Comparative studies show that recent vertical movements of the earth’s crust are of much greater magnitude than the average value given for such movements in the geologic past. This can be explained by referring specifically to conditions prevailing in California, which indicate that the earth is presently in an orogenic cycle. In California this orogeny, here called the Pasadenian, started in the middle Pleistocene and its after effects can still be observed.
Additional examples of Pleistocene orogenies are cited with particular reference to Pacific border regions. Great thrusts, dipping away from oceanic margins under adjoining continental regions, have been recognized on the basis of deep-focus earthquake studies. Phenomena closely related to these thrusts and cited in vertical succession from bottom to top are: (a) deep-focus and intermediate-focus earthquakes, (b) the circle of fire, (c) the belt of negative gravity anomalies (Meinesz zone), and (d) the deep-sea trenches. We are here dealing with German type mountain building—or block faulting—of rather large proportions. On the other hand, mountain building of the true Alpine type was no longer active during the Pleistocene. The last orogenies of this character involving rather limited areas occurred before the beginning of the Pleistocene (“Walachian Phase”).
A better understanding of recent geotectonics is gained by a discussion of the concept of progressive consolidation of the earth’s crust. This process is expressed in the regionally progressive reduction in size of crustal portions subject to folding, i.e. orthogeosynclines, in their replacement by consolidated, “cratonic” basements, and last but not least in the development of geomagmatic phenomena. Retrogressive phases, called regenerations, may interrupt this orderly regional progression, but are on the whole compensated for relatively rapidly by renewed consolidation. The progressive consolidation of the earth’s crust with specific reference to America is demonstrated by a series of examples.
The present climax of consolidation of the earth’s crust is indicated by the evidence that true orthogeosynclines no longer seem to exist and that mountain building is at best of the German type which can produce only minor compressive effects in the earth’s crust. Thus present crustal conditions represent a late phase of or even a sequel to earlier geologic history characterized by mountain building of the Alpine type with its accompanying magmatic phenomena.
How will the earth’s crust respond to future expectable compressive stresses? In answer to this question it is pointed out that at least once before, viz., following the Algoman orogeny at the close of Huronian time, the crust had achieved complete or nearly complete consolidation when it was unable to accommodate any further compressions of the Alpine type, a condition which seems to exist again at the present time. This erstwhile complete consolidation was followed by a major regeneration (“Algonkischer Umbruch” in German terminology) which initiated a great new system of orthogeosynclines. This interval represents the most profound break in the geotectonic history of the earth. It separated the Protogean1 (“Protogäikum”) and its orogenic development which came to a close in the Algoman phase, from the Neogean1 (“Neogäikum”) which encompasses the remaining geologic time to the present. Just as the “Algonkischer Umbruch” followed crustal consolidation of a high degree at the end of the Protogean, a comparably large regenerative event may again be impending at this time. We may then be on the threshold of a third tectonic era which would include hundreds of millions of years to come.
