Sedimentary basins and global tectonics

Abstract

Concepts of geosynclines developed from the study of ancient orogenic belts and sedimentary basins. Now, however, geophysical data, derived mainly from oceanic areas and continental margins, have led to the theory of plate tectonics which has largely replaced geosynclinal theory as the basis for understanding orogenic belts and sedimentary basins. A variety of basins can now be distinguished which have developed (1) on continental crust either as large downwarps, such as the Chad basin, or as rift basins following long-lived fault systems, such as the East African rift, (2) in association with ocean-floor spreading, as newly formed rifts, such as the Red Sea, at mid-oceanic spreading centres, as failed rifts, such as the Benue trough, and at continental margins either rifted or transform, (3) at subduction zones either as trenches, outer-arc, slope or back-arc-basins, the best examples being along the Indonesian margin, off New Zealand and Japan, (4) at collision zones such as the Himalayas (5) along transform/strike-slip belts, such as the onland and offshore Californian basins and the Dead Sea. In addition, thick sedimentary accumulations may form on oceanic crust as large submarine fans, such as the Bengal Fan, as sedimentary swells due to thermohaline currents, such as the Outer Ridge off the Blake Plateau, and as tectonically accreted wedges in fore-arc regions. Postulated examples of such features in the British Isles include the rifted margins of the Iapetus ocean and present Atlantic, the Southern Uplands accretionary prism, Midland Valley fore-arc basin becoming a series of strike-slip basins in Devonian times, the Lake District island arc and Welsh back-arc basin with sag basins and rift basins in the North Sea. The old geosynclinal terminology can now usually be abandoned provided it is realized that many sedimentary basins fall into more than one category and that interpretations of the ancient are never more than working hypotheses.