Late to post-Appalachian strain partitioning and extension in the Blue Ridge of Alabama and Georgia

Structural observations and U-Pb and 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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. The Alexander City fault zone is a middle greenschist facies, dextral strike-slip fault rather than a west-vergent thrust fault, as was previously thought. This fault zone is obliquely cut and extended by more east-trending, subvertical, cataclastic faults (Mesozoic?) characterized by intense quartz veining. These brittle faults resemble those in other parts of the Blue Ridge, Inner Piedmont, and Pine Mountain terrane and, together with the Goodwater-Enitachopco and Towaliga faults, they appear to form a broad graben-like structure across the entire piedmont. Shoulder rocks flanking the Alexander City fault zone contain earlier formed (peak to late peak metamorphic) dextral shear zones. A 369.4 ± 4.8 Ma U-Pb thermal ionization mass spectrometry date on zircon records the time of crystallization of a dike that cuts one of the shears bracketing the peak metamorphic fabric to between ca. 388 and 369 Ma and places a minimum age for right-slip shearing. Similar kinematics, geometries, tectonostratigraphic positions, and timing indicate that these Devonian shears are more southern counterparts to the system of Neoacadian dextral faults exposed in the North Carolina Blue Ridge. Kinematic analysis of the Goodwater-Enitachopco and Alexander City faults document that dextral strains in the Alabama and western Georgia Blue Ridge are partitioned much farther toward the foreland than is reported to the northeast, likely as a consequence of the southern Appalachian master decollement having passed obliquely across a several kilometer step up along the Cartersville transform. The top-to-the-south-southeast normal-slip component of movement along the Goodwater-Enitachopco fault is unusual, considering its position far toward the foreland. Loose timing constraints for this extensional event (late Carboniferous to Early Jurassic) leave room for several tectonic explanations, but we favor the following. (1) Late Pennsylvanian to Early Permian crustal thickening created a wedge of Blue Ridge rocks bound above by the Goodwater-Enitachopco, below by the decollement, and to the northwest (present-day direction) by a topographically steep mountain front. (2) Further convergence and crustal thickening caused this wedge to gravitationally collapse with southward-driven motion. (3) Mesozoic rifting reactivated some of the faults as the Gulf of Mexico began to open.