THE DETACHMENT FAULT CONTROVERSY OF THE VALLEY AND RIDGE IN THE SOUTHERN APPALACHIANS

Jim DeCinque


Spring 2016
Prepared for ES 767 Global Tectonics
Dr. James Aber
Emporia State University

Figure #1 - Outline of the Valley and Ridge Province in the Appalachians

Abstract
Introduction
Vertical or Horizontal Displacement
Thick versus Thin Skinned Tectonics
Thin Skinned Deformation
Plate Tectonics and Continental Collisions
Conclusion
References

Abstract

In the 1960’s a great debate ensued over thin or thick skinned tectonics in the Valley and Ridge of the Southern Appalachians. John Rodgers led the school of thin skinned tectonics and believed the autochthonous basement was not involved in folding and faulting within the Valley and Ridge. Rodgers was opposed by Byron Cooper who recognized syndepositional effects related to synclinal development in the Valley and Ridge. Not until the 1970’s with the advent of plate tectonic theory, deep seismic lines and continental collision tectonics did Leonard Harris and Robert Milici finally solve the detachment fault controversy.

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Introduction

Geologists, like all scientific investigators, usually develop profound theories one small step at a time. Plate tectonics and continental collisions are based on multiple ideas including Clarence Dutton’s theory of isostasy, Alfred Wegener’s drifting continents, Arthur Holmes’ convecting mantle and John Tuzo Wilson’s life cycles. Each theory provides the essential building blocks for understanding the mechanics of plate movements. Like all great theories, many hypothetical ideas and concepts must be pursued. Reviewing the history of some of these concepts helps to clarify the true skill of the men and women who pursued their ideas with great tenacity. The folds and thrust faults of the Southern Appalachian Valley and Ridge province were originally recognized by Henry D. and William B. Rogers in the 1830’s. James D. Dana developed a geosynclinal theory from these same mountains in the 1870’s. Bailey Willis and other geologists working for the newly formed USGS in the early 1890’s developed a tectonic framework for all future geologists to build upon.

In the 1950’s, John Rodgers began to argue for recognition of a decollement between the Valley and Ridge structures and basement. An alternate hypothesis recognizing basement involvement in structural development of the Valley and Ridge was defended by Byron Cooper. In 1977, Leonard Harris and others published “Characteristics of Thin-Skinned Style of Deformation in the Southern Appalachians, and Potential Hydrocarbon Traps.” Harris and others verified John Rodgers’ hypothesis that folds and thrust faults were rootless and ultimately remain above a low angle master detachment fault near the contact between sedimentary rock and the hard rock basement.

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Vertical or Horizontal Displacement

In the 1830’s and 1840’s, two brothers, Henry Darwin Rogers and William Barton Rogers began detailed geological mapping in the Southern Appalachians. Henry worked for the New Jersey and Pennsylvania Surveys while his brother William represented Virginia and West Virginia. Working together the brothers established a detailed geologic section for the Valley and Ridge. The detailed section allowed the Rogers brothers to decipher the complex structural geology of the region (see Figure 2). In 1842, at a meeting of the American Association for the Advancement of Science, the Rogers brothers summarized their theories for the formation of the folds and thrusts in the Valley and Ridge. The brothers believed both tangential and vertical forces driven by liquid lava beneath a shallow crust created the parallel fold and fault axes identified throughout the Valley and Ridge. The liquid lava contained gas under pressure and as the gas escaped through fractures to the southeast (Blue Ridge and Piedmont) it produced great waves in the liquid lava. These great waves propagated to the surface and produced the folds and faults visible in the Valley and Ridge.

Figure #2 - Rogers Cross Section Valley & Ridge

In 1857, James Hall hypothesized that folds and faults in the Valley and Ridge were a consequence of down-warping of thickly accumulating sediments in the Valley and Ridge basin. In 1873, James Dwight Dana formulated his geosynclinal hypothesis. Dana was the first to describe geosynclines as a basin created by the cracking and sinking of the cooling outer shell of the earth filled and with sediments.

In 1887, the USGS sent a group of geologist to the southern Appalachians to begin mapping 30-minute quadrangles for their new folio series. Headed by Bailey Willis, the geologists were able to define geologic structures in greater detail. Examples included recognition of warped thrust fault planes, large lateral displacement, and low angle fault movements. Using experimental models produced in a compression machine, Willis was able to prove that most of the structures in the Valley and Ridge were produced from tangential forces (Rodgers, 1949).

Figure #3 - Bailey Willis compression machine

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Thick versus Thin Skinned Tectonics

In 1964, John Rodgers outlined his "no basement" theory for the Valley and Ridge province. Rodgers used a “pile of rugs on a floor” to explain his concept of “no basement” involvement for the Valley and Ridge folding and faulting. As the rugs are pushed tangentially along a smooth floor, they fold, slip, and wrinkle (see Figure #4). Rodgers suspected in the Valley and Ridge that rocks slipped internally along shale or weaker zones above a harder rock basement similar to his rug and hard floor analogy (Rodgers, 1964). During the same time period, Byron Cooper was suggesting the basement was involved in uplifting and folding of the structures in the Valley and Ridge. In fact, Copper believed he recognized depositional changes within the original formations suggesting deformation at times pre-dated deposition of the sediments. Cooper stated "local patterns of stratigraphic variation indicate that major Appalachian folds originated during the Paleozoic deposition and grew to a large extent as the result of differential vertical down warp” (Cooper, 1964).

Cooper and Rodgers led the thick versus thin skinned debate in the 1960's. Cooper stated "continuing study of Appalachian strata forced me to reject the "no basement" hypothesis because of the overwhelming evidence indicating subtle tectonic local control of the stratigraphy by structural elements” (Cooper, 1964). Rodgers used studies from the Jura Mountains to support his no basement hypothesis. Including data from seismic, magnetic and gravity surveys Rodgers stated "the available data suggests to me, as in the Jura, that such irregularities are oriented at various angles to the visible folds and faults and are too small to have caused them” (Rodgers, 1964).

Figure #4 - Cross section example of no basement in the Valley and Ridge

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Thin Skinned Deformation

In 1977, Harris and Milici published “Characteristics of Thin Skinned Style of Deformation in the Southern Appalachians, and Potential Hydrocarbon Traps.” Harris described the Valley and Ridge geology as rootless folds and steeply dipping thrust faults joining a low angle thrust at the basement. Harris theorized the initial thrust followed the basement and sedimentary basin contact; the thrust traveled upward ascending through the original continental shelf and slope. Ultimately, the thrust ramped through Devonian and Pennsylvanian strata and splayed into the Cumberland Plateau province. The movement along the basement thrust (detachment fault), up slope and across several tectonic ramps, created the rootless anticlines and synclines evident in the Valley and Ridge province (see Figure #5). “Folds with wavelengths on the order of kilometers are thought to be largely the result of ramping of thrust faults rather than buckling” (Odom, 1980). Using seismic and deep hole drill data, Harris was able to verify John Rodgers theories of “no basement” involvement in the evolution of Valley and Ridge folding. Byron Cooper’s theory of basement involvement was eliminated as a viable hypothesis.

Figure #5 – Plate 5 from Harris & Milici depicting ramping of a section of the Valley and Ridge province above the basal decollement

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Plate Tectonics and Continental Collisions

During the Late Precambrian (750 m.y.), rifting began on the supercontinent of Rodinia. Over time, the Iapetus Ocean developed within the rift valley. The Iapetus basin was slowly filled with sediments from the Late Precambrian to the Pennsylvanian. The sediments accumulated from both the North American and African continents throughout the Paleozoic. In the southernmost Appalachians, some evidence indicates the detachment fault below the Valley and Ridge province began to develop in the Ordovician (480 m.y.) and terminated in the Permian (260 m.y.). Most Appalachian geologists believe thrusting and placement of the foreland over autochthonous basement is Alleghanian in age (see Figure #6).

In Middle to Late Ordovician times, the eastward movement of the African continent changed direction and moved west towards the North American continent; this change in direction initiated the closure of the Iapetus basin. Thrusting along and above the Iapetus Oceanic crust began to build thrust stacks on the North American shelf. From the Late Ordovician to the Early Carboniferous, the basin sediments, oceanic crust, and micro continents began to accumulate and move onto the North American continent (Hatcher, 2013, Higgins, 1988). In the Late Carboniferous, the intensity of the collision thrust the pre-existing North American shelf sequence over the former North American basement along an almost flat decollement. The forces of these collisions were intense and caused developing thrust faults to ramp along Paleozoic shales and other shaley formations within the Paleozoic shelf sequence. The thrusting caused the less competent strata to fold and slip as Rodgers described with his “pile of rugs” analogy.

Figure 6 – Plate tectonic and collisional history of the Iapetus Ocean and formation of the Southern Appalachians fold and thrust belt

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Conclusion

Since basement rocks are not exposed within the Valley and Ridge province, and in the 1960’s little seismic information was available, Cooper believed syndepositional effects of down warping in synclines indicated the basement was involved in the folding of the Valley and Ridge. However, Rodgers recognized thin skinned faults displace Pennsylvanian rocks and syndepositional deformation seemed unlikely or localized. Since Rodger’s and Cooper’s debate about thin or thick skinned deformation, new COCORP seismic lines and extensive deep drilling have all but verified the basement is not directly involved in Valley and Ridge folding and faulting. However, Thomas concludes by stating “large scale cratonward thrusting was a late phase in Appalachian orogenic history. The thrusts have overridden some earlier basement faults and have transported others. Within the fold and thrust belt, earlier structures, including possible basement faults, evidently have influenced the geometry of the late thrust faults.”

Over the last two hundred years geologists have continued to debate and hypothesize on basement interactions and the development of folds in the Valley and Ridge. Not until the advent of plate tectonic theories and a better understanding of sea floor spreading, continental drift and collision tectonics, was it possible for an accurate model of the Southern Appalachian Valley and Ridge detachment fault to be produced. Each hypothesis built the framework and produced a greater understanding and ultimate resolution of the detachment fault controversy in the Southern Appalachians.

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References

Bailey, C. (2008). Mistaken Identity. The William and Mary Blogs. Retrieved 3/25/16 from http://wmblogs.wm.edu/cmbail/mistaken-identity/

Bressan, D. (2015) Bailey Willis – The Man who made Mountains. History of Geology. Retrieved 3/25/16 from http://historyofgeology.fieldofscience.com/2015/02/bailey-willis-man-who-made-mountains.html

Condie, K.C., 1982, Plate Tectonics & Crustal Evolution. Pergamon Press, New York. p. 195-199.

Cooper, B N., 1961, Grand Appalachian field excursion: Virginia Engineering Experiment Station Extension Series, Guidebook 1, p. 1–119, 133–170, 182–187.

Cooper, B.N. 1964. Relation of Stratigraphy to Structure in the Southern Appalachians. In Lowry, W.D. (ed.), Tectonics of the Southern Appalachians, Virginia Polytechnic Institute, Department of Geological Sciences, Memoir 1. p. 81-114.

Fitcher, L.S. (2013) Appalachian Maps & Diagrams, SEPM Strata. Retrieved 3/25/16 from http://www.sepmstrata.org/page.aspx?pageid=455

Harris, L.D., and Milici, R.C., 1977. Characteristics of Thin Skinned Style of Deformation in the Southern Appalachians, and Potential Hydrocarbon Traps. U.S. Geological Survey Professional Paper 1018, 40 p.

Hatcher, R.D. (2013). Appalachians and Little Tennessee River Geologic History, Occasional Paper No. 23. McClung Museum of Natural History & Culture. Retrieved 3/25/16 from http://mcclungmuseum.utk.edu/appalachians-tennessee-river-geologic-history/

Higgins, M.W., Atkins, R.L., Crawford, T.J., Crawford, R.F., Brooks, R., and Cook, R.B., 1988. The Structure, Stratigraphy, Tectonostratigraphy, and Evolution of the Southernmost Part of the Appalachian Orogen. U.S. Geological Survey Professional Paper 1475. 173 p.

McGinley, M. (2014) Appalachian Blue Ridge Forests. The Encyclopedia of Earth. Retrieved 3/25/16 from http://www.eoearth.org/view/article/150144/

Odom, A.L., and Hatcher, R.D. Jr., 1980. Bedding Plane Thrusts --- Decollements in A Characterization of Faults in the Appalachian Fold Belt. U.S. Nuclear Regulatory Commission. p. 81-111.

Rodgers, J. 1949. Evolution of Thought on Structure of Middle and Southern Appalachians. Bull. Am. Assoc. Petrol. Geol., 33, p. 1643-1654.

Rodgers, J. 1964. Relation Basement and No-Basement Hypothesis in the Jura and the Appalachian Valley and Ridge. In Lowry, W.D. (ed.), Tectonics of the Southern Appalachians, Virginia Polytechnic Institute, Department of Geological Sciences, Memoir 1. p. 71-81.

Williams, T.A. 1983. Basement-cover relations in the Appalachian Fold and Thrust Belt. Geological Journal. Vol.18, p. 267-276.

Figure 1 – File retrieved via Wikimedia commons and is licensed under the Creative Commons Attribution ShareAlike 3.0, https://commons.wikimedia.org/wiki/File:Greatvalley-map.jpg

Figure 6 – File retrieved via United States Geological Survey, Geology of the National Parks, New York City Region,http://3dparks.wr.usgs.gov/nyc/images/fig53.jpg

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