CONTROVERSY ABOUT MOUNTAIN CHAINS FORMATION
Most mountain chains have roots built up of folded rock layers, a process named orogeny. Nowadays, the tendency is to believe that Plate Tectonics Theory (PTT) can explain all aspects of mountains formation. As you will see, orogeny remains a mystery.
To expose the controversy, we will use social rhetoric:
A Ph.D. student is asked to publish some results of his field works on tectonics. His wise supervisors ask him to read landmarks papers on 9 difficulties related to orogeny:
How the critical taper/subduction theory has serious shortfalls.
Long overthrust (thin skin tectonics) nappes cannot be moved by an applied force proposed by the taper/subduction theory.
Why over thrusting nappes never ran over an eroded surface?
Why the first stage of folding and over thrusting nappes is not affected by vertical tectonics alike the last orogenic stage?
The world orogeneses were all eroded into “peneplains”. Surprisingly, these flat surfaces were all synchronized to be lifted in late Cenozoic, regardless of their folding age. Then, these plateaus were eroded into modern mountains.
Why many of these peneplains are depleted of differential erosion?
Why, in many cases, there is a shortcoming of sediment deposit to account for the presume erosion?
In some places like in the Alpes, the theoretical reconstruction of the eroded volume indicates the preexisting of a 100 km high mega fold. How can plates collision build up slowly such a mega fold if erosion and gravity flow rates should defuse quickly this tilting movement?
Some minerals like coesite crystallize only at 100 km deep in the crust. How they ended up at the surface when the orogeny’s geometry doesn’t give any clue for such a mechanism?
This exercise will request this student to read many papers. He can also read The Origin of Mountains (Cliff Ollier, 2000) a book that summarizes some of these issues. This student will realize how recent PTT publications ignored these questions. Give the same challenge to any geo-students on earth, they will all come out of darkness learning the shortfalls of PTT.
■ Equally, imagine a veteran expert submitting a paper on these issues to a major geoscience journal. There is a high probability that the peer review editor would refuse this paper on the base that recent PTT publications already solved these controversies. Ollier’s monography was published in 2000 and so far, I haven’t seen any reply. These controversies are still pending.
■ Let’s look at the following paradox. Rock layers at great scale are weak. A large thrust sheet (say 100 km x 100 km x 300 m thick) has no internal force compare to the requested displacement forces. The internal resistance of the rock is made up of cohesive forces between minerals whose crystals are bounded by molecular forces. An applied force would never move such a plate but just crush it into small blocks. At a smaller scale, it is like moving a custard nappe on a table, and better, multi-colored custard layers. You can add dish soap as a lubricant. You will surely create a bulge of mixt custard layers. In the phenomenon of thick skin tectonic we name this bulge taper, wedge, accretionary prisms, backstop.
This bulge might even look like the tapered wedge create by a sand box experiment. BUT, never would you mount custard nappes one above the other on a long span like the orogeneses overthrust nappes. To be more realistic with your kitchen experiment, you can also slide a board topped with custard under another one to simulate a subduction/collision. So, how nature was able to do what we call thin skin tectonics? The answer is dramatic because the orogeny nappes were behaving like a body force. It was put in movement first. The deformation happens when the nappe decelerates and curb. Just tip the kitchen table until the custard nappes start to slide. There is no lateral applied force into play. Gravity applies its force on each custard particles. In orogeneses, the front toe curbs first, so deformed first, followed by a series of bumping coming from behind.
In his structural geology course at MIT, Burrell Clark Burchfiel says:
“The backstop in the sandbox experiment is probably the most unsatisfying part of the whole setup. What, in nature, corresponds to a vertical, unyielding wall? Early papers on critically tapered wedges had cartoons showing bulldozers pushing wedges in front of them, but this is surely just trading one suspect metaphor for another.
One thing to realize is that the critical taper models and sandbox experiments are meant to simulate or describe fold and thrust belts or accretionary prisms. That is, they are models of a small part of the anatomy of an entire mountain range, in particular, the exterior parts. The backstop then, is just the interior (hinterland) of the mountain range, and all the model requires is that this part of the mountain range consists of thicker crust and higher elevations. How that part of the range became thickened and whether sandbox experiments shed any light into this is beside the point.”
Our readers might not get this whole point. But here a statement reminding us how poorly orogeneses are understood. Wikipedia: “Professor Burchfiel joined the MIT faculty in 1976. Over his career he has written close to 200 papers and mentored more than 50 graduate students.” Here, I am not using the argument from authority. Attention to one authority help to evaluate how trustable is the majority consensus. For many geoscientists, you do not question the PTT. It is simple truth because it tells you a story in a nice package. So, why giving attention to PTT detractors when they ring the alarm bell even if they are reputable geoscientists.