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

Abstract

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.