Xin Qiao

Topic

Thermochronological records of tectonics along the northwestern North American margin: Vancouver Island, Canada

School of Earth and Ocean Sciences

Date & location

• Thursday, January 29, 2026
• 9:30 A.M.
• Clearihue Building, Room B017

Examining Committee

Supervisory Committee

• Dr. Ruohong Jiao, School of Earth and Ocean Sciences, University of Victoria (Co-Supervisor)
• Dr. Dante Canil, School of Earth and Ocean Sciences, UVic (Co-Supervisor)
• Dr. Lucinda Leonard, School of Earth and Ocean Sciences, UVic (Member)
• Dr. Eva Enkelmann, Department of Earth, Energy, and Environment, University of Calgary (Outside Member)

External Examiner

• Dr. Florian Hofmann, Geophysical Institute, University of Alaska Fairbanks

Chair of Oral Examination

• Dr. Paul Schure, Department of Economics, UVic

Abstract

How the oceanic plates west of North America evolved and shaped the continental margins remains a subject of active debate. The Eocene ridge-trench interactions between the Kula/Resurrection-Farallon ridge and the western North American margin initiated a complex tectonic regime involving plate fragmentation, ridge subduction and oceanic plateau accretion, the details of which remain debated. The Cenozoic history of Vancouver Island records the long-term response of the crust to changes in this plate configuration of the convergent margin. This thesis employs multiple low-temperature thermochronometers to reconstruct the thermal and exhumation history of crust that underlies Vancouver Island. The objective is to resolve the spatial and temporal variations in the cooling patterns and provides new insights into the paleo-plate configurations and convergence processes that have shaped the northwestern North American margin throughout the Cenozoic.

Across southern Wrangellia Terrane of Vancouver Island, apatite fission track (AFT) ages of 85−23 Ma and apatite (U-Th)/He (AHe) ages of 37−14 Ma reveal variable cooling patterns since the late Cretaceous. Thermal history modeling indicates accelerated Eocene cooling (4−5 °C/Myr) adjacent to major thrust faults, interpreted as a response to oroclinal bending following plateau accretion. In contrast, the west coast experienced minimal cooling (<0.5 °C/Myr) until ∼30 Ma, followed by a moderate phase (1.5−3 °C/Myr) linked to the establishment of the Cascadia subduction zone. These patterns suggest the presence of a widespread Eocene sedimentary cover, and the Leech River Complex of the Pacific Rim terrane being partly the outboard equivalent of this cover.

Along the strike of Vancouver Island, AHe, AFT and zircon (U-Th-Sm)/He (ZHe) ages and inverse thermal models show a stark contrast from north to south. The northernmost region experienced prolonged thermal quiescence before 40 Ma followed by slow exhumation, with ZHe, AFT and AHe ages of 147−101, 138−95 and 45−12 Ma, respectively. In contrast, the central-southern island records more rapid exhumation that decelerated after ∼50 Ma, with ZHe, AFT and AHe ages of 83−31, 61−17 and 45−12 Ma, respectively. These differing exhumation patterns are explicable if Cascadia subduction established earlier over central-southern Vancouver Island (∼50 Ma) than in the northernmost region (∼40 Ma), reflecting a northward migration of the Juan de Fuca plate boundary during the early stages of Cascadia subduction. This northern boundary is juxtaposed to the north with translational motion of the Kula/Resurrection plate. Then the Kula/Resurrection-Juan de Fuca ridge shifted northward ∼40 Ma following the Resurrection subduction beneath southern Alaska or the merging of the Pacific and Kula plates. This reorganization leads to the establishment of a uniform slow exhumation across Vancouver Island thereafter. The northern extent of the Eocene Cascadia margin can thus be constrained to have started off northern Vancouver Island.

Finally, thermal history reconstruction of Vancouver Island over 250 °C was also investigated using the novel approach of dating radiation damage in zircon using Raman spectroscopy. With its thermal sensitivity, age calculation method, and evaluation protocol established, zircon Raman dating emerges as a new approach to low-temperature thermochronology within the 260–370 °C closure interval. Intrusive samples with distinct and well-constrained thermal histories from Vancouver Island, including rapid cooling, multi-stage cooling, and prolonged residence at 2−4 km depth, are analyzed to evaluate the technique’s applicability as a thermochronometer in subduction margin settings. Five samples yield zircon Raman ages consistent with previously constructed thermal history models, whereas three samples show anomalously old outliers. The different Raman bands show different sensitivities to thermal disturbance, necessitating band-specific interpretation of the resulting ages. The results validate the application of zircon Raman dating for reconstructing various cooling paths and suggest that further methodological improvements should focus on low-damage samples, long-term thermal histories, and integration with other thermochronometers.

Collectively, this work provides a comprehensive thermochronological framework from Vancouver Island, providing views of how the island’s crust responded to the plate boundary evolution along the northwestern North America through the Cenozoic. The results reveal a coherent exhumation pattern associated with the Eocene ridge-trench interactions and transition from Farallon to Cascadia subduction. This work also demonstrates the value of zircon Raman dating in resolving thermal histories in active plate convergence settings.