GSN Regular Membership Meeting – Friday, May 16, 2025

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Contact: gsn@gsnv.org

GSN REGULAR MEMBERSHIP MEETING—Friday, May 16, 2025

TIME: Drinks @ 6 pm, Dinner @ 6:30 pm, Talk @ 7:30 pm

WHERE: ATLANTIS CASINO RESORT, RENO, NV

BALLROOM TO BE ANNOUNCED

GUEST SPEAKER: Sean Long, Professor, Washington State Univ., School of the Environment

TITLE: Reconstructing Cordilleran deformation in eastern Nevada and western Utah: Implications for crustal shortening and underthrusting of the craton

MEETING SPONSORS:  GEOTEMPS/GEOPROS & SWICK DRILLING

DINNER COST:  $60 per person

PLEASE MAKE ONLINE RESERVATIONS AND PAYMENTS  BELOW.

PLEASE RSVP FOR DINNER NO LATER THAN MONDAY, MAY 12, 2025

You do NOT need a dinner reservation to attend the talk.  All members are welcome to attend!

Please contact Laura Ruud at the GSN office for more information:  gsn@gsnv.org

Abstract:

The Jurassic-Paleogene North American Cordillera, a vast mountain belt constructed as a response to subduction of the Farallon plate, has inspired models for fold-thrust belts, foreland basins, and magmatic arcs that have been utilized globally. Constraining the geometry of contractional deformation and the magnitude of crustal shortening are critical to our understanding of the Cordillera. However, the complex overprint of Cenozoic extension hinders analysis of contractional deformation. To address this problem, over the past 13 years my research group has performed studies in eastern Nevada and western Utah aimed at reconstructing Cordilleran deformation, including Eocene-Oligocene paleo-geologic maps that allow correlation of regional-scale thrust faults and folds, retro-deformed cross-sections that illustrate the geometry of contractional structures, and investigations of syn-contractional sedimentary rocks. Here, I summarize the results of these efforts and other recent studies, and their implications for the geometry and magnitude of Cordilleran deformation.

On the west, the southern part of the Central Nevada thrust belt consists of three east-directed thrust systems that branch northward off of the Sevier fold-thrust belt, which accommodated 10-15 km of total shortening prior to intrusion of ~86-98 Ma granites. In the northern part of the Central Nevada thrust belt, the Eureka culmination (a regional-scale anticline) was constructed above east-directed thrust faults during ~114-99 Ma syn-contractional deposition of the Newark Canyon Formation. To the east, the 100-150 km-wide Eastern Nevada fold belt contains five, 100-250 km-long, north-trending folds that have crest-to-trough heights of 2-4 km. These folds locally deform rocks as young as early Jurassic and mid-Cretaceous, and accommodated ~15 km of total shortening. Moving eastward, the Western Utah thrust belt contains east-directed thrust faults and related folds that accommodated ~10 km of shortening, likely between ~93-77 Ma based on correlation with syn-metamorphic contractional structures in the Northern Snake Range. Collectively, these structural provinces accommodated ~35-40 km of shortening within the Sevier hinterland, much of which was likely contemporaneous with the 220 km of shortening accommodated in the Cretaceous-Paleocene Sevier fold-thrust belt in west-central Utah.

By the latest Cretaceous, a 200-250 km-wide orogenic plateau had been constructed in eastern Nevada and western Utah, as supported by reconstructions of extension (50-60 km crustal thicknesses) and paleoaltimetry (2.75-3.5 km elevations). This brings into question the dominant thickening process that built such a broad plateau. The Sevier fold-thrust belt deformed a Neoproterozoic-Jurassic sedimentary package that thickened westward from a 2-3 km-thick platform section that overlies the 40 km-thick unrifted craton to a 15-20 km-thick continental margin section that was deposited above middle-lower crust that was significantly thinned during Neoproterozoic rifting. Shortening in the Sevier fold-thrust belt translated this thick continental margin section 220 km eastward, which resulted in the relative westward underthrusting of an equivalent length of thick cratonic basement beneath the Sevier hinterland; this was likely the dominant thickening process that constructed the broad hinterland plateau. The reconstructed western edge of the underthrusted craton (which lies at the approximate longitude of Eureka and Elko) coincides with the western limit of 2.75-3.5 km paleoelevations, which supports this interpretation.

Biography:

Sean Long is a structural geology professor at the School of the Environment at Washington State University, where he has worked since 2015. Prior to this, he was a research professor at the Nevada Bureau of Mines and Geology at the University of Nevada, Reno, between 2010-2015. Sean is originally from northern Idaho, and he earned a B.S. degree in Mathematics from the College of Idaho in 2001, a M.S. degree in Geology from Idaho State University in 2004, and a Ph.D. in Geology from Princeton University in 2010. Sean’s research focuses on investigating the structural evolution of contractional and extensional mountain belts, including the Himalayan-Tibetan orogen (Bhutan and northwestern India), the North American Cordillera and Basin and Range Province (Nevada, Utah, and Idaho), and the Andes (Bolivia). Sean’s research approach involves integrating mapping-based field data with a diverse suite of datasets, including balanced cross-sections, regional structural reconstructions, geochronology, thermochronometry, metamorphic temperatures and pressures, and microstructural analyses. Major themes of Sean’s research include developing new approaches for large-scale structural reconstructions, performing field-based tests of the predictions of models for the long-term dynamics of mountain belts, and integrating microstructural and metamorphic datasets to investigate ductile shear zones.

Email: sean.p.long@wsu.edu;  Website: www.seanpatricklong.com

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