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.