WHAT IS THE NATURE OF THE BOUNDARY SEPARATING THE RIO GRANDE RIFT AND BASIN AND RANGE PROVINCE IN SOUTHERN NEW MEXICO?

The Rio Grande rift and the Basin and Range (BR) are adjacent extensional domains that have evolved contemporaneously in western North America. Although the rift is often considered to be the easternmost boundary of BR extension, there is geologic and geophysical evidence that suggests the two are discrete provinces. The Rio Grande rift has more active Quaternary faults associated with deep rift basins that cut across BR faults, and it contains positive gravity anomalies relative to the BR, suggesting a different deep crustal and/or mantle structure. Quaternary volcanism and higher heat flow are also present across the rift. Metamorphic core complexes, characteristic of BR extension, exist as far east as southern Arizona, where they were active from less than 30 to 12 Ma. Core complexes are absent from the Rio Grande rift, although it has been active for at least the last 30 million years and has continued extending to the present. 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 remains sparse in southern New Mexico. This study will obtain additional thermochronologic data from the southern Rio Grande rift and adjacent BR. Rock samples were collected from the footwalls of normal faults in eight mountain ranges across southern New Mexico and southeastern Arizona, and from these samples a total of 70 apatite and 42 zircon grains are being analyzed using apatite and zircon (U-Th)/He thermochronologic methods. The newly collected data will evaluate whether the time of extension from 25-10 Ma is observed across the southern rift and will also help to constrain the timing of extension along the Rio Grande rift – Basin and Range transition. The relatively low closure temperatures of these methods allow us to constrain the samples’ cooling histories from 200 °C to near-surface temperatures. Forward and inverse modeling of the thermochronologic data will be completed using HeFTy software, and will constrain when faults in different regions of the southern Rio Grande rift and adjacent BR were active.