Strain partitioning in an obliquely convergent orogen, plutonism, and synorogenic collapse: Coast Mountains Batholith, Biritish Columbia, Canada

We describe the crustal structure of the Coast Mountains batholith between 54° and 55°N, within the Canadian Cordillera, with emphasis on emplacement of the 7 km thick Kasiks sill complex (KSC). Kinematic patterns that developed during emplacement of the KSC are the result of interactions between magma transport, magma accumulation and regional deformation. The sills were emplaced during NW directed normal shearing and flattening of country rocks that host the KSC. A ∼2 km thick shallowly NE dipping mylonite zone cuts the eastern side of the KSC. Kinematic indicators within the mylonite zone record top to the east normal displacements. Structural analysis shows that mylonite formation occurred during subvertical shortening and east-northeast, subhorizontal extension. U'Pb zircon age dates show that ENE directed normal shearing along the eastern side of the KSC and WNW directed normal shearing within the KSC occurred contemporaneously between ∼54 and 51 Ma, indicating strong strain partitioning between the mylonite and the KSC. This pattern of strain partitioning is interpreted to have been driven by return flow of melt-laden crust in response to tectonic denudation of the upper crust. Seismic profiling shows that many of these structures extend to mid and lower crustal depths. Comparison of our results with other regions within the Canadian Cordillera indicates that orogen-scale right-lateral strike-slip faults deformed synchronously with wide spread magmatism and formation of extensional gneiss domes. Thus the crustal structure of the Coast Mountains batholith was the result of early Tertiary batholith construction during dextral oblique convergence and synorogenic collapse.