Ghazar Galoyan

Subductions, obduction and collision in the Lesser Caucasus (Armenia, Azerbaijan, Georgia), new insights

Abstract In the Lesser Caucasus three main domains are distinguished from SW to NE: (1) the autochthonous South Armenian Block (SAB), a Gondwana-derived terrane; (2) the ophiolitic Sevan–Akera suture zone; and (3) the Eurasian plate. Based on our field work, new stratigraphical, petrological, geochemical and geochronological data combined with previous data we present new insights on the subduction, obduction and collision processes recorded in the Lesser Caucasus. Two subductions are clearly identified, one related to the Neotethys subduction beneath the Eurasian margin and one intra-oceanic (SSZ) responsible for the opening of a back-arc basin which corresponds to the ophiolites of the Lesser Caucasus. The obduction occurred during the Late Coniacian to Santonian and is responsible for the widespread ophiolitic nappe outcrop in front of the suture zone. Following the subduction of oceanic lithosphere remnants under Eurasia, the collision of the SAB with Eurasia started during the Paleocene, producing 1) folding of ophiolites, arc and Upper Cretaceous formations (Transcaucasus massif to Karabakh); 2) thrusting toward SW; and 3) a foreland basin in front of the belt. Upper–Middle Eocene series unconformably cover the three domains. From Eocene to Miocene as a result of the Arabian plate collision with the SAB to the South, southward propagation of shortening featured by folding and thrusting occurred all along the belt. These deformations are sealed by a thick sequence of unconformable Miocene to Quaternary clastic and volcanic rocks of debated origin.

The Armenian Ophiolite: insights for Jurassic back-arc formation, Lower Cretaceous hot spot magmatism and Upper Cretaceous obduction over the South Armenian Block

Abstract Similar geological, petrological, geochemical and age features are found in various Armenian ophiolitic massifs (Sevan, Stepanavan and Vedi). These data argue for the presence of a single large ophiolite unit obducted on the South Armenian Block (SAB). Lherzolite Ophiolite type rock assemblages evidence a Lower–Middle Jurassic slow-spreading rate. The lavas and gabbros have a hybrid geochemical composition intermediate between arc and Mid Ocean Ridge Basalt (MORB) signatures which suggest they were probably formed in a back-arc basin. This oceanic sequence is overlain by pillowed alkaline lavas emplaced in marine conditions. Their geochemical composition is similar to plateau-lavas. Finally, this thickened oceanic crust is overlain by Upper Cretaceous calc-alkaline lavas likely formed in a supra-subduction zone environment. The age of the ophiolite is constrained by 40Ar/39Ar dating experiments provided a magmatic crystallization age of 178.7±2.6 Ma, and further evidence of greenschist facies crystallization during hydrothermal alteration until c. 155 Ma. Thus, top-to-the-south obduction likely initiated along the margin of the back-arc domain, directly south of the Vedi oceanic crust, and was transported as a whole on the SAB in the Coniacian times (88–87 Ma). Final closure of the basin is Late Cretaceous in age (73–71 Ma) as dated by metamorphic rocks.

Linking the NE Anatolian and Lesser Caucasus ophiolites: evidence for large-scale obduction of oceanic crust and implications for the formation of the Lesser Caucasus-Pontides Arc

In the Lesser Caucasus and NE Anatolia, three domains are distinguished from south to north: (1) Gondwanian-derived continental terranes represented by the South Armenian Block (SAB) and the Tauride–Anatolide Platform (TAP), (2) scattered outcrops of Mesozoic ophiolites, obducted during the Upper Cretaceous times, marking the northern Neotethys suture, and (3) the Eurasian plate, represented by the Eastern Pontides and the Somkheto-Karabagh Arc. At several locations along the northern Neotethyan suture, slivers of preserved unmetamorphozed relics of now-disappeared Northern Neotethys oceanic domain (ophiolite bodies) are obducted over the northern edge of the passive SAB and TAP margins to the south. There is evidence for thrusting of the suture zone ophiolites towards the north; however, we ascribe this to retro-thrusting and accretion onto the active Eurasian margin during the latter stages of obduction. Geodynamic reconstructions of the Lesser Caucasus feature two north dipping subduction zones: (1) one under the Eurasian margin and (2) farther south, an intra-oceanic subduction leading to ophiolite emplacement above the northern margin of SAB. We extend our model for the Lesser Caucasus to NE Anatolia by proposing that the ophiolites of these zones originate from the same oceanic domain, emplaced during a common obduction event. This would correspond to the obduction of non-metamorphic oceanic domain along a lateral distance of more than 500 km and overthrust up to 80 km of passive continental margin. We infer that the missing volcanic arc, formed above the intra-oceanic subduction, was dragged under the obducting ophiolite through scaling by faulting and tectonic erosion. In this scenario part of the blueschists of Stepanavan, the garnet amphibolites of Amasia and the metamorphic arc complex of Erzincan correspond to this missing volcanic arc. Distal outcrops of this exceptional object were preserved from latter collision, concentrated along the suture zones.

RADIOLARIAN EVIDENCE FOR EARLY CRETACEOUS (LATE BARREMIAN - EARLY APTIAN) SUBMARINE VOLCANIC ACTIVITY IN THE TETHYAN OCEANIC REALM PRESERVED IN KARABAGH (LESSER CAUCASUS)

Dating radiolarites in the ophiolitic sequences of Alpine mountain belts is important for the geodynamic reconstruction of Neo-Tethys in the Lesser Caucasus.However, radiolarian data from Karabagh are very rare. A moderately well-preserved radiolarian assemblage was obtained from radiolarites that overliepillow lavas cropping out west of the village of Vank in the mountainous region of Karabagh. The presence of the easily recognisable subspecies Aurisaturnaliscarinatus perforatus is of particular significance because it is the end member of a well studied lineage that went extinct in the early Aptian. Indeed, themiddle late Barremian - early early Aptian age range of this subspecies allows to suggest that submarine volcanic activity took place during this interval in theTethyan oceanic realm preserved in Karabagh. Given that this submarine volcanic event is more or less contemporaneous to the Aptian alkaline OIB-typelavas from the Vedi ophiolite, it is likely that the Karabagh lavas are also the expression of a volcanic activity associated to the emplacement of the sameoceanic volcanic plateau.

Thermochronometric evidence for Miocene tectonic reactivation of the Sevan-Akera suture zone (Lesser Caucasus): a far-field tectonic effect of the Arabia-Eurasia collision?

Abstract Low-temperature thermochronological data for the Eurasian foreland north of the Bitlis–Zagros suture zone suggest that the tectonic stresses related to the Arabian collision during mid-Miocene time were transmitted efficiently over large distances, focusing preferentially at rheological discontinuities. Since the late Middle Miocene a new tectonic regime has been active as the westwards translation of Anatolia is accommodating most of the Arabia–Eurasia convergence, thus precluding the efficient transfer of stress northwards. Apatite fission-track data from the central Lesser Caucasus show that a portion of this orogen underwent a discrete phase of cooling/exhumation at 18–12 Ma (late Early–early Middle Miocene) as a result of the structural reactivation of a segment of the Late Cretaceous–Palaeogene Sevan–Akera suture zone. This inference contradicts the notion that the post-collisional history of the study area was dominated by strike-slip tectonics with relatively minor dip-slip components. Reactivation and exhumation was focused along those segments of the suture zone at high angles to the inferred collisional stress field; the remaining areas were not exhumed enough to expose a new apatite partial annealing zone and thus retained the thermochronological record of a phase of Late Cretaceous cooling/exhumation associated with ophiolite obduction and the following continental collision along the suture zone.

Radiolarian evidence for the age of chert blocks from the Upper Cretaceous ophiolitic mélange unit of the Erakh area, Armenia

Abstract Two new and relatively well-preserved radiolarian faunas were extracted from blocks included in the Cretaceous ophiolitic mélange unit that crops out in the Erakh area south of Yerevan, Armenia. One of these radiolarian assemblages was extracted from cherts that are in stratigraphic contact with variolitic lavas. This radiolarian assemblage suggests a late Aalenian to middle Bajocian age (Unitary Association Zones 2–3) based on the co-occurrence of the species Parasaturnalis diplocyclis (Yao) and Transhsuum hisuikyoense (Isozaki & Matsuda). A second radiolarian assemblage was obtained from a block, several metres thick, of alternating pelagic limestones and cherts. It was characterized by the co-occurrence of the species Hemicryptocapsa capita Tan and Dicerosaturnalis dicranacanthos (Squinabol). The presence of these two species within the same assemblage correlates with Unitary Association Zone 17 and these samples are therefore assigned to the late Valanginian. These results suggest that submarine volcanic activity took place during the late Aalenian–middle Bajocian interval and that mixed carbonate and siliceous ooze accumulated during the late Valanginian. Previous biochronological data on Radiolaria from the Erakh mélange and the Vedi ophiolite are reviewed and their implications for the geodynamic and palaeoenvironmental evolution of the Tethyan ocean in Armenia are discussed.

New data on the tectonic evolution of the Khoy region, NW Iran

Abstract The Khoy region (NW Iran) is important in the clarification of the structural framework of the alpine belt between the Taurides, the Lesser Caucasus and the NW Iran belt. The area is well-known for these ophiolitic units. We present here new stratigraphic and structural data that can be used to reconstruct the tectonic evolution of this region and then try to establish connections between these belts. According to new dates from nannoplankton assemblages, the obducted ophiolite of the Khoy complex was thrust over a sheared Campanian olistostrome and lenses of amphibolite are included within the contact. The obduction event is also marked by erosion of the ophiolitic unit and the deposition of conglomerates, shales, sandstones and siltstones. Poorly extended Paleocene detrital deposits cover the Campanian–Maastrichtian rocks. The Eocene formations characterize a basin filled with volcanogenic and sedimentary layers. The Middle and Upper Eocene series unconformably overlie the ophiolites and their cover of Campanian–Maastrichtian and Paleocene deposits. This corresponds to a syn-orogenic basin formed after the collision between Eurasia and the Taurides–Anatolides–South Armenian microplate. The Oligocene–Miocene Qom Formation with basal conglomerates unconformably covers all the earlier formations, including the Palaeozoic formations, indicating intense shortening before its deposition. Compressional deformation continued and is manifested by numerous folds, mainly west-dipping thrusts and reverse faults cutting the Qom Formation, and by recent NW–SE dextral strike-slip faults. This illustrates the continuous shortening and uplift (with intense erosion) resulting from the advanced stage of the collision between Arabia and Eurasia. The structural location of the tectonic units suggests that the Khoy Gondwana-related basement was part of the South Armenian Block and that the Khoy allochthonous ophiolites were obducted on it from the Amasia–Stepanavan–Sevan–Hakari suture zone.