Northwestern New South Wales regional magnetics and gravity

Abstract

Bouguer gravity maps and aeromagnetic maps of the Koonenberry 1:500 000 map area in northwestern New South Wales have been compiled from Bureau of Mineral Resources data bases. Low regional magnetic field strength within the area of the Precambrian Adelaide Fold Belt extends about 90 kilometres north of the northernmost Precambrian outcrop, suggesting some continuity of crustal structure through this region. On the western edge of the area, in the vicinity of 30°30¢ S, magnetic response from shallow subcrop indicates that a detailed aeromagnetic survey in this vicinity could clearly delineate the shallow subsurface structure. The Bancannia Trough is underlain by strongly magnetised rock units, possibly dacite/andesite. These magnetized rock units are terminated at their southern end by an ENE trending lineament which extends through the Wonominta Block into the basement of the Darling Depression. ERTS-1 images indicate that this lineament also extends WSW through Broken Hill. Bouguer gravity values over the Wonominta Block are markedly higher than over the Adelaide Fold Belt, reflecting a probable convergent plate margin associated with the Wonominta Block. The full extent of the Koonenberry Fault (which in this area is a major component of the boundary between the Kanmantoo and Lachlan Fold Belts) is clearly delineated by the magnetic data. The few near-surface magnetized rock units east of the Koonenberry Fault generally give rise only to isolated anomalies, except in the restricted area of Ordovician metasediments surrounding the Tibooburra Granite. The Eromanga Basin is distinguished from the Darling Depression by higher Bouguer values (indicating shallower sedimentary basement depths), and basement structures with generally northwesterly trends (compared with no discernable basement trend directions in the Darling Depression).