Origin, accretion, and postaccretionary evolution of the Bridge River Terrane, southwest British Columbia

Abstract

The Bridge River terrane of southwestern British Columbia, Canada, is one of a number of crustal fragments in the western part of the North American Cordillera which originated elsewhere in continental margin or oceanic settings and have subsequently been incorporated into North America. Detailed geologic analysis of individual accreted terranes such as Bridge River is essential in order to elucidate the processes of continental accretion and the postaccretionary evolution of accreted crust. The Bridge River terrane was probably derived from a mid‐Mesozoic ocean‐margin setting and consists of three principal elements: (1) the Carpenter Lake assemblage (CLA), (2) Bridge River schists (BRS), and (3) the Shulaps ultramafic body. The CLA is mid‐Triassic to mid‐Jurassic in age and consists of ribbon cherts, both pillowed and tuffaceous greenstones, limestone olistoliths, and coarse volcaniclastic rocks which were metamorphosed in prehnite‐pumpellyite facies and are nonpenetratively deformed on a regional scale. Parts of the CLA are intensely faulted; the faulting in part records Mesozoic accretion‐related shortening but also includes structures which offset dikes of probable Tertiary age. The BRS, separated from the CLA by a late high‐angle fault, was metamorphosed in prehnite‐pumpellyite through lower amphibolite facies, is locally twice‐deformed, and contains large‐scale recumbent folds. A Mesozoic phase of deformation and metamorphism was related to thrust emplacement of the Shulaps ultramafic complex above the BRS. The thrust zone between the Shulaps and the BRS, characterized by a thick serpentinite melange, cuts lower plate metamorphic rocks, which preserve an inverted metamorphic gradient. Tertiary deformation resulted in recumbent folding of the earlier, thrust‐related fabric in the BRS, probably during a postaccretion strike‐slip deformational regime. Early phases of the deformation of all three elements of the Bridge River terrane probably record accretion. The current position and age of the Bridge River terrane imply accretion later than the Intermontane terrane and earlier than the Insular terrane. However, its incorporation into North America may have been closely linked to one or both of these other crustal blocks. Early structures in the Bridge River area have been strongly overprinted during Late Cretaceous and early Tertiary strike‐slip faulting and northward transport.