Thermal cycles and horizontal tectonics

Increasing the temperature through the Earth’s crust will mean that the rock wants to expand laterally. However, the crust is restrained from expanding by its interaction with the relatively stiff inner mantle and core. The result will be the development of massive in-plane compressive stresses. Taking the rock of the crust to have an average coefficient of thermal expansion =12x10^-6m/m/^oC, then an increase in average temperature of say 20^oC would, if unrestrained, induce a tensile strain of 240x10^-6. This would be typical of the changes occurring near the surface during glacial and interglacial periods. Over a continental landmass of roughly circular shape, having an in-plane radius of say a=3000 km, this unrestrained expansion would give rise to an outward, in-plane, radial movement of 720 m. This would represent the maximum level of in-plane displacement available to fold, shear, or otherwise distort the Earth’s crust when these upper strata are restrained by the underlying deep crust and adjacent plate during the heating or compression cycle. Such distortions would undoubtedly occur selectively as will be discussed later. They could certainly account for a lot of folding, shear faulting, fracture and associated seismic and volcanic activity within the crustal layers during the warming phase near the surface of the crust.

The level of compressive stress developed during the warming cycles will of course depend upon the average temperature and its gradient into the crust. The levels actually reached would in turn depend upon the forces needed to fail the particular area of crust, whether by folding, shearing, or whatever. But with no failure to relieve the compressive strain of 240x10^-6, a relatively hard rock having an elastic modulus of say E = 30x10⁺³ MPa, will develop a compressive stress of 7.2 MPa (720 Tonnes for every 1 square metre of rock). While this would be lower than the stress expected, on average, to be needed to crush the rock it could, when integrated over significant crustal depth, certainly be enough to induce various forms of geometric failure such as folding and faulting. And because the Earth’s crust is highly non-homogeneous local stress concentrations may well be enough to induce local crushing failures in the rock.

Over the longer period adjustments of geothermal gradient that might be expected during a warm age the depth of penetration of the surficial thermal waves will be that much greater. An idealised form of the thermal equilibrium reached over the full thickness of the crust during a typical warm age, will develop considerably greater levels of compressive restraint, as is suggested in Figure 5. Compressive stresses would be anticipated to show gradients through the crust thickness similar to the changes in geothermal temperature gradients – at least until depths where temperature starts to change the rheological properties so that there is a reduction in the effective stress changes. But these stress changes could reach levels of considerable significance to tectonic processes. If for example a 200^oC temperature change occurred at a particular level within the crust the horizontal compressive stresses if horizontal expansion is fully restrained could reach values of around 72 MPa, or if unrestrained could over the area of a typical continental landmass produce an outward motion of over 7 km. At these levels considerable tectonic activity becomes possible. 

During the warm-up phases at various temporal scales alluded to above, the kinematic adjustments within the crust involving sudden releases of stored energy would be expected to occur with compressive related failure modes. These failure modes would occur when the strain build-up reaches the level required for this or that particular failure mode to be induced; they could be expected to be progressive and accumulative, and to occur at different locations at different times over the entire period of the crustal warming. At the end of the warm-up period it might be anticipated that the greater part of the compressive energy will have been transferred into the distortions characterising the various failure modes, whether they be mountain building, crustal over-riding, shear faulting, strata folding or metamorphic adjustments. So by the start of the next cooling period the crust would consequently contain very little residual horizontal compressive stress.

As the crust cools over an ice age it will want to contract. Being again prevented from doing so by the effectively rigid inner core and mantle, tensile stresses will be developed. Over such a long period, adjustment of geothermal gradient might be expected to produce substantial levels of tensile restraint, as is suggested in Figure 6. Tensile stresses would again be anticipated to show gradients through the crust thickness similar to the changes in geothermal temperature gradients. Such a cooling period could be termed the tension cycle.

Reversing the above scoping calculations, a drop in average temperature of 200^oC at a given level within the crust will produce tensile stresses of around 72 MPa, more than sufficient to cause tensile cracking of the rock, opening-up fissures and rifts into which high pressure magma could be extruded. Adequate also to account for considerable shear distortion and slip on fault planes, often in a slip-stick fashion typical of that occurring in earthquakes. It is likely that the tensile fractures would be concentrated in those areas where the crust is at its weakest. With oceanic crust being apparently so much thinner than that of continental crust, at least in the present phase of the dynamic tectonic cycle to be elaborated later, it would be expected that most, but by no means all, of these fractures would be located on the ocean floors. While the dominant fractures would be anticipated to be concentric with the geographic centre of the continental “plate”, the restrained contractions could also produce radial fractures and transform faults.

 

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.
  
 
 

Influence of surface ice and water on geothermal flux

In suggesting that surface temperature changes experienced over typical glacial and interglacial periods, typically in the range of 20 to 110 ka, could penetrate far enough into the crust to interfere with the flow of geothermal energy I was in an earlier contribution (Croll, 2007a) clearly wrong. As Figure 1 shows, a cyclic thermal loading at the Earth’s surface with a periodicity of 110 ka will penetrate fully depths of just 1.75 to 3.5 km. These depths would be insufficient to trigger the forms of interaction with the flow of geothermal energy required to develop tectonic disturbances described in this earlier paper. However, over the periods of around 140 Ma, typifying the cyclic thermal loading seemingly responsible for the ice ages, no such problems exist. As the lower diagram of Figure 1 shows surface temperature variation having periodicity of 140 Ma are capable of full penetration to depths typical of both oceanic and continent and crust. This means that major changes to the heat transfer processes at the Earth’s surface, occurring over these lengths of geological time, will likely have significant effects upon the geothermal energy flux. And variations in the geothermal flux will in turn exert major controlling influences on the crust thickness. 

 

Figure 4 illustrates schematically how various surficial changes could play a role in determining the geothermal energy flux and consequently the crust thickness.

Ice sheets and crustal rise:
Figure 4(a)(1) shows a typical column of crust at or near a continental passive margin which is then overlain with a thick ice sheet Figure 4(b)(2). Note, for this illustration I have ignored the isostatic readjustment due to the weight of the ice, but this too will play a role as later discussion will attempt to show. The thermal gradient through the ice sheet will be determined by its thickness, and the average surface and base temperatures. Where permafrost exists, this too will over a very long period of time develop an equilibrium thermal gradient. This near surface thermal gradient will eventually exert a controlling influence over the level of geothermal heat flux. If for example the ice sheet, or combination of ice sheet and underlying permafrost, had a thickness of 2 km and an average temperature difference of -40^oC, over a period of time sufficient for thermal equilibrium to be reached through the thickness of the crust and ice sheet there will result a downward thermal gradient of +20^oC.km^-1.  Over an area of relatively thin crust, that might have perhaps been part of a continental passive margin with say a thickness of 10 km and a temperature difference of +1200^oC between top and bottom, the thermal gradient will be around 120^oC.km^-1. Note, for simplicity I have ignored any contributions to the geothermal gradient arising from the decay of radioactive elements within the crust – in any case these are thought to be relatively small in oceanic crust. This initial disparity in thermal gradient will

gradually be adjusted to ensure that the flux through the ice is matched by the flux through the crust. In other words depending upon the relative diffusivities of ice and crustal rock, the geothermal gradient in the crust will be lowered to match that controlled by the average ice thickness and temperature gradient. This is suggested in Figure 4(a)(3). The result of this adjustment over long geological periods (10’s Ma) will likely be some phase changes in the magma beneath the crust and a gradual thickening of the crust from below. Being lighter than the semi-liquid magma from which it has been formed, the now thickened crustal column will be forced upward through the effects of buoyancy, as suggested in Figure 4(a)(3). In other words there will be an isostatic readjustment for which the original free surface will have been elevated relative to its original location. At a given depth within the crust such a realignment of the geothermal gradient will be likely to produce very substantial changes in temperature, the consequences of which could be of major significance in many tectonic processes, some of which will be discussed later. A mechanism along these lines will for example be invoked later to explain how ocean crust of continental shelves could be elevated to become continental crust and continue to rise during the cyclic waxing and waning of ice sheets over the period of an ice age.

It might be objected that during an ice age there will not necessarily be continuous ice cover as seems to be implied in the above model. However, the long period component of the shorter period cyclic fluctuations of ice sheets will provide what could be regarded as a continuous average ice cover. That is, on average over the period of the ice age the effects will be as described. Equally, during any interglacial flooding of previously continental margin crust due to eustatic changes, it might be anticipated that the effects will be the opposite. Figure 4(b)(1) represents a column of crust that might perhaps have been exposed during an average sea level drop accompanying an ice age. During the warm age (sometimes referred to as the “hot house”), lasting for periods of circa 100 Ma between ice ages, this area may be flooded as depicted in Figure 4(b)(2). It might also be cyclically flooded during the glacial – inter-glacial fluctuations. With oceans seemingly providing more efficient crustal surface heat transfer mechanisms, there will initially be an incompatibility between the surface heat transfer from the crust and that associated with the geothermal gradient of the previous continental crust. Over time this increased heat flux will be reflected by an increased geothermal gradient and an associated decrease in the effective crustal thickness. In this case the reduced crustal thickness might be anticipated to have again arisen from the phase change at the lower crustal boundary to accommodate the increased geothermal flux and its associated geothermal gradient, as shown in Figure 4(b)(3). As a result of the increased overall density of the crustal column and its overlying water will there will be a lowering of the crust. This mechanism will be used to later to provide a possible explanation as to how continental crust could sink beneath the waves.            

What is being suggested is that during the very long period cycles associated with the ice ages (circa 140 Ma) and their intervening warm ages, major changes in geothermal gradients could give rise to a number of long term cyclic tectonic processes.  These will be explored more fully in later posts. 

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.
  

Influence of surface ice and water on geothermal flux

In suggesting that surface temperature changes experienced over typical glacial and interglacial periods, typically in the range of 20 to 110 ka, could penetrate far enough into the crust to interfere with the flow of geothermal energy I was in an earlier contribution (Croll, 2007a) clearly wrong. As Figure 1 shows, a cyclic thermal loading at the Earth’s surface with a periodicity of 110 ka will penetrate fully depths of just 1.75 to 3.5 km. These depths would be insufficient to trigger the forms of interaction with the flow of geothermal energy required to develop tectonic disturbances described in this earlier paper. However, over the periods of around 140 Ma, typifying the cyclic thermal loading seemingly responsible for the ice ages, no such problems exist. As the lower diagram of Figure 1 shows surface temperature variation having periodicity of 140 Ma are capable of full penetration to depths typical of both oceanic and continent and crust. This means that major changes to the heat transfer processes at the Earth’s surface, occurring over these lengths of geological time, will likely have significant effects upon the geothermal energy flux. And variations in the geothermal flux will in turn exert major controlling influences on the crust thickness. 

 

Figure 4 illustrates schematically how various surficial changes could play a role in determining the geothermal energy flux and consequently the crust thickness.

Ice sheets and crustal rise:
Figure 4(a)(1) shows a typical column of crust at or near a continental passive margin which is then overlain with a thick ice sheet Figure 4(b)(2). Note, for this illustration I have ignored the isostatic readjustment due to the weight of the ice, but this too will play a role as later discussion will attempt to show. The thermal gradient through the ice sheet will be determined by its thickness, and the average surface and base temperatures. Where permafrost exists, this too will over a very long period of time develop an equilibrium thermal gradient. This near surface thermal gradient will eventually exert a controlling influence over the level of geothermal heat flux. If for example the ice sheet, or combination of ice sheet and underlying permafrost, had a thickness of 2 km and an average temperature difference of -40^oC, over a period of time sufficient for thermal equilibrium to be reached through the thickness of the crust and ice sheet there will result a downward thermal gradient of +20^oC.km^-1.  Over an area of relatively thin crust, that might have perhaps been part of a continental passive margin with say a thickness of 10 km and a temperature difference of +1200^oC between top and bottom, the thermal gradient will be around 120^oC.km^-1. Note, for simplicity I have ignored any contributions to the geothermal gradient arising from the decay of radioactive elements within the crust – in any case these are thought to be relatively small in oceanic crust. This initial disparity in thermal gradient will

gradually be adjusted to ensure that the flux through the ice is matched by the flux through the crust. In other words depending upon the relative diffusivities of ice and crustal rock, the geothermal gradient in the crust will be lowered to match that controlled by the average ice thickness and temperature gradient. This is suggested in Figure 4(a)(3). The result of this adjustment over long geological periods (10’s Ma) will likely be some phase changes in the magma beneath the crust and a gradual thickening of the crust from below. Being lighter than the semi-liquid magma from which it has been formed, the now thickened crustal column will be forced upward through the effects of buoyancy, as suggested in Figure 4(a)(3). In other words there will be an isostatic readjustment for which the original free surface will have been elevated relative to its original location. At a given depth within the crust such a realignment of the geothermal gradient will be likely to produce very substantial changes in temperature, the consequences of which could be of major significance in many tectonic processes, some of which will be discussed later. A mechanism along these lines will for example be invoked later to explain how ocean crust of continental shelves could be elevated to become continental crust and continue to rise during the cyclic waxing and waning of ice sheets over the period of an ice age.

It might be objected that during an ice age there will not necessarily be continuous ice cover as seems to be implied in the above model. However, the long period component of the shorter period cyclic fluctuations of ice sheets will provide what could be regarded as a continuous average ice cover. That is, on average over the period of the ice age the effects will be as described. Equally, during any interglacial flooding of previously continental margin crust due to eustatic changes, it might be anticipated that the effects will be the opposite. Figure 4(b)(1) represents a column of crust that might perhaps have been exposed during an average sea level drop accompanying an ice age. During the warm age (sometimes referred to as the “hot house”), lasting for periods of circa 100 Ma between ice ages, this area may be flooded as depicted in Figure 4(b)(2). It might also be cyclically flooded during the glacial – inter-glacial fluctuations. With oceans seemingly providing more efficient crustal surface heat transfer mechanisms, there will initially be an incompatibility between the surface heat transfer from the crust and that associated with the geothermal gradient of the previous continental crust. Over time this increased heat flux will be reflected by an increased geothermal gradient and an associated decrease in the effective crustal thickness. In this case the reduced crustal thickness might be anticipated to have again arisen from the phase change at the lower crustal boundary to accommodate the increased geothermal flux and its associated geothermal gradient, as shown in Figure 4(b)(3). As a result of the increased overall density of the crustal column and its overlying water will there will be a lowering of the crust. This mechanism will be used to later to provide a possible explanation as to how continental crust could sink beneath the waves.            

What is being suggested is that during the very long period cycles associated with the ice ages (circa 140 Ma) and their intervening warm ages, major changes in geothermal gradients could give rise to a number of long term cyclic tectonic processes.  These will be explored more fully in later posts. 

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.
  

Celestial Driver of Earth Climate

In later posts it will be reasoned that a major driver of processes taking place on and deep within the Earth could derive from the Earth’s various interactions with our celestial neighbours. At short timescales we are all familiar with the ways in which fluctuations of solar radiation reaching the Earth surface can have such dramatic effects on weather and climate. Whether at periods measured in hours, those of circadian origin or those determined from seasonal changes, many surface phenomena appear to be driven by the cyclic changes of insolation. Many forms of concrete and asphalt failure (see Croll, 2006, 2007b, 2008b) are believed to derive from thermal mechanical interactions driven by short term variations of insolation energy. Analogous forms of behaviour are known (Leffingwell, 1915, Mackay et al, 2002), and believed (Croll, 2004, 2006, 2007c,d, 2008a) to derive from seasonal variations of insolation affecting the response of ice and permafrost. So it would not require a major leap of imagination to consider whether very long period fluctuations of insolation energy could also be playing a major role in determining certain geological phenomena near the surface and deep within the Earth’s crust.

Ice Ages over Phanerozoic:
In earlier paper (Croll, 2007a) I attempted to explain the cyclic nature of many important Earth tectonic processes in terms of the periodic nature of the glacials and interglacials occurring when the Earth is experiencing an ice age. At the time I had not fully appreciated just how long a surface thermal wave would take to penetrate to the lower boundary of the crust. Accordingly, I implied that major changes to the thermal conditions within the crust could be generated by the surface thermal perturbations arising from the glacial and inter-glacial cycles having periodicities in the range of 20s to 100s of ka. This was only true to a limited extent. As Figure 1 shows, a sinusoidal change in surface temperature having period of say 100 ka will fully penetrate typical crustal rocks to depths of only around 1.75 km to 3.50 km, depending upon the thermal diffusion and other properties of the rock. While enough to drive many near surface processes this degree of penetration would be insufficient to account for some of the deeper tectonic phenomena ascribed to these circa 100 ka cycles. In addition, I had not fully appreciated the evidence relating to the periodicity at which the Earth experiences ice ages. This has now become more readily available and does seem to make it plausible to contemplate a celestial driver for many of the important processes that have and continue to shape the Earth’s lithosphere. 

 

From the work of Shaviv (2002) and Shaviv and Veizer (2003) it appears that ice ages also display a periodicity thought by some to largely derived from the interaction arising from the Solar System’s orbital passage through the Milky Way. There is strong evidence to suggest that one of the primary drivers of long term Earth climate is changes in the Cosmic Ray Flux (CRF). In Figure 2 of Shaviv and Veizer (2003), reproduced here also as Figure 2, the geologically reconstructed smoothed temperature anomalies based upon running means of δ18O values found in calcite shells, as reported by Veizer et al (2000), are plotted over the Phanerozoic eon as the lower continuous red line. The periods over which low temperature anomalies occur, often lasting many 10’s Ma, can be seen to correspond closely to the paleoclimate reconstructions of the known ice ages, shown by the blue bars at the top of Figure 2. The upper continuous black curve of Figure 2 summarised the reconstructed CRF values determined by iron meteorite exposure ages reported by Shaviz, 2002. Based upon these CRF values a prediction can be made of the ΔT values; these are shown by the continuous black line in the lower part of Figure 2. What is remarkable is the degree to which the fluctuations in CRF values appear to determine the temperature anomalies. Apart from the period of circa 30 Ma occurring around 250 Mbp, the close correlation between CRF predicted values and ΔT geological reconstructions of the temperature variations cannot reasonably be put down to chance. It is interesting to observe that this relatively poor correlation around 250 Mbp corresponds with a time when a scarcity of fossils, during one of the largest extinction events in Earth history, meant that few δ¹⁸O measurements have been possible. Not repeated here, since it is not of central concern to the theme of these posts, are the relatively poor correlations between reconstructed pCO₂ levels and temperature anomalies. Some of the highest levels of pCO₂ are shown to occur (Figure 2, Shaviv and Veizer, 2003) at the periods of lowest recorded temperature anomalies and known ice ages around 440 Mbp and 170 Mbp. Values of CO₂ over these earlier ice ages

appear to have been considerably higher than those currently existing. Not unexpectedly, these findings triggered an almost immediate response from those committed to pCO2 being the primary driver of climate change, Royer et alia (2004). They pointed out that with appropriate adjustments to allow for the influence of seawater pH on δ18O values, the pCO2 levels become somewhat more closely aligned to the temperature anomalies. But all this is perhaps a discussion for another day!      

While the periodic ice ages over the past 600 Ma are the most obvious symptoms of this cyclic process there is also considerable paleantological evidence to suggest that they have been occurring, possibly with even more extreme variations in temperature, for very much longer than this. If as suggested above an important driver for the occurrence of ice ages is variations in cosmic ray flux, and if as seems likely such variations have been occurring for periods longer than the Phanerozoic, then it is more than likely the Earth has been experiencing ice ages over equally long periods. As shown in Figure 1 cyclic surface temperature changes with periods of the order of 120Ma could be expected to penetrate the full depth of the Earth's crust.

And of course there are now widely recognised shorter period variations in temperature anomalies that are determined by Earth’s interaction with our nearer celestial neighbours.  Glacials and inter-glacials during ice ages:Changes in the Earth’s elliptic orbit around the sun together with the cyclic changes in the tilt and precession of the axis of spin relative to the orbital plane, are regarded as the chief sources of the massive changes in climate that have seen the development of periodic glacials and inter-glacials over the duration of an ice ages.

This periodic model explaining the known cyclic changes of Earth temperatures was                                            
first put forward by Croll (1864, 1875) and although discredited in the next 60 or so years was finally given a more soundly based astronomical confirmation by Milankovic (1920, 1941). The so called Croll-Milankovic model is now regarded as a major source for the very large changes in level of solar radiation reaching the Earth’s surface and for the associated large changes in temperature gradient through the upper few km of the Earth’s crust. One of the direct consequences of these cycles can be seen in the evidence of permafrost penetration during a glacial period (Washburn, 1979). 
With periods of around ti = 20 ka for the variations of tilt angle and tp = 40 ka for the precession of the axis of spin, and around to = 110 ka for the change from the eccentric elliptic orbit to the basically circular orbit of the Earth, the intensity of the temperature changes at a given location on the Earth’s surface are anticipated to show a time dependence like those shown in Figure 3. The phasing of the orbital cycles along with the tilt and precession of the spin axis will result in changes being differentially experienced within the northern and southern hemispheres. How intense are these changes in temperature and how deep they penetrate into the Earth’s crust are still being debated, but as shown in Figure 1 they could be having significant influence down to depths of around 4 km. 

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.

 



Dynamic evolution of the Earth's crust

Evidence is overwhelming that the evolution of the Earth’s crust has been one of dynamic change, with dynamic tectonic evolution displaying a great deal of synchronicity over vast spatial domains. As Wezel (1992) expresses it “the whole tectonosphere, our planetary geosystem, is a dynamic and integrated network of morphotectonic structures that exist in a state of continual and interdependent mutation and transformation in relation to the changing environment” so that “there are no static structures in nature: they are instead the spatial manifestations of the underlying temporal processes.”

This appears to be borne out within the evidence of the sedimentary records. There seems to be wide recognition of distinct pulses in the rates and forms of sedimentary deposition that have occurred over wide geographical areas. Deep-sea drilling in both the Atlantic and Pacific have penetrating into distinct Mesozoic (circa 65 - 250Mbp‡) sequences to reveal correlated stratigraphic

units, Jansa et al (1979), which seem to closely resemble those typical of the Tethyan, Wezel (1985). At around 110Mbp a dramatic change in the nature of the sediments occurred. This saw the ending of the limestone formations and the sudden increase in clay and quartzose beds. At this time deposition rates are seen to accelerate and be accompanied by tectonic changes at large numbers of the world’s passive margins, De Graciansky et al (1987), involving widespread regional subsidence and tectonic mobility at the margins, Kent (1977) and Wezel (1985).    

Recording time-stratigraphic relationships of sequences in the North American craton, in which certain geological periods display non-depositional hiatuses while others show rapid and widespread deposition, Sloss (1964) commented that “in other words, let us not here argue the validity of the sequence concept, but, rather, proceed to a consideration of the alternation between depositional and non-depositional episodes of the craton and the significance of these events. Are the successive episodes repetitions of the same behavior? If differences exist, is there a systematic pattern of recurrence? What relationship, if any, appears to exist between cratonic events and those occurring in neighboring mobile belts?” He went on to conclude from his earlier work, Sloss (1963), that it should be “reiterated … that the stratigraphic record of the North American craton (late Precambrian to the present) is punctuated by six interregional, cratonwide unconformities. The unconformities are recognizable surfaces which subdivide the cratonic stratigraphic column into six vertically successive groupings of rocks, each identified by a geographic term in the manner of other lithostratigraphic units. These units are defined and described in the aforementioned paper, and their time-stratigraphic limits are indicated” very graphically in Figure 2 (Sloss, 1964).

All of this presents problems for both plate tectonics (PT) and the Expanding Earth (EE) hypotheses. With, say, the N American continent hypothesised by PT to be firmly locked within the embrace of mother of all continents, Pangaea, prior to 250Mbp, and with many of these unconformities outlined by Sloss also occurring prior to 250MBP, there appears to be no way of explaining the massive marine inundations needed to lay down the mega-sequences separated as they are by unconformities over this Cenozoic period prior to 250Mbp. It appears similar problems exist over most of the other current continental land masses. And if this is a problem for PT it would seem to be an equal problem for EE.         

There is other evidence to suggest that the pulses in epeirogeny, often triggering periods of mountain building activity, also occur over quite distinct periods of geological time. Synchronous periods of mountain building have been compared with paleoclimate temperature estimates over the Phanerozoic (Post & Illis, 2009) and noted by Kalander et alia (2011) to have a close correspondence with the geological evidence for the occurrence of ice ages, supporting the model outlined by Croll (2007). Ollier (2006b) has made detailed analysis of the spurt in epeirogenic events during the so called “neotectonic” period of the late Cenozoic. And evidence of such regional uplift in Plio-Pleistocene and its possible thermochronology has been very clearly identified as a global process from the study of marine and fluvial terrace structures (see for example Bridgeland (1988, 2010) and Westaway (2002, 2010)*. None of this seems compatible with the PT or EE models.

Over the next few postings I will examine each of these aspects of the dynamic processes of tectonic evolution. These postings will attempt to demonstrate that the phasing of periods of rapid build-up of sedimentary deposits followed by periods during which vast areas of continental landmass uplift or subside, are no accident. It will suggest to the extent consistent with the far from universally accepted data, that this phasing of tectonic activity is driven by forces derived from Earth’s interactions with our galaxy, solar system, and especially the Sun. Specifically, it will attempt to demonstrate that over deep geological timescales the phases during which continents undergo maximum rates of erosion and synchronously marine sedimentation experiences its greatest rates of build-up are closely locked-in to the periodicity of warm-ages. Furthermore, the thermal-mechanical model used to explain this locking-in process will also be suggested to account for the periods during which the Earth’s crust experiences its greatest rates of epeirogenic rise and fall.            

 

‡ Experienced geologists will of course not need my numerical interpretations of the conventionally used geological periods, but as someone not trained in the nomenclature I find it very frustrating reading geological works where numerical values are not given. It means a chart is required to translate to real time – would it not be easier to include real time translations as a matter of course?  

* The work of Bridgeland and Westaway was for me particularly illuminating. Unexpectedly, I had accepted for presentation at the Geological Society of London a paper dealing with the very long period cyclic rises and falls of continents (Croll, 2011). While cycling to the meeting I had experienced a pretty nasty crash, due to someone from a country where people drive on the right since she checked only to her left as she left the curb, while trying to avoid a pedestrian walking out in front of me while crossing Oxford Circus. As a result the first 2 presentations at the meeting were spent in the projection room trying to stem the flow of blood. With my presentation scheduled for the 3rd spot it was inevitably a pretty shaky performance but I have to say heartened by the presentations from Bridgeland and Westaway immediately before. Over widely dispersed geographic areas they described the evolution of marine and fluvial terraces have been laid down at periodicities of around 100ka showing strong synchronicity with the known glacial and inter-glacial periods. That monotonic epeirogenic uplift has been occurring across these glacial and inter-glacial periods appears to rule out isostatic rebound as a possible explanation for these gradually rising marine and fluvial terraces. Here again, there seems to be major conflicts in chronology with any predictions from PT.

 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.

  

Plate Tectonics and the inconvenient truth of Vertical Motion:

A major challenge for PT is to explain away the question put so succinctly by Bill Bryson in his wonderful “The short history of nearly everything” as to “how do ancient fossil clamshells get to the mountaintops?” To a PT follower the answer is simple. The oceanic portions of the plates during their 100 to 200 Ma outward journeys from the spreading zones, within the once unified continental land masses, will have accumulated deep sedimentary beds washed down from the adjacent eroding continental parts of the plates. It is into these sedimentary strata that the unwary clams will have become buried and fossilised. As the basalt base of these sedimentary beds is pushed back down into the mantle at the subduction zones, reheated and re-liquified to become once again part of the mantle, the overlying deep strata of sediments along with their clam shells will have been conveniently sheared off and shoved up to become part of the great mountain ranges that now so often bound the trenches into which the oceanic plates are supposed to be subducting.

If this is the PT explanation there do appear to be some serious questions that need addressing. First, it fails to recognise that most of the mountain ranges occurring within the interiors of the continents have resulted from regional uplift – what used to be called epeirogeny, a term that appears to have largely gone out of use - partly one suspects because it fits inconveniently within the PT model. Even the mountain ranges occurring at the margins of continents are often in areas where no subduction behaviour is evident. In all these cases there appears to be no credible mechanism within PT to explain their emergence. Second, and a corollary to the above, the great majority of continental seaboards are of a passive form in that there is no evidence of mountain building, volcanism and earthquakes often attributed to subduction. Indeed, compared with the lengths of the mid-ocean rifts there is a distinct shortage of subduction, raising the very simple question as to where all that ocean crust supposedly propelled out  in either direction from the mid-oceanic rifts, has actually gone? If new oceanic crust is being thrust out in either directions from the mid-oceanic rifts there would need to be roughly twice the length of subducting continental margins to account for this process. The mid-Atlantic rift is often cited as providing definitive evidence for PT. However, this fails to recognise that over almost its entire extent there are no subduction margins on either the eastern seaboard of North and South America or the western seaboard of Africa and Europe. In fact globally the total length of continental margins that could in any way be classified as having their origin in a subduction type of mechanism is considerably less than the total length of the mid-oceanic rifts thought to be their cause - and most certainly not twice their length. There would appear to be a distinct shortfall in the length of subducting margins to account for the supposed mechanics involved with plate tectonics - even if subduction is actually happening. To account for this incompatibility would require the plates to increase their velocities as they approach the inadequate lengths believed to be undergoing subduction (Ollier, 2006b). So all in all there must surely be a good many clamshells at the tops of mountains that have not been put there by the processes envisaged in the PT model.

 

But this surely is not the real problem. Within virtually all the great continental land masses that constitute the areas considered by PT to have once comprised the hypothesised supercontinents of Pangaea, and later Gondwana and Laurasia, there is ample evidence of vast areas for which marine sediments have been laid down within the time-frame believed to have been needed for these supercontinents to break up and commence their long journeys to occupy their current positions over the globe. Not only that but in many cases there is evidence of multiple marine incursions having occurred over this same period. These periods of marine deposition are at times separated by periods in which these mega-sequences of marine sediments have undergone regional uplift and subsequently been eroded to form mountain ranges or over longer periods planated surfaces. There is evidence of many such cycles having occurred over the past 600Ma. Perhaps nowhere illustrates these cycles of sedimentation followed by uplift, erosion and then subsidence more effectively than in the exposures carved-out by the Colorado River - more on this later. That this process has occurred many times over the past 600Ma or so is surely a major problem for PT and also other models  predicated upon the breakup of supercontinents, such Earth Expansion. It appears that the great majority of the mountain ranges have as their origin this process of epeirogeny rather than as a by product of PT invoked subduction.      

What this epeirogenic origin implies is that at some point over geological time a surface that may or may not have been planated experiences a regional uplift which to a large extent preserves the earlier surface morphology. The original uplifted surface may overlie strata that are essentially flat or strata that have at some earlier time been subjected to forces that have caused the strata to be folded. The uplifted surface may have previously been beneath the sea or on land. But once elevated the processes of erosion set-in, gouging out river or glacial valleys, creating ridges, cols, saddles, horns, and all the other surface features we associate with high mountains. But the key point is that it is seemingly rare for the evolving new mountain topology to preserve the geometry of the original surface. It is even rarer for the new mountain surface to reflect the morphology of the underlying strata, whether they are folded or largely planar. Put simply, the surface shape of mountains seldom reflects the geometry of the underlying strata.

All of this has been rather more eloquently argued by Ollier et al (2000, 2006a,b) who also point out that the geological epochs over which major epeirogenic events are said to have occurred usually bare no relationship to the periods required for PT to purportedly do its stuff. Describing all the mountain building that has taken place within what Ollier refers to as the “neotectonic period”, and this includes the raising of the European Alps, Apennines, Pyrenees, Caparthians, Caucasas, Urals, Himalayas, Tibet-Qinghai plateau, Sierra Nevada, Rockies, and many others, it is concluded that “mountains are created by the vertical uplift of former planes, independent of any folding of the rocks underneath”.  Indeed, Ollier provides evidence that most of the great mountain ranges of today have arisen after the late Miocene <25ma span=""> with the great majority emerging within the Pliocene.

 

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.

 

Some past explanations of crustal dynamics

There is convincing evidence that the earth’s crust has undergone periodic changes with timescales, both very long measured in 100’s of ma, and shorter measured in10’s or 100’s ka,. Over the very long term continents would on a periodic basis seem to sink to become ocean floors and ocean floors rise to again become continents. How many such cycles have occurred during the circa 4.5 ba of the Earth’s existence and when exactly a significant crust of the form we know it today actually formed to make such movements possible, seem to be largely unresolved. However, it appears more than conceivable that the number of such very long-term cycles could be many. It also seems clear that the PT model, dominated as it is by the tangential motions of the crust, is incapable of explaining the occurrence of these very long-term vertical tectonic cycles.

Within these long-term geological cycles there seem to be other shorter timescale processes at work. These shorter period processes could be responsible for the alternations between compressive and tensile actions occurring within the crust. Before going on to outline a model that could provide an explanation for this dynamical system, involving as it appears to do both horizontal and vertical motions, accompanied by both tensile and compressive actions, it may be useful to consider some of the previously postulated explanations, other than PT, that have been advanced for the development of the earth’s crust as we know it today.

One major differentiation of Earth models is between those that take the line that what we see today has been the result of a gradual evolution in which the processes at work in the past should be evident from those that are at work today. This so called “uniformist” model contrasts with those that see the evolution in terms of more discrete and often cataclysmic changes.

Cataclysmic models: 

Among the latter were those that tried to explain the Earth as we find it today in terms of a Biblical flood. This idea, prevalent in the 18^th and early 19^th C, incorporated the growing recognition that the match between the coastlines of the Americas and Africa/Europe was due to a rifting apart of the Atlantic following the flood referred to in the Bible. Others have suggested that the spin-off of the Moon left a great hole in what is now the Pacific Ocean with the great void so created being filled by the splitting apart of Americas and Europe/Africa to form the Atlantic Ocean (this view is associated with George Darwin nephew of Charles). Various other ideas have included the colliding or near colliding bodies, taking different forms but including the idea that the gravity field generated by the Earth’s capture of the Moon developed at an early stage the forces needed to drag the continents towards the equator, creating enormous mountain building forces (Taylor 1908). A variant was the idea that the close approach of Venus (Baker, 1911) created the gravity field needed to drag the Moon from the Earth, with other orbital interactions being elaborated by Velikovsky (1950).

Uniformist models:

In the former, more traditional, uniformist, view the models have included the shrinkage (contraction) model relying upon the idea that the shrinkage of the Earth’s interior against the crust created the compression forces needed to build mountains. This idea appears to have been first put forward by Newton (1681) using the analogy of the wrinkling of the skin of an ageing apple (must have come a few days after his gravity observations from the falling apple!). Jeffreys (1976) too has argued that since its inception the Earth as a whole has contracted while cooling. Clearly these models fail to account for the clear evidence that in certain places and at some times, the crust has been and continues to be torn apart by tensile actions. To accommodate the very clear evidence of tensile stretching action, and at the other extreme, the expansion model advocates that the tearing apart of the oceans has been the result of a massive increase in the diameter. Carey (1958) argued that the diameter of the Earth could have been increased by as much as 100%. How these expansions occurred has been explained in a number of ways. Others account for the increased diameter by suggesting changes in phase or molecular composition of the Earth’s matter, or on a more modest level through a gradual decline in the strength of the gravity force (Dicke, 1962). Each of these uniformist models fail to account for the massive compressions needed to either explain upward folding and mountain building or the downward folding to form ocean trenches. None of the models are able to account for clear spatial and temporal evidence of periodic cycles of tension and compression being involved.

 

An attempt to reconcile the clear evidence of periods of tension and other periods of compression, the mixed shrinkage and expansion model, recognises that during the earliest period of the Earth’s formation the largely gaseous materials gradually changed phase to become liquid and some eventually solid. This gradual compaction of the molecules would have been accompanied by a massive decrease in the diameter of the Earth. When later these dense liquids and solids were broken-down into less compact molecular forms the volume was once again increased. This latter period would cover the formation of the Earth’s crust, during which the breakdown in molecular forms would have started to produce the water that now forms such an important ingredient in the dynamics of the Earth. This view (see for example MacDonald, 1959) is attractive but is more concerned with the period prior to the dynamic crust of present interest. It would suggest however, that underlying any shorter periodicities there may continue to be a gradual expansion occurring as the average thickness of the crust and the associated volumes of free water and other low density molecules increase. Along similar lines the so called antimobilists believe that the Earth’s crust has been shaped by cycles of heating and cooling, causing expansion and contraction of the land masses. They took the opposite view to the mobilists who supported Wegener’s notions of continental plates in motion. The concept of a pulsating earth has also been advocated by Wezel (1992) and Dickins (2000).

 The truth, if and when found, will undoubtedly find that most of these models contain elements required to explain what has occurred. It seems evident that certain phenomena are associated with sudden and cataclysmic changes. It seems equally clear that other phenomena have been the result of gradually emerging processes. It is also very clear that whether one adopts a steady state or a transient model, the evolution of the Earth’s crust has been and remains a highly dynamical process. There is strong evidence that at a given location the crust has at times experienced tensile action and at others compression. This is incompatible with either the uniformist view or many of the prevailing notions of the nature of the Earth’s dynamical system. There is also unquestionable evidence that the various regions of the Earth’s crust have experienced, on a periodic basis, major changes in vertical elevation, which is also at odds with most of the past models and certainly at variance with PT.

 

What therefore might be an alternative model that could explain all of the essential processes known to have taken place and which continue to take place in the shaping of the earth’s crust?

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.

Some further problems with Plate Tectonics

Evidence of Tension and Compression:

At many locations within the earth’s crust there is evidence of both tensile and compressive actions having occurred at different times (Storetvedt, 1997). This is perhaps particularly in evidence at the mid-ocean “spreading zones” where the crust is supposed to be torn apart by a tension field normal to the stripe of new crust being formed by the intrusion of magma into the fissures. As previously observed these fissures occur in bands that are broadly parallel to the mid-oceanic ridge. And yet the existence of a ridge or mid oceanic mountain ranges, sometimes involving folded sedimentary deposits, is strongly suggestive of a compression field action normal to the ridge, and Antipov et al (1990) have suggested that thrust faults adjacent to the mid-Atlantic ridge are more likely to have been caused by compression rather than tension. Fracture patterns are also suggestive of compression related failures in the vicinity of the spreading zone. Zoback et al (1989) demonstrated that earthquake data at mid oceanic ridges is more strongly supportive of compression action than as supposed by PT from tension behaviour. It would appear that alternations of both tension and compression actions are experienced in locations where PT would indicate steadily developing tensile failure. PT appears to have little on offer to resolve these apparent contradictions (Pratt, 2000).

Vertical Tectonics:

It should not take long for even an untrained geologist to become concerned about the fact that many of the highest continental mountain ranges and some of the most extensive continental plateau, are formed from sedimentary rock that was once laid down at the bottom of an ocean floor. Equally, as observed above considerable areas of deep, ocean floor are composed of rock whose palaeontological evidence alone indicates that it once formed part of a continental land mass (Spencer, 1977). PT does not appear to have addressed these issues and seemingly would be hard pushed to provide an explanation for this very clear piece of geological reality. That marine sediments and fossils can be found near the highest peaks of the Himalayas or that shallow sediments and even land based fossils can be recovered from the depths of ocean crust, are inconsistent with any notions that currently form part of PT (Pratt, 2000). Explanations based upon changes in sea level, believed to be brought about by increased volumes of uplift at the mid-oceanic ridges, has been suggested by an acknowledged supporter of PT  to be an inadequate explanation, and that the scale of these movements “fit poorly into plate tectonics” (van Andel, 1994).

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.