Abstract

A detailed deformation history for central Vancouver Island was determined at Myra Falls volcanic-hosted massive sulfide camp with early ductile deformation overprinted by several distinct episodes of brittle deformation. Brittle structures were subdivided into separate groups based on their morphology, geometry, kinematics, and crosscutting relations. The central location of this study provides a link between previous deformation studies in northern and southern Vancouver Island. Late Paleozoic northeast–southwest compression (D1) produced open upright folds with variably developed north-northwest-striking axial planar cleavage zones (S1) and subhorizontal stretching lineations (L1) subparallel to F1 fold axes. Renewed northeast–southwest compression during the collision of Wrangellia and North America produced a second foliation (S2) in localized shear zones, slightly oblique to the dominant S1 foliation. These two events are recorded throughout Vancouver Island wherever the basement is exposed. Mid-Cretaceous northeast–southwest compression during D3 produced early steep conjugate strike-slip faults (D3a), overprinted by northeast- and southwest-dipping thrust faults and bedding-parallel shears (D3b). D3 structures have been previously recognized in northern Vancouver Island but not as far south as Myra Falls. North–south extension (D4) produced east, north, and east-southeast-striking normal faults. These faults consistently crosscut earlier D1–D3 structures and reactivate steep D3a faults. Normal faulting is correlated with the development of the Upper Cretaceous Nanaimo Basin, but no faults of this age have previous been reported from onshore studies. The youngest structures at Myra Falls are large northwest-striking, northeast-dipping thrust faults and steep west- to west-northwest-striking sinistral strike-slip faults formed during the D5 event. These faults are gouge-rich, wavy anastomosing structures, with cleaved wall-rock zones up to several metres wide. The D5 faults are correlated with Eocene deformation caused by the accretion of the Pacific Rim and Crescent Terranes along the southwestern margin of Vancouver Island. Myra Falls is the northernmost location to have been reported, at which the structures formed as part of the Cowichan fold and thrust belt.

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