Introduction
The existence of oroclines in the New England Orogen (NEO) has been accepted by many authors and have been incorporated in models proposed for the development of the NEO (Cawood et al. 2011; Rosenbaum and Rubatto 2012 and references therein). However, the evidence for two of the oroclines, namely Manning and Hastings is not well established. This discussion will assess the regional aeromagnetics, the features in the northern Hastings Block, proposed hinge area of the Manning Orocline and the polarity of forearc-accretion complexes to show that the Manning and Hastings oroclines do not exist.
The aeromagnetic image they use to delineate the oroclines does not have the clarity or definition to support the presence of these structures nor does the distribution of the serpentinites. The latter are assumed to be the same age and their outcrop pattern is thought to outline oroclinal structures. However, there is no continuity in the outcrops of serpentinite between the fore-arc and subduction-accretion complex around the oroclines and the serpentinite bodies are of different ages (Figure 1). Further, the orientation of bedding and the pattern of folding in the Hastings Block are not consistent with passive, high angle rotation of bedding around an orocline.
Figure 1. Simplified map of the serpentinite outcrops in the Hastings Block portion of the Southern New England Orogen.
The possible position of the edge of the Hastings Block is shown by the blue dashed line. The serpentinite bodies do not form a continuous line around the edge of the Hastings Block as expected if they were all of one age and were passively rotated around an orocline.
The model for the development of the Manning and Hastings Oroclines proposed by Glen and Roberts (2012), involves the formation of the Parrabel Dome a structure they believe formed as an asymmetric, hanging wall anticline on the steeply, northeast-dipping, Bagnoo Fault. This is in contradiction to the field data collected by the authors that shows the bedding on the southwestern limb of the Parrabel Dome adjacent to the Bagnoo Fault is not steeply dipping. These data also show that thrusting required for the formation of the hanging wall anticline did not occur. Rather, this fault records a sinistral, strike-slip movement with the northeast side being down inconsistent with the required thrust history. Further, the Parrabel Dome plunges very gently northwest unlike that expected by rotation around a vertical axis as is normally the case with oroclines.
There is no structural evidence in the subduction-accretion complex sequences of the Manning Orocline similar to that in the accretion-subduction sequences that define the Texas and Coffs Harbour oroclines. Finally, ocean-pointing vectors used by Glen and Roberts (2012) to determine the polarity of the fore-arc /accretion complex sequences and thus provide evidence for the Manning and Hastings Oroclines, are untenable in the Port Macquarie Block because the subduction-accretion sequences are older than those in the Tablelands Complex.
Detailed below are observations based on field data collected by the authors and data of other authors, that argue against the existence of the Manning and Hastings Oroclines.