Subduction accretion and orocline development in modern and ancient settings: Implications of Japanese examples for development of the New England Orogen of eastern Australia

摘要

The southern New England Orogen of eastern Australia is a component of the Palaeozoic palaeo-Pacific active margin of Gondwana (the Terra Australis Orogen) and is characterised by subduction complex terranes marked by orogenic curvature that are collectively termed the New England oroclines and include the Z-shaped coupled western Texas and eastern Coffs Harbour oroclines. This study addresses the problem of why these two oroclines were initiated and has involved examination of the accretionary development of the New England subduction complex based on comparisons with onland accretionary belts of southwest Japan and the active subduction zone associated with southwest Japan (the Nankai Trough). The main outcome is recognition that the Texas Orocline was nucleated during subduction of a seamount chain resulting in orogenic curvature of the Carboniferous subduction complex. Subduction of the seamount chain is shown by abundant limestone associated with ocean island basalts amongst the accreted turbidites in the core of the Texas Orocline. Further enhancement of the initial curvature of the Texas and adjacent Coffs Harbour oroclines continued in the Early Permian in a backarc setting influenced by rollback as argued by previous authors, although the role of this rollback process in orocline development has been overstated as no oceanic backarc basin developed during this process. The accretionary history of the subduction complex of the southern New England Orogen is resolved into two distinct phases: an earlier phase in the Devonian with accretion of an island arc backarc basin resulting in larger thrust slices analogous to those in the Honshu - Izu-Bonin-Marianas collision zone, and the Carboniferous phase involving accretion of dominantly trench-wedge deposits during which the Texas and Coffs Harbour oroclines were nucleated.