TWO SYNCHRONOUS THRUST FAULT SYSTEMS IN THE EARLY ALLEGHENIAN OROGENY

The Alabama Blue Ridge of the southern Appalachians contains metamorphic rocks juxtaposed along thrust faults that carry rocks of different metamorphic grade. The timing of these thrust faults is traditionally interpreted to break towards the hinterland, where younger faults progressively develop toward the southeast. To test the timing of thrust faults in the Alabama Blue Ridge, we present geologic mapping and U-Pb zircon from 14 samples of the northern Blue Ridge. U-Pb zircon analyses collected by laser ablation depth profiling of zircon rims from 8 samples reveal subpopulations of elevated U/Th (>10) zircon populations interpreted to reflect 370-345 Ma metamorphism in the Ashland Supergroup.

The Ashland Supergroup, a unit containing mid-to-upper amphibolite facies rocks, must have been loaded to mid-crustal conditions between 370 and 345 Ma, which coincides with burial and metamorphism of the Emuckfaw Group and Jackson’s Gap Group to the southeast. These assemblages to the southeast, however, are separated by the Wedowee Group, which experienced peak metamorphism at 331-320 Ma (Stowell et al, 2019). A ⁴⁰Ar/³⁹Ar muscovite cooling age from the Jackson’s Gap Group is ~315 Ma (Whitmore, 2018).

The simultaneous loading of rocks in the footwall and exhumation of rocks in the hanging wall of the Alexander City fault suggests thrust loading as a mechanism to drive metamorphism in the Alabama Blue Ridge. A similar timing relationship exists between the Talladega belt and overriding Ashland Supergroup, where older metamorphic rocks of the Ashland Supergroup were likely juxtaposed on top of the Talladega belt between 330 and 320 Ma, which we interpret to reflect thrust loading. Peak metamorphism for the Talladega belt is constrained to 334-320 Ma (McClellan et al, 2007).

These interpretations would require that two different thrust fault systems in different parts of the Alabama Blue Ridge were actively loading rocks to mid-crustal conditions and stacking metamorphic thrust sheets. After metamorphic stacking, the intermediate Goodwater-Enitachopco fault that separates the Ashland Supergroup and Wedowee Group placed the Wedowee/Emuckfaw thrust system above the Talladega-Ashland thrust system. The proposed timeline requires out of sequence thrust fault development, with two, synchronous thrust fault systems active 330-320 Ma within the Appalachian hinterland.