Discrete Shear Zones Research Articles

New geological insights on the Palaeoproterozoic Sembé-Ouesso Basin in northern Congo-Brazzaville – Implications for the Kibaran orogeny, Neoproterozoic rifting and Pan-African overprint

The Palaeoproterozoic Sembé-Ouesso Basin (SOB) is the northeasternmost of six foreland subbasins of the Eburnean orogen; it covers an area of about 64,000 km2 in the northern Republic of Congo, southeast Cameroun and the Central African Republic. The structural architecture and tectono-metamorphic history of the SOB have been studied in the Republic of Congo by airborne geophysical interpretation, geological mapping, and petrography, analysis of illite crystallinity and K-Ar geochronology.The oldest tectono-metamorphic event (D1/M1), which caused the main structuring of the SOB, coincides in age with the last (1.0–1.07 Ga) stage of Kibaran orogeny. Metamorphic conditions decrease from greenschist facies in the east to upper diagenesis in the west. D1 deformation started with west-vergent folding followed by N-S sinistral transpression in discrete shear zones. The tectonic style characterises the SOB as foreland in the external zone of an eastern concealed orogen that either represents the continuation of the Kibaran Belt or forms a separate, as yet unidentified unit.Subsequent Neoproterozoic extension is marked by two conjugate N-S/NNE-SSW and E-W/NW-SE fault systems and a local pull-apart structure (D2). Coeval mafic magmas were emplaced in the faults of both systems. At regional scale, the extension can be linked to Tonian rifting in the central part of the Congo Basin and within the West Congo Belt.The influence of the Neoproterozoic (600 Ma) Central African orogeny is mainly restricted to the north-western part of the SOB where the Dja nappe was thrust from the north over the Palaeoproterozoic rocks (D3/M2). Elsewhere, the Pan-African event had no thermal and only minor structural effects on the SOB.

Low temperature creep of garnet: Insights from blueschist-facies skarn of the Preveli nappe (Crete, Greece)

We present a comprehensive record of deformation, metamorphism, and the state of stress during interaction between garnet-rich skarn and infiltrating fluids in an Eo-Hellenic subduction zone (Preveli nappe, Crete). The structural data and deformation microfabrics of quartz, coupled with petrological constraints, suggest that subduction of the Preveli rocks occurred under epidote-blueschist-facies conditions (T = 360 ± 40 °C, P > 1.0 GPa) and differential stresses >70 MPa. Localized deformation of fine-grained monomineralic grandite of the skarn led to mode I fractures and discrete shear zones along which synkinematic ferri-winchite developed. Brittle-viscous deformation of grandite in high-strain domains was accommodated by cataclasis, incongruent dissolution precipitation creep (IDPC), and incipient dislocation creep (DC). Cataclasis led to strong grain-size reduction and increased permeability resulting in fluid infiltration and enhanced IDPC as is documented by serrated reaction seams along which actinolite and ferri-winchite grew in sequence while replacing grandite. Incipient DC of grandite is indicated by lattice distortion and possible subgrains. Brittle-viscous deformation of grandite occurred at high differential stress (>250 MPa) and was significantly controlled by externally derived fluids, which contributed to hydraulic fracturing and the redistribution and addition of mobile elements.

Review of the thermo-tectonic evolution of the Central Zone of the Limpopo Complex with implications for conflicting published geodynamic models

Abstract This paper addresses the credibility of published data utilised to underpin conflicting models recently proposed for the geodynamic evolution of the Limpopo Complex (LC), Southern Africa, in the Neoarchaean and Palaeoproterozoic. We are mainly focused on the tectonic and metamorphic processes that affected the Central Zone (CZ) of the LC, but also consider the significance of the timing of the thermo-tectonic interaction of the Southern Marginal Zone (SMZ) of the LC with the granite-greenstone terrane of the Kaapvaal Craton (KVC) at the position of the steep north-dipping Hout River Shear Zone (HRSZ). HRSZ-linked tectonism at the contact with the KVC is expressed as a narrow “hot-iron zone” directly dated at 2.72 to 2.69 Ga and developed in the footwall of the north-dipping HRSZ. HRSZ-linked tectonic activity intermittently continued up to 2.65 to 2.62 Ga with no structural-metamorphic or geochronological evidence that the SMZ and the rest of the KVC were affected by regional thermo-tectonic (orogenic) activity after ca. 2.68 Ga. The complementary evolution of the CZ at 2.72 to 2.62 Ga prior to emplacement at 2.612 Ga of the Bulai granitic pluton is expressed by two thermo-tectonic events, at 2.72 to 2.66 Ga and 2.65 to 2.62 Ga, respectively. The early 2.72 to 2.66 Ga event was associated with near-vertical exhumation of the CZ from a rising crustal-scale granulite diapir, accompanied by emplacement of steeply-dipping isoclinal folded and granoblastic-textured CZ granulites at the mid-crustal level (20 km depth). A moderately (~45°) northeast-directed shear deformational event, accompanied by extensive granitic diapirism, controlled the final emplacement of the CZ in the Neoarchaean at 2.65 to 2.62 Ga prior to intrusion of the 2.612 Ga Bulai pluton. This second tectono-thermal event is expressed by major sheared structural features that include mega-closed folds, mega-north-south trending folds, and the 29 km-wide southwest-northeast-trending and moderately southeast-dipping and northeast-verging Tshipise Straightening Zone (TSZ) that bounds the CZ in the south. The CZ was finally exhumed and emplaced at the upper crustal level 600 Myr later (at ca. 2.02 Ga) during a regional high-temperature Palaeoproterozoic thermal event associated with major near-vertical strike-slip shear zones that overprint Neoarchaean oblique slip shear zones that bound the CZ. Thermo-tectonic activity in the CZ that is associated with this mainly thermal event is recognised as discrete steeply-dipping narrow fabric-parallel shear zones dated at ca. 2.02 Ga that overprint older structures. A gravity-driven crustal-scale diapiric model is utilised to explain the evolution of the SMZ and CZ of the LC at 2.72 to 2.62 Ga during the Limpopo Orogeny. Data presented and discussed contradict alternative published models that propose a continent-continent collisional orogeny at 2.65 to 2.62 Ga involving the SMZ and KVC, followed 600 Myr later at 2.02 Ga by a transpressional orogeny associated with near-horizontal thrust tectonics.

Remote sensing-based deformation monitoring and geological characterisation of an active deep-seated rock slide (Tellakopf/Cima di Tella, South Tyrol, Italy)

This study presents a 5-year long field campaign combined with 75 years of remote sensing data on the active, deep-seated Tellakopf rock slide in South Tyrol, Italy. Detailed field data and remote sensing techniques from airborne and terrestrial laser scanning (ALS, TLS) and orthoimages from federal flight campaigns and a helicopter-based monitoring campaign, allowed the development of a geometrical-kinematical rock slide model to investigate the initial failure and temporal-variable deformation mechanisms. The rock compound slide formed in foliated, highly fractured metamorphic rock masses at the SE-facing Tellakopf slope, directly above the commuter-rich national road SS 41. A total volume of approximately 3–4 Mm3 was estimated based on geographic information system (GIS) analysis by comparing the pre-failure surface topography and the reconstructed basal shear zone. Multi-temporal deformation analyses based on orthoimages, ALS and TLS data show alarmingly high velocities of at least 9 m/month during the initial formation phase in 2014, followed by a continuous velocity reduction to the current mean annual values of 1–2.5 m/year. Results indicate internal slab formation along discrete shear zones displayed on surface as main and minor scarps. These slabs show a translational movement behaviour along a fully persistent, slightly curvilinear basal shear zone. Rock mass yielding and fracturing conditions extensive mass loss manifested by high rock fall activity and large rock scree deposits at the rock slide toe. The cause for rock slide formation is attributed to retrogressive processes caused by long-term stress release due to topographical and hydrogeological changes by adjacent rock slides.

Late Famatinian (440–410 Ma) overprint of Grenvillian metamorphism in Grt‐St schists from the Sierra de Maz (Argentina): Phase equilibrium modelling, geochronology, and tectonic significance

AbstractThe analysis of major and trace elements in zoned minerals is useful for deciphering parts of the tectonothermal evolution of polymetamorphic tarrain. We applied this approach to the Maz Metasedimentary Series in Western Sierras Pampeanas of Argentina, where polymetamorphism resulted in the overprinting of a Grenvillian basement (the Maz Complex) during the pervasive Rinconada tectonic phase of the Famatinian orogeny. The older metamorphism (M1) is assigned to the youngest Grenvillian metamorphic event recognized in this basement at c. 1035 Ma, whereas the Rinconada metamorphism (M2) was Silurian to early Devonian, essentially between 440 and 410 Ma. The latter resulted from oceanward migration of the orogenic front relative to earlier late Cambrian to Ordovician (490–470 Ma) tectonic phases of the Famatinian orogeny. The M1 and M2 metamorphic events have been recognized in a staurolite‐garnet schist from the Maz Metasedimentary Series. Most metamorphic minerals from this rock were formed during the M2 event which was of the Barrovian type (±kyanite). Part of the metamorphic P–T evolution is recorded in the complex compositional zoning of garnet porphyroblasts. Three types of garnet were identified based on texture and chemistry, including trace elements (REEs). Phase equilibrium analysis, compositional isopleth, and multi‐equilibrium thermobarometry were applied in order to establish the P–T history. M1 is represented by preservation of Grt1 ± Kfs ± Sil, with peak P–T condition of 790°C and 5.2 kbar, that is, granulite facies. This early metamorphic event was related to a deformational D1 episode represented by a relict S1 foliation. The latter is preserved as aligned inclusions in staurolite porphyroblasts and as relics of an older crenulated foliation in microlithons from the matrix. M2 followed a clockwise P–T path with three mineral growth stages. The earliest occurred at ~585°C and ~8.7 kbar and is represented by Grt2 ± St1 ± Bt1 + Qz. Grt2 was partially coeval with growth of St1, which was stable at ~625°C and 9.0 kbar. Grt2 + St1 are syn‐kinematic to the main S2 foliation (D2 episode). Subsequently, decompression (D3) started as St2 (+ Bt2 + Ms1 + Qz + Pl) crystallized, and garnet was partially consumed at ~612–620°C and ~7.3–7.7 kbar. St3 + Grt3 crystallized at ~608°C and ~6.8 kbar at the end of D3. Increasing P–T conditions during the earlier M2 growth stage suggest burial of the Maz Metasedimentary Series, probably linked to tectonic thickening by underthrusting (tectonic phase D2). Peak metamorphic conditions were attained during thrust stacking. The tectonic phase (D2) was responsible for the main S2 foliation, which was penetrative at all scales. The nearly isothermal decompression path is compatible with exhumation, probably resulting from extension along discrete shear zones (tectonic phase D3) that produced a mylonitic foliation S3.

Geology along the Bedretto tunnel: kinematic and geochronological constraints on the evolution of the Gotthard Massif (Central Alps)

The unlined Bedretto tunnel crosses large parts of the pre-Triassic basement of the Gotthard massif (Central Alps), giving the possibility to study late-Variscan plutonic rocks (Rotondo granite) and their Caledonian (poly-)metamorphic host rocks (Tremola and Prato series). The Rotondo granite consists mostly of an equigranular, fine-grained granite and to a lesser extent of a porphyritic granite. Commonly, the Rotondo granite is massive or only slightly foliated. Ductile deformation is localized along discrete shear zones composed of granitic or quartz-biotite-rich lithologies. This paper reviews the geology of the Bedretto tunnel with emphasis on the Rotondo granite and presents constraints based on kinematic, microstructural, and U–Pb geochronological evidence, which can be summarized as follows: (1) Both granitic and quartz-biotite-rich shear zones (QB-SZ) in the Rotondo granite generally dip moderately to steeply towards north and are related to top-to-south reverse shearing, indicating south-verging backthrusting during the exhumation of the Gotthard massif. (2) Zircons from both the equigranular and porphyritic Rotondo granite show overlapping {}^{206}hbox {Pb}/{}^{238}hbox {U}-age ranges of 285–319 Ma and 280–335 Ma, respectively, which indicate that both are part of the same late-Variscan magmatic episode. Almost no older inherited cores are reported. (3) In zircons from a QB-SZ, 30% of the concordant age spots scatter between 339 and 589 Ma. This suggests that the parent material of the QB-SZ is unrelated to the magmatic episode that formed the Rotondo granite, but rather that the QB-SZ represent sheared xenoliths within the granite.

Interactions between large-scale strike-slip intersecting faults: Implication from the Tan–Lu and Xiangfan–Guangji fault zones in eastern China

ABSTRACT The large-scale strike-slip Tan–Lu Fault Zone (TLFZ) and Xiangfan–Guangji Fault Zone (XGFZ) both terminate in the southeastern corner of the Dabie Orogen at an angle of almost 90°, and this corner therefore provides a very good natural laboratory for understanding the mechanism by which large-scale strike-slip faults terminate. We present new geochronological and structural data for the southeastern tip of the XGFZ and the southwestern tip of the TLFZ. The NW–SE-striking XGFZ records ductile shearing in its northwestern segment, characterized by discrete dextral shear zones that formed at temperatures of 350–400°C, as indicated by quartz c-axes fabrics and microstructures. In the southeastern segment of the XGFZ, WNW–ESE-trending thrusts are displayed. The NE–SW-striking TLFZ is characterized by discrete NE–SW-trending sinistral ductile shear zones in the Qianshan–Tongcheng segment, brittle left-lateral strike-slip faults in the Taihu–Qianshan segment, and thrusts to the south of Taihu. The trends of these thrusts change progressively southward from NE–SW to ENE–WSW and E–W. New zircon U–Pb dating results and previous cooling biotite 40Ar/39Ar ages constrain the timing of shearing in the XGFZ to 112–102 Ma (late Early Cretaceous), which is the same as the age of faulting in the TLFZ (110–102 Ma). The large-scale strike-slip TLFZ and XGFZ both terminate at their tips as thrusts. We suggest that interactions between the two faults led to the kinematic changing from strike-slip to thrusting, with this playing an important role in controlling the termination of these two large-scale intersecting strike-slip faults. The dextral shearing of the NW–SE-trending XGFZ, the sinistral shearing of the NE–SW-trending TLFZ, and the nearly E–W-trending thrust faults all indicate continental-scale N–S compression in eastern China during the late Early Cretaceous. This compression resulted from rapid NNW-ward oblique subduction of the Paleo-Pacific Plate.

Experimental Deformation of Opalinus Clay at Elevated Temperature and Pressure Conditions: Mechanical Properties and the Influence of Rock Fabric

The mechanical behavior of the sandy facies of Opalinus Clay (OPA) was investigated in 42 triaxial tests performed on dry samples at unconsolidated, undrained conditions at confining pressures (pc) of 50–100 MPa, temperatures (T) between 25 and 200 °C and strain rates (dot{varepsilon }) of 1 × 10–3–5 × 10–6 s−1. Using a Paterson-type deformation apparatus, samples oriented at 0°, 45° and 90° to bedding were deformed up to about 15% axial strain. Additionally, the influence of water content, drainage condition and pre-consolidation was investigated at fixed pc–T conditions, using dry and re-saturated samples. Deformed samples display brittle to semi-brittle deformation behavior, characterized by cataclastic flow in quartz-rich sandy layers and granular flow in phyllosilicate-rich layers. Samples loaded parallel to bedding are less compliant compared to the other loading directions. With the exception of samples deformed 45° and 90° to bedding at pc = 100 MPa, strain is localized in discrete shear zones. Compressive strength (σmax) increases with increasing pc, resulting in an internal friction coefficient of ≈ 0.31 for samples deformed at 45° and 90° to bedding, and ≈ 0.44 for samples deformed parallel to bedding. In contrast, pre-consolidation, drainage condition, T and dot{varepsilon } do not significantly affect deformation behavior of dried samples. However, σmax and Young’s modulus (E) decrease substantially with increasing water saturation. Compared to the clay-rich shaly facies of OPA, sandy facies specimens display higher strength σmax and Young’s modulus E at similar deformation conditions. Strength and Young’s modulus of samples deformed 90° and 45° to bedding are close to the iso-stress Reuss bound, suggesting a strong influence of weak clay-rich layers on the deformation behavior.

Structural characterisation, internal deformation, and kinematics of an active deep-seated rock slide in a valley glacier retreat area

This study presents a multi-disciplinary approach for combining geological-geomorphological field mapping with different surface deformation monitoring techniques (i.e. extensometer, tachymetry, photogrammetry, airborne laser scanning (ALS), terrestrial laser scanning (TLS), uncrewed aerial vehicle (UAV)) to evaluate the impact of structural features on the formation process and deformation behaviour of the deep-seated Marzellkamm rock compound slide. The investigated rock slide is located in a high-alpine, glacier retreat environment in Northern Tyrol (Ötztal Alps, Austria) and formed in a well-foliated, fractured metamorphic rock mass. The total volume of the rock slide of approximately 13 Mm 3 was estimated based on geographic information system (GIS) analysis by comparing the surface topography and reconstructed geometry of the basal shear zone. In the upper part of the rock slide, annual mean velocities of up to 0.3 m/year were measured by a combined tachymetric-global navigation satellite system (GNSS) procedure. Based on multi-temporal terrain model analyses, the highest sliding velocities were obtained at the central slope foot area, reaching more than 1 m/year. Geological-geomorphological field mapping showed that the rock slide can be subdivided into two large rock slide systems with different geometry, formation age, and current activity. Furthermore, structural and geomorphological features (i.e. main and minor scarps, graben-structures, downhill- and uphill-facing scarps, tensile fractures, trenches), as well as surface deformation data, indicate the formation of six rock slide slabs. These slabs move downwards at different velocities and are separated by discrete shear zones, where slope displacement primarily accumulates. Large offsets along these shear zones indicate that internal slab deformation has only limited influence on the overall rock slide behaviour. At the Marzellkamm rock slide, pre-existing tectonic fault zones and planes were reactivated and used as weakness zones, playing a crucial role in the overall rock slide geometry and internal separation of the rock mass into slabs. Field survey and monitoring data suggest that a fully persistent, curved, non-circular basal shear zone has developed. Based on comprehensive analyses of mapping and deformation monitoring data, a geological-geometrical and kinematical model of the Marzellkamm rock slide was developed, providing a basis for site-specific hazard assessment and numerical modelling. • Two rock slide systems of six rock slide slabs of different deformation behaviour • Significant influence of pre-existing geological structures on rock slide formation • Deformation is controlled by the magnitude of shearing on separating shear zones. • Field survey indicates a fully persistent, curved, non-circular basal shear zone. • Two rock fall/avalanche events occurred at the lower part of the most active slabs.

Interplay between melt infiltration and deformation in the deep lithospheric mantle (External Liguride ophiolite, North Italy)

Mantle peridotites from the External Liguride Jurassic ophiolites (Northern Apennines, Italy) show diffuse occurrence of pyroxenite bands, recording melt migration and crystallization at the lithosphere-asthenosphere boundary, during old, pre-Jurassic stages of their subcontinental lithospheric mantle evolution. We present coupled microstructural and geochemical study of profiles across various types of pyroxenite-peridotite layering in these ophiolites, aiming to constrain the relative timing and potential interplay between melt infiltration and mantle deformation. The mantle sequence is composed of lherzolite and harzburgite, occasionally interleaved with dunite, crosscut by centimeter- to decimeter-wide pyroxenite layers. The peridotites have a porphyroclastic texture and show a penetrative tectonic foliation subparallel to the pyroxenite layering. Peridotite-pyroxenite contacts are irregular at the grain scale. Olivine and pyroxenes in both peridotites and pyroxenites record moderate to strong crystallographic preferred orientations (CPO) with alignment of olivine [100] and pyroxenes [001] axes subparallel to the stretching lineation marked by olivine and pyroxenes elongation. This is compatible with coeval deformation of olivine and pyroxenes during a high-temperature, spinel lherzolite-facies deformation event. The major and trace element compositions of peridotites record a metasomatic imprint that decreases with distance from the pyroxenite layers, whereas the strength of the olivine CPO decreases from the country peridotites towards the pyroxenite layers. The parallelism between pyroxenite layers and the peridotite foliation, their irregular contacts, as well as the spatial correlation between CPO and geochemical changes, are consistent with syn- to late-kinematic emplacement of the pyroxenites. These observations point to a strong interplay between deformation and melt transport processes in the mantle, characterized by melt focusing in conduits parallel to the foliation and changes in the mantle deformation processes due to the presence of melts. Exhumation of this mantle section in the Jurassic resulted in partial replacement of the spinel-facies assemblage by a plagioclase-bearing assemblage. Topotaxial relationship between plagioclase and precursor spinel-facies minerals suggests that this exhumation was not associated with pervasive deformation of the peridotite, but rather accommodated by deformation localized in discrete shear zones, not sampled in the present study.

Integrated multi-temporal analysis of the displacement behaviour and morphology of a deep-seated compound landslide (Cerentino, Switzerland)

Deep-seated gravitational slope deformations (DSGSDs) are complex slope instabilities commonly involving long-term evolution with significant spatial and temporal variations in slope kinematics. The Cerentino compound landslide in southern Switzerland exemplifies the long-term evolution of DSGSDs as well as short-term movements causing substantial damage to infrastructure. Using an integrated analysis of surface geomorphology, displacement monitoring data, borehole core and cosmogenic nuclide dating, we discuss the preconditioning and driving factors, kinematic behaviour, and long-term evolution of the instability. We demonstrate that foliation orientation and lithological boundaries, together with slope geometry and alpine faults, likely played significant roles in preconditioning the slope for failure. Based on morphology mapping and hierarchical clustering analysis of 2007–2018 detrended surface displacement data, we show that the landslide is separated into up to eight compartments at surface, revealing differences in kinematic behaviour from very active at the toe to insignificant movements in the upper region of the landslide above the village of Cerentino. These surficial compartments likely correspond to secondary sliding zones within the landslide mass. The analysis of data from inclinometer and fibre optic strain sensors installed in a new deep borehole (230 m) provided insight into movement within the landslide mass. Results show vertical heterogeneity in total displacement, which appears to be controlled by an active discrete shear zone located at 107 m, by shallower secondary sliding surfaces, and by distributed movement, particularly in the upper 20 m of the instability. Extrapolation of cosmogenic nuclide dating results from five samples taken on the exposed upper sliding scar reveals a possible age of failure initiation of 5–6 ka, with long-term decelerating movement trends. These generally slow averaged movements show multi-annual responses to long-term changes in precipitation and locally rapid responses to strong recharge events. The rare combination of dating and detailed analysis of displacement allows us to demonstrate the behaviour of DSGSDs at long and short timescales, with implications for hazard assessment and mitigation of these complex failures.

Exhumation of eclogitic ophiolitic nappes in the W. Alps: New age data and implications for crustal wedge dynamics

The W. Alps high pressure ophiolitic terranes (Monviso and Rocciavre areas) are a natural laboratory to study processes such as nappe-stacking and crustal exhumation in the deep regions of subduction margins. We sampled each of the main shear zones, representing thrust contacts later reactivated as detachments, for petrological and Rb-Sr multi-mineral geochronological analyses. Three generations of white mica are commonly found in mylonitized metasediments with crystal cores formed during the high pressure event, a broad rim formed during decompression in the epidote blueschist facies and texturally late muscovite flakes locally lining the main foliation. Semi-brittle discrete shear zones commonly crosscut previous structures witnessing deformation at temperatures lower than 300°C during exhumation. In spite of this apparent structural heterogeneity, homogeneous deformation ages mostly ranging between 38 and 35 Ma were obtained for all the shear zones bounding these major ophiolitic bodies. Pseudosection modeling confirms that the bulk of the shearing occurred in the epidote blueschist facies around 400–450°C and 1.0–1.5 GPa. These findings suggest that the shear zones between high pressure ophiolitic nappes were being actively mylonitized during exhumation in the lower epidote blueschist facies (25–40 km depth) between 38 and 35 Ma while some of the units forming the internal crystalline massifs (e.g. the Dora Maira Ultra High-Pressure unit) were still buried at more than 100 km depth (3–4 GPa and 36-34 Ma, according to most recent peak burial estimates). The growth of a crustal-scale duplex in the W. Alps is seen here as a consequence of basal accretion events that followed the exhumation of eclogitized coherent crustal slices in a serpentinized subduction channel. We conclude that in the study area (i) the buoyancy-driven exhumation of subducted continental crust has not been the only and decisive trigger for the exhumation of eclogitized oceanic lithosphere and (ii) continental subduction imprint on crustal wedge dynamics may have not been as instrumental as previously thought.

Tectono-metamorphic evolution of subduction channel serpentinites from South-Central Chile

This work provides the first comprehensive petrological and multi-scale structural investigation of a suite of ultramafic bodies and enclosing metasediments from the Late Paleozoic accretionary prism of South-Central Chile. The targeted outcrops are located in the La Cabaña area and are characterized by the presence of three main types of ultramafic rocks: (1) partially serpentinized massive peridotites, (2) antigoritic schistose serpentinites, and (3) antigoritic mylonitic serpentinites. Partially serpentinized massive peridotites and schistose serpentinites recorded a common first hydration event with the formation of Fe2+ porous rim in chromites (at ~500 °C), followed by static partial transformation of olivine to lizardite (below ~300 °C) and formation of rims of Fe3+-rich chromite and magnetite around chromites. A second event of serpentinization linked to fluid passage through zones with focused deformation resulted in the partial transformation of partially lizarditized massive peridotites into antigoritic schistose serpentinites (at ~320–400 °C). This second event took place within the shallower portion of the serpentinitic subduction channel, near the base of the accretionary prism and was characterized by the formation of tubular folds made up of lenses of olivine-lizardite massive rocks embedded in a matrix of schistose serpentinites. Antigoritic mylonitic serpentinites encountered in the matrix of schistose serpentinites may locally contain metamorphic olivine (i.e., olivine-bearing mylonitic serpentinites), registering higher temperature conditions at medium pressures (~600 °C, 11 kbar) that prevailed during the initial immature stage of development of the subduction system. Moreover, some mylonitic serpentinites containing Ti-clinohumite, Ti-chondrodite and metamorphic olivine still record higher pressure conditions (>15 kbar, i.e., >50 km depth for ~500 °C), which are related with a deformational event taking place when the subduction system had already cooled. These medium- and high-P mylonitic serpentinites preserved kinematic indicators of burial, whereas their exhumation along the subduction channel towards the shallower schistose serpentinites near the base of the accretionary prism could take place through discrete shear zones possibly aided by large sheath folds. Once incorporated in the accretionary prism (c. 285 Ma ago), the serpentinites shared metamorphism and deformation with the enclosing metasediments. Subvertical shortening related to uplift processes within the accretionary system took place in the stability field of antigorite (>300 °C) and, later, a subhorizontal E-W shortening prevailed in more superficial conditions.

3D Modeling of the Epembe (Namibia) Nb-Ta-P-(LREE) Carbonatite Deposit: New Insights into Geometry Related to Rare Metal Enrichment

Geological 3D modeling delivers essential information on the distribution of enrichment zones and structures in (complex) mineral deposits and fosters a better guidance to subsequent exploration stages. The Paleoproterozoic Epembe carbonatite complex showcases the close relation between enrichment of specific elements (Nb, Ta, P, Total Rare Earth Element (TREE) + Y) and shear zones by structural modeling combined with geochemical interpolation. Three-dimensional fault surfaces based on structural field observations, geological maps, cross-sections, and drillhole data are visualized. The model shows a complex, dextral transpressive fault system. Three-dimensional interpolation of geochemical data demonstrates enrichment of Nb, Ta, P, and TREE + Y in small, isolated, lens-shaped, high-grade zones in close spatial distance to faults. Based on various indicators (e.g., oscillating variograms, monazite rims around the apatite) and field evidence, we see evidence for enrichment during hydrothermal (re-)mobilization rather than due to magmatic differentiation related to the formation of the alkaline system. This is further supported by geostatistical analysis of the three-dimensional distribution of Nb, Ta, P, and Light Rare Earth Elements (LREE) with respect to discrete shear zones.

From an ocean floor wrench zone origin to transpressional tectonic emplacement of the Sithonia ophiolite, eastern Vardar Suture Zone, northern Greece

In the Hellenides of northern Greece, the Sithonia back-arc ophiolite constitute an element of the Vardar suture zone against the Chortiatis island arc magmatic suite, the Melissochori Formation and the Serbo-Macedonian Massif further north at the Mesozoic continental margin of Eurasia. A granodiorite from the Chortiatis island arc magmatic suite crystallized at 160 Ma as derived from new U-Pb zircon geochronology and confirms the end of arc magmatic activity that started at around 173 Ma. Located southerly of the Chortiatis island arc magmatic suite, the Sithonia ophiolite had igneous life from 159 to 149 Ma, and the ophiolite interfinger with clastic-carbonate Kimmeridgian sediments. Magmatic structures (i.e., sheeted dykes) in the ophiolite witness for NE-trending rift axis, while the transform faults and fracture zones sketch NW–SE transcurrent transtension-like propagation of the rift-spreading center at Sithonia that is consistent with a dextral wrench corridor already proposed for the ophiolite origin in the eastern Vardar zone. The tectonic emplacement of the Sithonia ophiolite involved dextral ENE to SE strike-slip sense of shear and SW and NE reverse thrust sense of shear on mostly steep foliation S1, subhorizontal lineation L1 and associated variably inclined F1 fold axes. This structural grain and kinematics are shared by adjacent Chortiatis island arc magmatic suite and the Melissochori Formation. The coexistence of strike-parallel and thrust components of displacement along discrete dextral strike-slip shear zones and internal deformation of the mentioned units is interpreted to result from a bulk dextral transpressive deformation regime developed in greenschist-facies metamorphic conditions. The back-arc ocean floor previous structural architecture with faults and fracture zones where Kimmeridgian sediments deposited in troughs was used by discrete strike-slip shear zones in which these sediments involved, and the shear zones become the sites for strain partitioning of transpressional deformation. Available biostratigraphic and radiometric age constraints define latest Jurassic-earliest Cretaceous (Tithonian-Berriasian to early Valanginian) time frame for the Sithonia ophiolite northeastward tectonic emplacement accomodated by dextral transpression that led to the ophiolite accretion to the Chortiatis island arc magmatic suite and its trench-fill exposed in the Melissochori Formation and further north toward the Serbo-Macedonian margin of Eurasia.

The Grenvillian Namaqua fold belt adjacent to the western Kaapvaal Craton: 2. Archaean Craton and supercontinent connections

The Namaqua sector of the Namaqua-Natal Province consists of large tectonostratigraphic terranes, some of which includes Paleoproterozoic fragments inherited from the supercontinent Columbia. Most of the terranes became part of Rodinia during the Mesoproterozoic Namaqua Orogeny (1.3–1.0Ga, Grenvillian). The terranes in the western foreland of the Kaapvaal Craton (Kheis Subprovince) form a thin-skin fold and thrust belt (5–15km depth to sole thrust). Westerly directed thrusting towards the hinterland is reminiscent of thick-skin tectonics, associated with the exhumation of high grade facies terranes, such as the granulitic and migmatitic Grünau Terrane. Upper amphibolite Upington Terrane was thrusted from the west onto thrust sheets contiguous to Kaapvaal Craton, in turn over-ridden near Prieska by the Grünau Terrane. Severity of the ductile deformation is illustrated by regional shear zones, ubiquitous recumbent isoclinal folds and allochthonous mega sheath folds in the Grünau Terrane. Vergence reversal of fold and thrust structures along a narrow (10–20km) northwesterly trending zone (Vergence Inversion Zone, VIZ), represents a crustal scale pop-up structure coincident with the Namaqua Front, a paired low-high gravity anomaly linked to a mega-scale anticline-syncline pair caused by deep-seated thrusting of the Columbian Moho.Late Columbian extension (1.6–1.3Ga, diachronous with early Rodinian events) produced parautochthonous basins with volcano-sedimentary sequences, deformed during the Namaqua Orogeny. Early to mid-Columbian (2.3–1.9Ga) rocks form basement to the mobile belt, with Steinkopf Terrane as type outcrop area (probably time-equivalent with the Grünau and Pofadder Terranes, Sperrgebiet Domain, Rehoboth structural Province and part of the Congo Craton). The western boundary of the Kaapvaal Craton (and composite Kalahari Craton) exhibits structural wedge edges of Archaean Kaapvaal basement and Ventersdorp-Vaalian basins. Columbia-Kaapvaal Detachment (COLKAD), a décollement between Kaapvaal and Columbia cratons (footwall) and Olifantshoek and other Namaqua terranes (hanging wall) extends for >600km westwards.Namaqua crust was thickened (20–25km) by both structural stacking and sheeted intrusions (Namaqua tectogenesis) with clockwise PTt-paths in the west-central Namaqua sector (Olifantshoek, Grünau, Pofadder, Bladgrond Terranes). In the western part (Okiep/Garies Terrane), crust was dominantly thickened by massive sheetlike granitoid intrusions resulting in granulite grade and anti-clockwise PTt-paths. COLKAD-rooted inter/intra-terrane thrusts are subhorizontal except where rotated by subvertical (discrete) shear zones (e.g. Tantalite Valley and Brakbos-Doringberg-Dabep). Crustal thickening and granulite-uplift stimulated temperature rise with anatexis-palingenesis of early Columbian/supracrustal rocks. Kilometer-scale granitoid bodies, derived from predominantly Columbian crust (according to isotopic source rock models) that are emplaced along shear zones, caused a rise in the brittle to ductile transition zone (i.e. mid-crustal conditions) and low-pressure granulite facies.

Structural characteristics of the Yilan–Yitong and Dunhua–Mishan faults as northern extensions of the Tancheng–Lujiang Fault Zone: New deep seismic reflection results

The Tancheng–Lujiang Fault Zone (TLFZ) can be subdivided into three segments that exhibit sharp contrasts in their deep structures. A deep seismic reflection profile (length ~600km) across the north part of the TLFZ, which provides new constraints on the structural styles of the northern TLFZ, was recently completed by the Chinese Sinoprobe Project. Here, the TLFZ branches into the Yilan–Yitong Fault (YYF) to the west and the Dunhua–Mishan Fault (DMF) to the east. The YYF developed as an internal fault in the Songnen–Zhangguangcai massif, while the DMF serves as the tectonic boundary between the Nadanhada terrane and the Khanka massif. Both faults developed large-scale flower structures, with that of the YYF being negative and that of the DMF being positive with reverse faults. The Moho in the profile is at a depth of 25–39km and is offset by the faults. The north part of the TLFZ extends into the upper mantle as thin shear zones with the reflectors truncated in the middle/lower crust. This feature differs from most crustal-scale strike-slip faults that distribute over a discrete shear zone in the lower crust, such as the San Andreas Fault.

The Sperrgebiet Domain, Aurus Mountains, SW Namibia: A ∼2020–850 Ma window within the Pan-African Gariep Orogen

The Aurus Mountains in SW Namibia form a tectonic inlier (Sperrgebiet Domain) within the Neoproterozoic Gariep belt. We document the geological history from field mapping, whole-rock geochemical, metamorphic and U-Pb zircon isotope data. The calc-alkaline granodioritic orthogneisses of the Wasserkuppe Suite, dated at ∼2020Ma, are the predominant, oldest rocks, which are interpreted to form part of a ∼2Ga “Sperrgebiet arc”; the tectono-magmatic equivalent of the ∼250km wide Richtersveld arc to the southeast, but ∼100Ma older. ∼2Ga detritus is common throughout the western Namaqua belt, but its source was hitherto unknown. The suite intruded synchronously with amphibolite facies metamorphism and deformation, considered an early phase of the Palaeoproterozoic Orange River Orogeny. The Wasserkuppe Suite is intruded by alkali granite and microgranodiorite dated at ∼1885Ma, interpreted as equivalents of the Vioolsdrif Suite (Richtersveld arc). The Sperrgebiet Domain is thus seen as an early phase of a process of Palaeoproterozoic island-arc development at the margins of the Archaean Kaapvaal Craton/Rehoboth Province. It was cannibalised by the later Richtersveld arc which dominated the palaeogeography of this part of southern Africa 100m.y. later, and which forms the crustal infrastructure of much of Western Namaqualand.The Palaeoproterozoic arc rocks are overlain by a clastic metasedimentary sequence; the Aurus Schist. Detrital zircon geochronology indicates that these rocks were predominantly derived from both arcs, with minor Archean and younger ∼1800Ma detritus, of unknown source(s). Bodies of latest Mesoproterozoic (∼1000Ma) Warmbad Granite and pegmatite are mapped throughout the area and represent the only Namaquan rocks definitively identified. Evidence for Namaquan metamorphic or tectonic imprints are lacking, apart from a garnet-clinopyroxene metabasic inclusion in a quartz-feldspar leucogneiss which gave a typical “Namaquan” P-T estimate of ∼700–750°C at 6–7kbar. The leucogneisses, distributed throughout the area, were dated at ∼840Ma and are interpreted as metamorphosed felsic igneous rocks of the Richtersveld Suite, formed during early rifting of the proto-Gariep basin. The Sperrgebiet rocks are variably deformed by the Pan-African Gariep Orogeny that produced regionally-pervasive fabrics, predominantly partitioned into discrete high-strain shear zones in a NW-trending sinistral transpressive tectonic regime.

Rhyacian A-type tholeiitic granites in southern Brazil: Geochemistry, U–Pb zircon ages and Nd model ages

In the southern South American platform, 2.5 to 2.0Ga terranes, probably related to the Atlantica supercontinent, occur mainly as minor reworked inliers within Neoproterozoic, Brasiliano/Pan-African orogenic belts, as the Ribeira Belt in southern Brazil. The dispersion of such fragments has generated uncertainties about their geotectonic reconstruction, and their study has been supported mainly by elemental and isotope geochemistry. The southern Ribeira Belt lies between the Paranapanema and Luiz Alves cratons and contains reworked Neoarquean and Paleoproterozoic terranes which outcrop as basement nuclei in supracrustal sequences, as the Setuva Complex. The Água Comprida Suite, situated in the northern part of the Setuva Complex, consists of Amphibole-Biotite Syenogranite (ABS), Porphyritic Biotite Syenogranite (PBS), and Equigranular Biotite Syenogranite (EBS). All granites are foliated and intensively deformed. The oldest foliation (Sn) is marked by augen feldspars set in a recrystallized matrix, followed by a crenulation cleavage (Sn+1) which evolves to discrete shear zones. ABS is a metaluminous, reduced A-type granite with FeOt/(FeOt+MgO)>0.9, with high HFSE and REE contents, corresponding to magmas related to continental medium to high-K tholeiitic series. PBS and specially EBS are highly differentiated, metaluminous to peraluminous (EBS), oxidized granites. The increase of Al2O3 and Rb, and decrease of HFS and RE elements relative to ABS indicate their evolution from tholeiitic magmas. The Água Comprida Suite granites are cogenetic rocks evolved from a within-plate mantle source, marked by high Nb, Ta, and Y. The influence of previously metasomatised mantle sources is evidenced by negative Nb, Ti, and P anomalies. The age of ABS is 2187±26Ma, and that of PBS is between 2180±13 to 2186±22Ma. The Nd model age of 2.4Ga, and εNd(2.18Ga) between −0.23 and −0.27 support the interpretation of ABS being formed from juvenile material with a minor crustal component. The deviation of εNd(2.18Ga) for PBS and EBS (−0.69 to −2.65), plus the higher Nd model ages (2.4 to 2.5Ga) suggest crustal contamination. A post-collisional environment for the interaction of the mafic tholeiitic and felsic crustal magmas is suggested for the formation of the Água Comprida Suite, based mainly on geochemical features of these magmas. Post-collisional environments are still not described in the Rhyacian orogenies of southern Brazil, and this proposition may be an important contribution for the study of the final period of amalgamation of the Paleoproterozoic supercontinent in southern South America.

Reactivation of intrabasement structures during rifting: A case study from offshore southern Norway

Pre-existing structures within crystalline basement may exert a significant influence over the evolution of rifts. However, the exact manner in which these structures reactivate and thus their degree of influence over the overlying rift is poorly understood. Using borehole-constrained 2D and 3D seismic reflection data from offshore southern Norway we identify and constrain the three-dimensional geometry of a series of enigmatic intrabasement reflections. Through 1D waveform modelling and 3D mapping of these reflection packages, we correlate them to the onshore Caledonian thrust belt and Devonian shear zones. Based on the seismic-stratigraphic architecture of the post-basement succession, we identify several phases of reactivation of the intrabasement structures associated with multiple tectonic events. Reactivation preferentially occurs along relatively thick (c. 1 km), relatively steeply dipping (c. 30°) structures, with three main styles of interactions observed between them and overlying faults: i) faults exploiting intrabasement weaknesses represented by intra-shear zone mylonites; ii) faults that initiate within the hangingwall of the shear zones, inheriting their orientation and merging with said structure at depth; or iii) faults that initiate independently from and cross-cut intrabasement structures. We demonstrate that large-scale discrete shear zones act as a long-lived structural template for fault initiation during multiple phases of rifting.