Extraction of meaningful information on the timing of fault activity from clay gouges using radiometric dating methods, such as those based on the K-Ar system, can be challenging. One of the factors complicating interpretation of the radiometric dating results is the presence of multiple K-bearing components in the gouge material. In the current study, an attempt was made to develop a new interpretative method for K-Ar and 40Ar-39Ar dating, capable of handling a three-component mixture. In addition, the mineral composition of clay gouges from the Tatra Mountains (Poland), which has not been investigated before, is reported. The mineral compositions of the bulk clay gouge material and separated size fractions were determined by X-ray diffractometry and Fourier-transform infrared spectroscopy. The gouge samples were composed of quartz, dioctahedral mica (as a discrete phase and as a component of mixed-layered illite-smectite), and chlorite, commonly with plagioclase and more rarely with K-feldspar, dioctahedral smectite, calcite, anatase, or trace kaolinite. One feldspar-free sample containing three mica polytypes (1Md, 1M, and 2M1) was chosen for dating with the 40Ar-39Ar method. The results of 40Ar-39Ar dating were interpreted using three concepts: Illite Age Analysis (IAA), a method based on the MODELAGE software, and a newly developed three-component concept. The age values obtained with IAA were −14 Ma ± 31 Ma and 180 ± 91 Ma for authigenic (1Md) and inherited (1M + 2M1) components, respectively. The MODELAGE-based approach returned –4 ± 40 Ma and 165 ± 62 Ma. The three-component approach returned age values of polytypes as follows: 1Md, 15 ± 37 Ma; 1M, 135 ± 57 Ma; 2M1, 121 ± 56 Ma based on the medians and the interquartile ranges of non-normal distributions of Monte Carlo-simulated age values. The results obtained indicated that the 1Md polytype was probably formed during the most recent stage of fault activity, while 1M and 2M1 polytypes are of equal age, roughly.

Delimiting the Aravalli mountain range in the east, the Great Boundary Fault (GBF) occurs as a crustal-scale tectonic lineament in the NW Indian Shield. The structural and tectonic characteristics of the GBF are, as yet, not well-understood. We attempt to fill this gap by using a combination of satellite image processing, high-resolution outcrop mapping and structural analysis around Chittaurgarh. The study area exposes the core and damage zone of the GBF. Three successive phases of folding, F1, F2 and F3, are associated with deformation in the GBF. The large-scale structural characteristics of the GBF core are: (i) a non-coaxial refolding of F1 folds by F2 folds; and (ii) the parallelism between the GBF and F2 axial traces. In addition, numerous metre-scale ductile shear zones cut through the rocks in the GBF core. The damage zone is characterized by the large-scale F1 folds and the mesoscopic-scale strike-slip faults, thrusts and brittle-ductile shear zones. Several lines of evidence, such as the inconsistent overprinting relationship between the strike-slip faults and thrusts, the occurrence of en échelon folds and the palaeostress directions suggest that the GBF is a dextral transpression fault zone. Structural geometry and kinematic indicators imply a wrench- and contraction-dominated deformation in the core and damage zone, respectively. We infer that the GBF is a strain-partitioned dextral transpression zone.

Fluids can play a direct role in exhumation by influencing exhumation mechanisms and the driving processes for these mechanisms. In addition, the process of exhumation leads to the development of fluid-related features that in themselves may not drive exhumation. Fluids involved in exhumation are generally derived from dehydration reactions occurring during decompression, but at shallower crustal levels may also involve the introduction of exotic fluids. The composition of fluids attending exhumation are generally saline – CO2 mixtures, but N2, CH4, H2O mixtures have also been recorded. Studies of fluid features related to exhumation have found that fluids may contribute to density changes and the initiation of partial melting during decompression, as well as the development of extensive vein systems. However, the preservation of geochemical signatures related to fluid processes occurring prior to high-P and ultrahigh-P metamorphism indicates that large-scale pervasive fluid flow systems, in general, do not operate at any stage during the exhumation history. Large-scale channelled fluid flow may have operated in detachment faults and shear zones related to exhumation, and this requires further study. The most significant role of fluids during exhumation appears to be their controlling influence on the preservation of high-P or ultrahigh-P rocks.

We describe the geometry and kinematics of a Jurassic to Early Cretaceous transpressive sinistral strike-slip system within a metamorphic basement inlier of the Mesozoic magmatic arc near Bahia Agua Dulce at latitudes 31–32°S in north-central Chile and discuss possible relations with the Atacama Fault System further north. Sinistral transpression overprints structures of an accretionary system that is represented by the metamorphic basement. Sub-vertical semi-ductile NNW-striking strike-slip shear zones are the most conspicuous structures. Chlorite and sericite grew, and white mica and quartz dynamically recrystallized, suggesting low-grade metamorphic conditions during semi-ductile deformation. Folds at the 10–100 metre scale developed before and during strike-slip shearing. The folds are deforming a former sub-horizontal transposition foliation that originated during prior accretion processes. The folds have axes sub-parallel to the strike-slip shear zones and sub-vertical axial surfaces indicating a component of shortening parallel to the shear-zone boundaries, suggesting an overall transpressive deformation regime. Transpressive strike-slip deformation also affects Middle Triassic (Anisian) basal breccias of the El Quereo Formation. 40Ar–39Ar laser ablation ages of synkinematically recrystallized white mica in one of the shear zones provide an age of 174–165 Ma for the waning stages of semi-ductile strike-slip shearing. The semi-ductile shear zones are cut by mafic and rhyolite dykes. Two rhyolite dykes yield 40Ar–39Ar ages of 160.5 ± 1.7 Ma and 131.9 ± 1.7 Ma, respectively. The latter dyke has been affected by brittle faulting. Fault-slip analysis shows that the kinematics of the faulting event is similar to the one of the semi-ductile shearing event, suggesting that sinistral transpression continued after ~130 Ma. Timing, kinematics and geographic position suggest that the shear zones at Bahia Agua Dulce represent a southern continuation of the prominent Atacama Fault System that affected the Jurassic/Early Cretaceous arc over its ~1400 km length.

India–Asia collision resulted in crustal thickening and shortening, metamorphism and partial melting along the 2200 km-long Himalayan range. In the core of the Greater Himalaya, widespread in situ partial melting in sillimanite+K-feldspar gneisses resulted in formation of migmatites and Ms+Bt+Grt+Tur±Crd±Sil leucogranites, mainly by muscovite dehydration melting. Melting occurred at shallow depths (4–6 kbar; 15–20 km depth) in the middle crust, but not in the lower crust. 87Sr/86Sr ratios of leucogranites are very high (0·74–0·79) and heterogeneous, indicating a 100 crustal protolith. Melts were sourced from fertile muscovite-bearing pelites and quartzo-feldspathic gneisses of the Neo-Proterozoic Haimanta–Cheka Formations. Melting was induced through a combination of thermal relaxation due to crustal thickening and from high internal heat production rates within the Proterozoic source rocks in the middle crust. Himalayan granites have highly radiogenic Pb isotopes and extremely high uranium concentrations. Little or no heat was derived either from the mantle or from shear heating along thrust faults. Mid-crustal melting triggered southward ductile extrusion (channel flow) of a mid-crustal layer bounded by a crustal-scale thrust fault and shear zone (Main Central Thrust; MCT) along the base, and a low-angle ductile shear zone and normal fault (South Tibetan Detachment; STD) along the top. Multi-system thermochronology (U–Pb, Sm–Nd, 40Ar–39Ar and fission track dating) show that partial melting spanned ̃24–15 Ma and triggered mid-crustal flow between the simultaneously active shear zones of the MCT and STD. Granite melting was restricted in both time (Early Miocene) and space (middle crust) along the entire length of the Himalaya. Melts were channelled up via hydraulic fracturing into sheeted sill complexes from the underthrust Indian plate source beneath southern Tibet, and intruded for up to 100 km parallel to the foliation in the host sillimanite gneisses. Crystallisation of the leucogranites was immediately followed by rapid exhumation, cooling and enhanced erosion during the Early–Middle Miocene.

A transpressive crustal-scale dextral shear zone is documented in the Variscan Basement of northeastern Sardinia. It indicates the presence of a shear deformation parallel to the belt overprinting previous D1 structures related to nappe stacking and top-to-the-S and -SW thrusting. The L2 stretching lineation points to an orogen-parallel stretching and to a general change in the tectonic transport from D1 to D2. Phase D1 developed during initial frontal collision, whereas the D2 deformation was characterized by dextral shearing during the increasing curvature of the Ibero-Armorican arc. Transpressional deformation developed in a regime of decreasing pressure. It caused telescoping of the Barrovian isograds and the exhumation of the low- to medium-grade metamorphic rocks. In this sector of the Variscan belt, exhumation is due to continuing compression with an increasing component of horizontal displacement. The overall change of the shortening direction in a large sector of an orogenic belt, with the occurrence of increasing orogen-parallel displacement, may be regarded as a general mechanism affecting the exhumation of rocks and preventing the thickened collisional crust from undergoing a generalized gravitational collapse.