Geology and geochronology of the Emu Creek Block (northern New South Wales, Australia) and implications for oroclinal bending in the New England Orogen

Geological Society of America Bulletin

Wormald

2015

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.

Section: Provenance Of the Nambucca Blockmentioning

confidence: 85%

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

Shaanan

Geological Society of America Bulletin

Wormald

2015

“…Therefore, we think that detrital zircons defining late Permian to Triassic age peaks in the Gympie terrane were most likely sourced from the middle Permian to Triassic continental arc system to the west, which was built on the former accretionary complex of the NEO (Figure 1b) [Gust et al, 1993; Figure 1a). Detrital zircon ages in New Zealand are adapted from Adams et al [2007], while data from the NEO are taken from Korsch et al [2009a], Hoy et al [2014], and Shaanan et al [2015]. The plotted detrital zircon ages of the accretionary complex in Figures 5d and 6 just refer to LA-ICP-MS data of Korsch et al [2009a], which were conducted with an aim at tracing the provenance.…”

Section: Provenance Of the Gympie Terranementioning

confidence: 99%

Australian‐derived detrital zircons in the Permian‐Triassic Gympie terrane (eastern Australia): Evidence for an autochthonous origin

Yang

et al. 2015

Tectonics

The Tasmanides in eastern Australia record accretionary processes along the eastern Gondwana margin during the Phanerozoic. The Gympie terrane is the easternmost segment of the Tasmanides, but whether its origin was autochthonous or allochthonous is a matter of debate. We present U-Pb ages of detrital zircons from Permian and Triassic sedimentary rocks of the Gympie terrane with the aim of tracing the source of the sediments and constraining their tectonic relationships with the Tasmanides. Our results show that the Permian stratigraphic units from the Gympie terrane mainly contain Carboniferous and Permian detrital zircons with dominant age peaks at~263 Ma,~300 Ma,~310 Ma, and~330 Ma. The provenance ages of the Triassic sedimentary units are similar (~256 Ma,~295 Ma, and~328 Ma) with an additional younger age peak of~240 Ma. This pattern of provenance ages from the Gympie terrane is correlative to episodes of magmatism in the adjacent component of the Tasmanides (New England Orogen), indicating that the detrital zircons were dominantly derived from the Australian continent. Given the widespread input of detrital zircons from the Tasmanides, we think that the sedimentary sequence of the Gympie terrane was deposited along the margin of the eastern Australian continent, possibly in association with a Permo-Triassic continental arc system. Our results do not show evidence for an exotic origin of the Gympie terrane, indicating that similarly to the vast majority of the Tasmanides, the Gympie terrane was genetically linked to the Australian continent.

“…Evidence for the existence of these oroclines includes (1) a curved belt of early Paleozoic serpentinites (Korsch and Harrington, 1987), (2) curved structural and magnetic fabrics within the Devonian-Carboniferous subduction complex (Korsch and Harrington, 1987;Aubourg et al, 2004;Li et al, 2012;Li and Rosenbaum, 2014;Mochales et al, 2014), (3) the spatial distribution of the segmented DevonianCarboniferous fore-arc basin blocks (Korsch and Harrington, 1987;Glen and Roberts, 2012;Hoy et al, 2014) and (4) Block, Permian strata are exposed in the cores of two north-south trending, doublyplunging synclines (Gloucester and Myall;Figs. 2b,4).…”

Section: Geological Settingmentioning

confidence: 97%

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

Shaanan

Pisarevsky³

et al. 2015

Tectonophysics

Orogenic curvatures (oroclines) are common in modern and ancient orogens, but the geodynamic driving forces of many oroclines remain controversial. Here we focus on the New England oroclines of eastern Australia, the formation of which had been previously broadly constrained to the Early-Middle Permian. This time interval encompasses periods of both back-arc extension (at ~300-280 Ma) and subsequent contractional deformation (Hunter-Bowen Orogeny) that commenced at ~270 Ma along the paleo-Pacific and Gondwanan subduction plate boundary. We present new paleomagnetic data from volcanic rocks that were extruded during the transition from extension to contraction (at ~272 Ma), and we show that the oroclinal structure must have formed prior to the emplacement of the volcanic rocks. Our results thus indicate that oroclinal bending in the southernmost New England Orogen has been completed prior to the onset of Middle Permian contractional deformation. It is therefore concluded that the oroclines have likely formed during back-arc extension, and that a major contribution to the orogenic curvature was driven by trench retreat.