PALEOMAGNETISM AND TECTONIC ROTATION OF THE HASTINGS TERRANE, EASTERNAUSTRALIA

The Hastings Terrane comprises two or three major fragments of the are -related Tamworth Belt of the southern New England Orogen, eastern Aus tralia, and is now located in an apparently allochthonous position out board of the subduction complex. A palaeomagnetic investigation of man y rock units has been undertaken to shed light on this anomalous locat ion and orientation of this terrane. Although many of the units have b een overprinted, pre-deformational magnetizations have been isolated i n red beds of the Late Carboniferous Kullatine Formation from the nort hern part of the terrane. After restoring these directions to their pa laeohorizontal (pre-plunging and pre-folding) orientations they appear to have been rotated 130 degrees clockwise (or 230 degrees anti-clock wise) when compared with coeval magnetizations from regions to the wes t of the Hastings Terrane. Although these data are insensitive to tran slational displacements, a clockwise rotation is incompatible with mod els previously proposed on geological grounds. While an anti-clockwise rotation is in the same sense as these models the magnitude appears t o be too great by about 100 degrees. Nevertheless, the palaeomagnetica lly determined rotation brings the palaeoslopes of the Tamworth Belt, facing east, and the Northern Hastings Terrane, facing west before rot ation and facing southeast after rotation, into better agreement. A po le position of 14.4 degrees N, 155.6 degrees E (A(95) = 6.9 degrees) h as been determined for the Kullatine Formation (after plunge and beddi ng correction but not corrected for the hypothetical rotation). Revers ed magnetizations interpreted to have formed during original cooling a re present in the Werrikimbe Volcanics. The pole position from the Wer rikimbe Volcanics is at 31.6 degrees S, 185.3 degrees E (A(95) = 26.6 degrees). These rocks are the volcanic expression of widespread igneou s activity during the Late Triassic (similar to 226 Ma). While this ac tivity is an obvious potential cause of the magnetic overprinting foun d in the older units, the magnetic directions from the volcanics and t he overprints are not coincident. However, because only a few units co uld be sampled, the error in the mean direction from the volcanics mak es it difficult to make a fair comparison with the directions of overp rinted units. The overprint poles determined from normal polarity magn etizations of the Kullatine Formation is at 61.0 degrees S, 155.6 degr ees E(Ass = 6.9 degrees) and a basalt from Ellenborough is at 50.7 deg rees S, 148.8 degrees E (A(95) = 15.4 degrees), and from reversed pola rity magnetizations, also from the basalt at Ellenborough is at 49.4 d egrees S, 146.2 degrees E (A(95) = 20.4 degrees) These are closer to e ither an Early Permian or a mid-Cretaceous position, rather than a Lat e 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 pol arities, the age of these overprint magnetizations appears to be eithe r Early Permian or mid-Cretaceous.