WHY DID THE BOPHORUS OPEN WHERE It DID?

Since mythological times mankind contemplated how the Bosphorus (Bosphorus thracicus or the Strait of Istanbul) had originated. It was realised already in antiquity that its valley resembled a stream valley and that it now helped to disgorge the excess water of the Black Sea to the world ocean. Scientific speculation beginning in the eighteenth century agreed with the early ideas except that now it was thought an already existing fluvial valley had later been invaded by the sea. The debate became centred on the direction in which the paternal river had flowed and how many such rivers may actually have existed. Recently the Ryan and Pitman Black-Sea-flood hypothesis has led to a renewed flurry of research and combined with the marine geological work following the disastrous 1999 earthquakes in İzmit and Kaynaşlı produced a wealth of excellent new data on the geology of the Bosphorus. It seems however that in the excitement of gathering new information much hasty interpretation has led to some errors in assessing the new information and to a very narrow ‘marine-only’ approach to the problem of the origin of the Bosphorus. I here show that the Bosphorus was opened at a neutral space between two oppositely tilting peninsulae of Thrace and Bithynia (Kocaeli). This neutral space formed not only the lowest point in the topography, but also was located where the watershed in the north in the Thracian Peninsula moved to the south in the Bithynian Peninsula. The rivers on both peninsulae have northwest-southeast courses dictated by the post-tilting slope conditions helped along by the structures generated not only by the torsion, but also by the developing North Anatolian Fault Zone, but they flow in opposite directions. The basement sill depth of the Bosphorus is probably -90 m (not -70 as previously maintained), but the actual sea-bottom sill in the Holocene, formed from a now eroded Neogene fill, may have been considerably shallower and most likely located farther south than the basement sill, between Büyükdere and Beykoz. This new interpretation of the history of the Bosphorus greatly affects the interpretation of the stratigraphy of its sedimentary infill and the palaeoclimatic and palaeoceanographic inferences made on the basis of it.