ORIGIN OF THIN NORTHERN CANADIAN CORDILLERA LITHOSPHERE: RECENT DELAMINATION?

The Northern Canadian Cordillera (NCC) is a high-elevation accretionary orogen (~1.5 km above sea level) that formed through a series of terrane accretion and transpression events. The eastern limit of the NCC terranes is the Tintina Fault, to the east of which is the Mackenzie Mountains (MM) foreland belt and stable North American Craton (NAC). Geophysical observations indicate a change in lithosphere structure from the thin (~55 km) hot lithosphere below NCC to thicker lithosphere (>200 km) below NAC. The NCC is also characterized by the Northern Cordilleran Volcanic Province (NCVP), consisting of alkaline basalts with ages < 20 Ma. The origin of the thin NCC lithosphere is enigmatic. One proposal is that it is a long-lived feature associated with subduction; alternately, it may have formed more recently as a consequence of gravitational thinning. Here, we use 2D thermal-mechanical numerical models (SOPALE code) to investigate the dynamics of lithosphere thinning through delamination. The models start with a laterally variable lithosphere thickness (100 km for NCC vs. 200 km for NAC). Gravitational instability is initiated with a pre-existing lithospheric weak zone at the edge of the NCC. As this founders, it triggers detachment of the deep NCC lithosphere within the weak lower crust. This is aided by the presence of dense eclogite within the NCC crust. Delamination leads to thinning across the entire width of the NCC within ~5 Myr. A delamination event initiating at ~15 Ma can explain the present-day thin NCC crust and lithosphere, high surface heat flow, Moho temperature, and seismic structure. The models also predict that delamination is accompanied by magmatism, where magmas are primarily derived from mantle decompression melting and consistent with NCVP volcanism. Further, the delaminating lithosphere induces stresses in the overlying continent, which may explain ongoing deformation and seismicity in the MM (>600 km from the nearest plate boundary).