Zircon (U-Th)/He Thermochronologic Constraints On The Long-Term Thermal Evolution Of Southern New Mexico And Western Texas

Zircon (U-Th)/He (ZHe) dates are presented from eight samples (n = 55) collected from three ranges including the Carrizo and Franklin Mountains in western Texas and the Cookes Range in southern New Mexico. ZHe dates from Proterozoic crystalline rocks range from 6 to 731 Ma in the Carrizo Mountains, 19 to 401 Ma in the Franklin Mountains, and 63 to 446 Ma in the Cookes Range, and there is a negative correlation with eU values. These locations have experienced a complex tectonic history involving multiple periods of uplift and reburial, and we use a combination of forward and inverse modeling approaches to constrain plausible thermal histories. Our final inverse models span hundreds of millions of years and multiple tectonic events and lead to the following conclusions: (1) Proterozoic exhumation occurred from 800 to 500 Ma, coinciding with the break-up of Rodinia; (2) elevated temperatures at approximately 100 Ma occurred during final development of the Bisbee basin and are a likely result of elevated heat flow in the upper crust during continental rifting; (3) a pulse of cooling associated with Laramide shortening is observed from 70 to 45 Ma in the Cooks Range and 80 to 50Ma in the Franklin Mountains, whereas the Carrizo Mountains were largely unaffected by this event; and (4) final cooling to near-surface temperatures began 30-25Ma at all three locations and was likely a result of Rio Grande rift extension. These data help to bridge the gap between higher and lower temperature isotopic systems to constrain complex thermal histories in tectonically mature regions.