Evidence of Active Faulting in the Halfway Gulch-Little Jacks Creek Area of the Western Snake River Plain, Idaho

Publication Date

5-1997

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geology

Department

Geosciences

Supervisory Committee Chair

Charles Waag

Abstract

In the Halfway Gulch-Little Jacks Creek (HG-LJC) area of southwest Idaho, a fault-generated mountain front escarpment bounds the western Snake River Plain (WSRP) graben on the southwest. Numerous fault scarps near the base of the escarpment generally show down to the northeast normal offset of Pleistocene to Holocene alluvial gravels. Geomorphic analysis of four of these scarps indicates late Pleistocene to Holocene movement. The most conspicuous of the scarps is the 5.3 km long Halfway Gulch fault scarp. Fourteen topographic profiles measured across the Halfway Gulch fault scarp yield an average vertical offset of 7.7 m and maximum slope angles which average 26°. Scarp ages calculated from the 14 profiles using the scarp-degradation relationships of Bucknam and Anderson (1979) and Mayer (1984) are about 5-20 ka and 3-12 ka respectively for the HG fault scarp. Similar analyses of the Water Tank fault and the Parker Ranch fault, having vertical offsets of 3.7 m and 2.7 m, respectively, yield ages ranging from 12-60 ka. Although indications of composite scarps are not present in the topographic profiles, vertical-offset-versus-length ratios of all three scarps suggest that they are formed by multiple events. Therefore, their derived ages are likely to be older than the actual age of most recent movement. Analysis of these scarps suggests that faulting in the HG-LJC area is substantially younger than the commonly accepted age of most recent extensional faulting associated with WSRP development (a minimum of 100 ka; Othberg, 1994).

A trench excavated in unconsolidated Quaternary alluvial deposits across the Water Tank fault scarp reveals evidence of recurrent late Quaternary faulting. Because the Halfway Gulch fault scarp lies within an area set aside for wilderness designation, trenching was limited to the Water Tank fault, a scarp 8 km basinward from the main Halfway Gulch fault scarp. Age estimates for individual offset events on the Water Tank fault are based on soil calcium carbonate rates of accumulation derived from regional and local data. Stratigraphic, structural, and soil evidence from colluvial wedge deposits indicate five discrete episodes of surface rupture during the last 26 ± 8 ka. The calculated average recurrence interval for movement on the Water Tank fault during this time is 5,200 years. Comparison with displacements observed in historical earthquakes suggests that the moment magnitudes of the late Pleistocene to Holocene events were about Mw 6.7 ± 0.5.

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