Article

Le filtrage des anomalies gravimetriques; une cle pour la comprehension des structures tectoniques du Boulonnais et de l'Artois (France)

May 2001
Bulletin de la Societe Geologique de France 172(3):267-274

DOI:10.2113/172.3.267

Authors:

Royal Museum for Central Africa

The geology of the Boulonnais has been well studied since the early part of the last century [Gosselet and Bertaut, 1873; Olry, 1904; Pruvost and Delepine, 1921]. Extensive coal exploration added substantially to the general understanding of the geology of the region but as outcrop is poor, many questions remain. Gravity methods used in the analysis of geological structures have had a long and successful history in helping to study the earth's crust for scientific and applied objectives. Regional gravity data are particularly useful in mapping geographic distribution and configuration of density contrast of rocks. Previous gravity research shows the main trends of the structure. In most cases the regional Bouguer gravity hides the relationship between the geology and the shape of the anomaly caused by the perturbing body. New information can be obtained by filtering the maps. The purpose of filtering a map is to remove unwanted characteristics and enhance desirable characteristics that are diagnostic for the geology. Because of their simple mathematical forms, most potential field filters are in the spectral domain. It is advisable to transform the original unfiltered field to the spectral domain, apply the filter, then transform the filtered map back to the spatial domain for use in the interpretation. Several spectrally filtered versions of the original gravity map are used in this regional interpretation. In the case of the Boulonnais the most useful filters have been the horizontal component and the first vertical derivative. In the first instance computing the horizontal gradients of the gravity field permits us to localise the limit of the blocks and then the fault positions. The gravimetric field above a vertical contact of rock with different density shows a low on the side of the low density rocks and a high on the side of the high density rocks. The inflection point is located just on the contact of the two types of rocks. This contact can be outlined by locating the maxima of the horizontal gradient. In the case of a low dipping contact maxima stay close to the contact, but are displaced down dip. In the second instance the first vertical derivative acts as a booster for the short wavelength; this attenuates or destroys the effect of the regional field. The resulting map shows a better structure because in complex areas they give a better definition of the different bodies by separating their effects. In the case of the Boulonnais the first vertical derivative allows us to distinguish the depressed region from the uplifted one. The structural evolution of the Boulonnais-Artois area includes two main extensional events in the late Palaeozoic-early Cretaceous interval and an inversion in mid-late Palaeocene time. The new gravity data in combination with recent field and published data have provided a new insight into the structure of the Boulonnais-Artois area and a new interpretation is proposed. -- Fault patterns are oriented 110N and 040N in the Boulonnais and 140N in Artois areas. -- The linkage between the faults shows a relay geometry with transfer zones [cf. Morley et al., 1990 and Pea-cock and Sanderson, 1994]. The best example is located between Sangatte (near the tunnel) and Landrethun faults where overlapping synthetic faults with a relay ramp are imaged. -- There is no major continuous fault zone but a complex en echelon fault system. -- Linkage between Boulonnais and Artois fault is not well constrained. An important discontinuity between the two regions is apparent. This model underlines the importance of overlapping fault tips with the generation of transfer zones. These structures are also known in the Wessex and Weald basins [Stoneley, 1982; Chadwick, 1993] where heritage and inversion are significant.

Understanding the Deep Structure of the Essaouira Basin Using Gravity Data: Hydrogeological Inferences for a Semiarid Region in Central-Western Morocco

Article

Full-text available

Nov 2023

The Essaouira Basin, located in central western Morocco, faces a significant threat of water shortage due to both the substantially reduced rainfall caused by climate change and the continuously increasing demand for this essential resource. Groundwater resources are being increasingly exploited to meet the needs of the population, whether for agricultural or domestic purposes. Therefore, it has become necessary to intensify investigations across the entire basin, particularly through indirect methods such as geophysical techniques, to accurately delineate the productive zones. In this context, the present study was undertaken to investigate the deep structure of this basin with the aim of comprehending the functioning of its aquifer system. This study is based on the interpretation of gravity data covering the Essaouira Basin. In addition to their qualitative analysis, these data underwent a methodological approach involving transformations to extract meaningful insights. The observed anomalies were interpreted in terms of (i) thickness variations within the slightly folded sedimentary series of the basin; (ii) Paleozoic basement topography; and (iii) the presence of salt deposits. In fact, among the negative anomalies, some coincide with evaporitic deposits that are known either from the geological outcrops or the seismic surveys carried out in the Essaouira Basin within the framework of petroleum exploration programs, while others coincide with areas of increased thickness of sedimentary sequences. The latter include synclines and basement depressions, where the accumulation of groundwater tends to occur; as a result, they constitute suitable zones for the drilling of water extraction wells. Groundwater flows observed in some existing wells are consistent with this hypothesis. The results of the contact analysis approach implemented within the framework of the study reveals the Essaouira Basin is affected by a fault network whose main direction is parallel to the Atlantic margin (i.e., NNE-SSW). This implies that the extensional tectonic phase responsible for initiating the rifting of the Central Atlantic in the Triassic era has primarily impacted the structural configuration of this basin. This study demonstrates the strong potential of Geosciences 2023, 13, 345. https://doi.org/10.3390/geosciences13110345 https://www.mdpi.com/journal/geosciences Geosciences 2023, 13, 345 2 of 18 the gravity method as a tool to delineate the deep structure of sedimentary basins and to identify potentially productive groundwater zones. The final results will provide important support to decision makers in sustainable groundwater management, especially in vulnerable areas.

Origin and geomorphology of Dover Strait and southern North Sea palaeovalleys and palaeo-depressions.

Thesis

Jan 2017

David Garcia Moreno

Fault activity in the epicentral area of the 1580 Dover-Strait/Pas-de-Calais earthquake (North-western Europe)

Article

Full-text available

Apr 2015
GEOPHYS J INT

On 1580 April 6 one of the most destructive earthquakes of northwestern Europe took place in the Dover Strait (Pas de Calais). The epicentre of this seismic event, the magnitude of which is estimated to have been about 6.0, has been located in the offshore continuation of the North Artois shear zone, a major Variscan tectonic structure that traverses the Dover Strait. The location of this and two other moderate magnitude historical earthquakes in the Dover Strait suggests that the North Artois shear zone or some of its fault segments may be presently active. In order to investigate the possible fault activity in the epicentral area of the AD 1580 earthquake, we have gathered a large set of bathymetric and seismic-reflection data covering the almost-entire width of the Dover Strait. These data have revealed a broad structural zone comprising several subparallel WNW-ESE trending faults and folds, some of them significantly offsetting the Cretaceous bedrock. The geophysical investigation has also shown some indication of possible Quaternary fault activity. However, this activity only appears to have affected the lowermost layers of the sediment infilling Middle Pleistocene palaeobasins. This indicates that, if these faults have been active since Middle Pleistocene, their slip rates must have been very low. Hence, the AD 1580 earthquake appears to be a very infrequent event in the Dover Strait, representing a good example of the moderate magnitude earthquakes that sometimes occur in plate interiors on faults with unknown historical seismicity.

Neogene and Quaternary flexure neotectonic in the Somme valley.

Article

Full-text available

Jul 2000

Gravity study of the Western Bahira Basin and the Gantour Phosphatic Plateau, central Morocco: Interpretation and hydrogeological implications

Article

May 2022
J AFR EARTH SCI

The Western Bahira Basin (WBB), located in Central Morocco, is known for its large reserves of phosphate, which are currently being extracted through open pit mines within the Gantour Phosphatic Plateau (GPP). This mining activity, as well as the increasing agricultural development that this region has been experiencing during the last few decades, has subjected the groundwater in this semi-arid region to intense exploitation through pumping. Consequently, thorough knowledge of the underground water resources is essential for sustained activity. This study uses gravity data to investigate the deep geologic structure of the western Bahira and Gantour plateau area. Its main purpose is to improve the knowledge of this structure that influences the groundwater dynamics. The methodology involves, in addition to the qualitative and quantitative interpretation of the gravity data, the use of various filtering and analysis techniques to highlight the major geological structures. In fact, a qualitative analysis of the residual gravity map shows that the observed anomalies can be explained in terms of the bedrock topography of the study area. Positive anomalies spatially correlate with the outcrops of this bedrock in the Hercynian massifs of Jebilet and Rehamna, as well as with the Douar Rhirat structural high. The negative anomalies, on the other hand, tend to reflect increased thicknesses of sedimentary sequences, such as in the southern part of the WBB. Moreover, the Brikyine granite in the north of the study area is noticeably associated with the highest amplitude negative anomaly. Analysis of the residual gravity data using the total horizontal gradient, upward continuation, and Euler deconvolution techniques provides a quantitative assessment of the geologic structure. These methods help delineate various structures oriented in two main directions: N45E and N105E. Among these structures, several major faults have been identified. They are rooted at a maximum depth in excess of 2800 m. These faults are responsible for the general structure of the study area and the morphology of the basement. Furthermore, the 2D forward modeling, performed across the main negative gravity anomalies of the WBB, helps characterize the Paleozoic bedrock topography and provides more constraints on its variation. The resultant gravity models clearly show an asymmetric structure for the western Bahira trough. This depression is filled by sedimentary formations whose thickness at the depot-center can locally reach 1200 m. Additional analysis of piezometric data shows that the groundwater flows are controlled by the gravity structures highlighted in this study.

Geological structures mapping using aeromagnetic prospecting and remote sensing data in the karstic massif of Beni Mellal Atlas, Morocco

Article

Full-text available

Dec 2018

The current study exposes the results of aeromagnetic data interpretation and a combined analysis of 1-A sentinel radar image that cover the study area of Beni Mellal Atlas in order to describe the structural geometry and to understand its tectonic evolution.The map of the reduced to pole of residual magnetic field highlights various magnetic anomalies with high amplitudes that correspond to Jurassic-Cretaceous basaltic formations outcropping synclinal basins of Beni Mellal Atlas. The interpretation of magnetic data using tilt derivative (TDR), horizontal gradient technique coupled to upward continuation and Euler Deconvolution allows us to distinguish the fractures network that is affecting the study area. In order to complete this analysis, the Sentinel-A radar image is processed and filtered using SNAP ESA Sentinel-1 toolbox software to extract lineaments. The final structural map reveals four faults groups oriented respectively NE-SW, ENE-WSW to E-W, N-S and NW-SE Their depths estimated by the application of Euler deconvolution exceed 1500 m.Thoses faults have played a major role in structural evolution of Beni Mellal Atlas.

Contribution de la gravimétrie à l'étude de la structure des Hauts Plateaux (Maroc oriental)

Article

Full-text available

Résumé. La carte gravimétrique des Hauts Plateaux met en évidence des anomalies négatives et des anomalies positives séparées ainsi que des zones à fort gradient. Les hauts gravimétriques sont associés au socle et à ses hétérogénéités, alors que les bas gravimétriques sont la signature de bassins sédimentaires, de granites dans le socle et de dômes de sel. Pour cartographier les contacts géologiques, les failles et leurs caractéristiques spatiales, trois méthodes d'interprétation ont été utilisées : gradient vertical, gradient horizontal couplé au prolongement vers le haut et déconvolution d'Euler. L'application de ces traitements a permis de cartographier un grand nombre de linéaments gravimétriques matérialisant les discontinuités de densité. Les directions obtenues sont NE-SW, E-W et NW-SE, avec une dominance des directions NE-SW. Les solutions d'Euler indiquent des profondeurs pouvant atteindre 2400 m. La carte des linéaments établie permet d'améliorer la connaissance de la structure profonde des Hauts Plateaux et constitue un guide pour la prospection des ressources du sous-sol (eau et hydrocarbures). Structural setting of the High Plateaus; insights from gravity data analysis (Eastern Morocco). Extended English abstract. The purpose of the present work is to use gravity data to improve the knowledge on the structure of the High Plateaus, located between the Jerada mountains to the north, the eastern High Atlas to the south, the Rekkam to the west, and the Algerian border to the east (Fig. 1). Most part of the High Plateaus is covered by thick Quaternary deposits, outcrops of older rocks are exposed. From the structural point of view, according to the first draft of the northern High Plateaus, the area is affected by faults which led to the development of grabens and horsts (Fig. 2). In order to improve the knowledge on the deep structure of the study area, the gravimetric map was reinterpreted. The isogal curves of the Bouguer anomaly map of the High Plateaus (scale 1/200,000; d=2.5 g/cm 3) were digitized and a regular grid was calculated with a mesh of 250 m (Fig. 3). We used various filtering techniques to delineate the major geological structures such as faults and basins: vertical and horizontal derivatives, upward continuation, and Euler deconvolution (Fig. 4). The residual-gravity anomaly field (Figs. 5-6) map presents (positive/negative) anomalies separated by strong gradient zones. Interpretation makes it possible to associate the negative anomalies with deep basins, domes or granites (Tab. I). The positive anomalies are associated to basement uplift or outcrop. The statistical analysis of the anomalies allows identifying the major structural trends of the region (Fig. 5b). The vertical gradient allowed us to amplify short waves and to reduce interferences between anomalies (Fig. 7). The superimposition of horizontal derivative maxima from the Bouguer anomaly and from its upward continuation at several heights up to 4,000 m by steps of 250 m (Fig. 8) shows alignments outlining the contacts and gives information on their dip (Fig. 9a). The statistical analysis of contacts interpreted as faults shows five main trends: N00; N120; N70; N90 and N45, with a clear predominance of the last trend (Figs. 9b-c). Euler deconvolution was used to determine source depths and their location. Solutions are well grouped and correlate with various contacts previously inferred from horizontal derivative maxima. The values of sources depth can reach 2,338 m (Fig. 10). Superimposition of the interpreted major faults to the geological map, shown in figure 9, indicate that NE-SW and E-W system is well developed and represented by several faults with regional importance; the other fault systems are mainly N-S, NW-SE (Fig. 11).

APPORT DE LA GRAVIMETRIE A L’ETUDE DE LA STRUCTURE GEOLOGIQUE DE LA PROVINCE DU MAI-NDOMBE EN R.D. CONGO : IMPLICATION A L’EXPLORATION DES HYDROCARBURES

Article

Jun 2024

Willy Kilembe Biliki

This study is focused on the recognition of the geological structure of the province of Maï-Ndombe in DR Congo based on the analysis and interpretation of gravity data. These data were processed using regional-residual separation methods, horizontal and vertical derivatives, and upward continuation. The integration of all the geological information resulting from the interpretation of gravity maps has enabled us to develop a structural map that improves our knowledge of the geological structures of major petroleum interest in this area. Reading this structural map shows us that the Inongo high is the highest geological structure while the Lokoro depocenter is the deepest geological structure in this province. These two structures being close, a part of the oil formed in the Lokoro depocenter could easily migrate along the large faults to be trapped in the upper stages located in the Inongo high. As such, these two structures represent the most important oil targets in this region and should be the subject of much more seismic exploration.

Deep geometry of Triassic evaporite bodies associated with the Bazina structure, North Tunisia: new insights from gravity modeling

Article

Full-text available

Feb 2024
ACTA GEOPHYS

The Triassic outcrop in the Bazina structure is known among the largest massif in northern Tunisia. Its tectonic setting and sub-surface geometry are related to geodynamics and kinematics in North Africa and western Mediterranean domains. Gravity data processing and interpretation over the Hedil region highlight deep faulting and the geometry of Bazina structure and the neighboring ones. Enhancement of the residual anomaly, which represents the gravity response of sedimentary cover, mainly horizontal gradient map and its maxima, reveals varied oriented lineaments. The E–W conspicuous direction covers and delimits the outcropping Triassic body. However, the NE–SW major direction, highlighting eastward and southward of the Bazina structure, coincides with Cretaceous exposures. Furthermore, less extended N- and NW-trending features were highlighted. Limits and depth of deep gravity causative bodies were located using the Euler deconvolution method. 3D inversion and 2D forward gravity modeling combined with geological outcrops enabled the reconstruction of the Triassic bodies and surrounding structures. The study area reveals piercing diapiric structures and salt wrinkles. This Bazina evaporite exposure corresponds to a deep-rooted diapir marked by a southward evaporite lateral outpouring related to the Eocene Pyrenean compressive tectonic phase. The evaporite lateral flow should be controlled by reactivation of preexisting regional faults with its thermodynamic conditions and asymmetrical basin geometry.

New insight on the structural architecture of northern Tunisia with a multidisciplinary approach: Association of Miocene piggy-back basin and active Plio-quaternary out-of-sequence thrusts

Article

Dec 2023
MAR PETROL GEOL

In order to better understand the intricate structural architecture of northern Tunisia, we use a multidisciplinary approach that includes field observations, geomorphic, sedimentological, and gravity data across this area. We offer a model of progressive thrusting in which four deformation phases occurred in the context of plate convergence (i) Uplift of the Jebel Ichkeul Triassic succession via a NE-trending thrust fault during the Eocene; (ii) Advance of Numidian thrust sheets during the late Oligocene-Middle Miocene, causing the formation of the Lake Ichkeul/Jalta as being piggy-back basin; (iii) Initial out-of-sequence thrusting during the Late Miocene, resulting in the formation of the Sejnane and El Mejel basins with syntectonic Pliocene infill; and (iv) Subsequent out-of-sequence thrusting during the Early Pleistocene, leading to the formation of the Oued (River) Zyatine basin with a syntectonic Quaternary deposits. We used the gravity data analysis to mark the thicknesses of syntectonic deposits and their bounding thrust faults. The kinematic analysis indicates the occurrence of progressive multiphase out-of-sequence thrusting events with thrusts verging to the southeast. The combination of climate and mechanical fractures induced gradually long-term damage and changed the original landscape of the evidenced out-of-sequence thrusts. Furthermore, the existence of local normal faults in the backlimbs and reverse faults in the forelimbs, together with the drainage network type, earthquakes, the slope erosion, and the aggraditional terrace system, implies a gradual landscape evolution of thrust topography. This physical evolution occurred following an active tectonics across the exhibited out-of-sequence thrusting since the Pliocene.

Gravity Interpretations to Investigate the Deep Geological and Structural Status of Middle Atlas Chain Region, Morocco

Article

Oct 2023

The Moroccan Middle Atlas is an intracontinental chain characterized by a combination of tectonic, sedimentary, and magmatic events. These events influenced the Middle Atlas basement structure and its morphology. In order to improve our understanding of the subsurface and deep geological structures, a gravimetric investigation was carried out. A residual gravity map and its horizontal derivative were computed and interpreted. We present in this work the main findings: The residual gravity anomaly map shows fourteen positive anomalies which are well correlated with the undulated Paleozoic basement in the Middle Atlas or with the High Moulouya and the Merhraoua Anticlinorium in the Northeastern part of the Middle Atlas chain. It also exhibits sub-circular negative anomalies related to the thick subsurface Meso-Cenozoic cover of the Middle Atlas chain depocenters located in the center and southeastern parts of this chain. These anomalies are correlated with the Bou Anguer, Bekrit, Tazouta, Enjil, and Almis Mermoucha basins; the Bounacer mount; and the extension of Boumia granite in the High Moulouya. The horizontal gravity gradient map discloses that the study area is mainly affected by the NE-SW trends as major faults, while the N-S, NW-SE, and E-W are minor faults. They’re superposed with the Southern Middle Atlas Accident (SMAA), the North Middle Atlas Accident (NMAA), the Tizi N’tretten Accident (TTA), and their branches. They are due to the alpine neotectonic reactivation phases of the Paleozoic and Jurassic heritaged faults. A synthetic structural map displaying the lateral extension of most Middle Atlas faults was postulated. Consideration is also being given to explore the buried faults or unmapped ones. Our final map represents an efficient and effective tool to help decision-makers during mining exploration.

Contribution of gravity anomalies interpretation to the geology of the Jbel Saghro (Eastern Anti-Atlas, Morocco): implications for the impact of structural control on sedimentation distribution.

Article

Full-text available

Nov 2021
B GEOFIS TEOR APPL

The uses of geophysical methods in hydrogeological study of Grombalia plain (Northeastern Tunisia)

Article

Dec 2021

The Grombalia plain (Northeastern Tunisia) is one of the most important hydrogeologic systems in Tunisia. Despite the various geological, structural, and hydrogeological studies and due to the tectonic and sedimentary complexity affecting the region, the geometric characterization of the Grombalia Plain aquifer system remains incompletely known. So it is very important to conduct multidisciplinary research in order to understand the geometrical configuration of this system. This characterization is based especially on the analysis and the interpretation of deep oil wells, the seismic reflection sections, and the gravity data. Hydrogeophysical investigations show that the Grombalia aquifers are constituted by two major aquifers: (1) the Mio-Plio-Quaternary to Quaternary multilayer shallow to deep aquifer and (2) the Miocene-Oligocene deep aquifer. The structural isochron map inferred from seismic data as well as the gravity mapping based on different techniques of analysis and interpretation (total horizontal derivative, source edge detect filter, tilt angle filter, Euler Deconvolution solutions) shows more details about the deep structure and the limits of this hydrogeological system. Finally, the produced structural map of Top Langhian in Grombalia plain can be considered a very useful document for future deep-water targets exploration and their sustainable management. Also, this paper will be the key and the base for future hydrogeological modeling of the region.

Iraqi Geological Journal Exploiting Aeromagnetic and Gravity Data Interpretation to Delineate Massif Deposits of Rehamna Area (Western Meseta-Morocco)

Article

Full-text available

Oct 2021

The analysis of the magnetic signatures and gravity gradient values of the Rehamna Massif south of the Moroccan Western Meseta by using Geosoft Oasis Montaj 7.0.1 software, allowed us to detect several useful anomalies to be exploited and which are related to magmatic bodies and structural features within the study area. These data were analyzed by applying several techniques, including the horizontal gradient filters combined with the first vertical derivative. Subsurface structures; such as geological boundaries, faults, dykes and folds, were visualized as lineaments on geophysical maps, then results were compared with structural features provided by previous studies in the region. Thus, the Rehamna Massif structural map shows sets of linear features which may represent faults or boundaries of geological structures, which can be either faults or boundaries of geological structures, and they are mostly oriented in the directions: N-S, NNE-SSW, NE-SW, E-W with the predominance of the NNE-SSW to NE-SW directions. In addition, the super position of the minerals bearing beds or formations were distinguished from gravity and magnetic data processing results. Some of the recognized anomalies are related to the existence of precious metals which belong to the granitic bodies within the study area.

Phosphatic series under Plio-Quaternary cover of Tadla Plain, Morocco: Gravity and seismic data

Article

Full-text available

Aug 2019
CR GEOSCI

The Ouled Abdoun sedimentary basin in Morocco contains the largest phosphate reserves in the world. To the Northwest of the basin, the phosphate layers crop out or are at a shallow depth and are exploited in open pit mines. By contrast, towards the central and southwestern parts of the basin, the phosphate layers lay below the Plio-Quaternary sediments of the Tadla Plain. This study aims to characterize the distribution and structure of the phosphatic series at depth under this cover, in particular to guide the future exploration and exploitation of the phosphate deposits. The work is based on gravimetric and seismic data acquired in a previous oil exploration study, calibrated by borehole data. The analysis of the gravity data controlled by boreholes first reveals the existence of large regional faults in the basin, forming pronounced gradients in the gravity data. The faults are also seen in the seismic profiles. In the Southeast of the basin, the phosphatic series exhibit a stairway structure controlled by some of these regional faults. The drilling data and the isochron maps established from the seismic profiles show that the base of the phosphatic series is more than 500 m deep in the southeastern area of the basin. As the phosphatic layers get buried deeper, they also become thicker. The phosphate reserves thus increase towards the southeast, and our analysis identifies the areas where this reserve is larger. Our results eventually suggest that the mining methods currently adopted in the Khouribga mining district will need to be modified and adapted to the more complex situation of the phosphatic series deeply buried in the basin.

Imagerie sismique de la structure profonde de la marge Algérienne orientale (secteur de Jijel) : implications en terme de potentiel pétrolier

Thesis

Jun 2014

Mihoubi Abdelhafid

Cette thèse a été conduite dans le cadre du programme de coopération de recherche Algéro-française SPIRAL (Sismique Profonde et Investigations Régionales du Nord de l’Algérie). Ce projet vise à étudier la structure profonde de la marge algérienne par une approche combinée des techniques sismiques ; grand-angle et multi-canal. Le domaine couvert par la présente étude se concentre dans la région de Jijel dans la marge algérienne orientale. L’objectif principal de notre thèse est d'améliorer en profondeur l'imagerie de la marge algérienne en utilisant une combinaison de données sismiques grand-angle (OBS, sismomètres de fond de l'océan) et multi-canal (MCS). Le but de cette thèse est d'apporter de nouvelles connaissances pour répondre à quelques questions sur la nature de la croûte terrestre, la zone de transition continentale-océanique, la présence du sel messénien, sa distribution et sa relation entre les formations sédimentaires superficielles et les structures crustales. Dans cette étude, notre approche est une inversion jointe des enregistrements grand-angle et des données sismiques multi-canal. Nous avons conduit une série de tomographie des premières arrivées, une inversion jointe des arrivées réfractées et réfléchies ainsi qu’une modélisation gravimétrique. Etant donné que la solution du problème inverse n’est pas unique, deux programmes de tomographie ont été utilisés sur les mêmes données pour la même région d’étude à savoir : FAST (First Arrival Seismic Tomography) et Tomo2D. La tomographie a été suivie par une inversion jointe des arrivées réfractées et réfléchies suivant une approche basée sur la combinaison de la migration en profondeur « Kirchhoff » avant sommation (PSDM) des données de sismique réflexion multi-canal (MCS) et la modélisation directe des enregistrements grand-angle sur le fonds marin (OBS). Afin de vérifier la consistance du modèle de la vitesse avec les données gravimétriques, l’anomalie à l'air libre a été modélisée. Les résultats de l’imagerie conduite dans ce travail montrent la structure de la marge, la croûte continentale, la zone de transition continent-océan et la croûte océanique de la Méditerranée. La structure du modèle confirme les études antérieures basées sur des données bathymétriques, gravimétriques et magnétiques. Cette structure montre essentiellement : - un plateau continental étroit et pente continentale une très raide.- l’Expulsion du sel vers le nord impliquant la formation de diapirs au-dessus du flanc nord du bassin (plaine abyssale).- L’approfondissement et l’épaississement des séquences sédimentaires (bassin sédimentaire) près de la marge algérienne. Le modèle de vitesses obtenu et l’épaisseur des différentes unités structurales formant ce modèle apportent des arguments quantitatifs pour enrichir la connaissance de cette partie de la Méditerranée occidentale. Les couches sédimentaires dans le bassin montrent des vitesses sismiques allant de 1,9 km / s à 3,8 km / s. Les formations messéniennes ont été modélisées en utilisant une vitesse située entre 3,7 km / s à 3,8 km / s. La croûte continentale s’amincit sur une bande étroite de la marge dont la distance est d'environ 15 km. La vitesse de la croûte océanique dans cette région présente deux couches distinctes : l’une caractérisée par des vitesses variant de 4,7 km / s à 6.1 et l’autre de 6.2 à 7.1 km / s. La vitesse du manteau supérieur quant à elle a été modélisée par 7,9 km / s.

Contributions of gravity and field data on the structural scheme updating of the Tellian domain and its foreland (Nefza-Bizerte region, northern Tunisia)

Article

Full-text available

Mar 2018

The Nefza-Bizerte region, eastern part of the Tunisian Alpine chain, covers the thrust sheets domain called the Tell and its Atlassic foreland. The deep structures under the Tellian thrust sheets are not enough explored. The structural interpretation of magmatic rocks, Triassic outcrops and the depressions are still a subject of discussion. In this work, we intend to inves-tigate deep faults and their eventual role in magmatism and Triassic salt setting up and to explain the depression genesis. Analysis of the Bouguer anomaly map and its derivatives reveals the main gravity lineaments, organized in major NE- and NW-trending systems. The NE-trending system, dipping towards the NW, is the main component of the structural scheme and has controlled the tectonic evolution of this area. After the immobilization of the Tellian thrust sheets during the upper-most Langhian, the Tell and its Atlassic foreland were aﬀected by the Tortonian compressive event with a NW-trending maximum horizontal stress. The reverse kinematics of the NE-trending deep-seated faults created at their front continental environments ﬁlled later by post-nappes Neogene deposits. After the early Pleistocene, a NNW-directed compressional stress regime deformed the post-nappes Neogene series and generated NW-trending grabens. This coexistence of compression-extension continues until present day.

Validation of gravity data from the geopotential field model for subsurface investigation of the Cameroon Volcanic Line (Western Africa)

Article

Full-text available

Dec 2018
EARTH PLANETS SPACE

Belonging to the Cameroon Volcanic Line (CVL), the western part of Cameroon is an active volcanic zone with volcanic eruptions and deadly gas emissions. The volcanic flows generally cover areas and bury structural features like faults. Terrestrial gravity surveys can hardly cover entirely this mountainous area due to difficult accessibility. The present work aims to evaluate gravity data derived from the geopotential field model, EGM2008 to investigate the subsurface of the CVL. The methodology involves upward continuation, horizontal gradient, maxima of horizontal gradient–upward continuation combination and Euler deconvolution techniques. The lineaments map inferred from this geopotential field model confirms several known lineaments and reveals new ones covered by lava flows. The known lineaments are interpreted as faults or geological contacts such as the Foumban fault and the Pan-African Belt–Congo craton contact. The lineaments highlighted coupled with the numerous maar lakes identified in this volcanic sector attest of the vulnerability of the CVL where special attention should be given for geohazard prevention.

Hydrogeological framework and geometry modeling via joint gravity and borehole parameters, the Nadhour-Sisseb-El Alem basin (central-eastern Tunisia)

Article

Dec 2017
J AFR EARTH SCI

The Nadhour-Sisseb-El Alem basin, in the central-eastern part of Tunisia, is characterized by the scarcity of surface and subsurface water resources. Although the aquifer systems of this basin are not well understood, the scarce water resources are subject to a high rate of exploitation leading to a significant drop in the level of the water table. This work presents correlation of gravity data with hydrogeological data in order to improve the knowledge of the deep structures and aquifer systems. Various geophysical filtering techniques (e.g., residual anomaly, upward continuation, horizontal gradient, and Euler deconvolution) applied to the complete Bouguer anomaly, deduce the deep structures and geometry of the basin and highlight gravity lineaments that correspond to the tectonic features. The structural framework of the Nadhour-Sisseb-El Alem hydrogeological basin shows N-S to NNE-SSW and E-W oriented structures that should be related to tectonic deformations. In addition to the faults, previously recognized, new lineaments are highlighted by the present work. They correspond to NE-, NW-, E- and N- trending faults that have controlled structuring and geometry of the basin. 2D gravity forward modeling, based on the interpretation of geophysical, geological and hydrogeological data, led to a better understanding of the basin geometry and spatial distribution of the Campanian-Maastrichtian and Cenozoic potential aquifers. Three hydrogeological sub-basins identified include the Nadhour sub-basin in the north, the El Alem sub- Basin in the South and the Etrabelsia sub-Basin in the East. These sub-basins are marked by a thickening of deposits, are separated by the Sisseb-Fadeloun raised structure of Neogene and Quaternary thinned series. The results allow the determination of limit conditions for the basin hydrodynamic evolution and explain some anomalies on the quantity and quality of the groundwater. They provide a management guide for water resources prospection in Atlassic basins in North Africa.

Contribution de la gravimétrie à l'étude de la partie sud du bassin sédimentaire côtier onshore du Bénin

Article

Full-text available

Jan 2013

Le bassin sédimentaire côtier onshore du Bénin a fait l'objet de plusieurs études qui ont permis d'avoir des précisions sur son histoire géologique. Basée sur l'analyse et l'interprétation de données gravimétriques, la présente étude gravimétrique vise à améliorer la connaissance de la structure de la partie sud du bassin sédimentaire côtier onshore du Bénin. La carte des anomalies gravimétriques résiduelles réalisée par la méthode de la moyenne mobile en utilisant la carte d'anomalie de Bouguer, met en évidence aussi bien des zones d'anomalies négatives et positives, que des zones à fort gradient. Cette carte fournit des informations sur la variation de la densité du sous-sol. Les anomalies négatives peuvent refléter l'existence de bassins, de chenaux ou de l'effondrement du socle, tandis que les anomalies positives sont associées à une surrection du socle. Dans la localité de Sèhouè, l'anomalie positive orientée N-S à NE-SW a été liée à une remontée du socle alors que l'anomalie négative caractériserait le graben d'Allada. Le calcul du gradient horizontal a permis d'identifier, à l'Ouest de l'anomalie positive de la localité de Sèhouè, un linéament gravimétrique de direction NNE-SSW. Ce linéament est matérialisé par un contraste de densité et apparait comme la signature gravimétrique de la faille de Sèhouè. L'analyse spectrale réalisée sur trois profils choisis transversalement aux anomalies dans la région d'étude, a permis d'estimer les profondeurs moyennes des sources d'anomalies à 1870, 1820 et 2300 m de l'ouest vers l'est du bassin sédimentaire côtier onshore du Bénin confirmant ainsi l'affaissement du socle dans sa partie sud est. Mots clés-gravimétrie, anomalie, bassin sédimentaire côtier du Bénin, onshore, gradient horizontal, analyse spectrale, profondeur. Contribution of gravimetry to the study of the south part of the onshore coastal sedimentary basin of Benin Abstract – The onshore coastal sedimentary basin of Benin has been an object of several studies which provide insights into his geological history. Based on gravimetric data analysis and interpretation, the present gravimetric study aims to improve knowledge of the structure of the southern part of the onshore coastal sedimentary basin of Benin. The map of residual gravimetric anomalies realized by the method of the mobile average using Bouguer anomaly map, highlight as well zones of negative and positive anomalies, as zones of strong gradient. This map provides information of the density variation of the subsurface. The negative anomalies may reflect the existence of basins, channels or basement collapse, while the positive anomalies are associated to basement uplift. In Sèhouè locality, the positive anomaly of N-S to NE-SW direction was linked to basement uplift while the negative anomaly should characterize the Allada graben. The calculation of the horizontal gradient allows the identification in the West of the positive anomaly of Sèhouè locality, a gravimetric lineament of NNE-SSW direction. This lineament is materialized by a contrast of density and seems to be the gravimetric signature of the Sèhouè fault. The spectral analysis carried out on three profiles chosen transversely to the anomalies of the study area permits to evaluate the averages of depths of the sources of anomalies at 1870, 1820 and 2300 m from west towards the east part of the onshore coastal sedimentary basin of Benin confirming thus the profundity of the basement in its southern part.

Subsurface structure of Teboursouk and El Krib plains (dome zone, northern Tunisia) by gravity analysis

Article

Mar 2016
J AFR EARTH SCI

Gravity data was used to investigate sub-surface structure of the Teboursouk and El Krib plains belonging to the dome zone in the Northwest of Tunisia. Analysis of the gravity data included the computation of the Bouguer anomaly, the horizontal and vertical gravity gradients, the upward continuations, Euler deconvolution and analytic signal of high-resolution. The Bouguer anomaly map (d = 2.4 g cm−3) has provided information on the variation of the underground density and shown contrasting anomalous zones. The treatments applied to the Bouguer anomaly map have detected new deep faults and provided details on their dips and depths (exceeding 1500 m per places).

Rectificatif to Fonctionnement hydrogéologique du bassin de la bahira (maroc central): Apport de l'analyse des données géologiques et gravimétriques (Rev. can. sci. Terre, (2014), 51, (517-526))

Article

Full-text available

Jun 2014
CAN J EARTH SCI

The quality of surface water and groundwater in the eastern Haouz and Tassaout area, Morocco

Article

Full-text available

Jan 2013

The eastern Haouz and Tassaout area of Morocco is an intra-mountain plain characterized by a semi-arid climate, where there is an increasing demand for agricultural productivity. To meet this need, local irrigation is dependent on the supply of water from surface and aquifer sources. Piezometric mapping shows that groundwater recharge is driven by anastomosis in contact with limestones outcropping in the High Atlas Mountains, and by re-infiltration of irrigation water along the upper Tassaout River. Another recharge area is located further downstream in the plain bordering the Jebilet Mountains. The hydrodynamic behaviour of the groundwater in the eastern Haouz and the Tassaout area is controlled by a divide, as evidenced by gravity anomalies. Electrical conductivity measurements and geochemical analysis have been used as a basis for mapping groundwater quality. The salinization problem is mainly caused by the presence of shales and salt Triassic deposits, whereas alternative pollution is primarily due to water contamination by nitrate leaching.

Apport des filtrages des anomalies gravimétriques à l'étude des structures profondes du Maroc oriental

Article

Jan 2005

New evidence on the geologic setting of Medjerda Valley plain (northern Tunisia) from integrated geophysical study of Triassic evaporite bodies

Article

Full-text available

Dec 2015

Integrated gravity, two dimensional (2D) seismic and field data (lithostratigraphy and tectonic deformations) help to understand the structural setting of the Triassic evaporites in the Atlas Mountains of northern Tunisia. In the Medjerda Valley plain, Triassic outcrops are bounded by NE-, ENE-, and NW-trending faults. These faults have been reactivated and have controlled the basin framework. The gravity analysis included the construction of a gravity anomaly maps, and 2D gravity model. Corresponding gravity responses of the complete Bouguer anomaly, the residual gravity anomaly, and upward continued maps reveal that the Triassic evaporites do not have a neat gravity signature. 2D seismic profiles show rising structures of Triassic strata associated with Mesozoic and Cenozoic tectonic edifices. The seismic horizons, calibrated to outcrop and well data, reveal paleohighs and diapirs of Triassic strata that were existed during the Jurassic and Early Cretaceous. These structural features induced a lateral outpouring of evaporitic strata within Late Cretaceous strata. A NW–SE-trending gravity model, which crosses the Medjerda Valley plain, highlights geometry identified on seismic lines. In the Medjerda plain, Mesozoic extensional and transtensional movements were associated with rising of Triassic evaporites, thus resulting in diapiric structures. The Late Cretaceous-Eocene structural setting was marked by outpouring of locally extruded Triassic evaporites. The Tertiary and Quaternary times are marked by major contractional events, causing inversion of pre-existing tectonic edifices. This integrated geophysical study provides a greater understanding of the Thibar deep structure, and a new geometry model of the Triassic evaporite bodies in the North Tunisia. © 2015 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved.

New evidence on the geologic setting of Medjerda Valley plain (northern Tunisia) from integrated geophysical study of Triassic evaporite bodies

Research

Full-text available

Nov 2015

New evidence on the geologic setting of Medjerda Valley plain (northern Tunisia) from integrated geophysical study of Triassic evaporite bodies

Amira et al.2015

Data

Full-text available

Nov 2015

Dynamique et gestion d'un littoral à falaises, l'exemple du Boulonnais. Evolution morphotectonique et morphodynamique à long terme dans les domaines de socle (Appalaches, Massif central, Ardenne).

Thesis

Full-text available

Dec 2005

Guillaume Pierre

Le choix du littoral boulonnais comme terrain de recherche découle initialement de la prise de conscience récente des problèmes posés, en terme de responsabilité, par le recul des falaises dans la mesure où celui-ci expose des personnes et des biens. Cette prise de conscience a rapidement fait apparaître l'absence d'études synthétiques utiles à l'élaboration d'un PPR (Plan de Prévention des Risques naturels) sur le secteur. Les valeurs de recul parfois élevées (jusqu'à 60 m en 74 ans) qui caractérisent certains secteurs urbanisés (comme Equihen) ou très fréquentés (comme le cap Blanc-Nez) des falaises du Boulonnais, ainsi d'ailleurs que les méthodes de quantification du recul employées, qui s'appuient sur l'utilisation de plans cadastraux (les premiers datent du XIXème siècle) et sur les techniques d'analyses photogrammétriques à partir de photographies aériennes (les plus anciennes sont de 1929), ont naturellement orienté l'étude de ces falaises vers la description de leur dynamique récente. L'élaboration en cours d'un PPR nécessite d'ailleurs que soit connue l'ampleur des changements susceptibles de se produire dans un futur proche (100 ans), or à cette échelle de temps, il paraît raisonnable de supposer que la caractérisation de la tendance actuelle de l'évolution des falaises puisse servir de base à la prévision de leur dynamique à venir. L'analyse et la cartographie à grande échelle des valeurs et des processus du recul ont ainsi permis d'isoler une série de paramètres statiques - combinaisons diverses entre la structure et la topographie de l'ensemble falaise-plate-forme littorale, parfois perturbées par des facteurs anthropiques - induisant une variabilité spatiale de l'évolution des falaises. L'évolution à long terme a ensuite été abordée, l'essai d'application des taux de recul contemporains à un passé plus lointain amenant de fait à s'interroger sur la situation de départ et à replacer la dynamique actuelle de ces falaises dans une histoire qui, pour l'Holocène, débute avec le Subatlantique. Enfin, la question du rythme et des causes du recul a fait l'objet d'une étude détaillée, permettant d'évaluer le rôle respectif des paramètres climatiques, météo-marins et anthropiques dans l'instabilité des falaises et la variabilité temporelle de leur évolution. La pertinence de l'approche géomorphologique et son utilité dans le domaine de la gestion du risque lié au recul des falaises ont finalement été évaluées à partir de cas concrets. L'approche naturaliste choisie pour décrire la dynamique des falaises du Boulonnais est justifiée par l'extrême diversité des modalités d'évolution rencontrées sur cette portion de côte, diversité que reflète une large gamme de valeurs de recul et qui incite à aller trouver sur le terrain le (ou les) paramètre(s) responsable(s) de cette différenciation spatiale de l'évolution. Ce choix tient aussi à la volonté de partir d'une donnée mesurée, la valeur du recul, et de l'expliquer, plutôt que de partir d'un modèle théorique d'évolution qui ne trouve pas nécessairement de traduction concrète sur le terrain et n'est donc pas d'une grande utilité pour la gestion. - (L’A.)

Gravimetry contributions to the study of the collapsed structures in northeast Tunisia: hydrogeological implications

Article

Full-text available

Jan 2013

New mapping of Kilometric Anisotropies over the Continental crust of Madagascar: Minerals Associated Economic Implications.

Article

Jun 2009

Elisa Brice Randrianasolo

The objective of this thesis is to investigate the petrologic nature and geometry of the outcropping Madagascar granulitic crust. We have created a geological database of the southern third of Madagascar (in the Madagascar Laborde 1923 coordinate system). The data used are varied: field observations, satellite images, topographic, gravity, radiometric and aeromagnetic data. The analysis, processing, comparison and integration of these different datasets (after being made compatible) have enabled us to create precise and reliable new geological maps of the region, as well as new map representations. We will highlight particular the areas of major and minor vertical shear zones (SZ), fractures, dykes and lithological boundaries. Thus, we clarify the nature of the structural-lineaments, the so-called Bongolava – Ranotsara shear zone: the ductile structure oriented 140 °N is on a regional scale due to the North SZ deflections oriented N ° 157, and the “Ranotsara” fractures (orientation 140 ° N) does not exist in terms of regional fault, but is the results of faults relays with a size ≤ 60 km long which are oriented 115 °N, 140 °N and 160 °N. SZ are the sites of localisation of anomalies rich in incompatible elements (U-Th). They are also the location of concentration of oxides of magnetite and titanomagnetite, together with the disappearance of biotite. These observations emphasize the concentration of magmas and the merger within the SZ. Thus, the SZ are focus the concentration of the magmas that provide heat by advection. In southern Madagascar, the conditions of high temperature (800 ° C) were homogenous over a wide area. We establish a new lithological map, highlighting competency contrasts: SZ grow preferentially in the aluminous orthogneiss; aluminous metasediments and ampibolo-migmatites are more resistant and form boudins at different scales; all of the granites, syenites and stratoïdes amphibolo-migmatites are the most rigid blocks. We identify kilometers of dykes: basalt, dolerites and microgranite, they're preferentially oriented at 25 °N and 120 °N. By cross-correlating our data, we show that the position of economic mineralization is controlled by the kilometer scale anisotropies (SZ, hinges folds, lithological boundaries) that localize interchange and magmatic fluids. The primary mineralization types (metamorphic or metasomatic: corundum, iron and graphite) are located within the SZ. The secondary mineralization types (uranothorianite, gold) are explained by remobilizations in the SZ. The final mineralization is controlled by the geometry of river systems and faults. On the scale of Gondwana, the geometry of Neoproterozoic major SZ controls the geometries of faults and dykes (170 Ma-90 Ma) which are oriented in 160 °N. In four regions: Satrokala, Fenoarivo, Analavoka, Lavaraty; we identified magnetic and gravity "bulls eye" type anomalies which are located at the intersection of 160 °N oriented dykes with other ones oriented 30°N, 140 °N and 170 °N; and sometimes with SZ. These points correspond to circular topographic depressions (10 m - 2.5 km in diameter) isolated or in clusters. These areas are potential kimberlite intrusions. Their presence at the intersection of kilometric anisotropies supports the rapid ascent of silicate magmas, resulting in the possible presence of diamondiferous kimberlites.

Géométrie de l'aquifère du Haouz oriental et Tassaout amont. Maroc occidental: Approche géophysique et hydrogéologique

Article

Full-text available

Dec 2014

Cette étude vise à améliorer la connaissance de la géométrie et de l’hydrogéologie de l’aquifère du Haouz Oriental et du domaine de la Tassaout Amont en se basant sur l’analyse et l’interprétation des données de forages, de gravimétrie, de sismique réflexion et de piézométrie. L’examen de la carte gravimétrique de l’anomalie de Bouguer révèle l’existence d’un gradient régional souligné par des valeurs croissantes du Sud-Est au Nord-Ouest. Ce gradient a été déterminé puis soustrait des données initiales, ce qui a permis d’obtenir une carte résiduelle sur laquelle les anomalies sont plus clairement mises en évidence. Ces anomalies reflètent en général les zones d’affleurement du socle et les variations d’épaisseur de la couverture sédimentaire à l’échelle de la zone d’étude. Le profil de la sismique réflexion et le traitement des données de forages profonds confirment les résultats de la gravimétrie et montrent que la région est formée par un horst qui correspond aux affleurements du socle des Jebilets au nord du bassin du Haouz et une zone subsidente où la couverture est très développée au Sud de la plaine du Haouz. A ce niveau, l’épaisseur importante de la couverture favorise l’accumulation des eaux et assure l’alimentation du réservoir. La remontée du socle au Nord de la plaine impose une ligne de partage des eaux souterraines avec deux sens d’écoulement.

2010 JAES HAOUZ CENTRAL

Data

Full-text available

Feb 2015

ETUDE DES EAUX SOUTERRAINES DE LA PLAINE DU HAOUZ ORIENTAL ET DU DOMAINE DE LA TASSAOUT AMONT. MAROC OCCIDENTAL. APPROCHE GEOPHYSIQUE ET HYDROCHIMIQUE

Conference Paper

Full-text available

May 2013

Major faults, salt structures and paleo-ridge at tectonic nodes in Northern Tunisia: contribution of tectonics and gravity analysis

Article

Full-text available

Nov 2014

Structural analysis, fault kinematics, detailed geological mapping and gravity analysis provide a new approach regarding to the tectonic-model of the Bazina–Aouana structure (North–Eastern Tunisia). They show that the Bazina Triassic outcrops, such as several other structures, are related to the NE and E trending major faults junction, controlling its halokinesis, and pouring out to the NW of tectonic nodes in the collapsed compartment. During the Eocene and Tortonian compressive phases, the Triassic materials previously forming a intumescence in the NE and E major faults intersection have acted as a barrier on which collided the Aouana structure. Therefore, this structure is blocked between overthrust nappes and the Bazina paleo-ridge forming an imbricated zone overlapping the Triassic materials. This paper aims to present a new evolutionary geodynamic model for the Bazina–Aouana structure related to the tectonic node easing the Triassic salt mobilization, the paleo-ridge formation and the development of a collapsed zone. By comparing with previous works, this model is likely applicable to the important Triassic outcrops of Northern Tunisia.

Apport des filtrages des anomalies gravimétriques dans la détermination des accidents tectoniques majeurs de l’Anti-Atlas (Maroc)

Conference Paper

Sep 2006

The Anti-Atlas (Morocco) gravimetric map shows significant gravimetric anomalies bordered by high gradients due to density contrast of anomalies sources. Using the horizontal gradient method associated to upward continuation of this map permit to outline several gravimetric lineaments associated with. A structural map showing the spatial organization of the major faults affecting the Anti-Atlas domain has been established. Mots clés : Gravimetry; anomalies, horizontal gradient, upward continuation, faults, Anti-Atlas ; Morocco

Apport de la gravimétrie à l'étude des structures effondrées du Nord-Est de la Tunisie: Implications hydrogéologiques

Article

Full-text available

Aug 2013

Cette étude basée sur l’analyse des données gravimétriques a pour but d’améliorer la connaissance des structures effondrées du Nord-Est de la Tunisie. La carte de l’anomalie résiduelle a d’abord été calculée à partir de la carte de l’anomalie de Bouguer en enlevant un gradient régional. La résiduelle calculée fournit des informations sur la variation de la densité dans les bassins sédimentaires de l’Oued Chafrou-Khlidia, de Mornag et de Grombalia. Afin de mettre en évidence les différentes structures géologiques (failles, bassins, etc.) dans le secteur d’étude, différentes techniques (gradient vertical, gradient horizontal, prolongement vers le haut, déconvolution d’Euler) ont été appliquées à la carte gravimétrique des structures effondrées du Nord-Est de la Tunisie. Les résultats obtenus ont permis de dresser une carte structurale présentant le système de failles responsable de la structuration de la zone d’étude. Cette carte constitue un document très utile pour orienter les recherches hydrogéologiques futures qui seront menées dans ces plaines.Editeur Z.W. KundzewiczCitation Ouerghi, S., Rebai, N., Gabtni, H., Farhat, B., et Bouaziz, S., 2013. Apport de la gravimétrie à l’étude des structures effondrées du Nord-Est de la Tunisie: implications hydrogéologiques. Hydrological Sciences Journal, 58 (6), 1361–1373.

Structural control on Ypresian karstic aquifer revealed by an integrated geophysical and geological approach: the Mateur imbricated thrust zone (Northern Tunisia)

Article

Full-text available

Jul 2023

We present the characterization of a poorly studied karstic aquifer, documenting the geometry, resistivity properties and the control of structural discontinuities affecting the outcropping Ypresian limestone in the Mateur imbricated thrust zone in Northern Tunisia. The karst system was produced by meteoric diagenesis. Geophysical methods and field investigations were useful in extracting geologic features. First, our findings suggest that the N110 to N140 directions are the preferred dissolution paths oriented parallel to strike-slip and extensional faults. Second, the gravity data analysis (regional-residual separation, horizontal gravity gradient, and Euler deconvolution) show a particular tectonic node as a junction of NE-SW and NW-SE directions. Third, the 2D electrical resistivity models exhibit the various karstification typologies with high (more than 1000 Ω m), medium (between 200 and 1000 Ω m) to low (less than 100) electrical resistivity zones. These electrical resistivity models contributed to the establishment of the conceptual structural model of the global karst features. Finally, 2D ERT and 1D time domain electromagnetic analysis highlighted the presence of potential water zones integrated into the aquifer. Thematic collection: This article is part of the Karst: Characterization, Hazards & Hydrogeology collection available at: https://www.lyellcollection.org/topic/collections/karst

Spatial Analysis of Gravity Data in the Basement of the Yaoundé-Yoko Area from the Global Gravity Model: Implication on the Sanaga Fault (South-Cameroon)

Article

Full-text available

Jan 2023

Contribution of Gravity Data for Structural Characterization of the Ifni Inlier, Western Anti-Atlas, Morocco: Hydrogeological Implications

Article

Full-text available

May 2023

The Sidi Ifni region in southwest Morocco is mainly composed of crystalline rocks with limited groundwater storage capacity. These water resources drain in particular fault zones with high fracture permeability. The main objective of this study is to describe the geological structure of the region to optimize future drilling locations. The gravity data were processed using various techniques, such as total horizontal gradient, tilt derivative, and Euler deconvolution, in conjunction with the interpretation of the geological data, to create a new structural map. This map confirms the presence of many previously identified or inferred faults and identifies significant new faults with their respective trends and depths. Analysis of this map shows that major faults are oriented NNE-SSW and NE-SW, while minor faults are oriented E-W, NW-SE, and NNW-SSE. The superposition of the hydrogeological data and the structural map reveals that the high groundwater flow values in the boreholes are located in the vicinity of the major faults and talwegs. The structures deduced from the filtering and interpretation of the gravity data suggest that the hydrogeological system of the Ifni Inlier is controlled by its structures. To confirm this impact, a high-resolution electrical resistivity map (7200 Hz) was used, with penetration depths ranging from 84 to 187 m. Negative boreholes, located in high resistivity ranges corresponding to sound basement formations without fault crossings, showed high resistivity values. The positive holes, located in anomalies with low linear resistivity, revealed the impact of fault crossings, which drain water and tend to decrease the resistivity values of the formations. Therefore, these new structural maps will assist in planning future hydrogeological studies in this area.

Gravimetric contributions to the study of the collapsed structures in northeast Tadla Plain:structural and hydrogeological implications

Article

Oct 2022

This study was realized in the north-east of the plain of Tadla, in the region of Beni Mellal-khenifra, Morocco. The main aim of this research is to establish the geometry and hydrogeology characteristics of the region's aquifer system, based on the results of analysis and interpretation of deep-drilling data, and on gravimetric and electrical survey measurements. First, the regional gradient was determined, then the initial data was extracted, and based on the extracted data, we created the gravity map of the Bouguer, where the anomalies are clearer. The examination of the Bouguer anomaly's gravity map reveals the existence of a regional gradient. These anomalies are typically found in outcrop areas of the basement and differences in thickness of the sediment cover in the study area. The Electrical survey measurements, and the interpretation of the deep-drilling data, confirm the results of gravimetry survey, after applying different techniques such as horizontal gradiometer and upward extension, on the Gravimetric map. The founded results allowed us to draw up a structural map showing the system faults, which is accountable for the study area structure. The structural map is a very useful tool to guide future hydrogeological researches in the plain of Tadla.

Mapping of major tectonic lineaments across Cameroon using potential field data

Article

Full-text available

Dec 2022
EARTH PLANETS SPACE

The cartography of lineaments across a territory can be optimized using geophysical potential field data. In this study, land gravity and EMAG2 (Earth Magnetic Anomaly Grid) data were simultaneously used to identify and characterize the major lineaments that spread across Cameroon. The data were filtered using a multi-scale approach including horizontal and vertical gradient analyses. The Euler Deconvolution method was later applied to the filtered data to estimate the extension and depth of the identified lineaments. Results show that the main lineaments across Cameroon are laterally extended with a dominant N45°E orientation. Some of these lineaments correlated well with the geographical location of some known major tectonic structures found across the country. The depth of these lineaments varies between 1 and 35 km. Some of the identified faults are still active as their location correlated with the location of some recent earthquakes that occurred in Cameroon. This work, therefore, highlights some hidden tectonic features which knowledge generally precedes exploration for subsurface resources. Graphical Abstract

DELINEATION OF LINEAMENTS IN THE REGION OF OUADDAI (CHAD) USING GRAVITY DATA

Article

Full-text available

Sep 2021

The gravity map of Ouaddai in the eastern region of Chad exhibits important anomalies that are often delimitated by the high gravity gradients, resulting from the density contrasts between various anomaly sources. This presumes an important tectonic activity in zone. Because of its arid and desert nature, the region is water-poor or even lacking water resources. The main source of water in the region is made of very deep aquifers. The gravity analysis from this study helps to better understand the network of faults in the area. Many complementary approaches of the gravity data processing have been applied, namely the horizontal gradient coupled to the upward and downward continuation and the Euler method. Results from the data filtring have allowed highlighting faults network in various directions (SSW-NNE S-N E-W, NE-SW) and the main direction. In total, 37 major faults were detected with various lengths including 11 (F6, F9, F10, F12, F17, F29, F31, F34, F35, F37) for a cumulative total length of 353 km oriented towards Bongor basin and 15 others (F1, F4, F7, F8, F11, F13, F18, F20, F23, F25, F26, F27, F28, F30, F33), for a cumulative total length of 621 km, oriented towards Doba basin. These faults are of paramount importance with great potential impacts on the regions hydrocarbon reservoirs. Thise results in one hand confirmed some known faults from the previous investigations. In the other hand, the study helps to identify other new and unknown tectonic signatures.

THE CONTRIBUTION OF REMOTE SENSING AND AEROMAGNETISM TO GOLD PROSPECTING: THE CASE OF THE MEIGANGA ZONE, CAMEROON

Article

Full-text available

May 2021

In order to optimize gold prospecting in the Meiganga zone located in the Adamaoua region of Cameroon, aeromagnetic and remote sensing prospecting was carried out in the eastern and southern parts. The remote sensing approach on a Landsat 8 OLI/TIRS image highlighted areas of maximum gold concentration. Thus, ferric ion bearing minerals are located in the North-West, silicate minerals bearing ferrous ions are in the Centre while clay minerals are in the North-East and East. The principal component analysis revealed important structural information. The PCA Spatial Map (PC1, PC2, PC3) showed the plutonic formations composed of anatexis and anatexis granites, vegetation cover (at the date of image acquisition: February 22, 2019), areas of permanent water circulation or accumulation, and metamorphic and sedimentary formations namely gneisses, quartzites, schists and superficial clay formations. A Landsat SRTM (Shuttle Radar Topography Mission) image was also used to enhance the lineaments through the Sobel filter to highlight the geomorphological (cliffs, valleys, ...) and topographic (river network, ridge and drainage segment) structures. The aeromagnetic approach was also important. The study of the modified magnetic field (CM) showed 4 ranges: very high, high, medium and low. The Total Magnetic Anomalies (TMI) of the area are subdivided into 2 ranges large positive anomalies (221.1-103.0 nT) located in the lower part of NE-SW orientation, small positive anomalies (103.0-(-)89.7 nT) located in the upper part of NE-SW orientation. The reduced total magnetic anomaly at the equator shows a fairly similar distribution to the total magnetic anomaly with the large positive anomalies in almost the entire lower part. Superimposed on the geological map, Neoproterozoic pre- to syn-tectonic granitoids (C) are superimposed on the large positive anomalies and Neoproterozoic conglomerates, quartzites, sedimentary shales and volcanosedimentary rocks (A) and Neoproterozoic syn-tectonic granitoids (B) are superimposed on the large and small positive anomalies. The grid of the reduced residual equatorial anomaly (ARRE) confirms that the local geology is strongly magnetic (gneiss and quartzite). The filters of the derivatives allowed to establish a map of magnetic lineaments of major orientation N045Ã‚Â° and minor orientation N130Ã‚Â°. The horizontal gadient superimposed on the local maxima showed the presence of deep structures oriented NE-SW. The analytical signal superimposed on the local maxima highlights the metamorphic basement consisting of rocks with strong magnetism. The application of Euler deconvolution localizes the depth of the sources of linear anomalies.

Contribution of gravity data and Sentinel-1 image for structural mapping. Case of Beni Mellal Atlas and Beni Moussa plain (Morocco).

Article

Full-text available

Jan 2018

The present work is a combined study of gravity and Sentine-1 data for fracture mapping in the karstic massif of Beni Mellal Atlas and the adjacent plain of Beni Moussa. In order to locate the various faults that contribute to the study area structuring, the gravimetric contacts analysis method, based on the joint use of the horizontal gradient and the upward continuation at different altitudes, has been applied to the gravity data. To optimize the structural mapping in the study area, the gravimetric lineaments obtained were completed and correlated with the lineaments got from Sentinel-1 image. Four faults families of NE-SW; E-O; N-S and NWSE directions have been highlighted. There fault families are perfectly combined with the studied area’s surface water network, moreover, they corroborate with the previous geological and structural studies.

Analyse des données gravimetriques des accidents longitudinaux et structures associées de la Tunisie septentrionale, modélisation des failles profondes.

Thesis

Full-text available

Dec 2017

Nesrine Frifita

New insights into the structure of Om Ali-Thelepte basin, central Tunisia, inferred from gravity data; Hydrogeological implications

Article

May 2016
J AFR EARTH SCI

This work presents new results from gravity data analyses and interpretation within the Om Ali-Thelepte (OAT) basin, central Tunisia. It focuses on the hydrogeological implication, using several qualitative and quantitative techniques such as horizontal gradient, upward continuation and Euler deconvolution on boreholes log data, seismic reflection data and electrical conductivity measurements. The structures highlighted using the filtering techniques suggest that the Miocene aquifer of OAT basin is cut by four major fault systems that trend E-W, NE-SW, NW-SE and NNE-SSW. In addition, a NW-SE gravity model established shows the geometry of the Miocene sandstone reservoir and the Upper Cretaceous limestone rocks. Moreover, the superimposition of the electrical conductivity and the structural maps indicates that the low grade mineralisation of sampled water from boreholes is located around main faults.

Duration of the marine evolution and Holocene retreat of a cliffed coast, a case study in Nothern Boulonnais (France)

Article

Sep 2007

Guillaume Pierre

The analysis of the distribution of the Subboreal peat beds formed in fresh water behind ancient coastal barriers of Atlantic age (such as the Pierrettes spit in the Calaisis), and now exposed on the foreshore, and the caracterization of a cliff top Late Glacial deposit (the Formation d'Escalles) make it possible to reconstitute, along the littoral of Boulonnais, the shoreline frame corresponding to the initial situation of the Holocene cliff retreat. Cliffs are evolving under marine condition only since the beginning of the Subatlantic. On this relatively short period, values of historical retreat established by photogrammetric analysis over 60 years should account for the recent evolution of the shoreline, whose recession is comprise between a few meters (cape Gris-Nez) and some 300 m (cape Blanc-Nez). The straightening of the shoreline is of pre-Holocene age, and the development of the fronting shore platform, which consists of contemporary and/or partially inherited or even inherited units, is described.

Gravity acquisition in Belgium and the resulting Bouguer anomaly map. 2012

Article

Full-text available

Jan 2012

The Albian stage in its type area, the Aube (France): A synthesis in a global sedimentary context

Article

May 2014

The Aube department is the type locality of the Albian stage created by d'ORBIGNY (1842). Two formations are recognised in the clay facies (the "Gault" auct.) of the stratotype, the Argiles tégulines de Courcelles (82 m), which is overlain by the Marnes de Brienne (43 m). The boundary between the two formations is defined at the top of an indurated bed (hardground L'Étape) that is readily identifiable, both in the field, and from boreholes. The type area of the Albian stage is of great interest because of the size of the clay facies (more than 120 m) unique in the Anglo-Paris basin. Today, 82 % of the lithological succession is documented accurately from 16 outcrops. In two intervals of respectively 33 m and 28.50 m thick, composite sections are made of overlapping sections which are correlated in the field with the help of lithological marker beds. Six successive facies are described: in the Argiles tégulines de Courcelles and from bottom to top: 1 - phosphatic nodules bed; 2 - silty clays; 3 - clays and limestone beds; in the Marnes de Brienne: 4 - clay marls; 5 - gaize; 6 - clay marls. On a smaller scale, a decimetre to metre-scale rhythmicity can be identified in the sedimentation. This is reflected in the Argiles tégulines de Courcelles by pluri-decimetric cycles that start with a silty level and are bounded at the top by a bored surface. In the Marnes de Brienne, the metric cycles (except for the gaize where they are decimetric) exhibit alternating dark grey clay marl and a pale grey marl that herald the rhythmic chalks of the Cenomanian. Individual couplets probably represent the precession cycles (20 kyr). On the other hand, 13 lithoevents as phosphatic nodules beds, hardgrounds, glauconitic and sandy beds and ecoevents characterised by the brief abundance of some fossils. These marker beds are very useful for fine correlations across the basin. A comparison is proposed between sandy units of Yonne and clay facies of Aube. The continuity of several phosphatic nodules beds is shown from Yonne in the South West to Aube and Perthois in the North East, that is a distance exceeding 130 km. These marker beds are interpreted as Flooding Surfaces of 3rd-order cycles. Finally, a detailed study of the lithology, complemented with the analysis of the ammonite and inoceramid faunas, is used for a depositional sequence interpretation. As a result, eight sequences are identified in the upper half of the Argiles tégulines de Courcelles and in the Marnes de Brienne in the type area of the Albian stage, including two new sequences indexed AL 5a and AL 6a.

Estudios Geologicos Najib2-2010

Data

Full-text available

Feb 2015

Fault overlap zones within developing normal fault systems

Article

Full-text available

Jun 1995

Overlap zones between normal faults have been studied using a variety of 2D and 3D seismic reflection datasets. The overlaps are of two types, (i) relay zones in which displacement is transferred between the overlapping faults and (ii) non-relay overlaps in which displacement is not transferred. Overlap zones are continually formed and destroyed during the growth of a fault system. Overlap zones are formed either by interference between initially isolated faults or as a result of bifurcation of a single fault. The mode of overlap formation is reflected in the 3D geometry of the overlapping faults which may be either unconnected or linked at a branch-line or branch-point. Seismic reflection data from regions of growth faulting, and also sandbox analogue data, allow analysis of fault development through time. Reconstructions of the displacement distribution on some faults with sharp bends and associated hanging-wall splays, show that the bends originated as overlap zones which were later breached to form through-going faults. Depending on the displacements of relay-bounding faults, the effect of relay zones on hydrocarbon reservoirs may be to (a) provide structural closure, (b) form gaps in otherwise sealing faults or (c) increase reservoir connectivity.

Palaeomagnetic dating of brittle tectonics structures: Case studies on Ferques fault (Boulonnais, France) and two faults from the Holy Cross Mountains (Poland)

Article

Full-text available

Oct 1999

Calcite and sedimentary fills in fractures cutting the Upper Devonian carbonates in the Holy Cross Mountains (HCM) and Boulonnais (B) were dated palaeomagnetically by comparison with the Apparent Polar Wander Path (APWP) for Baltica in order to constrain age of faults. Here we present results obtained from calcite and sedimentary fills of two selected faults as case studies. Hematite-bearing elastics and calcite possessed well-defined components of natural remanent magnetization (NRM) that were preserved up to thermal demagnetization at ca. 500 °C. Two components in Rzepka calcite (HCM) and one component residing in elastics within Ferques Fault (B) could be determined. Comparison of directions of these components with reference directions calculated from the APWP for Baltica constrains the upper age limit for the Ferques Fault to Middle Triassic, while Rzepka and Jazwica faults may be no younger than Late Permian. Generally, the remanence from fracture fills may be useful for dating related tectonics, karst phenomena or mineralization processes.

Displacement gradient on fault surfaces

Article

Full-text available

Dec 1989
J STRUCT GEOL

The maximum displacements and the dimensions of single faults are systematically related and the variation in displacement, from a maximum at the centre of a fault to zero at an elliptical tip-line loop, is described by a simple theoretically derived expression. These relationships are used to examine the theoretical ranges and distributions of rates of change of displacement on fault surfaces. The dimensions and maximum displacement of a fault can be estimated from a limited number of displacement gradient measurements. Estimates are further constrained by a knowledge of the effective shear modulus of the rocks containing a fault. An understanding of displacement gradients can be applied to problems commonly encountered in mining operations and in the interpretation and use of seismic reflection data, in addition to field problems.A comparison of theoretical displacement gradients with measured displacement gradients from British Coalfield faults shows that (i) fault surface ellipticities are between 1.5 and 2.5 and (ii) gross displacement gradients, expressed as the ratio of maximum displacement/long axis radius of the fault ellipse, range from 0.02 to 0.002. The measured displacement gradients indicate that the shear moduli of the faulted rocks range from 3 to 11 GPa, which is within the known range for Coal Measure rocks.Displacement gradients along an array of fault segments are similar to those of single faults. For analysis of displacement gradients it is necessary to sum the discontinuous and continuous components of displacement; in extreme cases all the displacement may be accommodated by continuous deformation.

Modeling of bed contours and cross-sections adjacent to planar normal faults

Article

Full-text available

Dec 1989
J STRUCT GEOL

The geometry of beds in the volume surrounding a single blind planar fault depends on: (i) the axial ratio of the fault surface ellipse, (ii) the ratio between the maximum displacement and the maximum dimension of the fault surface, (iii) the displacement distribution on the fault surface from the point of maximum displacement to zero displacement at the tip-line loop (fault surface displacement profile) and (iv) the rate at which the displacement decreases to zero along a normal to the fault surface (reverse drag displacement profile). These relationships, which are either known or can be estimated, are incorporated in a computer model which generates structural contours on horizons intersected by a planar fault. Structural contour patterns also vary with the pre-faulting dip and strike of beds, the dip and strike of the fault and the level at which a bed intersects the fault surface. When the boundary conditions are relaxed to include synsedimentary faults, the ratio of hangingwall to footwall displacement varies systematically with fault dip.A synthetic structural contour map is matched with that of an actual fault. Model isopachs, based on simple depositional models, are generated for synsedimentary units. Cross-sections are generated for synsedimentary faults by sequential fault growth and sedimentary deposition, and show features similar to actual cross-sections. The soft-domino model is applicable primarily to dip-slip tectonic faults with maximum displacements of up to about 5 km; listric faults are not considered.

The Place of the Vertical Gradient in Gravitational Interpretations

Article

Jan 1936

H. M. Evjen

Aspects of basin inversion in southern Britain

Article

Apr 1993

R. A. CHADWICK

The Wessex Basin of southern England, a Permian to Cretaceous extensional basin, can be structurally divided into a set of constituent asymmetrical graben, bounded by major E-W-trending zones of en echelon syn-depositional normal faults. The graben were inverted in late Cretaceous and Tertiary times by compressive stresses oriented roughly north-south. Inversion structures fall into two related categories. Regional upwarps overlie earlier graben depocentres and were formed by bulk shortening of the graben-fill. Superimposed upon and geographically delimiting the regional upwarps, are roughly east-west trending linear zones of en echelon inversion structures. These coincide with the earlier graben-bounding faults and typically have the form of monoclinal or periclinal flexures each underlain by a partially reversed normal fault. The linear reverse fault/monocline inversion structures were a relatively inefficient method of basin shortening. Because upper crustal faults in the region steepen upward markedly, reversal of these faults under compression resulted in a shortening discrepancy at shallow depths. Locally, particularly in south Dorset, this was overcome by buckle-folding and low-angle reverse faulting which increased the amount of shortening attainable across the linear inversion structures. Elsewhere however, the shortening discrepancy was accommodated by bulk shortening and regional upwarp of the graben sedimentary-fill. This occurred preferentially in those graben containing young, poorly lithified and therefore weak sediments. Thus the early Cretaceous depocentres of the Weald and Channel basins were strongly upwarped, with axial uplifts of over 1000 m. Conversely, graben having older, more lithified sequences suffered little regional upwarp, shortening primarily by fault reversal along the linear inversion structures. The amount of crustal shortening which accompanied inversion was considerably less than the earlier crustal extension.

Extension tectonics in the Wessex Basin, Southern England

Article

May 1986

R. A. CHADWICK

The Permian to Cretaceous tectonic evolution of the Wessex Basin was controlled by horizontal tensional and vertical isostatic forces within the lithosphere. The gross morphologies of its constituent structures were governed by the location of Variscan thrust and wrench faults in the upper and middle crust, which suffered extensional reactivation in tensional stress fields oriented approximately NW-SE. Several episodes of crustal extension can be resolved, in early Permian, early Triassic, early Jurassic and late Jurassic/early Cretaceous times. These were characterized by the rapid subsidence of fault-bounded basins and commonly, by erosion of adjacent upfaulted blocks. Superimposed upon the fault-controlled subsidence, dominant during periods of fault quiescence, and becoming increasingly important with time, a component of regional subsidence is considered to have a thermal origin. This suggests that crustal extension was accompanied by some form of, not necessarily uniform, lithospheric thinning. Subsidence analyses assuming local Airey isostasy give cumulative crustal extension factors of 20–28% beneath the grabens. A more reasonable assumption of regional Airey compensation indicates basinwide crustal extension of 13–17%. which is consistent with BIRPS offshore deep seismic reflection data.

Structural analysis of the adjacent Acadian and Variscan fold belts in Belgium and northern France from geophysical and geological evidence

Article

Aug 1999
TECTONOPHYSICS

Two Palaeozoic fold belts are juxtaposed in Belgium and northern France. The northern belt of the Brabant Massif, traditionally considered a part of the European Caledonides, was folded during the Acadian orogeny in the early Devonian. Although it is largely concealed, gravity and magnetic maps give a general picture of the existing structures. A curved gravity trend is interpreted as an arc-shaped succession of granitic batholiths. Structures north and south of this arc are completely different. The northern subdomain, containing magnetic lower Cambrian metasediments, shows multiple thrust faults towards the south over the granites, whereas the southern subdomain, without magnetic rocks, is moulded around the granitic core. The southern Variscan belt is characterized in general by thrusting with a north, NW and NE vergence. During the Variscan orogeny the Brabant Massif acted as an obstacle to the general transport of the Ardennes Massif to the N–NW. The oblique convergence with the Brabant Massif caused differential displacement towards the northwest, the western areas having travelled farther, as seen from a pattern of WNW-oriented strike-slip faults.

Locating edges of source bodies for magnetic or gravity anomalies

Article

Jul 1986

Cordell and Grauch (1982, 1985) discussed a technique to estimate the location of abrupt lateral changes in magnetization or mass density of upper crustal rocks. The final step of their procedure is to identify maxima on a contoured map of horizontal gradient magnitudes. Attempts to automate their final step. The method begins with gridded magnetic or gravity anomaly data and produces a plan view of inferred boundaries of magnetic or gravity sources. The method applies to both local surveys and to continent-wide compilations of magnetic and gravity data.-from Authors

The structural development of the Wessex Basin

Article

Aug 1982

R. Stoneley

The Mesozoic and Cenozoic structures of S England form part of a system of disturbances which extends across the Channel into N France. They are reviewed in the light of published data from S Dorset. Basin development started with Permo-Triassic rifting, and is believed to have continued through the Jurassic and early Cretaceous with intermittent growth on deep-seated listric normal faults, probably associated with roll-over anticlines. This regime had ceased by the Aptian and the basin passed through a period of relative stability in the late Cretaceous, followed by tectonic inversion and, in the Miocene, by N compressive movement along the former normal faults. This partially restored the earlier separative displacement at depth, and probably accentuated pre-existing roll-over anticlines: it was expressed in the surficial Upper Cretaceous-Palaeogene sequence as monoclinal flexuring. The early rifting and the change from overall N-S crustal stretching to compression may reflect plate tectonic events in the N Atlantic region. -Author

The 'Basse-Normandie' duplex (Boulonnais, N France): Evidence for an out-of-sequence thrusting overprint

Article

Jan 1998
J STRUCT GEOL

The thrust-sheets outcropping in the ‘Basse-Normandie’ quarry (near the Hydrequent village, Boulonnais, N France) represents an exceptionally well exposed section of the NW European Variscan thrust front. These structures, developed in the footwall of the main Hydrequent thrust, have been often described as a classic example of a duplex structure. Only the lower part of the structure satisfies, however, the geometric and kinematic criteria defining a duplex (and more precisely an intraformational hinterland dipping duplex).The upper thrust-sheets of the imbricate stack exhibit a much more complex pattern of deformation than a simple piggy-back duplication of the same rock sequence. Restoration of these thrust-sheets (based upon the definition of two marker beds as well as the analyses of fold—thrust relationships and strain markers) argues for a late NE verging thrust event that progressed within the thrust system from the tip of the upper thrust sheets towards the Hydrequent thrust in a local break-back style of thrust propagation. This out-of-sequence thrusting event induced refolding and cross-cutting of the forelimb of a hangingwall anticline developed previously above the footwall ramp of the NNE verging basal thrust of the structure. Within the whole thrust system, the lower duplex represents only a minor structure developed during the initial phase of thrusting in the foreland of the major anticline as a frontal second-order duplex.The structural data presented in this paper illustrate the tectonic processes acting within the deformed zones lying in the footwall of major thrusts and emphasize the out-of-sequence style of thrust migration that arises from the sequential blocking of thrust propagation towards the foreland.

Le filtrage des anomalies gravimetriques; une cle pour la comprehension des structures tectoniques du Boulonnais et de l'Artois (France)

Abstract

No full-text available

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