PALEOMAGNETISM AND TECTONIC ROTATION OF THE HASTINGS TERRANE, EASTERN AUSTRALIA

The Hastings Terrane comprises two or three major fragments of the are-related Tamworth Belt of the southern New England Orogen, eastern Australia, and is now located in an apparently allochthonous position outboard of the subduction complex. A palaeomagnetic investigation of many rock units has been undertaken to shed light on this anomalous location and orientation of this terrane. Although many of the units have been overprinted, pre-deformational magnetizations have been isolated in red beds of the Late Carboniferous Kullatine Formation from the northern part of the terrane. After restoring these directions to their palaeohorizontal (pre-plunging and pre-folding) orientations they appear to have been rotated 130 degrees clockwise (or 230 degrees anti-clockwise) when compared with coeval magnetizations from regions to the west of the Hastings Terrane. Although these data are insensitive to translational displacements, a clockwise rotation is incompatible with models previously proposed on geological grounds. While an anti-clockwise rotation is in the same sense as these models the magnitude appears to be too great by about 100 degrees. Nevertheless, the palaeomagnetically determined rotation brings the palaeoslopes of the Tamworth Belt, facing east, and the Northern Hastings Terrane, facing west before rotation and facing southeast after rotation, into better agreement. A pole position of 14.4 degrees N, 155.6 degrees E (A(95) = 6.9 degrees) has been determined for the Kullatine Formation (after plunge and bedding correction but not corrected for the hypothetical rotation). Reversed magnetizations interpreted to have formed during original cooling are present in the Werrikimbe Volcanics. The pole position from the Werrikimbe Volcanics is at 31.6 degrees S, 185.3 degrees E (A(95) = 26.6 degrees). These rocks are the volcanic expression of widespread igneous activity during the Late Triassic (similar to 226 Ma). While this activity is an obvious potential cause of the magnetic overprinting found in the older units, the magnetic directions from the volcanics and the overprints are not coincident. However, because only a few units could be sampled, the error in the mean direction from the volcanics makes it difficult to make a fair comparison with the directions of overprinted units. The overprint poles determined from normal polarity magnetizations of the Kullatine Formation is at 61.0 degrees S, 155.6 degrees E(Ass = 6.9 degrees) and a basalt from Ellenborough is at 50.7 degrees S, 148.8 degrees E (A(95) = 15.4 degrees), and from reversed polarity magnetizations, also from the basalt at Ellenborough is at 49.4 degrees S, 146.2 degrees E (A(95) = 20.4 degrees) These are closer to either an Early Permian or a mid-Cretaceous position, rather than a Late Triassic position, on the Australian apparent polar wandering path. Therefore, despite their mixed polarity, and global observations that the Permian and mid-Cretaceous geomagnetic fields were of constant polarities, the age of these overprint magnetizations appears to be either Early Permian or mid-Cretaceous.