10 results found
Laurent V, Scaillet S, Jolivet L, et al., 2021, <sup>40</sup>Ar behaviour and exhumation dynamics in a subduction channel from multi-scale <sup>40</sup>Ar/<sup>39</sup>Ar systematics in phengite, Geochimica et Cosmochimica Acta, Vol: 311, Pages: 141-173, ISSN: 0016-7037
The behaviour of the phengite 40Ar/39Ar system has been explored at different scales on Sifnos and Syros, two key islands of the Upper Cycladic Blueschist system (UCB). These islands expose well-preserved eclogites and blueschists that were exhumed via extensional collapse of the Aegean subduction/accretionary wedge during the Eocene and Miocene. Both islands record the switch from convergence to extension via a nappe-scale thermal-kinematic reworking marked by a sharp (~2 km thick) metamorphic transition from preserved high-pressure low-temperature (HP/LT) eclogites and blueschists at the top of the stack (peak P-T conditions of 22 ± 2 kbar and 550 ± 25 °C), to retrogressed greenschists at the base (overprinted at P-T conditions of 12–10 kbar and 500-450 °C). Extensive mapping of regional- to grain-scale 40Ar/39Ar age variations along this gradient reveals that phengite ages consistently decrease from 51 Ma down to 18 Ma towards the base of the stack, in parallel with progressive strain localization during exhumation and reworking by the regional greenschist ductile overprint. Homogeneous grain-scale Ar distributions are recorded in the poorly deformed topmost HP samples as Eocene (51–37 Ma) plateau ages and homogeneous in situ UV ablation ages, and by Miocene (20–18 Ma) plateaus in the intensely deformed basal greenschist samples. The homogeneous eclogite and blueschist ages are interpreted to record protracted crystallization in the subduction channel between 51 and 37 Ma, while the homogeneous basal ages likely record complete resetting and/or neocrystallization at 20–18 Ma. All intermediate samples across the metamorphic transition display variably complex age spectra and discordant in situ ages. Sub-grain scale Ar disequilibrium is ubiquitous in the form of within-sample and within-grain variations, with in situ (≪1 mm) variations locally encompassing almost the entire age span recorded regionally (up to 26 Myr). T
Laurent V, Lanari P, Nair I, et al., 2018, Exhumation of eclogite and blueschist (Cyclades, Greece): Pressure-temperature evolution determined by thermobarometry and garnet equilibrium modelling, JOURNAL OF METAMORPHIC GEOLOGY, Vol: 36, Pages: 769-798, ISSN: 0263-4929
Rabillard A, Jolivet L, Arbaret L, et al., 2018, Synextensional Granitoids and Detachment Systems Within Cycladic Metamorphic Core Complexes (Aegean Sea, Greece): Toward a Regional Tectonomagmatic Model, TECTONICS, Vol: 37, Pages: 2328-2362, ISSN: 0278-7407
Laurent V, 2017, Strain localization in high-pressure low-temperature shear zones and the 40Ar/39Ar isotopic record
Exhumation mechanisms of high-pressure low-temperature (HP-LT) metamorphic rocks in subduction zones are complex and actively discussed. The study of fossilized subduction zones allows a better comprehension of these mechanisms, showing that exhumation of HP-LT rocks is mainly accommodated along crustal-scale ductile shear zones. This study aims at constraining the geometry, the kinematic and the timing of the tectonometamorphic history of the HP-LT Cycladic Blueschist Unit (CBU) cropping out in Greece. A main objective is to constrain the timing of strain localization at different scales during exhumation to better understand the mechanical behaviour of subduction zones. Three principal methods of investigation have been used, including i) a structural fieldwork that allows to characterize the geometry, the kinematic and the distribution of deformations, highlighting progressive strain localization during exhumation toward the base of the CBU and along shear zones, ii) a metamorphic petrology study aiming at determining the P-T evolution of the CBU, and iii) 40Ar/39Ar dating used to constrain the timing of the tectonometamorphic evolution of the CBU and the timing of strain localization within kilometre- to millimetre-scale shear zones showing different degrees of retrogression. We observe an obvious correlation between the intensity of finite deformation, the degree of retrogression and youngest mica ages. A major result of this thesis work is that the preservation of eclogite and blueschist-facies rocks does not necessarily imply fast exhumation rates. Our results instead suggest that the exhumation history of the CBU is relatively long, spanning over ca. 30 Ma. Consequently, it appears that the exhumation rate is not the main parameter controlling the degree of retrogression of HP-LT metamorphic rocks in the CBU compared to progressive strain localization during exhumation along a cold retrograde P-T evolution within the subduction channel.
Laurent V, Huet B, Labrousse L, et al., 2017, Extraneous argon in high-pressure metamorphic rocks: Distribution, origin and transport in the Cycladic Blueschist Unit (Greece), LITHOS, Vol: 272, Pages: 315-335, ISSN: 0024-4937
Roche V, Laurent V, Cardello GL, et al., 2016, Anatomy of the Cycladic Blueschist Unit on Sifnos Island (Cyclades, Greece), JOURNAL OF GEODYNAMICS, Vol: 97, Pages: 62-87, ISSN: 0264-3707
Laurent V, Jolivet L, Roche V, et al., 2016, Strain localization in a fossilized subduction channel: Insights from the Cycladic Blueschist Unit (Syros, Greece), TECTONOPHYSICS, Vol: 672, Pages: 150-169, ISSN: 0040-1951
Beaudoin A, Augier R, Laurent V, et al., 2015, The Ikaria high-temperature Metamorphic Core Complex (Cyclades, Greece): Geometry, kinematics and thermal structure, JOURNAL OF GEODYNAMICS, Vol: 92, Pages: 18-41, ISSN: 0264-3707
Jolivet L, Menant A, Sternai P, et al., 2015, The geological signature of a slab tear below the Aegean, TECTONOPHYSICS, Vol: 659, Pages: 166-182, ISSN: 0040-1951
Laurent V, Beaudoin A, Jolivet L, et al., 2015, Interrelations between extensional shear zones and synkinematic intrusions: The example of Ikaria Island (NE Cyclades, Greece), TECTONOPHYSICS, Vol: 651, Pages: 152-171, ISSN: 0040-1951
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