Lithospheric mantle duplex beneath the central Mojave Desert revealed by xenoliths from Dish Hill, California

Abstract

[1] Low-angle subduction of oceanic lithosphere may be an important process in modifying continental lithosphere. A classic example is the underthrusting of the Farallon plate beneath North America during the Laramide orogeny. To assess the relevance of this process to the evolution and composition of continental lithosphere, the mantle stratigraphy beneath the Mojave Desert was constrained using ultramafic xenoliths hosted in Plio-Pleistocene cinder cones. Whole-rock chemistry, clinopyroxene trace element and Nd isotope data, in combination with geothermometry and surface heat flow, indicate kilometer-scale compositional layering. The shallow parts are depleted in radiogenic Nd (e Nd = À13 to À6.4) and are interpreted to be ancient continental mantle that escaped tectonic erosion by low-angle subduction. The deeper samples are enriched in radiogenic Nd (e Nd = +5.7 to +16.1) and reveal two superposed mantle slices of recent origin. Within each slice, compositions range from fertile lherzolites at the top to harzburgites at the bottom: the latter formed by 25-28% low-pressure melt depletion and the former formed by refertilization of harzburgites by mid-ocean-ridge-basalt-like liquids. The superposition and internal compositional zonation of the slices preclude recent fertilization by Cenozoic extension-related magmas. The above observations imply that the lower Mojavian lithosphere represents tectonically subcreted and imbricated lithosphere having an oceanic protolith. If so, the lherzolitic domains may be related to melting and refertilization beneath mid-ocean ridges. The present Mojavian lithosphere is thus a composite of a shallow section of the original North American lithosphere underlain by Farallon oceanic lithosphere accreted during low-angle subduction.

CVM-S4.26

“…6.19). The SNB, together with the Southern California Batholith (SCB) (e.g., Luffi et al 2009) and the Peninsular Ranges Batholith (PRB) (e.g., Schmidt et al 2002) The S-velocity at 2-km depth in CVM-S4.26 shows good correlation with surface geology in this region (Fig. 6.19).…”

“…Laboratory measurements of P-velocity for the Pelona Schist range from 5.8 km/s for metaclastic rocks to 6.4 km/s for metabasaltic rocks (Pellerin and Christensen 1998) and there are indications that the proportion of the metabasalt increases with depth (e.g., Yan et al 2005). Geochemical data from xenoliths in the Mojave region show evidences of MORB-like material lying under western Mojave (Luffi et al 2009). It might be possible that substantial portions of oceanic crust from the Farallon plate were scrapped off and remain under western Mojave when the Farallon plate encountered the North American Craton during the Laramide shallow subduction (e.g., Luffi et al 2009).…”

“…6.2.6.1) and the San Gabriel terrane were immediately adjacent to the western Mojave block (e.g., Huffman 1972;Saleeby 2003). Collectively, the Mojave, Salinian, San Gabriel and the immediately adjacent southern Colorado River Desert basement complexes were termed the Mojave Desert-Salinian Batholith (MSB) in (Saleeby 2003) and the Southern California Batholith (SCB) in (Luffi et al 2009). Different from the Sierra Nevada Batholith to the north and the Peninsular Ranges Batholith to the south, the batholithic root under most of SCB was tectonically removed by the shallow segment of the Farallon plate during the Laramide orogeny from latest Cretaceous to early Paleogene (e.g., Coney and Reynolds 1977;Dickinson and Snyder 1978;Bird 1988;Saleeby et al 2003;Saleeby 2003).…”

“…6.79, I show comparisons of the S-velocity at 2-km depth in CVM-S4.26, CVM-S4 and CVM-H11.9 with a geologic map of the Mojave Desert region. Camp-Rock-Homestead Valley--Emerson Fault Zone might be due to the higher heat flow caused by Quaternary volcanism (e.g., Bonner et al 2003;Luffi et al 2009). Outcrops of the Pelona-Orocopia-Rand (POR) schists were found around the northern and southwestern boundaries of the Mojave block.…”

“…The tectonic erosion of the batholithic root and the denudation of the overlying mid-to upper-crustal felsic batholith tapered the thick batholithic column to a much thinner section and uplifted the remaining batholithic column and the underplated schist to upper-and mid-crustal depths (e.g., Malin et al 1996;Saleeby et al 2003;Saleeby 2003). From about 70 Ma to about 20 Ma, the mid-to lower-crust and mantle lithosphere under western Mojave were reconstructed (Luffi et al 2009). In Early Miocene between about 24 Ma and 18 Ma, the central Mojave experienced east-west extension and volcanism, which resulted in the central Mojave metamorphic core complex (e.g., Glazner et al 1989;Walker et al 1990;Martin et al 1993;Fletcher et al 1995;Glazner et al 2002).…”