GSA 2019 Annual Meeting Wrap-Up VOL. 30, NO. 1 | JANUARY 2020 Fate of the Lower Lithosphere during Shallow-Angle Subduction: The Laramide Example 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). GSA Today, v. 30, https://doi.org/10.1130/GSATG412A.1. Copyright 2019, The Geological Society of America. CC-BY-NC. 120˚W 116˚W 112˚W 38˚N COO SNBNBB pplateaplateaulaateau 36˚N SaliniS a alin CV a CC 34˚N B Xenolith locations B: Big Creek C: Cima Cv Cc: Camp Creek Cc Cv: Chino Valley D: Dish Hill C D X: Crystal Knob Mz SNBSNB - SCB EarlyEarly Cz subducsubduction megamegathrustthrust MzMz underplatedunderplated scschistshists X Late KsK subducubducttionion Cratonic/innerCratonic/inner passive marmargingin cruscrustt cchannelhannel bouboundariesndaries UnderplatedUnderplated Farallonrallon mantmantlele lithospherlithosphere Late K normal ffaultsaults MMzz wewedgedge + pC SCML (ora(orange)nge) + Mz arclogitearclogite (red)(red) Inactive retro-arc tthrustshrusts UUpperpper Mz forearcrearc basin (li(light)ght) + Jurassic ophiolite (darkk)) Figure 1. Fence diagram showing idealized lithospheric structure for beginning of Cenozoic time for sections across Sierra Nevada batholith (SNB)–Great Valley forearc, southern California batholith (SCB)–Colorado Plateau transition zone, and linking section across the southern SNB. Locations of sections shown on inset. Cz—Cenozoic; CO—Colorado; K—Cretaceous; Mz— Mesozoic; pC—Precambrian; SCML—subcontinental mantle lithosphere. The SNB is tilted into a southward deepen- the central Mojave demarcating a western eNd = −6.4 to −13.0 (Fig. 1; Luffi et al., ing section spanning paleodepths of ~10–35 schist-bearing domain, and an eastern 2009). These data indicate that ancient km (e.g., Nadin and Saleeby, 2008). Structural domain lacking significant schist and con- LC-SCML was not entirely sheared off and petrologic relations in the SCB and taining remnants of ancient LC-SCML from beneath the eastern to central Mojave southernmost SNB indicate that the base of (Fig. 1). region by Laramide flat-slab subduction. the batholith and underlying LC-SCML was Remnants of pre- to syn-Laramide man- sheared off at 30–35 km depth and replaced Laramide Imprints in Xenoliths tle lithosphere that constituted the mantle with trench materials (Grove et al., 2003; Xenolith suites of the SW Cordillera wedge for the SNB are present in late Saleeby, 2003; Chapman, 2017). record development of the LC-SCML prior Miocene xenolith suites from the central What was the fate of the sub-SCB to, during, and following the Laramide SNB (e.g., Ducea and Saleeby, 1998; LC-SCML? Do remnants of the displaced event. Proterozoic upper mantle and lower Ducea, 2001; Chin et al., 2012). Pressure- material exist, and if so, what is the rela- crustal xenoliths in the Colorado Plateau temperature-time constraints indicate that tionship between LC-SCML remnants and vicinity, in conjunction with Nd isoto- this fossilized mantle wedge extended to and underplated schist? Seismic data and pic data on mafic volcanic rocks of ~125 km depth and cooled rapidly follow- receiver function analysis provide some the region, record local preservation of ing the Late Cretaceous (Laramide) termi- answers to these questions, linking sur- LC-SCML beneath the region through nation of magmatism (e.g., Ducea and face exposures of schist directly to a Laramide time (Livaccari and Perry, 1993). Saleeby, 1998; Saleeby et al., 2003; Chin et regional flat fabric with NE-SW seismic Xenolith suites from the eastern and cen- al., 2012). Peridotites and garnet websterite anisotropy beneath thin (~30 km) Mojave tral Mojave region also provide evidence for dominate the base of the section and grade crust (Cheadle et al., 1986; Porter et al., underlying remnants of ancient LC-SCML. upward into an ~45-km-thick zone of gar- 2011). Additional constraints from geo- First, spinel peridotites from the Pliocene– net clinopyroxenite followed by garnet chemical data reveal an ~N-S–trending Quaternary Cima cones yield Re-Os model granulite at ~40 km paleodepth. Trace- boundary at ~116°W, west of which lacks a ages of 1.8–3.4 Ga (Fig. 1; Lee et al., 2001), element data and Sm-Nd