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Earthquake dynamics from pseudotachylyte microstructure
No full textData Repository for the production of Figure 3 and Table 1 of the manuscript "Earthquake dynamics from pseudotachylyte microstructure" by Aldrighetti S., Pennacchioni G. and Di Toro G
Effects of fluids on the seismicity of a fault. Analysis of exhumed structures in the Northern Adamello (Southern Alps, Italy)
No full textA comparison between two major cataclastic faults within the northern composite Adamello batholith (Southern Alps) is attempted. The two investigated faults have strike-slip kinematics and are related to the slip along the adjacent Tonale Line, a segment of the Periadriatic Lineament bordering to the north the Adamello. The exhumed fault sections were active 30Ma ago at 9-11km depth and 250-300°C ambient temperature. The Gole Larghe Fault Zone (GLFZ) has an E-W strike and cut across the Avio tonalites. The Passo Cercen Fault Zone (PCFZ) is located, a few kilometres north of GLFZ, within the Presanella pluton. Both faults consist of a swarm of subparallel green indurated cataclasites exploiting precursor sets of joints developed during the early stages of pluton cooling (PENNACCHIONI et alii, 2006); pseudotachylytes (frictional melts formed during coseismic slip on fault planes) are abundant along the PCFZ, but almost absent along the PCFZ. The field work arid the laboratory (geochemical and microstructural) analyses have highlighted the following main differences between the two faults: (i) Orientation: the PCFZ has direction N130°, almost parallel to the direction of the regional σ1 during the faulting; the GLFZ has orientation N105° and forms an angle of about 30° with σ1. (ii) Fault rocks: a) The GLFZ includes abundant pseudotachylytes, which are very rare along the PCFZ; b) Within the PCFZ, cataclasites are associated to diffuse macroscopic veins of epidote + Kfeldspar + quartz, which are rare along the GLFZ. Our hypothesis is that both faults were seismic (small amount of pseudotachylytes are present within the PCFZ), but processes inhibiting frictional melting were active along the PCFZ. The different orientation in the regional stress field determines a different component of normal stress on fault planes affecting the degree of fluid infiltration along the two fault zones. The orientation of the PCFZ to the regional stress field is favourable to exploitation of the precursor joints as hybrid mode of fractures. Hybrid shear-extensional fractures developing at depth of 9-11km imply high pore pressure and low differential stress; both factors act reducing the effective normal stress, and thus the shear strength along the fault planes. In condition of low shear strength, frictional heating is reduced; furthermore, fluid-rich conditions trigger processes such as fluid pressurization that inhibit frictional melting
Salt-rich aqueous fluids formed during eclogitization of metabasites in the Alpine continental crust (Austroalpine Mt. Emilius unit, Italian Western Alps).
No full textHigh salinity brines in eclogiotized metabasites as the result of rock hydration during high-pressure metamorphism (Mt. Emilius Austroalpine unit, Italian Western Alps).
No full textEarthquake dynamics from pseudotachylyte microstructure
Full text linkDuring an earthquake, most of the dissipation of the stored elastic strain energy occurs by fracturing processes and frictional heat along and adjacent to the seismic fault. Information on earthquake energy partitioning and dynamics can be retrieved from the analysis of exhumed faults containing pseudotachylytes (solidified frictional melts produced during seismic faulting). Here, microstructural analysis is carried out on an east-west striking pseudotachylyte-bearing fault of the dextral strike-slip Gole Larghe Fault Zone within the Adamello granitoid pluton (Italian Alps), exhumed from 8–11 km depth. FESEM cathodoluminescence analysis reveals a strong fragmentation of the wall rocks, invisible with other techniques, which decays in the first centimetres from the pseudotachylyte fault-parallel vein. In the northern block the microfracture density is on average low (7448 mm-2) and the microfractures strike preferentially E-W. In contrast, in the southern block microfracture density is on average high (12,120 mm-2), and the microfractures strike preferentially N-S. This asymmetric wall rock damage provides evidence that the microfractures developed as result of the dynamic stress field associated with earthquake rupture propagation. The measured surface area associated with wall rock fracturing and the volume of the pseudotachylyte allow the estimate of the energy dissipated in fracturing processes US (0.015–1.88 MJ m-2) and frictional heat Q (32 MJ m-2), respectively. The comparison between US and Q implies that frictional heat is the major energy sink during rupture propagation in these intracontinental earthquakes
Channelized aqueous fluids controlling eclogitization of granulitic protoliths from the Mt. Emilius Austroalpine unit (Western Alps)
No full textExploring the Ar isotope record of an early Miocene pseudotachylyte in an early Oligocene intrusion (Rieserferner pluton, eastern Alps)
No full textWe investigate the influence of unmelted clasts on the 40 Ar\\ 39 Ar age record of a Miocene pseudotachylyte within the early Oligocene Riesenferner pluton (eastern Alps). This case study is ideal to investigate the effect of inherited Ar retained in survivor clasts because, although seismic faulting was only slightly younger (b20 Ma) than the igneous source rock, the pseudotachylyte includes layers with significant variations in K content, of either dominant plagioclase and biotite microlites or dominant submicrometric muscovite matrix. Numerical modeling, assuming pure diffusive loss and a short-lived thermal pulse due to frictional heating, indicates that unmelted plagioclase clasts represent the most critical issue in interpreting Ar data. Plagioclase clasts of only a few to tens micrometers in size are expected to retain most of radiogenic Ar accumulated before coseismic faulting. A comparable behavior is only predicted for much larger (ten to fifty times larger) K-feldspar clasts. In-situ 40 Ar\\ 39 Ar laser analyses of low-K microlitic domains, free of discernable clasts, yield slightly, but significantly, older ages than those from the high-K matrix. Results suggest that older ages (up to ~4% on average) from low-K domains are due to contamination by plagioclase clasts and suggest an age of 17.12 ± 0.22 Ma for coseismic slip. This age dates the last stage of activity of the Giudicarie Fault during tectonic indentation of the Dolomites block of the southern Alps against the nappe stack of the eastern Alp
Extrinsic Anisotropy of Two-Phase Newtonian Aggregates: Fabric Characterization and Parameterization
Full text linkRocks of the Earth's crust and mantle commonly consist of different minerals with contrasting mechanical properties. During progressive, high-temperature (ductile) deformation, these rocks develop extrinsic mechanical anisotropy linked to strain partitioning between different minerals, amount of accumulated strain, and bulk strain geometry. Extrinsic anisotropy plays an important role in a wide range of geodynamic processes up to the scale of mantle convection. However, the evolution of grain- and rock-scale fabrics causing this anisotropy cannot be directly simulated in large-scale numerical simulations. For two-phase aggregates–a good rheological approximation of most Earth's rocks–we propose a method to indirectly approximate the extrinsic viscous anisotropy by combining (a) 3D mechanical models of rock fabrics, and (b) analytical effective medium theories. Our results confirm that weak inclusions induce substantial weakening by forming a network of weak thin layers with limited lateral connectivity. Consequently, even when the inclusion phase is extremely weak, structural weakening is not larger than 30–60%, less than in previous estimates. On the other hand, the presence of strong inclusions does not have a profound impact on the effective strength of the aggregate, and lineated fabrics only develop at relatively low viscosity contrasts. When rigid inclusions become clogged, however, the aggregate viscosity can increase over the theoretical upper bound. We show that the modeled grain-scale fabrics can be parameterized as a function of the bulk deformation and material phase properties and combined with analytical solutions to approximate the anisotropic viscous tensor
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