEIVER FUNCTIONS
The transverse ranges in southern California have been subject to much investigation because they do not appear to have an adequate crustal root to account for observed topography. Humphreys and Hager (1990) have found evidence of very localized subduction or a drip of lithospheric material beneath this region, as a result of convergence due to a kink in the San Andreas fault system, which can account for the observed topography. Using seismic tomography they imaged a high velocity anomaly (generally considered to be evidence for cooler mantle temperatures) directly beneath the transverse ranges to a depth of almost 300 km. While they do not suggest that this feature penetrates the upper mantle transition zone, we have found in receiver function images that there is an anomalously thick TZ directly beneath the proposed drip which would indicate a small localized low temperature anomaly. We interpret this feature as evidence that the drip feature penetrate the TZ or may be captured within the TZ. Our TZ image also suggests other localized down-welling features beneath southern California.