Abstract

The thermal effects associated with emplacement of the Willard thrust sheet within the Idaho-Utah-Wyoming thrust belt have been numerically modeled. Fluid-inclusion studies and mineralogy, including illite crystallinity, narrowly limit choice of permissible thermal models. Permissible models have initial thermal gradients between about 30 °C/km and 35 °C/km and fluid pressures less than lithostatic pressure.

Fluid inclusions were trapped in syntectonic veins within the basal part of the Willard thrust sheet and in cataclastically deformed basement within the footwall. Homogenization temperatures have a bimodal distribution in the Willard sheet with maxima at 180 °C and 260 °C and a unimodal distribution in the footwall with a maximum at 200 °C. These temperatures, along with fluid composition, determine fluid density and define isochores along which the fluids were trapped. Illite crystallinity and mineralogy record maximum temperatures in the range of 300 °C to 500 °C with somewhat lower maximum temperatures in the footwall.

Thermal models for emplacement of the Willard sheet indicate that the hanging wall undergoes initial rapid cooling, but the footwall undergoes initial warming due to tectonic burial. Later both areas undergo approximately isothermal decompression as erosion proceeds. P-T paths for reasonable parameter values intersect the modal isochores and are consistent with mineralogy, as required for a permissible model. The thermal model provides a reasonable framework for understanding the thermal history of the Willard sheet. The models are sensitive to thrust-sheet thickness, initial heat flow, and thermal conductivity. The thermal models are relatively insensitive to erosion history for reasonable erosion rates and to redistribution of heat-producing elements for the low values typical of sedimentary rocks.

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