THE NATURE OF DUCTILE DEFORMATION IN THE PHYLLITE-QUARTZITE UNIT (EXTERNAL HELLENIDES)

This work describes the nature of ductile deformation in the Phyllite-Quartzite (PQ) unit in terms of structural evolution and spatial variation of finite strain and vorticity of flow. The PQ unit is affected by at least three ductile deformation (D1, 2, 3) phases. However, the D2 is the dominant phase resulting in the formation of a penetrative foliation (S2) which is by far the most common structural feature in all scales of observation. A stretching lineation (L2), which trends perpendicular to the structural grain of the belt, is well-developed within the S2 plane. Numerous kinematic criteria clearly indicate west (or south)-directed transport of the PQ unit during D2. This phase is also characterized by a systematic non-linear increase of strain ratio (Rxz) with proximity to the Basal thrust. Spatial variation of kinematic vorticity number reveals an increase of pure shear component of D2 deformation towards the middle structural levels of the unit. These results are used to discuss the validity of various geodynamic models related to the exhumation of the PQ unit

Mechanisms of fault mirror formation and fault healing in carbonate rocks

The development of smooth, mirror-like surfaces provides insight into the mechanical behaviour of crustal faults during the seismic cycle. To determine the thermo-chemical mechanisms of fault mirror formation, we investigated carbonate fault systems in seismically active areas of central Greece. Using multi-scale electron microscopy combined with Raman and electron energy loss spectroscopy, we show that fault mirror surfaces do not always develop from nanogranular volumes. The microstructural observations indicate that decarbonation is the transformation process that leads to the formation of smooth surface coatings in the faults studied here. Piercement structures on top of the fault surfaces indicate calcite decarbonation, producing CO2 and lime (CaO). Lime subsequently reacts to portlandite (Ca(OH)2) under hydrous conditions. Nanoscale imaging and electron diffraction reveal a thin coating of a non-crystalline material sporadically mixed with nano-clay, forming a complex-composite material that smooths the slip surface. Spectroscopic analyses reveal that the thin coating is non-crystalline carbon. We suggest that ordering (hybridisation) of amorphous carbon led to the formation of partly-hybridised amorphous carbon but did not reach full graphitisation. Calcite nanograins, 100 nm) and new nanograins formed by back-reaction (secondary nanograins, <50 nm). Hence, we suggest that the new, secondary nanograins are not the result of comminution during slip but originate from pseudomorphic replacement of calcite after portlandite. The continuous coverage of partly-hybridised amorphous carbon on all samples suggests that calcite decarbonation products may develop across the entire fault surface, controlling the formation of carbonate fault mirrors, and may facilitate slip on a decarbonation-product glide film.This study was funded by the Dutch research organisation (NWO) with the project number ALWOP.2015.082

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Paleotethys was a highly mobile oceanic realm pinching into the supercontinent Pangea between Gondwana and Eurasia in the late Paleozoic/early Mesozoic. Published Paleotethyan reconstructions reveal that the time of Paleotethys closure and the position of its suture are highly debated. We present new magmatic and detrital zircon ages, separated from pre-Alpine basement and Permian to Triassic cover rocks exposed in the External Hellenides of Crete. These age data reveal Variscan and Cimmerian docking of microplates along the southern margin of Laurasia and help to constrain the time of Paleotethys closure.46% of detrital zircons from quartzite in the Variscan basement, are Pennsylvanian with concordant ages at 321. ±. 2. Ma, 310. ±. 3. Ma, and 300. ±. 3. Ma. The basement is unconformably overlain by arc-related volcanics of the Tyros Unit, magmatic zircons of which yielded a concordant U-Pb zircon age at 285. ±. 2. Ma. Thus, the metasediments of the basement, interpreted as former trench sediments, were deposited, metamorphosed and exhumed in latest Carboniferous to early Permian times (302-283. Ma). Magmatic activity during this late Variscan phase is also indicated by igneous boulders within Olenekian (meta)conglomerates of the Tyros Unit, which yielded concordant U-Pb zircon ages at 291. ±. 2 and 310. ±. 2. Ma. The late Variscan orogenic phase is attributed to the collision of the Gondwana-derived southern Minoan terrane (SMT) with Laurasia subsequent to northward subduction of Paleotethys lithosphere and Viséan collision of the northern Minoan terrane (NMT).Magmatic activity ceased during the late Permian, but revived in the Lower Triassic as is indicated by felsic volcanics (249. ±. 2. Ma, concordant U-Pb zircon) and by detrital zircons (242. ±. 3, 240. ±. 5. Ma, 237. ±. 3. Ma concordant U-Pb zircon) of the Tyros Unit. At the same time the Variscan chain was exhumed and removed as is shown by the detritus in the Lower to Middle Triassic Tyros sediments, which includes high-grade metamorphic rocks and detrital zircons with U-Pb ages ranging from 280 to 335. Ma.A significant change in the detrital components occurred in the Ladinian when the Variscan basement with its Permo-Triassic cover was thrust on top of clastic sediments, today represented by the Phyllite-Quartzite Unit s.str. The Phyllite-Quartzite Unit s.str. shows Cadomian and older - but no Variscan - detritus because of its position along the northern margin of the Cimmerian ribbon continent. Thus, in the eastern Mediterranean, Paleotethys was closed during the Ladinian and the related suture in the External Hellenides is situated between the Variscan basement (active margin in the north) and the Phyllite-Quartzite Unit s.str (passive margin in the south). Carnian crustal extension led to subsidence of the Variscan/Cimmerian chain, most parts of which merged below sea level. This is the reason why 90% of the detritus of the Carnian Tyros Beds are not related to the Variscan, but to the Cadomian and Grenvillian basement of the E-Gondwana derived Cimmerian ribbon continent

Mechanisms of fault mirror formation and fault healing in carbonate rocks

© 2019 Elsevier B.V. The development of smooth, mirror-like surfaces provides insight into the mechanical behaviour of crustal faults during the seismic cycle. To determine the thermo-chemical mechanisms of fault mirror formation, we investigated carbonate fault systems in seismically active areas of central Greece. Using multi-scale electron microscopy combined with Raman and electron energy loss spectroscopy, we show that fault mirror surfaces do not always develop from nanogranular volumes. The microstructural observations indicate that decarbonation is the transformation process that leads to the formation of smooth surface coatings in the faults studied here. Piercement structures on top of the fault surfaces indicate calcite decarbonation, producing CO2 and lime (CaO). Lime subsequently reacts to portlandite (Ca(OH)2) under hydrous conditions. Nanoscale imaging and electron diffraction reveal a thin coating of a non-crystalline material sporadically mixed with nano-clay, forming a complex-composite material that smooths the slip surface. Spectroscopic analyses reveal that the thin coating is non-crystalline carbon. We suggest that ordering (hybridisation) of amorphous carbon led to the formation of partly-hybridised amorphous carbon but did not reach full graphitisation. Calcite nanograins, 100 nm) and new nanograins formed by back-reaction (secondary nanograins, <50 nm). Hence, we suggest that the new, secondary nanograins are not the result of comminution during slip but originate from pseudomorphic replacement of calcite after portlandite. The continuous coverage of partly-hybridised amorphous carbon on all samples suggests that calcite decarbonation products may develop across the entire fault surface, controlling the formation of carbonate fault mirrors, and may facilitate slip on a decarbonation-product glide film

Constraints on the rheology of the lower crust in a strike-slip plate boundary: evidence from the San Quintín xenoliths, Baja California, Mexico

The rheology of lower crust and its transient behavior in active strike-slip plate boundaries remain poorly understood. To address this issue, we analyzed a suite of granulite and lherzolite xenoliths from the upper Pleistocene–Holocene San Quintín volcanic field of northern Baja California, Mexico. The San Quintín volcanic field is located 20 km east of the Baja California shear zone, which accommodates the relative movement between the Pacific plate and Baja California microplate. The development of a strong foliation in both the mafic granulites and lherzolites, suggests that a lithospheric-scale shear zone exists beneath the San Quintín volcanic field. Combining microstructural observations, geothermometry, and phase equilibria modeling, we estimated that crystal-plastic deformation took place at temperatures of 750–890 °C and pressures of 400–560 MPa, corresponding to 15–22 km depth. A hot crustal geotherm of 40 ° C km−1 is required to explain the estimated deformation conditions. Infrared spectroscopy shows that plagioclase in the mafic granulites is relatively dry. Microstructures are interpreted to show that deformation in both the uppermost lower crust and upper mantle was accommodated by a combination of dislocation creep and grain-size-sensitive creep. Recrystallized grain size paleopiezometry yields low differential stresses of 12–33 and 17 MPa for plagioclase and olivine, respectively. The lower range of stresses (12–17 MPa) in the mafic granulite and lherzolite xenoliths is interpreted to be associated with transient deformation under decreasing stress conditions, following an event of stress increase. Using flow laws for dry plagioclase, we estimated a low viscosity of 1.1–1.3×1020 Pa ⋅ s for the high temperature conditions (890 °C) in the lower crust. Significantly lower viscosities in the range of 1016–1019 Pa ⋅ s, were estimated using flow laws for wet plagioclase. The shallow upper mantle has a low viscosity of 5.7×1019 Pa ⋅ s, which indicates the lack of an upper-mantle lid beneath northern Baja California. Our data show that during post-seismic transients, the upper mantle and the lower crust in the Pacific–Baja California plate boundary are characterized by similar and low differential stress. Transient viscosity of the lower crust is similar to the viscosity of the upper mantle

Mantle strength of the San Andreas fault system and the role of mantle-crust feedbacks

In lithospheric-scale strike-slip fault zones, upper crustal strength is well constrained from borehole observations and fault rock deformation experiments, but mantle strength is less well known. Using peridotite xenoliths, we show that the upper mantle below the San Andreas fault system (California, USA) is dry and its maximum resolved shear stress (5–9 MPa) is similar to the shear strength of the upper, seismogenic portion of the fault. These results do not fit with any existing lithospheric strength profile. We propose the “lithospheric feedback” model in which the upper crust and lithospheric mantle act together as an integrated system. Mantle flow controls displacement and loads the upper crust. In contrast, the upper crust controls the stress magnitude in the integrated system. Crustal rupture transiently increases strain rate in the upper mantle below the strike-slip fault, leading to viscous strain localization. The lithospheric feedback model suggests that lithospheric strength is a dynamic property— varying in space and time—in actively deforming regions

U–Pb zircon and biostratigraphic data of high‐pressure/ low‐temperature metamorphic rocks of the Talea Ori : tracking the Paleotethys suture in central Crete, Greece

Inherited deformation microfabrics of detrital quartz grains and U–Pb (Laser ablation (LA)-ICPMS and ID TIMS) ages of detrital zircons separated from the Phyllite–Quartzite Unit s.l. of the Talea Ori, central Crete, suggest strikingly different source rocks. Albite gneiss of the lower Rogdia Beds includes Cambrian and Neoproterozoic rounded zircons with main U–Pb age peaks at 628 and 988 Ma. These and minor Paleoproterozoic and Archean peaks, together with the lack of Variscan-aged and Mesoproterozoic zircons, are similar to the age spectra obtained from the Phyllite–Quartzite Unit s.str. of the Peloponnesus and eastern Crete and from the Taurides. All of these zircons should be derived from the northeastern passive margin of Gondwana (Cimmeria). Metatuffites of the uppermost Rogdia Beds and metasandstone of Bali beach, on the other hand, include euhedral detrital zircons displaying a Variscan U–Pb age spectra at ca. 300 Ma with concordia ages at 291 ± 3, 300 ± 1 Ma (Rogdia) and 286 ± 3, 300 ± 3, 313 ± 2 Ma (Bali). Both types of metasediments and their zircons are similar to those of the pre-Alpine basement and overlying Tyros Beds of eastern Crete, revealing a provenance at the southern active margin of Laurasia. Thus, in central Crete the Paleotethys suture should be situated inside the Rogdia Beds. Magmatic zircons separated from a rhyolite boulder of the lower Achlada Beds yielded a concordant U–Pb zircon age at 242 ± 2 Ma placing a maximum age for the deposition of the (meta) conglomerate from which the boulder was collected. This age is compatible with an Olenekian-early Anisian age of the underlying Vasilikon marble suggested by new findings of the foraminifera Meandrospira aff. pusilla. Both the Achlada Beds and the Vasilikon marble can be attributed to the lower Tyros Beds of eastern Crete. The Alpine deformation led to a pervasive mylonitic foliation, which is affecting most of the studied rocks. This foliation results from D2 top-to-the-north shearing, which post-dates the growth of blue amphiboles (crossite)

Ductile nappe stacking and refolding in the Cycladic Blueschist Unit: insights from Sifnos Island (south Aegean Sea)

New geological and structural mapping combined with kinematic and amphibole chemistry analyses is used to investigate the deformation history of the Cycladic Blueschist Unit (CBU) on Sifnos Island (Cyclades, Aegean Sea). We concentrate on north Sifnos, an area characterized by exceptionally well-preserved eclogites and blueschists. Our data show that the early, main phase (D2) of ductile deformation in the CBU occurred synchronous with the transition from prograde to close-to-peak retrograde conditions. This deformation phase took place at middle Eocene and is related to ESE-directed thrusting that emplaced the metavolcano-sedimentary subunit over the Marble subunit. The subsequent exhumation-related (D3) deformation is characterized by gently NE-plunging folds and NE-directed contractional shear zones that formed parallel to the axial planes of folds. NE-directed shearing occurred under blue- schist and transitional blueschist-/greenschist-facies conditions during late Eocene–Oligocene and caused the restacking of the early nappe pile. We suggest that a mechanism of ductile extrusion of the CBU in a tectonic setting of net compression could explain better the recorded exhumation-related deformation than a mechanism of syn- and post-orogenic extension. Our new kinematic results in combination with previous works in the Cyclades area reveal a regional scale change in tectonic transport direction from (W)NW–(E)SE at Late Cretaceous–middle Eocene to (E)NE–(W)SW at late Eocene–Oligocene times. The observed change in transport direction may be governed by the relative motion of Africa with respect to Europe during Alpine orogeny

Tracking the late Paleozoic to early Mesozoic margin of northern Gondwana in the Hellenides : paleotectonic constraints from U–Pb detrital zircon ages

We report new detrital zircon U–Pb ages of nine quartzites sampled along the Phyllite–Quartzite unit sensu stricto (PQ unit s.s.), in the high-pressure belt of the southern Hellenides. The detrital zircon age spectra are dominated by two significant age peaks at ca. 600 and ca. 1000 Ma, which are typical for an east Gondwana provenance. The absence of zircons of Carboniferous–Triassic ages suggests that the depositional environment was isolated from Variscan and early Mesozoic sources. Our data are in support of paleogeographic configurations placing the protolith of the PQ unit s.s. south of the Paleotethys ocean and along the northern Gondwana margin. The zircon age spectra do not show significant variations with respect to the tectonostratigraphic position within the PQ unit s.s. and the position along the high-pressure belt. Combining the new (this study) with published detrital zircon ages, we suggest that the siliciclastic metasediments of the PQ unit s.s. on Crete, Kythera, and the Peloponnesus have a common paleogeographic origin