Abstract

The Lewis Hills massif (Bay of Islands Ophiolite, Newfoundland) preserves a deeply eroded transform fault. A major, low-angle, extensional mylonite zone is next to the transform assemblage and probably merges laterally into it. The mylonite developed in a high-temperature environment and occurs at the top of relatively rigid lithospheric mantle. The lithospheric mantle represents exhumed, older, arc-type basement of the Little Port Complex that is also exposed across the transform in the western Lewis Hills. It differs dramatically from an asthenospheric mantle unit of the Bay of Islands Complex, exposed in the eastern Lewis Hills, that formed in a spreading ridge environment.

The fossil ridge-transform segment and associated low-angle normal fault in the Lewis Hills formed at an inside-corner structural setting similar to recently discovered core complexes at oceanic ridge-transform intersections. However, the Lewis Hills segment developed in a setting where a spreading center propagated across a transform margin and rifted older arc-type lithosphere. Factors that contributed to formation of the low-angle detachment fault are the lithospheric nature of the mantle basement, rheological weakening and strain focusing by intrusive sills, and weak lateral coupling realized by the extensional transform assemblage.

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