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.