EVIDENCE FOR LITHOSPHERIC DRIP FEATURES BENEATH SOUTHERN CALIFORNIA FROM COMMON CONVERSION POINT STACKING OF RECEIVER FUNCTIONS

Common conversion point stacking of receiver functions has been applied to data from 21 seismic stations located throughout southern California to produce an image of the upper mantle transition zone. This method has gained wide acceptance in the investigation of the upper mantle transition zone (TZ) which is generally believed to be the depth range in the mantle where olivine undergoes a series of phase changes. The olivine to spinel structure phase change is generally believed to be responsible for the 410 km discontinuity. A phase transformation in the olivine mineral system to perovskite and magnesiowüstite is generally accepted to be associated with the 660 km discontinuity. An investigation of depth variations in the 410 and 660 km discontinuities is generally considered to be a means to identify variations in mantle temperatures associated with the respective phase change. That is that the 410 will generally become shallow in cooler regions of the mantle and deeper in warm regions. The 660 will behave oppositely in response to thermal variation in the mantle. As a result we would find a thick transition zone in cooler regions and a thin transition zone in warm regions.

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.