The word “stereosphere,” in contrast to “asthenosphere,” is proposed for the relatively strong outer shell of the earth, to replace “crust” which is now restricted to a chemical-petrologic meaning.
Orogenic belts arise from downbending of the stereosphere, possibly into constrictions of the contracting shell below it. The Gulf Coast geosyncline and the folded geosynclinal sediments of Cuba show that such sinking requires no “compensating” geanticline.
In the Western Alps and Southern Appalachians, considered typical, three major structure zones exist between foreland and hinterland: (1) an outer belt of shallow deformation, (2) a marginal belt of “peel thrusts”, and (3) an active belt of recumbent crustal folds of metamorphic and plutonic rocks.
Specific examples are given to justify the hypothesis of superficial deformation in the outer belt. The nature of the “peel thrusts” which distinguish the marginal belt is analyzed. The properties of the recumbent nappe folds of the Pennine Alps are sketched; they mark the “active zone” where the energy was liberated that produced the orogenic movement and tectonics. A reason is indicated for the conflict between the geophysicist’s and geologist’s estimates of surface shortening in orogeny.
Marginal and active zones are believed present in all orogenic belts. They vary because of differences in (1) intensity of deformation, (2) stage of development reached, (3) nature of magmatic processes (dominantly plutonic or volcanic).
Fracture patterns produced independently in the stereosphere interfere characteristically with orogenic belts. Examples are the great fracture belts that pass from the Eastern Pacific ocean floor into the Western United States, and others in Europe and the Mediterranean.
Everything points to basic differences in dynamics and magmatic activity between the orogenic belts and the nonorogenic regions.