Alain Bonneville

Surface heat flow and the mantle contribution on the margins of Australia

[1]xa0We present thermal data from 473 oil exploration wells in Australia and New Zealand. Approximately 2300 bottom-hole temperatures are corrected to form a homogeneous set along with 86 temperatures from reservoir tests. Thermal conductivity profiles are estimated from a set of geophysical well logs using a recently developed neural network approach. Retaining wells in which temperature and thermal conductivity data overlap over an interval greater than 1000 m, we estimate 10 heat flow values in the Taranaki basin of New Zealand and 270 values in the northwestern, western, and southern margins and in the intracontinental Canning basin of Australia. The values are in the range 30–80 mW m−2. As a result of several differences in the data and methods, our heat flow values are 10–20 mW m−2 lower compared to previously published estimates for the same wells in New Zealand. For Australia, our values are consistent with previously measured values and trends in the continental and marine regions. On the northwestern and southeastern margins, we interpret the variations as reflecting changes in the nature of the underlying basement. Consistent with onshore data, it is inferred that the Archean crust is depleted in radiogenic elements compared to Proterozoic regions and that recent volcanism affects the eastern Paleozoic area. After removing from surface heat flow the sediment contributions, including a permanent radiogenic heat component and a transient sedimentation effect, a simple crustal model suggests that mantle heat flow on the continental margin bordering the Pilbara craton is higher than below the craton itself. Moreover, heat flow corrected for the sediment contributions is markedly lower in the Petrel intracontinental basin than in the adjacent margin, although the crust is thinner below this latter region. As both are underlaid by the same basement, this observation may indicate that the mantle contribution is also higher below that margin. Such a higher mantle heat flow on old continental margins is consistent with experiments of fluid convection below an insulating lid and suggests that the thermal regime of the continental lithosphere never returns to its prerift state, as usually assumed by several thermomechanical models of evolution of continental margins.

Heat flow at the edge of continental lithosphere: from methodology to numerical modeling

In order to gain insights into the thermal regime of rifted margins, we successively developed a methodological part, estimated new heat flow values and performed numerical modeling. First, we developed a new method, based on the neural network technique, to estimate thermal conductivity from oil exploration data. The method was systematically applied on a large number of wells on Atlantic and Australian old margins, providing almost 600 new heat flow measurements. In all cases the mantle heat flow below the margins is comparable to that of the oceanic domain, and in some cases it is higher. Measurements on young margins also show unexpected high values. These results are in agreement with numerical simulations that set the continental margins at the scale of the mantle convection. These indeed show that heat flow increases permanently towards the edge of the continent. This changes significantly the subsidence evolution and the relations with the pre-existing thermal regime of the continent.