Response of continental magmatic arcs to regional tectonic changes recorded by synorogenic plutons in the middle crust: An example from the Coast Mountains of British Columbia

The Coast Mountains of British Columbia record an increase in magmatic activity, acceleration in exhumation rate, and a change from transpression to extension between ∼60 and 52 Ma. Structural analysis of fabrics in three mid-crustal plutons and country-rocks leads to conclusions about pluton emplacement mechanisms and strain partitioning during changing tectonic conditions. The Quottoon and Kitlope plutons (∼60 Ma) have steep foliations and lineations consistent with partitioned transpressional deformation. The Chief Matthew’s pluton (∼57–55 Ma) intruded during the formation of a sub-horizontal transposition foliation, and has radially distributed lineations consistent with sub-vertical flattening during extension. The change in orientation of the foliation represents an almost orthogonal rotation of the shortening direction from sub-horizontal to sub-vertical. The Chief Mathew’s pluton is interpreted to intrude initially into gently dipping fractures perpendicular to the steeply dipping foliation. These melt-filled fractures acted as conduits for melt, triggering horizontal flow, and eventually formed kilometer-scale sills. The steeply dipping fabrics of the Coast shear-zone provided pathways for melt to ascended from lower to middle-crustal depths. Partitioning deformation into three-dimensional domains of flattening, simple shear, and constrictional strain created the space for plutons. This pattern of deformation reflects the interaction of regional deformation with magma emplacement.