OBSERVATIONS REGARDING WASATCH FAULT VERTICAL SLIP RATES: INSIGHTS USING THE AGE OF SEDIMENTS IN TIMPANOGOS CAVE, UTAH

Timpanogos Cave is located on the steep south wall of American Fork Canyon about 3.8 km from the Wasatch fault, which is the longest normal fault in the United States. Cave openings are about 357 m above the American Fork River. When the cave was at river level it was partially filled with fine-grained fluvial sediments. The sediments and an interbedded carbonate flowstone layer yield a paleomagnetic and U-Th depositional age of 350 to 780 ka. The calculated river down cutting rates since 350 and 780 ka are 1.02 to 0.46 mm yr^-1, respectively. The vertical slip rate of the Wasatch Fault, since 350 to 780 ka, is inferred to be equal to the stream down cutting rate.

Cave sediment calculated slip rates are in the range of the 0-12 Ma Wasatch Range exhumation rate (~0.5-0.7 mm yr^-1), suggesting the long-term Wasatch fault slip rate remained stable through mid-Pleistocene time. However, the late-Pleistocene (0-250 ka) decelerated slip rate (~0.2-0.3 mm yr^-1) and the accelerated Holocene slip rate (~1.2 mm yr^-1) are consistent with the idea of episodic fault activity. Assuming the late-Pleistocene slip rate of ~0.2-0.3 mm yr^-1 represents an episodic slowing of fault movement and the long-term average vertical slip rate, including the late-Pleistocene and Holocene, should be ~0.6 mm yr^-1, there is a net late-Pleistocene vertical slip deficit of ~ 50-75 m. Accelerated Holocene slip only reduced the late Pleistocene net slip deficit by ~7.2 m. Because glacial loading calculations show that glaciation was only sufficient to lock the Wasatch fault for about 8% of the past 600 ka and the literature predicts that post-Bonneville enhanced slip only lasted for 5-6 kyr, the reduced late-Pleistocene slip rate and much of the accelerated Holocene rate need to attributed to factors other than Pleistocene glaciation. The late-Pleistocene slip deficit may mean that the current accelerated Wasatch fault slip rate will extend well into the future.