Abstract
AbstractZircon (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 50 Ma in the Franklin Mountains, whereas the Carrizo Mountains were largely unaffected by this event; and (4) final cooling to near-surface temperatures began 30–25 Ma 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.
Highlights
Thermochronology is a powerful tool to constrain the ages and durations of past geologic events because exhumation leads to cooling, the timing of which is recorded by different isotopic systems
A total of 55 individual grain Zircon (U-Th)/He (ZHe) dates are presented from three ranges in southern New Mexico and western Texas
We find that a forward modeling approach is advantageous for quickly comparing several dissimilar thermal histories, an inverse modeling approach can further refine these results and can test tens or hundreds of thousands of possible t-T paths
Summary
Thermochronology is a powerful tool to constrain the ages and durations of past geologic events because exhumation leads to cooling, the timing of which is recorded by different isotopic systems. Cooling ages are interpreted in context of closure temperatures of different minerals (e.g., [1]) in which diffusion of radiogenic daughter isotopes slows significantly below a known temperature range. There can be a gap in the dates obtained from high-temperature thermochronologic systems such as titanite U-Pb, hornblende 40Ar/39Ar, and mica 40Ar/39Ar, which record the timing of cooling from ~600 to 300°C [2], versus low-temperature systems such as apatite fission-track and (U-Th)/He, which record the timing of cooling below 120–60°C and 90–30°C, respectively [3, 4]. Zircon fission-track, sensitive to temperatures of ~270–210°C [11], is a widely used thermochronometer to partially fill this temperature range, there can still be a gap in the thermochronologic history between 40Ar/39Ar methods and apatite (U-Th)/He because of variations in closure temperature related to cooling rate
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