TIME TRANSGRESSIVE STRAIN RECORDED JURASSIC AND EARLY CRETACEOUS PLUTONS OF THE CENTRAL COAST PLUTONIC COMPLEX, BELLA COOLA, BRITISH COLUMBIA, CANADA

Plutons are powerful tools used to understand deformation within arcs and examine orogenic evolution. Magmatic fabrics (foliations and lineations) formed during deformation of partially crystallized magma may record increments of regional strain during the latest stages of crystallization. These fabrics are increasingly used to infer instantaneous paleo-strain fields and plate motions. The Bella Coola region is located in the central Coast Plutonic Complex, British Columbia and consists of early Jurassic through Eocene plutons that variably contain magmatic fabrics. The timing and mode of terrane displacement in the Canadian Cordillera during Mesozoic arc construction remains enigmatic and plutons exposed in the Bella Coola region reveal a time transgressive picture of the regional paleo-strain field from Jurassic thorough Late Cretaceous.

Mapping, microstructural analysis, digital image analysis and anisotropy of magnetic susceptibility (AMS) are used to quantify the orientation, shape and intensity of magmatic fabrics in Early Jurassic Howe Lake suite plutons and Early Cretaceous Desire suite plutons. Timing constrains for magmatic fabrics come from existing U-Pb geochronology. Howe Lake plutons show a NW-SE steeply dipping foliation and a sub-horizontal NW-SE lineation. The Desire Suite plutons consistently show a NW-SE steeply dipping magmatic foliation a moderate to shallowly SE plunging lineation. Plutons from the Desire suite are also cut by the Pootlass Shear zone, which is a series of northwest-southeast trending, moderately to steeply dipping shear zones initiated in the Early Cretaceous. Preliminary microstructural and electron backscatter diffraction (EBSD) data for the Pootlass Shear Zone reveal a strong quartz crystallographic preferred orientation with weak sinistral shear sense. Magmatic fabric and shear zone data are interpreted to reflect weakly sinistral transpression in the CPC during Jurassic through Early Cretaceous.