Journal of the Virtual Explorer

The dip values that we presented were not designed to make any point and we accept that the statistical data of Lennox et al. (2013) show that Carboniferous rocks have a lower average dip than Permian rocks in the northern hinge area of the Parrabel Anticline. In our new figure 2, we have plotted, as best we can, bedding data from the Hastings Block and Permian strata to the north and east. These data have been selected from the 1:25.000 scale compilation by Roberts and in figure 2 we have chosen representative bedding data located as close as possible to their to actual measured locations. These data show that dips in the Hastings Block are highly variable in both dip and dip direction.

In our discussion of the geometry of the Hastings Block (Glen and Roberts 2012, appendix of page 35 and figure 11), we followed Lennox et al. (1999) in suggesting that the Parrabel Anticline is doubly plunging, with a north-plunging part in the north and a south-plunging part in the south. Our north-plunging part corresponds to the northwest plunging D2 structures of Lennox et al. (2013). We pointed out that the Parrabel Anticline is a D2 fold, possibly nucleated on an earlier D1 fold(s). We cited Lennox and Roberts (1988) on the presence of refolded D1 folds and east-west S1 cleavage in the crestal area. Figure 4 of Lennox et al. (2013) shows the complexity of folding, which they subdivide, without elaboration, into three generations of folds. Interestingly, their new cross sections across the anticline (Lennox et al., 2013 figure 3) imply that all the folds belong to the one generation, in contrast to the three generations in their figure 4 (Lennox et al., 2013). We suggested that the plunge variation of the D2 Parrabel Anticline reflects folding of a previously deformed surface, thus explaining why the hinge of the Hastings orocline is not subvertical.

Lennox et al. (paragraph 1 page 2, 2013) argue that bedding in the western limb of the Parrabel Anticline is not steeply dipping. However, the bedding data in this limb in figure 2 show reasonably consistent steep dips in the Hyndmans Creek and Pappinbarra formations, although there are some shallow dips and some dips to the east rather than to the west. These steep dips are consistent with the findings of Lennox et al. (1988, p 74) who wrote that "The Parrabel Anticline, identified by repetitions of formations around its northern flanks, may be a dome because mapping north of Wauchope indicates that the formations young to the south (reference omitted add by us). This dome has steeply dipping western and southern flanks, (our emphasis), a broad crestal region and a gently dipping northern and northeastern southern flank."

Interestingly, the two cross sections across the Parrabel Anticline (CD and EF, figure 3 of Lennox et al. 2013), do not show a regional anticline in the Carboniferous rocks. Instead their sections suggest that the regional structure in the Carboniferous strata consists of anticline-syncline pairs. From their cross sections, it thus appears that Lennox et al. (2013) no longer support the concept of a Parrabel Anticline or dome in the Hastings Block. This is in contrast to Lennox and Roberts (1988), Lennox et al. (1999), and the text of Lennox et al. (2013).

This complexity of structure within the core of the Parrabel Anticline has been inferred by Lennox et al. (2013) to be a result of three generations of folding. Their figure 4 is the very reason we decided to base our structural interpretation on the shapes within Late Carboniferous and early Permian units on the limbs of the Hastings Block (our previous figure 5a, enlarged here as figure 2). Figure 2 of this reply shows that even where stratigraphic units are planar and macroscopically folded around a north plunging antiform, they possess internal dips that are both consistent and inconsistent with the regional structure.

Lennox et al. (2013, paragraph 1 page 1, paragraph 3 page 3 and point 3 page 6) dispute our interpretation that the Bagnoo Fault is a thrust. This highlights the differences between our work and the approach of Lennox et al. The interpretation of regional geometry and block dynamics by Glen and Roberts (2012) was based on variations in regional younging directions established from the stratigraphic template. We thus use the juxtaposition of different stratigraphic units either side of the Bagnoo Fault to argue for a contractional stratigraphic separation and thus a thrust-type solution.

Several points are relevant here. Firstly, a thrust fault recognized in this way may have undergone strike-slip kinematics as well reverse kinematics. Secondly, we adopted the Lennox et al. (1999) interpretation of multiple deformation phases, by suggesting that the Parrabel Anticline is a D2 structure superimposed on D1 structures. Indeed we have suggested that north-younging, east-west strata west of the Parrabel Anticline represent a relict D1 hinge. Glen and Roberts (2012, figure 11) suggested that, as a result, the Bagnoo Fault was a D1-D2 structure, having formed during the D1 deformation as a west-dipping thrust (at least in part) followed by reactivation as a D2 east block up thrust. D2 west-vergent thrusting is consistent with the presence west of the fault (in our footwall) of synclines outlined by the Kindee Conglomerate and the Nevann Siltstone.

Our D1-D2 history of the Bagnoo Fault implies complex reactivation. It is thus compatible with northeast-block down kinematics (our D1 event) recorded in the two references of the 1980s cited by Lennox et al. (2013). Reactivation is also compatible with the finding of Leitch and Feenan (1989), cited by Lennox and Offler (2010), who stated that "Reactivation of the NW to WNW-striking faults appear to have taken place later because quartz-kaolinite alteration of breccia in the NW-striking Bagnoo Fault has been produced during an influx of hydrothermal fluids from Triassic granitoids".