Abstract

This paper examines the nature of faulting along the Bartlett Springs fault zone, which is located in the Coast Ranges of northern California, about 25 km north of Clear Lake (and 50 km north of the Geysers Steam Field), 75 km east of the San Andreas fault, and generally 10 to 25 km west of the zone of Coast Range thrust faults. The Bartlett Springs fault appears to be restricted to an exposed 40 km long northwesterly trending belt of melange located within an undetermined thickness of Franciscan rocks. The fault forms the northwesterly end of a southeasterly trending continuous fault system, including the Green Valley fault and probably the Calavaras fault. A lineation of microearthquake epicenters along the belt of melange shows that the Bartlett Springs fault is active. Hypocentral locations and focal mechanisms were estimated carefully for a limited number of microearthquakes along the belt of melange that were recorded at nearby stations. The hypocenters, obtained by a group hypocentral algorithm, are located along a nearly vertical, 2- to 3-km-wide zone that extends downward to depths of about 12 km. Most of the focal mechanisms obtained indicate right-lateral, strike-slip displacements which are in accord with the inferred Bartlett Springs fault zone. The remaining focal mechanisms indicate predominantly dip-slip motions with fault planes of variable strike, most of which differ from the inferred fault. We interpret the Bartlett Springs fault as being consistent with a wide zone of shear stress at the lithospheric plate margin that extends from the San Andreas transform fault for a considerable distance eastward.

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