REGIONAL LARAMIDE DEFORMATION TIMING AND ITS IMPLICATIONS FOR CRETACEOUS-PALEOGENE TECTONIC PROCESSES

Laramide deformation timing is a key parameter needed to evaluate the tectonic processes that produced western USA foreland deformation. We interpret deformation timing by integrating apatite thermochronology (AT; arches), tectonic subsidence curves and decompacted stratigraphic accumulation rates (basins), and compiled published estimates (arches and basins). Data are plotted on five west-to-east transects from MT-WY-SD to northern AZ-NM in order to characterize the spatiotemporal distribution of Laramide deformation. This method of integrating the independent timing records of the arch and basin allows for more detailed characterization of how orogenesis proceeded (e.g., differentiating deformation onset vs. start of rapid exhumation). Results show that onset regionally swept WSW-ENE and started in the west at ~90-95 Ma in northwest AZ (Kingman arch) and ~85-90 Ma in southwest MT (Blacktail-Snowcrest arch) and proceeded inboard at 40-90 km/Ma until ~70-75 Ma in north-central NM (Sangre de Cristo arch-Raton basin) and ~60-65 Ma in northwest SD (Black Hills arch). Discrepancies in this regional record include: later onset in UT (based on existing published results), delay of rapid cooling/exhumation ~15 million years after onset in some WY ranges, and early ~90 Ma cooling in eastern WY ranges based on previously published AT data. These discrepancies and sweeping onset are interpreted to reflect the transition from Sevier/Coloumb wedge-driven forebulge migration (early ~90 Ma signals) to eventual eastward passage of the Farallon flat slab and intracratonic destabilization via slab dewatering (sweeping onset) and later slab delamination (as proposed by previous authors; late signals). We note that the modeled timing and trajectory of the conjugate Shatsky Rise (CSR) was associated with low amplitude Colorado Plateau structures and, hence, may have impeded significant upper crustal strain, possibly due to an inability to dewater and weaken the lithosphere. This alternative interpretation and whole-scale onset (AZ and MT at ~90 Ma) does not support a causal relationship between CSR passage and start of Laramide deformation. Continued work on deformation cessation may similarly help infer transitional tectonic processes from intracontinental contraction to extension and volcanism.