Geology of the 1:24,000 Tallassee, Alabama, Quadrangle, and its Implications for Southern Appalachian Tectonics
Type of DegreeThesis
Geology and Geography
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The 1:24,000 scale Tallassee, Alabama, Quadrangle, was mapped to characterize lithologies and structures in the southernmost exposures of the southern Appalachians. Four deformation events are recognized. D1 was associated with prograde amphibolite-facies metamorphism that produced the dominant schistosity and gneissosity, S1, isoclinal folds of compositional layering, S0, and a mineral lineation, L1. D2 was a lower amphibolite- to upper greenschist-facies event that occurred during late M1 metamorphism. D2 resulted in F2 isoclinal folds of S0/ S1 and conical folds, extensional shear bands (S2), L2 elongation lineation, and foliation boudinage. D3 was a greenschist-facies event that produced F3 kink folds and the map scale Tallassee synform and S3 crenulation cleavage. D4 resulted in tension gashes and small cataclastic zones. The base of the Dadeville Complex is marked by the pre- or syn-metamorphic Stonewall Line shear zone on the southeast limb, through the hinge zone, and on the northwest limb of the Tallassee synform. North of the Tallassee Quadrangle, late, post-M1 reactivation of the Katy Creek fault cut D1 fabrics and S0/S1 layering of the Dadeville Complex and excised a segment of the Stonewall Line shear zone. The Stonewall Line shear zone is interpreted to be a segment of the early stage of development of the Brevard zone. Within the study area, Jacksons Gap Group metasedimentary rocks are interleaved with quartzofeldspathic gneisses interpreted as mylonitized Farmville Metagranite, the latter becoming more abundant as the hinge zone is approached from along the synform’s southeast limb. Middle Ordovician Kowaliga, Zana, and Farmville granitoids are interpreted to continue through the buried parts of the Tallassee hinge zone, supporting correlation of the eastern Blue Ridge with portions of the Opelika Complex. The base of the Opelika Complex, i.e., the Towaliga fault, should be considered to be a segment of the Hayesville-Fries fault since it emplaces eastern Blue Ridge rocks upon Laurentian units (i.e., the Pine Mountain terrane). The presence of a voluminous mass of granitoids and migmatites at the base of the Dadeville Complex in this area is compatible with the concept of a ‘super migmatite’ zone, supporting Hatcher and Merschat’s (2006) interpretation for mid-crustal level channelized flow of the Inner Piedmont terrane. The Opelika Complex is not related to the Inner Piedmont, as is traditionally thought, but is continuous with the eastern Blue Ridge around the hinge zone of the Tallassee synform as originally suggested by Grimes et al. (1993a).