Opposing shear senses in a subdetachment mylonite zone: Implications for core complex mechanics

Global studies of metamorphic core complexes and low-angle detachment faults have highlighted a fundamental problem: Since detachments excise crustal section, the relationship between the mylonitic rocks in their footwalls and the brittle deformation in their hanging walls is commonly unclear. Mylonites could either reflect ductile deformation related to exhumation along the detachment fault, or they could be a more general feature of the extending middle crust that has been captured by the detachment. In the first case we would expect the kinematics of the mylonite zone to mirror the sense of movement on the detachment; in the second case both the direction and sense of shear in the mylonites could be different. The northern Snake Range decollement (NSRD) is a classic Basin and Range detachment fault with a well-documented top-east of displacement. We present structural, paleomagnetic, geochronological, and geothermometric evidence to suggest that the mylonite zone below the NSRD locally experienced phases of both east- and west-directed shear, inconsistent with movement along a single detachment fault. We therefore propose that the footwall mylonites represent a predetachment discontinuity in the middle crust that separated localized deformation above from distributed crustal flow below (localized-distributed transition (LDT)). The mylonites were subsequently captured by a moderately dipping brittle detachment that soled down to the middle crust and exhumed them around a rolling hinge into a subhorizontal orientation at the surface, producing the present-day NSRD. In this interpretation the brittle hanging wall represents a series of rotated upper crustal normal faults, whereas the mylonitic footwall represents one or more exhumed middle crustal discontinuities (LDTs). Citation: Cooper, F. J., J. P. Platt, E. S. Platzman, M. J. Grove, and G. Seward (2010), Opposing shear senses in a subdetachment mylonite zone: Implications for core complex mechanics, Tectonics, 29, TC4019, doi:10.1029/2009TC002632.