Fully developed ophiolite sequences are widespread in the Alpine System from the Betics to the Oman. Ophiolite occurrences vary from giant nappes, through slabs and blocks in melanges, to small clasts in flysell and wildflysch, Contact relations invariably involve tectonic juxtaposition of ultramafics against the country rocks. Where ophiolite sheets occur as high level nappes obducted onto shelf and (or) exogeosynclinal terranes (e.g., Oman), full, fresh ophiolite sequences are well preserved. Basal contact relationships of such sheets may involve a complex combination of mylonitic ultramafics, ophiolitic wildflysch, transported blue- schists, garnet amphibolites, greenschists, and rodingites. Such rocks may represent a variety of tectonic- metamorphic environments from subduction zones to wasting melanges developed and telescoped into juxtaposition during progressive obduction. High-level ophiolite nappes involve an obduction problem that is poorly understood. Such sheets may originate from Mediterranean Ridge-like welts, or from ophiolitic base- merits of arc-trench gaps that are driven from the advancing jaws of continental blocks, or by the development of subduction zones that are adjacent to passive continental margins and that have a continent-facing polarity. All such nappes appear to involve some gravity sliding, at least during the final stages of obduction. Some ophiolite assemblages (e.g., those of the Platta Nappe) are intensely shredded, deformed, and metamorphosed in narrow flat-lying zones between major crystalline nappe sequences. Such assemblages, whose emplacement postdates blueschist metamorphism, were probably driven from oceanic realms by the collision of continental blocks and may root, as do obducted high-level sheets, at considerable distances from their present outcrops. A third type of ophiolite occurrence is that of disjunct slabs in steeply dipping melange blueschist zones (e.g., Zagros Crush Zone) ; these appear to represent collisional sutures that did not involve extensive nappe development.
Two particular problems relate to Alpine ophiolites. First, the time interval between origin and emplacement is frequently very short. For example, in the Zagros Ocean of Asia Minor, ophiolite generation and emplacement appear to have occurred during one stage of the Late Cretaceous, suggesting the development of a convergent plate boundary very close to, and shortly after, an accreting plate boundary. Second, there is an almost complete absence of pre-Late Triassic ophiolites that could represent portions of the pre-Alpine Tethys. Alpine ophiolites are dominantly Mesozoic and represent sea-floor spreading along accreting plate boundaries within the Tethys in spite of the gross convergence of Africa and Europe. The likeliest candidate for a remnant of the early Tethys is the zone defined by the Black Sea, Colkhide Depression, and south Caspian area, on both sides of which active volcanic arcs faced an oceanic realm for most of the Mesozoic and JTertiary. Alpine ophiolites mainly represent smaller, younger oceans, some of which (e.g., Pontide Ophiolite Belt) pay have been rear-arc, or interarc, marginal basins.
The distribution and age of origin and emplacement of Alpine ophiolites are consistent with the relative motion of Africa and Europe from 178 my onward as deduced from Atlantic-plate accretion history. Most appear to represent short lengths of accreting margins that joined transform faults and subduction zones within a complex small-plate mosaic. Theoretically, dike orientation in sheeted diabase complexes would lie normal to the divergent motion vector across an accreting margin and thus, in favorable circumstances (e.g., Cyprus), correlate with the vector deduced from the Africa-Europe motion model. However, in most examples, the complex deformations and rotations developed during ophiolite emplacement prohibit such a correlation.
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Modern and Ancient Geosynclinal Sedimentation
The Kay Conference was held in Madison, Wisconsin, November 1972. This symposium volume contains the texts of papers presented at Madison. It is organized in a topical manner, and in most areas of discussion, modern analogues and ancient examples together provide a comparative basis for evaluating sedimentary models for geosynclines. In the 1970s students of both modern and ancient sediments have compiled an immense body of knowledge relevant to the geosynclinals concept. Moreover, the new theory of plate tectonics has required a complete reassessment of the geosynclines as well as orogenesis. The purpose of this volume is to evaluate by comparison of modern and ancient sediments a number of depositional models applicable to the great variety of strata seen in orogenic belts also called geosynclinal.