RELATIONSHIP BETWEEN LITHOSPHERE TEMPERATURES AND MOHO GEOMETRY IN SOUTHWESTERN CANADA

Geophysical observations in southwestern Canada reveal a pronounced contrast in crust and lithosphere properties at the Rocky Mountain Trench (RMT), the surface geological boundary between the Canadian Cordillera and stable craton. West of the RMT, the Cordillera is characterized by high surface heat flow, low mantle seismic velocity, high elevation, and low elastic thickness. These are consistent with a Moho temperature of 800-850°C throughout the Cordillera. In contrast, geophysical data for the adjacent craton indicate a 400-500°C Moho. There are also clear differences in the Moho geometry between the Cordillera and craton. The Cordillera Moho depth is remarkably uniform at 33±3 km depth despite a complex history of extensional faulting, shortening deformation, and terrane accretion. The flat Moho is evident over distances of 10’s of km from multichannel seismic reflection and 100’s of km from seismic refraction, tomography, and receiver functions. In Alberta, a new receiver function analysis, combined with existing reflection, refraction and tomography data, shows that the Moho depth varies from ~35 to 50 km over length scales of 150 to 250 km.

Here, we argue that the difference in Moho geometry between the Cordillera and craton is a consequence of the thermal contrast between the two regions. The Moho marks the boundary between low density crust and higher density underlying mantle. Perturbations to this boundary will undergo gravitational relaxation, with a timescale depending primarily on the viscosity of the crust and mantle at the boundary. Our theoretical analysis and numerical models show relaxation to a subhorizontal Moho in less than 20 Ma for lower crustal viscosities less than 5*10¹⁹ Pa s. A strong mantle lithosphere increases the timescale by a factor of 2 or more, if its viscosity is greater than 10²² Pa and/or its thickness is greater than 25 km. For the Cordillera, high temperatures imply a thin mantle lithosphere layer and viscosities of ~10¹⁹ Pa s in the lowermost crust. We conclude that the observed flat Moho below the Cordillera is the consequence of ductile flow of the hot lower crust. On the other hand, the lower craton temperatures result in a stronger crust and thicker mantle lithosphere. As a result, Moho perturbations can be sustained for 100 Ma or more.