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The Origin and Tectonic Significance of

the Basin and Range–Rio Grande Rift

Boundary in Southern New Mexico, USA

Jason W. Ricketts, Dept. of Earth, Environmental and Resource Sciences, The University of Texas at El Paso, 500 West University Ave.,
El Paso, Texas 79968, USA, jricketts@utep.edu; Jeffrey M. Amato, and Michelle M. Gavel, Dept. of Geological Sciences, New Mexico
State University, Las Cruces, New Mexico 88003, USA

ABSTRACT separated from the Basin and Range Province thermal boundary and reinforce the idea that
Cenozoic extension in the western United by the Colorado Plateau, and they exist as the southern Rio Grande rift is a separate
States occurred within two iconic domains: distinct structural entities, but in southern structural entity from the adjacent Basin and
the Basin and Range and Rio Grande rift. New Mexico, they merge to form an inter- Range Province.
These provinces merge in southern New connected zone of extension that continues
Mexico to form an interconnected zone of south into Mexico (Fig. 1). The existence of a THE RIO GRANDE RIFT–BASIN AND
extension, although the existence, location, discrete boundary between the two domains RANGE BOUNDARY IN SOUTHERN
and nature of the boundary between the two and the nature of this transition in southern NEW MEXICO
provinces are uncertain. In southern New New Mexico remain unclear, although There are three models for how to assess
Mexico, existing thermochronologic, geo- understanding the transition is crucial for the Rio Grande rift–Basin and Range
logic, and geophysical data sets, combined assessing how these two extensional prov- Province boundary (Fig. 1): (1) The northern
with thermal modeling of zircon (U-Th)/He inces evolved through time. The physio- Rio Grande rift is a separate entity from the
(ZHe) data, define a subvertical, 30–40-km- graphic expression of extension in southern Basin and Range Province, and is distin-
wide boundary that extends through the New Mexico suggests an indistinct or nonex- guished by its narrow width (dark orange in
lithosphere to depths of at least 100 km. istent boundary, favoring models where the Fig. 1); (2) the entire Rio Grande rift exists as
Thermal modeling indicates Proterozoic Rio Grande rift is the easternmost segment a separate entity along its entire length (light
basement in the upper crust of the south- of the Basin and Range Province (Eaton, and dark orange in Fig. 1); and (3) the two
eastern Basin and Range exceeded 225 °C 1982). This view is contentious, however, provinces are contiguous and coeval and
during Oligocene magmatism, resetting because thermochronologic (Gavel, 2019) thus the Rio Grande rift is just a localized
ZHe dates and creating a thermal boundary and geophysical (e.g., Keller et al., 1990; term for the Basin and Range Province on its
that coincides with independent geologic Averill and Miller, 2013; Feucht et al., 2019) eastern margin adjacent to the Colorado
and geophysical data sets. Although many data sets highlight important differences Plateau (thick dark green line in Fig. 1). We
aspects of this boundary are transient, oth- between the two provinces, supporting mod- investigate the nature of a possible boundary
ers may become permanent features to els where they exist as two separate, albeit in southern New Mexico, and use “transition
define a lithospheric-scale boundary prone contiguous entities. This nontrivial distinc- zone” to refer to a 40-km-wide zone that
to reactivation during future tectonism. tion has implications for the relative role includes major differences in the crust and
This assessment of the boundary supports of plate-boundary versus mantle processes lithosphere. Physiographic maps, which are
models in which the southern Rio Grande driving extension in western North America based on modern topography and drainage
rift is a separate structural entity from the (Dickinson, 2002). basins, lump the southern Rio Grande rift
adjacent Basin and Range, and this region A more complete understanding of the with the Basin and Range Province
provides an exceptional case study for boundary requires diverse data sets at dif- (Hammond, 1970). Most workers place the
understanding how extensional lithospheric- ferent scales to constrain its current charac- boundary at the eastern edge of the transition
scale boundaries evolve to become stable teristics and evolutionary history. Here we zone (e.g., Mack, 2004; van Wijk et al., 2018),
features of continents. use thermochronologic data, including apa- which coincides with the western edge of the
tite fission-track (AFT), apatite (U-Th)/He rift farther north, but it has not yet been
INTRODUCTION (AHe), and zircon (U-Th)/He (ZHe), together clearly documented as to why this is a mean-
The Rio Grande rift and Basin and Range with a synthesis of geologic and geophysi- ingful or geologically relevant location in
Province are two of the most iconic exten- cal data from southwestern New Mexico to southern New Mexico.
sional domains on Earth; the Basin and investigate the nature of the transition at a
Range Province is the archetypal example of lithospheric scale. We then document a pro- GEOLOGIC AND GEOPHYSICAL
a wide rift, and the neighboring Rio Grande nounced thermal boundary across the tran- MANIFESTATIONS OF A BOUNDARY
rift is one of the classic modern examples of sition preserved in ZHe data sets. When In southern New Mexico, independent
a narrow continental rift (e.g., Buck, 1991). viewed collectively, these independent data data sets highlight a subvertical boundary
For most of its length, the Rio Grande rift is sets reveal a complex and dynamic tectono- 30–40 km wide that extends to depths of at

GSA Today, v. 31, https://doi.org/10.1130/GSATG509A.1. CC-BY-NC.

4 GSA Today | October 2021