The Hat Creek fault is a major, young, north-striking, normal fault along the western boundary of extensional Basin and Range deformation in the Lessen region of northeastern California. Volcanic rocks of Quaternary and late Pliocene age are displaced a total of >500 m down to the west along west-facing, en echelon scarps now retreated to ∼35° slopes. Fresh, young scarps as much as 30 m high cut the Hat Creek Basalt (erupted between 15 and ∼40 ka) a few tens of meters west of the retreated scarps. Prior to the late 1980s, these young scarps were interpreted as lava slump scarps formed as the Hat Creek Basalt ponded against the older fault scarps and then drained away to the northwest. Numerous pieces of geologic evidence, however, show that the young scarps formed after the Hat Creek Basalt solidified and cooled and are true fault features formed by the youngest displacements of the Hat Creek fault. Structural details are remarkably well preserved along the series of left-stepping scarps cutting the Hat Creek Basalt. Near the central parts of individual segments, the fault is displayed as a single, vertical scarp. Near the ends of the segments, the scarp decreases in height and becomes a monodinal flexure on which the recent dip separation has been taken up by small-scale offset along columnar cooling joints in the basalt These monodinal flexures commonly rotate into east-west monoclines that join adjacent north-south segments. Displacement of outwash gravel overlying the Hat Creek Basalt shows that vertical separation on the Hat Creek fault has averaged ∼1.3 mm yr−1 for the past 15,000 yr. The Hat Creek fault thus represents a potential earthquake hazard, despite the low level and diffuse nature of modern seismidty in the region.

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