Discussion and conclusions
The collective set of observations from the southern New England Orogen is consistent with a geometrical model of a regional ear-shaped structure hundreds of km across. This geometry involves four major oroclines (Texas, Coffs Harbour, Manning and Nambucca/Hastings), and is different than previous interpretations, in which a simpler structure of two oroclines (Murray et al., 1987; Offler and Foster, 2008) or three oroclines (Cawood and Leitch, 1985; Korsch and Harrington, 1987) was assumed. Although the fieldtrip did not permit careful observations of all the hinge zones, it has provided an introduction to the major geological components that define the oroclinal structure, in particular the Devonian-Carboniferous convergent margin (subduction complex, forearc basin and magmatic arc), the serpentinite belt, and the early Permian belt of granitoids.
Tectonic reconstructions of the New England oroclines are still oversimplified and require further structural, geochronological and palaeomagnetic constraints. Three alternative schematic reconstructions are presented in Figure 15. The reconstruction of Offler and Foster (2008) explains the formation of the Texas-Coffs Harbour oroclines and assumes that the southern oroclines do not exist. The proposed model (Figure 15a) considers that the oroclines were generated by an offshore dextral strike-slip fault. Movement on such a transform fault was supposedly responsible for a progressive bending of the earlier convergent margin, which was pinned in the area around Tamworth. The model involves large ~N-S displacements, particularly in the area of the Coffs Harbour Orocline, with the supposed indentation of the Coffs Harbour Block onto the Nambucca Block. Offler and Foster (2008) concluded that the earlier E-W deformation in the Nambucca Block, dated at ~270-265 Ma, was directly related to this indentation, thus marking the last stage of oroclinal bending.
Figure 15. Alternative tectonic models for the formation of the New England oroclines.
(a) Oroclinal bending generated by an offshore dextral strike-slip faulting (modified after Offler and Foster, 2008). CHB, Coffs Harbour Block; NB, Nambucca Block. Note that the model only accounts for the formation of the Texas and Coffs Harbour oroclines. (b) Formation of the oroclines by buckling within a zone of sinistral transpression (modified after Cawood et al., 2011b). (c) A schematic model showing the progressive curvature of the plate boundary during subduction rollback, followed by further bending in a strike-slip setting (modified after Rosenbaum et al., 2012).
There are a number of problems with the model of Offler and Foster (2008). Firstly, their dextral strike-slip model does not explain the curvature of the southernmost oroclines (Manning and Nambucca oroclines). Secondly, as recently pointed out by Li et al. (2012a), the predicted strain from the strike-slip model (~500 km displacement and ~50% shortening) is considerably larger than the observed strain in the area of the Texas Orocline. Thirdly, Cawood et al. (2011b) have recently demonstrated that the dextral strike-slip model is not consistent with available palaeomagnetic data.
The model by Cawood et al. (2011b) (Figure 15b) is based on a synthesis of available but limited palaeomagnetic data from the southern New England Orogen. However, this dataset is insufficient for the construction of an unequivocal model (Pisarevsky et al., 2010), and therefore suffers from large uncertainties. The major assumption by Cawood et al. (2011b) is that oroclinal bending was governed by sinistral strike-slip tectonics (Figure 15b), which involved a very large northward displacement (~1600 km) of the Hastings Block. Glen and Roberts (2012) have recently argued that such large-scale displacements are unlikely. In my opinion, the major shortcoming of Cawood et al.’s (2011b) model is the fact that they attributed the entire process of oroclinal bending to transpressional tectonics, without explaining the plethora of evidence for syn-oroclinal extension.
An alternative model (Rosenbaum et al., 2012) is based on the assumption that subduction rollback and back-arc extension have played a primary role in controlling the process of oroclinal bending (Figure 15c). This process is similar to many modern examples, in which tight orogenic curvatures were obtained by along-strike variations in the rate of subduction rollback and slab segmentation (Barker, 2001; Schellart et al., 2002; Rosenbaum and Lister, 2004; Rosenbaum and Mo, 2011). In the New England Orogen, it is possible that earlier rollback-related curvatures, as well as other irregularities in shape of the convergent margin (e.g. Glen and Roberts, 2012; Li et al., 2012a), were subjected to further tightening during subsequent events of contraction and transpression.
The key to understanding the geodynamics of the New England Orocline, in my opinion, is the information from early Permian rocks. Evidence for early Permian extensional tectonics, high-temperature metamorphism and crustal melting supports the suggestion that this period was characterised by a retreating, perhaps Mediterranean or SW Pacific style, subduction boundary.