30% SHORTENING IN THE RHODOPE-PONTIDE FOREARC NEAR SINOP THAT CREATED A MARGINAL FOLDBELT STYLE

We have mapped a 900 km² area at a scale of 1/10,000 in northernmost Turkey, between the towns of Ayancık and Gerze south of Sinop. In the south, the area borders on Mt. Çangal and the Black Sea forms its northern limit. The area was chosen to assess the total shortening in the forearc area of the Rhodope-Pontide arc so as to bring certain constraints on the palaeotectonic reconstructions of the Cimmerian continent in this critical area of Cimmeride/Alpide overlap. The oldest unit in the area is a south-facing Atlantic-type continental margin carbonate platform consisting of the İnaltı Fm. The opening of the Black Sea is heralded by the rift deposits of the Çağlayan Fm. bearing mafic volcanic rocks near its top. The breakup unconformity is below a thin, but widespread band of pelagic sediments of the Kapanboğazı Fm. Above that the development of the magmatic arc begins with the andesitic volcanics of the Yemişliçay. This arc retreats northward by the Maastrichtian and shuts off by the time of the turbidites of the Ayancık Fm of Lower Eocene age. The turbiditic sedimentation continues until the medial Eocene and then the area gets deformed. No Oligocene is known from this region. Sedimentation resumed in the Miocene. Another deformational unconformity separates the Miocene from the Pliocene. We have combined our mapping with industry seismic sections and 5 wells. The whole area was deformed by flexural slip folding and accompanying thrusting with a double vergence: to the south in the south and to the north in the north. Because the Miocene is so limited it is not possible to assess how much shortening its deformation took up, but the total shortening, in piggy-back duplexes, in the area is at least 30%. Thus the original forearc width here was at least 140 km wide. This is almost as wide as the gas-bearing Thrace Basin farther west, the widest Rhodope-Pontide forearc known. It is concluded that a similar basin existed in the Central Pontides. This provides ample space both for the Cimmerian sliver (consisting of Palaeo-Tethyan subduction-accretion complexes) and the Alpide subduction-accretion complex.