A phenomenon which is becoming increasingly apparent in areas around the eastern Mediterranean is the movement of huge volumes of highly incompetent rock for great distances.
In the Greek Peloponnesos, thousands of square miles are covered by an allochthonous mass of Mesozoic and Lower Tertiary pelagic sedimentary rocks. These rocks rest on strata of the same age, but in a neritic facies. For this relationship to exist, the allochthon must have moved a distance in excess of 128 km, and judging from its internally contorted and even chaotic nature, its movement must have been due to its own mass being acted upon by the force of gravity. The most feasible way to explain the actual mechanics of the sliding is to say that the allochthon was propelled westward through folding in the underlying autochthon, folding so timed that a slope was successively created by the progressive development of fold limbs.
A large and important problem remains concerning the original site of deposition of the sediments which composed the allochthon. These sediments must have covered a considerable area. To explore this problem, two differing orogenic concepts are examined, one based primarily on vertical forces (a fixed crust hypothesis), and the other based primarily on lateral forces (a mobile crust hypothesis), to see which better explains the observed geology. Any hypothesis based on a fixed crust would mean that a large area of tectonic denudation must be situated somewhere to the west of the Peloponnesos. After a review of the geology of surrounding areas, however, it appears that the only conceivable area large enough to provide a source for the allochthon is 500 to 600 km away. With a hypothesis based on a mobile crust, especially one based on recent concepts of sea-floor spreading, this “space” problem presents no difficulty. The basin in which the sediments of the allochthon were deposited can be conveniently reabsorbed within the mantle. Vet this basin still must have been raised to a sufficient elevation to have enabled its contents to begin sliding westward.
Thus problems remain in explaining the geology of the Peloponnesos with either fixed or mobile crust theories of orogeny, at least as presently visualized. One conclusion that does seem warranted, however, is that long-distance gravity sliding of this type is not the result of simple vertical uplift. Intricate and complicated crustal movements are involved whose timing is controlled within narrow limits.