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Fate of the Lower Lithosphere

during Shallow-Angle Subduction:

The Laramide Example

Alan D. Chapman, Ojashvi Rautela, Geology Dept., Macalester College, St. Paul, Minnesota 55105, USA; Jessie Shields, Department
of Earth and Environmental Sciences, California State University, Fresno, California 93740, USA; Mihai N. Ducea, Dept. of
Geosciences, University of Arizona, Tucson, Arizona 85721, USA, and Faculty of Geology and Geophysics, University of Bucharest,
010041, Bucharest, Romania; and Jason Saleeby, Division of Geological and Planetary Sciences, California Institute of Technology,
Pasadena, California 91125, USA

ABSTRACT abundance of spinel peridotite xenoliths in and Shatsky conjugates, which were em-
Continental arc lower crust and underly- ca. 15 Ma and younger volcanic host rocks bedded in the Farallon plate as they sub-
ing mantle wedge assemblages native to the and the presence of a vertical high-seismic- ducted in Laramide time (Saleeby, 2003;
Mojave Desert were dislodged, transported velocity anomaly beneath the western Liu et al., 2010). The damage zone con-
eastward during Laramide shallow-angle Colorado Plateau suggest that arclogite has sists of the southern California batholith
subduction, and attached to the base of the been foundering into the mantle and being (SCB) of the Mojave Desert and the south-
Colorado Plateau transition zone (central replaced by upwelling asthenosphere since ernmost Sierra Nevada batholith (SNB;
Arizona, USA) and further inboard. We the early Miocene. Fig. 1, inset).
identify here two late Oligocene xenolith As emphasized below, the impact of oce-
localities from the transition zone (Camp INTRODUCTION AND anic plateaux is consistent with evidence
Creek and Chino Valley) that likely contain BACKGROUND for the shutdown of arc magmatism, deep
remnants of the missing Mojave litho- crustal exhumation, tectonic underplating
sphere. Geochemical, isotopic, and ther- The SW North American Cordillera of trench sediments, and—the focus of this
mobarometric data from garnet clinopy- The Laramide orogeny was a regional research—removal of the LC-SCML (e.g.,
roxenite, the dominant xenolith type at compressional event that evolved from Saleeby, 2003; Luffi et al., 2009; Chapman
both studied localities, strongly suggest a Late Cretaceous–early Paleogene contrac- et al., 2012; Chapman, 2017; Ducea and
continental arc residue (“arclogite”) rather tion of the SW margin of North America Chapman, 2018).
than a lower plate subduction (“eclogite”) to Eocene–early Oligocene deformation
origin. Zircon grains extracted from these up to 2,000 km inboard in the craton inte- Overview of the SCB Domain of the
nodules yield a bimodal age distribution rior (Saleeby, 2003; DeCelles, 2004; Laramide Corridor
with peaks at ca. 75 and 150 Ma, overlap- Copeland et al., 2017). A commonly cited The formerly contiguous SNB–SCB–
ping ages of continental arc magmas mechanism for the orogeny is intensified Peninsular Ranges batholithic belt was a
emplaced into the Mojave Desert (the traction and tectonic erosion of the lower- >2000-km-long NNW-trending granitic
southern California batholith) and suggest- most crust and upper subcontinental man- arc emplaced largely during three mag-
ing a consanguineous relationship. In con- tle lithosphere (LC-SCML) due to flatten- matic “flare-up” events at ca. 230–210 Ma,
trast, Mesozoic and early Cenozoic igneous ing of an ~500-km-wide segment of the ca. 160–150 Ma, and ca. 100–75 Ma (e.g.,
rocks from SW Arizona, with age peaks at subducting Farallon plate (Livaccari et al., Ducea, 2001). In contrast to the SNB to the
ca. 60 and 170 Ma, do not provide as close 1981; Bird, 1988; Saleeby, 2003; Axen et north and the Peninsular Ranges batholith
a match. In light of these results, we sug- al., 2018). Parts of the central Andean oro- to the south (Fig. 1), much of the ~500-km-
gest that a mafic keel to the southern gen are regarded as the best modern ana- long SCB is rootless, lying tectonically
California batholith: (1) formed in two dis- logue, where shallow slab segments coin- above underplated trench assemblages (the
crete (Late Jurassic and Late Cretaceous) cide with colliding aseismic ridges and Rand and related schists) that were trans-
pulses; (2) was transported along the Moho oceanic plateaux (e.g., Gutscher et al., ported inboard by shallow-angle subduc-
~500 km eastward along the leading edge 2000). Analysis of plate reconstructions tion (Jacobson et al., 1988; Grove et
of the shallowly subducting Farallon plate; for the Pacific-Farallon ridge led to the al., 2003; Chapman, 2017; Ducea and
and (3) was affixed to the base of the crust interpretation that the Laramide orogeny Chapman, 2018). These schists are exposed
in central Arizona. Titanite U-Pb and gar- resulted from the subduction of conjugate in the footwall of the shallowly dipping
net Sm-Nd ages spanning ca. 60–30 Ma massifs to the Hess and Shatsky oceanic Rand fault, interpreted as a remobilized
suggest that displaced arclogite remained plateaux (Livaccari et al., 1981; Liu et subduction megathrust (e.g., Cheadle et al.,
at >600 °C for tens of millions of years fol- al., 2010). Furthermore, an ~500-km-wide 1986; Chapman, 2017), beneath deep
lowing its dispersal and until entrainment Laramide deformation corridor parallels crustal level SCB assemblages and the
in host latite. The lack of arclogite and the subduction trajectory of inferred Hess southern SNB (Fig. 1).

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