The Sierra Ballena Shear Zone in the southernmost Dom Feliciano Belt (Uruguay): evolution, kinematics, and deformation conditions

The Sierra Ballena Shear Zone (SBSZ) is part of a high-strain transcurrent system that divides the Neoproterozoic Dom Feliciano Belt of South America into two different domains. The basement on both sides of the SBSZ shows a deformation stage preceding that of the transcurrent deformation recognized as a high temperature mylonitic foliation associated with migmatization. Grain boundary migration and fluid-assisted grain boundary diffusion enhanced by partial melting were the main deformation mechanisms associated with this foliation. Age estimate of this episode is >658 Ma. The second stage corresponds to the start of transpressional deformation and the nucleation and development of the SBSZ. During this stage, pure shear dominates the deformation, and is characterized by the development of conjugate dextral and sinistral shear zones and the emplacement of syntectonic granites. This event dates to 658–600 Ma based on the age of these intrusions. The third stage was a second transpressional event at about 586 to <560 Ma that was associated with the emplacement of porphyry dikes and granites that show evidence of flattening. Deformation in the SBSZ took place, during the late stages, under regional low-grade conditions, as indicated by the metamorphic paragenesis in the supracrustals of the country rocks. Granitic mylonites show plastic deformation of quartz and brittle behavior of feldspar. A transition from magmatic to solid-state microstructures is also frequently observed in syntectonic granites. Mylonitic porphyries and quartz mylonites resulted from the deformation of alkaline porphyries and quartz veins emplaced in the shear zone. Quartz veins reflect the release of silica associated with the breakdown of feldspar to white mica during the evolution of the granitic mylonites to phyllonites, which resulted in shear zone weakening. Quartz microstructures characteristic of the transition between regime 2 and regime 3, grain boundary migration and incipient recrystallization in feldspar indicate deformation under lower amphibolite to upper greenschist conditions (550–400°C). On the other hand, the mylonitic porphyries display evidence of feldspar recrystallization suggesting magmatic or high-T solid-state deformation during cooling of the dikes.     

Interaction between tectonic and erosion processes on the morphogenesis of an Alpine valley: geological and geophysical investigations in the lower Romanche valley (Belledonne massif, western Alps)

In the Belledonne massif, the steep Paleozoic Belledonne Middle Fault (BMF) separates micaschists, displaying numerous landslides, from amphibolites. The massif is incised by the lower Romanche river valley. When crossing the BMF, the valley widens into a lozenge-shaped basin recently interpreted as an active pull-apart type structure associated with a major N110 striking Quaternary fault. Multidisciplinary investigations were carried out in the basin to check if this model has implications on the seismic and landslide hazard assessment. This study demonstrated the existence of a N80 sinistral strike slip Séchilienne Fault Zone (SFZ). This fault zone is suspected to offset the BMF by 375 m across the basin. Geophysical experiments revealed that the bedrock depth increases strongly in the basin, up to 350 m. Our study invalidates the active pull-apart origin of the basin and suggests it results from Quaternary glacial and fluvial erosion processes, magnified by the intersection of two inherited structures, the BMF and the SFZ.     

Spaceborne remote sensing of greenhouse gas concentrations

Despite their primary contribution to climate change, there are still large uncertainties on the sources and sinks of the main greenhouse gases: carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). A better knowledge of these sources is necessary to understand the processes that control them and therefore to predict their variations. Indeed, large feedbacks between climate change and greenhouse gas fluxes are expected during the 21st century. Sources and sinks of these gases generate spatial and temporal gradients that can be measured either in situ or from space. One can then estimate the surface fluxes, either positive or negative, from concentration measurements through a so-called atmospheric inversion. Surface measurements are currently used to estimate the fluxes at continental scales. The high density of spaceborne observations allows potentially a much higher resolution. Several remote sensing techniques can be used to measure atmospheric concentration of greenhouse gases. These techniques have motivated the development of spaceborne instruments, some of them already in space and others under development. However, the accuracy of the current estimates is still not sufficient to improve our knowledge on the greenhouse gases sources and sinks. Rapid improvements are expected during the forthcoming years with a strong implication of the scientific community and the launch of dedicated instruments, optimized for the measurement of CO₂ and CH₄ concentrations.     

Forward kinematic modelling of a regional transect in the Northern Emirates using geological and apatite fission track age constraints on paleo-burial history

This paper aims to simulate the kinematic evolution of a regional transect crossing the Northern Emirates in the northernmost part of the Semail Ophiolite and the Dibba zone, just south of the Musandam Platform exposures. The studied section comprises, from top to bottom and from inner to outer zones, (1) the erosional remnants of the Semail Ophiolite, mainly made up of serpentinized ultramafics in the west and gabbros in the east, (2) high-grade metamorphic rocks which are currently exposed in the core of a nappe anticline near Masafi, (3) far-travelled Hawasina basinal units and Sumeini paleo-slope units of the Dibba Zone, (4) parautochthonous platform carbonates, which are currently well exposed in the Musandam area, and (5) a flexural basin filled with uppermost Cretaceous to Neogene sediments. Two main compressional episodes are generally identified, resulting first in the obduction of the Semail Ophiolite and then in the stacking of underlying platform carbonate units of the former Arabian passive margin, thus accounting for the present architecture of this transect: (1) first, deformation at the plate boundary initiated in the Late Cretaceous, resulting in the obduction of the Semail Ophiolite and the progressive accretion of the Hawasina and Sumeini tectonic wedge on top of the Arabian foreland, leading to a progressive bending of its lithosphere and development of a wide flexural basin; (2) compression resumed during the Neogene, leading to the tectonic stacking of the parautochthonous platform duplexes of Musandam and Margham trends, the development of out-of-sequence thrusts and triangle zones, refolding of the sole thrust of the former Late Cretaceous accretionary wedge and coeval normal (?) high-angle faulting along the contact between the Musandam and Dibba zones. However, seismic profiles and paleo-thermometers also help in identifying another erosional event at the boundary between the Paleogene Pabdeh and the Neogene Fars series. Evidenced by the local erosional truncation of the Pabdeh series in the vicinity of the frontal triangle zone (i.e. the inner part of the former Late Cretaceous foredeep), this Paleogene uplift/unroofing episode is tentatively interpreted here as an evidence for a continuum of compressional deformation lasting from the Late Cretaceous to the Middle Miocene although one may alternatively speculate that it was related to the detachment of the subducted slab. Although carbonate facies are usually not suitable for apatite fission track (AFT) studies, we were able to extract detrital apatites from quartz-bearing Triassic dolomites in the Musandam area. However, the yield and the quality were both poor and too few fission track lengths could be measured, making it difficult to interpret the meaning of the FT ages. The FT dates obtained in this study are therefore compared with those existing in the literature. Fortunately enough, for each sample, at least ten apatite crystals could be used for fission track dating, except for site 6 with only five datable apatite grains. The obtained apatite fission track dates between 28 and 13 Ma, much younger than the Triassic age of the series, are taken to represent reset fission track ages, implying erosion of an up-to-3-km-thick pile of Jurassic–Cretaceous carbonates and Hawasina allochthon during the Neogene. Apatite fission track dates from the ~95 M-old plagiogranites of the Semail complex (Searle and Cox, Geol Mag 139(3):241–255, 2002) obtained in this study and compared with those recently published provide evidences for more than one cooling event. An early unroofing of the ophiolite during the Late Cretaceous is revealed in fission track dates of 72–76 Ma at the top of the ophiolite in the east, which are coeval and also consistent with the occurrence of paleo-soils, rudists and paleo-reefs on top of serpentinized ultramafics in the west. High-pressure rocks at As Sifah in the southeast near Muscat revealed apatite fission track data ranging from ~46 to 63 Ma (Gray et al. 2006). The leucocratic part of the ophiolite (sample UAE 180) yielded comparable young apatite (40.6?±?3.9 Ma) and zircon (46.6?±?4.3 Ma) FT dates. A Cenozoic (~20–21 Ma) exhumation has been determined for the Bani Hamid metamorphic sole in northern Oman, applying low temperature geochronology and combining apatite FT and apatite (U–Th)/He analyses (Gray et al. 2006). In this study, young apatite fission track dates of 20 Ma have also been found but at the base of the ophiolite near Masafi, in the core of the nappe anticline, thus indicating a Neogene age for the refolding of the allochthon and stacking of underlying parautochthonous platform carbonate units. During the subsequent 2D forward Thrustpack kinematic modelling of the regional transect, these AFT data-set has been used, together with available subsurface information, to reconstruct the past architecture of the structural sections through time, accounting for incremental deformation along the various decollement levels, synorogenic sedimentation and erosion, as well as for successive bending and unbending episodes of the Arabian lithosphere.     

New constrains on the thickness of the Semail ophiolite in the Northern Emirates

Near-critical angle and refraction studies were performed at IFP as piggyback studies during a wider programme of crustal imagery operated by WesternGeco on behalf of the Ministry of Energy of the United Arab Emirates. The main objective is to illuminate the base of the Semail Ophiolite along part of a regional transect (D1) crossing the Northern Emirates from the Gulf of Oman in the east up to the Arabian Gulf in the west. Results confirm that the sole thrust of the ophiolite has been folded during the Miocene stacking of the underlying Arabian Platform. The thickness of the ophiolite grades from zero in the core of the Masafi tectonic window, up to a maximum of 1.7 km below the axial part of a successor basin which has been preserved on top of the serpentinite west of the current exposure of the main ultramafic bodies. Apatite grains extracted from plagiogranites of the Semail ophiolite also provide evidences for an early unroofing of the gabbros and plagiogranites during the Late Cretaceous, with cooling ages of 72–76 Ma at the top of the ophiolite in the east (not far from the Fujairah coast line), which are coeval and also consistent with the occurrence of Late Cretaceous paleo-soils, rudists and paleo-reef deposits on top of serpentinized ultramafics in the west. Younger cooling ages of 20 Ma have been also found at the base of the ophiolite near Masafi, in the core of the nappe anticline, thus providing a Neogene age for the refolding of the allochthon and stacking of underlying parautochthonous platform carbonate units. These results, together with the occurrence of a thick sedimentary pile illuminated below the metamorphic sole along the north-trending, strike-profile D2 running parallel to the axis of the Masafi window, should stimulate a renewal of the exploration in the central part of the Emirate foothills, where the ophiolite thickness is currently limited, and was already drastically reduced by the end of its Late Cretaceous obduction.     

Modern Sphagnumδ13C signatures follow a surface moisture gradient in two boreal peat bogs,James Bay lowlands,Québec

Carbon isotopic composition of Sphagnum macrofossils can potentially be used as a palaeohydrological tool for peat‐based climatic studies since a relationship between Sphagnum δ¹³C values and peatland surface moisture has been presented in previous studies. In order to verify this hypothesis, modern Sphagnum δ¹³C values were measured along a moisture (microtopographic) gradient in two boreal peat bogs. Isotopic measurements were performed on bulk material of S. fuscum, S. magellanicum, S. capillifolium and S. pulchrum. Isotopic variations found within and between Sphagnum species along the microtopographic gradient were compared using analysis of variance. A significant positive correlation (P < 0.0001) was found between Sphagnum δ¹³C values and their position along the surface moisture gradient. Results show that ¹³C‐depleted values are related to low water table depths (WTD), while ¹³C‐enriched values correspond to a water table that is close to the peat surface. Although the mechanisms underlying carbon fractionation processes in mosses are not well understood, we demonstrate that water resistance to CO₂ diffusion is an important fractionation process that is observed in bulk Sphagnum δ¹³C measurements, since drier and wetter samples exhibit consistent and very different isotopic signatures. Copyright © 2008 John Wiley & Sons, Ltd.     

Experimental constraints on storage conditions in the chemically zoned phonolitic magma chamber of the Laacher See volcano

Phase relations of three samples of the Laacher See Tephra (LST) have been determined experimentally as a function of temperature (760 to 880 °C), pressure (200, 300 and 400 MPa), water content of the melt and oxygen fugacity (ƒO₂). The crystallization experiments were carried out at ƒO₂=NNO buffer and at NNO=+ 2.3 log units. The melt water contents varied between 6 and more than 8 wt% H₂O, corresponding to water-undersaturated and water saturated conditions respectively. The synthetic products are compared to the natural phases to constrain pre-eruptive conditions in the Laacher See magma chamber. The major phases occurring in the LST have been reproduced. The stability of hauyne is favoured at high ƒO₂ (≈NNO + 2.3). The CaO contents in melt and plagioclase synthesized under water-saturated conditions are significantly higher than in the natural phases, implying that most of the differentiation of the phonolites took place under water-undersaturated conditions. However, this does not exclude the presence of a S-, Cl- and CO₂-rich fluid phase in the upper parts of the magma chamber. The phase relationships and the TiO₂ contents of melts show that the temperature was lower than 760 °C in the upper part of the magma column (probably down to 720 °C in the most differentiated levels) and that temperatures above 840–860 °C prevailed in the lower part. The variation of the X _Mg of ferromagnesian minerals observed in both natural and experimental phases reflects the strong variations in ƒO₂ in the lower magma chamber just prior to eruption (probably variation of about 2 log units). The most probable explanation for these ƒO₂ variations is the injection of an oxidized alkali-rich magma, containing Mg-rich phenocrysts, at the base of a chemically zoned and more reduced magma column prior to eruption. Although the amount of injected magma may not have been very important, it was sufficient to change the ƒO₂ conditions locally, explaining the heterogeneous X _Mg of ferromagnesian minerals and the formation of hauyne at the base of the chamber. Received: 30 May 2000 / Accepted: 12 August 2000     

La rétromorphose du talc dans les dolomies métallifères de Boukdema (Algérie); discussion du rôle des fluides

Résumé Les échantillons étudiés proviennent du gîte stratiforme de Boukdema (Algérie), où l'on observe notamment l'association dolomite-quartz-talcsphalérite. L'étude des inclusions fluides primaires des quartz contemporains, selon toute vraisemblance, de la dissolution partielle du talc, montre que ceux-ci se sont développés dans des solutions de salinités très variables (entre 4 et 28% en équivalent pondéral NaCl). L'hypothèse d'une ébullition des solutions (par suite d'une chute brutale de pression) rend compte de ce fait inhabituel. Dans le cadre d'un tel modèle, les corrections dues à la pression sont nulles et l'on peut en déduire à partir des mesures de températures d'homogénéïsation que la croissance du quartz s'est amorcée vers 250 °C. Compte tenu de cette indication thermométrique, on discute enfin du rôle relatif des facteurs physiques et chimiques (T, f_{CO 2}, a_Mg/a_Ca) sur la stabilité du talc dans le contexte minéralogique du gîte de Boukdema.

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The samples studied were collected in the Boukdema stratabound deposit (Algeria), where the association dolomite-quartz-talc-sphalerite occurs. Cryoscopic investigations of fluid inclusions in quartz indicate a wide salinity range in mineral forming fluids (4 to 28 weight % NaCl eq). Boiling of hydrothermal fluids possibly explains this unusual characteristic. In this hypothesis thermometric data need no pressure corrections and one may conclude that quartz growth (and simultaneously talc dissolution) occurred in the 250°–200 °C range. On the basis of this last indication, the relative role of T, f_{CO 2}, a_Mg/a_Ca, on talc stability is discussed in the mineralogical environment of Boukdema.