Central Gulf of Aden conjugate margins (Yemen-Somalia): Tectono-sedimentary and magmatism evolution in hybrid-type margins

International audience6 7 Magma-rich and magma-poor passive margins are generally studied separately. Yet, the spatial evolution from 8 one type to another is not well understood. Central Gulf of Aden margins are at the transition between these 9 two types of margins. Based on new seismic data, published wells and gravity data, we determine seismic 10 stratigraphy and structural patterns. We map the distinct crustal domains (continental, Ocean-Continent 11 Transition (OCT), oceanic domains) and propose a tectonic evolution of the Yemeni and Somalian conjugate 12 margins in an oblique rifting context. The most striking results are the significant segmentation, narrowness 13 and asymmetry of conjugate margins, as well as the gradual eastward decrease in the intensity of magmatic 14 activity of these hybrid-type margins. West of Bosaso-Hami Fracture Zone (BHFZ), the Hami-Ahl Medo segment 15 presents magmatic-type margin features in the distal Yemeni margin related to the strong influence from the 16 Afar plume: Seaward dipping reflectors develop since the late syn-rift stage (~21-18 Ma) near the Xiis-Al 17 Mukalla Fracture Zone. Conversely, east of BHFZ, the Bosaso-Sayhut margins segment is characterized by 18 magma-poor margins for which the exhumation stage is characterized by the unroofing of mantle along 19 multiple detachment faults in the OCT and shortly followed by diachronous volcanism (~18 Ma). The central 20 Gulf of Aden is affected by a major uplift during the final syn-rift stage and up to OCT formation. This event is 21 associated with the formation of erosional surfaces in the east and with gravitational deformation taking place 22 on decoupling surfaces at the base of the syn-rift sequence (lower Ghaydah Fm) that lead to the sliding of syn-23 rift units on top of the exhumed mantle rocks in the OCT. 24 2

Comportement non-standard des prismes d'accrétion tectonique (Implications structurales)

Les marges actives et leurs prismes d'accrétion montrent une grande diversité de géométries et de types de déformation interne. Des paramètres, tels que le taux de convergence et la sédimentation dans la fosse, sont les principaux facteurs déterminant s'il y a ou non accrétion. Les mécanismes de déformation interne sont encore peu connus, ainsi que la manière dont ils sont affectés par des facteurs de perturbation tels que la présence d'hétérogénéités internes, la sédimentation dans le prisme lui même ou le chevauchement des ride océaniques. Le but principal de ce travail est donc de mieux appréhender comment les facteurs de déformation interne des prismes d'accrétion sont affectés par des facteurs perturbants. Un comportement standard a été défini afin de comparer le comportement de prismes d'accrétion avec un modèle au fonctionnement idéal. Il a été montré que la résistance des couches internes ductiles par rapport à celle des couches fragiles, contrôle le style structurel à l'intérieur de prisme, particulièrement au-dessus des couches ductiles. Au contraire, le profil général du prisme n'est que peu affecté par ce paramètre. Le dépôt d'une plate-forme progradante au-dessus des écailles imbriquées du prisme produit l'arrêt de leur activité et le déplacement du front de déformation. La présence d'un niveau de décollement interne induit le découplement de la plate-forme par rapport aux structures compressives sous-jacentes. Ce phénomène produit à son tour le développement des failles normales sur la plate-forme et une augmentation du raccourcissement au-dessus du détachement.Active margins and their accretionary prisms show an important diversity of geometries and internal deformation patterns. Parameters like the convergence rate and sedimentation at the trench are the primary controls dictating whether there is accretion or not. Little is known about the internal deformation mechanisms and how are they affected by perturbation factors such as internal heterogeneities, sedimentation and oceanic seamounts. The main goal of this study is to understand how the internal deformation mechanisms on accretionary prisms are affected by these perturbation factors. A standard behaviour has been defined in order to compare the behaviour of an accretionary prisms with an ideal model of functioning. It has been shown that internal ductile layers and their relative strength with respect to the fragile layers, control the structural style inside the accretionary prism, particularly above the ductile layer. On the contrary the overall profile of the prism is little or no affected by this parameter. The deposition of a prograding platform above the imbricated thrust sheets of the prism induces the arrest of their activity and the translation of the deformation front. The presence of an internal detachment level enhance the decoupling between the platform and the underlying compressional structures. This induces in turn the development of normal faulting on the platform and the increase of the shortening above the detachment.RENNES1-BU Sciences Philo (352382102) / SudocSudocFranceF

Fault surface development and fault rock juxtaposition along deformation band clusters in porous sandstones series

International audienceOur field study examines two sites revealing the processes responsible for fault surface development and fault rock juxtaposition along normal faults in porous-sandstone–dominated formations. At the first site, we show that a cluster of cataclastic deformation bands made in an initially poorly consolidated sandstone localizes overprinting of a fault slip surface and brecciation during a subsequent tectonic episode, after a significant diagenesis of the formation induced by burial. Because the permeability of the clusters ranges between 6 × 100 and less than 5 × 10−1 md and because the breccia was highly dilatant, we deduce that the fault behaved as a baffle for cross-fault fluid flow at an early age of the formation and as a conduit after significant diagenetic evolution and subsequent fault surface development. At the second site, we show that the presence of clay-rich layers as thin as 80 cm (31 in.) are responsible for the initiation of a major fault slip surface in the underlying and overlying sandstone. The propagation of the fault prevents further cataclastic deformation and cluster development in these sandstones. Fault displacement juxtaposes fault surfaces, clusters of cataclastic deformation bands, clay-rich gouges, and different sedimentary units. Because both fault rocks have low permeability, their spatial juxtaposition provides a continuous baffle for cross-fault fluid flow. Our study shows that fault surface localization is related to an increase in the contrast of mechanical behavior between the cluster and the adjacent material (diagenetic hardening of the cluster or softening of the clay-rich gouge). Lithological contrasts and diagenesis are favorable conditions for localizing faulting and fault rock juxtaposition, allowing significant three-dimensional anisotropy of permeability during and/or after deformation. These processes must, therefore, be considered for fault-seal analyses in sandstone reservoirs

Clusters of cataclastic deformation bands in porous

International audienceClusters of cataclastic deformation bands represent potential barriers or baffles to reservoir fluid flow, and their processes of formation remain debated. In this work, we rely on an integrated field study at seven sites to describe the extent of clusters, their morphology and their density of deformation as a function of several parameters: the tectonic loading, the burial depth of deformation, the Andersonian stress regime and the lithology of the sandstone. We perform porosity, sorting and grain shape analyses of the deformed material to improve the understanding of microscopic process of cluster development. In agreement with previous works on cataclastic deformation bands, our results reveal that the tectonic loading constrains the extent and the morphology of the clusters. Extensional tectonics favors the formation in normal-fault Andersonian regime of series of hundreds of meter long, rather thin and dense clusters, forming kilometer long networks associated with faults. We find that the formation of major slip-surfaces (faulting) can occur at any stage of cluster thickening. Contractional tectonics favors the formation of strike-slip clusters of medium-thickness, and more rarely the formation of tens of meter long, low-density, thick thrust clusters. The clusters formed in contractional tectonic settings are sparse. Since 15%–79% of the bands/slip-surface of the clusters have permeabilities close to or lower than 1 × 101 mD, and because clusters can have significant dimensions, they may be significant baffles, particularly in extensional tectonics. Our analysis of the porosity and shape evolution of the clasts with increasing deformation corroborates the hypothesis of strain hardening of the band by combined increase of cohesion and friction induced by the cataclasis. We propose that the variability of hardening associated with the variable packing and related cementation in specific parts of the deformed material controls the development and the morphology of the clusters as a function of the tectonic loading

Amenaza sismica por método determinista en el triángulo de fallas Valera-Boconó-Oca en el occidente de Venezuela

&lt;p class="MsoNormal" style="line-height: normal; margin: 0cm 0cm 0pt; mso-layout-grid-align: none;"&gt;&lt;span style="font-family: " lang="EN-US"&gt;The development of several studies regarding active faults geology has allowed us to build a complete database wich is used together with the instrumental catalog, in the probabilistic seismic hazard assesment (PSHA). The geologic data is used here to develop a deterministic seismic hazard assesment (DSHA) and then the results are compared with former PSHA results to propose hybrid approaches that lead us to make good use of all the available information.&lt;/span&gt;&lt;/p&gt;&lt;p class="MsoNormal" style="line-height: normal; margin: 0cm 0cm 0pt; mso-layout-grid-align: none;"&gt;&lt;span style="font-family: " lang="EN-US"&gt;Maximum credible earthquakes and minimum actual distances to the rupture surfaces are used with the attenuation relationships to create a scenario describing the hazard in terms of the maximum acceleration. These strong ground motion values are the result of the maximum earthquake that could be triggered by the faults in the area. The whole zone, including three main cities and important economic activities, is covered in this study by a grid with points every 5 km. &lt;/span&gt;&lt;/p&gt;&lt;p class="MsoNormal" style="line-height: normal; margin: 0cm 0cm 0pt; mso-layout-grid-align: none;"&gt;&lt;span style="font-family: " lang="EN-US"&gt;The results are shown in acceleration maps, where the maximum values follow the active faults trends. When the return period of the maximum credible earthquake is included in the analysis this correlation changes and a more likely scenario is created. The pros and cons of both approaches were analized in terms of the available data for Venezuela. This has shown that more efforts must be done in the treatment and processing of the data and in the design and application of better methodologies as well.&lt;/span&gt;&lt;/p&gt;<br>&lt;p class="MsoNormal" style="line-height: normal; margin: 0cm 0cm 0pt; mso-layout-grid-align: none;"&gt;&lt;span style="font-family: "&gt;EI desarrollo de múltiples proyectos de geología de fallas activas en Venezuela ha permitido que el país cuente hoy día con una base de datos completa y de gran calidad, usada como complemento a la información instrumental en los cálculos de amenaza sísmica del país con métodos probabilistas. En el presente trabajo se utiliza esta información geológica disponible para realizar cálculos de amenaza con métodos deterministas para luego comparar los resultados y proponer la implantación de metodologías mixtas que permitan aprovechar mejor los datos, tanto en la ventana instrumental como en la geológica. Sobre la base de disponibilidad de información publicada y de complejidad en la geometría de las mayores estructuras, se ha escogido una zona del occidente del país limitada por las fallas de Valera, Bocono y Oca.&lt;/span&gt;&lt;/p&gt;&lt;p class="MsoNormal" style="line-height: normal; margin: 0cm 0cm 0pt; mso-layout-grid-align: none;"&gt;&lt;span style="font-family: "&gt;Se utilizan sismos máximos creibles de cada falla, distancias mínimas a la superficie de ruptura y las leyes de atenuación necesarios para crear un marco multiescenario combinando todas las estructuras que puedan, potencialmente, generar amenaza.&lt;/span&gt;&lt;/p&gt;&lt;p class="MsoNormal" style="line-height: normal; margin: 0cm 0cm 0pt; mso-layout-grid-align: none;"&gt;&lt;span style="font-family: "&gt;La zona de estudio, con un área aproximada de 45.000 km2 que incluye tres capitales de estado y una actividad industrial importante, es cubierta con una red espaciada cada 5 kilómetros para el cálculo de los valores de aceleración.&lt;/span&gt;&lt;/p&gt;&lt;p class="MsoNormal" style="line-height: normal; margin: 0cm 0cm 0pt; mso-layout-grid-align: none;"&gt;&lt;span style="font-family: "&gt;Los resultados se muestran en mapas con areas de isoaceleraciones donde se puede observar un control marcado de la geometría de las fallas activas sobre los valores de aceleración esperados, por encima de la sismicidad asociada a cada una de ellas.&lt;/span&gt;&lt;/p&gt;&lt;p class="MsoNormal" style="line-height: normal; margin: 0cm 0cm 0pt; mso-layout-grid-align: none;"&gt;&lt;span style="font-family: "&gt;A difercncia de los métodos de uso más extendido, el cálculo por métodos deterministas no permite la visualización de probabilidades de no excedencia, pero permite analizar escenarios de amenaza asociada a fallas que presenten poca sismicidad en la ventana instrumental.&lt;/span&gt;&lt;/p&gt

Geomechanical modelling of spent fluid reinjection in the Hengill geothermal field

Monitoring micro-seismicity during operations of a geothermal field is critical to the understanding of seismic hazard and changes in the reservoir. In the context of a geothermal project, induced earthquakes are an important tool to enhance the permeability and thus productivity of reservoirs and to image structure and processes. However, felt and/or damaging earthquakes are a major threat to societal acceptance and regulatory license to operate. With the adaptive data-driven tool ATLS (Adaptive Traffic Light System), we aim at managing and mitigating the risk posed by induced earthquakes during stimulation and operations, while at the same time ensuring and optimising the productivity.The demonstration site for the application of ATLS lies in the Hengill volcanic region located in the South-West of Iceland, host to two power plants (Hellisheiði and Nesjavellir) with a total production capacity of 423 MWe and 433MWth. The production of energy and heat is accompanied by reinjection of the spent geothermal water in dedicated areas, both to maintain production and to comply with legal requirements. These reinjection areas have been showing different seismic responses to drilling and injection operations. We investigate these different behaviours by performing numerical modelling for two of the reinjection regions. Two models are compared: TOUGH2-Seed, a full 3-dimensional stochastic simulator and an analytical model based on a cumulative density function linking maximum pressure in the reservoir and reactivation. Those two models fulfil two different aspects of the development of an ATLS, with the full 3D allowing an in-depth dive in the driving mechanisms of induced seismicity; and the analytical solution providing a robust and fast approximation of the forecast for real-time application. We show that both models can reproduce observed seismicity patterns in the Hengill geothermal field

A Late Pleistocene-Holocene natural seismograph along the Boconó Fault (Mérida Andes, Venezuela): the moraine-dammed Los Zerpa paleo-lake

International audiencehe Boconó Fault system is a major active tectonic feature accommodating an important part of the dextral relative motion between the Caribbean Plate and northern South-America. The main trace follows an axial valley running SW-NE within the Mérida Andes (northwestern Venezuela), and crosscuts a series of moraines related to late Pleistocene glaciers developments and retreats, at an altitude between 2600 and 5000 m. Several lakes were generated after the last retreat (between the Late Glacial Maximum –LGM– and the Younger Dryas re-advance), dammed by lateral and frontal moraines. Among them, the Los Zerpa moraine system yielded rich outcrops ranging from an upstream very coarse torrential to deltaic fill, to a downstream clayey-silty horizontal laminated lacustrine accumulation; a fore-set-type heterogeneous "prograding" body links the two sets. The whole system, as well as the surrounding moraines, underwent successive major earthquakes during the Late Glacial/lower Holocene period as evidenced by co-seismic scarps in the moraines, migrations of the outlet, and associated sagponds. Besides active faulting affecting both the moraines and the sedimentary fill, the latter –main purpose of our detail study– exhibits various evidence of strong disturbances which we relate to seismic shaking, such as: i) successive unconformities with co-seismic slips along fractures in the coarse proximal sediments; ii) successive dip changes, discontinuities, and slumps in the foreset-like set; iii) slumps with basal liquefaction, syn-sedimentary fractures, and instantaneous re-sedimentation in the fine-grained laminated accumulation. Lateral (temporal) correlations are established between the successive disturbances detected in the three situations; in turn, these sedimentary events are correlated with seismic activity of the Boconó Fault main trace. Thus, the whole paleo-lake may be considered as a natural seismograph which worked during several thousands years, after the end of the LGM and during early Holocene

Preliminary Modelling Activities for an Adaptive Traffic Light System for the Hengill Geothermal Field

Monitoring micro-seismicity during operations of a geothermal field is crucial for the understanding of seismic hazard and the reservoir evolution. Induced earthquakes in the context of geothermal projects are on the one hand an important tool to enhance the permeability and thus productivity of reservoirs and to image structure and processes. On the other hand, felt and in particular damaging induced earthquakes are a major threat to societal acceptance and regulatory license to operate. With the adaptive data-driven ATLS (Adaptive Traffic Light System), we aim at managing and mitigating the risk posed by induced earthquakes during stimulation and operations, while at the same time ensuring and optimising the productivity. RISC is developed as part of the EC Geothermica project COSEISMIQ (www.geothermica.eu/projects/coseismiq/) and will demonstrate the full potential of adaptive tools for risk mitigation and reservoir optimisation on the Hengill volcanic region. There, two power plants (Hellisheiði and Nesjavellir) have a total production capacity of 423 MWe and 433MWth, but to maintain this energy output, new production wells and reservoir stimulations are needed, often associated with induced seismicity. RISC integrates the entire processing chain, from automated picking of waveforms and data processing to seismic hazard and risk forecasting, including a strong seismicity modelling component. The seismicity forecasting module routinely computes and weights different models, ranging from purely statistical to fully coupled, 3D hybrid models. To implement such complex data-driven risk mitigation tool in Hengill, we performed a preliminary analysis of the past seismicity at two re-injection sites and provide preliminary numerical modelling for one of the sequences using the hybrid simulator TOUGH2-Seed. Future analysis will aim, on the one hand, at understanding the physical processes happening at the different sites, and on the other hand, at evaluating the performance of such code for future real-time application