THE BREVARD ZONE IN ATLANTA, GEORGIA AND THE MEERS FAULT OF SOUTHERN OKLAHOMA: CONJUGATE WRENCH FAULTS FORMED UNDER CONTRASTING TEMPERATURE CONDITIONS

Geologic investigations for recent deep tunnel projects in Cobb County and Atlanta, Georgia suggest that the Brevard Zone is a late Paleozoic, right-lateral wrench fault that is related more to the late Paleozoic wrench faults of southern Oklahoma than to the compressional tectonics that dominate the rest of the southern Appalachians. The principal difference is that the Brevard Zone formed in an already hot pile of amphibolite facies metamorphic rocks, whereas the southern Oklahoma system formed in a sedimentary basin. The Brevard Zone extends southwestward from Virginia, through Atlanta, and on into Alabama where it becomes buried under the Cretaceous onlap. The Brevard Zone cross cuts, albeit at a low angle, the structural, stratigraphic, and topographic grain of the Appalachians. The Brevard Zone in Atlanta contains both ductile and brittle deformational features. The ductile features are concentrated in a 2 km-wide belt of mylonite and ultramylonite. The brittle features consist of several persistent fracture sets that occupy a belt about 10 km wide that surrounds and includes the ductile belt. The fracture patterns mapped in the tunnels are consistent with a right-lateral wrench system. Together, the mylonite and the fractures show a continuous retrograde sequence ranging from amphibolite facies, through greenschist and zeolite facies, to post-metamorphic calcite pyrite, and gypsum. The mylonite and retrograde mineral assemblage are interpreted as being caused by the ambient temperature conditions in which the faulting happened to occur. The dropping temperature is reflected first in the change from ductile to brittle deformation, then by the various mineral assemblages occuring in the brittle fractures. The Brevard Zone appears to be the mirror image of the left-lateral wrench faults of Oklahoma. Both are of equivalent length and both have equivalent but opposite lateral offsets of around 35 to 45 km. The vertical offset of the Meers Fault is around 14 km (down to the northeast); the vertical offset of the Brevard Zone is not known. If both faults are projected beneath the Cretaceous onlap, they would meet near New Orleans at the intersection of the Mississippi embayment rift and the edge of the Paleozoic continental margin.