Exhumation of the central Wasatch Mountains, Utah: 2. Thermokinematic model of exhumation, erosion, and thermochronometer interpretation

The Wasatch fault is a similar to370 km long normal fault in Utah that marks the boundary between the stable Colorado Plateau to the east and the extending Basin and Range to the west. Understanding the thermokinematic evolution of this fault can provide insights into intracontinental extensional tectonics and deformation processes in other rift zones (e.g., East Africa Rift, Transantarctic Mountains). We explore the thermokinematics of footwall exhumation and erosion in the Cottonwood Intrusive Belt of the central Wasatch Mountains. Emphasis is placed on using low-temperature thermochronometers to quantify (1) the spatial and temporal variability of exhumation and erosion rates, (2) the geometry of footwall tilt, (3) the fault dip angle, and (4) the magnitude and duration of exhumation. These processes are investigated using two-dimensional (2-D) thermal-kinematic models coupled with cooling-rate-dependent kinetic models which predict exhumed apatite fission track (AFT) and (U-Th)/He ages. The range of model parameters considered includes footwall exhumation and erosion rates at the fault between 0.2 and 2.0 mm yr(-1), footwall tilt hinge positions between 15 and 40 km distance from the fault, a single planar normal fault with dip angles of 45degrees and 60degrees, and exhumation magnitudes of up to 15 km at the fault. Simulations include the formation of a low thermal conductivity sedimentary basin and erosion of heat-producing layers. Erosion maintains a constant topographic profile. The kinematic and exhumation history of the Wasatch Mountains is investigated by comparing model predicted thermochronometer ages to observed AFT, ZFT, and (U-Th)/He ages. Predicted and observed ages are compared using a reduced chi-square analysis to determine a best fit kinematic model for the Wasatch Mountains. The preferred model includes exhumation occurring on either a 45degrees or 60degrees dipping fault, a footwall hinge located a minimum of 20-25 km from the fault, and a step decrease (deceleration) in the footwall exhumation rate at the fault from 1.2 to 0.8 mm yr(-1) at around 5 Ma. The model also suggests an exhumation duration of similar to12 Myr+/-2 Myr.