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| Keystone Thrust | |
|---|---|
| Name | Keystone Thrust |
| Type | Thrust fault |
| Location | Wyoming, Idaho, Montana |
| Coordinates | 44°N 111°W |
| Age | Late Proterozoic–Paleozoic (involved rocks) |
| Orogeny | Sevier Orogeny, Laramide Orogeny |
Keystone Thrust The Keystone Thrust is a major thrust fault zone in the northern Rocky Mountains region of the western United States, exposing a complex juxtaposition of older Precambrian and Paleozoic rocks over younger Mesozoic and Cenozoic strata. It crops out in parts of Montana, Idaho, and Wyoming and forms part of a regional system of thin-skinned and thick-skinned imbricate faults associated with the Sevier Orogeny and later modification during the Laramide Orogeny. The structure has been central to studies that connect field mapping in the Bighorn Basin and Yellowstone National Park region to regional cross-sections through the Cordilleran orogen.
The Keystone Thrust lies within the northern flank of the Greater Yellowstone Ecosystem and adjacent to major physiographic provinces including the Absaroka Range, the Beartooth Mountains, and the Bighorn Mountains. It is spatially related to mapped thrusts such as the Harvard Thrust and the McCullough Peaks Fault and intersects regional strike-slip elements tied to the Lewis and Clark line and the Transcontinental Arch framework. The thrust's surface expression aligns with drainage divides near Yellowstone River tributaries and is documented on USGS maps for Park County, Wyoming, Carbon County, Montana, and neighboring counties.
Structurally, the Keystone Thrust exhibits a low-angle detachment plane that places older crystalline and sedimentary units over younger cover rocks, producing repetition and truncation of stratigraphic sequences similar to styles seen on the Mesa Verde-to-Uinta Basin transects. Recognizable features include mylonitic shear zones, fault-bend folds, duplex structures, and associated thrust sheets comparable to those described for the Sevier fold-and-thrust belt and the Wind River thrust system. Kinematic indicators such as slickenlines and asymmetric pressure shadows record top-to-the-east or top-to-the-northeast transport vectors, which correlate with regional transport directions inferred from work on the Sawatch Range and the Front Range.
The hanging wall of the Keystone Thrust commonly exposes metamorphic and igneous basement rocks correlated with Precambrian terranes like the Beartooth Complex and remnant Proterozoic shelves, whereas the footwall contains stacked sequences of Cambrian through Cretaceous sedimentary rocks including limestone, dolostone, shale, and sandstone. Important mapped units include lithologies equivalent to the Flathead Sandstone, the Madison Limestone, and the Morrison Formation, juxtaposed against Archean gneiss and schist similar to the Yellowstone Gneiss descriptions. Mineral assemblages in mylonite zones show chlorite, muscovite, and epidote overprints consistent with greenschist-facies metamorphism, analogous to mineral parageneses reported from the Sierra Nevada and Snake River Plain regions.
The tectonic history of the Keystone Thrust records multistage deformation beginning with Precambrian crustal assembly and subsequent Paleozoic passive-margin sedimentation tied to the Western Interior Seaway evolution. Major thrusting is attributed to Late Sevier Orogeny shortening, with later reactivation during the Laramide Orogeny and modification by Cenozoic extension related to the Yellowstone hotspot and Basin and Range transtension. Comparative thermochronology and balanced cross-section work link its exhumation history to regional studies in the Cordillera and tie timing constraints to isotopic ages obtained by groups working in the Bitterroot Range, the Absaroka Volcanic Province, and the Beartooth uplift.
The Keystone Thrust influences groundwater flow, spring emergence, and local soil development, affecting resources in the Bighorn Basin and grazing lands near Cody, Wyoming. Contact zones and fractured carbonate units associated with the thrust serve as potential reservoirs and conduits for hydrocarbons analogous to productive settings in the Williston Basin and Powder River Basin. Quarries and small-scale mineral occurrences—historically prospected for base metals and barite—are reported near thrust margins similar to mineralization documented in the Washakie Basin and Wyodak coalfield. Environmentally, the thrust-exposed relief affects habitats within the Greater Yellowstone Ecosystem and poses considerations for seismic hazard studies that reference paleoseismic records compiled for the Intermountain West.
Key studies of the Keystone Thrust began with early 20th-century geological reconnaissance by investigators associated with the U.S. Geological Survey and universities such as University of Wyoming and Montana State University, who produced foundational maps linking local structure to the broader Sevier fold-and-thrust belt. Later contributions include structural analyses and balanced cross-sections by researchers from Stanford University and University of California, Berkeley that integrated field mapping with seismic reflection data analogous to work on the Rocky Mountain Front. Thermochronology and geochronology efforts employing fission-track and (U–Th)/He methods involved collaborations with Arizona State University and the University of Arizona, refining exhumation timing comparable to results from the Wind River Range and Tetons. Recent multidisciplinary projects have combined remote sensing, LiDAR surveys, and GIS hosted at institutions such as the Smithsonian Institution and the National Park Service to better resolve surface expression and regional correlation with thrust systems across the northern Cordillera.
Category:Geology of Wyoming Category:Thrust faults