Geochronological evidence for repeated brittle reactivations of a pre-existing plastic shear zone: The Himdalen–Ørje deformation zone, Southern Norway

<p>It is well known that faults, once formed, become permanent weaknesses in the crust, localizing subsequent brittle strain increments. The case of repeated brittle reactivations localized along pre-existing plastic shear zones is less recognized, although this situation is frequently observed in many geologically old terranes.</p><p>We have studied the prolonged deformation history of the Himdalen–Ørje Deformation Zone (HØDZ) in SE Norway by combining K–Ar and <sup>40</sup>Ar–<sup>39</sup>Ar geochronology with structural analysis. The HØDZ consists of a large variation of deformation products from mylonites and cataclasites to pseudotachylites and fault gouge. Several generations of mylonites make up the ductile part of HØDZ, called the Ørje shear zone, a km-think SW-dipping shear zone within the late Mesoproterozoic Sveconorwegian orogen. <sup>40</sup>Ar–<sup>39</sup>Ar dating of white mica from one of these mylonites give a plateau age of c. 908 Ma, interpreted to constrain the timing of late-Sveconorwegian extensionial reactivation of the Ørje shear zone.</p><p>This mylonitic fabric is extensively reworked in a brittle fashion along the SW-dipping Himdalen fault, a 10–25 m thick fault zone of cataclasite, breccia, fault gouge and, in places, abundant pseduotachylite veins. <sup>40</sup>Ar–<sup>39</sup>Ar dating of pseduotachylite material gives several small plateaus between c. 375 and 300 Ma, whereas K-feldspar clasts from the cataclasitically deformed host rock carry a Caledonian signal (plateau at c. 435 Ma). K–Ar dating of three fault gouges constrain the timing of gouge development at c. 270 and 200 Ma. Two of the fault gouges also contain protolithic K-bearing mineral phases that overlap in age with the c. 375 Ma pseudotachylite <sup>40</sup>Ar–<sup>39</sup>Ar plateau age, consistent with field observations of the former reworking the latter.</p><p>In sum, the HØDZ records multiple Paleozoic and Mesozoic brittle reactivations of the early Neoproterozoic (and older) mylonitic Ørje shear zone. Most of the brittle deformation is interpreted to have accumulated during development of the Permian Oslo rift and its subsequent latest Triassic evolution. The suggested late Devonian (c. 375 Ma) initiation of brittle deformation does not have a clear tectonic association, but we speculate that it relates to strike-slip displacements caused by the Variscan orogen, as also suggested for the sub-parallel Tornquist zone to the south.</p>

To the Development of the Phenomenology of Fast Shear Flows of Grain Materials

Phenomenological approaches to describing the rheological behavior of granular materials under conditions of rapid and quasi-plastic shear deformations are considered. A unified approach to the phenomenological-logical description of the physical parameter, called the temperature of the granular medium, and the mechanisms of shear stress generation is proposed. A description is given of the mechanism for generating shear stresses under the action of a flow of pulses directed along the shear rate gradient and caused by transverse quasi-diffusion of particles. This mechanism is taken into account in the rheological model in addition to the traditional mechanism of generating kinetic shear stresses under the action of tangential shock pulses.

The influence of soft clay saturation characteristics on unconfined compressive strength in Guangdong-Hong Kong-Macao Greater Bay Area

In order to study the influence of saturated characteristics of soft clay on unconfined compressive strength, the soft clay of Guangdong-Hong Kong-Macao Bay Area is taken as the research object, comparing and contrasting on unconfined compression test of saturated clay and unsaturated clay in laboratory, studying the variation law of unconfined compressive strength and sensitivity of unsaturated and saturated soft clay. The test results show that: 1. During the failure of unsaturated soft clay samples, oblique fractures appear, showing brittle shear failure, while the saturated clay samples appear constant bulging in the middle, and finally a “cross” is broken in the central bulging part, showing plastic shear failure.2. The unconfined compressive strength of unsaturated clay is about 10kPa higher than that of saturated soft clay, and its corresponding sensitivity is also about 0.4 higher. It can be shown that the soil saturation has a certain influence on the soil strength. The difference between saturated soil and unsaturated soil is the existence of gas phase. To be more precise, the existence of gas phase in unsaturated soil, i.e. the existence of suction, makes the soil stronger and presents the brittle shear failure form, while the saturated soil basically presents the plastic shear failure form.

Anti-twinning in nanoscale tungsten

Nanomaterials often surprise us with unexpected phenomena. Here, we report a discovery of the anti-twinning deformation, previously thought impossible, in nanoscale body-centered cubic (BCC) tungsten crystals. By conducting in situ transmission electron microscopy nanomechanical testing, we observed the nucleation and growth of anti-twins in tungsten nanowires with diameters less than about 20 nm. During anti-twinning, a shear displacement of 1/3〈111〉 occurs on every successive {112} plane, in contrast to an opposite shear displacement of 1/6〈1¯1¯1¯〉 by ordinary twinning. This asymmetry in the atomic-scale shear pathway leads to a much higher resistance to anti-twinning than ordinary twinning. However, anti-twinning can become active in nanosized BCC crystals under ultrahigh stresses, due to the limited number of plastic shear carriers in small crystal volumes. Our finding of the anti-twinning phenomenon has implications for harnessing unconventional deformation mechanisms to achieve high mechanical preformation by nanomaterials.

Large-deformation finite-element modelling of earthquake-induced landslides considering strain-softening behaviour of sensitive clay

Large-scale landslides in sensitive clays cannot be explained properly using the traditional limit equilibrium or Lagrangian-based finite-element (FE) methods. In the present study, dynamic FE analysis of sensitive clay slope failures triggered by an earthquake is performed using a large-deformation FE modelling technique. A model for post-peak degradation of undrained shear strength as a function of accumulated plastic shear strain (strain-softening) is implemented in FE analysis. The progressive development of “shear bands” (the zone of high plastic shear strains) that causes the failure of a number of soil blocks is simulated successfully. Failure of a slope could occur during an earthquake and also at the post-quake stage until the failed soil masses come to a new static equilibrium. Upslope retrogression and downslope runout of the failed soil blocks are examined for varying geometries and soil properties. The present FE simulations can explain some of the conditions required for different types of seismic slope failure (e.g., spread, flowslide or monolithic slides) to be triggered, as observed in the field.