Exploring the Moho beneath the Northern Canadian Cordillera, with seismically constrained gravity inversion.

Abstract

The geometry and depth of the Moho beneath the Northern Canadian Cordillera and adjacent North American craton are modeled through the application of both cokriging and 3-D inversion of gravity data, integrated with sparse seismic depth estimates. Models require a regional density correction, with lower densities beneath the Cordillera than the craton. The lower densities are primarily attributed to a reduction in upper mantle density, ascribed to thermal expansion under regionally higher temperatures. The eastern margin of this low-density zone is broadly aligned with the rapid westward shallowing of the lithospheric-asthenospheric boundary. From the Cordillera to the North American craton, the Moho is broadly flat at a depth of ~32 km. A zone of deeper Moho (up to ~38 km) beneath the Mistry Creek embayment has a modeled mantle density that is of a colder cratonic signature, akin to the Mackenzie craton, and is interpreted to represent the preserved remnants of an old rift basin, that is a local focus of the diffuse seismicity. Southeast of the Fort Norman structure, seismicity is broadly focussed along the eastern edge of the low-density zone. Major structures such as the Denali and Tintina faults, with 100’s km of right-lateral displacement, separate zones of higher and lower upper mantle density, supporting the interpretation of their continuation into the upper mantle. Within the North American craton upper mantle density steadily increases towards the Great Bear magmatic zone, but increases more rapidly beneath the Slave craton in tangent with a deepening of the Moho.