【文章內(nèi)容簡(jiǎn)介】 oreover, coals formed in rapidly subsiding foreland basins are more likely to have high vitrite, clarite and ash contents than coals formed on cratonic shelves or in slowly subsiding cratonic basins. These coals are likely to be rich in dull coals consisting mainly of durite inertite. 2 Basin Formation as Part of Plate Tectonics The theory of plate tectonics, although primarily concerned with horizontal movements of the relatively rigid lithospheric plates (crust and uppermost mantle) over the softer asthenosphere (mantle), has also provided an explanation for the vertical movements that lead to subsidence and basin formation. The following crustal movements can be distinguished (after Dickinson 1974 and Fischer 1975): 1. Change in crustal thickness. According to the principle of isostasy thick lowdensity continental crust floats higher on heavy mantle material than thin highdensity oceanic crust. For example, an isostatically pensated continental crust of 50 km thickness extends 4 km above the sea, whereas a 6kmthin oceanic crust is covered by approximately 5 km of water (Holmes, 1965). Plate tectonics provides several mechanisms for both crustal thickening and thinning. The latter, which is of immediate interest here, is often exemplified in areas of continental rifting, where in the early stages of plate separation the crust along the rift zone is attenuated by extensional stepfaulting, thus forming rapidly subsiding grabens and halfgrabens. Erosional thining of anisostatically uplifted crustal portions also leads to subsequent subsidence. Uplift due to crustal thickening is of some interest in this context,because it creates potential source areas for coal measure sediments. This is particularly important in foredeeps, where basin formation is invariably coupled with uplift in nearby orogenic belts. Most examples of uplift due to crustal thickening are related either to the injection of magma into the crust or to continental collision. 2. Change in thermal regime. Convection currents in the plastic asthenosphere are responsible not only for horizontal plate movements but also for some vertical crustal motions which are independent of ceustal thickness. Upwelling magma from the mantle may cause uplift by forming heat bulges in the overlying crust, and new oceanic crust is formed where such mantle material is extruded along midoceanic rift zones. The latter are elevated above the sea floor because of thermal expansion of the affected crust which bees colder and denser with increasing age and distance fiom the rise crest. Areas of thermal tumescence within or along the margin of continental plates are subjected to erosional thinning, which accentuates their subsidence during the period of thermal decay. 3. Loading affects. When sediments accumulate on an isostatically pensated crust the additional load will create a disequilibrium which will be balanced by subsidence. This means that, whatever the initial cause for the creation of a depositional site, once sediments are beginning to accumulate, their weight and paction are in some measure responsible for the deposition of additional sediments. This, to some extent selfprepetuating process is particularly well shown by the flexural bending under load of the continental shelf margin (Walcott 1972). Other mon geotectonic sites for loadinduced subsidence and sedimentation are orogenic fordeep margins which are flexurally downwarped under the weight of the overriding thrust sheets generated in the adjacent fold belt (Price 1973。 Laubscher 1978。 Beaumont 1981。 Quinlan and Beaumont 1984). Additional loading of the downwardly flexed crust is provided by the mass of molasses sediments produced in the fold belt and transported into the developing foredeep. As has been discussed before, additional causes of sediment and coal formation are provided by subsurface salt migration and leaching, and eustatic sealevel changes, in particular by their interaction with crustal movements, which produce a variety of sedimentary responses in different tectonic domains. For example, rifting of oceanic crust has depositional consequences quite different from the separation of continental crust. The rifting of continental crust may lead to coal formation, but the rifting of oceanic crust is unlikely to lead to the formation of coal. A survey of the effects of the geotectonic setting of coalfields on peat accumulation and coal position requires therefore an understanding of the major crustal elements of Earth and their principal motions. A platetectonic interpretation of the main crustal elements in reference to their ability to provide suitable sites for the formation of coal is summarized in . This interpretation is based on the notion that the creation of new lithospheric crust along midoceanic rifts and the lateral movement of the lithospheric slabs towards subductionzones, where oceanic crust is consumed, produce three types of plate junctures. These are (after Dickinson 1974): 1. Divergent plate edges, where plate separation takes place and the developing gap is filled by upwelling mantle material welding new oceanic crust to the separating plates. 2. Convergent plate edges, where old crust is subducted into the mantle underneath the leading edge of the overriding plate. 3. Transform plate edges, where adjacent plates are laterally displace by movement along strikeslip faults. . The geotectonic setting of coalfields in reference to Curray’s (1975) platetectonic subdivisions of the earth. The identification of countries is by international country code. The tectonic subdivisions of coalfields used by von Bubnoff (1937) in Table can be broadly acmodated within the plate tectonic framewor