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Summary and Recommendation The revolution in earth sciences brought about by the the- ory of plate tectonics was possible largely because major concepts could be tested by drilling in the ocean floor. The more complex structures and longer histories of the continents are difficult to in- terpret in the framework of plate tectonics. Geophysical methods that explore the continental lithosphere from its surface indicate large structures that appear to have formed as a result of plate interactions, but whose existence and nature can only be demon- strated by direct sampling. The drill, so successful in recovering rock samples used to prove the fundamental hypotheses of plate tectonics for the oceanic lithosphere, is now considered a criti- cal research tool for unraveling the history of the continents and understanding the processes that formed them. Recent improvement in prospects for funding for a national program of dedicated research drilling prompted the Continental Scientific Drilling Committee to suggest that highest priority be given to research drilling to test the thin-skinned tectonic thrust- ing model for the southern Appalachians (Continental Scientific Drilling Committee, 1984). Results of surface geologic mapping (Hatcher, 1972), seismic reflection profiling (Cook et a/., 1979), and potential field studies (Hatcher and Zietz, 1980) are all consis- tent with the hypothesis that extensive sheets of Precambrian and
Paleozoic crystalline rock, generally 6 to 15 km thick, have been thrust at least 260 km to the west over lower Paleozoic sedimen- tary rocks of the proto-Atlantic continental margin. A dedicated research drill hole would be designed to study and ultimately pass through the crystalline overthrust to penetrate these sedimentary rocks, perhaps even reaching the underlying basement. The hole would be sited so that these goals could be achieved with a target depth of 10 km. The southern Appalachian research drill hole is being proposed to solve a major geologic problem, the thin-skinned thrusting hy- pothesis, the solution of which will have application to many of the important orogenic belts of the world. The hypothesis cannot be tested without adequate samples of rocks from all depths in the hole, especially the greatest depths. Success of the project depends upon our ability to identify rock units and structures en- countered in the hole. Samples for chemical and physical studies must be significantly larger than the grain size of the rock, free of contamination, and accurately fitted into the general geologic setting. Preferably the samples should also display a continuity and coherence sufficient for an observer to spot important small features such as veins, fractures, alteration zones, and critical con- tacts. Continuous drill core is ideal for this purpose, and it can be obtained in adequate diameter (preferably greater than 4 cm) and length for scientific study. In contrast, cuttings from drill holes in crystalline rocks tend to be fine grained, and logging techniques in such terranes are not yet calibrated with the sophistication typical of petroleum exploration in sedimentary basins. The Continental Scientific Drilling Committee therefore rec- ommends extensive coring as essential to achieving the scientific goals of the Southern Appalachian research project. Moreover, if nearly continuous core is recovered, a vast amount of new infor- mation can be obtained on the deformational and thermal history of the southern Appalachians and on the chemical and physical properties of the rocks. The equipment to core the entire 10-km hole can be assembled from existing technology. Only after rigorous evaluation of the cost of continuous core should a decision be made to core less than the entire hole. If that be the case, at least 30 percent should be cored, with emphasis on the bottom 2 km. Results of core studies will be used to improve interpretations of geophysical and well-logging
data for similar terranes and will be applicable to understanding physical and chemical processes within many other orogenic belts. This report concerns scientific problems that can be addressed through studies of core samples from the southern Appalachian scientific research drill hole, requirements for obtaining the core samples in suitable size and condition, and some approaches to the coring process. Although aimed at this specific target, the types of scientific problems and methods of study are common to many projects likely to be approached with the drill as a research tool. Properly curated and preserved in a repository, lengths of drillcore from a variety of depths and localities in the continental crust of North America will represent an enormously important scientific resource. Such a resource would have lasting value and would, in fact, perhaps gain in value as more and more sophisticated analytical instruments are manufactured and as new geological insights are developed. Obtaining sufficient amounts of core to achieve the goals of each drilling project should be one of the highest priorities of the Continental Scientific Drilling Program.
