PATTERNS IN CENOZOIC EXTENSION ALONG THE RIO GRANDE RIFT BASIN AND RANGE TRANSITION IN SOUTHERN NEW MEXICO FROM APATITE AND ZIRCON (U-TH)/HE THERMOCHRONOLOGY

In southern New Mexico, the southern Rio Grande rift can be distinguished from the adjacent Basin and Range Province by higher heat flow, deeper Cenozoic basins, and more Quaternary faulting and volcanism. In addition, the Basin and Range has experienced higher strain magnitudes resulting in the development of metamorphic core complexes through 10’s of kilometers of slip along low-angle normal faults, whereas extensional faults in the Rio Grande rift have not accumulated enough slip to expose ductilely-deformed rocks at the surface. Here, we investigate the nature of this boundary using low-temperature thermochronologic methods to explore possible differences in the timing and magnitude of extension.

A total of 42 new zircon (U-Th)/He (ZHe) and 23 new apatite (U-Th)/He (AHe) ages were collected from seven mountain ranges across the Rio Grande rift-Basin and Range transition zone in southeastern Arizona, southern New Mexico, and western Texas. AHe ages from the Basin and Range and Rio Grande rift are relatively young and range from 8-26 Ma. In the Basin and Range, ZHe data also show a narrow age range from 21-31 Ma despite large eU values that range from 120-1382 ppm. In contrast, samples from the southern Rio Grande rift yield ZHe ages that range from 19-649 Ma and eU values that range of 56-737 ppm. Forward and inverse modeling using HeFTy software suggests that Cenozoic cooling due to extension in the southeastern Basin and Range was sufficient to exhume rocks from temperatures >200 °C, whereas rocks in the southern Rio Grande rift cooled from maximum temperatures of 160-180 °C. Times of extension in the Basin and Range also preceded extension in the southern rift by several million years. These data provide important constraints on the style of extension in this region when coupled with previous data. For example, basins in the southern Rio Grande rift are up to 3 km deep, whereas basins in the southeastern Basin and Range contain less than 700 m of basin fill. We suggest that these observations are related to a difference in the style of extension in each region. Within the Basin and Range, extensional deformation produced shallow, broad basins and exhumed rocks from relatively deep crustal levels, while within the southern Rio Grande rift extension produced deep basins and exhumed rocks from relatively shallow levels.