Exploration Geophysics - Volume 10, Issue 2, 1979

Heat flow in the Moomba, Big Lake and Toolachee gas fields of the Cooper Basin is estimated from corrected bottom hole temperatures and an assumed bulk thermal conductivity of 5 × 10^–3 cal/cm sec °C. The Moomba and Big Lake fields have heat flows of 2.61 and 2.60 microcal/cm² sec C. Samples of basement granite from the Moomba and Big Lake fields yield heat production of 17.5 × 10^–13 and 24.2 × 10^–13 cal/cm³sec, respectively, which are sufficient to account for observed surface heat flow if the granite layer is between 7 to 10 km thick. Hydrocarbon maturation and coal rank (expressed as vitrinite reflectance) in the high heat flow Moomba-Big Lake region exhibits a different correlation to maximum palaeo-temperature and depth than in lower heat flow regimes. The degree of maturation may be dependent on the thermal energy available for metamorphism (i.e. heat flux), rather than the temperature of the basin.

The topographic anomaly dominates over contributions from subsurface bodies whose dimensions are comparable to the topography. Its removal from airborne and marine data is made doubly difficult by variations in topographic density over large distances, and the non-proximity of the observations to the anomaly sources. The smoothing inherent in non-proximity data makes the separation of anomaly components difficult, resulting in a high degree of ambiguity.

Various automatic methods of dealing with the topographic anomaly are discussed. None of them can restore gradients already missing in non-proximity data, and none are valid for general application. The blending of anomaly components, which can be easily demonstrated, means that there can be no general form of Nettleton's density profiling concept.

There is no general panacea for the topography problem Observing as close to the ground as possible will give the best results and multiple density profiling, based on Nettleton's principle is essential for understanding the ambiguity involved. Regardless of whether topographic density varies laterally or not, a set of multiple-density Bouguer profiles is the only known means of conveying the ambiguity resulting from the interaction between the topographic anomaly and other anomalies. This factor mitigates against contour analysis, where density profiles cannot be used.

The arc of the Western Alps started to develop during the main Alpine orogenic phase in Late Eocene to Early Oligocene time. This occurred as a side effect of the general north-south compression in the Central and Eastern Alps. Before this time no arc existed. A dominant east-west structural grain, expressed by the axes of the folds which originated during the Late Cretaceous, characterized the entire belt from the Dauphiné and Provence of southeastern France to the region which later became the Eastern Alps.

The final and most powerful curvature was the result of an east-southeast to west-northwest compression caused by the motion of the Italian-Dinaric microplate along the right-lateral Insubric line during Early Oligocene to Early Miocene time. The deformation produced in the Western Alps by these movements can best be explained by applying the slip-line field theory. Accordingly, the two systems of strike-slips faults observed in the Western Alps, in the Jura mountains and in the Dauphinl, are more or less parallel to the slip-lines created in the rigid plastic medium of the Western Alps by the motion of the northwestern prong of the Italian-Dinaric microplate, which acted as the indenter. It is thus postulated that by the recognized style of deformation the rock masses forming the arc of the Western Alps flowed toward north-northeast and south-southwest from an axis through the centre of the arc.