Southern continuation of the Coast shear zone and Paleocene strain partitioning in British Columbia-southeast Alaska

This paper documents the newly recognized southern continuation of the earl y Tertiary Coast shear zone, extending its known length by similar to 350 k m to more than 1200 km. Three sites along the shear zone in British Columbi a, Douglas Channel, Bella Coola, and Machmell River, have similar histories during the period ca. 65-55 Ma. The shear zone is 2-11 km thick and is def ined by well-developed mylonite zones that strike northwest and dip steeply northeast. Motion on the shear zone was predominantly reverse, with the no rtheast side up. Synkinematic plutons are common in the shear zone. Lower p late rocks, high-grade gneiss derived from an ancient continental margin as semblage, show little evidence of the extensive deformation and plutonism i n the shear zone. North of Bella Coola, high-grade gneiss forms the upper p late, but to the south weakly metamorphosed racks of Stikinia compose the u pper plate. Geochronologic data show that the shear zone was active between ca. 60 and 55 Ma at Douglas Channel, ca. 62 and 56 Ma at Bella Coola, and after 66 and before 56 Ma near the Machmell River. These features match tho se of the Coast shear zone in southeast Alaska and adjacent British Columbi a. Together the shear zones formed a continental-scale reverse ductile faul t in Paleocene time. Plate reconstructions show an oblique component to the dominantly dextral transcurrent Paleocene margin. Partitioning of This mot ion across the continental margin produced a regional system of strike-slip faults and contraction on the Coast shear zone.