Late- to post-Variscan tectonics and the kinematic relationship with W-Sn vein-type mineralisation: evidence from Late Carboniferous intramontane basins (Porto-Sátão syncline, Variscan Iberian belt)

Many studies have constrained that late-Variscan buckling produced the arcuate geometry of the Ibero-Armorican belt. Nonetheless, debate remains on the associated geodynamic framework. Poorly studied Late Carboniferous intramontane basins offer an excellent framework to decipher the timing and kinematics of the late- to post-Variscan tectonics. Understanding the latter also helps constrain the structural emplacement mode of contemporaneous W-Sn-Nb-Ta-Li mineralisation. In Iberia, the Porto-Sátão syncline is exemplary of such a Late Carboniferous intramontane basin. We present a structural analysis of the syncline, its basement and the associated W-Sn deposits. The regional structure is dictated by the Alcudian angular unconformity, caused by Cadomian tectonics (575-555Ma) and separating tilted Ediacaran and subhorizontal Lower Palaeozoic formations. Superposed Variscan deformation led to F1-F3 folds with steep and gentle plunges, respectively. The late-orogenic D3 fabric is locally affected by post-orogenic F4 kink folds and a S4 crenulation cleavage. W-Sn bearing vein systems occur along granite-hosted cone sheets, or exploit cross-fold joints associated with the F3 and F4 fold generations, revealing a close kinematic relationship between granite-related mineralisation and the late- to post-Variscan deformation style. This structural history is interpreted as a plate-scale geodynamic change from Late Carboniferous N-S (D3) to Early Permian WNW-ESE (D4) convergence.

Structure of the Golets Vysochaishy gold deposit (Northern Transbaikalia)

The article describes the fold-thrust structure of the Golets Vysochaishy deposit located at the Baikal-Patom Upland in the Marakan-Tunguska megasyncline. The latter is composed of terrigenous-carbonate carbonaceous rocks metamorphosed in greenschist facies conditions. The deposit is detected in the hanging wing of the asymmetric Kamenskaya anticline. In a cross section, the anticline is an S-shaped structure extending in the latitudinal direction. The main feature of the Golets Vysochaishy deposit is the development of interlayer sulfidization zones (pyrite, pyrrhotite), including gold-bearing ones. Its gold-ore zones tend to occur in layered areas of interlayer sliding in the rocks of the Khomolkhinskaya suite.Four structural markers revealed within the deposit area are indicative of repeated deformation processes: (1) sublatitudinal folding, cleavage of the axial surface and its subsequent transformation into schistosity; (2) crenulation cleavage; (3) interlayer sliding and rock breakdown with interlayer drag folds, parallel microfractures and polished slickensides; (4) large quartz veins and veinlets that cross cut the main structural elements in plan.

Structure and geochronology of Sargur schist belt, Western Dharwar Craton, southern India

<p>The Sargur schist belt (SSB) - one of the oldest supracrustal belt (>3.4 Ga) - occurs as discontinuous band along the south-eastern part of Western Dharwar Craton of Indian peninsula. It is a 320 km long belt present in form of lenses, sheets, enclaves, pockets, patches and disrupted layers within the peninsular gneisses, tectonically interleaved, deformed and metamorphosed together with the associated supracrustal rocks (Janardhan et al., 1978; Srikantappa et al., 1984, 1985; Bidyananda and Mitra, 2005; Jayananda et al., 2008). The SSB shows a wide variation in lithology ranging from metapelites, metamafites, metaultramafites, quartzites, calc-silicates etc. with a varying metamorphic grade from greenschist to granulite facies. The major rock types in the study area include garnet-biotite±muscovite±staurolite schist, talc-tremolite-chlorite schist, banded magnetite quartzite, micaceous quartzite, hornblende-biotite±garnet gneiss, amphibolite schist, pyroxene granulites, foliated/deformed granite etc. The fabric in schistose rocks is mainly defined by the shape preferred aggregates of biotite-muscovite (in metapelites) and tremolite-talc-chlorite/amphibole (in metamafites/ultramafites). Whereas the gneissic fabric is defined by the quartzo-feldspathic rich leucocratic layers and biotite-garnet-amphibole-pyroxene rich melanocratic layers.</p><p>In the northern part, the SSB trends roughly N-S but towards the southern part the fabric orientation changes to E-W, whereas the dip is nearly vertical through-out the belt. The belt has undergone at least three phases of deformations. In the northern part the most penetrative fabric is a crenulation cleavage S<sub>1</sub>. The S<sub>1</sub> fabric describes open asymmetric folds having sub-vertical N-S and NNE-SSW axial plane (S<sub>2</sub>). The F<sub>2</sub> fold plunges gentle to moderately towards NNE to SSW. A set of E-W trending shears (S<sub>3</sub>) truncating the S<sub>2</sub> axial zones are zonally developed. In the southern part, as the E-W trending Moyar shear zone approaches, the early fabrics are obliterated or brought into parallelism with the E-W trending penetrative S<sub>3</sub> fabric. U-Th-total Pb dating of texturally controlled metamorphic monazites have yielded mainly two different age peaks at 2.2-2.3Ga and 2.4-2.5Ga with few older ages of ~2.7Ga ages along the northern part while the sample from the southern part (near to the E-W trending Moyar shear zone) gave younger ages ranging from 700-850 Ma and 500-600 Ma.</p><p>From the integration of structural and chronological data the D<sub>2</sub> deformation corresponds to the E-W shortening during the East and West Dharwar Craton accretion is syn- to post-tectonic with respect to the 2.4-2.6 Ga monazite growth. The 700-850 Ma and 500-600 Ma monazite growths post-tectonic with respect to the D<sub>3</sub> deformation indicates that the Neoproterozoic accretionary events affected the whole Southern Granulite Terrain and recrystallize the monazites present in the Moyar shear zone.</p>

Structural Evolution of the Rio das Velhas Greenstone Belt, Quadrilátero Ferrífero, Brazil: Influence of Proterozoic Orogenies on Its Western Archean Gold Deposits

The Quadrilátero Ferrífero region is located in the extreme southeast of the Brasiliano São Francisco craton, Minas Gerais state, Brazil. It is composed of (i) Archean TTG granite-gneaissic terranes; (ii) the Archean Rio das Velhas greenstone belt; (iii) the Proterozoic metasedimentary and metavolcano-sedimentary covers. The Rio das Velhas rocks were deposited in the synformal NW–SE-directed Nova Lima basin. The Archean deformation converted the Nova Lima basin into an ample synclinorium with an eastern inverted flank. Archean orogenic gold mineralization within the Rio das Velhas greenstone belt rocks is controlled by NNW–SSE-directed, Archean regional shear zones subparallel to the strata of the Nova Lima synclinorium borders. Transamazonian and Brasiliano orogenies are superposed onto the Archean structures that control gold mineralization. In the eastern domain, Brasiliano fold-and-fault belts prevail, whereas in the western domain Archean and Transamazonian structures abound. The present study focus mainly is the western domain where the Cuiabá, Morro Velho, Raposos, Lamego and Faria deposits are located. Gold orebodies plunge to the E–NE and are tectonically controlled by the Archean D1–D2 deformation. The D3 Transamazonian compression—Which had a SE–NW vector sub-parallel to the regional mineralized Archean foliation/bedding—Buckled these structures, resulting in commonly open, synformal and antiformal regional folds. These are well documented near the gold deposits, with NE–SW axial traces and fold axes plunging to E–NE. Such folds are normal to inverted, NW-verging, with an axial planar foliation dipping moderately to the SE. The Transamazonian compression has only been responsible for the reorientation of the mineralized Archean gold ores, due to coaxial refolding characterized by an opposite tectonic transport. It has therefore not caused any other significant changes. Thrust shear zones, sub-parallel to the strong Transamazonian foliation, have given rise to localized metric segmentation and to the dislocation of gold orebodies. Throughout the region, along the towns of Nova Lima to Sabará, structures pertaining to the Brasiliano Araçuaí orogeny are represented only by gentle folding and by a discrete, non-pervasive crenulation cleavage. Thrust-shear zones and small-scale normal faults have caused, at most, metric dislocations along N–S-oriented planes.

Effects of tectonically induced fabrics on geomechanical properties of rocks, NW Pakistan

The Chakdara Granitic Gneisses (CGG) of the Indian plate and Kamila Amphibolite of the Kohistan Island Arc (KIA) along the Main Mantle Thrust (MMT) in Shigo Kas, Talash Dir Lower, indicate that tectonically induced foliations and lineations strongly affected the geomechanical properties of these rocks. The earlier S1 crenulated cleavages are well preserved in the microlithon of a well-developed ENE-WSW trending S2 crenulation cleavage. The pervasive S2 foliations, D2 fold axes, and L22 lineations are induced by NNW-SSE horizontal bulk shortening. The core samples obtained parallel and perpendicular to the main ENE-WSW trending S2 and L22 have higher and lower uniaxial compressive strength (UCS) values, respectively. The UCS and uniaxial tensile strength (UTS) average values of four core samples obtained parallel and perpendicular to the main S2 are 51.8 MPa and 12.21 MPa versus 45.65 MPa and 12.45 MPa, respectively. Core samples from the weakly foliated S-2 specimen shows little variation in the UCS and UTS values. The variation in the UCS values in the core samples cut perpendicular and parallel to the main tectonic fabric has been controlled by micro-shear zones at the contact zones of crenulated and crenulation cleavages and sigmoidal mica fish. The UCS values are higher in the core samples parallel to the pervasive S2 and L22 because the parallel shear on the sigmoidal crenulated cleavages in microlithon of the S2 and S2 mica fish counterbalance the parallel external applied load. However, the UCS values decrease in the core samples that were cut perpendicular to the pervasive S2 and L22 because the perpendicular shear on the sigmoidal crenulated cleavages in microlithon of the S2 and S2 mica fish enhances the external applied load, which lead to the failure of core samples.

Thermal and structural history of underplated rocks from subduction initiation to thermal steady state: Easton Metamorphic Suite and related units, WA, USA

<p>Rocks of the Easton Metamorphic Suite and San Juan Islands preserve an inverted metamorphic sequence with ultramafic rocks underlain by amphibolite and high-temperature blueschist juxtaposed above low-temperature blueschists. The sequence is interpreted as a metamorphic sole and younger accreted rocks that formed during and after the initiation of Farallon plate subduction beneath North America in Jurassic time. Thermobarometry, Ar/Ar dating, and structural observations constrain a relatively continuous deformation history and the rheology of rocks during subduction.  The data suggest HT metamorphism and accretion of oceanic crust at the initiation of subduction was followed by rapid cooling, underplating, exhumation, and later underplating and HP/LT metamorphism that persisted for >30 m.y. at a thermal steady state.</p><p>The earliest deformation event in the metamorphic sole at ~10 kbar, 760 °C formed S<sub>1</sub><sup>A</sup> in amphibolite followed by cooling through hornblende closure temperature by 167 Ma. Strain was variable, with high strain in amphibolite interlayered with quartzite and quartz-mica schist and weaker S<sub>1</sub><sup>A</sup> fabric in homogeneous blocks of amphibolite. Metasomatism due to contact with hot hangingwall rocks may have occurred before, during, and after S<sub>1</sub><sup>A</sup>, as locally preserved blackwall assemblages occur at the contact of relatively undeformed amphibolite and ultramafic rocks, but metasomatic assemblages also overprint hornblende-dominated fabrics. Recrystallization during isoclinal folding of amphibolite formed a second fabric (S<sub>2</sub><sup>A</sup>) at 590°C, >165 Ma.  S<sub>2</sub><sup>A</sup> is mylonitic where amphibolite blocks are in contact with quartzite, quartz-mica schist, and tremolite schist; foliation in the schists is discordant to and wraps blocks.  The S<sub>2</sub><sup>A</sup> event overlaps with the earliest metamorphism and strong deformation (S<sub>1</sub><sup>N</sup>) of high-grade Na-amphibole schist at ~530°C, 10 kbar, which cooled below 400°C by 165 Ma. We interpret the Na-amphibole schist to have been underplated as a lower metamorphic sole during this event. Retrograde metamorphism, cooling, and partial uplift to ~350°C, 7 kbar by 157 Ma is evidenced by a crenulation cleavage in the Na-amphibole schist (S<sub>2</sub><sup>N</sup>) during brittle deformation in the amphibolite and metasomatic schist evidenced by glaucophane-filled fractures in hornblende. </p><p>Younger accretion and exhumation events occurred as HP/LT conditions persisted, including underplating of regional phyllite at ~7 kbar, ~320°C from ~154-142 Ma and metavolcanic greenschist-blueschist at ~7 kbar, 360°C at ~140 Ma.  Exhumation to ~5 kbar, ≤300˚C occurred between ~140-125 Ma during later deformation of greenschist-blueschist and underplating of structurally lower metagraywacke and greenstone.  Low-T fabrics are characterized by early pressure solution cleavage followed by tight to isoclinal folding and local shearing with weak to strong recrystallization in the second cleavage.  Strain partitioning at this stage was high, with non-coaxial strain focused in phyllite and flattening fabric dominant in metagraywacke.  No deformation is evident in the high grade rocks at this time, showing the locus of strain had stepped to lower structural levels.  Meso-scale and microstructures throughout the deformation history are consistent with initial high-T deformation and limited rheological differences between lithologies, rapidly followed by weakening of metasomatized rocks and lower-T ductile and ductile-brittle deformation where strong strength contrasts favored strain partitioning into weaker units.</p>

Structural and geological mapping of the Gran Sometta-Tournalin ridge (Aosta Valley, Italy)

<p>We present a new geological and structural map of the Gran Sometta -Tournalin ridge (Valle d’Aosta). In this area we have Pennidic ophiolitic units of the Combin (Co) and Zermatt-Saas (ZS) zones. In addition, in this area the continental cover sequence of the Pancherot-Cime Bianche-Bettaforca (PCB) unit crops out, close to the base of the Combin zone. The PCB and Co are characterized by Alpine greenschist facies assemblages, while the ZS is characterized by eclogitic assemblages. The greenschist and HP complexes are juxtaposed along the extensional Combin Fault Zone.</p><p>Our detailed 1:5000 map allowed reconstructing in 3D, and with a high level of detail, the spatial and crosscutting relationships between metamorphic layering (e.g. calcschists and metabasites in the Co), ductile foliations and shear zones, semi-brittle features (e.g. extensional crenulation cleavage – ECC - along the Combin Fault Zone), and post-metamorphic brittle faults.</p><p>The metamorphic layering and foliations are sub-horizontal in this area, and the ECC associated to the Combin Fault results in large components of horizontal stretching. These features are crosscut by two sets of high-angle normal faults, of Oligocene and Miocene age (according to literature), and, thanks to the favourable exposure and numerous structural data, we have been able to reconstruct these structures and their relationships in 3D.</p>