Role of heat production in relative proportions of "rift-phase" and "sag-phase" subsidence.

 

The relative proportions of "rift-phase" and "sag-phase" subsidence associated with rifting can vary significantly depending on the amount (and vertical distribution) of crustal heat production. Increasing the crustal heat production in the pre-rift lithosphere causes a reduction in the amount of thermal subsidence, and leads to siginficant changes in the proportion of rift to sag sequences generated for an arbitrary riftiung event. The reason for this is that the burial of the existing crustal heat production beneath the acucmulating basin leads to significant, long-term, deep crustal and uppper mantle heating (10-100 C) relative to the pre-rift lithosphere (see inversion applet). This provides a mechanism that can account for some of the observed spatial variations in relative thickness of syn-rift and thermal (or sag-phase) subsidence observed in some basins.

For example, in the Mount Isa region, significant variations in the proportions of rift - sag phase subsidence have been observed between the Palaeoproterozoic Isa Basin and the adjacent Lawn Hill Platform (eg, Betts et al. 1998, Tectonics???). Available heat flow measurements suggest a profound change in curstal heat production across this transition with surface heat flow measurements of 80 mWm-2 in the Isa Basin and as low as 50 mWm-2 on the Lawn Hill Platform. Assuming that this variation in heat flow can be attributed to variations in crustal heat source distributions representative of scales of the order of several 100 kms, then the relative proportion of rift phase subsidence may vary from less than 50% in the lawn Hill platform to ~70% in the Isa Basin (see figure below).



The top-left panel shows the amount of rift phase subsidence generated for Isa Basin-fill parameters, for various stretching factors (assumed to be homogeneous in both crust and mantle parts of the lithosphere) and for different initial crustal heat source contributions, qc. The top right panel shows the thermal or sag-phase subsidence for an identical range in parameters, while the bottom left shows the rift phase subsidence shown as the percentage of the total subsidence (ie both rift and sag phase). The bottomn right panel shows the predicted steady state surface heat flow resulting form basin formation. The total thickness of sediment acucmalated in the Isan basin and lawn Hill platforms is of the order of 10km. The coloured regions shows the expected partitioning of rift and sag phase subsidence needed to produce about 10 km of sediment in the two differing environments, characterised by different modern-day heat flows. Note that slightly lower total stretching values are needed to generate 10 kms of subsidencne for Lawn Hill parameters than for the Isa basin.
More figures can be seen in the Numerical Image library under the Isan_basins directory

 

Thermal effects of basin development

The differential burial of high heat producing layers under an insulating sedimentary blanket has far-reaching consequences for the thermal structure of the underlying crust. Such a scenario may be relevant to Mount Isa where the developemnt of the Mount Isa Basin saw the accumulation of a thick sequence of fine-grained (insulating) clastic sedimentary rocks above high- heat producing basement.

Illustration of thermal effects of the accumulation of a localised thick sequecne of insulating sediments above a high heat producing laer. The thermal consequences at deep crustal levels reflect two distinct processes. Firstly, the insulating nature of the sediments (in this case k = 2 Wm-1K-1 - compared with a basement conductivity of 3 Wm-1K-1 - causes a large increase in upper crustal thermal gradients. Secondly, the increased depth to the high heat producing layers extends the depth extent of the steep upper crustal geothermal gradients.

 

An interesting outcome of the geometry shown above is the steep lateral thermal gradients that are associated with the main basin bounding structures (where lateral temperature gradients may be as high as 8íC/km - see image below). These gradients must have important implications for fluid flow in and around the basins in-as-much-as they provide a permanent "source" capable of localising fluid flow systems over very long geological periods. Is it any wonder that major mineralsation occurs along the major basin bounding faults of the Mt Isa Inlier?

 

Illustration of lateral temperature gradients (íC/km) associated with the thermal strutcure shown ain the previous slide)