Metal Deposits Associated with the Early Stages of Continental Rifting
As noted in previous chapters, rifting activity in oceanic and arc environments can be associated with the generation of certain types of metal deposits, but in this chapter metal deposits that tend to form in continental rift settings will be considered.
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Metal Deposits Associated with the Early Stages of Continental Rifting
7.1 General Considerations As noted in previous chapters, rifting activity in oceanic and arc environments can be associated with the generation of certain types of metal deposits, but in this chapter metal deposits that tend to form in continental rift settings will be considered. As stated so aptly by Mohr (1982), "sufficient unto rift valleys are the existing models thereof." Mohr stresses the complexity and variability of rift systems and the need to reexamine some the notions regarding rifting that have become entrenched in the minds of many workers. He demonstrates, for example, that the stratigraphic record, where legible, does not always support the notion of prior updoming, and the development of suites of exotic alkaline rocks is not a universal feature of rift development. The plethora of rifting models that have been proposed are perhaps an indication that extensional tectonics and rifting in continental environments can be manifest in a variety of ways. McKenzie (1978), for example, has suggested that formation of all large sedimentary basins must involve some degree of crustal thinning and therefore necessitate tensional tectonics. Burke et al. (1981) provide an excellent brief summary of continental rifts and stress that they are both more common than previously recognized and can develop in a variety of tectonic settings. Four basic types of rifts are recognized: (1) rifts formed by continental rupture, (2) failed rifts at Atlantic-type margins, (3) aulacogens, and (4) impactogens. Given the continuum of plate tectonic processes, the first three of these rift types represent variations on a single theme, and due to either Wilson Cycle or ensialic collision events (see Chap. 9), most rift sequences developed by continental rupture eventually become enmeshed in orogenic belts and may be difficult to recognize. Aulacogens, defined originally by Shatski (1947), are rifts striking into foldbelts but must have originally been failed rifts at Atlantic-type margins (Burke 1977). Impactogens also strike into foldbelts, but they differ from aulacogens in that they form as a consequence of a collision event, rather than during initial rifting that is eventually followed by collision (Sengor et a1.l978). An important additional type of rift system is that formed as a result of collision events, but this type, unlike impactogens, develops at considerable distances from the collisional orogen. Tapponnier et al.(1982 and references therein) demonstrate that this concept explains much of the rifting in eastern Asia, including the Baikal Rift, Siberia, and the Shansi Basin, China, in addition to major transform faults such as the Altyn Tagh and Red River F. J. Sawkins, Metal Deposits in Relation to Plate Tectonics © Springer-Verlag Berlin Heidelberg 1990
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Metal Deposits Associated with Continental Rifting
systems. The realization that intracontinental plate stresses may have their sources at distant plate boundaries, and not solely in the
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