Page 6 - i1052-5173-31-10+AR

Longitude from 240 to 50 °C (Guenthner et al., 2013).
109°W 108°W 107°W 106°W 105°W
1 A Mogollon Datil volcanic field Rio Grande rift Great Plains Compiled ZHe dates (Biddle et al., 2018;
Gavel, 2019; Reade et al., 2020) show dra-
Profile parallel rate (mm/yr) -1 0 1.45 ± 0.31 nstr/yr zone 8.54 ± 2.10 nstr/yr 0.68 ± 0.17 nstr/yr matic differences across the transition zone
and Basin and Range
with relation to eU (Fig. 3). In the Basin and
-2
all eU values. In contrast, east of and includ-
800 Range Province, ZHe dates are consistent for
700 B Basin and Range Rio Grande rift
600 ZHe Date (n = 118) transition ing the transition zone, ZHe dates have a wide
range, where oldest ZHe dates are correlated
500
Thermochronologic Date (Ma) 200 AFT Date (n = 65) 150 ± 174 Ma highest eU values. This observation suggests
400
with lowest eU and youngest ZHe dates have
AHe Date (n = 129)
300
that radiation damage in zircon is a primary
100
control on ZHe dates in this region.
80
Forward modeling allows for the calcula-
tion of ZHe date-eU curves from an input
25.4 ± 5.9 Ma
60
40
of testing the potential effects of reheating
during magmatism (see supplemental mate-
20 26.8 ± 5.7 Ma 22.2 ± 17.5 Ma thermal history, and here it provides a means
20.5 ± 7.2 Ma
15.2 ± 8.7 Ma rial for complete modeling details ). We use a
1
0
2 C basin boundary general thermal history of southern New
4 velocity (km/s) Mexico that includes crystallization at 1.6 Ga
6 Tomographic velocity model
8 234567 (Averill and Miller, 2013) and cooling to 350 °C at 1.45 Ga, based on
10 3 3 40 39
Ar/ Ar muscovite data (Amato et al., 2011),
velocity
20 velocity 2740 kg/m 3 2880 kg/m 3 2700 kg/m moho 15 °C at 500 Ma based on the age of the over-
(km/s)
(km/s)
2880 kg/m
30
Depth (km) 40 6 7 8 3280 kg/m 3 3250 kg/m 3 Anisotropic electrical resistivity lying Bliss Formation, and maximum reheat-
Velocity and gravity
models (Averill, 2007)
50
ing to 150 °C at 80 Ma from accumulation of
100 model oriented perpendicular to Paleozoic and Mesozoic sediment (Fig. 3A).
partial melt the rift axis (Feucht et al., 2019) We include two endmember Proterozoic cool-
log (resistivity (Ωm)
150 10 ing histories: multiple cooling pulses during
0 1 2 3 assembly of Rodinia (path 1; Ricketts et al.,
200 2021), and multiple pulses of cooling that
109°W 108°W 107°W 106°W 105°W coincide with assembly and then breakup of
Longitude
Rodinia (path 2; DeLucia et al., 2017).
Figure 2. E-W cross section across the transition. (A) Global positioning system velocities across the Resulting ZHe date-eU curves are roughly
profile line shown in Figure 1 (Murray et al., 2019). (B) Thermochronologic dates for all samples shown
in Figure 1. Average dates are calculated for each data set in the Basin and Range Province and Rio similar to observed ZHe dates for the
Grande rift (±1 standard deviation). (C) Stacked geophysical models for the crust and upper mantle. southern Rio Grande rift regardless of the
Note changes in scale with depth. AFT—apatite fission-track; AHe—apatite (U-Th)/He; ZHe—zircon
(U-Th)/He; nstr/yr—nanostrain/year. Proterozoic cooling history. Boot Heel volca-
nic field magmatism in southwestern New
Magnetotelluric data collected along an THE THERMAL IMPRINT OF Mexico occurred from 37 to 26 Ma based on
E-W transect through southern New Mexico A BOUNDARY 40 Ar/ Ar sanidine geochronology (McIntosh
39
show that the upper mantle is moderately ZHe dates from the Basin and Range and Bryan, 2000), and we test the effects of
resistive (30–100 Ωm; Feucht et al., 2019). Province (Fig. 2B) largely overlap with ages this event on ZHe dates (Fig. 3B). Calculated
The one exception is a zone age conductive of volcanic rocks in southwestern New ZHe date-eU curves only match the observed
material centered on 107.5°W at the eastern Mexico, suggesting they were likely reset by data for reheating temperatures of >225 °C
margin of the transition zone that is excep- magmatism (Gavel, 2019), and we use these and indicate that this thermal event did not
tionally pronounced at depths of 50–100 km data to model the heating effects of Oligocene affect ZHe dates in the southern Rio Grande
and that may also extend to depths >200 km magmatism in this region. Individual zircon rift. These results suggest that late Oligocene
(Fig. 2C). This feature is interpreted to be a grains from a single sample have variable clo- magmatism imprinted a major thermal
zone of lithospheric decompression melting sure temperatures due to accumulation of boundary that coincides with independent
(Feucht et al., 2019). Interestingly, this region varying amounts of radiation damage that is geologic and geophysical data sets.
of decompression melting is slightly asym- proportional to eU (eU = U + 0.235Th), where
metric beneath the southern Rio Grande rift low eU, high He retentivity grains typically DISCUSSION
and skewed to the west, as opposed to more correspond to oldest ZHe dates and high eU,
symmetric lithospheric thinning and mantle low retentivity grains yield youngest ZHe Age and Evolution of the Boundary
upwelling in central New Mexico (Wilson dates (Guenthner et al., 2013). These proper- The collective data sets suggest that the
et al., 2005). ties allow for thermal modeling of ZHe data southern Rio Grande rift is best explained
1 Supplemental Material. Full description of zircon (U-Th)/He modeling, inputs, and assumptions. Go to https://doi.org/10.1130/GSAT.S.14794197 to access the supple-
mental material; contact editing@geosociety.org with any questions.
6 GSA Today | October 2021