ROLE OF GRAVITY IN OROGENESIS

In elastico-viscous materials, the concept of “viscosity” makes model experiments possible, since time appears in its definition as an independent dimension when gravity is the only force acting on the material in both nature and the laboratory. Stitching wax (“shoemaker's pitch”) is discussed as a useful material for tectonic experiments. The limit of stress below which solid flow does not take place is very low in this material, permitting slopes of only a few degrees to be maintained. Material raised to greater relative elevations flattens under its own weight, as does ice in an ice cap. Laboratory experiments using stitching wax are described which show that such flattening produces superficial folding in stratified layers of the same material if zones of lower viscosity, such as petrolatum, are present among the layers. Good examples of disharmonious folding are figured.

In a second series of experiments, bars of layered stitching wax (without weak zones) were subjected to slow compression. When one part was kept a little warmer than the rest, a recumbent anticline was formed at the interface; the cooler “foreland” pushed under the uparching “welt,” and it in turn pushed the former down, flattening it. In this way, the features which in alpine mountains have been thought to call for either an actively overriding master thrust sheet (“traineau écraseur”) or an active downsucking (“engulfment”) were produced simultaneously. This leads to the conjecture that orogenic belts, at least of the alpine type, arise because, at the time of their formation, they are weaker than the normal crust. This thought is pursued further in the last chapter. The structural details of the recumbent folds produced in the experiments are instructive.

In the course of such close folding of the basement surface, the weaker materials of sedimentary mantle must be forced out onto the slope toward the foreland. The structural consequences are discussed hypothetically. The stronger of such detached sedimentary units pile up as “peel thrusts.” The weaker ones, such as sediments of flysch type and serpentine, must flatten out greatly to form “thrust flow sheets.” The weakest may continue to spread still farther as the foreland is warped (“tilt flow”). “Chaotic” structures are briefly mentioned. They seem to be special cases involving gaseous volcanic activity.

In the last chapter, geophysical observations are listed which suggest that the belts of deep-focus earthquakes, genetically tied to orogenic belts, are fracture zones produced by shrinkage in the “strictosphere,” which lies between the outer “stereosphere” and the deeper mantle. Volatiles carrying excess heat are thought to rise along these fracture zones into the Earth's crust, warming and weakening it, and thus localizing compression. To be worthy of consideration, this hypothesis like any other must automatically explain also the peculiarities of the pattern of orogenic belts. An experiment is described designed to show what pattern of cracks may form in a thin shell shrinking between solid boundaries. Significant similarities with the pattern of orogenic belts were found.