Contrast of metamorphic and structural histories across the work channel lineament, Coast Plutonic Complex, British Columbia

Abstract

A prominent topographic lineament along the western margin of the Coast Plutonic Complex of southeast Alaska and adjacent parts of British Columbia separates terranes of contrasting metamorphic histories where it enters British Columbia near Prince Rupert as the Work Channel lineament. The western terrane preserves a regional medium‐high pressure metamorphic facies series, increasing from chlorite grade in the west to kyanite + muscovite migmatite near the lineament. The metamorphic, structural and age relations of the western terrane are consistent with a model of crustal thickening accompanied by westward transport of higher over lower grade metamorphic units during the Jurassic. This terrane was uplifted and cooled by Cretaceous time (∼85 Ma). East of Work Channel lineament, high temperature, relatively low pressure assemblages are overprinted onto high pressure assemblages. The low pressure metamorphic assemblages developed in the Eocene during rapid uplift (2 mm/yr) of rocks which initially may have been metamorphosed at the same time and along the same PT gradient as those of the western terrane. Structures along the Work Channel lineament suggest vertical motion during uplift of the eastern terrane and after metamorphism of the western terrane. Because the metamorphic discontinuity across the lineament is 3–5 kbar at about 700°, the lineament in the Prince Rupert area marks a major crustal break.