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

Exhumation rates inferred from thermochronometers are dependent on spatial and temporal variations in temperature. In active extensional mountain belt s, the temperature field is complicated by tectonic and surface processes i ncluding: (1) lateral heat flow across large, range-bounding normal faults due to the juxtaposition of a cool hanging wall and a relatively warmer foo twall, (2) uplift and erosion of the footwall, (3) sedimentation and burial of the hanging wall, (4) lateral heat refraction around low thermal conduc tivity sediments deposited in the hanging wall basin. and (5) 3D temperatur e variations due to high-relief topography developed on the footwall. We ex plore these mechanisms through a series of 2D conductive thermal models des igned to investigate the effect of tectonics and topography on apatite fiss ion track (AFT) and (U-Th)/He thermochronometer data. Models were tuned to the geometry and kinematics of the Wasatch Mountains, Utah, USA. The princi pal 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 t he fault. surface morphology taken from USGS digital elevation models, and basin geometries inferred from seismic and gravity surveys. Predicted AFT a nd (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 a nd AFT closure temperature isotherms within 10km of the fault are advected upward 500 and 1000m, 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 ver sus age using ID thermal models. For simulations with a uniform vertical up lift rate and canyon and ridge topography, 1D and 2D exhumation rate differ ences were 20-70% for (U-Th)/He and similar to 10% for AFT data. Samples co llected perpendicular to fault strike up the range front are sensitive to t he exhumation rate and footwall tilt. Differences between 1D and 2D range f ront exhumation rates were 10-95% for (U-Th)/He and 10-40% for AFT data. Fu rthermore, 1D thermal models are incapable of deciphering footwall tilt. At simulated exhumation rates of 3. 4, and 5 mm per year predicted (U-Th)/H e and AFT ages form closely spaced, near vertical lines on a plot of elevat ion versus sample age, therefore, suggesting a decreased sensitivity of dat a to topography and surface processes at high exhumation rates. Despite lar ge eff ors in previously collected AFT data from the Wasatch mountains, our model suggests a revised fault-adjacent exhumation rate of 0.5-0.6 nun per year compared to previous estimates of 0.7 turn per year based on ID therm al models. (C) 2001 Elsevier Science B.V. All rights reserved.