Geothermal systems in terrains affected by polyphase deformation have reservoirs with a 3D geometry that is always difficult to predict. In this paper we describe a fossil exhumed geothermal system exposed in eastern Elba Island that developed in a polyphase folded and faulted setting, which can help us to understand how geothermal fluids circulate in geological bodies with inherited structures. Geothermal circulation at Elba allowed the deposition of Fe-ore deposits (haematite/magnetite and pyrite) and altered rock volumes, which represent tracers of the palaeo-fluid flow. Normal and oblique-slip faults dissected a polyphase folded metasiliciclastic succession and produced a secondary permeability in the range of 5 × 10−13 to 5 × 10−16 m2. From the permeable fault zones acting as feeder conduits, geothermal fluids permeated the hydraulically connected metasiliciclastic rock bodies previously deformed by two generations of folds. Geothermal fluids followed the already defined geometry, thus giving rise to apparent folded mineralized levels. Fluid migration into the metasiliciclastic rocks was possible due to their chemical aggression, which favoured the dissolution and reprecipitation of quartz, and Fe-oxide and sulphide deposition. Renewed fluids maintained their chemical properties (pH value and temperature, mostly). This conclusion provides inputs for reconstructing geothermal conceptual models and evaluating the geothermal potentiality of exploitable areas developing in similar geological settings.

Calcite veins are common in organic-rich mudrocks, but their genesis and ability to transmit fluids are debated. A combined microstructural and isotopic investigation of an array of calcite veins recovered in core from the Marcellus Formation reveals that the veins grew via a combination of continuous fibrous growth and punctuated fracture-opening increments. Continuous opening is the result of pressure-solution creep and involves no mechanical fracturing, but rather the growth of a pressure fringe around a pre-existing, sealed fracture. In contrast, incremental opening is accomplished by overpressured, mineral-saturated fluid, which repeatedly ruptures the rock at the cement / host-rock interface. Punctuated growth increments occurred repeatedly throughout an otherwise protracted, continuous growth history, indicating that the present structures preserve hybrid deformation conditions between brittle, fluid-assisted cracking and plastic strain. Stable isotopic signatures match those of a regional opening-mode fracture set that formed in response to catagenetic fluid overpressures amid a tectonically imposed (Alleghanian) stress field. It is concluded that calcite veins form as opening-mode hydraulic fractures and are susceptible to increments of brittle reactivation, even while inelastic growth processes widen and fill the veins with fibrous cement.

Faulted and fractured systems form a critical component of fluid flow, especially within low-permeable reservoirs. Therefore, developing suitable methodologies for acquiring structural data and simulating flow through fractured media is vital to improve efficiency and reduce uncertainties in modelling the subsurface. Outcrop analogues provide excellent areas for the analysis and characterization of fractures within the reservoir rocks where subsurface data are limited. Variation in fracture arrangement, distribution and connectivity can be obtained from 2D fractured cliff sections and pavements. These sections can then be used for efficient discretization and homogenization techniques to obtain reliable predictions on permeability distributions in the geothermal reservoirs. Fracture network anisotropy in the Malm reservoir unit is assessed using detailed structural analysis and numerical homogenization of outcrop analogues from an open pit quarry within the Franconian Basin, Germany. Several events are recorded in the fracture networks from the Late Jurassic the Alpine Orogeny and are observed to be influenced by the Kulmbach Fault nearby with a reverse throw of 800 m. The fractured outcrops are digitized for fluid flow simulations and homogenization to determine the permeability tensors of the networks. The tensors show differences in fluid transport direction where fracture permeability is controlled by orientation compared to a constant value. As a result, it is observed that the orientation of the tensor is influenced by the Kulmbach Fault, and therefore faults within the reservoirs at depth should be considered as important controls on the fracture flow of the geothermal system.

Widely distributed Mid-Neoproterozoic mafic rocks of the Qilian – Qaidam – East Kunlun region record the tectonic evolution of the northeastern Tibetan Plateau. This study presents whole-rock geochemistry, zircon U–Pb geochronology and Hf isotopes for the Xialanuoer gabbros of the central South Qilian Belt (SQB). Zircon laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) U–Pb dating indicates that the gabbros were emplaced at ca. 738 Ma, indicating they are contemporaneous with mafic magmatism elsewhere in the northeastern Tibetan Plateau. The gabbros have low SiO2, Cr and Ni contents and Mg# values, are relatively enriched in light rare-earth elements (LREEs) and depleted in high-field-strength elements (HFSEs; e.g. Nb and Ta), have no positive Zr or Hf anomalies and have relatively high Nb/Ta but low Nb/La ratios. These data indicate that the Xialanuoer gabbros formed from calc-alkaline basaltic magmas that were originally generated by the partial melting of an enriched mantle of type-I (EMI-type) enriched region of the lithospheric mantle, underwent little to no crustal contamination prior to their emplacement, and have within-plate basalt geochemical affinities. Combining these data with the presence of widespread contemporaneous continental rift-related magmatism and sedimentation in the North Qilian, Central Qilian, South Qilian, Quanji, North Qaidam and East Kunlun regions suggests that the northeastern Tibetan Plateau underwent Mid-Neoproterozoic continental rifting, which also affected other Rodinian blocks (e.g. Tarim, South China, Australia, North America and Southern Africa).

New palynological results from the Gafo Formation (southern sector of the Pulo do Lobo Domain, South Portuguese Zone) are integrated with recently studied sections and drillholes from the Portuguese and Spanish sectors. A total of 44 samples were studied, 27 of which were positive for palynomorph taxonomy. This research revealed well preserved palynological assemblages, including 73 spore species allocated to 28 genera, four acritarch genera, three prasinophyte algae genera plus common chitinozoan remains. Some additional forms were retained under open nomenclature. From this, the first complete age determination for the Gafo Formation in Portugal and Spain was achieved, indicating a middle Famennian (Grandispora gracilis–Grandispora famenensis, GF Biozone) to a late Famennian (Grandispora echinata, VH Biozone) age. A greywacke sample from the same Gafo Formation was dated by U–Pb zircon geochronology and a maximum depositional age of 369 ± 2.5 Ma was obtained. A correlation between these palynological and U–Pb zircon data and the palynoflora assemblages of previous authors was made, as well as with the ages of felsic volcanic rocks found intercalated in the Gafo Formation, confirming the complex stratigraphy of Pulo do Lobo Domain. The results are consistent with stratigraphic mapping and structural interpretations, allowing a middle–late Famennian age (GF and VH Biozone) to be assigned to the Gafo Formation sedimentary rocks. This work has also contributed to a reinterpretation of Gafo Formation depositional facies correlatives (e.g. the Santa Barbara Group in Spain) as being the same lithological unit.

In the Palghat–Cauvery Shear/Suture Zone of the Southern Granulite Terrane, the Sittampundi Layered Complex occurs as a mappable unit. The Sittampundi Layered Complex consists of mafic, ultramafic and anorthositic rocks with chromitite layers and has been interpreted as an arc/ophiolite complex that formed in an Archaean suprasubduction zone arc setting. In the Sittampundi Layered Complex, reddish-black metabasites occur as layers or boudins with a rim of amphibolite within anorthosite. The peak metamorphic assemblage of the metabasites is garnet + clinopyroxene + quartz + rutile ± plagioclase ± orthopyroxene, and symplectite consisting of amphibole and plagioclase formed around the garnet during retrograde metamorphism. The protolith of metabasite may have intruded in a suprasubduction zone arc setting during the late Neoarchaean (c. 2540–2520 Ma), and then underwent high-pressure granulite-facies peak metamorphism (900–800 °C and 11–14 kbar) in the early Palaeoproterozoic (c. 2460–2440 Ma), followed by amphibolite-facies metamorphism (550–480 °C and 5.5–4.5 kbar) in the middle Palaeoproterozoic (c. 1900–1850 Ma). The results obtained in this study, together with previous studies, indicate that: (1) the high-pressure granulite-facies metamorphism in the study area indicates that subduction occurred during the Archaean–Proterozoic boundary with a higher apparent average geothermal gradient (∼20–16 °C km−1) than the modern-day Earth, and (2) the apparent average geothermal gradient of the subduction zone was ∼29–14 °C km−1 during the Archaean–Proterozoic boundary, which was still too high to enter the realm of eclogite-facies metamorphism.

The Viggiano Mt. platform carbonates form a layered succession cross-cut by a dense array of pressure solution seams, and five sets of fractures and veins, which together form a sub-seismic structural network associated with polyphasic tectonic evolution. To assess the influence exerted by depositional and diagenetic heterogeneities on fracture geometry, distribution and multiscale properties, we present the results of stratigraphic, petrographic, mineralogical and mesoscale structural analyses conducted at the Viggiano Mountain, southern Italy. Based on rock textures and fossil associations, we documented that the Sinemurian–Pleinsbachian carbonates were deposited in a low-energy open lagoon, the Toarcian carbonates in a ramp setting rimmed by sand shoals, and the Cenomanian carbonates in a medium- to high-energy, lagoonal–tidal setting. Fracture-density (P20) and intensity (P21) values computed after circular scanline measurements show similar trends in both Sinemurian–Pleinsbachian and Toarcian carbonates, consistent with the bed and bed-package heterogeneities acting as efficient mechanical interfaces during incipient faulting. On the other hand, P20 and P21 do not show very similar variations throughout the Cenomanian carbonates due to pronounced bed amalgamation. Throughout the study area, the aforementioned parameters do not vary in proportion to the bed thickness, and show higher values within the coarse-grained carbonate beds. This conclusion is confirmed by results of linear scanline measurements, which focus on the P10 properties of the most common diffuse fracture set. The original results reported in this work are consistent with burial-related, physical–chemical compaction and cementation processes affecting the fracture stratigraphy of the Mesozoic platform carbonates.

This study corroborates interpretations suggesting that the Makrotantalon Unit on Andros represents a tectonic slice with Pelagonian affinity in the nappe stack of the Cycladic Blueschist Unit. Previously reported Cretaceous 40Ar–39Ar dates of a garnet-glaucophane schist from the Makrotantalon Unit could not be reproduced by Rb–Sr geochronology, but this is not an indication of contamination with excess Ar. Instead, the newly dated samples record disturbance of the Rb–Sr isotope system by partial recrystallization. Subsets of these phengite populations, representing the smaller grain-size fractions, yielded low-precision dates ranging from c. 21 to c. 15 Ma that document deformation-related resetting and recrystallization of a presumably Cretaceous white mica population. Although these Miocene dates cannot be linked with blueschist-facies metamorphism, they provide time constraints on the formation of shear zones that overprinted the original thrust contact during exhumation. The geological relevance of a Cretaceous high-pressure event is confirmed by a Rb–Sr date of c. 121 Ma for an epidote-glaucophane schist collected further away from the tectonic contact. The occurrence of a second blueschist-facies event in the Eocene is verified by Rb–Sr dates of two epidote-glaucophane schists (c. 40 Ma and c. 44 Ma) that can unambiguously be assigned to the Makrotantalon Unit.

The primary features (morphologies, structures, textures) of volcanic lava flows are determined by parameters such as composition, temperature, crystallinity, viscosity, flow velocity, strain rate and cooling rate. However, lava flows are open systems, and their primary features are strongly influenced by their emplacement environment. Among subaerial lava flows, those that solidify in a wet environment with rainfall develop very different internal structures (e.g. jointing patterns) and textures from those in a dry environment. Thus, the outcrop structures and textures of ancient lava flows, such as those forming continental flood basalt sequences thousands of metres thick, provide clues to their cooling histories and the palaeoclimates. Here we provide field, petrographic and geochemical data on large tholeiitic lava flows (sheet lobes) and associated dykes of the Saurashtra region in the northwestern Deccan Traps continental flood basalt province (India). The sheet lobes are dominantly pāhoehoe and rubbly pāhoehoe, and occasionally ‘a‘ā, with colonnade and entablature tiers. We show that the jointing patterns in the entablatures (irregular, chevron, rosette and skeleton jointing), and the textures of the sheet lobes and even some dykes (abundant glass, and quench crystals of plagioclase and Fe–Ti oxides) reflect convective heat removal, owing to widespread interaction with meteoric waters (rainfall) during solidification. The observations thus provide evidence for a wet climate in western India 65.5 million years ago.

Interactions of hydrothermal and deformation processes in active fault zones are important in light of seismic and geothermal activities. This study investigates different tectonites of the Grimselpass breccia fault, a hydrothermally active strike-slip structure in the Central Alps (Switzerland). We combine microstructural and geochemical investigations to decipher the interaction of grain-size reduction and associated dilation (volume gain) during frictional deformation with geochemical processes such as mass gain via precipitation, grain growth and healing. Three types of tectonites were investigated in the fault zone: (1) cockade-bearing tectonites, (2) cataclasites and (3) fault gouges. They differ in terms of microstructure of clasts, mass and volume gain/loss, as well as type and degree of cementation. Clast size distributions show decreasing median values from low-strain cataclasites to high-strain cataclasites. Mass/volume gains are up to 20 %, with samples of smallest clast sizes often showing the highest mass gain. In the case of cockade-bearing tectonites large volume and mass gain occur. In particular, the precipitation of SiO2-rich cements is inferred to (i) reduce permeability and (ii) induce a gain in strength of the respective fault rocks. Location, timing and degree of healing directly control the fault architecture by deactivating fault strands from the circulation of hydrothermal fluids. In this case, new rupturing events are necessary to reactivate locked fault strands. Such feedback cycles of episodic deformation, mass/volume and clogging preserved hydrothermal circulation and deformation over the last 3.4 Ma, representing a process chain for the long-term preservation of orogenic hydrothermal systems.

Lower-crust-derived adakitic rocks in the Gangdese belt provide important constraints on the timing of Tibetan crustal thickening and on the relative contributions of magmatic and tectonic processes. Here we present geochronological and geochemical data for the Wangdui porphyritic monzogranites in the western Gangdese belt. Zircon U–Pb dating yields emplacement ages of 46–44 Ma. All samples have high Sr (321–599 ppm), low Yb (0.76–1.33 ppm) and Y (10.6–18.3 ppm) contents, with high La/Yb (51.1–72.3) and Sr/Y (21.0–51.4) ratios, indicating adakitic affinities. The low MgO (0.97–1.76 wt %), Cr (7.49–53.6 ppm) and Ni (4.75–29.1 ppm) contents, as well as high 87Sr/86Sr(i) (0.7143–0.7145), low ϵNd(t) (−10.4 to −9.8) and zircon ϵHf(t) (−17.7 to 0.4) values, suggest that the Wangdui pluton most likely originated from partial melting of the thickened ancient lower crust. In combination with previously published data, despite the east–west-trending heterogeneity of crustal composition in the Gangdese belt, the La/Yb ratios of magmatic rocks reveal that both western and eastern segments experienced remarkable crustal thickening in the Eocene. However, in contrast to the thickened juvenile lower crust in the eastern segment formed by the underplating of mantle-derived magmas, tectonic shortening plays a more crucial role in thickening of the ancient basement in western Gangdese. In fact, such Eocene-thickened ancient lower-crust-derived adakitic rocks are widely distributed in the central Himalayan–Tibetan orogen. This, together with the extensive development of fold–thrust belts, suggests that tectonic shortening might be the main mechanism accounting for the crustal thickening associated with the India–Asia collision.

The North Purulia Shear Zone that dissects the granulite basement of the Chotanagpur Granite Gneissic Complex of the East Indian Shield exposes a deformed and metamorphosed nepheline syenite. The studied ‘foid-monzosyenite’ shows high abundances of large ion lithophile elements and high field strength elements with low abundances of compatible elements. Trace-element signatures show negative U, Th, Zr, Ti and Pb and positive Sr, Ba and Eu anomalies with respect to the primitive mantle. The chondrite-normalized diagram shows strongly fractionated rare earth element patterns ((La/Lu)N ∼23–87). Geochemical fingerprints suggest that the basanitic protolith was formed by low-degree partial melting of garnet peridotite in the sub-continental lithospheric mantle. The enriched large ion lithophile, high field strength element and light rare earth element concentrations (relative to primitive mantle) can be explained by a mixed mantle source with components from a previously deformed alkaline rock/carbonatite. Geochemical data do not support any significant crustal contamination and suggest variable fractionation of clinopyroxene, ilmenite, titanite and apatite from the parental melt. Petrological data are consistent with the view that the nepheline syenite magma was emplaced in a rift setting with a minimum temperature of 800–900°C, low fO2 conditions (below the fayalite–magnetite–quartz buffer) at a mid-crustal depth between 950 and 900 Ma. The continental rift zone, however, did not lead to the formation of an open ocean basin. Subsequently, the studied rock and its basement was deformed and metamorphosed in a continent–continent collisional setting at ∼900 Ma. Combining information from the other Indian occurrences with this study, it is demonstrated that the deformed alkaline rocks and carbonatite are potentially valuable for tracing the birth and demise of the palaeo-supercontinents.

Quantitative Miocene climate and vegetation data from the Siwalik succession of western Nepal indicate that the development of the Indian summer monsoon has had an impact, though in part, on vegetation changes. The climate and vegetation of the Lower (middle Miocene) and Middle (late Miocene–Pliocene) Siwalik successions of Darjeeling, eastern Himalaya, have been quantified. Reconstructed climate data, using the Coexistence Approach, suggest a decrease in winter temperatures and precipitation during the wettest months (MPwet) from the Lower to Middle Siwalik. The floristic assemblage suggests that Lower Siwalik forests were dominated by wet evergreen taxa, whereas deciduous ones became more dominant during the Middle Siwalik. The vegetation shift in the eastern Himalayan Siwalik was most likely due to a decrease in MPwet. The quantified climate–vegetation data from the eastern and western Himalayan Siwalik indicate that changes in the Indian summer monsoon had a profound impact on vegetation development during the period of deposition. We suggest that the decrease in winter temperature and summer monsoon rainfall during the Middle Siwalik might be linked with the Northern Hemisphere glaciation/cooling or a number of other things that were also going on at the time, including the continued rise of the Himalaya, and drying across the Tibetan region, which may have affected atmospheric circulation regionally.