98 research outputs found
The age, origin and emplacement of the Tsiknias Ophiolite, Tinos, Greece
The Tsiknias Ophiolite, exposed at the highest structural levels of Tinos, Greece, represents a thrust sheet of Tethyan oceanic crust and upper mantle emplaced onto the AtticâCycladic Massif. We present new field observations and a new geological map of Tinos, integrated with petrology, THERMOCALC phase diagram modelling, UâPb geochronology and whole rock geochemistry, resulting in a tectonoâthermal model that describes the formation and emplacement of the Tsiknias Ophiolite and newly identified underlying metamorphic sole. The ophiolite comprises a succession of partially dismembered and structurally repeated ultramafic and gabbroic rocks that represent the Moho Transition Zone. A plagiogranite dated by UâPb zircon at 161.9 ± 2.8 Ma, reveals that the Tsiknias Ophiolite formed in a supraâsubduction zone setting, comparable to the âEastâVardar Ophiolitesâ, and was intruded by gabbros at 144.4 ± 5.6 Ma. Strongly sheared metamorphic sole rocks show a condensed and inverted metamorphic gradient, from partially anatectic amphibolites at PâT conditions of ca. 8.5 kbar 850â600 °C, downâstructural section to greenschistâfacies oceanic metasediments over ~250 m. Leucosomes generated by partial melting of the uppermost sole amphibolite, yielded a UâPb zircon protolith age of ca. 190 Ma and a highâgrade metamorphicâanatectic age of 74.0 ± 3.5 Ma associated with ophiolite emplacement. The Tsiknias Ophiolite was therefore obducted ~90 Myrs after it formed during initiation of a NEâdipping intraâoceanic subduction zone to the northeast of the Cyclades that coincides with Africa's plate motion changing from transcurrent to convergent. Continued subduction resulted in highâpressure metamorphism of the Cycladic continental margin ~25 Myrs later
Kinematics of the Southern Rhodope Core Complex (North Greece)
The Southern Rhodope Core Complex is a wide metamorphic dome exhumed in the northern Aegean as a result of large-scale extension
from mid-Eocene to mid-Miocene times. Its roughly triangular shape is bordered on the SW by the Jurassic and Cretaceous metamorphic
units of the Serbo-Macedonian in the Chalkidiki peninsula and on the N by the eclogite bearing gneisses of the Sideroneron
massif. The main foliation of metamorphic rocks is flat lying up to 100 km core complex width. Most rocks display a stretching
lineation trending NEĂą SW. The Kerdylion detachment zone located at the SW controlled the exhumation of the core complex from
middle Eocene to mid-Oligocene. From late Oligocene to mid-Miocene exhumation is located inside the dome and is accompanied
by the emplacement of the synkinematic plutons of Vrondou and Symvolon. Since late Miocene times, extensional basin sediments
are deposited on top of the exhumed metamorphic and plutonic rocks and controlled by steep normal faults and flat-ramp-type
structures. Evidence from Thassos Island is used to illustrate the sequence of deformation from stacking by thrusting of the
metamorphic pile to ductile extension and finally to development of extensional Plio-Pleistocene sedimentary basin. Paleomagnetic
data indicate that the core complex exhumation is controlled by a 30ĂŻÂżÂœ dextral rotation of the Chalkidiki block. Extensional
displacements are restored using a pole of rotation deduced from the curvature of stretching lineation trends at core complex
scale. It is argued that the Rhodope Core Complex has recorded at least 120 km of extension in the North Aegean, since the
last 40 My
Thermotectonic evolution of an extensional dome : the cenozoic Osogovo-Lisets core complex (Kraishte zone, western Bulgaria)
The Kraishte region of Bulgaria is located at the junction of the Balkanides and Hellenides-Dinarides tectonic belts. Fission-track analysis on both apatites and zircons documents the Cenozoic exhumation of a Precambrian basement bounded by low-angle detachments. Late Eocene - Oligocene extension began prior to 47 Ma and was dominantly in a top-to-the-southwest direction, confirmed by the sense of younging of apatite and zircon ages. This crustal extension controlled the formation of half-graben sedimentary basins on the hanging walls of the detachments. Thermal modelling of these hanging wall units provides evidence for heat transfer across the detachments from a relatively warm rising footwall. From 32 to 29 Ma, pervasive magmatic activity resulted in the emplacement of rhyolitic to dacitic subvolcanic bodies and dykes, along with intrusion of the Osogovo granite. The results give evidence for extension in the southern Balkan older than, and separated from, the Miocene to Quaternary Aegean extension. This might reflect transtension during northeastward extrusion and rotation of continental fragments around the western boundary of Moesia. Eocene - Oligocene extension seems to have been controlled by the distribution of earlier thickening all around the Carpatho-Balkanic orocline, which is reflected by the Cretaceous emplacement of the Morava Nappe in the Kraishte
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