DEFINITION OF REGIONAL DEFORMATION OF ROCKSa

Deformation of rocks occurs at a variety of scales, from the

atomic to the scale of continents and entire tectonic plates.

Deformation at the continental to plate-scale produces some

distinctive regional structures. Cratons are large stable blocks

of ancient rocks that have been stable for a long time (since

2.5 billion years ago). Cratons form the cores of many continents

and represent continental crust that was formed in the

Archean Era. Most are characterized by thick continental

roots made of cold mantle rocks, by a lack of earthquakes,

and by low heat flow.

Orogens, or orogenic belts, are elongate regions that

represent eroded mountain ranges, and they typically form

belts around older cratons. They are characterized by abundant

folds and faults and typically show shortening and repetition

of the rock units by 20–80 percent. Young orogens

are mountainous—for instance, the Rocky Mountains have

many high peaks, and the slightly older Appalachians have

lower peaks.

Continental shields are places where ancient cratons

and mountain belts are exposed at the surface, whereas

continental platforms are places where younger, generally

flat-lying sedimentary rocks overlie the older shield. Many

orogens contain large portions of crust that have been

added to the edges of the continental shield through a

mountain building process related to plate tectonics.

Mountain belts may be subdivided into three basic types:

fold and thrust belts, volcanic mountain chains, and faultblock

ranges.

Fold and Thrust Belts

Fold and thrust mountain chains are contractional features,

formed when two tectonic plates collide, forming great

thrust faults and folding metamorphic rocks and volcanic

rocks. By examining and mapping the structure in the belts

we can reconstruct their history and essentially pull them

back apart, in the reverse of the sequence in which they

formed. By reconstructing the history of mountain belts in

this way, we find that many of the rocks in the belts were

deposited on the bottom of the ocean, or on the ocean margin

deltas, and continental shelves, slopes, and rises. When

the two plates collide, many of the sediments get scraped

off and deformed, forming the mountain belts, thus fold

and thrust mountain belts mark places where oceans have

closed.

The Appalachians of eastern North America represent a

fold and thrust mountain range. They show a detachment

surface, or decollement, folds, and thrust faults. The sedimentary

rocks in the mountain belt are like those now off the

coast, so the Appalachians are interpreted to represent a

place where an old ocean has closed.

Volcanic Mountain Ranges

Volcanic mountain ranges represent thick segments of crust

that formed by addition of thick piles of volcanic rocks, generally

above a subduction zone. Examples of volcanic mountain

chains include the Aleutians of Alaska, the Fossa Magna

of Japan (including Mount Fuji), and the Cascades of western

United States (including Mount Saint Helens). These mountain

belts are not formed primarily by deformation but by

volcanism associated with subduction and plate tectonics.

However, many do have folds and faults, showing that there

is overlap between fold and thrust types of mountain chains

and volcanic ranges.

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