Éditeur(s) : HAL CCSD Cambridge University Press (CUP)
Résumé : International audience
Andros is a key island for understanding both the timing of high-pressure–low-temperature(HP-LT) metamorphism and the dynamics of crustal-scale detachment systems exhuming high-gradeunits in the Cyclades (Greece). Using phengite 40Ar–39Ar geochronology coupled with thermobarometry,as well as data from literature, we constrain the pressure–temperature–time (P-T-t) paths of theMakrotantalon and Attic–Cycladic Blueschist units on Andros. Peak conditions of the HP-LT episodein the Makrotantalon unit are 550 °C and 18.5 kbar, dated at 116 Ma. We correlate this episode withEarly Cretaceous blueschist facies metamorphism recognized in the Pelagonian zone of continentalGreece. This is a new argument favouring a Pelagonian origin for the Makrotantalon unit. In the Attic–Cycladic Blueschist unit, the P-T-t path is characterized by: (1) exhumation after peak conditions inHP-LT conditions between 55 and 35 Ma; (2) isobaric heating at 7 kbar until 30 Ma; and (3) isothermaldecompression until 21 Ma. This thermal evolution and timing are similar to those of the neighbouringTinos Island, emphasizing major thermal re-equilibration at the transition between stable and retreatingsubduction. Modifications of the crustal thermal state played a major role in the evolution of the NorthCycladic Detachment System, below which Andros HP-LT units were exhumed.
ISSN: 0016-7568

insu-01114909
https://hal-insu.archives-ouvertes.fr/insu-01114909
DOI : 10.1017/S0016756814000661

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Since Deccan Traps extrusion ~ 65 Ma ago, thick weathering mantles have developed over Peninsular India on both the western coastal lowland and adjacent plateau separated by the Western Ghats Escarpment. Manganiferous lateritic profiles formed by supergene weathering of Late Archean manganiferous protores are exposed on paleolandsurface remnants on both sides of the escarpment. Petrological and geochemical characterizations of samples from those Mn lateritic profiles allowed identifying cryptomelane (K-Mn oxide) dated by 40Ar/39Ar geochronology. The ages obtained document major weathering periods, ca. 53-50 Ma, and ca. 37-23 Ma in the highland, and ca. 47-45 Ma, ca. 24-19 Ma and discrete weathering pulses at ~ 9 Ma and ~ 2.5 Ma in the lowland. Old ages of the highland (53-50 Ma) and the lowland (47-45 Ma) indicate synchronous lateritic weathering across the escarpment at a time the peninsula started to drift across the equatorial belt. Intense weathering periods at ca. 53-45 and ca. 37-23 Ma are interpreted to reflect the Early Eocene climatic optimum and the onset of Asian monsoon regimes, respectively. The ages further indicate that most of the dissection of the highland must have taken place after ~ 23 Ma, whereas the lowland was weakly incised essentially after ~ 19 Ma. Our results also document divergent erosion and weathering histories of the lowland and the highland after the Eocene, suggesting installation of a dual climatic regime across the Western Ghats escarpment.
ISSN: 0009-2541

ird-01419899
http://hal.ird.fr/ird-01419899
http://hal.ird.fr/ird-01419899/document
http://hal.ird.fr/ird-01419899/file/CHEMGE9048R2.va.pdf
DOI : 10.1016/j.chemgeo.2016.04.018

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : This study focuses on the petrology, geochronology and thermochronology of metamorphic rocks within the northern Menderes Massif in western Turkey. Metasediments belonging to the cover series of the Massif record pervasive amphibolite-facies metamorphism culminating at ca. 625–670 °C and 7–9 kbars. U–Th–Pb in situ ages on monazite and allanite from these metapelites record crustal thickening and nappe stacking associated with the internal imbrication of the Anatolide–Taurides platform during the Eocene. In addition, new 39Ar/40Ar single muscovite grain analyses on deformed rocks were performed in three localities within the northern Menderes Massif and ages range from 19.8 to 25.5 Ma. These mylonites may be related to both well-known detachments, Simav to the north and Alaşehir to the south, which accommodate Oligo–Miocene exhumation of the Menderes core complex. U–Th–Pb data on monazite grains (22.2 ± 0.2 Ma) from migmatites emplaced within the Simav detachment confirm these ages.
ISSN: 1437-3254

hal-01355967
https://hal.archives-ouvertes.fr/hal-01355967
DOI : 10.1007/s00531-015-1271-2

Éditeur(s) : HAL CCSD
Résumé : High Pressure - Low Temperature (HP-LT) rocks testify for burying and exhumation of crustal material in subduction zones. Their complete exhumation is the result of processes acting during (syn-orogenic) and after (post-orogenic) subduction is active, the latter partly obliterating the first. In the Aegean domain, the Attic-Cycladic Blueschist unit (ACBU) is a HP-LT unit exhumed in metamorphic core complexes (MCCs) in the back-arc domain of the Hellenic subduction zone. This study is an attempt to distinguish features related to syn-orogenic exhumation processes responsible for the first stages of exhumation This syn-orogenic "message" has to be read through the post-convergence structural and thermal overprint that accompanied the formation of the MCCs. Particularly, the geometry, the timing and the amount of the syn-orogenic exhumation has to be determined. We investigate these features with coupled field, metamorphic petrology and radiochronology studies First, based on observations on Ios (southern Cyclades), it has recently been proposed that syn-orogenic exhumation of the ACBU was accommodated by a basal thrust over the Cycladic Basement (CB) [Huet, et al., 2009]. The peak conditions, deduced from pseudosections calculation for the Ios main lithologies, are 500 °C-19 kbar for the ACBU and 550 °C-16 kbar for the CB. Exhumation until 400 °C-8 kbar is considered as syn-orogenic. Second, phengites populations, single grains and in situ 40Ar-39Ar geochronology has been carried out on samples from Tinos and Andros (northern Cyclades), Syros (central Cyclades) and Ios. The results gather in several clusters: 50-55 Ma, 40-45 Ma, 30-35 Ma, and 22-26 Ma. Based on these new P-T-t data and a synthesis of published data, we are able to refine the geometry and the timing of the exhumation of the ACBU. Data from the whole Cyclades exhibit little differences in ages and P-T conditions for the successive stages of exhumation. The peak of pressure (500±50 °C-18±1 kbar) has been reached at 50-55 Ma from Tinos to Ios. The onset of the exhumation occurred at 40-45 Ma in blueschist facies conditions, between a thrust at their bottom and a detachment at their top. The following decompression was accompanied by cooling until 30-35 Ma (400±50 °C-8±1 kbar). At that time, the boundary conditions switched to extension. Finally, we propose a model for the exhumation of HP-LT units accounting for these data. We also consider the rheological implications of the syn-orogenic structuration of the Aegean wedge on the post-orogenic evolution. B. Huet, L. Labrousse, and L. Jolivet [2009] Thrust or detachment? Exhumation processes in the Aegean: Insight from a field study on Ios (Cyclades, Greece), Tectonics, 28, 3, doi:10.1029/2008TC002397.
EGU General Assembly
Vienne, Austria

insu-00839054
https://hal-insu.archives-ouvertes.fr/insu-00839054
BIBCODE : 2010EGUGA..1214098H

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Alkali feldspar thermochronologic modeling with the 40Ar/39Ar method has generated marked advances in knowledge of the mechanisms for argon diffusion in feldspars. While the goal in many cases has been to extrapolate the observed and modeled argon behavior in feldspars to natural geological settings, scientific debate surrounding the true feasibility of such extrapolations and indeed, the validity of thermochronologic modeling in itself, have provided much impetus to improve laboratory techniques to test, and increase basic understanding of, argon diffusion. Two cornerstones for the debate over the feasibility of alkali feldspar thermochronology for modeling natural, geologic processes have been: 1. is volume diffusion the main mechanism for argon movement in feldspars? and 2. if volume diffusion is a viable mechanism, does argon then reside in discrete ‘domains' within the feldspar lattice? We describe a study of alkali feldspars from a profile through a well-controlled brittle fault zone in western Norway; the feldspars document argon loss during deformation and strongly suggest the existence of argon ‘domains' within the feldspars, at least during laboratory step heating. The progressive change in the character of argon diffusion is recognizable in the logr/ro diffusion data from the feldspars and is mimicked by physical changes observed optically in the feldspars through progressive degrees of brittle deformation. Modeling results indicate a reduction in size of the biggest domains and the appearance of smaller domains during the strongest stages of deformation. Whether or not this reveals the existence and the transformation of the domain structure in naturo is difficult to prove from our data alone, but interestingly, this behaviour corresponds directly to the physical (optical) appearance of more intense crack networks and subgrains in progressively more brecciated feldspars. Because the thermochronologic histories derived from modeling the feldspar data conform very well to the known tectonic history of the area, the feldspars appear to have successfully retained physical (optical and isotopic) records of episodic tectonic processes operating from ductile through lowtemperature brittle regimes in rocks with a Caledonian history overprinted by several later (younger) geologic events. However, because the ‘cold' brecciation is the last tectonothermal event recorded by these rocks, it is impossible to truly test for the existence of diffusion domains in naturo. Argon loss appears to have been effective only in the most highly brecciated (deformed) samples where the combination of the connected crack network, increased fluid flow and higher temperatures enhanced diffusion via fast diffusion pathways and thus, volume diffusion from the lattice. Only minor argon loss occurred in zones of lower brittle strain, although some development of cracks and brittle features is evident. Independent of the existence of diffusion domains, this study highlights the possible pitfalls when cooling histories are deduced from brecciated feldspars in which age and diffusion charateristics have been decoupled: while the geochronological memory has survived and is identical to that of nonbrecciated feldspars (suggesting no loss and minor effects of deformation), the diffusion characteristics have been completely transposed by brecciation and the appearance of new domains. Modeling feldspars with these latter characteristics would effectively utilise a new feldspar diffusion structure with an ‘old' (relict) age memory.
ISSN: 0016-7037

hal-00198283
https://hal.archives-ouvertes.fr/hal-00198283
https://hal.archives-ouvertes.fr/hal-00198283/document
https://hal.archives-ouvertes.fr/hal-00198283/file/KsparBreccia_Arnaud_Eide_GCA_2000.pdf

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Located on the margin of the west Alboran basin, the Gibraltar Arc (Betic-Rif mountain belt) displays post-Pliocene vertical movements evidenced by uplifted marine sedimentary basins and marine terraces. Quantification of vertical movements is an important clue to understand the origin of present-day relief generation in the Betic-Rif mountain chain together with the causes of the Messinian Salinity Crisis. In this paper, we present the results of a pluridisciplinary study combining an analysis of low temperature thermochronology and Pliocene basins evolution to constrain the exhumation history and surface uplift of internals units of the Rif belt (Northern Morocco). The mean (U-Th)/He apatite ages obtained from 11 samples are comprised between 14.1 and 17.8 Ma and display a wide dispersion, which could be explained by a great variability of apatite chemistries in the analyzed samples. No correlations between altitude and age have been found along altitudinal profile suggesting a rapid exhumation during this period. Thermal modeling using our (U-Th)/He apatite ages and geochronological data previously obtained in the same area (40Ar/39Ar and K/Ar data on biotite, zircon and apatite fission track) allow us to propose a cooling history. The rocks suffered a rapid cooling at 60-100 °C/Ma between 22.5 and 19 Ma, then cooled to temperatures around 40 °C between 19 and 18 Ma. They were re-heated at around 110 °C between 18 and 15 Ma then rapidly cooled and exhumed to reach the surface temperature at around 13 Ma. The re-heating could be related to a renewal in thrusting and burying of the inner zones. Between 15 and 13 Ma the cooling resumed at a rate of 50 °C/Ma indicating an exhumation rate of 0.8 mm/y considering an average 40 °C/km geothermal gradient. This exhumation may be linked to the extension in the Alboran Sea. Otherwise biostratigraphic and sedimentological analysis of Pliocene basins of the internal Rif provided informations on the more recent events and vertical movements. Pliocene deposits of the Rifian coast represent the passive infilling of palaeo-rias between 5.33 and 3.8 Ma. The whole coastal area was uplifted at slow average rates (0.01-0.03 mm/y) in relation with a northeastward tilting of 0.2-0.3° since the Lower-Pliocene. A late Pliocene to present extensional tectonics associated to uplift has been identified all along the coastal ranges of the Internal Zone of the Rif chain. This extension was coeval with the major late Pliocene to Pleistocene extensional episode of the Alboran Sea and appears to be still active nowadays. No significant late Messinian uplift was evidenced, thus calling into question the geodynamic models relating the closure of the marine gateways and the MSC to slab roll back.
ISSN: 0264-3707

hal-01053531
https://hal.archives-ouvertes.fr/hal-01053531
DOI : 10.1016/j.jog.2014.01.006

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
Geochronological, mineralogical, and geochemical analyses have been focussed on the Mont-Loze're- Borne plutonic complex and surrounding rocks (Ce'vennes, French Massif Central) in which B-W-Sn and As-Au-Sb mineralization is encountered. Two main results are highlighted: (1) the existence of a 301-306 Ma magmatohydrothermal event unrelated to the emplacement of the Pont-de-Montvert-Borne plutonic body at 316 Ma; (2) the magmatic and hydrothermal features are strongly associated, both in time and in space, thus demonstrating an intimate connection between mineralizing processes and magmatism in this part of the French Massif Central. We also show that mineralization and associated hydrothermal occurrences do not correspond to a simple and single geochemical signature and that a contamination model must be invoked in order to account for the complexity of isotopic results. This study demonstrates that the application of the O and H isotopic signatures as tracers of the source and nature of fluids in an orogenic context requires some specific care. Finally, a model of the tectonic-magmatic- hydrothermal evolution of the study area is suggested in which we discuss two alternative scenarios. The first one implies the existence of two different hydrothermal/ mineralizing events (Bo-W-Sn and As-Au-Sb ones). The second one suggests the same source for all hydrothermal and mineralized structures.
ISSN: 1437-3254

insu-00576546
https://hal-insu.archives-ouvertes.fr/insu-00576546
https://hal-insu.archives-ouvertes.fr/insu-00576546/document
https://hal-insu.archives-ouvertes.fr/insu-00576546/file/Chauvet-InternationalJournalEarthSciences-2012.pdf
DOI : 10.1007/s00531-011-0639-1

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
Investigation of an eclogite xenolith, discovered in a Cretaceous granite from the Central Domain of the Dabieshan massif in eastern China, yields new petrological insights into the high to ultrahigh-pressure metamorphism, experienced by the Qinling-Dabie orogen. Prior to inclusion as a xenolith in the granite during the Early Cretaceous, this eclogite xenolith had recorded a complex metamorphic evolution that complies with subduction and exhumation processes experienced by the continental crust of the South China Block. Well-preserved mineral parageneses substantiate the prograde and retrograde stages revealed by inclusions in porphyroblastic garnet and zoned minerals such as garnet, comphacite and amphibole in the matrix. The relatively low P/T re-equilibration during a latemetamorphic stage was textually inferred by the presence of aluminous and calcic-subcalcic amphiboles such as katophorite, edenite, taramite and pargasite as main matrix phases. According to our U/Pb, Rb/Sr and new 40Ar/39Ar geochronological results, namely 109 ± 1 and 112 ± 2 Ma plateau ages for muscovite and amphiboles, respectively, two successive but distinct cooling stages account for the thermal history of the granite–migmatite gneiss dome that forms the Central Dabieshan Domain. We argue that prior to the Cretaceous doming, the Central Dabieshan Domain experienced a tectono-metamorphic evolution similar to that observed in the high-pressure to ultra high-pressure units developed in the Southern Dabieshan Domain and Hong'an massif.
ISSN: 1437-3254

hal-00104224
https://hal-insu.archives-ouvertes.fr/hal-00104224
https://hal-insu.archives-ouvertes.fr/hal-00104224/document
https://hal-insu.archives-ouvertes.fr/hal-00104224/file/Lin-IntJEarthSciences-2007.pdf
DOI : 10.1007/s00531-006-0129-z

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Ash flow tuffs, or ignimbrites have been recently proposed to be a good material for palaeointensity determination. In this paper, we present a multidisciplinary study, combining geochronology, petrology, rock magnetism and palaeomagnetism, carried out on Oligocene to Early Miocene ignimbrites and related flows from the southern part of the Sierra Madre Occidental. Two new 40Ar/39Ar ages were determined for ignimbrites; 20.4 ± 0.2 Ma (the youngest age obtained so far in this area) and 29.2 ± 0.5 Ma. Density measurements, as a proxy for welding, proved to be extremely useful to estimate the emplacement temperature and the origin of the magnetizations carried by the ignimbrites. After alternating field and thermal demagnetizations, the mean palaeomagnetic pole (Lat = 66.8°N; Long = 180.5°E; Kappa = 142; A95 = 6.3°), calculated for the period 28-31 Ma, is in close agreement with our only Miocene determination. Comparison with the North America Synthetic Apparent Polar Path indicates a net counter-clockwise vertical axis rotation of about 10 ± 4° compared to stable North America, which occurred likely during the last extensional episode in the Late Miocene (ca. 12-9 Ma). Palaeointensity estimates, obtained with the Thellier-Coe method, are mainly questionable and should not be used for global interpretation. Therefore, these ignimbrites are not a viable material for reliable palaeointensity determinations.
ISSN: 0956-540X

hal-00854457
https://hal.archives-ouvertes.fr/hal-00854457
DOI : 10.1093/gji/ggt079