DEVELOPMENT OF THE RIO GRANDE RIFT SYSTEM FROM ASSESSMENT OF SPATIOTEMPORAL PATTERNS IN FAULTING AND MAGMATISM

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. We combine apatite (U-Th-Sm)/He (AHe) and zircon (U-Th)/He (ZHe) thermochronometric data with previously published AHe and apatite fission track (AFT) data to compile 14 vertical transects, spanning more than >800 km along the RGR axis. Thermal history modeling of these data 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 we suggest that extension is accommodated through Late Cenozoic magmatic injection. The faulting accommodation in the northern and southern RGR occurs along a N-S strike while the magmatic accommodation occurs along the Jemez lineament trending NE-SW. The physiographic differences between the three sections of the rift as well as the variations in deformation orientation and rift accommodation along strike appear to be related to lithospheric architecture. We infer that rift structure and geometry are at least partly controlled by inherited structure and/or lithospheric properties. We propose an evolutionary model for the RGR that involves 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).