Interpretation of low‐temperature thermochronometer ages from tilted normal fault blocks

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  1. Joseph P Colgan

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Description: 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. structural depth 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 to demonstrate that in these settings the regression of age-depth data is susceptible to significant errors (>10%) in the identification of the initiation and rate of faulting. 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 AHe ages comparable to (within ~10%) those derived from exhumation through fixed isotherms. We apply the fixed-isotherm model to published data from the Pine Forest Range, Nevada and the East Range, Nevada, by incorporating field and thermochronologic constraints into a Markov chain Monte Carlo model. The Pine Forest Range is well-constrained by field observations, and most model parameters are described by relatively narrow ranges of geologically reasonable values. 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. The geometry of the East Range fault block is less well-constrained by field observations, but the data nonetheless robustly support an approximately 6-fold reduction in extension rate at ~14 Ma, after faulting began at ~17 Ma with an extension rate of ~3 km/Myr. The absence of a preserved partial retention zone in the East Range sample set limits how well the model can predict fault dip and footwall geometry. 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.

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