Field trip B26
"NEOTECTONIC TRANSECT MOESIA - APULIA"
The field trip aims to demonstrate basic features of the neotectonic structure and deformation along a transect from the Moesian to the Apulian platform, through the mountain chains of the Balkanides and Hellenides.The trip will give a comprehensive idea about the Alpine geodynamics of the Balkan Peninsula, and largely coincides with Transect III of the TRANSMED Project. Emphasis will also be placed on seismic and geotechnical hazards,the geological heritage and its conservation, and on the archeological, cultural and historic heritage and geomythology. Important natural, cultural and historic monuments are included, such as the Vrachanski Balkan, Rila and Pirin national parks, the Rila Monastery, numerous ancient and medieval towns and monuments (Plovdiv, Hisar, Melnik, Thessaloniki, Vergina, Ioannina), the monastic rock complex of Meteora, and many others.The recently excavated Thracian temple and royal tomb (one of the most outstanding archaeological finds of the last 10 years) at Starosel will also be visited.
General geologic description
The Balkan Peninsula has a complex geological structure issued of Precambrian, Cadomian,Hercynian (Variscan), Early Alpine (Cimmerian) and Alpine s.s. tectonic movements.Although the field trip is showing mostly the structures formed during the extensional collapse of the Late Alpine orogen in Palaeogene and Neogene times, it will demonstrate also some of the older features. The Alpine structure of the peninsula is dominated by two principal elements: the metamorphic crustal fragments and the ophiolites issued from closed oceans.
The metamorphic crustal fragments (from the North to the South: Sredna-gora crystalline block; Rhodope massif, Pirin-Pangaion unit included; Osogovo-Lisets and Lisiya fragments in the core of the Strouma unit; pre-Ordovician fragments in the Morava unit; Serbo-Macedonian massif; Pelagonian massif) have mostly a Precambrian age, and are amalgamated and reworked in Cadomian times. In some of the units they are covered unconformably by Palaeozoic (Ordovician and younger) sedimentary formations, and are intruded by Hercynian and Alpine granitoids. The Alpine structure is divided in two parts by the Vardar (Axios) suture zone. To the ENE of this zone,the Alpine structure is characterized by pre-Alpine and Alpine continental crust of the Moesian platform and its southern margin, with development of arc basins and rifts. The Moesian platform is geomorphically a low plain(Danube plain) with Quaternary loess cover.The oldest sedimentary complexes (Precambrian, on Romanian territory, and Devonian, in Bulgaria) are crossed by boreholes.Uplift episodes and partial erosion are reflected in several regional unconformities: between the Upper Permian and Lower Triassic; in the Lower Jurassic; between the Lower and Middle Cretaceous; several unconformities in the Palaeogene and Neogene.
The cover consists of continental Lower Triassic red beds followed by Middle Triassic to Norian platform carbonates, and Carnian to Norian marine red beds; terrigenous and carbonate Jurassic, the marine transgression over the deeply eroded Triassic beginning often directly with Middle Jurassic; Upper Jurassic – Lower Cretaceous carbonate platform sequences; Upper Cretaceous shallow-marine carbonate and terrigenous deposits, and continental or marine Palaeogene; locally Badenian to Pontian Neogene marine sediments belonging to a transitional zone between the Central and Eastern Paratethys.
The transitional zone between the Balkan fold belt and the Moesian platform (Fore-Balkan) is characterized by a low degree of deformation, and by transitional Alpine facies. In the eastern parts of the zone,the Triassic and Jurassic rocks are deeply buried beneath the Cretaceous and Palaeogene (partially) cover. The Balkan (Stara-planina) zone is characterized by north-verging thrusting and folding in pre-Late Eocene and latest Oligocene times. The units (thrust sheets and anticlinoria) include fragments of units of Mid-Cretaceous and Late Cretaceous folding. The following complexes are distinguished: pre-Ordovician volcano-sedimentary complexes of oceanic(partially) and island-arc signature; Palaeozoic (Ordovician to Carboniferous)sedimentary formations intruded by Carboniferous granitoids; Permian red beds; a Triassic Peri-Tethyan sequence ending with folding and covered with unconformity by a Lower Jurassic to Kimmeridgian Peri-Tethyan sequence; Upper Jurassic - Lower Cretaceous carbonate sequence (carbonate platform); Upper Cretaceous shallow-water carbonate sequence;Danian to Middle Eocene continental and shallow-marine sediments; unconformable Upper Eocene molasse.
The Srednogorie zone has been set on after the Mid Cretaceous compressive event by crustal necking and rifting in the southern edge of the Moesian platform, and developed as a volcanic island arc of Late Cretaceous age. It was transformed in an orogen by north-verging folding and thrusting in latest Cretaceous times, and later thrusted northward over the Stara-planina zone. The Upper Cretaceous covers with unconformable depositional contact different complexes: slivers and rootless bodies of ultramafic (ophiolitic?) character and mantle (or oceanic crust) origin (parts of the Precambrian Prerhodopian Supergroup); the principal part of the Precambrian Prerhodopian Supergroup - a typical continental crust of granitic composition (migmatites, gneisses, metagranites); granitoid plutons of Carboniferous age (determined with Rb-Sr whole-rock isochrons) and mixed (I-type) and continental crustal signatures; Permian, Triassic and Jurassic sedimentary complexes with Peri-Tethyan (Balkanide) signature.
The Upper Cretaceous sedimentary formations are represented by coal-bearing continental to shallow-marine Cenomanian and Turonian, terrigenous to carbonate marine Coniacian, pelagic carbonates and shales, locally with radiolarites(Santonian – Campanian) and Lower-Middle Maastrichthian flysch. The Cenomanian? to Early Maastrichthian volcanics (from pikrites and basanites to andesites and latites) form a volcano-plutonic association with gabbro to monzonites and granodiorites of mantle signature.
The Late Cretaceous folding, uplift and erosion was followed by a Danian conglomeratic formation of fluviolacustrine origin that contains well-rounded pebbles from all older rocks. Upper Eocene and younger formations fill in syn- and post-sedimentation grabens.
The tectonic units South of the Srednogorie represented in Late Cretaceous times a frontal arc ("plateau") built up of Mid-Cretaceous tectonic units. This Morava-Rhodope zone consisted of several units: Rhodope s.l. (Rila-West Rhodope and Pirin-Pangaion subunits included), Strouma,Morava and Serbo-Macedonian massif.The Rhodope massif is a lenticular body of continental crust. It is built up of two metamorphic complexes designated as Prerhodopian and Rhodopian Supergroup. The Prerhodopian Supergroup represents a continental crust of granitic composition (migmatites, gneisses, metagranites) that contains amalgamated rootless bodies of ultrabasic and basic rocks.
The Rhodopian Supergroup originated from products (flysch-like sedimentary sequences, volcanogenic sequences, platform carbonates) of a Precambrian island arc. Both supergroups suffered Cadomian reworking, and were intruded by igneous bodies of granitic composition and Palaeozoic and Late Cretaceous age, in most cases bearing the signature of recycled and molten older continental crust. The metamorphic complexes are covered by non-metamorphic sedimentary and volcanic cover of Palaeogene age. Palaeogene monzonites and granitoids are unroofed during the Neogene in the most uplifted blocks (as the Pirin horst) whereas sedimentary formations of Neogene age have been deposited in the adjacent grabens. The basement of the Strouma unit consists of various gneisses, schists and amphibolites (including orthoamphibolites) of Precambrian age and Cadomian reworking. The Neoproterozoic - Cambrian Frolosh Formation(metadiabases, schists, metapsammites) contains rootless bodies of mafics and ultramafics (lherzolite, troctolite, gabbro, norite) and is intruded by the Strouma diorite formation (gabbrodiorite to granite). They are regarded as a volcanic arc association that contains some slices of oceanic crust or upper mantle. These Precambrian to Lower Palaeozoic rocks are directly covered by Permian red beds formed in the intramountain depressions of the Variscan orogen.
The Triassic section begins over Permian or its basement with Lower Triassic mature conglomerate and sandstone followed by Middle-Upper Triassic carbonates (limestones, dolomites) and Carnian – Norian marine red beds (red to purple shales, marls and conglomerates interbedded with oligomictic quartz sandstones and limestones). The whole unit has been folded and uplifted in latest Triassic time. The gradual Early Jurassic to Bajocian transgression occurred only in the northern, Trun subunit. The Jurassic sedimentation ended with the Upper Jurassic – Berriasian flysch of the Nish-Troyan flysch trough (called also Perimoesian marginal flysch basin) that continued to the East also within the future Srednogorie and Balkan zones. After the Mid-Cretaceous folding and thrusting, the unit has been united with the Morava and Rhodope units into a single Morava-Rhodope superunit that represented a frontal arc ("plateau") of the complex Late Cretaceous volcanic island arc.
The Late Cretaceous marine transgression of the Srednogorie basin penetrated only locally in the northeastern parts of the Strouma unit where a trough with flysch-like sedimentation existed in Campanian and Maastrichtian times. The Morava unit has been formed as a result of Mid Cretaceous thrusting of a ridge structure (Serbo-Macedonian massif) over the Triassic to Lower Cretaceous sediments of the Nish-Troyan flysch trough.The unit consists of several thrust sheets with different lithologies. Only pre-Alpine rock units are present. The basement rocks are very similar to that of the Strouma unit, and belong to two types: pre-Cadomian high-grade gneisses, migmatites and amphibolites intruded by Cadomian granites, and Neoproterozoic - Cambrian greenschist-facies rocks similar to the Frolosh Formation, with gabbro-diorites to diorites. Some of the thrust sheets exhibit Ordovician metasandstones and schists, Silurian – Devonian basinal schists, lydites and limestones, and Upper Devonian flysch-like sediments. The Ograzhden unit is a part of the Serbo-Macedonian massif. It consists of gneisses, migmatites and amphibolites amalgamated with mantle ultrabasic and basic rocks and intruded by Cadomian granites, aplites and pegmatites. The whole Ograzhdenian polymetamorphic and polydeformational complex has been reworked in late Cadomian times.
A special emphasis is laid on the extensional collapse processes in Late Cretaceous, Palaeogene and Neogene times.The Late Cretaceous orogen issued from the Srednogorie island arc has been subject of fast uplift and erosion already in latest Maastrichtian times,and of gradual peneplainization in the Danian. Exhumation of deeper levels occurred both on the former frontal arc ("plateau") of the Morava-Rhodope superunit, and in the Srednogorie itself. A new extension occurred in Palaeocene and Early to Middle Eocene times, in the Balkan area with local flysch basins, and with continental grabens with marine ingressions in the Rhodope region. Intracontinental collision processes formed the first fold and thrust belt of the Balkanides. Limited thrusting probably occurred in the Rhodope area,being more important in its southern part on Greek territory.
The extensional collapse of the Middle Eocene orogen occurred in Late Eocene and Early Oligocene times, and was most prominent in the area of the "plateau" – in the heterogeneous crustal areas of Srednogorie and Morava-Rhodope. Numerous small grabens were filled in by coarse terrigenous often coal-bearing sediments beginning with the Bartonian, and mostly, in Priabonian times. At the end of the Priabonian, marine ingression along the Strouma/Strymon fault belt and in the East Rhodope Mountains was almost coeval with outbursts of intense volcanic activity.
The volcanic activity has been of a bimodal composition probably deriving from mixed mantle and crustal sources (in the East Rhodope), of a trachyandesitic composition with a later more acidic tendency (along the Strouma belt and in the Mesta graben complex), and with aci (rhyolites) character in continental conditions (on dry land or in lacustrine environments), - in the Central and Western Rhodope where the continental crust reaches its maximum thickness. Thus, a complex fluviolacustrine system was drained towards the two marine gulfs. The field trip demonstrates the relations between the Palaeogene formations and their basement in the Padesh graben (a part of the gulf along the Strouma belt) and in the continental Padala graben (a part of the fluviolacustrine system near the source area).The marine regression in Mid- to Late Oligocene times led to a total reorganizing of the fluviolacustrine systems on the Balkan Peninsula. Almost all previous basins were dried out. A marine gulf became active along the eastern part of the Maritsa fault belt East of Plovdiv. In transtension conditions, a huge lake extended along the Strouma fault belt from Brezhani in SW Bulgaria (to be briefly visited) through Bobovdol, Pernik and East Serbia where drained in the Pannonian sea basin with a probable link to the East Carpathian basin.
This Paratethys system gave birth to the most important coal deposits in Romania, Serbia and Bulgaria. In earliest Miocene times, the system became reorganized due to dramatic transpression events.Most of the Balkan Peninsula has been a dry land in Early and Middle Miocene times. The evidence about the evolution in these times is scarce. Marine basins still existed in the Hellenides, and in the Pannonian basin of the Paratethys.The central and eastern parts of the Peninsula were affected by slow motions and denudation that resulted in formation of the principal planation surface (initial peneplain, orthoplain).Due to the balance between uplift, accumulation and erosion, no traces of sediments formed in these times have been preserved. Only recently,palaeobotanical evidence has been published about Ottnangian to Karpatian age of some formations with very limited occurrence in the West Rhodopes.Considerable changes in this regime began in the second half of Middle Miocene time.Marine and brackish basins flooded the surrounding areas:Ionian/Adriatic, Pannonian, Precarpathian, Euxinian(Black Sea) and later, the Aegean basin. Normal faulting (mostly along the older fault belts) led to relief contrasts and triggered faster erosion and denudation in the horsts and sediment deposition in the grabens thus initiating a new pattern of fluviolacustrine systems centered on big palaeorivers along the Maritsa and Strouma/Strymon fault belts. The areas with the thickest continental crust (West and Central Rhodopes) became again involved in differential uplift and extension. Times of relative quiescence are documented by mountain steps (pediments, oroplains). The fault amplitudes changed in time, with a climax in the time span Pontian – Pliocene when the thickest terrigenous sediments were deposited parallel with the fast climate changes marked by the disappearance of savannahs and their big mammals (Pikermi faunas), and coming of the Ice Age in the highest mountains (Rila, Olympos,Pirin, Belasitsa, Vitosha, etc.).