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1 Introduction TESTING THE THIN-SKINNED THRUSTING HYPOTHESIS The southern Appalachians are made up of several subpar- allel belts or terranes (Figure 1). From northwest to southeast these are the foreland composed of the Cumberland Plateau and Valley and Ridge province, the Blue Ridge, Inner Piedmont includ- ing the Chauga Belt, Avalon Terrane composed of the Charlotte and Carolina Slate belts, and the Brunswick Terrane (Hatcher et ai, 1982). These belts are separated in most cases by faults that dip to the southeast. The proposed drill site would be lo- cated in the Inner Piedmont such that underlying Blue Ridge rocks and the intervening Brevard fault zone would be encoun- tered before parautochthonous sedimentary rocks and Grenville basement are penetrated (Figure 2). A wide variety of rock types can be expected, from weakly metamorphosed sedimentary rocks to granitic gneisses. All are likely to be anisotropic, and rocks in the vicinity of fault zones may be mylonitic. Because the surface geology is well known, the succession of rock types to be expected in drilling can be anticipated. Existing geophysical surveys and those now being carried out in the site selection process allow these projections to be made with a comparatively high degree of
FIGURE 1 Major subdivisions of the southern Appalachians and detail of the proposed site study area. AA"âabbreviated section shown in Figure 2.
BLUE RIDGE Blue Ridge Thru Modantaly lo itrongly â¢nliottoplc m«dlum to hlgh grada ortho- and Strongly anlaotroplc low grad* phyllonllai . lmpura carbonalaa. Martiltai, mylonlloi, gunltlc gn*l»o> Uod«ralaly lo itrongly anlaotroplc madlum lo hlgh grada ortho- and Uodaralaly anlaotroplc low grada Waakly lo modaralaly anliotroplc granltlc and mallc gnaliiai and pluloni TD 10 km (35,000 ft) FIGURE 2 Enlarged portion of Transect Section E-5 (Figure 1) showing the possible tectonic units and generalized lithologies likely to be encountered if the deep hole were drilled directly on the line. The striped area is the Blue Ridge thrust sheet containing a highly deformed and transported mass of basement (black). Arrows above surface indicate the approximate locations of six proposed shallow boreholes projected into the line of section. The entire section is 55 km in length. There is no vertical exaggeration. Location of this section is shown as A-A' in Figure 1. confidence. Confirmation of the thin-skinned thrusting hypoth- esis will depend on identification of key groups of rocks in the hole. Since all of the rocks in the thrust sheet have been meta- morphosed, fossils will not be present. Rocks beneath the master detachment thrust should be of lower metamorphic grade and thus may contain fossils. Correlations of high grade metamorphic rocks with surface exposures to the west will have to be made on the basis of stratigraphic succession of rock types, chemical and iso- topic compositions, radiometric ages, and similarity of thermal and deformational histories. Preliminary identification of rock units can be made at the drill site by geologists familiar with the area. Cuttings a few mil- limeters in size probably would be sufficient for this purpose, but the prospect of obtaining such coarse material from hard crys- talline rocks is slim. Core samples are superior, but they may
TABLE 1 Major geologic targets to be cored in a southern Appalachian research drill hole. Target Estimated Depth (km) Inner Piedmont Amphibolite Granitic Gneiss 0-4 Biotite Gneiss Chauga Belt NW SE Henderson Gneiss Chauga River Fm. Poor Mountain Fm. 4-5 Brevard Fault Zone Blue Ridge Talullah Falls Fm. Grenville Basement (allochthonous) 5-8 Sole Thrust 8 Platform Sedimentary Rocks Knox Group and pre-Knox rocks 8-9 Grenville Basement (autochthonous) 9 not be representative of lithologically heterogeneous units unless sufficiently long intervals are cored continuously. Table 1 gives sug- gested minimum lengths of core needed from each major geologic unit and fault zone thought to be present. Most units probably could be identified with 3 meters of core. Recovery of contact zones between units will require much longer cored intervals un- less groups of units have been tectonically thinned on one limb of a fold. In the Chauga Belt, for instance, 30 meters of core might transect as many as three stratigraphic units. Although microstructures and geometric relationships between lithologies cannot be elucidated from cuttings, they may be critical to iden- tification of stratigraphic units. Once the response of specific rock units is known, logs may simplify correlation. Advance calibration of logging tools could be done in other suitable drill holes in the appropriate terranes. In a limited coring program, anticipating where cores should be taken on the basis of on-site examination of cuttings is risky and could result in failure to core critical intervals. Other methods of downhole sampling would have to be employed,
TABLE 2 Minimum lengths of core needed for various types of studies or laboratory measurements. Core diameter assumed to be at least 2.5 cm unless otherwise noted. Tabulated minimum lengths for physical property measurements assume fine- to medium-grained rocks with internal structures on a scale significantly less than core size. Investigation Minimum length (m) Identification of 10 geologic units Study of contact 10 and fault zones Geochemistry 0.4-8 (2.5 cm dia.)* 0.04-0.8 (7.6 cm dia.)* Deformation 10 Metamorphic less than 10 petrology Geochronology 1.5-7 (2.5 cm dia.)* 0.2-0.7 (7.6 cm dia.)* Thermal conductivity 0.01* Seismic velocity 0.1* Magnetic properties 0.02* Electrical properties 0.02 Hydraulic properties 0.02 Elastic and deformational properties 0.02 * excluding Brevard fault zone and sole thrust per sample necessitating interruption of drilling or postponement of sampling until completion of the hole but without setting casing. Continuous core doubtless would be ideal for answering the fundamental scientific questions posed by the southern Appala- chian hole. Less extensive coring, such as 30 percent of the hole, may be a viable option if cuttings are sufficiently coarse for un- ambiguous field identification and if later downhole sampling is possible (Table 2). If the entire hole is not cored, coring should emphasize the base of the overthrust sheet and the platform sedi- ments below. Perhaps as much as 1000 meters of core are needed from this interval in order to extract the maximum amount of information on the timing, mechanics, and conditions of over- thrusting. Coring at shallower levels should emphasize ductile fault zones that separate terranes. Such zones may be only a few meters thick, so that approximately 10 meter cores would be ade- quate to sample their entire thicknesses, provided that the depths of these zones can be predicted accurately.
