The Cocorp Southern Applachian Traverse
F.A. Brown, L.D. Brown, S. Kaufman, J.E. Oliver, 1983. "The Cocorp Southern Applachian Traverse", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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The COCORP southern Appalachian seismic reflection traverse provides evidence that the crystalline rocks at and near the surface well east of the Valley and Ridge were thrust westward above Late Precambrian and Early Paleozoic miogeoclinal strata. These data thus imply that the "thin-skinned" style of deformation is applicable to the crystalline "cores", as well as the often documented forelands, of many orogenic belts.
The traverse, which includes some 700 km (435 mi) of profile (including cross lines) from near Madisonville in the alley and Ridge of eastern Tennessee to near Savannah in the Coastal Plain of Georgia, is the first crustal-scale seismic reflection profile in the public domain to transect an entire orogen. From west to east the major elements of this part of the Appalachians include the alley and Ridge, Blue Ridge, Brevard zone, Inner Piedmont, Charlotte belt, Carolina slate belt, Eastern Piedmont (Kiokee belt, Augusta fault, Belair belt), and Coastal Plain (Figure 1). With the exception of a gap of about 60 km (37 mi) in the Blue Ridge, the seismic traverse is continuous across all of these belts.
Generalized interpretations were previously presented (Cook et al, 1979; 1981) and a more detailed discussion is developed in Cook et al (1982). Several of the major discoveries which resulted from these profiles are briefly noted here.
The data were recorded in two phases of field acquisition (Figure 1). The first phase, recorded in 1978 and 1979, resulted in the completion of the traverse from the alley and Ridge to the east side of the Carolina slate belt. The second phase resulted in the extension of the profiles across the Eastern Piedmont and Coastal Plain.
Nearly 350 km (217 mi; including cross-lines) of data were acquired in the first phase of profiling which led to some important fundamental discoveries (Cook et al, 1979). Of primary significance is the observation that nearly horizontal reflections, mot likely from Paleozoic sedimentary layers, could be traced at 1. 5 to 2.0 secs (about 4.5 to 6 km, 13,500 to 18,000 ft) from the alley and Ridge to beneath the Blue Ridge (Figures 2A, 3A). Furthermore, similar horizontal reflections are observed beneath the eastern Blue Ridge, Brevard zone, and Inner Piedmont (Figures 2B, 3B). In spite of the 60 km (37 mi) data gap in the Blue Ridge, the reflections on the west probably correlate with those on the east. Several independent lines of evidence also lead us to believe that the layers beneath the Brevard zone and Inner Piedmont are similarly Precambrian and/or Paleozoic metasedimentary layers. These have been described in Cook et al (1979; 1982).
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Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces
Until a few decades ago, structural and regional geology were traditionally the preserve of field geologists. They usually mapped areas of outcropping deformed rocks and supplemented their work by laboratory studies of rock deformation and by theoretical work. Structural geology became tied to the geology of uplifts, folded belts, and underground mines, all of which were accessible to direct observation. Since World War II we have witnessed a tremendous development of geophysics in oceanography and in petroleum geology. Academic geophysicists in oceanography led their geological colleagues into modern plate tectonics and industry geophysicists developed reflection seismology into a superb structural mapping tool that penetrated the subsurface.
Today we are facing a situation where instruction and textbooks in structural geology are almost entirely dedicated to rock deformation, analytical techniques in detailed field geology and summaries of plate tectonics. Illustrations based on reflection seismic profiles are virtually absent in textbooks of structural geology. These texts illustrate only the parts of the proverbial elephant, together with some conjecture, but without ever offering a glimpse of the whole elephant.
Some of the reason cited for the relative scarcity of published reflection profiles are: 1) the confidentiality of exploration data; 2) difficulties in the photographic reduction and reproduction of seismic profiles for a book format; 3) the two-dimensional nature of vertical reflection profiles; and 4) the obvious distortions in reflection profiles that are typically recorded in time.
The AAPG leadership felt that it was time to attempt to correct the situation and to produce this picture and work atlas. The first volumes, of what may become a series of volumes, are addressing an audience that includes: petroleum geologists concerned with structural interpretations; exploration companies that provide in-house training; the AAPG continuing education program; and academic colleagues interested in updating their curricula in structural geology by inclusion of reflection profiles from the “real world” in their teaching.
The atlas is not meant to be a textbook in reflection seismology (instead we listed some at the end of this introduction) nor a text in structural and/or regional geology. Our intent is simply to provide a teaching tool.