Geothermal Mechanism for Epeirogenic Uplift

Previous studies have noted the close link between periods of mountain building with climate cycles^14,15,16. Recorded periods of mountain building¹³ are summarised by the bar charts at the top of Fig 9. The 4 recorded periods of widespread mountain building during the Phanerozoic, often occurring synchronously over widely dispersed geographical domains, are shown to correspond closely with the periods of ice-house climate conditions. Tentative explanations for how ice-house conditions could result in spurts of uplift required for mountain building and subsequent erosion have again focused on how changes in surface disposition of ice and water could influence the rate of geothermal heat loss as expressed by the geothermal flux^13,14. Reductions in the geothermal heat flux caused by the insulating effects of the development of deep surface ice sheets and permafrost would result in a lowering of the geothermal gradient and a concomitant increase in lithosphere thickness brought about by aggradation, due to phase change at the lower lithosphere-mantle boundary. Associated reductions in average density within the lithosphere and mantle could then be expected to result in regional uplift of the crust. This is summarised in Fig 11.

                   (a)                                  (b)                                  (c)      

Fig 11 Mechanism for epeirogenic uplift and sediment erosion based upon (a) the high geothermal gradient due to high heat flux into the overlying ocean during hot-house conditions which with (b) the development of thick ice-sheets and permafrost after transition to ice-house climate reduces the geothermal heat flux which over a few Ma (c) readjusts to a lower geothermal gradient and increased crustal thickness through aggradation of lower crust while crust continues to rise during ice erosion. 

 A variant and perhaps more prevalent situation than that of the above model for epeirogenic uplift and sediment erosion would be as follows. Again starting with the situation of Fig 11a which depicts a high geothermal gradient due to high heat flux into the overlying ocean during hot-house conditions, a lowering of mean sea level (msl) at the onset of an ice-house period might expose the previous sea bed to sub-aerial conditions - the situation of Figure 11b but without the overlying ice sheet. This would be  followed by a long term thickening of the crust due to a lowering of the geothermal heat flux as indicated in Fig 11c - again without the overlying ice sheet but as a result of the reduced heat flow into the overlying atmosphere. This lowered geothermal gradient and associated thickening of the lithosphere over a few Ma Fig 11c would result from phase changes and consequent aggradation of lower crust. 

It might be observed that such a mechanism for the raising of a previous, largely horizontal, seabed helps to explain why so frequently vertical tectonics produces uplifted peneplains. Frequently, as I hope later posts will describe, mountain building results from erosion of such uplifted peneplains.