The Coast - Insular boundary revisited: Mid-Jurassic ductile deformation on Quadra Island, British Columbia
A research poster presented at Resources for Future Generations 2018 in Vancouver, BC.The boundary between the Insular and Coast geomorphological belts of the North American Cordillera is well exposed at Open Bay, on Quadra Island, British Columbia. On the western side of the boundary at Open Bay is the upper Triassic-aged Quatsino limestone and Karmutsen basalts of the Wrangellia terrane. On the eastern side of the boundary are intrusive phases of the Coast Batholith. At Open Bay the Wrangellian Quatsino limestone (Upper Triassic) has been subjected to strong ductile deformation resulting in re-folded transposed bedding. A U-Pb crystallization age from a syn-deformational basaltic dike that both cross-cuts isoclinal folds and is folded by more open folds, constrains the age of ongoing deformation around 164.8 ± 1.6 Ma. Two undeformed intrusive phases cut both the limestone and the strongly deformed intrusions; U-Pb crystallization ages of the undeformed intrusive phases, a hornblende quartz diorite pluton and a plagioclase-phyric dacite dike, are constrained at 160 ± 1.6 and 158.7 ± 0.8 Ma, respectively, indicating that ductile deformation at Open Bay was complete by ca. 160 Ma. These age constraints on ductile deformation are consistent with Middle- to Late- Jurassic contractional deformation recorded on the West Thurlow Island, Hernando Island, Texada Island, Bowen Island. Together with Quadra Island, these locations define a NW-trending linear structural corridor within the Georgia Basin that may extend as far north as the Queen Charlotte Islands. The timing of this deformation approximately coincides with the end of Bonanza Volcanism and the onset of the intrusion of the Coast Batholith.