The lowest structural levels of the Istanbul Zone are exposed in the Sunnice Massif, north of Bolu. They comprise the greenschist-facies Ediacaran calc-alkaline bimodal Yellice metavolcanics, which are intruded by the Dirgine granitoid. Amphibolite-facies mafic and subordinate ultramafic rocks of the Cele mafic complex, hitherto termed the Cele meta-ophiolite, include hornblende gneisses of island arc meta-tholeiitic and transitional to calc-alkaline metagabbroic compositions, and were previously thought to be part of the Istanbul Zone basement. Together with minor serpentinized troctolite and harzburgite lenses, they are exposed in a broadly antiformal structure beneath the Yellice metavolcanics. Although these rocks structurally underlie, and, on account of their higher metamorphic grade, were previously thought to be older than the Ediacaran-Cambrian rocks of the Istanbul Zone basement, their oldest isotopic dates are Permian, indicating that they are instead much younger than the overlying rocks, from which they must be divided by an (inferred) Istanbul Zone basal thrust. Instead, these mafic rocks may be linked to the Almacrk Complex, and hence interpreted as the under-thrust crustal base to a Permo-Triassic active continental margin attached to the Gondwana-derived Sakarya Zone now located south of the Istanbul Zone. Triassic and Jurassic metamorphic ages record compressional events marking a Cimmerian event closing the Palaeotethys Ocean. (C) 2012 Elsevier B.V. All rights reserved.