Abstract

Well-defined high-angle faults were exposed in 1969 by trenching across scarps in alluvial fans along mountain ranges north of Arco and Howe, Idaho. Faulting along both the Arco and Howe scarps disrupts well-developed calcic soils that are considered to be about 30,000 yr old. On the other hand, both scarps are interrupted by gaps occupied by unfaulted alluvial fans that probably are younger than 15,000 yr old.

The Arco scarp coincides with a zone of closely spaced faults in alluvial fans along the western foot of the Lost River Range. From Arco, the scarp extends northward about 16 km (10 miles), where it merges into an unstudied scarp in bedrock. At a trench across the scarp 10 km (6 miles) north of Arco, multiple movements in the fault zone have resulted in aggregate vertical displacement of at least 12 m (40 ft). Measured offsets of stratigraphic units exposed in the trench indicate at least two episodes of vertical movement on individual faults. One episode of movement caused a minimum offset of 4.6 to 6.1 m (15 to 20 ft), and another offset was more than 3 m (10 ft).

The Howe scarp coincides with a zone of closely spaced faults in alluvial fans along the western foot of the Lemhi Range. A southern part of the scarp trends northwest along the southern part of the range a distance of 15 km (9 miles), and a northern part trends north at least another 6 km (4 miles). These parts of the scarp are separated by a bedrock ridge 3 km (2 miles) wide in which the scarp is indistinct. At a trench aross the scarp about 11 km (7 miles) north of Howe, multiple movements in the fault zone have resulted in aggregate vertical displacement of at least 15 m (50 ft). Measured offsets of stratigraphic units exposed in the trench indicate five or more episodes of vertical movement on individual faults, ranging from 0.3 m (1 ft) to more than 3 m (10 ft).

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