Authors: Christopher D. Henry and Charles H. Thorman
Series: Open-File Report 15-8
Format: plate: 41 x 29 inches, color, includes 3 cross sections; text: 12 pages, b/w
The Pequop Summit 7.5-minute quadrangle covers the northern Pequop Mountains and is just north of newly recognized Carlin-type gold deposits in a geographic and geologic setting distinct from those of long-known deposits to the west. Mapping was done from Summer 2014 to Summer 2015; digital cartography, cross sections, and unit descriptions were done in Fall to Summer, 2014–2015.
Major results of this work are as follows:
1) The map area contains a >5-km-thick section of Cambrian through Permian sedimentary rocks. A major unconformity between Permian and Mississippian rocks documents an episode of uplift and erosion in the Late Pennsylvanian.
2) All Paleozoic rocks were contractionally deformed and metamorphosed during the Mesozoic. Contraction was accomplished by a combination of thrust and attenuation faults; west-northwest-striking, probable tear faults; folding within less competent units; and intense brecciation of competent units.
3) Eocene igneous rocks include locally derived andesite-dacite lavas and rhyolite intrusions that are geochemically unrelated to the lavas but may have been the heat source for the Carlin deposits.
4) Cenozoic extension includes a middle Miocene pulse of ~40° east-tilting along west-dipping normal faults in the central and southern Pequop Mountains and modern faulting along range-bounding faults.
5) The northern Pequops are west tilted, and a northeast-striking, extensional anticline formed between oppositely dipping normal fault systems there.
Building on the stratigraphic-structural understanding from STATEMAP mapping, we are using other funds to further resolve the thermal and uplift history of the Pequop Mountains and to test conflicting interpretations of metamorphism and contraction, similarly conflicting interpretations of extension and exhumation, and the relation of deformation and magmatism to the origin of Carlin deposits.
This map was prepared as part of the STATEMAP component of the National Cooperative Geologic Mapping Program in cooperation with the U.S. Geological Survey.
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