Introduction: The basic premise is that continental crust can not subduct, because of its composition, because it is too light and 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 contractional 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. For every mile of average mountainous elevation over a region, a corresponding root of 5 miles typically exists below.

Major collisional orogens of the world:

• orogeny vs. orogen: event vs. the mountain belt.
• Caledonides (Norwegian, East Greenland, Svalbardian):
  • characterized by thick nappe stacks.
  • Ordovician/ Silurian peak.
  • Devonian extensional collapse.
• Appalachians/Hercynides: Alleghananian culmination from 320-275 Ma, collision with Africa.
• Urals: Permian closure.
• Alps: complex
• Himalayas:
  • started some 40-45 Ma ago, ongoing.
  • includes Tibetan plateau.
  • is this orogen typical?

Suture zones:

• fundamental asymmetry inherited from subduction process. Defines foreland vs. hinterland. Has there been an active-active continental margin collision?
• contact of passive margin rocks with exotic terrane or accretionary wedge material.
• marked by ophiolites (problem is can also have ophiolites from prior accretion tectonics, and not all suture zones have ophiolites). Most foreland suture zone?
• marked by melange against continental material in absence of ophiolite.

Processes of continental collision: 3 major processes shape the orogen.

• crustal thickening.
• indentation and escape tectonics.
• gravitational collapse - gravitational spreading.

Processes of continental thickening:

• mountain roots, isostasy and crustal thickening. Simple modeling of isostasy in Excel.
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• 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, 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.
• folding and thrusting:
  • basement vs. cover, two very different mechanical units that behave differently.
  • 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 differeent ways. In other words one level is detached from the other. The authochthonous 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 figure to the right one can see the fold hinge for one of the many large 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.
  • 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 shortening.
• pervasive flow or penetrative and semi-homogeneous strain:
  • in the deeper metamorphic realm.
• lithospheric underplating.
• question as to the exact geometry: delamination and insertion of crust (+or - lithosphere) between overriding crust and lithosphere?

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, 3cm/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 in the hinterland (more interior) portion of the orogen?

We will take 15 minutes in class to complete a) for one of the sections and then compare results. The other can be done at your leisure sometime during next week.

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.

• importance of wedges in geology, mechanics of wedges.
• models for indentation (handouts/overheads).
  • active research consortium focused on indentor and escape tectonics.
  • link to satellite image of Alps, as a micorcontinental indentor into Europe.
• what determines the indentor vs. the indentee?
• Himalayas as an example.

Process of gravitational collapse:

• 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.
• 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 rocks.
• late stage collapse.
• post-orogenic collapse: spectacular example are the Devonian basins along the Caledonide axis. Extensional tectonic denudation - Devonian conglomerates depositional on top of mantle rock!!!

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 tuite 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.
  • high sedimentation rates, terrestrial clastics dominant.
  • sediment sometimes known as molasse (Siwaliks in Himalayas).
  • may initiate as submarine turbidite trough (flysch).
  • 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 with Alps.
  • 
  • 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:
  • further description of example in Alps.
  • with foreland propagation a foreland basin can become a piggyback basin.
• intermontane basins:
  • basins within an orogen.
  • terrestrial and high altitude.
  • long term preservation potential?
  • origin? extensional, or structural pile-ups (e.g. duplexes).

Select references:

• Allen, P. A. & Homewood, P., 1986, Foreland Basins; Blackwell Scientific Publications, Oxford, 453 p.
• 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.