Abstract

Metamorphic core complexes of North America are asymmetric when viewed perpendicular to the direction of tectonic transport. On one side, mylonitic fabrics and the overlying detachment fault dip gently in the direction of upper-plate transport, and the mylonitic rocks have a down-dip sense of shear. On the opposite side, the mylonitic zone rolls over to a dip opposed to the direction of upper-plate transport, diverges downward from the detachment fault, and dies out upward across a mylonitic front. New kinematic data show that the backdipping mylonitic zone in most complexes is a downfolded continuation of the main mylonitic zone. Folding of the mylonitic zone was accompanied by the formation of antithetic shear zones and occurred during the late stages of mylonitization.

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