THE THERMOCHRONOLOGIC RECORD OF TECTONISM AND MAGMATISM ACROSS THE RIO GRANDE RIFT-BASIN AND RANGE TRANSITION (Invited Presentation)

The Basin and Range and Rio Grande rift are two archetypal regions of extension in western North America. In southern New Mexico, the two are physiographically similar, but the rift can be distinguished by higher heat flow, deeper Cenozoic basins, decreased upper mantle velocities, thinner crust, differences in electrical resistivity, and more active Quaternary faulting and volcanism. Here, we investigate possible differences in the timing of cooling in mountain ranges that lie along a transect from southeastern Arizona, through southern New Mexico, and into western Texas using combined apatite and zircon (U-Th)/He (AHe and ZHe) methods.

A total of 108 new ZHe and 49 new AHe dates were collected from eight mountain ranges. AHe dates from the Basin and Range and Rio Grande rift are relatively young and range from 8-40 Ma. In the Basin and Range, ZHe data also show a narrow date 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 dates that range from 6-731 Ma and eU values that range of 53-1729 ppm. These data document times of Neogene extension and also capture additional information related to previous pulses of magmatism and periods of tectonism that occurred throughout the Phanerozoic. In the Basin and Range, AHe and ZHe dates are similar to nearby Ar/Ar ages of volcanic rocks such that magmatism may have had a strong influence on the observed thermochronologic data. In the southern Rio Grande rift, Proterozoic ZHe dates require that these rocks have remained at temperatures <250 °C. In addition, intrasample single grain ZHe dates vary by hundreds of millions of years and show well-defined negative correlations with eU values, which is the cumulative result of multiple superimposed tectonic/magmatic events, culminating in the development of the Rio Grande rift. Coupled AHe and ZHe thermochronology highlights important differences between the Rio Grande rift and Basin and Range and also provides valuable information on previous tectonic and magmatic events. Future work will include combining AHe/ZHe data with Ar/Ar dates to constrain the long-term thermal history of Proterozoic rocks exposed in the southern Rio Grande rift, which will increase our understanding of extensional processes in active and ancient environments.