Sinistral Strike-slip Shear Zones Research Articles

On the crustal framework of the central Alto Pajeú Terrane (Borborema Province, NE Brazil): Geophysical and structural data constrain late Neoproterozoic transpression in Western Gondwana

The structural framework of the Alto Pajeú Terrane, Borborema Province (NE Brazil) records a complex tectonic evolution related to the Neoproterozoic amalgamation of Western Gondwana. Combined, multiscale geophysical and litho-structural datasets for the Central Borborema Province constrain the regional crustal architecture. Magnetic and radiometric lineaments mostly follow NE-SW and E-W trends that correlate to strike-slip shear zones and associated subvertical mylonitic foliations. Regional, outcrop-scale and microstructural kinematic indicators consistently assign sinistral and dextral motion sense for NE-SW and E-W shear zones, respectively. Four sectors with contrasting litho-geophysical and structural patterns are juxtaposed across major shear zones: the Alto Pajeú, Alto Moxotó and Piancó-Alto Brígida terranes, plus a small portion of the Northern Borborema Province. Expressive magnetic lineaments highlight the boundary between the Alto Pajeú and Alto Moxotó terranes, represented by a pair of sinistral strike-slip shear zones. At outcrop scale, relic thrust surfaces (top-to-NW vergence) with associated flat-lying foliation and down-dip mineral stretching lineations affecting Tonian orthogneisses and metapelites disclose a contractional episode of uncertain age. Transposition of the shallowly-dipping foliation in highly sheared domains imply it is older than the shear-related deformation. The shear zone system and associated structures record the transpressional regime that characterizes the late stages of the Brasiliano/Pan-African Orogeny in the region (ca. 575–565 Ma). Microstructures of a mylonitic orthogneiss from the Juru-Belém shear zone reveal greenschist to amphibolite facies deformational conditions, with increasing importance of subgrain rotation mechanisms at higher strain rates and/or temperature. Microstructural similarities between this rock and tectonites along the southern Patos lineament suggest that bordering and internal regions of the Alto Pajeú terrane were subject to similar thermodynamic conditions during the late Brasiliano transpression.

Sinistral strike-slip shearing along the Jiali shear zone around the Eastern Himalaya syntaxis region: Evidences for oligocene eastward limited translation of Tibet

In and around the Eastern Himalaya Syntaxis (EHS), the metamorphic complex and its structural boundaries are key tectonics to investigate the kinematics and thermal evolution of the continental collision. We provide new structural, kinematic and geochronological data along the northeastern boundary of the EHS, namely the Jiali strike-slip shear zone. Structures and various kinematic indicators record sinistral strike-slip shearing in the zone. Based on the microstructural features, crystallographic preferred orientation patterns of quartz and their slip systems, coupled with Raman Spectroscopy of Carbonaceous Materials (RSCM) geothermometry, the ductile sinistral shearing fabrics formed at upper greenschist and amphibolite facies (>500 °C). Two hornblende, one muscovite and five biotite separates from the mylonites in the shear zone were dated by the 40Ar/39Ar method. A hornblende plateau age of 23.7 Ma is interpreted as representing or being close to the time of sinistral movement. Muscovite and biotite 40Ar/39Ar ages from 22.9 to 16.9 Ma represent cooling ages following sinistral shearing. It is concluded on the basis of these data that the Jiali sinistral strike-slip shear zone was active in latest Oligocene. This sinistral movement on the Jiali shear zone is considered to link with the Ailao Shan-Red River shear zone, and form a unique, continuous structural boundary around the EHS. The Jiali-Ailao Shan-Red River sinistral shear zone and Gaoligong dextral shear zone formed a huge conjugate strike-slip shear pair around the EHS to accommodate the N–S shortening corresponding to northward indentation of the folded Indian continent. The conjugate strike-slip shear systems also contribute to transporting Tibet southeastward in a limited scale.

Description and hierarchy of ductile deformation events in the Camalaú region, state of Paraíba, central portion of the Alto Moxotó Terrane, Borborema Province, Brazil

The Borborema Province (NE Brazil) has a long history of deformation, magmatism, and metamorphism during the Precambrian Eon. Thus, it represents a natural laboratory to investigate widely developed ductile-deformation markers, such as major fold belts and large-scale shear zones. This paper describes the structural evolution of a complex folded area of the Alto Moxoto Terrane, Central Borborema Province. In this region, para-derived rocks of the Sertânia Complex are intruded by orthogneisses and migmatitic orthogneisses. The Geophysical magnetometric expressions of the Borberma Province are characterized by strong aligned and folded magnetic lineaments and integrated structural analysis allowed us to identify three ductile deformation episodes. Brittle tectonics is also present, but not described. D1 stage represents the basement structural framework restricted to local structural windows of ancient orthogneisses and migmatites that are not mappable at the working scale. D2 deformation is widespread throughout the Central Borborema Province, producing nappes that mark top-to-NW tectonic vergence. Foliation and lineation attitudes are compatible with progressive deformation from tangential to strike-slip tectonics (i.e. D2 to D3). The latter is associated with the NE-SW Xinxo and Congo-Cruzeiro do Nordeste sinistral strike-slip shear zones, producing refold patterns that resemble Type-3 interference geometry. Based on airborne magnetic geophysical imaging interpretations and mesoscopic observations, we suggest that progressive deformation strongly affected the region and overprinted at large-scale early formed rocks.

Timing of Paleozoic Exhumation and Deformation of the High-Pressure Vestgӧtabreen Complex at the Motalafjella Nunatak, Svalbard

The Vestgӧtabreen Complex exposed in the Southwestern Caledonian Basement Province of Svalbard comprises two Caledonian high-pressure units. In situ white mica 40Ar/39Ar and monazite Th-U-total Pb geochronology has resolved the timing of the tectonic evolution of the complex. Cooling of the Upper Unit during exhumation occurred at 476 ± 2 Ma, shortly after eclogite-facies metamorphism. The two units were juxtaposed at 454 ± 6 Ma. This was followed by subaerial exposure and deposition of Bullbreen Group sediments. A 430–400 Ma late Caledonian phase of thrusting associated with major sinistral shearing throughout Svalbard deformed both the complex and the overlying sediments. This phase of thrusting is prominently recorded in the Lower Unit, and is associated with a pervasive greenschist-facies metamorphic overprint of high-pressure lithologies. A c. 365–344 Ma geochronological record may represent an Ellesmerian tectonothermal overprint. Altogether, the geochronological evolution of the Vestgӧtabreen Complex, with previous petrological and structural studies, suggests that it may be a correlative to the high-pressure Tsäkkok Lens in the Scandinavian Caledonides. It is suggested that the Vestgӧtabreen Complex escaped to the periphery of the orogen along the sinistral strike-slip shear zones prior to, or during the initial stages of continental collision between Baltica and Laurentia.

A „Kapos-vonal” középső szakaszának szerkezeti-mélyföldtani viszonyai és neotektonikai jellegei a legújabb geofizikai vizsgálatok tükrében

Tanulmányunkban a Kapos-vonal középső szakaszának legújabb geofizikai vizsgálatainak eredményeit tekintjük át. Munkánkkal tavaly ősszel elhunyt Kollégánk, Barátunk, Tanárunk és Mesterünk — Prof. Dr. Horváth Ferenc — emléke előtt tisztelgünk. A kutatási területen elérhető korábbi és újonnan mért 2D/3D reflexiós szeizmikus és fúrási adatok integrációjával komplex adatbázist alakítottunk ki, amelynek értelmezésével földtanilag és geometriailag egyaránt konzisztens, új 3D földtani-tektonikai modellt hoztunk létre. A modell a vizsgált terület földtani felépítésének alapvonásait szem előtt tartva öt meghatározó jelentőségű földtani horizont (prekainozoos aljzat-tető, alsó-miocén tető, középső-miocén tető, Endrődtető, Algyő-tető) és az értelmezett vetők téradatrendszerét tartalmazza. A kialakított 3D modell alapján elvégeztük a korábbi vizsgálatokkal nyert földtani-szerkezeti kép felülvizsgálatát, és azt szükség szerint módosítottuk, illetve pontosítottuk/kiegészítettük.A kutatás mélyföldtani szempontból legfontosabb eredményei közé tartozik a kristályos és mezozoos aljzat képződmények szerkezeti, elterjedési és kifejlődési viszonyainak reambulációja a lemélyült új szerkezetkutató fúrások és a szeizmikus adatrendszer integrált értelmezése alapján. Ugyancsak alapvető jelentőségű a markáns kora-miocén riftesedés kimutatása, amely során intenzív, többfázisú mészalkáli vulkáni működés, illetve egyidejű kontinen tális üledékképződés zajlott a létrejött, helyenként akár 2 km mélységű süllyedékekben. Mindez tágabb kontextusban a Pannon-medence korábban kevésbé ismert (és hangsúlyozott) kora-miocén kialakulási szakaszának jelentőségére hívja fel a figyelmet. A vetőtérképezés eredményei alapján a szűkebb kutatási terület meghatározó vetőzónái a KÉK–NyDNy-i csapású „Kapos-vonal”, továbbá az ÉK–DNy-i csapású Dunaszentgyörgy–Harta és az ezen kutatás során először térképezett Bonyhádi vetőzóna, amelyeket mind meredek dőlés (≥60–70°) jellemez. A Bonyhádi vetőzóna délnyugati irányban a Mecsek északi pikkelyzónájához csatlakozik. A terület szerkezeti képét összességében a fentiekkel többnyire (közel) párhuzamos törések uralják, míg az előbbiekre kb. merőleges ÉÉNy–DDK-i csapású, szerkezetileg kevésbé jelentős törések főként a Kapos-vonaltól délre jelentkeznek.Eredményeink alapján geometriai-szerkezeti értelemben egységes Kapos-vonal nem létezik: e vetőzóna a kutatási területen ugyanis egy nyugati („Kapos-Ny”) és egy keleti szegmensre („Kapos-K”) bontható, amelyek mind dőlés irányban, mind az észlelt neotektonikus aktivitásban — összhangban a korábbi szakirodalmi adatokkal — markánsan különböznek. A vizsgált területen élénk neotektonikus aktivitást mutató Kapos-K, Dunaszentgyörgy–Harta és Bonyhádi vetőrendszerek regionális léptékben egy ÉK–DNy-i és KÉK–NyDNy-i csapású elemekből felépülő, széles nyírási zónát körvonalaznak, amely a neotektonikus fázisban balos eltolódásként működött. Ezt igazolja az egyes vetőzónákban megfigyelt tipikus „virágszerkezetet” mutató belső struktúra, továbbá a kapcsolódó másodlagos formaelemek (Riedel-törések) is.A kialakított 3D modell eredményeinek figyelembevételével telepített nagyfelbontású 2D és pszeudo-3D sekélygeofizikai vizsgálatok, valamint az ezek eredményei alapján kijelölt árkolás és sekélyfúrások adatai bizonyították a Dunaszentgyörgy–Harta vetőzóna negyedidőszaki, sőt az árkolás eredményei szerint késő-negyedidőszaki aktivitását, hiszen a kutatóárokban feltárt késő-pleisztocén végi futóhomok tektonikus eredetű deformációja jelentkezett. A fiatalnegyedidőszaki tektonikai aktivitást a kutatási területen számos további korábbi, illetve a jelen kutatáshoz kapcsolódó, de e tanulmányban nem részletezett neotektonikai megfigyelés is alátámasztja. Mindezen eredmények a szak irodalomban a Kapos-vonal keleti szakaszáról (Duna–Tisza köze és Tiszántúl) közölt tektonikai adatokkal és értelmezésselteljes összhangban állnak.A tanulmányunkban ismertetett eredmények a Paks II földtani kutatási programjának (FKP) keretében 2015–2016 során elvégzett földtani-geofizikai kutatásokhoz kapcsolódnak.

Structural analysis of the central and southwestern parts of the Neoproterozoic Salalah Crystalline Basement, Sultanate of Oman

The Salalah Crystalline Basement (SCB) is the largest Precambrian exposure in Oman located on the southern margin of the Arabian Plate at the Arabian Sea shore. It is thought to represent one of the eastern-most exposures of the Arabian-Nubian Shield. We used detailed structural analysis to establish the deformational history of the SCB by focusing on its central and southwestern parts. This part of the SCB is dominated by the Juffa complex (820 ± 10 Ma) which is made-up of two-mica gneisses, amphibolites and ultramafic lenses, and the Sadh complex (816 ± 12 Ma) which is composed of biotite and amphibolite gneisses. Our work found that the SCB evolved through four deformational events that shaped its final architecture: (1) Folding and thrusting event that resulted in the emplacement of the Sadh complex atop the Juffa complex. This event resulted in the formation of possibly N-verging nappe structure. Associated with this event was the development of mesoscopic asymmetrical tight folds and thrusts with various orientations because of folding during subsequent event. However, the dip of these structures fluctuates between SE and SW suggestive of an initial N-verging nappe structure; (2) Regional folding event around SE- and SW-plunging axes that deformed the regional fabric developed during the N-verging nappe structure. This event produced map-scale SE- and SW-plunging antiforms resulting in shaping the Juffa and Sadh complexes into a semi-dome structure; (3) Strike-slip shearing event that produced a conjugate set of NE-trending sinistral and NW-trending dextral strike-slip shear zones. The NW-trending dextral strike-slip shear zones dissected the northeastern part of the Juffa-Sadh semi-dome whereas the NE-trending sinistral strike-slip shear zones dissected the southwestern part of the semi-dome. Strong mylonitic fabric, stretching lineation and associated kinematic indicators were developed during this event; and (4) Localized SE-directed extension manifested by top-to-the-southeast kinematic indicators and the development of stretching lineation in the form of stretched amphibole crystals overprinted on older planar fabric. This is particular apparent in the hinge of the map-scale SE-plunging antiform.

Structures, kinematic analysis, rheological parameters and temperature-pressure estimate of the Mesozoic Xingcheng-Taili ductile shear zone in the North China Craton

The NE to ENE trending Mesozoic Xingcheng-Taili ductile shear zone of the northeastern North China Craton was shaped by three phases of deformation. Deformation phase D1 is characterized by a steep, generally E–W striking gneissosity. It was then overprinted by deformation phase D2 with NE-sinistral shear with K-feldspar porphyroclasts forming a subhorizontal low-angle stretching lineation on a steep foliation. During deformation phase D3, lateral motion accommodated by ENE sinistral strike-slip shear zones dominated. Associated fabrics developed at upper greenschist metamorphic facies conditions and show the deformation characteristics of middle- to shallow crustal levels. In some parts, the older structures have been in turn overprinted by late-stage sinistral D3 shearing. Finite strain and kinematic vorticity in all deformed granitic rocks indicate a prolate ellipsoid (L-S tectonites) near plane strain. Simple shear-dominated general shear during D3 deformation is probably of general significance. The quartz c-axis textures indicate prism-gliding with a dominant rhomb <a> slip and basal <a> slip system formed mainly at low-middle temperatures. Mineral deformation behavior, quartz c-axis textures, quartz grain size and the Kruhl thermometer demonstrate that the ductile shear zone developed under greenschist facies metamorphic conditions at deformation temperatures ranging from 400 to 500 °C. Dislocation creep is the main deformation mechanism at a shallow crustal level. Fractal analysis showed that the boundaries of recrystallized quartz grains had statistically self-similarities. Differential stresses deduced from dynamically recrystallized quartz grain size are at around 20–39 MPa, and strain rates in the order of 10−12 to 10−14 s−1. This indicates deformation of granitic rocks in the Xingcheng-Taili ductile shear zone at low strain rates, which is consistent with most other ductile shear zones. Hornblende-plagioclase thermometer and white mica barometer indicate metamorphic conditions of medium pressures at around ca. 3–5 kbar and temperatures of 400–500 °C within greenschist facies conditions. The main D3 deformation of the ENE-trending sinistral strike-slip ductile shearing is related to the roll-back of the subducting Pacific plate beneath the North China Craton.

Extent, kinematics and tectonic origin of the Precambrian Aswa Shear Zone in eastern Africa

The Aswa Shear Zone (ASZ) is a fundamental Precambrian lithospheric structure that has been shaped by many tectonic events in eastern Africa. It separates the Saharan Metacraton in the northeast from the Northern Uganda terrane (which represents part of the Northeastern Congo block of the Congo craton) to the southwest. Nonetheless, its tectonic evolution is not fully understood. We used high-resolution airborne magnetic and radiometric data over Uganda integrated with Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) in South Sudan to assess the extent, kinematics and contribute to the understanding of the tectonic origin of the ASZ. (1) Our results showed that the ASZ extends in a NW–SE direction for ~550km in Uganda and South Sudan. (2) The airborne magnetic and radiometric data revealed a much wider (~50km) deformation belt than the 5-10km of the exposed surface expression of the ASZ. The deformation belt is defined by three NW-trending sinistral strike-slip shear zones bounding structural domains with magnetic fabric showing splays of secondary shear zones and shear-related folds. These folds are tighter close to the discrete shear zones with their axial traces becoming sub-parallel to the shear zones. A similar fold pattern is observed in South Sudan from the SRTM DEM. We interpreted these folds as due to ENE–WSW contraction associated with the sinistral strike-slip movement. (3) To the northeast, the magnetic patterns and radiometric signatures suggest the presence of a series of W-verging nappes indicative of strong E–W to NE–SW contraction deformation. (4) We relate the evolution of the ASZ to E–W to NE–SW Neoproterozoic oblique collision between East and West Gondwana. The deformation related to this collision was partitioned into E–W to NE–SW contraction resulting in W-verging thrusts in the east and a sinistral strike-slip movement along the NW-trending ASZ with the strain localized at the boundary between the Saharan Metacraton and the Northern Uganda terrane.

Tectonic evolution and provenance of the Santa Bárbara Group, Camaquã Mines region, Rio Grande do Sul, Brazil

Cu- and Pb–Zn-hosting sedimentary units of the upper part of the Camaquã Basin (Ediacaran-Lower Ordovician) in the Dom Feliciano Belt of southernmost Brazil were formed during the late stages of the West Gondwana amalgamation and were controlled by large left-handed strike-slip shear zones. Integration of structural geology, stratigraphy and thermochronology allow recognition of five structural events (D1, Ediacaran-Lower Cambrian, through D5, Cretaceous). D1 structures are related to a N30E-trending, sinistral strike-slip shear zone that controlled the deposition of the mineralized sedimentary unit and its overlying units, the Santa Barbara and Guaritas Groups, respectively, in a transtensional setting. Based on U–Pb in situ methods, it is possible (a) to establish a maximum depositional age of 566 ± 6.9 Ma for the basal section of the Santa Barbara Group and, therefore, a minimum age of ca. 566 Ma for D1, and (b) to recognize two main zircon populations, Neoproterozoic and Paleoproterozoic, with sources from the eastern and southern parts of the Dom Feliciano Belt and reworking of older units of the Camaquã Basin. The D2 structures are mainly N-trending shear zones that developed after the deposition of the Guaritas Group during the Cambrian. During the Phanerozoic (post-Cambrian), the recognized structures were connected to compressional and extensional events that affected West Gondwana and the South America Platform. Thermochronological fission track analyses on apatite revealed four main age populations. The first three are interpreted to have formed during tectonic processes at the Gondwana Margin, namely the Famatinian and Gondwanides orogenies, and can be related to the D3 and D4 tectonic events in the basin. The last age population formed from thermal heating by the Upper Cretaceous continental flood basalts, which are represented in the area by volcanic intrusions, that were related to the separation of Africa and South America.

On the occurrence of gold mineralization in the Pala Neoproterozoic formations, South-Western Chad

The Pala region, in southwestern Chad, belongs to the northern part of the Central African Pan-African Fold Belt. It is made up of greenschist-facies schists and is characterized by bimodal, mainly mafic, magmatism. This schist unit named Goueigoudoum Series is intruded by pre- to post-tectonic plutonic rocks dated between 737 and 570Ma and dykes of quartz. Gold is mined artisanally from alluvial deposits and primary chalcopyrite–pyrite-bearing quartz veins, brecciated and silicified zones and shear zones. The majority of the mineralized shear zones and some quartz veins generally trend N–S to NNE–SSW or NW–SE and are interpreted as extensional shear fractures related to regional NE–SW-trending sinistral strike–slip shear zones. The geological context of the Pala region clearly indicates hydrothermal fluids formed along active continental margins during collisional orogenesis, and subsequent associated fluid migration typically occurred during strike–slip events. Although the origin of fluids may be varied (magmatic, metamorphic or meteoric fluids, Proterozoic seawater, or sedimentary basin formation waters), the distribution of the mineralizations along the granitoid intrusions suggests that magmatism played a major role in the dynamics of the mineralizing fluids.

Caracterização geoquímica e estrutural do Granodiorito Cruzeiro do Sul: magmatismo shoshonítico pós-colisional neoproterozoico em zona de transcorrência, região de Quitéria, RS

The Cruzeiro do Sul Granodiorite is located in the Eastern portion of the Sul-rio-grandense shield. It forms one ENE-WSW elongate body of approximately 4 km² within a sinistral strike slip shear zone. The rocks were originally considered as part of the Arroio dos Ratos Complex, but they were recently mapped as a separate unit because igneous textures are widely preserved and they are intrusive in the gneissis complex rocks. The Cruzeiro do Sul Granodiorite is thus a porphyritic horblende-biotite granodiorite in a mafic-rich (M' ~ 20), medium-grained matrix. The magmatic foliation is marked by the shape alignment of feldspar megacrystals and mafic minerals. A solid-state, mylonitic structure is well-developed mainly near the contacts. Leucocratic veins are found both concordant and discordant to the main foliation, and they may give rise to a composite banding within high-strain zones, together with elongate mafic microgranular enclaves and synplutonic dykes of tonalitic to dioritic composition. The mylonitic foliation trends EW to ENE-WSW with medium to steep dip, and contains a shallow plunging stretching lineation, which indicates a transcurrent kinematic for this shear zone. Structural evidences, such as the parallelism and progressive evolution of mylonitic foliation relative to the magmatic one, as well as the elongated form of granodioritic bodies and the concordant character of planar structures with the ENE shear zone, are interpreted as an indication of syntectonic emplacement. Moreover, the absence of metamorphic foliation reworked by mylonitic shows that the Cruzeiro do Sul Granodiorite registers only one phase of deformation, which would have occurred during their emplacement. Thus, we have concluded that the Cruzeiro do Sul Granodiorite has no relation with the EW transversal event, and low dip angle responsible for the deformation of Arroio dos Ratos Complex, but is linked to the transcurrent event of Dorsal de Canguçu Shear Zone. The structural and compositional features of Cruzeiro do Sul Granodiorite magmatism are consistent with its post-collisional character, and the shoshonitic affinity is given by its high Sr contents, regular rare earth element chondrite-normalized patterns and the abundance of light rare earth regarding the heavy rare earth elements. Patterns of trace elements with enrichment in Ba and Rb, and impoverishment of High-Field Strength in relation to Large Ion Lithophile Elements is also an important feature of the rocks of shoshonitic affinity. Their medium- to high-K content, and metaluminous to slightly peraluminous characters are possibly indicative of some crustal contamination during their differentiation. The conditions of P and T, calculated with the geobarometer Alt-Hb and Plg-Hb thermobarometer were estimated at about 4.3 to 5.3 kbars and crystallization temperatures on the order of 720 to 760°C. The feldspar microstructures suggest deformation temperatures consistent with the amphibolite facies.

Ediacaran high-pressure collision metamorphism and tectonics of the southern Ribeira Belt (SE Brazil): Evidence for terrane accretion and dispersion during Gondwana assembly

The Curitiba Terrane represents a major segment of the southern Ribeira Belt (SE Brazil), which was derived from the collision between the São Francisco, Congo, Paranapanema and Luís Alves Cratons during the Neoproterozoic (Brasiliano/Pan-African Orogeny). The tectonic setting and the metamorphic records of two major juxtaposed units from the Curitiba Terrane, a Neoproterozoic shallow continental-shelf metasedimentary assemblage (Turvo-Cajati Formation) and an Archaean to Paleoproterozoic TTG-type orthogneiss assemblage (Atuba Complex), were investigated. Migmatitic paragneisses from the Turvo-Cajati Formation underwent a deep collision metamorphism. Conventional geothermobarometry and petrological modelling in the system NCKFMASHTi indicate peak metamorphic conditions between 670 and 810 °C at 9.5–12 kbar. Metamorphic paths calculated from zoned garnet and plagioclase using the Gibbs method of differential thermodynamics indicate distinct evolution for two major groups of migmatites from the Turvo-Cajati Formation: (i) kyanite migmatites evolved from low-temperature eclogite to high-pressure granulite facies conditions following near isobaric heating; (ii) sillimanite migmatites underwent near isothermal decompression and apparently evolved from high-temperature eclogite facies conditions. Chemical dating of monazite indicates that the peak metamorphism of the Turvo-Cajati Formation occurred at 589 ± 12 Ma, followed by a greenschist facies metamorphic overprint at 579 ± 8 Ma related with late transcurrent shear zones. 40Ar– 39Ar biotite ages indicate that the Turvo-Cajati Formation cooled below 250–300 °C at 555 ± 4 Ma. P– T data and petrological evidence of rocks from the Atuba Complex suggest a retrograde metamorphic path with cooling from 730 to 630–650 °C at 6–7 kbar. Available K–Ar and 40Ar– 39Ar data indicate that the Atuba Complex had cooled to below 300–500 °C between ca. 590 and 580 Ma. Geochronological data indicate that the main metamorphism of the Turvo-Cajati Formation and the Atuba Complex are coeval, but very contrasting metamorphic signatures reflect formation in different parts of a collisional suture. The integration of structural and petrological data indicates that the structural pattern of the Curitiba Terrane is related to Ediacaran westward directioned nappes during the late- to postmetamorphic period. This is concomitant with a main, crustal-scale, strike-slip regime, dominant throughout the Ribeira Belt. The nappe stack was later deformed by cylindrical folds with E–W trending sub-horizontal axes parallel to the synthrusting stretching lineation and was dismembered and dispersed by late sinistral strike-slip shear zones. The late tectonic assembly of the Ribeira Belt was controlled by significant postcollision terrane dispersion along major strike-slip shear zones.

Geochemistry of Ferriferous, High-K Calc-Alkaline Granitoids from the Banefo-Mvoutsaha Massif (NE Bafoussam), Central Domain of the Pan- African Fold Belt, Cameroon~!2009-11-12~!2010-01-05~!2010-03-11~!

The ferriferous high-K calc-alkaline rocks of the Banefo-Mvoutsaha Massif, Pan-African Belt, Central Cameroon, were synkinematically emplaced in a sinistral strike-slip shear zone of Pan-African age. The rock sequences consist of orthogneises made of coarse-grained granites, quartz-monzonites, and medium- to fine-grained granodiorites, ranging from ca. 55 to 75 wt.-% SiO2. They display characteristics of shoshonitic and high-K calc-alkaline series. Orthogneises are metaluminous to peraluminous and show characteristics of I-type granitoids from a ferriferous series. Trace element distribution patterns reveal rocks enriched in LILE when compared to HSFE and distinctively depleted in Th, Nb, Ba, Sr, Ti and Ta. The data indicate that this granitic rock assemblage did not result from the simple differentiation of a common parental magma, but show that Banefo-Mvoutsaha plutonic rocks were derived from different crustal protoliths. Major and trace element composition are consistent with the magmatism which may have involved remelting of (1) a composite metagreywackes protolith in the upper crust and (2) amphibolitised high-K calc-alkaline basaltic andesites in the central domain of the PANEFB (Pan-African North Equatorial Fold Belt). The plutonic rocks of Banefo-Mvoutsaha area resemble other Neoproterozoic high-K calc-alkaline syntectonic plutons in Western Cameroon. They also display strong similarities with high-K calc-alkaline plutons of the Pernambuco shear zone in NE Brazil and Eastern Nigeria.

Petrogenesis of peraluminous magmas from the Akum-Bamenda Massif, Pan-African Fold Belt, Cameroon

Peraluminous intrusives of the Akum-Bamenda Massif, Pan-African Belt, Central Cameroon, were synkinematically emplaced in a Pan-African sinistral strike-slip shear zone. The rock sequences consist of medium-grained leucogranites, fine-grained leucogranites, and orthogneisses of biotite granite composition; in aggregate, they cover a range from about 65 to 74 wt.% SiO2, defining a continuous chemical evolutionary trend and displaying characteristics of the high-K and medium calc-alkaline series. Leucogranites are strongly peraluminous (A/CNK > 1.1) and plot in the field of S-type granites, whereas orthogneisses are metaluminous and plot in the field of I-type granitoids. Major and trace element compositions and the Rb/Sr isotopes of the leucogranites indicate crustal derivation by remelting of a composite metapelite + metagreywacke protolith similar to the metasedimentary rocks of the central domain of the Cameroon Pan-African North-Equatorial fold belt.

Manifestations of the Late Carboniferous and Early Permian stages of formation of nappe-fold structures in the southern framework of the Siberian platform (East Sayany, South Siberia)

The geological structure of the Tunka Goltsy (the Tunka Range) of the East Sayany is characterized by a complex nappefold structure, composed mainly of Paleozoic terrigenous and carbonate rocks and their metamorphosed analogues (1-3). It is generally rec� ognized that the nappefold structure of the East Sa� yany, including its southeastern segment, regarded as the Tunka terrain (3) or Ilchirskaya zone (4), formed in the Ordovician as a result of collision between the Tuva-Mongolian microcontinent and the Siberian continent. As referred to in (5), the Ordovician-Mid� dle Paleozoic deformations over the entire vast terri� tory of Central Asia, from the Olkhon zone of the Pribaikalie to the North Kazakhstan, were manifested as a result of the closing of the oceanic basin and the subsequent collision between the Kazakhstan- Baikalian complex continent (including the Tuva- Mongolian microcontinent) and the Siberian conti� nent. In the Ordovician the Olkhon nappeoverthrust zone was formed along the southeastern framework of the Siberian Craton. In addition, the metamorphism was manifested over the entire vast territory of the East Sayany that could probably be connected with nappe formation. In the Late Ordovician-Silurian, the oblique slipthrust structures, magmatism, and meta� morphism were manifested in the Sangilen highlands and Tuva. Later, the deformations continued. In the Late Devonian-Early Carboniferous, the dextral strikeslip fault Charysh-Terektinskaya zone was formed; in the Late Carboniferous, the Kurayskaya and Kuznetsko-Teletsko-Bashkaus sinistral strike� slip shear zones were formed. In the Late Carboniferous-Permian, the collision between the East European, Kazakhstan-Baikalian, and Siberian continents took place (5, 6). In East Kazakhstan, the sinistral strikeslip movements took place in the band of more than 400 km width along the Chara ophiolite zone and the Irtysh and Northeastern shear zones. The age of deformation rejuvenates regu� larly to the east towards the internal part of the Sibe� rian continent; the movement amplitude along shears decreases in the same direction from several thousand to a hundred kilometers. In the late Palaeozoic in the Tunka Goltsy and the adjacent regions, a number of geological events of Late Carboniferous and Early Permian age are distin�

The Sha’it–Nugrus shear zone separating Central and South Eastern Deserts, Egypt: A post-arc collision low-angle normal ductile shear zone

The northerly dipping Sha’it–Nugrus shear zone (SNSZ) is the boundary separating the Central Eastern Desert from the South Eastern Desert of Egypt. The hangingwall of this shear zone is composed of low-grade metavolcanics and ophiolitic nappes of the Central Eastern Desert, while the footwall consists of South Eastern Desert high-grade metapsammitic gneisses (Migif-Hafafit gneissic complex). The SNSZ is about 700 m thick and represents the shear foliated lower parts of the hangingwall and upper parts of the footwall. A significant part of the SNSZ has been truncated by a later normal fault along Wadi Sha’it, however the SNSZ is well-preserved along Wadi Nugrus. Features of the SNSZ include shear-related schistosity (termed Ss), mylonite zones, sheared syn-kinematic granitoid intrusions, diverse metasomatism and metamorphic effects (higher T overprinting of hangingwall lithologies and retrogression of footwall lithologies). Shear-sense indicators clearly show top-to-N or NW displacement sense. SNSZ structures overprint arc collision related nappe structures (∼680 Ma) and are therefore post-arc collision. SNSZ syn-kinematic intrusives have been dated at ∼600 Ma. The SNSZ is deformed (regionally and locally folded and thrust dissected) during later NE–SW compressive tectonism. The SNSZ had an originally approximately E–W strike, low-angle N-dip and a normal shear sense, making this an example of a low-angle normal ductile shear (LANF) or detachment fault. The steep NE dip of Ss foliations and low-pitching slip lineations along Wadi Nugrus are due to NW–SE folding of the SNSZ, and do not indicate a sinistral strike-slip shear zone. The normal shear sense activity is responsible for juxtaposing the low-grade Central Eastern Desert lithologies against South Eastern Desert gneisses. A displacement of 15–30 km is estimated on the SNSZ, which is comparable to LANF displacements in the Basin and Range province of the western USA. Frictional resistance along this shear was probably reduced by high magmatic fluid pressure and hydrothermal fluid pressure. The vastness and diversity of the hydrothermal activity along this shear zone is a characteristic of other LANFs in the Eastern Desert, e.g. at Gabal El-Sibai, and may be Gabal Meatiq. The SNSZ formed during the Neoproterozoic extensional tectonic phase of Eastern Desert that began ∼600 Ma, and followed arc collision and NW-ward ejection of nappes.

Tracing along-strike structural continuity in the Neoproterozoic Allaqi-Heiani Suture, southern Egypt using principal component analysis (PCA), fast Fourier transform (FFT), and redundant wavelet transform (RWT) of ASTER data

We have used the visible and near infrared (VNIR) and short wave infrared (SWIR) bands of the advanced space-borne thermal emission and reflectance radiometer (ASTER) to trace along-strike structures in the ophiolite-decorated Neoproterozoic Allaqi-Heiani Suture in southern Egypt. The suture is E-trending in the west, changes to N-trending in its central part, and E-trending further east. The western part of the suture is defined by a fold and thrust belt dominated by an autochthon in the south followed by a southern, central and northern allochthons that were emplaced top-to-the-south. Tracing along-strike structures in the Allaqi-Heiani Suture has been a challenging task due to structural complexity and change of structural style. Principal component analysis (PCA), fast Fourier transform (FFT), and redundant wavelet transform (RWT) are used to identify ophiolite components such as talc carbonate schist, gabbro, and serpentinite as well as Neoproterozoic ductile structures to trace along-strike continuation in the Allaqi-Heiani Suture. (1) The PCA is performed on the 9 × 9 covariance matrix of the ASTER VNIR and SWIR bands. PC5 is selected because the ophiolite components and the Neoproterozoic ductile structures are best emphasized. (2) The FFT is performed on PC5 to reduce noises and mosaicking boundaries of the ASTER scenes. (3) The RWT is performed to enhance the contrast between ophiolite components and the surrounding rock types. This work helped in tracing structures along the Allaqi-Heiani Suture and revealed structural features that have not been identified before: (1) Some folds associated with the S-verging fold and thrust belts are periclinal folds that might be associated with nappe emplacement. (2) Truncation of structures associated with the S-verging fold and thrust belts occurs along NNW-trending sinistral strike-slip shear zones that swing at their extreme ends to become parallel to the truncated structures. (3) Presence of N-trending folds close to the NNW-trending sinistral strike-slip shear zones. (4) Presence of N- and NE-trending brittle faults younger than the Neoproterozoic ductile structures. Nappes identified in the west are traced into the central part of the suture allowing for subsequent modification by younger structures. The difference in structural styles along the suture might be due to development of frontal and lateral ramps along the traces of the orogen front. This was followed by superimposition of E–W shortening to produce N-trending folds that are localized close to the lateral ramps. Subsequently, N- and NE-trending brittle faults were developed and deformed early Neoproterozoic ductile structures.

Restored transect across the exhumed Grenville orogen of Laurentia and Amazonia, with implications for crustal architecture

New 40 Ar/ 39 Ar analyses from a transect across the major tectonic units of the southwest Amazon craton document the heterogeneous effects of the late Mesoproterozoic collision with the Grenville margin of North America. Basement rocks of the Amazon and adjacent Paragua cratons mostly preserve pre-Grenvillian ages (older than 1.3 Ga). Localized iso- topic age resetting at 1.18-1.12 Ga is caused by Grenvillian activation of widespread, sinistral strike-slip shear zones in the Amazon basement. In the Nova Brasilandia belt between these two cratons, new 40 Ar/ 39 Ar data record cooling through 920 Ma after the granulite facies deformation of this suture zone. Regional cooling rates calculated from compiled U/Pb, 40 Ar/ 39 Ar, and Rb/Sr thermochronologic data are used to establish post- Grenvillian exhumation patterns for the southwest Amazon and the North American belt. Paleodepths calculated for 1.0 Ga along a transect of the restored 1300-km-wide belt vary from uniformly deep levels (15-30 km) exposed in North America to shallower levels (5- 15 km) observed in the southwest Amazon. We interpret this difference as reflective of a change in tectonic architecture, i.e., thrust-dominated deformation in Laurentia versus strike-slip dominated deformation in the Amazon, with a commensurate variation in crust- al thickness. This interpretation explains the widespread preservation of both pre- Grenvillian ages and collisional ages from the Amazon craton, in contrast with the more homogeneous array of cooling ages from the North American Grenville Province marking the postorogenic extensional collapse of an overthickened crust. The asymmetrical oro- genic architecture from the reconstructed Grenville belt mirrors cross sections proposed for modern orogenic belts where deep-crustal rocks are not yet exposed.

Neoproterozoic deformation in the northeastern part of the Saharan Metacraton, northern Sudan

The northeastern part of the Saharan Metacraton is dominated by medium to high-grade gneisses and migmatites, disrupted by belts of low-grade volcano-sedimentary sequences representing arc assemblages and highly dismembered ophiolites, and intruded by A-type granitoids. This part of the Saharan Metacraton is affected by three Neoproterozoic deformation events: (1) Emplacement of S- to SE-verging nappes of ophiolites and passive margin sediments. The most prominent of these nappes is the Atmur-Delgo fold and thrust belt which extends westward from the eastern margin of the Saharan Metacraton to just east of the Third Cataract Nile. This belt is interpreted as manifesting the closure of a restricted oceanic basin between the Bayuda and Halfa Terranes at ∼700–650 Ma. (2) Development of N- to NE-trending folds as a result of E-W shortening that accompanied NW-SE oblique collision between the Saharan Metacraton and the Arabian-Nubian Shield at ∼650–590 Ma. This event, although dominantly localized along the Keraf Suture at the boundary between the Saharan Metacraton and the Arabian-Nubian Shield, also resulted in deformation within the northeastern part of the Saharan Metacraton. N- to NE-trending folds occur either as broad belts (Wadi Halfa Fold Belt) or narrow and discrete zones (Third Cataract Shear Zone). Thrust stacking and folding thickened the Saharan Metacraton lithosphere as shown by numerous gneissic domes, such as the Delgo Dome, in the northern part of the Bayuda Terrane. Crustal thickening also generated A-type granitoids that intrude the northeastern part of the Saharan Metacraton. E-W shortening culminated in the development of ∼590–550 Ma, N- to NNW-trending sinistral strike-slip shear zones, sometimes coinciding with the N-trending fold belts as in the case of the Third Cataract Shear Zone. (3) Thickening of the Saharan Metacraton lithosphere triggered orogenic collapse with N-trending moderate-angle normal-slip faults developed after 550 Ma. Neoproterozoic deformation and igneous activity in the northeastern part of the Saharan Metacraton are interpreted as due to processes that involve collision, lithospheric mantle delamination, and regional extension.

A Large Ductile Sinistral Strike‐Slip Shear Zone and Its Movement Timing in the South Qilian Mountains, Western China

Abstract There is a large ductile shear zone, 2 km wide and more than 350 km long, in the South Qilian Mountains, western China. It is composed of volcanic, granitic and calcareous mylonites. The microstructures of the ductile shear zone show nearly E‐W extending subvertical foliation, horizontal and oblique stretching lineations, shearing sense from sinistral to oblique sinistral strike‐slip from east to west, “A” type folds and abundant granitic veins. Measured lattice preferred orientations (LPOs) of the mylonitic and recrystallized quartz of the granitic mylonite in the west segment suggest a strong LPO characterized by the dominant slip systems {10times10} &lt;c&gt; formed at high temperature (&gt;650°C). K‐feldspar of the mylonite shows an 39Ar/40Ar high‐temperature plateau age of 243.3±1.3 Ma, and biotite, 250.5±0.5 Ma, which represent the formation age of the ductile shear zone. The 39Ar/40Ar plateau ages of 169.7±0.3 Ma and 160.6±0.1 Ma and the 39Ar/40Ar isochron ages of 166.99±2.37 Ma and 160.6±0.1 Ma of biotites in the mylonite represent the subsequent deformation age. These ages indicate that this ductile shear zone is similar to the Altun and South Kunlun sinistral ductile shear zones in its ages of movement, formation, reactivation and duration.