UTILIZING COOLING HISTORIES TO DETERMINE THE GEOMETRY, SEQUENCE AND RATE OF FAULTING

Mineral cooling histories are a common tool to investigate the magnitude and rate of rock exhumation either based on tectonic or climatic drivers. These exhumational histories are typically calculated in two different ways; 1) estimating the closure temperature and depth, either through assuming a nominal closure temperature and gradient or through 1-D thermal modeling, 2) calculating the exhumation gradient through an age-elevation relationship. The cooling history of low-temperature thermochronometers, particularly apatite fission track (AFT) and apatite (U-Th)/He (AHe), are sensitive to topographic relief and thus vary (to a first order) with elevation. In compressional settings, significant lateral transport modifies both the subsurface thermal field as well as the pathway of rocks to the surface. Because of this, 1-D models may yield incorrect long-term exhumation rates, which are spatially offset from where exhumation was focused. We suggest that the lateral transport of rocks and the resulting exhumation pattern can be exploited to determine the geometry of subsurface structure and the rate at which faults move. The thrusting of rocks up and over footwall ramps imparts a broad U-shape cooling curve in the thermochronometric data with the youngest ages focused at the top of the ramp. Multiple ramps complicate the shape of the cooling curve, but in a predictable way. Thus the sub-surface geometry (location and magnitude of modern and past ramps) controls the across-strike pattern of cooling ages for any given thermochronometer system. Changes in thermal properties alter the absolute age, and changes in velocities alter both the slope and age of the cooling signal, but neither effects its first-order shape. Using a kinematic model that allows for sequential displacement on structures and a thermal model to calculate the resulting thermal field and thermochronometer age, we show how across strike cooling curves can be used to determine the geometry of structures, the locations of ramps and rate of faulting in Bhutan Himalayas and the Bolivian Andes. We also illustrate the sensitivity of these ages to estimates of topography with time.