Paper No. 17
Presentation Time: 1:00 PM


MCLAUGHLIN, Robert J. and MORING, Barry C., U. S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025,

The Bartlett Springs Fault Zone (BSFZ) in the northern California Coast Ranges is the northeasternmost creeping right-lateral fault of the transform margin, and it extends to the subduction margin north of the Mendocino triple junction. New framework geologic mapping north of Wilbur Springs and south of Round Valley confirms an earlier history as part of the ancestral Cascadia subduction margin and re-activation by the modern San Andreas strike-slip system.

The present BSFZ dips steeply to the NE and is transpressional, with a prominent hanging wall antiform locally mapped along the crest of Pacific Ridge on the NE side of the fault zone. The fault is aligned along a N43°W-oriented, 54 km long, 3-8 km wide belt of ophiolitic mélange composed of Coast Range ophiolite (CRO) interleaved with, or overlain by ca.150-131 Ma Great Valley Group (GVG) strata. These rocks are folded into a synform NE of the BSFZ near Wilbur Springs, but toward the NW, shearing along the BSFZ increasingly obscures the synformal geometry. In Rice and Round Valleys on opposite sides of the BSFZ, the ophiolitic mélange is structurally overlain by GVG rocks that in turn are unconformably overlain by cross-bedded, littoral to inner shelf Paleocene-Eocene strata containing conglomerate clasts derived from deeply subducted Franciscan rocks. The ~66-49 Ma Paleogene strata and underlying GVG plus ophiolitic mélange section may initially have been preserved in a graben 48-24 Ma, during regional extension. Earlier extension that significantly thinned the upper crust, was taken up largely in the CRO and lower GVG during ~70-50 Ma unroofing of the Franciscan Complex that in turn, was driven by subduction. In Round Valley, these rocks are unconformably overlain by littoral to estuarine mid-Miocene (ca. 15 Ma) deposits. We interpret the ophiolitic mélange belt to have been dextrally sheared along the BSFZ since deposition of Miocene strata in Round Valley. Based on matching of GVG and Paleogene rocks underlain by ophiolitic mélange SW of the BSFZ at Round Valley, with similar Paleogene and older rocks NE of the fault at Rice Valley, we tentatively suggest 38-40 km of dextral translation since the mid-Miocene (~15 Ma), yielding a long term rate of about 2.6 ± 0.1 mm/yr.