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Mid- to upper-crustal fault zones often possess arrays of shear surfaces whose traces on sections perpendicular to the fault surface and parallel to the ac-plane conform with one or more of the ‘Riedel shear’ orientations. These shear surfaces often are oblique to the transport plane, however, so arrays exhibit monoclinic rather than orthorhombic symmetry. In a mudstone-dominated mélange in Humber Arm Supergroup strata in the Bay of Islands, Newfoundland, and in serpentinites from the base of the Bay of Islands complex, shear surfaces have orientations inclined to major fault-zone boundaries and an inferred ac-plane for macroscopic fault-related deformation. Deformation in these zones exhibits an overall monoclinic symmetry. The 3D, asymmetric character of shear surface fabrics suggests that a factor other than stress or the symmetric strain rate tensor controlled their formation. The velocity gradient tensor in a steady, non-coaxial shearing flow possesses a symmetry consistent with monoclinic fabrics. Shear surfaces in asymmetric arrays may initiate with predictable orientations relative to the velocity gradient tensor and then rotate toward flow apophyses, which identify stable positions in steady, 3D flows.

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