STRUCTURES AND MICROSTRUCTURES ALONG THE BREVARD FAULT ZONE

The Brevard fault zone represents a major tectonic lineament of the Southern Appalachians that extends from eastern Alabama northeast into Pennsylvania. The lineament has been the subject of intensive studies and debates for over 100 years but its nature and significance are still unclear. Interpretations of the structure are manifold and range from an infold to an overthrust, a normal fault, a strike-slip fault or an alpine-type root zone. The complex nature of the Brevard zone probably results from several tectonic events throughout the Appalachian orogenesis. In order to elucidate the deformation history of the fault zone three sections were investigated in detail by microstructural studies and X-ray quartz texture analyses. The first section, close to Douglasville (GA), consists of graphitic schists and mica quartz feldspar schists interlayered with quartzites forming right lateral en-echelon folds on a kilometer scale. Small scale kinematic analyses on augen structures suggest dextral shear along the fault zone. The second section, close to Atlanta (GA), is highly deformed and consists of metapelites, mafic rocks as well as partially feldspar-rich quartzites, which are sheared into mylonites. Foliation shows consistent northeast strike and dip ranging from 50 to 70 degrees to the southeast. The third section, close to Rosnan (NC), is characterized by a well developed deformation gradient which increases from the Henderson Gneis into quartz-rich ultramylonites in the center of the Brevard zone. All sections are characterized by sub-horizontal mineral lineations. We used this lineation and the dominant foliation as a reference system for our quartz texture analysis. Quartz textures are considered to reflect the kinematics of the last deformation increments. The preliminary results of our X-ray texture analyses reveal quartz c-axes distributions which range from centered point maxima, indicating high temperature shear, to asymmetric single girdles, reflecting advanced dextral shear. These results suggest that the Brevard fault zone was dominated by dextral strike slip displacement during the last stage of ductile deformation. This is also in agreement with our observations on the outcrop scale.