Interplay of plutonism and regional deformation in an obliquely convergent arc, southern Coast Belt, British Columbia

The Coast Plutonic Complex is an extensive zone of continental growth that formed along the Mesozoic convergent margin of northwestern North America. The orogeny creating this belt involved terrane accretion and assembly, massive upward transfer and emplacement of sial in the form of batholiths constituting a magmatic arc, and transformation of broad tracts of sedimentary and volcanic rocks into crystalline continental crust, all operating in more or less the same space and time. The mechanisms and interplay of these orogenic processes are well displayed in the Harrison Lake area of the southern Coast Belt, British Columbia. Great structural relief across the area exhibits a present-day architecture defined by thin, thrust-stacked terrane sheets and early concordant pluton sheets folded on a macroscopic scale, all truncated by oblique dextral-reverse faults and overlain by later floored plutons. Construction of this complex began with terrane assembly on orogen-normal thrusts during a lull in plutonism in the Early Cretaceous. Low-grade metamorphism during this event indicates only modest crustal thickening. Subsequent plutons intruded into the assembled terranes appear to be composites of sheets. Early pluton sheets are now steeply dipping due to folding but were likely intruded as horizontal bodies. Large ovoid post folding plutons are mostly subhorizontal floored bodies, at least in part sheeted. These plutons are underlain by Barrovian mineralogic aureoles that indicate downward vertical displacement of 10 km or more during plutonism, suggesting pluton emplacement by vertical inflation. Magmatic fabrics in these bodies, and the discordant relation of plutons to regional structures, preclude emplacement in active fault zones. Penetrative strain aureoles flanking plutons are mostly limited to zones a few hundred meters wide, and regional tectonic structures are widely preserved. Tectonic deformation of the arc is characterized by contraction and strike-slip, not orogen-normal extension. Plutons played a greater role than terrane accretion in causing crustal thickening and high-grade metamorphism.