Collapsing Continental Rises: Actualistic Concept of Geosynclines—A Review
Robert S. Dietz, John C. Holden, 1974. "Collapsing Continental Rises: Actualistic Concept of Geosynclines—A Review", Modern and Ancient Geosynclinal Sedimentation, R. H. Dott, Jr., Robert H. Shaver
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In the 1950's, geosynclinal theory was dominated by the tectogene concept and Marshall Kay's synthesis. These and earlier concepts were derived from field study of tectonized geosynclines on land. In 1959 C. Drake, Maurice Ewing, and G. Sutton, applying the data of marine geophysics, recognized that sedimentary prisms now being laid down along the eastern margin of the United States may represent nascent miogeosynclines and eugeosynclines. They assumed that there is a close parallel with Kay's model and included in their interpretation a shelf-edge basement high that supposedly is equivalent to the tectonic borderland and, also, a toe of sialic crust underlying the continental rise that supposedly makes the rise ensialic. The eugeosyncline then would be elevated eventually to continental level largely by sialization of oceanic crust and without horizontal translation of the prism.
Between 1963 and 1967, we have developed what may be called an actualistic concept of geosynclines that is based upon sea-floor spreading and collapsing continental rises. This, too, was based upon Kay's model, except that gross surgery was applied. The seaward half of the miogeosyncline was deleted, as though it never existed and making it a wedge that thickened out, so to speak, like the modern terrace wedge. Also omitted was the tectonic borderland; instead, a continental slope was inserted between the miogeocline and eugeocline. (For simplicity and since none of these sedimentary prisms are really synclinal in form, we prefer the terms miogeocline and eugeocline.) In this model, the miogeoclinal sediments were deposited ensialically on a downflexing continental margin and the eugeoclinal sediments ensimatically on oceanic crust. There seemed to be insufficient reason to equate the shelf-edge basement high with a tectonic borderland or to insert a sialic toe beneath the continental rise. Tectonization was envisioned as the result of underthrusting of the continental margin (sub- duction), which collapsed the continental rise, magmatized it, and inserted allochthonous crust and mantle rock within the eugeocline.
Our model is explicitly concerned with the mio-eugeoclinal couplet of the Atlantic type, such as would form marginal to a rift ocean on the trailing edge of a drifting continent, With the rapid development of plate tectonics and especially with the recognition of opening and closing ocean basins, much sophistication has recently been added to geosynclinal theory by J. Dewey, J. Bird, A. Mitchell, H. Reading, W. R. Dickinson, and many others.
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Modern and Ancient Geosynclinal Sedimentation
The Kay Conference was held in Madison, Wisconsin, November 1972. This symposium volume contains the texts of papers presented at Madison. It is organized in a topical manner, and in most areas of discussion, modern analogues and ancient examples together provide a comparative basis for evaluating sedimentary models for geosynclines. In the 1970s students of both modern and ancient sediments have compiled an immense body of knowledge relevant to the geosynclinals concept. Moreover, the new theory of plate tectonics has required a complete reassessment of the geosynclines as well as orogenesis. The purpose of this volume is to evaluate by comparison of modern and ancient sediments a number of depositional models applicable to the great variety of strata seen in orogenic belts also called geosynclinal.