CAPÍTULO 5: ESTUDIO DE FACTIBILIDAD
5.3 A NÁLISIS DE COSTOS Y BENEFICIOS
Subsurface relationships in the study area are illustrated in four cross sections oriented roughly perpendicular to the strike of dominant planar fabrics (Fig. 3-2;
Plate 1). Although Ramsay (1967) demonstrated the inefficiency of cross sections for interpreting fabric data from polydeformed terranes, clearly the constricted NE-SW alignment of structures here permits their use. Parallel cross sections A-Aʼ, B-Bʼ, and C-Cʼ were constructed primarily to depict the relationships of macroscale fault zones in the northeastern IP. Relative steep dips depicted for thrust faults are interpreted to be the result of later folding and transposing events.
Section A-Aʼ
Section A-Aʼ (Fig. 3-2; Plate 1) illustrates the subsurface geometries of the BCFZ and PSSZ in the southwestern part of the study area (Fig. 3-21). The BCFZ juxtaposes eastern and western IP lithologies. New mapping from this study has shown that F1 sheath folds in the Brindle Creek thrust sheet (PSSZ) are truncated at depth by the BCFZ as depicted in this section. F2 and F3 tight to isoclinal folds fold hanging wall and footwall lithologies together along the BCFZ. Metagraywacke and schist units are thought to be conformable, and may be genetically related. Contact relationships between metasedimentary and metaigneous lithologies are depicted as concordant.
Toluca Granite lithologies are folded with boudinaged limbs. Walker Top Granite bodies are shown as layer-parallel sills folded into F1 map-scale sheath folds and exhumed in shear zones. Macroscale sheath folds are refolded and transposed by later F and F
Section B-Bʼ
Section B-Bʼ (Fig. 3-2; Plate 1) traverses the Neoacadian BFZ and the BCFZ.
Four major fault zones, the TCFZ, MSFZ, BCFZ, and PSSZ, are depicted in this section (Fig. 3-21). Differences in F1 fold style between eastern and western IP terranes are best illustrated in this section. Western IP F1 folds exist as map-scale anticline-syncline pairs in the footwall of the discontinuous TCFZ, while eastern IP F1 folds occur as map-scale sheath folds. Macromap-scale folds in the western IP are recognized by the repetition of stratigraphic units in the Tallulah Falls Formation. The TCFZ is recognized by strong mylonitic fabrics in the Henderson Gneiss near contacts. Dismembered bodies of Henderson Gneiss are characteristic of the western IP in the area and are likely the products of a boudinaged early Neoacadian thrust sheet containing the Henderson Gneiss. Neoacadian A-type subduction and deformation likely folded and dismembered the TCFZ. Shear sense indicators also show a large component of dextral strike-slip movement between the Henderson Gneiss and underlying lithologies, suggesting that further detachment of the Henderson Gneiss bodies could have occurred during a later orogen-parallel deformational event. The MSFZ is primarily a strike-slip fault and truncates all earlier western IP structures. Folding of the MSFZ likely occurred post D4 and after fault development.
The folded BCFZ is shown to the southeast and truncates F1 structures in the PSSZ. The PSSZ is shown to have a shallower dip than in section A-Aʼ, which can be attributed to late folding events. F2 and F3 folds refold F1 fabrics. Evidence for late strike-slip motion exists along all fault zones. Foliation form lines and intrusive contacts between the Toluca Granite and metasedimentary lithologies illustrate map-scale F5 refolds of earlier (F2 and F3) fold generations.
Section C-Cʼ
Section C-Cʼ also includes a segment of the Neoacadian BFZ and the BCFZ.
This section best illustrates the juxtaposition of eastern and western IP lithologies along the folded BCFZ. As in section B-Bʼ, macroscale folds in the Tallulah Falls Formation are truncated by the Henderson Gneiss in the tightly folded TCFZ. Map relationships infer truncation of the TCFZ by the MSFZ near the western boundary of the study area.
Therefore, the MSFZ here is shown to truncate the TCFZ and associated lithologies.
Map patterns clearly indicate the isoclinally folded nature of the BCFZ and its truncation of the PSSZ. Map-scale folds plunge to the southwest and expose the Silurian Brooks Crossroads Pluton in an embayment on the BCFZ. An additional body of Walker
Top Granite is exposed near the eastern boundary of the Moravian Falls quadrangle and is interpreted in this section to be fault bounded based on mylonitic textures near contacts.
However, insufficient exposure and outcrop area do not permit a more detailed structural interpretation. Late (F5) open folds can be seen to refold earlier fold generations, and NE-plunging folds in Toluca Granite bodies are inferred to be present in the subsurface here.
CONCLUSIONS
1. Four map-scale shear zones, the TCFZ, the MSFZ, the BCFZ, and the PSSZ, are present in the study area and are orogen parallel, striking northeast-southwest across the Moravian Falls quadrangle.
2. Six successive deformation events can be recognized in the northeastern IP, roughly corresponding to those outlined by previous investigators.
3. Eastern and western IP terranes experienced separate D1 events, as indicated by deformational style and geochronologic constraints, before being deformed and metamorphosed together during D2.
4. Truncated macroscale F1 folds in the TCFZ footwall must have formed earlier than the crystallization age of the Henderson Gneiss (~490 Ma).
5. Development of the TCFZ must postdate the age of the Henderson Gneiss, and is likely related late Taconian to early Neoacadian thrust emplacement.
6. The Walker Top and Toluca Granites occur exclusively southeast of the BCFZ and must have intruded eastern IP lithologies before BCFZ emplacement.
7. Map patterns show laterally continuous tabular bodies of Walker Top Granite occurring in map-scale shear zones throughout the study area, suggesting that these bodies are present in the lowermost portions of the Brindle Creek thrust sheet and are exhumed along fault zones as the thrust sheet is successively imbricated to the SE.
8. Developments of both the PSSZ and the BCFZ must postdate the crystallization age of the Walker Top Granite (~407 Ma).
9. Truncation of PSSZ by the BCFZ further constrains the timing of PSSZ development to 407-360 Ma.
10. Penetrative D structures in the Moravian Falls quadrangle were developed during
12. L2 mineral lineations parallel hinge lines of SW-directed F2 sheath folds and NW-vergent F3 folds in eastern and western IP terranes, suggesting their simultaneous development during initial NW- followed by SW-, and finally NW-directed thrust sheet emplacement during the Neoacadian. These fabrics define a flow lineation developed in response to high-grade metamorphism and migmatization during the Neoacadian, causing SW-directed crustal flow and extrusion of material.
Ubiquitous top-to-the-SW shear sense indicators further support this hypothesis.
13. Persistent D4 folds and shear fabrics indicate SW-directed transport and overprint earlier NW-vergent structures.
14. The MSFZ is a dextral strike-slip fault, which truncates NW-vergent D2 and D3 fabrics, and likely coincides with SW-directed transport during D4.
15. Dismemberment of the Henderson Gneiss body in the northeastern IP could have occurred either during Neoacadian thrust emplacement (D2 and D3) or early dextral reactivation of the Neoacadian BFZ (early D4), but must have occurred before the development of the MSFZ.
16. The transition from ductile to brittle deformation in the northeastern IP likely took place over the course of D4 as indicated from the transition from passive to flexural fold styles.
17. Foliation patterns permit subdivision of the study area into three distinct domains.
18. The zone of constricted flow corresponds to domain I and encompasses most of the study area. Domain II is recognized by a deflection of foliations near the BCFZ. Domain III shows similar patterns to domain I, but is separated by differences in dip direction and magnitude.
19. Mineral lineations are consistently oriented throughout the study area and no curvature or deflection of linear fabrics is apparent. The proximity of the BFZ to the study area may account for this apparent constriction of fabrics.
20. Kinematic indicators show a dominance top-to-the-SW shear sense most likely resulting from dextral strike-slip movement during D2 and D4. A significant component of orogen-parallel simple shear would be required to overprint NW-vergent fabrics developed during earlier deformational events. However, overprinted F2 and F3 folds are asymmetric and indicate initial NW-directed transport.
21. Structures in the study area are overprinted by two younger generations of open folds and joint sets.
22. Structural observations and analyses provide evidence for a qualitative model in
which initial NW-directed convergence and subduction of eastern IP lithologies evolved to NW-directed terrane accretion and orogenesis and were ultimately followed by SW-directed orogen parallel transport between thrust sheets.
Structures produced during these events are successively overprinted by later brittle fabrics developed during extension and uplift.
REFERENCES CITED
Bier, S. E., 2001, Geology of the southwestern South Mountains [M. S. thesis]:
Knoxville, University of Tennessee, 162 p.
Bream, B. R., 1999, Geology of the Glenwood and Sugar Hill quadrangles, North Carolina, and structure of the northeast end of the Henderson Gneiss [M. S. thesis]:
Knoxville, University of Tennessee, 155 p.
Bream, B. R., 2003, Tectonic implications of geochronology and geochemistry of para- and orthogneisses from the southern Appalachian crystalline core [Ph. D.
dissertation]: Knoxville, University of Tennessee, 310 p.
Bream, B. R., Hatcher, R. D., Jr., Miller, C. F., and Fullagar, P. D., 2001, Geochemistry and provenance of Inner Piedmont paragneisses, NC and SC: Evidence for an internal terrane boundary: Geological Socieety of Americal Abstracts with Programs, v. 33, no. 2, p. A-65.
Bryant, B., and Reed, J. C., 1970, Geology of the Grandfather Mountain window and vicinity, North Carolina and Tennessee: U. S. Geological Survey Professional Paper 615, 190 p.
Carrigan, C. W., Bream, B. R., Miller, C. F., and Hatcher, R. D., Jr., 2001, Ion microprobe analyses of zircon rims from the eastern Blue Ridge and Inner Piedmont, NC-SC-GA:
Implications for the timing of Paleozoic metamorphism in the southern Appalachians:
Geological Society of America Abstracts with Programs, v. 33, no. 2, p. A-7.
Davis, T. L., 1993, Lithostratigraphy, structure, and metamorphism of a crystalline thrust terrane, western Inner Piedmont, North Carolina, [Ph.D. dissertation]: Knoxville, Tennessee, University of Tennessee, 245 p.
Engelder, T., 1982, Is there a genetic relationship between selected regional joints and contemporary stress within the lithosphere of North America?: Tectonics, v. 1, p. 161-Fullagar, P. D., Goldberg, S. A., and Butler, J. R., 1997, Nd and Sr isotopic 177.
characterization of crystalline rocks from the southern Appalachian Piedmont and Blue Ridge, North and South Carolina, in Sinha, A. K., Whalen, J. B., and Hogan, J. P., eds., The nature of magmatism in the Appalachian orogen: Boulder, Colorado, Geological Society of America Memoir 191, p. 165-179.
Garihan, J. M., Preddy, M. S., and Ranson, W. A., 1993, Summary of mid-Mesozoic
Giorgis, S. D., 1999, Inner Piedmont geology of the northwestern South Mountains near Morganton, North Carolina [M. S. thesis]: Knoxville, University of Tennessee, 191 p.
Giorgis, S. D., Mapes, R. W., and Bream, B. R., 2002, The Walker Top Granite: Acadian granitoid ore eastern Inner Piedmont basement?, in Hatcher, R. D. Jr., and Bream, B. R., eds., Inner Piedmont geology in the South Mountains-Blue Ridge Foothills and the southwestern Brushy Mountains, central-western North Carolina: Carolina Geological Society Guidebook, p. 33-44.
Goldsmith, R., Milton, D. J., and Horton, J. W., Jr., 1988, Geologic map of the Charlotte 1 degree x 2 degree quadrangle, North Carolina and South Carolina: U. S. Geological Survey Map I-1251-E, scale 1:250.000.
Griffin, V. S., Jr., 1969, Migmatitic Inner Piedmont belt of northwestern South Carolina:
South Carolina Division of Geology, Geologic Notes, v. 13, p. 87- 104.
Griffin, V. S., Jr., 1971, The Inner Piedmont belt of the southern crystalline Appalachians:
Geological Society of America Bulletin, v. 82, p. 1885-1898.
Harper, S. B., and Fullagar, P. D., 1981, Rb-Sr ages of granitic gneisses of the Inner Piedmont belt of northwestern North Carolina and southwestern South Carolina:
Geological Society of America Bulletin, v. 92, p. 864-872.
Hatcher, R. D., Jr., 1969, Stratigraphy, petrology, and structure of the low rank belt and part of the Blue Ridge of northwesternmost South Carolina: South Carolina Division of Geology, Geologic Notes, v. 13, p. 105-141.
Hatcher, R. D., Jr., 1972, Developmental model for the southern Appalachians:
Geological Society of America Bulletin, v. 83, p. 2735-2760.
Hatcher, R. D., Jr., 1993, Perspective on the tectonics of the Inner Piedmont, southern Appalachians, in Hatcher, R. D., Jr., and Davis, T. L., eds., Studies of Inner Piedmont geology with a focus on the Columbus Promontory: Carolina Geological Society Guidebook (North Carolina Geological Survey), p. 1-16.
Hatcher, R. D., Jr., 1998, Structure of the Appalachian Inner Piedmont: Geological Society of America Abstracts with Programs, v. 30, p. A-17.
Hatcher, R. D., Jr., 2001, Rheological partitioning during multiple reactivation of the Palaeozoic Brevard Fault Zone, southern Appalachians, USA, in Holdsworth, R. E., Strachan, R. A., Macloughlin, J. F., and Knipe, R. J., eds., The nature and significance of fault zone weakening: Geological Society of London Special Publication 186, p.
255-269.
Hatcher, R. D. Jr., 2002, An Inner Piedmont primer, in Hatcher, R. D., Jr., and Bream, B.
R., eds., Inner Piedmont geology in the South Mountains-Blue Ridge Foothills and the southwestern Brushy Mountains, central-western North Carolina: North Carolina Geological Survey, Carolina Geological Society Annual Field Trip Guidebook, p. 1-Hatcher, R. D., Jr., and Hooper, R. J., 1992, Evolution of crystalline thrust sheets in the 18.
internal parts of mountain chains, in McClay, K. R., ed., Thrust Tectonics: London, Chapman and Hall, p. 217-234.
Hatcher, R. D., Jr., and Merschat, A. H., in press, The Appalachian Inner Piedmont: An exhumed strike-parallel, tectonically forced orogenic channel, in Law, R. D., Searle, M., and XXX, eds., Geological Society of London Special Publication.
Heyn, T., 1984, Stratigraphic and structural relationships along the southwestern flank of the Sauratown Mountains anticlinorium [M.S. thesis]: Columbia, University of South Carolina, 192 p.
Hill, J. C., 1999, Geology of the Marion East quadrangle, North Carolina, and the stratigraphy of the Tallulah Falls Formation in the Chauga belt [M.S. thesis]:
Knoxville, University of Tennessee, 188 p.
Holcombe, R., 2005 Geoorient, v. 9.2, computer program.
Hopson, J. L., and Hatcher, R. D., Jr., 1988, Structural and stratigraphic stetting of the Alto allochthon, northeast Georgia: Geological Society of America Bulletin, v. 100, p.
339-350.
Kalbas, J. L., 2003, Geology of part of the southwestern Brushy Mountains, Inner Piedmont [M. S. Thesis]: Knoxville, University of Tennessee, 208 p.
Knapp, J. H., 2001, Cenozoic tectonic evolution of southeastern North America:
Evidence for passive margin uplift?: Geological Society of America Abstracts with Programs, v. 33, no. 2, p. A-82.
Lemmon, L. E., 1973, Geology of the Bat Cave and Fruitland and the origin of the Henderson Gneiss, western North Carolina [Ph. D. dissertation]: Chapel Hill, University of North Carolina, 145 p.
Lemmon, L. E., and Dunn, D. E., 1973a, Geologic map and mineral resources of the Fruitland quadrangle, North Carolina: North Carolina Department of Natural Resources and Community Development GM 202-NW, scale 1:24,000.
Lemmon, L. E., and Dunn, D. E., 1973b, Geologic map and mineral resources of the Bat Cave quadrangle, North Carolina: North Carolina Department of Natural Resources and Community Development GM 203-NW, scale: 1:24,000.
Lemmon L. E., and Dunn, D. E., 1975, Origin and geologic history of the Henderson Gneiss from Bat Cave and Fruitland quadrangles, North Carolina: Geological Society of America Abstracts with Programs, v. 7, p. A-509.
Mapes, R W., 2002, Geochemistry and geochronology of mid-Paleozoic granitic plutonism in the southern Appalachian Piedmont terrane, North Carolina-South Carolina-Georgia [M. S. thesis]: Nashville, Vanderbuilt University, 150 p.
McConnell, K. I., 1990, Geology and geochronology of the Sauratown Mountains anticlinorium, northwestern North Carolina [Ph.D. dissertation]: Columbia, University of South Carolina, 232 p.
Merschat, A. J., 2003, Inner Piedmont tectonics in the southwestern Brushy Mountains, North Carolina: Field and laboratory data revealing 3-D crustal flow and sillimanite I and II metamorphism [M.S. thesis]: Knoxville, University of Tennessee, 199 p.
Merschat, A. J., and Kalbas, J. L., 2002, Geology of the southwestern Brushy Mountains, North Carolina Inner Piedmont: A summary and synthesis of recent studies, in
Hatcher, R. D., Jr., and Bream, B. R., eds., Inner Piedmont geology in the South
Merschat, A. J., Hatcher, R. D., Jr., and Davis, T. L., 2005, The northern Inner Piedmont, southern Appalachians, USA: Kinematics of transpression and SW-directed mid-crustal flow: Journal of Structural Geology, v. 27, p. 1252-1281.
Odom, A. L., and Fullagar, P. D., 1973, Geochronological and tectonic relationships between the Inner Piedmont, Brevard zone, and Blue Ridge belts, North Carolina:
American Journal of Science, v. 273, p. 133-149.
Osberg, P. H., Tull, J. F., Robinson, P., Hon, R., and Butler, J. R., 1989, The Acadian orogen, in Hatcher, R. D., Jr., Thomas, W. A., and Viele, G. W., eds., The
Appalachian-Ouachita orogen in the United States: Boulder, Colorado, Geological Society of America, The Geology of North America, v. F-2, p. 179-232.
Overstreet, W. C., Whitlow, J. W., White, A. M., and Griffits, W. R., 1963a, Geologic map of the southern part of the Casar quadrangle, Cleveland, Lincoln, and Burke Counties, North Carolina, showing areas mined for monazite and mica: U.S. Geological Survey Mineral Investigations Field Studies Map MF-257, scale 1:24,000.
Overstreet, W. C., Yates, R. G., and Griffite, W R., 1963b, Geology of the Shelby quadrangle, North Carolina: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-384, scale1:62,000.
Passchier, C. W., and Trouw, R. A. J., 1998, Microtectonics: New York, Springer-Verlag, 289 p.
Passchier, C. W., Myers, J. S., and Kroner, A., 1990, Field geology of high-grade gneiss terranes: New York, Springer-Verlag, 150 p.
Ramsay, J. G., 1967, The folding and fracturing of rocks: London, McGraw-Hill, 568 p.
Rankin, D. W., Espenshade, G. H., and Neuman, R. B., 1972, Geologic map of the west half of the Winston-Salem quadrangle, North Carolina, Virginia, and Tennessee: U.S.
Geological Survey Miscellaneous Investigations Map I-907 A, scale 1:250,000.
Reed, J. C., Jr., and Bryant, B., 1964, Evidence for strike-slip faulting along the Brevard Zone in North Carolina: Geological Society of America Bulletin, v. 75, p. 1177-1196.
Roper, P. J., and Dunn, D. E., 1973, Superposed deformation and polymetamorphism, Brevard zone, South Carolina: Geological Society of America Bulletin, v. 84, p. 3373-3386.
Sinha, A. K., and Glover, L., III, 1978, U/Pb systematics of zircons during dynamic metamorphism: Contributions to Mineralogy and Petrology, v. 66, p. 305-310.
Vinson, S. B., 1999, Ion probe geochronology of granitoid gneisses of the Inner
Piedmont, North Carolina and South Carolina [M. S. Thesis]: Nashville, Vanderbilt University, 84 p.
Williams, S. T., 2000, Structure, stratigraphy, and migmatization in the southwestern South Mountains, North Carolina [M.S. thesis]: Knoxville, University of Tennessee, 111 p.
Wilson, C. G., 2006, Evolution of the Neoacadian orogenic core of the southern Appalachians, USA: Evidence from the Gilreath quadrangle, North Carolina [M.S.
thesis]: Knoxville, University of Tennessee.
Worley, B. A., and Wilson, C. J. L., 1996, Deformation partitioning and foliation
reactivation during transpressional orogenesis, an example from the Central Longmen
Shan, China: Journal of Structural Geology, v. 18, p. 395-411.
Yanagihara, G. M., 1994, Structure, stratigraphy, and metamorphism of a part of the Columbus Promotory, western Inner Piedmont, North Carolina [M.S. thesis]:
Knoxville, University of Tennessee, 214 p.