Abstract

New data on the geochronology of upper Quaternary units in the Willow Creek drainage bear on the timing of the last prehistoric surface faulting event along the Thousand Springs segment of the Lost River fault. Surficial effects of this event were paralleled by the 1983 MS 7.3 Borah Peak earthquake. Radiocarbon dated till (8,470 ± 160 B.P.) at Doublespring Pass can be traced to the geomorphically youngest alluvial surface cut by the fault. The alluvial surface is related to deglaciation and thus postdates 8,470 B.P.; the faulting also postdates this time. Aeolian/sheetwash sediments draped over the fault scarp are 14C dated as 530 ± 80 B.P., providing an upper age bracket for faulting. Geomorphological analysis of presence-absence relations of regionally widespread Mt. Mazama tephra (6,850 14C yr B.P.) suggests that the faulting event may postdate this tephra fall, and thus lies in the age range 6,850 to 530 B.P. Based on the amount of pre-1983 offset (2.1 to 2.6 m) and a maximum of 6,850 yr between events, minimum short-term slip rates range from 0.3 to 0.4 m/1,000 yr along this segment.

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