References cited in the Earth tectonics posts

References in blogs over the period 12 August to 7 September, 2016                          

 Ahmad, F., (1990): The bearing of paleontological evidence on the origins of the Himalayas, in Barto- Kyriakidis, (Ed.). Critical aspects of the plate tectonic theory, 1, 129-142, Athens, Greece: Theophrastus Publications, S. A.

Andel, T H, van, (1994). New Views on an Old Planet: a History of Global Change, Cambridge University Press.

Antipov, M P, Zharkov, S M, Kozhenov, V Y A, Pospelov, I I. (1990). Structure of the mid Atlantic ridge, Int Geology Review, 32, 468-478.

Beloussov, V. V., (1990). Present trends in present-dat geosciences. In In Barto-Kyriakidis, (Ed.). Critical aspects of the plate tectonic theory, vol 1, pp3-15, Athens, Greece: Theophrastus Publications, S. A.

Bridgland, D.R. (1988): The Plestocene fluvial stratigraphic and palaeo-geography of Essex, Proc. of the Geologists’ Assoc., 99, 291-314.

Bridgland, D.R. (2010): The record from British Quaternary river systems within the context of global fluvial archives, Journal of Quaternary Science, 25, 433–446.

Bullard, E.C., Everett, J. E., Smith, A. G., (1965). The fit of the continents around the Atlantic, in A Symposium on Continental Drift, Blackett, P. M. S., Bullard, E. C., and  Runcorn, S. K. (ed). Phil Trans., Series A, Royal Society London, A258, 41-51.

Chatterjee, S. and Hotton, N., (1986). The paleoposition of India. J. SE Asian Earth Sciences, 1, 145-189.

Chekunov, A. V., Gordienko, V. V. and Guterman, V. G., (1990). Difficulties of plate tectonics and possible alternative mechanisms.  In Barto-Kyriakidis, (Ed.). Critical aspects of the plate tectonic theory, vol 1, pp397-433, Athens, Greece: Theophrastus Publications, S. A.

Choi, D.R., Vasil’yev, B.I. and Bhat, M.I., (1992). Paleoland, crustal structure and composition under the northwestern Pacific Ocean. In, Chatterjee, S. and Hotton, N.III (eds.), New Concepts in Global Tectonics, 179-191. Texas Tech Univ. Press, Lubbock.

Croll, James (1864): On the physical causes of change in climate during geological epochs, Philosophical magazine, 28, 121-137.

Croll, James (1875): Climate and time in their geological relations: a theory of secular changes in Earth’s climate, London, Daldy and ,Tsbister, pp574.

Croll, J. G. A. (2004): An alternative model for “Pingo” formation in permafrost

regions, Paper presented at 21st Int. Congress of Theoretical and Applied Mechanics,

ICTAM-04, Warsaw, 15-21 Aug., 2004.

Croll, J G A. (2005). Mechanics of glacial flow, submitted for possible publication to J of Glaciology.

Croll, J. G. A. (2006): From asphalt to the Arctic: new insights into thermo-mechanical ratchetting processes, III Int Conf on Computational Mechanics Solids, Structures and Coupled Problems in Engineering, Elsevier, Lisbon, Portugal, 5-8 June.

Croll, J. G. A. (2007a): A new hypothesis for Earth lithosphere evolution, New Concepts in Global tectonics, Newsletter, 45, December 34-51.

Croll, J. G. A. (2007b): A new hypothesis for the development of blisters in asphalt pavements, Int. J Pavement Engineering, 9(1), 59-67.

Croll, J. G. A. (2007c) Mechanics of thermal ratchet uplift buckling in periglacial morphologies, Proceedings of the SEMC Conference, Cape Town, September, 2007.

Croll, J. G. A. (2007d) Thermal ratchet uplift buckling and periglacial morphologies, Int. Conference Cryogenic Resources of Polar Regions, Salekhard City, Russia, 17-22 June, 2007.

Croll, J. G. A. (2008a) Dynamics of patterned ground evolution, Proc 9th Int. Conference on Permafrost, Fairbanks, Alaska, 30 June – 3 July, 2008.

Croll, J. G. A. (2008b) Possible role of thermal ratchetting in alligator cracking of asphalt pavements, Int. J of Pavement Engineering, 10(6), 2009, 447-453.

Croll, J. G. A. (2008c) Thermally induced pulsatile motion of solids, Proc Royal Society, A, London, 465(2103), 2009, 791-807.

Croll, J. G. A. and Jones, E. W. J. (2006): Thermal ratchetting in periglacial environments, Asian Conf. on Permafrost, Lanzhou, China, 7-9 August.

De Graciansky, P. C., …. (1987): Organic-rich sediments and paleoenvironmental reconstructions of the Cretaceous North Altantic, in Marine Petroleum Source rocks : Geological Society Special Publication, ed. Brooks, J. and Fleet, A. J., 26, 317-344.

Dickens, J M. (2000). Major global changes in the development of the earth during the Phanerozoic, New Concepts in Global Tectonics Newsletter, 16, 2-4.

Dickens, J.M., Choi, D.R. and Yeates, A.N., (1992). Past distributions of oceans and continents, in Chatterjee, S. and Hotton, N. III (eds.), New Concepts in Global Tectonics, 192-199, Texas Tech Univ. Press, Lubbock.  

Dietz, R.S. and Holden, J.C., (1970). The breakup of Pangaea, Scientific American, 223,30-41.

Grant, A. C., (1980). Problems with plate tectonics: the Labrador sea, Bulletin of Canadian Petroleum Geology, 28, 252-278.

Jansa, L. F., Enos, P., Tucholke, B. E., Gradstein, F. M. , Sheridan, R. E., (1979): Mesozoic-Cenozoic sedimentary formation of the North-American basin, western North Atlantic, in “Deep drilling results in the Atlantic Ocean, American geophysical Union, 3, 1-57.

Jeffreys, H., (1976). The Earth: Its Origin, History and Physical Constitution (6^th Edition), Cambridge University Press.

Kalenda, Neumann et al. (2011) Tilts, global tectonics and earthquake prediction, research monograph, in press.

Kent, P. E., (1977). The Mesozoic development of aseismic continental margins,  J Geol. Soc., London, 134, 1-18.  

Leffingwell, E. de K. (1915): Ground-ice wedges, the dominant form of ground-ice on the north coast of Alaska, J of Geology, 23, 635-654.

Mackay, J. R. and Burn, C. R. (2002) The first 20 years (1978-179 to 1998-1999) of ice-wedge growth at the Illisarvik experimental drained lake site, western Asrctic coast, Canada, Can. J Earth Sciences, part1, 39(1), 95-111, and part 2, 39(11), 1657-1674.

Meyerhoff, H H, & H A. (1974). Tests of plate tectonics, Plate Tectonics – Assessment and Reassessment (memoirs 23), ed Kahle, American Association of Petroleum Geologists.

Meyerhoff, A.A., Kaman-Kay, M., Chen, C., Tanner, I., (1991): China – stratigraphy, paleogeography and tectonics, Dordrecht, Kluwer.   

Meyerhoff, H H, Boucot, A J, Meyerhoff, H D and Dickins, H D. (1996). Phanerozoic Faunal and floral realms of the earth, memoir 189, Geological Society of America.

Milankovic, M. (1920): Théorie mathématique des phénomènes thermiques produits par la radiation solaire (Mathematical theory of thermic phenomena caused by solar radiations),

monograph published by the Yugoslav Academy of Sciences and Arts by Gauthiers-Villards, Paris.

Milankovic, M. (1941): Kanon der Erdbestrahlung und seine Anwendung auf das Eiszeitenproblem, Royal Serbian Academy. English translation: Canon of Insolation of the Earth and Its Application to the Problem of the Ice Age, 1969, Israel Program for Scientific Translations and published for the U.S. Department of commerce and the National Science Foundation, Washington, D.C.

Ollier, C. D., Pain, C. F., (2000): The origin of mountains, Routledge, London, pp324.

Ollier, C. D. (2006a): A plate tectonics failure: the geological cycle and conservation of continents and oceans. Annals of Geophysics, supplement to vol. 49, 1, 427-437.

Ollier, C. D. (2006b): Mountain uplift and the Neotectonic Period. Annals of Geophysics, supplement to vol. 49, 1, 427-437.

Pratt, D., (2000): Plate tectonics: A paradigm under threat, J Scientific Exploration, 14(3), 307-352.

Post, G., Illis, B. (2009): Searching the PaleoClimate record for estimated correlations: temperatureCO2 and sea level, see Kalenda et al. .  

Royer, D. L., Berner, R. A., Montanez, I. P., Tabor, N. J. and Beerling, D. J. (2004): CO₂ as a primary driver of Phanerozoic climate, GSA Today, 14, 3, 4-10.

Shaviv, N. J. and Veizer, J. (2003): Celestial driver of Phanerozoic climate? GSA Today, July, 4-10.

Shaviv, N. J. (2002): The spiral structure of the milky way, cosmic rays, and ice age epochs on Earth, New Astronomy, 8, 39-77.

Sloss, L.L. (1964): Tectonic Cycles of the North American Craton, in Symposium on cyclic sedimentation: Kansas Geological Survey, ed. Merriam, D. F., Bulletin 169, 449-459.

Smiley, C J. (1992). Paleoflors, faunas, and continental drift: some problem areas, New Concepts in Global Tectonics, ed Chatterjee and Hotton, Texas Techn Univ Press, 241-257.

Smith, A.G. and Hallam, A., (1970). The fit of the southern continents, Nature, 225, 139-144.

Smoot, N C, Meyerhoff, A A. (1995). Tectonic fabric of the Atlantic Ocean floor: speculation vs reality, J Petroleum Geology, 18, 207-222.

Spencer, E. W., (1977). Introduction to the structure of the Earth, 2^nd Edition, McGraw Hill, New York.

Storetvedt, K.M., (1997). Our evolving planet: earth history in new perspective, Alma Mater Forlag AS, Bergen, Norway, pp456.

Timofeyev, P.P. et al., (1992). Equatorial segment of the mid-Atlantic ridge as a possible structural barrier between the north and south Atlantic, USSR Academy of Science, Transactions (Doklady) Earth science sections, 312, 133-135.

Udintsev, G.B. (ed.), (1996). Equatorial segment of the mid-Atlantic ridge, IOC Technical series, 14, UNESCO.

Veizer, J., Godderis, Y. and Francios, L. M. (2000): Evidence of decoupling of atmospheric CO₂ and global climate during the Phanerozoic eon. Nature, 408, 698-701.

Veizer, J. (2005): Celestial climate driver: A perspective from four billion years of the carbon cycle. Geoscience Canada, 32(1), 13-28.

Voisey, A.H., (1958). Some comments on the hypothesis of continental drift, In, “Continental drift” – a symposium, Hobart, Tasmania, 162-171.

Velikovsky, I., (1950). Worlds in collision, Macmillan, pp450.

Washburn, A. L. (1979): Geocryology: A survey of periglacial processes and environments, Edward Arnold, pp406.

Westaway, R. (2002): Long term river terrace sequences: evidence for global increases in surface uplift rates in the late Pliocene and early Middle Pleistecene caused by flow in the lower continental crust induced by surface processes, Netherlands Journal of Geosciences, 81, 305-328.

Westaway, R. & Bridgland, D.R. (2010): Causes, consequences and chronology of large-magnitude palaeoflows in Middle and Late Pleistocene river systems of northwest Europe, Earth Surface Processes and Landforms, 2010.

Wezel, F-C. (1985): Facies anossiche ed episodi geotettonici globali: Giornale di Geologia (Selli Volume), 47, 281-286.

Wezel, F-C.(1992): Global change: shear-dominated geotectonics modulated by rhythmic Earth pulsations, in New Concepts in Global Tectonics, ed. Chatterjee, S and Hotton, N., Texas Univ Press, Lubbock, 421 - 439.  

Zoback, M L, & M D, and Compilation Working Group. (1989). Global patterns of tectonic stress, Nature, 341, 291-298.



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 remarks on the alternative earth tectonics hypothesis

The preceding few blogs have attempted to cover some of the apparent weaknesses of the Plate Tectonics (PT) explanation for how the Earth’s lithosphere has dynamically evolved throughout the eons of geological time. It has focussed upon the particular problems relating to the ways in which sediments are laid down and mountains are formed. There appears to be compelling evidence that mountains are generally the result of erosion of regionally uplifted crust, which is quite out of line with the mechanism envisaged by PT. Moreover, the evidence I have been able to gather suggests that these periods of regional uplift occur over quite short geological timescales (measured in 10’s Ma), whereas PT regards continents as having been impacted into one another at timescales measured over 100’s Ma. There also appears to be good grounds for supposing that the regional uplifts occur synchronously over very different regions of the Earth’s surface. For this to happen in the PT model would require some remarkable coincidences.  

Sedimentary evidence seems to provide a picture of crust rising and falling, sometimes above and sometimes below sea level, severely challenging the PT notion that continents have over many 100’s Ma had essentially unchanged plan forms, albeit they have broken away from their precursor super-continents, and been propelled into different spatial locations. Some of the important evidence I have been able to gather shows that the nature of sedimentary deposit also displays a cyclical form, with periods in which the forms of the sediments and their rates of deposition display globally synchronous characteristics. Moreover,  many cycles of sedimentary deposition, epeirogenic rise, continental erosion, epeirogenic subsidence, appear to have occurred over the time it is believed to have taken for the assembly, breakup and global journeys of the fragmented supercontinents envisaged in the PT hypothesis. PT seems to have very little useful to say as to why this might be the case.

Based upon my own reading of the current situation a new hypothesis has been presented that appears to provide an alternative explanation for what has and continues to help shape the Earth’s crust. It is based upon observations of how cyclical thermal processes are at work, albeit at greatly reduced spatial and temporal scales, in controlling both natural and man-made surface morphologies. There is growing evidence that long (10’s to 100’s ka) and very long (10’s to 100’s Ma) cyclic changes in thermal conditions on the Earth could be similarly playing an important role in global tectonics. Some of the earlier posts have argued that a great many features of the dynamic evolution of the lithosphere can be explained in terms of massive cyclic changes in thermal regime within the crust, including phase changes of lower lithospheric rock.

These earlier posts have suggested that the driving forces for these thermal changes are closely related to those responsible for glacial and interglacial episodes arising within the geologically recurring ice ages. Waxing and waning of thick ice sheets and the associated changes in sea levels are suggested to induce major changes in geothermal heat flux and its associated geothermal gradients. The consequential changes in lithospheric temperature are shown to provide models capable of explaining the cyclically developed massive horizontal tension and compression forces, and their associated kinematic and rheological consequences within the upper regions of the crust. But over longer geological periods these cyclic changes in surface ice and water and the thermal insulation they provide, have been argued to cause alternations in the thermal regimes and possibly associated phase changes at the lower lithosphere boundary, sufficient to induce substantial changes to the thickness of the Earth’s crust. Consequential isostatic and eustatic changes then help to explain the all too evident vertical motions of ocean and continental crust, including explanations of how ocean crust can be raised to form new continental crust and vice versa - issues which are left poorly accounted for by PT.

There appears to be some evidence in support of this new theory. The pulses of mountain building have been compared with paleoclimate temperature estimates (Post & Illis, 2009) and noted by Kalander et alia (2011) to coincide rather well with the geological evidence on the periods ice ages have occurred. Figure 11 reproduces Figure 2 but has had added the periods when major regional mountain forming episodes have been recorded. There does appear to be a remarkable coincidence between the ice age pulses and those of mountain building, including that of the Plieno-Pliestocene (Ollier, 2006b). And although the precise dating seem a little less clear, there does appear to be more than circumstantial evidence that the pulses in different forms and rates of sedimentation may also have strong links with the ice ages (Sloss, 1964; Wezel, 1992). Over shorter geological periods the seemingly synchronous and global development of river terraces (Bridgeland, 1988, 2010; Westaway, 2002, 2010) are strongly indicative of the work of the glacial – inter-glacial cycles being superimposed upon long term epeirogenic uplift.

All in all there does appear to be a compelling case that in contrast with the supposed monotonically evolving processes envisaged by PT, much of the dynamic of Earth lithosphere evolution has been one of cyclical processes. The preceding blogs have attempted to explain the close agreement between the phasing of the cycles of geological evolution and the forcing of climate caused by Earth’s interactions with the sun (whether directly or indirectly as controlled by variations in cosmic ray flux). They have also proposed a thermal-mechanical models for how this dynamic process might be working.     

 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.
  
   

                                

 

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 see 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 7(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.
  

Some further remarks on the alternative earth tectonics hypothesis

The preceding few blogs have attempted to cover some of the apparent weaknesses of the Plate Tectonics (PT) explanation for how the Earth’s lithosphere has dynamically evolved throughout the eons of geological time. It has focussed upon the particular problems relating to the ways in which sediments are laid down and mountains are formed. There appears to be compelling evidence that mountains are generally the result of erosion of regionally uplifted crust, which is quite out of line with the mechanism envisaged by PT. Moreover, the evidence I have been able to gather suggests that these periods of regional uplift occur over quite short geological timescales (measured in 10’s Ma), whereas PT regards continents as having been impacted into one another at timescales measured over 100’s Ma. There also appears to be good grounds for supposing that the regional uplifts occur synchronously over very different regions of the Earth’s surface. For this to happen in the PT model would require some remarkable coincidences.  

Sedimentary evidence seems to provide a picture of crust rising and falling, sometimes above and sometimes below sea level, severely challenging the PT notion that continents have over many 100’s Ma had essentially unchanged plan forms, albeit they have broken away from their precursor super-continents, and been propelled into different spatial locations. Some of the important evidence I have been able to gather shows that the nature of sedimentary deposit also displays a cyclical form, with periods in which the forms of the sediments and their rates of deposition display globally synchronous characteristics. Moreover,  many cycles of sedimentary deposition, epeirogenic rise, continental erosion, epeirogenic subsidence, appear to have occurred over the time it is believed to have taken for the assembly, breakup and global journeys of the fragmented supercontinents envisaged in the PT hypothesis. PT seems to have very little useful to say as to why this might be the case.

Based upon my own reading of the current situation a new hypothesis has been presented that appears to provide an alternative explanation for what has and continues to help shape the Earth’s crust. It is based upon observations of how cyclical thermal processes are at work, albeit at greatly reduced spatial and temporal scales, in controlling both natural and man-made surface morphologies. There is growing evidence that long (10’s to 100’s ka) and very long (10’s to 100’s Ma) cyclic changes in thermal conditions on the Earth could be similarly playing an important role in global tectonics. Some of the earlier posts have argued that a great many features of the dynamic evolution of the lithosphere can be explained in terms of massive cyclic changes in thermal regime within the crust, including phase changes of lower lithospheric rock.

These earlier posts have suggested that the driving forces for these thermal changes are closely related to those responsible for glacial and interglacial episodes arising within the geologically recurring ice ages. Waxing and waning of thick ice sheets and the associated changes in sea levels are suggested to induce major changes in geothermal heat flux and its associated geothermal gradients. The consequential changes in lithospheric temperature are shown to provide models capable of explaining the cyclically developed massive horizontal tension and compression forces, and their associated kinematic and rheological consequences within the upper regions of the crust. But over longer geological periods these cyclic changes in surface ice and water and the thermal insulation they provide, have been argued to cause alternations in the thermal regimes and possibly associated phase changes at the lower lithosphere boundary, sufficient to induce substantial changes to the thickness of the Earth’s crust. Consequential isostatic and eustatic changes then help to explain the all too evident vertical motions of ocean and continental crust, including explanations of how ocean crust can be raised to form new continental crust and vice versa - issues which are left poorly accounted for by PT.

There appears to be some evidence in support of this new theory. The pulses of mountain building have been compared with paleoclimate temperature estimates (Post & Illis, 2009) and noted by Kalander et alia (2011) to coincide rather well with the geological evidence on the periods ice ages have occurred. Figure 11 reproduces Figure 2 but has had added the periods when major regional mountain forming episodes have been recorded. There does appear to be a remarkable coincidence between the ice age pulses and those of mountain building, including that of the Plieno-Pliestocene (Ollier, 2006b). And although the precise dating seem a little less clear, there does appear to be more than circumstantial evidence that the pulses in different forms and rates of sedimentation may also have strong links with the ice ages (Sloss, 1964; Wezel, 1992). Over shorter geological periods the seemingly synchronous and global development of river terraces (Bridgeland, 1988, 2010; Westaway, 2002, 2010) are strongly indicative of the work of the glacial – inter-glacial cycles being superimposed upon long term epeirogenic uplift.

All in all there does appear to be a compelling case that in contrast with the supposed monotonically evolving processes envisaged by PT, much of the dynamic of Earth lithosphere evolution has been one of cyclical processes. The preceding blogs have attempted to explain the close agreement between the phasing of the cycles of geological evolution and the forcing of climate caused by Earth’s interactions with the sun (whether directly or indirectly as controlled by variations in cosmic ray flux). They have also proposed a thermal-mechanical models for how this dynamic process might be working.     

 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.
  
   

                                

 

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.
  

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.