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.

Cenozoic extension in the western UnitedStates occurred within two iconic domains: the Basin and Range and Rio Grande rift. These provinces merge in southern New Mexico to form an interconnected zone of extension, although the existence, location, and nature of the boundary between the two provinces are uncertain. In southern New Mexico, existing thermochronologic, geologic, and geophysical data sets, combined with thermal modeling of zircon (U-Th)/He (ZHe) data, define a subvertical, 30-40-kmwide boundary that extends through the lithosphere to depths of at least 100 km. Thermal modeling indicates Proterozoic basement in the upper crust of the southeastern Basin and Range exceeded 225 °C during Oligocene magmatism, resetting ZHe dates and creating a thermal boundary that coincides with independent geologic and geophysical data sets. Although many aspects of this boundary are transient, others may become permanent features to define a lithospheric-scale boundary prone to reactivation during future tectonism. This assessment of the boundary supports models in which the southern Rio Grande rift is a separate structural entity from the adjacent Basin and Range, and this region provides an exceptional case study for understanding how extensional lithosphericscale boundaries evolve to become stable features of continents.

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<div>Table S1: Summary of all of the geochronology (dates and locations) for the samples in this study. Table S2: Thermal history model inputs; Table S3: Apatite fission track data used in HeFTy modeling.<br></div><div><br></div><div><br></div><div><br></div>

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