Authors: Yonkee, A; Weil, AB
Source: LITHOSPHERE, 9 (5):828-850; 10.1130/L622.1 OCT 2017
Publication Type: Journal Article
Abstract: Minor fault kinematics, fracture sets, anisotropy of magnetic susceptibility (AMS) fabrics, and remanent paleomagnetism within Triassic to Jurassic red beds and limestone along the Dallas-Derby-Sheep Mountain fold system and northeast flank of the Laramide Wind River arch record patterns of progressive deformation from early layer-parallel shortening (LPS) to large-scale fault propagation and fold linkage. The fold system comprises a series of doubly plunging, left-stepping anticlines with segmented southwest-vergent forelimbs cut by reverse faults and a gently northeast-dipping backlimb. Anticlines are linked across structural saddles (relay zones) that are locally cut by steep east-striking oblique-slip faults and thrust faults that accommodated shortening transfer. Minor wedge faults within limestone show consistent relationships with respect to bedding around anticline limbs, recording early LPS prior to and synchronous with initial fold growth. Within red beds, LPS produced microkinked phyllosilicate grains that define AMS K-max (axes of maximum susceptibility) lineations perpendicular to shortening directions. LPS directions estimated from minor faults and AMS trend approximately west-southwest-east-northeast within the backlimb, subparallel to the regional Laramide shortening direction, and partly refract across arcuate forelimbs. Paleomagnetic data record local counterclockwise rotations in forelimbs associated with map-view curvature and eastward-striking faults. Fracture sets and previously published calcite-twin strain data show more complex patterns related to evolving stress-strain fields, from pre-Laramide (intraplate) westnorthwest- east-southeast compression, to early Laramide west-southwest-east-northeast LPS, to later Laramide fault propagation and local stress refraction. Reverse faults in basement rocks propagated upward into anticline forelimbs, and laterally with partial linkage by eastward-striking faults that formed along basement weaknesses.