Triassic asymmetric subduction rollback in the southern New England Orogen (eastern Australia): the end of the Hunter-Bowen Orogeny

Abstract

The New England Orogen, the youngest subduction-related component in the Australian continent, records a prolonged history of west-dipping subduction from the Devonian to the Triassic. From mid-late Permian (ca 265 Ma) to Upper Triassic (ca 235 Ma), the New England Orogen was subjected to pronounced contractional deformation (Hunter-Bowen Orogeny) and widespread I-type calc-alkaline magmatism. We obtained zircon U-Pb ages from seven granitic samples within the I-type magmatic system. The new SHRIMP U-Pb data ranging from 255 to 215 Ma, combined with previous geochronological data from the southern New England Orogen, suggest that magmatism during the Hunter-Bowen Orogeny was spatially distributed along a NNE-SSW belt that was likely associated with a west-dipping Andean-type subduction zone. In contrast, younger magmatism (235-215 Ma) is aligned along a N-S belt farther east. Assuming that magmatism was subduction-related and was spatially distributed parallel to the subduction zone, we interpret the spatio-temporal change in magmatism as an indicator for eastward arc migration, possibly in response to slab rollback. The trench position is not well constrained but was possibly located between the Australian continent and the Lord Howe Rise. We propose a model involving asymmetric slab rollback, possibly in response to pinning of the northern part of subduction zone by the Gympie Terrane accretion. Our model invokes that this phase of tectonic activity marked the transition from contractional deformation in the continental margin to extensional tectonics that represents the earliest phase of the Mesozoic rifting of eastern Australia.