Page 8 - GSATJanuary2020

emplaced into the central and eastern Mojave A 155 Ma Magmatic arc
Desert crust (Barth et al., 2008; Needy et al., Sr <0.706 Sr >0.706 x’
i
i
2009; Chapman et al., 2018). Following an x Farallon plate N. American crust MSL
Early Cretaceous lull in arc activity, the sub- ~ ~ ~ ~ ~ ~ ~ ~ ~~ ~ ~ ~ ~ ~ ~ ~ ~ 50
SCB root experienced a Late Cretaceous SOML ~ ~ ~ SCML
pulse of growth associated with increased 100
magmatism in the SCB (Wells and Hoisch, 150 km
2008; Needy et al., 2009; Chapman et al.,
2018), as recorded by ca. 80–70 Ma zircon B 120 Ma Magmatic lull in arc
ages in group 1 and 2 xenoliths. These Late
Cretaceous additions to existing SCB root ~ MSL
material and concomitant plutonism at higher SOML ~ ~ ~ ~ ~ ~ ~ 50
levels of the crust likely resulted from delam- SCML 100
ination and foundering of LC-SCML, desta- 150 km
bilized by slab shallowing-related lateral
stresses, and ensuing upwelling of hot asthe-
nosphere (Leventhal et al., 1995; Wells and C 75 Ma Delamination magmatism Nevadaplano
Hoisch, 2008). MSL
Shallow-angle subduction likely com- Shatsky conjugate
menced at the plate margin in the ca. 90–75 SOML LC-SCML failure 50
and translation
Ma time interval, based on plate motion 100
modeling and the timing of schist under- 150 km
plating (e.g., Grove et al., 2003; Liu et al.,
2010; Chapman, 2017). The leading edge of
the shallowly subducting segment likely D 65 Ma Laramide magmatism
reached the central Mojave Desert, then MSL
~500 km inboard from the margin, no later 50
than 70 Ma, assuming an orthogonal con-
verge rate of 100 km/m.y. (Engebretson et Imbricated SOML 100
al., 1985; Copeland et al., 2017). At this 150 km
point, schist underplating continued while
the slab shallowed quickly, shutting off E 20 Ma Incipient SAf CV, CC
magmatism in the SCB, dislodging its Ig. are-up; MCC CO Plateau
LC-SCML, and tectonically bulldozing MSL
these materials along the Moho and up to 50
Arclogite
~500 km farther inboard to the Colorado delamination 100
Plateau transition zone (Axen et al., 2018). Imbricated SOML 150 km
In the process, strong coupling along the
subduction interface drove significant
thickening of foreland crust and the forma- F Today SAf Basin and Range SC CO Plateau
tion of a vast orogenic plateau (DeCelles, Paci c plate ~ ~ ~ MSL
2004; Henry et al., 2012; Copeland et al., ~ ~ ~ ~ ~ ~ ~ ~ ~~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~ ~ ~ 50
2017; Chapman et al., 2018). (It should be 100
noted that lower lithosphere is intact NE of SOML
the Colorado Plateau transition zone; 150 km
hence, deformation along the Laramide Figure 4. Cross sections corresponding to x–x' locations in Figure 3. Vertical equals horizontal scale.
corridor as far inboard as the Black Hills of Colors correspond to those in Figure 3. See text for details. CC—Camp Creek; CO—Colorado; CV—
Chino Valley; Ig.—ignimbrite; LC—lower crust; MCC—metamorphic core complexes; MSL—mean sea
South Dakota was likely due to horizontal level; SAf—San Andreas fault; SC—San Carlos; SCML—sub-continental mantle lithosphere; SOML—
end-loading, as suggested by Livaccari and sub-oceanic mantle lithosphere.
Perry [1993].) Meanwhile, magmatism
swept inboard, forming ca. 75–55 Ma gra-
nitic stocks and associated copper mineral- A LASTING SCAR: CENOZOIC al., 1988; Erdman et al., 2016). This obser-
ization in the transition zone (Coney and DELAMINATION AND EXTENSION vation is readily explained by the well-
Reynolds, 1977; Vikre et al., 2014). We IN THE SW UNITED STATES documented Eocene to Miocene westward
suggest that the combination of magma- Xenolith garnet Sm-Nd and titanite U-Pb sweep of magmatism (e.g., Coney and
tism and crustal thickening–related radio- ages span the entire Paleogene, indicating Reynolds, 1977; Copeland et al., 2017),
genic heating precipitated Latest Cre- that displaced arclogite remained hot (>600 including the ca. 25 Ma arclogite xenolith-
taceous–early Cenozoic zircon growth in °C) for tens of millions of years following hosting latite, that accompanied rollback
dislodged arclogite. its dispersal (Dodson, 1973; Esperança et and tearing of the Farallon slab. This time

8 GSA Today | January 2020