Abstract

Integrated structural, anisotropy of magnetic susceptibility (AMS), and paleomagnetic analyses of sedimentary cover rocks along variably oriented major faults and en echelon fold systems in the southern Bighorn Arch, Wyoming, were undertaken to test kinematic and mechanical models of Laramide thick-skin deformation. The Laramide foreland is characterized by an anastomosing network of basement-cored arches and associated cover folds that trend overall NW-SE, but in detail are curved, range from N-S to E-W trending, and form both right- and left-stepping en echelon systems. Development of variably trending Laramide arches has been variously attributed to temporal changes in stress directions, wrench faulting, and localization of deformation related to basement heterogeneities during regional SW-NE shortening. Within the southern Bighorn Arch, widespread but limited layer-parallel shortening (LPS) was accommodated mostly by minor faults with conjugate wedge and strike-slip geometries early in the deformation history. LPS directions vary from perpendicular to acute with local fold structural trends, consistent with a single shortening episode. Although internal strain is limited, weak AMS lineations defined by kinked and rotated phyllosilicates are widely developed and consistently perpendicular to LPS directions. Structurally restored paleomagnetic declinations record only limited, non-systematic vertical-axis rotations, indicating that wrenching was not an important component of the deformation field during development of the southern Bighorn Arch and that curvature of the arch was a primary feature. Palinspastically restored LPS directions are on average WSW-ENE, but display local deflections related to heterogeneities of underlying basement blocks and proximity to major faults, some of which were localized along Precambrian shear zones and igneous dikes. Crustal shortening patterns across the Laramide foreland are interpreted to reflect deformation partitioning in response to a single far-field shortening direction partly related to flat-slab subduction along with effects of pre-existing basement weaknesses and strain softening during progressive faulting.

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