Geothermal Mechanism for Epeirogenic Subsidence

 It is possible the following will be a little repetitive of earlier posts. But it is perhaps worth reiterating in the current context.

An explanation as to how the transition from ice-house to hot-house conditions could trigger an extended period of sedimentation has focused on the adjustments to the geothermal flux occurring when low lying continental crust is inundated by rising sea levels as global ice sheets and extensive permafrost melts^13,14. This is summarised in Fig 10 for a situation that might arise at high latitudes. An icesheet over the very long average ice cover during an ice-house period will have resulted in a thickening of the lithosphere associated with a low geothermal flux as depicted in Fig 10a. After ice and permafrost melt, sea level rise and oceanic inundation, Fig 10b, will eventually lead to an increased geothermal heat flux caused by the greater heat transfer capacities of sea water, enhanced by mixing due to tides and currents. Over time this will result in steeper geothermal gradients and a concomitant decrease in crustal thickness brought about by phase change, re-magmafication, at the lower lithosphere-mantle boundary, Fig 10c. This crustal thinning will in turn see an increase in average density within the lithosphere and upper mantle which in turn would be expected to result in regional subsidence. Such a model, involving as it does wasting of crust at the lower lithosphere-mantle boundary, starts to account for how km scale sedimentary sequences can be continuously added from above. As suggested, this requires maintenance of a consistent equilibrium thermal gradient even as sediments are added – a process much more likely given the relatively thin nature of oceanic crust.

To aid visualisation of isostacy these crustal columns are drawn relative to the mean magmatic level (mml) – the free surface of an idealised magmatic fluid.  

                                                 (a)                                                    (b)                                                   (c)

Fig 10 Mechanism for epeirogenic subsidence and sediment accumulation based upon (a) the low geothermal gradient due to insulation effects of overlying ice-sheets and permafrost during ice-house conditions which with (b) rise in mean sea level after transition to hot-house climate floods upper surface of the crust which over a few Ma (c) readjusts to a higher geothermal gradient and decreased crustal thickness through degradation of lower crust allowing accumulation of sediments from above. 

A slight variant of the above postulated mechanism for subsidence, followed by initiation and continuation of sedimentation would be as follows. A very low-lying continental area resulting from extensive sub-aerial erosion, at the tail end of an ice-house period but at a low enough latitude to be not covered with ice sheets, would after the melting of the ice and permafrost at the start of a hot-house period find itself being inundated by sea level rise as in Fig 10b. In this case the low geothermal gradient prior to the inundation would have been the consequence of the low heat loss into the overlying atmosphere. But once started the sedimentation process would again continue while the lithosphere experiences ongoing phase change at the lower lithosphere boundary, as suggested in Fig 10c.