Meandarra Gravity Ridge: symmetry elements of the gravity anomaly and its relationship to the Bowen–Gunnedah–Sydney basin system

Abstract

The Meandarra Gravity Ridge is one of the major gravity features in eastern Australia. The source(s) for this gravity anomaly is problematic because the ridge, a gravity high, coincides for much of its length with the thickest parts of the Bowen and Gunnedah Basins, areas that would normally be expected to be regional gravity lows. Furthermore, the Meandarra Gravity Ridge is continuous across numerous structural elements and regional terranes, and there is no obvious expression of its source in the surface geology. Construction of a series of gravity profiles across the Meandarra Gravity Ridge in this study has provided detailed analyses of changes in the size and shape of the Bouguer anomaly along the ridge axis. These data suggest that the ridge is composed of a series of offset colinear anomalies, and demonstrate that the magnitude and symmetry of the Bouguer anomaly are different for each segment of the ridge. 3D-mesh diagrams of the total-field Bouguer gravity illustrate that the ridge may be subdivided into at least five segments, each of which is characterised by particular gravity values and shapes for the anomaly profile. 2D gravity modelling of selected Bouguer anomaly profiles suggests that the source for the Meandarra Gravity Ridge anomaly is a dense mafic body in the upper crust. This source has a preferred density contrast of +0.25 tm⁻³, and probably consists of a pile of mafic volcanics up to 9 km thick. The size and geometry of the modelled anomalous mass supports a rift-type origin for the source of the Meandarra Gravity Ridge. Our modelling predicts that the magnitude and symmetry of the Meandarra Gravity Ridge gravity anomaly are controlled by the geometry of an underlying mafic source and not by interference effects due to density contrasts between sedimentary and metamorphic rocks of adjacent orogens.

Key Words:

• Bouguer gravity
• gravity anomalies
• mafic rocks
• Meandarra Gravity Ridge

Acknowledgements

We thank Geoscience Australia's Gravity Section for the provision of the gravity data from the Australian National Gravity Database, and Malcolm Nicoll and Kevin Wake-Dyster for help with manipulation of gravity grids and with production of colour images using Petrosys software. We also thank Paula Waschbusch and Bruce Goleby for discussions on aspects of the modelling, and Cec Murray and Erwin Scheibner for helpful reviews of the manuscript.