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GEOREF RECORD

Rift and grain in basement; thermally triggered snapshots of stress fields during erosional unroofing of the Rocky Mountains of Montana and Wyoming

Donald U. Wise
Rift and grain in basement; thermally triggered snapshots of stress fields during erosional unroofing of the Rocky Mountains of Montana and Wyoming
Rocky Mountain Geology (December 2005) 40 (2): 193-209

Abstract

Even though "rift" and "grain" (R/G) are New England quarrymens' terms for ease-of-fracture, somewhat similar structures appear in basement exposures throughout the Rocky Mountains of Montana and Wyoming. However, the nature, origin, and structural utility of these subtle features are generally unrecognized in the western uplifts. R/G represent directions of easy mesoscopic splitting produced by closely spaced microscopic fracture planes in quartz, now healed into thin zones of very tiny fluid inclusions. At hand-specimen to outcrop-scale these represent strength anisotropies that can exert major controls in the orientation of a host of younger types of joints and other fractures, one of many types of tectonic heredity. In the Rocky Mountain uplifts, strike of R/G can remain constant over 100's of km (super 2) , commonly with strikes parallel to adjacent mountain fronts. A late to post-Laramide age is indicated by dips remaining vertical even in strongly tilted mountain fronts. It is argued that these microfractures were produced by stresses generated by differential thermal contraction along the several crystallographic axes of quartz grains. Thermally induced grain-scale stresses directionally augmented by even weak regional stress fields produced the observed systematic regional patterns. Temperatures well above 150 degrees C have been documented for some microcrack origins, but these Rocky Mountain examples are limited by stratigraphic cover and association with eroding mountain fronts to formation at temperatures below 80 degrees C, possibly as low as 50-60 degrees C, and depths as shallow as 1-2 km. Some of the R/G orientational domains represent older stress fields, but the mountain-front parallel ones are essentially snapshots of the last gasps of dying Laramide stress fields. The relationships suggests formation by volumetric expansion and interchange of sigma (sub 1) , sigma (sub 2) , and sigma (sub 3) stress fields as decreasing confinement by adjacent basin fill allowed the uplifted mountain mass to begin gravitational spreading.


ISSN: 1555-7332
EISSN: 1555-7340
Coden: WUGGAO
Serial Title: Rocky Mountain Geology
Serial Volume: 40
Serial Issue: 2
Title: Rift and grain in basement; thermally triggered snapshots of stress fields during erosional unroofing of the Rocky Mountains of Montana and Wyoming
Author(s): Wise, Donald U.
Affiliation: University of Massachusetts, Department of Geosciences, Amherst, MA, United States
Pages: 193-209
Published: 200512
Text Language: English
Publisher: University of Wyoming, Dept. of Geology and Geophysics, Laramie, WY, United States
References: 31
Accession Number: 2006-019307
Categories: Structural geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. block diag., sketch maps
N44°10'00" - N45°49'60", W111°00'00" - W109°00'00"
N43°30'00" - N45°30'00", W108°30'00" - W106°30'00"
N44°30'00" - N49°00'00", W116°01'60" - W104°01'60"
N41°00'00" - N47°30'00", W73°30'00" - W67°00'00"
N41°00'00" - N45°00'00", W111°04'60" - W104°04'60"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, University of Wyoming. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200610
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