Strike-slip Shear Research Articles

Indentation tectonics of the Fanshi Block in the Trans-North China Orogen

The Trans-North China Orogen (TNCO) is a tectonic zone that records significant magmatic, metamorphic, and tectonic events during the Paleoproterozoic. The central part of the TNCO comprises several Precambrian blocks (the Hengshan Block, the Wutai Block, the Fuping Block, and the Fanshi Block) that are separated by three Paleoproterozoic ductile shear zones as tectonic boundaries. The Hengshan Block and the Fuping Block are represented by a group of large-scale gneissic domes, whereas the Wutai Block characterized by the ENE–WSW-trending synclinorium, and the framework of the Fanshi Block is dominated by strike-slip ductile shear zones superimposed on tight folds. We present detailed structural analysis of the Zhujiafang ductile shear zone in the central Hengshan, the Wangjiahui ductile shear zone at the northern margin of the Wutaishan, and the metamorphic complex between these two shear zones. The macro- and microstructures indicate that the ductile shear zones show different kinematics. The Zhujiafang ductile shear zone is E–W-trending and record sinistral strike-slip shearing, and the Wangjiahui ductile shear zone is WSW–ENE-trending and record dextral strike-slip shearing. Both of shear zones have similar deformation ages according to the 40Ar/39Ar dating results, and they cut through all the gneissosity and the fold developed in these metamorphic rocks. The kinematic difference between the Zhujiafang and the Wangjiahui ductile shear zones clearly define the Fanshi Block sandwiched between them, which is characterized by movement from west to east. The Fanshi block differs from the Hengshan Block and the Wutai Block in terms of deformation history, metamorphism, and magmatism. In addition, a series of NE-trending ductile–brittle thrust nappes inside the Wutai Block under the influence of the indention of the Fanshi Block inside. Therefore, we propose that the Fanshi Block was indented to the TNCO from west to east, accompanied by strong strike-slip ductile shearing and thrusting during the late Paleoproterozoic.

Quartzite crests in Paleoproterozoic granites (Anti-Atlas, Morocco); a hint to Pan-African deformation of the West African Craton margin

South of the Pan-African suture zone of the Anti-Atlas, the Agadir Melloul inlier exhibits several crests of nearly vertical conglomeratic quartzite beds inserted in Paleoproterozoic granites. These enigmatic rocky walls have been considered as Early Neoproterozoic platform sediments pinched within the 2.03 Ga-old granites during brittle faulting events at ca. 570 Ma. The data presented here contradict this interpretation. The quartzite crests correspond to parts of tight synclines pinched along brittle-ductile, strike-slip shear zones involving the granitic basement itself. This deformation occurred in low-grade greenschist facies conditions at T = 260–280 °C. The detrital zircon grains from the vertical quartzite beds are mostly grouped around 2 Ga, with the youngest grains at 1.8 Ga. Thus, the studied quartzites exhibit the same detrital zircon barcode as the other siliciclastic platform formations of the southwestern Anti-Atlas (Taghdout Group) whose age has been recently constrained between 1.8 and 1.750 Ga. 40Ar-39Ar age measurements were unsuccessful to determine the age of crystallization of small syn-tectonic white mica grains within the quartzite samples, but the large white mica grains of granite samples outside the quartzite-bearing shear zones yielded ages of ∼1.777 and 1.830 Ga. We interpret the enigmatic quartzite structures as the result of two major events that the Paleoproterozoic foreland of the Pan-African mobile belt likely suffered, i.e., an extensional event around 800-750 Ma, coeval with the opening of the Pan-African Ocean, and a compressional event during its final closure at about 650–600 Ma. We propose that the quartzite formations detached then from the granitic basement of the Pan-African foreland in the frame of a complex, thick- and thin-skinned tectonics.

The evolution of quartz veins during the tectonometamorphic development of the Brusque Metamorphic Complex, Brazil

Microstructural analysis and characterization of quartz veins hosted in rocks from the Brusque Metamorphic Complex in the region of Brusque, State of Santa Catarina, have been performed to establish the relations between fluid regimes, tectonic styles and deformation/recrystallization mechanisms. The analysis was based on observations of structural overprinting, spatial structural relations, the origin of the quartz veins and microtectonics studies. Five types of veins were identified: meter-long veins parallel to the regional foliation (V1-veins); massive, meterwide veins present in thermal aureoles (V2-veins); millimeter-wide, erratic veins, also present in thermal aureoles (V3-veins); tabular, undeformed and NW striking (V4-veins); millimeter-wide erratic veins restricted to brittle reactivation of strike-slip shear zones (V5-veins). The regional foliation, developed under garnet zone metamorphic conditions, was more effective in vein production when compared to the steeply dipping mylonitic foliation corridors developed under chlorite zone conditions. Pressure solution was the main deformation mechanism during the regional foliation development. However, granoblastic and decussate textures in hornfels reveal an influence of grain boundary area reduction mechanism. The granoblastic texture in hornfels would have inhibited fluid circulation during dehydration reactions, increasing fluid pressure and promoting massive (V2-veins) and erratic hydraulic fracture-related (V3) veins. In dextral NEtrending strike-slip shear zones, tabular quartz veins (V4-veins) are parallel to tension gashes. Reactivation of strike-slip shear zones under brittle conditions has produced local brecciated millimeter-wide quartz veins (V5-veins). This study underscores the important role of fluids during orogenic evolution near the brittle-plastic transition of the crust, and demonstrates how combined vein and microtextural analysis can reveal the tectonometamorphic history of lowgrade metamorphic rocks.

A Neoarchean K-rich granitoid belt in the northern North China Craton

The Archean metamorphic basement in the southeastern part of Eastern Hebei to Western Liaoning provinces, northern North China Craton, is dominated by Neoarchean K-rich granitoids, with a small amount of Neoarchean supracrustal rocks and Na-rich dioritic-tonalitic-trondhjemitic-granodioritic gneisses, and minor Paleo- to Mesoarchean rocks. The Paleo- to Mesoarchean lithological signatures include TTG gneisses and supracrustal rocks at the Caozhuang and Caochang areas. The Neoarchean K-rich granitoids form a ca. 350 km long belt extending in a NE direction, and primarily consist of porphyritic monzodioritic-quartz monzodioritic-granodioritic-monzogranitic (MQGM) gneisses and secondary monzogranite-syenogranites and charnockites, with minor Neoarchean TTG gneiss enclaves. Based on their major mafic mineral phases, the MQGM gneisses can be divided into an amphibole-dominated group and a biotite-dominated group. Zircon U-Pb isotopic dating reveals that the Caozhuang trondhjemitic gneisses, amphibole-dominated MQGM gneisses, and biotite-dominated MQGM gneisses were emplaced at ∼2.90 Ga, 2.55–2.53 Ga, and 2.55–2.53 Ga, respectively. The amphibole-dominated porphyritic MQGM gneisses are geochemically analogous to sanukitoids, and the sanukitoid magma was derived from the partial melting of lithospheric mantle metasomatized by dehydration fluids and melts from subducted slabs and related sediments, following the fractionation of amphibole and clinopyroxene during their magmatic evolution. The magmatic precursors of the biotite-dominated MQGM gneisses were most likely generated by the partial melting of medium- to high-K mafic-intermediate rocks and locally generated by the partial melting of metagreywackes at medium pressures. Within the K-rich granitoid belt, the 2.55–2.53 Ga porphyritic MQGM gneisses experienced 2.53–2.51 Ga E-W extension and strike-slip shearing deformation in the Jinzhou-Xingcheng structural domain, and diapiric upwelling by the formation of dome structures in the Jielingkou-Anziling-Suizhong structural domain. All these structures then suffered a top-to-the-SE-NEE thrusting in the northern margin during 2.51–2.50 Ga. The 2.53–2.51 Ga dioritic-tonalitic-trondhjemitic-granodioritic gneisses and charnockites in the Taipingzhai-Santunying-Zunhua structural domain firstly underwent diapiric upwelling during 2.51–2.50 Ga, then together with the 2.52–2.50 Ga volcanic-sedimentary rocks in the Shuangshanzi-Qian’an-Lulong structural domain overprinted by nearly N-S-striking dextral strike-slip shearing deformation during 2.50–1.80 Ga. The temporal-spatial relationships, petrogenesis, and structural patterns of these various Neoarchean lithological assemblages confirm that the Neoarchean K-rich granitoid belt in the northern North China Craton was most likely developed in an active continental margin setting, and our studies suggest this involved progressively southeastward multi-stage retreating oceanic slab subduction and collisions between an oceanic arc and an ancient continental nucleus.

Episodicity of stress state in an overriding plate: Evidence from the Yalu River Fault Zone, East China

The ca. 700-km-long Yalu River Fault Zone (YRFZ) in East China, adjacent to the Pacific Ocean, underwent a polyphase evolution during the Cretaceous when it controlled the development of rift basins interrupted by several shortening events. The East China continent lies in an overriding plate with respect to the subducting Paleo-Pacific Plate during the Cretaceous. The YRFZ is ideal for studying the episodicity of stress state in the overriding plate. To constrain the polyphase evolution of the YRFZ, structural observations, fault-slip data measurements and LA–ICP–MS zircon U–Pb dating on Cretaceous volcanic rocks and sandstones were undertaken in this study. The first deformation (D1) is characterized by sinistral strike-slip shear in the earliest Cretaceous. The D2 event is featured by normal faulting deformation along the fault zone, which led to development of rift basins during the rest of the Early Cretaceous. Sinistral faulting (D3) developed again in the earliest Late Cretaceous, followed by dextral normal faulting (D4) and rift basin development during the rest of the Late Cretaceous, and finally reverse dextral faulting (D5) at the end of the Cretaceous. The fault-slip data show that compressional directions during D1, D3 and D5 faulting events are N–S, N–S and E–W respectively. Extensional directions during D2 and D4 faulting events are NW–SE and N–S. The zircon U–Pb ages indicate that the Early Cretaceous basins (D2 event) controlled by the YRFZ were active between 131 and 100 Ma, and the Late Cretaceous basins (D4 event) were active between 97 and 70 Ma. These U–Pb ages, together with previous geochronological data, show that the D1 and D3 episodes of compression each lasted 3 Ma, D2 extension lasted 31 Ma, and D4 extension 27 Ma. These data indicate an episodicity in the stress state with longer periods of extension and shorter periods of compression. A slab-driven model with relatively long periods of low-velocity subduction alternating with shorter periods of high-velocity subduction could account for the episodicity of stress state in the overriding plate from D1 to D5.

Structural analysis of the central and southwestern parts of the Neoproterozoic Salalah Crystalline Basement, Sultanate of Oman

The Salalah Crystalline Basement (SCB) is the largest Precambrian exposure in Oman located on the southern margin of the Arabian Plate at the Arabian Sea shore. It is thought to represent one of the eastern-most exposures of the Arabian-Nubian Shield. We used detailed structural analysis to establish the deformational history of the SCB by focusing on its central and southwestern parts. This part of the SCB is dominated by the Juffa complex (820 ± 10 Ma) which is made-up of two-mica gneisses, amphibolites and ultramafic lenses, and the Sadh complex (816 ± 12 Ma) which is composed of biotite and amphibolite gneisses. Our work found that the SCB evolved through four deformational events that shaped its final architecture: (1) Folding and thrusting event that resulted in the emplacement of the Sadh complex atop the Juffa complex. This event resulted in the formation of possibly N-verging nappe structure. Associated with this event was the development of mesoscopic asymmetrical tight folds and thrusts with various orientations because of folding during subsequent event. However, the dip of these structures fluctuates between SE and SW suggestive of an initial N-verging nappe structure; (2) Regional folding event around SE- and SW-plunging axes that deformed the regional fabric developed during the N-verging nappe structure. This event produced map-scale SE- and SW-plunging antiforms resulting in shaping the Juffa and Sadh complexes into a semi-dome structure; (3) Strike-slip shearing event that produced a conjugate set of NE-trending sinistral and NW-trending dextral strike-slip shear zones. The NW-trending dextral strike-slip shear zones dissected the northeastern part of the Juffa-Sadh semi-dome whereas the NE-trending sinistral strike-slip shear zones dissected the southwestern part of the semi-dome. Strong mylonitic fabric, stretching lineation and associated kinematic indicators were developed during this event; and (4) Localized SE-directed extension manifested by top-to-the-southeast kinematic indicators and the development of stretching lineation in the form of stretched amphibole crystals overprinted on older planar fabric. This is particular apparent in the hinge of the map-scale SE-plunging antiform.

The Sangmelima granite-greenstone belts (South Cameroon): Integration of remote sensing and aeromagnetic data for structural interpretation

The integration of geology, geophysics and remote sensing data was used to map the Sangmelima granite-greenstone belt terrain located in the South Cameroon. The region was not easily accessible, because of dense vegetation and thick lateritic overburden. Aeromagnetic data were help in mapping new possible small greenstone belts and structural features in this area. Many of the identified lineaments are interpreted to represent fracture/fault zones, some of strike-slip faults. The NE–SW, NW–SE, N–S and E–W are the main revealed from remote sensing processing; while the NE–SW, E–W to ESE–WNW, NW–SE and N–S are the dominant structural trends revealed from aeromagnetic data. We distinguish three deformation events within the Sangmelima area: D1 is recorded by a sub-vertical penetrative S1 foliation bearing steeply plunging stretching lineations. D2 overprints D1 structures with tight-to-isoclinal folds with associated foliation, N160°−175°E and N05°−25°E ductile shear zones, indicating ENE–WSW shortening. The D3 strike slip shearing is likely the result of the Eburnean deformation phase. The mafic dykes are striking NE–SW, N–S and NW–SE, and field evidence and airborne magnetic data suggest that some of them intrude preexisting structures. Many faults/fractures might have originated during previous deformation phases and they were only reactivated during the pan-African events.

Correction to: Early uranium mobilization in late Variscan strike-slip shear zones affecting leucogranites of central western Spain

Correction to: Early uranium mobilization in late Variscan strike-slip shear zones affecting leucogranites of central western Spain

Early uranium mobilization in late Variscan strike-slip shear zones affecting leucogranites of central western Spain

Mineralization related to the post-collisional reorganization in Iberia occurs along strike-slip shear zones (Sn, Au, ± U), in extremely differentiated leucogranites and pegmatites (Sn, Nb, Ta, Be, Li), in skarns (W, ± Au, ± U), and in quartz-veins with greisen (W, Sn, ± Au, ± U). Episyenites/albitites with uraninite mineralization in strike-slip shear zones yield U–Pb ages of 304 ± 6 Ma (2σ) that are identical within error with the 40Ar/39Ar age of micas grown during fabric development (306 ± 3 Ma, 2σ), suggesting shearing and formation of episyenite/albitite and uraninite mineralization are directly related. Mineral chemistry of uraninite from episyenite and deformed muscovite-rich leucogranite host are indistinguishable and has immobile element signatures typical for uraninite that formed from a hydrothermal or metamorphic fluid. The mineral chemistry of uraninite hosted in the deformed muscovite-rich leucogranite, together with the large time span between granite emplacement (347.6 ± 1.9 Ma, U–Pb zircon) and uraninite (chemical age 300 ± 3 Ma) excludes a magmatic origin of uraninite. Mass balance indicates that leaching of rock-forming minerals (especially muscovite and monazite) of the muscovite-rich leucogranite during extensive metasomatism (e.g. albitite or episyenite formation) could provide most uraninite-hosted U. The precipitation of the uraninite-pyrite association with carbonaceous matter in episyenite/albitite was related to local reducing conditions and/or fluid unmixing processes, analogue to hydrothermal uraninite from retrograde scheelite skarn and W ± Au quartz veins in the Iberian Massif. Chemical ages of uraninite from episyenite and muscovite-rich deformed granite demonstrate that episyenitization is not a late-stage Variscan process, but a post-orogenic one. Metal mobilization is structurally controlled. Chemical dating of uraninite may provide a first-order age constraint in unravelling the post-Variscan tectonic history.

Source tectonic dynamics features of Jiuzhaigou Ms 7.0 earthquake in Sichuan Province, China

On August 8, 2017, a Ms 7.0 earthquake occurred 5 km to the west of Jiuzhaigou National Park, causing 25 deaths and injuring 525. The objective of this study was to explore the seismogenic fault of the earthquake and tectonic dynamics of the source rupture. Field investigations, radon activity tests, remote sensing interpretations, and geophysical data analyses were carried out immediately after the earthquake. The Jiuzhaigou earthquake occurred at the intersection of the northern margin of the Minshan uplift belt and the south part of the Wenxian–Maqin fault in the south margin of the West Qinling geosyncline. There are two surface rupture zones trending northwest (NW), which are ground coseismic ruptures caused by concealed earthquake faults. The rupture on the southwest is the structure triggering the earthquake, along the Jiuzhaitiantang–Epicenter–Wuhuahai. The other one on the northeast (Shangsizhai–Zhongcha–Bimang) is a reactivation and extension of the secondary fault trending NW. The source rupture of this earthquake is a strike-slip shear fracture associated with the fault plane trending NW 331° and steeply dipping 75°, which is continuously expanding at both ends. The tectonic dynamics process of the source rupture is that the “Jiuzhaigou protrusion” is left-lateral sheared along the seismogenic fault in the NW direction. Finally, the Maqin fault and the arc fault system at the top of the “Wenxian protrusion” will be gradually broken through sometime in far future, as well as earthquaketriggered landslides will be further occurred along the narrow corridor between the seismogenic faults. The research results revealed the basic geological data and tectonic dynamic mechanism in this earthquake.

Evidence for deep crustal seismic rupture in a granulite-facies, intraplate, strike-slip shear zone, northern Saskatchewan, Canada

Evidence for deep crustal seismic rupture in a granulite-facies, intraplate, strike-slip shear zone, northern Saskatchewan, Canada

The Palaeoproterozoic Grythyttan Field in the Svecofennian Orogen, West Bergslagen, Central Sweden: structure, stratigraphy and age

A number of discrete slate belts of limited size occur in the Paleoproterozoic volcano-sedimentary Bergslagen Group of western Bergslagen in the Fennoscandian shield of south central Sweden. The Grythyttan Slate Belt, studied for more than a century, forms a single basin with the nearby Saxan Slate Belt. We use the lithostratigraphy of the Grythyttan Slate Formation of the Grythyttan belt, based on basin scale sedimentary facies associations of the volcaniclastic sediments in time and space, as a method to interpret overall tectonic structure. Contrary to traditional views, we reconstruct the GSB as a single overturned limb of a km-scale anticline with horizontal axis in the hanging wall of an east vergent thrust fault, reactivating a listric extensional fault. The fold connects the GSB to the neighboring SSB. Folding and thrusting were related to tectonic closure of a volcanic back- or intra-arc basin. The early folds were subsequently affected by strike-slip shearing and folding around vertical fold axes, which partitioned preferentially into the least competent lithologies (slates and marbles), significantly modifying the map appearance of the slate belt. The late shearing and folding resulted from accretion of Bergslagen onto the Fennoscandian continental margin during the late, Svecobaltic phase of the Svecofennian orogeny. The GSB forms a thin, intraformational wedge in the Bergslagen Group and represents a relatively short interlude with a conglomeratic alluvial fan and turbiditic volcaniclastics followed by more felsic volcanic rocks resembling those of the Bergslagen Group. We suggest that the GSB, and by inference the other slate belts, stem from calderas, as either terrestrial volcanic lakes or shallow submarine eruption centers. The conglomerates are pre-orogenic, not a post-orogenic molasse, as traditionally conceived, and their clast fabric has resulted from sedimentary processes only. Newly determined SIMS U-Pb zircon ages constrain the age of the GSB at ca. 1895 Ma.

Reappraisal of the kinematic history of Nugrus shear zone using PALSAR and microstructural data: implications for the tectonic evolution of the Eastern Desert tectonic terrane, northern Nubian Shield

Nugrus shear zone (NSZ) is a NW-trending semiductile-semibrittle high-strain zone exhibiting obvious sinistral sense of shear. It is regarded by some workers to represent the boundary between the Central Eastern Desert (CED) and South Eastern Desert (SED) of Egypt. A variety of Neoproterozoic lithologies are remarkably affected by the NSZ, encompassing high-grade gneisses, metavolcanics and metavolcaniclastics, gneissose granite, and ophiolitic melange. Visual interpretation and automated lineament extraction have been applied on the local sigma-filtered level 1.5 PALSAR data. Results of directional filtering and line algorithm techniques support the dominance of NW and NE structural trends in the study area. Microstructural and finite strain analyses have been done for the oriented samples of gneisses, metavolcanics, metavolcaniclastics, and gneissose granite using CSS, Fabric 8, and EllipseFit software. The Rf/ϕ and Fry methods demonstrate that the strain ratios of the collected samples are closely compatible and consistent. The core of Hafafit culmination gave 1.5 strain ratio with top-to-the-NW sense of shear, whereas the two rims show 1.7 strain ratio with top-to-the-NE sense of shear. Samples collected along the flanks of Wadi Nugrus show the two types of shear but the NW is dominant with 1.8 strain ratio and sinistral sense of shear, whereas those collected from Wadi Abu Rashid and Wadi Sikiat display 1.4 strain ratio with dextral sense of movement. Finite strain results along with field relations and observations prove that the NSZ and environs experienced a prolonged tectonic history involving at least three deformations (Dl, D2, and D3). D1 phase involves NE-SW shortening and is documented by large-scale NW-trending, NE-dipping, imbricate thrust sheets with NE plunging stretching lineation. D2 was a transpressional phase (660–645 Ma), coeval with the Najd Shear System (NSS), and expressed by the NW- to NNW-trending, steeply dipping, strike-slip shear zones with frequent subvertical foliation and subhorizontal lineation. Exhumation of gneissic domes in Hafafit area was related to this phase. The D3 phase (645–540 Ma) was responsible for the formation of NE-oriented shear with dextral sense of movement. Whether the NW and NE shears are conjugate pairs or not will be a matter of much discussion.

Structural and hydrothermal evolution of a strike-slip shear zone during a ductile-brittle transition, Sierra Nevada, CA

The incursion of meteoric-hydrothermal fluids into the ductile crust can be evaluated with the stable isotope ratios of minerals in exhumed ductile shear zones, but the incursion mechanisms are yet to be well-demonstrated. Field structural and microstructural relations and regional oxygen isotope patterns from an exhumed, Late Cretaceous shear zone-fault system in the central Sierra Nevada, CA, are used to reconstruct the deformation history and develop insights into fluid incursion mechanisms. A ductile-to-brittle transition was partially accommodated by a strike-slip duplex involving flexural slip-accommodated folding along faults. Two main NNW-striking brittle fault strands overlap, the northern fault curves into the southern fault, which is sealed by a >4 m wide composite quartz vein. Based on structural relationships, we interpret the transition from ductile to brittle deformation to have occurred during cooling of the adjacent Late Cretaceous Tuolumne Intrusive Complex. Quartz-sealed fault veins in the pendant have δ18O values ranging from −3.2 to 14.5‰, and are interpreted to have precipitated partially or fully from meteoric-hydrothermal fluids, including quartz veins that have been recrystallized. The lowest values are only found within the region of the jog. The δ18O values for pseudotachylyte matrix in one sample from this fault system are δ18O = 3.3‰, δ2H = −137‰. Together, these data indicate meteoric-hydrothermal incursion into a deforming, seismogenic brittle-ductile transition.

Migmatization, Granite Generation and Melt Accumulation in the Himalayan Orogenic Channel, Central and Eastern Bhutan

In Central and Eastern Bhutan Himalaya, the Great Himalayan Sequence (GHS) reveals mesoscopic structures within the migmatite–leucogranite association due to crustal anataxis above the Main Central Thrust (MCT). The first phase of dominant melting generates stromatitic migmatite along the main foliation during high grade of metamorphism, possibly by dehydration melting. Subsequent ductile strike–slip shearing caused in situ melting in dilatational sites to produce structureless, non-foliated patchy leucogranite leucosome as well as in boudin necks and post-tectonic patches. In addition, melt-enhanced deformation caused doming of accumulated melt and subsidiary ductile shear zones on either margins of these domes. Surrounded by biotite-rich melanosome, leucosomes destroy the pre-existing foliation during new anatectic phase, which post-dates earlier stromatitic migmatite. These migmatites are the snapshot of mutual relations between newly-developed migmatite and leucogranite melt, and signify the transportation of Himalayan Orogenic Channel to the extreme south in Central and Eastern Bhutan over the Lesser Himalayan sedimentary belt along the MCT.

Multiple Paleozoic magmatic-orogenic events in the Central Extremadura batholith (Iberian Variscan belt, Spain)

The Central Extremadura batholith located in the southeast part of the Central Iberian Zone (e.g. Iberian Autochthonous domain of the Iberian Variscan belt) was originally thought to have been generated entirely during Carboniferous igneous activity. However, some recent geochronological work has shown the existence of Ordovician plutonic rocks. The aim of this study is to re-examine the age of granitic rocks in the Central Extremadura batholith and complement this information with new field and geochemical data. This data set is used: to constrain the relative timing of plutons emplacement, as well as deformation and metamorphism preserved in the host rocks; to track deep crustal rocks and granitic magma sources; and to discuss prevailing tectonic evolutionary models for the Paleozoic evolution of the Iberian Variscan belt. We use geochemical and SHRIMP U-Pb zircon geochronology data of three granitic plutons (Ruanes, Plasenzuela and Albala) from the Central Extremadura batholith to track magmatic sources and provide a better understanding of temporal and spatial relationships between deformation and magmatism in the Iberian Variscan belt. Ruanes tonalite dated at 464 ± 2 Ma is peraluminous, magnesian and calc-alkaline, as typical of a magmatic arc setting. We report, for the first time, the occurrence of a Middle Ordovician intrusion spatially and temporally related to host deformed rocks of the Central Iberian Zone (e.g. the Iberian Autochthonous domain), which reached high-grade metamorphic conditions. Plasenzuela two-mica leucogranite is strongly peraluminous and of anatectic origin and includes a Neoproterozoic and Ordovician population of inherited zircon grains. This granite possibly derived from the partial melting of a crustal source composed of Neoproterozoic metapelites and metagreywackes (Schist-Greywacke Complex) and/or Lower Ordovician gneisses (Ollo de Sapo Formation), both including greywackes of volcano-sedimentary origin and peraluminous composition. The crystallization age of 330 ± 7 Ma obtained for the syn-kinematic Plasenzuela two-mica leucogranite constrains the functioning of D2 dextral strike-slip shear zones within the Iberian Autochthonous domain. The age of 309 ± 2 Ma obtained for the Albala cordierite-bearing monzogranite matches the age interval of the calc-alkaline magmatic suite post-dating the main Variscan D1–D3 structures in the Iberian Autochthonous domain. The new data presented in this study make it possible to recognize multiple Paleozoic magmatic-orogenic events (e.g. Caledonian, Variscan and Cimmerian) in the Central Extremadura batholith. During the Ordovician, the emplacement of intermediate magmas at shallow depths gave rise to extensive metamorphism due to heat transfer to the host rocks. The onset of this Ordovician plutonic–metamorphic complex in the Iberian Autochthonous domain is contemporaneous with the development of an active continental margin probably related to the subduction of the Iapetus–Tornquist Ocean (i.e. the Caledonian orogeny). During the Lower Carboniferous, these D2 strike-slip domains acted as lateral margins of large-scale gravitational collapses associated with the SE-direct transport of low-angle extensional shear zones (i.e. the Variscan cycle). The emplacement of Upper Carboniferous arc type granitic rocks is interpreted in the context of the amalgamation of Pangaea and the spatial proximity of Iberia relative to the Eurasian active margin in the course of Paleotethys subduction (i.e. the Cimmerian orogeny).

Structure of Masuleh Shear Zone: Evidence for Early–Middle Jurassic Dextral Shear Along Paleo-Tethys Suture Zone in the Western Alborz, NW Iran

The Paleo-Tethys suture zone in northern Iran was formed when the Paleo-Tethys Ocean, (between Gonwana-derived Alborz Microcontinent and the Turan Plate), closed during the Eocimmerian orogeny and after they collided together in the Mid-Late Triassic. The NW-striking Boghrov-Dagh basement Fault Zone that lies in the vicinity of Masuleh village and the southern boundary of Gasht Metamorphic Complex is a part of the Eocimmerian suture zone in the Western Alborz. Along this part of the suture zone, tourmaline leucogranites intruded in metamorphic rocks. We recognize three distinct deformation stages (D1 to D3) in the study area especially in the Masuleh Shear Zone. D1 which was synchronous with formation of the main metamorphic minerals, such as sillimanite and staurolite under medium- to high-grade metamorphic conditions probably during the Hercynian event and a NE-directed shortening. The slaty cleavage in metamorphosed Upper Paleozoic rocks and crenulation cleavage and folds in the older rocks were produced due to D2 deformation during the Eocimmerian event under greenschist facies conditions. The Masuleh Shear Zone formed as a result of a ductile strike-slip shear during the Early-Middle Jurassic Mid-Cimmerian D3 event with a pure dextral to transtension shear sense at low to locally medium-grade conditions. All of the D3 structural features agree with a NNW-directed compression and an ENE-directed extension caused by overall dextral shear parallel to the Masuleh shear zone and the Boghrov-Dagh Fault Zone. Based on the available evidence, especially cross-cutting relationships between structural fabrics and rock units, emplacement of the Gasht-Masuleh leucogranites occurred after the D2 collisional event coeval to the possible slab break-off and before the D3 event, between Eocimmerian and Mid-Cimmerian movements.

The tectonic history of a crustal-scale shear zone in the Tanzania Craton from the Geita Greenstone Belt, NW-Tanzania Craton

In this contribution, we present for the first-time field based evidence of a crustal scale shear zone from the southern margin of Geita Greenstone Belt. The Geita Shear Zone is a broad (∼800 m wide) ductile, high-strain, deformation zone that can be traced for at least 50 km along the southern margin of the Geita Greenstone Belt. It is near vertical, trends ∼E-W and separates the mafic volcanics of the Kiziba Formation, to the north, from the TTG gneisses that crop out south of the shear zone. The shear zone is hosted almost entirely by the TTG gneisses and is characterised by a well-developed mylonitic foliation near the greenstone margin that transitions into a gneissic foliation and eventually becomes a weakly developed foliation further south. It contains approximately equal amounts of dextral, sinistral, and asymmetric shear sense indicators suggesting that the shear zone accommodated mainly flattening strain while the mineral stretching lineation defined by quartz and feldspar ribbons and stretched biotite selvages plunges shallowly W. A series of younger, sub-vertical, NW trending brittle-ductile, strike-slip shear zones truncate and displace the Geita Shear Zone with dextral displacement in the order of 2–4 km. Deformed tonalite interpreted to predate the shear zone yielded U-Pb zircon ages of ∼2710 Ma while synshearing granodiorite samples have zircon ages between 2680 Ma and 2660 Ma. The ∼2630 Ma age of the undeformed Nyankumbu granite is interpreted to mark the minimum age of movement on the shear zone. The presence of 3000 Ma and 3200 Ma zircon xenocrysts in the tonalite and granodiorite opens the possibility that older basement rocks underlie the greenstone belts in the northern half of Tanzania Craton. Whether or not the greenstone belts were erupted on older basement, thrusted on top of older basement rocks or incorporated older basement fragments has profound implications for the tectonic framework and evolution of the Tanzania Craton.

Kinematic analysis of secondary faults within a distributed shear-zone reveals fault linkage and increased seismic hazard

Complex multifault earthquake ruptures involving secondary faults emphasize the necessity to characterize their seismogenic potential better and study their relationship with major faults to improve the seismic hazard assessment of a region. High-resolution geophysical data were interpreted to make a detailed characterization of the Averroes Fault and the North Averroes Faults, which are poorly known secondary right-lateral strike-slip faults located in the central part of the Alboran Sea (western Mediterranean). These faults appear to have evolved since the Pliocene as part of a distributed dextral strike-slip shear zone in response to local strain engendered by the diverging movement of the Carboneras Fault to the north, and the Yusuf and Alboran Ridge faults to the south. In addition, the architecture of these faults suggests that the Averroes Fault may eventually link with the Yusuf fault, thus leading to a higher seismogenic potential. Therefore, these secondary faults represent a hitherto unrecognized seismogenic hazard since they could produce earthquakes up to moment magnitude (Mw) 7.6. Our results highlight the importance of the role played by secondary faults in a specific kinematic framework. Their reciprocal linkage and their mechanical relationship with the main faults could lead to future complex fault ruptures. This information could improve fault source and earthquake models used in seismic and tsunami hazard assessment in this and similar regions.

Domino structures evolution in strike-slip shear zones; the importance of the cataclastic flow

The Porto-Tomar-Ferreira do Alentejo dextral Shear Zone is one of the most important structures of the Iberian Variscides. In its vicinity, close to Abrantes (Central Portugal), a localized heterogeneous strain pattern developed in a decimetric metamorphic siliceous multilayer. This complex pattern was induced by the D2 dextral shearing of the early S0//S1 foliation in brittle-ductile conditions, giving rise to three main shear zone families. One of these families, with antithetic kinematics, delimits blocks with rigid clockwise rotation surrounded by coeval cataclasites, generating a local domino structure.The proposed geometrical and kinematic analysis, coupled with statistical studies, highlights the relation between subsidiary shear zones and the main shear zone. Despite the heterogeneous strain pattern, a quantitative approach of finite strain was applied based on the restoration of the initial fracture pattern. This approach shows the importance of the cataclastic flow coupled with the translational displacement of the domino domain in solving space problems related to the rigid block rotation. Such processes are key in allowing the rigid block rotation inside shear zones whenever the simple shear component is a fundamental mechanism.