The long-term burial and exhumation history of basement blocks in the footwall of the Wasatch fault, Utah

Abstract

Thermochronologic studies of the Santaquin and Farmington Canyon crystalline basement complexes, exposed in the footwall of the Wasatch fault in Utah, provide rare opportunities to investigate the long-term tectonic, burial, and exhumation history of this region. Both complexes underwent amphibolite-facies metamorphism at ∼1700 Ma, followed by a complex pressure-temperature-time history. By 740–770 Ma, exhumation had brought both complexes to the surface from a depth of ∼9–10 km (3–3.5 kbar), followed by reburial by passive margin, Oquirrh Basin, and foreland basin sedimentation from Neoproterozoic through early Cretaceous time. The final structural pathway to present-day surface exposure of both complexes began in early Cretaceous time, with crustal contraction along the Sevier belt and resultant structural stacking. Structural breaching of the thrust culminations and final cooling of the crystalline complexes occurred as a result of Tertiary through Holocene extension and accompanying normal faulting. Inferred exhumation rates for the last 10–15 my are on the order of 0.3–0.6 mm/yr, although recent slip rates across the Wasatch fault appear to be several times higher. This suggests that: (1) periods of enhanced slip on the Wasatch fault from Miocene to present time may have been punctuated by periods of quiescence; and (2) the fault now may be experiencing an episode of rapid slip. Alternatively, strain may have been partitioned into multiple fault strands at a boundary between the Provo and Nephi segments.