Interpretation of subsurface shape of granites in the eastern lachlan fold belt using aeromagnetic data

Abstract

Comparison of aeromagnetic and geological maps of the Canberra and Bega 1:250 000 sheet areas indicates that most of the granites in the area have an associated magnetic anomaly. Susceptibility measurements on hard samples confirm that the granites are magnetic and susceptibility values vary from 0.001?0.05 S.I. The granites have been grouped on chemical criteria into I-types which are of purely igneous origin and S-types which have been derived by partial melting of sedimentary rocks. The I-type granites are far more prominent than the S-types in the magnetic anomaly map. Two dimensional magnetic modelling over the I-type granites indicates that most have the form of broad elongate domes with shallow dipping sides. However a few have steeply dipping sides and some of the less magnetic have sides which dip inwards so that the intrusion becomes narrower at depth. A depth to the base of the granites of 3 kms below sea level fits the larger wavelength features adequately. It is hypothesised that the I-type granites were intruded through relatively small feeder pipes and that they spread out laterally in a sill-like manner. Those on the Canberra sheet apparently moved laterally for considerable distances and penetrated sufficiently close to the surface to give rise to extrusive volcanic activity. Whereas most of those on the Bega sheet remained at depth as a large body and did not move laterally for any great distance.