Conclusions
1. Late Devonian to Late Carboniferous volcanism in the forearc basin of the southern New England Orogen involved a diachronous change in the nature of volcanism from intermediate to felsic in different volcanic centres: at the ~Devonian-Carboniferous boundary (~360 Ma) in the Tamworth belt, but Visean to Namurian (~325 Ma) in the Hastings Block. This change was associated with uplift in the forearc basin in both regions, recorded by a change to shallow and non-marine deposition, above an unconformity and short time break in the Hastings Block. The youngest arc rocks are 305 Ma in the Tamworth belt.
2. The complex Manning and Hastings oroclines exist. Together with the simpler Texas and Coffs Harbour oroclines to the north, they form four oroclinal hinges that may have begun forming in the latest Carboniferous, and which had completed, either intermittent or continuous, oroclinal bending, except for minor local deformation in the latest early Triassic, by 267-264 Ma. Deformation in the earliest Permian was synchronous with deposition of sedimentary rocks containing Late Devonian cherts from the subduction complex, mixed with /recycled volcanic + granite detritus.
3. If the early Permian turbidites of the Nambucca block are largely in situ and continuous (across ?rift margin faults) with shallow- water sediments in the hinge of the Parrabel anticline (Hastings Block), then the unconformity above latest Carboniferous strata indicates that the Hastings Block had already begun rotating, and was largely coupled to the Nambucca block, by the earliest Permian.
4. Final formation/modification of the Manning orocline occurred in the middle Permian by 267-264 Ma, and was accompanied by strong D2 faulting, that produced NNE to NNW-trending thrusts fanning around hinge (forming discontinuities in outcrop patterns) and overprinting older D1 orogen-parallel structures. The resultant NNW-trending structurally repeated strips of forearc stratigraphy in the hinge of the orocline lie at high angles to the fault between the forearc basin and the subduction complex, implying mechanical separation between the two units. This D2 event is similar in age to final formation of the Texas orocline, that is overprinted by the ~265 Ma Kiaman palaeomagnetic reversal.
5. East-west shortening, completed by ~267 Ma, predates resumption of subduction magmatism as measured by emplacement of I-type granites of the New England batholith, beginning at ~ 255 Ma. Unless there was a ~10 myr lag time in melt generation and emplacement, D2 east-west shortening does not appear to be linked to the resumption of subduction and thus to development of stronger coupling across the plate boundary.
6. The Hastings orocline was formed by 90-120° anticlockwise fold rotation of the Hastings Block within the deforming forearc basin. Stratigraphy and facies changes suggest that the Hastings Block was not exotic but formed in the Devonian to Carboniferous forearc basin, close to, but not in contact with, the Tamworth belt. Changes in regional vergence in two cross sections suggest that the missing connections may have been overthrust by the eastern part of the Tamworth belt by ~60 km.
7. The opposite senses of rotation vergence for the two pairs of oroclines, anticlockwise for the Manning and Hastings and clockwise for the Texas and Coffs Harbour, have led to published models in which they formed due to dextral or sinistral displacement on a major fault located either onshore or offshore. However, we suggest that the oroclines reflect buckle folds responding to variable shortening within the forearc basin and subduction complex. These variations are continuous along axial traces with smaller amplitude changes in amounts and directions of shortening on the inboard margin of the forearc basin.
8. Changes in shortening occur in an otherwise linear belt some 2000 km long that became the site of renewed subduction magmatism in the late Permian, with arc and backarc granite roots cutting across oroclines and emplaced into subduction complex rocks.
9. The timing of the bulk of orocline formation, during inferred rollback of the plate boundary, leads us to suspect that the changes in amounts and directions of maximum shortenings developed in response to recesses and/prominences in the continental margin, rather that just to plate boundary-induced variations.