Geodynamic studies of southwestern Canada: Subduction zone processes and backarc mantle dynamics

Abstract

Geodynamic models allow insights into the processes that control lithosphere structure and evolution. Here, we highlight geodynamic studies along a profile through southwestern Canada, from the Cascadia subduction zone into the Laurentian craton. Geophysical and geological observations show distinct changes in thermal structure along this profile. One major change is between the cool forearc and hot volcanic arc. This marks the transition from (1) a stagnant forearc mantle that is cooled by the subducting Juan de Fuca Plate to (2) an advection-dominated arc region, where high temperatures arise from mantle flow driven by the subducting plate (corner flow). High temperatures occur for 400-500 km east of the arc to the Rocky Mountain Trench (RMT), where lithosphere thickness increases from 60-70 km below the Cordillera to >200 km below the craton. The timing of Cordillera lithosphere thinning is debated. A long-lived (>100 Ma) thin lithosphere inherited from earlier tectonics requires vigorous convection of a weak, hydrated mantle. Conversely, thinning may have occurred through gravitational removal of the lower lithosphere in the Eocene. Models show that a removal event only allows for a short-lived thin lithosphere (~25 Myr), owing to conductive cooling. Even if there was Eocene delamination, the present-day thin lithosphere requires small-scale convection in the Cordillera mantle. The thermal contrast across the RMT is enhanced by edge-driven convection at the Cordillera-craton lithosphere step. The step itself is an enigmatic feature, and its long-term preservation requires that the craton mantle lithosphere is strong (dry) with moderate chemical depletion.