Dextral Research Articles

Mesoarchaean to Neoproterozoic (3.2–0.8 Ga) crustal growth and reworking in the Aravalli Craton, northwestern India: Insights from the Pur-Banera supracrustal belt

The Banded Gneissic Complex (BGC) of the Aravalli Craton in Rajasthan, northwestern India, forms the Archaean basement to the major, extensively studied, Proterozoic-age metasedimentary sequences of the Aravalli and Delhi Supergroups. However, the poorly documented deformation history of the basement and other supracrustal sequences, and insufficient geochronological work, limit a holistic understanding of the evolution of the Aravalli Craton. Here we present structural analysis and zircon and monazite geochronology of the relatively poorly studied Pur-Banera supracrustal belt in the Bhilwara region of south-central Rajasthan. The belt comprises paragneisses, carbonates, calc-gneisses, banded iron formations, garnetiferous schists, and amphibolites, deposited over Aravalli Supergroup rocks and locally also granite. A single detrital zircon core from the Pur-Banera paragneisses yields a 207Pb/206Pb age of 3159 ± 8 Ma (1σ), indicating a Mesoarchaean protolith. Magmatic zircon grains from the granite, here termed the Malola Granite, provide a 207Pb/206Pb crystallisation age of 2538 ± 11 Ma (95% confidence). The Pur-Banera supracrustals have experienced three phases of deformation, D1–D3, each of which produced a tectonic fabric. Zircon and monazite geochronology provides complementary ages. The detrital zircons, derived from both magmatic and metamorphic protoliths, provide ages of 3.2–1.7 Ga. Monazite ages of 1.3–0.8 Ga record multiple post-depositional tectonothermal events. Textural and compositional evidence and U–Th–total Pb monazite ages indicate that D1 deformation occurred at ca. 1.30 Ga as a result of NW–SE-directed shortening, forming upright folds on the primary foliation that resulted in a penetrative fabric in the gneisses and schists. Subsequent NNE–SSW-directed deformation (D2) between 1.13 and 1.05 Ga produced WNW–ESE-trending vertical folds in the gneisses and crenulations in the schists. Asymmetrical folds developed due to N–S dextral shearing at 0.85–0.80 Ga constitute the last phase of deformation (D3). The Pur-Banera supracrustal rocks probably represent a rift-fill sequence, of Mesoproterozoic age (thus temporally equivalent to the Delhi Supergroup), deposited on an Archaean and Palaeoproterozoic basement, and repeatedly reworked during 1.30–0.80 Ga.

Structures, Deformation Mechanisms and Tectonic Phases, Recorded in Paleoproterozoic Granitoids of West African Craton, Southern Part: Example of Kan’s Complex (Central of Côte d’Ivoire)

The granito-gneissic complex of Kan is located in the central part of the Paleoproterozoic domain of Cote d’Ivoire. It consists essentially of migmatitic and mylonitic gneisses with basic intrusions and xenoliths. This Proterozoic domain belongs to the Man Leo shield, southern part of West African craton (WAC). The present study, essentially based on a structural analysis at outcrop scale, aims to identify deformation mechanisms and tectonic phasesrecorded in the granito-gneissic complex of Kan. Deformation mechanisms include: (1) flattening, (2) constriction, (3) simple shear (4), rotation (5), brittle shear, and (6) extension. The Kan complex deformation occurred during four major tectonic phases named D1, D2, D3 and D4. D1 corresponds to WNW-ESE compression. It led to the formation of NS to NNE-SSW foliation, of stretching lineation, and of folds with sub-horizontal axes. It is accompanied by N170° and N10° sinistral shear zones, which constitute globally a NS major transcurrent shear zone in the central part of Cote d’Ivoire. D1 is also marked by N90° dextral shear zones. Tectonic phase D2 is associated with EW compression. It is marked by N50° dextral and N110° sinistral transcurents shear zones. D3 corresponds to NNE-SSW compressive phase and is responsible of N110° crenulation cleavage formation. (D4) constitutes a brittle deformation phase. It correspondsto posteburnean deformation in the Proterozoic crust of Cote d'Ivoire. Generally, these deformation phases are similar to D2 and D3 reported in the Man-Leo shield and that are part of regional collisional phase referred to as Eburnean orogeny in the WAC.

Ductile and brittle deformation in Singapore: A record of Mesozoic orogeny and amalgamation in Sundaland, and of post-orogenic faulting

Singapore bedrock geology is dominated by late Permian to Triassic arc magmatism and a genetically related, essentially Middle to Upper Triassic, marine to fluvial volcano-sedimentary inner forearc succession. These Mesozoic strata are deformed into a pattern of NE-translated ductile–brittle deformation structures during the latest Triassic to earliest Jurassic collision and amalgamation of the Sibumasu continental block with the southern part of the Sukhothai Arc. The subduction-related magmatic complex represented in Singapore by the granitic to gabbroic plutons of the Bukit Timah Centre likely acted as a backstop to thrusting at this time. Collisional tectonics drove progressive shortening and steepened earlier-formed inclined asymmetrical folds, culminating in the regional-scale development of a non-coaxial, NE-vergent and NE-facing, fold and thrust system. In Singapore, the Murai Thrust and Pasir Laba Thrust are identified as major elements of this system; both are associated with SW-dipping thrust-imbricate duplex slices. Two distinct early Cretaceous (Berriasian and Barremian) sedimentary successions overstep these collisional tectonic structures. An array of mostly NE–SW and ENE–WSW trending faults and fractures acts as an important control on bedrock unit distribution across Singapore and are most likely generated by Cenomanian dextral shear stress. That stress locally reactivated faults initiated during orogeny, or even earlier. Knowledge of the geotechnical impact of these structural features is critical to both future development and ongoing management of the subsurface in Singapore.

40Ar/39Ar age of rock deformations across the Bazhenov suture zone (eastern border of the Middle Urals)

Research subject. This article presents data on the nature of rock deformations in the Bazhenov suture zone. The data was obtained by 40Ar/39Ar dating of mica samples from schists and blastomylonites, which were collected acrossan area extending approximately for 100 km along the Bazhenov suture. This suture separates the Eastern zone of the Middle Urals dipped under the cover of the West Siberian plate from exposed geologic structures of the Urals.Methods.The character of the deformations was studied by means of direct geological observation. The age of the deformations was determined by mica dating using the 40Ar/39Ar method.Results. The deformations were found to have occurred in several phases. The initial phase, which led to the formation of a band of blastomylonites and rocks showing a varying degree of schistosity with a width of over 10 km in particular areas, is likely to have had the character of left-lateral fault. The upper age limit of this deformation phase is determined by the intrusion of the subalkaline rocks of the Petukhovskii complex (280 Ma), which had not been affected by any deformation processes. The subsequent type of deformations had been left-lateral strike-slip faults of submeridianal strike and subvertical dip, which were represented by low-thickness (usually about 10 cm, rarely up to 0.5 m) zones of intensely deformed rocks cutting the schistosity of the previous phase at an acute angle. The final phase of the deformations is shown to be a sloping fault. The dislocation planes of this type were represented by slickensides and chlorite incrustations. Despite the apparent polychronicity of the deformation processes, which were established within the Bazhenov suture zone by our geological observations, 40Ar/39Ar age dating identified only one event. The average value of five conducted tests was about 251 Ma. Apparently, this age should be associated with the most recent geological processes having occurred at temperatures sufficient to fully overload the K-Ar isotopic system of the studied rocks, such as the deformations caused by strike-slip dislocations.Conclusions. The strike-slip fault dislocations dated in this work occurred approximately 25 Ma after the completion of the tectonic activity in the Serov-Mauk fault zone, which is located to the west of the Bazhenov zone. In addition, the time of these dislocations very nearly coincides with that of the formation of the grabens of meridional strike at the base of the West Siberian plate, which took place about 250 Ma ago. This supports apreviously made conclusionon the similar character of the geological development of the eastern border part of the Urals and the adjoining basement of the West Siberian plate.

Strain Pattern Analysis of Mylonites From Sitampundi-Kanjamalai Shear Zone, Thiruchengode, South India

This study aims to investigate the petrography and strain pattern of mylonites from parts of N-S trending Sitampundi-Kanjamalai Shear Zone (SKSZ) around Thiruchengode. The petrographic study indicates the presence of recrystallized quartz, K-feldspar, plagioclase, biotite and some hornblende. The kinematic analysis of Mylonites was done with the help of shear sense indicators such as recrystallized type quartz (quartz ribbon) around the cluster of feldspar, S-C fabric shows dextral shear sense and some sinisterly shear sense in some parts of SASZ which can be considered as a product of partitioning of both strain and vorticity between domains. These all indicates the simple shear extension along E-W direction and the mylonitic foliation shows the pure shear compression along N-S direction. Further the study of bulk strain analysis by Flinn plot method using L and T section of mylonite shows k<1 which lies in the field of flattening zone of finite strain. The kinematic vorticity number is calculated by Rxz/β method which gives the value of 0.36 indicating the general shear. The rigid grain graph shows that the pure shear component is more ­­­­dominant than the simple shear component. The analysis leads to the conclusion that the mylonite has experienced a high temperature shearing of above 700°cat deep crustal level.

Mesozoic evolution of the eastern Pamir

AbstractWe present field and analytical results from the Tashkurgan and Waqia valleys in the southeastern Pamir that shed new light on the tectonic evolution and terrane architecture of the region. Field mapping of metasedimentary and igneous units along the Tashkurgan and Waqia valleys in the Southeast Pamir, integrated with metamorphic petrology, garnet-biotite thermometry, and zircon U/Pb isotopic analysis, help identify major structures and terrane boundaries in the region, as well as compare structural units across the Miocene Muztaghata gneiss dome. South of the Muztaghata dome, the gently northwest-plunging synformal Torbashi thrust klippe juxtaposes amphibolite facies Triassic Karakul-Mazar terrane schist and gneiss structurally above (1) greenschist facies Triassic Karakul-Mazar terrane metasedimentary rock in the north, and (2) lower-amphibolite facies schist in the south that are interpreted to be Gondwanan-derived crust (Central or South Pamir terrane). Farther south, the Rouluke thrust fault imbricates the Gondwanan crust, placing early Paleozoic schists over Permian marble and slate. Exposure of the Torbashi thrust sheet terminates in the southeast, and with it the surface exposure of the Triassic Karakul-Mazar terrane, leaving the Paleozoic Kunlun terrane juxtaposed directly against Gondwanan terrane crust. Based on lithologic and isotopic similarities of units north and south of the Muztaghata gneiss dome, we document the existence of a regionally extensive thrust nappe that stretched across the northern and eastern Pamir, prior to being cut by Miocene exhumation of the Muztaghata dome. The thrust nappe links the Torbashi thrust in the southeast Pamir with the Tanymas thrust in the northern Pamir, and documents regionally extensive exposure of lithologically continuous units across the northeast Pamir. While timing of emplacement of the Torbashi thrust klippe and displacement on the Rouluke fault to the south is not well constrained, we interpret shortening to be Cretaceous in age based on previously published cooling ages. However, a component of Cenozoic shortening cannot be ruled out.A key observation from our mapping results is that the surface exposures of the Karakul–Mazar–Songpan Ganzi terrane are not continuous between western Tibet and the Pamir, which indicates tectonic and/or erosional removal, likely sometime in the Mesozoic. Furthermore, our documentation of the Jinsha suture in the southeast Pamir on the eastern side of the Karakoram fault shows deflections of terranes across the Himalayan-Tibetan orogen were not primarily accommodated along discrete, large displacement faults (>400 km) faults. Instead, oroclinal bending of the northern Pamir, and dextral shear along the Pamir margins, may be largely responsible for the northward deflection of terranes.

An Analysis of Distributed Strike‐Slip Shear Deformation of the Qaidam Basin, Northern Tibetan Plateau

AbstractThe interior structure of the Qaidam Basin records kinematic information related to the regional deformation of the northern Tibetan Plateau. Based on balanced cross sections and depth structure maps covering two individual anticlines in the western Qaidam Basin, we calculate shortening and strike‐slip amounts on the structures. Strain analyses reveal a complex deformation mechanism for them, comprising ~7°–10° clockwise rotation and oblique convergence, with block rotation having contributed more to the sinistral slip than has oblique convergence. Considering the similarities in geometry, kinematics, and onset time of the pervasive northwest trending structures within the basin, we propose that they were developed during distributed dextral shear that has affected the Qaidam Basin since the early Miocene. The bulk shear strikes at ~N55°E, which is consistent with the northeastward decrease in gravitational potential energy away from the central Tibetan Plateau, and is facilitated by the movement on the Altyn Tagh and eastern Kunlun faults.

Extensional magmatism in a continental collision zone,Tafresh area, western central Iran: structural, geochemical and mineralogical considerations

The Tafresh area, located on the middle part of Urumieh-Dokhtar magmatic arc, is a suitable area for considering the effects of tectonic structures on development of the volcanic facies in the magmatic provinces. The main structure in the area is a dextral fault (so called Rahjerd Fault) that has produced the local extensional basins and also the parallel dyke swarms (or feeder dykes) on its both sides. So, the pyroclastic deposits, basaltic-andesitic lava flows, parallel dyke swarms were appeared by explosive subaqueous and then effusive subaerial eruptions and eventually a dioritic stock intruded in the volcanic pile. Petrologically, the magmatic rocks are belonged to the calcalkaline suite that had been changed and evolved in the magma chambers of local extensional basins in a collisional tectonic setting. The mineral chemistry and geochemical modeling as well as the coexistence of different mineral paragenesis of plagioclases (An47-72 and An17-37), pyroxenes (diopside and pigeonite) and amphiboles (magnesiohastingsite and magnesiohornblende) reveal that the AFC process is a dominant process in the magma chamber. Also, according to geothermobarometric calculations, the investigated volcanic rocks of the area could be grouped into two types: one type with higher P-T (>6Kbar and about 900 °C) including Eocene pigeonite-magnesiohastingsite bearing andesites (PHA) and Miocene andesites and other with lower P-T (1-4 Kbar and 600- 800 °C) including the Eocene diopside-magnesiohornblende bearing andesites (DHA) and the dioritic stock.

Repeated megaturbidite deposition in Lake Crescent, Washington, USA, triggered by Holocene ruptures of the Lake Creek-Boundary Creek fault system

AbstractLake Crescent, a 180-m-deep, glacially carved lake located on the Olympic Peninsula in western Washington, USA, overlies the Lake Creek-Boundary Creek fault zone, a system of structures with at least 56 km of late Pleistocene to Holocene surface rupture. Investigation of the lake’s sediment, including a reflection seismic survey and analysis of piston cores, reveals evidence that the fault beneath the lake has ruptured four times in the past ∼7200 years, producing unusually thick deposits termed megaturbidites. The earthquakes triggered rockslides that entered the lake and caused displacement waves (lake tsunamis) and seiches, most recently ca. 3.1 ka. Seismic reflection results from beneath the depth of core penetration reveal at least two older post-glacial ruptures that are likely to have similarly affected the lake. The stratigraphy of Lake Crescent provides insight into the behavior of a fault system that partially accommodates regional clockwise rotation and contraction of the northern Cascadia forearc through oblique dextral shear, and highlights the potential for disruption to critical infrastructure, transportation corridors, and industry on the North Olympic Peninsula during future surface-rupturing earthquakes. Our results illustrate the potential synergism between lacustrine paleoseismology and fault-scarp trench investigations. More precise dating of strong earthquake shaking afforded by continuous accumulation of lake sediment improves earthquake histories based on trenched fault scarp exposures, which are commonly poorly dated.

Geometric and tectonic analysis of Ad-Damm mega-scale fold: implication of Neoproterozoic Transpressive Regime in the west-central Arabian Shield

A mega-scale fold is exposed near Jeddah city in the western-central part of the Arabian Shield. This fold extends NE for about 35-km long and 12-km width and occupies the northwestern block of a distinct linear feature known as Ad-Damm Shear Zone. It is composed essentially of foliated, sheared, and mylonitized metamorphic and plutonic rocks. Para and ortho-gneisses were an amphibolite facies condition whereas greenschist facies units were represented by metasedimentary and metavolcanics. Image processing and field-based structural analysis, in addition to micro structural investigations of the mega-scale fold, indicating that the ductile dextral-sense of shear was overprinted by a brittle regime associated with the evolution of Ad-Damm shear zone. The Ad Dam mega-scale fold is a shear-related structure, north plunging overturned anticline. Mylonitization and dextral shear-related structures were developed during a transpressional regime. The Ad-Damm fold is a structural system that illustrates the evolution history of the Arabian Shield associated with the development of the NE-wrench fault. This fault started as an early plastic followed later by a brittle tectonic regime. The present study indicates that Ad-Damm shear zone and several sub-parallel dextral shears of NE-trends which located in the western side of the Arabian Shield represent a conjugate antithetic shear to the NW- trending sinistral shear of the Najd fault system.

Metamorphic evolution of the Loma Marcelo skarn within the geotectonic context of the crystalline basement of the Ventania System (Argentina)

This study describes the mineralogical and isotopic changes that carbonate xenoliths experienced under multiple metamorphic events and hydrothermal fluid circulation during the evolution of the Ventania System basement. The high reactivity of carbonate xenoliths allowed the preservation of mineral assemblages corresponding to at least three metamorphic events in the resulting Loma Marcelo skarn. The Ventania System basement is composed of Neoproterozoic granites and ignimbrites, Early Cambrian granites, and Middle Cambrian rhyolites. Carbonate xenoliths were incorporated during the intrusion of a calc-alkaline granite with an LA-ICP-MS U-Pb crystallization age of 621.6 ± 2.2 Ma. The intrusion induced pyroxene–hornfels facies metamorphism in the carbonate xenoliths and the associated metasomatism generated calcic and magnesian skarns depending on the protolith composition. Garnet, clinopyroxene, wollastonite, bytownite, and meionite were formed in the calcic skarn (CaS), whereas forsterite and spinel were formed in the magnesian skarn (MgS). Crystallization of Early Cambrian alkaline granites was accompanied by intense hydrothermal activity that was responsible for low temperature (≤300 °C) F-metasomatism in the skarn, as evidenced by the presence of F-rich vesuvianite (CaS) and chondrodite (MgS), among other minerals. Vesuvianite was formed from calc-silicate mineral assemblages of the previous metamorphic event, whereas chondrodite was formed by replacement of forsterite. The low temperature formation of these typical high-grade minerals could be an evidence of mineral formation under disequilibrium conditions favoured by the high reactivity of hydrothermal fluids. Neopalaeozoic basement mylonitization under greenschist facies metamorphism was accompanied by hydrothermal fluid circulation. This event promoted extreme mobility of chemical elements in the basement rocks and epidotization (CaS) and serpentinization (MgS) in the Loma Marcelo skarn. The elongated and boudinaged shape of the skarn bodies, parallel to the mylonitic foliation, is a consequence of dextral shearing that affected the basement rocks. Additionally, almost pure grossular crystallized post-tectonically in the CaS. Carbonates of the Loma Marcelo skarn are depleted in 13C and 18O (δ13CV-PDB = −2.5/−10.1‰; δ18OV-SMOW = +7.3/+14.0‰) relative to carbonate sedimentary rocks. The δ13C and δ18O variations can be attributed to the interaction between large amounts of hydrothermal fluids (W/R = 30–50) and Neoproterozoic carbonate sedimentary rocks.

ADRIA AS PROMONTORY OF AFRICA AND ITS CONCEPTUAL ROLE IN THE TETHYS TWIST AND PANGEA B TO PANGEA A TRANSFORMATION IN THE PERMIAN

It has been almost 60 years since the first results from the Early Permian Bolzano Quartz Porphyries from the Trento Plateau of northern Italy (Southern Alps) showed paleomagnetic inclinations steeper than inclinations from broadly coeval units from central Europe. This experimental discrepancy, confirmed ever since at varying levels of magnitude and certitude, implied that northern Italy had paleolatitudes too northerly relative to Europe to be considered part of the European continent. On the other hand, it became progressively more apparent that paleomagnetic data from northern Italy were more compatible with data from Africa than with data from Europe, and this observation revived and complemented Argand’s original concept of Adria as a promontory of Africa. But if Adria was part of Africa, then the paleolatitude anomaly of Adria relative to Europe translated into a huge crustal misfit of Gondwana relative to Laurasia when these landmasses were forced into a classic Wegenerian Pangea as typified by the Bullard fit of the circum-Atlantic continents. This crustal misfit between Gondwana and Laurasia was shown to persist in the ever-growing paleomagnetic database even when data from Adria were provisionally excluded as non-cratonic in nature. Various solutions were offered that ultimately involved placing Gondwana to the east (allowing it to be more northerly) relative to Laurasia and envisaging a dextral shear occurring in the Tethys (Mediterranean) realm between these supercontinental landmasses. This shear or transformation was initially thought to occur as a continuum over the course of the Mesozoic–Cenozoic (the so-called ‘Tethys Twist’) but soon afterwards when plate tectonics came into play and limited the younger extent, as a discrete event during the post-Triassic, Triassic or most probably – as in the latest and preferred reconstructions – the Permian between a configuration of Pangea termed B – with the northwestern margin of Africa against southern Europe – to a configuration termed Pangea A-2, with the northwestern margin of Africa against eastern North America, that is more proximal in shape to the classic Pangea A-1 that started fragmenting in the Jurassic with the opening of the Atlantic Ocean. The Permian timing and presumed locus of the ~2300 km dextral shear is supported by rotated tectonic domains in Sardinia and elsewhere along the interface between Lauarasia and Gondwana. The concept of Pangea B and its transformation into Pangea A developed therefore in close conjunction with the concept and paleomagnetic support of Adria as a promontory of Africa, and has ramifications to many aspects of tectonics, climate change and biogeography yet to be explored.

Active Convergence of the India‐Burma‐Sunda Plates Revealed by a New Continuous GPS Network

AbstractThe Rakhine (Arakan)‐Bangladesh megathrust, along which the Indian and Burma plates collide, is assumed by some to be inactive/aseismic due to the lack of notable interplate earthquakes in the modern instrumental catalog. However, geological and historical evidence of the great 1762 Arakan earthquake suggest the megathrust can produce M~8 events that could adversely affect the lives of millions of people in the region. To investigate the seismogenic potential and determine the slip budget of the megathrust, we first need to solve for India‐Burma‐Sunda relative plate motions. We present a new set of 24 GPS velocities (2011–2017) from the Myanmar‐India‐Bangladesh‐Bhutan continuous GPS network. We use the new velocities and those from previously published studies to explore the geometries and slip rates of three major faults (Rakhine‐Bangladesh megathrust, Churachandpur‐Mao Fault, and Sagaing Fault) that accommodate the India‐Burma‐Sunda plate motion. Our results suggest that the three major faults we studied are likely fully coupled; the modern shortening rate across the Burma plate is 12–24 mm/year, while the total dextral shear rate is 25–32 mm/year. The possibly fully coupled shallow megathrust, and splay faults that may sole into it, are geodetically invisible while they are not slipping. However, we can identify the transition from coupling to steady creep on the deeper extension of the megathrust; we use this to show active oblique India‐Burma convergence and to map along‐strike and along‐dip variations in dip‐slip and strike‐slip motion. This implies that the megathrust is currently accumulating strain which will eventually be released in earthquakes.

Dikes Deformation in Lakhshak Pluton: Microscopic Evidence from the Northeast of Zahedan, Southeastern Iran

Based on geological classification of Iran, Oligocene Lakhshak pluton is situated in the Sistan suture zone that composed of siliciclastic and ophiolitic melanges. The Lakhshak pluton is a host of numerous northeast dikes intruded the siliciclastic sediments. The pluton is granodiorite, granite, diorite, quartz diorite in composition. The dikes occur along shear zones and show moderate to high deformations. Dikes are mainly diorite and granodiorite in composition and consist of plagioclase, biotite, hornblende and quartz. The deformation evidences are quartz microboudines, mica fish (especially in biotite), and fractures in minerals, inflation in quartz, asymmetric porphyroclasts, preferred orientation and rotation of minerals. These evidences show that the dikes suffered from brittle to ductile deformation. Dynamic recrystallization of quartz and feldspar grains is mainly similar as bulge type. Mylonitic structures in the dikes probably formed between 300 and 500°C. Oriented thin sections, three-dimensional block diagram and field studies show dextral shears in dikes.

Geochemistry and geochronology of orthogneisses across a major transcurrent shear zone (East Pernambuco shear zone, Borborema Province, Northeast Brazil): Tectonic implications

The East Pernambuco shear zone (EPSZ) is a dextral shear zone bounding the Rio Capibaribe and Pernambuco-Alagoas domains of the Neoproterozoic Borborema Province (NE Brazil). This study reports new geological data from orthogneisses across the central segment of the EPSZ. Two samples from the Bezerros orthogneiss to the north of the EPSZ yielded 207Pb/206Pb weighted mean ages of 2111 ± 7 Ma and 2104 ± 8 Ma. The c. 2.1 Ga age is taken as the age of crystallization of the protolith. It is undistinguishable from the age of the 2.13–2.09 Ga-old Vertentes Complex, which is the main Paleoproterozoic unit of the Rio Capibaribe Domain, and of orthogneisses that occur immediately south of the EPSZ. Like the Vertentes Complex, the Bezerros orthogneiss has a subduction-zone geochemical signature (e.g., pronounced negative Nb-Ta anomalies), suggesting intrusion in a magmatic arc setting. South of the EPSZ, a migmatized granitic orthogneiss and the Encruzilhada de São João orthogneiss yielded 206Pb/238U weighted mean ages of 645 ± 3 Ma and 639 ± 5 Ma, respectively. Ages of 615 ± 17 Ma and 618 ± 12 Ma obtained at the rims of zircon grains from Encruzilhada de São João and migmatized granitic orthogneisses, respectively, are interpreted as dating the acquisition of the low-angle gneissic fabric. The age of the EPSZ is bracketed by the 591 ± 4 Ma age of a monazite grain from the Bezerros orthogneiss and by the of 575 ± 17 Ma age of a young zircon grain in the Encruzilhada de São João orthogneiss. Rather than being a terrane boundary, the EPSZ more likely represents an intracontinental shear zone that nucleated at preexisting crustal heterogeneities.

Deformation-driven emplacement-differentiation in the Closepet pluton, Dharwar Craton, South India: an alternate view

Abstract In the Late Archean north-trending Closepet pluton, trains of euhedral K-feldspar phenocrysts and matrix-supported idiomorphic K-feldspar crystals in the central part of the pluton define oblique-to-pluton margin steep-dipping east/ENE-trending magmatic fabrics. The magmatic fabric is defined by phenocryst-rich and phenocryst-poor layers, with the euhedral porphyries continuous across the layers. The fabrics are near-orthogonal to the gently-dipping gneissic layers in the host gneisses. The fabrics curve adjacent to locally-developed north/NNE-trending melt-hosted dislocations parallel to the axial planes of horizontal/gently-plunging north-trending upright folds in the host gneisses. In the pluton interior, both fabrics in the intrusives formed at supra-solidus conditions, although the volume fraction of melts diminished drastically due to cooling/melt expulsion. At the pluton margin, the north-trending fabric is penetrative and post-dates magma solidification. Within the pluton, the major element oxides, rare earth elements, anorthite contents in plagioclase, and (Mg/Fe + Mg) ratios in biotite decrease with increasing SiO 2 from phenocryst-rich (up to 75% by volume) granodiorite to phenocryst-poor (<15 vol%) granite that broadly correspond to minimum melt composition. The chemical-mineralogical variations in the pluton is attributed to deformation-driven ascent of magma with heterogeneous crystal content, ascending at variable velocities (highest in crystal-poor magma) along oblique-to-pluton margin east/ENE-trending extensional fractures induced by dextral shearing.

New Insights on the Role of the Strike-Slip Tectonics in the Late Miocene-Quaternary Evolution of Sicily

Understanding the influence of the Late Miocene-Quaternary opening of the Tyrrhenian Basin on the evolution of the external sectors of the Africa paleo-margin in Sicily, actually, represents a hard challenge, even though several, and contrasting, models have been proposed in the last decades. One open problem refers to localizing the main regional scale tectonic lineaments of Sicily that accommodated the hundreds of kilometers of lateral displacement, due to the Tyrrhenian Basin opening. In the present work, we present the results rising from a detailed field mapping carried out in relevant vast areas of central Sicily. These data enabled to reconstruct a Neogene-Quarternary kinematic evolutionary model of the collision belt of Sicily. We analyzed the northern tectonic margin of the Caltanissetta Through, which represents a structural depression hosting a thick allochthonous tectonic wedge, on which lay different unconformable thrust-top basin deposits. In more details, our study aims to reconstruct the tectono-sedimentary Late Tortonian-to-Quaternary evolution of this tectonic wedge, revealing that regional E-W-trending dextral shear zones deform and cut the NE-oriented, SE-verging, thrust-and-fold belt. The strike-slip tectonics thus controlled the deposition of different sedimentary cycles on the thrust-top basins and governed the tectonic inversion of the external sectors of the Africa paleo-margin.

The geologic record of the exhumed root of the Central African Orogenic Belt in the central Cameroon domain (Mbé – Sassa-Mbersi region)

We present structural, petrological, geochemical and geochronological data compiled in a new geological map on the high-grade metamorphic rocks exposed in the Mbé – Sassa-Mbersi region located along the Tcholliré-Banyo Shear Zone, at the northern edge of the Central Cameroon domain.The region exposes a complex assemblage of metabasic (hornblendite, metagabbro, amphibolite) and metasedimentary (garnet paragneiss, calc-silicate gneiss) rocks alternating with ubiquitous migmatitic intermediate to felsic gneisses. U-Pb ages on detrital zircon in paragneiss point to the contribution of Archean, Paleoproterozoic and Neoproterozoic sources, and suggest a maximum deposition age of 725 ± 12 Ma for their sedimentary protolith. The protolith of some amphibolites was emplaced during the Neoproterozoic as indicated by U–Pb ages on magmatic zircon at 600 ± 4 and 599 ± 6 Ma. Plutonic rocks with I-type chemical signatures were emplaced discontinuously over a period of ca. 80 Ma. The magmatic evolution started with the intrusion of magnesian medium-K tonalites and trondhjemites at 651 ± 3 Ma, followed by magnesian high-K hornblende-biotite granites and ferroan shoshonitic biotite granites until 599 ± 2 Ma. Finally, pegmatite dykes were emplaced along dextral shear zones at 573 ± 2 Ma. The metamorphic rocks show a penetrative shallow-dipping foliation partially to totally transposed in a NE–SW trending steep-dipping foliation bearing a shallow-dipping lineation. These ductile syn-migmatitic fabrics, developed under high-grade metamorphic conditions in the presence of melt, between the metamorphic peak at ca. 1.3 GPa–770 °C, and the following isothermal decompression down to ca. 0.8 GPa. In situ U–Pb dating of metamorphic zircon rims from a garnet migmatitic paragneiss yields an age of 582 ± 4 Ma. Localization of high-grade deformation is indicated by the development of the Tcholliré-Banyo Shear Zone, characterized by mutually crosscutting subvertical and shallow-dipping mylonitic to ultramylonitic shear zones associated with sinistral and top-to-the SW kinematic criteria, respectively. These data indicate that the Central Cameroon domain exposed in the Mbé – Sassa-Mbersi region, represents the exhumed mid-to lower part of the former orogenic root of the Pan-African Central Africa Orogenic Belt that has undergone partial melting, lateral flow and intrusion of mafic to felsic calc-alkaline magmas between 650 and 580 Ma. The waning stage of the Pan-African orogeny is marked by the emplacement of syntectonic pegmatitic granite at ca. 575 Ma in subvertical E-W trending retrogressive dextral shear zones.

The Jbel Saghro Au(–Ag, Cu) and Ag–Hg Metallogenetic Province: Product of a Long-Lived Ediacaran Tectono-Magmatic Evolution in the Moroccan Anti-Atlas

The Jbel Saghro is interpreted as part of a long-lived silicic large igneous province. The area comprises two lithostructural complexes. The Lower Complex consists of folded metagreywackes and N070–090°E dextral shear zones, which roughly results from a NW–SE to NNW–SSE shortening direction related to a D1 transpressive tectonic stage. D1 is also combined with syntectonic plutons emplaced between ca. 615 and 575 Ma. The Upper Complex is defined by ash-flow caldera emplacements, thick and widespread ignimbrites, lavas and volcaniclastic sedimentary rocks with related intrusives that were emplaced in three main magmatic flare ups at ca. 575, 565 and 555 Ma. It lies unconformably on the Lower Complex units and was affected by a D2 trantensive tectonic stage. Between 550 and 540 Ma, the magmatic activity became slightly alkaline and of lower extent. Ore deposits show specific features, but remain controlled by the same structural setting: a NNW–SSE shortening direction related to both D1 and D2 stages. Porphyry Au(–Cu–Mo) and intrusion-related gold deposits were emplaced in an earlier stage between 580 and 565 Ma. Intermediate sulfidation epithermal deposits may have been emplaced during lull periods after the second and (or) the third flare-ups (560–550 Ma). Low sulfidation epithermal deposits were emplaced late during the felsic alkaline magmatic stage (550–520 Ma). The D2 stage, therefore, provided extensional structures that enabled fluid circulations and magmatic-hydrothermal ore forming processes.

Tectonostratigraphy-volcanic of Gajah-Ijo-Menoreh Tertiary volcanic formations in Kulon Progo mountain area, Yogyakarta-Indonesia

Kulon Progo Mountains areas are formed by three Tertiary volcanic bodies that formed during Oligocene-Miocene times. Each volcano shows different tectonic control and stratigraphic result. This research conducted through interpretations of volcanic bodies, construction of stratigraphic columns, construction of geological cross-sections, supported by lithological-structural data from the field and some previous researchs. There are four volcanism periods in Kulon Progo Mountains, namely Late Eocene Volcanism, Early Oligocene Volcanism/Gajah, Late Oligocene Volcanism/Ijo and Late Miocene Volcanism/Menoreh. Gajah shape shows there are collaps volcano at the southern part and it was happened before Ijo emerge. Volcanism of the Eocene-Gajah-Ijo period is related to the Palaeogene extensional rifting period. Ijo volcano covers the southern part of Gajah Volcano. Menoreh volcano emerges at Late Miocene and covers Gajah volcano and Jonggrangan Formation. Menoreh Volcanism is related to the period of Neogene Compressional Wrench that produces a sinistral fault with left-stepping left lateral fault pattern.