Complex subduction and small-scale convection revealed by body-wave tomography of the western United States upper mantle Brandon Schmandt ⁎, Eugene Humphreys Department of Geological Sciences, University of Oregon, Eugene, OR 97403, USA abstract article info Article history: Received 15 April 2010 Received in revised form 21 June 2010 Accepted 24 June 2010 Available online 16 July 2010 Editor: R.W. Carlson Keywords: upper mantle tomography subduction small-scale convection New high-resolution P- and S-wave tomography of the United States upper mantle from the Pacific Coast to the Great Plains reveals strong multi-scale heterogeneity closely correlated with tectonic and magmatic activity. We invert teleseismic travel-time residuals from the EarthScope Transportable Array and more than 1700 additional temporary and permanent stations for 3-D velocity perturbations to a depth of 1000 km. The inversion uses recent advances in western U.S. crust models to better isolate the mantle component of travel- time residuals, and frequency-dependent 3-D sensitivity kernels to map travel-time residuals, measured in multiple frequency bands, into velocity structure. In addition to separate V P and V S models, we jointly invert the two datasets for V P /V S perturbations by imposing a smoothness constraint on the δlnV S /δlnV P field. The joint inversion helps us identify regions where partial melt is probable. The amplitude of V P ,V S , and V P /V S variations is greatest in the upper 200 km of the mantle and the form of velocity anomalies suggests a provincially heterogeneous lithosphere and the occurrence of widespread small-scale convection. Partially molten mantle is inferred beneath Yellowstone and the eastern Snake River Plain (SRP), the Salton Trough, and the Clear Lake volcanic field. The inferred depth extent of partial melt is consistent with a generally hydrated upper mantle and elevated temperatures beneath the eastern SRP and Yellowstone. Despite continuous subduction since the Cretaceous, the distribution of sub-lithospheric high-velocity anomalies is dissected (similar to other recent studies). Based on our new tomography models, western U.S. geologic history, and plate–tectonic reconstructions, we infer patchy and incomplete removal of the flat-subducting Laramide slab and slab tearing associated with Eocene accretion in the northwestern U.S. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The western United States provides an excellent region for advancing our understanding of mantle heterogeneity caused by subduction (Sigloch et al., 2008), small-scale convection of the lithosphere (Bird, 1979; Zandt et al., 2004), and plumes (Schutt and Dueker, 2008; Smith et al., 2009) both because these processes appear to be active there and because the seismic data are unparalleled. In particular, the transportable array (TA) component of EarthScope's USArray is providing a fundamental advance in data coverage, allowing for continuous high-resolution imaging from the Pacific coast to the western Great Plains. Seismologists have long recognized that subduction beneath western U.S. occurs into an anomalously low-velocity upper mantle (Romanowicz, 1979; Grand, 1994); on average, mantle seismic velocities between 100 and 200 km depth are among the lowest on Earth (Lebedev and van der Hilst, 2008). Imaged within this generally low-velocity mantle are small-scale high- velocity structures that exhibit seismic contrasts as great as that observed between average craton and the tectonically active western U.S. (Humphreys and Dueker, 1994). That the western U.S. upper mantle is vigorously active is implicated by geologic study, which has shown that the western third of the U.S. is undergoing post-Laramide orogenic collapse with accompanying volcanism (Coney and Harms, 1984; Burchfiel et al., 1992), and it has been uplifted into one of Earth's great plateaus. The elevated western U.S. interior is comprised of distinctive tectonic and geomorphic provinces (Fig. 1), including the highly extended and magmatically altered Basin and Range, the Laramide-contracted and unextended Colorado Plateau and Rocky Mountains, and the tilted and intact Great Plains. It appears that large portions of the Great Plains, Rocky Mountains and Colorado Plateau have been uplifted in part since the Laramide orogeny (Heller et al., 2003), with evidence for uplift continuing to the present (Riihimaki et al., 2007; Karlstrom et al., 2008). This indicates young and ongoing mass redistribution at depth. Ongoing transition of the westernmost North America plate margin from subduction to transform (Atwater, 1970) has been used to predict a triangular slab-free area beneath most of the Basin and Range and southern Rocky Mountains (Dickinson and Snyder, 1979), although geologic evidence for complex subduction since ~ 80 Ma suggests that the actual slab distribution may be more complicated Earth and Planetary Science Letters 297 (2010) 435–445 ⁎ Corresponding author. E-mail address: bschmand@uoregon.edu (B. Schmandt). 0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2010.06.047 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl