Episodicity during orogenesis

Abstract

Abstract Deformation events and episodes of metamorphic mineral growth are usually regarded as relatively local phenomena. It is not expected that specific events and episodes within an orogenic sequence should exactly correlate over large distances. There is no obvious reason, for example, to assume that deformation and/or metamorphic events in the Western European Alps would directly correlate with events taking place in the Aegean continental crust, c. 1000 km distant. Yet linked episodes of deformation and metamorphism appear to take place at the same time over large distances, even in these apparently unrelated segments of the same orogenic belt. This large-scale episodic behaviour appears to be associated with switches in tectonic mode, from compressional orogenesis to extensional tectonism, and may be the result of orgenic surges and/or periods of lithospheric extension following accretion events. The effect of these switches is greatest in back-arc environments, in the over-riding plate above major subduction zones. In these environments, roll-back of the subducting lithospheric slab after individual accretion events ensures that the amount of lithospheric extension after each accretion event is large. As a result this is where coherent high-pressure metamorphic terranes formed in the preceding accretion event are exhumed, and where remnants of newly emplaced ophiolite sheets are stranded by newly formed detachment faults.