Miocene Central Volcanoes, NW New South Wales: Genesis over a Lithospheric Cavity (?)

Abstract

Basalt fields and central volcanoes form a curved south-migrating trace through NW New South Wales. A segment of East Australian intraplate volcanism, it traces Australia’s northern plate motion north over a mantle plume system. This created the western New England basalt field (24 – 21 Ma), Nandewar central volcano (19 – 18 Ma), Warrumbungle central volcano (18 – 15 mya), Mount Canobolas central volcano (13 – 11 Ma) and minor alkaline eruptions near Oberon (10 – 9 Ma). This ‘boomerang-shaped’ segment initially swelled south-westerly with increasing mantle melting and basaltic evolution. After initial fluid basaltic outpourings in New England, it formed two large central volcanoes along its outward curve before bending southerly to form a smaller central volcano and a scattered tail of small late-eruptions. This volcanic trace did not match Australia’s linear plate motion trend between 24 – 9 Ma. Neither did it correspond with adjacent plume trend seen in the leucititic lavas to the west, the coastal NSW plume volcanoes and Tasman Sea submarine plume chains to the east. Recent seismic tomography has revealed ‘cavities’ within the underlying lithosphere-asthenosphere boundary (LAB). The anomalous NW New South Wales plume upwelling underwent diversion by its interaction along the western edge of a LAB ‘cavity’.