Plateau uplift in western Canada caused by lithospheric delamination along a craton edge

Abstract

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 combination of lithospheric delamination, lower crustal flow and regional shortening 1-4 . Although the rates, timing and mechanisms driving plateau uplift remain controversial, lithospheric delamination continues 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 mant...

Large variations in lithospheric thickness of western Laurentia: Tectonic inheritance or collisional reworking?

“…3. In particular, the paths located in the cratonic region are characterized by pronounced high velocities, whereas the paths located in the Cordillera show conspicuous low velocities (Bao et al, 2014). Fig.…”

“…Cook, 1995). The Phanerozoic evolution of this part of our study region is the subject of a recent study using the same tomographic dataset considered here (Bao et al, 2014).…”

“…Next, we applied frequency-time analysis (Levshin and Ritzwoller, 2001) to the selected seismograms to verify the quality of the group-velocity spectrum and to identify the most reliable period range with robust dispersion measurements. Finally, we measured interstation phase velocity dispersion using a cross-correlation method (Bao et al, 2011(Bao et al, , 2014Yao et al, 2005Yao et al, , 2006. The robustness of measurements can be influenced by effects such as off-great-circle propagation and scattering (Yoshizawa and Kennett, 2002).…”

Peering beneath the Canadian crust

“…This pulls the edge of the root downwards (e.g. the Canadian Cordillera, Bao et al 2014) into the convecting mantle.Key to understanding the formation and evolution of cratonic roots is detailed knowledge of their deep structure, but this cannot be achieved easily with traditional field geology alone. A crucial tool in unravelling the evolutionary history of cratonic roots is seismology.…”

“…This pulls the edge of the root downwards (e.g. the Canadian Cordillera, Bao et al 2014) into the convecting mantle.…”