Nature of extensional accretionary orogens

Abstract

Extensional accretionary orogens form by creation and destruction of large arc/back arc basin systems, generated by extension and sediment infilling during prolonged slab retreat, but episodically thickened by basin inversion during short‐lived (∼10 Ma), orogenic contraction events. They are characterised by widespread, syntectonic, silicic, and minor basaltic magmatism, regional low‐P, variable‐T metamorphism, and by the enigmatic development of rift basins throughout the peak orogenic history. These orogens have features associated with retreating subduction boundaries and contrast markedly with those formed by terrane accretion, such as the Canadian Cordillera. The Paleozoic Lachlan orogen example from eastern Australia shows that Silurian‐Devonian synorogenic basalts and gabbros were intimately associated with rifting and granite emplacement, but they formed during a period of repeated orogenic contraction. Moreover, primitive basaltic compositions have oceanic affinities, indicating generation under lithosphere that was <30 km thick. Only in the final stages of orogeny (Middle Devonian), after at least three major crustal contraction events, did the lithosphere thicken to ∼80 km or more, leading to stabilization of the orogen. Extensional accretionary orogens grow by magmatic and sedimentary additions during extension, caused mainly by asthenospheric melting and rift basin formation/sedimentation, augmented by localized and repeated crustal thickening events. Orogenic contraction leaves an indelible structural imprint which may obliterate the prior‐formed extensional structures. The orogen remains hot, despite repeated thickening events, because of ongoing extension, which promotes advective heat transfer into the crust by basalt injection and crustal melting. Rapid switching to contraction, possibly during intermittent arrival of buoyant oceanic plateaus, inverts the thermally softened basins and forms localized fold‐thrust belts in which the penetrative foliations record the peak metamorphism. Their most diagnostic features are the presence of basaltic rocks and rift basins throughout the orogenic contraction history.