For many years the terms orogeny and mountain building have been used so loosely that they no longer convey definite meanings. Because of this lack of precision, hypotheses of diastrophism are often based on false premises.
Folding and thrusting, often referred to as tectogenesis or orogeny, is best displayed by stratified rocks. Some geologists believe that folds and thrusts are due to vertical movements accompanied by lateral spreading, that the strata are stretched an amount commensurate with the intensity of the folding, and that the opposite sides of the sedimentary packet do not move toward each other. Most geologists, however, agree that during folding and thrusting the strata are shortened and that the two opposite sides of the sedimentary packet move toward each other by an amount commensurate with the intensity of the folding. But it is not clear to what extent the entire crust in the deformed belt is shortened. In one extreme view, the two opposite ends of the deformed belt do not move toward each other, and the folds and thrusts result from gravity sliding. At the other extreme, the entire crust is believed to be shortened to the same extent as the sedimentary strata.
An excellent example of broad vertical movements (epeirogenesis) that has never been sufficiently emphasized is in eastern North America, involving the Appalachians, Coastal Plain, and Continental Shelf. During the Mesozoic and Cenozoic the Fall-Line surface was depressed as much as 20,000 feet beneath the Continental Shelf, whereas it was uplifted at least 8000 feet over the Appalachians. The northeastern two-thirds of the Appalachian-Ouachita belt has been going up since the Jurassic, whereas the southwestern third has been going down. Thus the present Appalachians are unrelated to the late Paleozoic folding. Certain metamorphic minerals—such as kyanite—or certain metamorphic assemblages—such as jadeite and quartz—form at confining pressures found only at depths of tens of miles, implying tremendous erosion wherever such minerals or assemblages are exposed.
Broad vertical movements, accompanied by high-angle faulting, are illustrated by the fault-block mountains of the Basin and Range Province and by such features as the Rhine graben and the Rift Valleys of Africa.
Although the nature of the displacement along the San Andreas fault has been known for more than 50 years, only in recent decades have other large strike-slip faults been recognized. But it is now the fad to assign all kinds of faults and even nonexistent faults to the strike-slip category. In recent years seismologists have emphasized the importance of strike-slip faults. The author suggests that in the Fairview Peak-Dixie Valley, Nevada, earthquakes there is evidence that strike-slip movements are invading an area previously characterized by Basin-Range structure. The geologic record indicates that large strike-slip faults have been distinctly subordinate to folding, broad vertical movements, and broad vertical movements accompanied by high-angle faulting.
Possible displacement of the crust or the entire earth relative to the axis of rotation has been emphasized in recent years. Continental drift, if it occurs, is one such type of movement. Slipping of the entire crust over the mantle is another. Much research has been accomplished in paleomagnetism in recent years, but the data are too scanty and conflicting to permit any definite conclusions.
Among the possible causes of diastrophism are (1) contraction of the earth, (2) convection currents, (3) formation of large pockets of magma, (4) sialic material leaking out of the mantle, (5) conversion of sial to mantle by change of low-pressure minerals to high-pressure minerals, or the reverse, and (6) serpentinization or deserpentinization of the upper part of the mantle.
Mountain building is merely one manifestation of vertical movements of the crust. In the past mountain building has been erroneously considered by many to be primarily the result of folding and thrusting. Certainly many modern ranges are the result of vertical uplift unrelated to folding. Many such uplifts are accompanied by high-angle faulting to produce fault-block mountains. Blocks caught between strike-slip faults may be squeezed upward if the blocks move toward each other. Mountainous uplifts have also resulted from folding and thrusting. Conversely, in some folded belts erosion appears to have kept pace with the rise of the folds so that no mountains developed.