Structure and Dynamics of the Mantle Beneath Western Canada
Creators: Yu, Tai-Chieh
Publisher: name: University of Alberta Library
Year: 2022

<p>The Canadian Cordillera (mountain belts) in western Canada has a high elevation (~1.5 km above sea level), thin crust (~35 km) and thin lithosphere (50-70 km). In contrast, the Laurentian Craton to the east has a low elevation (near sea level) and thicker crust and lithosphere (~40 km and &gt;200 km, respectively). However, the structure and dynamics of the uppermost mantle beneath the cordillera and craton are unclear. <br>
The first part of my research investigates the mantle structure of southwestern Canada using seismic tomography and magnetotelluric data for the Cascadia subduction zone backarc and adjacent Craton. Both shear wave velocity (VS) and electrical resistivity are sensitive to the temperature and olivine water content of the mantle. I developed a joint analysis of VS and resistivity to quantify the temperature and olivine water content at 75-150 km depth. These depths correspond to the sublithospheric mantle for the cordillera and the lithospheric mantle for the craton. I found that the cordilleran mantle is either hydrated (~1600 ppm H/Si) and warm (~1240 °C) or drier (~600 ppm H/Si) and hot (~1370 °C); in contrast, the craton mantle is relatively dry (&lt;500 ppm H/Si) and cool (~960 °C). These conditions imply that the cordillera sublithospheric mantle viscosity is in the range 1019 of 1021 Pa s, whereas the craton mantle lithosphere viscosity is in the range 1022 of 1024 Pa s. At present-day, there is a sub-vertical boundary between the cordilleran and craton mantle. My results indicate that the cordillera sublithospheric mantle is weak enough to undergo small-scale and edge-driven convection, and that the cordillera-craton boundary may be unstable, as the craton mantle lithosphere is susceptible to deformation.<br>
The second part of my research investigates the mantle dynamics in the Northern Canadian Cordillera (NCC) to test the hypothesis that the thin NCC lithosphere developed recently through gravitational thinning via delamination. I developed 2D thermal-mechanical models to investigate the consequences of delamination. Removal was triggered by a combination of (1) weak zone in the mantle lithosphere and (2) an eclogite layer in the lowermost crust, assumed to have formed due to shear zone or previous arc volcanism at the western edge of the NCC and an earlier episode of crustal thickening, respectively. The weak zone created a low viscosity conduit that enabled the dense eclogite layer to decouple from the crust. Delamination resulted in the formation of a high-elevation region with a thin crust, thin lithosphere and high heat flow, in good agreement with present-day NCC observations. The models predict that removal was accompanied by widespread mantle melting and changes in crustal stress. Hence, a delamination event at 15 Ma provides a new explanation for the enigmatic Northern Cordilleran Volcanic Province and crustal earthquakes in the Mackenzie Mountains.<br>
The last part of my research uses generic models to study the dynamics and surface expressions of delamination. The models investigate an area with earlier crustal thickening, where there is an eclogite layer in the lowermost crust. The density and strength of this layer play a crucial role in triggering delamination. Three styles of delamination are identified: (1) slab-like removal occurs for strong (cool) lithosphere and creates a migrating wave of surface subsidence followed by uplift, (2) stringy delamination occurs for a weaker (hotter) lithosphere, and (3) delamination with drips occurs for the weakest (hottest) lithosphere. For Styles 2 and 3, delamination causes surface uplift but minimal subsidence. All three styles result in a wide region of thin, hot lithosphere and mantle decompression melting. The models also show that delamination induces stresses in the overlying crust, creating areas of extension/compression and crustal thickening. These results suggest that the wide range of surface observations for areas with delamination may reflect differences in lithospheric rheology and temperature.</p>