Late To Post-Appalachian Strain Partitioning And Extension In The Blue Ridge Of Alabama And Georgia

Structural observations and U-Pb and 40Ar/39Ar isotopic age dates are reported for shear zones and metamorphic rocks in the southernmost Appalachian Blue Ridge. Two major mylonite zones, the Goodwater-Enitachopco and Alexander City fault zones, have retrograded peak amphibolite facies fabrics and assemblages in rocks of the ancient Laurentian margin. Both faults are within a zone of transition between Carboniferous (Alleghanian) west-directed thrusts in the foreland and synchronous strike-parallel dextral shear zones in the hinterland. The 40Ar/39Ar hornblende and muscovite dates record late Mississippian cooling and exhumation from the Late Devonian (Neoacadian orogeny, 380–340 Ma) peak. Retrograde mylonites of the Goodwater-Enitachopco fault are of two types. Earlier formed, type 1, upper greenschist to lower amphibolite facies shears are roughly coplanar with the dominant schistosity of the country rock, and show northwest-southeast stretching. Later formed, type 2 shears are discrete, steeply dipping, middle to upper greenschist facies shear zones that cut across the type 1 shears, displacing them in an oblique dextral and normal slip sense. A 366.5 ± 3.5 Ma U-Pb SHRIMP-RG (sensitive high-resolution ion microprobe–reverse geometry) date on zircon from a prekinematic trondhjemite dike that is cut by a type 2 shear zone places a maximum age on the time of movement along the Goodwater-Enitachopco fault. The 40Ar/39Ar cooling dates place a minimum on the timing of extensional movement along the type 1 shears of between ca. 334 and 327 Ma. The Goodwater-Enitachopco fault coincides at depth with a basement step up that has been interpreted as a Cambrian rift fault formed along the ancient Laurentian margin, possibly reflecting its reactivation during Mesozoic rifting of Pangea.