Mantle structure and dynamics at the eastern boundary of the northern Cascadia backarc

• Published in: Journal of geodynamics 2023-03, Vol.155, p.101958, Article 101958
• Main Authors: Currie, Claire A., Mallyon, Deirdre A., Yu, Tai-Chieh, Chen, Yunfeng, Schaeffer, Andrew J., Audet, Pascal, Gu, Yu Jeffrey
• Format: Article
• Language: English
• Subjects: Backarc; Canadian cordillera; Cascadia; Craton; Edge-driven convection; Numerical models
• ISSN: 0264-3707
• DOI: 10.1016/j.jog.2022.101958

The tectonics of southwestern Canada are dominated by the Cascadia subduction zone. The northern Cascadia backarc encompasses a > 400 km wide region of the Southern Canadian Cordillera. Geophysical observations, including seismic tomography and surface heat flow, show that the backarc is characterized by a hot, thin lithosphere (60–70 km). The eastern limit of the backarc approximately underlies the Rocky Mountain Trench, where there is an abrupt eastward increase in lithosphere thickness to the ∼250 km thick North American (Laurentian) Craton. Seismic tomography studies show that the transition in lithosphere thickness occurs over a horizontal distance of 50–100 km, resulting in a subvertical to west-dipping lithosphere step, with a dip angle of 75–90°. Using numerical models, we show that such a structure can be readily destabilised by internal buoyancy forces, edge-driven convection, and shearing by regional mantle flow. To maintain a subvertical step for > 50 Myr, the lowermost craton mantle lithosphere must be both dry and moderately chemically depleted. The observed westward dip may reflect partial lateral extrusion of the lowermost craton lithosphere, as well as shearing from west-directed mantle flow associated with the Cascadia subduction zone. The models also show that the backarc mantle must be relatively weak, such that vigorous convection maintains the hot, thin lithosphere. This also provides a mechanism to explain the observed lateral seismic gradient between the low-velocity backarc mantle and high-velocity craton. Our models demonstrate that the eastern limit of the Cascadia backarc is a region of active mantle flow, including possible slow deformation of the craton edge. •The northern Cascadia backarc is bounded by a ∼190 km step in lithosphere thickness.•The step induces vigorous mantle convection, promoting high backarc temperatures.•Step stability depends on the composition of lowermost craton lithosphere.•Step geometry consistent with dry, depleted craton and west-directed mantle flow.