Variations in structure and petrology in the Coastal Complex, Newfoundland: Anatomy of an oceanic fracture zone

The Coastal Complex of western Newfoundland is perhaps the world's best example of an ancient oceanic fracture zone preserved within an ophiolite complex. The structural style, metamorphic zonation, and magmatic history of the Coastal Complex contrasts sharply with that of the contiguous Bay of Islands Complex. The exposures of the Coastal Complex provide cross sections of the fracture zone at various vertical and lateral positions. A reconstruction of these sections reveals structural and petrological variations that are a function of depth as well as position along strike. For example, deep crustal sections show a history of strike-slip deformation that is fairly homogeneously distributed across a 4 km wide high-strain zone. High-temperature metamorphic rocks, partial melting relationships, and peridotite intrusive bodies are also present. Diabase dykes cut the entire assemblage. At shallow crustal levels, deformation is concentrated along relatively narrow, well-defined faults and shear zones that overall have an anastomosing aspect. High-grade metamorphic rocks occur only locally and have in most places suffered retrograde effects. Felsic intrusive bodies occur locally and serpentinite mélange is widespread. Volcanic rocks non-conformably overlie a tectonically brecciated horizon that transects the entire assemblage. The most striking lateral variations are the discontinuous nature of lithologic units and the apparent bifurcation of major high-strain zones along strike. Oceanic fracture zones and their ophiolite analogues probably display a wide range of geological relationships that vary with depth, with position along strike, and in scale. Thus, the Coastal Complex should probably be viewed only as a single example of how exceedingly complex and variable the oceanic lithosphere may be near transform faults and their non-transform extensions. Clearly, similar ophiolitic assemblages should not be considered when attempting to model processes along purely extensional plate boundaries.