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Basement-cover geometry of Laramide fault-propagation folds

Eric A. Erslev and James L. Rogers
Basement-cover geometry of Laramide fault-propagation folds (in Laramide basement deformation in the Rocky Mountain foreland of the Western United States, Christopher J. Schmidt (editor), Ronald B. Chase (editor) and Eric A. Erslev (editor))
Special Paper - Geological Society of America (1993) 280: 125-146

Abstract

The geometry of Laramide foreland uplifts near the basement-cover interface provides a critical test of models for basement-involved foreland structures. Limits and controls on basement folding are well defined by deflections of the basement-cover unconformity in the northeastern Front Range of Colorado, Rattlesnake Mountain anticline of northern Wyoming, and northern Teton Range of northwestern Wyoming. Where basement overlies sedimentary strata on a single reverse fault, basement folding is largely limited to the area within 1 km of the fault. In this area, distributed high-angle faulting bends the unconformity in the hanging-wall basement tip up to 20 degrees toward the fault. Folding of the hanging-wall tip increases with progressive fault slip and decreased fault dip. Smaller structures commonly show little basement folding, contradicting the fold then fault sequence predicted by the fold-thrust structural model. The footwall basement is either undeformed or bent downward by hanging-wall loading, with upward folding limited to the fault"s narrow zone of cataclastic flow. Where fault splays break the basement into one or more fault-bounded wedges, overlying strata form more open folds without necessitating penetrative basement folding. The hanging-wall basement commonly shows little folding because the weak basement tip, defined by the main fault and the hanging-wall unconformity, is often left behind in the footwall as a subthrust wedge, filling the gap between the flexed sedimentary strata and the footwall basement block. The downward-narrowing zones of deformation with their variably curved fold surfaces and heterogeneous strain are consistent with fault-propagation folding by distributed shear in a triangular shear zone (trishear). Heterogeneous trishear models predict either extensive basement folding in the hanging wall (footwall-fixed trishear) or no basement folding in the hanging wall (hanging-wall-fixed trishear). These trishear modes form a continuum of basement behavior, with the strength of the hanging-wall basement tip controlling the shear-zone location and the extent of basement deformation. Structures with lower angle thrusts show more basement folding because their basement tips are narrowly tapered and weak, fixing the triangular shear zones with respect to the footwall. Structures with higher angle reverse faults show less basement folding because their basement tips are blunt and stronger, fixing the triangular shear zones with respect to the hanging wall.


ISSN: 0072-1077
EISSN: 2331-219X
Coden: GSAPAZ
Serial Title: Special Paper - Geological Society of America
Serial Volume: 280
Title: Basement-cover geometry of Laramide fault-propagation folds
Title: Laramide basement deformation in the Rocky Mountain foreland of the Western United States
Author(s): Erslev, Eric A.Rogers, James L.
Author(s): Schmidt, Christopher J.editor
Author(s): Chase, Ronald B.editor
Author(s): Erslev, Eric A.editor
Affiliation: Colorado State University, Department of Earth Resources, Fort Collins, CO, United States
Affiliation: Western Michigan University, Department of Geology, Kalamazoo, MI, United States
Pages: 125-146
Published: 1993
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
ISBN: 0-8137-2280-2
References: 56
Accession Number: 1993-046851
Categories: Structural geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects., geol. sketch maps
N37°00'00" - N41°00'00", W109°00'00" - W102°00'00"
N41°00'00" - N45°00'00", W111°04'60" - W104°04'60"
Secondary Affiliation: Colorado State University, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 1993
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