Main Shear Zone Research Articles

Syndeformational granite crystallisation along the Mount Magnet Greenstone Belt, Yilgarn Craton: evidence of large-scale magma-driven strain localisation during Neoarchean time

The Neoarchean is characterised by a global-scale period of crust formation, extensive mafic to granitic magmatism, crustal reworking and intracrustal partial melting. Since a dramatic strength drop is associated with the presence of melt in crystallising or melting rocks, Neoarchean continental deformation is thought to have been largely accommodated by shearing of high-grade gneisses and by syntectonic granitic magmatism. However, direct field examples of such strain localisation in hot and partially molten granites are relatively scarce. The Archean Yilgarn Craton, which is made up of volumetrically dominant granites and granitic gneiss, represents a natural laboratory to study the effect of melting on continental deformation. This paper describes meso- and microstructural evidence of large-scale, synmagmatic deformation recorded during crystallisation of a Neoarchean granitic complex. Two main kilometre-scale synmagmatic shear zones have been identified, and they are inferred to have been active during granite crystallisation. Structural evidence suggests that the dome-shaped structure that characterises the granitic complex represents a primary feature developed during pluton crystallisation. In the Mount Magnet greenstone belt, early structures predate granite emplacement, while the kilometre-scale, Boogardie synform developed during pluton crystallisation in response to granite doming. The studied complex represents a natural example of strain localisation within a low-viscosity corridor that may have accommodated large amounts of strain during the Neoarchean, as predicted by numerical experiments and tectonic models.

High-pressure granulites from Cariré, Borborema Province, NE Brazil: Tectonic setting, metamorphic conditions and U–Pb, Lu–Hf and Sm–Nd geochronology

In the northwest region of the Ceará Central Domain, Borborema Province, an elongate area of mylonitic diorites and granodiorites is structurally controlled by NE–SW-trending strike-slip faults. Granitic material and layers and lenses of felsic and mafic granulites occur along the main shear zones. We present and discuss petrographic, geothermobarometric, and geochronological (LA-MC-ICPMS U–Pb and Lu–Hf in zircon and Sm–Nd whole-rock) data for these rocks. Felsic granulites are composed of orthopyroxene+clinopyroxene+plagioclase+quartz±hornblende±garnet±biotite±feldspar, while the mafic granulites comprise clinopyroxene+plagioclase+garnet+hornblende±rutile±quartz. The P–T conditions recorded range from 9.4 to 8.3kbars and from 804 to 870°C for the felsic granulites and from 13.6 to 10.2kbars and 910 to 750°C for the mafic granulites. U–Pb zircon ages of granodiorite host rocks are 2157 and 2044Ma. The felsic granulites yield a U–Pb age of 2110Ma, while the mafic granulites yield U–Pb zircon ages of 613 and 589Ma for the metamorphic recrystallization. Sm–Nd and Lu–Hf TDM ages for the mafic granulites are similar (1.6–1.3Ga) and share positive values of εNd and εHf, which suggests that these mafic rocks were derived from a juvenile Mesoproterozoic crust. In addition, the felsic granulites and the granodiorites have similar TDM-Nd and U–Pb ages (with positive values of εNd), indicating a Paleoproterozoic mantle source for these rocks.

A Paleozoic subduction complex in Korea: SHRIMP zircon U–Pb ages and tectonic implications

The Wolhyeonri complex in the southwestern margin of the Korean Peninsula is divided into three lithotectonic units: Late Paleozoic Zone I to the west, Middle Paleozoic Zone II in the middle and Early Paleozoic Zone III to the east. Zones II and III display characteristics of continental arc magmatic sequence. Zone II is dominated by mafic metavolcanics, whereas zone III is characterized by the presence of dismembered serpentinite bodies including chaotic mélange. These zones are proposed to have been formed in a convergent margin setting associated with subduction. Here we present zircon SHRIMP U–Pb ages from the various units within the Wolhyeonri complex which reveal the Paleozoic tectonic history of the region. The Late Carboniferous ages obtained from the main shear zone between the Wolhyeonri complex and the Paleoproterozoic Gyeonggi massif are thought to mark the timing of continental arc magmatism associated with the subduction process. In contrast, Zone I with Neoproterozoic arc magmatic remnants might indicate deposition in a forearc basin. The Wolhyeonri complex also preserves strong imprints of the Triassic collisional event, including the presence of Middle Triassic high-pressure metabasites and eclogites near the eastern boundary of the Zone III. These range of radiogenic ages derived from the Wolhyeonri complex correlate well with subduction and accretion history between the North and South China cratons. Similar geochronological features have also been indentified from the Qinling, Tongbai–Xinxian, and northern Dabie areas in east-central China. The existence of Paleozoic coeval subduction in East Asia prior to the Triassic collision is broadly consistent with a regional tectonic linkage to Gondwana.

Late Miocene–Pliocene deceleration of dextral slip between Pamir and Tarim: Implications for Pamir orogenesis

The timing of the late Cenozoic collision between the Pamir salient and the Tien Shan as well as changes in the relative motion between the Pamir and Tarim are poorly constrained. The northern margin of the Pamir salient indented northward by ~ 300 km during the late Cenozoic, accommodated by south-dipping intracontinental subduction along the Main Pamir Thrust (MPT) coupled to strike-slip faults on the eastern flank of the orogen and both strike-slip and thrust faults on the western margin. The Kashgar–Yecheng transfer system (KYTS) is the main dextral slip shear zone separating Tarim from the Eastern Pamir, with an estimated cumulative offset of ~ 280 km at an average late Cenozoic dextral slip rate of 11–15 mm/a (Cowgill, 2010). In order to better constrain the slip history of the KYTS, we collected thermochronologic samples along the eastward-flowing, deeply incised, antecedent Tashkorgan–Yarkand River, which crosses the fault system on the eastern flank of the orogen. We present 29 new biotite 40Ar/ 39Ar ages, apatite and zircon (U–Th–Sm)/He ages, and apatite fission track (AFT) analysis, combined with published muscovite and biotite 40Ar/ 39Ar and AFT data, to create a unique thermochronologic dataset in this poorly studied and remote region. We constrain the timing of four major N-trending faults; the latter three are strands of the KYTS. The westernmost, the Kuke fault, experienced significant dip-slip, west-side-up displacement between > 12 and 6 Ma. To the east, within the KYTS, our new thermochronologic data and geomorphic observations suggest that the Kumtag and Kusilaf dextral slip faults have been inactive since at least 3–5 Ma. Long-term incision rates across the Aertashi dextral slip fault, the easternmost strand of the KYTS, are compatible with slow horizontal slip rates of 1.7–5.3 mm/a over the past 3 to 5 Ma. In summary, these data show that the slip rate of the KYTS decreased substantially during the late Miocene or Pliocene. Furthermore, Miocene–present regional kinematic reconstructions suggest that this deceleration reflects the substantial increase of northward motion of Tarim rather than a significant decrease of the northward velocity of the Pamir.

Geophysical signature of the vien-type uranium mineralization of Wadi Eishimbai, Southern Eastern Desert, Egypt

The application of various geophysical tools with different responses succeeded in fixing U-mineralization in Wadi Eishimbai area. The area was studied using detailed ground spectrometric, magnetic, and filtered very low-frequency electromagnetic (VLF-EM) surveys. The interpretation of the obtained spectrometric maps clearly reflects the sharp increase of equivalent uranium (eU) content. Meanwhile, K and Th contents show sharp decreases. The eU/equivalent thorium (eTh) ratio correlates positively with eU concentrations and negatively with eTh concentrations, indicating an increase in U potentiality than the surrounding granite. The N–S shear zone displays an eU content ranging from 20 to 140 ppm. The ENE-trending lamprophyre is characterized by elongated uranium anomalies trending in the E–W direction, with values >90 ppm. Equivalent uranium content of the brecciated granite attains values up to 700 ppm. The ground magnetic and VLF-EM surveys played important roles in providing structural information which are proven useful in geological mapping and mineral exploration for the discovery of uranium mineralization in the study area. This study follows the expected subsurface extension of the Sela shear zone under Wadi sediments. The ground total magnetic intensity map shows a relatively narrow and an elongated shape for the lamprophyre anomaly extending for about 600 m in the Wadi toward the western direction. VLF-EM contour maps of the two used frequencies (17.1 and 28.5 kHz) show excellent agreement, indicating that the shear zone is distinguished with slightly strong conductivity westwards as an extension of the main shear zone. It is elongated in an ENE–WSW trend and extends in the western direction, referring to the existence of conductive materials. Most of the NW/SE-trending faults cause sudden changes in the magnetic and VLF-EM contour spacing over an appreciable distance, which suggests a discontinuity in depth due to their left-lateral strike-slip displacements. The interpreted faults, with an ENE–WSW trend representing the main trend of Sela shear zone through which hydrothermal solutions flowed, cause high alteration and uranium mineralization.

Characterization of gold mineralization in Garin Hawal area, Kebbi State, NW Nigeria, using remote sensing

Garin Hawal area, Kebbi State, NW Nigeria is part of the Neoproterozoic to Early Phanerozoic terrane separating the west African and Congo Cratons. Three main gold-bearing shear zones were detected in the study area from the processed Landsat ETM+ images and extensive ground investigation. Field and petrographical studies indicate that the Neoproterozoic rocks are represented by a highly folded and faulted belt constituted of hornblende, muscovite and graphite schist. They are intruded by granondiorites and late to post granitic dykes. Extensive alteration zones were identified using high resolution QuickBird image along Garin Hawal shear zone. The alteration zones and associated quartz veins are generally concordant with the main NE–SW regional structural trend and are dipping to the NW. Geochemical studies indicate that the gold content reaches 8 g/t in the alteration zones, while it reaches up to 35 g/t in the quartz veins. Mineralogical studies indicate that the alterations are strongly potassium-enriched. Pyrophyllite, kaolinite, illite, gypsum and quartz also occur. The main ore minerals are gold, chalcopyrite, arsenopyrite, pyrite, galena and iron oxides. This study indicates that the alteration zones and the associated quartz veins in the muscovite schist are promising and need more detailed exploration for Au and Ag mineralization to evaluate their potential.

Geometry and kinematics of Mykonos detachment, Cyclades, Greece: Evidence for slip at shallow dip

[1] The eastern part of Mykonos island (Cyclades, Greece) shows the detailed internal structure of a two-branch shallow-dipping extensional detachment system of Miocene age. This paper shows that the last stage of slip along the detachment, during the deposition of syn-rift sediments, occurred with a very low dip and proposes that this geometry prevailed since the initiation of extension. Extensional deformation during and after a 13 Ma old granite intrusion is taken up by two main shallow-dipping shear zones: (1) The lower Livada detachment, which extends laterally across Tinos island, is a ductile structure located at the interface between the granite and the Upper Cycladic Nappe metabasites. A pervasive top-to-the-NE shearing deformation is observed throughout the granite in this island with a strong gradient toward the intrusive contact. Later brittle faults, shallowly and steeply dipping, rework the ductile deformation with the same overall shear sense. (2) The upper Mykonos detachment is brittle and separates the metabasites from late Miocene sandstones and conglomerates. The detachment fault dips 12–15° toward the NNE, and its sense of shear is consistent with that of the Livada detachment. Soft-sediment deformation during the time of detachment faulting and the presence of steep normal faults that root into the detachment gouge indicate a shallow depth of deformation. Rotations about vertical and horizontal axes can be reconstructed within and below the detachment zone, indicating that the overall direction of extension is NNE/SSW with a sense of shear toward the NNE and the dip of the fault is throughout very low. The two detachments have accommodated several tens of kilometers of horizontal extension during the formation of the Aegean Sea, which emphasizes the importance of low-angle extensional faults and shear zones in extensional tectonics.

Tectonic evolution of the Northern Sicanian-Southern Palermo Mountains range in Western Sicily: insight on the exhumation of the thrust involved foreland domains

Geological mapping and structural analysis, coupled with extensive samplings on terrigenous Tertiary covers, enable us to reconstruct the structural setting as well as the tectonic evolution of the northern Sicanian-southern Palermo Mountains range in western Sicily. The analyzed region is a segment of the Apenninic-Maghrebian Orogen characterized by the occurrence of two regional superimposed tectonic edifices; the Apenninic-Maghrebian Chain (AMC) and the Pelagian Sicilian Thrust Belt (PSTB) which constitute the uppermost and lowermost structural level respectively. The different structural associations generally suggest a multiphase tectonic history in which we distinguished three main deformational events. The oldest one is characterized by the development of low-angle foreland-verging large thrust contacts. These usually propagated along Lower-Middle Miocene terrigenous levels and leads to the staking of the AMC. As a whole the AMC structured during Middle Miocene and tectonically overrode the Pelagian foreland since Tortonian time. The subsequent tectonic phase developed as effect of continental collision with the propagation of south verging high-angle thrust faults. These involve mainly the underlying Pelagian foreland giving rise to the imbrications at depth of the PSTB. The more recent tectonic phase starts since Middle Pliocene with the activation of NW-SE oriented right-lateral traspressive shear zones and associated double verging E-W striking thrust contacts. These latter occurred mainly in the overlapping area between major shear zones and are responsible for the final uplift of the previously imbricate PSTB. During this tectonic stage the previously E-W oriented structural features suffered local clockwise rotations in the neighbours of the main right-lateral shear zones. In this newly proposed structural framework, we interpreted the Mt. Kumeta and Rocca Busambra carbonate ridges as a Pliocene push-up type antiformal structures grown and exhumed at restraining stepovers in a contest of regional strike-slip kinematics

Relationships between displacement and distortion in orogens: Linking the Himalayan foreland and hinterland in central Nepal

Research Article| July 01, 2010 Relationships between displacement and distortion in orogens: Linking the Himalayan foreland and hinterland in central Nepal Kyle P. Larson; Kyle P. Larson † Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario K7L 3N6, Canada †Current address: Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada; e-mail: kyle.larson@usask.ca. Search for other works by this author on: GSW Google Scholar Laurent Godin; Laurent Godin Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario K7L 3N6, Canada Search for other works by this author on: GSW Google Scholar Raymond A. Price Raymond A. Price Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario K7L 3N6, Canada Search for other works by this author on: GSW Google Scholar GSA Bulletin (2010) 122 (7-8): 1116–1134. https://doi.org/10.1130/B30073.1 Article history received: 13 Apr 2009 rev-recd: 21 Aug 2009 accepted: 22 Aug 2009 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Kyle P. Larson, Laurent Godin, Raymond A. Price; Relationships between displacement and distortion in orogens: Linking the Himalayan foreland and hinterland in central Nepal. GSA Bulletin 2010;; 122 (7-8): 1116–1134. doi: https://doi.org/10.1130/B30073.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Greater Himalayan sequence rocks exposed in the Manaslu–Himal Chuli Himalaya can be separated into distinct upper and lower parts. Deformation recorded in both parts occurred at temperatures ranging between ∼450 °C and ∼640 °C and is characterized by almost equal coaxial and noncoaxial components. Across the upper Greater Himalayan sequence, peak metamorphic temperatures are essentially isothermal, whereas corresponding metamorphic pressure estimates across the same section decrease downward with an apparent gradient of 620 bars/km. In the lower Greater Himalayan sequence, however, both metamorphic pressure and temperature decrease with structural depth. The abnormal pressure gradient in the upper Greater Himalayan sequence is attributed to ∼50% vertical thinning during southward displacement, while the inverted gradient in the lower portion is interpreted to be the result of coeval exhumation and downward expansion of the Main Central thrust shear zone and the progressive incorporation of more rock into the Greater Himalayan sequence. Deformation in the upper portion of the Greater Himalayan sequence was characterized by extending flow, i.e., extension in the direction of flow, whereas deformation in its lower portion was characterized by compressing flow, i.e., compression in the direction of flow. Extending flow is a distinctive feature of displacement and distortion in deep orogenic hinterlands, while compressing flow is emblematic of displacement and distortion in orogenic foreland regions. The transition between the upper and lower parts of the Greater Himalayan sequence therefore represents a fundamental transition between hinterland-style deformation, involving processes such as lateral midcrustal flow, and foreland-style deformation, involving critical-taper thrust-fold wedge development. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A study of mylonites from parts of the Salem-Attur shear zone (Tamil Nadu) and its tectonic implications

The E-W running Salem-Attur shear zone demarcates the tectonic boundary between Archaean Dharwar Craton in the north and Proterozoic Southern granulite terrane in the south. This study reveals that the shear zone is a low angle thrust. The thrust zone is around 10 m thick and it merges with the main shear zone along the strike. The thrust is developed on charnockite near Odyarpatti, which is retrograded into schists. Further, it is marked by gently dipping mylonitic foliation and subhorizontal lineation. The S-C fabric, mantled porphyroclasts and intragranular faults indicate northeasterly slip along the thrust. Recumbent shear folds SF 1 are developed within the thrust zone. The thrust has been folded by late stage F 2 fold which has brought variation in the orientation of the mylonitic foliation from subhorizontal to vertical attitude; the mylonitic lineations have been rotated to subvertical orientation also. Additionally, the F 2 crenulations and shear cleavages and intersection lineations are superimposed on the mylonitic fabric. Thrusting along the Salem-Attur shear zone is probably the cause for upliftment of the charnockites to the upper crust. Post-upliftment stage has witnessed brittle deformation in the form of development of shear fractures in NNE-SSW and E-W directions. Pseudotachylites are emplaced along these fractures.

The geology and mineralisation at the Golden Pride gold deposit, Nzega Greenstone Belt, Tanzania

The Golden Pride gold deposit (∼3 Moz) is located in the central part of the Nzega Greenstone Belt at the southern margin of the Lake Victoria Goldfields in Tanzania. It represents an inferred Late Archaean, orogenic gold deposit and is hosted in intensely deformed meta-sedimentary rocks in the hanging wall of the approximately E–W striking Golden Pride Shear Zone. The hanging-wall sequence also includes felsic (quartz porphyritic) to mafic (lamprophyric) intrusions, as well as banded iron formations. Hydrothermal alteration phases associated with mineralisation are dominated by sericite and chlorite. Two main ore types can be distinguished, chlorite and silica ore, both occupying dilational sites and structural intersections in the hanging wall of the main shear zone. Sulphide minerals in both ore types include pyrrhotite, arsenopyrite, pyrite and accessory sphalerite, galena, sulphosalts and Ni–Co–Bi sulphides. Gold and tellurides are late in the paragenetic sequence and associated with a secondary phase of pyrrhotite deposition. Sulphur isotope compositions range from −6 to 7 per mil and are interpreted to reflect contributions from two distinct sources to the mineralising fluids in the Golden Pride gold deposit. A redox change, potentially induced by the intrusion of mafic melts, together with structural elements in the hanging wall of the Golden Pride Shear Zone, are interpreted to be the main controls on gold mineralisation in this deposit.

Fluid Evolution and Metallogenic Dynamics during Tectonic Regime Transition: Example from the Jiapigou Gold Belt in Northeast China

AbstractThe Jiapigou gold belt, one of the most important gold‐producing districts in China, is located in the northern margin of the North China Craton (NCC). The tectonic evolution of the gold belt is closely related to the Siberian Plate (SP) in the north, Yangtze Craton (YC) in the south and Pacific Plate in the east. In order to investigate the nature of the tectono‐fluid‐metallogenic system, the authors investigated the relationships among the tectonic regimes, fluid evolution and metallogenesis. This paper examined the corresponding spatial–temporal relationship between the ore‐controlling tectonic regime and hydrothermal fluid evolution in the Jiapigou gold belt. There are two types of gold mineralization: disseminated ores that are distributed within the NW‐trending main ductile shear zone and gold‐bearing quartz veins and minor disseminated ores that are distant to the ductile shear zone. The fluid inclusions in quartz contain a large amount of CO2. Metamorphic fluids of middle to high temperatures and pressures and meteoric waters of low temperatures and pressures mixed together during mineralization. A proposed ore‐forming model is as follows: in the pre‐ore phase, the collision of SP and NCC resulted in the NS‐trending compression of the ore belt. This formed the NE‐trending and NW‐trending shear faults and EW‐trending folds. During the ore‐forming phase, the collision of YC and NCC resulted in dextral shearing of the NW‐trending Jiapigou fault and the NE‐trending Green faults. High‐pressure fluids caused by the compression flowed into the dilatant zone. This may have caused both phase separation of CO2‐bearing fluids and the mixing of meteoric waters, metamorphic waters and magmatic source fluids and finally resulted in the disequilibrium of the ore fluids and precipitation of ore minerals.

Alpine tectonic evolution and thermal water circulations of the Argentera Massif (South-Western Alps)

Three groups of thermal springs with temperatures close to 70 °C discharge both in the core (at Bagni di Vinadio and Terme di Valdieri) and on the external margin (at Berthemont-Les-Bains) of the Argentera Massif. Detailed structural field analysis carried out on the hydrothermal sites allows us to delineate both a model of Alpine tectonic evolution of the Argentera Massif and the patterns of hydrothermal circulation that were active during its final exhumation. The observed fault rock assemblages provide information relative to deformation that occurred in viscous, frictional-to-viscous and frictional crustal regimes. During the Early Miocene, the Bersezio Fault Zone and the Fremamorta Shear Zone, two main mylonitic shear zones, mainly accommodated regional transpression and provided pathways for fluid flow promoting mineral reactions in greenschist facies. During the Late Miocene–Early Pliocene, frictional-to-viscous deformation affected the massif, which underwent predominant transpression in the internal sectors and extension on the external margin. During the Plio-Pleistocene, deformation in frictional condition accompanied the final exhumation of the massif in a transpressive regime and resulted in the development of the NW–SE striking cataclastic zones. The hydraulic properties of these structures mainly influence the patterns of the active thermal circulations and the localization of the recharge and discharge zones. At Berthemont these faults represent conduits, whereas at Vinadio and Valdieri they form complex systems of conduits and barriers. In these two latter sites, the cataclastic faults compose flower structures that constrain laterally the thermal fluid flows while intensely fractured granites sited at depth constitute a highly-transmissive geothermal reservoir. Less permeable migmatitic gneisses overlaying the granites prevent a massive infiltration of the cold fluids at depth. This context favours within the high-permeability fractures granites the development of buoyancy-driven flows which combined with topographically-driven flows, provided the conditions for the upflow of the high-temperature waters.

The relationship between forceful and passive emplacement: The interplay between tectonic strain and magma supply in the Rosses Granitic Complex, NW Ireland

The Rosses Granitic Complex, NW Ireland, part of the late Caledonian (c. 400 Ma) Donegal Batholith, provides the opportunity to study the interplay between relative tectonic strain and magma supply rates in the wall of a major tectonic shear zone. Anisotropy of magnetic susceptibility (AMS) measurements, structural analysis and examination of key intrusive relationships were used to assess the accommodation of magma, associated deformation and magma flow pathways in this complex. In this case the varying emplacement styles, switching from forceful to passive, indicate that relative tectonic strain and magma supply rates were not constant. In the earliest component (a suite of microgranite sheets), AMS reveals subtle fabrics discordant to the sheet margins and is interpreted as post-emplacement deformation fabrics. The concordance of these fabrics to the next component of the complex, the main pluton (G1 and G2 monzogranite), indicates that this deformation was caused by the forceful emplacement of the pluton. AMS fabrics in G1 and G2 reveal a dome shaped foliation with an east-west lineation, indicating an east-west oriented magma transport direction. Outcrops of small stocks or cupolas similar to G2, east of the main pluton, link it to similar lithologies in the Main Donegal Granite further to the east via a partially exposed lateral feeder. This suggests east to west emplacement. The next component, a suite of subvertical porphyritic felsite dykes, is shown (from AMS and visible shearing fabrics) to have been emplaced passively under east-west tension. The final component comprises G3 and G4 of the main pluton, which passively cut all earlier components and contains significant amounts of aplite, pegmatite and greisen. These are interpreted to be cupolas or stocks emanating from an unexposed sheet probably similar in composition and mode of emplacement to G1 and G2. Thus, a general model is put forward where initially forceful subhorizontal sheets extending laterally from the nearby Main Donegal Granite shear zone, gave way to passive emplacement where the roofs of these and subjacent sheets failed. The control on siting of the complex can be related to lower crustal structures, which control upper crustal shear zones. The reason for the switch in emplacement style is suggested to be due to a waning magma supply, in a tectonic strain field where the extensional component was oriented east-west. The waning magma supply then possibly due to switching of the site of emplacement of magma, supplied by the Main Donegal Granite shear zone, to another member of the Donegal Batholith – the Trawenagh Bay Granite.

Transpressive dextral shear in the Italva-Itaperuna section, Northern State of Rio de Janeiro, Brazil

Structural analysis carried out on a segment of the Neoproterozoic Ribeira Belt, southeastern Brazil, show that it represents part of the transpressive dextral orogen related to the Central Mantiqueira Province. NNE-trending and steeply dipping regional mylonitic belts form anastomosed geometry, and describe a map-scale, S-C-like structure that is characterized by their deflection towards NE near the Além Paraíba Lineament. Lithological and structural control related to deformation partition were responsible for the formation of felsic mylonitic granulites with S-type granites lenses developed in ductile shear zones, alternated with less deformed intermediate to basic granulites associated with charnockites. The dextral shear sense indicators are consistent with transpressive deformation in the region and are common especially at the border of the main shear zones. The presence of S-type leucogranite may lead to variations of linear and planar relationships, which result in local extension zones. These elements are consistent with oblique continental collision considering the São Francisco Craton as a stable block.

Contractional tectonics and magma paths in volcanoes

This study aims to contribute a possible explanation for magma migration within volcanoes located in contractional tectonic settings, based on field data and physically-scaled experiments. The data demonstrate the occurrence of large stratovolcanoes in areas of coeval reverse faulting, in spite of the widely accepted idea that volcanism can develop only in extensional/transcurrent tectonic settings. The experiments simulate the propagation of deformation from substrate reverse faults with different attitudes and locations into volcanoes. The substrate fault splits into two main shear zones within the volcano: A shallow-dipping one, with reverse motion, propagates towards the lower volcano flank, and a steeper-dipping one, with normal motion, propagates upwards. In plan view, the reverse fault zone is arcuate and convex outwards, whereas the normal fault zone is rectilinear. Structural field surveys at volcanoes located in contractional settings show similar features: The Plio–Quaternary Trohunco and Los Cardos–Centinela volcanic complexes (Argentina) lie above Plio–Quaternary reverse faults. The Late Pleistocene–Holocene El Reventador volcano (Ecuador) is also located in a coeval contractional tectonic belt. These volcanoes show curvilinear reverse faults along one flank and rectilinear extensional fracture zones across the crater area, consistent with the experiments. These data consistently suggest that magma migrates along the substrate reverse fault and is channelled along the normal fault zone across the volcano.

Geodynamic evolution of Mt. Riiser-Larsen, Napier Complex, East Antarctica, with reference to the UHT mineral associations and their reaction relations

Abstract Mt. Riiser-Larsen is the largest outcrop in the Archaean–early Proterozoic Napier Complex, East Antarctica. The area is structurally divided into the Main and the Western Blocks by the subvertical Riiser-Larsen Main Shear Zone (RLMSZ) of about 200 m width composed of mylonite and pseudotachylite. Mineral parageneses including sapphirine+quartz and osumilite, diagnostic of ultrahigh-temperature (UHT) metamorphism, are found in Mg-rich aluminous, siliceous and quartzo-feldspathic gneiss layers in both the Main and the Western Blocks of the Mt. Riiser-Larsen area. Some of the sapphirine–quartz associations are accompanied by retrograde reaction textures, which include growth of cordierite and/or garnet between sapphirine and quartz in the Main Block, and of orthopyroxene+sillimanite in the Western Block. These textures indicate the reaction (1) sapphirine + quartz ( + orthopyroxene ) = cordierite and (2) sapphirine + quartz ( + orthopyroxene ) = garnet in the Main Block and (3) sapphirine + quartz = orthopyroxene + sillimanite in the Western Block. Phase equilibria and P – T pseudosections for sapphirine+quartz-bearing associations suggest that these three reactions took place during a temperature drop from 1100 °C to 1000 °C at pressures of 0.6–0.8 GPa in the Main Block and 0.8–0.9 GPa in the Western Block. The geological structure and distribution of the UHT rocks provide an insight into the vertical extent of the>1000 °C UHT metamorphic zone: a minimum thickness of 4–5 km of the UHT-metamorphosed layers, which become deeper towards the west in the Main Block. The Western Block represents a c. 0.1–0.3 GPa ( c. 3–10 km) deeper structural level than the Main Block. In addition to the extent of the horizontal distribution of UHT metamorphism in the Napier Complex, our results on the vertical component provide new constraints for modelling the heat source and tectonic process of the unusually high-temperature regional metamorphism in the late Archaean–early Proterozoic. Electron microprobe monazite U–Th–Pb dating for hydrated and mylonitized sapphirine–quartz gneiss gave a wide spectrum of monazite age distribution between 2300 and 800 Ma, suggesting the tectonic uplift and juxtaposition of the two blocks in the Mt. Riiser-Larsen area later than the mid–late Proterozoic.

Fluid immiscibility and gold deposition in the Birimian quartz veins of the Angovia deposit (Yaouré, Ivory Coast)

The Paleoproterozoic terranes (Birimian) of West Africa are well known to host numerous economic gold mineralizations. The Angovia gold mineralization is located in a brecciated and mylonitic zone within the Birimian greenstones. The sulfide–gold mineralization is mainly represented by gold associated with pyrite and chalcopyrite. A fluid inclusion study undertaken on mineralized quartz veins revealed the presence of aqueous-carbonic (CO 2–H 2O) fluids, the association of carbonic (CO 2) and early aqueous fluids, followed by later aqueous (H 2O-salt) and finally nitrogen-rich fluids. Entrapment of the initial homogeneous aqueous-carbonic fluids prior to fluid immiscibility depicts the evolution of the P–T conditions during the exhumation of the terranes after the peak of green-schist metamorphism. The CO 2 rich-fluid occurs especially in gold-bearing quartz, and are considered as the main evidence of the ore-forming process in the gold-bearing quartz veins. It is considered as a product of immiscibility of the CO 2–H 2O parent. The volatile fraction of carbonic and aqueous-carbonic fluid inclusions is dominated by CO 2, containing minor amounts of N 2, even smaller amounts of CH 4 and sporadically, H 2S. The aqueous-carbonic fluids have moderate salinity (3–10 wt.% eq. NaCl). Late aqueous and N 2 – (CH 4–CO 2) fluids are considered as later, unrelated to the main ore stage, and were trapped during the cooling of the hydrothermal system from 300 to 200 °C. The immiscibility has been favored by a strong pressure drop, the main trapping P–T conditions being 320–370 °C and 105–135 MPa. The mineralizing process is likely related to the immiscibility event, which was probably favored by the release of the fluid pressure after fracturing along the main shear zones. The ore process is likely to have occurred along the main shear zones or related secondary structures affected by cycling of the fluid pressure and quartz sealing–fracturing processes. The superimposed process can also explain the relative complexity of the quartz textures and fluid inclusion microfractures, and the rather wide range in the density of both parent fluid and CO 2-dominated fluid.

Geological records of the Lhasa-Qiangtang and Indo-Asian collisions in the Nima area of central Tibet

A geological and geochronologic investigation of the Nima area along the Jurassic–Early Cretaceous Bangong suture of central Tibet (∼32°N, ∼87°E) provides well-dated records of contractional deformation and sedimentation during mid-Cretaceous and mid-Tertiary time. Jurassic to Lower Cretaceous (≤125 Ma) marine sedimentary rocks were transposed, intruded by granitoids, and uplifted above sea level by ca. 118 Ma, the age of the oldest nonmarine strata documented. Younger nonmarine Cretaceous rocks include ca. 110–106 Ma volcanic-bearing strata and Cenomanian red beds and conglomerates. The Jurassic–Cretaceous rocks are unconformably overlain by up to 4000 m of Upper Oligocene to Lower Miocene lacustrine, nearshore lacustrine, and fluvial red-bed deposits. Paleocurrent directions, growth stratal relationships, and a structural restoration of the basin show that Cretaceous–Tertiary nonmarine deposition was coeval with mainly S-directed thrusting in the northern part of the Nima area and N-directed thrusting along the southern margin of the basin. The structural restoration suggests >58 km (>47%) of N–S shortening following Early Cretaceous ocean closure and ∼25 km shortening (∼28%) of Nima basin strata since 26 Ma. Cretaceous magmatism and syncontractional basin development are attributed to northward low-angle subduction of the Neotethyan oceanic lithosphere and Lhasa-Qiangtang continental collision, respectively. Tertiary syncontractional basin development in the Nima area was coeval with that along the Bangong suture in westernmost Tibet and the Indus-Yarlung suture in southern Tibet, suggesting simultaneous, renewed contraction along these sutures during the Oligocene-Miocene. This suture-zone reactivation immediately predated major displacement within the Himalayan Main Central thrust system shear zone, raising the possibility that Tertiary shortening in Tibet and the Himalayas may be interpretable in the context of a mechanically linked, composite orogenic system.

Late Orogenic doming in the eastern Betic Cordilleras: Final exhumation of the Nevado‐Filabride complex and its relation to basin genesis

The geometry, timing, and kinematics of late orogenic extension in the Betic Cordilleras pose the problem of a decoupling of upper crustal and lower crustal deformation regimes. Perpendicular directions of extension in metamorphic domes and nearby sedimentary basins remain unexplained. This paper puts kinematic constraints on the final exhumation of the Nevado‐Filabride complex, focusing on the formation of metamorphic domes and their relations with the adjacent basins. Structural fabrics and kinematic indicators below the main shear zones as well as their relations with both published changing metamorphic P‐T conditions and geochronological data were studied. Our approach describes (1) a consistent top‐to‐the‐west shear parallel to dome axes of during D2 (i.e., during decompression) with distributed ductile flow and the onset of strain localization along major shear zones, (2) further strain localization along the major shear zones under greenschist facies conditions, during D3 leading to S‐C′ mylonites formation accompanied with a rock strong thickness reduction, (3) the divergence of shear direction on either limbs of domes during D3 showing the appearance of the dome geometry, and (4) a local evolution toward N‐S brittle extension (D4) in the upper plate and formation of sedimentary basins. Continuous ductile to brittle top‐to‐the‐west shear is compatible with the slab retreat hypothesis from the Miocene; the formation of domes which adds gravitational forces responsible for the final stages of exhumation is thus characterized by important kinematics changes necessary to explain coeval N‐S opened basins. Later, from the upper Tortonian, a contractional event (D5) amplified the earlier domal structures forming the present north vergent folds.