Structural, Geochronological, and Thermochronological Constraints on the Evolution of Orogenic Infrastructure in the Thor-Odin – Pinnacles Area of Southeastern British Columbia.

The Thor-Odin dome is a basement-cored tectonothermal culmination in the southeastern Canadian Cordillera containing high grade metamorphic rocks, tectonically overlain by a heterogeneous tract of polydeformed medium to high grade metamorphic rocks exposed in the footwall of the Columbia River fault system. The Thor-Odin and Frenchman Cap domes comprise Paleoproterozoic “basement” gneiss infolded with Proterozoic to Paleozoic metasedimentary “cover” rocks. Detrital zircon geochronology from three samples of the inferred basal quartzite within the Thor-Odin dome contain two separate maximum depositional age populations at <1.65 Ga and <1.1 Ga, suggesting two temporally separate quartzite units, neither of which is correlative with the basal quartzite in the Frenchman Cap dome, c. 1.85 Ga. A Neoproterozoic to Cambrian panel structurally overlies the dome, as evidenced by a <600 Ma quartzite on Mount Symonds. Middle Jurassic, or younger, quartzites are present at Plant Creek, representing a transposed terrane boundary and the accretion of rocks on to the Laurentian margin. Four tectonothermal domains were defined based on timing of metamorphism, deformation and cooling, structural and petrographic data, and 40Ar/39Ar thermochronology. The structurally highest domain (Whatshan – Pinnacles) was polyfolded in the Late Cretaceous. The second highest domain (South Fosthall) was penetratively deformed in the mid-crust at c. 73 – 64 Ma. Deformation ended in the rocks of the South Fosthall area after c. 64 Ma, but continued in the structurally lower third domain (North Fosthall – Cariboo Alp). The North Fosthall– Cariboo Alp domain became suprastructure relative to the deforming rocks in the structurally lowest domain (Thor-Odin dome) after c. 58 Ma. The Thor-Odin dome remained in the infrastructure until c. 52 Ma. Exhumation and cooling through 500 ⁰C in the upper part of the structural section was ongoing during the last stages of transposition and folding in the dome in the Late Paleocene to Early Eocene. All the rocks cooled through 300 ⁰C together between 52.5 – 51 Ma. Extensional structures were active at all structural levels by c. 51 Ma, reflecting a regional switch from transpression to transtension, expressed as crustal scale extension and exhumation via the east-dipping Columbia River and west-dipping Okanagan Valley extensional fault systems.