GEOLOGIC AND GEOPHYSICAL CHARACTERIZATION OF THE BARTLETT SPRINGS FAULT ZONE (CALIFORNIA): IMPLICATIONS FOR FAULT BEHAVIOR AND EVOLUTION OF A MAJOR STRIKE-SLIP FAULT

We use joint geologic-geophysical mapping and modeling to characterize the structure of the Bartlett Springs fault zone, the easternmost of three major strike-slip faults of the northern San Andreas system. Combining geologic mapping with gravity and magnetic anomalies provides a way to characterize rock types, in particular serpentinite, that promote creep along the fault zone, and thus we can differentiate between geodetic models that assume different depth extents of creep. The fault zone can be divided into two parts according to its magnetic signature at a point about 5 km southeast of Bartlett Springs. Southeast of that point, the fault zone traverses a complicated pattern of magnetic highs (amplitudes up to 700-800 nT and ~15 km wide) sourced primarily from serpentinized ultramafic rocks of the Coast Range ophiolite (unit Jsp). The highs indicate that this part of the fault zone is underlain by extensively more Jsp than is exposed at the surface. Preliminary modeling across the Wilson Valley area indicates that Jsp extends to depths of 10 km on the southwest side of the fault, whereas the northeast side of the fault has Jsp extending to depths of 2-6 km. North of Bartlett Springs, the fault is marked by narrower (2-4 km wide), lower-amplitude (100-200 nT) magnetic anomalies, again sourced by Jsp. In many places, the magnetic anomalies are truncated by various strands of the fault zone. Modeling indicates Jsp to depths of about 5 km, except at Lake Pillsbury where Jsp extends from 2 to 11 km depth. Here this deep body may be the source of high-temperature serpentine mineralogy found in fault gouge, in contrast to low-temperature serpentine mineralogy in nearby outcrops of flat-lying ophiolitic mélange. Upward movement of the high-temperature serpentinite may have been facilitated by a releasing step in the fault at Lake Pillsbury. The step in the mapped neotectonic fault strand is about 1 km wide; a coincident gravity low suggests that an older releasing step is up to 3 km wide. In Round Valley (50 km to the northwest), the mapped neotectonic strands make a restraining bend 2 km wide; a gravity low not only is much wider but indicates an older releasing geometry at this location. These relationships suggest that the fault zone may be straightening with increasing offset.