Unravelling the deformation history of the Northern Hastings Block, southern New England Orogen

Abstract

The Northern Hastings Block (NHB) consists of Carboniferous, complexly folded, weakly cleaved, extensively faulted, fore-arc sequences out of position in the New England Orogen. It shares part of deformation history with the adjacent Nambucca Block (NB) and Southern Hastings Block (SHB). It was subjected to prolonged N-S and later E-W shortening.

Four generations of macroscopic and mesoscopic folds and associated cleavage have formed within the NHB region according to their cross-cutting relationships. Fault analysis shows faults of a similar orientation in the NHB did not form at the same time. Some fault movement is related to Late Carboniferous emplacement of Hastings Block, but most faults formed and moved after cleavage and fold formation. Limited fault movement occurred after Triassic granite emplacement.

The extreme variability in the orientation of bedding from fault block to fault block recognized in this study, highlights the shuffling of fault blocks between the NHB and SHB post-Parrabel Dome formation. The extreme disruption of sequences between the NHB and SHB, are considered to be due to the rotation and translation or because of a fault network produced in a restraining fault-bend along strike-slip faults. Gravity and magnetic worm analysis revealed a possible new boundary between the NHB and SHB.

Four cross sections across the NHB based on field data are presented and have been 2D restored following the principles of structural balance. Based on these sections, the Parrabel Dome is a NW and SE-plunging dome which is box-like in the core and more open towards the north and south. The total horizontal shortening suggests only moderate cleavage development could occur consistent with field observations.

A 3D structural model is created from a comprehensive field dataset which shows a gross picture of the shape of the NHB. This model has aided fault block reconstruction and provided information on the direction and movement of fault blocks.

The formation of the Hastings Block was not due to large-scale ‘oroclinal’ folding in the Manning and Hastings area as suggested by previous authors but has been transported northward between two major fault systems. The NHB at this time underwent anticlockwise rotation.