Pre- and post-orogenic extensional settings for burial-type metamorphism

Abstract Extensional basins are formed in a variety of geotectonic settings and in some cases the infills of such basins show the effects of very low grade, burial-type metamorphism. It is proposed that extensional basins with a low grade metamorphic infill can be divided into pre- and post-orogenic types with a different causal mechanism for the metamorphism. In the pre-orogenic setting the extensional process driven by mantle upwelling provides both the mechanism for basin formation and the development of the metamorphism in response to the enhanced thermal flux in such settings. The typical mineral parageneses and facies series found in these low grade metamorphic terranes are more compatible with the counter-clockwise pressure-temperature-time loops postulated for such settings, rather than the clockwise loops associated typically with convergent settings. In the post-orogenic settings, extensional basins formed in over-thickened continental crust during orogenesis have infills in which very low grade metamorphism is also developed. It is proposed that such metamorphism is not linked to the extensional process itself. Instead, it is linked to crustal heating during the process of thickening and orogenesis and is expected to be found only in orogenic belts where widths are in excess of 500 km.