THE PLATE-TECTONIC SIGNIFICANCE OF INVERSION CYCLES DURING OROGENESIS

The architecture of an orogenic belt is fundamentally affected by the nature of the tectonic mode switches that have taken place during its evolution. To classify an orogen on this basis, if we neglect wrench-related movements, there are two main end-member types: a) orogens dominated by “pull-push” inversion cycles; b) orogens dominated by “push-pull” inversion cycles. These two modes of evolution can be related in turn to the behaviour of adjacent subduction zones. The subduction hinge is either “pushed back” by the advance of the over?riding plate, or it “pulls back” faster than the over-riding plate is able to adjust. In consequence a simple terrane-stacking model can be proposed that adequately describes the geometric (and thermal) effects of inversion cycling, and which allows explanation of enigmatic large-scale tectonothermal structures found in classic mountain belts. Orogens subjected to “push-pull” sequences are characterized by intervening episodes of high-pressure metamorphism. Orogens subjected to “pull-push” sequences are characterized by intervening episodes of high-temperature metamorphism.