The ADCOH (Appalachian UltraDeep COre Hole) Project

The ADCOH Project to drill the first ultradeep hole in the United States for scientific purposes was funded by the National Science Foundation under a Grant to Professor Robert D. Hatcher, Department of Geological Sciences, University of Tennessee at Knoxville. Geophysical data acquired by, or under the supervision of, Virginia Tech for this project included heat flow determinations in four holes, and a number of regional reflection seismic lines. The ADCOH site area was particularly attractive for a deep scientific hole because abnormally high temperatures or fluid pressures were not anticipated at depths of 10-12 km, and the excellent (in hindsight) reflectivity of the Inner Piedmont and surrounding areas would have provided a remarkable reference seismic section for future studies elsewhere in the southeastern part of the Appalachian Mountain System. The preliminary site selection project was a resounding success overall; however, funding for the deep, 10-12 km hole planned near Westminster, South Carolina, never materialized. Many publications resulted from the preliminary site study.

ADCOH Hole 1 ADCOH Hole 3 ADCOH Hole 2 ADCOH Hole 4 ADCOH Seismic Line 1 ADCOH Seismic Line 3

Adcoh-1 Adcoh-2 Adcoh-3 Adcoh-5 Adcoh-6

There are many references to the ADCOH site study. Only results from the geothermal and regional reflection seismic lines are reported here. The following discussion of the seismic data is taken primarily from Coruh and others (1987).

The Appalachian Ultradeep Core Hole (ADCOH) Project Site Study seismic reflection data from South Carolina, North Carolina, and Georgia, are of the highest quality yet obtained in a continental crystalline terrane. Acquisition of the ADCOH seismic data was conducted using a 120-channel recording system and five vibrators. Spread configuration was a symmetrical split-spread with offsets up to 4150 m and a receiver group interval of 67 m. An up-sweep of 24 s length with a linear frequency band of 14 to 56 Hz was used throughout to give 8 s of full-correlation time. Seismic data processing with multi-pass velocity and statics analysis resulted in excellent quality sections.

Interpretation of the ADCOH lines introduced new important elements for subsurface models for the southern Appalachians, where large-scale overthrusting of crystalline thrust sheets has been suggested. The Blue Ridge master decollement overlies a deformed sequence of early Paleozoic strata, as thick as 4 - 5 km. The Brevard fault zone appears to splay from the master decollement at a depth of 6 km; thrusts of lesser importance occur at about 9 km. The interpreted thickness of the Blue Ridge thrust sheet (2-4 km) is less than previously estimated, but is about identical to the thickness of 3 km of the metamorphic allochthon in central Virginia. The ADCOH seismic data also suggest the presence of Eocambrian-Cambrian(?) rift basins in the Grenville basement. Reflections from within the Grenville basement provide new evidence for the lack of seismic transparency of the upper crust in areas that have not undergone Mesozoic extension.

The following example of the regional seismic data is from the northwesternmost end of Line 3. See also the correlation of these data with seismotectonic structures along the Savannah River Corridor by Domoracki and others (1999).

ADCOH NW end of Line 3

References

Çoruh, C., Costain, J.K., Hatcher, R.D., Jr., Pratt, T.L., Williams, R.T., and Phinney, R.A., 1987, Results from regional vibroseis profiling: Appalachian ultra-deep core hole site study, in Deep Seismic Reflection Profiling of the Continental Lithosphere, edited by D. Matthews and C. Smith, Special Issue of the Geophysical Journal of the Royal Astronomical Society, v. 89, pp. 147-156.

Costain, J. K. and E. R. Decker, 1987, Heat flow at the proposed Appalachian Ultradeep Core Hole (ADCOH) Site: Tectonic Implications, Geophys. Res. Letters, v. 3, no. 14, 252-255.

Costain, J.K., Çoruh, C., and Boyer, S.E., 1993, Synchronous thrusting and folding near the Blue Ridge master decollement: An interpretation from reflection seismic data, Geological Society of America Abstracts With Programs, v. 25, No. 6, p. A-178.

Domoracki, W.J., Stephenson, D.E., Çoruh, C., and Costain, J.K., 1999, Seismotectonic structures along the Savannah River Corridor, South Carolina, U.S.A., v. 27, pp. 97-118. (Refers to Line 3 figure above.)

Hatcher, Jr., R.D., Costain, J.K., Çoruh, C., Phinney, R.A., and Williams, R.T., 1987, Tectonic implications of new Appalachian ultradeep core hole (ADCOH) seismic reflection data from the crystalline southern Appalachians, in Deep Seismic Reflection Profiling of the Continental Lithosphere, edited by D. Matthews and C. Smith, Special Issue of the Geophysical Journal of the Royal Astronomical Society, v. 89, pp. 157-162.

Hatcher, Jr., R.D., R.T. Williams, S.H. Edelman, J.K. Costain, C. Çoruh, R.A. Phinney, K. R. Chowdury, E.R. Decker, M.D. Zoback, D. Moos, and R.N. Anderson, 1988, The Appalachian ultradeep core hole (ADCOH) project, In Deep Drilling in Crystalline Bedrock, v. 2, (A. Boden and K.G. Eriksson, eds.), Springer Verlag, Berlin, 117-154.

Hubbard, S.S., Çoruh, C., and Costain, 1991, Paleozoic and Grenvillian structures in the southern Appalachians: Extended interpretation of seismic reflection data, Tectonics, v. 10, p. 141-170.


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Regional Geophysics Laboratory, Department of Geological Sciences, Virginia Polytechnic Institute and State University, 1046 Derring Hall, Blacksburg, VA 24061-0422. Comments to: costain@vt.edu

Last updated: 05/15/97