REGIONAL TIMING OF LARAMIDE DEFORMATION ONSET, INTERIOR WESTERN USA

Timing datasets from many of the major Laramide foreland arches and basins are analyzed on five west-to-east transects in order to deduce deformation onset spatiotemporally. Transects are populated with new and previously published data of three types: arch thermochronology (apatite fission track and U-Th/He), basin subsidence and stratigraphic accumulation histories, and compiled literature estimates of onset for both arches and basins. This integrated method allows for detailed characterization of Laramide deformation timing based on different data types that leads to comprehensive tectonic interpretations. Onset commenced ca. 95-85 Ma in both northwest Arizona and southwest Montana, then swept east-northeastward at 30-65 km/Ma to north-central New Mexico at 75-70 Ma, north-central Colorado at 70-65 Ma, and westernmost South Dakota at 65-60 Ma. Deviations to this onset sweep include: ca. 90-80 Ma cooling signals within the Wyoming foreland; rapid cooling signals that post-date stratigraphic evidence for onset; and apparently synchronous onset at ca. 75 Ma for Colorado Plateau arches.

Two major implications arise from our synthesis. (1) Synchronous onset (AZ and MT) indicates that Laramide deformation was not driven solely by subduction of the postulated conjugate Shatsky Rise (CSR) oceanic plateau. Models for the CSR trajectory, subdued structural relief, and later Colorado Plateau signals may in fact suggest that it impeded deformation. (2) The rate of eastward-sweeping onset is less than or equal to half the rate of recent estimates for Farallon-North America convergence. Therefore, we suggest a model for Laramide orogenesis whereby upper crustal strain was established by compressional stress transfer from the Nevadaplano hinterland and/or plate margin end loading, and attribute time-transgressive onset to be the result of progressive foreland destabilization via slab dehydration as a portion of the Farallon slab flattened beneath North America (with or without basal traction). Such a model is reconcilable within a larger plate tectonic framework in that the mechanisms responsible for both the Sevier fold-thrust belt and Laramide basement-involved foreland arches need not be separated from one another, allowing for a holistic view of an evolving plate margin from 95-45 Ma.