UNRAVELING THE EARLY LPS-STRESS HISTORY OF THE LARAMIDE FORELAND: INTEGRATING FIELD STUDIES OF MINOR FAULTS WITH STRESS INVERSION AND AMS STUDIES

Integrated analysis of fracture patterns in Triassic red beds, stress inversion of minor fault data from thin limestone intervals, anisotropy of magnetic susceptibility (AMS) measurements, and paleomagnetic studies, constrain regional patterns of layer-parallel shortening (LPS) and stress fields across Laramide foreland uplifts. Multiple fracture sets in red beds and limestone reflect a complex stress history, including fold-related sets. Minor faults with overall conjugate wedge and strike slip geometries accommodated early LPS and show consistent relationships with bedding suggesting subsequent tilting around large-scale fold limbs. Early faults from fold backlimbs record overall WSW-ENE directed LPS, which closely matches directions recorded by weak, but consistent AMS lineations. AMS lineations are defined by kinked, rotated and intersecting phyllosilicate fabrics, and are consistently perpendicular to LPS inferred from stress inversion of minor faults. In detail, LPS directions display subtle regional deflections that broadly correlate with overall regional structural grain. Stress inversion indicates low stress ratio (sig2 ~ sig3) in most areas. Forelimbs display more complex relations, including younger fault sets, minor changes in internal shortening directions during folding, and minor vertical-axis rotations recorded by paleomagnetic data. LPS in the Laramide foreland is interpreted to partly reflect basal traction during flat slab subduction beneath cratonic lithosphere, with spatial-temporal variations in stress direction related to inherited basement anisotropy, evolving fault systems and resulting fold geometry. Orientations of palinspasticly restored LPS across the Laramide foreland are consistent with an integrated tectonic system with local stress refraction and wrench shear, but not with a multi-stage or multi-directional shortening model for Laramide deformation.