Normal fault thermal regimes and the interpretation of low-temperature thermochronometers

Abstract

Exhumation rates inferred from thermochronometers are dependent on spatial and temporal variations in temperature. In active extensional mountain belts, the temperature field is complicated by tectonic and surface processes including: (1) lateral heat flow across large, range-bounding normal faults due to the juxtaposition of a cool hanging wall and a relatively warmer footwall, (2) uplift and erosion of the footwall, (3) sedimentation and burial of the hanging wall, (4) lateral heat refraction around low thermal conductivity sediments deposited in the hanging wall basin, and (5) 3D temperature variations due to high-relief topography developed on the footwall. We explore these mechanisms through a series of 2D conductive thermal models designed to investigate the effect of tectonics and topography on apatite fission track (AFT) and (U–Th)/He thermochronometer data. Models were tuned to the geometry and kinematics of the Wasatch Mountains, Utah, USA. The principal parameters in our model are exhumation and burial rates ranging from 0.2 to 5.7 mm per year at the range front and decreasing with distance from the fault, surface morphology taken from USGS digital elevation models, and basin geometries inferred from seismic and gravity surveys. Predicted AFT and (U–Th)/He ages were generated using cooling rate dependent annealing and diffusion kinetic models. Results indicate after 10 million years of exhumation, footwall (U–Th)/He and AFT closure temperature isotherms within 10 km of the fault are advected upward 500 and 1000 m, respectively, from their initial position. The upward advection of isotherms and the 2D nature of the thermal regime can result in erroneous exhumation rates calculated from plots of sample elevation versus age using 1D thermal models. For simulations with a uniform vertical uplift rate and canyon and ridge topography, 1D and 2D exhumation rate differences were 20–70% for (U–Th)/He and ∼10% for AFT data. Samples collected perpendicular to fault strike up the range front are sensitive to the exhumation rate and footwall tilt. Differences between 1D and 2D range front exhumation rates were 10–95% for (U–Th)/He and 10–40% for AFT data. Furthermore, 1D thermal models are incapable of deciphering footwall tilt. At simulated exhumation rates of 3, 4, and 5 mm per year predicted (U–Th)/He and AFT ages form closely spaced, near vertical lines on a plot of elevation versus sample age, therefore, suggesting a decreased sensitivity of data to topography and surface processes at high exhumation rates. Despite large errors in previously collected AFT data from the Wasatch mountains, our model suggests a revised fault-adjacent exhumation rate of 0.5–0.6 mm per year compared to previous estimates of 0.7 mm per year based on 1D thermal models.