The structural evolution of the Northern Hastings Block and southern Nambucca Block, southern New England Orogen, eastern Australia

The Hastings Block is a weakly cleaved and complexly folded and faulted terrain made up of Devonian, Carboniferous and Permian sedimentary and volcanic rocks. The map pattern of bedding suggests a major boundary exists that divides the Hastings Block into northern and southern parts. Bedding north of this boundary defines an upright box-like Parrabel Anticline that plunges gently northwest. Four cleavage/ fold populations are recognised namely: E-W-striking, steeply dipping cleavage S-1 that is axial surface to gently to moderately E-or W-plunging; F-1 folds that were re-oriented during the formation of the Parrabel Anticline with less common N-S-trending, steeply dipping cleavage S-2, axial surface to gently to moderately N-plunging F-2 folds; poorly developed NW-SE-striking, steeply dipping cleavage S-3 axial surface to mesoscopic, mainly NW-plunging F-3 folds; and finally, a weakly developed NE-SW-striking, steeply dipping S-4 cleavage formed axial surface to mainly NE-plunging F-4. The Parrabel Anticline is considered to have formed during the D-3 deformation. The more intense development of S-2 and S-3 on the western margin of the Northern Hastings Block reflects increasing strain related to major shortening of the sequences adjacent to the Tablelands Complex during the Hunter-Bowen Orogeny. The pattern of multiple deformation we have recorded is inconsistent with previous suggestions that the Hastings Block is part of an S-shaped orocline folded about near vertically plunging axes.