When and how quickly do magma systems look like those associated with rifting? A case study from the Southern Rocky Mountains and northern Rio Grande Rift

The onset of continental rifting induces significant tectonic, magmatic, and metallogenic transitions. Building a better understanding of how and when rifting begins is an important aspect of geological and mineral exploration models. However, defining exactly when rifting begins, or how quickly the lithosphere transitions into rifting, remains challenging. The northern Rio Grande Rift (RGR) is a classic example of this problem. It is widely accepted that rift-related uplift and basin development was widespread during the Oligocene, and that rifting was superimposed onto Cretaceous-Eocene silicic magma centers associated with the Laramide orogeny. Defining which silicic magmas might be related to rifting versus those more closely related to the end-stages of the Laramide orogeny is challenging, because many ages of pre- and syn-rift magma systems in the RGR were derived from chronometers prone to thermal perturbations and/or low precision. The large uncertainties of ages and complex patterns of magmatism during the transition from the Laramide orogeny to rifting obscure our ability to identify the onset of the RGR and spatio-temporal expression of magmatism. A growing dataset of high-precision ages for pre- and syn-rift magma systems in and around the northern RGR is building a more complete picture of how rifting in the region began, and we present new geochronology from this geologic period. However, even with these new data, there remains a paucity of high-quality geochronology throughout the region. We constructed a spatio-temporal model of silicic magmatism with a weighted bootstrap simulation that better accommodates locations with limited geochronologic quality. This approach allows us to utilize existing geochronological data, while putting a stronger emphasis on newly acquired high-precision ages. Our model indicates that the orientation of magmatism shifted into alignment with the N-S faulting that accommodated uplift in the northern RGR. This shift in magmatism occurred between 40-35 Ma, ahead of the record of thermochronologic uplift due to Rio Grande rifting. Our results emphasize that the N-S ancient crustal structures that controlled the geometry of the northern RGR may also have acted as focal planes for magmatism millions of years before rift-basin development.