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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 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.

The Basin and Range and Rio Grande rift (RGR) are regions of crustal extension in southwestern North America that developed after Laramide-age shortening, but it has not been clear whether onset and duration of extension in these contiguous extensional provinces were the same. We conducted a study of exhumation of fault blocks along a transect from the southeastern Basin and Range to across the RGR in southern New Mexico. A suite of 128 apatite and 63 zircon (U-Th)/He dates (AHe and ZHe), as well as 27 apatite fission-track (AFT) dates, was collected to investigate the cooling and exhumation histories of this region. Collectively, AHe dates range from 3 to 46 Ma, ZHe dates range from 2 to 288 Ma, and AFT dates range from 10 to 34 Ma with average track lengths of 10.8–14.1 µm. First-order spatiotemporal trends in the combined data set suggest that Basin and Range extension was either contemporaneous with Eocene–Oligocene Mogollon-Datil volcanism or occurred before volcanism ended ca. 28 Ma, as shown by trends in ZHe data that suggest reheating to above 240 °C at that time. AHe and ZHe dates from the southern RGR represent a wider range in dates that suggest the main phase of cooling occurred after 25 Ma, and these blocks were not reheated after exhumation. Time-temperature models created by combining AHe, AFT, and ZHe data in the modeling software HeFTy were used to interpret patterns in cooling rate across the study area and further constrain magmatic and/or volcanic versus faulting related cooling. The Chiricahua Mountains and Burro Mountains have an onset of rapid extension, defined as cooling rates in excess of >15 °C/m.y., 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 occurred from ca. 23 to 9 Ma. Measured average track lengths are longer in Rio Grande rift samples, and ZHe dates of >40 Ma are mostly 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 in the southern RGR occurred ca. 25–7 Ma, similar to what has been documented in the northern and central sections of the rift. Although rapid cooling occurred throughout southern New Mexico, thermochronological data from this study with magmatic and volcanic ages suggest rapid cooling was coeval with magmatism in the Basin and Range, whereas in the Rio Grande rift cooling occurred during an amagmatic gap. These observations support a model where an early phase of extension was facilitated by widespread ignimbrite magmatism in the southeastern Basin and Range, whereas in the southern Rio Grande rift, extension started later and continues today and may have occurred between local episodes of basaltic magmatism. These differences in cooling history make the Rio Grande rift tectonically distinct from the Basin and Range. We infer based on geologic and thermochronological evidence that the onset of extension in the southern Rio Grande rift occurred at ca. 27–25 Ma, significantly later than earlier estimates of ca. 35 Ma.

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Multiple thermochronologic methods (apatite fission-track and (U-Th)/He) have been used to place important time constraints on periods of Cenozoic extension in the Rio Grande rift and Basin and Range Province. However, Proterozoic-aged rocks in this region have experienced a much longer thermal history that is not typically recorded by these data. To investigate long-term (>1 Ga) thermal histories, we combine GIS analysis of past deformational events with zircon (U-Th)/He (ZHe) thermochronology to document periods of pre-Cenozoic burial and exhumation. Recent advancements in the understanding of differential radiation damage and helium retention in zircon suggest that long-term thermal histories experienced since their formation in the Proterozoic, may be extracted from the rocks. We analyzed samples along a transect across the Rio Grande rift -Basin and Range transition zone in west Texas, southern New Mexico and south-eastern Arizona. This region serves as natural laboratory to investigate the Proterozoic-Cenozoic thermal histories of rocks exposed in fault block uplifted mountain ranges. Prior to recent exhumation in the Cenozoic, Rio Grande rift and Basin and Range extension, the Rio Grande rift -Basin and Range transition zone has been affected by multiple deformation events, including the Ancestral Rocky Mountains, Jurassic rifting and the Laramide Orogeny. GIS was used to combine published deformational maps of New Mexico to construct a deformational sequence map of southern New Mexico, illustrating areas that experienced uplift and/or burial during the Ancestral Rocky Mountains, Laramide Orogeny and Rio Grande rift. For each sample location, forward models and predictive age-eU plots were constructed using HeFTy software that include periods of exhumation and/or burial due to each tectonic event. Inverse modeling of ZHe data will be done using HeFTy software to compare to forward models and predicted age-eU plots, to test whether ZHe data record long and complex cooling histories. Preliminary data from some of these ranges yield ZHe ages that range from 19-649 Ma and show a negative correlation with eU, suggesting that ZHe data obtained from this region are an important record of pre-Cenozoic tectonic exhumation. Results from study will constrain long-term timing, magnitude and rates of cooling experienced in these fault blocks across the Rio Grande rift -Basin and Range transition zone, and may yield important insight into the timing and duration of deformation related to the Ancestral Rocky Mountains, Jurassic rifting, Laramide Orogeny, and Cenozoic extension.