Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The 40Ar/39Ar geochronological technique has significantly contributed to the study of crustal deformation. However, the frequent occurrence of extraneous argon in high-pressure terrains can make the interpretation of 40Ar/39Ar ages problematic. This contribution attempts to: i) characterize the distribution and origin of extraneous argon in the Cycladic Blueschist Unit (CBU, Greece) by applying the 40Ar/39Ar dating technique on three Cycladic islands, Syros, Tinos and Ios, which span a complete transect of the CBU and ii) discuss regional implications for the tectonometamorphic evolution of the CBU. Our results, combined with ages available in the literature, show that periods of peak-burial conditions (55–49 Ma) and syn- (45–38 Ma) to post-orogenic exhumation (37–21 Ma) are well constrained in the CBU by the 40Ar/39Ar method. Moreover, Variscan apparent ages are preserved in the Cycladic basement of Ios, indicating that the 40Ar/39Ar system was not completely reset during the Eocene metamorphic episode. Additionally, our 40Ar/39Ar data show that the occurrence of extraneous argon is heterogeneously distributed in the CBU. While high-pressure rocks of Syros and Tinos are only locally affected, extraneous argon is clearly evidenced in the CBU of Ios, yielding geologically meaningless ages up to 900 Ma. On each island, the most affected ages were obtained in microstructures where fluid circulation is enhanced such as shear bands or strain shadows around garnets. We suggest that the Cycladic Basement, a polymetamorphic continental unit, behaved as an argon reservoir that has been mobilized by fluid circulation at the Eocene. These argon-enriched fluids have then heterogeneously contaminated metamorphic rocks when percolating through the CBU. Finally, we claim that a minimum closure temperature of 550 °C must be accepted for the phengites of the CBU and conclude that volume diffusion is not the main process resetting the argon isotopic system in high-pressure rocks compare to recrystallization enhanced by fluid circulation and deformation.
ISSN: 0024-4937

Droits : http://creativecommons.org/licenses/by-nc-nd/
insu-01424268
https://hal-insu.archives-ouvertes.fr/insu-01424268
https://hal-insu.archives-ouvertes.fr/insu-01424268/document
https://hal-insu.archives-ouvertes.fr/insu-01424268/file/Valentin-Lithos-Accepted-2016.pdf
DOI : 10.1016/j.lithos.2016.12.013

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We review the geodynamic evolution of the Aegea-Anatolia region and discuss strain localisation there over geological times. From Late Eocene to Present, crustal deformation in the Aegean backarc has localised progressively during slab retreat. Extension started with the formation of the Rhodope Metamorphic Core Complex (Eocene) and migrated to the Cyclades and the northern Menderes massif (Oligocene and Miocene), accommodated by crustal-scale detachments and a first series of core complexes (MCCs). Extension then localised in Western Turkey, the Corinth Rift and the external Hellenic arc after Messinian times, while the North Anatolian Fault penetrated the Aegean Sea. Through time the direction and style of extension have not changed significantly except in terms of localisation. The contributions of progressive slab retreat and tearing, basal drag, extrusion tectonics and tectonic inheritance are discussed and we favour a model (1) where slab retreat is the main driving engine, (2) successive slab tearing episodes are the main causes of this stepwise strain localisation and (3) the inherited heterogeneity of the crust is a major factor for localising detachments. The continental crust has an inherited strong heterogeneity and crustal-scale contacts such as major thrust planes act as weak zones or as zones of contrast of resistance and viscosity that can localise later deformation. The dynamics of slabs at depth and the asthenospheric flow due to slab retreat also have influence strain localisation in the upper plate. Successive slab ruptures from the Middle Miocene to the Late Miocene have isolated a narrow stripe of lithosphere, still attached to the African lithosphere below Crete. The formation of the North Anatolian Fault is partly a consequence of this evolution. The extrusion of Anatolia and the Aegean extension are partly driven from below (asthenospheric flow) and from above (extrusion of a lid of rigid crust).
ISSN: 0040-1951

insu-00715950
https://hal-insu.archives-ouvertes.fr/insu-00715950
https://hal-insu.archives-ouvertes.fr/insu-00715950/document
https://hal-insu.archives-ouvertes.fr/insu-00715950/file/Jolivet-Aegean_Tectonophysics-2012.pdf
DOI : 10.1016/j.tecto.2012.06.011

Éditeur(s) : HAL CCSD
Résumé : International audience
Mylonite bodies are exhumed extensions at depth of shallow structural level brittle faults. They are characterised by intense strain localisation involving large volumes of middle to lower continental crust. The analysis of shear zone geometry and microstructures is a useful tool to decipher deformation kinematics, though finite strain estimation is still relatively qualitative. Timing of the deformation has also been the focus of numerous studies using the complete spectrum of geochronometers. However, only rare examples provide geochronological data that can be unambiguously interpreted and cooling, recrystallisation and neocrystallisation are difficult to tell from each other. Thus, time-scales over which major mylonite zones develop and remain active under ductile conditions as well as their strain rate often remain poorly documented. We investigated two sections across the South-Armorican Shear Zone (SASZ), a crustal-scale pluri-km thick dextral shear zone active during a single tectonic event of late-Variscan age and associated with syntectonic granite emplacement.. Finite strain profiles on these cross-sections, quantified from average Quartz grain-size in 34 samples, show a strong deformation partitioning towards the center of the SASZ, with a decrease, for each section, from 150µm in slightly deformed zones to less than 7-5 µm in the core of the ultramylonite. Furthermore, toward the core of the SASZ, development of a penetrative shear band network progressively eliminates lenses of less deformed rocks and results in homogenisation of the Quartz grain sizes. In parallel, we performed in-situ UV-laser Ar/Ar dating on the fabric-forming white micas. White micas population can be decomposed into inherited muscovites carried by the folation and newly-formed, syn-tectonic substituted phengites located along the shear bands yielding 2 age groups for each sample. Along the profiles, the difference between these two ages evolves from 10-15 Ma in weakly deformed domains to zero toward the SASZ core. In addition, mylonite-forming phengites yielded the same age irrespectively of their position on the section or of the rock strain.. On the regional point of view, this study brings deformation-ages for the late-orogenic evolution of the South Armorican domain. Strain accumulated along the SASZ in a time-span extending to 300-298 Ma, i.e. contemporaneously with the activity of nearby major extensional shear zones. These results support a new model of crustal-scale shear zone activity, where the whole body of the shear zone is active throughout its history (defined as the largest age difference through the section), i.e. the actively deforming zone does not get progressively narrower. The strain distribution nevertheless evolves with time, with an increasingly large fraction partitioned in the ultramylonitic core.
EGU General Assembly
Vienne, Austria

insu-00839320
https://hal-insu.archives-ouvertes.fr/insu-00839320
BIBCODE : 2010EGUGA..12.9881A

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Regional-scale deformation taking place in a strongly anisotropic, yet homogeneous metapelitic protolith during an apparently single tectonic event was systematically investigated as a function of the distance to the main tectonic contact (i.e., the Filabres shear zone, a major detachment in the Betic Cordilleras, Spain). The density of C3' shear bands (or extensional crenulation cleavage) reworking the earlier S2 schistosity increases exponentially towards the contact, in parallel with the decrease in the size of the shear domains. Systematic variations in angles and shape ratios are also reported. Deformation and age patterns, however, suggest that this spectacular trend at least partly results from a progressive localization of the deformation through time. This fossilized shear strain gradient was thus produced somewhat diachronously. Such shear strain patterns nevertheless provide a mean to constrain the rheological properties for such weak lithologies and a mean to better understand crustal deformation.
ISSN: 0191-8141

insu-00545043
https://hal-insu.archives-ouvertes.fr/insu-00545043
https://hal-insu.archives-ouvertes.fr/insu-00545043/document
https://hal-insu.archives-ouvertes.fr/insu-00545043/file/JSGagardetal2.pdf
DOI : 10.1016/j.jsg.2010.11.011

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In East Asia, widespread extensional sedimentary basins together with the close association of Metamorphic Core Complexes and magmatism are prominent features of the large-scale wide rift system developed during late Mesozoic times. This region thus appears as a proper place to study continental extension as well as questions about the link between continental extension and magmatism. This paper primarily provides pioneer exhumation time-constraints on the Linglong MCC (Jiaodong Peninsula) including cooling and deformation ages. MCC cooling at mid-crustal levels occurred at ca. 143 Ma. Besides, last ductile deformation occurred at ca. 134 Ma while final exhumation stages, under brittle 2 conditions, occurred as late as ca. 128 Ma. Continuous crustal stretching were then recorded by the emplacement of a synkinematic pluton in the upper crust at ca. 128 Ma that cooled fast below an intracrustal shear zone crossing the ductile-brittle transition at ca. 123 Ma. The ca. 120-118 Ma age cluster is ascribed to the fast cooling of undeformed plutons marking the end of extension that lasted, in the area, over a minimum period of ca. 30 My. Combining other MCC exhumation constraints and the onset of subsidence in the sedimentary basins, total duration of late Mesozoic extension in East Asia could be estimated at ca. 60 My, related with a rather long process for extension from 160 to at least 100 Ma. East Asian continental extension is heterogeneously distributed in space and time as revealed by fundamental differences between two end-member classes of migmatite-cored MCC already described in other wide rift systems. Extension seems to have first favoured partial melting which subsequently, in turn, maintained continental extension.
ISSN: 1342-937X

insu-00752551
https://hal-insu.archives-ouvertes.fr/insu-00752551
https://hal-insu.archives-ouvertes.fr/insu-00752551/document
https://hal-insu.archives-ouvertes.fr/insu-00752551/file/Charles_et_al_Gondwana_Research_2012.pdf
DOI : 10.1016/j.gr.2012.10.011

Éditeur(s) : HAL CCSD
Résumé : International audience
In the external Western Alps, the collision started with the shortening of the proximal European (Dauphinois) margin. One peculiar feature is the crustal thickening that finally led to the exhumation of the basement External Crystalline Massifs (ECM) during the Neogene. In these massifs, various age of shortening, structural style and sequence of deformation have been proposed leading to different kinematics along the Alpine arc that in turn do not imply the same crustal/lithospheric strength. In this contribution, we present new structural and microstructural data along with Ar/Ar ages from field studies in the Oisans-Grandes Rousses massifs, between Grenoble and Briançon). From balanced cross-sections, we show that about half of the external zone crustal shortening is accommodated within the ECM with similar Ar/Ar ages in all massifs. Furthermore, the shear zones present very large thicknesses (several hundred meters) and do not reactivated either the inherited Liassic normal faults or the Variscan foliation. Finally, the main shortening structures are localized beneath inherited Liassic syn-rift basins. These results suggest that the crust was weak during its shortening. Such behavior is most likely due to the combination of several factors: the crust burial (below the overthrusting internal units) down to about 10km, the presence of weak inherited syn-rift basins, and the strain softening in shear zones that evolve in mica-rich phyllonites. These results are consistent with the studies in the Aar massif and should most likely apply to the Mont Blanc massif, whose relationships with its cover are still debated.
EGU General Assembly
Vienne, Austria

insu-00839426
https://hal-insu.archives-ouvertes.fr/insu-00839426
BIBCODE : 2013EGUGA..15.8837B

Éditeur(s) : HAL CCSD
Résumé : International audience
Mylonite-bearing crustal shear zones are exhumed extensions at depth of (potentially seismogenic) brittle faultsat shallow structural level. By concentrating deformation, shear zones play a major role in the rheology and mechanicalbehavior of the continental crust. Analysis of shear zone geometry and microstructures is a useful toolto decipher deformation kinematics, though finite strain estimation is still relatively qualitative. Timing of thedeformation has also been the focus of numerous studies using the complete spectrum of geochronometers. However,only rare examples provide geochronological data that can be unambiguously linked with deformation. Thus,time-scales over which major mylonite zones develop and remain active under ductile conditions as well as theirstrain rates often remain poorly documented. Several sections across the South Armorican Shear Zone (SASZ),a crustal-scale, several km-thick dextral shear zone were investigated by structural, petrological and Ar/Ar radiochronologicalmethods. Finite strain profiles on these sections showed a strong deformation partitioning towardsthe ultramylonite core of the SASZ, with a decrease, of both the C’/S mean angle (from ca. 45 to less than5) and the Quartz grain-size (from ca. 150 to 5 m). In parallel, conventional and in-situ Ar/Ar datings on thefabric-forming white micas were performed on compositionally complex white-micas. Inherited magmatic muscovitescarried by the foliation and newly-formed, syn-tectonic substituted phengites located along the shear bandsyielded 2 age groups for the less deformed samples. Along the shear zone cross-section, the difference betweenthese two ages evolves from 10-15 Ma in weakly deformed domains to zero toward the SASZ core. In addition, thevery fine-grained syn-tectonic phengites yielded the same ca. 300-298 Ma age irrespectively of their distance to thecore of the shear zone, even in slightly deformed rocks distant by several km. These results support a new modelof crustal-scale shear zone activity, where the whole body of the shear zone is active throughout its history, i.e.the actively deforming zone does not get progressively narrower. The strain distribution nevertheless evolves withtime, with an increasingly larger fraction partitioned in the ultramylonitic core. Besides, combination of the finitestrain estimations and the Ar/Ar ages enables us to assess the range of strain rates associated with mylonitization.
EGU General Assembly 2014
Vienne, Austria

insu-01565057
https://hal-insu.archives-ouvertes.fr/insu-01565057

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Oisans Massif, located in the external zones of the western Alps, experienced significant shortening during the Alpine collision. While a series of major top-to-the west shear zones was recently described, the general low grade of the metamorphism has not attracted much petrological and geochronological studies. This paper provides combined temperature and age constraints on the evolution of the Oisans Massif. Temperature was estimated with the Raman Spectrometry of Carbonaceous Material (RSCM) method and chlorite geothermometry. Maximum temperature reached by the Mesozoic cover (Tmax) from Grenoble to the Galibier pass (E-W) and from Saint Jean de Maurienne to Embrun (N-S) yielded almost constant ca. 330 °C temperatures all through the Massif. Temperatures however strongly decrease either westward towards the top of the Vercors sedimentary sequence or eastward towards the Penninic Frontal Thrust. Age constraints were retrieved using 40Ar/39Ar in situ analyses performed on variously strained samples from Alpine shear zones. Over strain gradients, incipient Alpine recrystallizations progressively develop at the expense of former Variscan parageneses. A combined textural and EPMA approach permitted to identify newly formed chlorite and phengite that unequivocally grew in response to deformation or to low grade metamorphism. Chlorites recorded temperatures from ca. 350–150 °C during the activity of shear zones. In parallel, 40Ar/39Ar in situ experiments enabled dating deformation using both synkinematic phengites crystallized below the closure temperature of white-micas and former Variscan muscovite whose isotopic system have been, at least partially re-opened. Activity of top-to-the-west shear zones responsible for the shortening and thickening of the Oisans massif thus occurred between 34 and 33 and 25 Ma. Integrating these new age-constraints, Tmax estimates, and published geological data on the Oisans Massif and neighbouring areas allow proposing a new shortening scenario for the external zones. From ca. 34–33 Ma, the Oisans Massif was buried as a rigid block below more internal units and reached a temperature of ca. 330 °C. Shearing progressively localized along a series of top-to-the-W shear zones in the basement until 25 Ma. Deformation then localized along a major thrust at the base of the ECM massif and propagated along the basement-cover interface below the Vercors massif after 16 Ma.
ISSN: 0264-3707

Droits : info:eu-repo/semantics/OpenAccess
insu-01093878
https://hal-insu.archives-ouvertes.fr/insu-01093878
https://hal-insu.archives-ouvertes.fr/insu-01093878/document
https://hal-insu.archives-ouvertes.fr/insu-01093878/file/GEOD1326RA.pdf
DOI : 10.1016/j.jog.2014.09.004

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Mechanism and kinematics of the Late Mesozoic continental extension event of eastern Asia are still debated. In order to better constrain its evolution, two granodioritic plutons of the Jiaodong Peninsula have been chosen as targets for a time-constrained palaeomagnetic study. Indeed, plutons are devoid of visible deformation, did not experience rotation along horizontal axis and are precisely dated by U/Pb and 40Ar/39Ar methods. Multidomain (MD) magnetite has been identified as the principal magnetic remanent carrier. The interpolation of existing and new U/Pb and 40Ar/39Ar ages revealed that characteristic remanent magnetisation was acquired in a narrow range of 116 ± 2 Ma. Twenty out of 27 sites present stable magnetic directions calculated from high-temperature or high-coercive components. The observations of the solo normal magnetic polarity for this palaeomagnetic collection and of the magnetic remanent age consistent with the Cretaceous Normal Superchron (CNS) argue that the characteristic magnetic remanence may be considered as primary. Detailed field observations of the intrusive relationship between the plutons and country rocks and Anisotropy of Magnetic Susceptibility (AMS) study reveal the absence of the subsequent deformation of plutons, or rigid rotation of plutons along a horizontal axis. Two palaeomagnetic poles have been therefore calculated from these plutons. Among 12 out 15 Cretaceous palaeomagnetic poles, including the two new poles provided by this study, from the Jiaodong Peninsula and on both sides of Tan-Lu and Muping-Jimo faults are statistically consistent. As a result, the Jiaodong Peninsula behaved as a rigid block as internal deformation appears negligible. The remaining three derived poles are probably due to the secular variation or/and the vicinity of fault zones near of the palaeomagnetic sampling site. Thus, they can not be applied to the peninsula-scaled tectonics. Comparison of these time52 constrained Cretaceous palaeomagnetic results of the Jiaodong Peninsula with those of North China Block (NCB) indicate that the Jiaodong Peninsula was rigidly attached to NCB since, 3 at least, the Cretaceous as residual difference is clearly insignificant with respect to the error brackets. These new palaeomagnetic results confirm that the relative clockwise (CW) rotation of Eastern Liaoning-Korea Block (ELKB) with respect to NCB does not affect the Jiaodong Peninsula as a rigid block, nevertheless, some tectonic structures have been reactivated since Cenozoic and local rotations could be produced. This palaeomagnetic study reveals that the Late Mesozoic extension in Eastern Asia is heterogeneously expressed in time and space. The complete understanding of the mechanism(s) of this extensional event needs further multidisciplinary investigations.
ISSN: 0031-9201

insu-00599546
https://hal-insu.archives-ouvertes.fr/insu-00599546
https://hal-insu.archives-ouvertes.fr/insu-00599546/document
https://hal-insu.archives-ouvertes.fr/insu-00599546/file/Charles_et_al_PEPI2011.pdf
DOI : 10.1016/j.pepi.2011.05.006

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
The Tian Shan Mountains constitute central Asia's longest and highest mountain range. Understanding their Cenozoic uplift history thus bears on mountain building processes in general, and on how deformation has occurred under the influence of the India-Asia collision in particular. In order to help decipher the uplift history of the Tian Shan, we collected 970 samples for magnetostratigraphic analysis along a 4571-m-thick section at the Jingou River (Xinjiang Province, China). Stepwise alternating field and thermal demagnetization isolate a linear magnetization component that is interpreted as primary. From this component, a magnetostratigraphic column composed of 67 polarity chrons are correlated with the reference geomagnetic polarity timescale between ~1 Ma and ~23.6 Ma, with some uncertainty below ~21 Ma. This correlation places precise temporal control on the Neogene stratigraphy of the southern Junggar Basin and provides evidence for two significant stepwise increases in sediment accumulation rate at ~16–15 Ma and ~11–10 Ma. Rock magnetic parameters also undergo important changes at ~16–15 Ma and ~11–10 Ma that correlate with changes in sedimentary depositional environments. Together with previous work, we conclude that growth history of the modern Tian Shan Mountains includes two pulses of uplift and erosion at ~16–15 Ma and ~11–10 Ma. Middle to upper Tertiary rocks around the Tian Shan record very young (<~5 Ma) counterclockwise paleomagnetic rotations, on the order of 15° to 20°, which are interpreted as because of strain partitioning with a component of sinistral shear that localized rotations in the piedmont.
ISSN: 0278-7407

insu-00376345
https://hal-insu.archives-ouvertes.fr/insu-00376345
https://hal-insu.archives-ouvertes.fr/insu-00376345/document
https://hal-insu.archives-ouvertes.fr/insu-00376345/file/2007TC002137.pdf
DOI : 10.1029/2007TC002137

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Tarim and Junggar basins in central Asia are capped by a thick pile of conglomerates, called the Xiyu Formation, that are commonly linked to a change in climate and/or accelerated uplift near the Plio-Pleistocene boundary. In order to better understand their origin and significance, we carried out a combined structural and magnetostratigraphic study in the Quilitage syncline (southern Tianshan), where the base of the Xiyu conglomerates is observed at both sides of the syncline. A balanced cross-section shows that, even at a local-scale, the base of the Xiyu conglomerates cannot be regarded as a single continuous stratigraphic layer. On the southern flank of the Quilitage syncline, we collected 172 samples collected for magnetostratigraphic dating identify 17 polarity chrons that date the new section from 5.2 to ~ 1.7 Ma and constrain the base of the Xiyu conglomerate here at ~ 1.7 Ma. This is 4.2 Ma younger than the age of the Xiyu previously found on the northern limb of the same syncline. Together with other magnetostratigraphic studies carried out around the Tianshan, our study unambiguously demonstrates that the onset of deposition of the Xiyu conglomerates is diachronous, and that the conglomerates are systematically younger toward the basin. Consequently, the Xiyu Formation should not be considered as a chronostratigraphic marker related to any particular tectonic or climatic event, but is instead a prograding gravel wedge that has prograded over the underthrusting forelands. A synthesis of chronologic and structural results yields progradation rates over the last 10 Ma on the order of ~ 2.0 mm/yr and ~ 3.9 mm/yr south and north of the Tianshan Mountains respectively. These rates are comparable to the shortening rate across the Tianshan range, suggesting that underthrusting is the main factor governing the progradation rate of the Xiyu Formation.
ISSN: 0012-821X

insu-00414151
https://hal-insu.archives-ouvertes.fr/insu-00414151
https://hal-insu.archives-ouvertes.fr/insu-00414151/document
https://hal-insu.archives-ouvertes.fr/insu-00414151/file/Charreau-EPSL-2009.pdf
DOI : 10.1016/j.epsl.2009.07.035

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
The current structure of the South Armorican Domain in the French Variscan Belt, provides an excellent record of the late orogenic evolution, as documented by the complex interactions between extensional structures and partial melting. In the Golfe du Morbihan, the South Brittany Migmatite Belt (SBMB), tectonically overlain by non-melted metamorphic units, was studied by combined structural and 40Ar/39Ar methods. The recognition of successive deformation stages points to a strain continuum initiated at depth at the end of a partial melting event, and continued to near-surface brittle faulting, encompassing ductile strain localization over a major top-to-the-ESE extensional shear zone (SSZ) consistent with an overall WNW-ESE orogen-parallel stretching. This multidisciplinary study making the link between structure, texture, microchemistry and 40Ar/39Ar geochronology allow distinguishing between cooling, recrystallization and syn-tectonic crystallization ages. The first extensional insights, witnessed by dyke intrusion were dated at ~319 ± 6 Ma immediately after the SBMB partial melting. Strain localization along the SSZ, occurred until ~302-298 Ma when SBMB crossed the ductile-brittle transition. Moreover, in situ 40Ar/39Ar analysis, of syn-tectonic white micas sampled in the ductile and brittle structures yield similar ~300 Ma. Formation or reactivation of the SSZ may have occurred during the rapid cooling of the SBMB settled at ~305 Ma by conventional 40Ar/39Ar analysis. Large-scale late orogenic extensional event of the previously overthickened Variscan crust thus lasted over ~20 ± 3 Myr in the same extensional tectonic regime guided by the South Armorican Shear Zone.
ISSN: 0278-7407

insu-00628699
https://hal-insu.archives-ouvertes.fr/insu-00628699
https://hal-insu.archives-ouvertes.fr/insu-00628699/document
https://hal-insu.archives-ouvertes.fr/insu-00628699/file/2010TC002788.pdf
DOI : 10.1029/2010TC002788