Thermal cycles and Vertical Tectonics

 

Over the circa 4.5 Ga years it has taken to develop the Earth’s crust and its associated water volume, the average thickness and therefore the volume of the crust has been gradually increasing. It might be safe to assume that over a shorter time frame, measured in terms of say a few 10’s of million years, the crustal volume remains effectively constant. Hence, the cyclic creation of new crust, often at mid-oceanic spreading zones, must be balanced by the loss of older crust. However, this does not necessitate a model envisioned by PT where the new crust is being continuously pushed out only to be lost again in subduction zones by being thrust back down into the molten magma. Instead, the present model suggests a very different form of mass balance that could help to explain the very long period vertical motions experienced by both continental and sea floor crust. This new model could provide an explanation for why continents sink and become thinner ocean floors, and then build-up, rise and become continents again, only to be eroded and sink back to become ocean floor. It suggests a driving force for this cyclic process which is partially based upon the periodic occurrence of ice ages involving as they do massive variations in the disposition of surface ice and water. And it will be argued that these very long term cycles of changing surface ice  and water distribution have the effect of altering the geothermal flux which in its turn controls the thickness of the lithosphere.  Changes in the thickness of the crust, having density less than that of the mantle, will affect its buoyancy and result in changes in vertical elevation.  

Vertical movements of continental and ocean floor crust are clearly on time scales that are orders of magnitude greater than the periodic thermal cycles responsible for aspects of the horizontal tectonics described above. Whereas much of the near surface horizontal tectonics can be measured in 10’s of thousands of years the former would appear to be in terms of 10’s of million years. That being so hundreds of short term thermal cycles could occur within the longer term periods required for the vertical motions of continents. The regular staircases of river terraces, synchronised to the Croll-Milankovic cycles, described by for example Bridgeland (1988, 2010) and Westaway (2002, 2010), are certainly indicative of such processes taking place. Here it is suggested that the driving forces for these long period vertical motions could also be derived from the thermal cycles arising from the same causes as the ice ages and the periodic glacials and inter-glacials occurring within the ice ages. 

 

A previous attempt to explain the epeirogonic events considered the possibility of regional uplift buckling (Croll, 2007a). While such a process may or may not affect regional vertical motions of the crust further reflection has suggested a much more likely cause, but once again this alternative cause is suggested to derive from the actions of periodic thermal loading. Figure 8 shows a schematic depiction of an area typical of a passive continental margin. During a warm age sea levels will be high and continental erosion will have created a well formed continental shelf. A subsequent typical glacial within an ice age will produce a drop in sea level which in certain circumstances will see the ice sheet advance out over the continental shelf. With many such cycles occurring over the ice age the region would experience average conditions that could have a significant influence on the outward flow of geothermal energy. This has been discussed in an earlier blog in relation to Figures 4. But let us consider a little more carefully what might happen to regions of crust overlain at regular intervals with such ice sheets, and how this might help explain why certain ocean crust can rise to become part of continents and how continental crust can sink to become ocean floor.

                                          
Figure 9 shows at time (0) (for indications of where on the geological thermal cycle these times are located see the middle plot of Figure 8(b)) an imaginary column of crust (depicted in yellow) overlying a semi-liquid mantle (shown in red). This is of course a highly simplistic representation of the subtle and complex properties of the lithosphere, the asthenosphere and mantle, but should be enough to illustrate the hypothetical thermal-mechanics that might be involved. After a long period associated with a warm age, erosion has by time (1) lowered the margin of the continental surface to a little above warm age sea level, just prior to the onset of an ice age. Over the duration of the ice age there will be an averaged lowering of the sea level and an isostatic adjustment for which the crust will have sunk under the weight of the ice overburden between times (1) to (2). During the course of the ice age there will be frequent incursions of ice cover like that depicted at time (2). And over the period of the ice age there will be an averaged lowering of the sea level and a regular isostatic adjustment in which the surface of the crust will have sunk under the weight of the ice overburden. At an early stage of the ice age, time (2), let us suppose the thickness and temperature differentials between the upper and lower ice surface are associated with a geothermal gradient similar to that pre-existing in the crust. This means that by time (3) there will have been no thermal readjustments. However, when the ice sheets melt and the sea level rises, at the start of the warm period of time (4), there will be some isostatic rebound. But due to the weight of the now over lying sea depth, this isostatic rebound will be less than the original subsidence. As a result the crust surface will be left beneath the mean sea level. Because of the higher geothermal heat flux from the sea bed, over time the crust might thin in relation to the increased geothermal gradient, time (5). The result will be a crust surface that is now further below sea level. Lowering of the crust between times (1) and (4) for this illustrative model depends only on isostatic adjustments, with just the additional subsidence between times (4) and (5) being caused by readjustment of the thermal gradient. And since isostatic adjustments seem to occur at greatly reduced timescales to those arising from thermal gradient changes, it might be anticipated that the above cycles could also take place over the much shorter glacial – inter-glacial cycles.

If incursions of ice sheets can cause continents to sink below the waves, what is it that might explain how sea floor can rise to become once again part of continental land mass? Figure 10 adopts a similar convention to Figure 9. In this case consider at time (1) that a

shallow column of seabed crust within the continental shelf becomes grounded in an ice sheet at time (2), having a thickness that even with isostatic depression due to the weight if the ice sheet, has the free surface of the ice sheet above the lowered sea level. Depending upon the average thickness of the ice over the ice age and the temperatures at its upper and lower surface, the thermal gradient through the ice may be very much lower than that pre-existing in the sea bed. After a time sufficient for the ice overburden and the crust to reach a new thermal equilibrium at time (3), the now realigned thermal gradient will have caused considerable aggradation at the lower crust boundary, as a result of phase change forced on the magma. The lowering of average density of the thickened crust at time (3) relative to that at time (2) will cause the original seabed level to rise to a level that could be above the sea level during the subsequent warm age. Hence, when the ice disappears at the beginning of the warm age, time (4), the crust will now be above sea level. Taking into account the isostatic rebound when the ice is removed, the sea bed will now have emerged to become part of the continental land mass, time (5). Further readjustments of the geothermal gradient during the warm age, might see the crust being gradually thickened and in the process leading to a further rise of surface level, time (6).

It should be pointed out that the examples chosen for the discussion of Figures 4, 9 and 10 are merely illustrative. Many other scenarios are possible in different circumstances. 

Much of the above post has been taken from the paper "On the Causes of Vertical Motions of Lithosphere", James G A Croll, Frontiers meeting, Geological Society of London, November, 2011.