Mylonitic Shear Zones Research Articles

Indosinian tectonics in Vietnam

In Vietnam, the Triassic Indosinian collision affected coevally the Truong Son belt and the Kontum Massif,which were not independent tectonic units, but parts of the same Gondwana-derived Indochina continental block. This thermotectonic event took place synchronously throughout Vietnam, during the Lower Triassic 250–240-Ma time interval, as demonstrated by numerous geochronological data, combining Ar–Ar and U–Pb dating methods. Structural and kinematic investigations, in the Indosinian metamorphic rocks, reveal that the collisional process resulted from a consistent northwest-striking convergence of Indochina with respect to the adjacent blocks. It is suggested that this motion was taken up by a pair of opposite subduction zones: to the north, beneath South China, and to the west, beneath western Indochina, along the Song Ma and Po Ko sutures, respectively. Tectonic markers, calc-alkaline subduction-related volcanism and granitic intrusions and the generation of high-pressure rocks that have been recently discovered support this geodynamic setting, at least along Po Ko. Along the northwest-trending Song Ma zone, the obliquity of the convergence during subduction and subsequent collision resulted in the development, within the Truong Son Belt, of a set of subparallel dextral mylonitic shear zones, under amphibolite-facies metamorphism. The intermediate segments remained weakly metamorphic or even almost devoid of metamorphism. Along Po Ko, the convergence was near-orthogonal, with a left-lateral strike-slip component; the ongoing continental subduction resulted in the reworking of the Kontum granulitic basement and the development of Indosinian HP granulitic conditions; the subsequent extension-related exhumation operated approximately in the same northwestwards direction. This Indosinian evolution, applied on a continental crust that had been probably affected, as in South China, by a Caledonian-related event, as judged by the general unconformity of the Lower Devonian sediments, the widespread occurrence of magmatic crystallisation ages of ca 450 Ma (Ordovician-Silurian), and by the approximately similar age of the primary granulitic episode in the Kontum Massif. The similarities of the Devonian facies over central, northern Vietnam and South China imply a land connection, possibly as a consequence of a Caledonian collision along Song Ma, but this does not preclude a further oceanic opening and a closure during the Indosinian.

Lower Carboniferous post-orogenic granites in central-eastern Sierra de Velasco, Sierras Pampeanas, Argentina: U–Pb monazite geochronology, geochemistry and Sr–Nd isotopes

The central-eastern part of the Sierra de Velasco (Sierras Pampeanas, NW Argentina) is formed by the large Huaco (40 × 30 km) and Sanagasta (25 × 15 km) granite massifs and the small La Chinchilla stock (2 × 2 km). The larger granites intrude into Ordovician metagranitoids and crosscut Devonian (?) mylonitic shear zones, whereas the small stock sharply intrudes into the Huaco granite. The two voluminous granites are biotitic-muscovitic and biotitic porphyritic syeno- to monzogranites. They contain small and rounded tonalitic and quartz-dioritic mafic microgranular enclaves. The small stock is an equigranular, zinnwaldite- and fluorite-bearing monzogranite. The studied granites are silica-rich (SiO2 >70%), potassium-rich (K2O >4%), ferroan, alkali-calcic to slightly calk-alkalic, and moderately to weakly peraluminous (A/CNK: 1.06–1.18 Huaco granite, 1.01–1.09 Sanagasta granite, 1.05–1.06 La Chinchilla stock). They have moderate to strong enrichments in several LIL (Li, Rb, Cs) and HFS (Nb, Ta, Y, Th, U) elements, and low Sr, Ba and Eu contents. U–Pb monazite age determinations indicate Lower Carboniferous crystallization ages: 350–358 Ma for the Huaco granite, 352.7 ± 1.4 Ma for the Sanagasta granite and 344.5 ± 1.4 Ma for the La Chinchilla stock. The larger granites have similar ɛNd values between −2.1 and −4.3, whereas the younger stock has higher ɛNd of −0.6 to −1.4, roughly comparable to the values obtained for the Carboniferous San Blas granite (−1.4 to −1.7), located in the north of the sierra. The Huaco and Sanagasta granites have a mainly crustal source, but with some participation of a more primitive, possibly mantle-derived, component. The main crustal component can be attributed to Ordovician peraluminous metagranitoids. The La Chinchilla stock derives from a more primitive source, suggesting an increase with time in the participation of the primitive component during magma genesis. The studied granites were generated during a post-orogenic period in a within-plate setting, possibly as a response to the collapse of the previous Famatinian orogen, extension of the crust and mantle upwelling. They are part of the group of Middle Devonian–Lower Carboniferous granites of the Sierras Pampeanas. The distribution and U–Pb ages of these granites suggests a northward arc-parallel migration of this mainly post-orogenic magmatism with time.

Multiscaling of shear zones and the evolution of the brittle‐to‐viscous transition in continental crust

A new method is introduced for quantifying the scale and the intensity of strain localization from maps of natural shear zones. The method employs autocorrelation functions to determine local areal scales of geometric homogeneity. These homogenization scales are used to calculate scale‐dependent localization fractions of deformed rock. The strain localization intensity is quantified from measurements of mean relative to maximum shear strain. This approach is used to analyze shear zones on different scales from an exposure (Cap de Creus, Spain) of the fossil brittle‐to‐viscous transition (BVT). Changes in the scaling characteristics of shear zones are interpreted to reflect a time sequence of localization during the evolution of the continental BVT. We show that shear zone scaling is related to inherited anisotropies (older schistosity, lithological layering, pegmatite bodies) and to the predominant mode of deformation (brittle, viscous). The length‐to‐width ratio of shear zones increases with their length up to the meter scale and decreases for larger length scales as they evolve from isolated shear fractures to interconnected mylonitic shear zones. Variations in strain localization intensity calculated along a single shear zone indicate that such shear zones weakened from their brittle tips to their mylonitic centers, thus driving their propagation and growth to larger scales. Our results imply that the BVT evolves by “network widening,” a process whereby strain localizes on progressively larger scales until a dense network of weak, mylonitic layers tens to hundreds of meters wide and hundreds to thousands of meters long forms subparallel to the regional shearing plane.

A natural example of crystal-plastic deformation enhancing the incorporation of water into quartz

Water content of quartz in and around a greenschist facies mylonitic shear zone located in the western Adirondacks was analyzed by micro-FTIR spectroscopy. The shear zone is within a pegmatitic dike, which cuts across a granitic gneiss. The thickness of the shear zone varies along strike from 15 cm wide and encompassing all of the pegmatite dike at its northern most exposure to 5 cm wide approximately 10 m south, along strike. Microstructures, including quartz ribbons and recrystallized grains, indicate quartz and feldspar within the mylonite underwent dislocation creep. Infrared spectral analysis was carried out using a Nicolet micro-FTIR on mylonitic quartz ribbons, pegmatitic quartz and gneissic quartz. A small aperture size (56 μm by 50 μm) for the IR beam allowed optically clear regions of the quartz grains to be analyzed without any contribution from grain boundaries. The smallest dimension of the quartz ribbons is 0.3 mm, whereas the pegmatitic quartz has a grain size of 3 to 5 cm. Results show mylonitic quartz ribbons contain the most water (320 H:10 6 Si average, range of 50 to 1120 H:10 6 Si); pegmatite quartz contains much less water (30 H:10 6 Si average, range of 20–40 H:10 6 Si) and the gneissic quartz contained an intermediate amount (200 H:10 6 Si average, range of 20 to 870 H:10 6 Si). These data indicate that water was preferentially incorporated into the deformed quartz ribbons.

High PGE contents and extremely abundant PGE-minerals hosted in chromitites from the Veria ophiolite complex, northern Greece

More than 100 small, isolated chromite occurrences form elongated lenses that follow the NW–SE trend of the host serpentinized dunite within a 60 km 2 area 15 km SE of Veria, northern Greece. They occur only within mylonite shear zones, characterized by a strong overprint of late brittle deformation. Detailed investigation of chromitites by scanning electron microscope with energy-dispersive X-ray analysis shows that chromite is dominantly of high-Cr composition, with the atomic Cr/(Cr + Al) ratio ranging from 0.55 to 0.84, and an average value of 0.7. Interstitial primary silicates in chromitites have been replaced by secondary phases consisting of mostly Cr-andradite, Cr-chlorite and serpentine flakes in lesser amounts. The most salient feature is the high total platinum-group element (PGE) content (up to 25 ppm), coupled with the presence of extremely abundant solitary platinum-group element minerals (PGM) within massive chromite samples located along a shear zone. These PGM fragments are angular, rarely corroded grains or fine-grained aggregates and occur in a matrix of Cr-garnet. They are mostly Os–Ru–Ir–Fe alloys containing small amounts of Cr, Co ± Mn, followed by irarsite. Laurite (RuS 2) with variable Os and Ir contents occurs as remnants surrounding by Ru–Os–Ir oxides of varying composition. Available geological, mineralogical and geochemical data indicate a prolonged multistage evolution (at least four stages) for the PGE mineralization. The relatively high IPGE enrichment in chromitites seems to be related to post-magmatic processes encompassing a long period of deformation episodes, beginning with plastic deformation in the asthenospheric mantle and culminating with brittle deformation in the crust. Metasomatic fluids have, during a process of rodingitization and serpentinization at a low state of sulfidation, substantially modified the composition of primary laurite towards Fe-, Cr-, Co-, and Mn-bearing Fe–Os–Ir–Ru alloys and oxides. The most prospective locations for chromitite–PGM targets seem to exclusively be small chromite occurrences that are found along shear zones within ophiolite complexes. These shear zones post-date the initial magmatic deposition of the host chromitites.

Precollisional, multistage exhumation of subducted continental crust: The Sesia Zone, western Alps

The Sesia Zone within the Tertiary arc of the western Alps is a relic of the subducted part of the Adriatic continental margin along the SE border of the Tethyan ocean. The Sesia Zone comprises three basement nappes which individuated during Late Cretaceous (65–80 Ma) subduction to different depths at high‐pressure (HP, blueschist, eclogite facies) conditions (peak pressures of 1.0–1.2, 1.0–1.5, and 1.5–2.0 GPa). The thrusts bounding these nappes developed where the crust was previously thinned during Jurassic rifting. Crustal‐scale shear zones partly overprinted these early thrusts and exhumed coherent slices of crust containing HP rocks. Initial exhumation of the internal part of the accreted margin involved thrusting (D1) and transpressional shearing (D2) along a subvertical, E‐W trending mylonitic shear zone under retrograde blueschist‐ to greenschist‐facies conditions. This exhumation was nearly isothermal to a depth of about 25 km, where the basement nappes were juxtaposed. Subsequent exhumation of these nappes to a common depth of about 15–20 km occurred in the footwall of a greenschist‐facies, top‐SE extensional shear zone (D3) preserved in some of the highest mountain peaks of the Sesia Zone. New Rb‐Sr mineral ages constrain D2 to have occurred at about 60–65 Ma and D3 at about 45–55 Ma. Thus top‐SE extensional exhumation was broadly coeval with Eocene, SE directed subduction of the Liguro‐Piemont oceanic lithosphere beneath the Adriatic margin. Slow cooling and erosional denudation of the Sesia Zone from 45 to 30 Ma occurred in the hanging wall of the Gressoney extensional shear zone (D4), which itself contributed to the exhumation of Eocene HP and ultra‐HP oceanic rocks in its footwall. By 30 Ma, HP rocks of the Sesia Zone were intruded by shallow granitic plutons which were eroded and redeposited within volcanoclastic sediments. Oligo‐Miocene Insubric backfolding and thrusting (D5) only exhumed northeastern parts of the Sesia Zone, where HP metamorphism is absent or was overprinted by Tertiary temperature‐dominated metamorphism. Most exhumation of continental HP rocks in the Sesia Zone therefore preceded Tertiary Alpine collision and coincided with Late Cretaceous to Early Tertiary subduction of the Adriatic and Tethyan lithosphere. The transition from D2 to D3 in the Sesia Zone is interpreted to mark a change from high‐stress, oblique SE directed subduction and accretion of the distal Adriatic continental margin to NW retreating, low‐stress subduction of the Liguro‐Piemont oceanic lithosphere.

Deformation and recrystallization mechanisms in mylonitic shear zones in naturally deformed extrusive Eocene–Oligocene rocksalt from Eyvanekey plateau and Garmsar hills (central Iran)

Microstructural processes in mylonitic shear zones from extrusions of Eocene–Oligocene rocksalts from Garmsar hills and Eyvanekey plateau (central Iran) are described and analyzed. The halite samples were studied by transmitted light microscopy of gamma-irradiated thin sections, subgrain size palaeopiezometry of polished and chemically etched samples and texture measurements by electron backscatter diffraction (EBSD). The studied shear zones are thin (<5 cm), subhorizontal and bedding parallel. The protomylonite comprises 2–6 mm sized grains, occasionally rich in primary fluid inclusions indicative of their primary non-recrystallized state. Abundant, well-developed subgrains suggest that the protomylonite was deformed mainly by dislocation processes. Elongated subgrains at grain edges point to recrystallization by fluid-assisted grain boundary migration. Recrystallized, strain-free grains are common. The material in the mylonitic zones is fine grained (∼0.6 mm) with strong shape-preferred orientation. Microstructures such as oriented fibrous overgrowths and growth banding (observed in gamma-irradiated sections) suggest that the principal deformation mechanism was solution-precipitation creep (non-conservative grain boundary migration and grain boundary sliding accommodated by solution-precipitation). Crystal fabrics measured by EBSD show a weak crystallographic preferred orientation that is consistent with solution-precipitation accommodated grain boundary sliding. Using published flow laws, the strain rate in the fine-grained mylonites is about 10 −10 l/s, which is in a good agreement with earlier in situ measurements on glacier salt flow rate.

Metamorphism of Precambrian–Palaeozoic schists of the Menderes core series and contact relationships with Proterozoic orthogneisses of the western Çine Massif, Anatolide belt, western Turkey

The tectonic setting of the southern Menderes Massif, part of the western Anatolide belt in western Turkey, is characterized by the exhumation of deeper crustal levels onto the upper crust during the Eocene. The lowermost tectonic units of the Menderes Massif are exposed in the Çine Massif, where Proterozoic basement orthogneisses of the Çine nappe are in tectonic contact with Palaeozoic metasedimentary rocks of the Selimiye nappe. In the southern Çine Massif, orthogneiss and metasedimentary rocks are separated by the southerly dipping Selimiye shear zone, preserving top-to-the-S shearing under greenschist facies conditions. In contrast, in the western Çine Massif, the orthogneiss is deformed and mylonitic near the contact with the metasedimentary rocks. The geometry of the mylonite zone and the observed shear directions change from north to southwest. In the north, the mylonite zone dips shallowly to the north, with top-to-the-N shear sense indicators showing northward thrusting of the orthogneiss over the metasedimentary rocks. In the southwest, the mylonite zone resembles a steep N–S striking strike-slip shear zone associated with top-to-the-SSW sense of shear. Overall, the geometry of the mylonite shear zone is consistent with northward movement of the orthogneiss relative to the metasedimentary rocks. Different shear senses are attributed to strain partitioning.AFM diagrams and P–T pseudosections with mineral parageneses of metasedimentary rocks of the Selimiye nappe and metasedimentary enclaves within the orthogneiss of the Çine nappe indicate a single Barrovian-type metamorphism. An earlier higher pressure phase is evident from staurolite–chloritoid inclusions in garnets of the Çine nappe, suggesting a clockwise P–T path. A similar path is inferred for the Selimiye nappe. Index minerals and the sequence of mineral parageneses point to a single amphibolite facies metamorphic event affecting metasedimentary rocks of both nappes, which predates Eocene emplacement of the high pressure–low temperature Lycian and Cycladic blueschist nappes. Northward thrusting of the orthogneiss onto the metasedimentary rocks of the Selimiye nappe is coeval with amphibolite facies metamorphism. Recently postulated polymetamorphism cannot be supported by this study. Petrological data provide no evidence for burial of the lower units of the Menderes Massif to depth greater than 30 km during closure of the Neo-Tethys. A major pre-Eocene tectonic event associated with top-to-the-N thrusting and Barrovian-type metamorphism could lend support to the idea of a Neo-Tethys (sensu stricto) suture south of the Menderes Massif and below the Lycian nappes.

The Broodkop shear zone, southeast Vredefort dome (South Africa): a Dominion-related extensional feature?

Research Article| September 01, 2006 The Broodkop shear zone, southeast Vredefort dome (South Africa): a Dominion-related extensional feature? Cristiano Lana; Cristiano Lana Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Private Bag 3, P.O. Wits 2050, Johannesburg, South Africa, Impacts and Astromaterials Research Centre, Imperial College London, Royal School of Mines, South Kensington Campus, United Kingdom, SW7 2AZ, e-mail: c.lana@imperial.ac.uk Search for other works by this author on: GSW Google Scholar Roger L. Gibson; Roger L. Gibson Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Private Bag 3, P.O. Wits 2050, Johannesburg, South Africa, e-mail: gibsonr@geosciences.wits.ac.za Search for other works by this author on: GSW Google Scholar Wolf Uwe Reimold Wolf Uwe Reimold Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Private Bag 3, P.O. Wits 2050, Johannesburg, South Africa Museum für Naturkunde, Humboldt University, Invalidenstrasse 43, 10115 Berlin, e-mail: uwe.reimold@museum.hu-berlin.de Search for other works by this author on: GSW Google Scholar Author and Article Information Cristiano Lana Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Private Bag 3, P.O. Wits 2050, Johannesburg, South Africa, Impacts and Astromaterials Research Centre, Imperial College London, Royal School of Mines, South Kensington Campus, United Kingdom, SW7 2AZ, e-mail: c.lana@imperial.ac.uk Roger L. Gibson Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Private Bag 3, P.O. Wits 2050, Johannesburg, South Africa, e-mail: gibsonr@geosciences.wits.ac.za Wolf Uwe Reimold Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Private Bag 3, P.O. Wits 2050, Johannesburg, South Africa Museum für Naturkunde, Humboldt University, Invalidenstrasse 43, 10115 Berlin, e-mail: uwe.reimold@museum.hu-berlin.de Publisher: Geological Society of South Africa First Online: 09 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2006 Geological Society of South Africa South African Journal of Geology (2006) 109 (3): 265–278. https://doi.org/10.2113/gssajg.109.3.265 Article history First Online: 09 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 Cristiano Lana, Roger L. Gibson, Wolf Uwe Reimold; The Broodkop shear zone, southeast Vredefort dome (South Africa): a Dominion-related extensional feature?. South African Journal of Geology 2006;; 109 (3): 265–278. doi: https://doi.org/10.2113/gssajg.109.3.265 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 SocietySouth African Journal of Geology Search Advanced Search Abstract The Broodkop shear zone is an approximately 1 km-wide, vertical, northeasterly-trending, mylonitic shear zone that separates upper amphibolite- to granulite-facies gneisses (≥670°C, 4.5 to 5.5 kbar) from mid-greenschist-facies (~400°C) greenstones in the southeastern part of the Vredefort dome. The mylonitic fabric transposes 3.09 to 3.08 Ga peak-metamorphic migmatitic structures but shows progressive overprinting of shear-related amphibolite-facies assemblages by greenschist-facies assemblages; indicating initiation of deformation close to the metamorphic peak and continued shearing during cooling. Shear-sense indicators are consistent with southeastern-side-up dip-slip movement. Removal of Late Archaean and Proterozoic rotation effects, particularly those associated with formation of the Vredefort dome, suggests that the shear zone formed as a shallowly northwesterly-dipping extensional feature associated with exhumation of the mid-crustal amphibolite-to-granulite-facies footwall rocks and transport of the greenschist-facies hanging wall rocks to the northwest. Several kilometres of throw along the shear zone is suggested by the juxtaposition of rocks with markedly different metamorphic grades. Assuming an initial shallow dip for the shear zone, this suggests in excess of 10 km of dip-slip movement, making the Broodkop shear zone a crustal-scale feature. The timing and geometry of the shear zone suggest that it may represent a mid-crustal manifestation of the rifting event that led to the deposition of bimodal volcanics and associated sediments of the 3074 ± 6 Ma Dominion Group. The Broodkop shear zone provides an appropriate mechanism to explain how these volcanics could be deposited unconformably on mid-crustal gneisses within only a few million years of the high-grade metamorphism. It provides a link between the final major pulse of greenstone-TTG formation in the core of the Vredefort dome and the initiation of a series of large sedimentary and volcanic basins that dominated the Late Archaean history of the Kaapvaal craton. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Exhumation History of a Garnet Pyroxenite-bearing Mantle Section from a Continent-Ocean Transition (Northern Apennine Ophiolites, Italy)

Garnet clinopyroxenite and garnet websterite layers occur locally within mantle peridotite bodies from the External Liguride Jurassic ophiolites (Northern Apennines, Italy). These ophiolites were derived from an ocean–continent transition similar to the presentday western Iberian margin. The garnet clinopyroxenites are mafic rocks with a primary mineral assemblage of pyrope-rich garnet þ sodic Al-augite (Na2O 2.5 wt %, Al2O3 12.5 wt %), with accessory graphite, Fe–Ni sulphides and rutile. Decompression caused Na-rich plagioclase (An50–45) exsolution in clinopyroxene porphyroclasts and extensive development of symplectites composed of secondary orthopyroxene þ plagioclase (An85–72) þ Al-spinel ± clinopyroxene ± ilmenite at the interface between garnet and primary clinopyroxene. Further decompression is recorded by the development of an olivine þ plagioclase-bearing assemblage, locally under syn-kinematic conditions, at the expense of two-pyroxenes þ Al-spinel. Mg-rich garnet has been also found in the websterite layers, which are commonly characterized by the occurrence of symplectites made of orthopyroxene þ Al-spinel ± clinopyroxene. The enclosing peridotites are Ti-amphibole-bearing lherzolites with a fertile geochemical signature and a widespread plagioclase-facies mylonitic foliation, which preserve in places a spinel tectonite fabric. Lu–Hf and Sm–Nd mineral isochrons (220 ± 13 Ma and 186.0 ± 1.8 Ma, respectively) have been obtained from a garnet clinopyroxenite layer and interpreted as cooling ages. Geothermobarometric estimates for the high-pressure equilibration have yielded T 1100 C and P 2.8 GPa. The early decompression was associated with moderate cooling, corresponding to T 950 , and development of a spinel tectonite fabric in the lherzolites. Further decompression associated with plagioclase–olivine growth in both peridotites and pyroxenites was nearly isothermal. The shallow evolution occurred under a brittle regime and led to the superposition of hornblende to serpentine veining stages. The garnet pyroxenitebearing mantle from the External Liguride ophiolites represents a rare tectonic sampling of deep levels of subcontinental lithosphere exhumed in an oceanic setting. The exhumation was probably accomplished through a two-step process that started during Late Palaeozoic continental extension. The low-pressure portion of the exhumation path, probably including also the plagioclase mylonitic shear zones, was related to the Mesozoic (Triassic to Jurassic) rifting that led to continental break-up. In Jurassic times, the studied mantle sequence became involved in an extensional detachment process that resulted in sea-floor denudation.

Networking of shear zones at the brittle-to-viscous transition (Cap de Creus, NE Spain)

A crustal-scale shear zone network at the fossil brittle-to-viscous transition exposed at Cap de Creus, NE Spain evolved by coeval fracturing and viscous, mylonitic overprinting of an existing foliation. Initial fracturing led to mylonitic shearing as rock softened in ductilely deformed zones surrounding the fractures. Mylonitic shear zones widened by lateral branching of fractures from these shear zones and by synthetic rotation of the existing foliation between the fractures and shear zones. Shear zones lengthened by a combination of fracturing and mylonitic shearing in front of the shear zone tips. Shear zones interconnected along and across their shearing planes, separating rhomb-shaped lozenges of less deformed rock. Lozenges were subsequently incorporated into the mylonitic shear zones by widening in the manner described above. In this way, deformation became homogeneous on the scale of initial fracturing (metre- to decametre-scale). In contrast, the shear zone network represents localisation of strain on a decametre-length scale. The strength of the continental crust at the time of coeval fracturing and viscous shearing is inferred to have decreased with time and strain, as fracturing evolved to mylonitic shearing, and as the shear zones coalesced to form a through-going network subparallel to the shearing plane. Crustal strength must therefore be considered as strain- and scale-dependent.

Extensional shear zones, granitic melts, and linkage of overstepping normal faults bounding the Shuswap metamorphic core complex, British Columbia

An investigation of a normal-fault system in the southern Canadian Cordillera documents extensional shear zones exhumed from the middle crust, including the root of a transfer zone between overstepping fault segments in which shearing was enhanced by leucogranitic melts. The western margin of the Shuswap metamorphic core complex is delimited by west-dipping, ductile-brittle normal faults of the Eocene Okanagan Valley fault system. Migmatites with gently dipping mylonitic fabrics have been exhumed in the footwalls of the Okanagan-Eagle River and Adams-North Thompson fault segments. The mylonitic fabrics formed in upper amphibolite facies, continued to evolve in greenschist facies, and display asymmetric features that consistently indicate westward movement of the hanging wall. The Shuswap Lake transfer zone is a 45-km-wide left stepover between the two fault segments where a domed mylonitic shear zone has been exhumed and cut by high-angle brittle faults. Mylonites in the transfer zone display the same sense of shear as the mylonites that are associated with the overstepping fault segments. All of these mylonites are interpreted as having formed in a mid-crustal shear zone in which the fault system was rooted. The mylonites in the transfer zone are distinct, however, in that they formed in leucogranite (the Pukeashun granite). Structural relationships imply that the leucogranitic melts were emplaced during extensional shearing and that their distribution may have been influenced by preexisting structures in both the footwall and the hanging wall of the system. The melts in turn controlled the evolution of the transfer zone, facilitating the processes of heterogeneous extension, footwall doming, and differential exhumation.

Syn‐deformational migmatites and magmatic‐arc metamorphism in the Xolapa Complex, southern Mexico

AbstractThe Xolapa Complex (XC) is the largest plutonic and metamorphic mid‐crustal basement unit in Mexico and represents an ancient continental magmatic‐arc. A complete range from metatexite to diatexite migmatitic structures has been produced during a single high‐grade metamorphic event. However, structural relics reveal the existence of early Cpx + Pl + Qtz ± Opx and Grt + Opx + Pl + Qtz ± Cpx pre‐migmatitic metamorphic assemblages. Field relationships and microstructural observations allow us to constrain five pre‐, syn‐ and post‐migmatitic deformational phases. It is argued that migmatitic structures and minor anatectic granites were developed during ductile recumbent folding and shear structures related to the D2–D3 phases. Late post‐migmatitic ductile‐brittle deformation is evidenced by the development of NNE trending transpressional thrusting (D4), and E–W left‐lateral mylonitic shear zones (D5). Biotite‐breakdown melting in felsic rocks and amphibole‐breakdown melting in mafic rocks, as well as geothermobarometric results, indicate that metamorphism took place at temperatures from 830 to 900 °C and pressures ranging from ≥6.3 to 9.5 kbar. Late migmatitic assemblages equilibrated in the highest temperature range along a clockwise P–T path. The relationships between the large diversity of migmatitic structures and the progressive production of melt suggest that feedback relations prevailed as a time‐marker during a contractional regime. Deformation, metamorphism, and plutonism of the XC show that this terrane evolved as a north‐east‐verging thrust system with synkinematic metamorphism and partial melting, during the Late Cretaceous – Palaeogene. The tectonothermal history of XC is analogous to a Cordilleran metamorphic magmatic‐arc formed in an accretionary tectonic framework. This new model provides constraints on the exhumation mechanism and thermal evolution of southern Mexico.

OCCURRENCE AND SIGNIFICANCE OF SERPENTINITE-HOSTED, TALC- AND AMPHIBOLE-RICH FAULT ROCKS IN MODERN OCEANIC SETTINGS AND OPHIOLITE COMPLEXES: AN OVERVIEW

This paper reviews the occurrence and significance of talc- and amphibole-rich fault rocks developed in mafic-ultramafic sequences and evaluates their role in deformation and alteration of the oceanic lithosphere from different tectonic settings (from spreading mid-ocean ridges up to orogenic belts). Recently, talc and amphibole-rich fault rocks have been sampled and studied from detachment fault surfaces along slow and ultra-slow spreading mid-ocean ridges, and constraining the conditions of deformation and strain localization during the evolution of oceanic core complexes. These rocks are documented not only in oceanic core complexes, but also in other oceanic fracture zones where ultramafic rocks are exposed on the seafloor, while only few occurrences have been reported in ophiolite sequences. Samples recovered in situ in oceanic settings record heterogeneous deformation (crystal-plastic to cataclastic) under greenschist- facies conditions and are commonly restricted to localized shear zones (< 200 m) and are associated with intense talc-amphibole metasomatism. The presence of mechanically weak minerals, such as talc, serpentine and chlorite, may be critical to the development of such fault zones and may enhance unroofing of upper mantle peridotites and lower crustal gabbroic rocks during seafloor spreading. Talc in particular may be influential in lubricating and softening mylonitic shear zones and can lead to strain localization and focused hydrothermal circulation along such faults. The rheology of these rocks, and its evolution during dehydration reactions could play an important role also in subduction-zone processes and during the formation of ultramafic orogenic belts. Here, we review the occurrence and significance of talc- and amphibole-rich fault rocks in different tectonic settings on the seafloor and evaluate their role in deformation and alteration of the oceanic lithosphere.

Genesis of Mayouom kaolin deposit (western Cameroon)

Geological, mineralogical and geochemical studies were carried out on Mayouom kaolin deposit located within a mylonitic shear zone about 30 km north of Foumban town (western Cameroon), in order to define the ore genesis and its economic interest. Two facies (sandy kaolin and sand-poor kaolin) were characterised by different techniques: description of outcrops and pits, X-ray diffraction, differential scanning calorimetric and thermogravimetric analyses, scanning electron microscopy, Fourier transform infrared spectrometry and bulk chemical analyses. Sand-poor kaolin (76–85% kaolinite and 2–9% quartz) forms 1 to 2 m thick veins which crosscut sandy kaolin (48–59% kaolinite and 36–47% quartz). All the samples have the following mineral phases in common, close to an advanced argillic alteration system: kaolinite + quartz + illite/muscovite + anatase ± hematite ± Ba, Sr-hydroxyapatite ± zircon ± pyrite ± florencite. Both sandy and sand-poor kaolins present kaolinite mineral with a homogeneous shape and a good crystallinity. Results from the different analytical techniques point out a hydrothermal alteration of feldspar- and mica-rich rocks as petrogenetic origin of kaolins. Sandy kaolin comes from mylonite basement while sand-poor kaolin is related to the alteration of magmatic intrusive shape veins. Due to its high kaolinite content (up to 85%) and the low iron mineral content (< 1.5%), Mayouom kaolin is a suitable raw material for white burning industrial clays. Moreover, the advanced argillic alteration mineral assemblage can be considered as a rough indicator for exploration of related ores in neighbouring areas and along the northern Foumban shear zone.

Chemical and mineralogical data of the metalliferous mineralization from S. Carlo mine (Peloritani mts, Ne Sicily, Italy)

The mineralization processes in the Peloritani Belt (Southern Sector of the Calabria- Peloritani Arc) prevalently developed during the Variscan orogenesis producing Pb, Zn, Fe, As, Sb, Cu, Ag, W, etc. polymetalliferous ore-bearing horizons. This paper focuses on the polymetalliferous mineralization recognised in the ancient S. Carlo Mine, which has already been subject of some studies and is part of an important discordant vein deposits system that are widespread in the Mandanici Unit (MaU). This Unit is characterized by a Variscan low-P, polyphasic and plurifacial metamorphic basement, exhibiting a prograde zoning, from chlorite zone of greenschist facies to oligoclase-almandine zone of amphibolite facies. The Variscan main foliation (Fv2) is irregularly cut by mineralized veins of decimetric to metric width. They are also perpendicular to the Alpine mylonitic shear zones of metric thickness developing along the sub-horizontal tectonic contacts between the tectono-stratigraphic units. These vein deposits formed along late-Alpine systems of fractures and faults, after Peloritani nappe emplacement. Minerographic study reveals a metalliferous mineral association mainly composed of tetrahedrite associated with, in order of decreasing abundance, chalcopyrite, bournonite, pentlandite, stromeyerite, arsenopyrite, scheelite, galena, sphalerite, pyrite, bismuthinite, boulangerite, jamesonite, covellite, bornite and argentite. Quartz, siderite and ankerite among non-metalliferous minerals are predominant. This work has been supported by mineralogical studies and chemical analyses carried out by Atomic Absorption and Inductively Coupled Plasma-Mass Spectrometry on powdered and separated samples of minerals. Geochemical data (major and trace elements) have allowed a detailed characterization of the minerals. They have revealed that the most significant minerals with Au contents around 1 ppm are tetrahedrite, sphalerite, chalcopyrite and bournonite. The presence of this noble metal in all minerals is important and shows that the mineralizing event is late and widespread. The Ag content is generally about 2000 ppm in all metalliferous minerals, and is higher than those found in the same minerals of other Peloritani deposits in which this content is around 1000 ppm. Sphalerite and carbonates show very low Ag content. Chalcopyrite, sphalerite, bournonite and tetrahedrite show high contents of Ni, As, Bi. Cd and Mn are concentrated in sphalerite. Fe content in sphalerite is on average > 6%, in agreement with pressure-temperature conditions (pressure from 2 to 3 kbar and temperature from 420 to 550 C).

Three-dimensional model and late stage warping of the Plattengneis Shear Zone in the Eastern Alps

The Plattengneis shear zone is a 250–600 m thick, flat lying, Cretaceous, eclogite facies, mylonitic shear zone, with north-over-south transport direction, that is exposed over almost 1000 km 2 in the Koralpe region along the eastern margin of the Alps. Although the shear zone is one of the largest in the Alps, its role in the Eoalpine metamorphic evolution and the subsequent exhumation of the region, remain enigmatic and its large-scale geometry is not well understood. The outcrop pattern suggests that the shear zone is made up of a single sheet that is folded into a series of open syn- and antiforms with wavelengths of about 10 km. Eclogite bodies occur above, within and below the shear zone and there is no metamorphic grade change across the shear zone. In the south, the fold axes strike east–west and plunge shallowly to the east. In the north, the fold axes are oriented in north–south direction and form a dome shaped structure of the shear zone. Total shortening during this late stage warping event was of the order of 5%. Indirect evidence constrains this folding event to have occurred between 80 and 50 Ma and the fold geometry implies that the final exhumation in the Koralpe occurred somewhat later than further north. Interestingly, the shear zone appears to strike out of the topography in the south and dip into the topography in the north, so that north of the shear zone only hanging-wall rocks are exposed and south of it only foot-wall rocks. Possibilities for the geometric relationship of the Plattengneis shear zone with the surrounding south dipping detachments are discussed.

Autochthonous and allochthonous strata of the El Callao greenstone belt: Implications for the nature of the Paleoproterozoic Trans-Amazonian orogeny and the origin of gold-bearing shear zones in the El Callao mining district, Guayana shield, Venezuela

The tectonic style of the Paleoproterozoic Trans-Amazonian orogeny in the Guayana shield was considered by previous workers to represent non-actualistic tectonics in which rising masses of granitoid magma of the Supamo complex deformed 2.1–2.0Ga supracrustal belts of volcanic and sedimentary rocks to form greenstone belts. Subsequent collision with the dominantly granulite-grade Imataca complex deformed the greenstone belts. However, the base of the greenstone belts is everywhere marked by a major mylonitic shear zone that placed rocks of the greenstone belts atop a regional unit of quartzite that sits unconformably upon both the Imataca and Supamo complexes. The simplest interpretation of these relationships is that the greenstone belts are allochthonous with respect to the Imataca and Supamo complexes. As the quartzite oversteps the contact between the Imataca and Supamo complexes, juxtaposition of the two complexes must predate deposition of the quartzite and deformation of the Trans-Amazonian orogeny.The quartzite and the thrust fault are used as paleohorizontal markers to unfold the greenstone belt in the El Callao area where auriferous shear zones, which clearly predate the thrusting, show reverse sense of movement. When restored to their pre-folding orientation they are normal faults. This, and the tholeiitic to calc-alkaline volcanic packages, are compatible with an extensional forearc to arc setting for both the mineralization and the volcano-sedimentary sequences.East–northeasterly trending folds of both the allochthon and the autochthon, coupled with the northerly thickening of the quartzite, suggest that the allochthon was part of an unknown craton that formerly lay to the north. A second regional fold set, which trends northerly, refolds the earlier structures. It is possibly related to collision of the San Carlos terrane with the amalgamated Imataca–Supamo–Pastora terranes. The unconformably overlying 1.98Ga Cuchivero arc terrane provides a minimum age for the two collisional events.

Zircon geochronology and compositional record of late- to post-kinematic granitoids associated with the Bavarian Pfahl zone (Bavarian Forest)

The northwest-striking Pfahl zone, Bavarian Forest, is a mylonitic shear zone that is associated with brittle-ductile deformation fabrics and a conspicuous hydrothermal quartz mineralization. Two granites from this shear zone yield U–Pb and Pb–Pb evaporation ages between 321–329 Ma and two granodiorites give concordant 238U–206Pb and 235U–207Pb ages of 325±3 Ma and 326±3 Ma, respectively. Zircon populations of the granitoids show sub-types clustering around S20, S10 (granite) and S22 to S24 (granodiorite) testifying different magma affinity. Compositional and isotopic characteristics indicate that the granites and granodiorites were coeval melts, but not differentiates of a single parent magma. The granodiorites were derived from a source with higher time-integrated 87Rb/86Sr and lower 147Sm/144Nd ratios than the granites. One granite body is transected by the shear zone but the main mass of the granite is largely undeformed. This finding suggests that granite intrusion predates the final stage of ductile deformation along the Pfahl shear zone.

Paleocene–Eocene record of ophiolite obduction and initial India‐Asia collision, south central Tibet

Uppermost Cretaceous to Eocene marine sedimentary sequences occur both to the south and north of the Yarlung Zangbo suture in south central Tibet. They consist of Indian‐margin strata of the northern Tethyan Himalaya and Asian‐margin strata of the Gangdese forearc. Both assemblages are characterized by major changes in depositional environment and sedimentary provenance at ∼65 Ma and an appearance of detrital chromium‐rich spinel of ophiolite affinity (TiO2 generally &lt;0.1 wt%) during the Paleocene. Ophiolitic melange exposed along the suture could have provided a source for detrital spinel. The melange occurs in the hanging wall of a north dipping, south directed mylonitic shear zone which includes a tectonic sliver of mafic schist. Amphibole from the schist yields 40Ar/39Ar ages of ∼63 Ma, which we attribute to cooling during slip along the shear zone and southward obduction of the melange. Melange obduction was coeval with the development of an angular unconformity within the Gangdese forearc basin to the north (between late Maastrichtian time and ∼62 Ma). Upper Paleocene to middle Eocene sandstones in the northern Tethyan Himalaya yield 200–120 Ma U‐Pb detrital zircon ages and 190–170 Ma 40Ar/39Ar detrital mica ages. These detrital grains were most likely sourced from regions north of the Yarlung Zangbo suture, suggesting that onset of India‐Asia collision in south central Tibet is middle Eocene or older in age. Collectively, our results support previous suggestions that oceanic rocks were obducted onto the northern margin of India during latest Cretaceous–earliest Tertiary time. Coeval changes in Gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between India and Asia at ∼65 Ma. Alternatively, in concert with the conventional view of Eocene collision initiation, the obducted oceanic rocks may be of intraoceanic origin, while coeval changes in Gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean‐continent convergence following the demise of the intraoceanic subduction zone.