RESPONSE OF SYNOROGENIC PLUTONS IN THE MIDDLE CRUST OF CONTINENTAL MAGMATIC ARCS TO REGIONAL TECTONIC CHANGES: AN EXAMPLE FROM THE COAST MOUNTAINS OF BRITISH COLUMBIA

The Coast Mountains of British Columbia, Canada, record an increase in magmatic activity, acceleration in the exhumation rate, and a change from transpression to extension between ~60 and 52 Ma. We studied three mid-crustal plutons located at the southern termination of the central gneiss complex that intruded during this time period. We derive conclusions about the role of preexisting structures on melt migration, pluton emplacement mechanisms, and strain partitioning during changing tectonic conditions. The study is based on structural analysis of fabrics developed in the plutons and country-rocks. The Quottoon and Kitlope plutons (~60 Ma) have nearly-vertical foliations and lineations consistent with vertical flow of magmas during N-S shortening with a flattening component to strain, in a transpressional regime. The Chief Matthew’s pluton (~57 Ma) intruded with sub-horizontal foliation and radially distributed lineations consistent with sub-vertical flattening, during extension. The change in orientations of the structures represents an almost orthogonal rotation of the shortening direction in the area. We envision the initiation of the transposition of the foliation by the development of melt-filled tension fractures perpendicular to the foliation consistent with the orientation of the regional strain. These structures locally change the orientation of the incremental strain axis and attract melt, trigger horizontal flow, and eventually form sub-horizontal plutons, like the Chief Matthew’s pluton. The Coast shear-zone is present in the study area and acted as the conduit that focused and fed the melts that formed the sub-vertical plutons. The space for the intrusion of plutons in an overall compressional environment is created by partitioning the deformation around the plutons into domains of flattening, simple-shear, and constrictional strain. Patterns of deformation around the plutons are strongly three dimensional and reflect the interaction of magma transport and regional deformation.