Continent-continent collisional tectonics: final closure
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 ,
- mark the contact between the two plates, typically where see your first exotic terrane or accretionary wedge material parked next to passive margin sediments. Typically a thrust fault contact.
- fundamental asymmetry inherited from subduction process. Defines foreland vs. hinterland. Has there been an active-active continental margin collision, and how will that differ?
- marked by ophiolites (problem is can
also have ophiolites from prior accretion tectonics, and not
all suture zones have ophiolites). Can conceive of multiple suture zones, with the most foreland suture zone as the most important?
- marked by melange against continental material in
absence of ophiolite.
Processes of continental collision: 3 major
processes shape the orogen with time. Each is describe in more detail below.
- crustal thickening.
- indentation and escape tectonics.
- gravitational collapse - gravitational spreading.
Major collisional orogens of the world
Reminder of some terminology: orogeny vs.
orogen = event vs. the mountain belt.
(Norwegian, East Greenland, Svalbardian):
- characterized by thick nappe stacks.
- Ordovician/ Silurian peak of crustal thickening.
- Devonian extensional collapse (more on down below).
- In the figure
to the right one can see the fold hinge for one of the many medium sized
recumbent fold nappes in the Caledonide orogen. This is from
a mountain just a little south of Tromsø. The glaciers
have carved the fjords such as seen in the background and this
makes for excellent 3-D exposure of the structures.The cliff
exposure with the fold hinge is probably several hundred meters
tall. Fold nappes can be much larger than this.
- Appalachians/Hercynides: Alleghananian
culmination from 320-275 Ma, collision with Africa: more info at link.
through Triassic-Jurassic closure, earlier in the southern part and later in the northern Novoya Zemyla northern portion.
- ophiolites and other exotic terranes identified, with Devonian to Carboniferous periods of terrane accretion.
- axial dextral strike-slip tectonics (indicating oblique convergence) has been identified for the southern portion. .
- Mesozoic basin development on both sides, with significant hydrocarbon accumulations. It is argued that in the Triassic deltas sourced in the Urals prograded across the entire Barents Shelf and into the Svalbard area.
- Image to right is of Novoya Zemyla with the Barents Sea to the left and the Kara Sea to the right (both basins with thick Mesozoic sections). Note the significant curvature that links this part with the southern part of the Urals. Does this curvature reflect orogenic deformation or the original shape of the continental margin? A close look shows the orogenic grain topographically expressed, with strong curvature, that is variable in its degree along strike (i.e. localized sharper bends). As you might guess from the image and its location, this is a remote area, and more difficult to map and so less is known about the detailed geology. Image source: NASA Visible Earth - http://visibleearth.nasa.gov/view.php?id=67852 .
- complex, bivergent (thrusting outward from both margins).
- Jura Mountains - foreland thin-skinned fold and thrust belt
- strong curvature, consistent with indentor tectonics.
- main phase - Tertiary in age.
- GeodynAlps: website focused on tectonics of the Alps - http://www.geodynalps.org/ .
- arguable that this is still terrane accretion tectonics, with an Italian continental terrane, and Africa and Europe yet to have closed the intervening Mediterranean oceanic basin.
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 .
- started some 40-45 Ma ago, ongoing.
- includes Tibetan plateau.
- is this orogen typical?
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.
- Above geophysical image (seismic) depicts both major thrust faults and the root beneath the San Gabriel elements. This crustal thickening is not due to continent-continent collision. Instead it reflects localized crustal contraction associated with the San Andreas plate boundary. Crustal thickening and associated crustal roots can be developed in several tectonic settings. Image from USGS Larse Fact Sheet (as found at http://www.scec.org/news/00news/spot001018.html).
- USGS model of crustal thicknesses from http://earthquake.usgs.gov/research/structure/crust/ .
This is a low resolution model, and in reality much more detailed crustal structure exists. Note that some older orogens, such as the Urals, still have both a topographic expression and a crustal root.
The Himalayas stand out as an active orogen with a thick root.
However, the Caledonides stand out as not preserving any root.
- folding and thrusting:
- basement vs. cover, two very different mechanical
units that behave differently.
- thin-skinned style in cover strata, characterized by extensive detachments.
- thrust stacks versus fold nappes, different styles.
- Cross section showing thin-skinned character of thrust stack associated with the Appalachian mountains. The blue layer has been repeated several times by cutting and stacking the pieces. In a thin-skinned style sub-horizontal detachments dominate, and the rocks at different levels are deforming in very different ways. In other words one level is detached from the other. The autochthonous basement rocks referred to here are basically undeformed, while a stack of thrust sheets exists above. Interestingly, here the rocks involved are crystalline basement rocks, whereas in many other localities stratified platform cover rocks tend to show this structural style. From USGS site http://3dparks.wr.usgs.gov/nyc/images/fig37.jpg .
- In the image below the ridges are the result of a layer that is more difficult to erode, but is also folded. In arid area like this it is fairly easy to see the structures, and to create an interpretative map from the image. The folds here are some how associated with the closure of the Mediterranean, as Africa moves toward Europe. Image source is from NASA Visible Earth - http://visibleearth.nasa.gov/view_rec.php?id=2470 , where more detailed information can be found.
- cross section balancing, and estimates of
- pervasive flow or penetrative and semi-homogeneous
- channel flow in the lower crust between upper brittle crust and lower stronger mantle rocks.
- in the deeper metamorphic realm: upper amphibolite facies, migmatites.
- lithospheric underplating.
- question as to the exact geometry: delamination
and insertion of crust (+or - lithosphere) between overriding
crust and lithosphere?
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
- mechanics of wedges in geology important.
- models for indentation (handouts/overheads).
- what determines the indentor vs. the indentee?
as an example.
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
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.
- processes of weakening the root:
- heating of the thickened crust (reestablishment
of a perturbed geothermal gradient), deformation heating (??).
- partial melting of thickened crust: would
provide a dramatic weakening. Some geophysical evidence that
have zones of partial melt beneath portions of the Himalayas.
- lithospheric delamination.
- introduction of fluids (little exploration
of this aspect). Do see fluids that are expulsed from the front
of the orogen, and these should dewater and strengthen these
- late stage collapse.
- post-orogenic collapse: spectacular example
are the Devonian basins along the Caledonide axis. Extensional
tectonic denudation - Devonian conglomerates depositionally on
top of mantle rock!!! Development of metamorphic core complexes.
- At what scale can gravitational collapse occur. It has been used for smaller scale topographic welts, including for the Absaroka volcanic pile and development of the Heart Mountain detachment in Wyoming. Another example is shown below and was taken from: http://pubs.usgs.gov/sir/2004/5206/SIR5206-508.html#figurecaption67372480 .
In this case extensive fractures in the crystalline basement rocks may have reduced the strength of topographic feature so that it spread, with deformation supported by cataclastic flow.
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.
- foreland basins:
- basins that lie between the front of the
mountain chain and the adjacent craton.
- typically had foreland migrating depositional
axes with time as the thrust belt grows.
- high sedimentation rates, terrestrial clastics
- sediment sometimes known as molasse (Siwaliks
- may initiate as submarine turbidite trough
- basin accommodation space created by tectonic
loading and depression of the crust. Shape is a function of the
flexural rigidity of the lithosphere.
- forebulge as
a secondary source, as cause of erosion and disconformity production.
The forebulge also migrates with time.
- classic examples are Molasse basins associated
- Schematic diagram of a foreland basin. Image source USGS site: http://pubs.usgs.gov/pp/pp1624/ .
Consider the likely evolution of this static cross section image, and it becomes clear that the foreland basin sediments can easily become involved in the thrusting, and incorporated into the wedge. Also, all sorts of aspects of the mountain belt/welt history get preserved in the sediments of the foreland basin.
- piggy-back basins:
- intermontane basins:
- basins within an orogen.
- typically terrestrial and high altitude.
- long term preservation potential?
- origin? Two possibilities: extensional, or in between structural pile-ups
- Allen, P. A. & Homewood, P., 1986, Foreland
Basins; Blackwell Scientific Publications, Oxford, 453 p.
- Brown, D. et al., (2008, August) Mountain building processes during continent–continent collision in the Uralides. Earth-Science Reviews, 89 (3-4), 177–195.
- Ducea, M. N., 2016, Understanding continental subdcution: A work in progress (Research Focus); Geology, 44, p. 239-240.
- Hsü, K. J., 1983, Mountain Building
Processes; Academic Press, New York, 263 p.
- Maher, H. D. Jr., 1994, The Role of Extension
in Mountain-belt Life Cycles: Journal of Geological Education;
v. 42, p. 212-219.
- Miyashiro, A., Aki, K., Sengor, C., 1982,
Orogeny, John Wiley & Sons, 242 p.
- Rast., N. & Delaney, F. M., (eds.), 1983,
Profiles of Orogenic Belts; Geodynamic Series Volume 10, AGU
& GSA, 310 p.
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.