Micro- to nanostructural analysis of deformation along the Muddy Mt. thrust, Nevada, U.S.A.

The Muddy Mt. thrust, Nevada, U.S.A, is an iconic contractual fault juxtaposing Paleozoic carbonates onto the autochthonous Mesozoic Aztec sandstone.  The minimum thickness of the thrust sheet is estimated at 4-5 km.  In contrast to many thrusts that exhibit shale-carbonate interfaces, the Muddy Mt. thrust has a mineralogically simple bimaterial interface defined by the carbonate-silicate interface.  The deformation associated with the thrust exists as a variable cataclastic zone on the order of 10-100m. Previous work has documented the extensive evidence of brittle deformation at a range of scales.  Questions at hand during such studies have included the systematic variation in fracture intensity; the contribution of fluid pressure, if any, to the thrust mechanics; the nature of deformation and permeability changes in the footwall porous sandstone and measurement of temperature transients along discrete slip surfaces.

Structures within the deformation zone, and along the thrust interface provide evidence of both seismic and interseismic displacements.  The latter comprise simple fracture, brecciation, shattered dolomite, extensive and cyclic cataclasis and fragment reduction producing tectonic foliation with evidence of cannibalized fragment, gouge injection, discrete slip surfaces and, notably, intracrystalline (plastic) deformation of carbonate aggregates.  Given that macroscopic structures, such as those listed, ultimately depend on atom-to-grain scale processes, this contribution examines the fine-scale textural attributes of aggregates proximal to the thrust interface that in many cases have grain/particle sizes sizes less than a micrometre.  The objective is to extract micromechanical behaviour that can be relevant to the displacement record of crustal-scale faults.  The microstructures and microfabrics of the interface rocks have been examined by analytical STEM, EBSD of samples prepared by focussed ion-beam techniques, with longer range elemental variations determined by mXRF.