LATE QUATERNARY NORMAL FAULTING OF THE HAT CREEK BASALT, NORTHERN CALIFORNIA

The Hat Creek fault is a major, young, north-striking, normal fault al ong the western boundary of extensional Basin and Range deformation in the Lassen region of northeastern California. Volcanic rocks of Quate rnary and late Pliocene age are displaced a total of > 500 m down to t he west along west-facing, en echelon scarps now retreated to similar to 35 degrees slopes. Fresh, young scarps as much as 30 m high cut the Hat Creek Basalt (erupted between 15 and similar to 40 ka) a few tens of meters west of the retreated scarps. Prior to the late 1980s, thes e young scarps were interpreted as lava slump scarps formed as the Hat Creek Basalt ponded against the older fault scarps and then drained a way to the northwest. Numerous pieces of geologic evidence, however, s how that the young scarps formed after the Hat Creek Basalt solidified and cooled and are true fault features formed by the youngest displac ements of the Hat Creek fault. Structural details are remarkably well preserved along the series of left-stepping scarps cutting the Hat Cre ek 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 monoclinal flexure on whi ch the recent dip separation has been taken up by small-scale offset a long columnar cooling joints in the basalt. These monoclinal flexures commonly rotate into east-west monoclines that join adjacent north-sou th segments. Displacement of outwash gravel overlying the Hat Creek Ba salt shows that vertical separation on the Hat Creek fault has average d similar to 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 seismicity in the region.