One kilometre-thick ultramylonite, Sierra de Quilmes, Sierras Pampeanas, NW Argentina

We describe a 1 km-thick ultramylonite forming the high strain base of the >3.5 km-thick El Pichao shear zone in the Sierra de Quilmes. This shear zone thrusted granulite facies migmatites onto amphibolite facies rocks during the 470 Ma Famatinian orogeny. Strain grades upwards from ultramylonites to weakly sheared migmatites across the 3.5 km-zone and the mylonitic rocks define a geochemical field narrower than the protolith, suggesting they underwent mixing and homogenization through shearing. Ultramylonites this thick are uncommon. The width of a shear zone, in the absence of significant compositional rheological contrasts controlling strain localization, is controlled by the balance between shear heat generation and diffusion. Under typical crustal conditions a strain rate of 10−12 s−1 is required to form a 1 km-thick ultramylonite, and this is achieved when large movement velocities are imposed across the shear zone. We postulate that the El Pichao shear zone and its thick ultramylonite accommodated a significant fraction of convergence velocities driving the orogeny, and that the wide mylonitic shear zones characteristic of the Cambrian–Ordovician deformation of the Sierras Pampeanas result from the convergent movement being taken up by only a few active major shear zones

Porous Melt Flow in Continental Crust—A Numerical Modeling Study

International audienceAbstract In continental crust, rapid melt flow through macroscopic conduits is usually envisaged as the most efficient form of melt transport. In contrast, there is growing evidence that in hot continental crust, grain‐scale to meso‐scale porous melt flow may operate over long distances and over millions of years. Here, we investigate the dynamics of such porous melt flow by means of two‐dimensional thermo‐mechanical numerical models using the code ASPECT. Our models are crustal‐scale and describe the network of pores through which the melt flows by permeability that depends on the spacing of the pores. Our results suggest that assuming realistic material properties, melt can slowly migrate in the hot and thick continental crust through pores with a characteristic spacing of 1 mm or larger. Despite its low velocity (millimeters to centimeters per year), over millions of years, such flow can create large partially molten zones in the middle‐lower crust and significantly affect its thermal state, deformation, and composition. We examined the role of the permeability, melt and solid viscosities, the slope of the melting curve and temperature conditions. We obtained contrasting styles of melt distribution, melt flow, and solid deformation, which can be categorized as melt‐enhanced convection, growth of partially molten diapirs and melt percolation in porosity waves. Our numerical experiments further indicate that grain‐scale porous flow is more likely in rocks where the melt productivity increases slowly with temperature, such as in metaigneous rocks

Thermal and mechanical evolution of collisional and accretionary orogens: a volume in honour of Karel Schulmann—an introduction

International audienceProfessor Karel Schulmann (Fig. 1) is a scientific leader in the domain of structural geology, tectonics of collisional and accretionary systems and geodynamic processes in general. His publication record is impressive both in terms of quality and quantity (190 refereed papers and over 6200 citations). Karel Schulmann has basically founded the modern basement geology community in the Czech Republic and greatly contributed to the development of this discipline in France. This volume is an outcome of a conference in honour of Karel Schulmann’s 60th birthday entitled Thermal and mechanical evolution of collisional and accretionary orogens held in Třešť, Czech Republic from August 31st to September 2nd, 2018. This conference featured over 50 contributions from colleagues, former students and friends all around the world and 17 selected contributions are presented in this volume

One kilometre-thick ultramylonite, Sierra de Quilmes, Sierras Pampeanas, NW Argentina

We describe a 1 km-thick ultramylonite forming the high strain base of the >3.5 km-thick El Pichao shear zone in the Sierra de Quilmes. This shear zone thrusted granulite facies migmatites onto amphibolite facies rocks during the 470 Ma Famatinian orogeny. Strain grades upwards from ultramylonites to weakly sheared migmatites across the 3.5 km-zone and the mylonitic rocks define a geochemical field narrower than the protolith, suggesting they underwent mixing and homogenization through shearing. Ultramylonites this thick are uncommon. The width of a shear zone, in the absence of significant compositional rheological contrasts controlling strain localization, is controlled by the balance between shear heat generation and diffusion. Under typical crustal conditions a strain rate of 10−12 s−1 is required to form a 1 km-thick ultramylonite, and this is achieved when large movement velocities are imposed across the shear zone. We postulate that the El Pichao shear zone and its thick ultramylonite accommodated a significant fraction of convergence velocities driving the orogeny, and that the wide mylonitic shear zones characteristic of the Cambrian–Ordovician deformation of the Sierras Pampeanas result from the convergent movement being taken up by only a few active major shear zones.Fil: Finch, M. A.. Monash University; AustraliaFil: Weinberg, R. F.. Monash University; AustraliaFil: Fuentes, María Gabriela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Investigaciones en Energia No Convencional. Universidad Nacional de Salta. Facultad de Ciencias Exactas. Departamento de Física. Instituto de Investigaciones en Energia No Convencional; ArgentinaFil: Hasalová, P.. Monash University; AustraliaFil: Becchio, Raul Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Investigaciones en Energia No Convencional. Universidad Nacional de Salta. Facultad de Ciencias Exactas. Departamento de Física. Instituto de Investigaciones en Energia No Convencional; Argentin

Late Paleozoic-Mesozoic tectonic evolution of the Trans-Altai and South Gobi Zones in southern Mongolia based on structural and geochronological data.

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Monazite Dating of Prograde and Retrograde P−T−d paths in the Barrovian terrane of the Thaya window, Bohemian Massif

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The Effect of melt infiltration on metagranitic rocks: The Snieznik dome, Bohemia Massif

Highly deformed banded phengite-biotite metagranite from the Snieznik dome in the Bohemian Massif has been modified locally to have stromatic, schlieren or nebulitic textures typical of migmatites. This occurred mostly along subvertical deformation zones at eclogite-facies conditions, at a scale of several centimetres to several metres, mostly parallel to the foliation. The transition from banded to migmatite types of orthogneiss is marked by an increase in the amount of phases interstitial along grain boundaries in the dynamically recrystallized monomineralic feldspar and quartz aggregates, and by increasing consumption of recrystallized K-feldspar grains by fine-grained plagioclase and quartz, as well as myrmekite (intergrowth of PlQz). The new minerals are in textural equilibrium with phengite. The myrmekite, quartz and feldspars can be coarse-grained (grain size 05cm). These features are considered to be the result of grain-scale melt infiltration that caused dissolution-reprecipitation along grain boundaries in the presence of phengite. The infiltration was pervasive at the grain scale, but localized at hand-specimen to outcrop scales. All the rock types have the same mineral assemblage of Grt+Ph+Bt+Ttn+Kfs+Pl+Qz+Rt+Ilm; they have similar garnet, phengite and biotite compositions, and based on mineral equilibria modelling we infer equilibration at a pressure of 15-17GPa and a temperature of 690-740C. Because the rocks are inferred to be H2O-undersaturated and above the temperature conditions of the wet solidus, infiltration must have involved a hydrous melt, as opposed to an H2O fluid. Stability of melt-bearing mineral assemblages and mineral compositions are almost independent of the melt proportion in the system, thus explaining the identical assemblage and mineral compositions observed in all the migmatite types. This precludes the estimation of the amount of melt infiltrated. Migmatite textures, however, suggest that variable degrees of melt-rock interaction occurred, being low in the banded migmatite types and higher in the nebulitic and schlieren types. Retrograde equilibration was largely restricted to retrograde zoning in phengite, garnet and plagioclase, and crystallization of biotite around phengite and garnet, presumably in a continuous reaction consuming melt. This may have occurred down to 07-10 GPa. According to Sr-Nd isotope data, the infiltrating melt is probably derived from similar rocks, structurally beneath the observed ones. The infiltration may have facilitated exhumation of a 2 km wide structural domain from 17 to 07 GPa, within which are the subvertical deformation zones along which the infiltration occurred

P–T–t–D record of crustal-scale horizontal flow and magma-assisted doming in the SW Mongolian Altai.

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