QUANTIFYING THE UPPER-CRUSTAL THERMAL STRUCTURE OF THE NEVADAPLANO: PRELIMINARY DEPTH-TEMPERATURE RELATIONSHIPS FROM THE SCHELL CREEK AND DEEP CREEK RANGES

The thermal architecture of orogenic crust can greatly influence the style and magnitude of contractional deformation. During the Late Cretaceous (~70-90 Ma), the hinterland plateau of the Cordilleran orogen in Nevada (the ‘Nevadaplano’) experienced a tectonothermal event that produced variably-distributed greenschist- and amphibolite-facies metamorphism at upper-crustal levels. This has been interpreted as the shallow thermal expression of lower-crustal anatexis triggered by either lithospheric delamination or crustal thickening. Quantifying the conditions and spatial distribution of this event is critical for understanding the thermal evolution of the Nevadaplano.

In order to quantify the peak temperatures attained in the upper crust in eastern Nevada, we performed Raman spectroscopy of carbonaceous material (RSCM) thermometry on Neoproterozoic-Mississippian sedimentary rocks collected from the Schell Creek and Deep Creek Ranges. Published tectonic reconstructions show that Neoproterozoic-Mesozoic rocks in these ranges were relatively flat-lying prior to mid-Cenozoic volcanism and were not duplicated by thrust faults; therefore, stratigraphic depths are utilized to approximate pre-extensional structural depths. Temperatures in the Deep Creek Range are between ~520-625 °C at depths of 8-13 km, decrease rapidly to ~380 °C at 6.5 km, and decrease to 280 °C by ~4.5 km. Temperatures in the Schell Creek Range are between ~470-525 °C at depths of 9-11.5 km and decrease to ~300 °C at 7 km. Published conodont alteration indices from both ranges are between 1 and 2 in the upper 1.5 km, bracketing temperatures between 50-140 °C. These temperature patterns corroborate published field observations of the stratigraphically-downward appearance of chlorite, biotite, and garnet.

Our data imply the potential for significant rheological weakening of the plateau crust during Late Cretaceous thickening and eastward translation, including elevating the quartz crystal-plastic transition to as shallow as ~5-7 km and the possibility for partial melting of pelitic rocks at mid-crustal depths. Further research will include RSCM data collection from additional ranges and integration of temperature data with retro-deformed cross sections, in order to place our samples in a detailed pre-extensional context.