INTERPRETATION OF LOW-TEMPERATURE THERMOCHRONOMETER AGES FROM TILTED NORMAL FAULT BLOCKS

Low-temperature thermochronometry is a widely-used tool for dating the timing and rate of slip on normal faults. Rates are often derived from suites of footwall thermochronometer samples, but simple 2D regression of age vs. paleodepth fails to account for the fact that rocks collected at similar elevations today experienced curved particle trajectories and variable velocities during fault slip. We present a simple formulation of the thermal evolution of a rotating fault block driven by a constant extension rate. We show that advection of heat and perturbation of geothermal gradients by topography influence the thermal histories of exhumed particles, but for a range of geologically reasonable fault geometries and rates these effects produce Apatite U-Th/He ages comparable to those derived from rotation through fixed isotherms. We apply the fixed-isotherm version of the model to a natural example from the Pine Forest Range, Nevada, by incorporating field and thermochronologic constraints into a Markov-Chain Monte Carlo model. Most model parameters are described by relatively narrow ranges of geologically reasonable parameters, and the model suggests an average slip rate of ~1.1 km/Myr and an onset of faulting ca. 9-10 Ma, compared to rates of 0.3-0.8 km/Myr and initiation ca. 11-12 Ma derived from visual inspection of the data alone. This model is conducive to Bayesian parameter estimation to quantify the geological uncertainty in the geometry of the tilted fault block, and its simplicity and flexibility allow application to a wide variety of normal faults where cooling ages already exist or could potentially be collected.