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0 1000 Rio Grande rift ZHe (n = 98) young alluvium (Mack, 2004). In the south-
100 A path 1 path 2 Figure 3B 800 Basin and Range ZHe (n =20) ern Rio Grande rift, the timing of initial
extension was inferred by Amato et al.
Temperature (ºC) 300 40 age southern New Mexico ZHe Date (Ma) 600 Paleozoic (22%) (2019) to have begun by ca. 27 Ma when
200
Proterozoic (73%)
Ar/ Ar muscovite cooling
39
Cenozoic (5%)
the voluminous Uvas basalts were erupted
400
400
lighter shades are
(Clemons, 1979), which was coeval with
(Amato et al., 2011)
500
sedimentary (19%)
600
(Boryta, 1994). Extension was active through
1
700 Crystallization 2 3 4 5 200 0 50 μm grain size deposition of the Thurman Formation
the late Quaternary as evidenced by long fault
1800 1400 1000 600 200 0 0 400 800 1200 1600 2000
Time (Ma) eU (ppm) scarps that bound major rift flank uplifts.
0 1000 Thermochronologic data from the study
B Earlier thermal area offer an opportunity to further con-
50 history uses path 2 800 strain the times of extension and compare to
Temperature (ºC) 150 Volcanism 150 °C ZHe Date (Ma) 600 the sedimentary record and regional tectonic
100
history. Inverse thermal history modeling of
Boot Heel
175 °C
400
AHe, AFT, and ZHe data in southern New
200
200 °C
250 (McIntosh and 225 °C 200 Mexico suggests that main cooling in the
Bryan, 2000)
250 °C southeastern Basin and Range Province was
300 0 from 35 to 14 Ma (Gavel, 2019). In contrast,
100 80 60 40 20 0 0 400 800 1200 1600 2000
Time (Ma) eU (ppm) thermal modeling documents distinctly
younger cooling from 25 to 5 Ma in the
Figure 3. (A) Forward modeling and calculated zircon (U-Th)/He (ZHe) date-eU curves compared to a
compilation of ZHe dates from the southern Rio Grande rift and Basin and Range Province (Biddle et southern Rio Grande rift east of the transi-
al., 2018; Gavel, 2019; Reade et al., 2020), where Basin and Range Province data are west of 108°W tion (Fig. 1) to create a complex and highly
longitude and Rio Grande rift data are east. ZHe date-eU curves are calculated from a thermal history
using the helium diffusion model of Guenthner et al. (2013). 1—assembly of Rodinia; 2—breakup of dynamic lithospheric boundary that formed
Rodinia; 3—Ancestral Rocky Mountains; 4—Laramide orogeny; 5—Neogene exhumation. (B) Testing during diachronous pulses of extension.
the effects of magmatic reheating in the Boot Heel volcanic field. Grain size used on modeling is the Initial Cenozoic development of the bound-
average of all zircon grains.
ary occurred from 35 to 25 Ma when the
as an active extensional province that devel- become more pronounced in independent southeastern Basin and Range Province
oped adjacent to the generally inactive data sets, suggesting that many of the dif- experienced voluminous magmatism in the
southeastern Basin and Range Province ferences across this boundary are controlled Boot Heel volcanic field and coeval exten-
(Fig. 1). The data do not support models by differences in the timing of extension sion (Fig. 4; McIntosh and Bryan, 2000;
where the northern Rio Grande rift is sepa- from the Basin and Range Province to the Gavel, 2019), and extension had not yet initi-
rate from the Basin and Range of southern Rio Grande rift. ated in the Rio Grande rift. Evolution of the
New Mexico (dark orange in Fig. 1) or that Constraining the timing of extension boundary dramatically slowed from 25 to
the entire Rio Grande rift is the easternmost across the transition zone provides context 14 Ma when extension in both provinces
arm of the larger Basin and Range Province for how and when the boundary evolved. occurred. Thermochronologic data indicate
(dark green line in Fig. 1). Many of the Regionally, the easternmost metamorphic that the main phase of rapid extension ended
observed manifestations of a boundary in core complexes in southern Arizona are the at 14 Ma in the Basin and Range Province,
southern New Mexico can be understood in Pinaleño, active from 29 to 19 Ma (Long et although slower extension likely continued
the context of active extension in the south- al., 1995), and the Catalina-Rincon, active until 6 Ma. During this time, formation of
ern Rio Grande rift and a relative lack from 27 to 20 Ma (e.g., Davy et al., 1989). the boundary may have continued again as
thereof in the Basin and Range Province. Post-detachment extension in the region Rio Grande rift extension outpaced Basin
Active lithospheric extension produces persisted until the onset of seafloor spread- and Range Province extension. However, a
higher mantle conductance through partial ing in the Gulf of California at 6 Ma (e.g., crucial difference at this stage is that contin-
melting, higher strain rates, active faulting, Lizarralde et al., 2007). The sedimentary ued formation of the boundary was due to
young volcanism, thinner crust, decreased record of extension in southwestern New active extension to the east of the boundary
upper mantle velocities and densities, and Mexico is relatively unexplored compared (Rio Grande rift) rather than extension to the
possibly deeper basins within the southern to the central and northern segments of the west (Basin and Range Province). Formation
Rio Grande rift, where the westernmost Rio Grande rift because there are few out- of the boundary accelerated again at 6 Ma
expression of each of these features defines crops of Miocene and older basin strata. when Basin and Range Province extension
a 30–40-km-wide subvertical boundary Available data suggest that initial Basin and dramatically decreased and continued to the
that extends through the lithosphere (Fig. Range Province extension in this region present. Thermochronology is thus consis-
2). At the surface, we place the boundary at was under way during the Oligocene based tent with the available sedimentary record
the eastern edge of the transition zone, on thickness of sedimentary basin fill and and, when viewed within the context of the
because this coincides with changes in basin sparse age control from interbedded volca- regional tectonic framework, reveals impor-
depth, Quaternary faulting, volcanism, active nic rocks (Mack, 2004). Although main tant differences in the timing of extension
strain rates, and bulk crustal conductance extension is thought to have ceased by across the transition zone, where boundary
(Fig. 1). As active extension continues in the 6 Ma, minor Quaternary extension is evi- evolution occurred in two discrete pulses
southern Rio Grande rift, the boundary will dent by short fault segments that offset and continues today.
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