INHERITED STRUCTURE FROM THE ANCESTRAL ROCKY MOUNTAIN AND LARAMIDE OROGENIES INFLUENCE RIO GRANDE RIFT DEVELOPMENT

Low-temperature thermochronometric data from the southern Rocky Mountains and Rio Grande rift (RGR) in Colorado and New Mexico, USA reveal exhumation timing, rates and magnitudes of exhumation related to both the Laramide Orogeny and subsequent extension within the RGR. Apatite (U-Th-Sm)/He (AHe), and zircon (U-Th)/He (ZHe) thermochronometric data were collected in vertical transects and analyzed with previously published AHe and apatite fission track (AFT) through inverse thermal history modeling in QTQt. Throughout the Rocky Mountains these data record cooling primarily related to the Laramide orogeny with the exception of the RGR. AHe, AFT, and ZHe data from fault-bounded ranges in the RGR, which can be divided into northern, central, and southern sections, shows that exhumation initiated contemporaneously in the northern and southern sections at ca. 25 Ma. In contrast, thermochronometric data from the central section chronicles exhumation related to the Laramide Orogeny rather than rift related exhumation. Major structures associated with rift formation in the northern and southern sections of the RGR are spatially coincident with recognized north-south oriented Ancestral Rockies structures (the Central Colorado Trough, Orogrande and Estancia Basins, and Frontrange, Apishipa Sierra Grande, and Pedernal uplifts). Conversely, the central, non-exhumed section of the RGR is coincident with the southwest-northeast striking Jemez lineament, which is recognized as a possible boundary between the Yavapai and Mazatzal terranes. The central RGR is also spatially coincident with a south-to-north transition from thinner to thicker lithosphere that accompanies a change from a wide rift with multiple faults accommodating extension to a narrow rift focused on single basin-bounding structures. We propose that rift structures in the RGR preferentially localize along pre-existing crustal weaknesses inherited from earlier orogenic events including the Ancestral Rocky Mountain and Laramide orogenies, and that rift style and extension mechanisms are controlled by lithospheric-scale structure.