Diapirism and structural thickening in an Early Palaeozoic subduction complex, southeastern New South Wales, Australia

A major problem in the study of subduction complexes has been to distinguish between processes of mud diapirism, deformation of unlithified sediments, frontal accretion and underplating. In the Batemans Bay area, Cambro–Ordovician rocks of the south coast of New South Wales (eastern Australia) contain common mud-rich mélanges that occur amongst an imbricated succession of Early to Middle Ordovician quartz turbidites, Late Ordovician black mudstone and chert, Cambrian limestone and mafic volcanics. An early set of fluidal obliquely trending folds are restricted to the Late Ordovician black mudstone and chert and probably formed by gravitational down-slope movement on a lower trench slope. Features indicative of deformation of unlithified sediments are still preserved and include mud injections, delicate small-scale faulting and the absence of grain-scale deformation. Tight to isoclinal north–south folds with axial planar west-dipping cleavage overprint all units and formed during the younger main-phase deformation that post-dated early deformation in the prism. The mélange has been strongly overprinted by this main-phase deformation. Early mélange formation is attributed to mud diapirism involving intrusion along early accretionary faults, bedding planes and in pipe-like features resulting in chaotic outcrop and map patterns.