Summary It is very important to have good subsurface data in order to understand the nature and behaviour of thrust faults. Deep crustal seismic reflection profiling is the best technique currently available to make detailed subsurface studies of such important problems as the attitude and extent at depth of major faults, and hence deduce the mode of deformation and tectonic forces producing them. The Consortium for Continental Reflection Profiling (COCORP) is collecting large quantities of seismic reflection data from the deep crust and upper mantle in many parts of the U.S.A. Areas of major thrusting which have been profiled so far by COCORP include the Wind River Mountains in Wyoming and the Southern Appalachians of Georgia and Tennessee. Seismic profiles have been very successful in delineating a major thrust fault of moderate dip underlying the Wind River Mountains, thus demonstrating that compressional tectonics were dominant in their formation. In Georgia and Tennessee the seismic profiles demonstrate that the major tectonic feature of the Southern Appalachians is a relatively thin overthrust sheet, which may have moved at least 260 km. Deep crustal seismic reflection profiling thus appears to be an indispensable tool for the study of areas in which thrusting and nappe formation have occurred.
Correlation of potential field data to regional geological features within the Lithoprobe southern Canadian Cordillera transect corridor allows characterization of anomaly patterns according to their likely sources. Long-wavelength Bouguer gravity anomalies are attributed to isostatic effects of topography, which in most areas is compensated. Two notable exceptions occur: in the Foreland belt a large positive isostatic anomaly is likely due to mechanical support of topography formed as Cordilleran thrust sheets were emplaced over the thick craton, and on the west coast, isostatic anomalies are related to active subduction. Long-wavelength magnetic anomalies in the Foreland belt are associated with cratonal basement beneath the thrust sheets, and these can be followed westward to near the Omineca belt. A prominent positive magnetic anomaly along the western Coast belt is probably associated with mafic rocks generated during subduction. Elsewhere, relatively short wavelength gravity and magnetic anomalies correlate well with either plutons (both gravity and magnetic), volcanics (primarily magnetics), or faults (magnetics) within the region of accreted terranes.
Seismic reflection data obtained across the Quebec Appalachians using the VIBROSEIS (trademark Conoco) technique were recorded with parameters that allowed harmonic distortion arrivals to interfere with layered reflections. The data exhibit reflections from layered miogeoclinal rocks dipping eastward beneath the allochthonous rocks of the orogen; the layering appears to terminate beneath the Notre Dame Anticlinorium. However, as the apparent termination of the layers also occurs at the arrival times of high-amplitude noise harmonics, it may have no geological significance. Precambrian Grenville crust, which probably underlies the layered sediments, extends at least as far east as the apparent termination, and may extend much farther. Examination of the Bouguer gravity field in relation to the seismic reflection data shows that a major gravity change is due to density differences that occur considerably west of the eastern limit of Precambrian Grenville crust. The gravity thus shows no correlation with surface structures proposed as suture zones. An actualistic model incorporates subduction of a passive (Atlantic-type) margin beneath an arc terrain during the Ordovician.
New deep seismic reflection data coupled with regional stratigraphic correlations, drill-hole information, and potential field data are interpreted to provide images of Middle Proterozoic Wernecke Supergroup (meta-)sedimentary layers that were uplifted during tectonic development of the ca. 0.9–1.3 Ga Racklan Orogen in Canada's western Northwest Territories. The reflection data are located at the eastern front of the Mackenzie Mountains portion of the Canadian Cordillera and on the western flank of the Fort Simpson structural trend that is a prominent Proterozoic structure in the subsurface throughout the region. Along three parallel profiles, layers that are correlated with thick Wernecke Supergroup sedimentary rocks produce prominent reflections between about 3.0 and 9.0 s (about 7.5 and 23 km) that were arched prior to deposition of younger Proterozoic (probably Mackenzie Mountains Supergroup) and Phanerozoic sedimentary rocks. The strata are considered to be Wernecke basin sedimentary rocks that were uplifted during deformation associated with the development of the Racklan Orogen.
Research Article| September 01, 1980 Comment and Reply on ‘Thin-skinned tectonics in the crystalline southern Appalachians; COCORP seismic-reflection profiling of the Blue Ridge and Piedmont’: REPLY F. A. Cook; F. A. Cook 1Department of Geological Sciences, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar D. S. Albaugh; D. S. Albaugh 1Department of Geological Sciences, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar L. D. Brown; L. D. Brown 1Department of Geological Sciences, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar S. Kaufman; S. Kaufman 1Department of Geological Sciences, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar J. E. Oliver; J. E. Oliver 1Department of Geological Sciences, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar R. D. Hatcher, Jr. R. D. Hatcher, Jr. 2Department of Geology, Florida State University, Tallahassee, Florida 32306 Search for other works by this author on: GSW Google Scholar Geology (1980) 8 (9): 403–404. https://doi.org/10.1130/0091-7613(1980)8<403:CAROTT>2.0.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation F. A. Cook, D. S. Albaugh, L. D. Brown, S. Kaufman, J. E. Oliver, R. D. Hatcher; Comment and Reply on ‘Thin-skinned tectonics in the crystalline southern Appalachians; COCORP seismic-reflection profiling of the Blue Ridge and Piedmont’: REPLY. Geology 1980;; 8 (9): 403–404. doi: https://doi.org/10.1130/0091-7613(1980)8<403:CAROTT>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract No Abstract Available. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Relict subduction zones that are interpreted on 10 Lithoprobe deep seismic reflection profiles in Canada provide key observational evidence that lithosphere plate interactions have operated with essentially the same resultant geometry for at least 2.7 Gyr. Subduction zone reflections are generally found beneath known collisional belts, and their orientations are often consistent with the locations of exposed arcs, sutures, and subduction complexes. They project from the lower crust into the upper mantle and provide key geometric relationships between the crust, Moho, and upper mantle. The interpretation of subduction zone reflections as being syntectonic establishes a minimum age for the overlying lower crust and mantle wedge and precludes inferred formation of the Moho and upper mantle by postorogenic delamination in these areas. The orientation of subduction zone reflections may be synthetic or antithetic to exposed sutures, indicating juxtaposition by subcretion or by delamination, respectively. The mantle wedge above the subduction zone reflections may exhibit sparse reflectivity. Subduction zone reflections commonly dip toward the craton, indicating a preferred dip of subduction during the last stages of convergence within an orogen, or else a higher likelihood of preservation beneath older domains. Interpreted ages of the Canadian examples range from Recent to Neoarchean (∼2.7 Ga), providing observational evidence that plate tectonics or similar processes have been active since at least the Neoarchean. The locations and geometries of these relict subduction zones provide a snapshot of accretion boundaries in the mantle and establish a framework for the Neoarchean‐Recent accretionary architecture of the North American lithosphere.
Deep reflection surveys carried out by the Consortium for Continental Reflection Profiling (COCORP) are revealing major structures within the continental crust in a variety of tectonic settings across the U.S. In many cases, these structures bear directly upon such geologic problems as the state of stress during large scale crustal deformations, the migration of magma, and the nature of the crust‐mantle transition zone. COCORP, formed during the U.S. Geodynamics Project and funded by the National Science Foundation, has surveyed eight different localities along lines totalling over 800 km in length. The VIBROSEIS technique has been used in all cases and has proven to be a flexible and effective tool for deep crustal exploration. Among the more significant observations made thus far are: (1) the details of the extensional structure of the Rio Grande Rift; (2) evidence for at least 21 km horizontal and 13 km vertical displacement along the Wind River Thrust, which dips uniformly at about 30 degrees and extends to a depth of at least 25 km; (3) evidence for at least 225 km displacement along a major subhorizontal thrust system beneath the southern Appalachians, in which the Brevard Zone appears to be rooted; and (4) the apparently discontinuous nature of the Moho or crust‐mantle transition zone, which has been observed to some degree at most of the sites studied. Taken together, the results of all surveys analyzed thus far demonstrate a much larger degree of heterogeneity in crustal structure than is indicated by more conventional geophysical techniques for exploration of the basement.
Nearly 270 km of crustal seismic reflection data obtained by Lithoprobe in the southern Canadian Cordillera provide a geometric link between the Rocky Mountain foreland thrust and fold belt, the Purcell anticlinorium, and the extensional regime superimposed on the crystalline core zone. Autochthonous North American basement and its overlying deformed and transported cover can be traced from the thrust and fold belt, beneath the Rocky Mountain trench, to 20 km depth beneath the central part of the Purcell anticlinorium. The Purcell anticlinorium is cored by foreshortened Proterozoic supracrustal rocks that were carried northeastward on a series of west dipping imbricate thrust faults. These faults crop out within and east of the anticlinorium and converge downward with subhorizontal detachments above the autochthonous North American basement. Beneath the western Purcell anticlinorium and Kootenay Arc, reflections associated with the Purcell stratigraphy and its underlying crystalline basement terminate at about 20 km depth and may be truncated against the east dipping Eocene Slocan Lake fault zone. The Slocan Lake fault zone is clearly imaged from the surface to about 12 km depth and can probably be followed discontinuously to about 25 km depth. A west‐dipping, high amplitude reflection from beneath the Valhalla gneiss complex outlines the domal geometry of the complex and is probably related to an east verging compressional shear zone.
