EXTENSIONAL STRAIN, CONTRACTIONAL STRAIN, AND PEAK THERMAL CONDITIONS IN THE WHITE PINE RANGE, EASTERN NEVADA: INSIGHTS FROM A STRUCTURAL RECONSTRUCTION INTEGRATED WITH RSCM THERMOMETRY

Quantifying strain magnitude and thermal conditions within mountain belts can provide insight into their evolution. In the hinterland of the Sevier thrust belt in Nevada, a complex overprint of Cenozoic extension obscures the geometry of Cordilleran contractional deformation. In this study, we perform a structural analysis of the White Pine Range in eastern Nevada, which lies within the newly-defined Eastern Nevada fold belt. We utilize published mapping to draft a cross-section, which we retro-deform to quantify extension and shortening. In addition, we use Raman spectroscopy on carbonaceous material (RSCM) thermometry and published conodont alteration indices (CAI) to estimate peak temperature versus restored stratigraphic depth.

In the White Pine Range, an 8 km-thick section of Cambrian-Pennsylvanian sedimentary rocks is deformed by N-striking, high-angle (≥50°) normal faults. Retro-deformation of all normal faults, combined with restoration of ~12° of eastward tilting of a Paleogene sub-volcanic unconformity, yields 8.0 km of extension, or 48%. The pre-extensional geometry consists of three N-trending folds, which have limb dips typically ≤30°. Line length measurement indicates that folding accommodated 1.7 km of shortening, or 9%.

Four samples of Cambrian and Ordovician limestone and shale were collected, which restore to pre-extensional depths between 6.0-7.6 km. RSCM thermometry yielded temperatures of 277±39°C at a depth of 6.0 km, 283±35°C at 6.2 km, 465±31°C at 6.4 km, and 565±38°C at 7.6 km. The lowest two samples are interpreted to have been heated by Cretaceous intrusions, which intrude similar stratigraphic levels within ~1 km map distance of these samples. However, the two highest samples are not in proximity to any mapped intrusions, and are interpreted to record representative thermal conditions for these depths. Their temperatures are best-fit by a ~46°C/km geothermal gradient, which is corroborated by a compilation of published CAI values. These data, when combined with published evidence for ~50-60°C/km geothermal gradients in other ranges in eastern Nevada, provide further support that elevated temperatures were locally attained at shallow-crustal levels, and that Cretaceous magmatism played a key role in the transfer of heat to the upper crust during Cordilleran orogenesis.