A blueschist belt formed during the mid-Cretaceous (similar to 88 Ma) high-pressure metamorphism of a passive continental margin sequence of shale, siltstone, quartzite and limestone occurs immediately south of the Tethyan suture in northwest Turkey. A transect across the Tethyan suture was mapped to constrain the exhumation of blueschist-facies assemblages. The presence of jadeite, glaucophane and lawsonite in the blueschists indicates P-T conditions of 20 kbar and 430 degrees C. The blueschists are tectonically overlain by a Cretaceous oceanic accretionary complex showing an incipient blueschist metamorphism. A peridotite slab with subvertical layering lies along a low-angle fault contact over the blueschists and the accretionary complex. Isolated slices of Early Cretaceous (101 +/- 4 Ma) garnet-amphibolites occur at the base of the peridotite. The garnet-amphibolites, which show an incipient blueschist-facies overprint, have probably formed during the oceanic subduction in the foot-wall of the subduction zone and later underplated the hanging wall. Blueschists and peridotites are abruptly truncated in the north by the steeply dipping suture fault, that juxtaposes unmetamorphosed and slightly deformed Triassic to Jurassic sediments of the northern continent against the peridotite and the accretionary complex. Tectonic juxtaposition of the lower-pressure accretionary complex and peridotites over the higher-pressure blueschists implies the removal of a 45-km-thick oceanic mantle sequence above the blueschists. Regional geological and geometric constraints suggest that the exhumation of the blueschists was achieved by a combination of two simultaneously acting mechanisms. The first was the detachment of the upper crustal blueschist sequence from its basement in the subduction zone and its incorporation to the hanging-wall followed by buoyant ascent, The second mechanism is the progressive shallowing of the subducted continental lithosphere resulting in the thinning of the mantle wedge above the subducted slab. The blueschists were partly exposed prior to the Paleocene continent-continent collision. Post-collisional Eocene (52-48 Ma) calc-alkaline plutons, that were emplaced in the blueschists and the overlying peridotite at similar to 10 km depth, are interpreted to have formed by crustal anatexis from advective heating by mantle melts. These have probably been generated during the upwelling of the asthenosphere under the shallowing continental lithosphere. (C) 1998 Elsevier Science B.V. All rights reserved.
