Geological Setting
The geology of eastern Australia is considered to be a type-example of a subduction-related accretionary orogen (Cawood and Buchan, 2007). The resulting assembly of Palaeozoic orogens, normally referred as the Tasmanides (Glen, 2005), makes a large component of the Australian continent (Figure 1a) and generally becomes younger from west to east. The New England Orogen is the easternmost and youngest component of the Tasmanides. It has developed in a supra-subduction environment from Devonian to Triassic, although recycled components from an earlier orogen (Cambrian and Ordovician) are also found (Aitchison et al., 1994; Fukui et al., 1995).
The southern New England Orogen occupies the area between Brisbane and Newcastle (Figure 1b). The majority of the rocks in this area are associated with the forearc of an arc related to a Devonian-Carboniferous subduction zone (Leitch, 1974). These rocks are overlain by early Permian sedimentary basins, which were deposited simultaneously with the emplacement of S-type granitoids (Shaw and Flood, 1981; Korsch et al., 2009). A later stage of voluminous calc-alkaline, I-type, magmatism took place from late Permian to Triassic (260-220 Ma), possibly in a continental arc setting (see Bryant et al., 1997).
The Devonian to Carboniferous forearc region is expressed in forearc basin rocks (predominantly of the Tamworth Belt) and the subduction complex rocks of the Tablelands Complex (Figure 1b). The Tamworth Belt comprises fluvial to shallow marine sedimentary rocks deposited on a shelf that was deepening from west to east (Roberts and Engel, 1987). The southern continuation of the Tamworth Belt is the Rouchel, Gresford and Myall blocks. Other blocks attributed to the forearc basin are the Hastings Block (Korsch, 1977; Roberts et al., 1995b) the Emu Creek Block (Cross et al., 1987), and possibly also the small inlier of Mt Barney in southeast Queensland (Olgers et al., 1974) (Figure 1b). The Tablelands Complex consists of variably deformed and metamorphosed deep marine volcaniclastic turbidites, cherts and argillites, mafic volcanic rocks, and olistostromal deposits (Leitch and Cawood, 1980; Cawood, 1982; Fergusson, 1984). Metamorphic conditions vary from prehnite-pumpellyite/lower greenschist-facies to high-grade (amphibolite-facies) metamorphic complexes (Korsch, 1978; Stephenson and Hensel, 1982; Phillips et al., 2008; Danis et al., 2010; Craven et al., 2012).
The other elements of the Devonian to Carboniferous convergent margin, namely the arc and back-arc regions, are not exposed in the southern New England Orogen, with the exception of a few occurrences of arc-related rocks in external parts of the Tamworth Belt (e.g. Nerong Volcanics). The majority of the volcanic arc was originally positioned west of the Tamworth Belt (Korsch, 1984), but was subsequently overthrust by the forearc basin (Glen and Roberts, 2012).
The contact between the Tamworth Belt and the Tablelands Complex is a tectonic contact, the Peel-Manning Fault System (Figure 1b), along which, serpentinites and high-pressure rocks are exposed (Benson, 1913; Aitchison et al., 1994; Och et al., 2003). These rocks, which are Cambrian-Ordovician in age, are the oldest component in the New England Orogen, with metamorphic ages that are roughly similar to rocks from the Lachlan Orogen in Victoria (Phillips and Offler, 2011, and references therein).
Early Permian rocks in the southern New England Orogen are predominantly S-type granitoids and clastic rift-related sedimentary successions. The former occurs mainly in two granitic suites, the elongate Bundarra Granite in the west, and the granitoids of the Hillgrove plutonic suite (Figure 1b). The two granitic suites, along with a number of additional granitoids, were emplaced simultaneously at 295-290 Ma (Cawood et al., 2011a; Rosenbaum et al., 2012), and their map-view structure delineates the shape of the New England oroclines (Figure 1b). Granite emplacement was contemporaneous with the early development of back-arc extensional basins (Korsch et al., 2009), possibly in response to subduction rollback (Jenkins et al., 2002; Rosenbaum et al., 2012). Clastic sedimentation occurred in a number of peripheral basins (Sydney, Gunnedah and Bowen Basins), which were subsequently inverted into foreland basins, as well as in other more internal basins (e.g. Nambucca Block, Figure 1b). Although these basins are thought to represent an early Permian backarc environment (Korsch et al., 2009), the exact architecture of the convergent margin remains speculative due to the lack of information on the early Permian arc and forearc regions.
Following the emplacement of the early Permian S-type granitoids, magmatic activity ceased in the southern New England Orogen from 280 to 260 Ma (with the exception of the ~267 Ma Barrington Tops Granodiorite, Figure 1b). Magmatism resumed at ~260 Ma, forming the broad NE-SW New England batholith of subduction-related granitoids and volcanic rocks (1b). At about the same time (from ~265 Ma to ~235 Ma), the region was subjected to ~E-W contractional deformation, normally referred as Hunter-Bowen Orogeny. At the end of this period of contractional deformation, from 235 Ma onwards, the focus of magmatic activity shifted eastward, possibly in response to asymmetric subduction rollback (Li et al., 2012b).
During the Mesozoic, eastern Australia experienced widespread basin formation and sedimentation. Jurassic and Cretaceous continental clastic sediments and coal measures were deposited in the Clarence-Moreton and Surat Basins unconformably on top of the earlier rocks (Figure 1b), thus obscuring the connection between the southern and the northern segments of the New England Orogen.
The youngest geological components in the New England region are Cenozoic basalts (Figure 1b), which are predominantly lava flows derived from a number of central volcanoes. The origin of this volcanism has been attributed to a hotspot beneath the northward moving Australian plate (Vasconcelos et al., 2008).