SERPENTINITE IN A CREEPING TRACE OF THE BARTLETT SPRINGS FAULT, NORTHERN CALIFORNIA

The Bartlett Springs Fault (BSF) is an active, right-lateral strike-slip fault that is part of the San Andreas Fault System in northern California. Its slip rate currently is estimated to be 6 ± 2 mm/yr, and along a segment that crosses Lake Pillsbury, half the surface slip (3.1 ± 0.2 mm/yr) is taken up by creep. An exposure of this fault segment near Lake Pillsbury consists of a ~1.5 m-wide zone of sheared serpentinite that has risen buoyantly through late Pleistocene to Holocene (?) fluvial deposits to the surface. The serpentinite consists of porphyroclasts of ultramafic and crustal rocks, including Franciscan lawsonite-bearing metagraywacke, distributed in a sheared and foliated, fine-grained matrix that varies from light-greenish to dark-bluish gray in color. The lighter-colored portions are dominated by antigorite serpentinite; antigorite in the ultramafic porphyroclasts forms fan-shaped arrays of bladed crystals as much as 0.5 mm in length. The darker-colored portions are rich in porphyroclasts containing one or more of the minerals talc, chlorite, and tremolite in a sheared matrix of the same minerals. The mineral assemblage of the darker areas is characteristic of metasomatic reaction zones formed between ultramafic and quartzofeldspathic crustal rocks at greenschist-facies conditions. Incipient low-temperature alteration accompanying shear has produced Mg-rich smectitic clays. Serpentinite-matrix mélange outcrops extensively in the Lake Pillsbury area, characterized by the low-temperature assemblage lizardite + chrysotile + calcite + magnetite. The serpentinite blocks in the mélange are largely mesh- and bastite-textured lizardite pseudomorphic after primary olivine and pyroxene, respectively. An associated ophiolitic mélange unit is interleaved with basal Great Valley Sequence sedimentary rocks. The antigorite serpentinite in the BSF contrasts markedly with ultramafic rocks in these other units and has been tectonically entrained in the fault from considerable depth. The mode of emplacement, textures, and mineral assemblages of the BSF serpentinite are very similar to those observed in the creeping traces of the San Andreas Fault encountered in the SAFOD drillhole, suggesting a common origin for creep at both locations.