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5 Deformational History Rock samples recovered from a deep drill hole can yield a wealth of information on geologic structures and rock mechanics within terranes and at their tectonic contacts. Models of defor- mation mechanisms and related physical and chemical processes that operate in orogenic belts such as the Appalachians are de- rived from observation and interpretation of rock textures at many scales. Optical, scanning-electron, and transmission-electron mi- croscopy provide information beyond that obtained from study of macroscopic structures. Some discussions of deformation textures in rocks are given by Spry (1969), Vernon (1975), Nicolas and Poirier (1975), and Hobbs et al. (1976). Interpretation of deformation textures is based on knowledge of discrete grain-scale deformation mechanisms, each of which is dominant at a particular set of physical conditions (pressure, temperature, fluid properties), has a particular rheology (flow law), and produces characteristic textures or microstructures (e.g., Tullis et al., 1982). Experimental deformation studies under con- trolled conditions have succeeded in reproducing many of the microstructures observed in naturally deformed rocks, and they show how textures vary as a function of lithology, finite strain, presence of water, and other parameters. Although recent work 19
20 has contributed to increased ability to interpret natural deforma- tion textures, some caution is necessary because rocks can have complex deformation histories, and their textures may reflect only the last increment of deformation. Relict textures that measure evidence of earlier deformations, however, are commonly present. Some examples of the sorts of information that can be obtained from a study of deformation textures of rocks likely to be recovered from a southern Appalachian research drill hole are given below. GRAIN-SCALE DEFORMATION MECHANISMS The chief mechanisms are fracture, cataclastic flow, pressure solution (the dissolution of material from highly stressed interfaces and redeposition on low stress interfaces), dislocation creep, and grain boundary sliding. Identification of the dominant mechanism allows an inference to be drawn about the mechanical behavior (rheology) as well as the physical conditions of deformation. Com- plex textures may give evidence for more than one episode of deformation, occurring under different conditions and at different times. For instance, grains flattened by dislocation creep may be offset by later grain-scale faults, indicating a cooling accompanying uplift or perhaps an increase in fluid pressure. Such information complements that obtained from geochronology and metamorphic petrology. Knowledge of the operative grain-scale deformation mecha- nisms is important for understanding deformation on larger scales, such as the mechanism of emplacement of thrust sheets and the degree of basement involvement in orogenic belts. For example, Schmid (1975) has convincingly shown that the Glarus thrust in the Alps was emplaced on a 1-meter thick layer of calcite my- lonite that underwent grain boundary sliding and behaved in SL macroscopically superplastic fashion. This mechanism allowed for extremely sharp strain localization. At present there is much de- bate about the apparent tendency for a number of major thrust sheets to be located within dolomite layers (Burchfiel et a/., 1982), because this rock type is traditionally thought to be very strong. However, it appears that dolomite may have an anomalously low friction coefficient, especially in the presence of water (Weeks and Tullis, 1984), and some evidence of this is given by the textures of its gouge products. These results have important implications for the situation in the southern Appalachians, particularly with
21 regard to the master sole thrust which is thought to occur in dolomitic rocks. CONDITIONS OF DEFORMATION Deformation textures can be used as rough indicators of de- formation temperature, based on a number of calibrations be- tween experimental studies and natural occurrences. For exam- ple, quartz shows the beginnings of syntectonic recrystallization at about 300° C, and it is completely recrystallized at only moderate strains from 350° C to higher temperatures; the recrystallized grain size increases with temperature. Feldspar, on the other hand, does not show syntectonic recrystallization until a temperature of about 450°C is achieved. Some textures, such as filled veins and fibrous overgrowths, can be used to infer high fluid pressures. These fluid properties should be consistent with results of petrologic studies. TYPE AND AMOUNT OF FINITE STRAIN Studies of deformation textures determine the amount of strain that different rocks have undergone. Similarly, if the strain occurred by simple shear, the orientation and sense of shear can be defined (Simpson and Schmid, 1983). There are a number of ways in which the type and amount of finite strain can be de- termined. These involve strain markers, such as grain, pebble, and fossil shapes, or foliations and lineations. Deformation under some conditions, such as dislocation creep accompanied by syntec- tonic recrystallization, produces equigranular textures that do not indicate any strain; in such cases, measurement of the preferred crystallographic orientation of quartz can be useful in determining the type of strain and its approximate magnitude. Deformation by pressure solution results in textural relations that can supply information about the strain history of rocks. Curved quartz fibers precipitated on rigid grains, such as magnetite or pyrite, provide evidence of changes in orientation of the rock with respect to the stress axes over time. Evidence for strain history is also gleaned from foliations and lineations, which may show deformation after formation; e.g., an early foliation may be subsequently crenulated. Such textures may be preserved by mineral inclusions in porphy- roblasts (Bell and Rubenach, 1983). Because of their small size, lack of preservation of fractures and veins, and lack of information
22 on circulation, cuttings would be inappropriate for strain analy- sis. Core samples are required to carry out useful studies of rock deformation. TYPE AND MAGNITUDE OF DEFERENTIAL STRESS This information can be obtained from orientations of exten- sion and shear fractures, both macroscopic and grain-scale, and from the orientation of crystallographic twins in grains of calcite, dolomite, or pyroxene. In addition, twins can give an indication of the magnitude of the stress (Tullis, 1980). Recent paleopiezomet- ric techniques can be applied to rocks that have been deformed by steady state dislocation creep in the absence of subsequent an- nealing. Experiments and theory show that in such cases there is a relation between stress magnitude and recrystallized grain size, subgrain size, and dislocation density (Christie and Ord, 1984). Finally, if direct measurements of in-situ stress in the bore hole are to be made as accurately as possible, oriented core is essential to provide rock samples on which to make the relevant elastic tests. DEFORMATION OF THE SOUTHERN APPALACHIAN OROGEN Given that information on deformation mechanisms, amounts of strain, and stress parameters can be provided by study of defor- mation textures in core samples, what are some of the important geologic problems to be solved or processes to be studied in an ultra-deep drill hole in the southern Appalachians? One of the most important functions of a cored hole will be to provide cali- brations of surface geophysical and down-hole logging techniques that are at present poorly known for crystalline rocks. A ma- jor question concerns the seismic expression of mylonite zones: do they produce a definite and unique signal? In addition, is there evidence for transport of the Inner Piedmont as a thrust sheet over relatively undeformed basement, and if so, how sharp is the detachment surface and what mechanisms of deformation were dominant? Did the deformation involve lithologic control of strain? To what extent was basement involved in the deformation? Other fault zones besides the sole thrust will be encountered and should be sampled in a drilling program. Is the Brevard zone a
23 sharply defined mylonite zone of finite width at depth, similar in character to what is seen in surface outcrops? One cannot accurately predict in advance where the struc- turally significant sections will be, yet thorough sampling of duc- tile fault zones is necessary if the maximum amount of information on deformation is to be obtained. Continuous core through these zones and representative core samples from less strained sections will be needed. Furthermore, only through coring will there be ex- act positional control on samples, not only to place rocks in proper sequence in the hole but, equally important, to allow determina- tion of the spatial orientations of structural features. Core of any diameter that might be obtained will be adequate for microstruc- tural studies as long as a high percentage of recovery is achieved through important intervals (Table 2). Orientation of structures with respect to vertical can be easily determined in normal core, but the azimuths of these features cannot. The problem of obtain- ing samples of known orientation in three dimensions can be solved by sidewall sampling of critical intervals after macroscopic exami- nation of core. Such oriented samples would also be of great value for paleomagnetic studies and physical property measurements.
