Sinistral Kinematics Research Articles

Structural and Geochronological Constraints on the Collision Between the Chinese Altai and the West Junggar in Central Asia: Implication for Deformation Response and Geodynamic Evolution of Arc‐Arc Collision in 4D

AbstractArc‐arc collision represents one of crucial geological processes to shape mountain belts on the Earth. The deeply‐eroded Central Asian Orogenic Belt (CAOB) is characterized by the collision of multiple arc systems and provides a natural laboratory to understand arc‐arc collision in 4D given the exposure of crustal rocks at different levels. Here we focus on the ∼NW‐SE Irtysh fault zone, which represents a major suture zone within the CAOB to bound arc systems of the Chinese Altai and the West/East Junggar in NW China. Our structural work, for the first time, delineates the spatial position of the western segment of the Irtysh Fault between the Chinese Altai and the West Junggar, and demonstrates a phase of brittle strike‐slip deformation with sinistral kinematics in the Permian. An earlier episode of ∼NE‐SW shortening event affected both southern Chinese Altai and northern West Junggar, forming ∼NW‐SE fold structures. Both ∼NE‐SW shortening and sinistral strike‐slip faulting overlap in time with the collision of the Chinese Altai and the West Junggar during the latest Carboniferous to Permian as constrained by new detrital zircon data, thus indicating a transition from orthogonal collision to oblique convergence between the Chinese Altai and the West Junggar. A comparison with ductile deformation at deep structural levels along the eastern segment of the Irtysh Fault between the Chinese Altai and the East Junggar allows for investigation of arc‐arc collision in 4D, which highlights deformation decoupling across different crustal levels, with an intermediate episode of orogen‐parallel flow recorded within deep crustal rocks.

On the anatomy and structural control of a dyke swarm that fed caldera-forming ignimbrite eruptions

The evolution of eruptive vents related to calderas is not fully understood. We focus on a structural, rock-magnetic and geochemical investigation of a 314 Ma rhyolite dyke swarm associated with the late-orogenic Altenberg–Teplice Caldera, Bohemian Massif, eastern Variscan belt. The whole-rock major element, trace element and Nd–Pb isotope geochemistry along with the published U–Pb zircon geochronology link the extra-caldera dyke swarm with intra-caldera ignimbrites. The magnetic fabrics determined using the anisotropy of magnetic susceptibility are interpreted to record a continuum from magma ascent to emplacement and eruption during sinistral shearing. The latter evidences an interplay with regional tectonics associated with the activity of crustal-scale shear zones. The sinistral kinematics and strike of the dyke swarm, the elongation of caldera intrusive units, and the kinematics of major caldera faults are consistent with the dextral Riedel shear system, where the dykes correspond to antithetic R’ or X shears. Such a kinematic configuration implies that the maximum and minimum principal stresses were oriented roughly north–south and east–west, respectively. The relation between the stress field and the caldera elongation and orientation is not typical. We suggest that a pre-existing mutually perpendicular set of cross-cutting structural lineaments largely controlled the magma chamber and caldera formation. Supplementary material : The whole-rock major, trace element and isotope geochemical tables, magnetic fabrics source data and details of methods are available at https://doi.org/10.6084/m9.figshare.c.6715893

2019 Mw 6.0 Mesetas (Colombia) Earthquake Sequence: Insights From Integrating Seismic and Morphostructural Observations

AbstractThe boundary between the Northern Andean block and the South American plate is a first‐order tectonic structure in Colombia with historically M > 7 earthquakes. However, details about individual sections of the system remain unknown. We illuminate the seismotectonic of the Algeciras fault by investigating an earthquake sequence that started on 24 December 2019. Using seismic networks at region, we estimate foreshocks and aftershocks focal mechanisms, local stress field, kinematic slip models of the largest events, and Coulomb stress changes. We integrate seismological and morphostructural observations to characterize source properties and reinterpret local faults. Two mainshocks (doublet of Mw 6.0 and 5.8) occurred within 16 min, rupturing just a few kilometers from each other. Discrimination of causative faults among the centroid moment‐tensor nodal planes is difficult because the focal zone is a complex tectonic environment. The focal mechanisms and the local stress field obtained are consistent with a regional NE trending dextral transpressive shear. The relocated aftershocks show a cluster with an L‐shaped pattern concentrated in a ∼7 × 7 km area. Our model defines the Algeciras fault with two structural styles for its northern termination. The NW part is characterized by a duplex‐style of right‐lateral strike‐slip with inner secondary faults of the same sense of movement, and the SE zone by a domino‐style system with inner minor faults of sinistral kinematics. The earthquake doublet is in the zone characterized by the duplex style. In contrast, the southern part of the aftershocks is located in the zone characterized by the domino style.

Continental magmatic arc rotation during the assembly of western Gondwana

The Ceará Central Domain (CCD) is part of the Borborema Province and is delimited by two major NE-SW-trending strike-slip zones associated with the Transbrasiliano and Senador Pompeu lineaments. These two strike-slip zones formed in Neoproterozoic/Early Cambrian times under ductile conditions and underwent several brittle-ductile and brittle reactivation events from 540 Ma onwards, which affected Precambrian and Phanerozoic rocks, respectively, causing ductile and brittle-ductile and brittle deformation. The Transbrasiliano Lineament is an NNE-SSW-trending, 4000 km-long structure marked by strong magnetic anomalies. It is interpreted to represent a mega-suture active during the Gondwana amalgamation and/or the post-collisional shearing. The Santa Quitéria Continental Magmatic Arc, located between both lineaments, consists of a Neoproterozoic (830-620 Ma) gneissic-migmatitic association affected by collisional metamorphism (640-615 Ma). This arc has been dissected, dislocated, and rotated under brittle-ductile and brittle conditions by a set of secondary strike-slip faults, possibly interconnected with the two major Transbrasiliano and Senador Pompeu lineaments, and most likely have been contemporarily active throughout history. The roughly NNW-SSE-trending Tauá Fault, located southwest of CCD, appears to relate to both fault systems. A closer inspection of gamma-ray spectrometric data suggests a complex network of interconnecting E-W-trending brittle transcurrent faults, some of which show dextral and/or sinistral kinematics, displacing the Santa Quitéria Continental Magmatic Arc and Canindé-Independência units. We suggest a paleo-dextral ductile deformation of the Santa Quitéria Continental Magmatic Arc resulting from the coeval transcurrent activity along Transbrasiliano and Senador Pompeu lineaments. These strike-slip mega-shear zones developed an intricate pattern of the minor (first-, second-, and third stages) interconnecting strike-slip faults, along which the Santa Quitéria Continental Magmatic Arc has been displaced and rotated, under conditions that changed from ductile to brittle-ductile to brittle through time.

Dynamics of Early Neoproterozoic accretion, west-central India: II ~1.65 Ga HT-LP and ~0.95 Ga LT-HP metamorphism in Godhra-Chhota Udepur, and a tectonic model for Early Neoproterozoic accretion

The N/NNE-striking Aravalli Delhi Fold Belt (ADFB) and the E-striking Central Indian Tectonic Zone (CITZ) converge at the Godhra-Chhota Udepur (GC) sector, west-central India. Analyses of mesoscale deformation structures and metamorphic phase equilibria in the basement and supracrustal rocks are integrated with geochronological-geochemical data (accompanying article) to address the dynamics of the Early Neoproterozoic CITZ-ADFB accretion. In the GC sector, ~1.65 Ga granulite facies anatectic gneisses, ~2.5 Ga and 1.03–1.02 Ga granitoids, and greenschist to amphibolite facies allochthonous supracrustal rocks constitute a tectonic mélange (D2 deformation). The lithodemic units are traversed by networks of W/WNW-striking steep-dipping transpressional shear zones with sinistral kinematics (D3). The shallow-dipping mélange with top-to-the south kinematics is intruded by post-D2 to syn-D3 0.95–0.93 Ga granitoids. Mn-NCKFMASH P-T pseudosection analyses of the anatectic gneisses with pre-D2 garnet + cordierite-bearing leucosomes suggest the rocks evolved along a clockwise P-T path in the range of 5–6 kbar and 680–720 °C. By contrast, the Early Neoproterozoic (0.95–0.93 Ga) chlorite + phengite (Si up to 3.32 apfu) + clinozoisite + quartz ± biotite ± garnet schists in the mélange attained pre/syn-D2 peak metamorphic conditions (10–12 kbar, 450–500 °C). NCKFMASH pseudosection analyses of the ~0.95 Ga phengite-bearing schists indicate the supracrustal rocks evolved along a high-P, low-T clockwise path; phengite-poor micas defining the D3 fabric (Si up to 3.04 apfu) attest to post-peak decompression in the schists. The ~2.5 Ga ADFB granites accreted with the ~1.65 Ga HT-LP anatectic gneisses, ~1.03 Ga granites, and the ~0.95 Ga LT-HP supracrustal rocks of the CITZ during D2 thrusting. The crustal convergence continued with the emplacement of post-D2 0.95–0.93 Ga granitoids that culminated with the nucleation of W/WNW-striking D3 transpressional shear zones. This broad contemporaneity among felsic plutonism , LT-HP metamorphism in the supracrustal rocks, and the D2-D3 shortening are interpreted to be the result of a switch in subduction polarity between 1.03 and 0.93 Ga during oblique ADFB-CITZ convergence. • ~1.65 Ga HT-LP clockwise granulite metamorphism in CITZ basement anatectic gneiss. • ~0.95 Ga LT-HP clockwise metamorphism in the CITZ allochthonous supracrustal rocks. • E-striking CITZ & N-striking Aravalli Delhi Fold Belt (ADFB) accreted at ~0.95 Ga. • ~0.95 Ga metamorphism coeval with crustal shortening and granitoid emplacement. • Early Neoproterozoic oblique accretion related to subduction polarity reversal.

Morfostrukturní doklady ponásunového porušení Pavlovských vrchů, Vnější Západní Karpaty

The article presents geomorphological and structural evidences of post-thrust limestone brittle deformation in the area of Pavlov Hills on the westernmost margin of the Outer Western Carpathians, Czech Republic. Airborne LiDAR-derived digital terrain model with 1 m resolution was used for this analysis, as well as aerial photographs and fieldwork. Combining these methods, we document a distinct N-S directed fault zone that intersects or delineates the majority of the Pavlov Hills Jurassic limestone nappe outliers. Data revealed an almost continuous fault zone of the north-south direction, which ranges from Horní Věstonice in the North, to Mikulov in the South, and extends further south to Austria. The thrusted Jurassic limestone bodies are cut by the fault zone, which tectonically crushed the limestone in its core and cores of the secondary fault branches. The map pattern of the fault zone suggests directional anastomotic branching and reattaching with the production of lenticular tectonic slices. We interpret the fault as a prominent sinistral shear zone. This is indicated by block displacement on the Svatý kopeček Hill and also by the orientation of the accompanying subvertical Riedel shears with identified horizontal lineation. The activity of the fault zone onset tightly after the nappe thrusting at the final stages of the accretion wedge formation, which is suggested from the observed sinistral kinematics. The main movement along the fault is thus most probably of a late Miocene age.

Exhumation of the Ronda Peridotite During Hyper‐Extension: New Structural and Thermal Constraints From the Nieves Unit (Western Betic Cordillera, Spain)

AbstractThe Ronda peridotite (Betic Cordillera, Southern Spain) is the largest alpine‐type peridotite massif worldwide. Yet, the emplacement mechanism of these mantle rocks is still a highly debated topic. In this study, we aim at better constraining their context of exhumation by focusing on the Nieves Unit, which is mostly composed of Mesozoic metasediments displaying HT metamorphism and deformation along the contact with peridotite. The topmost parts of the peridotites and the bottom parts of the Nieves Unit share the same planar/linear fabric parallel to the contact accompanied by component of sinistral kinematic indicators. Raman Spectroscopy on Carbonaceous Material geothermometry then shows a contraction of peak‐temperature conditions within metasediments of the Nieves Unit, as well as a north‐eastward decrease along strike, highlighting a strong thinning of the unit and a significant obliquity of the isograds to the contact with the peridotites. Local magnetite ores and ophicalcite bodies are finally observed nearby the contact, suggesting a hyper‐extension context for the mantle exhumation. Indeed, the Nieves Unit, the Ronda peridotite and the Jubrique Unit, where a strong thinning has also been documented, are all in contact at a triple junction between the western contact of the peridotite and the Nieves/Jubrique contact. Altogether, these features support the idea that the western shear zone limiting the peridotites from the Nieves and Jubrique units is a tilted large‐scale extensional detachment that has exhumed mantle rocks from beneath the continental basement (Jubrique Unit) and its Mesozoic cover (Nieves Unit) during an episode of hyper‐extension.

Geology and geochemistry of deformed granites from the Querari Complex, Rio Negro Province, NW-Amazonian Craton, Brazil

The Querari Complex corresponds to the basement rocks from Uaupes Domain and represents one of the least studied units in the northwest of the Rio Negro Province, Amazonian Craton. In this paper, we present new field geology, petrographic, and geochemical data of deformed granites from the Querari Complex. Based on our results, we suggest the differentiation of the Querari Complex into two magmatic facies. The Pana-Pana Facies comprise mainly porphyritic syeno to monzogranites with biotite as the main mafic mineral, and titanite, epidote, and allanite as accessory phases. The rocks of the Pana-Pana Facies have calc-alkaline and metaluminous affinities. The Matapi Facies have a syenogranite composition, coarse to porphyritic texture with magmatic muscovite and biotite, normative corundum higher than 1% (1.85-2.06), and mostly a peraluminous character. Both facies are generally enriched in silica and have a small spectrum of variation. (67.5 to 70.1 wt.% Pana-Pana Facies - 71.1 to 73 wt.% Matapi Facies), but they differ by the high Al2O3, Fe2O3, MgO, CaO, Sr, and Ba contents in the Matapi Facies. Geological and geochemical compositions suggest that the magmatic precursors of these rocks of Matapi Facies may be derived from partial melting of metasedimentary sources, whereas the Pana-Pana Facies are probably derived from a meta-ignea source of intermediate composition, such as tonalitic gneisses. The rocks of the Querari Complex would have been generated in the collisional tectonic settings during the Paleoproterozoic (between 1778 and 1740 Ma). During this 1.78-1.74 Ga tectono-magmatic event (D1), the magmatic fabric (S0) and the parallel S1 foliation have been developed, with NE-SE orientation. A subsequent tectonic event (D2) was responsible for the folding S1 foliation, creating the S2 axial planar foliation with an approximate orientation E-W. This D2 event is thought to represent a Calymmian (1.52 - 1.48 Ga) tectono-metamorphic event, associated with widespread I- and S-type granite emplacement in the Rio Negro Province. The deformation textures, as well as the mineral paragenesis for the Pana-Pana and Matapi facies, reveal temperature conditions at upper amphibolite facies, up to 700oC. Finally, the D3 tectono-metamorphic event comprises the reactivation of many NE-SW ductile shear zones with sinistral kinematic, creating a mylonitic foliation (S3) in the rocks of the Querari Complex. This D3 tectono-metamorphic event is probably linked to the intracratonic K´Mudku tectono-metamorphic event (between 1317 and 1198 Ma) of the Amazonian Greenvillian-Sunsas continental collision.

Fault inversion in central Iran: Evidence of post Pliocene intracontinental left lateral kinematics at the northern Iranian Plateau margin

Structural inversion of strike–slip faults in response to plate kinematic changes and interactions of large–scale basement blocks is a significant phenomenon in continental collision zones. In the Alborz Mountains, at the northern margin of the Iranian Plateau, Late Cenozoic kinematic of major fault systems changed in the late Pliocene from dextral to sinistral strike–slip. This kinematic change has been attributed to the clockwise rotation of the rigid South Caspian Block. However, the spatial extent and distribution of this kinematic change toward the south and into the Iranian Plateau is controversial. Here we present a detailed structural analysis of the West Saveh fault system located at the northwestern margin of the Iranian Plateau to unravel the regional distribution of this kinematic change. The West Saveh fault system is composed of the WNW–ESE trending Kushk–e Nosrat, Khalkhab, Saveh, and Nashveh faults. We identified several different sets of strike–slip related structures, which are classified into three categories based on their cross–cutting relationship and superimposition of kinematic indicators. We document dextral, dextral transpression and sinistral kinematics and their relative timing since Miocene time in the West Saveh fault system. Our structural analysis indicates recent slip sense inversion from dextral transpression to sinistral strike–slip, which possibly occurred in post Pliocene time. This kinematic change is regional and can be explained by the clockwise rotation of the South Caspian Block similarly to the kinematic change documented in the Alborz Mountains. We propose that the kinematic change due to South Caspian Block rotation is not restricted to the Alborz Mountains, but also affected the Iranian Plateau during post Pliocene time.

Quartz textural analysis from an anastomosing shear zone system: Implications for the tectonic evolution of the Ribeira belt, Brazil

Strain localization and influence of grain-size reduction processes were investigated from field and microstructural observations, and from quartz textural analysis using EBSD, in rocks deformed along an anastomosing network of shear zones in the Ribeira belt in Brazil. Rocks deformed along the Lancinha shear zone (LSZ) display high percentage of recrystallized grains, in which quartz recrystallized grain size from rocks in the southern LSZ reaches a maximum of 50 μm, and in the northern LSZ a maximum of 200 μm. The dominant serrated grain boundaries and the widespread (50–85%) presence of very small (>10–50 μm) recrystallized quartz grains, suggest that BLG was the dominant recrystallization mechanism responsible for grain-size reduction in the south portion of the LSZ. In the northern LSZ the presence of large recrystallized grains with sizes close to subgrains, suggests that grain-size reduction occurred by SGR, and the presence of lobate grain boundaries point to GBM recrystallization, which led to an increase in recrystallized grain size. This suggests grain-size reduction by dynamic recrystallization as a major factor leading to weakening and strain localization. Evidence of intracrystalline deformation and the well-developed quartz crystallographic fabrics suggest that strain localization in the LSZ occurred by dislocation creep deformation mechanisms. Dislocation creep is recorded by dominant activation of basal <a> slip and secondary contribution of rhomb <a> slip, in the southeast of the LSZ, and by activation of rhomb <a> + basal <a> and prism <a> in the northeast. These slip systems together with microstructural observations suggest that plastic deformation occurred at around 400 °C in the southern region, and at around at >400–500 °C in the northern region. S–C foliation, quartz <c> axis distribution and several kinematic indicators suggest sinistral sense of shear along the LSZ, and strain partitioning during progressive deformation. The dominance of sinistral indicators observed in the LSZ are also found in parallel shear zones to the east, such as the Putunã and Serra do Azeite. The sinistral kinematics in this overall dextral transpressional orogenic system may be explained by dual sense of shear on shear zones bounding extruding blocks, likely formed during progressive orogenic evolution instead of the polyphase evolution previously suggested for this southern part of the Ribeira belt.

Structural evolution of the eastern segment of the Irtysh Shear Zone: Implications for the collision between the East Junggar Terrane and the Chinese Altai Orogen (northwestern China)

The tectonic evolution of the Central Asian Orogenic Belt (CAOB) involved multiple episodes of arc accretions/collisions, but the reconstruction of these tectonic processes remains relatively poorly constrained. Evidence for a collision between the intra-oceanic island arc system of the East Junggar Terrane and the active margin of the Siberian Craton (Chinese Altai Orogen) is recorded in the eastern segment of the Irtysh Shear Zone (northwestern China). Field observations from the Qinghe area show that this segment of the Irtysh Shear Zone consists of four NW–SE sinistral mylonitic zones, and domains bounded by these mylonitic zones show variable fold patterns. In the northern part of the Irtysh Shear Zone (southern Chinese Altai Orogen), three generations of structures (DCA1–DCA3) are recognized. The earliest generation of foliation (SCA1) is only recognized locally and is transposed to the orientation of the dominant DCA2 foliation (SCA2). The latter is associated with a shallowly plunging stretching lineation, which is sub-parallel to the NW–SE hinge of DCA3 folds. These third-generation folds (DCA3) show a steeply-dipping axial plane trending NW–SE. In the southern part of the Irtysh Shear Zone (northern East Junggar Terrane), the structural pattern is simpler and only involves a single generation of penetrative foliation (SEJ1). New U–Pb detrital zircon and 40Ar/39Ar data provide constraints on the timing of collision and deformation. These results suggest that the collision occurred after the Early Carboniferous, with the timing of sinistral shearing constrained to the Early–Middle Permian. The combination of sinistral shearing and NW–SE DCA3 folds likely represents an episode of transpressional deformation, which was driven by oblique collision between the East Junggar Terrane and the Chinese Altai Orogen during the Early–Middle Permian. The occurrence of original sub-horizontal SCA2 foliation and associated orogen-parallel stretching lineation might indicate that following the initial collision (after the Early Carboniferous), the southern Chinese Altai Orogen was subjected to orogen-parallel extension. In a larger-scale context, sinistral kinematics along the Irtysh Shear Zone in northwestern China and northeastern Kazakhstan, together with coeval dextral strike-slip deformation farther south, might reflect an eastward escape of orogenic materials, possibly in response to the Permian convergence of the Siberian, Baltic, and Tarim cratons.

Textures of auriferous quartz-sulfide veins and 40Ar/39Ar geochronology of the Rushan gold deposit: Implications for processes of ore-fluid infiltration in the eastern Jiaodong gold province, China

The Rushan gold deposit, located in the eastern part of the world-class Jiaodong gold province, contains the largest gold resource within a single fault-controlled quartz vein in China. Processes involved in auriferous quartz vein deposition are not clear, and the precise gold mineralization age remains controversial for this deposit. New 40Ar/39Ar dating data on sericite from ores and altered rocks suggest that the Rushan gold deposit formed between ca. 122 Ma and 117 Ma, which overlap or are slightly younger than those of gold deposits in northwestern Jiaodong province. The age differences between samples from different locations indicate that the quartz veins on the margin of the orebody may be younger than those in the center, suggesting a single and protracted gold mineralization event lasting for about 5 My. Differences between gold mineralization ages and emplacement/cooling ages of the ore-hosting Kunyushan granite imply that they are not genetically related and that the Kunyushan granite is unable to provide any thermal input to the mineralization system. This also applies to the Sanfoshan monzonite as it marginally postdated gold mineralization and lacks any spatial association with it.Internal textures of high-angle shear veins and low-angle extension veins indicate that episodic fluctuations from supralithostatic to hydrostatic fluid pressure triggered the multiple ore-controlling fault ruptures induced by breach of supralithostatic fluids under a seismogenic regime. In this model, the controlling fault starts to slip with both reverse and sinistral kinematics, and dilatational zones are created. The ore-forming fluids then migrate upwards along the fault, due to the large pressure difference between the dilatational zones and the deep parts of the fault, causing hydraulic fracture in the host rocks and earlier quartz veins. Large volumes of quartz, pyrite, and other ore components are precipitated due to fluid immiscibility caused by pressure fluctuations, and new quartz veins form along both sides of the earlier quartz veins. The fault is now sealed by these new quartz veins and the fluid pressure starts to build until the next breach cycles. Orebodies with complex internal textures were thus deposited incrementally in these cycles, closely fitting the classic fault-valve model, in this case under a weak regional NW-SE transpressional stress.

Transpressional deformation during Ediacaran accretion of the Paranaguá Terrane, southernmost Ribeira Belt, Western Gondwana

The Paranaguá Terrane is mainly constituted by an Ediacaran arc-related granitic complex, spread in a NE-SW trending elongated stripe, located in the Southern Ribeira Belt, South-Southeastern Brazil. The basement of these granites occurs as a disrupted folded belt composed of metasedimentary and gneissic-migmatitic rocks. The integration of structural and geophysical (gamma spectrometry and magnetometry) data from the Paranaguá Terrane and the adjacent tectonic units, suggests that the deformation in the study area is partially controlled by the geometry of the Luis Alves Cratonic Block, as well as the northern part of Dom Feliciano Belt. The irregular shape of this cratonic unit associated with N-NW tectonic transport direction caused strain partitioning in the area and the development of two structural domains: southern and northern Paranaguá. The southern part is characterized by sinistral transpressional shear zones, while the northern sector is dominated by thrust shear zones that represent a large frontal collision with oblique components. The hierarchy of the structural and microstructural features in the Paranaguá Terrane associated with geochronologic data suggest two deformational phases (D1 and D2) during a transpressive collisional system. The D1 progressive deformational stage is correlated to the main period of granitic crystallization during the Ediacaran and was divided in two moments: i) early D1 (640–610) associated with the regional metamorphism and thrust and folding tectonics with N-NW tectonic transport and; ii) late D1 (610-580 Ma) linked to the strain partitioning and development of the southern (simple-shear dominated) and northern (pure-shear dominated) structural domains with sinistral kinematics, granite emplacement and high-temperature mylonites. The D2 deformation stage (540–500 Ma) is mainly represented by the low-temperature mylonites and deformation under retrograde conditions.

Major fault zones in the Austroalpine units of the Kreuzeck Mountains south of the Tauern Window (Eastern Alps, Austria)

The Kreuzeck Mountains are located between the Tauern Window in the north and the Southalpine unit in the south. They expose an Eoalpine (Cretaceous) nappe stack of Adria derived continental crust. The Koralpe–Wolz Nappe System in the footwall partly experienced Cretaceous eclogite-facies metamorphism, while the Drauzug-Gurktal Nappe System in the hanging wall remained at anchizonal to greenschist-facies conditions in Cretaceous time. From the Late Cretaceous onward the Austroalpine units in the Kreuzeck Mountains were affected by brittle/ductile shearing and brittle deformation. This contribution is based on mapping of the southeastern part of the Kreuzeck Mountains at the 1:10000 scale in combination with structural and petrological investigations and Rb–Sr biotite dating. The new data indicate that the boundary between the Koralpe–Wolz Nappe System and the overlying Drauzug-Gurktal Nappe System consists of three segments. It includes (1) the south dipping Wallner Normal Fault representing a more than 500 m wide shear zone responsible for the Cretaceous exhumation of the eclogite-bearing Polinik Complex to shallow crustal levels (2) the Oligocene to Miocene Ragga-Teuchl Fault and (3) the Groshalsgraben Fault. The latter is mechanically related to the Molltal Fault and shows a dextral offset. The Drauzug-Gurktal Nappe System is from bottom to top composed of the Strieden, the Gaugen and the Goldeck complexes, which are transgressively overlain Permo-Triassic (meta)sediments. It is dissected by the steeply north dipping Lessnigbach Shear Zone with a dominant top to the south reverse sense of shear and the ENE–WSW trending, sub-vertical Blassnig Shear Zone showing sinistral kinematics. Both faults were active in the Late Cretaceous, but might have been initiated in the Late Jurassic.

Reativações pós-silurianas do Lineamento Transbrasiliano na porção sul da Bacia do Parnaíba

O Lineamento Transbrasiliano (LTB) apresenta direção NE-SW. Cerca de 900 km desse sistema interceptam o substrato da Bacia do Parnaíba (BPar), inferido, a partir de dados geológicos e geofísicos, como uma zona de cisalhamento plástica transcorrente dextral. Este trabalho objetivou abordar a assinatura estrutural e os marcadores estratigráficos associados às reativações do LTB na região sul da bacia, ao leste de Palmas (TO). No embasamento cristalino, um estágio tardio do LTB se expressa como uma zona dúctil-frágil também de cinemática dextral de idade ediacarana-cambriana, no NW do Ceará. Nas unidades litoestratigráficas da BPar são distinguidos eventos de reativação em regime frágil ou hidroplástico. Na região de estudo, um evento mais antigo registra cinemática transcorrente sinistral expressa principalmente como bandas de deformação e falhas com direção NE, combinadas a estruturas dilatacionais (incluindo juntas e falhas normais) ou conjugadas/antitéticas de rejeito direcional ou oblíquo, com orientações que variam de NNE a NNW, todas observadas nos litotipos das Formações Sambaíba e Pedra de Fogo, bem como nas mais antigas. Ao sudeste de Alto Parnaíba (MA) são observadas evidências de atividade tectônica sindeposicional durante o Permiano superior. Um segundo conjunto de estruturas composto por falhas normais associadas a uma distensão N/NNW, impressas nos corpos básicos de idade eojurássica da Suíte Mosquito, que, por sua vez, é capeada por depósitos com seixos das vulcânicas, correlacionados à Formação Corda. Um terceiro conjunto de estruturas, caracterizado por falhas normais ou oblíquas com direção NE, registra distensão NW, sendo associada ao evento de rifteamento da Margem Leste brasileira, durante o Cretáceo Inferior. Finalmente, um quarto evento, de ocorrência restrita, envolve distensão NE e está registrado em arenitos correlacionados ao Grupo Urucuia, implicando uma idade máxima neocretácea.

Sinistral reactivation of the Transbrasiliano Lineament: Structural and geochronological evidences in the Cariré Granulite Zone, Borborema Province – NE Brazil

The Precambrian basement in the region of Cariré, northeast Ceará Central Domain, NE Brazil, comprises essentially dioritic gneisses and granodiorites formed under high-grade metamorphism conditions. The granulites occur frequently as metric bodies or as boudins in mylonitic/cataclastic orthogneiss matrix, within shear zones related to Transbrasiliano Lineament. Geological mapping in this region confirmed a mylonitic foliation trending ∼ N20E and sub vertical dip (∼70°) towards NW or SE. Hornblende, biotite and orthopyroxene crystals define the lineation of the main extensional direction, with sub horizontal S-SW plunge (∼5-10°). Many outcrops show a large variety of structural elements linked to ductile deformation and typical structural elements representing the transition from ductile to brittle domains. Felsic and mafic dykes of various dimensions and both ductilely and brittlely deformed are found frequently. Analysis of kinematic indicators in the lineament shows both dextral and sinistral shear sense, although ductile dextral shear is more frequent. Structures developed in shallower crustal conditions, including micro-shear, tension gashes and faults trending N12E-N20E, show sinistral kinematics. Zircon crystals obtained from felsic dykes, dislocated by faults parallel to the regional trend of the Transbrasiliano Lineament, display four groups of concordant ages, the most recent of which is ca. 460 Ma. Our data suggest that throughout its complex history, the Transbrasiliano Lineament has been reactivated under brittle conditions from the Upper Ordovician onwards.

Exhumation of a migmatite complex along a transpressive shear zone: inferences from the Variscan Juzbado–Penalva do Castelo Shear Zone (Central Iberian Zone)

High-grade metamorphic rocks associated with S-type granites are recorded in the Central Iberian Zone, Iberian Variscides. Though most of these occur as inferred metamorphic core complexes affiliated with detachment faults, others, such as the Figueira de Castelo Rodrigo–Lumbrales Anatectic Complex, crop out between low-grade metamorphic rocks separated by steeply-dipping strike-slip shear zones, such as the Juzbado–Penalva do Castelo Shear Zone. Our structural analysis has been able to constrain two major deformation stages during the Variscan D 3 : (a) D 3a ductile deformation event, with clear sinistral kinematic criteria; and (b) D 3b thrusting ductile–brittle deformation event. The petrological investigation confirmed the jump in metamorphic grade between the host rocks and the anatectic complex. P–T estimates on calc-silicate rocks interlayered with the metapelites of the anatectic complex provided minimum metamorphic peak conditions of T = 761 ± 50°C and P = 5.0 ± 1.0 kbar. However, petrological modelling results show that P–T conditions must have exceeded T &gt; 800°C. Both structural and geothermobarometric data support a two-step model for the exhumation of the Anatectic Complex, including a 5 – 8 km vertical exhumation along a 65 – 100 km horizontal displacement due to a simple shear-dominated transpression mechanism during the Variscan D 3 events. Supplementary material: The petrology data, mineral chemistry analyses of the calc-silicate units and EPMA analytical conditions, and the P–T modelling methodology can be found at https://doi.org/10.6084/m9.figshare.c.3785648

Late Variscan deformation in the Iberian Peninsula; a late feature in the Laurentia–Gondwana dextral collision

The Late Variscan deformation event in Iberia is characterized by an intraplate deformation regime induced by the dextral oblique collision between Laurentia and Gondwana. This episode in Iberia is characterized by NNE–SSW brittle to brittle–ductile strike-slip faults, which are considered by the classic works as sinistral strike-slip faults. However, the absence of Mesozoic formations constraining the age of this sinistral kinematics led some authors to consider it as the result of Alpine reworking. Structural studies in Almograve and Ponta Ruiva sectors (SW Portugal) show that NNE–SSW faults have a sinistral kinematics and are occasionally associated with E–W dextral shears. Moreover, this kinematics is related to the late deformation episodes of Variscan orogeny. In Almograve sector, the Late Variscan structures are characterized by NNE–SSW sinistral kink bands, spatially associated with E–W dextral faults. These structures are contemporaneous and affect the previously deformed Carboniferous units. The Ponta Ruiva Sector constrains the age of deformation because the E–W dextral shears affect the Late Carboniferous (late Moscovian) units, but not the overlying Triassic series. The new data show that the NNE–SSW and the E–W faults are dynamically associated and result from the Late Variscan deformation episode. The NNE–SSW sinistral faults could be considered as second-order domino structures related to first-order E–W dextral shears, linked to Laurentia–Gondwana collision during Late Carboniferous–Permian times.

Age, tectonic evolution and origin of the Aswa Shear Zone in Uganda: Activation of an oblique ramp during convergence in the East African Orogen

The Aswa Shear Zone (ASZ) is a major NW–SE trending structure of over 1000 km length in East Africa. In Uganda, the ASZ is a steeply NE-dipping, up to 11 km wide mylonitic shear zone that shows multiple stage brittle reactivation. On outcrop-scale, the fabric in the ASZ is characterized by a well-developed NW–SE striking and subvertical or steeply NE or SW dipping mylonitic foliation and a subhorizontal to moderately NW- or SE-plunging stretching lineation. Sinistral kinematics and fabric are very consistent along strike. The strain is heterogeneously distributed and partitioned into lens-shaped lower strain zones dominated by folding and characterized by pure shear, which are surrounded by high strain zones, some of them thick ultramylonites, with intense simple shear combined with flattening and strong transposition of pre-existing fabrics. Ductile shearing occurred during bulk E–W shortening, commenced at amphibolite facies conditions and continued with similar kinematics at greenschist and even lower grade conditions. A number of (sub-)parallel shear zones occur to the NE and SW of the main zone at a distance of up to 20–45 km. They show similar fabrics and kinematics and are thus related to activity along ASZ reflecting strain partitioning into simple shear and pure shear domains on a regional scale.Samples of mylonitic gneisses from the shear zone have been analyzed with U–Pb LA-MC-ICPMS and show Neoarchaean crystallisation ages between 2.66 and 2.61 Ga. Timing of ductile sinistral shearing is poorly constrained by lower intercept ages of 686 ± 62 and 640 ± 44 Ma. The fabric and structural relationship of the ca. 660 Ma Adjumani Granite exposed in the northern segment of ASZ suggest that the age of shear activity can be further limited to ca. 685 and 655 Ma.The Aswa Shear Zone is interpreted as an intra-cratonic, crustal-scale structure close to the northeastern margin of the Congo Craton, possibly inherited from previous continental extension. Early Aswa Shear Zone activation is linked to underthrusting of the Congo Craton and coeval high-grade metamorphism and intense deformation in the orogen interior. During E–W convergence between ca. 690 and 650 Ma, the NE-dipping ASZ was activated as an oblique ramp leading to deflection of the transport direction and concentration of non-coaxial strain and sinistral shear along the shear zone system. During progressive convergence, between ca. 645 and 620 Ma, sinistral shearing along ASZ changed to ductile–brittle deformation mechanisms, while thrusting took place in Pan-African belts in eastern and western Uganda. Late-orogenic brittle sinistral reactivation of the ASZ can be regarded as the result of continent collision and closure of the Mozambique ocean further to the east, that potentially caused lateral escape manifested in NW–SE striking sinistral shear zones in Kenya and the southern Arabina-Nubian Shield between 620 and 570 Ma.

Fault Geometry and Active Stress from Earthquakes and Field Geology Data Analysis: The Colfiorito 1997 and L’Aquila 2009 Cases (Central Italy)

The fault segmentation pattern and the regional stress tensor acting since the Early Quaternary in the intra-Apennine area of central Italy was constrained by integrating two large geological and seismological fault-slip data sets collected for the areas struck by the two most energetic seismic sequences of the last 15 years (Colfiorito 1997, M w 6.0 and L’Aquila 2009, M w 6.1). The integrated analysis of the earthquake fault association and the reconstruction of the 3D shape of the seismogenic sources were exploited to identify homogeneous seismogenic volumes associated with subsets of geological and focal mechanism data. The independent analysis of geological and seismological data allowed us to observe and highlight similarities between the attitude of the long-term (e.g., Quaternary) and the instantaneous present-day (seismogenic) extensional deformations and to reveal their substantial coaxiality. Coherently, with the results from the kinematic analysis, the stress field inversion also noted a prevailing tensional seismotectonic regime associated with a subhorizontal, NE–SW, minimum stress axis. A minor, very local, and shallow (<5 km) strike-slip component of the stress field was observed in the Colfiorito sector, where an inherited N–S oriented right-lateral fault was reactivated with sinistral kinematics. Instead, an almost total absence of strike-slip solutions was observed in the L’Aquila area. These results do not agree with those indicating Quaternary regional strike-slip regimes or wide areas characterized by strike-slip deformation during the Colfiorito and L’Aquila seismic sequences.