Fixism

one of the two schools of thought in tectonics, based on the assumptions that the continents are immobile, or fixed, on the earth’s surface and that vertical tectonic movements played the principal role in the development of the earth’s crust. Fixism was one of the leading schools of thought in geology until the mid-1960’s, when the mobilist assumptions became widely accepted (seeTECTONIC THEORIES). Fixism is based on the hypotheses that plates, cratons, anticlinoria, and other sources of the denudation of terrigenous material develop lineally; that abyssal fractures exist for a very long time; and that magmatism of the same type is exhibited for a long time in similar regions.

Fixists, such as V. V. Belousov and the American scientist H. A. Meyerhoff, deny the mobilist assumption that horizontal movements of the large plates of the lithosphere are possible and postulate only insignificant horizontal movements (by a few dozen kilometers) of relatively small regions of the crust along overthrusts and strike-slip faults. The insignificant horizontal movements are assumed to result from vertical movements. A component of the fixist conception is the assumption that the formation of the ocean basins was a result of the subsidence of the crust without substantial extension but with transformation of the continental crust into a thinner oceanic crust (seeBASIFICATION). This assumption is opposed to the mobilist assertion that the formation of the ocean basins is a consequence of the moving apart of the continents.

According to the fixist theories, the major differences in tectonic conditions at the earth’s surface are determined by differences in endogenic conditions, that is, in conditions, within the earth.