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Abstract

Are there analogs of flysch presently accumulating in today's oceans, and can the term appropriately be used to designate recent sequences? The thesis of this study is that unconsolidated deposits can be designated as flysch if such sequences meet the geographic, tectonic, and sedimentary conditions stipulated; the Hellenic Arc in the eastern Mediterranean Sea is proposed as one type area of modern flysch sedimentation. This region comprises an internal trough and andesitic volcanic arc, an island and ridge arc, and the external Hellenic Trench; the arcuate Mediterranean Ridge lies to the south. Geophysical studies and deep drilling support a model of compressional convergent plate juncture, that is, underthrusting of the African plate beneath the Eurasian plate.

Ponded sediment to which the term flysch can be applied is found in numerous depressions south and north of the Peloponnesus-Crete-Rhodes belt; circular to linear lows occur at depths from less than 400 m to just over 5,000 m and range in size from about 10 km2 to 10,000 km<sup>2</sup> Seismic profiles reveal thick (<0.2 to 2.0 km) stratified sediment wedges in marginal depressions, basin plains, and some trenches. Tectonic deformation decreases from north to south, and sediment is derived from uplifted cordilleras to the rear of depositional zones, not unlike the classic Alpine Flysch model.

Very high resolution subbottom profiles show distinct thin strata in the upper 30 to 60 m of section; cores in the upper 5 to 10 m of section penetrate thin graded sand turbidites and laminated and homogeneous muds. Sedimentation involves fluvial input, bypassing of narrow shelves, and transport down steep margins, via canyons, to basin and trench plains. Profiles and cores reveal typical base-of-slope sequences, including tur- bidites and slumps, and strata offset by contemporaneous faulting and folding. Intense earthquake activity coupled with the relatively small size and topographic isolation of depressions have resulted in rapid ponding rates. Clays presumably move downslope by turbid flows and are also deposited from circulating water masses. The predominance of clay over sand results in a flysch marneux facies, not unlike Studer's original Flysch in the Simmental. In contrast, the silt- and sand-enriched deposits south of the Mediterranean Ridge derived from North African sources, and particularly, the Nile, more closely resemble the coarser flysch gréseux facies (Tertiary Numidian, Annot Sandstone, etc.) exposed in the circum-Mediterranean.

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