TRANSFORM FAULTS, SEISMIC ANISOTROPY, AND LITHOSPHERIC STRUCTURE

Several geologic observations, taken together, suggest a comprehensive role for transform faults along rifted continental margins. Some large-offset transform faults of the Mesozoic Atlantic margin of North America are aligned with large-offset transform faults of the late Precambrian–early Cambrian Iapetan margin of Laurentia, suggesting tectonic inheritance of transform orientation and location. Along ancient and modern continental margins, from synrift to passive-margin to subsequent orogenic settings, distributions of anomalously thick sediment document anomalous crustal subsidence systematically located at zones of lithospheric weakness along transform faults at rifted margins and projecting into continental crust. Intracratonic fault systems parallel orientations of rift and transform segments of the continental margin, posing the question of how crustal extension and transform motion propagate into continental crust far inboard from the rifted margin. The nature and scale of transform-parallel inheritance, subsidence, and fault propagation suggest a lithospheric scale of large transform faults; seismic data provide for resolution of the deep lithospheric structure.

Recent models interpret ductile extension of the mantle lithosphere to be the dominant process in continental rifting and supercontinent breakup; brittle extensional structures at continental rifts in the shallow crust are a consequence of ductile flow, stretching, and thinning of the deeper lithosphere. Abrupt offsets of the rift at transform faults in the crust require a complementary mechanism in the mantle lithosphere. Seismic anisotropy documents a transform-parallel lithospheric fabric in the mantle lithosphere beneath and parallel with crustal transform faults, indicating a transform-parallel zone of distributed shear in the ductile mantle lithosphere. Transform-parallel flow (expressed in distributed-shear fabric) in the ductile lithosphere offers a comprehensive mechanism for transform offsets of rifted margins of continental crust, transform-parallel zones of lithospheric weakness, and propagation of intracratonic fault systems into continental crust, consistent with a coherent system of lithospheric extension and transform motion through the brittle crust and mantle lithosphere.