Journal of the Virtual Explorer

Neither the Manning or Hastings Oroclines are clear on the grey scale 1VD aeromagnetic imagery (Figure 1C in Glen and Roberts 2012). The proposed oroclines have been highlighted by a dashed blue line but it is not obvious that the position of this line corresponds to a clear boundary between sequences of different aeromagnetic signature.

Although many authors support the presence of these oroclines, there is some disagreement amongst them as to the location of the hinge zone of the Manning Orocline. For example, Rosenbaum (2010) and Rosenbaum and Rubatto (2012) on the basis of the distribution of Permian granites, delineate a hinge zone approximately 60-65km west of that proposed by Glen and Roberts (2012) and Cawood et al. (2011) who locate the hinge zone near Mt George. As indicated previously, the authors provide no evidence to support the presence of the hinge zone near Mt George. Figure 2 shows a map, cross section and stereoplot of the area north of Mt George where the orocline should be located. The Devonian Myra Beds in this area are tightly mesoscopically folded but their map pattern is not consistent with a steeply northwest-plunging orocline. The location of the hinge zone proposed by Rosenbaum and Rubatto (2012) is just as contentious because it is based on the distribution of granites of the same age in their current position that is the result of faulting subsequent to emplacement (Landenberger et al. 1995). This has not been taken into consideration in their delineation of the fold hinge. Further, S₅ the last fabric to form within and adjacent to the Tia Granodiorite (Dirks et al. 1992) prior to oroclinal bending is not folded. Evidence for this should be present because this granodiorite body occurs in the hinge of the proposed Manning Orocline.

The distribution of serpentinites has been used by Glen and Roberts (2012) to delineate the Manning and Hastings Oroclines. However, close examination shows that the serpentinites are not as continuous as required to outline these structures (Figure 1). For example, they do not wrap around the Hastings Block but rather form pod-like bodies near and sometimes away from the block boundary. Further, very few serpentinites are present on the northern margin of the Hastings Block, none on the northeast side of this block and here is no continuity with those in the Port Macquarie Block. Finally, the ages of the serpentinites and associated protoliths have not been established to be the same for all the exposures around these structures. For example, plagiogranites in the northern part of the Peel-Manning Fault System are of Cambrian age (U-Pb; 530 Ma; Aitchison and Ireland 1995) and hornblende cumulates at Glenrock Station and Pigna Barney, are of Silurian age (Rb-Sr; 425 Ma; Sano et al. 2004; U-Pb; 436 Ma; Kimbrough et al. 1993). By contrast, near Yarras on the western margin of the Hastings Block, plagiogranites are Devonian in age (Figure 1; U-Pb; 377 Ma; Aitchison and Ireland 1995) and serpentinites at Port Macquarie that are linked with those at Yarras (Figure 1C; Glen and Roberts 2012) are Silurian in age (K-Ar; 427 ± 8 Ma; Cr-rich white micas formed during serpentinisation; Och et al. 2010).

One of the approaches used by Glen and Roberts (2012) to show evidence for these oroclines was to determine the orientation of ocean-pointing vectors which is based on the spatial relationship between forearc basin sediments and subduction complex rocks. This is a valid approach if the sequences are the same age throughout but this is not so. The caption of Figure 1B implies that the rocks of the subduction-accretion complex in the Port Macquarie Block (PMB) are the same age as the subduction-accretion sequences in the Tablelands Complex. However, these rocks are of Middle-Late Ordovician age (Och et al. 2007) and thus older than the sequences to the west.

Furthermore, the deep water vector asserted to point east in the Hastings Block/Port Macquarie Block area (Figure 1B) depends on the sequences in both blocks being the same age or at least comparable ages. This is not the case as the rocks in the Hastings Block on the margin with the Port Macquarie Block are mainly Devonian in age (Mile Road Beds or Touchwood Fm, Pickett et al. 2009), whereas those of the PMB are (?) Neoproterozoic to Ordovician in age (Och et al. 2007). Therefore to define an ocean-pointing vector using rocks of different ages is not valid.

It is asserted (p.9) that Early Permian (Asselian and Sakmarian) rocks around the northern margin of the Hastings Block deepen northwards (across faults) into mainly Artinskian turbiditic strata of the Nambucca Block. However, the geology of this area is more complex than this interpretation suggests. For example, the early Permian (Asselian and Sakmarian) Youdale B and A units give way to the Artinskian Commong Formation that overlies the Yessabah Limestone. Above this limestone are the Warbro/Parrabel beds that are in faulted contact with the Nambucca Beds. This transition therefore, does not simply represent a deepening trend as evidenced by the presence of limestone in the Artinskian succession.

Based on their interpretation of seismic sections offshore from Newcastle and Sydney, Alder et al. (1998) and Breeze (2009) delineated three major structures, namely the Offshore Syncline, Offshore Uplift and Newcastle Syncline. They suggested that the Offshore Uplift has been thrust NNE over Permian strata in the Newcastle Syncline (Fig. 9B) and a similar overthrust relationship between the Offshore Uplift and Offshore Syncline (Fig. 9C). The features in these seismic sections do not have sufficient clarity to ascertain whether their interpretations are correct. A similar criticism could be levelled at the interpretation of the aeromagnetic data in this area by Glen and Roberts (2012) who state that the “core of the Offshore Uplift has similar properties to….the Carboniferous continental margin arc volcanics…” but no evidence is given to justify this interpretation.