DEVELOPMENT OF EN ECHELON FOLD SYSTEMS ON THE FLANKS OF TWO MAJOR LARAMIDE ARCHES

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. Variable trends and development of en echelon systems have been variously attributed to wrench faulting, compartmentalized deformation related to basement heterogeneties, and temporal changes in stress directions. In order to better understand the kinematic and mechanical evolution of variably trending arches in the Laramide foreland, patterns of layer-parallel shortening (LPS) and vertical-axis rotations were quantified for two en echelon fold systems located along the flanks of the Wind River and Bighorn arches using integrated structural, anisotropy of magnetic susceptibility (AMS), and paleomagnetic analyses. Within these fold systems, limited LPS was accommodated mostly by minor faults with conjugate wedge and strike-slip geometries early in the deformation history. LPS directions in gentler fold limbs vary from perpendicular to acute with fold structural trends. Although internal strain is limited, weak AMS lineations defined by kinked and rotated phyllosilicates are widely developed and consistently perpendicular to LPS directions. Palinspastically restored LPS directions, corrected for minor vertical-axis rotations, are on average WSW-ENE within gentler fold limbs of the two systems, but display local deflections that appear partly related to geometry of underlying basement blocks. Steep forelimbs display more complex relations, including younger fault sets that developed during evolving stress states and localized vertical-axis rotations. Minor fault kinematics in steep fold limbs record localized stress changes near propagating basement faults, which were important in partitioning bulk strain in the Laramide foreland. Early LPS and subsequent folding are interpreted to partly reflect basal traction during flat slab subduction beneath thick cratonic lithosphere, with detailed spatial-temporal variations in stress trajectories related to basement heterogeneities and evolving fault systems.