REGIONAL ONSET OF LARAMIDE DEFORMATION TIMING AND INFERRED TECTONIC PROCESSES AS EVIDENCED BY INTEGRATED ARCH-BASIN THERMOCHRONOLOGY AND STRATIGRAPHY

Laramide basin stratigraphy is a record of accompanying arch formation and exhumation. We integrate this basin stratigraphic record (where possible) with low-temperature apatite fission track and apatite (U-Th)/He thermochronology from many of the major Laramide arches from Montana to New Mexico in order to discern the timing and character of deformation across the Rocky Mountain and Colorado Plateau region. Onset, duration, rate, and cessation of deformation for each individual range can be interpreted from HeFTy continuous time-temperature path models and stratigraphic accumulation curves, and allow for (1) a comprehensive characterization of Laramide deformation timing and (2) inferences on the likely tectonic processes responsible for late Cretaceous-Paleogene inboard contractional deformation. Initial results suggest that deformation onset swept from west to east leading to continued contraction and deformation of the continent after initial propagation of this deformation front. This may be most consistent with a holistic plate model by which high topography of the Cordillera transferred compressional stresses past the front of the Sevier belt, and protracted inboard contractional strain was further aided by flat slab propagation and fluids from the underlying Farallon slab that led to crustal destabilization. Upper crustal strain within the Laramide foreland due to passage of the conjugate Shatsky rise is deemed unlikely by these initial results. Deformation was perhaps localized via mid- to lower-crustal detachments sourced from laterally continuous lithospheric anisotropies. This interpretation is consistent with geometric and kinematic studies of the foreland that show regional WSW-ENE-directed Laramide shortening, and does not require >30° time-transgressive changes in the orientation of this maximum compressive stress. Furthermore, such characteristic patterns in deformation timing suggest that tectonic models that argue against flat slab subduction (e.g., westward subduction) are not favorable or are in need of revision to incorporate deformation timing. Continued work will further elucidate deformation onset to test this tectonic model and will incorporate cessation to better understand the change from contraction to extension in Cenozoic western North America.