LOW THERMAL GRADIENTS AND DEEP TECTONIC BURIAL IN THE EASTERN GREAT BASIN FROM LRCM DATA

The ranges of the eastern Great Basin, from the Snake Range to the Fish Creek Range, are critical to understanding the tectonic evolution of the area. They lie along the crest of the pre-Miocene zone of high elevation and crustal thickness, yet very few Mesozoic contractional structures have been documented. We have used Laser Raman spectroscopy of Carbonaceous Material (LRCM), in which the structure of carbon in the rock is evaluated and used to estimate peak metamorphic temperatures, to demonstrate that temperatures across the area outside the Snake Range Metamorphic Core Complex exceeded 300°C, and at the deepest structural levels reached 550°C. These peak temperatures are hotter than would be achievable through solely stratigraphic burial beneath the 10.5 km thick stratigraphic sequence using the Late Cretaceous thermal gradient of 25°C/km determined in the core complexes. It has been proposed that these temperatures are a result of an elevated Late Cretaceous thermal gradient, as there is little field evidence for large scale thrust duplication that would bury the rocks to the depths needed. This hypothesis should be testable by measuring temperatures from a transect that goes through as much continuous stratigraphy as possible. If an elevated thermal gradient is the source of the high temperatures, that should be detectable from the temperature increase moving down through the stratigraphic section. Preliminary data indicate thermal gradients broadly consistent with those indicated by the core complexes. Taken together with our peak temperature determinations, this requires that the rocks have been buried tectonically to depths substantially greater than their stratigraphic depth, possibly beneath a major thrust sheet more than 10 km thick that has been largely removed or obscured by erosion and Neogene extension.