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Leaders: Anne Rassios, Dimitri Kostopoulos and Yildirim Dilek XIX Congress of the Carpathian Balkan Geological Association Thessaloniki, Greece, 23 26 September 2010 Exhumation of the West Pelagonian margin and Pindos basin emplacement Leaders: Anne Rassios, IGME-Kozani, [email protected]; Dimitri Kostopoulos, University of Athens, [email protected]
FOREWARD
To quote Alan Smith: “The geology of Greece is complex.” To quote Annie Rassios: “Yes, and it never seems to get any simpler.”
The purpose of this fieldtrip is to show newly documented sites within West Macedonia in the light of innovative interpretative models of core complex exhumation. The majority of sites relate to interpretation of the west Pelagonian margin as parts of a classic gneiss dome, and the relation of this complex with the Mesohellenic ophiolites (in this case, Vourinos). The fieldtrip route also passes world-class sites in other disciplines such as sedimentation and active tectonics that should not be missed: these impressive sites lack comprehensive study, and they, too, need new interpretative analyses.
Thus, the approach of this fieldtrip is to introduce “cutting edge” interpretation into the regional geologic studies of the area. These interpretations update the classic geo-tectonic framework into the more recent exhumation paradigm, and, we hope, further refine a model of the complex geology of Greece. None of these interpretations are “etched in stone” as yet, and we expect that some of our opinions may change during the course of this fieldtrip. We will be extremely disappointed if we fail to learn anything from the participants.
To formulate and document a new model for the region will take many years of study. It will take more researchers than are presently working in the area. We invite you to take an active role in our research, but be warned – there are no established prototypes. The investigation of the rocks of West Macedonia will constrain and describe mechanisms of how these complex tectonic processes work. Our research approach is a multi- disciplinary study taking advantage of the full spectrum of geologic topics. Ultimately the model will be told by the rocks, and not vice versa.
We are open to any and all ideas – especially new and innovative ideas. Rassios 2010. page 2 Guidebook to “Exhumation of the West Pelagonian margin and Pindos basin emplacement”
FRAMEWORK GEOLOGY For the purposes of this fieldtrip, only the geology of West Macedonia will be described in detail, with particular emphasis on the western Pelagonian margin and ophiolitic complexes. A geologic map of West Macedonia (below) shows a distribution of rock types that span geologic history from recent to ~700 million years in the past. Rassios 2010. page 3 Guidebook to “Exhumation of the West Pelagonian margin and Pindos basin emplacement”
Post Mesohellenic deposits include uppermost Miocene to Quaternary cover. These include in majority clastic sediments deposited following cessation of Alpine tectonics (cessation of deformation of the Mesohellenic trough). Early deposits are deltaic – shallow lacustrine; mountains of the region were “islands” in this environment. Within this time period, about 0.5 – 1 million years BP, a peneplain (locally referred to as the Myrsini Surface) was established that, today, can be observed at ~600 m elevation over much of Grevena, Kastoria, and parts of Kozani. In areas of recent block faulting, the Myrsini Surface is found up to 700 m in elevation. Deeply incised streams cutting the Myrsini surface demonstrate regional uplift approaching 150m in areas of Grevena. Much of the Myrsini surface occurs over a section of repetitive paleosol formations that in some areas extend to the Pliocene boundary. Pleistocene deposits are, in general, high-energy stream and river deposits with loess: the glacial activity in the region has yet to be thoroughly investigated or documented. Some major talus and breccia deposits of the region are generated by Pleistocene-era landslides (eg Mount Vounassa carbonate talus formation) or periglacial activity (Imera giant cross beds).
The post-molasse period includes the region of the greater Ptolemaiida Basin: this consists of a ~100 km x 15km NW-trending graben that initiated in the latest Alpine period. With upper Miocene – Recent uplift, the original basin has broken into a series of smaller grabens and horsts. Each graben provided a separate environment conducive to lignite formation: lignite formation continues today in the Lake Hematidis graben. Neogene basin deposits are lacustrine marls with lignite interlayers. Marine sediments are totally lacking. Fossil studies related to lignite deposits include freshwater mollusks, flora and mammals. Grading from Neogene to the latest Quaternary deposits, fluvial and terrestrial sediments predominate.
Mesohellenic Sediments date from late Eocene to upper Miocene. These include molasse deposits (in particular in the eastern regions) but grade to deltaic and flysch in the western part of the region. The greatest depth of these sediments occurs within the Mesohellenic trough where they attain ~4,500 m thickness. Much of this thickness is due to syn-deformational deposition during downwarp of the trough. The molasse sediments thin away from the central portions of the trough; remnants of Tertiary molasse deposits have been located within the Pindos south of Konitsa and correlate with molasse deposits east of Mt Vourinos. A transgression (coarse conglomerates) separates the lower from upper Miocene that occurs throughout the region and extends to the conglomerates of Meteora.
The preponderance of Mesohellenic sediments are shales, clays, marls, and arkose sandstones. The basal unit invariably consists of conglomerates with large cobbles to boulders. Where these overlie ultramafic terrains, the basal conglomerate consists of a distinctive welded serpentine-sand matrix with rounded cobbles of ophiolitic origin. Silica-enriched (secondary; syn conglomerate formation) serpentine rinds occur around ultramafic and serpentinized cobbles. The age of the basal conglomerate varies across the region from mid-Eocene (eg Kranea basin) to Miocene (eg Vourinos area). Layers of carbonate-rich shales and limestone occur at sporadic levels within the section, including several reefy intervals (eg Dotsikos ~30 mya, Sarakina ~6-8 mya?). Rassios 2010. page 4 Guidebook to “Exhumation of the West Pelagonian margin and Pindos basin emplacement”
The Mesohellenic trough is a NW trending structure that includes the Tertiary sediments described above as well as ophiolitic rocks, the Avdella sedimentary accretionary mélange and other “basement” formations; magnetic maps detect ophiolitic signatures to depths of 25 km. The eastern basement of the Mesohellenic trough is Pelagonian; the western basement is Apulian. The trough itself marks a deep suture between these two continental fragments. The origin of this trough as a constrictional “piggy back” basin was put forward by Jones and Robertson in the l990’s, and Rassios notes that cross sections through the Pindos require the Mesohellenic to be bordered by steep reverse faults. The depth of the trough probably requires the interpretation of its shallow portions as the sedimentary/tectonic response of overburden formations to a far more important basement constriction (ie right-lateral transtensional collision) at depth. A Miocene-aged Mesohellenic fault zone and angular unconformities resulting from movement of this fault will be visited on the field trip (Taxiarchis – Egnatia stop). Rassios 2010. page 5 Guidebook to “Exhumation of the West Pelagonian margin and Pindos basin emplacement”
Mesohellenic deposits along the Pindos margin of the trough are noted for an abundance of syn-depositional structures including channeling features, gravity slides, slump structures and folds, and dewatering structures. Tectonism was vigorous during the deposition of these sediments.
The present field trip does not extend to the region including Pindos flysch (late Cretaceous to Eocene). This deep water turbidite includes distal facies and substantial pelagic carbonates. It seems to represent an older, more distal, continuation of the Mesohellenic sediments, with deposition interrupted by a tectonic phase including a) the SW back-movement of the Pindos nappe ophiolite and b) initial faulting and deformation accompanying the formation of the Mesohellenic trough.
Cretaceous limestone in the region of the fieldtrip is prominently marked by a rudistid-bearing, reefal formation marking a Maestrichtian transgression. Towards the Pindos of West Macedonia, the Cretaceous limestone is tectonically disrupted and incorporated preferentially into complex shear zones; the limestone units of the Pindos are so dislocated from their original strato-tectonic series that it cannot be determined if they originally comprised a single continuous unit, or “reefal islands” on relatively topographically high paleogeographic positions. The strato- tectonic continuity is better in the east of the region, where the rudistid limestones grade to flysch – clastic carbonate units by the uppermost Cretaceous.
Reference to this transgressive formation as a major strato-tectonic horizon will be made at several stops: essentially, formations (eg ophiolitic) below the reefal limestone can be rotated to their pre-upper Cretaceous position when the topography of the Cretaceous contact and its internal layering is well exposed. It has thus been found that post-mid Jurassic to pre-Cretaceous ophiolitic rotations vary from 60 CCW (eg Vourinos) to ~15 CW (eg Rodiani).
Lower Cretaceous to upper Jurassic formations are rare, but significant when they crop out within West Macedonia. In the Pindos area, the lower Cretaceous is highly deformed, but apparently reaches thicknesses of 150m. It has been correlated with the Alpine “first flysch” units. Its strato-tectonic position in the Pindos is as a unit within the base of the Pindos tectonic nappe (the age of the nappe dates to the Eocene). In the east of the West Macedonian region, lower Cretaceous rocks are thin units, sometimes not present at all, beneath the upper Cretaceous transgressive limestone and overlying ophiolitic or Pelagonian material. In the east, the lower Cretaceous includes lateritic deposits immediately above ultramafic units, and both carbonate and flysch units above this horizon. The high topographic location of some lower Cretaceous in the east may be related to exhumation of Pelagonia. Rassios 2010. page 6 Guidebook to “Exhumation of the West Pelagonian margin and Pindos basin emplacement”
The thickest interval of lower Cretaceous appears to be in the Rodiani area, where an apparent continuous section from upper Jurassic sediments is preserved. Important lower Cretaceous sediments of the Vermion area include discontinuous lateritic deposits overprinted by transextensional deformation.
The uppermost Jurassic sediments of the region are red to grey carbonates including calpionellid microfossils and, in many outcrops, macroscopic belemnite tests. In both the Pindos and Vourinos ophiolitic terranes, calpionellid limestones lie directly on uppermost ophiolitic rocks. In the Vourinos ophiolite, a small rotation (~12 ) between the ophiolitic lavas and Jurassic oceanic sediments has been shown. In the eastern region, calpionellid limestones similar to those of the west occur, as well as conglomeritic Jurassic units that have, heretofore, been included as the “Tectonic Nappe of Vermion.” An important stop will be made in these sediments to put forward an interpretation as an olistolithic formation, probably forming synchronous to early exhumation of Pelagonia.
Belemnites in Jr Calpionellid limestone. Langadakia, Grevena Rassios 2010. page 7 Guidebook to “Exhumation of the West Pelagonian margin and Pindos basin emplacement”
All ophiolites of the West Macedonian region are similar in age, that is, ~168 - 171 my as derived from analyses of zircons in plagiogranites from the Pindos Nappe and Vourinos ophiolites. The ophiolites can be considered in four general groups: The Pindos Nappe ophiolite The Pindos Mesohellenic marginal ophiolites The Vourinos Ophiolite “Rootless” ophiolites Rassios 2010. page 8 Guidebook to “Exhumation of the West Pelagonian margin and Pindos basin emplacement”
On the present field trip, the Pindos ophiolites will not be visited. Salient points of the general ophiolitic geology of the Pindos are as follow: The Pindos Nappe ophiolite includes the Dramala complex and related rocks comprising a complete ophiolitic lithosphere preserved as overlapping imbricates within a younger nappe sheet. The obduction base of the Dramala ophiolite has been preserved intact in several localities. Footwall to the amphibolite sole is the mid-late Jurassic aged Avdella sedimentary accretionary mélange; reversed zonal metamorphism extending from the sole into this mélange is well documented at the Loumnitsa locality. Age of the amphibolite – garnet amphibolite sole is ~164 – 169 my with physical conditions reaching at least 840 ± 50 °C at 5.2 ± 0.5 kbar. Kinematic indicators within the high-temperature to brittle facies within the mantle section of the ophiolite show NE topping directions. The ophiolitic section is contained in a NE topping stacked ramp pile along its eastern margin.
The nappe as a whole includes all formations older than the Pindos flysch (latest Cretaceous-Eocene), that is: the ophiolitic members and sole, Avdella mélange, Duo Dendro (lower Cretaceous) carbonates, and upper Cretaceous reefal limestones. This already internally complex packet was detached from the main Mesohellenic ophiolite nappe, and thrust or slid to the SW over Eocene Pindos flysch. Much of this younger “back” movement was facilitated by reactivation of older NE trending tear systems in the Jurassic-Cretaceous packet. The Eocene movement of the nappe is a brittle phenomena that in some places appears to be a low-angle thrust, and elsewhere a gravity slide. The original geographic position of the Pindos nappe ophiolite is inconclusive for the time being: ophiolitic members bear a stronger geochemical signature in common with the Rodiani complex east of Vourinos than to Vourinos itself. However, the general exceptional preservation of internal ophiolitic relations and structures would suggest that the nappe was transported “gently” and/or not over a great distance. The Pindos nappe can not be observed to extend directly into the Mesohellenic trough, though in several localities it is imbricated against Mesohellenic marginal ophiolites. Rassios 2010. page 9 Guidebook to “Exhumation of the West Pelagonian margin and Pindos basin emplacement”
The magnetic maps of the region shows continuation of ophiolitic signature rocks between the Pindos and Vourinos range, presumably with extreme downwarping and thickening of the ophiolitic signature rocks beneath the Mesohellenic trough. Rassios 2010. page 10 Guidebook to “Exhumation of the West Pelagonian margin and Pindos basin emplacement”
Left: Magnetic map of Greece (IGME)
Right: Detail of magnetic data from area above Mesohellenic Trough (IGME, Memou and Skianis 1991). Rassios 2010. page 11 Guidebook to “Exhumation of the West Pelagonian margin and Pindos basin emplacement”
The Pindos Mesohellenic marginal ophiolites crop out along the western margin of the Mesohellenic trough, and extend into the trough as “basement” formations. These include: the Dotsikos Strip ophiolite and its northern continuation past Zouzouli to the Albanian border; the ophiolite units to the south and east of the Kranea basin, and the eastern nappe of the Mesovouni peridotite (Trikala). All these ophiolitic remnants bear petrologic/geochemical resemblance to both Vourinos and to the Pindos nappe ophiolites. In the Dotsikos strip ophiolite, mantle and cumulate rocks reminiscent of Vourinos crop out in association with Avdella mélange, but no sole formations are intact.
The Vourinos Ophiolite is one of the best documented ophiolitic complexes on global scale. Its complete lithospheric section was photographed by Brunn in 1938 (a panoramic view site to be visited on the fieldtrip), and Brunn later made the first observation that rocks such as the Vourinos complex were like to those described in the initial studies of the Mid-Atlantic ridge. The Vourinos section described by Moores still highly biases the Penrose definition of ophiolites and generalized models of oceanic lithosphere. Zimmerman first documented an ophiolitic “sole” at Vourinos (the term “sole” was adopted later), and first described reverse metamorphic zoning beneath an ophiolitic contact – emplacement of oceanic lithosphere was pioneered at Vourinos by these two researchers. The recognition of ophiolitic cumulates, distinct magma chambers, and the “petrologic moho” was first made at Vourinos. Early development of supra-subduction zone chemistry took advantage of the Vourinos section.
The study of Vourinos has also greatly aided in deciphering mechanisms of emplacement and internal deformation within the ophiolite dating to the slab environment between ridgecrest and emplacement. Developed to aid chrome mineral exploration, the detailed tectonic and microtectonic analyses of ore districts (Xerolivado, Voidolakkos, Rizo, Aetoraches among others), document a deformation history spanning ductile to brittle fields. In all temperature regimes, the structures of Vourinos reconcile continual strain in a decreasing TP environment. High temperature fabrics (mantle layering and foliation, dunite body contacts) are folded reconciling NE topping directions. This deformation continues to lower temperature ductile structures (eg sheath folds in SE Xerolivado), ductile-brittle shears, and early brittle structures imprinting on the entire section. Mylonite zones of 0.5 - 1 m scale thickness are mapped in peridotites and represent deformation at temperatures ~800 C and conform to a ductile thrust geometry. Structures formed at all temperatures parallel the basal morphology of the ophiolite itself, implying a developmental relation between them and ophiolitic emplacement. Post-ophiolitic structures, such as the ~EW trending Tertiary-recent fault system crossing the ophiolitic terrain, often take advantage of older Jurassic shear zones (reactivated), but can be traced to younger formations (as at the Pilori roadcut locality). Structures constrained to the ophiolitic slab era reconcile NE motion within a ductile thrust nappe. Kinematic indicators within the sole formations, including brittle fabrics imprinting older ductile fabrics, agree with NE emplacement by topping or by lateral shear deformation along the nappe margins. These deformations cannot be traced into younger (Tertiary) formations. Rassios 2010. page 12 Guidebook to “Exhumation of the West Pelagonian margin and Pindos basin emplacement” Rassios 2010. page 13 Guidebook to “Exhumation of the West Pelagonian margin and Pindos basin emplacement”
A “rootless” ophiolite is a piece of oceanic lithosphere that is no longer contiguous with an ophiolitic complex emanating and emplaced from a plate tectonic suture zone. In the fieldtrip area these most frequently consist of scraps of serpentinites or highly altered lavas. Rarely are complete “Steinmann” associations represented: contacts between lithologic divisions appear sedimentary/low-temperature tectonic in nature. In the West Macedonian region, “rootless” ophiolites include the Rodiani complex east of Vourinos, the Zindani complex, and numerous smaller ophiolitic fragments “floating” on a Pelagonian basement.
Rodiani has generally been interpreted as a strato-tectonic continuation of Vourinos, despite significant geochemical variations between sub- lava associations. The two complexes are separated by Tr-Jr platform carbonates of the Pelagonian margin complex: the emplacement sole of Vourinos occurs along the west of these carbonates, but no intact sole remains in Rodiani. Serpentinized peridotite retaining an originally mylonitic fabric of Rodiani implies the section was at or closely overlying the basal sole. The mantle and crustal rocks (serpentinized peridotite, chrome ores, gabbroic and ultramafic cumulates) petrologically and geochemically more resemble rocks of the Aspropotamos section of the Pindos nappe ophiolite than similar rocks of Vourinos. A theoretic structural cross section connecting the Vourinos and Rodiani requires a thinning of section from ~14 km (Vourinos) to less than 1 km (Rodiani), while at the same time rotating the pre-Cretaceous orientation of the presumed slab from steeply dipping to the west (Vourinos) to gently dipping to the east (Rodiani). Frankly, intervening folding in the Tr-Jr carbonates have not been well documented, and it is doubtful that such folds show the greater antiformal structure thought to separate the two ophiolites.
Views of antigoritic massive serpentine (a. and b.) and general structural cross section (c) of the Zindani serpentinite body. Rassios 2010. page 14 Guidebook to “Exhumation of the West Pelagonian margin and Pindos basin emplacement”
The Zindani complex appears as a structural continuation of Rodiani towards the south. A genetic association with Vourinos and/or with the Pelagonian complex has never been established. Composed nearly entirely of antigorite, the Zindani serpentinite complex is imbricated with slices of Pelagonian schist, and apparently was emplaced over this schist (based on relict mylonitic fabric) though no amphibolite sole is preserved. Antigorite requires higher temperatures (~450 C - 550 C) and pressures than “normal” ophiolitic serpentine (lizardite and microchrysotile) and its presence is possibly promoted during shearing conditions. However, a “Zindani-type” complex is not known elsewhere in association with ophiolitic bodies, and appears to be quite unique. We speculate that this unique serpentinite complex may be due to post- ophiolitic emplacement conditions associated with Pelagonian exhumation.
Smaller “rootless” ophiolites crop out between Mt Olympos in the east of the region (Levadi ophiolite) and Vourinos. Only Levadi retains some primary lithospheric structures. In a broad sense, these ophioltic fragments are underlain by Pelagonian carbonates, schist, or gneiss, but in no situations are sole assemblages preserved. Many of these ophiolitic fragments are extremely altered to low-temperature mineral associations and interdeformed (as are a few meters of ophiolite passed on Egnatia highway, nearing Veria): no remnant determination of pseudostratigraphy is possible in these sorts of sections. A “thin” ophiolitic section will be visited along the route of the field trip; this particular site is located ~12 km east of Vourinos, and contains ~3 m of fragments of ophiolitic material (serpentine, lavas, Upper Jr sediments) mocking a Steinman Trinity association above a schistose pebbly mudstone. This association is problematic, and more likely to be an olistolithic collection as are other so- called “ophiolites” in the area. The retention of primary constrictional structures within these ophiolites is problematic: all are strongly overprinted by brittle extension that is not pervasive either into footwall or hanging wall formations, and is lacking from Mesohellenic ophiolitic complexes. We would like to speculate that this deformation is related to sub-formational exhumation of Pelagonia, but are still in documentation phase. In at least one area studied (Komnina), a listral fault in peridotite probably is a reactivated constrictional shear; the area in general is characterized by transextensional structures. Certainly, when present in this and other areas (Rodiani, Vateron localities for example), syn-depositional sedimentary structures are very apparent in the lower Cretaceous sediments. What is surprising in all the “rootless” ophiolites is their lack of intraformational constrictional shears and ramping structures. Rassios 2010. page 15 Guidebook to “Exhumation of the West Pelagonian margin and Pindos basin emplacement”
BELOW: General map of the Levadi Ophiolite after Nance, 1976.
RIGHT: a. Map of detailed study area at Komnina documenting transtensional overprint on Jurassic-pre-upper Cretaceous formations. b. Base of ophiolitic unit serpentinites on schistose pebbly mudstones and mylonitic Tr-Jr carbonate platform rocks of the Pelagonian unit. c. Representative outcrop of ophiolitic rocks showing intense veining. d. Listric fault within serpentinite. The photo is rotated to show the pre- upper Cretaceous geometry of this fault. Rassios 2010. page 16 Guidebook to “Exhumation of the West Pelagonian margin and Pindos basin emplacement”
The Pelagonian rocks of West Macedonia are an association ranging in age from ~700 my to upper Jr that, with respect to this field trip, we will be comparing to models of gneiss domes and exhumation.
The Pelagonian Complex is an assemblage of lightly to severely metamorphosed rocks with a cover of platform carbonates. The metamorphic section includes ~700 my Proterozoic inclusions apparently “welded” during collision in a generally Hercynian (upper Paleozoic) matrix. Nearly all Hercynian rocks are derived from the intense deformation of granodiorites. Old sedimentation on Hercynian terranes gave rise to quartzites and meta-greywackes, also highly metamorphosed. Some rocks of probable oceanic crust affinity have been interpreted as Paleozoic ophiolites, though none are dated and no adequate geochemical analyses have been made. Initial studies suggest these may all be rifting assemblages.
The rifting of Pangaea split this older terrane: dikes of altered basalts (now amphibolite) intrude schistose hosts; irregular bodies of highly deformed meta-basalt and acidic intrusions are likely rifting related. Analyses of these lavas in the Aliakmon valley indicate WPB (ie, rifting) geochemical affinity. Stepping towards the rifting margin from the East towards Vourinos, more detail emerges with lessening regional metamorphism: sedimentation in the Deskati – Aliakmon Valley areas includes Triassic rocks, at least in part deposited over a sedimentary unconformity with the Hercynian continent: this sedimentation includes members compatible in origin with a rapidly deepening ocean basin: radiolarian cherts, red (Antiqua Rosso) limestones, tuffaceous sediments, greywackes and pebbly mudstones (upper Jr in age).
The entire continental Pelagonian section is overlain by platform carbonates fossil dated as Tr-Jr in age. These carbonates include algal bearing zones. Towards the east, the platform carbonates are nearly all marbles, and highly (mylonitic) deformed. Towards the west along the Pelagonian margin, carbonates are less likely to be marble, and less likely to be deformed. In the Siatista – Vourinos area, zones of mylonitic Tr-Jr carbonates appear to parallel the emplacement zone of Vourinos. The distribution of carbonate type and deformation fabric has yet to be documented over the Pelagonian metamorphic complex. The greater occurrence of mylonite overlying the gneissic Pelagonian metamorphics suggests these tectonic fabrics to be linked, most likely through exhumation processes. Rassios 2010. page 17 Guidebook to “Exhumation of the West Pelagonian margin and Pindos basin emplacement” Rassios 2010. page 18 Guidebook to “Exhumation of the West Pelagonian margin and Pindos basin emplacement”
EXHUMATION PROCESSES AND THE ROLE OF OPHIOLITES
For the purpose of this fieldtrip, a very general introduction to gneiss domes and exhumation processes is presented below.
LEFT: A very general model of an “exhuming” gneiss dome and overlying overburden. In a broad sense, correlation of fieldtrip stops to this model are made to invigorate discussion:
Migmatitic Gneiss: Stop 1.2 Layered Gneiss: Stop 1.3 Schist-Gneiss: Stops 1.2 & 2.2 Schist: Stop 2.2 Phyllite: Stop 2.7 Displaced overburden: Stop 1.6 Syndepositional (?): Stop 1.7
Associated granite: As yet, reliable dating of the Florina granite has not been made. If as suggested it has a Cretaceous date, then it could be part of the gneiss dome model. However, correlation with this young date seems contraindicated by its relation to Hercynian tectonics. The story has not yet been worked out. Rassios 2010. page 19 Guidebook to “Exhumation of the West Pelagonian margin and Pindos basin emplacement”
The exhumation of a gneiss dome can be effected by multiple mechanisms. However, by whatever causative means, domes are exhumed via shear zones that extend to great depths into continental lithosphere. The erosion of the remnants of such a shear zone can lead to the dome formation’s resemblance to a thrust nappe. Possibly, this could explain older attempts to “explain” the Pelagonian complex as such a nappe.
Ophiolitic rocks are present in the regions surrounding many metamorphic core complexes as well as in the dome “overburden” formations. The Pelagonian complex is rimmed by the Pindos Basin ophiolitic root zone to the west, and Vardar ophiolitic root zone to the east (as delineated by magnetic surveys shown above). “Rootless” ophiolites are common outcrops above the Pelagonian formation. The juxtaposition of ophiolitic rocks with gneiss domes on a global scale has given rise to several “cause and effect” models of exhumation: a. The obduction of a relatively “heavy” oceanic lithospheric nappe of ~10-14 km thickness onto a continental crust of 30-40 km thickness could cause initial depression of the “lighter” continental material to mantle levels, initiating melting, and a gravitational instability that requires the doming of the continent to higher levels again. Only in this model are ophiolites the “sole” causative factor. The low likelihood that a single ophiolitic nappe could be obducted without substantial thinning or imbrication over the required distances involved in this model is mechanically not likely. In the case of Greece, for example, a “single” obducted nappe containing Vardar and Pindos ophiolites together would have had to be emplaced further than 400km distance: mechanically, this is not feasible. As well, the single nappe model would be required to explain high temperature sole formations on the Pindos side (Vourinos, Othris, Pindos ophiolites) that indicate derivation near the oceanic source. b. During collision of oceanic with continental tectonic plates, initial subduction of continental crust could result in a similar gravitationally unstable condition reconciled by subsequent exhumation. In this case, ophiolitic rocks are present as evidence of collision, back- arc spreading, and supra-subduction environments. Ophiolites are not the “causative” factor in exhumation, but occur as evidence of tectonic processes that could initiate continental subduction resulting in exhumation. c. Transcurrent shear along plate margins could depress continental crust and “emplace” oceanic crust at high levels. Again in this model, ophiolitic rocks are present as evidence of tectonic processes, and are not in themselves a causative factor.
Ophiolites, then, could be considered “suspicious witnesses” rather than “suspects” in explaining the initiation of exhumation of metamorphic core complexes. Rassios 2010. page 20 Guidebook to “Exhumation of the West Pelagonian margin and Pindos basin emplacement”
For now, no PT path has been established for the Pelagonian core complex. Such a study would greatly aid understanding exhumation processes, but would not be able to diagnose the root cause of formation of the gneiss dome. To do this requires, in addition, knowledge of the regional tectonics hosting the core complex. One purpose of this field trip is to question the role of the geology of West Macedonia within the exhumation model, and discuss possible causation of exhumation.
Some key areas that influence our interpretation of exhumation of Pelagonia include the following: Z The ductile normal fault of Polyfito (stop 1.4) conforms in geometry and position to the as part of the shear system responsible for Pelagonian uplift: the fault occurs at migmatitic depths, and in comparison with PT conditions in nearby gneissic areas, should represent deformation at a depth of at least ~25 km. We put forward that it represents the root of the shear zone (or one of the shear zones) responsible for uplift of the complex. As mentioned in the description in the guide to the site, the age of this deformation has not yet been determined. A migmatitic vein between this locality and Veria has given a preliminary zircon date of 140 mya, but is still under study. The “strato-tectonic” age given to exhumation places it within the upper-Jr to mid-Cretaceous. The ductile fault at Polyfito could belong to the present exhumation cycle of Pelagonia, or it could be a relict of an older Hercynian exhumation somehow preserved through the younger exhumation. The latter situation does not seem possible. Z The small “IGME” ophiolitic assemblage (stop 1.6) is typical of many of the “rootless” ophiolites cropping out above the Pelagonian complex. The juxtaposition of the ophiolitic members (serpentinized peridotite, pillow lavas, sediments) represents a secondary process, most likely listric emplacement from a Pelagonian “topographic high” created during exhumation. Z We include a stop at the striated pebble formation (stop 1.7) to initiate discussion on interpretation of what has previously been mapped as the “Vermion nappe” within an exhumation framework. The striations themselves are problematic: they could be caused by deformation within an active olistostrome emplacement through sedimentary “rubbing”, or they could be more “tectonic” in origin, deformed as part of the overburden above the rising Pelagonia, but below younger overburden formations (that is, a piercement or roof zone assemblage). We hope to demonstrate that the assemblage is representative of an extensional environment. Z Our visit to the Pelagonian rocks near Deskati and adjacent to Vourinos show the “footprint” of ophiolitic emplacement, that is, compression. The well-dated sole of this emplacement is older than the exhumation window put forward by Sharpe and Robertson. Pelagonian rocks within this “compressive footprint” are those of the peripheral zone of the Pelagonian complex, including lightly metamorphosed sediments, preserved rifting assemblages, and sedimentary provenance schists of the upper Paleozoic. Ghikas and Myhill (in prep.) report a compressive/extensional transition ~1.5 km from the ophiolitic sole. Whether structures below this contact in the Pelagonian relate to Hercynian or emplacement era deformation is still under study. Z The “coincidence” of the association of Vourinos with Pelagonia ought to be one of the keys in determining the causative factor of Pelagonian exhumation. In Vourinos, the emplacement vector ranges from 90 - 40 from the north to south against the Pelagonian margin. The Zavordas shear zone including the emplacement sole and highly deformed rifting complex may mark part of what is Rassios 2010. page 21 Guidebook to “Exhumation of the West Pelagonian margin and Pindos basin emplacement”
originally transcurrent shear along plate margins. Could this be the initial key tectonic force leading to subsequent exhumation of Pelagonia?
Final Remarks: The stops on this field trip should be considered the tip of the iceberg in generating an interpretation of the regional exhumation – ophiolite connection. The interpretation of the Pelagonian complex in light of contemporary gneiss dome – exhumation models is long overdue. Fortunately, the general description of the metamorphics of the complex by D. Mountrakis has laid the groundwork for today’s reinitiation of its study. He may not agree with all we have to say in this guidebook, but without his work in the area, we wouldn’t be able to say anything about it at all.
It is to him, that this guidebook is dedicated.
Annie Rassios Dimitrios Kostopoulos
GENERAL REFERENCES
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Grivas, E., Rassios, A., Konstantopoulou, G., Vacondios, I., and Vrahatis, G., 1993. Drilling for “blind” podiform chrome orebodies at Voidolakkos in the Vourinos ophiolite complex, Greece. Economic Geology 88, 461-468. Harkins, M., Green, H., and Moores, E., 1980. Multiple intrusive events documented from the Vourinos ophiolite complex, northern Greece. Am. Journ. Sci. 280-A, 284- 290. Iishi, K., and Saito, M., 1973. Synthesis of Antigorite. American Mineralogist 58, 915-919. Jackson, E.D., Green, H.W., and Moores, E.M., 1975, The Vourinos ophiolite, Greece: cyclic units of lineated cumulates overlying harzburgite tectonite: Bulletin of the Geological Society of America, v. 86, p. 390–398. Jones, G., and Robertson, A., 1991. Tectonostratigraphy and evolution of the Mesozoic Pindos ophiolite and related units, northwestern Greece. Journal of the Geological Society, v. 148-2, 267-288. Konstantopoulou, G., 1992. Structural criteria in locating chromite ores: evidence from the Rizo district, Vourinos ophiolite, Greece. Bull. Geol. Soc. Greece, XXVIII/2, 381-392. Mavrides, A., 1980. A propos de l’age de mise en place tectonique du cortege ophiolitique du Vourinos (Grece). In Panayiotou, A., ed., Ophiolites: Proceedings of the International Ophiolite Symposium, Cyprus, 1979, 349-350. Mavrides, A., 1994. Knidi Sheet, of Geological Map of Greece (1:50,000 scale), Inst. Mining and Mineral Exploration, Athens. Mavrides, A., Skoutsis-Coroneou, V., and Tsalia-Monopolis, S., 1979. Contribution to the geology of the suppelagoinan zone (Vourinos area, West Macedonia). In 6th Colloquium on the Geol. of the Aegean Region, Athens. 175-195. Memou, G., and Skianis, G., 1993. Interpretation of aeromagnetic data in the Pindos-Vourinos region --Part One: qualitative analysis. Internal Report Inst. Mining and Mineral Exploration, Athens, 11 pp. Moores, E.M., 1969, Petrology and structure of the Vourinos ophiolitic complex of northern Greece: Geological Society of America Special Paper, v. 118, p. 74. Mountrakis, D., 1984. Structural evolution of the Pelagonian Zone in northwestern Macedonia, Greece. in Dixon, J., and Roberstson, A. (eds) The Geological Evolution of the Eastern Medterranean. Geological Society of Lindon Special Paper 17, 581-590. Mountrakis, D., 1985. Geology of Greece (in Greek). University Studio Press Thessaloniki, 207 pp. Myhill, R., 2008. Metamorphic Development beneath the Pindos-Vourinos Ophiolitic Slab. Master’s Thesis, University of Cambridge. 124pp. Nance, R., 1976. The Livadi Mafic-Ultramafic Complex and its Metamorphic Basement, N.E. Greece. PhD Thesis, University of Cambridge, 183pp. Naylor, M.A., and Harle, T.J., 1976, Palaeogeographic significance of rocks and structures beneath the Vourinos ophiolite, northern Greece: Journal of the Geological Society (London), v. 132, p. 667–675. Pearce, J., Lippard, S.J., and Roberts, S., 1984, Characteristics and tectonic significance of supra-subduction zone ophiolites, in Kokelaar, B.P., and Howells, M.F., eds., Marginal basin geology: Special Publication Geological Society of London: Oxford, Blackwell Scientific Publications, p. 77–94. Pichon, J., and Brunn, J., 1985. An inverted metamorphism beneath the Vourinos ophiolite suite, Greece. Ofioliti 10 2/3, 363-374. Pichon, J., and Lys, M., 1976. Sur l’existence d’une série dy Jurassique superieur a Cretace inferieur, surmontant les ophiolites, dan les collines de Krapa (Vourinos, Grece). C.R. Acad. Sci., Paris, D 282, 523-526. Rassios, A., 2004, 2008. A Geologist’s Guide to West Macedonia, Greece. Grevena Development Agency (An.Gre), 120pp. Rassios, A., 1981. Geology and Evolution of the Vourinos Complex, Northern Greece. PhD. Thesis, Univ. of Calif. (Davis), 499 pp. Rassios, A., and Kostopoulos, D., 1990. The geochemistry of dunite and its relation to the position of chromitites in the Vourinos ophiolite complex, Greece. in Malpas, J., Moores, E., Panayiotou, A., and Xenophontas, C., eds., Ophiolites: Oceanic Crustal Analogues; Proceedings of the Symposium “Troodos ‘87”, Nicosia, Cyprus, Geol. Surv. Dept., 593-604. Rassios, A., Beccaluva, L., Bortolotti, V., Mavrides, A., and Moores, E.M., 1983, The Vourinos ophiolitic complex: a field excursion guidebook: Ofioliti, v. 8, no. 3, p. 275– Rassios 2010. page 23 Guidebook to “Exhumation of the West Pelagonian margin and Pindos basin emplacement”
292. Rassios, A., and Dilek, Y., 2008. Rotational deformation in the Jurassic Mesohellenic ophiolites, Greece, and its tectonic significance. LITHOS-01812; No of Pages 17 Rassios, A., Grivas, E., Konstantopoulou, G., and Vacondios, I., 1994, The geometry of structures forming around the ductile-brittle transition in the Vourinos-Pindos-Othris oceanic slab: Bulletin of the geological Society of Greece, v. XXX, no. 2, p. 109–121. Rassios, A., Moores, E., and Green, 1983. Magmatic structure and stratigraphy of the Vourinos ophiolite cumulate zone, northern Greece. Ofioliti 8(3), 377-410. Rassios, A., Smith, A. and Kostopoulos, D. 2009. Ophiolites 2008 Guidebook: Link between the Mesohellenic Ophiolites and the Pelagonian Margin. In: (Eds.) Anne Rassios, Alan G Smith, and Dimitrios Kostopoulos, Ophiolites 2008 Guidebook: Link between the Mesohellenic Ophiolites and the Pelagonian Margin, Journal of the Virtual Explorer, Electronic Edition, ISSN 1441-8142, volume 34, paper 1. Ring, U., Brandon, M., Willett, S., and Lister, G., 1999. Exhumation processes. Geological Society, London, Special Publications 1999; 154, 1-27. Roberts, S., Rassios, A., Wright, L., Vacondios, I., Vrahatis, G., Grivas, E., Nesbitt, R., Neary, C., Moat, T., and Konstantopoulou, G, (1988). Structural controls on the location and form of the Vourinos chromite deposits. In Boissonas, J., and Omenetto, P., eds., Mineral Deposits within the European Community, Springer-Verlag, Berlin, 249-266. Ross, R.V., Mercier, J.C., Avé Lallemant, H.G., Carter, N.L., and Zimmerman, J., 1980, The Vourinos ophiolite complex, Greece: the tectonite suite: Tectonophysics, v. 70, p. 63–83. Sharp, I., and Robertson, A., 2006. Tectonic–sedimentary evolution of the western margin of the Mesozoic Vardar Ocean: evidence from the Pelagonian and Almopias zones, northern Greece. Robertson, A. & Mountrakis, D. (eds) Tectonic Development of the Eastern Mediterranean Region. Geological Society, London, Special Publications, 260, 373–412. Smith, A., and Woodcock, N., 1976. Emplacement model for some “Tethyan” ophiolites. Geology 4, 653-656. Smith, A.G., 1979, Othris, Pindos and Vourinos ophiolites and the Pelagonian zone, 6th Colloquium on the geology of the Aegean region: Athens, Greece, p. 1369–1374. Smith, A.G., 1993, The tectonic significance of the Hellenic-Dinaric ophiolites: Special Publication of the Geological Society of London, v. 76, p. 213–243. Spray, J.G., and Roddick, J.C., 1980, Petrology and 40Ar/39Ar geochronology of some Hellenic sub-ophiolitic metamorphic rocks: Contributions to Mineralogy and Petrology, v. 72, p. 43–55. Spray, J.G., Bebien, J., Rex, D.C., and Roddick, J.C., 1984, Age constraints on the igneous and metamorphic evolution of the Hellenic-Dinaric ophiolites, in Dixon, J.E., and Robertson, A.H.F., eds., The geological evolution of the Eastern Mediterranean: London, Geological Society, p. 619–627. Vergely, P., 1977. Discussion of the paleogeographic significance of rocks beneath the Vourinos ophiolite, northern Greece. J. Geol. Soc. London 133. 505-507. Zimmerman, J., 1968. Structure and Petrology of Rocks Underlying the Vourinos Ophiolitic Complex, Northern Greece. PhD. Thesis, Princeton University. Zimmerman, J., 1969. The Vourinos Complex--an allochthonous alpine ophiolite in Northern Greece. Abstr. Geol. Soc. Am. Ann. Mtg., 245. Zimmerman, J., 1972. Emplacement of the Vourinos ophiolitic complex, northern Greece: Memoir of the Geological Society of America, v. 132, p. 225–239.