Crustal Architecture

Several statements are made by Glen and Roberts (2012) in this section that we consider to be incorrect. These are discussed below.

(1) It is stated on p.14 that Late Carboniferous strata have dips of 35-50º and early Permian strata 28-60º. This is an over simplification because data collected from the Carboniferous strata in the area covered by the Kemps Pinnacle and Sherwood 1:25,000 sheets, show dips varying from 2º to 90º (av. = 34º; n = 253) for the Kemps Pinnacle sheet and 2º to 73º (av. = 26º; n = 176) for the Sherwood sheet (Lennox unpubl. data). By contrast, data for the early Permian on the same two sheets shows dips varying from 33º to 87º (av. = 54º; n = 27) for the Kemps Pinnacle sheet and from 24º to 87º for the Sherwood sheet (av. = 58º; n = 30). Clearly the Carboniferous rocks have a lower average dip than the early Permian sequences in the northern hinge area of the Parrabel dome.

(2) The authors claim that folds in the hinge zone of the Hasting Orocline plunge north (p. 35). However, this is incorrect because macroscopic F1 folds in the hinge zone of the Parrabel Dome plunge east or west (Figure 4; Lennox et al. 1999). Early almost east-west trending macroscopic folding in the northern Hastings Block is overprinted by northwest-southeast trending macroscopic folds as shown in Figure 4. All bedding readings for the Northern Hastings Block which incorporates the Parrabel Dome a D2 structure, indicate that overall this structure plunges gently northwest (11º → 302º).

Figure 4. Macroscopic folding in the Northern Hastings Block shows three generations of folds.

Macroscopic folding in the Northern Hastings Block shows three generations of folds.

Early east-west macroscopic folds are shown in blue and have been refolded by northwest-southeast trending folds in red. The black coloured macroscopic folds on the northeast limb of the Parrabel Dome are probably third generation folds. The sinistral dip-slip movement observed on the Bagnoo Fault is consistent with the drag highlighted by the outcrop pattern of the Kindee Conglomerate shown south of the fault.

(3) According to Glen and Roberts (2012; p.15) the Bagnoo Fault shows thrust movement. However, our studies and those of other authors based on kinematic indicators and drag of units (e.g. Kindee Conglomerate in Figure 4) has revealed that sinistral, dip slip movement of 1-2km with down throw to the northeast has occurred (Figure 4; Feenan 1984, Spackman 1989).

(4) In Figure 7 of Glen and Roberts (2012), the trends of what are referred to as F1 folds are shown with NW-SE trending axial plane traces and F2 with meridional trending axial planes. Collins (1991) considers that the meridional trending fold traces are related to D1 and NW trending fold traces parallel to the Hunter Thrust to D2 which is in contradiction to the interpretation of Glen and Roberts (2012). We favour the interpretation of Collins (1991) because E-W contractional deformation associated with the Hunter-Bowen Orogeny commenced at ~265 Ma and the thrusting with which the NW trending folds are associated post 254 Ma. This is based on the occurrence of tuffs of this age in the footwall of the Hunter Thrust.

(5) Figure 10 C. In this figure, cleavage in the accretion complex is shown to have formed initially in the Latest Carboniferous. This is contrary to the observations of other authors who have shown that cleavages of this age formed during the HT/LP event in sequences close to the ~300 Ma Hillgrove Suite intrusives. These cleavages are superimposed on earlier subduction-related cleavages (e.g. Morand 1982; Dirks et al. 1992) which in some sequences may be as old as 346 Ma (Phillips unpubl. results)

(6) P. 25. 1st paragraph. The early shortening is supposed to have been responsible for the “sporadically developed cleavage in the Tamworth belt”. However, there are two cleavages developed in the Tamworth Belt, one that is parallel and strongly developed adjacent to the Peel-Manning Fault System (PMFS) that could be attributed to the E-W shortening associated with the Hunter-Bowen Orogeny and the second, oblique to the PMFS, that is due the sinistral movement on the PMFS (Cao and Durney 1993).

(7) In Figure 5, the Majors Creek Formation is shown as Visean in age whereas Roberts et al. (1995) indicates it is Namurian in age. This affects the correctness of Figure 3 where Namurian rocks should be shown in a pale pink colour (as per the Kullatine Fm) whereas the Majors Creek Formation on this figure is shown uncoloured (consistent with its proposed age in Figure 5 as Visean). Why is the Majors Creek Formation Visean rather than Namurian?