Title: Understanding the Development of the Rio Grande Rift through Faulting, Magmatism, and Landscape Evolution

Abstract:

Analysis of low-temperature thermochronometric data in the Rio Grande rift (RGR) in New Mexico and Colorado, USA provides the means to assess the timing of fault initiation, as well as patterns in growth and linkage of rift faults. Evaluating spatiotemporal patterns in faulting and rift-related magmatism reveals insights into processes behind extension accommodation and helps to distinguish between possible rift models. Apatite (U-Th-Sm)/He (AHe), zircon (U-Th)/He (ZHe) and apatite fission track (AFT) thermochronometric data were combined to compile 14 vertical transects, spanning more than >800 km along the RGR axis. Thermal history modeling of these data, in the program QTQt, reveals contemporaneous rift initiation at ca. 25 Ma in both the northern and southern RGR with continued fault initiation, growth, and linkage progressing from ca. 25 to ca. 15 Ma. The central RGR, however, shows no evidence of Cenozoic fault-related exhumation as observed with thermochronometry and instead chronicles exhumation related to the Laramide Orogeny rather than rift related exhumation. Rift structure and geometry appear to be partly controlled by inherited structure and/or lithospheric properties. These data and observations lead to the proposal of a new evolutionary model for the RGR involving initiation of fault-accommodated extension by oblique strain followed by block rotation of the Colorado Plateau, where extension in the RGR is accommodated by a combination of faulting (southern and northern RGR) and magmatism (central RGR). Furthermore, initial detrital zircon U-Th-Pb data from samples in the Miocene-Pliocene Dry Union formation along with detailed stratigraphic characterization suggest a complex fluvial evolution of the modern and proto-Arkansas River indicating the potential that rift structures also played a role in evolution of the river.