The Basin and Range and Rio Grande rift (RGR) are regions of crustal extension in southwestern North America that formed in the Paleogene after Laramide-age shortening. The timing of extension in the area of southern New Mexico where these two provinces blend is uncertain, which also calls in to question the developmental relationship between the two provinces. A suite of 96 apatite and 43 zircon (U-Th)/He dates (AHe and ZHe) and 16 apatite fission track (AFT) dates have been collected from an east-west transect across southern New Mexico and easternmost Arizona to investigate the cooling and exhumation histories of the southeastern Basin and Range, the southern Rio Grande rift, and the transition zone between them. AHe dates range from 3–22 Ma, ZHe dates range from 2–649 Ma, and AFT ages range from 10–34 Ma with average track lengths of 10.8–14.1 µm. First-order spatiotemporal trends in the combined dataset suggest that Basin and Range extension was either contemporaneous with Eocene/Oligocene Mogollon-Datil volcanism or occurred before volcanism ended around 28 Ma, as shown by trends in ZHe data that suggest reheating to above ca. 240 °C at that time. AHe and ZHe dates from the southern RGR represent a wider range in ages that do not coincide with Paleogene volcanism, and were likely not reheated. Time-temperature models created by combining AHe, AFT, and ZHe data were used to observe patterns in cooling rate across the study area. The Chiricahua Mountains and Burro Mountains have an onset of rapid extension, defined as cooling rates in excess of > 15 °C/My, at ca. 29–17 Ma. In the Cookes Range, a period of rapid extension occurred at ca. 19–7 Ma. In the San Andres Mountains, Franklin Mountains, Caballo Mountains, and Fra Cristobal range, rapid extension at ca. 23–9 Ma. Measured average track lengths are longer in Rio Grande rift samples and ZHe ages of > 40 Ma are only present east of the Cookes Range, suggesting different levels of exhumation for the zircon partial retention zone and the AFT partial annealing zone. The main phase of fault-block uplift occurred ca. 22–10 Ma, similar to what is observed in the northern and central sections of the Rio Grande rift. Although rapid cooling occurred throughout southern New Mexico, comparison with spatiotemporal patterns in magmatism suggest it was driven by isothermal relaxation following magmatism in the Basin and Range, whereas in the Rio Grande rift it was driven by fault-related exhumation. Differences in cooling history, crustal thickness, electrical resistivity, sedimentation, and mantle heterogeneity make the Rio Grande rift tectonically distinct from the Basin and Range, although the two provinces may have evolved together in the early stages of Cenozoic extension in the western U.S.
|Commitee:||Burgette, Reed, Hampton, Brian, Buenemann, Michaela, Ricketts, Jason|
|School:||New Mexico State University|
|School Location:||United States -- New Mexico|
|Source:||MAI 81/7(E), Masters Abstracts International|
|Keywords:||Geochronology, Low-temperature thermochronology, Rio Grande rift, Tectonics|
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