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International Geology Review
Vol. 52, Nos. 4 – 6, April– June 2010, 423–453
Structure and tectonics of subophiolitic mélanges in the western
Hellenides (Greece): implications for ophiolite emplacement tectonics
Constandina Ghikasa, Yildirim Dileka* and Anne E. Rassiosb
a
Department of Geology, Miami University, Oxford, OH 45056, USA; bInstitute of Geology and
Mineral Exploration, Lefkovrisi, 50100 Kozani, Greece
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(Accepted 3 April 2009)
The Jurassic Vourinos ophiolite is part of the Western Hellenide ophiolite belt in
Greece and rests tectonically on the Pelagonian microcontinent. The Vourinos and
coeval Pindos ophiolite to the west display suprasubduction-zone geochemical
affinities, and represent remnants of oceanic lithosphere formed in a rifted incipient
arc – forearc setting within the Pindos Basin. In structurally descending order, and from
west to east, the subophiolitic mélange beneath the Vourinos ophiolite contains the
Agios Nikolaos Formation (ANF) and a rift assemblage, both of which display ENEvergent thrust faults, shear zones, and folds. The ANF comprises schistose mudstone
with pebbles, cobbles, and boulders of arenite and wacke derived from the crystalline
basement of Pelagonia. Imbricated along ENE-directed thrust faults and metamorphosed up to lower amphibolite facies, the ANF represents continental rise deposits of
the rifted Pelagonian margin. The rift assemblage includes blocks of basaltic lavas,
ribbon chert, micritic cherty limestone, metagabbro, dolerite dikes, and serpentinite
breccia that are commonly in thrust contact with each other and are tectonically
imbricated with the Pelagonian carbonates; however, primary intrusive and
depositional contacts are locally well preserved. Gabbro and dolerite dikes are locally
intrusive into the recrystallized carbonates and metapelitic rocks of the Pelagonian
microcontinent. Lavas display mid-ocean ridge basalt within plate basalt affinities and
represent Upper Triassic rift units that erupted during the separation of Pelagonia from
Apulia. Gabbro, dolerite, and serpentinite breccia are the products of a magmatic rifting
episode prior to the onset of seafloor spreading in the Pindos Basin. The Vourinos
subophiolitic mélange thus consists of passive margin and rift assemblages that were
tectonically overridden by the Vourinos ophiolite in the Middle Jurassic. The Avdella
mélange beneath the Pindos ophiolite to the west includes a chaotic matrix including
ophiolitic clasts and blocks, reefal to pelagic limestones, and olistostromal turbidites
that range in age from Triassic to Jurassic – Cretaceous. The Avdella mélange rests
tectonically on a Cretaceous – Eocene flysch unit and the Cretaceous – Eocene shelf and
slope deposits of the Apulian margin along west-directed thrust faults. The
evolutionary history of the Vourinos and Avdella subophiolitic mélanges indicates
diachronous tectonic emplacement of the Mesohellenic oceanic slab, first in the east as
a result of a trench-continent (Pelagonia) collision in the Middle Jurassic, then in the
west due to the Eocene collision of Apulia with Eurasia.
Keywords: subophiolitic mélange; Western Hellenides of Greece; Pindos Basin;
Pelagonian microcontinent; passive margin sequence; rift lavas; suprasubduction-zone
ophiolite
*Corresponding author. Email: [email protected]
ISSN 0020-6814 print/ISSN 1938-2839 online
q 2010 Taylor & Francis
DOI: 10.1080/00206810902951106
http://www.informaworld.com
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Introduction
Jurassic to Cretaceous ophiolites in the Alpine orogenic belt (Figure 1) represent
fragments of oceanic lithosphere generated during the Wilson cycle evolution of Tethyan
basins (Dilek and Flower 2003). These ocean basins developed in response to successive
episodes of the break-up of Gondwana and evolved as latitudinal, east –west-oriented
basins separated by discrete continental fragments. Most eastern Mediterranean ophiolites
are spatially associated with both Triassic –Jurassic volcanic rocks and subophiolitic
mélanges that rest tectonically on passive margin sequences of ribbon continents
(i.e. Pelagonia in the Balkans and Tauride platform in southern Turkey; Figure 1). These
extrusive rocks display within-plate-type alkaline basalt (WPB) to transitional and
mid-ocean ridge basalt (MORB) chemistry and are intercalated with hemipelagic to
pelagic sedimentary rocks. They appear to represent rift-related magmatic pulses along
Figure 1. Simplified geological map of the Balkan Peninsula and the Aegean region, showing the
distribution of Tethyan ophiolites (in green) and active plate boundaries (modified from Dilek and
Flower 2003). AC, Antalya Complex; IA, Isparta Angle; IAESZ, Izmir – Ankara – Erzincan Suture
Zone; IPO, Intra-Pontide ophiolites; KF, Kefalonia Fault; NAFZ, North Anatolian Fault Zone; MO,
Mirdita ophiolite; PO, Pindos ophiolite; VO, Vourinos ophiolite. Black arrow shows the
convergence direction of the African lithosphere at the Hellenic Trench.
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425
the northern periphery of Gondwana (Juteau 1980; Pe-Piper 1982, 1998; Pamic 1984;
Shallo et al. 1990; Jones and Robertson 1991; Dilek and Rowland 1993; Malpas et al.
1993; Saccani et al. 2003), prior to the onset of seafloor spreading and oceanic crust
formation in the Tethyan basins (Dilek and Flower 2003).
Subophiolitic mélange units form thin (several hundred metres) slivers of thrust sheets
sandwiched between the metamorphic soles or upper mantle peridotites in the upper plate
and the continental margin units in the lower plate along Tethyan suture zones (Shallo
1990; Wakabayashi and Dilek 2003; Dilek et al. 2005, and references therein). Oceanic
rocks within the mélanges include ophiolitic material, fragments of seamounts, rift lavas,
and high-grade metamorphic rocks (mainly metamorphic sole rocks), whereas detrital
material and blocks in the mélange are generally derived from continental margin units
(shelf, slope, and rise sedimentary rocks) as well as continental rocks (Hsü 1968; 1974,
1988; Liou et al. 1977; Cloos and Shreve 1988a, 1988b). The mélange matrix may consist
of mudstone, sandstone, or serpentinite depending on the mélange-forming processes
(Phipps 1984; Raymond 1984; Yilmaz and Maxwell 1984; Dilek 1989; Shallo 1990; Polat
et al. 1996; Dilek et al. 1999; Scherreiks 2000). In most cases, subophiolitic mélange units,
continental margin rocks, and ophiolitic peridotites are tectonically imbricated in the
direction of ophiolite emplacement. Subophiolitic mélanges provide crucial information
about the tectonic, metamorphic, and sedimentary processes and their P – T conditions
during advancement of ophiolitic thrust sheets onto continental margins; they can also
constrain the maximum age of the timing of ophiolite emplacement. Because they contain
material from the rift – drift tectonics of the basins prior to their terminal closure, these
subophiolitic mélanges can also give us significant clues about the tectonic, sedimentary,
and magmatic processes and events involved in the early stages of the geodynamic
evolution of the basins and their continental margins.
In this paper, we document the internal stratigraphy and structure of a subophiolitic
mélange associated with the Jurassic Vourinos ophiolite in the Western Hellenides in
northern Greece. We then analyse our data and interpretations in a regional tectonic
framework to derive some conclusions about the tectonic processes involved in the
formation of the Vourinos ophiolite and its subophiolitic mélange and in the emplacement
of the ophiolite – mélange units onto the Pelagonian continental margin. The Vourinos
subophiolitic mélange is typical of most mélanges associated with the Tethyan ophiolites
in the eastern Mediterranean region, and therefore represents a case study of mélange
formation that involves rift – drift –collision tectonics of restricted marginal basins in the
Tethyan realm and beyond.
Ophiolite geology of the Hellenides
The NW-trending Neotethyan ophiolites in the Hellenides occur in two distinct zones
bounding the Pelagonian ribbon continent (Figure 2(a)). The Vardar Zone ophiolites, also
known as the ‘Innermost Hellenic ophiolites’ (Smith 1993) or the ‘Eastern Hellenic
ophiolites’, are located east of Pelagonia and are Jurassic –Early Cretaceous in age
(Bébien et al. 1986; Mussallam and Jung 1986; Robertson 2002). The ophiolites to the
west of the Pelagonian microcontinent, called the ‘Western Hellenic ophiolites’, are nearly
coeval or slightly older than those in the Vardar Zone and are spatially associated with
Triassic –Jurassic volcano-sedimentary units and mélanges (Smith 1993; Robertson and
Karamata 1994; Bortolotti et al. 2005; Dilek et al. 2005; Saccani and Photiades 2005).
The Western Hellenic ophiolites in Greece and Albania, their northward continuation into
Kosovo and Serbia, and the Dinaric ophiolites in Bosnia and Croatia collectively form
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the Pindos Zone ophiolites in the western Balkan Peninsula (Robertson and Karamata
1994; Hoeck et al. 2002; Smith and Rassios 2003; Koller et al. 2006; Dilek et al. 2008).
These ophiolites show bidivergent emplacement onto Pelagonia in the east and Apulia in
the west (Figure 2(b)).
The root zones of the Pindos and Vardar Zone ophiolites and their possible genetic
relations have been a source of debate in discussions of Balkan geology. Some researchers
argue that both the Pindos and Vardar Zone ophiolites were derived from a Mesozoic
ocean basin located east of the Pelagonian microcontinent, and that the Jurassic ophiolites
in the Pindos Zone thus represent far-travelled nappe sheets with their roots in the Vardar
Zone (Collaku et al. 1990, 1991; Ricou et al. 1998; Bortolotti et al. 2005; Gawlick et al.
2007; Schmid et al. 2008). These interpretations imply that the Pelagonian microcontinent
was not rifted apart from Apulia, and that it was an outlier of the Apulian passive margin at
all times throughout the Mesozoic. The alternative and more widely accepted models
suggest that the Pelagonian block was an insular continental entity separating the Pindos
and Vardar (and/or Maliac) Basins within the Neotethyan realm during much of the
Mesozoic (Smith 1993; Robertson and Shallo 2000; Robertson 2002; Stampfli and Borel
2004; Dilek et al. 2005, 2007; Saccani and Photiades 2005). The structure and tectonic
evolution of the subophiolitic mélanges in the Pindos Zone provide important constraints
on the validity of these different geodynamic models.
Vourinos and Pindos ophiolites: Mesohellenic oceanic slab
The Vourinos and Pindos ophiolites constitute the eastern and western edges, respectively,
of a larger Jurassic ophiolitic body, known as the Mesohellenic ophiolite (Rassios and
Dilek 2009) in the Western Hellenides (Figure 2(b)). These two ophiolites are exposed
. 30 km apart, although magnetic and drilling data indicate that they are part of a single
mafic – ultramafic slab, which is continuous in the subsurface beneath the Palaeogene –
Neogene sedimentary cover of the Mesohellenic Trough (Saccani and Photiades 2004;
Rassios and Moores 2006; Rassios and Dilek 2009). This ophiolite belt continues to the
NW into Albania (SE Albanian and Mirdita ophiolites) and to the SE into Southern Greece
(Figure 1). Both the Vourinos and Pindos ophiolites overlie subophiolitic mélanges
displaying some lithological similarities.
The Pindos and Vourinos ophiolites are coeval in age and display suprasubductionzone affinities (Kostopoulos 1989; Smith and Rassios 2003; Pe-Piper et al. 2004; Saccani
and Photiades 2004; Beccaluva et al. 2005; Rassios and Dilek 2009), although they also
show major differences in their internal structure and geochemical fingerprints. The Pindos
mantle section is harzburgitic to lherzolitic, indicating comparatively fertile mantle
compositions (Nicolas and Boudier 2003; Beccaluva et al. 2005; Rassios and Moores 2006).
R
Figure 2. (a) Simplified geological map of the west – central Balkan Peninsula and Adriatic Sea
region, showing major tectonic zones and ophiolite occurrences. Key to lettering for different
Jurassic ophiolites in the region (from north to south): Trp, Tropoja; Krb, Krrabi; Kuk, Kukesi; Puk,
Puke; Skd, Skenderbeu; Blq, Bulqize; Shp, Shpati; She, Shebenik; Vo, Voskopoja; Pin, Pindos;
Vou, Vourinos; Koz, Koziakas; Oth, Othris (modified after Dilek et al. 2007). (b) Tectonic
cross-section depicting the crustal architecture of the Apulia –Pelagonia collision zone and the root
of the Mesohellenic ophiolite (Pindos (PO) to the west and Vourinos (VO) to the east with their
underlying mélange units). Palaeogene – Neogene sedimentary rocks of the Mesohellenic Trough
(shown in yellow) cover the keel of the mafic – ultramafic slab that constitutes the root zone of the
Mesohellenic ophiolite. MHT, Mesohellenic Trough; VM, Vourinos mélange (modified after
Rassios and Dilek 2009).
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A nearly 1-km-thick extrusive sequence in Pindos shows MORB to island arc tholeiite (IAT)
and boninitic compositions (Capedri et al. 1980; Kostopoulos 1989; Jones and Robertson
1991; Jones et al. 1991; Pe-Piper et al. 2004; Saccani and Photiades 2004; Beccaluva et al.
2005; Dilek and Furnes 2009). The Pindos ophiolite is underlain by the subophiolitic Avdella
mélange (Kostopoulos 1989; Jones and Robertson 1991), which we describe in a later section.
The Vourinos ophiolite comprises a large mantle section of depleted harzburgite and
dunite with economically viable chromite bodies, a petrologic Moho, 4 –5-km-thick mafic
and ultramafic cumulates, sheeted dikes, a thin extrusive sequence consisting mainly of
pillow lavas, and a Cretaceous pelagic limestone cover (Jackson et al. 1975; Harkins et al.
1980; Vrahatis and Grivas 1980; Rassios 1981; Rassios et al. 1983; Grivas et al. 1993;
Liati et al. 2004; Rassios and Moores 2006; Sharp and Robertson 2006). All lithological
units in the Vourinos ophiolite have been tilted or overturned to the west, so that the
structural progression of units from top to bottom is from west to east, with the upper
mantle peridotites occurring at the easternmost rim of the ophiolite nappe (Figure 3).
The Vourinos dikes and lavas are mainly IAT in composition, but boninitic dikes and lavas
occur as late-stage magmatic products both in the sheeted dike complex and the extrusive
rocks (Rassios and Dilek 2009, and references therein). Around the eastern rim of the
Figure 3. Geological map of the Vourinos ophiolite. Numbered boxes show subophiolitic mélange
localities that are investigated in detail in this study.
Figure 4. Panoramic view (to the north) of the Vourinos ophiolite, its subophiolitic mélange exposed in the Agios Nikolaos Valley, and the Pelagonian carbonate
platform.
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ophiolite, the subophiolitic mélange occurs as a discontinuous sliver between the ophiolite
and the Pelagonian microcontinent (Figure 3; Jones and Robertson 1991; Ghikas et al.
2007; Rassios and Dilek 2009).
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Vourinos subophiolitic mélange
A 300– 500 m-thick mélange unit occurs structurally underneath the Vourinos ophiolite
and on top of the Pelagonian microcontinent (Figure 4), and is exposed discontinuously
around the entire margin of the ophiolite. This study has focused on four major localities
around the rim of the Vourinos ophiolite, where the representative sections of the Vourinos
subophiolitic mélange are exposed. These localities include: (1) the Agios Nikolaos Valley,
(2) North of Skoumtsa, (3) Frourio, and (4) the Aliakmonas River Valley (Figure 3).
The Vourinos subophiolitic mélange locally contains numerous blocks and thrust sheets
of various rock types, ranging in size from several metres to several kilometres. These
subunits are typically less than 100 m thick, and thus the surface expression of the mélange
resembles a collection of thin lensoidal blocks and thrust sheets with a strong preferred
alignment and imbrication in the direction of ophiolite emplacement to the NE (Figure 5).
Most mélange subunits show an internal fabric (bedding, foliation, or shearing) and the
contacts between the subunits are most commonly defined by reverse and thrust faults.
Both the contacts and the internal fabric elements in the subunits approximately parallel
the dip of the bounding thrust faults of the mélange. The general emplacement direction of
the Vourinos ophiolite is to the northeast (Figure 5; Rassios and Moores 2006; Rassios and
Dilek 2009), but locally the contractional deformation associated with this northeastward
movement has been overprinted by oblique-slip deformation. Along the north–
northwestern edge of the ophiolite (Mt. Vourinos, Agios Nikolaos Valley, and North of
Skoumtsa), the Vourinos subophiolitic mélange shows mainly contractional deformation
with sinistral oblique-slip overprint and duplexing (Figure 6). In the central region
(Frourio), the deformation is manifested in major ENE-vergent thrust faults with
second-order, conjugate dextral and sinistral oblique-slip faults (Figure 7). Along the
southern edge of the ophiolite nappe (Aliakmonas River Valley), the southeast-vergent
thrust faults show strong dextral strike-slip components (Figure 8).
The Vourinos subophiolitic mélange consists mainly of two major units, the Agios
Nikolaos Formation (ANF) and the Rift Assemblage. Below, we describe the internal
structure and stratigraphy of these two mélange units.
Agios Nikolaos Formation (ANF)
The ANF, as originally named by Naylor and Harle (1976), is the most extensive subunit
of the mélange and can be characterized as a ‘broken formation’. It is a brown, foliated to
R
Figure 5. Roadcut maps along the Zavordas Road traversing the basal thrust of the Vourinos
ophiolite in the Aliakmonas River Valley. Directly underneath the serpentinized and sheared
harzburgite is a 0.5 m-thick amphibolite unit making up the metamorphic sole. The ANF beneath the
amphibolite comprises mostly pervasively foliated mudstone, sandstone, and conglomerate that are
metamorphosed to greenschist facies. Sandstone clasts are elongated and mudstone displays pencil
cleavage. Cleavage planes flow around greater than 1-m-long blocks and boulders in the sandstone
conglomerate. Both the sheared serpentinite and the ANF are imbricated along E – SE-directed thrust
faults and duplex structures. Panels A –C are continuous from WSW (top) to NE (bottom).
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schistose mudstone containing pebble-, cobble-, and boulder-size clasts of sandstone
(arenite and wacke) mainly composed of quartz and plagioclase grains (Figure 9).
The provenance of the sandstone material is interpreted to be the Pelagonian hinterland
based on the presence of quartz, feldspars, muscovite, and detrital zircons (Mountrakis
1986; Lips et al. 1998; Most et al. 2001; Anders et al. 2006). The ANF has little or no
ophiolitic or Pelagonian carbonate blocks within it; however, the mudstone matrix is
locally tectonically mixed with reworked serpentinite mud and silt along shear zones.
Most blocks are lensoidal in shape and/or bounded by thrust faults. Both lithologically and
structurally, the ANF is analogous to the Lichi Mélange associated with the East Taiwan
ophiolite in Taiwan (Liou et al. 1977).
In most areas, the ANF has undergone lower greenschist-facies metamorphism, as
evidenced by the presence of abundant quartz – sericite – calcite – chlorite in the matrix
rocks. Higher grades of metamorphism are seen lower in the structural section near the
contact with the Pelagonian platform carbonates, where the ANF becomes a phyllite with
a strongly developed foliation (Figure 10) and actinolite – albite – chlorite –epidote
mineral assemblages. Phyllitic foliation (cleavage) in the matrix generally dips to the west
(SW or NW) with quartz elongation and/or stretching lineation also plunging to the west
(Figure 10). Both transposed bedding and phyllitic cleavage are folded around NW – SEtrending fold axes, which plunge at moderate to steep angles in these directions. Tight to
isoclinal folds and crenulation folds are commonly overturned to the NE. The sandstone
clasts are recrystallized to quartz veins, boudins, and lenses, and are commonly strongly
folded and elongated (Figure 9(c)). Adjacent to the Pelagonian micocontinent, the ANF
becomes a schist.
Rift assemblage
Discrete blocks and thrust sheets of igneous rock assemblages and associated sedimentary
rocks are locally abundant within the Vourinos subophiolitic mélange (Figures 6 and 8).
The rock types include basaltic lavas, serpentinite breccia, gabbro, dolerite, jasper, ribbon
chert, cherty carbonate, metapelitic rocks, and mylonitized pelagic carbonate, as lensoidal
or phacoidal bodies typically bounded by thrust faults. However, these lithologies locally
have intrusive and/or depositional contacts and/or are interlayered with each other. These
blocks occur along the northern and southern margins of the ophiolite, such as North of
Skoumtsa, in the Agios Nikolaos Valley, and in the Aliakmonas River Valley, but are not
observed at the Frourio locale on the eastern margin of the Vourinos ophiolite (Figure 7).
They are tectonically intercalated with the ANF (Figures 6 and 8).
Volcanic units and associated sedimentary rocks
Volcanic rocks are widely distributed in the mélange, particularly in the Aliakmonas River
Valley, the Agios Nikolaos Valley, and North of Skoumtsa. They occur as large blocks of
meta-tuff, pillow lavas, and hyaloclastites, varying in size from 10- to 100-m-long
(Figure 11(a,b); Zimmerman 1968; Ross and Zimmerman 1996). The basaltic lava
outcrops along the Aliakmonas River are strongly coloured in purple, dark red, pink,
and green, and are spatially associated with pink micritic limestone or cherty limestone
(Figure 11(c)). Calcite and chlorite fill brittle fractures as secondary minerals. The phyric
basaltic lavas contain acicular plagioclase microlites and granular clinopyroxene in a
vitrified glassy groundmass. Chlorite occurs as the replacement of olivine. Some of these
basaltic lava blocks contain hemipelagic cherty ribbon limestone (1 – 3 cm thick ribbons)
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Figure 6. Geological map of the Agios Nikolaos Valley and structural cross-sections. See text for
discussion.
C. Ghikas et al.
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434
Figure 7. Geological map of the Frourio region and a structural cross-section. See text for
discussion.
interlayered with siltstone and tuffaceous sandstone (, 0.5 cm thick layers). These rocks
are locally strongly folded and recrystallized.
Basaltic lava outcrops in the Agios Nikolaos Valley farther north occur as thin tectonic
wedges bounded by thrust faults (Figure 8), similar to the lava units along the Aliakmonas
River. However, they do not visually resemble the lavas to the south, and their associated
sedimentary units also differ from their counterparts to the south. These volcanic rocks in
the north are mostly massive lava flows and are brown to grey-green, aphanitic, and
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Figure 8. Geological map of the Aliakmonas River – Zavordas Monastery region and structural
cross-sections. Serpentinite breccia, metagabbro intrusions, dolerite dikes, basaltic lavas, and
associated pelagic limestone collectively constitute the Triassic rift assemblage of the Pelagonian
margin. See text for discussion.
commonly heavily weathered, although few pillow shapes are still locally discernable.
The sedimentary rocks associated with these northern volcanic units are metalliferous red
jasper and pelagic limestone. These sedimentary rocks occur either as continuous
interlayers or as large (0.5 m long) lenses within the volcanic units. The jasper interlayers
are highly folded and crenulated.
Intrusive units and associated sedimentary rocks
The intrusive rock units in the mélange include gabbro stocks and dolerite dikes that are
commonly associated with a serpentinite breccia (Figure 12). They are intrusive into and/or
tectonically overlie the Pelagonian carbonate platform units, and are also tectonically
intercalated with metapelitic rocks, micritic limestone, and chert. They have been intensely
deformed via folding and thrust faulting, but their original contact relationships have
largely survived this contractional deformation. Some of the originally intrusive contacts
were partly reactivated as thrust faults during emplacement of the Vourinos ophiolite.
Gabbroic and doleritic intrusions are best seen along the Aliakmonas River (Figure 8).
They occur structurally at the bottom of the Vourinos subophiolitic mélange, just above the
contact with the intensely deformed and recrystallized Pelagonian platform carbonates and
metapelites. This area of the mélange does not contain the ANF. Gabbroic stocks
and doleritic dikes are intrusive into finely foliated phyllite and marble, psammitic schist,
and mylonitized micritic limestone that constitute the outermost units of the Pelagonian
microcontinent. Locally, the sedimentary and igneous contacts between these rock
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units are well preserved. The Pelagonian marble units contain dasycladacean algae,
algal oncolites, and loferites, suggesting a neritic to littoral environment of deposition
for their protoliths, which ranged in age from Middle Triassic to Upper Jurassic
(Zimmerman 1972).
The gabbroic intrusions here are typically less than 100 m thick, with weakly foliated
to submylonitic fabric parallel to the NE-oriented strike of the outcrop. They appear as
lozenge-shaped, highly sheared intrusive bodies with significant grain size change in short
distances. Mylonitic high-T foliation in the gabbros is folded and crosscut by discrete
narrow mylonitic shear zones. Strong mineral lineation and the fold axes of parasitic
z-folds in these local shear zones all plunge to the west (, 2708). These gabbroic intrusions
are cut by numerous subparallel doleritic dikes (Figure 12(b)), which also extend into the
adjacent ultramafic breccia outcrops. Gabbro and dolerite rocks show pervasive secondary
alteration and lower amphibolite facies metamorphism evidenced by extensive
replacement of pyroxene by hornblende and actinolite. Some gabbro outcrops have
been completely altered to serpentinite, chlorite, and oxidized ferromagnesian minerals.
Epidosite is also common, occurring as thick bands parallel to the foliation, or as
disseminated individual grains.
Serpentinite
There are two types of reworked serpentinite types in the Vourinos mélange. The first one
strongly resembles the serpentinized harzburgite in the upper mantle section of the
Vourinos ophiolite in terms of its mineralogical and textural features, and still contains
relict pyroxene and olivine crystals although highly sheared. This sheared serpentinite is
found directly beneath the ophiolite or as rare thrust sheets within the ANF. It was derived
mainly from the Vourinos ophiolite during its tectonic emplacement, which locally
incorporated blocks of highly sheared harzburgite into the mélange.
The second type of reworked serpentinite bears no resemblance to peridotite rocks
from the ophiolite. It is black and contains rounded to angular clasts of serpentinite
cemented together by secondary serpentine and carbonate minerals. Outcrops of this
serpentinite are typically found in a structurally lower position in the mélange (Figure 8),
near the contact with the mylonitized carbonates and metapelitic rocks of the Pelagonian
microcontinent. All these rocks are strongly deformed with refolded folds, thrust
imbrication, and z-type parasitic folds. Stretching lineations in the mylonitized metapelitic
schists trend , 808 with westerly plunges. Fold axes of the z-folds are locally subvertical
and the shear sense indicators suggest a component of dextral strike –slip deformation
(Figure 8).
Similar serpentinite breccia occurrences have been reported from the Gödene Zone of
the Antalya Complex in SW Turkey (Figure 1), where foliated and reworked serpentinites
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Figure 9. Outcrop photos of the ANF exposed in the Aliakmonas River Valley. (a) Serpentinized
basal harzburgite (Hzb) of the Vourinos ophiolite (left) is thrust over the ANF (right), which in this
locality constitutes the structurally uppermost unit in the Vourinos mélange. (b) General appearance
of the brown, scaly mudstone with numerous small to large clasts of quartzofeldspathic sandstone in
the ANF, here metamorphosed to lower greenschist facies. Hammer for scale. (c) A floating fold
nose of an isoclinally folded sandstone – quartzite (QTZ) clast in a pervasively cleaved mudstone –
phyllite within the ANF. Sharpie pen for scale. Bedding in the original mudstone has been transposed
to phyllitic foliation.
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Figure 10. Equal-area lower-hemisphere stereoplots of various fabric elements in the Vourinos
mélange. (a) North of the Skoumtsa section: Black circles, poles to phyllitic foliation; open circles,
poles to quartz elongation and/or stretching lineation; red arrows, poles to fold axes in the phyllite.
(b) Frourio section: Black circles, poles to phyllitic foliation. (c) Aliakmon River Section: black
circles, poles to phyllitic foliation; black stars, poles to doleritic dikes; black arrows, poles to quartz
elongation and/or stretching lineation. See text for discussion.
439
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Figure 11. Field occurrence of lava units within the Vourinos subophiolitic mélange. (a) Basaltic
pillow lava block in a pebbly mudstone – phyllite of the ANF in Mt Vourinos northwest of the Agios
Nikolaos Valley. Pillow lavas are mostly aphyric, green-brown in colour. (b) Pillow lava block from
the Aliakmonas River Valley, with purple-pink coloured, visible pillows. (c) Close-up image of lava
flows in (b) showing thin beds of pink chert interlayers between the lava flows. ANF, Agios Nikolaos
Formation; CHRT, chert; LV, basaltic lava.
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are tectonically intercalated with Upper Triassic volcanic– sedimentary passive margin
assemblages (Marcoux 1976; Robertson and Woodcock 1981; Dilek and Rowland 1993).
Anastomozing lenses of fault-bounded serpentinite bodies in the Gödene Zone also
include lozenges of gabbro, dolerite, and harzburgite, and are spatially associated with
mafic pillow lavas, volcanic breccias, Upper Triassic pelagic limestones, and radiolarites.
Dilek and Rowland (1993) interpreted the Gödene Zone as part of the rifted continental
margin of the Tauride carbonate platform (or microcontinent).
Metamorphic sole unit
Thin, discontinuous exposures of high-grade metamorphic rocks occur between the
subophiolitic mélange and the overlying Vourinos ophiolite (Figures 7 and 8). Best
examples of these metamorphic thrust sheets are found on the Zavordas Road and Fruorio,
where a few metres- to 20-m-thick amphibolite, garnet-micaschist, and quartzite are
exposed structurally beneath the Vourinos peridotites (Zimmerman 1972; Ghikas 2007;
Myhill 2009). The contacts between these amphibolite and the mélange units are
invariably faulted and sharp, but the crenulation cleavage, tight to isoclinal and overturned
folds, and microfaults all show east-directed tectonic transport similar to those in the
overlying peridotites and the underlying mélange units.
Typical peak-metamorphic mineral assemblages in the Zavordas amphibolites include
hornblende – ferropargasite – plagioclase – garnet – sphene – ilmenite – quartz, with retrogressive chlorite, biotite, and phengite rimming hornblende and infilling fractures in
garnet (Myhill 2009). Peak temperatures of 6708C (^ 358C) for the Zavordas garnet –
hornblende pairs and 770 ^ 1008C for the Frourio garnet micaschists have been obtained
from the Vourinos subophiolitic metamorphic rocks (Myhill 2009). 40Ar/39Ar dating of the
Vourinos amphibolites has revealed a cooling age of 171 ^ 4 Ma (Spray 1984; Spray et al.
1984). Moores (1969) and Spray and Roddick (1980) interpreted the high-grade rocks
beneath the Vourinos ophiolite as a metamorphic sole. The geochemistry of the metabasic
rocks in the Vourinos sole indicates both MORB and WPB affinities (Spray and Roddick
1980; Jones and Robertson 1991), similar to the trace-element compositions of the
metalavas in the rift assemblage units within the structurally lowermost part of the
mélange (see below).
Neogene cover of the Vourinos mélange
Both the Vourinos ophiolite and the subophiolitic mélange are unconformably overlain by
a lower Miocene conglomerate (Tsotyli Conglomerate) of the Palaeogene– Neogene
sedimentary sequence of the Mesohellenic Trough (Figure 13). This matrix-supported and
poorly sorted conglomerate consists almost entirely of ophiolitic material, and the clast
size ranges from greater than 1 m to cm scale (Figure 13) A regional compressional
episode in the late Miocene (Vamvaka et al. 2006) produced several SW-dipping thrust
faults, tectonically stacking the conglomerate, the ophiolite, and the mélange along
R
Figure 12. Photos displaying original igneous structures within the rift assemblage outcrops in the
Aliakmonas River Valley. (a) Gabbro (upper half of photo, Gab) in intrusive contact with
serpentinite breccia (lower half of photo, Srpt breccia). (b) Doleritic fine-grained dike (Dol)
crosscutting the gabbro (Gab) and its internal fabric at an oblique angle. (c) Dolerite – microgabbro
dike (Dol– Gab) intrusive into the recrystallized limestone (marble) of the Pelagonian margin.
Figure 13. Panoramic roadcut image of the Miocene Tsotyli conglomerate, subophiolitic mélange, and serpentinized peridotite of the Vourinos ophiolite. Clasts
in the conglomerate are made entirely of ophiolitic material and range in size from more than 1 m to cm scale. (a) and (b) depict the close-up photos of the contact
relations among these three units. (a) Miocene conglomerate rests unconformably on both the serpentinized peridotite and the mélange (ANF). Serpentinite is in
thrust contact with the ANF. (b) The unconformity is reactivated as an east-vergent thrust fault. Both the conglomerate and the ANF have been imbricated along
NE-vergent thrust faults farther east in this outcrop.
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northeast-vergent thrust faults (Figure 13). These late Cenozoic thrust faults parallel
the orientation of the older emplacement faults and the foliation planes in sheared
peridotites and serpentinites in the ophiolite. Locally, the original depositional contact
between the conglomerates and the Vourinos mélange was also activated as a thrust fault
(Figure 13).
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Geochemistry of the rift lavas in the Vourinos mélange
A detailed geochemistry and petrogenetic history of the rift lavas and dikes in the
Vourinos mélange is presented elsewhere (Dilek et al. in preparation). We report here the
relevant geochemical features of these metabasic rocks. The metalavas in the mélange
straddle the boundary between the tholeiitic and calc-alkaline fields on the AFM diagram,
and overlap compositionally with the Triassic lavas from Albania and southern mainland
Greece (Figure 14(a); Pe-Piper 1998; Saccani et al. 2003; Monjoie et al. 2008). Two main
groups of basaltic lavas, high-Ti and low-Ti, have been recognized within the rift
assemblage. The high-Ti group lavas show WPB to MORB geochemical signatures,
whereas low-Ti group lavas display an IAT affinity, suggesting a subduction zone
influence in their melt evolution (Figure 14(b,c)). The incompatible element-enriched
high-Ti group lavas are likely to have been derived from an E-MORB mantle source in a
rift tectonic environment (Dilek et al. in preparation). The geochemical features of the
low-Ti group lavas suggest that the enriched mantle source was influenced by subduction
zone-derived fluids (Dilek et al. in preparation). We interpret this subduction zone
influence as an inheritance from earlier Palaeotethyan subduction events in the region that
produced the late Palaeozoic continental arc magmatism in the Rhodope, Strandja, and
Sredna Gora zones in today’s eastern Greece and Bulgaria (Pe-Piper 1998; Bonev and
Dilek 2010).
Regional tectonics of the Vourinos mélange
In order to understand the regional geology of the Vourinos subophiolitic mélange within
the tectonic framework of the Western Hellenides and the Pindos Basin, it is important to
compare it with its counterpart on the west, the Avdella mélange (Figure 2(b)).
The Avdella mélange contains abundant clasts ranging in size from 1 cm to 2 km
(Kostopoulos 1989; Jones and Robertson 1991). Major components of this mélange
include thick shallow turbidite and detrital sequences, and olistostromal blocks composed
of Triassic basaltic lavas (Pe-Piper 1982), Jurassic lavas, dolerite, gabbro, and
serpentinized peridotites of an ophiolitic origin, Upper Triassic ‘ammonitico rosso’,
pelagic limestone, Triassic and Jurassic radiolarian cherts, and metamorphic sole rocks
(i.e. amphibolite, greenschist, and micaschist). Volcanoclastic turbidites and debris flows,
composed in many localities of ophiolite-derived detritus, occur both as blocks within the
mélange and make up relatively coherent sequences within the mélange stratigraphy.
These lithologies range in age from Mid-Triassic to Upper Jurassic. The mélange matrix is
heterogeneous (Kostopoulos 1989), and is composed of shale, mudstone, and terrigeneous
sandstone (Kostopoulos 1989; Ross and Zimmerman 1996). This polygenetic composition
is reminiscent of the lithological make-up of other subophiolitic mélanges farther south in
the Hellenic ophiolite belt, such as the Pagondas and Komi Leibadi mélanges of Crete
(Scherreiks 2000).
The Avdella and Vourinos subophiolitic mélanges bear many lithological similarities
and occupy analogous tectonic positions beneath the western and eastern margins of the
Mesohellenic oceanic slab (respectively). However, they differ significantly regarding
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Figure 14. (a) AFM diagram showing the geochemical affinity of the rift lavas in the Vourinos
mélange (after Irvin and Baragar 1971). (b) Zr vs. Zr/Y diagram (after Pearce and Norry 1979).
(c) MnO –P2O5 – TiO2 diagram (after Mullen 1983). (d) Ti vs. V diagram (after Shervais 1982).
Samples of rift-related basalts from the Albanides (data from Monjoie et al. 2008) and southern
mainland Greece are also included for comparison (see text for discussion). Light green (left-facing
blank) triangles, OIB samples (Triassic) from the Albanides (Monjoie et al. 2008); green (rightfacing filled) triangles, BAB samples from Albanides and Othrys ophiolite in Greece (Monjoie et al.
2008); purple blank rhombs, rift lavas from the Vourinos mélange; purple filled triangles, Triassic
basaltic lavas from the Argolis Peninsula (Saccani et al. 2003); green (top-facing, blank) triangles,
basaltic lavas from the Hellenides (Pe-Piper 1998).
their internal structural architecture and the processes involved in their formation.
The Avdella mélange contains blocks of rocks associated with the initial rifting of
Pelagonia and Apulia, as well as with the development of a mature carbonate platform (preApulian platform), the deposition of thick shallow-marine turbidite and detrital sequences
(shelf and slope deposits of the pre-Apulian passive margin), and the emplacement of the
Pindos ophiolite (Jones and Robertson 1991; Rassios et al. 2009). With this association, the
Avdella mélange can be characterized as a ‘block-in-matrix’ type, polygenetic mélange,
the evolution of which involved both sedimentary and tectonic processes (Raymond 1984).
The Avdella mélange was emplaced westwards onto the Cretaceous –Eocene shelf and
turbidite deposits of the pre-Apulian platform (Rassios et al. 2009).
The Vourinos mélange, by contrast, is not a chaotic, ‘block-in-matrix’ mélange.
It consists mainly of tectonic imbricates of a pebbly mudstone (ANF) with
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Pelagonia-derived detrital material, and a Triassic rift assemblage including serpentinite
breccia, meta-gabbro, dolerite dikes, pillow lavas, and chert. Many of these tectonically
incorporated blocks show their own internal fabric features (i.e. bedding, foliation, and
pre-existing folding) overprinted by shearing and folding associated with the emplacement
of the Vourinos ophiolite and the subsequent eastward imbrication of the ophiolite,
mélange, and the overlying Mesohellic sedimentary cover in the late Cenozoic
(Rassios and Dilek 2009; Vamvaka et al. 2010). The ANF is derived almost exclusively
from mudstone and sandstone formed in a depocentre that developed west of Pelagonia
following the early episode of continental rifting in the Mid-Triassic (Zimmerman 1968;
Naylor and Harle 1976; Ghikas et al. 2007). The rift assemblage represents the incipient
rift sequence of the western continental margin of the Pelagonian microcontinent.
We interpret these igneous lithologies and associated sedimentary units collectively as
‘rift assemblages’ that formed during the initial rifting of the Pelagonian microcontinent
from Apulia in the Late Triassic – Jurassic and prior to the onset of seafloor spreading in
the Pindos Basin. Olistostromal blocks or olistoliths are not present in the Vourinos
subophiolitic mélange, and blocks of either the ophiolite or the Pelagonian carbonate
platform are rare to be absent within it. The Vourinos mélange, therefore, can be best
described as a tectonic mélange (Raymond 1984).
Origin of the Vourinos mélange and implications for the Mesohellenic ophiolite
The internal structure and origin of the Vourinos mélange are important not only for its
own provenance and tectonic history, but also for the root zone and the geodynamic
evolution of the overlying Vourinos ophiolite, which shares its geological history. Because
the Vourinos subophiolitic mélange and all its subunits display lithologies and fabric
elements consistent with an origin within the Pindos Basin and with tectonic accretion
beneath an eastward-advancing ophiolite nappe, the Vourinos ophiolite itself must be
rooted in the Pindos Basin. This interpretation rules out the Vardar Zone origin of the
Vourinos ophiolite (and hence the Mesohellenic oceanic slab) and its underlying mélange.
The ANF originated as a mudstone –sandstone unit deposited in a continental rise
setting west of the newly formed Pelagonian continental margin during the Late Triassic –
Early Jurassic. The sediment was derived from the interior of Pelagonia, where the
Palaeozoic and Precambrian schists and gneisses of its basement were exposed (Yarwood
and Dixon 1977; Mountrakis 1986; Koroneos et al. 1993; Schermer 1993; Lips et al. 1998;
Most et al. 2001; Mposkos et al. 2001; Anders et al. 2006). The lava blocks in the mélange
have WPB to E-MORB geochemical signatures, consistent with their continental rift
origin (Pe-Piper 1998; Saccani et al. 2003; Dilek et al. in preparation). These extrusive
rocks are also in depositional contacts with marine sedimentary rocks including
hemipelagic chert and limestone.
Nearly, all blocks in the Vourinos mélange are oriented with their long axis parallel to
the strike of faults bounding the mélange between the ophiolite and the Pelagonian margin.
Ductile and brittle deformation structures observed within the mélange, such as folds,
foliation, mylonitic and cataclastic shear zones, and brittle thrust faults, are all parallel to
the deformation structures within the lower sections of the overlying Vourinos ophiolite
(Figure 5). Mylonitic bands and shear zones within the mélange blocks and the matrix are
generally parallel to each other and to the direction of NE thrusting. In all locales within
the Vourinos mélange, the general direction of tectonic transport is eastwards.
Recrystallized limestone and metapelitic rocks of the Pelagonian microcontinent
structurally beneath the Vourinos ophiolite and mélange show east-vergent, tight to
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isoclinal folds that are commonly overturned to the east. Tectonic models for the root zone
and emplacement of the Vourinos ophiolite must, therefore, also include this information.
The Vourinos subophiolitic mélange represents a parautochthonous oceanic
assemblage – it was formed in an ocean basin at the western edge of the Pelagonian
continent; it is now situated along a suture zone between the Vourinos ophiolite, which is a
remnant of the Pindos oceanic lithosphere, and the western edge of the Pelagonian
microcontinent. Hence, the Mesohellenic oceanic slab is also a parautochthonous unit.
Therefore, a far-travelled nappe origin of the Mesohellenic ophiolite rooted in the Vardar
Zone east of Pelagonia is not supported by our observations and tectonic model.
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Tectonic history of the Pindos Basin and formation of the Vourinos mélange
Apulia and Pelagonia were part of the northern edge of Gondwana in the latest Permian –
Early Triassic. Extension during the Late Triassic caused rifting of Apulia and Pelagonia
and formation of the rift assemblage now preserved in the structurally lower section of the
Vourinos mélange (Figure 15(a,b)). Deposition of mudstone and sandstone, precursor to
the ANF, occurred in the rift basin at this time. Detrital sediments of this incipient rift
basin were derived from the crystalline basement of the Pelagonian microcontinent. As the
rift –drift tectonics of the Pindos Basin evolved and the passive margin of the Pelagonian
microcontinent fully developed, the ANF was deposited in the continental rise
environment of this Pelagonian passive margin (Figure 15(c)).
In the mid-Jurassic (175 – 165 Ma), the Pindos Basin started collapsing via intraoceanic subduction (Figure 15(d)) as a result of a collision-driven regional compressive
stress regime in the Tethyan realm farther east (Dilek et al. 2005, 2007). As subduction
continued and began to rollback to the east, suprasubduction-zone oceanic crust formation
in the Pindos Basin evolved in an east-facing infant arc – forearc environment (Saccani and
Photiades 2004; Beccaluva et al. 2005; Dilek et al. 2005, 2008). Mafic and ultramafic
rocks in the Mesohellenic ophiolite show, in general, a west-to-east MORB to SSZ to
boninitic progression in chemical affinity (Smith and Rassios 2003; Pe-Piper et al. 2004;
Beccaluva et al. 2005; Dilek et al. 2008), suggesting progressive depletion of the mantle
wedge peridotites through repeated melting episodes during SSZ oceanic crust evolution.
The arrival of the Pelagonian continental margin at the trench started an episode of arc –
continent collision in the Late Jurassic– Early Cretaceous that terminated the subduction
rollback cycle and magmatism (Figure 15(e)). The Vourinos ophiolite was then displaced
from its igneous environment of formation and was tectonically accreted onto the western
edge of Pelagonia along an ENE-directed lithospheric-scale thrust fault.
By the Early Cretaceous, the west-dipping subduction in the Pindos Basin was halted
by the collision and partial subduction of Pelagonia, and the Vourinos ophiolite (eastern
half of the Mesohellenic oceanic slab) was thrust over the Pelagonian microcontinent,
along with imbricate thrust sheets of the ANF (continental rise deposits) and the rift
assemblage units in this structurally descending order (Figure 15(f)). The western half of
the Pindos Basin remained open, and passive margin conditions along the Apulian margin
to the west persisted until early Tertiary time (Meço and Aliaj 2000; Dilek et al. 2005).
R
Figure 15. Tectonic model for the formation and evolution of the Pindos Basin. See text for
discussion. A and P in continental blocks refer to Apulia and Pelagonia, respectively. X and o refer to
post-collisional strike-slip components of deformation within the Mesohellenic Trough and in the
Pelagonian hinterland (x, away; o, towards). WPB, within-plate basalt.
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Continued continental convergence between Apulia and Pelagonia caused underplating of
the Apulian continental margin beneath the Pindos Basin during the latest Mesozoic –
Early Palaeogene. The western half of the Mesohellenic ophiolite slab, the Pindos
ophiolite, was then detached and thrust over the Apulian margin, tectonically accreting
and overriding rift-related rocks and turbidite deposits along the western margin of the
basin (Figure 15(f)). This sequence of events along the eastern margin of Apulia during the
latest Cretaceous– Eocene developed the Avdella mélange.
Throughout the early Cenozoic, Pelagonia and Apulia underwent oblique continental
collision, creating regional E– W crustal-shortening and broad-dextral deformation in a
, NW – SE-trending zone in the Pelagonian hinterland (Dilek et al. 2005). As the Pindos
basin contracted due to this collision and the Pindos ophiolite overrode the Apulian
continental margin, west-directed thrust faults propagated through the Apulian carbonate
platform (Figure 15(g)), flysch deposits accumulated within foreland-migrating flexural
basins, and thin-skinned thrust packages developed over the Apulian basement during the
Eocene – Oligocene. The remnant Pindos Basin gradually transitioned from a marinedominated environment to a coastal, deltaic, and finally terrestrial alluvial depocentre –
the Mesohellenic Trough. The Mesohellenic Trough was bounded and compartmentalized
by oblique-slip normal faults that created sub-basins, which were subsequently filled by
Miocene –Pliocene sediments (Figure 15(g); Vamvaka et al. 2006).
Conclusions
The Triassic– Jurassic Vourinos subophiolitic mélange consists of passive margin units
and rift assemblages that formed prior to the onset of seafloor spreading in the Pindos
Basin. The phyllitic matrix of the ANF was originally deposited as a pebbly mudstone and
sandstone on the continental rise setting of the rifted Pelagonian margin, and was then
deformed and metamorphosed during ophiolite emplacement. Clasts and blocks supported
by the phyllitic matrix were mostly derived from the crystalline basement of the
Pelagonian microcontinent. Igneous rocks of the rift assemblage are locally intrusive into
the recrystallized limestone and metapelites of the Pelagonian margin and have WPB to
MORB compositions. Original igneous and sedimentary contacts are well preserved in the
rift assemblage. The continental rise deposits and the rift assemblages were imbricated
along east-directed thrust sheets during the emplacement of the Vourinos ophiolite onto
the Pelagonian margin in the Middle –Late Jurassic. The internal structure and the
evolutionary history of the Vourinos mélange represent a tectonic mélange character.
The Triassic –Cretaceous Avdella mélange beneath the Pindos ophiolite to the west has a
longer, polyphase history of development, and can be described as a block-in-matrix,
polygenetic mélange, whose evolution involved both sedimentary and tectonic processes.
The shear sense indicators in the mélange units, the lower structural sections of the
Vourinos ophiolite, and the Pelagonian continental margin rocks consistently show top-tothe-ENE shearing associated with the ENE tectonic transport during the initial stages of
closure of the Pindos Basin. This tectonic phase was marked by ophiolite emplacement.
These structural observations, coupled with the existence of the parautochthonous igneous
and sedimentary rift units along the western margin of Pelagonia, rule out those
interpretations and models suggesting the westward tectonic transport of the Vourinos
ophiolite and its subophiolitic mélange from the Vardar Zone to the east of Pelagonia.
The Pindos ophiolite and the Avdella mélange beneath it were displaced westwards and
emplaced onto the pre-Apulian platform carbonates as a result of the oblique collision
between Eurasia and Apulia later in the Eocene.
International Geology Review
449
Acknowledgements
This paper is part of Constandina Ghikas’ MS thesis work at Miami University and has been funded
in part by research grants from the Geological Society of America, Institute of Geology and Mineral
Exploration of Greece (Kozani), and the Department of Geology at Miami. We have benefited from
our discussions on the geology of the Vourinos and Pindos ophiolites and their subophiolitic
mélanges with Alan G. Smith, Dimitrios Kostopoulos, Eldridge Moores, Bob Myhill, and John
Wakabayashi. Constructive and thorough reviews by Andrea Festa, Nancy Lindsley-Griffin, and
Kathleen Nicoll were helpful to improve the paper.
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