DEFORMATION MECHANISMS AND KINEMATICS OF THE HILLABEE SHEAR ZONE, ALABAMA BLUE RIDGE

In the Alabama Blue Ridge, low-grade metasedimentary rocks of the Talladega belt, which represent the outermost preserved portions of the southern Laurentian margin, are structurally overlain by the Middle Ordovician Hillabee Greenstone (HG), of probable arc/back-arc affinity. The contact between the HG and the underlying rocks is a ductile fault, the Hillabee shear zone (HSZ), a pre-to synmetamorphic zone of ductile shearing that variably affected both hanging wall and footwall rocks. The HSZ is constrained to have formed some time after deposition of fossiliferous rocks in the underlying Talladega Group (~360 – 350 Ma), but prior to or during metamorphism of the Talladega Group footwall units. 40Ar/39Ar data indicate that this metamorphic episode occurred no later than ~320 Ma. The best gauge of the conditions and kinematics of the HSZ are mineral assemblages and microstructures in HG metadacite in the hanging wall, and in footwall quartzite. Mylonitic textures are particularly well developed in metadacite, and growth of aligned acicular actinolite, chlorite, and epidote between fractures in hornblende porphyroclasts constrain conditions of mylonitization to greenschist facies. Quartz-rich segregations consist of larger relict grains that display very prevalent subgrain development and less common deformation lamellae, surrounded by smaller recrystallized grains (core and mantle structure). A mineral stretching lineation, visible on the mesoscopic scale, was produced by separation and extension of fractured hornblende segments. Similar quartz microstructures are present in footwall quartzite. Preliminary analysis of oriented samples of metadacite and quartzite from the Millerville area (Clay Co.) indicates dominant top-to-NW thrust motion, changing to strike-slip in a map-scale bend in the HSZ. Latest movement on the HSZ can be tracked by a distinctive set of mesoscopic folds that occur in foliated rocks throughout the area. The folds overprint the dominant S2 foliation, and are typically asymmetrical, kink-like in geometry, and locally produce a strong crenulation cleavage. In areas of intense strain the folds are highly noncylindrical, with hinges that tighten and curve toward the shear direction indicated by shear bands, foliation "fish", and asymmetrical veins.