LONG-TERM SLIP RATES FROM DISPLACED BASALT FLOWS ON THE HURRICANE FAULT IN UTAH, AND EVIDENCE FOR A PROPOSED NEW FAULT SEGMENT

The Hurricane fault is one of the longest and most active of several west-dipping normal faults in southwestern Utah and northwestern Arizona. A high rate of Quaternary activity on the fault is indicated by the geomorphology of the high, steep Hurricane Cliffs, a fault-line scarp, and by displaced Quaternary basalt flows (hundreds of meters), and alluvium and colluvium (meters to tens of meters) at several locations along its length. The Utah Geological Survey has developed new paleoseismic information that helps characterize the fault's behavior in Utah and provides information critical to earthquake-hazard assessment. Along the Hurricane fault in Utah, scarps formed on unconsolidated Quaternary deposits are rare. Trenching at the best available site was unsuccessful due to large boulders in the subsurface. Unable to trench, we developed long-term fault slip rates by correlating and dating Quaternary basalt flows displaced by the fault, and partially constrained the timing of the most recent surface-faulting event(s) (MRE) by dating detrital charcoal in faulted and unfaulted young alluvial-fan sediments that overlie the fault. We used trace element geochemistry to correlate displaced basalts at five locations, evaluated back-tilting of hanging-wall basalts using paleomagnetic indicators, and obtained radiometric 40Ar/39Ar ages for several basalt samples. Slip rates corresponding to those ages range from 0.21 to 0.57 mm/yr, which likely reflect both differences in the time period over which the slip was measured and fault segmentation. North of Murie Creek, two young alluvial fans overlying the Hurricane fault are not faulted. Detrital charcoal from those fans yielded radiocarbon ages of 420+40 and 1710+40 yr B.P., respectively, indicating that the MRE is older than those ages. At Murie Creek, the alluvial fan is displaced across a 2.5 meter-high (single event?) scarp. Detrital charcoal there gave a radiocarbon age of 1220+40 yr B.P., indicating that the MRE is younger than that age. Murie Creek is at a pronounced right bend in the fault. The presence of young faulting there, and the absence of evidence for young faulting to the north, suggests that the fault bend represents a previously unrecognized seismogenic segment boundary. The new, northern segment is named the Cedar City segment and is ~ 20 kilometers long.