Serpentinite and creep along the Bartlett Springs Fault at Lake Pillsbury

Diane Moore

Earthquake Science Center

Date & Time
Building 3, Rambo Auditorium

An exposure of a creeping segment of the Bartlett Springs Fault (BSF), part of the San Andreas System in northern California, consists of a ~1.5 m-wide zone of serpentinite-bearing fault gouge sandwiched between late Pleistocene fluvial deposits. The fault gouge is a heterogeneous mixture of pale-colored antigorite serpentinite and dark bluish-gray zones rich in talc, chlorite, and tremolite-actinolite that are characteristic of metasomatic reaction zones that form between ultramafic and crustal rocks. The serpentinite entrained in the fault differs mineralogically and texturally from extensive nearby outcrops of low-temperature lizardite-chrysotile serpentinite associated with the Coast Range ophiolite. The antigorite and metasomatic minerals in the BSF gouge are stable at temperatures above 250°-300°C. They were tectonically incorporated into the fault and rose buoyantly to the surface from depths near the base of the seismogenic zone. Low-temperature alteration of the fault rocks accompanying near-surface shear has produced minor to moderate amounts of Mg-rich saponitic smectite clays. The mineral assemblages, textures, and mode of emplacement of the BSF serpentinitic gouge correspond to a younger, lower-offset equivalent of the creeping San Andreas Fault encountered in the SAFOD (San Andreas Fault Observatory at Depth) drillhole. This suggests a common origin for creep at both locations, and one that may be applicable to other creeping faults of the San Andreas system in northern California.

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