CONSTRAINING TIMES OF EXTENSION IN THE SOUTHERN RIO GRANDE RIFT USING APATITE AND ZIRCON (U-TH)/HE THERMOCHRONOLOGY

The Rio Grande rift is a region of lithospheric extension and a major geologic feature in the western United States, but fundamental questions remain about its formation. Several conflicting models for the activation of the rift have been put forward that continue to be debated. In particular, some argue that the Rio Grande rift is not a distinct feature, but merely a segment of the larger Basin and Range extensional province. Existing low-temperature thermochronologic data indicate that a period of synchronous extension occurred across the entire length of the rift from ~25-10 Ma. However, these studies were more focused on the northern and central sections of the rift, and thermochronologic data remain sparse in southern New Mexico. This study aims to collect additional thermochronologic data from the southern Rio Grande rift and adjacent Basin and Range to evaluate whether the timing of extension from 25-10 Ma is also observed across the southern rift. It will also help to define the nature of the Rio Grande rift – Basin and Range transition. Rock samples will be collected from the footwalls of normal faults across southern New Mexico to record times of rapid cooling due to extensional faulting. They will be evaluated using apatite (U-Th)/He (AHe) and zircon (U-Th)/He (ZHe) thermochronologic methods. The relatively low closure temperatures of these methods allows us to constrain the samples’ cooling histories from 200 °C to near-surface temperatures. Forward and inverse modeling of the thermochronologic data will be accomplished with HeFTy software, and will constrain when faults in different regions of the southern Rio Grande rift and Basin and Range were active.