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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 Cretan Detachment represents a major tectonic contact between an Alpine high‐pressure Lower Nappe System in the footwall and an Alpine unmetamorphosed Upper Nappe System in the hanging wall. Interestingly, the kinematics and the tectonic nature of this contact is still highly controversial and has been interpreted either as a top‐to‐the S thrust or as a normal fault with top‐to‐the N and/or top‐to‐the S displacement sense. Whereas some models suggest exhumation of the high‐pressure rocks synchronous with out‐of‐sequence thrusting or Himalayan‐type extrusion tectonics, other models propose that Crete represents a metamorphic core complex that exhumed below the extensional Cretan Detachment. In this work, we show that the Cretan Detachment in eastern Crete is an up to 100‐m‐thick cataclastic low‐angle fault zone, which localized below the Tripolitza Unit (Upper Nappe System) within the upper part of the Upper Violet Slates (top of Lower Nappe System) cutting downsection to lower structural levels. The unequivocal kinematic indicators in this fault zone clearly suggest a top‐to‐the N displacement of the hanging wall. New low‐temperature geochronological data and numerical modeling suggest rapid cooling of the rocks in the footwall of the Cretan Detachment at circa 15 Ma. Top‐to‐the S ductile shear sense toward lower structural level in the Lower Nappe System support the idea that the Cretan Detachment facilitated the exhumation of the high‐pressure rocks in an extrusion tectonic setting. Therefore, in the early to middle Miocene, subduction‐related extrusion in Crete was simultaneously active with back‐arc extension‐related metamorphic core formation in the Cyclades. Cretan Detachment is an extrusion‐related low‐angle normal fault that operated mainly under brittle conditionsA suite of clear shear sense indicators from brittle fault rocks demonstrates that the kinematic of the Cretan Detachments is top‐to‐the NLow‐temperature geochronology and numerical modeling indicates that the Cretan Detachment was active in the Middle Miocene

Exhumation of the high‐grade metamorphic Seve Nappe Complex and its emplacement between lower‐grade nappes has been related to wedge extrusion in the central Scandinavian Caledonides. To test this hypothesis, the kinematic evolution of the Caledonian nappe pile is studied by systematic structural mapping in central and northern Jämtland, Sweden. Structural data, combined with petrological and quartz microstructure observations, document pervasive top‐to‐the‐ESE, foreland‐directed shearing under progressively decreasing metamorphic grade across the entire nappe pile. Mylonitic foliation, foliation‐parallel boudinage, and abundant top‐to‐the‐ESE and rare, scattered top‐to‐the‐WNW shear‐sense indicators imply foreland‐directed general shear. This deformation regime caused exhumation by concurrent thrusting and vertical ductile thinning. We propose a specific succession of in‐ and out‐of‐sequence thrusts that generated the metamorphic zonation. Our model envisions in‐sequence propagation of thrusts during exhumation of the Seve Nappe Complex, related to subduction of Baltica beneath a volcanic arc within Iapetus. Concurrently, Iapetus subducted beneath Laurentia farther to the west. When Iapetus was closed, Baltica subduction stepped westward and continued beneath Laurentia. The back stepping of subduction at the onset of continental collision caused out‐of‐sequence propagation of the orogenic wedge. Thrusting cut downsection across the existing tectonostratigraphy, emplacing units of lower metamorphic grade above the high‐grade Seve Nappe Complex. This imbrication generated the present metamorphic zonation of the Caledonian nappe pile during sustained convergence between Laurentia and Baltica. Top‐ESE shear is pervasive across the metamorphically zoned Caledonian nappe pileMylonitic fabrics indicate sustained foreland‐directed general shear, causing exhumation by concurrent thrusting and vertical thinningBack stepping of Baltica subduction resulted in out‐of‐sequence propagation of orogenic wedge

We present field‐based structural, geochronological, and petrological evidence for a major new Oligocene extensional shear zone, the Evia Shear Zone (ESZ), on the island of Evia in the NW Aegean Sea. Strain related to this structure occurs in a diffuse zone structurally underlying the nappe contact between the Cycladic Blueschist Unit and Basal Unit, within rock previously attributed to the Tsakei Unit or Basal Unit metaflysch. Structures within the ESZ record stretching in both the Xand Ydirections under ductile and brittle‐ductile conditions, indicative of a component of oblate coaxial strain during overall top‐to‐E general shear. New white mica 40Ar/39Ar geochronology from the ESZ shows that the structure was accommodating ductile deformation in the late Oligocene. Thermodynamic modeling shows that rock within the ESZ records greenschist facies retrogressive conditions of 310 ± 15°C and 7 ± 1 kbar. Zircon (U‐Th)/He thermochronology demonstrates that both the CBU and Basal Unit had exhumed into the brittle crust by the middle Miocene, slightly earlier than indicated by low‐temperature thermochronology for the HP‐LT units throughout the Aegean. We interpret these data as evidence that the Basal Unit underwent rapid exhumation via syn‐orogenic processes persisting until at least the early Miocene, facilitated in part by normal‐sense displacement along the ESZ. The occurrence of structures representative of coaxial deformation in the shear zone alludes to a major role for ductile thinning in controlling exhumation of HP rocks in the Cyclades, perhaps influencing the relative rates of exhumation observed across the region. Exhumation of stacked HP nappes in the Cyclades occurred under ductile syn‐orogenic oblate coaxial strainLow‐temperature geochronology reveals rapid unroofing of the lower nappe into the shallow crust over a 10–15 Myr intervalHeterogeneous distribution of coaxial strain within an orogen may control diachronous exhumation of HP rock Exhumation of stacked HP nappes in the Cyclades occurred under ductile syn‐orogenic oblate coaxial strain Low‐temperature geochronology reveals rapid unroofing of the lower nappe into the shallow crust over a 10–15 Myr interval Heterogeneous distribution of coaxial strain within an orogen may control diachronous exhumation of HP rock

Here we introduce the Minimized Power Geometric (MPG) model which predicts the viscosity of any polyphase rocks deformed during ductile flow. The volumetric fractions and power law parameters of the constituting phases are the only model inputs required. The model is based on a minimization of the mechanical power dissipated in the rock during deformation. In contrast to existing mixing models based on minimization, we use the Lagrange multipliers method and constraints of strain rate and stress geometric averaging. This allows us to determine analytical expressions for the polyphase rock viscosity, its power law parameters, and the partitioning of strain rate and stress between the phases. The power law bulk behavior is a consequence of our model and not an assumption. Comparison of model results with 15 published experimental data sets on two‐phase aggregates shows that the MPG model reproduces accurately both experimental viscosities and creep parameters, even where large viscosity contrasts are present. In detail, the ratio between experimental and MPG‐predicted viscosities averages 1.6. Deviations from the experimental values are likely to be due to microstructural processes (strain localization and coeval other deformation mechanisms) that are neglected by the model. Existing models that are not based on geometric averaging show a poorer fit with the experimental data. As long as the limitations of the mixing models are kept in mind, the MPG model offers great potential for applications in structural geology and numerical modeling. The MPG model predicts the viscosity of polyphase rocks having power law phasesThe MPG model provides a better fit with experimental data than existing modelsThe MPG model offers good potential in structural geology and numerical modeling

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. New bedrock mapping identifies a low‐angle fault system in central Attica, correlated to the West Cycladic Detachment SystemWhite mica 40Ar/39Ar and zircon (U‐Th)/He geochronology suggest this fault system was active from the late Oligocene through the middle MioceneThe architecture and timing of these faults suggest new Cycladic‐style detachment branches can form without plutonism

A <3 mm thick, intensely brecciated layer associated with Marinoan (c. 635 Ma) glacial striations on a Neoproterozoic sandstone pavement is documented. Only instantaneous solidification could have preserved the breccia in situ. Glacier speeds of c. 0.3 m s-1 would have induced peak flash temperatures sufficient (>1130 °C) to cause partial frictional welding of the deformed quartzitic bedrock beneath an ice-loaded clast. These results support suggestions that glacial seismicity is due to episodic very rapid shifts of large ice masses over the substrate (stick--slip movement). They also indicate that significant effective normal stresses occur at the base of glaciers. [ABSTRACT FROM AUTHOR]