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8: Progressive development of basement-involved foreland thrust belts: Results from the NSF/EarthScope Bighorn Project applied to the Laramide orogeny in the Colorado Front Range

By
Eric A. Erslev
Eric A. Erslev
Department of Geology and Geophysics, Department 3006, 1000 E. University Avenue, University of Wyoming, Laramie, Wyoming 82071-2000, USA
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Karen Aydinian
Karen Aydinian
Department of Geology and Geophysics, Department 3006, 1000 E. University Avenue, University of Wyoming, Laramie, Wyoming 82071-2000, USA
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Laura Kennedy
Laura Kennedy
Weatherford Laboratories, 16161 Table Mountain Parkway, Golden, Colorado 80403, USA
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Published:
January 01, 2016

Abstract

Lithosphere-scale seismic experiments, structural geometries, and minor structures in the Bighorn region of the northern Rockies show that Laramide deformation was controlled by horizontal shortening driven by detachment in the lower crust. This field trip visits three exposures in the Colorado Front Range that helped generate the detachment hypothesis, and uses observations from both areas to generate a thrust belt model for basement-involved foreland orogens.

Starting north of Boulder, a traverse following the trace of the classic Six Mile fold reveals minor structures showing early layer-parallel shortening overprinted by a progressively tightening, non-self-similar fault-propagation fold. The overlying petroliferous Niobrara marls show how sequential deformation predicts the unit’s natural fractures, which are critical to the success of individual wells in this major resource play.

South of Boulder in Eldorado Canyon (known worldwide for near-vertical climbing), the trip traverses excellent exposures of the Paleozoic strata in the hanging-wall above the Golden thrust fault. Out-of-the-basin and into-the-basin thrusts allow the restoration of the Rocky Flats 2D seismic line just to the south. From a revealing overlook, Front Range and Bighorn (using the seismic and structural results from the newly completed Bighorn Project) arch geometries can be generalized into a 4D (3D space + time) model for basement-involved foreland orogens. The final stop visits the enigmatic Boulder-Weld County fault system in the Denver Basin to the east to discuss the relative importance of regional tectonic setting (e.g., low-angle subduction), stress, lithospheric rheology, gravity, and fluids to the formation of basement-involved foreland thrust belts.

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Contents

GSA Field Guide

Unfolding the Geology of the West

Stephen M. Keller
Stephen M. Keller
Colorado Geological Survey Colorado School of Mines 1801 19th Street Golden, Colorado 80401, USA
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Matthew L. Morgan
Matthew L. Morgan
Colorado Geological Survey Colorado School of Mines 1801 19th Street Golden, Colorado 80401, USA
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Geological Society of America
Volume
44
ISBN print:
9780813756448
Publication date:
January 01, 2016

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