Deformation of the Roberts Mountains Allochthon in north-central Nevada

Abstract

During the Antler orogeny in Late Devonian and Early Mississippian time, early and middle Paleozoic siliceous rocks, largely chert and shale, were thrust eastward for 90 to 160 km over coexisting carbonate rocks. Minor and major structures of two small areas of the allochthon at Battle Mountain and in the southern Tuscarora Mountains were studied in order to characterize the deformation and test the consistency of the movement plan with respect to the large eastward displacement. In the Battle Mountain area, the lower Paleozoic Scott Canyon and Valmy Formations were deformed in the Antler orogeny but were unaffected by later tectonism during late Paleozoic or early Mesozoic. In the southern Tuscarora Mountains area, the Ordovician and Silurian siliceous rocks deformed in the Antler Orogeny were deformed by later, possibly Mesozoic, folding and thrusting.Most of the minor folding visible in the allochthon is in the cheret, but proportionally more of the strain was taken up in the shale and argillite, both poorly exposed but predominant rock types. Most minor folds, concentric in form, plunge at small angles to the north-northeast and south-southwest with steeply dipping or vertical axial planes. The b-fabric axis, parallel to these folds, is identical apparently to the B-kinematic axis. The horizontal component of tectonic shortening of the allochthon, N. 70°-75° W. both in the Battle Mountain area and in the southern Tuscarora Mountains area, is therefore consistent with an eastward direction of movement of the allochthon. Folds with westnorthwest trends locally present in the allochthon, may have formed in the direction of tectonic transport. In the southern Tuscarora Mountains, local strain in and below the allochthon was different from the prevailing strain in the allochthon, and tectonic shortening was locally at large angles to the accepted direction of movement of the allochthon.

Structural transect across the north-central Carlin trend, Eureka County, Nevada

“…However, fold axes in the trend, particularly near mineralized areas, plunge at shallow angles to the NW (Madrid, 1987;Madrid and Bagby, 1986;Volk and Zimmerman, 1991;Peters and Evans, 1995). The NW-trending fold axes were postulated by Evans and Theodore (1978) to be due to Jurassic tectonism, which is compatible with the timing of other tectonic events proposed by Ketner and Smith (1982), Ketner (1987) and Thorman and others (1991b) in northeastern Nevada.…”

“…The gold deposits in the Carlin trend are aligned along a NW-trending belt (Roberts, 1960(Roberts, , 1966Thorman and Christensen, 1991), but definitive geologic parameters have not been shown to account for this alignment. Previous workers in the Carlin trend (Evans and Theodore, 1978) have documented fold axes in the region of the Carlin trend, which plunge at low angles to the NE and SW. This orientation is consistent with orientations documented by Oldow (1984) who noted a relative consistency to deformation symmetry in the Roberts Mountains allochthon and attributed this symmetry to movement and emplacement of the allochthon.…”

Reactivated Palaeozoic normal faults: controls on the formation of Carlin-type gold deposits in north-central Nevada

“…1) comes from regional inspection of fold axes. Evans & Theodore (1978) first demonstrated that the Carlin trend corresponds to a zone of anomalously trending fold axes. Outside the Carlin trend fold axes trend mostly NNE, fairly typical of folds associated with the Antler Orogeny and subsequent compressional events.…”

Three‐Dimensional Evolution of the Early Paleozoic Western Laurentian Margin: New Insights From Detrital Zircon U‐Pb Geochronology and Hf Isotope Geochemistry of the Harmony Formation of Nevada

“…The metamorphic grade of RMA rocks is generally greenschist facies or lower (Gehrels, Dickinson, Riley, et al, ). RMA strata are highly deformed and include imbricated older‐over‐younger thrust sheets (e.g., Evans & Theodore, ; Noble & Finney, ; Oldow, ). Together, they have been interpreted as a westward thickening subduction assemblage (e.g., Dickinson, ; Speed & Sleep, ).…”