Strain partitioning and batholith emplacement at the root of a transpressive magmatic arc

Abstract

An integrative study of the high-pressure Ecstall batholith and its country rocks (Prince Rupert area, British Columbia) reveals the interplay of deformation and plutonism produced in the largest Cordilleran magmatic arc (i.e. the Coast Plutonic Complex) during mid-Cretaceous convergence between the Farallon oceanic plate and North America. The results emphasize the interference between three strain fields: (1) an early (>92 Ma) crustal wedge produced by orogen-perpendicular, SW-vergent thrusting, (2) an orogen-parallel sinistral strike-slip shear zone active until 87 Ma, and (3) the lateral expansion of the batholith (93–91 Ma) against the shear zone. The batholith expanded in a direction oblique with respect to the shear zone trend (+20°) by extruding country rocks against its head in a direction normal to its expansion direction during sinistral, strike-slip partitioned transpression. The batholith's emplacement combined far-field batholith boundary-normal translation, extrusion-related rotation and radial and concentric elongation in its structural aureole. This case study may typify the process by which a modern magmatic arc grows longitudinally at depth during transpression, the direction and sense of growth being determined by the direction of plate motion relative to the magmatic arc and the degree of strike-slip partitioning of transpression.