Mantle upwellings and convective instabilities revealed by seismic tomography and helium isotope geochemistry beneath eastern Africa

International audienceThe relationship between intraplate volcanism and continental tectonics has been investigated for North and East Africa using a high resolution three-dimensional anisotropic tomographic model derived from seismic data of a French experiment ''Horn of Africa'' and existing broadband data. The joint inversion for seismic velocity and anisotropy of the upper 400 km of the mantle, and geochemical data reveals a complex interaction between mantle upwellings, and lithosphere. Two kinds of mantle upwellings can be distinguished: The first one, the Afar ''plume'' originates from deeper than 400 km and is characterized by enrichment in primordial 3 He and 3 He/ 4 He ratios higher than those along mid-ocean ridges (MOR). The second one, associated with other Cenozoic volcanic provinces (Darfur, Tibesti, Hoggar, Cameroon), with 3 He/ 4 He ratios similar to, or lower than MOR, is a consequence of shallower upwelling. The presumed asthenospheric convective instabilities are oriented in an east-west direction, resulting from interaction between south-north asthenospheric mantle flow, main plume head and topography on the base of lithosphere

Evolution of a low convergence collisional orogen: a review of Pyrenean orogenesis

The Pyrenees is a collisional orogen built by inversion of an immature rift system during convergence of the Iberian and European plates from Late Cretaceous to late Cenozoic. The full mountain belt consists of the pro-foreland southern Pyrenees and the retro-foreland northern Pyrenees, where the inverted lower Cretaceous rift system is mainly preserved. Due to low overall convergence and absence of oceanic subduction, this orogen preserves one of the best geological records of early orogenesis, the transition from early convergence to main collision and the transition from collision to post-convergence. During these transitional periods major changes in orogen behavior reflect evolving lithospheric processes and tectonic drivers. Contributions by the OROGEN project have shed new light on these critical periods, on the evolution of the orogen as a whole, and in particular on the early convergence stage. By integrating results of OROGEN with those of other recent collaborative projects in the Pyrenean domain (e.g., PYRAMID, PYROPE, RGF-Pyrénées), this paper offers a synthesis of current knowledge and debate on the evolution of this immature orogen as recorded in the synorogenic basins and fold and thrust belts of both the upper European and lower Iberian plates. Expanding insight on the role of salt tectonics at local to regional scales is summarised and discussed. Uncertainties involved in data compilation across a whole orogen using different datasets are discussed, for example for deriving shortening values and distribution

Tectono-magmatic evolution of Central Afar since 5 Ma: late syn-rift and break-up processes

Central Afar (Ethiopia) is an active example of the final stages of continental rifting. The Stratoid magmatic series (ages between 5 and 1 Ma) were emplaced in a large fissural volcanic province, following an episode of thinning by normal faulting and detachment at 5-6 Ma (Stab et al., 2016). The Gulf Basalt series (0.9-0.4 Ma) later emplaced in more restricted areas attesting for the localisation of the deformation. Current active magmatic axes are even more localized and the most recent lava geochemistry attests for very little crustal contamination (Ayalew et al., 2018) along with recent dyking episodes. This suggests that Central Afar is currently in a late syn-rift stage, possibly close to continental break-up with divergence accommodated by magmatic accretion. The detailed study of the tectono-magmatic evolution of the region will allow us to better constrain the break-up processes active during volcanic margin formation.Our new mapping of Central Afar has consisted in defining Stratoid sub-series to better follow the interplay between magmatism and deformation during continent-ocean transition. This map is supported by field data, new mapping using satellite multispectral images, and new Ar/Ar dating. We defined three new units: the old Stratoid (5-3 Ma), the intermediate (3-2 Ma) and the young Stratoid (2-1 Ma). This mapping shows that the localisation processes started during the old Stratoid emplacement, which we interpret as an equivalent of Seaward Dipping Reflectors described in magma-rich margins. The detailed mapping of the normal faults in Central Afar is used to quantify the amount of deformation through space and time and discuss the mechanism of divergence accommodation (dyke vs normal faults) in order to track the timing and controlling parameters of the eventual switch from rifting to break-up processes. In the next future, we will study the chemical signature of each series to determine the evolution of magma sources and conditions of melting during the Stratoid phases we defined. Moreover, new dates will provide much needed data on this volcanic series's continuous vs discrete (with pulses) nature

Variability of magmatic and cosmogenic 3 He in Ethiopian river sands of detrital pyroxenes: Impact on denudation rate determinations

International audienceIn-situ cosmogenic 3 He is a robust tool for determining denudation rates or exposure ages of lavas bearing mafic phenocrysts. However, analyses are often complicated by the presence of several helium sources. In particular, in old magmatic rocks with high radiogenic 4 He contents, discriminating cosmogenic 3 He from magmatic 3 He is not straightforward since these varieties may vary largely between aliquots. We sampled sands from the Tekeze and Mile rivers, both draining the basaltic Ethiopian highlands, an area where erosion patterns are intimately linked to the development of the Western Afar margin and to heterogeneous monsoon precipitation. From each river we analyzed~15 aliquots of pyroxenes having variable grain sizes (0.3 mm up to N 1 mm). The total 3 He is both higher and more scattered in the bigger grains. Crushing of these largest grains and subsequent melting of the powder tends to produce more homogeneous 3 He values, suggesting that magmatic 3 He hosted in inclusions is responsible for most of the inter-aliquot variability. We also performed a Monte Carlo simulation based on a numerical denudation model of the two watersheds. The simulation confirms that cosmogenic 3 He variability cannot be responsible for the observed scatter since the cos-mogenic 3 He variability is averaged away and unobservable in aliquots of~200 grains. A compilation of previously published data also indicates that magmatic helium can be significantly variable, even between pre-crushed aliquots. Hence, magmatic helium, unlike cosmogenic 3 He, is highly variable, even in the case of aliquots of hundreds of grains. We suggest this is due to a strong nugget effect, possibly due to large fluid (or melt)-inclusions contained in phenocrysts. In addition, the fact that small and big grains have comparable radiogenic 4 He concentrations suggests that grain fragmentation during river transport is responsible for the lower magmatic helium content of the smallest grains. Therefore, one should preferably use small grain (0.2-0.5 mm) granulometry for in-situ cosmogenic 3 He analysis in mafic phenocrysts. Using the measured cosmogenic 3 He, we calculate basin-averaged denudation rates of 70 ± 20 and 57 ± 5 mm kyr −1 , for the Mile and for the Tekeze river, respectively. These values are coherent with long-term denu-dation rates previously proposed from low-temperature thermochronology

An Alternative Protocol for Single Zircon Dissolution with Application to (U-Th-Sm)/He Thermochronometry

International audienceZircon (U‐Th‐Sm)/He (ZHe) thermochronometry is a powerful tool that has been widely used in geology to constrain the exhumation histories of orogens. In this study, we present an alternative protocol for dissolving zircon grains for determination of parent nuclides. This new alkali fusion procedure developed at the SARM (Service d'Analyse des Roches et des Minéraux) in Nancy, France, is fast (requiring only 2 d, including cleaning steps) and offers several advantages over conventional methods by avoiding: (i) use of HF pressure dissolution and (ii) complete removing of grains from the metal microvials. After dissolution, U, Th and Sm were measured using an ICP‐MS. We tested the new procedure on two different ZHe reference materials, the Fish Canyon Tuff and Buluk Tuff; these provided precision values for ZHe‐age estimations of 9 and 6% (1s), respectively. In addition, using this method, zircons from the Buluk Tuff are shown to be chemically more homogenous and more suitable for assessing the uncertainty of the entire integrated procedure

Thermochronological and structural constraints for alpine exhumation in the Axial Zone, Pyrenees

International audienceThe Pyrenean belt was formed by the convergence between European and Iberian plates, which followed the Mesozoic rifting. The deformation in the Axial Zone is accommodated by a stacking of several tectonic units characterized by a complex pre-collisional history especially associated to the Variscan orogeny and possibly to the Mesozoic rifting. Published data allow to characterize the first order exhumation pattern (mainly from the AFT data, exhumation peak around 30 Ma) but not discuss the thermal and structural evolution in details. Is there an early phase of distributed shortening before the initiation of the crustal thrusts? Is there a thermal influence of the Mesozoic rifts in the Axial Zone? When did the exhumation start in the Axial Zone? We performed a multiscale analysis of Pyrenean structures (Alpines), particularly near the ECORS profile in the Axial Zone. Low temperature thermochronological (ZFT, ZHe, including laser ablation depth profile from surface of grain coupled to AFT from the literature) are coupled to a chlorite-phengite thermobarometric analysis. Their relative intercalibration will help us to constrain and model the exhumation in the plutonic massifs and deformation in paleozoic metasediments. Preliminary results show the Alpine shortening is distributed in the Axial Zone, while it seems more localized in the West. In the western part, the deformation is particularly clear in plutonic massifs, while along the ECORS profile it seems restricted to metasediments. Geochronological data will help us to date the deformation which has been recently described as mainly Variscan. Thermochronological data on zircon fission track highlight a complex exhumation story with Cretaceous ages in several units of the Axial Zone, which could be associated to the rifting in the North Pyrenean Zone. In the northern part of the Axial Zone, the Gavarnie unit, late Paleocene/early Eocene ZFT ages are obtained, which is significantly older than previously thought but more consistent with the foreland basins record. Moreover, ZFT ages also suggest fast exhumation rates during Eocene times. Thermal inverse modelling with QTQt provide the first complete and detailed results on the exhumation pattern of the Axial Zone, which is much more complex than previously thought. Such new data will allow new estimations of the (sequential) Cenozoic denudation, a discussion of the wedge thermicity and of its rheology. This study is included to the Orogen research project, a tripartite partnership between academy and industry (Total, BRGM, CNRS)

Thermochronological and structural constraints for alpine exhumation in the Axial Zone, Pyrenees

International audienceThe Pyrenean belt was formed by the convergence between European and Iberian plates, which followed the Mesozoic rifting. The deformation in the Axial Zone is accommodated by a stacking of several tectonic units characterized by a complex pre-collisional history especially associated to the Variscan orogeny and possibly to the Mesozoic rifting. Published data allow to characterize the first order exhumation pattern (mainly from the AFT data, exhumation peak around 30 Ma) but not discuss the thermal and structural evolution in details. Is there an early phase of distributed shortening before the initiation of the crustal thrusts? Is there a thermal influence of the Mesozoic rifts in the Axial Zone? When did the exhumation start in the Axial Zone? We performed a multiscale analysis of Pyrenean structures (Alpines), particularly near the ECORS profile in the Axial Zone. Low temperature thermochronological (ZFT, ZHe, including laser ablation depth profile from surface of grain coupled to AFT from the literature) are coupled to a chlorite-phengite thermobarometric analysis. Their relative intercalibration will help us to constrain and model the exhumation in the plutonic massifs and deformation in paleozoic metasediments. Preliminary results show the Alpine shortening is distributed in the Axial Zone, while it seems more localized in the West. In the western part, the deformation is particularly clear in plutonic massifs, while along the ECORS profile it seems restricted to metasediments. Geochronological data will help us to date the deformation which has been recently described as mainly Variscan. Thermochronological data on zircon fission track highlight a complex exhumation story with Cretaceous ages in several units of the Axial Zone, which could be associated to the rifting in the North Pyrenean Zone. In the northern part of the Axial Zone, the Gavarnie unit, late Paleocene/early Eocene ZFT ages are obtained, which is significantly older than previously thought but more consistent with the foreland basins record. Moreover, ZFT ages also suggest fast exhumation rates during Eocene times. Thermal inverse modelling with QTQt provide the first complete and detailed results on the exhumation pattern of the Axial Zone, which is much more complex than previously thought. Such new data will allow new estimations of the (sequential) Cenozoic denudation, a discussion of the wedge thermicity and of its rheology. This study is included to the Orogen research project, a tripartite partnership between academy and industry (Total, BRGM, CNRS)

Thermochronological and structural constraints for alpine exhumation in the Axial Zone, Pyrenees

International audienceThe Pyrenean belt was formed by the convergence between European and Iberian plates, which followed the Mesozoic rifting. The deformation in the Axial Zone is accommodated by a stacking of several tectonic units characterized by a complex pre-collisional history especially associated to the Variscan orogeny and possibly to the Mesozoic rifting. Published data allow to characterize the first order exhumation pattern (mainly from the AFT data, exhumation peak around 30 Ma) but not discuss the thermal and structural evolution in details. Is there an early phase of distributed shortening before the initiation of the crustal thrusts? Is there a thermal influence of the Mesozoic rifts in the Axial Zone? When did the exhumation start in the Axial Zone? We performed a multiscale analysis of Pyrenean structures (Alpines), particularly near the ECORS profile in the Axial Zone. Low temperature thermochronological (ZFT, ZHe, including laser ablation depth profile from surface of grain coupled to AFT from the literature) are coupled to a chlorite-phengite thermobarometric analysis. Their relative intercalibration will help us to constrain and model the exhumation in the plutonic massifs and deformation in paleozoic metasediments. Preliminary results show the Alpine shortening is distributed in the Axial Zone, while it seems more localized in the West. In the western part, the deformation is particularly clear in plutonic massifs, while along the ECORS profile it seems restricted to metasediments. Geochronological data will help us to date the deformation which has been recently described as mainly Variscan. Thermochronological data on zircon fission track highlight a complex exhumation story with Cretaceous ages in several units of the Axial Zone, which could be associated to the rifting in the North Pyrenean Zone. In the northern part of the Axial Zone, the Gavarnie unit, late Paleocene/early Eocene ZFT ages are obtained, which is significantly older than previously thought but more consistent with the foreland basins record. Moreover, ZFT ages also suggest fast exhumation rates during Eocene times. Thermal inverse modelling with QTQt provide the first complete and detailed results on the exhumation pattern of the Axial Zone, which is much more complex than previously thought. Such new data will allow new estimations of the (sequential) Cenozoic denudation, a discussion of the wedge thermicity and of its rheology. This study is included to the Orogen research project, a tripartite partnership between academy and industry (Total, BRGM, CNRS)

Thermochronological and structural constraints for alpine exhumation in the Axial Zone, Pyrenees

International audienceThe Pyrenean belt was formed by the convergence between European and Iberian plates, which followed the Mesozoic rifting. The deformation in the Axial Zone is accommodated by a stacking of several tectonic units characterized by a complex pre-collisional history especially associated to the Variscan orogeny and possibly to the Mesozoic rifting. Published data allow to characterize the first order exhumation pattern (mainly from the AFT data, exhumation peak around 30 Ma) but not discuss the thermal and structural evolution in details. Is there an early phase of distributed shortening before the initiation of the crustal thrusts? Is there a thermal influence of the Mesozoic rifts in the Axial Zone? When did the exhumation start in the Axial Zone? We performed a multiscale analysis of Pyrenean structures (Alpines), particularly near the ECORS profile in the Axial Zone. Low temperature thermochronological (ZFT, ZHe, including laser ablation depth profile from surface of grain coupled to AFT from the literature) are coupled to a chlorite-phengite thermobarometric analysis. Their relative intercalibration will help us to constrain and model the exhumation in the plutonic massifs and deformation in paleozoic metasediments. Preliminary results show the Alpine shortening is distributed in the Axial Zone, while it seems more localized in the West. In the western part, the deformation is particularly clear in plutonic massifs, while along the ECORS profile it seems restricted to metasediments. Geochronological data will help us to date the deformation which has been recently described as mainly Variscan. Thermochronological data on zircon fission track highlight a complex exhumation story with Cretaceous ages in several units of the Axial Zone, which could be associated to the rifting in the North Pyrenean Zone. In the northern part of the Axial Zone, the Gavarnie unit, late Paleocene/early Eocene ZFT ages are obtained, which is significantly older than previously thought but more consistent with the foreland basins record. Moreover, ZFT ages also suggest fast exhumation rates during Eocene times. Thermal inverse modelling with QTQt provide the first complete and detailed results on the exhumation pattern of the Axial Zone, which is much more complex than previously thought. Such new data will allow new estimations of the (sequential) Cenozoic denudation, a discussion of the wedge thermicity and of its rheology. This study is included to the Orogen research project, a tripartite partnership between academy and industry (Total, BRGM, CNRS)