A COMPARISON OF SHORT-TERM AND LONGER-TERM QUATERNARY TECTONIC UPLIFT RATES FROM MARINE TERRACES, PALOS VERDES HILLS, LOS ANGELES COUNTY, CALIFORNIA

The Palos Verdes Hills area near Los Angeles, California, has long been famous among geomorphologists because this crustal block hosts 13 marine terraces. This stairstep-like landscape is the result of periodic, interglacial high-sea stands superimposed on steady tectonic uplift. Uplift of the crustal block is hypothesized to be the result of compression generated by a restraining bend of the northwest-trending Palos Verdes Hills fault, situated to the northeast. A late Quaternary uplift rate is calculated by measurement of the elevation of the marine terrace dated to the last interglacial period, anchored by six uranium-series ages on fossil corals, ranging from ~114 ka to ~119 ka. The shoreline angle for this terrace lies at an elevation of ~70 m. Assuming, from tectonically stable coasts, that this terrace formed at a paleo-sea level of +6 m, late Quaternary uplift rates of 0.56-0.54 m/ka are implied. This range of uplift rates is relatively high compared to other parts of the California coast where uplift is controlled by tectonic processes other than restraining bends. The second-highest terrace in the Palos Verdes Hills is highly eroded, but retains part of its old marine platform very close to the ancient shoreline angle, at an elevation of 379 m. Twelve fossil mollusks yielded calibrated Sr-isotope age estimates of 0.680 Ma to 0.870 Ma, a range which spans three major interglacial periods of the middle-early Quaternary. Paleo-sea levels for these older interglacials are unknown, but were likely close to present. If so, a longer-term uplift rate range of 0.56-0.44 m/ka is implied, in broad agreement with the late Quaternary uplift rate. It is concluded from these observations that tectonic uplift rates in this part of California may have changed very little over the course of the Quaternary, indicating a long-term control of uplift by activity of the Palos Verdes Hills fault.