TECTONICS, VOL. 13, NO. 1, PAGES 127-138,FEBRUARY 1994 Postnappe stacking extension and exhumation of high-pressure/ low-temperature rocks in the islandof Crete, Greece Charalampos Fassoulas, Adamantios Kilias, and Demosthenis Mountrakis Department of Geology and Physical Geography, Aristotle University, Thessaloniki, Greece Abstract. The island of Crete consists largely of nappes of contrasting lithologies and metamorphism thatwere stacked southward duringan Oligocene-early Miocene N-S compression, with the lower nappes undergoing a high- pressure/low-temperature (H'P/LT) metamorphism. This was followed by a N-S Miocene crustal extension that caused thinning of the nappes and uplift of theHP/LT metamorphic rocks. Ductile conditions took placein the lower nappes associated with a retrograde greenschist metamorphism, while semiductile to brittle conditions tookplace in the upper nappes. A majornormal detachment faultseparates thelower and theupper nappes. The quartz texture analyses and the symmetry of structures indicate bulk coaxial deformation, while the results of strainanalysis suggest bothconstrictional andflattening strains. A younger NE-SW compression affected boththe thinned nappe pile andthe late Miocene- Pliocene sedimentary basins. Finally, a Pliocene NE-SW extension led to furtheruplift andexhumation of theHP/LT metamorphic rocks.This cyclictectonometamorphic process of alternate compression andextension tookplace during the migration of the Hellenic orogenic belt toward the most external units, including successive tectonic events. INTRODUCTION On the island of Crete, 100-150 km behindthe active Hellenicsubduction zone (AHSZ), (Figure la), Alpine tectonics juxtaposed high-pressure/low-temperature (HP/LT) metamorphic rocks with unmetamorphosed sediments (Figures 1, and2) [Seidel et al., 1982;Bonneau, 1984]. The complex nappe pile of central Crete(Figure l d, and Table 1) is made up of a varietyof sedimentary andmetamorphic units originating bothin the external andinternal Hellenides zones [Bonneau, 1984]. The trend of the Hellenic isopic zones in mainland Greeceis approximately NW-SE, while in the area of Crete it bends toward east (Figure la). The Plattenkalk unitand the Phyllite-quartzite, Gavrovo, and Pindos nappes originate in the external zones, whereas theVatos-Arvi- Miamou, Asteroussia, and Ophioliticnappes originate in the internal Hellenides zones (Figure la) [Bonneau, 1984]. As a consequence of the closure of thesouthern branch of theTethys ocean,which started in the Late Jurassic, several tectonic events affected the Hellenides zones. The Eocene compression created the HP/LT units exposed in the Olympus Copyright 1994by the American Geophysical Union. Paper number 93TC01955. 0278-7407/94/93TC-01955510.00 and theCyclades area(Figurela) [Altherr et al., 1979; Schermer et al., 1989], while the Oligocene compression stacked the nappes that constitute the nappe pile of Crete [Epting et al., 1972; Creutzburg andSeidel, 1975; Fytrolakis,1980; Greiling, 1982;Seidel et al., 1982; Bonneau, 1984; Hall et al., 1984]. Unmetamorphosed Alpine sedimentary units,crystalline rocks andophiolitic bodies were thrusted southward onto the externalunits [Seidelet al., 1982; Bonneau, 1984]. A HP/LT metamorphism affected the lowermost nappes in lateOligocene-early Miocene [Seidel, 1978; Seidelet al., 1982]. In central Crete the late Oligocene compression wassuperimposed by a majorMiocene extensional eventwhichresulted in the uplift of the HP/LT metamorphic rocks [Kiliaset al., 1993]. In thispaper we present structural, kinematic, and strain analyses of this majorpost-Oligocene tectonic event. The role of compression followed by the same trending extension is emphasized as an important factor in thestructural evolution of central Crete. GEOLOGICAL FRAMEWORK The petrological and metamorphic features of the nappes constituting the nappe pile of central Crete are presented in Table. 1. According to their tectonostratigraphic position and their tectonometamorphic history these nappes are divided into two major groups, the upper andthe lower nappes (Figures1, and 2). The lower nappes, constituting the Psiloritis metamorphic corecomplex [Kiliaset al., 1993], consist of thePlattenkalk series andthe Phyllite-quartzite nappe. The degree of the HP/LT metamorphism which affected the lower nappes in late Oligocene-early Miocene(Table 1) is increasing from west Crete to east[Seidel, 1978]. Especially for the areaof central Crete it is suggested that only remnants of the HP/LT metamorphism existbecause of a subsequent overprint by a greenschist metamorphism [Seidel et al., 1982]. A mineral paragenesis of whitemica, chlorite, actinolith, calcite, albite, andquartzwas observed in phyllites and metabasalts, indicating a greenschist metamorphism [Kilias et al., 1993]. It is likely thatthe ages of 18 Ma estimated for thePhyllite- quartzite nappe [Seidel et al., 1982]belong to whitemicas of the retrograde greenschist metamorphism. In eastern Crete, Frantz [1992] estimated 1- to 2- kbar pressure and 270-320 ø C temperature conditions for the retrograde metamorphism. The upper nappes consist from base to topof the unmetamorphosed Gavrovo andPindos nappes andthe upper metamorphic nappes, namely,the Vatos-Arvi-Miamou, Asteroussia, and Ophiolitic nappes (Table 1).