Structural Relations in the Subduction Complex of the Paleozoic New England Fold Belt, Eastern Australia

Abstract

The New England Fold Belt of southeastern Australia is divided by the Peel Fault System into a western magmatic arc-frontal arc succession and an eastern subduction complex assemblage. The western part of the subduction complex consists of repeated slices of a sequence which, where most complete, comprises dolerite overlain by basalt (sometimes pillowed), radiolarian chert, siltstone and, in the younger thicker slices, sandstone. The basal contact of this sequence is always faulted. A planar veined surface separates overlying basic igneous rocks from a sheared mass composed mostly of sedimentary material in which are imbedded tectonically derived blocks of basic igneous rock. This sheared zone passes gradationally down into less deformed sedimentary rocks of the underlying sequence. Slices, each of which is usually a few hundred meters thick, young constantly westward (toward the magmatic arc) forming a stacked steeplydipping sequence tens of kilometers thick. The distribution and inter-relationship of lithologies within the subduction complex is consistent with a model involving the progressive off-scraping and accretion of the upper segments of oceanic lithosphere by imbricate thrust faulting, rather than the subduction of this material beneath the arc. The subduction complex underwent regional orogenic deformation at the termination of underthrusting in the Early Permian. Structures produced during this event are analogous to those recognized in other inferred subduction complexes, perhaps suggesting that the structural fabric of some of these complexes is the result of both deformation following an episode of subduction as well as deformation at the time of underthrusting.