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On February 7th, 2021, a rockslide of about 20 Mio m³ detached in a height of 5600 m asl. from the northern flank of Mount Ronti (Chamoli district, Uttarakhand state, India), turned into a rock mass fall and produced a debris flow. When the rock mass hit the Ronti Gad valley after a fall height of 1800 m the rock mass mixed with melting dead ice together with snow and ice avalanche material of previous debris flows. The debris flow destroyed hydroelectric infrastructure between 10 - 20 km down the valley killing 204 people either working at or visiting the power plants. By combining remote sensing, structural geology and kinematics/mechanical analysis of the rockslide, we demonstrate that a 600 m wide and almost 800 m long block of quartzite, bordered laterally by two joints and a newly formed tension crack on the top detached from an underlying layer of biotite-rich paragneisses. Assuming full hydrostatic heads in both joints and in the tension crack as well as 75% of the full hydrostatic head in the lower boundary surface between quartzites and paragneisses, the rock block analysis yields a friction angle of 32° for both joints, which is a plausible value of the friction angle of joints in quartzites. The detachment of the block has been the result of the widening of the tension crack on top, of a progressive propagation of the lateral joints together with a catastrophic failure of the detachment plane at the border between quartzites and paragneisses. At the time of the failure, all discontinuities must have been almost completely filled with water raising the question, if the frequency of rockslides in the Himalayas is increasing as temperatures rise and permafrost is thawing due to climate change.

The paleogeographic position of the central Dodecanese Islands at the transition between the Aegean and Anatolian plates plays a considerable role in understanding the link between both geologically unique domains. In this study, we investigate the tectonic history of the central Dodecanese Islands and the general correlation with the Aegean and western Anatolian and focus on the poorly studied islands of Kalymnos and Telendos. Three different major tectonic units were mapped on both islands from bottom to top: (1) The Kefala Unit consists of late Paleozoic, fossil-rich limestones, which have been deformed into a SE-vergent fold-and-thrust belt sealed by an up to 200-m-thick wildflysch-type olistostrome with marble and ultramafic blocks on a scale of tens of meters. (2) The Marina Basement Unit consists of a Variscan amphibolite facies basement with garnet mica schists, quartzites, and amphibolites. (3) Verrucano-type formation violet shales and Mesozoic unmetamorphosed limestones form the Marina Cover Unit. Correlation of these units with other units in the Aegean suggests that Kalymnos is paleogeo-graphically located at the southern margin of the Pelagonian domain, and therefore it was in a structurally upper tectonic position during the Paleogene Alpine orogeny. New white mica 40Ar/39Ar ages confirm the Carboniferous deformation of the Marina Basement Unit followed by a weak Triassic thermal event. Single-grain white mica 40Ar/39Ar ages from pressure solution cleavage of the newly defined Telendos Thrust suggest that the Marina Basement Unit was thrusted toward the north on top of the Kefala Unit in the Paleocene. Located at a tectonically upper position, the units exposed in the central Dodecanese escaped subduction and the syn-orogenic, high-pressure metamorphism. However, these units were affected by post-orogenic extension, and the contact between the Marina Basement Unit and the non-metamorphic Marina Cover Unit has been reactivated by the cataclastic top-to-SSW, low-angle Kalymnos Detachment. Zircon (U-Th)/He ages from the Kefala and Marina Basement Units are ca. 30 Ma, which indicates that exhumation and cooling below the Kalymnos Detachment started in the Oligocene. Conjugate brittle high-angle normal fault systems, which resulted in the formation of four major WNW-ESE-trend-ing graben systems on Kalymnos, localized mainly in the Marina Cover Unit and probably rooted in the mechanically linked Kalymnos Detachment. Since Oligo-Miocene deformation in the northern Dodecanese records top-to-NNE extension and the Kalym-nos Detachment accommodated top-to-SSW extension, we suggest that back-arc extension in the whole Aegean realm and transition to the Anatolian plate is bivergent. © 2022 Geological Society of America. All Rights Reserved.

The bedrock of Hymittos, Attic peninsula, Greece, exposes a pair of low-angle crustal-scale ductile-then-brittle detachment faults. The uppermost detachment fault separates sub-greenschist facies phyllite and marble of a Pelagonian Zone hanging wall, from greenschist facies metasedimentary schist, calc-schist, and marble correlated to the Cycladic Blueschist Unit. A second, structurally lower detachment fault subdivides the metamorphic rocks of the Cycladic blueschist unit footwall into middle and lower units. There is a marked step in metamorphic grade between the sub-greenschist facies uppermost package, and the middle-to-upper greenschist facies middle and lower packages. A suite of new white mica 40Ar/39Ar and zircon (U-Th)/He dates indicates accommodation of deformation along these faults occurred from the late Oligocene to the late Miocene with both faults active during the middle Miocene. The structures have clear top-S/SSW kinematics determined from flanking folds, sigmoids, shear bands, stair-stepping of strain shadows on porphyroclasts, and SCC' fabrics. The ductile-to-brittle deformation of the structures, morphology of the massif, and the increase in metamorphic grade suggest these low-angle structures are part of a major, crustal-scale extensional complex, located at the northwest end of the West Cycladic Detachment System, that accommodated Miocene bivergent exhumation of Attic-Cycladic metamorphic core complexes in the central Aegean. Taken together, the above data suggest that multiple coeval detachment branches may form in areas with high strain gradients to accommodate the mechanically necessary termination of Cycladic-style detachment systems. © 2020. American Geophysical Union. All Rights Reserved.

The Lavrion district of the Attica Peninsula, Greece, exposes the West Cycladic Detachment System (WCDS), a low-angle crustal-scale extensional fault system separating tectonostratigraphic units of the Cycladic Blueschist Unit. New multiple single-grain fusion and step-heated white mica 40Ar/39Ar ages integrated with existing (U–Th)/He ages and independent paleothermometry resolves a syn- to post-orogenic deformation history. A structurally higher unit records Oligocene greenschist facies deformation that evolved into brittle conditions by the middle Miocene, and shares a similar history to Serifos at the southern end of the detachment system. The structurally lowest unit remained ductile until the late Miocene, preserving pervasive post-orogenic structures, similar to along-strike structures at the centre, deepest part of the fault. The similarities of structural styles and the timing of deformation across > 150 km of strike length of the detachment system indicates tens of kilometers of offset and extraordinary potential for correlating observations along Cycladic-style detachment systems. © 2019 John Wiley & Sons Ltd

We present field and geochronologic evidence for a Miocene top-to-SSE detachment system on Santorini, which exhumed the Cycladic Blueschist Unit (CBU) below a package of low-grade Upper Triassic marbles that contain well-preserved Megalodon fossils forming a syncline with metaflysch in the core. White mica bundles from the mylonitic foliation in the CBU yield 40Ar/39Ar dates of 25–19 Ma and zircon (U–Th)/He dates are 11–8 Ma. A weakly foliated granitic intrusion that cross-cuts the CBU is 8.5 Ma based on zircon U–Pb geochronology. Detrital white mica from the metaflysch yields Jurassic to early Palaeogene single grain 40Ar/39Ar dates, with a dominant Palaeocene signature. Zircon (U–Th)/He dating similarly reveals dispersed ages between 36 and 15 Ma suggesting Miocene metamorphic temperatures were insufficient (

Seismic anisotropy data are often used to resolve rock textures and deformation in the crust based on compilations of rock properties that may not be representative of the local geology. In this paper, we use teleseismic receiver functions jointly with in situ rock property data to constrain the seismic structure and anisotropy of the crust in the Cyclades, Greece, located in the back‐arc region of the Hellenic subduction zone. The receiver function data indicate that the Moho is relatively flat at 25 km depth toward the south and deepens to 33 km in the north, consistent with previous studies, and reveal an intracrustal discontinuity at depths varying from 3 to 11 km, mostly observed in the south central Aegean. Harmonic decomposition of the receiver functions further indicates the presence of both shallow and deep crustal anisotropy related to crustal structures. We model synthetic receiver functions based on constraints from in situ rock properties that we measured using the electron backscatter diffraction technique. Our results indicate that the shallow upper crustal layer is characterized by metapelites with ~5% anisotropy, underlain by a 20 km thick and possibly anisotropic layer of high‐pressure rocks comprising blueschist and eclogite and/or restitic crust resulting from magmatism. This study demonstrates the importance of rock textural data in the interpretation of seismic velocity profiles.

Si tratta di una Enciclopedia che comprende tutti i settori del sapere scientifico. Si compone di oltre 130 volumi. Il Tema "Geology" è pubblicato in 5 volumi sia in formato cartaceo che come e-Books.

Dating the duration and the age of different stages of deformation of crustal rocks is essential for establishing quantitative geodynamic models. This study combines U–Th–Pb dating of monazite and microfabric data of quartzo-feldspathic rocks that make up large parts of the continental crust. Depending on strain rate and fluid availability, deformation of these rocks is accomplished by the mechanisms dislocation creep (DC) or dissolution precipitation creep (DPC). The behavior of the U–Th–Pb system of monazite, however, depends on the deformation mechanism activated in the hosting metamorphic rocks. In quartzo-feldspathic rocks deformed by DC, old predeformative monazite grains may survive high temperatures, intense mylonitization, and high strain. Such predeformative monazite grains commonly show a patchy appearance of compositional domains, reflecting intra-grain transport of material and coupled dissolution–reprecipitation replacement processes. The various patchy zones may show a wide range of apparent, geologically inaccurate Th/Pb and U/Pb ages that are the result of incomplete removal of in-situ grown radiogenic Pb from the patchy domains and the redistribution of Th and U among the domains.

In rocks exhumed from depth corresponding to (U)HP conditions, fluid-induced dissolution– precipitation processes have significant bearing on element and isotope transport, on mineral reactions and hence on rheology. The U-Th-Pb system in geochronometers of such rocks — in this case, titanite and monazite — responds to fluid-mineral interaction; this response is dependent on the composition of the interacting fluid and on the deformation mechanism activated in the hosting rock. Tracking and dating the fluid-mineral-deformation histories using simultaneously measured in-situ U-Th-Pb and trace element data (Laser Ablation Split Stream ICPMS; LASS) and also Sr isotope data, is exemplified by the study of (A) pre- and post-UHP titanite from calc-silicates (Dabie UHP unit) and (B) monazite from relict granulites (Rhodope UHP domain). A) A pre-UHP titanite megacryst (XAl 0.18-0.19) has been partly replaced in a series of reactions that gave chemically and/or Sr-isotopically distinct product minerals. During stage I (subduction associated with rock deformation), rutile formed at the rim of the titanite. Following stage I, rutile back-reacted to form titanite by dissolution-precipitation reactions (stage II). The new titanite has relatively high XAl (0.21-0.27) and low HFSE contents. It is in turn replaced during stage III by titanite, showing low XAl (0.09-0.15) and relative HFSE enrichment. Channelized fluid infiltration (stage IV) resulted in titanite replacement by allanite. These four stages record in a single titanite the mineral-fluid reaction history from deep subduction to exhumation. Variable Sr isotopic composition demonstrates that fluids from different sources were present during the various stages of fluid-rock interaction. In rim portions completely replaced during the dissolution precipitation reaction stages, U-Th-Pb dating yield ages of c. 224 Ma (stage II) and c. 214 Ma (stage III). In the calc-silicate host rock pervasive fluid infiltration during exhumation has not been accompanied by ductile deformation. B) A HP granulite with the pre-deformative granulite mineral assemblage garnet, rutile, kyanite, K-feldspar, plagioclase, quartz has been pervasively deformed at high dislocation creep strain at high temperature and pressure conditions. The granulite is a relict within migmatitic rocks, migmatized in the presence of fluid during decompression at c. 42 Ma (Liati et al., 2011). Rutile and biotite yield U-Pb and Rb-Sr ages of 39–36 Ma, probably reflecting cooling following the migmatitization. Monazite in the HP granulite reacted with fluid by fluid-induced dissolution– precipitation reactions, resulting in resetting of U-Th-Pb ages at c. 54 Ma. Trace-element data are used to link the ages to the fluid-mineral-reaction-history and to identify monazite domains that have been completely reset during deformation.

The Cretaceous eo-Alpine collisional event in the European Eastern Alps is generally accepted to induce W-NW- directed thrusting both in basement and in sedimentary cover units. This study presents the first evidence of eo-Alpine W-NW directed normal kinematics along the Schneeberg Normal Fault Zone, which separates eo-Alpine high-pressure rocks in a footwall position from pre-Alpine basement rocks in a hanging wall position. New Garnet Sm-Nd data indicate that exhumation of the high-pressure rocks along the normal fault zone started around 95 Ma ago and continued up to low greenschistibrittle conditions at 76 Ma, as indicated by a Rb-Sr age from a low temperature mylonite. The occurrence of pre-Alpine basement rocks both in the hanging wall and the footwall of eo-Alpine high-pressure rocks suggests exhumation by extrusion processes. Despite the displacement or removal of parts of the lower portion of the high-pressure unit by Tertiary strike-slip faults, eo-Alpine top-to-ESE thrusting, as expected for the structurally lower part of an extruding wedge, was found at and below the base of the eo-Alpine high-pressure rocks. A Rb-Sr age of 77 Ma from a greenschist facies mylonite in this thrust shear zone shows the contemporaneity of deformation at the base and the top of the wedge. The tectonic transport direction within the extruding wedge was E-SE, opposite to the W-NW direction so far reported for the eo-Alpine event in the Eastern Alps. The contemporaneity of opposite tectonic transport directions during continental subduction may be explained by a double-vergent wedge model with a narrow zone of ductile flow, where the high-pressure rocks were exhumed. (c) 2005 Elsevier B.V. All rights reserved