AN INVESTIGATION OF FOOTWALL MYLONITES IN THE HARCUVAR METAMORPHIC CORE COMPLEX

Metamorphic core complexes are a fundamental mode of large-magnitude extension. However, the exact mechanisms of core complex formation remain controversial, specifically the initial dip of the bounding detachment fault, the timing and duration of core complex formation, and the role of footwall mylonites in the development of core complexes. The Harcuvar core complex, which is located in western Arizona, is thought to have developed during Oligo-Miocene extension via 50 km of top-NE displacement on both ductile and brittle faults (Reynolds and Spencer, 1985). The goal of this study is to evaluate the role of the footwall mylonites in the overall development of the core complex and determine whether they match expectations of a gently dipping simple shear zone model.

We collected and analyzed samples along a NE-SW (deep to shallow) transect across the footwall. Kinematic indicators suggest a top-northeast (normal) sense of shear is present throughout the range, however, syn-deformational temperatures inferred from petrographic observations of these samples vary from greenschist to mid-upper amphibolite grade without an observable spatial pattern. The degree of footwall strain is also highly variable and does not follow a spatial pattern. Rf/Phi strain measurements generally support a dominance of non-coaxial strain with some exceptions.

Taken together, these observations suggest a more complex evolution for the footwall mylonite zone than previously believed. Temperature observations imply either that Oligo-Miocene temperatures were higher then originally hypothesized or that the mylonites formed during multiple phases. If formation of the footwall mylonites was poly-phase, kinematic observations suggest that earlier phases were also extensional in nature. We speculate that the Harcuvar core complex may have begun to form much earlier than previously recognized, perhaps during the early Tertiary. A poly-phase history would also complicate attempts to understand the geometry, strain magnitude, and total displacement of the shear zone. New EBSD and 40Ar/39Ar data are anticipated to bring new constraints on the timing and tectonic significance of the Harcuvar footwall mylonites.