A B S T R A C TNew observations from the Island of Tinos, Greece, allow a better definition of the structural position of the Alpine (Eocene) blueschist belt exposed in the islands of the Aegean Sea. These blueschists, over a significant part of the Aegean sea, are delimited from below by a lowangle thrust fault, while from above they are delimited by a low-angle, normal-type fault which omits a substantial crustal interval. Both underlying and overlying rocks were not affected by the high? metamorphism. The rapid uplift and exhumation of the high? rocks was therefore mainly the result of fault movements rather than erosion and whole-crust uplifting. The low-angle normal fault apparently had a major role in the uplift of the blueschists.TERRA Now (1989) 1, 182-187. tacts. These observations support pre-

Abstract. Eclogite-facies rocks exposed in Mediterranean back arcs are delimited from above by low-angle normal faults and detachments. Nevertheless, our work demonstrates that these extensional structures associated with back are extension played only a limited role in removing the overburden from above the eclogites. Extension in Mediterranean back arcs began in the late Oligocene or early Miocene, but the pressure -temperature -time (P-T-t) paths of eclogite-facies rocks exposed in these areas indicate that a major part of the overburden, several tens of kilometers, has been removed from above these rocks prior to the Oligo-Miocene. We show that the time period bracketed between the peak of eclogite metambrphism (Eocene in the central Aegean, probably Upper Cretaceous in Corsica and the Betics) and the onset of back arc extension in the Oligo-Miocene was characterized by thrust faulting. In the central Aegean, Corsica, and the Betics, eclogite-bearing units were partly unroofed and then overthrusted lower-pressure units. We emphasize that, with one exception (Tinos island, Greece), the entire inventory of extensional contacts operated subsequently to the overthrusting of the eclogites above the lower-grade sequences. Thus Mediterranean back arc extension lags behind a major part of the denudation process, and is superposed on orogenic wedges that contain eclogite-facies rocks at relatively shallow structural levels. We emphasize that the mode of occurrence of eclogites in Mediterranean back arc regions involves a continuum of in -situ crustal accretion below the eclogites, widespread P-T paths that show cooling or isothermal decompression, and lower-grade rocks at the bottom of the structural pile. Thus instead of reflecting wholecrust back arc extension, the tectonic style associated with the denudation of Mediterranean eclogites better fits an active accretionary-wedge setting. This is similar to the mode of oc-• On leave at Laboratoire de G6ology, Ecole Normale Sup6rieure, Paris.Copyright 1997 by the American Geophysical Union.Paper number 97TC02003.0278-7407/97/97TC-02003512.00 currence of eclogite-facies rocks in mountain belts, such as the western Alps, where decompression was synorogenic and back arc extension played no role.

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The Alpine orogenic belt of the Hellenides has been strongly reworked by ductile and brittle extensional tectonics. Extensional structures have affected the central Aegean region and obliterated much of the original orogenic architecture since at least early Miocene times. In the area of Almyropotamos (on the island of Evia, flanking the western part of the Aegean) a unique remnant compressional nappe stack involving Tertiary metamorphic rocks has been preserved. This nappe sequence comprises a high-pressure rock unit on top of a lower grade unit. The upper unit (South Evia Blueschist Belt) is thought to be the westward continuation of the Cycladic blueschist belt metamorphosed at high-pressure conditions during Late Cretaceous-Eocene times. The underlying unit (the Almyropotamos Unit) is a continental margin sequence covered by a flysch and containing Lutetian nummulites, indicating that this unit accumulated sediments until at least late Eocene times.In the present study we analyse the petrology of the Almyropotamos nappe stack and define the P-T conditions of each of the different rock units exposed there. The presence of glaucophane, lawsonite rimmed by epidote, and jadeite (70 mol. %) suggest that peak P-T conditions in the South Evia Blueschist Belt reached approximately 10-12 kbar and 350-450°C. Unlike previous studies, which estimated that the underlying Almyropotamos Unit reached only greenschist-facies conditions, glaucophane relics and Si-rich phengites were found by us in this unit. These indicate that high-pressure metamorphism and crustal thickening in this part of the Aegean lasted until at least the late Eocene or early Oligocene. We note that in this respect the architecture of southern Evia resembles that of northern Greece (Olympos, Ossa). Our structural data indicate that rock units in the Almyropotamos area record different folding phases, with the South Evia Blueschist Belt having a more complex fold history than the underlying Almyropotamos Unit. The entire nappe stack shares large-scale folds which are E-W trending, and locally overturned-to-the-south, and which may represent (at present coordinates) N-S contraction and nappe transport.

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