EXHUMATION OF BLUESCHISTS ALONG A TETHYAN SUTURE IN NORTHWEST TURKEY

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 as semblages. The presence of jadeite, glaucophane and lawsonite in the b lueschists indicates P-T conditions of 20 kbar and 430 degrees C. The blueschists are tectonically overlain by a Cretaceous oceanic accretio nary complex showing an incipient blueschist metamorphism. A peridotit e 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 bl ueschist-facies overprint, have probably formed during the oceanic sub duction in the foot-wall of the subduction zone and later underplated the hanging wall. Blueschists and peridotites are abruptly truncated i n the north by the steeply dipping suture fault, that juxtaposes unmet amorphosed and slightly deformed Triassic to Jurassic sediments of the northern continent against the peridotite and the accretionary comple x. Tectonic juxtaposition of the lower-pressure accretionary complex a nd peridotites over the higher-pressure blueschists implies the remova l of a 45-km-thick oceanic mantle sequence above the blueschists. Regi onal 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 i ncorporation to the hanging-wall followed by buoyant ascent, The secon d mechanism is the progressive shallowing of the subducted continental lithosphere resulting in the thinning of the mantle wedge above the s ubducted slab. The blueschists were partly exposed prior to the Paleoc ene continent-continent collision. Post-collisional Eocene (52-48 Ma) calc-alkaline plutons, that were emplaced in the blueschists and the o verlying peridotite at similar to 10 km depth, are interpreted to have formed by crustal anatexis from advective heating by mantle melts. Th ese have probably been generated during the upwelling of the asthenosp here under the shallowing continental lithosphere. (C) 1998 Elsevier S cience B.V. All rights reserved.