The Cenozoic history of the retreating Hellenic subduction system in the eastern Mediterranean involves subduction, accretion, arc magmatism, exhumation, normal faulting, and large-scale continental extension from ∼60 Mya until the Recent. Ages for high-pressure metamorphism in the central Aegean Sea region range from ∼53 Ma in the north (the Cyclades islands) to ∼25−20 Ma in the south (Crete). Younging of high-pressure metamorphism reflects the southward retreat of the Hellenic subduction zone. The shape of pressure-temperature-time paths of high-pressure rocks is remarkably similar across all tectonic units, suggesting a steady-state thermal profile of the subduction system and persistence of deformation and exhumation styles. The high-pressure metamorphic events were caused by the underthrusting of fragments of continental crust that were superimposed on slab retreat. Most of the exhumation of high-pressure units occurred in extrusion wedges during ongoing lithospheric convergence. At 23–19 Mya large-scale lithospheric extension commenced, causing metamorphic core complexes and the opening of the Aegean Sea basin. This extensional stage caused limited exhumation at the margins of the Aegean Sea but accomplished the major part of the exhumation of high-grade rocks that formed between 21 and 16 Mya in the central Aegean. The age pattern of extensional faults and contoured maps of fission-track cooling ages do not show a simple southward progression. Our review of lithologic, structural, metamorphic, and geochronologic data is consistent with a temporal link between the draping of the subducted slab over the 660-km discontinuity and the large-scale extension causing the opening of the Aegean Sea basin.
Structural, metamorphic, and geochronologic work shows that the Ampelos/Dilek nappe of the Cycladic blueschist unit in the eastern Aegean constitutes a wedge of high-pressure rocks extruded during early stages of orogeny. The extrusion wedge formed during the incipient collision of the Anatolian microcontinent with Eurasia when subduction and deep underthrusting ceased and the Ampelos/Dilek nappe was thrust southward over the greenschist-facies Menderes nappes along its lower tectonic contact, the Cycladic-Menderes thrust, effectively cutting out a ∼30- to 40-km-thick section of crust. The upper contact of the Ampelos/Dilek extrusion wedge is the top-to-the-NE Selçuk normal shear zone, along which the Ampelos/Dilek nappe was exhumed by ∼3040 km. Detailed Rb-Sr and 40Ar/39Ar dating of mylonites demonstrates that both shear zones operated between 42 and 32 Ma. There is no evidence for episodic motion during the ∼10 Myr life span of the shear zones, suggesting that both shear zones operated in a steady, nonepisodic fashion. Our data provide supporting evidence that simultaneous thrust-type and normal sense shearing can accomplish the early exhumation of deep-seated rock
We show that the Styra Nappe of the Cycladic Blueschist Unit on Evia constitutes a wedge of high-pressure rocks extruded during early stages of orogeny. The nappe pile on Evia was assembled during D 2 top-to-the-SSW-directed thrusting (in restored Oligocene coordinates), which emplaced the Styra and Ochi nappes of the Cycladic Blueschist Unit above the Almyropotamos Nappe between c . 33 and 21 Ma. Peak metamorphism of the Almyropotamos Nappe at c . 23 Ma occurred at lower metamorphic pressure, showing exhumation of the Styra Nappe during underthrusting and burial of the Almyropotamos Nappe. This exhumation was largely accomplished by the D 2 top-to-the-NNE-displacing Mt. Ochi normal-sense shear zone. Normal shearing commenced at c . 33 Ma under peak high-pressure metamorphism in the Styra and Ochi nappes. Fission-track dating indicates slow cooling after D 2 in the Styra Nappe. Subsequently, the former thrust contact between the Almyropotamos Nappe and the Styra Nappe was reactivated as a D 3 top-to-the-ESE extensional shear zone and this extensional phase led to the formation of a number of Middle to Late Miocene graben. Our main conclusion is that there is strong evidence for an Oligocene extrusion wedge accomplishing the early exhumation of the Styra Nappe, which demonstrates the importance of extrusion wedges for the initial exhumation of the Cycladic Blueschist Unit.
We apply low-temperature thermochronology, Rb/Sr geochronology, petrological data, and structural mapping to constrain the timing and kinematics of the Ios metamorphic core complex. Top-to-north extension in the lower plate Headland Shear Zone was active at 18-19 Ma under metamorphic conditions of 475-610 8C and 0.65-1.1 GPa. The South Cyclades Shear Zone/Ios Detachment Fault (SCSZ/IDF) system shows top-to-south extensional shear active at c. 19 Ma at 380-550 8C, with local top-to-north bands. Extensional shear above the SCSZ/IDF is dominantly top-to-south to top-to-SW. PT estimates from an eclogite boudin constrain Eocene high-pressure metamorphism to 430-560 8C and 1.21 + 0.42 GPa to 0.66 + 0.37 GPa. Similar low-temperature thermochronometric ages across Ios demonstrate that ductile extensional movement ceased by c. 15 Ma. Exhumation to shallow crustal levels took place between c. 15 and 9 Ma at cooling rates of up to 120 8C Ma 21 with a slow down to ,20 8C Ma 21 between 12 and 9 Ma, most likely accommodated by extensional slip at rates of c. 3 km Ma 21 along the top-to-SW Coastal Fault System. We propose a model of bivergent extension for exhumation of the Ios core complex between 19 and 9 Ma, with Ios forming a secondary antithetic top-to-south to top-to-SW extensional fault system to a more dominant top-to-north Naxos/Paros detachment system.
The late Palaeozoic western Tianshan high-pressure /low-temperature belt extends for about 200 km along the south-central Tianshan suture zone and is composed mainly of blueschist, eclogite and epidote amphibolite/greenschist facies rocks. P±T conditions of ma®c garnet omphacite and garnet±omphacite blueschist, which are interlayered with eclogite, were investigated in order to establish an exhumation path for these high-pressure rocks. Maximum pressure conditions are represented by the assemblage garnet±omphacite±paragonite±phengite±glaucophane±quartz±rutile. Estimated maximum pressures range between 18 and 21 kbar at temperatures between 490 and 570 uC. Decompression caused the destabilization of omphacite, garnet and glaucophane to albite, Ca-amphibole and chlorite. The posteclogite facies metamorphic conditions between 9 and 14 kbar at 480±570 uC suggest an almost isothermal decompression from eclogite to epidote±amphibolite facies conditions. Prograde growth zoning and mineral inclusions in garnet as well as post-eclogite facies conditions are evidence for a clockwise P±T path. Analysis of phase diagrams constrains the P±T path to more or less isothermal cooling which is well corroborated by the results of geothermobarometry and mineral textures. This implies that the high-pressure rocks from the western Tianshan Orogen formed in a tectonic regime similar tò Alpine-type' tectonics. This contradicts previous models which favour`Franciscan-type' tectonics for the southern Tianshan high-pressure rocks.
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