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NEO general geology
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Figure 2.1. This figure shows a highly generalized representation of the distribution of New England, Lachlan, and Thomson orogen strata, together making up the eastern one-third of the Australian continent (modified from Leitch, 1975).
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The New England Orogen (NEO) is part of the Tasman orogen, which makes up the eastern one-third of the Australian continent, along with the Lachlan and Thomson orogens (Figure 2.1). The NEO developed on the eastern margin of Gondwana (Figure 2.2), mostly during middle to late Paleozoic and earliest Mesozoic time. The oldest rocks in the orogen are Cambrian strata of probable marine origin, though rocks within the New England Orogen are predominantly mid-Devonian through Triassic in age, excluding subsequent deposition of later Mesozoic strata associated with post-orogenic extensional basin development.
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Figure 2.2. Map showing the distribution of continental blocks within the supercontinent of Gondwana during the late Paleozoic (from Coney et al., 1990)
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Silurian to middle Devonian (~440 Ma - 385 Ma)
Conflicting ideas exist concerning the tectonic development of the eastern margin of Gondwana prior to mid-Devonian time, though many authors seem to believe that an island arc, probably with west-dipping associated subduction, collided with the eastern margin of Gondwana sometime during the middle Devonian. A distinct series of rock exposures in the southern NEO, commonly referred to as the Calliope or Silverwood terrane (Figure 2.4), consists of arc volcanics of Silurian to mid-Devonian age, providing some support for this idea.
middle Devonian to late Carboniferous (~385 Ma - 285 Ma)
Around middle Devonian time, presumably following collision, a continental margin arc began to form above the crust of eastern Gondwana. This new arc was related to west-dipping subduction and continued to develop through the remainder of the Devonian and up until late Carboniferous time. Active arc volcanism was associated with coeval deposition of strata in both forearc and backarc basins as well as the formation of an accretionary wedge complex (Figure 2.3).
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Figure 2.3. This diagram schematically shows the temporal evolution of the eastern margin of Australia, from a subduction-related arc/forearc/accretionary wedge complex during the middle to late Paleozoic to its present faulted configuration. The late Paleozoic subduction setting was disrupted by a period of extension in early Permian time, followed by exaggerated compression in the latest Paleozoic that is commonly referred to as the Hunter-Bowen orogeny (from Fergusson, 1991).
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latest Carboniferous to middle Permian (~300 Ma - 265 Ma)
The continental margin arc system continued to develop until approximately late Carboniferous or earliest Permian time, when a thermal or tectonic event of hotly disputed nature (see tectonic models) forced the subduction system into a state of extension. This period in the history of the New England Orogen is characterized by the development of volcanic and plutonic rocks of highly variable chemistries and lithologies, as well as the apparent formation of numerous extensional basins throughout the orogen. Extensional basin development may have included early formation of the Sydney, Gunnedah, and Bowen basins (Coney et al., 1990; Holcombe et al., 1997a). Intrusion of S-type granites is also associated with this time period (see geochemistry).
middle-late Permian to early Triassic (~265 Ma - 235 Ma)
Whatever the thermal or tectonic event that occurred in the late Carboniferous or early Permian, by late Permian time the eastern margin of Gondwana had resumed a subduction zone character, although supposedly with an increased convergence rate compared to mid-Devonian through late Carboniferous subduction. This increased convergence is related to what is thought to be the major phase of deformation in the New England Orogen, persisting around 30 million years and termed the Hunter-Bowen orogeny. During this time period, the Sydney and Bowen basins developed as foreland basins, bounded on the east in at least some areas by actively deforming, discontinuous fold-thrust belts, such as the Gogango overfolded zone and folded areas within the Bowen basin in the northern New England Orogen (Figure 2.4). The Gogango overfolded zone and portions of the accretionary wedge strata experienced the most intense deformation during this time, particularly a unit known as the Nambucca slate belt in the southern NEO (Fergusson, 1991; Holcombe et al., 1997b). This terrane is believed to have possibly consisted of an extensional basin during the early Permian, filled with a thick package of sediments that was easily deformed and metamorphosed during late Paleozoic to early Mesozoic compression (Leitch, 1988).
The Hunter-Bowen event is associated with the development of a west-directed system of thrusts that may have also involved east-directed backthrusting. Thrust systems within the Tamworth terrane imply shortening of around 75-85 km, an amount sufficient to emplace forearc and accretionary wedge strata on the arc strata in the southern NEO (Woodward, 1995). In addition to thrusting, the Hunter-Bowen orogeny may have been partly or wholly responsible for development of large-scale, oroclinal folding and widespread strike-slip faulting that is present in the southern New England Orogen and thought to have accommodated approximately 450-500 km of N-S directed shortening (Finlayson and Collins, 1993). This oroclinal folding is not present in the northern New England Orogen, although this area may have been affected by strike-slip faulting. Such faults are difficult to identify in most of the New England Orogen because their expected north-south trend is parallel to the general structural grain of the orogen. Current geological data does not provide information that is detailed enough to determine whether these folds developed in association with the Hunter-Bowen event or in relation to processes occurring during the enigmatic late Carboniferous to early Permian tectonic activity (see active research).
early Triassic to Cenozoic (235 Ma - present)
Following the Hunter-Bowen event, the New England region again experienced a transition to extension and eventually became a passive margin in the Jurassic. In Cretaceous time, a brief renewal of subduction is though to have occurred just before latest Mesozoic to Cenozoic tectonism and magmatism that was associated with the break-up of Gondwana. This later tectonic activity involved rifting of several pieces of eastern Australia in association with opening of the Tasman and Coral Sea basins (Figure 6.4). These later events have disrupted structures that developed during earlier orogenesis and may have largely erased any impact of the Hunter Bowen orogeny and prior deformation from the New England region lithosphere (Figure 4.9).
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Current surface geology
Present-day exposures of middle Paleozoic to early Triassic units are separated into northern and southern portions (sometimes referred to respectively as the Yarrol and New England Orogens, though most authors refer to them collectively as the New England Orogen) by strata deposited in a large, Mesozoic extensional basin known as the Clarence-Moreton basin (Figure 2.4). Within the northern and southern portions of the NEO, approximately N-S trending faults that are mostly gently east-dipping, with some west-dipping, separate terranes of different tectonic affinity (Figure 4.4). In particular, in the southern NEO, the Peel-Manning fault system separates the accretionary wedge from the forearc basin strata, and the Hunter-Mooki fault system separates the forearc from backarc and foreland basin deposits of the Bowen, Sydney, and Gunnedah basins to the west. Thrust faults also form contacts between equivalent tectonic units in the northern NEO, with the Yarrol fault separating forearc strata from accretionary wedge strata, though in general faults in this area are less commonly referred to with specific names. In addition, some thrust contacts separate volcanic arc strata from forearc strata on the east and basin deposits on the west. The arc terrane is known as the Connors-Auburn arc, sometimes referred to as the Proserpine terrane, and is only exposed in the northern portion of the New England orogen. The Yarrol and Tamworth belts are, respectively, the northern and southern exposures of the forearc basin strata. The accretionary wedge is comprised of a series of terranes that are often referred to generally as the Tablelands complex. Backarc basin strata are less continuously present, but, where present, they are west of the Connors-Auburn arc; one example is the Drummond Basin in the northern New England orogen (Allen, 2000). The Bowen, Sydney and/or Gunnedah basins may have also acted as backarc basins behind the active subduction zone prior to the Hunter Bowen orogeny, when these basins comprised part of the foreland for and experienced sedimentation from the rising mountains to the east (Woodward, 1995).
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Figure 2.4. Generalized terrane map of the New England Orogen. Click on image to enlarge.
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Table of NEO terranes and references for ages of strata
| Terrane Name |
Age of strata/ sediments |
Tectonic Environment |
Source (see below) |
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Beenleigh |
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Devonian(?)-Carboniferous |
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accretionary wedge |
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Calliope |
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Late Silurian-Middle Devonian |
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Upper Silurian-Middle Devonian |
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oceanic island arc |
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Queensland Geology |
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Coney et al., 1990 |
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Connors-Auburn |
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Late Devonian-Early Carboniferous |
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Upper Devonian-Lower Carboniferous |
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continental margin arc |
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Queensland Geology |
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Coney et al., 1990 |
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D'Aguilar |
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Devonian(?)-Carboniferous |
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accretionary wedge |
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Esk Trough |
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early Triassic |
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Late Permian-Early Triassic |
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extensional basin |
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Holcombe et al., 1997b |
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Korsch et al., 1997 |
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Gogango overfolded zone |
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heavily deformed forearc basin |
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Gympie |
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Permian-Early Triassic |
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Carboniferous(?)-Early Triassic |
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arc, forearc basin |
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Queensland Geology |
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Coney et al., 1990 |
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Hastings Block |
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late Devonian-Permian (?) |
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arc, forearc basin, originally part of Tamworth terrane |
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Nambucca |
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possible pull-apart basin or obducted oceanic strata |
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Tamworth-Yarrol |
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Late Devonian-Middle Permian |
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Devonian-Lower Carboniferous |
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arc, forearc basin |
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Queensland Geology |
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Coney et al., 1990 |
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Texas (also called Wandilla) |
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Late Devonian-Carboniferous |
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Carboniferous |
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ocean floor (accretionary prism) |
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Queensland Geology |
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Leitch & Scheibner, 1987 |
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