Provenance of the Early Permian Nambucca block (eastern Australia) and implications for the role of trench retreat in accretionary orogens

Abstract

The New England orogen of eastern Australia is characterized by tight orogenic curvatures (oroclines). Oroclinal bending commenced in the Early Permian during a period of extension that involved crustal melting, widespread emplacement of S-type granitoids, high-temperature metamorphism, exhumation of metamorphic complexes, extensional faulting, and development of rift basins. One of these basins is the Early Permian Nambucca block, which is situated in the "core" of the oroclinal structure, but its origin and time of deposition are poorly constrained. Here, we present new U-Pb ages of detrital zircons from the Nambucca block, which include age populations as young as 299 and 285 Ma, confi rming the Early Permian deposition of the succession. Additional Devonian-Carboniferous and Precambrian ages indicate that detritus was mainly derived from the New England subduction complex and cratonic Gondwana. The range of ages suggests that the Nambucca Basin received detritus from both arc and continent and that deposition occurred in a back-arc setting. Given the coeval formation of the Nambucca Basin and the New England oroclines, we propose that this back-arc extensional basin was controlled by trench retreat, which resulted in "Mediterranean-style" orogenic curvatures along the plate boundary of eastern Gondwana. The recognition of a genetic link between oroclinal bending and back-arc extension may explain how accretionary orogens, such as the eastern Australian Tasmanides, were able to obtain an anomalous width without a substantial contribution of accreted exotic terranes. A similar mode of tectonism may have played an important role in other accretionary orogens.

Different styles of modern and ancient non-collisional orogens and implications for crustal growth: a Gondwanaland perspective

“…6C, panel L). Replotting all of the U-Pb zircon age data of Shaanan et al (2015) shows a dominant age peak of 329 Ma, with younger smaller age peaks at 259 and 224 Ma and an older shoulder at 341-353 Ma (Fig. 19A, without showing the shoulders they recorded at 299 and 285 Ma).…”

“…Latest Carboniferous arc magmatism terminated at ϳ305 Ma and was followed by plate boundary rollback (Holcombe et al 1997), tracked by the nature of volcanism changing from calcalka- Li et al (2015), and Nambucca Basin, replotted from Shaanan et al (2015). (B) Hf zircon data from Gympie Arc Terrane compared to New England Orogen granitoid data of Kemp et al (2009).…”

Paleomagnetic data from the New England Orogen (eastern Australia) and implications for oroclinal bending

“…This is a characteristic feature of backarc regions (Currie and Hyndman, 2006). The transition from a fore-arc region during the Carboniferous to a back-arc environment in the Early Permian was attributed to the onset of eastward trench retreat (Jenkins et al, 2002;Shaanan et al, 2015).…”

The geometry, kinematics, and reactivation history of major fault systems in eastern Australia: implications for deformation phases from the Permian to Cenozoic

“…During the early Permian, the New England Orogen was subjected to extensional tectonism, possibly in a backarc setting (Jenkins et al, 2002;Korsch and Totterdell, 2009;Rosenbaum et al, 2012;Shaanan et al, 2015). Extensional deformation involved the development of widespread rift basins (Korsch and Totterdell, 2009) and the simultaneous emplacement of a large volume of S-type granitoids (Shaw and Flood, 1981;Hensel et al, 1985).…”

“…These rocks are intruded by Early Permian S-type granitoids (298-288 Ma) (Shaw and Flood, 1981;Cawood et al, 2011b;Rosenbaum et al, 2012), and locally overlain by Early Permian clastic sedimentary successions (e.g., Nambucca Block; Figure 2.1a). The latter basins have likely formed in a backarc extensional setting (Shaanan et al, 2015) that was also responsible for rifting in the Sydney, Bowen, and Gunnedah basins (Korsch et al, 2009a). The Sydney, Bowen, and Gunnedah basins were subsequently transformed into foreland basins and continued to receive sediments until the Middle Triassic (Glen and Beckett, 1997;Glen, 2013).…”