Deep-Sea Pelagic Sediments and Ophiolite Assemblaces
The earth's great mountain belts contain sediments and volcanic complexes of kinds that are well known in the stable cratonic regions and of kinds that are exposed only (or almost only) in the mountain belts. The latter kinds include ophiolites, great sequences of pillow basalts and (or) andesites, bedded, generally radio- larian cherts, uniform shale sequences containing pelagic biotas and representing long time spans, pelagic carbonate sequences, great bodies of turbidite flysch, olistostromes, and metamorphic rocks that are not dealt with here. The sediments are commonly red colored.
For a century, explanations for these rocks have been sought along two different lines (paradigms) : In one they are viewed as the products of mobile belts and orogeny--specifically, as the contents of a special generative trough or complex of troughs, the eugeosyncline of Hans Stille and of Marshall Kay. In the other, which we may term the oceanic model and which was stated most clearly by R. S. Dietz and J. C Holden, they are viewed as the normal rocks of the oceans and oceanic margins brought to the mobile belts by plate subduction and incorporated into the continental margins by orogeny.
Progressive exploration of the oceans, first by dredging and coring and more lately by drilling, has shown that most of these rocks indeed correspond closely to the rocks of the great ocean floors. This seems to be particularly true when sediments of the same ages are compared.
Only the great andesite sequences and olistostromes remain as rock types linked to mobile belts or convergent plate margins. A good case can be made for a distant oceanic derivation of some ophiolite-sediment sequences in mountains (e.g., Elba), and therewith the eugeosynclinal model seems to have outlived its usefulness.
At the same time, a model substituting the main ocean basins for the eugeosyncline is even less realistic. Through the work of D. E. Karig and others, confirmed by legs 6 and 7 of the Deep Sea Drilling Project, it has become essentially certain that juvenile oceanic or semioceanic crust overlain by oceanic type sediment can grow in mobile belts to produce the interarc and marginal or small-ocean basins. Yet other sites of crustal growth are likely (e.g., Canary Islands). The ophiolites and sediments of such sites have a better chance of unmetamorphosed incorporation into mountains than does the main sea floor. The ophiolite complexes of Cyprus (Mamonia, Troodos) and of Kandahar, Afghanistan, should be viewed in this light.
Furthermore, the odd sediments of the mountain belts commonly form parts of otherwise normal or mio- geosynclinal successions (e.g., Martinsburg flysch, Jurassic sediments of Northern Limestone Alps), and the great andesite sequences occur either on oceanic crust (Antilles) or on sial (Andes). Olistostromes are most likely trench generated.
Thus, in principle, the odd rocks of the mountain belts represent a wide range of generative settings, including crust-generating ocean ridges and their flanks, abyssal plains, trenches, volcanic arcs, tectonic welts, interarc basins, and the downwarped edges of continents. Tectonic and paleogeographic reconstructions will remain extremely uncertain until we have learned to recognize these constituents more clearly.