D2 Shear Research Articles

Polyphase deformation of staurolite-bearing metapelite in the Chinese Altai: Recording a Devonian-Permian tectonic regime switch in the southwestern Central Asian Orogenic Belt

Recent studies revealed that the Chinese Altai may have experienced two major tectonothermal events in the Devonian and Permian but their mutual relationships remain poorly constrained. In this study, we carried out a comprehensive analysis of macro- and micro-structural features of the Ordovician schists that were subjected to Barrovian-type metamorphism in the Chonghuer area of the northwestern Chinese Altai. Field structural and microstructural analyses revealed that metamorphic foliation S1 was folded by the recumbent fold F2, forming the sub-horizontal foliation S2. The S2 fabric was later reworked by the D3 transpressional shearing along the narrow sillimanite-bearing zone to form the foliation S3. Monazite U–Pb ages obtained from the staurolite schist located close to the D3 shear zone, as well as the zircon U–Pb ages from both folded and unfolded pegmatites that cross-cut granite, indicate that the D3 deformation took place during the Permian period, approximately 257–250 Ma. The EBSD study of quartz in the matrix and staurolite porphyroblasts conducted along the NE-SW cross-section allowed us to: 1) constrain the medium-temperature D2 and high-temperature D3 regimes and, 2) reconstruct foliation intersection axis (FIA) patterns both inside and outside the D3 deformation zone. The FIA between Devonian D1 and D2 preserved in staurolite porphyroblast was initially oriented in a N–S direction, which was ultimately reoriented to a NW-SE direction after undergoing the D3 shear deformation. This result is further reinforced by the numerical stochastic modelling, which quantifies the influence of the Permian HT sinistral transpression on the reworking of the Devonian FIA fabrics. Micro-structural data of this study verifies the significant tectonic switch from the Devonian E-W contraction to the Permian NNW-SSE transpression. Quantitative EBSD-based FIA analysis proves to be an effective approach for the structural analysis of polycyclic orogenic systems.

Emplacement of a lamprophyric sheet swarm under ductile transpression in continental crust. Insights from a Variscan magmatic complex in the West Armorican belt, Western France

We present structural data from a Variscan magmatic complex of dominantly lamprophyric (kersantite) and associated felsic (microgranodiorite) sheets - the West Armorican swarm (WAS) - injected into Paleozoic metasedimentary series at the western extremity of a transpressional system in the Armorican belt, Western France. The comparative structural analysis of nearly 100 individual intrusive sheets and their host-rocks demonstrates that the WAS complex is a syntectonic magmatic structure which intruded during the onset of regional shortening via successive pulses alternating with three strain episodes (D1-D2-D3). The moderate/high dipping attitude of a great majority of intrusions, along with their prominent orientation parallel to the regional cleavage (S1), i.e. orthogonal to the direction of shortening, challenge theoretical predictions about magma-filled fractures in contractional settings. These apparently atypical features are reconciled in a kinematic model which emphasizes the influence of both host-rock planar anisotropies (S1 cleavage and D3 shear patterns) and structural inheritance on magma pathways, and instead hypothesizes the negligible role of the ambient stress field. Comparisons are attempted with syncollisional lamprophyre systems present in the European Variscides, and more especially in the SE Bohemian massif.

Multisensor Satellite Data and Field Studies for Unravelling the Structural Evolution and Gold Metallogeny of the Gerf Ophiolitic Nappe, Eastern Desert, Egypt

The gold mineralization located in the southern Eastern Desert of Egypt mostly occurs in characteristic geologic and structural settings. The gold-bearing quartz veins and the alteration zones are confined to the ductile shear zones between the highly deformed ophiolitic blocks, sheared metavolcanics, and gabbro-diorite rocks. The present study attempts to integrate multisensor remotely sensed data, structural analysis, and field investigation in unraveling the geologic and structural controls of gold mineralization in the Gabal Gerf area. Multispectral optical sensors of Landsat-8 OLI/TIRS (L8) and Sentinel-2B (S2B) were processed to map the lithologic rock units in the study area. Image processing algorithms including false color composite (FCC), band ratio (BR), principal component analysis (PCA), minimum noise fraction (MNF), and Maximum Likelihood Classifier (MLC) were effective in producing a comprehensive geologic map of the area. The mafic index (MI) = (B13-0.9147) × (B10-1.4366) of ASTER (A) thermal bands and a combined band ratio of S2B and ASTER of (S2B3+A9)/(S2B12+A8) were dramatically successful in discriminating the ophiolitic assemblage, that are considered the favorable lithology for the gold mineralization. Three alteration zones of argillic, phyllic and propylitic were spatially recognized using the mineral indices and constrained energy minimization (CEM) approach to ASTER data. The datasets of ALSO PALSAR and Sentinel-1B were subjected to PCA and filtering to extract the lineaments and their spatial densities in the area. Furthermore, the structural analysis revealed that the area has been subjected to three main phases of deformation; (i) NE-SW convergence and sinistral transpression (D2); (ii) ~E-W far field compressional regime (D3), and (iii) extensional tectonics and terrane exhumation (D4). The gold-bearing quartz veins in several occurrences are controlled by D2 and D3 shear zones that cut heterogeneously deformed serpentinites, sheared metavolcanic rocks and gabbro-diorite intrusions. The information extracted from remotely sensed data, structural interpretation and fieldwork were used to produce a gold mineralization potential zones map which was verified by reference and field observations. The present study demonstrates the remote sensing capabilities for the identification of alteration zones and structural controls of the gold mineralization in highly deformed ophiolitic regions.

Polyphase deformation-controlled giant Renli Nb–Ta deposit, South China: Constraints from systematic structural analysis

The Early Cretaceous Renli Nb–Ta deposit was recently discovered near the southern margin of the Late Jurassic–Early Cretaceous Mufushan composite granitic batholith, central Jiangnan Orogenic Belt, and represents the largest known rare metal deposit in South China. Substantial breakthroughs have been made in prospecting the pegmatite-dike-type of Nb–Ta mineralization in the deposit, especially from the perspective of geochemical exploration. However, there is still a lack of systematic structural studies in this area which has seriously hindered the in-depth understanding of the emplacement of pegmatite dikes and the distribution patterns of Nb–Ta ore bodies. In order to shed new light on this key issue, this study presents new and detailed structural data from the Mufushan region. The results indicate that the Renli and adjacent areas have experienced several deformational events, including D1 Neoproterozoic NE–SW contraction and upright-folding, D2 latest Late Jurassic to Early Cretaceous regional extension accompanied by granitic magma intrusion, and D3 Early Cretaceous brittle normal faulting and fracturing with granitic magma emplacement and Nb–Ta mineralization. The D1 event, corresponding to the Jiangnan Orogeny, formed regional-scale NW–SE-striking upright folds with ubiquitous subvertical S1 cleavages and subhorizontal L1 crenulation lineations. The D2 event represented orogenic exhumation and produced a domal structure cored primarily by gneissic/mylonitic rocks and granites and mantled by schists, which dominated the bulk structural architecture of the Mufushan area. This was followed by the D3 event which controlled the formation of numerous extension fractures in the study area. The Nb–Ta-bearing magmatic-hydrothermal fluids migrated along the pre-existing D2 shear zone and subsequently filled in the widespread D3 extension fractures to form pegmatite dikes. The D2 to D3 phases can be considered as a progressive exhumation event from plastic to brittle regimes. The D2 and D3 structures dictate the spatial distribution of Nb–Ta ore bodies.

Orogenic reworking and reactivation in Central Iberia: A record of Variscan, Permian and Alpine tectonics

Interference between orogenic systems and deformation phases within them may lead to reworking and reactivation of previous structures. The eastern sector of the Spanish-Portuguese Central System holds evidence of two orogenic systems, Variscan and Alpine, plus a stage of Permian extension. We perform an integrated structural analysis to identify reworking and reactivation processes throughout the geological record. The Variscan record starts with crustal thickening (D1; E-verging overturned folds). A second phase features the intra-orogenic collapse of an overthickened crust (D2; top-to-the-SE ductile extensional shear zone), which produced intense structural reworking at the core of the shear zone and moderate reworking at its hanging wall. During subsequent strike-slip tectonics, crustal thickening parted transpressional deformation into a dextral shear zone and upright folds (D3). Variscan deformation did not reactivate previous structures, but exploited a weak rheological boundary defined by contrasted lithologies (sedimentary versus igneous rocks) to accommodate D2 shearing. Reactivation played a role afterwards: Variscan strike-slip shear zone acted as a transfer fault to accommodate Permian extension (post-orogenic collapse), and then Alpine contraction. The Permian extension record is blurred by Alpine inversion, although the trend of Alpine structures in Central Iberia, and the Spanish-Portuguese Central System, may result from Permian structural inheritance.

Paleozoic Geodynamics and Architecture of the Southern Part of the Mongolian Altai Zone

AbstractThe Mongolian Altai Zone of the Central Asian Orogenic Belt has been traditionally interpreted as a mosaic of Paleozoic magmatic arcs, back‐arcs, and Precambrian continental terranes. In order to define its architecture and its tectonic evolution, three domains previously interpreted as terranes were investigated. The findings show that the Northern and Central domains are formed by a metamorphic sequence characterized by Barrovian S1 fabric transposed by recumbent folds and dominant sub‐horizontal amphibolite facies S2 schistosity. The latter is associated with the intrusions of late Devonian syntectonic granite sheets and anatexis in the south. The Southern domain is formed by early Permian migmatites and anatectic granites separated from the metamorphic envelope by amphibolite to green‐schist facies D3 shear zone cross‐cutting S2 fabrics. All domains have been reworked by E‐W upright folds associated with axial‐planar greenschist facies cleavage, reflecting the final mid‐Permian to Triassic D4 shortening. Lithological, geochemical, and U‐Pb zircon analyses of metasediments of all domains indicate that they are formed by Ordovician mature quartzite derived from Precambrian basement intruded by Cambrian‐Ordovician continental arc and Silurian immature graywacke which originated through erosion of an oceanic arc. Altogether, the whole sequence represents a fore‐arc basin in front of a migrating arc affected by thickening and late Devonian extension. The Southern domain is interpreted as an early Permian core complex amplified by mid‐Permian to Triassic compression. The apparent “terrane” architecture of the Mongol Altai Zone originated due to Devonian and Permian heterogeneous reworking of a giant Ordovician to Silurian fore‐arc basin.

Structural and kinematic analysis of the Cuonadong dome, southern Tibet, China: Implications for middle-crust deformation

The Cuonadong Dome is located at the eastern part of the North Himalayan gneiss domes. Structural and kinematic analysis in the middle unit of the Cuonadong Dome provide significant new insights into the structural deformation of the middle crust in the Northern Himalaya. Field observations, microscopic kinematic and EBSD analyses, and deformation temperature estimates from the mid-crustal rocks of the Cuonadong Dome show high-strain D2 (top-to-N) kinematic fabrics with a downward-to-north progression from dominantly top-to-north shear in the garnet zone, to solely top-to-north shear in the kyanite/sillimanite zone. A combination of mineral assemblages, microstructures, and quartz crystal preferred orientation patterns indicates deformation temperatures associated with D2 kinematic fabrics increase with structural depth from ∼450 °C in garnet zone to > 630 °C in the kyanite/sillimanite zone. These values broadly overlap temperature range deduced from metamorphic petrography (450–630 °C). Microtextural relations indicate that peak metamorphism occurred post-D1 and pre-D2 deformation. Combined with the ∼1000-m-thick ductile deformation zone, the interpreted deformation temperatures yielded an average thermal field gradient of ∼ 180 °C/km over 1.05 km of structural thickness. Based on the geochronologic data, midcrustal D2 extensional deformation in the Cuonadong Dome initiated as early as early Oligocene (32 Ma) and ended during the middle Miocene (∼14 Ma). Comparative studies of D2 ductile deformation in midcrustal rocks from other North Himalayan gneiss domes and the South Tibetan Detachment System indicate that D2 shear zone in the Cuonadong Dome represent the northern part of the South Tibetan Detachment System, and highlight temporal and spatial variation in midcrustal D2 deformation patterns over the southern Tibet during the Himalayan orogeny.

Early Neoproterozoic Tectonics in the Godhra–Chhota Udepur Sector: Evidence for Two-Stage Accretion in the Great Indian Proterozoic Fold Belt

Abstract The Great Indian Proterozoic Fold Belt (GIPFOB) is a curviplanar highly-tectonized zone of Precambrian crystalline rocks. In the GIPFOB, the N/NNE-striking western arm (the Aravalli Delhi Fold Belt, ADFB) and the E-striking southern arm consisting of the Chottanagpur Gneiss Complex (CGC) and the central/southern domains of the Satpura Mobile Belt (SMB) converge at the Godhra-Chhota Udepur sector. To investigate the tectonics of the sector, we combine the results of analyses of mesoscale and regional structures, U-Pb (zircon) geochronology, and monazite chemical dating to constrain the convergence. The sector is dominated by an ensemble of shallow-dipping granitoid mylonites (D2 deformation) and recumbently folded anatectic granulite-facies basement gneisses interleaved with allochthonous greenschist/epidote-amphibolite facies supracrustal rocks thrust top-to-the-south. The shallow-dipping carapace is traversed by a network of E-striking steep-dipping shear zones with sinistral and N-down kinematics (D3 deformation). The D3 shear zone hosted granitoids exhibit E-striking suprasolidus deformation fabrics and chessboard microstructures. In the shallow-dipping carapace, the partly overlapping stretching lineations associated with D2-D3 deformations share low-angle obliquities with the W/WNW plunging hinges of D2 recumbent folds and the upright/moderately-inclined D3 folds in the basement gneisses and the supracrustal rocks. The transition from thrust-dominated (D2) to wrench-dominated (D3) deformation involved flipping of Y and Z strain axes for similar orientations of orogen-parallel stretching caused by N-S shortening. U-Pb LA-ICP-MS (zircon) and monazite chemical dates suggest the D2-D3 deformation and felsic plutonism occurred at 0.95–0.90 Ga, the pre-D2 high-grade metamorphism in the anatectic gneisses at 1.7–1.6 Ga. The 0.95–0.90 Ga structures in the Godhra-Chhota Udepur are identical to those in CGC-SMB in the southern arm and terminate the N/NNE-striking structures in the ADFB. We suggest the GIPFOB comprises two Early Neoproterozoic accretion zones, e.g., the western arm (ADFB) and the younger (GC-SMB-CGC) southern arm.

Exhumation of an anatectic complex by channel flow and extrusion tectonics: structural and metamorphic evidence from the Porto–Viseu Metamorphic Belt, Central-Iberian Zone

Structural and metamorphic analyses of the Porto–Viseu Metamorphic Belt (PVMB), an anatectic complex located in the Central-Iberian Zone (CIZ), Iberian Variscan Massif, constrain the kinematic features of the polyphase Variscan deformation, as well as the boundary conditions associated with the exhumation and emplacement of the belt onto the CIZ’s lower-grade metasediments. A sharp metamorphic contrast is evident between the lowest grade rocks of the anatectic complex—garnet-staurolite mylonitic schists with a minimum metamorphic peak at P = 5.7 kbar and T = 635 °C—and the host chlorite-biotite metasediments of the CIZ. Additional P–T estimates for metatexites in the PVMB provided P = 7.6–7.9 kbar and T = 770–810 °C, confirming high-grade metamorphic peak conditions. The temperature jump between the anatectic complex and the low-grade host rocks exceeds 400 °C if the anatectic core of the belt (metatexites, diatexites, and S-type granites) is considered, implying a vertical difference of 10–17 km. The PVMB is limited in the NE by a D2 shear zone displaying normal kinematics, which has been explained so far by extensional tectonics. However, the SW boundary is limited by a D2 shear zone showing reverse kinematics. This pair of coeval and antithetical ductile shear zones bordering the PVMB, associated with the increase of the metamorphic grade towards the axial domain of the belt, is consistent with anatectic channel flow, followed by extrusion tectonics of the PVMB within an overall D2 compressive setting.

Metallogeny of a Pan-African oceanic arc: VHMS and gold deposits in the Ariab-Arbaat belt, Haya terrane, Red Sea Hills (Sudan)

Gold in the Ariab district (Red Sea Hills, Sudan) is mined in gossans developed over volcanogenic massive sulphide (VHMS) deposits formed in the context of a Tonian ensimatic arc of the Haiya terrane, in the Neoproterozoic Arabo-Nubian Shield. All the primary gold occurrences of the Ariab belt, whatever their style (VHMS-, quartz lode- or shear-zone-hosted), appear, however, to have been syn-to late-kinematic, i.e., formed in the context of the final Pan-African collision leading to the Arabian-Nubian Shield (D3 event), and therefore are post-VHMS. Gold is thus found associated with either galena or bismuth telluride in late syn- to post-metamorphic sets of (micro)cracks, mostly within earlier pyrite or arsenopyrite. Gold deposition as native gold or electrum occurred when an intense fluid circulation of metamorphic CO2-H2O rich fluids underwent significant decompression from lithostatic to hydrostatic pressures at temperatures decreasing from 400 to 430 °C at around 8 km depth down to 300 °C. Although no clear evidence of volatile unmixing was observed, the decompression, together with a temperature decrease due to dilution by aqueous rich fluids, favoured gold deposition.The late-D3 gold event in the Ariab belt has all the makings of an orogenic gold system, notably similar to the Late Carboniferous gold event in the West European Variscan belt. Thus, it is possible to propose that, at the end of the collision events, fluids released from the newly formed terrane root were channelled towards the upper crust through the major D3 shear zones such as the Oko shear zone. The fluids outpouring from these major drains were then conveyed through fault plays and damaged zones, up to specific chemical traps within massive sulphide ore bodies or quartz veins systems. Thus, the Ariab gold district could be classified as a representative of the class of orogenic gold deposits.

Episodic Crustal Extension and Contraction Characterizing the Late Mesozoic Tectonics of East China: Evidence From the Jiaodong Peninsula, East China

AbstractDuring the Late Mesozoic, East China is characterized by widespread magmatism, thrusting and folding, extensional doming, strike‐slip faulting, and block rotation. The Jiaodong Peninsula provides a key area located in East China to understand the episodic intracontinental extension and contraction and associated granitoid emplacement. Based on our structural analysis, magnetic fabric measurement, and gravity modeling, polyphase deformation and magma emplacement have been recognized within the Queshan‐Kunyushan‐Yuangezhuang‐Sanfoshan (QKYS) massif of the central Jiaodong Peninsula. A significant Late Jurassic D1 event, developed in the northern margin of the massif, was expressed by a high‐temperature, top‐to‐the‐NE shearing. Late Jurassic plutons display magnetic fabrics corresponding to the D1 structural fabrics and several NW–SE‐trending feeder zones at depth. These results link the syn‐kinematic emplacement of Late Jurassic plutons with regional NE–SW extensional tectonics. In the south of the massif, a lower‐temperature, top‐to‐the‐SW contractional deformation (D2) resulted from NE–SW contraction. The D3 shear zone with a top‐to‐the‐WNW kinematics is a rolling‐hinge type detachment fault that exhumed the massif, indicating NW–SE regional extension. Finally, Early Cretaceous plutons emplaced into the upper crust with a fast‐cooling rate and formed inverted drop shapes with concentric magnetic foliations and variably oriented magnetic lineations. In light of the previous geochronological results, the timing of these tectonic events is discussed. The tectonic evolution of the QKYS massif indicates a process from crustal thickening to lithospheric foundering in response to the Late Mesozoic plate convergences.

Lateral subhorizontal middle to lower crustal flow in response to continental collision: Evidence from the Diancang Shan complex along the Ailao Shan-Red River belt, Southeastern Tibetan Plateau

How middle to lower crust in Southeastern Tibetan Plateau flowed in response to the Indian-Eurasian continental collision remains hotly debated in the last decades. Metamorphic complexes along crustal mobile zones, e.g., the Diancang Shan (DCS) complex along the Ailao Shan-Red River (ASRR) metamorphic belt may provide key knowledge on our understanding of the middle-lower crustal flow during the collision. In this contribution, integrated structural, microstructural, kinematic, and thermo-chronological studies of the DCS complex reveal that the complex constitutes a destructed A-type dome. High-grade metamorphic rocks of Proterozoic protoliths form the core (Unit I) that is covered by low-grade metamorphic rocks of Mesozoic age in the limbs (Unit II). The eastern limb of the DCS dome is cut by a normal shear zone (Unit III). Three phases (D1, D2 and D3) of Cenozoic deformation structures are recognized in the complex. The D1 and D2 structures constitute the dominant structural style of the Units I and II. The D1 structures consist of penetrative S1 foliations, mineral lineations (L1), symmetrical intrafolial folds (F1) and tectonic lenses of granitic and quartz veins in rocks from the Units I and II. Mylonitic foliations (S2//S1), NNW-SSE plunging subhorizontal stretching lineations (L2), widespread A-type folds (F2), asymmetric porphyroclasts and tectonic lenses form the dominant structural styles in the D2 deformation. D3 structures are new foliations S3 and hot striae L3 in the normal-slipping shear zone (Unit III). The S2//S1 foliations define the domal framework of the DCS complex, which is truncated by the D3 shear zone. The dome is NW-SE elongated and has a hinge line paralleling to the L2//L1 stretching lineations. Synthetic analysis of structural and microstructural styles, and kinematic vorticity data estimated by rotated rigid porphyroclast method (0.38–0.69) and oblique grain-shape/quartz c-axis-fabric method (0.72–0.98) implies that the D1 and D2 deformations formed a progressive process from early pure shear-dominated (D1) to simple shear-dominated (D2) shearing. Shear sense indicators from different parts (Units I and II) of the dome are compatible with top-to-the southeast shearing during D2 deformation. It is shown that the D1-D2 deformation was characterized by a successive process from penetrative flow (D1) and ductile shearing at middle to lower crustal levels to doming (D2) that was coeval with exhumation of the DCS complex since 27 Ma or earlier. We conclude that these processes, i.e., the penetrative flow, ductile shearing, doming and exhumation of the DCS complex, were attributed to lateral subhorizontal middle to lower crustal flow along the ASRR metamorphic belt in the Southeastern Tibetan Plateau during the Indian and Eurasian plate collision.

Structural setting, wall rock alteration and gold mineralisation of the Mt. Percy gold deposit, Kalgoorlie, Western Australia

Field mapping, drill-core logging, petrography, laser ICP-MS analysis on pyrite, chlorite geothermometry and geochemical analysis of hydrothermally altered rocks reveal two distinct styles of gold mineralisation at Mt. Percy. The first style is D2 shear zone-hosted and correlates with Fimiston-style mineralisation documented in the Golden Mile. Laser ICP-MS analyses on pyrite indicate high As concentrations (average of 5823 ppm for pyrite in Hannans Lake Serpentinite and 635 ppm in feldspar-quartz porphyry). Chlorite geothermometry reveals average temperatures of 262 ± 50 °C and 357 ± 50 °C in distal and proximal alteration zones, respectively. Geochemical analyses reveal (1) positive gold-silver and gold-antimony relationships; (2) lack of a base metal-gold relationship; (3) systematic increase of carbonatisation and alkali saturation indices from distal to proximal alteration zones; and (4) addition of K, Ca, Na and V, with corresponding depletion of Mg, Fe, P and REE. The second style is related to D4 sheeted quartz vein sets and correlates with Mt. Charlotte-style mineralisation documented in the Mt. Charlotte deposit. Laser ICP-MS analyses on pyrites reveal low As concentrations (average of 51 ppm for pyrites in feldspar-quartz porphyry and 870 ppm in Devon Consols Basalt). Chlorite geothermometry reveals an average temperature of 325 ± 50 °C on vein selvage in the Devon Consols Basalt. High-grade gold mineralisation (> 50 g/t Au) and extensive (< 50 m wide) hydrothermal alteration zones coincide with D2 shear zones and D4 fault-controlled sheeted quartz vein sets, and particularly in areas where both of these structures intersect porphyry stocks.

Evolution of structures and hydrothermal alteration in a Palaeoproterozoic supracrustal belt: Constraining paired deformation–fluid flow events in an Fe and Cu–Au prospective terrain in northern Sweden

Abstract. An approximately 90 km long Palaeoproterozoic supracrustal belt in the northwestern Norrbotten ore province (northernmost Sweden) was investigated to characterize its structural components, assess hydrothermal alteration–structural geology correlations, and constrain a paired deformation–fluid flow evolution for the belt. New geological mapping of five key areas (Eustiljåkk, Ekströmsberg, Tjårrojåkka, Kaitum West, and Fjällåsen–Allavaara) indicates two major compressional events (D1 and D2) have affected the belt, with each associated with hydrothermal alteration types typical for iron oxide–apatite and iron oxide Cu–Au systems in the region. Early D1 generated a regionally distributed, penetrative S1 foliation and oblique reverse shear zones that show a southwest-block-up sense of shear that formed in response to NE–SW crustal shortening. Peak regional metamorphism at epidote–amphibolite facies broadly overlaps with this D1 event. Based on overprinting relationships, D1 is associated with regional scapolite ± albite, magnetite + amphibole, and late calcite alteration of mafic rock types. These hydrothermal mineral associations linked to D1 structures may form part of a regionally pervasive evolving fluid flow event but are separated in this study by crosscutting relationships. During D2 deformation, folding of S0–S1 structures generated F2 folds with steeply plunging fold axes in low-strain areas. NNW-trending D1 shear zones experienced reverse dip-slip reactivation and strike-slip-dominated movements along steep, E–W-trending D2 shear zones, producing brittle-plastic structures. Hydrothermal alteration linked to D2 structures is a predominantly potassic–ferroan association comprising K-feldspar ± epidote ± quartz ± biotite ± magnetite ± sericite ± sulfides. Locally, syn- or post-tectonic calcite is the main alteration mineral in D2 shear zones that intersect mafic rocks. Our results highlight the importance of combining structural geology with the study of hydrothermal alterations at regional to belt scales to understand the temporal–spatial relationship between mineralized systems. Based on the mapping results and microstructural investigations as well as a review of earlier tectonic models presented for adjacent areas, we suggest a new structural model for this part of the northern Fennoscandian Shield. The new model emphasizes the importance of reactivation of early structures, and the model harmonizes with tectonic models presented by earlier workers based mainly on petrology of the northern Norrbotten area.

The Vaasa Migmatitic Complex (Svecofennian Orogen, Finland): Buildup of a LP‐HT Dome During Nuna Assembly

AbstractTectonic evolution of the Paleoproterozoic Vaasa migmatitic complex (VMC) in the central part of the Svecofennian accretionary orogen is deciphered using aeromagnetic and gravity maps, deep seismic and magnetotelluric profiles, and structural and metamorphic data. The VMC is a semicircular structure with migmatitic rim and granitic core composed of several subdomes. It evolved in three main tectonic events (D1–D3). The D1 event (ca. 1.89–1.88 Ga) corresponds to the stacking of supracrustal rocks and the formation of an inverted metamorphic gradient. Anatexis at LP‐HT metamorphic conditions enabled the material to flow. The D2 event (ca. 1.88–1.87 Ga) corresponds to large‐scale folding of the partially molten crust within an orocline. It is marked by folds with an E‐W vertical axial planar foliation. The late D3 event resulted from mass redistribution owing to mechanical instabilities within the hinge of the orocline. It is marked by vertical shearing (ca. 1.87–1.85 Ga) in the marginal parts of the complex and along the granitoid subdomes. The seismic reflection profile (FIRE 3a) and magnetotelluric profiles (MT‐PE and MT‐B2) image large‐scale D1 stacking structures within an accretionary prism. Near vertical breaks in crustal‐scale reflectivity and conductivity models are interpreted as D3 shear zones. The VMC is an example of early mass and heat transfer within a collage of hot supracrustal rocks in an accretionary belt. Partial melting enhanced the flow of material, the production, and rise of magma as well as exhumation, marked by magmatic domes in the hinge of the orocline.

Hydrothermal alteration and mineralization in the Faina greenstone belt: evidence from the Cascavel and Sertão orogenic gold deposits

The Faina greenstone belt is located in central Brazil and contains the Cascavel and Sertão gold deposits (500,000 oz Au total resource and past production). The bulk of the gold is hosted in quartzite and biotite schist at Cascavel, and carbonaceous schist and banded iron formation at Sertão. The metamorphic grade of the host rocks is greenschist facies.Cascavel is a vein-hosted gold deposit with orebodies mainly controlled by F2 fold hinges, with minor mineralization in D3 shear zones. Hydrothermal alteration in quartzite is characterized by distal quartz-K-feldspar ± muscovite, intermediate quartz-fuchsitic mica-biotite-pyrite, and proximal quartz-white mica-pyrite ± chalcopyrite. The biotite schist is characterized by distal quartz-chlorite-calcite and proximal white mica-biotite-tourmaline-siderite-pyrite. Native gold is located within intensely deformed quartz veins, and locally as free gold disseminated in proximal alteration zones.Sertão is a shear zone-hosted gold deposit with orebodies controlled by D3 shear zones including gold-bearing shear veins and surrounding hydrothermally altered wallrocks. Hydrothermal alteration is characterized by distal quartz-ankerite-chlorite-pyrite ± chalcopyrite and proximal quartz-white mica-siderite-pyrite-arsenopyrite ± chalcopyrite. Gold is in equilibrium with arsenopyrite and within arsenopyrite disseminated in the proximal alteration zone.Chlorite geothermometry suggests temperatures of 335–407 °C and 328–397 °C for the gold-bearing hydrothermal fluids in the Sertão and Cascavel deposits, respectively. Arsenopyrite geothermometry on arsenopyrite within gold-bearing veins in the Sertão deposit yields temperatures between 310 and 480°C.Structures controlling hydrothermal alteration and gold mineralization in both deposits are developed during a regional D2 event and subsequently deformed during a D3 event. Hydrothermal alteration minerals are consistently replacing metamorphic minerals. The alteration assemblages in both deposits are similar, albeit their modal mineralogy is controlled by the host rock composition. Therefore, both the Cascavel and Sertão deposits are interpreted as orogenic, mesozonal gold systems comparable with other orogenic gold systems in Brazil and worldwide.

The architecture of the Buem Structural Unit: Implications for the tectonic evolution of the Pan-African Dahomeyide Orogen, West Africa

The Buem Structural Unit (BSU) occurs about 50 km west of the high-pressure (HP) rocks of the Pharusian suture zone (PSZ) of the Dahomeyide Orogen, West Africa. Shale and sandstone intercalations form the base of the BSU, followed by serpentinised peridotite, gabbro, massive and pillow basalts (fragment of oceanic crust), jasper/chert, and limestone/dolostone, with sandstone and diamictite forming the top of the BSU. The package was deformed at lower greenschist-facies conditions during the Pan-African orogeny. The N-S-trending BSU has been affected by three deformational events. The main deformation event D2, which transposed bedding-parallel D1 fabrics, produced a steeply ENE-dipping S2 tectonic foliation. S2 is axial planar to upright or west-verging isoclinal and chevron F2 folds. Top-to-the-west D2 shear zones in the sandstone, shale and serpentinised peridotite are associated with a down-dip ENE-plunging L2 stretching lineation. The mafic-ultramafic rocks occur as D2 tectonic slices, marking the boundaries between thrust sheets within the BSU. The D2-related N-S trending structural and short-wavelength (<1 km) magnetic grain of the BSU is superimposed on ENE-trending long-wavelength (>1 km) magnetic anomalies that are continuous along trend with the regional structures of the West African Craton (WAC) and are interpreted as signals from the basement rocks of the BSU. D2 deformation is interpreted to result from overthrusting of the BSU onto the southeastern margin of the WAC during collision of the WAC and Benino-Nigerian Shield (BNS). Meter-scale, steep and open F3 kink and crenulation folds with steeply SE-dipping axial planes are sporadically observed throughout the entire BSU. D3 deformation is inferred to reflect a period of NW-SE-shortening. Deformation history of the BSU shares structural similarities with the one of the PSZ. We, therefore, interpret the BSU as an allochthonous fold and thrust belt with a geodynamic evolution involving subduction of the WAC beneath the BNS and the ensuing collision between the two terranes. The tectonic framework of the BSU suggests a geodynamic evolution of the Dahomeyide Orogen evolving from initial accretion of oceanic crust and deposition of passive margin sedimentary sequences, and their inversion due to continent-continent collision during the Pan-African orogeny.

Late tectonic reorientation of lineaments and fabrics in the northern Eastern Ghats Province, India: Evaluating the role of the Mahanadi Shear Zone

Abstract South of the MSZ that cuts across the Eastern Ghats Province (EGP) of India, lineament trends and early granulite facies fabrics rotate from NNE-SSW in the south towards the WNW-ESE striking, northerly dipping Mahanadi Shear Zone (MSZ) in the north. Using field, microstructural and EBSD techniques, we demonstrate that D1 and D2 planar fabrics are reoriented by an E-W trending, northerly dipping D3 extensional shear zone that has a subordinate sinistral, strike-slip component. D3 microstructures are pervasive, with quartz and feldspar microstructures characteristic of amphibolite facies metamorphism. Monazite chemical ages from the D3 shear zone yield ages in the range 730–680 Ma. The MSZ, representing D4 deformation in the area, is a predominantly dextral strike-slip deformation. During D4, microstructures evolve exclusively through plastic deformation of quartz and culminates in S4 mylonitic fabric formation in high D4 strains zones under greenschist facies conditions. These microstructures are restricted to the MSZ and localized high D4 strain zones, unlike the D3 microstructures. We conclude that the earlier, post-granulite extensional deformation D3, and not the MSZ, was responsible for the swing in lineament and fabric trends in the northern EGP.

Pan-African structural evolution of Paleoproterozoic basement gneiss and Cu-Co mineralized shear zones in the Domes Region of the Lufilian Belt, Mwombezhi Dome, Zambia

In the Domes Region of the Lufillian Belt, world-class stratiform Cu (±Co, ±Ni, ±U, ±Pb, ±Zn) deposits are closely associated with tectonized pre-existing basement unconformities, viz. the pre-Katanga-basement décollement (KD). In the KD footwall on the Mwombezhi Dome, the Lumwana Mine exploits mineralization within <60 m thick, laterally-extensive shear zones beneath kilometer-scale thrust sheets of Paleoproterozoic gneiss and schist. New field data and structural analysis highlight preferential strain localization, kinematics and sequential reactivation of multiple shear zone generations (D1-3) under high- to medium-grade, metamorphic conditions and clockwise P-T-t path during the Lufillian Orogeny (ca. 550-500). Regional, progressive, top-to-the-NNE fore-thrusting is assigned to D1, while D2 represents localized, opposite, top-to-the-SSW kinematics attributed to under-thrusting by dome-scale fore-thrusted basement slivers. Subsequent D3 shear zones transposed D1-2 structures and represent overall top-to-the-NNE extensional detachment and NNE-SSW-elongation along a maximum principal extension axis (L3) which defines kilometer-scale shear- or asymmetrical-boudins of competent gneiss in the KD footwall. A pre-existing NNE-SSW prolate dome asymmetry (L1-2) dictates a downlimb-directed sense of shear on the short axes of D3 asymmetrical boudins (WNW-ESE). D3 shear zones are preferentially localized along relict pre-Katanga anisotropy and D1-2 mylonitic horizons, while Cu–Co bearing ore shoots are associated with passive fold hinges in these asymmetrical boudin necks.

Kinematics, strain pattern and geochronology of the Salem-Attur shear zone: Tectonic implications for the multiple sheared Salem-Namakkal blocks of the Southern Granulite terrane, India

Structural mapping, strain analysis, and a variety of geochronological studies were carried out to determine the tectonothermal evolution of the Salem-Attur shear zone in the Southern Granulite terrane of South India. The Salem-Namakkal blocks containing the shear zone consisted of quartzofeldspathic gneiss, charnockite and mafic granulite, and had undergone multiple phases of magmatism spanning over a period of 3.2–0.5 Ga. The rocks were deformed by four phases of deformation D1–D4. The D1 deformation was characterized by isoclinal and recumbent NE-SW trending F1 fold with a pervasive subhorizontal axial planar granulitic fabric, S1, and associated quartzofeldspathic leucosomes. Granulite metamorphism was dated at ca. 2.5–2.3 Ga. The F1 fold and S1 fabric were coaxially refolded by tight to isoclinal, upright to steeply inclined NE-SW trending F2 folds during D2 deformation. The D2 deformation was associated with F2 axial planar shear zones, crenulations and leucosomes, S2 fabric. Large-scale D2 shear zones characterized by high-temperature ductile shear fabric with a vertical flow host syntectonic syenite pluton which was dated at ca. 2.5–2.4 Ga. A P-T condition of 7 kb/600 °C was inferred for the D2 deformation. The D3 deformation was characterized by NW-SE to E-W trending F3 folds and the Salem-Attur shear zone. The shear zone was a greenschist to amphibolite facies shear zone being characterized by mylonitic foliation and dominantly down-dip stretching lineation defined by quartz, biotite and hornblende minerals and dated at ca. 2.0 Ga. It indicated N-NNE vergence of thrusting with the mean kinematic vorticity number, Wm, as 0.7 suggesting general simple shear strain with 50% pure shear component. The D4 deformation was manifested as NNE-SSW striking strike-slip faults and NW-SE striking extensional normal faults. Pseudotachylite veins having an age of 1.9 Ga injected during strike-slip faulting and granite-pegmatite veins showing age of 0.8–0.5 Ga intruded during normal faulting. The Salem-Namakkal blocks thus recorded a long-lived shearing history. We suggest that the Salem-Attur shear zone and other shear zones such as Palghat-Cauvery, Moyar, Bhavani, Karur-Kambam-Painavu-Trichur and Achankovil shear zones, were Paleoproterozoic intraterrane shear zones which were overprinted by Meso-Neoproterozoic-Cambrian ductile and brittle deformations.