THE 1954 RAINBOW MOUNTAIN FAULT, CENTRAL NEVADA: ANOTHER MODERATE-SLIP RATE FAULT IN THE WESTERN BASIN AND RANGE

Three Rainbow Mountain-Stillwater earthquakes (M6.6, M6.4, and M6.8) were the first events of the 1954 central Nevada seismic belt sequence, producing surface faulting along a 70-km-long rupture zone west of the Stillwater Range in central Nevada. Chronostratigraphic mapping and exploratory trenching in the rupture zone show that the paleoseismic history of the Rainbow Mountain fault is similar to many other active faults of the western basin and Range, with evidence of two earlier Holocene events and a moderate slip rate. Faulting displaced lacustrine and intra-lacustral subaerial deposits of the late Pleistocene-Holocene Lake Lahontan allostratigraphic sequence (Morrison, 1991). Detailed (1:12,000) surficial mapping indicates that the primary 1954 ruptures occurred along a wave-modified paleo-scarp nested within a sequence of post-highstand (<13 ka) recessional shorelines on the east flank of Rainbow Mountain. Trenches exposed evidence of two older events displacing alternating sequences of lacustrine and subaerial deposits that we radiocarbon dated to determine the slip history of the fault. The age of the triultimate event is bracketed by faulted subaerial alluvial fan deposits dated at 14.5-17.8 ka and a deepwater clay dated at 11.8 ka. The penultimate event is bracketed by faulted subaerial deposits of late Sehoo age, dated at 8.1-9.9 ka, and unfaulted late Sehoo shoreline gravels dated at 6.3 ka. The elevation of the 6.3 ka shoreline at the trench site (1228 m) indicates that the late Sehoo (S3) lacustral cycle of Lake Lahontan is younger than previously known and in part accounts for the obscure surficial record of faulting. Reverse faulting relations observed in the trenches together with mapped historic offsets indicate that all events had significant components of right-oblique slip, and cumulative post-14.5-17.8 ka displacement is estimated to be 6.6 m when adjusted for fault dip and lateral offset yielding a slip rate 0.4-0.5 mm/yr. At the southern end of the rupture zone in Fourmile Flat, two older events having 3.0 m of cumulative net offset are recognized in lake deposits containing a 14.7 ka tephra, yielding a vertical slip rate of 0.2 mm/yr.