Collaborative Research: Testing Models for Extension in the Rio Grande Rift-Basin and Range Transition Zone of Southern New Mexico

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The Basin and Range province and the Rio Grande rift in the western United States are two tectonic domains where the lithosphere has slowly pulled apart over the past 25 million years. The stretching is distributed across wide areas in the Basin and Range and is more focused along a narrow zone from southern New Mexico to northern Colorado in the Rio Grande rift, which in many ways resembles other significant rift zones around the world. A variety of tectonic models have been proposed to explain the extension in these two domains. Some models attribute their development to the same mechanisms whereas others implicate different mechanisms for each domain. In this project, a research team from the University of Texas El Paso and New Mexico State University aims to distinguish which tectonic forces drive extension in the two provinces where they merge in southern New Mexico. To do so, they will determine the uplift ages for many of the ranges in the region in order to determine if the Basin and Range province and the Rio Grande rift experience the same extension history. Desired societal outcomes will be achieved through the full participation of students and researchers of Hispanic heritage in the research project.The Rio Grande rift is one of the world?s best exposed and most thoroughly studied continental rifts, yet there still remains considerable uncertainty and debate on the driving forces that produced it and why it formed when and where it did. Many researchers view the Rio Grande rift as the easternmost segment of the adjacent Basin and Range province, but others view it as a separate structural entity. Despite the observation that the southern Rio Grande rift physiographically resembles the adjacent Basin and Range, it can be distinguished by higher heat flow, thinner crust, deeper basins, and an abundance of Quaternary normal faults. These observations lead to uncertainty as to whether these two structural domains evolved contemporaneously or not, as well as different interpretations of the underlying causes(s) driving crustal extension in the southwest U.S. The objective of this project is to constrain times of extension in the region where these two structural domains merge to test models for formation of the Rio Grande rift. Specifically, the project aims to be addressed are: (1) the timing of the main phase of extension in the Basin and Range Province of southeastern Arizona/southwestern New Mexico and the southern Rio Grande rift of southern New Mexico; (2) the difference in style/timing of extension between the Rio Grande rift and Basin and Range; and (3) the main driver of extension in the southern Rio Grande rift. To these ends, the research team will use apatite (U-Th)/He, apatite fission-track, and zircon (U-Th)/He thermochronologic methods on samples collected from thirteen individual fault block uplifts in the Rio Grande rift-Basin and Range transition of eastern Arizona, southern New Mexico, and western Texas. These results will fill a data gap between the Basin and Range and the northern Rio Grande rift. For each sample, U and Th zoning profiles will be obtained for apatite and zircon, and inverse models will be produced that incorporate all new thermochronologic data, zoning information, as well as existing geologic constraints. These data will be able to place very tight constraints on each samples' time-temperature history. The resulting models will delineate times of rapid exhumation from times of tectonic quiescence at each location, which will be the principle method for testing between models for extension.