Brittle Shear Zones Research Articles

The Lavrion Pb-Zn-Fe-Cu-Ag detachment-related district (Attica, Greece): Structural control on hydrothermal flow and element transfer-deposition

The impact of lithological heterogeneities on deformation, fluid flow and ore deposition is discussed based on the example of the Lavrion low-angle detachment partly accommodating gravitational collapse of the Hellenides orogenic belt in Greece. The Lavrion peninsula is characterised by a multiphase Pb-Zn-Fe-Cu-Ag ore system with a probable pre-concentration before subduction followed by progressive remobilisation and deposition coeval with the development of a low-angle ductile to brittle shear zone. The mylonitic marble below the detachment shear zone is composed of white layers of pure marble alternating with blue layers containing impurities (SiO2, Al2O3, carbonaceous material). Ductile mylonitic deformation is more pervasive in the less competent impure blue marble. We propose that localised deformation in the impure marble is associated with fluid circulation and dolomitisation, which in turn causes an increase in competence of these layers. Mineralised cataclastic zones, crosscutting the mylonitic fabric, are preferentially localised in the more competent dolomitic layers. Oxygen and carbon isotopic signatures of marble invaded by carbonate replacement deposits during ductile to ductile-brittle deformation are consistent with decarbonation coeval with the invasion of magmatic fluids. Mineralised cataclastic zones reflecting brittle deformation evolve from low 13C to low 18O signatures, interpreted as local interaction with carbonaceous material that trends toward the contribution of a surface-derived fluid. These features indicate that the Lavrion area records a complex deposition history influenced by the evolution of fluid reservoirs induced by the thermal and mechanical evolution of the marble nappe stack. Ore remobilisation and deposition associated with the activity of the low-angle detachment is (i) firstly related to the intrusion of the Plaka granodiorite leading to porphyry-type and carbonate replacement mineralisation during ductile-brittle deformation and (ii) then marked by progressive penetration of surface-derived fluids guided by strain localisation in the more competent levels leading to epithermal mineralisation associated with brittle deformation.

Development of branching brittle and ductile shear zones: A numerical study

AbstractContinental collision zones are usually associated with large‐scale strike‐slip shear zones. In most cases, these shear zones are complex and consist of multiple strands, varying in width, length, and total displacement. Here we present 2‐D numerical models to simulate the formation of such shear zones at different depth levels within the crust, under either brittle (frictional/plastic) or ductile conditions. Localization of shear zones is initiated by a material contrast (heterogeneity) of the material parameters. We systematically test the rate of strain weakening in brittle and in ductile regimes to understand its influence on the development of shear zone networks. Our simulations suggest that the development of antithetic faults in a brittle shear zone system is closely linked to a decrease in the angle of friction during deformation. In general, variation of the strain weakening also has a significant influence on ductile shear zones. Numerical results show that the geometry and thickness of the localized high strain zone are especially affected by weakening mechanisms during deformation. Furthermore, the interconnection and interaction of the shear strands lead to a more complex kinematic pattern, which lead to a local change in the maximum principal stress axis. These interaction of shear strands may explain the occurrence of shear‐related structures (e.g., folds) or differing characteristics of shear zones, such as the thickness of shear zones or the orientation of the faults to the stress field, which are consistent with field observations.

Size distribution and roundness of clasts within pseudotachylytes of the Gangavalli Shear Zone, Salem, Tamil Nadu: An insight into its origin and tectonic significance

Gangavalli (Brittle) Shear Zone (Fault) near Attur, Tamil Nadu exposes nearly 50 km long and 1–3 km wide NNE–SSW trending linear belt of cataclasites and pseudotachylyte produced on charnockites of the Southern Granulite Terrane. Pseudotachylytes, as well as the country rock, bear the evidence of conjugate strike slip shearing along NNE–SSW and NW–SE directions, suggesting an N–S compression. The Gangavalli Shear Zone represents the NNE–SSW fault of the conjugate system along which a right lateral shear has produced seismic slip motion giving rise to cataclasites and pseudotachylytes. Pseudotachylytes occur as veins of varying width extending from hairline fracture fills to tens of meters in length. They carry quartz as well as feldspar clasts with sizes of few mm in diameter; the clast sizes show a modified Power law distribution with finer ones (<1000 \({\upmu }\)m\(^{2})\) deviating from linearity. The shape of the clasts shows a high degree of roundness (>0.4) due to thermal decrepitation. In a large instance, devitrification has occurred producing albitic microlites that suggest the temperature of the pseudotachylyte melt was >1000\(^{\circ }\hbox {C}\). Thus, pseudotachylyte veins act as a proxy to understand the genetic process involved in the evolution of the shear zone and its tectonic settings.

Carpathian Shear Corridor – A strike-slip boundary of an extruded crustal segment

The Carpathian Shear Corridor (CSC), a morphostructurally distinctive ENE-WSW brittle shear zone, is a prominent dynamic interface of crustal fragments shifted during an oblique collision process combined with lateral extrusions in the Late stages of the Western Carpathians tectonic evolution. This tectonics was due to convection in the upper mantle, driven mainly by slab-pull forces related to a subductional process in front of prograding Carpathians. The CSC separates the marginal segment of the Western Carpathians, already firmly attached to the European plate, from the southern still eastwardly moving block. This process led to structural transpositions, anomalous rotation of small blocks and tilting and uplift/subsidence events, resulting in a tectonic style of horst and intramountaine basin alternations within the corridor. Preliminary paleomagnetic data indicate anomalous CCW block rotations within this corridor, and AFT ages indicate Early and Late Miocene (ca 24–22Ma and ca 10–7Ma) fault controlled exhumation events triggered by increased shear zone activity. Deep seismic sections, magnetotelluric and gravity data show that CSC follows a frontal ramp of the Western Carpathians thrust over the foreland. The CSC remains an active strike-slip shear zone, and therefore the most important earthquake risk-zone in the Slovakian portion of the Western Carpathians. It presents a lateral ramp transform boundary of eastwardly extruding crustal segment during the Miocene and up to the recent time.

The Piúma lineament, southern Espirito Santo: structural expression and tectonic significance

ABSTRACT: The Piúma lineament, located in the Southern Espírito Santo State, Brazil, is one of the most prominent linear morphostructural features of the northern Mantiqueira Province. The lineament stands out in satellite images and topographic maps, extending for about 70 km in a N50°W direction, between the towns of Iconha and Conceição do Castelo (Espírito Santo). In order to investigate the nature and role played by the Piúma lineament in the tectonic evolution of the Mantiqueira Province and adjacent Campos Basin, we carried out a detailed structural analysis based on fieldwork, as well as geophysical data processing and interpretation. The results we obtained indicate that the Piúma lineament is a brittle shear zone characterized by N50W-trending and SW-dipping shear fractures, faults and joints. These structures record two distinct and incompatible motions, thereby recording the operation of two tectonic regimes: NNE-SSW extension and E-W transtension. Comparing our results with the literature on the Southeastern Brazilian Continental Margin, we conclude that the Piúma lineament more likely developed after the Ediacaran Brasiliano orogenic event in response to an NNE-SSW extension, experiencing later a reactivation as normal dextral to dextral transtensional zone, probably in connection with the South Atlantic rifting in the Lower Cretaceous.

Coseismic horizontal slip revealed by sheared clastic dikes in the Dead Sea Basin

Despite the hazard caused by near-surface destructive horizontal displacements during earthquakes, field evidence for coseismic slip along horizontal discontinuities is exceptionally rare, mainly due to the lack of adequate exposure and markers. However, within the seismically active Dead Sea Basin, the late Pleistocene Lisan Formation contains vertical clastic dikes that are sheared laterally at maximum depths of 15 m, and thereby provide unique profiles of such horizontal displacement. In order to investigate how coseismic horizontal shearing is distributed near the surface, we document an ∼1-m-thick brittle shear zone, consisting of up to 11 slip surfaces that can be traced for tens of meters in the Lisan Formation. Displacements along individual slip surfaces are up to 0.6 m, and the total displacement across the shear zone is up to 2.0 m. Displacement profiles and gradients indicate that the brittle shear zone formed by simple shear, and deformation was associated with slip partitioning and transfer between primary and secondary slip surfaces. Evidence for concurrent displacement along slip surfaces during a single event indicates that the brittle shear zone was formed during a coseismic event subsequent to 30 ka. We consider the mechanical effect of seismic-wave–related transient stress, which, when added to the initial static effective stress, may result in concurrent horizontal shear failure along detrital-rich layers in the Lisan Formation. The exceptional quality of exposures and markers enables us to document, for the first time, the details of near-surface horizontal shearing, and indicates that displacement along horizontal bedding planes is a viable mechanism to absorb coseismic deformation in well-bedded near-surface strata.

Petrology and mineralogy of the La Peña igneous complex, Mendoza, Argentina: An alkaline occurrence in the Miocene magmatism of the Southern Central Andes

The La Peña alkaline igneous complex (LPC) is located in the Precordillera (32°41′34″ S - 68°59′48″ W) of Mendoza province, Argentina, above the southern boundary of the present-day flat-slab segment. It is a 19 km2 and 5 km diameter subcircular massif emplaced during the Miocene (19 Ma) in the Silurian-Devonian Villavicencio Fm. The LPC is composed of several plutonic and subvolcanic intrusions represented by: a cumulate of clinopyroxenite intruded by mafic dikes and pegmatitic gabbroic dikes, isolated bodies of malignite, a central intrusive syenite that develops a wide magmatic breccia in the contact with clinopyroxenite, syenitic and trachytic porphyries, a system of radial and ring dikes of different compositions (trachyte, syenite, phonolite, alkaline lamprophyre, tephrite), and late mafic breccias. The main minerals that form the LPC, ordered according to their abundance, are: pyroxene (diopside, hedenbergite), calcium amphibole (pargasite, ferro-pargasite, potassic-ferro-pargasite, potassic-hastingsite, magnesio-hastingsite, hastingsite, potassic-ferro-ferri-sadanagaite), trioctahedral micas (annite-phlogopite series), plagioclase (bytownite to oligoclase), K-feldspar (sanidine and orthoclase), nepheline, sodalite, apatite group minerals (fluorapatite, hydroxylapatite), andradite, titanite, magnetite, spinel, ilmenite, and several Cu–Fe sulfides. Late hydrothermal minerals are represented by zeolites (scolecite, thomsonite-Ca), epidote, calcite and chlorite.The trace element patterns, coupled with published data on Sr-Nd-Pb isotopes, suggest that the primary magma of the LPC was generated in an initially depleted but later enriched lithospheric mantle formed mainly by a metasomatized spinel lherzolite, and that this magmatism has a subduction-related signature. The trace elements pattern of these alkaline rocks is similar to other Miocene calc-alkaline occurrences from the magmatic arc of the Southern Central Andes.Mineral and whole-rock chemical compositions support the interpretation that a first batch of a tephritic magma produced a cumulate of clinopyroxenite (clinopyroxene + magnetite + apatite) and a residual melt that crystallized as malignite at a shallow emplacement level (<5 km). Fractional crystallization in a deep chamber, coupled with rock assimilation, produced successive magma pulses that gave the composite central syenite, the syenitic and trachitic porphyries, and trachytic dikes. The latest rocks (phonolites) reflect an extreme fractionation with Ca, Al and K removal by feldspars. Mingling relationships between tephrites-basanites and trachytes-phonolites, plus compositional variations linked to the reabsorption surface observed in the K-feldspar from the phonolitic dikes, suggest recharge events with local mixing in the late-stages of the LPC evolution.The emplacement of the LPC melts at a shallow crustal level was favored by the aperture of extensional NNW-SSE fractures genetically linked to a local brittle shear zone, active at 18–19 Ma during a strong compressional event, that initiated the regime of flat-slab subduction in this part of the Andes.

九州南部北薩地方西目地域の仏像構造線相当の地体構造境界

The Nishime area of Hokusatsu District, southwest Kyushu, Japan, is underlain by a chert-clastic complex comprising the Nitao and Momigi Units of Chichibu Belt; a mélange of sandstone, chert, and limestone clasts in an argillaceous matrix, corresponding to the Omae Unit of Chichibu Belt; and coherent sandstone-mudstone formations that belong to the Saiki Subgroup of the Shimanto Belt. These pre-Paleogene bedrock formations are mostly covered by Neogene and younger formations. On the basis of detailed geologic mapping of this area, we propose two precise locations for the boundary between the Omae Unit and the Saiki Subgroup. This boundary is traditionally called the Butsuzo Tectonic Line (BTL). The first proposed location is on the Ushinohama coast, where a brittle shear zone separates the Saiki Subgroup from the mélange of the Omae Unit. Here, deformation structures in the shear zone indicate northeastward thrust of the hanging wall of the Omae Unit. The second location is at Chugenda. Although direct contact is not exposed at the Chugenda location, a mélange similar to the hanging wall of the Ushinohama coast shear zone is juxtaposed against coherent beds of sandstone and mudstone. We suggest that a geotectonic boundary corresponding to the BTL runs through the Ushinohama coast and Chugenda.

Localisation of ductile and brittle shear zones along the Szentlőrinc-1 well in the Mecsekalja Zone using quartz microstructural and well-log data

Quartz is among the most common minerals in the Earth’s crust and is stable within a wide range of temperature and pressure conditions. As its microstructure is sensitive to different deformation mechanisms, quartz may present information about the structural evolution of many different rock types. The Szentlőrinc-1 well with its drill cuttings brought to the surface from approximately 2 km depth provides an exclusive chance to investigate the shear zone beneath. The drill cutting collection includes only a few small rock grains, and more than 80 % of the material consists of tiny (<1 mm) single quartz grains. In this study, three microstructurally extreme quartz grain types were separated during microscopic analysis: grains with undulose extinction (U), grains with subgrains (S), and grains with recrystallized grains (R). Moreover, numerous microstructurally transitional grains were measured, which represent combinations of the above extremes. The characterisation of single quartz grain microstructures along the whole well enables identification and localisation of the ductile shear zones inside the crystalline complex. This information was combined with well-log data, which could provide information about the brittle deformation. Using these logs, brittle shear zones can be localised along the well. When comparing depths and extensions of the deformed horizons, a coincidence of the brittle and ductile zones becomes clear. This behaviour may suggest two different evolution schemes: it could be caused by primarily evolved softened regions, or it could be described by a detachment fault model.

Structure of the Variscan metamorphic complexes in the central transect of the Posada-Asinara Line (SW Gallura region, Northern Sardinia, Italy)

This study represents the first detailed tectono-metamorphic map of the metamorphic complexes cropping out in the inner portion of the Variscan belt in north-central Sardinia Island (Italy). The Main Map is at a 1:30.000 scale and covers an area of c. 148 km2. It is based on 1: 10.000 scale classic field mapping and represents an overview of the lithological and structural complexities documented in the metasedimentary and migmatite domes cropping out along the central transect of the Posada-Asinara Line (PAL). The PAL is a crustal scale discontinuity that divides migmatites from the metasedimentary sequences affected by greenschist- to amphibolite-facies metamorphism. The map shows the orientations of the superimposed foliations, fold axes and mineral lineations on the basis of geometric crosscutting relationships and, for the first time, the location of ductile-brittle and brittle shear zones developed during the long-lived activity of the Posada-Asinara Line.

Applying a general triclinic transpression model to highly partitioned brittle-ductile shear zones: A case study from the Torcal de Antequera massif, external Betics, southern Spain

Oblique convergence and subsequent transpression kinematics can be considered as the general situation in most convergent and strike-slip tectonic boundaries. To better understand such settings, progressively more complex kinematic models have been proposed, which need to be tested against natural shear zones using standardized procedures that minimise subjectivism. In this work, a protocol to test a general triclinic transpression model is applied to the Torcal de Antequera massif (TAM), an essentially brittle shear zone. Our results, given as kinematic parameters of the transpressive flow (transpression obliquity, ϕ; extrusion obliquity, υ; and kinematic vorticity number, Wk), suggest that the bulk triclinic transpressive flow imposed on the TAM was partitioned into two different flow fields, following a general partitioning type. As such, one flow field produced narrow structural domains located at the limits of the TAM, where mainly dextral strike-slip simple-shear-dominated transpression took place (Outer domains, ODs). In contrast, the remaining part of the bulk flow produced pure-shear-dominated dextral triclinic transpression at the inner part of the TAM (Inner domain, ID). A graphical method relating internal (ϕ, Wk) to far-field (dip of the shear zone boundary, δ; angle of oblique convergence, α) transpression parameters is proposed to obtain the theoretical horizontal velocity vector (V→), which in the case of the TAM, ranges between 099 and 118. These results support the applicability of kinematic models of triclinic transpression to brittle-ductile shear zones and the potential utility of the proposed protocol.

The Fluid Dynamics of Solid Mechanical Shear Zones

Shear zones in outcrops and core drillings on active faults commonly reveal two scales of localization, with centimeter to tens of meters thick deformation zones embedding much narrower zones of mm-scale to cm-scale. The narrow zones are often attributed to some form of fast instability such as earthquakes or slow slip events. Surprisingly, the double localisation phenomenon seem to be independent of the mode of failure, as it is observed in brittle cataclastic fault zones as well as ductile mylonitic shear zones. In both, a very thin layer of chemically altered, ultra fine grained ultracataclasite or ultramylonite is noted. We present an extension to the classical solid mechanical theory where both length scales emerge as part of the same evolutionary process of shearing the host rock. We highlight the important role of any type of solid-fluid phase transitions that govern the second degree localisation process in the core of the shear zone. In both brittle and ductile shear zones, chemistry stops the localisation process caused by a multiphysics feedback loop leading to an unstable slip. The microstructural evolutionary processes govern the time-scale of the transition between slow background shear and fast, intermittent instabilities in the fault zone core. The fast cataclastic fragmentation processes are limiting the rates of forming the ultracataclasites in the brittle domain, while the slow dynamic recrystallisation prolongs the transition to ultramylonites into a slow slip instability in the ductile realm.

Near N–S paleo-extension in the western Deccan region, India: Does it link strike-slip tectonics with India–Seychelles rifting?

This is the first detailed report and analyses of deformation from the W part of the Deccan large igneous province (DLIP), Maharashtra, India. This deformation, related to the India–Seychelles rifting during Late Cretaceous–Early Paleocene, was studied, and the paleostress tensors were deduced. Near N–S trending shear zones, lineaments, and faults were already reported without significant detail. An E–W extension was envisaged by the previous workers to explain the India–Seychelles rift at ~64 Ma. The direction of extension, however, does not match with their N–S brittle shear zones and also those faults (sub-vertical, ~NE–SW/~NW–SE, and few ~N–S) we report and emphasize in this work. Slickenside-bearing fault planes, brittle shear zones, and extension fractures in meso-scale enabled us to estimate the paleostress tensors (directions and relative magnitudes). The field study was complemented by remote sensing lineament analyses to map dykes and shear zones. Dykes emplaced along pre-existing ~N–S to ~NE–SW/~NW–SE shears/fractures. This information was used to derive regional paleostress trends. A ~NW–SE/NE–SW minimum compressive stress in the oldest Kalsubai Subgroup and a ~N–S direction for the younger Lonavala, Wai, and Salsette Subgroups were deciphered. Thus, a ~NW/NE to ~N–S extension is put forward that refutes the popular view of E–W India–Seychelles extension. Paleostress analyses indicate that this is an oblique rifted margin. Field criteria suggest only ~NE–SW and ~NW–SE, with some ~N–S strike-slip faults/brittle shear zones. We refer this deformation zone as the "Western Deccan Strike-slip Zone" (WDSZ). The observed deformation was matched with offshore tectonics deciphered mainly from faults interpreted on seismic profiles and from magnetic seafloor spreading anomalies. These geophysical findings too indicate oblique rifting in this part of the W Indian passive margin. We argue that the Seychelles microcontinent separated from India only after much of the DLIP erupted. Further studies of magma-rich passive margins with respect to timing and architecture of deformation and emplacement of volcanics are required.

Analysis of relationship between belts of concentrated remote sensing linear structures and gold deposits as well as prospecting prognosis in Xiaoqinling region

Xiaoginling region has densely- distributed magama hydrothermal gold deposits. The structural framework in this region is the Neoarchean gneiss dome,where intense magmatic activities occurred in Yanshanian period. The gold deposits are closely related to the distribution of quartz veins and altered mylonite. In the ETM+remote sensing image,the deposits often find expression in linear,parallel linear intensive belts and flexible- brittle shear zones. Consequently,the distribution of remote sensing linear structures and the intensive belts of linear structures are very significant for the gold ore prediction in this region. On such a basis and with the ETM+remote sensing image as the basic map,the authors made interpretation of the linear structure intensive belts and circular structure of Yanshanian granite bodies in Xiaoginling region. The gold deposit distribution and metallogenic regularity were analyzed based on remote sensing interpretation,the maps showing the favorableness of the metallogenic remote sensing characteristic conditions for gold deposits in this region were compiled,and the remote sensing prospecting prediction of gold resources in this region was made. These results will provide some reference for the prospecting of gold deposits in Xiaoqinling region and the assessment of mineral resource potential in Henan Province.

Late Quaternary tectonics in the inner Northern Apennines (Siena Basin, southern Tuscany, Italy) and their seismotectonic implication

Defining the most recent Quaternary tectonics represents a challenging task for neotectonic, palaeoseismological and seismotectonic studies. This paper focuses on an integrated approach to reconstructing the latest Quaternary deformation affecting the northern part of the Siena Basin (inner Northern Apennines, i.e., southern Tuscany, Italy) near the town of Siena, and to discuss the seismological implications. Field work and structural and stratigraphic analyses, coupled with the interpretation of reflection seismic lines, have been combined to define the geometry, kinematics and age of mesoscopic to map-scale faults which have affected the mainly Quaternary continental and Pliocene marine deposits. The resulting dataset describes a tectonic setting characterized by coeval SW- and NW-trending transtensional and normal faults, respectively, dissecting alluvial sediments younger than 23.9±0.23ka. Seismic interpretation sheds light on the geometrical setting of the faults at deeper levels, down to 1–2km, and provides support for the presence of a wide brittle shear zone defined by conjugated fault segments, locally giving rise to an asymmetrical negative flower-like structure. Faults and their damage zones have controlled (and still control) the discharge of gas vents (mainly CO2 and H2S) and hydrothermal circulation (which deposits travertine) since at least 23.216±0.124ka. The resulting complete data set provides support for our description of the Neogene–Quaternary tectonics which were active until the late Quaternary, providing additional information about the seismotectonic framework of an area characterized by low seismicity and generally low-magnitude earthquakes (M<4), but having experienced significant seismic events over the last few centuries.

Contemporary tectonic stressing rates of major strike-slip faults in the Tibetan Plateau from GPS observations using Least-Squares Collocation

Abstract We invert the stressing rates of major strike-slip faults in the Tibetan Plateau using GPS measurements by the method of Least-Squares Collocation. From the comparison results of the influence of different distance intervals and covariance functions on the fit to GPS velocity field and the statistics of variance and covariance, we choose 40 km as the distance interval and exponential function as the covariance function to calculate the stressing rates. The results show that the root-mean-square error of GPS observations is only 0.061 cm. The stressing rates of the faults in the eastern Tibetan Plateau are generally higher than those in the west. The faults, whose maximum stressing rates are larger than 2.0 kPa/yr, account for about 41.4% of selected faults. Specifically, the stressing rate of the northwest segment of the XSHF is the highest with a value of 7.05 kPa/yr. Our stressing rates of the KLF and XSHF are consistent with published values or deduced results from published stress drops and earthquake recurrence intervals. The comparison results of stressing rates with fault slip rates show that the ratios of the stressing rates to the slip rates range broadly from 0.4 to 7.5 bar/m, which may be due to different kinds of data and methods used to calculate the slip rates besides varying petrophysical properties in the Tibetan Plateau. However this variation of the same fault is less distinct than that in different faults. The GYFF, HYF, JLF, LCRF and RRF with low slip rates and high stressing rates may be located in brittle shear zones. Inversely, the ATF, KLF, KRKRF and KRKXF may be located in ductile zones. Additionally, the statistical results between the stressing rates and the seismicity rates show that the seismicity roughly grows with the tectonic stressing rates. And we deduce that the KRKXF and MCRCF with high seismicity rates and low stressing rates are the strike-slip faults mostly prone to rupture in the Tibetan Plateau.

Emplacement of the La Peña alkaline igneous complex, Mendoza, Argentina (33° S): Implications for the early Miocene tectonic regime in the retroarc of the Andes

The La Peña alkaline complex (LPC) of Miocene age (18–19 Ma) lies on the eastern front of the Precordillera (32°41ʹ34ʺS, 68°59ʹ48″W, 1400–2900 m a.s.l.), 30 km northwest of Mendoza city, Argentina. It is a subcircular massif of 19 km2 and 5 km in diameter, intruded in the metasedimentary sequence of the Villavicencio Formation of Silurian-Devonian age. It is the result of integration of multiple pulses derived from one or more deep magma chambers, which form a suite of silicate rocks grouped into: a clinopyroxenite body, a central syenite facies with a large breccia zone at the contact with the clinopyroxenite, bodies of malignite, trachyte and syenite porphyry necks, and a system of radial and annular dikes of different compositions. Its subcircular geometry and dike system distribution are frequent features of intraplate plutons or plutons emplaced in post-orogenic settings. These morphostructural features characterize numerous alkaline complexes worldwide and denote the importance of magmatic pressures that cause doming with radial and annular fracturing, in a brittle country rock. However, in the LPC, the attitude of the internal fabric of plutonic and subvolcanic units and the preferential layout of dikes match the NW–SE extensional fractures widely distributed in the host rock. This feature indicates a strong tectonic control linked to the structure that facilitate space for emplacement, corresponding to the brittle shear zone parallel to the N–S stratigraphy of the country rock. Shearing produced a system of discontinuities, with a K fractal fracture pattern, given by the combination of Riedel (R), anti-Riedel (R′), (P) and extensional (T) fracture systems, responsible for the control of melt migration by the opening of various fracture branches, but particularly through the NW–SE (T) fractures. Five different pulses would have ascent, (1) an initial one from which cumulate clinopyroxenite was formed, (2) a phase of mafic composition represented by dikes cross-cutting the clinopyroxenite, (3) a malignite facies that causes a small breccia in the clinopyroxenite, (4) a central syenite facies that develops breccias at the contact with the clinopyroxenite and, finally, (5) porphyry necks and a system of radial dikes intruding all units. At the moment of the emplacement different mechanisms would have acted, they summarized in: 1) opening of discontinuities synchronous to the magma circulation as the principal mechanism for formation of dikes and conduits; 2) stoping processes, that play an important role in the development of the breccia zone and enabling an efficient transference of material during the emplacement of the syenitic magma and 3) shear-related deformation (regional stress), affected the internal fabric of the facies, causing intracrystalline deformation and submagmatic flow, which is very evident in the central syenite intrusive. The kinematic analysis of shear planes allows proposing that emplacement of the LPC took place in a transtensive regime, which would have occurred in the back-arc of the Andes orogen, during a long period spanning from Miocene to the present, of the compressive deformation responsible, westward and at the same latitude, for the development of the Aconcagua fold and thrust belt.

Microfabric diversity and grain shape analysis of fault rocks from the selected areas of the Western Tatra Mountains

Fault rocks in brittle and brittle-ductile shear zones played a key role in the evolution of the Western Tatra Mountains crystalline rocks (Poland–Slovakia). Microfabrics of these rocks, including grain shape analyses, were investigated in the six areas of the Western Tatra Mountains. Based on studies of thin sections, 14 types of fault rock microfabric are distinguished, according to the following criteria: (a) the presence and abundance levels of a cataclastic matrix and (b) the presence and form of a preferred orientation features. General tendencies observed in these areas indicate southwards increasing non-coaxial deformation as well as the domination of ultracataclasites or ultramylonites to phyllonites in areas with negative relief (e.g., sedlo Zabrat’ Pass, Dziurawa Przełęcz Pass). A model of shear zone evolution embracing following three stages is proposed: (1) deformation partitioning and block-controlled cataclastic flow, (2) matrix-controlled cataclastic flow, (3) selective leaching and deposition of silica, leading to the formation of softened and hardened deformation domains respectively. These microstructural observations were supported by statistical analyses of the grain shape indicators (compactness, isometry, ellipticity, solidity, convexity). Two trends of relationships between compactness and convexity were noted: the first, horizontal on the correlation diagrams, was interpreted as an effect of rapid cataclasis and then sericitization, the second, with a strongly negative correlation coefficient, was considered as an effect of long-term cataclastic flow. The different microfabric data and microstructural interpretations described in this paper are consistent with a new model of the tectonic history of the Western Tatra Mountains evolution, with an important role for a non-coaxial deformation during Alpine orogeny in brittle and brittle-ductile conditions.

Origin of hydrous fluids at seismogenic depth: Constraints from natural and experimental fault rocks

Fluids control the mechanical behavior of fault zones during the seismic cycle. We used geochemical, mineralogical, microstructural, hydrogen isotope compositions and Fourier Transform Infrared (FTIR) investigations to characterize the origin of hydrous fluids involved in ductile and brittle shear zones at the bottom of the seismogenic crust. Natural samples were collected from exhumed mylonitic shear zones and cataclasite–pseudotachylyte bearing faults in the northern Adamello (Italian Southern Alps), which were active at 9–11 km depth. Pseudotachylytes, solidified coseismic friction-induced melts, testify to ancient seismogenic behavior of the faults. Natural pseudotachylytes were compared with artificial pseudotachylytes produced in high velocity friction experiments simulating seismic slip.Mylonites have mineralogical, elemental and hydrogen isotope compositions (−80‰<δD<−78‰) similar to the host tonalite (−77‰<δD<−73‰), within the analytical error of ±5‰. Cataclasites have instead mineralogical (chlorite, epidote, K-feldspar, no biotite), major and trace elements (enrichment in K2O, Ba, Rb; depletion in CaO, Na2O, SiO2) and hydrogen isotope (−69‰<δD<−60‰) compositions suggesting interactions with a crustal metamorphic fluid. Pseudotachylytes are composed of high temperature minerals (plagioclase, biotite, dmisteinbergite, cordierite, and scapolite) and have elemental compositions resulting from mixing of tonalite and cataclasite. Pseudotachylytes have complex microstructures, including: (i) microlitic domains, with well crystallized micrometric biotite, which have hydrogen isotope composition (−81‰<δD<−59‰) similar to cataclasites and tonalite; and (ii) cryptocrystalline domains, with poorly crystallized biotite, which have very high water content, release water upon heating at T>50°C and have low δD value (−93‰). The hydrogen isotope composition of bulk samples is dominated by the composition of cryptocrystalline domains (−103‰<δD<−88‰), where most of the water is hosted. Their hydrogen isotope composition is compatible with adsorption of present day rainfall water (δD=−95‰). Artificial pseudotachylytes have the same hydrogen isotope compositions of the starting tonalite (−76‰<δD<−74‰) or cataclasite (−68‰<δD<−62‰), with a slight decrease of the δD values in some samples (−85‰<δD<−81‰).The first ingression of a crustal metamorphic fluid occurred in cataclastic faults. Natural pseudotachylytes, when not contaminated by present day rainfall water, have a hydrogen isotope composition similar to tonalite and cataclasite, as reproduced in dry high velocity friction experiments. The fluids dissolved in coseismic melts are most likely derived from the breakdown of hydrous minerals of cataclasite and tonalite undergone melting, and we could not identify the infiltration of an external fluid during earthquakes.

Geometry and kinematics of the Montelanico-Carpineto Backthrust (Lepini Mts., Latium) in the hangingwall of the early Messinian thrust front of the central Apennines: implications for the Apennine chain building

The paper updates knowledge on the Montelanico-Carpineto Backthrust, which is confined in the Lepini Mts. thrust sheet, at the hangingwall of the Neogene Latina Valley Thrust Front.The local stratigraphic setting consists of a pre-orogenic succession of Mesozoic carbonates from the Latium-Abruzzi platform Auct. The carbonates are paraconformably overlain by Miocene ramp limestones, which evolve upwards into planktonic marls. The Sub-Ligurian unit thrusted over this dominantly carbonate succession.In the Latina Valley, siliciclastic turbidites (Frosinone Formation Auct.), which evolved from the above-mentioned Miocene units, constitute the upper Tortonian foredeep deposits, now preserved in the footwall of the Lepini Mts. thrust sheet. In the Segni-Montelanico area, conglomerates, unconformably overlying the carbonatebedrock and coeval with the Frosinone Formation, are interpreted as late Tortonian thrust-top deposits (Gavignano and Gorga unit). The Lepini Mts. ridge has two series of distinctive tectonic features: the first is related to the Neogene shortening events due to the Apennine chain building; the other results from Plio-Quaternary extensional events associated with the collapse of the Tyrrhenian margin.The Montelanico-Carpineto Backthrust, as evidenced by the structural survey, is a compressive fault plane dipping 45°-50° towards NE with a SW-verging dip-slip kinematics. The fault has an offset of about 700 m, ramp geometries and a cut-off of about 20°. The Montelanico-Carpineto Backthrust and the Latina Valley Thrust Front define a pop-up structure. The presumable age of the Montelanico-Carpineto Backthrust is the early Messinian, because the lateTortonian Gavignano and Gorga unit is involved in its brittle shear zone. This implies that the formation of the Montelanico-Carpineto Backthrust and of the related pop-up structure is coeval with the early Messinian thrusting of the Lepini Mts. thrust sheet upon the geodynamic propagation of the central Apennine chain-foredeep-foreland system.