A Recent Challenge for Plate Tectonics

Over October 2017 a meeting of very distinguished geologists, all having strong associations with the development of plate tectonics (PT), was held at the Geological Society London. Its purpose was to celebrate 50 years of PT achievement¹. Not unexpectedly, this meeting spent most of its time in a form of congratulatory mode highlighting the considerable achievements that had been made over this 50 years - in many cases by the attendees in the room. However, one of the attendees was brave enough to raise what appears to be a very serious challenge for PT, and one which was not wildly out of line with the issues that this blog was addressing back in 2016.

The nature of this challenge was to ask the attendees at this meeting to provide plausible explanations for the largely unresolved observations of kilometre scale, cyclic, rises and falls of continental and oceanic crust². The meeting was reminded that these kilometre scale cycles of burial and exhumation exhibit synchronicity over wide spatial domains and often occur in regions where PT models suggest passive tectonic activity³. That such motions occur and so often result in major unconformities in the sedimentary sequences were the cornerstones of James Hutton’s awakening of the field of geology⁴ in the closing decades of the 18^th C. And yet, even with the undoubted advances in the field of geological science over the past 50 years, the noticeable silence on this challenge from delegates suggested there remains no satisfactory explanation for these fundamental processes.

Having come across this meeting report sometime in 2019 encouraged me to return to my own concerns relating to this very issue, with the result that a short paper was written and just happened to be ready for submission when UCL launched a new open environment publication venture. The submission was entitled “Phanerozoic Climate and Vertical Tectonic Cycles” and as it transpired turned out to be the very first paper to be submitted to this new publishing outlet. The next few blog posts will be concerned with providing a slightly amended version of this partially published paper - I say partly published due to the referees not being happy with aspects of the paper, so although it appears somewhere in the online files it was not actually put forward for final publication*. This paper took as an important test of the suggested mechanism for vertical tectonics, outlined in earlier posts, the evidence so explicitly revealed in the geological records of the Colorado Plateau. So the next few postings will first summarising the observational evidence from the Grand and Bryce Canyon areas that demonstrate so clearly the cyclic nature of the periods when this massive area of the N American craton was beneath the ocean having deep sediments buried and other periods when it was back above sea level and subject to erosive actions. This evidence will allow identification of the unconformities in the sedimentary sequences where there is considerable missing time in the sedimentary and paleontological records. It will be indicated how, although the exact timing of uplift and erosion associated with these major unconformities are difficult to assess, the age of sediments immediately above provide vital temporal markers for the onset of subsidence and/or associated sea level rise. By reconsidering the much studied sedimentary sequences of the Grand and Bryce Canyon areas the next few posts will attempt to show that the at least over the Phanerozoic eon (the past 540Ma) the initiation of new pulses of deposition seem to occur at times when earth climate is emerging from ice-house to hot-house** conditions. Furthermore, it will be indicated that the recorded periods in which global occurrences of epeirogeny have occurred appear to correlate closely with the end of hot-house periods and the onset of ice-house global climate conditions. Finally, the tentative thermo-geodynamic explanations for this apparent causal link between global climate and vertical tectonics, briefly covered in earlier posts, will be further elaborated. While it might reasonably be claimed that any correspondence between vertical tectonics and long term climate on the Colorado Plateau is hardly sufficient to claim a generic such relationship, it is worth keeping in mind, and evidence will be provided for this, that such kilometre scale cycles of burial and exhumation at this particular location of recognised^6,7 passive tectonic activity are well documented as exhibiting synchronicity over very wide spatial domains across N America. 

 * https://journals.uclpress.co.uk/ucloe/plugins/isolinear/article/1979/version/1/

** In earlier posts I used the term "ice-age" to indicate the very long periods (many 10's Ma) during which other climate drivers produce alternating periods of "glacials" (measured in 10's kA) when ice and permafrost are features of Earth's the mid- to high- latitudes and other periods "interglacials" (also measured in 10's kA) when there are ice sheets and permafrost in only very high latitudes. Because the term "ice-age" is commonly used to refer to "glacials" I will in the future try to avoid the confusion and use the term "ice-house" in reference to these very long term ice prevalent periods. And to preserve some symmetry in terminology, the very long periods when Earth is pretty much free of ice and permafrost (again lasting for many 10's Ma) I will adopt the commonly used label "hot-house". We are currently living in an interglacial within an ice-house period.