Coastal Complex, western Newfoundland: An Early Ordovician oceanic fracture zone

Abstract

Oceanic crust and mantle adjacent to fracture zone extensions of ridge-ridge transform faults consist of two strips on either side of and parallel with the fracture zone. One strip with a transform-fault deformation history is welded to a younger strip that has not had a transform history. Observations of oceanic fracture zones and theoretical models indicate that both strips have complex petrologic and structural histories and relationships quite different from oceanic crust and mantle generated at ridge segments away from fracture zones. Geologic relationships predicted by our simple models are matched in a remarkably precise way in ophiolite assemblages in western Newfoundland. The Coastal Complex, a hitherto enigmatic, varied, and complex assemblage of sedimentary and ultramafic and mafic igneous rocks is believed to have acquired its complexity during movement through a Late Cambrian ridge-ridge transform domain and past a ridge termination. The Bay of Islands Ophiolite Complex is, in one region, in direct continuity with the Coastal Complex and is believed to have originated on the nontransform side of the ridge termination past which the Coastal Complex was moving. This Late Cambrian–Early Ordovician fracture zone may have nucleated the medial Ordovician obduction site, the younger, higher Bay of Islands Complex riding across the older, lower Coastal Complex side and, locally, carrying with it and preserving strips of the older fracture zone assemblage. The high frequency of fracture zones along some accreting plate margins in modern oceans suggests the likelihood that ophiolite complexes will contain parts of oceanic crust and mantle preserving a fracture zone history. This likelihood is further enhanced if fracture zones nucleate obduction zones. Many published accounts of ophiolite complexes, particularly in the Alpine system of Europe and the Middle East, include descriptions of petrologic and structural relationships that accord well with relationships observed in the Coastal Complex and also may have been developed in oceanic fracture zones.