Continent-continent collisional tectonics: final closure

Content:


Introduction: The basic premise is that continental crust can not subduct, because of its composition and lower density and because it is to thick. So when all the oceanic crust between two continents is recycled, subducted, the two impinging continents become involved in a different type of convergent plate boundary. Instead the continental crust thickens by a variety of mechanisms, creating major mountain belts. The mountains are just the erosional signature of a crustal welt that extends deeper to form a crustal root. For every mile of average mountainous elevation over a region, a corresponding root of 5 miles typically exists below. This is influenced by isostasy and the relative density of continental crust versus mantle density.

Recent work on what are known as rocks with ultra-high pressure (UHP) metamorphic mineral assemblages does question the basic assumption that continental lithosphere can't subduct (e.g. Ducea 2016).

In this very simplified geologic map from the USGS one can pick out many of the collisional orogens discussed below including the Appalachians, Caledonides, Himalayas, and Urals. Image source: https://commons.wikimedia.org/wiki/File:World_geologic_provinces.jpg ,

Suture zones:

Processes of continental collision: 3 major processes shape the orogen with time. Each is describe in more detail below.


Major collisional orogens of the world

Reminder of some terminology: orogeny vs. orogen = event vs. the mountain belt.

NASA Visible Earth Image of the Alps, displaying the topography of the Alpine orogeny. Note how the mountain belt is curved (forming an orocline) especially at its west end. Also note the distinctive flattish area to the south, which is in part an active sedimentary basin. Image source: http://visibleearth.nasa.gov/view.php?id=62563 .

Himalayas:


Processes of continental thickening

Mountains have roots. The principle of isostasy is that the the crust is 'floating', and orogenic scale topography is supported by a root below, which requires crustal thickening. Simple modeling of isostasy in Excel.


Exercise: Calculating shortening in a mountain belt. The cross section(s) provided are from the Valley and Ridge foreland fold-thrust belt of the Appalachians. This represents the leading edge of the Appalachian orogen on the North American craton. With them we can explore a bit the nature of crustal shortening.

a) We will start with a line length estimate of the shortening. Choose a distinctive bed or contact in the cross section that you can trace throughout. Likely some of it will be missing in the air (removed by erosion) and you will need to complete (restore) the geometry in the air. Then take a string and using the scale devised measure the total length of that bed or contact for the section. Compare it to the length of the section. The difference is the amount of shortening.

b) Assume the convergence rate that produced this shortening was the same as for the Himalayas, 3 cm/yr. How much time would be needed in that case to produce the above shortening?

c) Is the amount of shortening likely increase or decrease or stay the same if you include more of the hinterland (more interior) portion of the orogen?

This is an exposure of Triassic strata in Svalbard folded during Tertiary deformation. Exposures such as this allow for construction of detailed cross sections and the on estimation of line length shortening. Values for the Spitsbergen Tertiary orogen are typically in the 25% range.


Processes of indentation and escape tectonics

Image of major faults active in Afghanistan region. Note how the strike-slip faults define a wedge. An interesting question is as to how the wedge behaves with time. Extracted from Ruleman, C.A., Crone, A.J., Machette, M.N., Haller, K.M., and Rukstales, K.S., 2007, Map and database of probable and possible Quaternary faults in Afghanistan: U.S. Geological Survey Open-File Report 2007-1103, 39 p., 1 plate pubs.usgs.gov/of/2007/1103/downloads/pdf/of07-1103_508.pdf.

YouTube computer animation of movements on East side of Himalayas, depicting activation of escape tectonics - https://www.youtube.com/watch?v=DkC2jxN6244 .

Interesting YouTube animation of computer model for drainage and topography development in associated with a rigid indentor - https://www.youtube.com/watch?v=eqJju90SIxk .


Process of gravitational collapse

The basic idea is that mountain welt collapses underneath its own weight, spreading out and creating extension in the upper part of the welt. Hedge-hog tectonics is an informal name for how a wedge bounded by a low-angle normal fault on the top and an even shallower coeval thrust fault at its base, can "escape" towards the foreland.


The sedimentologic response - basins associated with continental collisions

Overall mountain belts are sites of net erosion, but local exceptions can exist within a mountain, and adjacent areas can often sag due to the crustal loading associated with thrust emplacement and crustal thickening. The results are a suite of sedimentary basins.


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Course materials for Plate Tectonics, GEOL 3700, University of Nebraska at Omaha. Instructor: H. D. Maher Jr., copyright. This material may be used for non-profit educational purposes with appropriate attribution of authorship. Otherwise please contact author.