DETECTION AND CONSEQUENCES OF OPEN FRACTURE NETWORKS IN THECRYSTALLINE SOUTHERN APPALACHIANS OF THE SOUTHEASTERN UNITED STATES

COSTAIN, J.K., costain@vt.edu, Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061,and 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, TM 37831.

Brace (1980, 1984) concluded that: 1) values of permeability determined in drill holes (2-3 km) in crystalline rocks range from about 10-11 to 10-7 cm2, 2) over some interval in nearly all of the boreholes, permeability was 10-14 to 10-9 cm2, and 3) permeabilities inferred from earthquake migration and other large-scale crustal phenomena range, for crystalline rocks, from 10-12 to 10-10 cm2, and are thus about the same as the more permeable intervals in boreholes. Brace also concluded that, in areas where crystalline rocks extend to the surface, the pore pressure will be equal to the hydrostatic fluid pressure to depths of at least 10 km. Data from water-bearing fractures from 227 wells in crystalline rocks in Coastal Maine (Loiselle and Evans, 1995) indicate that there is no evidence that fracture yield or fracture density decrease with depth in at least the upper 180 m. Intercrystalline porosity is absent in these rocks, and porosity is entirely a result of brittle fracturing. Groundwater flow in these rocks is localized along fractures.

Precision temperature logs are particularly useful for indicating the depths to permeable fracture zones. Although a display of logs of temperature versus depth does not obviously reveal temperature anomalies, the derivative (the geothermal gradient) of such a log clearly does. In Lancaster County, SC, the Virginia Tech geothermal drilling program clearly identified permeable intervals over the depth interval 260-325 m in the Africa-North America collision-related Alleghanian Liberty Hill-Kershaw granite pluton. Precision temperature logs obtained by Virginia Tech in crystalline basement rocks beneath the Savannah River Site in South Carolina clearly indicated subhorizontal fracture zones at depths of 450-600 m that correlated well with pump tests conducted over these same intervals. We suggested earlier that the 450-600 m depth might be related to a paleosurface on the base of the Coastal Plain sediments.

Thus, the available data do not justify imposing limits on well depths when drilling for water-bearing fractures, supporting the conclusions of Daniel (1989). Indeed, Costain and Bollinger (1991) showed a correlation between long-term variations in the elevation of the water table and periods of intraplate seismicity.