THE ORIGIN AND TECTONIC SIGNIFICANCE OF THE BASIN AND RANGE–RIO GRANDE RIFT BOUNDARY IN SOUTHERN NEW MEXICO, USA (Invited Presentation)

Cenozoic extension in the western U.S. occurred within two iconic domains: the Basin and Range and Rio Grande rift. These provinces merge in southern New Mexico to form an interconnected zone of extension, although the existence, location, and nature of the boundary between the two provinces is uncertain. A suite of 128 apatite and 63 zircon (U-Th)/He (AHe, and ZHe), as well as 27 apatite fission-track (AFT) dates were collected to investigate the cooling history of this region. Collectively, AHe dates range from 3-46 Ma, AFT dates range from 10-34 Ma, and ZHe dates range from 2-288 Ma. Thermal modeling of thermochronologic data indicates that fault-block cooling of ranges in the Basin and Range west of and including the Cookes Range and Florida Mountains occurred from 35-14 Ma. In contrast, southern Rio Grande rift extension occurred during a distinctly younger timeframe of 25-5 Ma, similar to what has previously been documented in the central and northern sections of the rift. Additional ZHe thermal modeling, including a compilation of published data, indicates Proterozoic basement of the southeastern Basin and Range exceeded 225 °C during Oligocene magmatism, resetting ZHe dates and creating a sharp thermal boundary, whereas southern Rio Grande rift ranges were relatively unaffected by this event. These data, combined with geologic and geophysical datasets, define a subvertical, 30–40-km-wide boundary that extends through the lithosphere to depths of at least 100 km. This assessment of the boundary supports models in which the southern Rio Grande rift is a separate structural entity from the adjacent Basin and Range, and this region provides an exceptional case study for understanding how extensional lithospheric-scale boundaries evolve to become stable features of continents.