Constraining timing of extension in the southern Rio Grande Rift and basin and range using apatite and zircon (U-Th)/He thermochronology

Abstract

A bstrAct — We sampled rocks for zircon (ZHe) and apatite (AHe) (U-Th)/He thermochronology from seven mountain ranges across the Rio Grande rift-Basin and Range transition zone in southeastern Arizona, southern New Mexico, and western Texas. Individual AHe ages (n=23) range from 8–26 Ma, and ZHe ages (n=42) range from 19–649 Ma. Samples from the Basin and Range province, west of the Cookes Range and the Florida Mountains (southwestern New Mexico), have a small spread in ZHe ages, whereas samples from the southern Rio Grande rift yield a wide range of ZHe ages that is related to a corresponding spread in effective uranium (eU). Forward and inverse modeling suggests that cooling from temperatures >200°C in the southeastern Basin and Range may have been due to a combination of tectonic exhumation and mid-Cenozoic igneous activity, whereas extension in the southern Rio Grande rift exhumed rocks from depths corresponding to temperatures <200°C. However, basins in the southern Rio Grande rift are up to 3 km deep, whereas the southeastern Basin and Range is characterized by basins with less than 700 m of basin fill. While further work is needed to fully understand the effects of normal faulting vs. igneous activity on thermochronologic data in southwestern New Mexico, these observations may be a reflection of different styles of extension in each region, separated by a N–S trending boundary in southern New Mexico. Core complex style extension involving low-angle normal faults and mid-crustal detachments are common within the Basin and Range. In contrast, extension in the southern Rio Grande rift may have been accomplished through high-angle faults, which would favor the formation of deep basins, but which did not exhume