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The P-T gradient in a Late Eocene low-T high-P metamorphic belt in northern New Caledonia increases from SW to NE. Metapelites in the pumpellyite-prehnite and blueschist zones contain lawsonite, Mg-carpholite, Fe-stilpnomelane and Fe-glaucophane. Thermodynamic calculations indicate a progression of metamorphic conditions from less than 0·3 GPa and 250°C in a kaolinite-bearing rock in the SW, up to 1·5 GPa and 410°C in a lawsonite-glaucophane-bearing sample in the NE of the Diahot terrane. Through a multi-method investigation of phyllosilicates, organic matter and fluid inclusions, we demonstrate that the evolution of organic matter and illite crystallinity depends strongly on the evolution of the P-T path with time. In addition, we show that the illite-muscovite b cell dimension provides a robust estimate of maximum pressure reached in low-temperature domains with polyphase metamorphic histories, despite subsequent high-temperature-low-pressure events. Fluid inclusion study reveals an isothermal decompression in the Diahot terrane

Abstract U-series disequilibrium measurements carried out on thermogenic travertine samples from a 12.6 m-long core and a 10 m-thick section from southeastern Morocco yielded finite ages ranging from 500 ka to the present-day, as well as two clusters determined to be older than 500 ka. The calculation of initial 234 U/ 238 U activity ratios in all samples younger than 500 ka shows high, reasonably constant values, with an average of 5.172 ± 0.520 (one standard deviation). Assuming that this value prevailed for periods older than 500 ka, we derived ages of up to approximately 1.2 Ma using the initial 234 U excess decay. Our results indicate that the two older clusters have ages of 776 ± 14 ka for samples from between 8 and 10.1 m and 1173 ± 22 ka for deeper samples respectively. The palaeomagnetic record of the core shows normal polarity inclinations from the surface to around 9 m followed by reverse polarity inclination and antipodal declinations. The inversion is attributed to the Brunhes-Matuyama transition. 234 U excess ages for the interval corresponding to the part of the core where the polarity inversion occurred are in the range of 735 ± 51 to 794 ± 54 ka, with an arithmetic mean value of 776 ± 14 ka for the B-M transition. This age is in good agreement with that determined previously using other dating methods.

In the basement fractured reservoirs, geometric parameters of fractures constitute the main properties for modeling and prediction of reservoir behavior and then fluid flow. This study aims to propose geometric description and quantify the multiscale network organization and its effect on connectivity using a wide-ranging scale analysis and orders scale classification. This work takes place in the Noble Hills (NH) range, located in the Death Valley (DV, USA). The statistical analyses were performed from regional maps to thin sections. The combination of the length datasets has led to compute a power law exponent around −2, meaning that the connectivity is ruled by the small and the large fractures. Three domains have been highlighted in the NH: (1) domain A is characterized by a dominance of the NW/SE direction at the fourth order scale; (2) domain B is characterized by a dominance of the E/W and the NW/SE directions at respectively the fourth and third order scales; (3) domain C is also marked by the E/W direction dominance followed by the NW/SE direction respectively at the fourth and third order scale. The numerical simulations should consider that the orientation depends on scale observation, while the length is independent of scale observation.

International audience

ABSTRACT Continental shelves generally supply large‐scale mass‐wasting events. Yet, the origin and significance of shelf‐derived mass‐transport deposits (MTDs) for the tectonostratigraphic evolution of subduction complexes and their trench‐slope basins have not been extensively studied. Here, we present high‐resolution, outcrop‐scale insights on both the nature of the reworked sediments, and their mechanisms of development and emplacement along tectonically active margins, by examining the Middle Miocene shelf‐derived MTDs outcropping in the exhumed southern portion of the Hikurangi subduction margin. Results show that periods of repeated tectonic activity (thrust propagation, uplift) in such compressional settings not only affect and control the development of shelfal environments but also drive the recurrent generation and destruction of oversteepened slopes, which in turn, favour the destabilisation and collapses of the shelves and their substratum. Here, these events produced both large‐scale, shelf‐derived sediment mass‐failures and local debris flows, which eventually broke down into a series of coalescing, erosive, genetically linked surging flows downslope. The associated MTDs have a regional footprint, being deposited across several trench‐slope basins. Recognition of tectonic activity as another causal mechanism for large‐scale shelf failure (in addition to sea‐level changes, high‐sedimentation fluxes) has implications for both stratigraphic predictions and understanding the tectonostratigraphic evolution of deep‐marine fold‐and‐thrust belts.

The study of geomagnetic excursions is key for understanding the behavior of the magnetic field of the Earth. In this paper, we present the geomagnetic record in a 2.29-m-long continuous core sampled in a flowstone in Liguria (Italy) and dated to the Lower Brunhes. The cored flowstone developed from Marine Isotopic Stage (MIS) 13 to MIS 7, according to 21 U-series dates. The mean growth rate is closely related to glacial and interglacial isotopic stages. Magnetic remanence was measured using u-channel and deconvolved. Four geomagnetic excursions were recorded at the same location, in a single flowstone, during interglacial MIS 11 and 13; Basura 1, 2, 3 and 4, at depths of 213 cm, 181, 160 and 92 cm, respectively. Due to the uncertainties of U-Th dating, the timing of the three events, namely Basura 1, 2 and 3 overlaps. The Basura 4 is well-dated to 417 + -7/8 ka and is clearly distinguishable from the others. It should therefore be considered as a possible excursion.

ABSTRACT Concepts of the interaction between autogenic (e.g., flow process) and allogenic (e.g., tectonics) controls on sedimentation have advanced to a state that allows the controlling forces to be distinguished. Here we examine outcropping and subsurface Neogene deep-marine clastic systems that traversed the Hikurangi subduction margin via thrust-bounded trench-slope basins, providing an opportunity to examine the interplay of structural deformation and deep-marine sedimentation. Sedimentary logging and mapping of Miocene outcrops from the exhumed portion of the subduction wedge record heavily amalgamated, sand-rich lobe complexes, up to 200 m thick, which accumulated behind NE–SW-oriented growth structures. There was no significant deposition from low-density parts of the gravity flows in the basin center, although lateral fringes demonstrate fining and thinning indicative of deposits from low-density flows. Seismic data from the offshore portion of the margin show analogous lobate reflector geometries. These deposits accumulate into complexes up to 5 km wide, 8 km long, and 300 m thick, comparable in scale with the outcropping lobes on this margin. Mapping reveals lobe complexes that are vertically stacked behind thrusts. These results illustrate repeated trapping of the sandier parts of turbidity currents to form aggradational lobe complexes, with the finer-grained suspended load bypassing to areas downstream. However, the repeated development of lobes characterized by partial bypass implies that a feedback mechanism operates to perpetuate a partial confinement condition, via rejuvenation of accommodation. The mechanism proposed is a coupling of sediment loading and deformation rate, such that load-driven subsidence focuses stress on basin-bounding faults and perpetuates generation of accommodation in the basin, hence modulating tectonic forcing. Recognition of such a mechanism has implications for understanding the tectono-stratigraphic evolution of deep-marine fold and thrust belts and the distribution of resources within them.

The Soultz-Sous-Forêts geothermal site (France) operates three deep wells for electricity production. During operation, scales precipitate within the surface installation as (Ba, Sr) sulfate and (Pb, As, Sb) sulfide types. Scales have an impact on lowering energy production and inducing specific waste management issues. Thus scaling needs to be reduced for which a thorough characterization of the scales has to be performed. The geothermal brine is produced at 160 °C and reinjected at 70 °C during normal operation. In the frame of the H2020 MEET project, a small heat exchanger was tested in order to allow higher energy production, by reinjecting the geothermal fluid at 40 °C. Samples of scales were analyzed by XRD and SEM-EDS, highlighting that mostly galena precipitates and shows various crystal shapes. These shapes can be related to the turbulence of the flow and the speed of crystal growth. Where the flow is turbulent (entrance, water box, exit), crystals grow quickly and mainly show dendritic shape. In the tubes, where the flow is laminar, crystals grow more slowly and some of them are characterized by well-developed faces leading to cubes and derived shapes. The major consequence of the temperature decrease is the increased scaling phenomenon.

Analysis of offshore seismic lines suggests that a strong relationship exists between tectonic structures and fluid migration in accretionary prisms. However, only few field analogues of plumbing systems and their tectonic frameworks have been investigated in detail until now. The uplifted accretionary prism of the Hikurangi Margin (North Island, New Zealand) exposes early to late Miocene mudrocks in coastal cliffs of Cape Turnagain and in the Akitio syncline, south-east of the Pongaroa city. These outcrops display tubular carbonate concretions corresponding to complex subsurface plumbing networks of paleo-seeps within Miocene trench slope basins. We present here, new results on the spatial distribution of these tubular carbonate concretions, with particular attention to their relation to tectonic structures. In the Pongaroa area, tubular carbonate concretions in lower Miocene mudrocks occur along a N-S trend, while in middle Miocene strata they occur along a NNE-SSW direction. The N-S trend parallels a major fault zone ( i.e. the Breakdown fault zone), which separates two wide synclines, the Waihoki and the Akitio synclines. During the Early-Middle Miocene, the Breakdown fault zone controlled the evolution of the Akitio trench slope basin constituting its western edge. The NNE-SSW strike parallels the axis of the Akitio syncline and is also parallel to the present-day subduction front. Our results therefore show that tubular concretions are parallel to post-Middle Miocene second order folding and thrusting in the northeastern limb of the Akitio syncline. In the Cape Turnagain area, tubular concretions occur in the western limb of the Cape Turnagain syncline, in the footwall of the major seaward-verging Cape Turnagain fault. This suggests that fluid migrations may occur not only in the crests of anticlines, as observed offshore for present-day plumbing system of cold seeps, but also in the footwalls of thrust faults. All these observations show that the spatial distribution of tubular concretions is controlled by regional tectonic structures with paleo-fluid migrations related to major deformation episodes of the accretionary prism. Thus, we distinguish three episodes events that likely triggered fluid migration leading to the formation of the tubular concretions: (1) In the Early Miocene, shortly after the onset of development of the Akitio trench slope basin, on its inner (western) edge; (2) During the late Middle Miocene, during an extensional deformation episode on the western limb of the Akitio trench slope basin; (3) At the end of the Late Miocene, during a second major shortening period at the footwall of major thrust fault, such as in the Cape Turnagain area.

International audience

This paper presents the main features of a multiple-criteria decision-making tool for economic and environmental assessment of enhanced geothermal systems projects. The presented holistic approach takes into account important influencing factors such as technical specifications, geological characteristics, spatial data, energy and heat prices, and social and environmental impact. The multiple-criteria decision-making approach uses a weighted decision matrix for evaluating different enhanced geothermal systems alternatives based on a set of criterions which are defined and presented in this paper. The paper, defines and quantifies new criterions for assessing enhanced geothermal systems for a particular site. For evaluation of the relative importance of each criterion in decision making, the weight is associated with each of the listed criterions. The different scenarios of end-use applications are tested in the case study. Finally, in the case study, the data and statistics are collected from real geothermal plants. The case study provides results for several scenarios and the sensitivity analysis based on which the approach is validated. The proposed method is expected to be of great interest to investors and decision makers as it enables better risk mitigation.

Only few data from geothermal exploited reservoirs are available due to the restricted accessibility by drilling, which limits the understanding of the entire reservoir. Thus, analogue investigations are needed and were performed in the framework of the H2020 MEET project. The Noble Hills range, located along the southern branch of the Death Valley pull-apart (CA, USA), has been selected as a possible granitic paleo-reservoir. The aim is to characterize the pervasive alteration processes affecting this granite, away from the influence of the faults, in terms of mineralogical, petrophysical and chemical changes. Various methods were used as petrographic, geochemical and petrophysical analyses. Mineral changes, clay mineralogy, bulk rock chemical composition, calcite content and porosity were determined on different granite samples, collected in the Noble Hills granite, far from the faults and in the Owlshead Mountains, north of the Noble Hills, considered as its protolith. In order to complete the granite characterization, the metamorphic grade has been studied through the Noble Hills granite body. This complete characterization has allowed distinguishing the occurrence of three stages of alteration: (1) a pervasive propylitic alteration characterized by calcite-corrensite-epidote-K-white mica assemblage, (2) a more local one, only present in the Noble Hills granite, producing illite, kaolinite, illite/smectite, calcite and oxides, characteristic of the argillic alteration, which overprints the propylitic alteration and (3) weathering evidenced by the presence of montmorillonite in the Owlshead Mountains, which is considered as negligible in both granites. Alteration was also outlined by the correlation of the loss on ignition, representing the hydration rate, to porosity, calcite content and chemical composition. Moreover, the Kübler Index calculated from illite crystals allowed to identify a NW-SE temperature gradient in the Noble Hills.

Fracture connectivity within fractured granitic basement geothermal reservoirs is an important factor controlling their permeability. This study aims to improve the understanding of fluid–rock interaction processes at low to moderate regional strain. The Noble Hills range (Death Valley, CA, USA) was chosen as a naturally exhumed paleo geothermal reservoir. A series of petrographic, petrophysical, and geochemical investigations, combined with a fracture distribution analysis, were carried out on samples collected across fracture zones. Our results indicate that several generations of fluids have percolated through the reservoir. An increase of (1) the alteration degree; (2) the porosity values; and (3) the calcite content was observed when approaching fracture zones. No correlation was identified among the alteration degree, the porosity, or the calcite content. At a local scale, samples showed that the degree of alteration does not necessarily depend on the fracture density or on the amount of the strain. It is concluded that the combined influence of strain and coeval fluid–rock interaction processes drastically influence the petrophysical properties of fracture zones, which in turn impact geothermal production potential.

Abstract Recognition of a pair of pre-Neogene markers together with analysis of published data indicate ∼40 km of dextral slip across the southern Death Valley fault zone, California, USA. Stratigraphic overlaps on fault rocks indicate much of the dextral slip predates the late Miocene, placing a significant fraction of the dextral slip in the same time window as regional extension and challenging interpretations that the modern strike-slip system became active post–6–3 Ma. However, these results are consistent with regional evidence that dextral transtension began by ca. 12 Ma.

Author: Kempe, Ulf et al.; Genre: Journal Article; Finally published : 2018; Keywords: Zircon, strain, EBSD, Raman, disorder, metamict, substitution; Title: Substitution-induced internal strain and high disorder in weakly radiation damaged hydrothermal zircon from Mt. Malosa, Malawi

The shift from fossil fuels to bioenergy has led to increased wood ash output. Since a large proportion of this waste ends up in landfills, recycling wood ash in construction materials is one of sustainable approaches of managing this by-product. However, the amount of biomass ashes currently recycled in building materials is still limited. This study therefore explored the feasibility of valorising four different types of wood ashes in large quantities in the production of lightweight building materials. Thirty pastes with different levels (80, 90, 95 and 100 wt%) of wood ash and (0, 5, 10 and 20 wt%) of natural hydraulic lime (NHL) or ordinary Portland cement (PC) were developed. Partial replacement of wood ash (WA) by NHL or PC accelerated the setting and improved the mechanical properties of blended pastes. The initial and final setting times decreased with increasing NHL or PC content in the mixes. The strength of the pastes increased with increasing NHL or PC levels and curing time. However, wood bottom ash (WBA) mixes were controversial because their mechanical properties gradually weakened over time. For WA-NHL blends, the 28-day flexural and compressive strength ranged from 0.02 to 1.12 MPa and from 0.05 to 2.59 MPa, respectively. On the other hand, for WA-PC pastes, the flexural and compressive strengths at 28 days varied from 0.07 to 2.72 MPa and from 0.13 to 5.49 MPa, simultaneously. These results prove that wood ash can be used effectively as a main component of binding matrices for lightweight building materials. • Intrinsic characteristics of wood ashes govern the properties of end materials. • Wood ash-PC blends set faster and are more robust than wood ash-NHL pastes. • Wood ash-based pastes containing 20 wt% PC show the best strength results.

Abstract Pre‐Late Cretaceous terranes from the central part of New Caledonia have been metamorphosed under very low‐grade conditions by two high‐pressure/low‐temperature events. The present study investigates the metamorphic patterns with phyllosilicate crystallinities, electron microprobe analyses and petrography. The first metamorphic event is of Late Jurassic age and is characterized by very low (anchizone) to low‐grade (epizone) conditions with a decrease of the illite Kübler Index (KI) and the chlorite Árkai Index (ÁI) values from northeast to southwest. This trend is also confirmed by chlorite thermometry. In the south of the area, un‐metamorphosed sediments (diagenetic KI values) are observed in the Senonian ‘formation à charbons’, post‐dating the metamorphism in this region. The second metamorphism is an Eocene high‐pressure event, which overprints the Late Jurassic metamorphism in the northern part of the studied area. In this zone, the pattern of KI and ÁI indicates another gradient with increasing metamorphic conditions from southwest to northeast. Temperatures calculated by chlorite thermometry also indicate an evolution from southwest to northeast with slight increase of temperature from 298 ± 8°C to 327 ± 16°C. In both metamorphic zones, the K‐white mica b cell dimension calculated on micas analyzed at electron microprobe are in good agreement with high‐pressure/low‐temperature metamorphic conditions (b 0 > 9.04 Å). A combination of chlorite thermometry and K‐white mica b cell dimension allows estimation of a minimum pressure of 1.3 GPa in the Eocene zone (in excellent agreement with the 1.5 GPa registered in the northern part of New Caledonia) and a minimum of 1.1 GPa in the Late Jurassic metamorphic part.

ABSTRACT Clay minerals and organic matter occur frequently in fault zones. Their structural characteristics and their textural evolution are driven by several formation processes: (1) reaction by metasomatism from circulating fluids; (2) in situ evolution by diagenesis; and (3) neoformation due to deformation catalysis. Clay-mineral chemistry and precipitated solid organic matter may be used as indicators of fluid circulation in fault zones and to determine the maximum temperatures in these zones. In the present study, clay-mineral and organic-matter analyses of two major fault zones – the Adams-Tinui and Whakataki faults, Wairarapa, North Island, New Zealand – were investigated. The two faults analysed correspond to the soles of large imbricated thrust sheets formed during the onset of subduction beneath the North Island of New Zealand. The mineralogy of both fault zones is composed mainly of quartz, feldspars, calcite, chabazite and clay minerals such as illite-muscovite, kaolinite, chlorite and mixed-layer minerals such as chlorite-smectite and illite-smectite. The diagenesis and very-low-grade metamorphism of the sedimentary rock is determined by gradual changes of clay mineral ‘crystallinity’ (illite, chlorite, kaolinite), the use of a chlorite geothermometer and the reflectance of organic matter. It is concluded here that: (1) the established thermal grade is diagenesis; (2) tectonic strains affect the clay mineral ‘crystallinity’ in the fault zone; (3) there is a strong correlation between temperature determined by chlorite geothermometry and organic-matter reflectance; and (4) the duration and depth of burial as well as the pore-fluid chemistry are important factors affecting clay-mineral formation.

A drainage system was installed in 2008 on the macrotidal beach of Quend-Plage, close to Abbeville (Somme, northern France), following a period of significant erosion of recreational areas. The “Direction départementale des territoires et de la mer” (French Coastal Department Authority) has requested a biannual survey in order to validate the beach drainage setup and its efficiency. This paper presents the methodology used for this survey, and the response of the coastal system to this soft engineering method for preventing erosion. These five years of drainage operation have strongly modified the morphology of the beach. Three main modifications occurred: (i) accretion of the upper beach and foredune, (ii) erosion of the lower and middle beach and (iii) a slight shift in directions of the beach bars and troughs. These morphological changes finally led to the stabilization of the beach.

Abstract A collection of large data sets from different orogenic belts was compiled for a correlation between organic matter (OM) versus clay mineral (CM) indices calibrated with the vitrinite reflectance, (VR) vs Kübler-Indices (KI) method. Data selection was based on a normal geothermal gradient (25 to 35 °C/km) as determined in previous studies, e.g. by maturity modelling and clay mineral reaction progress calculations. In the Lower Austroalpine (Eastern Switzerland, European Alps) a 20 myr lasting metamorphic overprint caused an OM–CM thermal equilibrium among the indices used. The observed correlation enables to determine gradual changes in metamorphic factors such as pressure, temperature and time causing sensitive shifts of the gradient slope in the range of normal gradients. For New Caledonia, an identical correlation has been determined. Prior to re-equilibration of the VR/KI indices, sediments in New Caledonia of diagenetic to incipient metamorphic grade underwent a high-pressure subduction event. VR/KI indices are in or close to equilibrium, while slight differences in OM vs CM indices allow for a better understanding of polyphase conditions, especially with respect to pressure. Temperature estimations are identical despite of their poly-phase metamorphic history, which was mainly controlled by the last orogenic thermal event lasting > 5 to < 10 myr. In the eastern Helvetic Alps and Northern Calcareous Alps similar correlations were found with slightly different slopes. Comparison between different regions is possible when using KI standardization and same data discrimination. In both parts of the Alps a complex thermal history of short durations (< 5.0 myr for the Northern Calcareous Alps to 10 myr for the Helvetic Alps) caused similar VR/KI trends, but disequilibrium is suggested by weaker regression parameters. The following correlation is calculated for a moderate geotherm (55 to 74 mWm 2 , mean = 61 mWm 2 ) and normal temperature gradient conditions (25 to 35 °Ckm −1 ): KI = 1.134e −0.305VR , (R 2 = 0.880, n = 462) with VR given as %R max , KI as Δ°2θ (limited to values between 0.2 to 1.0 Δ°2θ). With increasing depth (z) a VR gradient of 1.4 ± 0.2%R max km −1 is determined and a KI gradient of 0.09 ± 0.002 Δ°2θ km −1 is observed. The study illustrates that a normal geotherm can be described by VR/KI correlation, even if different heating episodes may occur. For the detection of a poly-phase or plurifacial thermal history, several indices of clay minerals and organic matter with very different kinetics should be used, as e.g. demonstrated by strong differences in smectite content at equal VR/KI values versus structural depth. A specific interest is given to the correlation of vitrinite like solid bitumen reflectance as an alternative method to VR, the persistent preservation of liptinite macerals and the stability range of clay minerals and sub-greenschist facies critical minerals compared with VR/KI data. Until now, despite the Alps in this study, systematic liptinite maceral studies have not been published in other orogenic settings.