GEOLOGIC FRAMEWORK GOVERNING THE GEOMETRY AND NATURE OF OPEN FRACTURES IN THE CRYSTALLINE SOUTHERN APPALACHIANS OF THE SOUTHEASTERN UNITED STATES

HATCHER, R.D., JR., bobmap@utk.edu, Department of Geological Sciences, University of Tennessee, Knoxville, TN 37996­1410 and Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831; and COSTAIN, J.K., costain@vt.edu, Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061.

The crystalline southern Appalachians comprise all but the westernmost Blue Ridge, and all of the Piedmont. This assemblage records a complex history of crust formation that spans most of the Paleozoic and culminated with the collision of Africa and North America in the late Carboniferous Permian, constructing one of the world's great mountain chains. The diverse array of exposed rock types and structures present permits us to subdivide the crystalline southern Appalachians into the western Blue Ridge of proximal Laurentian origin; the Piedmont terrane of distal Laurentian origin, separated from the western Blue Ridge by the Hayesville fault and suture, and the Carolina exotic terrane of African/European origin. The older structures and rock types may locally form conduits for ground water, but later brittle fractures form the main plumbing system for ground water in the crystalline Appalachians. These fractures formed during late Paleozoic and post-Paleozoic times as the upper crustal rocks cooled and the ductile-brittle transition readjusted to new thermal gradients related not to orogenesis but to the trailing margin tensional environment that developed from Late Triassic time until the Late Cretaceous to the present, when compression again prevailed, but this time related to ridge push.

A diverse array of systematic fractures was formed as a product of the late Paleozoic to Recent brittle tectonic history. Most of these fractures are planar, have steep dips (>70°), and cluster into four principal sets: N45°W, N45°E, N10°W, and ~EW. This steeply dipping array is hypothesized to form the principle open fracture system for conducting ground water into the subsurface to depths where major quantities of water have been encountered in subhorizontal sheeting (unloading) nontectonic fractures. Such unloading subhorizontal fractures have been identified by temperature logs in the ADCOH area at depths of 150-200 m. This type of occurrence may have first been noted by LeGrand in 1959. The steeply dipping systematic array also provides weak zones for alignment of surface drainage systems that in places modify an otherwise random (dendritic) drainage system into a rectangular and locally trellis system.

The depths at which open fractures close in crystalline rocks are unknown at present; however, others (e.g., Brace in 1980 and 1984) have long suggested that hydrostatic fluid pressures are present in open fracture systems in crystalline rocks to depths of at least 10 km. The systematic recharge fractures of tectonic origin together with the nontectonic subhorizontal unloading fractures complete the regional plumbing system in crystalline terranes, and provide a major reservoir system that may yet be largely unexplored worldwide.