Assessing controls on organic matter enrichments in hemipelagic homogenous marls of the Cretaceous Vocontian Basin (France): an unexpected variability observed from multiple organic-rich levels

The Marnes Bleues Formation from the Vocontian Basin (Southeastern France) shows many organic rich levels, some concomitant to oceanic anoxic events OAE1a and OAE1b. These organic-rich levels are scattered through a thick homogeneous succession of marls, poor in organic matter (OM). Through a multi-parameter approach, the organic-rich levels from the Aptian-Albian were characterized. Our results show that all OM-rich levels exhibit variable characteristics, such as OM nature (marine vs. continental), sedimentation and accumulation rates, redox conditions, surface-water productivity and relative sea level, but they all show low to modest enrichments in OM. Furthermore, all the levels share in common the fact that they formed under conditions of normal to low productivity and oxic to suboxic conditions. Thus, our results strongly suggest that, in the absence of high productivity and anoxic bottom conditions, the other factors reputed to favor OM accumulation only led to sporadic and low enrichments in organic contents. It is as if such factors could only enhance OM accumulation but could not induce it alone. What was true for the Vocontian Basin may be extended to other settings, regardless of their time of deposition or location.

Passive imaging of collisional orogens: a review of a decade of geophysical studies in the Pyrénées

This contribution reviews the challenges of imaging collisional orogens, focusing on the example of the Pyrenean domain. Indeed, important progresses have been accomplished regarding our understanding of the architecture of this mountain range over the last decades, thanks to the development of innovative passive imaging techniques, relying on a more thorough exploitation of the information in seismic signals, as well as new seismic acquisitions. New tomographic images provide evidence for continental subduction of Iberian crust beneath the western and central Pyrénées, but not beneath the eastern Pyrénées. Relics of a Cretaceous hyper-extended and segmented rift are found within the North Pyrenean Zone, where the imaged crust is thinner (10–25 km). This zone of thinned crust coincides with a band of positive Bouguer anomalies that is absent in the Eastern Pyrénées. Overall, the new tomographic images provide further support to the idea that the Pyrénées result from the inversion of hyperextended segmented rift systems.

Re-assessing copper and nickel enrichments as paleo-productivity proxies

Copper (Cu) and nickel (Ni) are elements frequently enriched in sedimentary deposits rich in organic matter (OM). In the marine environment, they are mainly supplied to the sediments in association with sedimentary OM. In current environments, a good correlation between the intensity of phytoplankton productivity and the quantities of Cu & Ni transferred to sediments made it possible to establish paleo-productivity calculations based on the contents of ancient sediments in these two metals. The present study is a re-evaluation of the value that can be attributed to these two metals as paleo-productivity proxies. The approach adopted here is based on the examination of a large database already available in the scientific literature. The choice was made to favor the examination of a large amount of data by simple means: comparisons of total organic carbon (TOC) content, enrichment in Cu & Ni (or even other trace metals), and value of the Fe:Al ratio that makes it possible to assess the availability of reactive iron. The basic idea is that the examination of a large number of geological formations thanks to the large database makes it possible to encompass all kinds of paleo-environmental settings, thus comprising an extreme variety of the factors conventionally involved in the mechanisms of accumulation of OM. The aim is to identify strong trends, valid in a large number of paleo-situations, which will have to be carefully taken into account in future detailed paleo-environmental reconstructions. It emerges from this study that, in many cases, Cu and Ni cannot be considered as faithfully reflecting the quantity of OM initially deposited. Several factors acting on the loss of Cu and Ni can be retained, and among them, (1) a rapid loss linked to the decomposition of the OM before the conditions conducive to sulfate-reduction set in; (2) a low abundance of reactive iron which limits the quantity of pyrite liable to form, which significantly hampers Cu & Ni fixation in sediments. If Cu & Ni are not reliably retained in the sediments, that is, proportional to the quantity of OM supplied to the sediment, the paleoenvironmental reconstitutions involving the concentrations of these metals may provide underestimated values of paleoproductivity. An interesting clue is the Fe:Al ratio that makes it possible to quickly know whether the values of the Cu & Ni enrichments are likely to be "abnormally" low.

The role of extensional detachment systems in thinning the crust and exhuming granulites: analogies between the offshore Le Danois High and the onshore Labourd Massif in the Biscay/Pyrenean rifts

Large uncertainties remain about the architecture, timing and role of the structures responsible for high degrees of crustal thinning and the exhumation of mid-crustal granulites in the Pyrenean and Biscay rift systems. Both, Le Danois High in the North Iberian margin and the Labourd Massif in the Western Pyrenees preserve evidence of extensional detachment faults and include exhumed granulites, which are locally reworked in syn-rift sediments. In this study, we compare the crustal structure and their link to the overlying sediments at the two sites based on the interpretation of high quality 2D seismic reflection profiles offshore and field observations and published geological cross-sections onshore. New reported seismic and field observations support the interpretation that the Le Danois High and the Labourd Massif are capped by extensional detachment systems, advocating for a similar tectonic evolution of the two sites. We propose that the two detachment systems were responsible for high degrees of crustal thinning and the exhumation of the pre-rift brittle-ductile transition and associated mid-crustal granulites during Aptian to Cenomanian extension, leading to the formation of the Le Danois and Labourd crustal tapers. Tilted and uplifted during the Alpine convergence, the two basement blocks lay at present in the hanging-wall of major Alpine thrusts. Their position at overlapping, en-echelon hyperextended rift segments at the end of rifting, and the occurrence of shortcutting structures linking neighbouring rift segments, can explain the preservation of the rift-related detachment systems. This study not only proposes for the first time analogies between the offshore Le Danois High and the onshore Labourd Massif, but it also demonstrates the importance of extensional detachment systems in thinning the crust and exhuming mid-crustal granulites at the seafloor in the Biscay and Pyrenean domains during Aptian to Cenomanian extension.

Review of the syn-rift to early post-rift depositional systems of the Cretaceous Mauléon rift: sedimentary record of continental crust hyperextension and mantle denudation (Western Pyrenees)

— The Mauléon basin, in the northwestern Pyrenean belt, is related to Early Cretaceous rifting and continental breakup. Here we review the evolution of depositional environments in the hyperextended Mauléon rift basin during Albian and Cenomanian time. This review includes the lithostratigraphy, regional distribution, boundaries, age and facies sedimentology of the basin’s syn-rift formations and their members. We construct paleogeographic maps to elucidate (1) the 3D distribution of sedimentary facies and depositional environments during the Albian and Cenomanian from the Iberian proximal margin to the hyperextended domain and (2) the link between major extensional structures and sedimentation during rifting and continental breakup. The Mauléon rift was supplied during most of the Albian by sediments from the Iberian proximal margin. The southern margin had a steep and abrupt topographic boundary related to a northward crustal rollover along the south-dipping Saint-Palais detachment. This feature controlled the deposition of base-of-slope conglomerates at the base of the margin that abruptly gave way to low-density turbidites, then hemipelagic deposits in the hyperextended domain. During latest Albian to Early Cenomanian time, continental breakup occurred in the eastern Mauléon basin and the vergence of the detachment systems reversed. Minor debris-flow deposits formed at the foot of fault scarps associated with the newly formed north-dipping detachments. Elsewhere, sediment from deltaic systems to the west in the Saint-Jean-de-Luz area deposited low-density turbidites in the hyperextended domain. During the post-rift stage, the flux of coarse sediment from the detachment footwall gradually declined as deformation waned, and low-density turbidites expanded onto the hyperextended domain from the European Late Cretaceous carbonate platform. These paleogeographic reconstructions, in addition to offering a synthetic view of the evolution of sedimentary environments during rifting, offer new insight into the post-rifting exhumation of the lower crust and mantle.

The role of inheritance in forming rifts and rifted margins and building collisional orogens: a Biscay-Pyrenean perspective

The aim of this paper is to provide a conceptual framework that integrates the role of inheritance in the study of rifts, rifted margins and collisional orogens based on the work done in the OROGEN project, which focuses on the Biscay-Pyrenean system. The Biscay-Pyrenean rift system resulted from a complex multistage rift evolution that developed over a complex lithosphere pre-structured by the Variscan orogenic cycle. There is a general agreement that the Pyrenean-Cantabrian orogen resulted from the reactivation of an increasingly mature rift system along-strike, ranging from a mature rifted margin in the west to an immature and segmented hyperextended rift in the east. However, different models have been proposed to explain the preceding syn-rift evolution and its influence on the subsequent reactivation. Results from the OROGEN project show a sequential reactivation of rift inherited decoupling horizons and identify the specific role of exhumed mantle, hyperextended and necking domains during reactivation. They also highlight the contrasting fate of segment centres vs. segment boundaries during convergence, explaining the non-cylindricity of internal parts of collisional orogens. Results from the OROGEN project also suggest that the role of inheritance is more important during the initial stages of subduction and collision, which may explain the complexity of internal parts of orogenic systems. In contrast, once tectonic systems get more mature, orogenic evolution becomes mostly controlled by first-order physical processes as described in the Coulomb Wedge theory for instance. This may account for the simpler and more continuous architecture of external parts of collisional orogens. It may also explain why most numerical models can reproduce mature orogenic and rift architectures with better accuracy compared to the initial stages of such systems. Thus, while inheritance may not explain steady-state processes, it is a prerequisite for comprehending the initial stages of tectonic systems. The new concepts developed from the OROGEN research are now ready to be tested at other orogenic systems that result from the reactivation of rifted margins, such as the Alps, the Colombian cordilleras and the Caribbean, Taiwan, Oman, Zagros or Timor.

Cenozoic mountain building and topographic evolution in Western Europe: impact of billion years lithosphere evolution and plate tectonics

The architecture and nature of the continental lithosphere result from billions of years of tectonic and magmatic evolution. Continental deformation over broad regions form collisional orogens which evolution is controlled by the interactions between properties inherited from hits long-lasting evolution and plate kinematics. The analysis of present-day kinematic patterns and geophysical imaging of lithosphere structure can provide clues on these interactions. However how these interactions are connected through time and space to control topographic evolution in collision zones is unknown. Here we explore the case of the Cenozoic mountain building and topographic evolution of Western Europe. We first review the tectono-magmatic evolution of the lithosphere of Europe based on the exploitation of geological, geochronological and geochemical constraints from ophiolites, mafic rocks and xenoliths data. Combined with the analyses of low-temperature thermochronological and plate kinematic constraints we discuss the key controlling parameters of the topography. We show that among the required ingredients is the primary effect of plume-, rift- and subduction-related metasomatic events on lithosphere composition. Those main events occurred during the Neoproterozoic (750-500 Ma) and the late Carboniferous-Permian (310-270 Ma). They resulted in the thinning and weakening of the sub-continental lithospheric mantle of Europe. Contrasting lithosphere strengths and plate-mantle coupling in Western Europe with respect to the cratonic lithosphere of West Africa Craton and Baltica is the first-order parameter that explain the observed strain and stress patterns. Subsequent magmatic and thinning episodes, including those evidenced by the opening of the early Jurassic Alpine Tethys and the CAMP event, followed by late Jurassic and early Cretaceous crustal thinning, prevented thermal relaxation of the lithosphere and allowed further weakening of the European lithosphere. The spatial and temporal evolution of topographic growth resolved by the episodes of increased exhumation show two main periods of mountain building. During the late Cretaceous-early Cenozoic (80-50 Ma) contractional deformation was distributed from North Africa to Europe, but the topographic response to the onset of Africa-Eurasia convergence is detected only in central Europe. The lack of rapid exhumation signal in southern Europe and north Africa reveal that the initial continental accretion in these regions was accommodated under water in domains characterized by thin continental or oceanic crust. The second phase of orogenic uplift period starts at about 50 Ma between the High Atlas and the Pyrenees. This second key period reflects the time delay required for the wider rift systems positioned between Africa and Europe to close, likely promoted by the acceleration of convergence. Tectonic regime then became extensional in northern Europe as West European Rift (WER) opened. This event heralds the opening of the Western Mediterranean between Adria and Iberia at ca. 35 Ma. While mature orogenic systems developed over Iberia at this time, the eastern domain around northern Adria (Alps) was still to be fully closed. This kinematic and mechanical conditions triggered the initiation of backarc extension, slab retreat and delamination in the absence of strong slab pull forces. From about 20 Ma, the high temperature in the shallow asthenosphere and magmatism trapped in the mantle lithosphere contributed to topographic uplift. The first period (80-20 Ma) reveals spatially variable onset of uplift in Europe that are arguably controlled by inherited crustal architecture, superimposed on the effect of large-scale lithospheric properties. The second period marks a profound dynamic change, as sub-lithospheric processes became the main drivers. The channelized mantle flow from beneath Morocco to Central Europe builds the most recent topography. In this study, we have resolved when, where and how inheritance at lithospheric and crustal levels rule mountain building processes. More studies focus on the tectonic-magmatic evolution of the continental lithosphere are needed. We argue that when they are combined with plate reconstructions and thermochronological constraints the relative impact of inheritance and plate convergence on the orogenic evolution can be resolved.

Three-dimensional shear velocity structure of the Mauleon and Arzacq basins (Western Pyrenees)

We present a 3-D shear wave velocity model of the Mauleon and Arzacq basins from the surface down to 10~km depth. This model is obtained by inverting phase velocity maps for periods from 2 to 9~s measured on coherent surface wavefronts extracted from ambient seismic noise by matched filtering. This new model, which is found in good agreement with local earthquake tomography, reveals the architecture of the Mauleon and Arzacq basins which were poorly imaged by conventional reflection seismic data. Combining these new tomographic images with surface and subsurface geological information allows us to trace major orogenic structures from the basement to the surface. In the basin, the models are successfully imaging first-order folds and thrusts at kilometric scale. The velocity structure within the basement and its geometrical relationship with the base of inverted rift basins supports a progressive northward exhumation of deep crustal and mantle rocks in the hanging wall of north-vergent Pyrenean thrusts. Our tomographic models image in 3-D orogen-perpendicular structures responsible for crustal segmentation as the Saison and Barlanes transfer zones. We propose that these steep structures consist in tear faults that accommodate the deepening of the Mauleon basin basement from west to east. To the west, this basement made of former hyper-extended rift domains (including mantle rocks) is anomalously sampled within the hanging-wall of north-directed orogenic thrusts, explaining its shallow attitude and its best preservation in comparison to the eastern segment of the study area. Eastward, the vertical shift of the basement makes that the former Mauleon basin hyper-extended rift basement remained in a footwall situation in respect of orogenic thrust and was underthrust. The comparison of the tomographic models obtained with surface wave tomography and local earthquake tomography shows that each approach has its own advantages and shortcomings but also that they are very complementary in nature, which would suggest to combine them in joint inversions to further improve passive imaging of the shallow crust and sedimentary basins.

From section to landscape(s) : reconstructions of environmental and landscape changes since the 7th millennium BCE around the site of Wakarida (Ethiopia) using chronostratigraphy

Geoscience is a very useful tool for approaching the relationships between societies and their environment. However, it is not always possible to apply very advanced techniques or to perform a large number of analyses. This paper aims to show that chronostratigraphic studies can provide satisfying answers to the main geoarchaeological questions, “only” using sedimentological, stratigraphic and chronological analyses. The objective here is to move from the study of stratigraphic sections, uncovered in valley bottoms filled by sedimentary deposits, to the reconstruction of landscape and environmental changes, in connection with the archaeological site of Wakarida (northern Tigray, Ethiopia). The actual landscape around Wakarida is made of steeply sloping valleys, filled with accumulations of sediments several meters thick. These deposits are cultivated by the inhabitants thanks to the building of agricultural terraces. Archaeological excavations have uncovered an urban settlement in Wakarida, dating between the classical Aksumite (150-400/450 CE) and the post-Aksumite periods (800/850 CE), and more ancient (pre- and proto-Aksumite periods) structures in the area under survey. Such structures raise the question of the interactions of the populations with their environment through time. More precisely, they question their role in the deposition of the sediments. To tackle these questions, a method based on a combination of field work, sedimentological analyses and dating has been employed. Such a chronostratigraphic study made it possible to identify several phases in the establishment of the current landscapes around the Wakarida site. During Early Holocene, valleys were gradually filled by alluvial and/or colluvial processes of low energy. During Middle Holocene, ablation processes alternated with deposition, showing the effects of climatic and possible anthropogenic processes. From the 1st millennium BCE onwards, the impact of population on their environment is visible, but especially around the 14th – 17th centuries. An important part of these conclusions is based on the attention given to chronological inversions and the fact that these dates, "false" at first glance, reveal ablation and deposition phases within sedimentary cascades. It therefore seems important, from a methodological point of view, not to reject these dates a priori, but to consider them by giving them their full place in the chronostratigraphic reflection.

Structural style of the Languedoc Pyrenean thrust belt in relation with the inherited Mesozoic structures and with the rifting of the Gulf of Lion margin, southern France

The “Wilson cycle” involves reactivation of rifting structures during convergence-driven inversion, then thrust reactivation during post-orogenic dismantling and extension. Classic documented examples of the Wilson cycle, such as in the pyrenean orogen, are based on sequential sections normal to the orogen. However oblique convergence/divergence that involves strain partitioning, and arcuate segments of the orogen prevent simple tectonic restorations. Languedoc region (southern France) provides a case study of a complex poly-phased deformation involving a range of reactivated structures and cross-cutting relationships, acquired in response to different stress-regimes of varying orientations. We analyse and correlate the onshore-offshore structures of the Languedoc, based on reassessment of existing and newly acquired subsurface data. New results in the previously poorly documented coastal area point to the existence of unrecognized major structures that improves onshore-offshore correlations. Our results show i) the part played by the Mesozoic (early Jurassic, then mid-Cretaceous) extensional phases in the development and the localization of pyrenean-related contractional structures; ii) the control of the later Oligocene rifting of the Gulf of Lion. Restoration of the Pyrenean shortening and Oligocene rifting, constructed along sections of relevant orientation (i.e. close to perpendicular to each other) indicate minimum shortening of 26 km and extension of 14km, respectively, in the Languedoc foreland. Integration of the Pyrenean structural framework of Languedoc reveals a wide, NE-trending transfer zone linking the Iberian Pyrenees to Provence.