Paleoseismology of Utah, Volume 5

Neotectonic deformation along the East Cache fault zone, Cache County, Utah

Special Study 83, 1994, 37 pages, $5.00

ABSTRACT

The Quaternary history of the East Cache fault zone was investigated utilizing surficial geologic mapping, backhoe trenching of fault scarps, surveying the Bonneville highstand shoreline, and measuring quantitative geomorphology of the Bear River Range front. Fault scarps displace deposits of the Bonneville lake cycle and younger sediments for 8 kilometers (4.8 mi) along an area east of Logan, Utah. To the north and south, the less youthful appearing range front either shows no clear evidence of late Quaternary faulting, or exhibits only pre-Bonneville-lake-cycle faults in rare exposures.

Based on geomorphology and age of latest faulting, the fault is divisible into a northern section greater than 26 kilometers (>15.6 mi) long, a central section 16 kilometers (9.6 mi) long (with post-Bonneville fault scarps), and a southern section greater than 14 kilometers (>8.4 mi) long. The northern section appears to have been the least active in the late Quaternary, and no fault scarps were found across any late Quaternary surfaces in that section. An apparent 20-meter (65.6 ft) displacement of a pre-Bonneville pediment at the southern end of the northern section implies a long-term slip rate of only about 0.05-0.10 mm/yr (0.002-0.004 in/yr).

In contrast, two surface-faulting events have occurred in the central section since occupation of the Bonneville highstand shoreline (15.5 thousand years ago). Radiocarbon and thermoluminescence (TL) age estimates, and quantitative pedology from two trenches, suggest the earlier event (displacement 1.4-1.9 m; 4.6-6.2 ft) occurred between 13 and 15.5 thousand years ago, and a later event (displacement 0.5 m-1.2 m; 1.6-3.9 ft) occurred about 4 thousand years ago. Evidence for an event earlier during the Bonneville transgression is equivocal. Long-term slip rates based on an 8.5 meter (27.9 ft) displacement of pre-Bonneville alluvium are as high as 0.06 mm/yr (0.002 in/yr), depending on age of the alluvium.

Quaternary faults also occur on the southern section, but TL age estimates indicate that latest faulting occurred between about 26 and 46 thousand years ago. Long-term slip rates based on pediment offsets may be as high as 0.07 mm/yr (0.003 in/yr).

The elevation of the Bonneville highstand shoreline was surveyed at 82 locations along the East Cache fault zone to an accuracy of roughly I meter (3.3 ft), but surveying revealed no discrete displacements where the shoreline crossed inferred fault segment boundaries. There was no elevation difference between reaches where post-Bonneville faulting of up to 4.2 meters (13.8 ft) had occurred, and reaches where no faulting had occurred.

However, three elevation downwarps (amplitudes of 2.5-5.3 m; 8.2-17.4 ft) occurred spatially coincident with large Bonneville/Provo delta complexes. The crust underneath Cache Valley was Theologically modeled as a beam subjected to point depositional loading by the deltaic piles. Model results show that the amplitude of the shoreline elevation anomalies can be predicted assuming reasonable deltaic weights and crustal strength parameters, but the observed radii of deformation are much smaller than predicted.

The unique faceted spur structure of the central segment extends across section boundaries to the north and south, indicating that, in a long-ten-n sense, uplift events may not have terminated at present segment boundaries. Though ambiguous, facet spacing and pattern suggest that the western splay fault of the northern section has developed recently, transferring displacement away from the eastern splay. In contrast, young facets of the central segment extend through the southern section on the eastern splay, suggesting that some Quaternary rupture events may involve both sections (combined length >29 k-m; >17 mi).