Batholith emplacement at mid-crustal levels and its exhumation within an obliquely convergent margin

Emplacement of the central part of the Coast Mountains batholith of northern coastal British Columbia occurred within a regime characterized by oblique convergence between the Farallon/Kula and North American plates. We use new structural, kinematic and U-Pb isotopic data to show that the locations, geometry, and mechanisms of pluton emplacement within this batholith were controlled by displacements within a network of normal faults and transtensional shear zones. These data also show that the most active period of pluton emplacement, from ~67 to ~51 Ma, coincided with a change in style of deformation within the batholith. Prior to ~67 Ma plutons were emplaced within an arc dominated by regional-scale contractional shear zones. In contrast, emplacement of 67-51 Ma plutons occurred in an arc increasingly dominated by normal faults with arc-parallel to oblique displacement and by sinistral transtensional shear zones. We have identified and mapped the structure of three plutonic complexes composed of 67 to 51 Ma plutons: the Khyex sill complex, Arden Lake plutonic complex and Quottoon plutonic complex. Shear-zone-controlled emplacement of plutons within the batholith accounts for the widely different orientations and structural features that characterize plutons within these three complexes. During and after this latest Cretaceous-Paleogene period of intense plutonic activity and accompanying deformation, the deep roots of the batholith were rapidly unroofed by ductile normal faulting prior to 50 Ma.