Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Kazdag Massif exposes an extensional metamorphic dome in the Biga Peninsula of northwest Turkey, bounded on both flanks by detachments and/or shear zones. The northern flank is marked by the extensional Alakeci shear zone (ASZ), with poorly known P-T-t path. We therefore focus on metamorphic conditions and temporal history of the ASZ to determine its tectono-metamorphic evolution. ASZ mylonites were derived from both the footwall Kazdag Massif core metamorphic rocks and the hanging wall Cetmi melange lithologies. The mylonitic fabrics in the ASZ depict a top-to-the NNE shearing, parallel to the NNE-plunging stretching lineation and NNW-dipping mylonitic foliation. This geometry implies normal sense movement i.e. north-side down-dip extensional displacement along this flank of the Kazdag Massif. The northward transition from ductile to brittle-ductile shear regime through the ASZ shows that the non-coaxial deformation occurred at decreasing temperatures and degree of metamorphism. The paragenesis in equilibrium of the mylonitic gneisses and schists contains quartz + feldspar + muscovite + biotite + garnet staurolite, with late retrogressive chlorite after biotite and garnet. Calculated thermo-barometric conditions for ASZ rocks yielded pressures between 6.9 and 5.7 kbar and temperatures between 706 and 587 degrees C, which constrain the peak metamorphism. The mylonitic rocks supplied isochron Ar-39/Ar-40 mica ages between 26.97 and 24.19 Ma, which we interpret to date the cooling of the mylonites following the P-T decrease across the ASZ. The metamorphic data support the structural record and strengthen the extensional character of the ASZ. The transition from sillimanite-bearing footwall Kazdag Massif core gneisses in the deeper structural levels to chlorite schists; towards the shallow structural levels at the top of the ASZ, is consistent with an extensional exhumation of the metamorphic pile. These data allow us to determine the peak P-T conditions and the temporal evolution in the northern flank of the Kazdag Massif, where Late Oligocene extensional exhumation was assisted by NNE-directed ductile-brittle ASZ, which had operated from amphibolite to greenschist facies. The ASZ, together with similar time-constrained ductile-brittle activity of SW-directed detachment along the southern flank of the massif, collectively indicates a bivergent mode of extension in the western Kazdag Massif. The latter is likely influenced by magmatic activity spatially and temporally related to these extensional zones. At the regional scale, the bivergent tectono-metamorphic pattern of the Kazdag Massif is similar to those observed on other places of the north-central Aegean domain.
ISSN: 0024-4937

hal-00445246
https://hal.archives-ouvertes.fr/hal-00445246
DOI : 10.1016/j.lithos.2009.02.010

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In Western Mediterranean, the Rif belt in Morocco is part of the Gibraltar Arc built during the Tertiary in the framework of Eurasia-Africa convergence. The structural and metamorphic evolution of the internal units of this belt as well as their timing, crucial to constrain the geodynamic evolution of the Alboran Sea, is still largely debated.Our study on the Ceuta Peninsula (Northern Rif) provides new structural, petrological and geochronological data (U-Th-Pb, Ar-Ar), which allow to precise the tectono-metamorphic evolution of the Lower Sebtides metamorphic units with:(1) a syn-metamorphic thrusting event developed under granulite facies conditions (7–10 kbar and 780–820 °C). A major thrust zone, the Ceuta Shear Zone, drove the emplacement of metapelites and peridotitic lenses from the Ceuta Upper Unit over the orthogneisses of the Monte Hacho Lower Unit. This compressional event ended during the Upper Oligocene.(2) an extensional event developed at the boundary between amphibolite and greenschist facies conditions (400–550 °C and 1–3 kbar). During this event, the Ceuta Shear Zone has been reactivated as a normal fault. Normal ductile shear zones contributed to the final exhumation of the metamorphic units during the Early Miocene.We propose that the compressional event is related to the formation of an orogenic wedge located in the upper plate, in a backward position, of the subduction zone driving the geodynamic evolution of the Alboran domain. In this context, the episode of lithospheric thinning could be related to the opening of the Alboran basin in a back-arc position.Furthermore, unlike the previous models proposed for the Rif belt, the tectonic coupling between mantle peridotites and crustal metamorphic rocks occurred in Ceuta Peninsula at a depth of 20–30 km under high temperature conditions, before the extensional event, and thus cannot be related to the back-arc extension.
ISSN: 0040-1951

insu-01682810
https://hal-insu.archives-ouvertes.fr/insu-01682810
DOI : 10.1016/j.tecto.2017.11.028

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The deep structure and deformation mechanisms of the Longmen Shan thrust belt (Sichuan, China), at the eastern border of the Tibetan plateau, were largely debated after the devastating Mw 7.9 Wenchuan earthquake (2008). Recent geophysical studies and field investigations have been focused on the active motion of the major Beichuan fault, which ruptured during the earthquake. However, the total exhumation across the fault still remains unclear. In the hanging wall of the Beichuan fault, the South China block is exhumed in the Pengguan massif. Close to the Beichuan fault, the rocks of the Pengguan massif underwent greenschist facies metamorphism associated with brittle-ductile deformation. No metamorphism is observed in the footwall of the fault. In this study, we characterize and date the metamorphic history recorded in the hanging wall of the Beichuan fault in order to constrain the depth and timing of exhumation of the rocks of the Pengguan massif along the fault. A high-resolution petrological approach involving chemical analyses and X-ray maps was used to analyze the micrometric metamorphic minerals. The P-T conditions of the greenschist facies metamorphic event were estimated by an inverse multi-equilibrium thermodynamic approach. The results, 280 ± 30 °C and 7 ± 1 kbar, suggest that the rocks of the Pengguan massif were exhumed from ca. 20 km depth. Our results underline the importance of the thrusting component in the long-term behavior of the Beichuan fault and provide a minimal depth at which the fault is rooted. In situ laser ablation 40Ar/39Ar dating of metamorphic white mica revealed that the greenschist overprint occurred at 135–140 Ma.The Pengguan massif was therefore partially thrusted along the Beichuan fault during the Lower Cretaceous, long before the Eocene-Miocene exhumation phase which is well-constrained by low-temperature thermochronology. Our results provide the first independent depth information for the exhumation history of the Pengguan massif and reveal a previously undocumented Lower Cretaceous tectonic event that marks the onset of the thick-skinned deformation in the external domain of the Longmen Shan (East of the Wenchuan Fault).
ISSN: 0743-9547

hal-01685556
https://hal.archives-ouvertes.fr/hal-01685556
DOI : 10.1016/j.jseaes.2017.04.003

Éditeur(s) : HAL CCSD
Résumé : The Karakorum Fault zone (KFZ), lying in the west of the Tibetan plateau, is an important dextral strike-slip fault and the important northern geological boundary of the west Himalaya. In the AyilaRi' gyu Range-Gai Basin area lying in the southeast of the KFZ, Ar-40/Ar-39 thermochronologic analysis was performed on the syn-tectonic minerals in the, metamorphic rocks in the Karakorum ductile shear zone, with the base of petrology and deformation structure researches. According to the microstructure features, the minerals in the shear zone recorded the continuous deformation from high ( > 600 degrees C) to low temperature ( < 250 degrees C) appeared as that the nearly horizontal dextral shearing has transformed into the oblique dextral normal slip, made the greenschist facies of deformation stack oil the early medium-high temperature deformation, indicated the uplift during the slip. The thermochronologic revealed the continuous shear process from the early Miocene to 4Ma at least, which containing three rapid cooling stage: from 25 similar to 22Ma to 21 similar to 18Ma, the deformation locally stopped or slower in the, shallow high temperature shear process; from 15Ma to 12Ma, the rapid uplift of the AyilaRi' gyu Range, the initial formation of the Gar Basin together with the main deep incision of river during the strip-slip of the KFZ; since 9Ma, the AyilaRi' gyu Range has increased the rapid uplift and the Gar Basin has formed finally. According to the different levels of deformation and the dextral displacement of the surface landform with different ages, the long term slip rate and the uplift rate indicated the intense dextral strike slip and uplift associated since the late Quarternary Period.
Acta Petrologica Sinica

hal-00411572
https://hal.archives-ouvertes.fr/hal-00411572

Éditeur(s) : HAL CCSD Springer Verlag (Germany) Springer Verlag
Résumé : International audience
La Cabaña peridotite is part of a dismembered ophiolite complex located within the metamorphic basement of the Coastal Cordillera of south-central Chile, and is the only location in Chile were Cr-spinels have been described so far. The La Cabaña peridotite is part of the Western Series unit, which comprises meta-sedimentary rocks, metabasites, and serpentinized ultramafic rocks. This unit has been affected by greenschist-facies metamorphism with reported peak PT conditions of 7.0–9.3 kbar and 380°–420 °C. Within La Cabaña peridotite Cr-spinels are present in two localities: Lavanderos and Centinela Bajo. In Lavanderos, Cr-spinel occurs in small chromitite pods and as accessory/disseminated grains with a porous or spongy texture in serpentinite, whereas in Centinela Bajo Cr-spinel is present as accessory zoned grains in partly serpentinized dunites, and in chromitite blocks. All Cr-spinels display variable degrees of alteration to Fe2+-rich chromite with a variation trend of major elements from chromite to Fe2+-rich chromite similar to those observed in other locations, i.e., an increase in Fe2O3 and FeO, a decrease in Al2O3 and MgO. Cr2O3 content increases from chromite to Fe2+-rich chromite in chromitite pods from Lavanderos and chromitite blocks from Centinela Bajo, but decreases in ferrian chromite zones in accessory grains from Centinela Bajo. The minor element (Ti, V, Zn, Ni) content is mostly low and does not exceed 0.4 wt.%, with the exception of MnO (<0.9 wt.%), which shows a correspondence with increasing degree of alteration. Cr# (Cr/Cr + Al) versus Mg# (Mg/Mg + Fe2+) and Fe3+/Fe3++Fe2+ versus Mg# plots are used to illustrate the Cr-spinel alteration process. Overall, the Cr-spinels from Lavanderos (chromitite pods and disseminated grains) exhibit Cr# values ranging from 0.6 to 1.0, Mg# (Mg/Mg + Fe2+) below 0.5, and (Fe3+/Fe3++Fe2+) <0.4. Cr-spinels from chromitites in Centinela Bajo have Cr# and Mg# values that range from 0.65 to 1.0, and 0.7-0.3, respectively, and (Fe3+/Fe3++Fe2+) < 0.4. Accessory Cr-spinels from Centinela Bajo have Cr# and Mg# values that range from 0.55 to 1.0, and 0.6- < 0.1, respectively. The (Fe3+/Fe3++Fe2+) ratio is less than 0.4 in chromite cores and Fe2+-rich chromite, and >0.5 in ferrian chromite and Cr-magnetite. Interpretation of the data obtained and Cr-spinel textures indicate that the alteration of Cr-spinel is a progressive process that involves in its initial stages the reaction of chromite with olivine under water-saturated conditions to produce clinochlore and Fe2+-rich chromite. During this stage the chromite can also incorporate Ni, Mn, and/or Zn from the serpentinization fluids. As alteration progresses, Fe2+-rich chromite loses mass resulting in the development of a spongy texture. In a later stage and under more oxidizing conditions Fe3+ is incorporated in chromite/Fe2+-rich chromite shifting its composition to an Fe3+-rich chromite (i.e., ferrian chromite). Depending on the fluid/rock and Cr-spinel/silicate ratios, Cr-magnetite can also form over Fe2+-rich chromite and/or ferrian chromite as a secondary overgrowth. The compositional changes observed in Cr-spinels from La Cabaña reflect the initial stages of alteration under serpentinization conditions. Results from this study show that the alteration of Cr-spinels is dependent on temperature. The degree and extent of alteration (formation of Fe2+-rich and/or ferrian chromite) are controlled by the redox nature of the fluids, the Cr-spinel/silicate and the fluid/rock ratios.
ISSN: 0930-0708

hal-01172266
https://hal.archives-ouvertes.fr/hal-01172266
DOI : 10.1007/s00710-014-0335-5

Éditeur(s) : HAL CCSD
Résumé : International audience
The Kazdağ Massif exposes a metamorphic dome in the Biga Peninsula of northwest Turkey. An extensional origin has been proposed for the dome, limited on both flanks by detachments and/or shear zones. The northern flank is bounded by the extensional Alakeçi Shear Zone (ASZ), whose P-T-t path is still poorly known. We therefore focus on its metamorphic conditions and related temporal history to precise its tectono-metamorphic evolution. The local tectonostratigraphy in structurally ascending order comprises: (i) the high-grade metamorphic core rocks of the Kazdaǧ Massif (gneisses and micaschists intercalated with amphibolites and marbles); (ii) the two kilometer-thick ASZ; (iii) the overlaying unmetamorphosed pre-Cenomanian accretionary Çetmi mélange; and (iv) Neogene sedimentary and volcanic cover rocks. ASZ mylonites were derived from both the core rocks and the mélange lithologies. From the north to the south the mylonitic fabrics in the ASZ depict a top-to-the N-NNE shearing, parallel to the NNE-plunging stretching lineation and NNW-dipping mylonitic foliation. This geometry implies normal sense movement i.e. north-side down-dip extensional displacement along this flank of the Kazdaǧ Massif. The northward transition from ductile to brittle-ductile regime through the ASZ shows that the deformation occurred at decreasing temperatures and degree of metamorphism. The paragenesis in equilibrium within the mylonitic gneisses and schists contains Qtz + Fs + Ms + Bt + Grt ± St ± Sill, with late retrogressive chlorite after biotite and garnet. Four samples of ASZ rocks yielded pressures between 6.9-5.7 kbar and temperatures between 706-587°C. Three samples from the mylonitic rocks supplied in situ isochron 36Ar/40Ar mica ages between 31.2-24.2 Ma, which we interpret to date the cooling of the mylonites following the P-T decrease across the ASZ. The metamorphic and structural results support the extensional character of the ASZ, and sketch transition from sillimanite core gneisses in the deeper structural levels to chlorite schists towards the top of the shear zone. These new data allow to precise the peak P-T conditions and the temporal evolution in the northern flank of the Kazdaǧ Massif, where Late Oligocene extensional exhumation was assisted by NNE-directed ductile-brittle ASZ, which had operated from amphibolite to greenschist facies. At the regional scale, this tectono-metamorphic pattern is similar to those observed on other places of the north Aegean domain.
IOP Conference Series
Austin, TE, United States

hal-00642725
https://hal-brgm.archives-ouvertes.fr/hal-00642725
DOI : 10.1088/1755-1307/2/1/012012

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We provide new insights into the geochemistry of serpentinites from the Alpine orogenic wedge representing a paleo-subduction zone. These serpentinites are derived from similar oceanic protoliths, but they have experienced different metamorphic conditions related to three different structural levels of the paleo-subduction zone ((1) obducted: Chenaillet ophiolite, (2) accretionary wedge: Queyras Schistes lustrés complex and (3) serpentinite channel: Monviso ophiolite). Metamorphism undergone by these three units is well defined, increasing eastward from sub-greenschist to eclogite facies conditions, and allows us to examine trace element behavior from the oceanic ridge environment to subduction. Serpentinites first record moderate trace element enrichment due to seawater interaction resulting in the replacement of olivine and pyroxene by chrysotile and lizardite below 300 °C. In the sediment-dominated accretionary wedge, serpentinites are strongly enriched in fluid-mobile-elements (B, Li, As, Sb, and Cs) and act as a trapping system following the metamorphic gradient (from 300 to 390 °C) up to total replacement of the lizardite/chrysotile assemblage by antigorite. Under higher temperature conditions (T > 390 °C), no enrichment was observed, and some fluid-mobile elements were released (B, Li, Cs, and Sr). Moreover, in the serpentinite channel (T > 460 °C), most of the fluid-mobile elements are absent due to the scarcity of metasediments which prevent geochemical exchange between metasediments and serpentinites. This is also due to the onset of antigorite breakdown and the release of fluid-mobile elements. Thus, we emphasize that the geochemistry of Alpine serpentinites is strongly dependent on (1) the grade of metamorphism and (2) the ability of metasediments to supply fluid-mobile elements. We conclude that serpentinites act as a trap-and-release system for fluid-mobile elements in a subduction context.
ISSN: 0009-2541

hal-00996839
https://hal.archives-ouvertes.fr/hal-00996839
DOI : 10.1016/j.chemgeo.2013.02.008

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
New petrographic and microstructural observations, mineral equilibria modelling and U/Pb (monazite) geochronological studies were carried out to investigate the relationships between deformation and metamorphism across the Rehamna massif (Moroccan Variscan belt). In this area, typical Barrovian (muscovite to staurolite) zones developed in Cambrian to Carboniferous metasedimentary rocks that are distributed around a dome-like structure. First assemblages are characterized by the presence of locally preserved andalusite, followed by prograde evolution culminating at 6 kbar and 620 °C in the structurally deepest staurolite zone rocks. This Barrovian sequence was subsequently uplifted to supracrustal levels, heterogeneously reworked at greenschist facies conditions, which was followed locally by static growth of andalusite, indicating heating to 2.5–4 kbar and 530–570 °C. The 206Pb/238U monazite age of 298.3 ± 4.1 Ma is interpreted as minimum age of peak metamorphic conditions, whereas the ages of 275.8 ± 1.7 Ma and 277.0 ± 1.1 Ma date decompression and heating at low pressure, in agreement with previous dating of Permian granitoids intruding the Rehamna massif. The prograde metamorphism occurred during thickening and associated horizontal flow in the deeper crust (S1 horizontal schistosity). The horizontally disposed metamorphic zones were subsequently uplifted by a regional scale antiform during ongoing N–S compression. The re-heating of the massif follows late massive E–W shortening, refolding and retrograde shearing of all previous fabrics coevally with regionally important intrusions of Permian granitoids. We argue that metamorphic evolution of the Rehamna massif occurred several hundred kilometres from the convergent plate boundaries in the interior of continental Gondwanan plate. The tectonometamorphic history of the Rehamna massif is put into Palaeozoic plate tectonic perspective and Late Carboniferous reactivation of (Devonian)–Early Carboniferous basins formed during stretching of the north Gondwana margin and formation of the Palaeotethys Ocean. The inherited heat budget of these magma-rich basins plays a role in the preferential location of this intracontinental orogen. It is shown that rapid transition from lithospheric stretching to compression is characterized by specific HT type of Barrovian metamorphism, which markedly differs from similar Barrovian sequences along Palaeozoic plate boundaries reported from Variscan Europe.
ISSN: 0263-4929

hal-01468009
https://hal.archives-ouvertes.fr/hal-01468009
DOI : 10.1111/jmg.12214

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : The role of the Karakorum fault zone (KFZ) is debated. South of 33°N, ongoing dextral-oblique slip along the SW edge of the Gar basin exhumes metamorphic and magmatic rocks of the Ayilari range. Minerals have recorded a continuum of deformation from temperatures >600–400°C down to <250_C. The 40Ar/39Ar ages, the oldest being 21.2 ± 1.0 Ma, yield minimum estimates for the initiation of the KFZ. These are in agreement with the U-Th/Pb ages constraining the onset of deformation at ca 25–22 Ma. Thermochronologic results show slow cooling for the period ca 21– 14 Ma, followed by rapid cooling between 14 and 4 Ma. These data demonstrate that right-lateral motion was in progress in the early Miocene and that shear continued at least until 4 Ma, pointing to >20 Ma of deformation along the fault. Greenschist facies deformation superimposed upon the medium- to high-grade deformation marks a kinematic change from pure dextral to dextral-normal motion associated with the onset of rapid cooling. At the regional scale, the coexistence of transtension in the Gar basin with transpression documented along the Pangong range farther north suggests another example of the ‘‘zipper tectonics'' model developed along the Red River fault. The kinematic shift induced the rise of the Ayilari range starting at 16–12 Ma and the incision of major river courses. The Indus River might have become captive of the relief at this time. The river's 120 km of apparent offset implies dextral motion at a long-term rate of ca 8.5 ± 1.5 mm/yr.
ISSN: 0278-7407

hal-00197561
https://hal.archives-ouvertes.fr/hal-00197561
https://hal.archives-ouvertes.fr/hal-00197561/document
https://hal.archives-ouvertes.fr/hal-00197561/file/Valli_Karakorum_thermochron_Tectonics_07_pre_print.pdf
DOI : 10.1029/2005TC001913

Éditeur(s) : HAL CCSD Society of Economic Geologists
Résumé : International audience
The ~8-Moz Sadiola Hill gold deposit is located in the Kédougou-Kénieba inlier, a window of deformed ca. 2200 to 2050 Ma rocks that crop out in eastern Senegal and western Mali. The geology of the inlier differs from other Paleoproterozoic granite-greenstone belts and sedimentary basins by the abundance of carbonate rocks. The Sadiola Hill gold deposit occurs within 3 km of the Senegal-Mali shear zone, and country rocks in the region have undergone polycyclic deformation. The lithostratigraphy of the open pit consists of impure limestones overlain by younger sequences of detrital sedimentary rocks, which include arenite, wacke, and siltstone. The sedimentary rocks have been subjected to regional greenschist-facies metamorphism and have been intruded by multiple generations of synkinematic, calc-alkaline stocks and dikes. The bulk of the ore is hosted within a N-S- to NNW-trending, 10- to 50-m-wide, brittle-ductile dilational shear zone defined as the Sadiola shear zone. Gold lodes also occur along an array of steep NNE-trending shear zones. Geometric and kinematic analyses indicate that the ore-hosting structures were undergoing sinistral displacement at the time of mineralization, locally defined as the D3s NNW-SSE-shortening event. Hydrothermal alteration is polyphase and includes an early high-temperature calc-silicate phase (i.e., porphyroblastic growth of actinolite-tremolite) followed by a potassic phase (i.e., biotite-calcite-quartz ± K-feldspar-tourmaline-actinolite) that was synchronous with ore mineral deposition. Paragenetic studies reveal a multistage ore development that includes an As-rich sulfide stage, followed by an Au-Sb stage. The ore is associated with a metal enrichment suite of Au-As-Sb ± Cu-Fe-W-Mo-Ag-Bi-Zn-Pb-Te. The Sadiola Hill deposit shares a similar relative timing and structural setting to that of other world-class orogenic gold systems in the West African craton (e.g., Ashanti, Loulo), with gold deposition occurring during a period of transcurrent tectonics, after the cessation of region-wide compressional deformation. The high-temperature paragenesis at Sadiola Hill is atypical of gold mineralization in the craton and indicates that the late Eburnean tectono-magmatic activity between ca. 2080 and 2060 Ma played an important role in the dynamics of hydrothermal fluid circulation along the Senegal-Mali shear zone.
ISSN: 0361-0128

hal-01583358
https://hal.archives-ouvertes.fr/hal-01583358
DOI : 10.2113/econgeo.112.1.23

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : Structural, metamorphic, and geochronologic work shows that the Ampelos/Dilek nappe of the Cycladic blueschist unit in the eastern Aegean constitutes a wedge of high-pressure rocks extruded during early stages of orogeny. The extrusion wedge formed during the incipient collision of the Anatolian microcontinent with Eurasia when subduction and deep underthrusting ceased and the Ampelos/Dilek nappe was thrust southward over the greenschist-facies Menderes nappes along its lower tectonic contact, the Cycladic-Menderes thrust, effectively cutting out a ∼30- to 40-km-thick section of crust. The upper contact of the Ampelos/Dilek extrusion wedge is the top-to-the-NE Selçuk normal shear zone, along which the Ampelos/Dilek nappe was exhumed by ∼30–40 km. Detailed Rb-Sr and 40Ar/39Ar dating of mylonites demonstrates that both shear zones operated between 42 and 32 Ma. There is no evidence for episodic motion during the ∼10 Myr life span of the shear zones, suggesting that both shear zones operated in a steady, nonepisodic fashion. Our data provide supporting evidence that simultaneous thrust-type and normal sense shearing can accomplish the early exhumation of deep-seated rocks
ISSN: 0278-7407

hal-00405013
https://hal.archives-ouvertes.fr/hal-00405013
DOI : 10.1029/2005TC001872

Éditeur(s) : Gauthier-villars
Résumé : The aim of the 20 Nautile dives of the recent Kanaut cruise was the study of the Kane Fracture Zone from its intersection with the Mid-Atlantic Ridge up to 80 km to the west. The dives have been conducted along four massifs located along the southern wall of the fracture valley. The flanks of the massifs consist of slightly tilted peridotites, gabbros and basalts, which have been exposed along normal and strike-slip major faults. No dike complex similar to that observed along the ZF Vema was observed The observed sections show pervasive cataclastic deformation and greenschist-facies metamorphic overprint.
Lors de la récente campagne Kanaut du Nautile (novembre-décembre 1992), 20 plongées ont eu pour objectif I’étude du mur sud de la Zone de fracture Kane, depuis son intersection orientale avec la dorsale médio-Atlantique jusqu’h environ 80 km de celle-ci. Les plongées réalisées SUT quatre massifs successifs bordant la vallée transformante, permettent d’observer des sections comprenant des péndotites, des gabbros et des basaltes, mis 21 l’affleurement B la faveur d’accidents normaux décrochants et d’un léger basculement. Les coupes observées ne montrent pas de complexe filonien épais, comme celui observé le long de la ZF Vema par exemple. L’ensemble de ces séries est fortement cataclasé et métamorphisé dans le facies des schites verts.
Comptes Rendus de l'Academie des Sciences Serie II (1251-8069) (Gauthier-villars), 1993-12 , Vol. 317 , N. 12 , P. 1641-1648

Droits : Académie des Sciences
http://archimer.ifremer.fr/doc/00183/29470/27886.pdf
http://archimer.ifremer.fr/doc/00183/29470/

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Field and petrostructural investigations in the Central Mauritanides provide new precisions on the polyorogenic character of the nappe edifice of this belt. The upper structural unit exposed in the Gaoua area includes anatectic gneisses and high-grade metasediments affected by medium temperature, high-pressure metamorphism and cut by granitoids. They represent a basement unconformably overlain by a monometamorphic cover, the Gaoua Group, of assumed lower Paleozoic age. Late Paleozoic regional metamorphism of metapelites from this cover is characterized by pyrophyllite-kyanite-chloritoid assemblages equilibrated at T around 420 degrees C and P = 1 GPa. In the lower structural unit exposed in the south around Boufkerine-Farkaka, metapelites of the Gadel Group display slightly retrogressed garnet-kyanite-staurolite-rutile assemblages that equilibrated at T around 600 degrees C and P >= 1.2 GPa before the intrusion of 639 Ma old plutons. The Gadel Group locally overlies in unconformity polycyclic gneisses. This continental assemblage represents an outboard terrane inserted between two monocyclic greenschist facies terranes: remnants of oceanic lithosphere in the east and arc-derived metasediments and metavolcanics cut by 670 Ma old calc-alkaline plutons in the west. All units have been involved in late Paleozoic nappes emplaced to the east above the West African craton and its late Neoproterozoic to Cambro-ordovician cover. Though an east-directed vergence is locally recorded in the Neoproterozoic units, most early E-W trending stretching and mineral lineations are synchronous with greenschist facies metamorphism developed during the Late Paleozoic and roughly coeval with the Applachian nappe system.
ISSN: 0899-5362

hal-00424501
https://hal.archives-ouvertes.fr/hal-00424501
DOI : 10.1016/j.jafrearsci.2008.09.004

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Slices of polycyclic metasediments (marbles and meta-cherts) are tectonically amalgamated with the polydeformed basement of the Dent Blanche tectonic system along a major Alpine shear zone in the Western Alps (Becca di Salé area, Valtournenche Valley). A combination of techniques (structural analysis at various scales, metamorphic petrology, geochronology and trace element geochemistry) was applied to determine the age and composition of accessory phases (titanite, allanite and zircon) and their relation to major minerals. The results are used to reconstruct the polyphase structural and metamorphic histories, comprising both pre-Alpine and Alpine cycles. The pre-Alpine evolution is associated with low-pressure high-temperature metamorphism related to Permo-Triassic lithospheric thinning. In meta-cherts, microtextural relations indicate coeval growth of allanite and garnet during this stage, at ~ 300 Ma. Textures of zircon also indicate crystallisation at HT conditions; ages scatter from 263 to 294 Ma, with a major cluster of data at ~ 276 Ma. In impure marble, U-Pb analyses of titanite domains (with variable Al and F contents) yield apparent 206Pb/238U dates range from Permian to Jurassic. Chemical and isotopic data suggest that titanite formed at Permian times and was then affected by (extension-related?) fluid circulation during the Triassic and Jurassic, which redistributed major elements (Al and F) and partially opened the U-Pb system. The Alpine cycle lead to early blueschist facies assemblages, which were partly overprinted under greenschist facies conditions. The strong Alpine compressional overprint disrupted the pre-Alpine structural imprint and/or reactivated earlier structures. The pre-Alpine metamorphic record, preserved in these slices of metasediments, reflects the onset of the Permo-Triassic lithospheric extension to Jurassic rifting.
ISSN: 0024-4937

hal-00758123
https://hal.archives-ouvertes.fr/hal-00758123
DOI : 10.1016/j.lithos.2012.05.007

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Serpentinites from subduction environments represent an important sink for fluid-mobile elements. In order to constrain geochemical behavior of fluid-mobile elements hosted by serpentine phases during subduction processes, we carried out a geochemical study (trace elements and Pb isotopes) of a series of serpentinites and cumulates from the accretionary wedge of Greater Caribbean (Cuba and Dominican Republic). The trace element compositions of the primary and alteration-related phases were analyzed in situ using LA-HR-ICP-MS techniques. The studied samples represent parts of the subducted proto-Atlantic oceanic lithosphere, which has experienced low to high grade metamorphism (greenschist to eclogite facies), before being exhumed; a subset of these samples were derived from the mantle wedge. This sampling provides the opportunity to trace the chemical mobility of fluid-mobile elements during prograde metamorphism along a cold geotherm in an oceanic subduction setting.
ISSN: 0009-2541

hal-00720895
https://hal.archives-ouvertes.fr/hal-00720895
DOI : 10.1016/j.chemgeo.2012.04.009

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
Dating the timing of motion on crustal shear zones is of tremendous importance for understanding the assembly of orogenic terranes. This objective is achieved in this paper by combining petrological and structural observations with novel developments in in situ U-Th-Pb geochronology of allanite. A greenschist facies shear zone within the Mont Blanc Massif is documented. Allanite is synkinematic and belongs to the mylonitic assemblage. LA-ICP-MS U-Th-Pb isotope analyses of allanite reveal high contents and highly radiogenic isotopic compositions of the common-Pb component. The use of measured Pb-isotope compositions of associated minerals (feldspars and chlorite) is critical for accurate common-Pb correction, and provides a powerful mechanism for linking allanite growth to the metamorphic assemblage. A mean 208Pb/232Th age of 29.44 ± 0.95 Ma is accordingly taken for synkinematic allanite crystallisation under greenschist facies conditions. This age reflects the timing of the Mont Blanc underthrusting below the Penninic Front and highlights the potential of directly dating deformation with allanite.
ISSN: 0954-4879

hal-00963545
https://hal.archives-ouvertes.fr/hal-00963545
DOI : 10.1111/ter.12066

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Hengshan–Wutaishan area forms part of the Trans-North-China Belt developed in response to the final amalgamation of the North China Craton. The Hengshan–Wutaishan domain and the adjacent Fuping massif constitute the largest and the most representative rock exposure of the belt. A structural study allows us to redefine the lithological and tectonic units that compose the architecture of the Hengshan–Wutaishan domain and to propose that the Trans-North-China Belt was built up through a polyphase tectonic evolution within the period 1900–1800 Ma. The first event (D1) corresponds to the emplacement of lower and upper nappes herein called the Orthogneiss and Volcanites Unit (OVU) and the Low Grade Mafic Unit (LGMU), respectively. The OVU mainly consists of rocks formed in an arc system and metamorphosed under amphibolite facies conditions whereas the LGMU represents oceanic crust rocks with an arc component that underwent greenschist facies metamorphism. The syn-metamorphic D1 deformation is characterized by a NW–SE stretching and mineral lineation with a top-to-the SE sense of shear that corresponds to the sense of nappe motion. U–Th–Pb EPM chemical dating on monazite from three metapelites of the OVU gives isochron ages of 1884 ± 11 Ma, 1886 ± 5 Ma and 1887 ± 4 Ma interpreted as the age of the prograde amphibolite facies metamorphism coeval with nappe stacking during the D1 event. The D2 event corresponds to a widespread crustal melting of the OVU and underlying basement rocks in Hengshan related to the exhumation of deep structures about 20–30 Ma after nappe emplacement. The migmatite contains blocks of retrograded eclogite or HP-granulitic restites, and is characterized by the development of a pervasive foliation and lineation. A late D2 event represents the folding stage of the migmatitic foliation during the completion of a dome. The Orthogneiss and Volcanics Unit and the Low Grade Mafic Unit are unconformably covered by weakly metamorphosed to unmetamorphosed sediments belonging to the Hutuo Supergroup. These rocks were deformed by a ductile D3 event, characterized by a NW–SE trending stretching lineation, south verging folds associated with an axial planar slaty cleavage and an E–W crenulation lineation. Lastly, a left-lateral wrenching along the Zhujiafang Shear Zone separates the northern and southern parts of Hengshan. Our structural study shows that there is no significant difference between the southern part of Hengshan and the lower part of Wutaishan, both areas belong to OVU and show a structural continuity. We suggest that this polyphase deformation developed in response to a north-westward subduction of an old ocean, named the Lüliang Ocean, beneath the Western Block, followed by collision with a micro-continent named the Fuping Block at not, vert, similar1890–1880 Ma.
ISSN: 0301-9268

insu-00151594
https://hal-insu.archives-ouvertes.fr/insu-00151594
https://hal-insu.archives-ouvertes.fr/insu-00151594/document
https://hal-insu.archives-ouvertes.fr/insu-00151594/file/Trap-PrecambrainResearch-2007.pdf
DOI : 10.1016/j.precamres.2007.03.001

Éditeur(s) : HAL CCSD
Résumé : International audience
Sub-continental lithospheric mantle rocks are exhumed at the foot of magma-poor distal passive margins as aresponse to extreme stretching of the continental crust during plate separation. Remnants of the Northern Iberianpaleo-passive margin are now exposed in the North-Pyrenean Zone (NPZ) and represent field analogues to studythe processes of continental crust thinning and subcontinental mantle exhumation. The NPZ results from theinversion of basins opened between the Iberia and Europa plates during Albo-Cenomanian times. In the westernNPZ, the ‘Chaînons Béarnais’ ranges display a fold-and-thrust structure involving the Mesozoic sedimentarycover, decoupled from its continental basement and associated with peridotite bodies in tectonic contact withPalaeozoic basement lenses of small size. Continental extension developed under hot thermal conditions, asdemonstrated by the syn-metamorphic Cretaceous ductile deformation affecting both the crustal basement and theallochthonous Mesozoic cover.In this study, we present structural and geochemical data providing constraints to reconstruct the evolution ofthe northern Iberia paleo-margin. Field work confirms that the pre-rift Mesozoic cover is intimately associatedto mantle rocks and to thin tectonic lenses of crustal basement. It also shows that the pre-rift cover was detachedfrom its bedrock at the Keuper evaporites level and was welded to mantle rocks during their exhumation atthe foot of the hyper-extended margin. The crust/mantle detachment fault is a major shear zone characterizedby anastomosed shear bands defining a plurimetric phacoidal fabric at the top of the serpentinized mantle. Thedetachment is marked by a layer of metasomatic rocks, locally 20 meters thick, made of talc-chlorite-pyrite-richrocks that developped under greenschist facies conditions. Raman Spectroscopy on Carbonaceous Materials(RSCM), performed on the Mesozoic cover reveal that the entire sedimentary pile underwent temperatures rangingbetween 200C and 480C.We show that: (i) at the site of mantle rocks exhumation, the boudinaged pre-rift sediments have undergone drasticsyn-metamorphic thinning with the genesis of a S0/S1 foliation and, (ii) the Paleozoic basement has been ductilelydeformed, into thin tectonic lenses that remained welded to the exhumed mantle rocks. Therefore the overallcrustal rheology appears dominated by shallow levels having a ductile behavior. This rheology is related to thepresence of a thick pre- and syn-rift decoupled cover acting as an efficient thermal blanket. This new geologicaldata set highlights important characteristics of ductile-type hyper-extended passive margin that cannot be obtainedfrom the study of seismic lines. Finally, we stress that studying field analogues represents a major tool to betterunderstand the mechanisms of extreme crustal thinning associated with mantle exhumation and their structuralinheritance during tectonic inversion.
Geophysical Research Abstracts
Vienne, Austria

insu-01510103
https://hal-insu.archives-ouvertes.fr/insu-01510103

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
Located east of Guadeloupe, the island of La Désirade exhibits the oldest rocks of the Lesser Antilles arc and the eastern Caribbean plate. An old magmatic basement is composed of (1) late Jurassic ophiolitic complex with meta-basaltic pillow lavas and interbedded radiolarites, (2) acid igneous complex comprising meta-quartz-diorite and meta-rhyolitic lavas flows, and (3) meta-diabasic/microdioritic dyke swarm complex. We present and discuss the discovery of synmetamorphic thrust tectonics in this island. Based on detailed structural analysis we evidenced two main compressive events. The first event (D1) is characterized by pervasive folding associated with thrust development. In the northeastern part of the island, a major thrust fault, the Grand Abaque Thrust, has been discovered which displaces for several kilometers the acid igneous complex onto the ophiolitic unit. The second deformation event (D2) is featured by a main pervasive cleavage S2 observed at the regional scale associated with upright folding (F2) and by the development of two sets of conjugated steeply dipping strike-slip shear zones, dextral 130°N and sinistral 20°N striking, respectively. Microstructural observations indicate that Greenschist facies metamorphism is clearly contemporaneous with the development of D1 and D2 superimposed tectonic structures. Ar/Ar geochronology was performed on two bulk of adularia minerals sampled in the northeast volcanic complex within a deformed zone related to a D2 event. These analyses yield well-defined and concordant plateau ages at 106.2 ± 1.7 Ma and 107.2 ± 1.8 Ma. Thus, shortening and thickening tectonics occurred during Lower Cretaceous (Albian). In the available framework of Caribbean geodynamics, the synmetamorphic thrust tectonics we discovered in the French Lesser Antilles is related to the collision between an Aptian-Albian oceanic plateau and the Andean-Cordilleran east-dipping subduction zone. Our results support subduction polarity reversal during Lower Cretaceous.
ISSN: 0278-7407

hal-00620555
https://hal.archives-ouvertes.fr/hal-00620555
DOI : 10.1029/2011TC002875

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Eastern Hoggar is by far the least well known part of the Tuareg Shield. It is composed, from west to east, of the Aouzegueur, Edembo and Djanet Terranes. The Djanet Terrane is the easternmost Hoggar terrane and comprises a greenschist-facies clastic sedimentary sequence, the Djanet Group, intruded by granitoids. Laser ablation (LA-) ICP-MS U-Pb zircon ages of detrital zircons from these sediments yield a large range of ages similar to ages known in Central and Western Hoggar; the youngest is 590 +/- 10 Ma (2 sigma), which is the maximum age of deposition of the Djanet Croup. The Djanet Group is intruded by the Djanet Batholith (571 +/- 16 Ma), by high-level subcircular plutons such as the Tin Bedjane Pluton (568 +/- 5 Ma) and finally by the felsic Tin Amali Dyke Swarm (558 +/- 5 Ma), all ages being SHRIMP U-Pb zircon. The deposition and metamorphism of the Djanet Group thus occurred between 590 Ma (the age of the youngest detrital zircons analyzed in the Djanet Group) and c. 570 Ma (the age of intrusive granitoids). Nd T-DM two-stage model ages (1.30-2.03 Ga), initial Sr-87/Sr-86 ratios (0.7035-0.7095) and epsilon(Nd) (-3 to -11) of the three intrusive suites, all being high-K calc-alkaline in composition, indicate mainly an old, Rb-depleted continental source. Migmatization in the adjacent Edembo Terrane is dated at 568 +/- 4 Ma (U-Pb zircon SHRIMP age), contemporaneous with the intrusion of the Djanet plutons. Rocks and events in the Edembo and Djanet Terranes are all intracontinental. We propose that the Djanet Terrane lies on the boundary of a craton located to the north-east, that we defined here as the Murzuq craton. The existence of the Murzuq craton is supported by sedimentary and geophysical data. We suggest that the deformation of Eastern Hoggar occurred 575-555 Ma, due to the indentation of the Murzuq craton and has no link with the older convergence with the West African craton as was the case for the central and western parts of the Tuareg Shield. This determines a late Ediacaran intracontinental Murzukian event that probably also occurred to the east in the Tibesti area.
ISSN: 0301-9268

hal-00512833
https://hal.archives-ouvertes.fr/hal-00512833
DOI : 10.1016/j.precamres.2010.05.011