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TECTONIC EVOLUTION OF THE OLYMPUS- OSSA MOUNTAINS: EMPLACEMENT OF THE BLUESCHISTS UNIT IN E....

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TECTONIC EVOLUTION OF THE OLYMPUS-OSSA MOUNTAINS: EMPLACEMENT OF THE BLUESCHISTS UNIT IN EASTERN THESSALY AND EXHUMATION OF OLYMPUS-OSSA CARBONATE DOME AS A RESULT OF TERTIARY EXTENSION (CENTRAL GREECE)

ADAMANTIOS KILIAS*

ABSTRACT

High pressure metamorphic rocks in the area of carbonate unit of the foreland. The emplacement Olym us-Ossa (Eastern Thessaly, Central Creece), was possible due to changes in the rate or in the were f rlned in Eocene under a pressure of 6-9 Kb direction of plate convergence. Large scale antithetic and at mperature of 300°-350° C at a depth greater normal movement of masses towards NE or SW, than 20 Km, during pla1te convergence and nappe took place during the second stage causing the final stacking. Kinematic and strain analyses of the high uplift and exhumation of the high pressure rocks pressur metamorphic belt and its surroundings and the underlying sediments, of the Olympus-Ossa sho hat the uplift history of the high pressure unit, in the form of a tectonic window. This extension rocks is related to an intense Upper Eocene/ regime is related with a large scale underplating of Oligocen to Miocene subhorizontal stretching and cold continental material of the foreland under the subverti I I thinning. It took place in two stages: extending nappe pile of the upper plate, during the During th first stage, the high pressure metamorphic continuing plate convergence, as well as to the rocks and he overlying nappe pile were emplaced formation of the thrust belts and nappes in the over the nearly unmetamotphosed Olympus-Ossa external Hellenides.

I,

INTRODC('"'TION Through a rapid uplift, mechanism «50 million HPfLT mcks formed as a consequence of a rapid years ENGLAND and THOMPSON 1984) without burial of hm,ic rocks and sediments at a depth > any major contribution of heat and retrograde 20 km, ill ,he subduction zone of the Apulian postburial heating, the HP/LT parageneses were plate under the Pelagonian continental block, during preserved in places, so that nowadays these rocks the Eocem: (GODFRIAUX 1968, PAPANIKOLAOU form the inner belt of the blueschists of the 1987, scHLmSTEDT et a1. 1987, SCHERMER et Hellenides. Distinctive occurences are preserved al. 1989, KH,IAS et al. 1991 b). in the Olympus-Ossa area in eastern Thessaly, also -_._------known as the Ambelakia unit (GODFRIAUX 1968, ASSllC. Professor, Department of Geology, Aristotle University of KATSIKATSOS et a1. 1982, SCHERMER 1990, Thcssaloniki

OPYKTOL ni\OYTOL/MINERALΨηφιακή WEALTHΒιβλιοθ ή96/1995κη Θεόφ ραστος - Τμήμα Γεωλογίας. Α.Π.Θ. 7 KILIAS et al. 1991 h), as well as in the areas of strain analysis (Rf;cr> method, LISLE 1985; Fry Pelion, (FERRIERE j 977), Euboea (KATSIKATSOS method, HANA & FRY 1979; quartz c-axis diagrams 1977), and the Cyclades (ALTHERR et al. 1979, LISTER & HOBBS 1980). BLAKE et al. 1981, SCHLIESTEDT et al. 1987, AVIGAD 1990) (Fig. I.). GEOLOGICAL SETTING The geology of the Olympus-Ossa area has been studied by m(\)1y investigators. Jt attracts the geologists' interest, since numerous questions concerning thE' evolution of the Hellenides are closely associated with the evolution of the suture zone between the 100ver Apulian plate and the upper PE'lagonian continental fragment. This is actually reflected in the structure and tectonic evolution of the blueschists of the area (fig. 1). The blueschists unit in the Olympus-Ossa area consists of alternations of metabasites and metasediments which was subducted below the Pelagonian continental block, during the Eocene and was metamorphosed undeT HPjLT conditions I (SCHERMER et al. 1989, SCHERMER 1990, KILlAS '~. 1._. 0 _ et aL 199 la). During the Oligocene, it was progressively affected by a retrograde low grade FIGURE 1: Location of' the study area in the Hellen­ metamorphism, (KATSIKATSOS et al. ]982, KILIAS ides. Development of the two HP/LT metamorphic belts et aL 1991b). in (lie Hellenic orogen: The inner one of eocenic age This HPjLT-rock unit is emplaced over the slightly imd tlie ollter one of Late Oligocene/Early Miocene metamorphosed to nonmetamorphosed carbonnte age. sediments of the Olympus-Ossa unit, ranging from Wherever the post-Eocene reheating of the crust the Triassic to the Eocene, and terminating in a was significant, as in the case of the Cyclades the flysch of late Eocene age (GODFRIAUX 1968). high pressure parageneses were intensely The Olympus-Ossa unit belongs to the "lower overprinted by high temperature barrovian type plate", which acted as the foreland during the metamorphism (ANDRIESSEN et al 1979, ALTHERR Tertiary collision of the southern branch in the et a1. 1982, WIJBRANS & McDOUGALL 1988). Alpine orogene (GODFRIAUX 1968,JACOBSHAGEN In this paper a new concept for the uplift and et a1. 1978, KILl AS et a!. 1991 a). Nowadays it emplacemel1t regime of the blueschists of eastern appears in the form of a tectonic window. A similar Thessaly is snggested. It includes the overlying tectonostratigraphy is recognized in the tectonic Pelagonian nappes, and the nearly nonmeta­ windows of Kranca, west of Olympus-Ossa (KILlAS l1lorphosecl carbonate Olympus-Ossa unit of the & MOUNTRAKIS 1987, KILIAS et al. 1991a) and foreland (Fig. 2, 3). Furthermore, we offer an of Rizomata further to the north (KILlAS & interpretation of the exhumation mechanism of MOUNTRAKIS 1985) (Fig. 2, 3). the high pressure rocks and the underlying The pile of the Pelagonian nappes constitutes sediments of the foreland in the form of a tectonic the "upper plate" of the Tertiary collision lone, window. which was originally emplaced over the blueschists To reach our conclusions we have used the (GODFRIAUX 1968, YARWOOD & DIXON 1977, following: a) the symmetry and geometry of all JACOBSHAGEN et a1. 1978, KILlAS 1980, KILlAS recognizable structures of the blneschists and of 1991, NANCE 1981, KATSIKATSOS et al. 1982, the Tectonic units directly underlying and overlying MIGJROS 1983, MOUNTRAKIS 1983, KILlAS & .them, b) the modern methodology of kinematic MOUNTRAKIS 1987/1989, DOUTSOS et al. 1993). aIlalysis (RAMSAY & HUBER 1983/1987,SIMPSON The unit of ophiolitic rocks (Eohellenic nappe. & SCHMID 1983), c) the relations betvv'een JACOBSHAGEN et al. 1976, VERGELY 1984), meTamorphism and tectonics and d) the method of together with the Cretaceous transgreSj';ive carbonate

Ψηφιακή Βιβλιοθήκη Θεόφραστος - ΤOPYKTOI.μήμα Γεω λni\OYTOL!MINERALογίας. Α.Π.Θ. WEALTH 96/1995 I------;;;O...-.~------·..,__;:=:;__------__, N --.on F '; ,,:j"" 0 Neogene-Oulrternary Lowen ~01:~A; PLATE l ~ B euechlBt9 unit HPIlT rocl

2'~ o Pelagonlan nappe

A--AI crose aec1lon

A detachment rone -­ Al­ nonn81 low angle feutlB - w1lt1 NE dIp _ 1Bclonlc boundary be1wa

movement eenee of ltle tactonlc top dOW11WBld. " -+­ antIcllnale ~- "'incllnele

~ ~ nOlTTlal hIgh ; 6I1gla faut! PI (: ; 14;/~ I· ANT1C~1AMI ..

~ A D2 coaxlal Row .­ non-oorodaJ now _

: -­ S8 & LAf [>

lOkm. arroW!! Indleata .hear 6eflsa of ~,e tec10nlc top, t>ased On klnemlrtlc Indlkato",

FIGURE 2: Geological map of the Olympus-Ossa area. Compilarion of shear criteria for D2 deformation. Arrows indicate the normal opposed sence of movement of the nappe pile in tile central Hellenides during (he Oligocene­ Miocene. Geological mapping: BRUNN 1956, GODFRIAUX 1968, SCHMITT 1983, MIGIROS 1983. MIGJROS e-t al. 1985, KILIAS & MOUNTRAKIS 1985, 1987, 1989, SCHERMER 1990, KILIAS et al 1991a,b). which terminates in Eocene flysch (GODFRIAUX fabric L2 stretching lineation, defined hy elongated 1%8, MERCIER; J968, FERRIERE 1982, and ruptured minerals and mineral aggrt'gales CLEMENT 1983), is recognized as tI,,' uppermost mainly develops with a NE-SW trend (Fig. 5). The nappe (Fig. 2,3). L2-stretching lineation west of the carbonate Olympus-Ossa window plunges SW while, on the contrary, east of the window it plunges NE, DATA AND OBSERVATIONS following the general trend of the S2-schistosity Geometry and kinematics of the deformation (Fig. 3,4). The B-axes of the isoclinal folds usually An Sl-schistosity and 81- intrafolial isoclinal develop parallel to the L2 stretching lineatiou. folds, as well as sheath folds, forming representativt' (KILlAS el al. 1991a, b). structures of the earliest 0 1-event are iclen tifled During this second D2-event all kinematic in the blueschist unit. A kinematic analysi~ of this indicators and the strain analysis, register either a initial stage is not possible since the 0 I-structures major component of coaxial deformati.on or are almost entirely affected by the second 02­ movement of the tectonic top towards SW. In places, penetrative ductile event (Photo la). 02 causes an diplacement develops also antithetic to the overal intense transposition of the Sl-schistosity in S2, top to SW sense of shear, due to downwards glide so that S 1 and S2 develop almost parallel and they along steep NE dipping glide surface (Fig. 9). cannot often be distinguished from each other (Fig. A map scale evolution of the sense of shear 4, Photo la, b, c). The associated with the S2 indicators is presented in Fig. 2. Coaxial strain

OPYKTOL: nI\OYTOL:/MINERALΨηφιακή WEALTHΒιβλιοθή 96/1995κη Θεόφ ραστος - Τμήμα Γεωλογίας. Α.Π.Θ. 9 3~ w E

Mesohellenic trough Axios basin 1500

o

A A 52-schistosity - detachment zone « . normat low ngl,e fau ts ith NE dip (LAF) low anQlle norma shea.r zone (S8) normal igh angle fau ts __. • ~ ~ ..J

FIGURE 3: Geological cross .~ection along the nappes of the cenlr;JI Hellenides, from the Axios basin to the MesohelJenic trough. The asymmetry of the Olympus and Ossa carbonate domes (lower plate), is also shown. Symbols ilS in fig. 2.

BISOkl • • +•• + -: ..•

normal low angle shear zones B constr and fault

+ I ~... I ~ t J ( FIGURE 4: Stereographic projection of the tectonic structures of the Olympus-Ossa area (Schmidt equal-area net, a . lower hemisphere). Ii

10 OPYKTOI n/\OYTOI/MINERAL WEALTH 96/1995 Ψηφιακή Βιβλιοθήκη Θεόφραστος - Τμήμα Γεωλογίας. Α.Π.Θ. o seems to be predominate mainly in the lower parts to that of the bluschist unit. during the D2-event. of the blueschist unit whereas the hanging wall The 52 mylonitic fabric and the boundary between parts, of the blueschists seems to have suffered upper and lower plate, are always parallel (Fig. 3). non-coaxial flow. The results obtained from the strain analysis The coaxiality of the deformation is stressed by (Rf/(I:>- and Fry-methods) of the blueschists and the development .of symmetrical conjugate shear the gneissic mylonites of the Pelagonian nappe, zones, associated with the main 52 foliation, with revealed a considerable thinning, perpendicular to an opposite sense of shear, (Photo I b), symmetrical the 52 schistosity, as well as a subhorizontal boudinage of competent layers. and symmetrical development of the maximum stretching (x-axis) elongation of clasts and mineral aggregates. (Photo with a NE-SW trent (KlLIAS et al. 1991 b, SFEIKOS 1b). et aL 1991, KILIAS 1991, SFEIKOS 1992). Southwestwards sense of shear of the tectonic The low angle ( 10 0 to 30') conjugate shear zones top show: S-C structures (LISTER & SNOKE 1984), (S8), with a movement of top downwards and the asymmetricaly rotated clasts of rigid minerals or low angle normal faults (LAF), with a NW-SE strike, mineral aggregates in their surrounding ductile are also important tectonic structures in the matrix (PASSCHIER & SIMPSON 1986), extensional blueschist unit. These structlltes persist with a shear bands of high shear strain formed similar intensity also in the Pelagonian and ophiolite synchronously with the main 52-foliation (PLATT nappes. & VISSERS 1980), asymmetric mica fish (LISTER The geometry of these discrete exteJlsional shear & SNOKE 1984), asymetric boudins (SIMPSON & zones is illustrated in figure 3 and Photo Ie, d, e. SCHMIDT 1983, HOOPER & HATHCER 1988), The listric surfaces of these zones terminate in S-Z-type folds in heterogeneous shear zones (VAN the main detachment zone of the two plates. The DEN DRIESSCHE & BRUNN 1987). A top to SW kinematics and geometty of these extensional shear large scale movement in the Mt. Olympus region zones exhibit a symmetry similar to that of the was also described by previous workers (KILIAS ductile D2-st.retching. This is indicated by tbe NE­ et 31 1991, SFEIKOS et al 1991, SCHERMER 1993). SW striations on their sliding planes. It is recognized The partitioning of coaxial and non-coaxial flow that a top to the SW sense of shear prevails to the cinematics is also demonstrated by comparison of west of the Olympus-Ossa dome, while a NE sense Cjllartz-c-axis fabrics. Fig. 5 schows quartz-c-axis of movement prevails to the east (Fig. 2, 3). Similar fabrics with orthorhombic symmetry (1,2,4: coaxial) conditions of kinematics are also preserved in the and monoclinic symmetry top to SW (3: non­ Kranea and Rizomata tectonic windows (Fig. 2, 3) coaxial). (KILIAS et al. 1991a, SFEIKOS et a1. 1991). Progressively, durIng the development of the D2­ Asymmetrical open folds or kink zones with event there develops locally, a compression parallel vergence sometimes NE and sometimes SW, often to the Y-axis of the finite strain ellipsoid. Thus accompany this downward sliding of the rocks (Fig. typical compressional structures (thrust faults and asymmetric open folds) are produced orientated 4). parallel to the maximum stretching. (Fig. 2, 8). A large scale downwards slide of rock masses The D2-deformation is also conveyed, with similar along these extensional conjugate shear structures, symmetry and kinematics, to the Pelagonian nappes resulted in the juxtaposition of the tectonically of the upper plate, over the blueschist uni t (KILL'\S higher units of the nappe edifice with the lower et al. 1991 a,b, SFEIKOS et a!. 1991, SFEIKOS ones (Fig. 3). Thus, we see normal displacements 1992). between the difference tectonic nappes, in places A significant component of coaxial flow also associated with an omission or intense thinning, prevails in the underlying Olympus-Ossa lower of whole nappes. For example, we see the plate and particularly towards the boundaries with pe1agonian basement in the western side of the the blueschists or the overlying nappes of the upper Olympus carbenate dome, in direct tectonic contact plate. In the sediments of the Olympus-Ossa unit with the rocks of the underlying Olympus-Ossa (Photo 2) develop two coeval sets of shear bands unit. In contrast, in the eastern side of the Olympus and abundant symmetric boudinage of competent dome the ophiolitic nappe mOllving downwards to layers. The maximum extension is NE-SW, similar the NE is emplaced directly on the blueschist unit

OPYKTOI ilAOYTOUMINERALΨηφι αWEALTHκή Βιβλ ι96/1995οθήκη Θ εόφραστος - Τμήμα Γεωλογίας. Α.Π.Θ. 11 or still on the lowermost Olympus-Ossa unit (Fig. 2, 3). It is recognized that the movement of rock masses towards NE at the eastern side of the Olympus­ Ossa, Kranea and Rizomata windows displays a greater throw (Fig. 2, 3). The preservation of the uppermost ophiolitic nappe in two narrow zones along the eastern sides of the Olympus-Ossa and Kranea carbonate domes, in direct tectonic contact with the rocks of the lower plate units, confirm this asymmetrical collapse of the nappes (Fig. 2, 3). The asymmetrical distribution of_these conjugate extensional shear zones on either side of the Olympus-Ossa and Kranea tectonic windows, also combined with the D2-kinematic regime previously _._------_----! descri bed, produces a mega-domino structure ·in FIGURE 5: Quarrz-c-axis fabrics in samples from the the nappe pile of the inner Hellenides, in the . blue schist unit. The distriburion symmetry of the qu­ wider Olympus-Ossa area. This asymmetry is artz c-axis plot appearing in diagrams 1, 2 & 4, indic­ reflected also in the asymmetry of the Olympus­ ales a coaxial deformation regime. X, Y, Z, are the Ossa and Kr,lIlea carbonate domes, as demonstrated finite strain ellipsoid axes, X > y > z. by the steeper development of their intense fractured eastern side (Fig. 2, 8, 9).

Finally, younger high angle normal fanlts, in ----_._------_._-----­ 03 Fig. 6). PROGRAM FAULT (CAPUTO & CAPUTO, 198() b1. The resuJrs of the right-diedrons method. Metamorphism-tectonics relations. Timing of b2. Orientation of tile three principal axes of stress deformation ellipsoid (s 1>s2>s3) Some microscopic and mesoscopic features show INVERSE METHOD (ANGELIER 1979). that D 1 in the blueschist unit took place c. Position of the three main axes of the stress ellips­ simultaneously with the HPjLT metamorphism (Ml­ oid, R=el/ipl:icity, F=fllJctuation.

12 Ψηφιακή Βιβλιοθήκη Θεόφραστος - ΤμήOPYKTOiμα Γεωλο γnAOYTOL/MINERALίας. Α.Π.Θ. WEALTH 96/1995 metamorphism). These are as follows: (1) 51­ that the uplift and exhumation of the blueschists schistosity is defined by elongate minerals of the occurred rapidly M I-high pressure paragenesis such as: blue The Olympus-Ossa lower plate underlying the amphibole, epidote, white mica, chlorite, and blueschists does not exhibit a similar tectonometa­ remnants of lawsonite (Photo la), (2) tension gashes, morphic process. Although tectonically it comprises D I microshear zones and pull apart ruptures in a deeper tectonic unit, it completely lacks the high augite minerals, in the basic members of the unit, pressure syn D I metamorphic fabric, while the are filled up with blue amphibole minerals, epidote 52-mylonitic foliation is characterized only by low and chlorite, (3) growth of blue amphibole in the pressure low temperature minerals (GODFRIAUX pressure shadows of pyroxene and amphibole 1968, KATSIKATSOS et a!. 1982) crystals having rotated during DI in the basic White mica, chlorite, and dynamically recrystal­ members of the unit. lized quartz develop along: the low angle conjugate Metamorphic P-T conditions derived from mineral extensional shear zones (S8) and the low angle equilibria (LIOU et a!. 1985) and compositions normal faults (LAF) of the blueschists unit and (SCHERMER et al. 1989 and KILIAS et al. 199tb) the overlying tectonic nappes. The local indicate T- 300°-350° C and P- 5-8 kbr for the development of these extensional shear structures, syn-D I M 1 event (Fig. 7). their style. the syntectonic nature of the low grade The penetrative mylonitic fabric aud the Ll minerals observed along their surfaces, as well as stretching lineation are defined by the growth of their symmetry in relation to the D2 mylonitic the following minerals (Photo la, b, c): white mica, fabric (as it was described above), indicate that chlorite, actinolite, stilpnomelane, albite, and these extensional zones must constitute a dynamically recrystallized quartz, which determine progressive stage of the ductile D2-deformation. a syn-D2 M2-metamorphism under conditions of This stage took place at least in its ini tial steps, in a tow grade greenschist phase (Fig. 7, KATSIKATSOS a still hot environment (1' reaching at least 250' et a!. 1982, KILIAS et al. 1991b). A sigmoidal C). development of the internal 5i-texture in rotated The high pressure syn-D I M 1 metamorphism albite crystals and the concordant transition of the evolved during the Middle-Late Eocene (55-40 5i-texture to the external 5e=52 texture, indicates million years), (GODFRIAUX 1968, KATSIKATSOS the syntectonic nature of the M2-albite. Frequently et a1. 1982, SCHERMER et al. 1989). Consequently also blue amphibole is observed reorientated within the younger syn-D2 M2 metamorphism must have the 52-fabric, following the L2-stretching lineation taken place during or after the Late Eocene. (Photo Ia, b). ArlAr data on microcline samples from the Comparisous of microfabrics under the footwalls of the low angle normal shear zones microscope have shown that the greenschist phase showed that the cooliug of the metamorphic rocks, partly overprints the high pressure M I metamorphism. Thus M2 in places causes a Kb Km decomposition of the blue amphibole into actinolite or chlorite. Furthermore, a breakdown is often observed of blue amphibole into aggregates of 8 er chlorite and stilpnomelane. Albite porphyroblasts, 1,1­ syntectonic with 52 fabric, were probably derived 6 !;­ in part from breakdown of sodic amphibole. 4 These relations of replacement of the M 1­ flf parageneses under the conditions of the greenschist 2 phase of M2, indicate that the syn-M2 D2 deformation took place outside the field of the 100 200 300 400 T high pressure M I-metamorphism (Fig. 7). The local preservatiou of the blue amphibole on the 52­ FIGURE 7: Pressure-temperature-time-deform

95 OPYKTOL: n;\OYTOL:/MINERALΨηφιακ ήWEALTH Βιβλιοθ ή96/1995κη Θεό φραστος - Τμήμα Γεωλογίας. Α.Π.Θ. 13 during crustal stretching and tbinning, started in EVIDENCE FOR CRUSTAL EXTENSION

early [Q middle Miocene time (- 16-23 Ma, The geometry and style of structural and map SCHERMER et al. ]989). relations as well as the metamorphism-tectonics Consequently there remains an interval within relations described here, indicate that during the the Upper Eocene to Oligocene, during which the D2 tectonics and its progressive stage of the low syn-D2 M2 metamorphism of the low greenschist angle normal shear zones the blueschists unit and phase must have taken place. the overlying nappes are simultaneously subjected The age of the younger normal faults, which to a total thinning il1 the vertical sense, and to a may be still active, is characterized by the subhorizontal stretching. Some of the criteria are syntectonic basins filled with Miocene and younger summarized below: sediments (BRUNN 1956, GODFRIAUX 1968, (I) Progression of the ductile D2-structures LALECHOS & SAVOYAT 197-9, BENDA & STEFFE 'S overprinted by brittle structnres with similar 1982, MIGIROS et al 1985, TRIANTAFILLTS et al. summetry, and thus decreasing pressure conditions 1987, CAPUTO 1990) and bounded, to the west (Fig. 7). (2) The mylonitic rocks exhibit a normal and east of the Olympus-Ossa window, by these sense of shear relative to the present orientation high angle normal faults. of the mylonitic foliation, (3) A sign ificant

,.. ::. ,.' »,,:,:­ :::: .. ~LYMPOS-OSSA Pelagonlan nappes--- ·:!::::::~:::~\I~~~~I··!;·li·!~ii'!:i";:;:!:::::... T cene A D d~ (sW)x~ Compression extension ~ Masohellenlo trough 'Z --.y I ~ _~_UpperEocene/O igo­ cene to Miocene /~ Upper Eocene/Oligocene + ••••••••9;reenSchlst overprint ~ Y-axis 2/8 ~ S8

FIGURE 8: Illustration ofthe Tertiary tectonic evolutiolJ, of the eastern Thessaly blueschist unit and of the adjacent lInderlying (Olympus-Ossa ulJit) and overlying (Pelagonian and Ophiolitic nappes) units. A Nappe stacking and crustal overrhickening. Development of the HPjLT metamorphism ,in the subducted materials, B. Emplacement of the blueschist unit and the overlying nappe pile, simultaneously with a subhorizontal stretching and vertical thinning of the overthickened pile nappe, which took place under a regional pla.te convergence regime. Compression subparallel to the Y-axis of the finite strain ellipsoid also develops. C. Opposed downwards nappe sliding and exhumation of the blueschist rocks and the Olympus Ossa carbonate dome.

14 Ψηφιακή Βιβλιοθήκη Θεόφραστος - ΤμήμαOPYKTOI Γεωλογί αfl;\OYTO[/MINERALς. Α.Π.Θ. WEALTH 96/1995 AP r~ PIITOtZ>?AH

a

c

1,2 nlm b

d

PHOTO 1: Thin sections iflllstrating the main tectonic structures and the symmetry of deformation of the blues­ t chists lin it in eartem Thessaly (location of the samples in Fig. 2, Ph la): a. Sl and 52 mylonitic fabric. 51 is defined d by elongated blue amphibole (Ola). Bille amphibole deformed during D2 and reorientated along the S2. Here, top If to SW-movement. GIi-white mica. b. Conjugate shear bands and symmetrical development during D2 of amphibole if clasts (Amphi). Significant component of coaxial deformation. S1 lie paraflel to S2 fabric due transposition. c. S 1 schistosity with blue amphibole (Gla) development oblique to S2 fabric. Top to SW sense of shear. NE dipping low angle, extensional shear bands. d. Low angle, asymmetric, NE dipping, shear bands, indicaeing normal northe­ astwards sense of shear.

1[) OPYKTOL ni\OYTOI/MINERALΨηφιακ WEALTHή Βιβλιο θ96/1995ήκη Θεό φραστος - Τμήμα Γεωλογίας. Α.Π.Θ. 15 progressive stage of the low angle normal shear zones, the blueschists unit and the overlying nappes are sil11ullaneollSly subjected to a total thinning in the vertical sense, and to a subhorizontal stretching. Combining these with the results of the strain­ analysis (KTLlAS et a1. 1991 b, SFEIKOS et a!. 199 I, SFEIKOS 1992), it was found that the structure of the upper plate nappes underwent a vertical thinning by half or even more of its initial thickness, during the D2-deformation and its progressive stages.

DISCUSSION

PHOTO 2: Symmetl ic boudInage of competent I develop perpendicular to the S2-myloni{ic fabric. interpreted as having evolved in the period of the Coaxiill deformation. Tectonic L'ontiict between bluesc­ hists iwd Olympus-Osse Unit at the SE side of the middle-upper Eocene, during the subduction of Olympus dome (exactly location of the sample in Fig the blueschist unit, under the Pelagonian continental -).l' block. This event took place in a regime of plate convergence and compression. Thus during thls component of coaxial extension subparallel to the Eocene period the stacking of the nappes and the layering with associated 52 mylonitic fabric nearly overthickening of [he continental crust, must have parallel to the boundaries of the nappes as well as been combined with the burial of the blueschist to the main detachment zone betwt'Gn the lmit under the Pelagoniau (Fig. 8A). blueschists and the lower plate. (4) The Thc absence of HP/LT mineral parageneses from subhorizontal development of the maximuJll [he Olympns-Ossa, Kranea, and Rizomata units of stretching (X-axis) and the corresponding almost the lower plate, suggests that these units have never vertical development of the maximum thinning participated in the high pressure compressional (Z-axis) of the D2 final strain ellipsoid in the deep tectonics of the blueschists. This means that, whole pile of mylonitic rocks. (5) The jnstaposition, although the Olympus-Ossa, Kranea, and Rizomata after the peak of the M2 metamorphism, of the units comprise the lower tectonic units of the inner uppermost nappes over the HP/LT blueschist unit Hellenides, they have not been snbducted to a or directly over the lower Olympus-Ossa plate, by great depth. sliding along the semiductile low angle extensional shear zones. (6) The normal low angle shear zones .....-...-. ·--(S-~-X_----- I --- \ exhibit a southwest-directed down-dip sense of shear to the west of the Olympus-Ossa dome and a ___ --~-::-- ~\ 'Z northeast-directed down-dip sense of shear to the --~~~- east of the dome. (7) Parallelism between the mylonitic L2 elongarion lineation and the striation S8---:=~~~ on the sliding plane of the low angle extensional j] shear zones. (8) The possible connection of the ---JI""" s ductile D2 deep tectonics, with its relatively --_._-- --­ b contemporary syntectonic basins of the upper plate FIGURE 9: Reconstruction of tile initial domino-stru­ tJ at the higher tectonic levels, which exhibit a similar cture development of the nappes in the Olympus-o.%iJ Sl symmetry with a maximum extension of a NE-SW region, during the nappe emplacement. Progressively, a( direction (e.g. the Mesohellenic trough BRUNN the gradual uplift of the underlying lower plate of 01­ (I 1956, PAPANTKOLAOU et a1. 1988). ympus-o.ssil was the result of the opposed eSCilpe of the In thls way during the D2 tectonics and its overlying nappes. th 16 Ψηφιακή Βιβλιοθήκη Θεόφραστος - ΤμήOPYKTOL:μα Γεωλ οnJ\OYTOL!MINERALγίας. Α.Π.Θ. WEALTH 96/1995 01 Moreover, we should assume that the right after the compression stage and the deep emplacement of the nappes with the blueschists at tectonics, the unit of the blueschists and the their base, over the nonmetamorphosed rocks of overlying pile nappes were emplaced over the the lower plate, occurred during or after the Late sediments of the Olympus-Ossa, Kranea, and Eocene, after the peak of the high pressure Rizomata units, in a regime of subhorizontal ductile metamorphism. stretching and simultaneous subvertical thinning The problem is: how these deep crustal high of the crust, covering the cold foreland units pressure rocks have been emplaced over the outwards (Fig. 8B). This proposal is in contrast to sediments of the nonmetamorphosed foreland at the previous works by SCHERMER et a1. 1989, higher tectonic levels, wi thout any significant SCHERMER 1990, which interpret the emplacement change of the high pressure mineral parageneses. mechanism of the blueschists as the result of thrust An emplacement mechanism of the high pressure tectonics after the peak of the high pressure rocks, according to the model of "buoyant uprise", metamorphism. by return flow in the subduction zone (ENGLAND The stretching regime during the emplacement & HOLLAND 1979, CLOOS ]982, ERNST 1984) of the nappes also warrants, to a large extent, the does not seem well supported: the blueschist unit lack of high pressure parageneses in the rocks of is not a well defined unit with an opposite sense the lower Olympus-Ossa, Kranea, and Rizomata of shear at its upper and lower boundaries and the units since, according to this pattern these do not stretching lineations are nearly subhorizontal participate in the deep underthrusting tectonics throughont the study area. Furthermore, the during the nappe stacking. On the contrary, if the surronndings of the blueschists have been intensely emplacement of the blueschists and the overlying deformed during the D2-deformation, so that the unit had developed simultaneously with the nappe possiblity of motion of the rising blueschist body stacking, the lower unit should have normally through an otherwise static medium is also rejected. undergone the most intense high presstlre deep Density ontrasts between the blueschist unit and tectonics. the COt1llTry rocks cannot be regarded as a driviug However, due to the contiuuing convergence of force for t Ie buoyant uprise, it relative proportions the plates compression and nappe stacking migrate of metabaRlleS and metasediments are taken into further out at the front of the stretching masses. account. Thus, the Upper Eocene-Oligocene thrust tectonics Low angle or horizontal stretching lineation on of the external Hellenides (BRUNN 1956) develops steep foliatia planes as required by the "wrench simultaneously with the crustal stretching and faulting model" for emplacemeut of high pressure thiuuing in the inner Hellenides (Fig. 8B). rocks at higher lectonic levels (KARIG 1980), are In DAVIS et a1. 1983, PLATT 1986 it is mentioned not often observed in the field. Steep S2 foliation that the slowing down of the plate convergence tterns with NE-SW trending subhorizontal rate, the change of direction of the plate ching lineation, as for example along the NW convergence, or even a roll-back of the subduction boundary of the Olymous carbonate dome, should zone, may cause the stretchi.ng of the overthickened be regarded as the result of the late updoming of wedge and its emplacement over the cold orogenic the Olympus carbonate dome or the perpendicular foreland. to the maximum stretching compression (Fig. 2 & Such changes in the kinematics of the plate 8B). convergence in the Alpine orogenic system have On the other hand, the geometry of the kinematics been described by many investigators (DEWEY et and the deformation regime of the D2-tectonics, a1. 1973, SAVOSTIN et a1. 1986, MEULENKAMP sufficiently warrant the emplacement of the et a1. 1988) so that shape adjustments of the scheme blueschists and the overlying pile of nappes over of the Alpine orogenic wedge are likely, resulting the foreland, as a result of the subhorizontal in the stretching of the Eocene nappe pile and its stretching of an overthickened accretional wedge, emplacement over the foreland. according to the "underplating and extension" model Exhumation of the blueschists and the lower plate (PLATT 1986). Thus, we suggest that during the initial stages of The continuous underplating of cold sialic the D2-deformation, at the Upper Eocene-Oligocene, material of the foreland under the stretching wedge, 5 17 OPYKTO[ nAOYTOL/MINERALΨηφιακ ήWEALTH Βιβλιοθ ή96/1995κη Θεό φραστος - Τμήμα Γεωλογίας. Α.Π.Θ. combined with the considerable slide downwards blueschists (DURR 1975, DURR et aL 1978) must of masses along the low angle extensional shear be associated with the development of a new zones with an opposite sense of shear are the cause subduction cycle. A significant crustal thinning of the rapid upward rebound of the blueschists aud stretching accompanying this Oligocene­ and the lower plate dnring the Oligocene-Miocene. Miocene thermal overprint of the Cydadic Massif At the same time, a further thiuning and collapse (LISTER et al 1984, BUICK 1991, FAURE et al. of the nappe pile takes place', finally resulting in 1991). The HPjLT phyllite-quartzite unit of Mani the uplift and exhumation of the lower plate in and Creta offers clear evidence of the existence of the form of the Olympus-Ossa, Kranea, and this new orogenic cycle of Oligocene-Miocene age Rizomata tectonic windows in the inner Hellenides (Fig. 1, WACHENDORF et aL 1980, SEIDEL et aL .as is proposed also by SCHERMER 1990, 1993, 1982, MEULENKAMP et al. 1988, KILLA.S et al. and KILlAS et al. 1991 a,b (Fig. 8C, 9). 1992, FASOULAS et al. 1993). This event, however, This Oligocene-Miocene stretching and thinning did not take place further north in the mainland of the inner Hellenides in a NE-SW direction is of Greece, where the orogenic process was definitely also supported by the simultaneous evolution and concluded with the emplacement of the blueschists filling of the Mesohellenic trough with Oligocene­ and the overlying nappe pile over the cold foreland Miocene sediments. (BRUNN 1956, PAPANIKOLAOU and its subsequent uplift and exhumation. et a!. 1988). Therefore, this trough initially formed at the tectonically higher levels of the Eocene CONCLUSIONS nappe pile simultaneously with the ductile D2­ Structures, kinematic analysis, strain analysis and deformation at depth, east and behind of the metamorphism-tectonics relations of the 01ympus­ simultaneous (Oligocene-Miocene) imbrication of Ossa blueschists, show that their tectonic evolution the external Hellenides (Fig. 8B) began in the Eocene with a rapid subduction and Moreover, considerable ophiolitic masses of the burial under the Pelagonian with the result that upper tectonic units during the same Oligocene­ they were metamorphosed under high pressure low Miocene period moving dowuwards to SW are temperature couditions. Nappes stacking and crustal emplaced over the Oligocene imbricated flysch of thickening takes place during this period. the external Hellenides (MOUNTRAKIS et aL 1992 The postburial history of the blueschists is a, b). characterized by an Upper Eocene-Oligocene ductile The syn-D2 uplift of the blueschists was not deformation under conditions of lower greenschist accompanied by any major reheating of the system. facies metamorphism. During this event the 1 This is an uncommon case of the P-T-t paths blueschists together with the overlying nappe pile evolution that is proposed from SPEAR & of the Pelagonian stretchends, and thinnends cover SELVERSTONE 1983, and ENGLAND & THOMPSON the cold sediments of the Olympus-Ossa unit of 1984. the foreland. This event can indeed be interpreted if we take The underplating of the cold plate beneath the B into account a tectonic sceuario where the nappe pile and the continuous supply of cold blueschists are detached from a relatively hot foreland materials to the subduction zone, (- 350°C) substratum to be emplaced Ol1to colder progressively causes the uplift and further tectonic crust that has not suffered deep burial and heating. thinning of the whole nappe pile, with no significant Thus, the preservation of the high pressure reheating, during the Oligocene-Miocene. As a result parageneses of the blueschists and the presence of of the significant opposite escape of nappe masses Cr nonmetamorphosed to slightly metamorphosed during this upward isostatic movement, the foreland units, under the intensely metamorphosed Olympus-Ossa unit is revealed in the form of an rocks of the nappes of the inner Hellenides, can extensional dome. also be accounted for. The stretching and emplacement of the blueschists The Oligocene-Miocene annealing (ATHER et al. and t~e overlying nappe pile over the foreland in CA 1982, HOlCK 1991), of the Cyclades blueschists, a total compression regime, due to the continuing whose HPjLT metamorphism also dates to the convergence of the plates, can be attributed either Eocene (ANDRIESSEN et a!. 1979), as well as the to a retardation of the rate of convergence of the preseuce of metamorphosed units below the plates, during the Oligocene. Ψηφιακή Βιβλιοθήκη Θεόφραστος - Τμήμα Γεωλογίας. Α.Π.Θ. 18 OPYKTOI nI\OYTOI!MINERAL WEALTH 96/1995 OPY REFERENCES

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(1980): Geodynamik des Mittelsgriechischen at' Pehlgonian Nappes in the Kranea area, North Deckenstapels (nordliches Dikti-Gebirge). -Geot. Tessaly, Greece. -Bull. Soc. Geol. or Greece, 25/1, Forsch., 59, 1-72., Univ. Berlin, Berlin. 101-115, Geol. Soc. or Greece, Thessaloniki. SFEIKOS, A. (1992): Geology, analysis of deformation WlJBRANS, I & McDOUGALL, I. (1988): MetamOlphic and kinematics of the Pelagonian nappe system, evolution of the Attic Cycladic Metamorphic Belt Kamvounia mountains (North Thessaly, Greece).­on Naxos (Cyclades, Greece) utilizing 40Ar/J9Ar TGA, A 12, 110 p., Uni~'. Tubingen. Tubingen. age spectrum measurements.- 1. Metamorph Geol., SPEAR, F & SELVERSTONE, 1. (1983): Quantitative 6, 571-594, Blackwell Scientific Publications, P-T pilths from zoned minerals: theory and tectonic Oxford. applications. -Contr. Miner. Petrol., 83, 348-357, YARWOOD, G & D1XON, 1. (1977): Lower Cretaceous Springer Verlag, Heidelberg. and younger thrusting in the Pelagonian rocks of TRIANTAFILL1S, E., CHORIANOPOULOU, P & the High-Pieria.- VI Call. Geol. Aegean region, VAPTI-MATA.RAGA, ,\1 (1987): Geological map A.thens, 269-280., Geol, Soc. of Greece, Athens.

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