DIFFERENT COOLING HISTORIES IN THE BASIN AND RANGE AND RIO GRANDE RIFT OF SOUTHERN NEW MEXICO REVEALED USING LOW TEMPERATURE THERMOCHRONOLOGY

The Basin and Range province and Rio Grande rift (RGR) form a complex region that records a major transition in the tectonic history of North America from Laramide shortening to Cenozoic crustal extension. Driving mechanisms for this episode are still highly debated and include changes in stress field, widespread small-scale mantle convection, and growth of the San Andreas transform. A suite of 88 apatite (AHe) and 33 zircon (ZHe) (U-Th)/He ages 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 and southern Rio Grande rift and the transition zone between them. AHe ages range from 3–22 Ma, ZHe ages range from 2–649 Ma, and AFT ages range from 10–34 Ma with average track lengths of 10.8–14.1µm.

First-order spatiotemporal trends in the dataset suggest that Basin and Range extension was contemporaneous with the end of Oligocene Mogollon-Datil volcanism, evidenced by trends in ZHe data that suggest reheating to above ~240 °C at that time. AHe and ZHe dates from southern RGR represent a wider range in ages that predate Paleogene volcanism, and were likely not reheated. Time-temperature models created from combining AHe, AFT, and ZHe data were used to observe patterns in cooling rate across the field area. The Chiricahua Mountains and Burro Mountains have an onset of rapid (cooling rates > 15 °C/My) extension at ca. 29–17 Ma. In the Cooke’s Range a similar period of rapid extension is observed at ca. 19–7 Ma. In the San Andres Mountains, Caballo Mountains, and Fra Cristobal range, rapid extension is observed ca. 23–9 Ma. Measured average track lengths are longer in Rio Grande Rift samples and ZHe ages of >40 Ma have only been observed east of the Cooke’s range, suggesting different exhumation conditions of the zircon partial retention zone and the AFT partial annealing zone. We interpret the following: 1) Rapid extension occurred along the entire length of the Rio Grande Rift at ca. 22-10 Ma. 2) While rapid cooling is observed throughout southern New Mexico, it was driven by post-volcanic cooling in the Basin and Range, and fault related cooling in the Rio Grande rift. 3) That differences in cooling history, crustal thickness, electrical resistivity, sedimentation, and mantle heterogeneity make the Rio Grande rift tectonically distinct from the Basin and Range.