Large-scale transpressive shear zone patterns and displacements within magmatic arcs: The Coast Plutonic Complex, British Columbia

Abstract

The Coast Plutonic Complex is the largest magmatic arc of the North American Cordillera, extending from northwestern Washington State to eastern Alaska. It forms the transition between two tectonic domains that are suspected to have undergone several phases of large (several thousands of kilometers) orogen-parallel displacement during the Mesozoic and early Cenozoic. A compilation of fabric data, published isotopic ages, and new structural observations shows that the western Coast Plutonic Complex was affected by subvertical, orogen-parallel, crustal-scale shear zones. These shear zones mainly reflect sinistral transpression and were sequentially active from ~110 to 87 Ma during the intrusion of voluminous batholiths. Sinistral shearing was roughly coeval with the development of the thrust belts flanking the Coast Plutonic Complex (between ~101 and ~85 Ma), suggesting plate-scale transpression was a first-order process in the construction of the Coast Mountains orogen. These shear zones separate panels with distinct plutonic and cooling histories, suggesting the sinistral displacements between crustal blocks were large (greater than tens to hundreds of kilometers). This transpressive shear system likely reflects the Jurassic to early Late Cretaceous migration of outboard Cordilleran terranes to the south suggested by paleomagnetic evidence and plate reconstruction models. This example from the Coast orogen shows how transpression is partitioned between a thermally weakened magmatic arc and outwardly vergent fold-and-thrust belts. Our analysis further shows that the ~2000-km-long Late Cretaceous to early Tertiary Coast shear zone has a minimum extent toward the south to at least 51°30'N.