INVESTIGATING THE SEVIER TO LARAMIDE TRANSITION STRESS/STRAIN HISTORY IN THE COLORADO PLATEAU: A DETAILED LOOK AT THE SAN RAFAEL SWELL AND ITS KINEMATIC EVOLUTION

The Colorado Plateau (CP) underwent Laramide thick-skin deformation, but with lower shortening magnitudes (<3%) and more variable fold trends compared to the classic Wyoming-Colorado area, indicating the CP acted as semi-rigid block with localized deformation likely related to basement weaknesses. The western CP was also located near the leading edge of the Sevier thrust belt, which influenced foreland basin sedimentation and likely transmitted stresses inboard, resulting in a composite paleo-stress/ strain field. A key area of this interaction is the San Rafael Swell (SRS), a N- to NE trending, doubly plunging, ~110 km long by 50 km wide uplift with a curved eastern forelimb and broad gently dipping backlimb. Albian to Santonian strata in the area record foreland deposition, whereas local thickness changes in upper Campanian to Paleogene strata record Laramide thick-skin foreland breakup.

Paleo-stress/ strain directions determined for 65 new sites around the SRS, plus 16 sites complied from the literature, reveal distinctive regional variations. Shortening directions were estimated from orientations of minor faults and tectonic stylolites, and from anisotropy of magnetic susceptibility fabrics in redbeds. Paleo maximum stress (σ1) directions are subparallel to bedding around the SRS, recording early layer-parallel shortening (LPS). Overall, LPS was widespread and oriented ESE on the western back limb, limited and oriented E to ENE in the central part, and more concentrated and oriented E to ESE on the steeper forelimb. This pattern is interpreted to reflect transmission of stress from the eastward propagating Sevier thrust belt into western parts of the CP, including the backlimb of the SRS, during the mid-Cretaceous. The concentrated development of LPS fabrics along the forelimb and their correlation with the SRS’s curved structural trend likely record increased Laramide stress above an upward propagating basement fault system. A 3D basement fault model, informed by trishear fault-fold relationships on a series of cross sections, is input in an elastic dislocation boundary element stress model to estimate paleo-stress variations across the SRS. Comparison of LPS patterns with the model is consistent with interactions between Sevier and Laramide stress fields and local stress perturbations from slip on the basement fault system.