GSN 2020 Symposium Virtual Lecture Series
Begins Thursday, May 28, 2020 at 7 PM, PDT

A message forwarded from Laura Ruud, Geological Society of Nevada (GSN) Executive Manager: “The GSN 2020 Symposium technical committee is developing a virtual lecture series that features speakers from the 2020 technical program discussing topics consistent with the symposium theme: “Vision for Discovery: Geology and Ore Deposits of the Basin and Range”. The lecture series will appear on the Zoom platform on a bi-monthly schedule during the remainder of 2020 and, possibly, the first quarter of 2021. The lectures will be recorded and posted for those who cannot make the livestream.

Chuck Thorman, USGS Emeritus, will present the first lecture at 7 PM, Thursday, May 28, 2020. The lecture will have new findings and interpretations of the structural setting of the Pequop Mountains, host to the newest large greenfield gold discovery in the Basin and Range.”

Regional Tectonics Based on Conodont CAIs and Burial Depths, as Viewed from the Pequop Mountains, NE Nevada – An Unbiased Opinion(?)

Charles Thorman (U.S. Geological Survey Emeritus), cthorman@comcast.net
Charles Sandberg (U.S. Geological Survey Emeritus), conodonts@comcast.net
Chris Henry, Andrew Zuza, and Mike Ressel (Nevada Bureau of Mines and Geology, University of Nevada, Reno)

BiographyChuck Thorman: Chuck has spent much of his career in the Basin and Range, beginning in 1959-1962 on his thesis in the Wood Hills and northern Pequops. From there he went with Humble Oil, where he was their structural geologist in southern Oklahoma in the Ardmore-Anadarko Basins. He then taught for six years before ending up with the USGS in Liberia, Brazil, and the Western U.S. (mostly in AZ, N.M., UT, and NV) from 1971 to 1995. Since retiring from the USGS in 1995, he has remained as a Scientist Emeritus. His first love is field work, for that is the source of all of the basic data that the rest of geologic disciplines rely on for their endeavors. Geology is a fantastic detective story, and he has found it to be what keeps him enjoying life. That plus all of the wonderful friendships that have developed as a result of working in/enjoying this wonderful science. What more can one ask for?

Abstract: The Pequop Mountains form an eastern segment of the Ruby Mts–East Humboldt Range Metamorphic Core Complex in NE Nevada. Geobarometric pressures in metasedimentary lower Paleozoic and Neoproterozoic rocks, based on metamorphic mineral assemblages, have been interpreted to indicate the Pequops were buried more than twice normal stratigraphic depths (~20 km). This has led to interpretations that the entire stratigraphic section in the Pequops was duplicated by a theoretical ~10 km-thick (Windemere) thrust sheet, with a ~69 km of SE-directed transport rooted NW of the Wood Hills, to account for the interpreted geobarometric pressures. Our work demonstrates that such a fault does not exist and that Paleozoic and Neoproterozoic strata were never buried to more than their original, normal stratigraphic depths.

Paleozoic strata are continuous down section, from Permian limestones, into regionally metamorphosed greenschist to lower amphibolite facies, Lower Ordovician to Neoproterozoic strata. Conodonts collected from Ordovician through Permian strata show a normal decrease in temperature up section from Color-Alteration Indexes (CAIs) of 5 (~300°–480°C, Lower Ordovician) to 3.5–4 (~150°–300°C, Devonian-Mississippian boundary) to 1– 2 (~50°–140°C, Pennsylvanian and Permian). This suggests a relatively steep geothermal gradient (~>45°C).

Hydrothermally altered conodonts at several localities have CAIs ranging from 4.5 to 7 (~250° to >500°C). These are found in deformed rocks and generally are associated with quartz and calcite veins. Unaltered conodonts are commonly found within 10 to 20 m of these altered conodonts. 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 to Jurassic rocks. Most small to moderate folds and faults indicate top-to-SE displacement. Intrusion of a currently undated, but almost certainly Jurassic (~160 Ma) lamprophyre along a major thrust suggests deformation occurred during the Middle to Late 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 at present.

Permian strata are overlain by Triassic and Lower Jurassic rocks from the mapped area in the Pequops southward 70 km to Currie. Regionally, Triassic rocks in NE Nevada have CAIs of 1 to 2 across a ~73,000 km2 area, indicating that these rocks were not buried more than their normal stratigraphic depths. This suggests that if appreciable thickening of the upper crust occurred during Late Jurassic to Paleocene time, it would most likely be situated below the exposed Paleozoic-Neoproterozoic sections. Following up on the Triassic scenario, CAIs in the carbonate sequence indicate similar circumstances far to the south in eastern Nevada, again indicating that deep burial of these rocks has not taken place.

Related NBMG publications:
Pequop Mountains
Wood Hills
Oxley Peak Area
Pequop Summit
Herder Creek
Independence Valley NE
Independence Valley NW
Windermere Hills

More links:
Geologis Maps Web Application
RuGGEd Virtual Field Trip

NMEC 8th Annual Great Basin Rendezvous
September 1113, 2020
Nevada Mineral Exploration Coalition (NMEC)
Friday, September 11 – Sunday, September 13, 2020

Hosted this year at the Royal Peacock Opal Mine.

In the spirit of boundless optimism characteristic of all explorers, the Nevada Mineral Exploration Coalition currently believes the 8th Annual Great Basin Rendezvous will be held as scheduled on September 11, 12, and 13, 2020 at the Royal Peacock Opal Mine southwest of Denio, Nevada. More information will be provided as we approach the date. However, if the COVID-19 emergency is still with us, we will cancel. Refunds will be issued if that is the case.

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