The blanketing effect in sedimentary basins
The deposition of cold sediments, called thermal blanketing, is studied when the sedimentary basin is considered as part of the lithosphere. A general one-dimensional temperature equation is obtained which accounts for blanketing, the movement of the lower boundary of the lithosphere, and the eventual stretching of the sediments and the lithosphere. These processes appear explicitly as convective terms in the model, which is a generalization of a previous temperature equation. The description of these processes by means of convective terms is a major result in the paper; it makes these processes more amenable to quantitative investigations. The temperature equation is studied in two different settings; one where the lithosphere remains unstretched, and another where the lithosphere is subjected to constant strain rate stretching. The parameter controlling thermal blanketing in these models is the Peclet number. The transient period towards a stationary state, and the stationary state representing maximal blanketing are studied. Analytical expressions for various stationary states are obtained. When the lithosphere remains unstretched it is concluded that a necessary condition for strong blanketing is a sedimentation rate of the order of 250 m Myr-1. Where the sediments contribute to the heat flow by decay of radioactive isotopes, the radioactive heat production divided by the sedimentation rate should be at least 10-9 (W m-3)/(m Myr-1) for the heat production to compete with blanketing. During stretching of the lithosphere it is shown that for stretching factors (p) larger than 2, the convective effect of stretching the sediments contributes more to the reduction of the heat flow than the blanketing effect. Blanketing counteracts the increased heat flow caused by the stretching of the lithosphere, and the upwelling hot asthenosphere. With sedimentation rates inferred from isostatic calculations it is found that the strain rate times the initial crust thickness should be more than 0.25 km Myr-1 for the blanketing effect to be noticeable. It is also shown that sediment infill which follows thermal subsidence, after a period of stretching, is not capable of blanketing.