Cordilleran metamorphic core complexes exhibit Tertiary extensional deformation, expressed either as detachment faults or detachment faults plus ductile mylonitic shear zones. Extension directions, as indicated by stretching lineations within mylonite and striations along detachment faults, fall into regional groups in which the directions are similar in trend throughout each group. Asymmetric fabrics on both small and large scale give a sense of shear and indicate that tectonic vergence within groups is directed outward from a central axis. The regional consistency of extension directions implies a regional control of extension in metamorphic core complexes.

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