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Exploring the tectonic and thermal evolution of the northern Snake Range metamorphic core complex, eastern Nevada: Insights from field studies and potassium feldspar 40Ar/39Ar thermochronology

  • Author(s): Wrobel, Alexander Joseph
  • Advisor(s): Gans, Phillip B
  • et al.
Abstract

Metamorphic core complexes of the North American Cordillera record episodes of Mesozoic burial followed by high magnitude crustal extension. This dissertation combines field investigation and potassium feldspar 40Ar/39Ar thermal modeling at a classic cordilleran metamorphic core complex, the northern Snake Range MCC, to understand a) the structures and fabrics associated with Mesozoic shortening, burial, and deep-seated metamorphism of footwall units, and b) the exhumational/cooling history of deeply buried footwall rocks. Chapter 1 presents geologic mapping and cross sections of km-scale Late Cretaceous fold and thrust relationships of the Northern Snake Range Fold and Thrust System exposed in the footwall of the northern Snake Range metamorphic core complex. Specifically, we describe the O’Neil Peak Recumbent Syncline – a NNW-trending, non-cylindrical, eastward-opening, inclined to recumbent syncline – and its overlying anticlinal closure, the O’Neill Peak Anticline (OPA), that effectively double the structural thickness of middle-upper Cambrian lower plate strata from ~3.5 km to ~8km. West of the O’Neil Peak Recumbent Syncline, pervasive isoclinal folding in lower Cambrian schistose units and local thrust duplication of lower and middle Cambrian units of the Eightmile Thrust System reflect significant layer parallel shear and shortening in stratigraphically deeper units. Crosscutting intrusive relationships and a well-developed synmetamorphic axial planar fabric indicate the O’Neil Peak Recumbent Syncline and Eightmile Thrust System are broadly coeval with Late Cretaceous prograde metamorphism of footwall units. Detailed cross sections are used to create an interpretive pre-extensional reconstruction of the Northern Snake Range that integrates the Northern Snake Range Fold and Thrust System with associated supracrustal shortening structures now exposed in the Confusion Range to the east. We conclude that the Northern Snake Range Fold and Thrust System and overlying structures accommodated ≥10km of horizontal shortening, resulting in ≥ 7 km of structural burial of the deepest structural levels in the NSR footwall. The structural burial estimate from our reconstruction satisfies the minimum burial depth proposed by thermobarometric studies of lower plate metamorphic units at the northern Snake Range metamorphic core complex. Chapter 2 employs multiple-diffusion domain (MDD) modeling of a large (n=28) geologically well-constrained K-feldspar 40Ar/39Ar thermochronology dataset from the Northern Snake Range and surrounding environs to provide new insights into MDD parameter space and test the reliability of MDD model cooling histories. Here, we describe the variability of kinematic and domain parameters across the dataset, provide a set of best-practices from optimizing the internal consistency of MDD model cooling histories, and propose a schema for extracting geologically reasonable T-t paths directly from a sample’s experimental age spectrum. Diffusion domain size distributions within a single sample typically span 4-5 orders of magnitude, and the distribution of Ar among domains is related to a sample’s original grain-size. All K-feldspar samples require ≥8 diffusion domains to adequately model experimental results. Activation energies (Ea) of Ar diffusion in K-feldspar range from 42-54 kcal/mol with a mean (±1σ) of 48.4 ± 1.6 kcal/mol. The MDD cooling history dataset is more internally consistent if all cooling histories are calculated using an Ea that falls within ±1σ of the mean Ea of the data set, suggesting that much of the observed variation in Ea is likely due to analytical error and not natural variation. We use these results to construct an internally consistent dataset of MDD cooling histories, and use these cooling histories to explore quantitative relationships between the shape of an age spectrum and its MDD cooling history. We find quantifiable correlations between a) the slope of an age spectrum and the MDD model cooling rate, b) the initial slope of an age spectrum and the minimum MDD model cooling temperature recorded by that sample, and c) the cumulative fraction of 39Ar released during a step heating experiment and the effective MDD model geologic cooling temperature of the sample – a curve we term the temperature spectrum. These relationships between the age spectrum and MDD model cooling history are sufficiently systematic and quantifiable that we develop a schema for extracting T-t histories directly from K-Feldspar age spectra.

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