Abstract

The Stansbury Mountains in the eastern Basin and Range Province are bounded on the west by the Stansbury fault, which offsets Quaternary alluvial fans. The fault is approximately 45 kilometers (28 mi) long and has two structural segments which very likely are rupture segments. The segment boundary is located at a cross fault at Pass Canyon. Fault scarps are confined to a single trace on the south segment, but occur across a complex fault zone on the north segment. Total throw and heave on the north segment may approach 5.3 and 4.4 kilometers (3.3 and 2.8 mi), respectively. The average vertical separation rate since mid-Miocene time is estimated to be 0.07 ± 0.02 mm/yr (0.003 ± 0.001 in/yr) for the north segment. Assuming a constant rate of displacement accumulation since 18,000 years ago, the next surface-faulting event should generate a scarp at least 1.25 meters (4.1 ft) high, consistent with an earthquake of M_S = 6.8-6.9. An earthquake of such magnitude could have serious effects on Salt Lake City and surrounding towns and facilities.

Maximum scarp angle versus scarp height plots suggest no surface faulting has occurred since the Lake Bonneville highstand (14,500 ¹⁴C yr B.P., or about 18,000 years ago). This agrees with the results of an earlier study, as well as with field evidence that shoreline processes have modified fault scarps in the north. However, different trends for these data on the north and south segments suggest the segments can rupture separately. The latest surface-rupture event on the south segment could be more recent than on the north segment. Maximum scarp angle versus scarp height studies of multiple-event fault scarps probably yield maximum, not approximate, relative ages for latest surface-rupture events.

Rough correlations exist among range crest elevation, basin depth inferred from the complete Bouguer gravity anomaly, fault geometry, Quaternary scarp heights, and patterns of asymmetrical stream drainages. These correlations tentatively suggest that relative displacement, and displacement rates overall, are highest on the south segment of the Stansbury fault. Local highs in displacement (rates) may occur adjacent to topographic highs on the south and north segments, and minimal displacement (rates) are suggested opposite topographic lows.

The Stansbury Mountains are also interesting from a regional tectonic perspective. Aligned major structures and breaks in the Stansbury and adjacent ranges suggest pre-existing crustal features exert structural control. Crystalline basement structure apparently affects segmentation and geometry of Cenozoic faults in the Wasatch Front region, perhaps confirming its influence in the Stansbury Mountains.