Estimation of driving pressure ratio and paleostresses from veins in the Pelling-Munsiari shear zone, Sikkim Himalayan Fold Thrust Belt

<p>We analyze veins from the deepest exposure of the regionally folded Pelling-Munsiari thrust (PT), the roof thrust of the Lesser Himalayan duplex, in the Sikkim Himalaya. The PT is exposed as a discontinuous, ~970 m thick quartz-mica mylonite zone near Mangan (27°29′ N, 88°31′ E), and records progressive deformation path where shallow crustal deformation features overprint deeper crustal deformation structures. The mean mylonitic foliation is north easterly oriented in the studied location (mean ~31°, 042°).  Based on the angular relationship with respect to the mylonitic foliation, we recognize three different fracture- and associated vein-sets at the outcrop scale. These are low-angle set (<30° with respect to the mylonitic foliation), moderate-angle (30°-60°) and high-angle set (>60°).The high-angle fracture set overprints the mylonitic foliation and is the youngest set. These are also the most dominant fracture set (~58 %), followed by the moderate-angle (~32%) and low-angle (~10%) sets. Interestingly, the low-angle vein set (mean orientation ~ 29°, 054°) is the most  dominant set (~61%), followed by the moderate-angle set (~26%; mean orientation  ~ 19°,  055°),  and the high-angle set (~13% ; mean ~23°, 340°).Field analysis indicates that ~95% of low-angle, ~71% of moderate-angle and ~ 40% of high-angle fracture-sets form veins. Some of the low- and moderate-angle veins are locally folded along with the mylonitic foliation. The co-efficient of variation (C<sub>v</sub>) of spacing of both the fracture and vein sets are less than 1, indicating that these follow anti-clustered distribution. The poles to the veins indicate two distinct patterns. The low- and moderate-angle veins define girdle distribution, implying pore fluid pressure (P<sub>f</sub>) exceeded intermediate principal stress axis (σ<sub>2</sub>), whereas the high-angle set shows a clustered distribution indicating σ<sub>2</sub> exceeded P<sub>f</sub>. A preliminary study reveals presence of blocky texture in the low- and moderate-angle veins with quartz growing at high angles with respect to the vein walls. The average thickness of the low-angle, moderate-angle, and high-angle veins, measured along appropriate scan-lines are ~ 0.92 cm, ~1.03 cm and ~0.64 cm respectively. As the low- and moderate-angle vein-sets are the most dominant sets and both show girdle distribution, we estimated a driving pressure ratio (R' ~0.35-0.6) and stress ratio (ɸ~0.251) for these veins.  The estimated paleostresses from these veins are σ<sub>1</sub> (28°, 058°), σ<sub>2</sub> (2°, 327°), σ<sub>3</sub> (62°, 233°).</p>

Brittle-ductile deformation in high-pressure continental units and deep episodic tremor and slip

<p>The geological record of deep seismic activity in subduction zones is generally limited due to common rock overprinting during exhumation and only a few regions allow studying well-preserved exhumed deep structures. The Northern Apennines (Italy) are one such area, granting access to continental units (Tuscan Metamorphic Units) that were subducted to high-pressure conditions, were affected by brittle-ductile deformation while accommodating deep tremor and slip and then exhumed back to surface, with only minor retrogression.</p><p>Our approach is based on detailed fieldwork, microstructural and petrological investigations. Field observations reveal a metamorphosed broken formation composed of boudinaged metaconglomerate levels enveloped by metapelite displaying a pervasive mylonitic foliation. Shear veins occur in both lithologies, but are more common and laterally continuous in the metapelite. They are mostly parallel to the foliation and composed of iso-oriented stretched quartz and Mg-carpholite (XMg>0.5) fibres, which are single-grains up to several centimetres long. These fibres define a stretching direction coherent with that observed in the metaconglomerate and metapelite, which is marked by K-white mica and quartz. Thermodynamic modeling constrains the formation of the high-pressure veins and the mylonitic foliation to ~ 1 GPa and 350°C, corresponding to c. 30-40 km depth in the subduction channel.</p><p>Shear veins developed in subducted (meta)sediments are a key indicator of episodic tremor and slip (e.g. <sup>1</sup>). We propose that these structures reflect the repeated alternation of localised brittle failure, with shear veins development, and more diffuse viscous deformation. These cycles were probably related to the fluctuation of pore pressure that repeatedly reached lithostatic values. Concluding, these structures can be considered the geological record of episodic tremors and slip occurring at >30 km of depth in the Apenninic subduction channel.</p><p>1. Fagereng, Å., Remitti, F. & Sibson, R. H. Incrementally developed slickenfibers — Geological record of repeating low stress-drop seismic events? Tectonophysics <strong>510</strong>, 381–386 (2011).</p><p>This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 839779.</p>

Cyclic brittle-ductile behaviour recorded in exhuming high-pressure continental units of the Northern Apennines.

<p>Exhumation of subducted high-pressure units is favoured by relatively narrow, high-strain shear zones, where most metamorphic and deformational processes occur. Unfortunately, these are commonly overprinted and/or partly or fully obliterated along the exhumation path by younger fabrics or by metamorphic re-equilibration. Their identification and characterization are, therefore, of primary importance when aiming at reconstructing the deepest (and thus earliest) tectonometamorphic history of high-pressure crustal units.</p><p>The Northern Apennines (Italy) offer the opportunity to study a unique setting where continental units (Tuscan Metamorphic Units) were subducted to high-pressure conditions and then exhumed and juxtaposed against non-metamorphic units (Tuscan Nappe). We have studied a well exposed section in the Monticiano-Roccastrada Unit of the Mid Tuscan Ridge (MTR), where a mesoscopic (~20 m length and 5 m high) compressional duplex deforms the Palaeozoic-Triassic quartz-rich metasandstones, metaconglomerates and minor metapelites of the Monte Quoio - Montagnola Senese Unit with a top-to-the-NE sense of shear (Arenarie di Poggio al Carpino Formation; Casini et al., 2007).</p><p>Our approach is based on detailed fieldwork, microstructural and petrological investigations. Field observations reveal severe strain partitioning within the duplex between metapelite levels, corresponding to 10-50 cm thick high-strain zones, and metasandstone levels, which form relatively strain-free metric horses. Early generations of quartz veins are highly transposed (sheath folds occur) parallel to the metapelitic high-strain shear zones. Veins are composed of iso-oriented quartz, forming up to several centimetre long single-grain ribbons, Mg-carpholite (XMg~ 0.65) needles and K-white mica marking the stretching lineation. Carpholite in the transposed veins invariably defines the stretching direction of shear zones. These high-P veins coexist with a later generation of less deformed, oblique quartz veins. The mylonitic foliation in the metapelites is defined by quartz, chloritoid, pyrophyllite and K-white mica forming a stretching lineation coherent with the one visible in the veins. Geometrical, cross-cutting and petrographic relations suggest that there has occurred cyclic deformation between brittle and viscous conditions, with the veins forming broadly syn-mylonitic shearing. Thermodynamic modeling results suggest >0.8 GPa and ~350°C for the formation of both the high-pressure veins and the mylonitic foliation.</p><p>Shear zones were subsequently folded about the NNW-SSE axis of the regional antiform associated with the MTR. Later brittle overprinting is represented by quart-filled tension gashes and localized C’ planes, mostly within the more competent metasandstone levels, indicating top-to-the-SW reactivation. In summary, our results suggest a cyclic brittle-ductile behaviour occurring at high pressure conditions. This could potentially reflect the repeated alternation between aseismic creep (viscous) and coseismic slip (brittle) during the first stages of the exhumation history of this portion of the northern Apennines, from lower to middle crustal levels in a compressional top-to-the-NE setting. Dating of K-white mica is ongoing to constrain the geodynamic scenario of such shear zone.</p><p> </p><p>Casini, G., Decandia, F.A., Tavarnelli, E., 2007. Analysis of a mesoscopic duplex in SW Tuscany, Italy: implications for thrust system development during positive tectonic inversion. Geol. Soc. London, Spec. Publ. 272, 437–446.</p><p> </p>

Early Devonian sinistral strike-slip in the Caledonian basement of Oscar II Land advocates for escape tectonics as a major mechanism for Svalbard terranes assembly

<p>The Svalbard’s Southwestern Basement Province in contrary to the Northwestern and Eastern Basement Provinces is commonly correlated with the Pearya Terrane or Timanides and bears a complicated internal structure. Here, we present new data from Oscar II Land supporting the model of Svalbard’s Basement being divided into the Laurentia and Barentsia plates in the late-Caledonian period.</p><p>In Oscar II Land the enigmatic Müllerneset Formation is tectonically juxtaposed against the remaining greenschist facies metamorphosed basement. It consists of Mesoproterozoic to Neoproterozoic metapelites and metapsammites that experienced a polymetamorphic history. The progressive amphibolite facies event M1 of unknown age reached the pressure-temperatures conditions of 5-7 kbar at 500-560 °C. The subsequent greenschist facies overprint (M2) is associated with mylonitization strongly pronounced across the whole Müllerneset Formation. Mylonitic foliation S2 dips steeply to the SW and it is associated with a stretching lineation dipping moderately-to-shallowly to the SE. In the western part of the unit, monazite is growing within the S2 foliation and related shear bands mainly replacing allanite. Th-U-total Pb dating of homogenous monazite population yielded a weighted average age of 410 ± 7 Ma with MSWD = 0.26 and p = 0.997. In the western part, where mylonitic foliation is less prevalent, monazite growths within M1 porphyroblasts and within the S2 foliation. Th-U-total Pb dating revealed an array of ages between 480 – 280 Ma with no correlation of chemical or structural features allowing divisions into subgroups.</p><p>Dating results indicating an early Caledonian signal should be attributed to the progressive M1 event. Uniform monazite age of 410 ± 7 Ma in the western part represents the timing of the M2 greenschist facies overprint. Younger ages obtained in the eastern part suggest fluid related disturbance of Th-U-Pb system during late Caledonian, Ellesmerian and Eurekan events. The timing of monazite growth during the M2 event is identical with the 410 ± 2 Ma <sup>40</sup>Ar/<sup>39</sup>Ar cooling age reported by Dallmeyer (1989). Geochronological evidence combined with structural observations suggests that the Müllerneset Formation in the Early Devonian was tectonically exhumed on the NW-SE trending left-lateral strike- to oblique-slip shear zone. Similarly oriented tectonic zones within the Southwestern Basement Province, in the Berzeliuseggene unit and the Vimsodden-Kosibapasset Shear Zone are also of similar age. This set of anastomosing shear zones is roughly parallel to the proposed orientation of the suture between Barentsia and Laurentia (Gudlaugsson et al. 1998). The documented Early Devonian sinistral displacement may mark the western boundary of the Barentsia microplate laterally extruded during the final Caledonian collision in a style similar to present day Anatolian Plate escape.</p><p>This work is funded by NCN research project no. 2015/17/B/ST10/03114, AGH statutory funds 16.16.140.315 and RCN Arctic Field Grant no. 282546.</p><p>Dallmeyer, R. D. (1989). Partial thermal resetting of<sup> 40</sup>Ar/<sup>39</sup>Ar mineral ages in western Spitsbergen, Svalbard: possible evidence for Tertiary metamorphism. Geological Magazine, 126(5), 587-593.</p><p>Gudlaugsson, S. T., Faleide, J. I., Johansen, S. E., & Breivik, A. J. (1998). Late Palaeozoic structural development of the south-western Barents Sea. Marine and Petroleum Geology, 15(1), 73-102.</p>

A new Miocene detachment from Antiparos Island (Cyclades, Greece)

<p>Numerous studies throughout the world have focused on the structure and evolution of metamorphic core complexes and the exhumation of subducted rocks in back-arc areas of orogenic belts. The Cycladic Islands (central Aegean Sea, Greece) are key areas for studying mechanisms of high-pressure rock exhumation. In that domain, the highly attenuated upper plate is preserved only as sparse extensional allochthons in the hanging wall of crustal-scale detachments. Several detachment systems have been identified on a number of islands indicating overall bivergent extension during the late Oligocene–Miocene. The island of Antiparos is situated at the center of the Cyclades, SW of the larger Paros Island where the top-to-N Paros-Naxos Detachment has exhumed pre-Alpine basement and metamorphosed Permian-Mesozoic rocks of the Cycladic Blueschist Unit (CBU). The tectonostratigraphic relationship of an enigmatic element, the Dryos Unit, remains unclear.</p><p>Detailed mapping in Antiparos revealed the existence of a sub-horizontal normal fault along the eastern coast of the island. This fault juxtaposes CBU in the footwall against the Dryos Unit and scarce (?)late Miocene clastic sediments in the hanging wall.</p><p>The CBU occupies most of the island and consists of marble alternating with schists and gneiss layers. The earlier HP assemblages are totally overprinted by mainly amphibolite facies metamorphism. An axial plane foliation to NE-SW isoclinal folds is accompanied by NE-SW stretching lineation. As indicated by recrystallization of feldspars and high-grade deformation mechanisms these structures formed under amphibolite facies conditions. Towards the detachment the foliation is reworked by a brittle-ductile mylonitic foliation and a brittle-ductile S -C’ fabric can be observed.</p><p>Numerous kinematic indicators such as σ- and δ-clasts, Riedel shears, flanking structures S-C’ fabric, observed within the ultramylonitic rocks of the footwall and the mylonites/cataclasites of the hanging wall indicate top-to-NE sense of shear, comparable to the sense of shear in the Paros-Naxos detachment.</p><p>The Dryos Unit is observed only along the central eastern coast of Antiparos, above the low-angle detachment and comprises lower grade (greenschist facies) metabasite, calc-phyllite and pink marble. Deformation in the structurally upper part is characterized by intense refolding and a steep axial plane foliation. At the structurally lower part a strong mylonitic foliation prevails, overprinted by intense cataclastic deformation. The stretching lineation is mostly NW-SE but in the lower part and towards the detachment it rotates to NE-SW. The late Miocene sediments are found adjacent to the Dryos rocks in two localities, comprising mainly sandstone, mudstone and conglomerate in which, large blueschist clasts are abundant.</p><p>The new data presented in this study combined with existing data from Paros Island substantially add to the continuation and structure of the complex Paros-Naxos detachment system, domed at an island scale. Furthermore, it suggests that most probably the Dryos Unit is not an upper part of the Cycladic Blueschist Unit but belongs to a different unit, possibly of Pelagonian origin.</p>

Gneisses (Serizzo and Beola) of the Verbano–Cusio–Ossola District (Piedmont, northern Italy): possible candidates for designation as Global Heritage Stone Resources

The Verbano–Cusio–Ossola quarrying district (Piedmont, northern Italy) produces many different ornamental stones (granites, gneisses, marbles); two important categories are represented by Serizzo and Beola gneisses. The Serizzo, a group of foliated orthogneisses, is the most important and extensively exploited ornamental stone, largely used to produce columns since the end of fifteenth century and used for many parts of the Duomo di Milano. Beola is the name of a group of heterogeneous orthogneisses with mylonitic foliation and strong mineralogical lineation, easy to split into thin slabs with hammer and chisel, occurring in the middle Ossola Valley. The quarries of Beola are probably the oldest of the Ossola Valley (since the Roman period), and the Beola trade probably started at the end of the thirteenth century. In general, Beola and Serizzo gneisses have been used for ornamental purposes and for the construction of churches, palaces and monuments, widely documented in many towns and villages of the Ossola Valley and in northern Italy. This contribution reviews the history and distinctiveness of these materials, their importance in local and national culture, and their present international diffusion. Both stones are recommended as Global Heritage Stone Resources within a Global Heritage Stone Province.

Neoarchean crustal shear zones and implications of shear indicators in tectonic evolution of Bundelkhand craton, central India

The gneisses and granitoids emplaced along E-W sub-vertical crustal shear zones are represented as important tectonic units in Bundelkhand craton of central India. The tonalite-trondhjemite-granodiorite (TTG) gneisses (3.5-3.2 Ga; oldest unit), and streaky to mafic gneisses structurally deformed in D 1 deformation. The metabasic, felsic, banded iron formation and metasedimentaries of greenstone complex exposed in central part, have characteristics of three sets of folding (F 1 -F 3 ). These gneisses associated with migmatite, amphibolite, quartzite, and schist were evolved in D 2 compressive phase, which are not occurring in northern part of craton. The K-rich Neoarchean granitoids (2.6-2.49 Ga) were intruded as granitic complex (D 3 magmatic phase) and the E-W strike-slip Raksa-Garhmau shear zone reported as important tectonic unit, were evolved in asyn-to post-tectonic D 3 phase. The dolerite dykes (ca. 2.0 Ga) were emplaced along NW-SE fractures in extension setting during D 4 magmatic event. The NE-SW riedel shears occupied by giant quartz veins (reefs) evolved in Paleoproterozoic during D 5 endogenic activity. The relationship between macro and microstructural fabrics has been documented within mylonitic foliation, stretching lineation, S-C planes and rotated fabrics, reflect mesoscopic shear indicators, as noted in three types of mylonitic rocks. i) The rotated porphyroclasts of quartz, feldspars and asymmetric pressure shadows showing strong undulose extinction, deformation lamellae, and dynamic recrystallization are characteristic features of protomylonite where altered orthoclase and kinked plagioclase are noticed. ii) Mylonite, a distinct mylonitic foliation represented by parallel orientation of elongated quartz and feldspar with flakes of mica. iii) The ground matrix of recrystallized quartz with few protoliths of quartz and feldspar are observed, important features of ultramylonite. The asymmetric microstructures viz. σa and σb mantled porphyroclasts, othermicrostructures show progressively deformed by crystal plastic (non-coaxial) strain softening under low to moderate temperature conditions. The sinistral top- to- SW sense of shear movement was dominant. The microfractures/ microfaults, kinking and pull apart structures observed in K- feldspars and are indicative of overprinting of brittle deformation on ductile shearing.

Structural Controls of Ore Mineralization in a Polydeformed Basement: Field Examples from the Variscan Baccu Locci Shear Zone (SE Sardinia, Italy)

The Baccu Locci mine area is located in a sector of the Variscan Nappe zone of Sardinia (the Baccu Locci shear zone) that hosts several type of ore deposits mined until the first half of the last century. The orebodies consist of lenses of Zn–Cu sulphides, once interpreted as stratabound, and Qtz–As–Pb sulphide ± gold veins; the implication of structural controls in their origin were previously misinterpreted or not considered. Detailed field mapping, structural analyses, and ore mineralogy allowed for unraveling how different ore parageneses are superimposed each other and to recognize different relationships with the Variscan structures. The sulphide lenses are parallel to the mylonitic foliation, hosted in the hinges of minor order upright antiforms that acted as traps for hydrothermal fluids. The Qtz–As–Pb sulphide veins crosscut the sulphide lenses and are hosted in large dilatational jogs developed in the hanging wall of dextral-reverse faults, whose geometry is influenced by the attitude of reverse limbs of late Variscan folds. The ores in the Baccu Locci shear zone are best interpreted as Variscan orogenic gold-type; veins display mutual crosscutting relationships with mafic dikes dated in the same district at 302 ± 0.2 Ma, a reliable age for the mineralizing events in the area.

Dating protracted fault activities: microstructures, microchemistry and geochronology of the Vaikrita Thrust, Main Central Thrust zone, Garhwal Himalaya, NW India

The timing of shearing along the Vaikrita Thrust, the upper structural boundary of the Main Central Thrust Zone in the Garhwal Himalaya, was constrained by combined microstructural, microchemical and geochronological investigations. Three different biotite–muscovite growth and recrystallization episodes were observed: a relict mica-1; mica-2 along the main mylonitic foliation; and mica-3 in coronitic structures around garnet during its breakdown. Electron microprobe analyses of biotite showed chloritization and a bimodal composition of biotite-2 in one sample. Muscovite-2 and muscovite-3 differed in composition from each other. Biotite and muscovite 39Ar–40Ar age spectra from all samples showed both inter- and intra-sample discrepancies. Biotite step-ages ranged between 8.6 and 16 Ma and muscovite step-ages between 3.6 and 7.8 Ma. These ages cannot be interpreted as ‘cooling ages’ because samples from the same outcrop cooled simultaneously. Instead, the Ar systematics reflect sample-specific recrystallization markers. Intergrown impurities were diagnosed by the Ca/K ratios. The age data of biotite were interpreted as a mixture of true biotite-2 (9.00 ± 0.10 Ma) and two alteration products. The negative Cl/K–age correlation identified a Cl-poor muscovite-2 (>7 Ma) and a Cl-rich, post-deformational, coronitic muscovite-3 grown at ≤5.88 ± 0.03 Ma. The Vaikrita Thrust was active at least from 9 to 6 Ma at c. 600°C; its movement had ended by 6 Ma.Supplementary material: Thermometric and 39Ar–40Ar data are available at https://doi.org/10.6084/m9.figshare.c.4069076

Lithologies, structure and basement-cover relationships in the schist belt of the Dom Feliciano Belt in Uruguay

ABSTRACT: This work is the result of a multiyear effort to use field geology to describe lithologies, to establish contact relationships and to create a sketch of the tectonic evolution of the Meso- to Neoproterozoic metasedimentary successions within the Schist Belt of the Dom Feliciano Belt. This low-grade metamorphic cover rests on the high-grade metamorphic basement of the La China and Las Tetas complexes. This basement is Archean-Paleoproterozoic in age. The Schist Belt is overlapped unconformably by the Barriga Negra formation. The Lavalleja complex and the Barriga Negra formation both deformed together during the D2 deformation event (~ 570 -540 Ma), but the Barriga Negra only partially recorded the D2 transpressive event, whereas the Lavalleja complex was affected by both the D1 tangential event and the D2 event. Event D1 would have developed a fold nappe with vergence to the south. This hypothesis is supported by different structures: (i) recumbent and upright folds oriented E-W, (ii) subhorizontal mylonitic foliation in marbles (calc-schists), (iii) stretching lineations plunging towards the SW in metaconglomerates of the Las Tetas Complex, and (iv) a reworking of the subhorizontal foliation parallel to the Sarandí del Yí strike-slip shear zone.