GRAVITY MODELS OF THE PALOS VERDES AND CABRILLO FAULTS

A high-resolution gravity survey in the San Pedro and Los Angeles Harbor region was used to better determine the geometry and evolution of the Palos Verdes and Cabrillo faults. Almost 125 new gravity measurements were collected and indicate a 35 mgal positive gravity high between the Palos Verdes and Cabrillo faults. The large positive gravity anomaly is due to uplift of the Catalina Schist basement rock and low-density sedimentary rocks that thicken towards and north of the Palos Verdes fault. Density measurements indicate an almost 1 g/ density difference between the basement rocks and sedimentary cover. Such a large density difference is needed to reproduce basin thickness where constrained by drill cores. The Palos Verdes fault itself sits at an inflection in gravity data. This inflection is important because the highly productive Wilmington Oil Field sits in this region.

Five new gravity models have been constructed to better quantify the 3-D geometry of the Palos Verdes and Cabrillo faults. Our gravity models constrain the origin of the gravity high and deformational fold structures located between the Palos Verdes and the Cabrillo faults. Immediately south of the Palos Verdes fault, the Gaffey Anticline/Syncline is a required feature of the models and along strike fold amplitude decreases to the west and east of this structure. The gravity models also constrain the dip and vertical offset along the two faults. The Cabrillo fault has little vertical offset, and may act as a deformational backstop for the crustal block between it and the Palos Verdes fault. Models indicate that the Palos Verdes fault has 500-1000m of vertical high angle thrust offset. However, offset is accommodated mainly by rotation and folding of the crustal block in-between the Palos Verdes and Cabrillo faults, and not discrete vertical offset along the Palos Verdes fault itself.

Estimates of strike-slip displacement along the Palos Verdes fault are 2-3 mm/yr. It is also estimated to have been active in its current geometry over the past several million years suggesting total displacements on the order of 5-10 km. If such estimates of strike-slip displacement are correct, then the Palos Verdes fault is primarily strike-slip in nature with less than 10-20 percent of its offset accommodated by vertical thrust motion.