6,961 research outputs found
Fluid-mediated transition from dynamic rupturing to aseismic slip at the base of the seismogenic continental crust
No full textISSN:0012-821XISSN:1385-013XISSN:1385-013
Early-Oligocene low-angle normal faulting in the Eastern Alps
No full textDuring Early-Oligocenic cooling of the Rieserferner pluton (RFP, Eastern Alps) through amphibolite-greenschist facies temperature conditions, a set of shallowly (15-25 ◦ ) E-dipping joints, epidote and quartz veins developed. These joint and veins were exploited as top-to-the-East normal ductile shear zones, then overprinted by a set of steeper brittle-ductile shear zones with the same orientation and kinematics. Strong fluid-rock interaction, feldspar destabilization and chlorite+white mica+calcite veins characterize these brittle-ductile mylonites. As a whole, the orientation, kinematics and meso-microstructural evolution of these shear zones are similar to that described for the Katschberg normal fault (KNF, Genser and Neubauer, 1989), a regional scale low-angle normal fault (LANF). Therefore, some genetic relationship between the two might be outlined.
The characteristics, timing and the comparison of this set of structures with the KNF allow us to investigate both regional geology topics and mechanical processes related to LANF, in particular:
1) The KNF is miocenic in age (23 Ma). Comparing field, microstructural and published thermo-chronological data, we infer that structures in the RFP developed between 30 and 26 Ma, defining, therefore, the first occurrence of exhumation tectonics in the future Tauern Window region. In addition, diffuse extensional structures might be responsible for the “regional E-down tilting” inferred from thermochronological data (Steenken et al., 2002).
2) Given the limited amount of accommodated strain, these structures might represent the incipient stages of deformation and nucleation (on brittle precursors) of a regional-scale LANF. Nucleation probably occurred at very low-angle, given the small amount of supposed “regional E-down tilting” (5 ◦ , Steenken et al., 2002) and the unlikely occurrence of rolling-hinge model rotations. The transition from shallow- to high-angle dips probably represents the evolution in geometry of fault system during footwall uplift and decreasing confining pressure. Veins and fluid-rock interaction suggest the occurrence of high pore-fluid pressure.
3) Different weakening mechanisms contributed to slip along misoriented planes: (i) reaction weakening processes in mylonites exploiting epidote veins; (ii) inherently weak quartz with respect to the host granitoid during quartz vein shearing; (iii) feldspar-to-mica reaction during later stage brittle-ductile mylonite development. The results of rheological models about shear zone deformation will be discussed focussing to the strength evolution and to the LANF-slip/seismicity conundrum
Structural Evolution of Periadriatic Plutons and its implications on solid-state deformation of granitoid rocks
Full text linkExhumed granitoid plutons are an ideal natural research target for studying the processes of nucleation and evolution of ductile and brittle deformation structures. Granitoid plutons, unaffected by later tectono-metamorphic cycles, preserve pristine deformation structures developed during cooling from magmatic to host rock ambient temperatures, that can be assumed as representatives of structures that at different structural levels of the continental crust. The main focus of this Ph.D. project is the analysis of deformation structures of the Rieserferner pluton – one of the major Periadriatic intrusions. The aim of the study is two-fold: (i) reconstruct the tectonic framework during the different stages of the pluton structural evolution, and (ii) determine the processes controlling localized ductile strain at different scales and the environmental conditions at which they occurred.
The structural evolution during pluton cooling consists of 5 main deformation stages, that have been bracketed in time and thermal conditions according to microstructural and textural analysis, literature and field data: (i) steeply dipping joints, leucocratic dykes and quartz-feldspar veins and associated ductile shear zones; (ii) shallowly dipping joints with associated epidote and quartz veins and ductile shear zones; (iii) steeply dipping mafic dykes and calcite-white mica-bearing brittle-ductile faults; (iv) steeply dipping pseudotachylyte-bearing cataclastic faults; and (v) zeolite-bearing faults. Integrating new field, microstructural and geothermo-chronological data with published data we have related the deformation sequence to the Tertiary tectonics of the Eastern Alps. (i) three main ductile deformation stages developed during Oligocene, followed by two brittle deformation stages during Miocene; (ii) paleostress inversion from kinematic analyses suggest a complex stress field variation during the structural evolution mainly due to switch in relative magnitudes of principal stress components; (iii) the described paleostress evolution reflect the sequence of tectonic processes occurred during Oligocene and Miocene at the scale of the Eastern Alps, from slab break-off to indentation and lateral escape tectonics.
Microstructural investigations were mainly focused on the analysis of ductile shear zones exploiting epidote- and quartz-rich veins. Softening and localization in quartz veins was mainly controlled by grain size reduction by recrystallization. EBSD mapping and image analyses have shown that different crystallographic orientations of quartz vein crystals controlled the evolution of microstructures and crystallographic preferred orientations (CPO) during vein-parallel simple shear up to high shear strains ( ≈ 10). Recrystallization by Subgrain Rotation (SGR) lead to the development of fine-grained ultramylonitic quartz veins, in which, the observed CPO banding have been inherited from the original crystallographic orientation of the vein crystals. Localization of ductile strain within heterogeneous shear zones exploiting epidote veins was mainly obtained through myrmekite development and following shearing. EBSD investigations suggest that myrmekite induced a switch in the dominant deformation mechanism, from dynamic recrystallization by SGR to diffusion-assisted grain boundary sliding (GBS) during shearing of plagioclase + quartz aggregates.
Thermodynamic modelling was aimed to define the temperature-pressure-fluid conditions under which deformation these processes occurred. Pseudosections computed for the chemical systems NaCaKFMASHO and MnNaCaKFMASHO suggest that: (i) the epidote-veining event in the RFP likely occurred at temperatures between 520°C and 490°C at water-saturated conditions; (ii) the main deformation phase likely occurred at 460±40°C and 0.35 ± 0.05 GPa, lasting probably during pluton cooling down to 350°C at slightly under-saturated water-conditions
Structural evolution of the Rieserferner pluton in the framework of the Oligo-Miocene tectonics of the Eastern Alps
No full textThe structural evolution of the Oligocene Rieserferner pluton (Eastern Alps), developed during pluton cooling and exhumation, includes localized deformation structures consisting of: (i) steeply dipping joints, leucocratic dykes and quartz-feldspar veins, and related ductile shear zones (formed at ∼600 °C); (ii) shallowly dipping joints with associated epidote and quartz veins, and related ductile shear zones (∼450 °C); (iii) steeply dipping mafic dykes and calcite + white mica-bearing brittle-ductile faults (∼300 °C); (iv) steeply dipping pseudotachylyte-bearing faults (< 250 °C); and (v) zeolite-bearing faults (< 200 °C). The structures (i-iii) and (iv-v) can be dated to the Oligocene and Early Miocene, respectively. The kinematic analysis suggests a change in local shortening direction from N290°to N350°, interpreted to reflect a change from transtension in the Early Oligocene, associated with a N060°-directed plate divergence, to transpression in the Miocene, associated with a (N)NW-directed plate convergence. This evolution reflects the sequence of transient and long-term geodynamic processes occurring in the Eastern Alps during the Tertiary, including slab break-off of the subducted European slab, the indentation of the Dolomites promontory against the Alpine stack of nappes, and the exhumation and lateral escape tectonics of the Alpine belt
Strength of Dry and Wet Quartz in the Low-Temperature Plasticity Regime: Insights From Nanoindentation
Full text linkAt low-temperature and high-stress conditions, quartz deformation is controlled by the kinetics of dislocation glide, that is, low-temperature plasticity (LTP). To investigate the relationship between intracrystalline H2O content and the yield strength of quartz LTP, we have integrated spherical and Berkovich nanoindentation tests at room temperature on natural quartz with electron backscatter diffraction and secondary-ion mass spectrometry measurements of intracrystalline H2O content. Dry (<20 wt ppm H2O) and wet (20–100 wt ppm H2O) crystals exhibit comparable indentation hardness. Quartz yield strength, which is proportional to indentation hardness, seems to be unaffected by the intracrystalline H2O content when deformed under room temperature, high-stress conditions. Pre-indentation intracrystalline microstructure may have provided a high density of dislocation sources, influencing the first increments of low-temperature plastic strains. Our results have implications for fault strength at the frictional-viscous transition and during transient deformation by LTP, such as seismogenic loading and post-seismic creep
Large displacement numerical study of 3D plate anchors
Full text linkThis paper discusses a numerical study carried out in the frame of the development of a new application of plate anchors for landslide stabilization. The plates are positioned on ground surface of the slope and linked to a deeper stable layer with a steel grouted bar, thus acting as discontinuous elements that contrast the slope movement. This technique is less expensive compared to standard retaining structures, especially in medium and deep landslides. Moreover, plate anchors can bear large displacements of the unstable moving mass without losing efficiency. Evaluating the stabilizing force and its optimization in relation to the plate shape and interspace are of great interest. Numerous studies have investigated the bearing capacity of rectangular and circular thin plates at small strains, but the performance of alternative shapes, such as cones or truncated cones, has never been considered. The numerical study here presented applies the Material Point Method to investigate the behaviour of plate anchors with different 3D shapes at large displacements. The numerical model is validated with the results of some small-scale laboratory tests. The pull-out resistance, the soil stress and displacement fields around the plate, as well as the group effect have been investigated, thus obtaining preliminary indications for the design of these elements
The rheology of sheared myrmekite during mylonitization of a granitoid
No full textStrain-induced replacement of K-feldspar by myrmekite is a very efficient reaction-softening mechanism in granitoids. We investigated the granitoid mylonites of the Rieserferner pluton (Eastern Alps), that underwent deformation at 420-460 ◦ C during pluton cooling, to establish the mechanism of weakening associated with myrmekite development. Myrmekite development results in both grain size reduction and phase mixing as a consequence of heterogeneous nucleation of quartz and plagioclase. The grain size reduction promotes grain-size sensitive creep mechanisms such as diffusion-assisted grain boundary sliding in plagioclase. This process is coupled with heterogeneous nucleation of quartz in pores developed by creep cavitation of plagioclase + quartz aggregates. K-feldspar is occasionally re-deposited in strain shadows and creep cavitation pores due to precipitation from K-rich fluids.
Rheological flow laws have been calculated for pure quartz, pure feldspar, and plagioclase + quartz layers of the Rieserferner mylonites following Platt (2015). Results show that sheared myrmekite diffusion-assisted grain boundary sliding can accommodate strain rates up to four orders of magnitude higher than pure quartz layers deforming by dislocation creep. Rheological models for plagioclase + quartz and for pure feldspar aggregates show that strain rates of diffusion-assisted grain boundary sliding in the former are comparable to those of diffusion creep in the latter. Therefore, diffusion creep and grain-size sensitive processes play a key role during mylonitization and contribute significantly to bulk rock softening. Our results have strong implications for the definition of rheological models of K-feldspar-rich lithologies during deformation at mid-upper crustal conditions
[Poesia] Três poemas de Alberto Secama
No full textThree poems by Alberto Secama. About the author: Alberto Secama is an Angolan poet who has poems published on many websites and on facebook:https://www.facebook.com/Xungurra/abouthttp://www.pordentrodaafrica.com/cultura/africa-em-verso-rio-kwanza-por-alberto-secamahttp://www.pordentrodaafrica.com/cultura/africa-em-verso-zong-por-alberto-secamahttp://www.pordentrodaafrica.com/cultura/coluna-africa-em-verso-o-sol-la-fora-por-alberto-secamaTres poemas de Alberto Secama. Sobre el autor: Alberto Secama es un poeta angoleño que tiene poemas publicados en varios sitios y en el facebook:https://www.facebook.com/Xungurra/abouthttp://www.pordentrodaafrica.com/cultura/africa-em-verso-rio-kwanza-por-alberto-secamahttp://www.pordentrodaafrica.com/cultura/africa-em-verso-zong-por-alberto-secamahttp://www.pordentrodaafrica.com/cultura/coluna-africa-em-verso-o-sol-la-fora-por-alberto-secamaTrês poemas de Alberto Secama. Sobre o autor: Alberto Secama é um poeta angolano que possui poemas publicados em vários sites e no facebook:https://www.facebook.com/Xungurra/abouthttp://www.pordentrodaafrica.com/cultura/africa-em-verso-rio-kwanza-por-alberto-secamahttp://www.pordentrodaafrica.com/cultura/africa-em-verso-zong-por-alberto-secamahttp://www.pordentrodaafrica.com/cultura/coluna-africa-em-verso-o-sol-la-fora-por-alberto-secam
- …
