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W E Our analysis in southern New Mexico is
35-25 Ma: Initial extension/magmatism in the Basin and Range essentially a 2D cross-sectional view of the
boundary, and further data sets to the south
Magmatism resets
ZHe dates in are needed to test whether the transition zone
upper crust crust has an overall NS or NW trend and coincides
with major Proterozoic boundaries. Based on
available data, we therefore suggest that ini-
lithospheric mantle tial development of the boundary occurred
Region of during the late Eocene with Basin and Range
decompression Boundary emerges at the eastern Province extension and resulted from sepa-
melting edge of extension and volcanism. rate driving mechanisms from the Basin and
25-6 Ma: Active extension in Rio Grande rift and Basin and Range Range Province to the Rio Grande rift.

Preservation Potential in the
Initial Rio Grande rift extension at 25 Ma
Geologic Record
If active extension is the underlying cause
Mid-crustal
Crustal thinning beneath the intrusion for most of the observed differences across
axis of the Rio Grande rift the transition, then some boundary features
No growth of the boundary due to lack of magmatism coupled are transient, such as differences in heat
with active extension on either side of the boundary. Basin flow, conductance, and upper mantle veloci-
and Range extension slows after 14 Ma.
ties and densities on either side of the bound-
ary. These features will likely vanish when
Present: Active extension in the Rio Grande rift high heat flow extension ceases. In contrast, permanent
Quaternary boundary features include changes in basin
faulting and
volcanism depth, changes in the style of extension
mid crustal (presence or absence of metamorphic core
welt complexes), different patterns in volcanism,
Boundary is Region of differences in the timing of faulting, and the
warm Moho
well-established decompression (900-1000 °C) thermal imprint on thermochronologic data
and composed of transition zone melting
both transient and warm upper sets (Fig. 4). These permanent features will
permanent features mantle become more pronounced as Rio Grande rift
extension continues. Permanent and tran-
Future: No extension in the Rio Grande rift or Basin and Range
sient boundary features are similar to the Rio
deeper Grande rift boundary in central New Mexico.
basins
In this well-defined segment of the rift, the
boundary is demarcated by differences in
thinned crust mantle velocities (West et al., 2004), crustal
younger older thickness (Wilson et al., 2005), surface heat
ZHe dates ZHe dates flow (Reiter et al., 2010), and extensional
at surface at surface
Boundary is defined by basins bounded by normal faults. The transi-
permanent crustal features tion from thinned lithosphere with these
characteristics to adjacent unaffected litho-
Figure 4. Schematic W-E cross sections showing the evolution of the Basin sphere over short distances is a classic and
and Range Province–Rio Grande rift boundary. ZHe—zircon (U-Th)/He.
definitive description of a continental rift
boundary, and this has been documented in
These independent data sets document the and Range Province was more influenced other rifts worldwide (e.g., Achauer and
timing of formation of a near-vertical, litho- by plate boundary effects (Bird, 2002). Masson, 2002; Corti, 2009).
spheric-scale boundary in southern New Cenozoic development of the boundary may Lithospheric-scale boundaries are long-
Mexico, but do not address the origins of also have been superimposed upon N-S– lived features of continents that can form
this feature. It may have emerged during trending or NW-trending extensional Neo- through a multitude of major tectonomag-
Oligocene magmatism in the Boot Heel vol- proterozoic structures that likely formed matic events. Once established, these fea-
canic field, which modified the chemical within an overall convergent tectonic setting tures are prone to reactivation (e.g., New
structure of the lithosphere and created a during Grenville orogenesis (e.g., Karlstrom Madrid fault zone; Hurd and Zoback, 2012)
sharp thermal gradient that influenced and Humphreys, 1998; Timmons et al., and are therefore influential in guiding the
extensional tectonism on either side. This 2001). However, evidence for their existence style and geometry of future deformation
model may indicate separate driving mecha- or history is cryptic in southwestern New (Karlstrom and Humphreys, 1998). Across
nisms, where mantle processes are responsi- Mexico, and such structures have been more the Colorado Plateau, Rocky Mountains, and
ble for Rio Grande rift extension and mag- thoroughly documented in central and north- Midcontinent regions, there are numerous
matism (Ricketts et al., 2016) and the Basin ern New Mexico (Karlstrom et al., 2004). examples of Ancestral Rocky Mountain and

8 GSA Today | October 2021