T201 From the Micro to Macro in Metamorphic Geology: Constraining Tectono-Metamorphic Processes with High-Resolution Approaches

Thursday, 29 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

UPPER-CRUSTAL THERMAL STRUCTURE OF THE SEVIER HINTERLAND PLATEAU, EASTERN NEVADA AND WESTERN UTAH: CONSTRAINTS FROM RSCM AND CAI THERMOMETRY INTEGRATED WITH STRUCTURAL RECONSTRUCTIONS

Paper number: T201-16
Program: Technical Sessions
Time: 4:50 PM – 5:00 PM

Authors

Abstract

The style and magnitude of deformation within orogenic crust can be greatly influenced by its thermal structure. The hinterland plateau of the Cordilleran orogen in eastern Nevada and western Utah experienced a tectonothermal event between ~90-70 Ma, resulting in variably-distributed, greenschist- to amphibolite-facies metamorphism and granitic magmatism at upper-crustal levels. This event has been interpreted as the shallow thermal expression of lower crustal anatexis following lithospheric delamination. Here, we investigate the thermal history of the hinterland plateau by quantifying peak temperature conditions of the upper crust along an east-west transect. We performed Raman spectroscopy of carbonaceous material (RSCM) thermometry on Neoproterozoic-Mississippian rocks collected from six ranges in eastern Nevada and western Utah and compiled published conodont alteration indices (CAI) from Cambrian-Triassic rocks from these six ranges and three additional ranges. We retro-deformed cross sections of each range to place the RSCM and CAI data in a pre-extensional depth context, which allows quantifying paleo-peak geothermal gradients.

From west to east, peak temperatures down to ~8-10 km define geothermal gradients of 20-25 °C/km in the Fish Creek and Pancake Ranges, and 33-39 °C/km in the White Pine and Egan Ranges. Temperatures are highest in the Schell Creek and Deep Creek Ranges, attaining 300-400 °C at 5-7 km depths, 475-525 °C at 10-13 km, and 575-625 °C at 13-17 km, defining gradients of 41-46 °C/km. Temperatures are locally elevated proximal to granitic intrusions in the White Pine and Deep Creek Ranges. To the east, cooler gradients of 24-29 °C/km were obtained down to ~8 km in the Confusion and House Ranges.

Our results, when combined with published data from other ranges that define localized gradients up to 50-60 °C/km, define spatially heterogeneous heating of the upper crust. We interpret that granitic magmatism was the primary driver for heating in eastern Nevada, where high thermal gradients imply the possibility for partial melting as shallow as ~18-20 km. Our data are inconsistent with thermobarometry from the Snake Range that define 20-25 °C/km gradients down to 20-30 km depths; this further highlights the debate over possible tectonic overpressures in Cordilleran core complexes.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-355044

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