The Miocene Salzachtal‐Ennstal‐Mariazell‐Puchberg (SEMP) strike‐slip fault in Austria allows study of the internal structure of a fault zone from the near surface to ∼30 km depth. As it enters the Tauern Window along the Rinderkarsee shear zone, the SEMP fault passes from a dominantly brittle to a dominantly ductile structure. The shear zone consists of three 1‐ to 100‐m‐wide zones of brittle‐ductile and ductile deformation separated by 500‐m‐wide zones of less deformed rocks. The southern shear zone is mylonitic, with ductile amphibole and plagioclase; weak crystal preferred orientations imply that the main deformation mechanism was dislocation‐accommodated grain boundary sliding. The northern and central shear zones are characterized by discrete millimeter‐wide shear zones with ductile quartz, muscovite, and biotite and brittle feldspar. Shear zone nucleation at the grain scale involved dislocation creep and the transformation of plagioclase to muscovite; strain then localized in muscovite‐rich grain boundary shear zones that linked to form throughgoing shear zones.
Ultramylonites and margarite-bearing quartz-feldspar S-C mylonites, containing amphibolite lenses with symplectitic texture, were encountered in a borehole (Bajánsenye-B-M-I) close to the west of the Transdanubian Central Range Unit. These rocks demonstrate a ductile, horizontal extensional shear zone attaining a thickness of 300 m. Microstructural data, mineral parageneses and mineral chemistry of these rocks indicate a multistage metamorphic evolution, which is consistent with that of the Koralm-Pohorje basement. The youngest mylonitic event (Early Tertiary) took place in the Bajánsenye mylonites at 430-450oC (greenschist-facies); it rejuvenated coarse-grained muscovite crystals of eo-Alpine age (Early-Middle Cretaceous). The radiometric data presented in this paper demonstrate for the first time an important Early Tertiary tectonic zone in this area.
A combined petrological, geochemical, and geochronological (Rb–Sr and Sm–Nd whole-rock, U–Pb and Lu–Hf zircon, and Ar–Ar hornblende) study on a section of pre-Witwatersrand basement drilled at the northwestern margin of the Witwatersrand Basin has revealed new insights into the nature and tectonic setting of the likely source area for some of the Mesoarchaean auriferous Witwatersrand sediments. The protoliths of intersected altered granite and hornblende metagabbro are of indistinguishable age (3062 ± 5 Ma) and have very similar geochemical signatures. Trace element characteristics typical of calc-alkaline magmatism and evidence of variable contamination with older crust (subchondritic ϵNd and ϵHf in zircon) point to an active continental margin setting. The Ar–Ar hornblende ages are within error of the magmatic crystallization age or slightly older. Alteration of presumably primary magmatic hornblende to magnesio-hornblende immediately after gabbro emplacement during late magmatic autometasomatism is suggested. The presence of hydrous melts (>4 wt % H2O), comparable with fertile Au-bearing magmatic–hydrothermal mineralizing systems in Phanerozoic volcanic arcs, is inferred. Thus, a kind of hinterland is proposed for the Witwatersrand that compares favourably with the tectonic domains that are known to host the majority of post-Archaean gold deposits. Later retrograde hydrothermal alteration at c. 2720 and 2630 Ma led to variable Pb loss in zircon and the resetting of the whole-rock Rb–Sr isotope system whereas the Ar–Ar and Lu–Hf isotope systems in the hornblende and zircon grains, respectively, were not significantly affected. Comparison with published data suggests that these alteration events are the same as those that affected the Witwatersrand Basin fill associated with major early Ventersdorp flood basalt volcanism and possibly a pre-Transvaal thrusting event in response to contractional deformation in the Limpopo Belt.
Abstract Hydrothermal polymetallic veins of the Gemeric unit of the Western Carpathians are oriented coherently with the foliation of their low‐grade Variscan basement host. Early siderite precipitated from homogeneous NaCl‐KCl‐CaCl 2 ‐H 2 O brines with minor CO 2 , while immiscible gas–brine mixtures are indicative of the superimposed barite, quartz–tourmaline and quartz–sulphide stages. The high‐salinity aqueous fluid (18–35 wt%) found in all mineralization stages corresponds to formation water modified by interaction with crystalline basement rocks at temperatures between 140 and 300°C. High brominity (around 1000 ppm in average) resulted from evaporation and anhydrite precipitation in a Permo‐Triassic marine basin, and from secondary enrichment by dissolution of organic matter in the marine sediments at diagenetic temperatures. Sulphate depletion reflects thermogenic reduction during infiltration of the formation waters into the Variscan crystalline basement. Crystallization temperatures of the siderite fill (140–300°C) and oxygen isotope ratios of the parental fluids (4–10‰) increase towards the centre of the Gemeric cleavage fan, probably as a consequence of decreasing water/rock ratios in rock‐buffered hydrothermal systems operating during the initial stages of vein evolution. In contrast, buoyant gas–water mixtures, variable salinities and strongly fluctuating P–T parameters in the successive mineralization stages reflect transition from a closed to an open hydrothermal system and mixing of fluids from various sources. Depths of burial were 6–14 km (1.7–4.4 kbar, in a predominantly lithostatic fluid regime) during the siderite and barite sub‐stages of the north‐Gemeric veins, and up to 16 km (1.6–4.5 kbar, in a hydrostatic to lithostatic fluid regime) in the quartz–tourmaline stage of the south‐Gemeric veins. The fluid pressure decreased down to approximately 0.6 kbar during crystallization of sulphides. U‐Pb‐Th, 40 Ar/ 39 Ar and K/Ar geochronology applied to hydrothermal muscovite–phengite and monazite, as well as cleavage phyllosilicates in the adjacent basement rocks and deformed Permian conglomerates corroborated the opening of hydrothermal veins during Lower Cretaceous thrusting and their rejuvenation during Late Cretaceous sinistral transpressive shearing and extension.
The Himalayan crystalline core zone exposed along the Sutlej Valley (India) is composed of two high‐grade metamorphic gneiss sheets that were successively underthrusted and tectonically extruded, as a consequence of the foreland‐directed propagation of crustal deformation in the Indian plate margin. The High Himalayan Crystalline Sequence (HHCS) is composed of amphibolite facies to migmatitic paragneisses, metamorphosed at temperatures up to 750°C at 30 km depth between Eocene and early Miocene. During early Miocene, combined thrusting along the Main Central Thrust (MCT) and extension along the Sangla Detachment induced the rapid exhumation and cooling of the HHCS, whereas exhumation was mainly controlled by erosion since middle Miocene. The Lesser Himalayan Crystalline Sequence (LHCS) is composed of amphibolite facies para‐ and orthogneisses, metamorphosed at temperatures up to 700°C during underthrusting down to 30 km depth beneath the MCT. The LHCS cooled very rapidly since late Miocene, as a consequence of exhumation controlled by thrusting along the Munsiari Thrust and extension in the MCT hanging wall. This renewed phase of tectonic extrusion at the Himalayan front is still active, as indicated by the present‐day regional seismicity, and by hydrothermal circulation linked to elevated near‐surface geothermal gradients in the LHCS. As recently evidenced in the Himalayan syntaxes, active exhumation of deep crustal rocks along the Sutlej Valley is spatially correlated with the high erosional potential of this major trans‐Himalayan river. This correlation supports the emerging view of a positive feedback during continental collision between crustal‐scale tectono‐thermal reworking and efficient erosion along major river systems.
