LETTERS PUBLISHED ONLINE: 19 OCTOBER 2014 | DOI: 10.1038/NGEO2270 Plateau uplift in western Canada caused by lithospheric delamination along a craton edge Xuewei Bao * , David W. Eaton and Bernard Guest Continental plateaux, such as the Tibetan Plateau in Asia and the Altiplano–Puna Plateau in South America, are thought to form partly because upwelling, hot asthenospheric mantle replaces some of the denser, lower lithosphere 1–4 , making the region more buoyant. The spatial and temporal scales of this process are debated, with proposed mechanisms ranging from delamination of fragments to that of the entire lithosphere 1–4 . The Canadian Cordillera is an exhumed ancient plateau that abuts the North American Craton 5 . The region experienced rapid uplift during the mid-to-late Eocene, followed by voluminous magmatism 6 , a transition from a compressional to extensional tectonic regime 7 and removal of mafic lower crust 8 . Here we use Rayleigh-wave tomographic and thermochronological data to show that these features can be explained by delamination of the entire lithosphere beneath the Canadian Cordillera. We show that the transition from the North American Craton to the plateau is marked by an abrupt reduction in lithospheric thickness by more than 150 km and that asthenosphere directly underlies the crust beneath the plateau region. We identify a 250-km-wide seismic anomaly about 150–250 km beneath the plateau that we interpret as a block of intact, delaminated lithosphere. We suggest that mantle material upwelling along the sharp craton edge 9 triggered large-scale delamination of the lithosphere about 55 million years ago, and caused the plateau to uplift. Orogenic plateaux are broad, high-standing, low-relief regions that develop in mature continental mountain belts. Plateaux are important because they affect climate 2 , orogenesis 3 and tectonic plate interactions 10 . Modern examples include the Tibetan Plateau and the Altiplano, which developed in the Cenozoic by some combi- nation of lithospheric delamination, lower crustal flow and regional shortening 1–4 . Although the rates, timing and mechanisms driving plateau uplift remain controversial, lithospheric delamination con- tinues to be a leading mechanism explaining their formation and maintenance, albeit with debated temporal and spatial scales 1–4 . Fossil plateaux can provide important insights into orogenic processes. In the Eocene, the interior of the Canadian Cordillera was perhaps the highest-standing mountain belt on Earth 11 and, in terms of crustal architecture, is one of the best-studied recent orogens. In this region, thickened crust that developed during orogenesis is no longer present 12 ; rather, present-day mountainous topography reflects the isostatic response to regional thermal structure 5 . In this study, we integrate published thermochronology results with new high-resolution teleseismic tomography. We use fundamental-mode Rayleigh waves recorded during the period 2006–2013 at 86 broadband seismograph stations to construct a new three-dimensional tomographic shear-velocity (V s ) model to depths of ∼300 km. Figure 1a shows V s at 105 km depth extracted from our tomographic model, highlighting an abrupt transition from the high-velocity mantle of the North American Craton to the low-velocity mantle of the Cordillera. The boundary between these domains coincides with the southern Rocky Mountain Trench (RMT), a conspicuous topographic lineament (Fig. 1b). In cross- section (Fig. 2), the craton edge seems to be subvertical, delineating a step change in thickness of the seismological lithosphere from >200 km beneath the craton to <50 km beneath the Cordillera. As shown in the Supplementary Information, the geometrical expression of this feature is virtually unchanged along the length of the RMT; moreover, synthetic tests, coupled with phase-velocity dispersion curves for closely separated paths on either side of the RMT, show that the location, orientation, depth extent and velocity contrast of the craton edge are robust elements of our tomographic model. The RMT also coincides with a major change in upper-mantle composition and surface heat flux 5 . The change in thermal state of the crust is clearly expressed by truncation of aeromagnetic anomalies (Fig. 1c), which originate from magnetized domains of Precambrian age at depths of 20–25 km in the craton 13 . Although structural interpretations of crustal seismic data indicate that corre- sponding Precambrian domains extend as a vestigial wedge in the lower crust for hundreds of kilometres west of the RMT (ref. 12), the termination of magnetic anomalies occurs where crustal tem- perature surpasses the Curie limit for magnetite (585 ◦ C), consistent with nearby xenolith data 14 . Furthermore, spinel lherzolite xenoliths show that, west of the RMT, the upper mantle is fertile in composi- tion 14 (that is, similar to Mid-Ocean Ridge Basalt, MORB), in con- trast to melt-depleted compositions of nearby cratonic xenoliths 15 . Medium- to high-temperature thermochronological data record variations in cooling rate that constrain exhumation history across the RMT. Ar/Ar and K/Ar cooling ages from hornblende, biotite and k-feldspar, and fission track ages from zircon and apatite (see Supplementary Information) show that, west of the RMT, the Cordillera experienced rapid cooling (∼10–20 ◦ C Myr −1 ) from ∼500 ◦ C to ∼100 ◦ C during the Eocene (about 56–34 Ma). In contrast, fission track ages from zircon and apatite to the east of the RMT, in the Late Cretaceous to Eocene Foreland Belt that accommodated ∼180 km of shortening 16 , show relatively slow cooling (1–2 ◦ C Myr −1 ) from ∼110 ◦ C to 20 ◦ C between ∼80 Ma and ∼25 Ma. Moreover, cooling ages obtained for the footwalls and hanging walls of major Eocene normal faults west of the RMT are synchronous, within analytical uncertainty 17 , implying that the Eocene cooling in the Cordillera was primarily caused by erosion associated with large-scale plateau uplift rather than local tectonic unroofing. The period of rapid cooling west of the RMT coincides with the 59–52 Ma onset of regional extension and magmatism in the Cordillera, whereas thrusting continued until ∼50 Ma in the Foreland Belt; extension in the Foreland Belt started at 52–49 Ma (ref. 16). This overlap of extension in the hinterland west of the RMT, with contraction in the foreland to the east of the RMT, is similar to what is observed around the actively extending Tibetan Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada. *e-mail: xubao@ucalgary.ca 830 NATURE GEOSCIENCE | VOL 7 | NOVEMBER 2014 | www.nature.com/naturegeoscience © 2014 Macmillan Publishers Limited. All rights reserved