New Mexico Geological Society Annual Spring Meeting — Abstracts

The Rio Grande rift extends north-south for approximately 1,000 km from central Colorado to at least southern New Mexico and western Texas, and possibly farther into northern Mexico. Structurally the rift is a relatively narrow feature that is composed of a series of north-south-trending flank uplifts that lie in the footwalls of major rift-related normal faults. In southern New Mexico, however, the physiographic expression of the rift widens and becomes more similar to the adjacent Basin and Range Province. In the narrow segment of the rift in Colorado and northern New Mexico, the timing of extension in many localities has been constrained utilizing apatite fission-track (AFT) and (U-Th)/He (AHe) thermochronologic methods, but very little thermochronologic data yet exists for the southern Rio Grande rift in southern New Mexico or western Texas.

To extend the existing thermochronologic database to the south, five samples were collected from the eastern flank of the Franklin Mountains of western Texas for AHe thermochronology. Samples were collected from the Proterozoic Red Bluff granite at varying elevations. A total of 26 single-grain AHe dates were obtained from these five samples, with ages ranging from 7.9 ± 2 Ma to 40 ± 4.0 Ma. eU values for these samples are very restricted from 4-27 ppm such that correlations between AHe age and eU are not observed. Thermal history models were produced for four of the samples; the fifth sample is not suitable for modeling because of a range of AHe ages that cannot be explained by different eU concentrations. Based on thermal history modeling constraints, one sample remained at temperatures above 40 °C until 23 Ma and cooled to near-surface temperatures by 10 Ma, two samples remained at temperatures above 40 °C until 18 Ma and cooled by 8 Ma, and the fourth sample remained at temperatures above 40 °C until 15 Ma and cooled by 8 Ma. Although these timing constraints are rough and could be improved with the addition of higher-temperature thermochronologic data, the AHe ages and thermal history models are consistent with results from the northern and central segments of the rift that preserve a strong pulse of exhumation from 25-10 Ma.

These data, when viewed within the context of existing AFT and AHe data from Colorado and New Mexico, indicate that the southern Rio Grande rift of western Texas evolved coevally with the more well-defined central and northern segments of the rift. These data do not support models of a northward-propagating rift or rotation of the Colorado Plateau as a primary driver of extension. Rather, models to explain the rift must account for synchronous extension along its length from northern Colorado to Texas as indicated by thermochronologic data.