Andean analogue for Late Carboniferous volcanic arc and arc flank environments of the western New England Orogen, New South Wales, Australia

Abstract

Late Carboniferous continental conglomerates interbedded with silicic ignimbrite sheets outcrop along more than 400 km of the western margin of the southern portion of the New England Orogen. Farther east, the coeval sedimentary facies are volcanogenic shallow marine and turbidite deposits. The volcanic source terrain, no longer exposed, was located to the west of the existing conglomerate-ignimbrite sequences and was underlain by continental crust which is, in part, represented by the northern Lachlan Fold Belt. The regional Late Carboniferous palaeogeography was similar to that of the present-day western continental margin of South America. The geology of the oceanward-flank of the Andean arc in northern Chile and a section of the Late Carboniferous continental sequence near Currabubula are comparable in detail. The Andean stratovolcanoes and ignimbrite centres thus provide the means of reconstruction of the Late Carboniferous volcanic source terrain. The geological record of both of these continental margin volcanic arcs, preserved in deposits of the arc flanks, is shaped by volcanism, especially the eruption of voluminous ignimbrites, and by uplift, deformation and glaciation centered on the arc. For the arc sections considered, diversity in the flank sequences arises because these controls vary in importance spatially and during the life of the arc (20–30 Ma). For the entire Andean arc, arc-parallel variations in the sites of active volcanism and its character appear to be related to differences in the continental crust thickness and the circumstances of subduction of oceanic crust, particularly the dip of the Benioff Zone. By analogy, variation in the age, duration and style of volcanic activity along the late Palaeozoic magmatic arc of the western New England Orogen perhaps reflects the former existence of significant differences in crust thickness and in the angle of subduction.