Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 2-5
Presentation Time: 9:40 AM


THORMAN, Charles H.1, SANDBERG, Charles A.1, HENRY, Christopher D.2, ZUZA, Andrew V.2 and RESSEL, Michael W.2, (1)Scientist Emeritus, U.S. Geological Survey, Box 25046, MS 973, Denver, CO 80225, (2)Nevada Bureau of Mines and Geology, University of Nevada, Reno, NV 89557

Geobarometic pressures in the Pequop Mts Paleozoic (Pz) metasedimentary rocks, which comprise an eastern segment of the Ruby Mts–East Humboldt Range Metamorphic Core Complex, imply burial more than twice stratigraphic depths. This has led to interpretations that the rocks were duplicated by a regional thrust that is not preserved. However, Triassic strata in eastern NV that would have been in the footwall of this thrust are part of an area having conodont Color-Alteration Index (CAI)s of 1 to 2 over a >51,800 km2 area, indicating burial to only normal stratigraphic depths. CAIs in basal Pz in the Pequops indicate metamorphism under normal stratigraphic depth. Paleozoic strata are continuous down section into regionally metamorphosed greenschist to lower amphibolite, Lower Ordovician to upper Precambrian strata. Conodonts in Ordovician through Permian strata show a normal decrease in temperature from CAIs of 5 (~300°–480°C, Lower Ordovician.) to 3.5–4 (~150°–300°C, Devonian-Mississippian boundary) to 1 to 2 (~50°–140°C, Pennsylvanian & Permian). These regional and local low CAIs in upper Pz and Triassic strata preclude their burial to more than normal stratigraphic depths from thicknesses of overlying Permian to Jurassic sequences in the Pequops and adjacent ranges. This suggests a relatively steep geothermal gradient (~>45°C).

Hydrothermally altered conodonts at many localities have CAIs ranging from 4.5 to 7 (~250° to >500°C). These are found in deformed rocks and commonly are associated with quartz and calcite veins. Conodonts indicating normal depth of burial temperatures have smooth, shiny surfaces, whereas hydrothermally heated conodonts have granular textured surfaces.

Detailed mapping in the northern Pequops and published maps of adjacent areas demonstrate NW-SE contraction of the Paleozoic-Jurassic rocks. Most small to moderate folds and faults indicate top-to-SE displacement. Intrusion of currently undated, but almost certainly Jurassic (~160 Ma) lamprophyre along a major thrust suggests deformation during the Mid-Jurassic Elko orogeny. A large-scale NW vergent syncline in the southern Pequops and large-scale NW vergent folds in the Wood Hills indicate top-to-NW displacement and may reflect back folding. The relative timing and relation of SE and NW vergence is not well understood.