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This article presents a morpho-structural map at 1:50,000 scale of the confluence area between three regional units: (1) the Burgalesa Platform (Mesozoic; Cantabrian Mountains), (2) the Duero Basin and (3) the Ebro Basin (Cenozoic foreland basins). The map covers an area of 527 km2 in north Iberia. Additionally, structural lineaments such as joints are well preserved and documented in the Upper Cretaceous sediments of the Burgalesa Platform, represented by four main sets regarding their orientation. The foreland basins are incised in Miocene materials by rivers that have developed a complex drainage system controlling the deposition of the most recent alluvial sediments, following the four structural orientations. The map, together with the analysis of orthophotos and a Digital Terrain Model, and outcrop-scale observations, have led us to document the influence of the Miocene to present-day compression and strike-slip tectonic regime on the configuration of the drainage system and the geomorphological features.

Tavani et al. (2022) raise in their comment some important points that must be dealt with in any model of thrust belts involving inversion and salt tectonics processes. The Southern Basque-Cantabrian Basin is indeed a paradigmatic case for which the different interpretations of the geological, seismicand borehole data have resulted in contradictory tectonic scenarios. Moreover, there is intense interest in the processes that have configured the underground storage systems. We kindly appreciate this opportunity to openly discuss the different views., This work is part of the projects “SENSE: Assuring integrity of CO2 storage sites through ground surface monitoring” (PCI2019-111943-2), REViSE-Betics-PID2020-119651RB-I00 and RTI2018-093613-B-I00 founded by the Spanish Science Ministry and “3D geological modelling of the Basque-Cantabrian Basin” projects funded by the Spanish Geological Survey (CN IGME-CSIC), and the Junta de Andalucía funded projects ARNM-005-UGR18 and FIPS, PY20-01387.

Geologic gas storage is a sustainable strategy to store energy in underground reservoirs in order to be used under demand. Injection and withdrawal gas operations could trigger induced seismicity according to the tectonic constrains of the reservoir. The study of the active stress/strain fields shows the properties of the fault patterns that can be involved in induced seismicity. In our work, we have determined the strain/stress field in the Yela underground gas storage, an Upper Cretaceous carbonate reservoir located in central Spain. Slickensides on fault planes were measured from Cretaceous, Miocene and Quaternary rocks. Results show a strain field with ey (maximum horizontal shortening) oriented NW-SE. This field was compressional during the early Miocene, switching off to extensional from late Miocene to present-day. Our results suggest that NNW-SSE and NW-SE oriented faults could act as an effective pathway for gas leakage and prone to trigger induced seismicity. Moreover, 141 small earthquakes (Mmax 2.3) were recorded during underground operations in Yela. Peaks of earthquakes appear seasonally from November to February, in coincidence with the injection operations prior to the winter withdrawal. Working at its maximum gas storage volume, earthquake peaks decreased in intensity and magnitude from 2017 to 2021, and show a time lag of 3 months from the month of maximum volume injection to the earthquake occurrence

The SW Iberian margin is an oblique passive margin developed during the Mesozoic, as part of the opening of the westernmost Ligurian Tethys. The Algarve Basin developed on the SW Iberian margin and its sedimentary record was strongly controlled by tectonics. The Algarve Basin was dominated during the Mesozoic by WSW–ENE trending extensional faults and their associated depocenters, segmented by oblique NW–SE transfer zones. Two major NW–SE trending transfer zones connected the Algarve Basin to the Gorringe Bank in the west and to the Betic Basin in the east. The SW Iberian margin was later inverted during Late Cretaceous to Cenozoic times, due to convergence between Africa and Iberia. Inversion of the Algarve Basin and the SW Iberian margin was controlled by both the WSW–ENE and NW–SE trending structures formed during the passive margin phase. The evolution of the Algarve Basin during the Mesozoic and up to the Paleogene is documented in this paper through paleogeographic reconstructions based on field geology and reinterpretation of offshore seismic and well data.

One of the concerns of underground CO2 onshore storage is the triggering of induced seismicity and fault reactivation by the pore pressure increasing. Hence, a comprehensive analysis of the tectonic parameters involved in the storage rock formation is mandatory for safety management operations. Unquestionably, active faults and seal faults depicting the storage bulk are relevant parameters to be considered. However, there is a lack of analysis of the active tectonic strain field affecting these faults during the CO2 storage monitoring. The advantage of reconstructing the tectonic field is the possibility to determine the strain trajectories and describing the fault patterns affecting the reservoir rock. In this work, we adapt a methodology of systematic geostructural analysis to underground CO2 storage, based on the calculation of the strain field from kinematics indicators on the fault planes (ey and ex for the maximum and minimum horizontal shortening, respectively). This methodology is based on a statistical analysis of individual strain tensor solutions obtained from fresh outcrops from the Triassic to the Miocene. Consequently, we have collected 447 fault data in 32 field stations located within a 20 km radius. The understanding of the fault sets' role for underground fluid circulation can also be established, helping further analysis of CO2 leakage and seepage. We have applied this methodology to Hontomín onshore CO2 storage facilities (central Spain). The geology of the area and the number of high-quality outcrops made this site a good candidate for studying the strain field from kinematics fault analysis. The results indicate a strike-slip tectonic regime with maximum horizontal shortening with a 160 and 50∘ E trend for the local regime, which activates NE–SW strike-slip faults. A regional extensional tectonic field was also recognized with a N–S trend, which activates N–S extensional faults, and NNE–SSW and NNW–SSE strike-slip faults, measured in the Cretaceous limestone on top of the Hontomín facilities. Monitoring these faults within the reservoir is suggested in addition to the possibility of obtaining a focal mechanism solutions for micro-earthquakes (M

The Iberian Peninsula is surrounded to the north by the convergence margin between Eurasia and the former Iberian plates (North and Northwest Iberian margin), and to the south by a transform plate boundary between Eurasia and Nubia (Gulf of Cádiz) to a shear-compressive indentation of Nubia northwards in the Alborán Sea. These margins are affected by historic and present-day seismicity, which are linked to active tectonic structures deforming the seafloor of the margins. The main objective is to better understand their development in the framework of the present plate organization and thus evaluate the seismic hazard around Iberia. Therefore, we carried out an extensive geophysical characterization of submarine faults, focusing on those that show seabed morphological expressions, by mapping them with high-resolution swath bathymetry data, high-resolution parametric sub-bottom profiles and multichannel 2D seismic profiles. Their activity and distribution are in good agreement with the geodetic and seismological observations.Our results show that the present-day active tectonics and its related deformation, including seismicity and tsunami-affected coastal areas, are mainly located in the south Iberian margin, around the boundary between the Eurasian and Nubia tectonic plates. The submarine active faults are represented in this margin by a large strike-slip fault system and fold-thrust systems, in response to the NW-SE convergence between the aforementioned tectonic plates. The different orientation and distribution of submarine faults, and the fault type from focal mechanism of seismic events, led us to identify simple and pure shear zones from the Alborán Sea to the east, to the Gibraltar Arc and Gulf of Cadiz to the west. This suggests a strain partitioning model along the plate boundary in response to the present-day shear stress orientation.Deformation is also documented in the NW Iberian margin. Thrust fault systems with high seismic activity were identified and mapped along Iberian ocean-continent transition around the Galician and Portuguese margins, reflecting the re-activation of former Cenozoic faults. Deformation in this margin is also derived from the westward motion of the Iberian oceanic domain and the clockwise rotation of the Iberian continental domain with respect to the Eurasian plate.

Oblique rift systems form when the axis of rifting is not orthogonal to the direction of tectonic extension, normally due to pre-existing zones of weakness that influence the location and orientation of new faults. Irrespective of the regional-scale obliquity, most individual extensional faults will tend to nucleate according to the orientation of the tectonic stress orientations, and therefore normal to the direction of maximum extension. Transfer faults in oblique systems will tend to form parallel to the direction of extension and, in contrast to orthogonal rifting, will play a major role in the architecture and development of the rift and its sedimentary basins.An intriguing feature in oblique rift systems is the formation of reverse structures evocative of wrench tectonics during the syn-rifting stage. This stems from the orientation of geological structures relative to the direction of tectonic extension. Even slight changes in tectonic transport direction or stress orientations during the development of the rift system can lead to events of transpression or transtension along transfer structures. Because of the relevance of transfer structures in oblique systems, transpression can result in the appearance of discontinuities in the sedimentary record that are often interpreted as, somewhat incongruent, inversion events.Oblique structures also play a crucial role during the full inversion of the rift system during convergence, particularly so because tectonic shortening will strike at an angle to the orientation of faults. Irrespective of the evolution of oblique rifting and inversion, the initial fault pattern is also normally preserved in fully inverted systems involved in fold-and-thrust systems. In many of cases, when the original rift obliquity is not well understood, the characteristic rhomboidal pattern is interpreted to relate to wrench tectonics. In this presentation we will review evidence from Iberia, Northwestern Africa and the Eastern Alps to discuss the role that obliquity plays in rift development and its inheritance in fold-and-thrust belts with different degrees of inversion.

The long-term managing from the geological hazard point of view of the Hontomín onshore pilot-plant for CO2 storage, located in Spain and recognized as the first and only key-test facility in Europe, is one of the main objectives stated in the ENOS European project. This project is led and funded by the European Network of Excellence on the Geological Storage of CO2 (CO2GeoNet).The complex geological emplacement of the Hontomín Carbon capture and storage plant is considered rather relevant to analyse the impact of fracturing and both local and regional strain field on the reservoir parameters. The reservoir of Hontomín pilot-plant is formed by highly fractured Middle Jurassic dolomites with associated secondary porosity. This parameter is one of the main concerns when managing CO2 storage and monitoring.In order to characterize the fracture pattern and its implications on a proper CO2 monitoring, we characterized the surface structural elements through the study area and their relationship with fractures affecting the reservoir porosity. The methodology followed in this work is mainly based on detailed geological mapping (field work complimented with orthophoto analysis), adding missing information from previous works. This analysis does not increase the cost for long-term monitoring, given that they are low-cost and the results are acquired in a few months.The main structural trend in the study area, concerning faults with a wide range of displacement and metric to decametric folds, follows a regional E-W orientation. On the other hand, fractures show two main sets of trends, from NW-SE to NE-SW.This fracturing pattern, considered as a conjugate fracture system, corresponds to the tectonic stress recorded in both Mesozoic and Cenozoic sedimentary successions where the Hontomín pilot-plant is placed. Riddle faults formed from a nearby regional right-lateral strike slip fault (Ubierna Fault) are the responsible structures for the fracture system affecting the area and the reservoir. Moreover, this fracturing pattern is in agreement with local to regional active tectonic field from Cenozoic times to present-day, when the Ubierna Fault recorded its maximum right-lateral displacement (15 km).Secondary porosity within the reservoir can be produced from this fracture pattern, highly increasing the permeable migration paths for CO2 migration after the injection. Therefore, we state that a combination between fracture analysis and structural and tectonic study, should be considered as mandatory in the monitoring phases of the CO2 plume, during and after injection operations.

One of the concerns of underground CO2 onshore storage is the triggering of Induced Seismicity and fault reactivation. Hence, a comprehensive analysis of the tectonic parameters involved in the storage rock formation is mandatory for safety management operations. Unquestionably, active faults and seal faults depicting the storage bulk are relevant parameters to be considered. However, there is a lack of analysis of the active tectonic strain field affecting these faults during the CO2 storage monitoring. The advantage of reconstructing the tectonic field is the possibility to determine the strain trajectories and describing the fault patterns affecting the reservoir rock. In this work, we adapt a methodology of systematic geostructural analysis to the underground CO2 storage, based on the calculation of the strain field and defined by the strain field from kinematics indicators on the fault planes (ey and ex for the maximum and minimum horizontal shortening respectively). This methodology is based on statistical analysis of individual strain tensor solutions obtained from fresh outcrops. Consequently, we have collected 447 fault data in 32 field stations located within a 20 km radius. The understanding of the fault sets role for underground fluid circulation can also be established, helping for further analysis about CO2 leakage and seepage. We have applied this methodology to Hontomín onshore CO2 storage facilities (Central Spain). The geology of the area and the number of high-quality outcrops, made this site as a good candidate for studying the strain field from kinematics fault analysis. The results indicate a strike-slip tectonic regime with the maximum horizontal shortening with N160° E and N50º E trend for local regime, which activates NE–SW strike-slip faults and NE–SW compressional faults. A regional tectonic field was also recognized with N–S trend, which activates E–W compressional faults. Monitoring of E–W faults within the reservoir is suggested in addition with the possibility of obtaining focal mechanism solutions for micro earthquakes (M

The Llert syncline is located in the South-central Pyrenees, between the eastern termination of the EW-trending Cotiella Basin and the north-western limb of the NS-trending Turbón-Serrado fold system. The Cotiella Basin is an inverted upper Coniacian-lower Santonian salt-floored post-rift extensional basin developed along the northern Iberian rift system. The Turbón-Serrado fold system consists of upper Santonian – Maastrichtian contractional salt-cored anticlines developed along an inverted transfer zone of the Pyrenean rift system. Based on field research, this paper presents a 3D reconstruction of the Llert syncline in order to further constrain the transition between these oblique salt-related structures. Our results suggest that the evolution of the Llert syncline was mainly controlled by tectonic shortening related to the tectonic inversion of the Cotiella Basin synchronously to the growth of the Turbón-Serrado detachment anticline, and by the pre-compressional structural framework of the Pyrenean rift system. Our contribution provides new insight into the geometric and kinematic relationships of structures developed during the inversion of passive margins involving salt.

Underground gas storage is one of the solutions required for reducing CO2 emissions and ensure the transition to a renewable power supply. The Hontomín pilot-plant in N-Spain is a well-studied CO2 facility located in the Burgalesa Platform, a fold belt in the Southern Basque-Cantabrian Basin. Previous works agree that crustal extension and evaporite migration controlled the subsidence in the area. The discrepancies lie in the salt expulsion models proposed during syn-rift and post-rift stages and the basin deformation during Cenozoic contraction. The models range from thin-skinned tectonics, basement-involved thrusting, strike-slip faulting, and interference between thick- and thin-skinned thrusting with lateral extrusion. There is multiple working hypothesis often with contradictory predictions of the underground structure. Understanding these structural styles is critical for identify the main salt structures suitable for gas storage in the Burgalesa Platform. We integrate multidisciplinary data from 2D seismic to wellbore, gravity and field data, allowing us to construct three restored cross-sections that illustrate the process of progradational loading and salt expulsion towards the SW during the Early Cretaceous, which led to the development of the Hontomín and other trap structures within the salt overburden. Differential loading occurred diachronously along strike during the Cenozoic, younging to the SE by evacuation of the remaining salt, which caused diapirism and salt welding. We found that most of the structures in the cover were formed by salt migration, whereas Cenozoic shortening caused the rejuvenation of salt-structures and local diapir squeezing. Shortening in the basement is consumed in the frontal thrust that overrides the Duero foreland and in the partial reactivation of the former Mesozoic transfer faults as transpressional faults. The results of this work have implications for a further understanding of the tectonic evolution of the external area of rift structures, and as tectonic analogues in other salt-bearing basins., This work is part of the projects “SENSE: Assuring integrity of CO2 storage sites through ground surface monitoring” (PCI2019-111943-2), REViSE-Betics-PID2020-119651RB-I00 and RTI2018-093613-B-I00 founded by the Spanish Science Ministry, the “Tectonic map of the Iberian Peninsula and the surrounding cordilleras” and “3D geological modelling of the Basque-Cantabrian Basin” projects funded by the Spanish Geological Survey (CN IGME-CSIC), and the Junta de Andalucía funded projects ARNM-005-UGR18 and FIPS, PY20-01387.

Este trabajo propone un modelo cinemático 3D de la confluencia de las placas tectónicas Norteamericana y Pacífica con las placas de Rivera y Cocos, centrado en la zona de convergencia de la Fosa Mesoamericana en el área de México. A partir del análisis de más de 1300 mecanismos focales de terremotos (M ≥ 5.5) obtenidos de la base de datos en abierto del programa Global Centroid-Moment-Tensor (CMT), se han aplicado diversas técnicas de geología estructural para obtener la distribución espacial de los tensores de deformación (ey, ex, ez) y del factor de forma (k’) de dichos tensores. Estas técnicas consisten en el estudio de los Diedros Rectos de los mecanismos focales y de la aplicación del Modelo de Deslizamiento, basados en la cinemática del deslizamiento sobre el plano de falla teórico asociado a cada sismo. Este análisis permite realizar un estudio tridimensional de la deformación mediante el agrupamiento espacial 3D de los diferentes tensores (análisis cluster), agrupando zonas homogéneas en estilos de deformación, junto con los diagramas triangulares de clasificación de mecanismos focales (Kaverina) en relación con la geometría de la subducción y de la presencia de flujos mantélicos. También se han calculado perfiles profundos de la deformación y del factor de forma (k’) en relación a la geometría de la subducción y las principales estructuras tectónicas. Los resultados muestran una zonación 3D influenciada por el ángulo de subducción y el aumento de la velocidad de convergencia hacia el SE, en relación al acoplamiento mecánico de la subducción, apareciendo menos terremotos inversos en la convergencia de la Placa de Rivera con la Placa Norteamericana, frente a la Placa de Cocos con la Placa Norteamericana. La geometría 3D propuesta es congruente con la distribución espacial de grandes terremotos, el ángulo de subducción, la presencia de corrientes mantélicas a 100 km de profundidad y la presencia de terremotos en fallamiento normal. Este modelo también podría explicar la presencia de terremotos anómalos como terremotos lentos y silenciosos en el Gap de Guerrero.

The SW Iberian margin developed as a passive margin during Mesozoic times and was later inverted during the mainly Cenozoic Alpine orogeny. The initial syn-rift deposits include a Lower Jurassic evaporite unit of variable thickness. In the onshore, this unit is observed to thicken basinward (i.e., southward), in fault-controlled depocenters, and salt-related structures are only present in areas of thick initial evaporites. In the offshore, multiple salt-structures cored by the Lower Jurassic evaporites are interpreted on seismic reflection data and from exploratory drilling. Offshore salt structures include the allochthonous Esperanca Salt Nappe, which extends over an area roughly 40 × 60 km. The abundance of salt-related structures and their geometry is observed to be controlled by the distribution of evaporite facies, which is in turn controlled by the structure of rift-related faulting. This paper presents a comprehensive study of salt tectonics over the entire onshore and offshore SW Iberian passive margin (southern Portugal and Gulf of Cadiz), covering all aspects from initial evaporite composition and thickness to the evolution of salt-related structures through Mesozoic extension and Cenozoic basin inversion.

The SW of Iberia is currently undergoing compression related to the convergence between Nubia and Iberia. Multiple compressive structures, and their related seismic activity, have been documented along the diffuse Nubia-Iberia plate boundary, including the Gorringe bank west of the Gulf of Cadiz, and the Betic-Rif orogen to the east. Despite seismic activity indicating a dominant compressive stress along the Algarve margin in the Gulf of Cadiz, the structures at the origin of this seismicity remain elusive. This paper documents the contractional structures that provide linkage across the Gulf of Cadiz and play a major role in defining the present-day seismicity and bathymetry of this area. The structures described in this paper caused the Neogene inversion of the Jurassic oblique passive margin that formed between the Central Atlantic and the Ligurian Tethys. This example of a partially inverted margin provides insights into the factors that condition the inversion of passive margins.

The West and SouthWest Margins of Iberia started their formation as intra-continental rifts during initial break up of Pangea in Triassic times. The tectono-stratigraphic record of the Algarve, Alentejo and Lusitanian basins and their offshore prolongation documents the syn-rift, post-rift and passive margin phases of the rifting process as well as three magmatic cycles of tholeiite to alkaline affinities. Although the Ocean-Continent Transition has been investigated using deep ocean drilling, seismics, gravimetry and magnetics its nature and location are still matters of debate. The tectonic inheritance of the Paleozoic Orogeny had great influence in the geometry of the rift basins and development of the Neo-Tethys and Atlantic Oceans intersection. Salt tectonics strongly controlled both syn-rift and post-rift basin tectonics and led to the formation of an allochthonous salt nappe in the Algarve Basin.

19 pages, 7 figures, The Alpine orogeny is well recorded onshore and offshore by tectonic inversion of the Mesozoic rift basins. Large scale linear seamounts (more than 250 km long and with up to 5 km of uplift) involving oceanic and continental lithosphere were carried on top of thrusts, such as the Gorringe seamount and the Estremadura Spur in the SouthWest and West Iberia Margin, respectively. The SouthWest Iberia Margin also recorded the westward migration of the Gibraltar Oceanic slab as the westwards propagation of the Neo-Tethys subduction. Rotation of the tectonic compression from NW-SE to WNW-ESE in Pliocene times caused the development of large scale dextral wrench faults as the present day Africa-Iberia plate boundary. Neotectonics of this plate boundary caused large to mega-scale destructive earthquakes and tsunamis

The Algarve Basin is a Meso-Cenozoic sedimentary basin overlying Carboniferous basement, located in the southwestern margin of the Iberian Peninsula. Its structure reveals a protracted tectonic history comprising various pulses of Mesozoic extension followed by Cenozoic compression. This work deals with the structure along the northern margin, where the Mesozoic extensional structures and Cenozoic inversion structures crop out. The strike of the extensional structures ranges from E–W to N–S, as controlled by a shift from Tethyan-dominated extension in the east to Atlantic-dominated extension in the west. Contractional structures are inverted extensional structures, following their same trends. It is argued that the thickness of the Hettangian evaporite layer exerts a strong control on the structural style throughout the basin during the extensional and inversion episodes. The basin is affected by thick-skinned deformation along the northern margin, where salt is thin or absent, basement involved fault systems and short-cut structures. Basinward, as the Hettangian salt thickens, the margin is affected by thin-skinned deformation, with listric and down-to-the-basin growth faults, diapirism and salt-cored detachment folds. The aim was to discuss the key tectonic features, the relevance of salt, and understand the nature, timing, and significance of all these structures in the regional tectonic evolution.

The southern Portuguese margin recorded significant extension and subsidence during the Jurassic, resulting in the formation of the Algarve basin. Recent studies have revealed the existence of oceanic crust, of potentially Jurassic age, underlying significant portions of the Gulf of Cadiz to the south and west of the Algarve basin, but the exact relationship between this basin and the oceanic crust is still ambiguous. In this study we integrate data derived from field outcrop studies, interpretation of commercial seismic, and gravity and magnetic modelling to better understand the transition from the rifted continental crust underlying the Algarve basin to the oceanic crust of the Gulf of Cadiz. This integrated approach reveals that the Gulf of Cadiz oceanic crust is related to the WSW-ENE trending Jurassic passive margin of southern Portugal, and is separated from the Algarve Basin by a domain of highly thinned continental crust.

The SW of Iberia is currently undergoing compression related to the convergence between Iberia and Africa. Multiple contractional features have been documented both immediately west and east of the Gulf of Cadiz in recent years. Significant seismic activity reflecting mainly compression has been observed between these two domains, but the features associated to this seismicity remained elusive. This paper documents the contractional structures that provide linkage across the Gulf of Cadiz and play a major role in defining the present-day bathymetry of this area.