Crustal Thermal Processes and the Interpretation of Thermochronometer Data

Abstract

Many thermochronology studies focus on extracting the thermal history of a sample using fission-track length distributions and track annealing models (e.g., Ketcham 2005, and references therein). Recent developments in measuring the 3He/4He concentration profile across apatite grains (e.g., Shuster and Farley 2005) offer a new approach for quantifying sample cooling histories over a broader range of temperatures with the (U-Th)/He system. The motivation behind calculating thermal histories from thermochronometer data has traditionally been to date events such as the onset of exhumation or fault motion, erosion, or for quantifying hydrocarbon maturation in sedimentary basins. The calculation of thermal histories from thermochronometer data is a routine aspect of most thermochronology studies and produces valuable geologic information. A relevant question for thermochronology studies is what can be learned from forward modeling crustal thermal fields from principles of heat conduction and advection beyond what is already learned from the thermal history extracted from the data? Forward modeling of crustal thermal fields requires a physically based model for heat transfer in the geologic setting of interest. Specific geologic processes such as magmatism, fault motion, fluid flow, as well as the kinematic, topographic, and erosional evolution of an orogen can significantly influence the thermal history of thermochronometer samples. These processes can be simulated with thermal models and then compared to thermochronometer derived cooling histories and ages. Thus, predicted thermal histories and thermochronometer ages generated under a known set of conditions can be compared to observed thermal histories and ages to quantify the geologic processes associated with sample cooling. To answer the question posed earlier, forward modeling crustal thermal fields allows robust, or at least constrained, interpretations of what geologic events could have occurred to produce an observed suite of thermochronometer ages. Hence, comparison of thermochronometer data with model predicted thermal histories …