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Paleomagnetic and structural evidence for oblique slip in a fault-related fold, Grayback monocline, Colorado

Joya Tetreault^1,, Craig H. Jones², Eric Erslev³, Scott Larson³, Mark Hudson⁴ and Steven Holdaway⁵

¹ Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA
² Department of Geosciences, University of Colorado, Boulder, Colorado 80309, USA
³ Department of Geosciences, Colorado State University, Fort Collins, Colorado 80523-1482, USA
⁴ U.S. Geological Survey, Lakewood, Colorado 80225, USA
⁵ Chevron USA Production Company, P.O. Box 36366, Houston, Texas 77236, USA

Correspondence: E-mail: joya{at}unm.edu

Significant fold-axis-parallel slip is accommodated in the folded strata of the Grayback monocline, northeastern Front Range, Colorado, without visible large strike-slip displacement on the fold surface. In many cases, oblique-slip deformation is partitioned; fold-axis-normal slip is accommodated within folds, and fold-axis-parallel slip is resolved onto adjacent strike-slip faults. Unlike partitioning strike-parallel slip onto adjacent strike-slip faults, fold-axis-parallel slip has deformed the forelimb of the Grayback monocline. Mean compressive paleostress orientations in the forelimb are deflected 15°–37° clockwise from the regional paleostress orientation of the northeastern Front Range. Paleomagnetic directions from the Permian Ingleside Formation in the forelimb are rotated 16°–42° clockwise about a bedding-normal axis relative to the North American Permian reference direction. The paleostress and paleomagnetic rotations increase with the bedding dip angle and decrease along strike toward the fold tip. These measurements allow for 50–120 m of fold-axis-parallel slip within the forelimb, depending on the kinematics of strike-slip shear. This resolved horizontal slip is nearly equal in magnitude to the ~180 m vertical throw across the fold. For 200 m of oblique-slip displacement (120 m of strike slip and 180 m of reverse slip), the true shortening direction across the fold is N90°E, indistinguishable from the regionally inferred direction of N90°E and quite different from the S53°E fold-normal direction. Recognition of this deformational style means that significant amounts of strike slip can be accommodated within folds without axis-parallel surficial faulting.

Key Words: oblique slip • folds • Laramide • vertical-axis rotations • Rocky Mountain foreland