MID-CRETACEOUS TO EARLY TERTIARY TRANSPRESSION AND OROGEN-PARALLEL DISPLACEMENTS WITHIN THE COAST PLUTONIC COMPLEX, WESTERN BRITISH COLUMBIA
Chardon, Dominique, Andronicos, Christopher L.,Hollister, Lincoln S.
Key words: Cordillera; transpression; terranes; displacements; magmatism
The Coast Plutonic Complex (CPC) of western British Columbia andsoutheast Alaska is the largest magmatic arc of the North AmericanCordillera. It forms the transition between two tectonic domains that are suspected to have undergone several phases of large orogen-parallel displacements during the Mesozoic and Early Cenozoic. A compilation of rock fabric data allows us to reconstruct the large-scale structural pattern of the batholith from 52 to 56 N. These data, together with published isotopic ages (U-Pb and K-Ar) show that the CPC was affected by subvertical, orogen-parallel, crustal-scale shear zones of Mid-Cretaceous to Early Eocene age.
Early shear zones affected the western part of the CPC and record sinistral transpression. They were sequentially active, from west to east, from 110 to 87 Ma. The shear zones separate crustal panels with distinct plutonic and cooling histories suggesting large orogen-parallel displacements. Sinistral shearing was roughly coeval with the development of the thrust belts along both sides of the CPC, suggesting transpression was a first order process in building the Mid-Cretaceous Coast orogen.
The Coast shear zone (CSZ), affecting the middle part of the CPC, is the longest tectonic feature of the map area, extending from SE Alaska to at least 51 30 N. Detailed field analysis in the vicinity of Prince Rupert, British Columbia, shows that dextral transpression took place before 59 Ma in the vicinity of the CSZ. Dextral transpression was partitioned between the 15 km wide CSZ and a wide zone of shallowly dipping gneissic fabrics along the eastern side of the CSZ that define a major reverse deformation zone. The regional structural pattern and isotopic ages suggests that dextral transpression along the CSZ could have started as early as 83 Ma.
The Mid-Cretaceous sinistral transpressive shear zone pattern is consistent with the southward displacement of outboard terrains relative to North America. Dextral transpression along the CSZ during the Late Cretaceous reflects northward displacement of the Insular Superterrane relative to the Intermontane superterrane, consistent with the Baja B. C. hypothesis.
