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What does a fold and thrust belt bell tell us about what occurred during an Orogeny?
Question Date: 2012-02-13
Answer 1:

Mainly a fold and thrust belt bell tell us that there was net compression across the plate discontinuity, and that the rocks making up the top plate were of the properties to allow this type of deformation to occur.

Answer 2:

Fold and thrust belts can tell us a lot about the amount of shortening, the direction of convergence, and something about the timing of deformation during Orogenesis. We can quantify the amount of shortening in a Progeny by study fold and thrust belts. We basically unfold all of the folds and get rid of all of the faults to restore a cross section of the belt to its pre-Progeny configuration. It is important to keep in mind that not all parts of a convergent margin are undergoing net shortening (forearc and backarc regions usually extend).

We can get information about the direction of convergence from the orientation of the Oregenic belt. For example, the generally north-south trend of the Appalachians tells us that there was east-west convergence during the progenies that formed those mountains. We also study the orientation and sense of slip on faults as well as the geometry of asymmetric folds to tell in which direction things were moving (e.g., the direction of subduction).

We can also tell something about the timing of deformation if we can constrain the age of the rocks that are being deformed. That is, we can obtain a maximum age of deformation the deformation has to be younger than the deformed rocks. This works particularly well in some deformed main-arc and forearc sedimentary/volcanic stratigraphic sequences. Active volcanoes are almost ubiquitous along convergent margins where subduction is occurring and volcanic/ volcaniclastic rocks are very easy to date using Ar/Ar or U-Pb geochronology.

Answer 3:

A fold-and-thrust belt happens when two plates crash into each-other and buckling happens in the superficial and sometimes not-so-superficial crust in the zone of impact. This effect is similar to what happens if you take a layer-cake and squeeze it from opposite ends; you will see the layers buckling and, when the strain you impose becomes great enough, they will break and slide past each-other. Generally speaking, what it means is that the mountain range you're looking at occurred as a result of some form of plate collision, which is true of most mountain ranges (even volcanic mountain ranges like the Cascades or the Andes involve a lot of fold-and-thrust activity that leads to non-volcanic ridges and peaks within the range as well as volcanoes).

Answer 4:

Fold-and-thrust belts are evidence of what we call 'thin-skinned' orogeny -- where most of the mountain-building is the result of folding and faulting of the uppermost rock layers (e.g. the sedimentary rocks overlying the continental basement rocks). You can think of it like taking a pizza and pushing all the cheese and toppings together without affecting the crust beneath. The Sevier orogeny of the western U.S. is a classic example of thin-skinned fold-and-thrust belt formation. Fold-and-thrust belts are now forming in front of the Andean and Himalayan mountains as well.

Alternatively, thick-skinned orogenies involve uplift of basement rocks (usually granite and gneiss) along deep, steeply-dipping fault zones (often the result of reactivation of pre-existing crustal-scale weaknesses). The Wind River range of Wyoming and the Uinta Mountains of Utah are both great examples of thick-skinned, basement-cored orogens that formed during the Laramide orogeny, NOT by a fold-and-thrust belt.

However, both thin- and thick-skinned mountain building can take place simultaneously. It is possible that the basement-cored uplifts are more related to increased crustal buoyancy resulting from flat-slab subduction, whereas fold-and-thrust belt formation is a more common result of compressional deformation of thick sequences of sedimentary rock above the basement.

Hope that helps,

Answer 5:

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