TS11.1

The Zagros fold-and-thrust belt in Southern Iran is famous for its spectacular outcrops of salt diapirs. Most of these diapirs already existed prior to the onset of the Zagros orogeny, but tectonic shortening caused their reactivation and extrusion of the salt. Thus, the diapir exposures often provide access to intense internal deformation of the Hormuz salt series and its entrained interlayers. However, highly soluble evaporites (mainly halite) were already dissolved in many of the exposures leaving behind degraded ‘caprock’, which is built of a multi-compositional residuum of less soluble minerals and rocks. Based on geological field studies on two iconic salt diapirs in Southern Iran, the Karmostaj (Gach) and the Siah Taq diapir, we ascertained that the caprock is also intensively deformed. The accessible part of the caprock is roughly 200 m thick and consists of a fine-grained, laminated gypsum containing fragments of brecciated carbonates and siliciclastics.  Especially in the down- and mid-slope regions of the salt exposure, this mixture is sheared and folded, but also dissected by thrust faults. Since such deformation processes in the caprock were not described before, there is a lack in explanations for the timing, the depth of formation and the structural evolution of these structures. For instance, it is unclear if the ductile shearing of the relatively competent gypsum matrix and the brecciation of the clasts took place near the surface or in larger depths (a few hundreds of meters), where confining pressure is higher.

In this study, we want to classify the observed structures in the caprock, characterize deformation mechanisms and differentiate typical deformation domains. Based on that, we speculate about the timing and structural evolution of the caprock deformation and suggest that three scenarios can be imagined: (1) Pre-extrusion deformation: The caprock exposed today was buried by a thicker caprock package and, therefore, is compacted and mechanically strong.  With the onset of the Zagros orogeny, tectonic shortening of the buried diapir caused lateral deformation before the salt extrusion. (2) Syn-extrusion deformation: The caprock is relatively young and was mechanically weak after its formation. Thus, it was deformed during diapir extrusion and, then, solidified during degradation of the salt. (3) Post-extrusion deformation: The caprock was mainly formed after salt extrusion, but it remained relatively immobile. The caprock matrix is occasionally weakened by the infiltration of meteoric water, and continued to be deformed due to gravitational gliding even after the dissolution of the rock salt.  In order to test these hypotheses, we intend to carry out analogue experiments in which we try to model a squeezed diapir. In a parameter study, the thickness and the material of the covering layer simulating the caprock will be varied to assess possible differences in the deformation patterns.

How to cite: Závada, P., Bruthans, J., Adineh, S., Warsitzka, M., and Zare, M.: Intense deformation of the caprock on salt extrusions in the Iranian Zagros Mountains – Insights from geological mapping and analogue modeling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14873, https://doi.org/10.5194/egusphere-egu21-14873, 2021.

The external Alps record a whole Wilson cycle that began at early Mesozoïc times by an extensional phase leading to the deposition of thick marine deposits upon an upper Triassic basement including a thick salt layer. Several diapiric structures (e.g. Astoin, the Barre de Chine ; Célini et al., 2020) are the witnesses of this important salt activity during deposition and the subsequent deformation through the Lower Jurassic. Otherwise, Triassic salt allowed thrusting on several decollement levels and emplacement of major thrusted units, such as the “Nappe de Digne” or the Authon thrust sheet, during the alpine phase s.s, initiated at the Oligocene-Miocene boundary. Between these two periods, the external Alps story is more uncertain and none salt activity has been clearly demonstrated except westwards in the Vocontian basin. In the whole South-East basin, only few clues, as bipyramidal quartz found in Priabonian deposits in the western Baronnies suggest a potential salt activity at surface during the Paleogene. However, in the St-Geniez areas, some Oligocene sediments, located at the vicinity of salt structures suggest a potential diapiric growth during this period. Indeed, some stratigraphic gypsum beds are found in an Oligocene lacustrine series, directly thrusted by the Authon thrust sheet.  None evaporite environments are described in the whole region at Oligocene times, which suggest a possible recycling of Triassic evaporites.

In order to determine if theses deposits are related to a Paleogene salt activity, a multi-analytical approach was used. First, a field study allowed characterizing the facies and the sedimentary filling and defining the stress regime during the deposit, by kinematic inversion on fractures which indicates a constant N-S compression during the Oligocene. The presence of halophilic fauna at the base of the lacustrine series of the St-Geniez area attests for saline influences during deposit. Moreover, 4km to the SW, a wedge in the conglomerates of the alpine continental molasse (so called red molasse) resting directly on Sorine’s Triassic diapir was put forward. Cargneules and dolomites from the Triassic constitute an important part of the reworked material. These observations indicate that the Sorine's diapir was active during the deposition of the Oligocene series. Then, a precise chemostratigraphic framework was determined by use of δ¹³C and δ¹⁸O isotopic data on the lacustrine limestones. ⁸⁷Sr/⁸⁶Sr isotopic ratio on gypsum beds of the lacustrine series aimed at determining their ages and a possible Triassic evaporite sourcing. Our results gave an age ranging from 6 to 23 Ma, which does not correspond with the Oligocene age of the overlying and underlying sediments. Moreover, the large variation in isotope ratios suggests that this gypsum did not come from primary precipitation but from leaching of a pre-existing evaporite source. In conclusion, field observations, together with geochemical analyses, made it possible to highlight the relationships between tectonics, salt tectonics and sedimentation and also to reconstruct the paleogeography of the region at the end of the Paleogene.

References

Célini, N., Callot, J.P., Ringenbach, J.C., Graham, R. (2020,). Jurassic salt tectonics in the SW sub-Alpine fold and thrust belt. Tectonics

How to cite: Hamon, A., Mehl, C., Huyghe, D., Révillon, S., and Callot, J.-P.: Salt activity and diapirism during the Paleogene in the Baronnies Orientales (South-East basin, France) : paleogeographic and structural implications., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15420, https://doi.org/10.5194/egusphere-egu21-15420, 2021.

Dissolution of halite rock can significantly impact underground constructions (e.g., caverns for energy storage and abandoned caverns) and above ground constructions (e.g., highways and buildings) potentially causing a threat to human life from land subsidence and sinkhole hazards, instability to underground construction and pollutant release. In this work, we explore and quantify changes in elastic and hydromechanical properties during dissolution of halite rock by migration of water.

We evaluated the impact of dissolution on the geophysical properties of pristine (non-fractured) and fractured halite samples (with ~2.7% dolomite), using a synthetic (seawater-like) brine solution (3.5wt% NaCl). The dissolution test commenced by setting an initial effective pressure of 15 MPa (with minimum pore pressure of 0.1 MPa), equivalent to a depth of ~720 m below ground level. This confining pressure of 14.9 MPa ensured the adequate contact between sample and the ultrasonic instrumentation (P- and S-wave sensors), and the set of electrodes for electrical resistivity. The test procedure was set to investigate the effect of increasing pore pressure from 0.1 to 14 MPa on dissolution. This procedure was only successful for the non-fractured sample, as dissolution rapidly occurred in the fractured sample during the initial stage of the test.

The non-fractured halite shows that P-wave velocity increases with increasing inlet pore pressure initially, followed by a lower pore fluid sensitivity stage. After this stage, the P-wave and the Vp/Vs ratio reduce and then ultrasonic velocities tend to their original values when effective pressure tends to zero. These results suggest that capillary pressure effects are initially increasing the bulk properties of the rock by filling the micro-pores, while dissolution is occurring locally, nearby the inlet-flow port, and therefore invisible to our geophysical tools. The small porosity fraction of 1.1% allows the saturating fluid to rapidly equilibrate with the surrounding halite within the pores, slowing down the dissolution process. In a close halite system with a local and continuous brine supply source, local dissolution may allow pressure increase up to the overburden stress and affect the geomechanical integrity of the reservoir by a combined fracturing-dissolution process.

How to cite: Dale, M. S., Falcon-Suarez, I., and Marín-Moreno, H.: Geophysical response to dissolution of undisturbed and fractured evaporite rock during brine flow, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15572, https://doi.org/10.5194/egusphere-egu21-15572, 2021.

Salt and related structures have a strong influence on the formation of extensional basins during lithospheric stretching and thermal subsidence at rifted margins. Salt significantly influences as well the structural styles and kinematics of fold-and-thrust belts. We aim to characterize the structure of inverted minibasins and salt-influenced fold-and-thrust belts, but the challenge is to understand, and to match, the present day contractional structures with reasonable pre-orogenic configurations. Yet, we still lack proper understanding on the development of these salt-sediment systems and particularly, how salt tectonics is initially triggered and evolves through space and time. Two fundamental triggering mechanisms on rift to passive margin salt tectonics are known: (1) extension by gravitational collapse, and (2) differential loading. Key questions are: do these mechanisms occur at the same time or does one commonly follow the other? Which one is first and which one dominates? Does it depend on the location and timing of deformation on the passive margin? Which are the stratigraphic evidences and structural geometries that may help us to answer these questions? Recognizing the initial structural geometries of these minibasins once they have been incorporated into a fold and thrust belt is challenging but of paramount importance.

In this contribution we address some of these questions by showing a brief historical review of concepts and show end-member analogue models of fold-and-thrust belts developed from the inversion and incorporation of rift to passive margin salt basins. Our work is inspired by field observations from the Pyrenees and the Northern Calcareous Alps, as well as from present day continental margins.

How to cite: Granado, P., Santolaria, P., Wilson, E., Ferrer, O., and Muñoz, J. A.: SALT: from rifted margins to fold-and-thrust belts. Insights from analogue modelling and case studies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3240, https://doi.org/10.5194/egusphere-egu21-3240, 2021.

Salt-bearing passive margin basins offshore SE Brazil have been and remain attractive for hydrocarbon exploration and production. In the Campos Basin, major reservoir types include post-salt turbidites, which are located in structural traps related to thin-skinned faulting above salt anticlines and rollers. Classic models of gravity-driven salt tectonics commonly depict kinematically linked zones of deformation, characterised by updip extension and downdip contraction, separated by a weakly deformed zone associated with downdip translation above a relatively smooth base-salt surface. We use 2D and 3D seismic reflection and borehole data from the south-central Campos Basin to show that this does not adequately capture the styles of salt-detached gravity-driven deformation above relict, rift-related relief. The base-salt surface is composed of elongated, broadly seaward-dipping ramps with structural relief reaching c. 2 km. These ramps define the boundary between the External High and the External Low, basement structures related to the rift tectonics. Local deformation associated with the base-salt ramps can overprint and/or influence regional, margin-scale patterns of deformation producing kinematically-variable and multiphase salt deformation. We define three domains of thin-skinned deformation: an updip extensional domain, subdivided into subdomains E1 and E2, an intermediate multiphase domain and a downdip contractional domain. The multiphase domain is composed of three types of salt structures with a hybrid extensional-contractional origin and evolution. These are: (i) contractional anticlines that were subjected to later extension and normal faulting; (ii) diapirs with passive and active growth later subjected to regional extension, developing landward-dipping normal faults on the landward flank; and, lastly, (iii) an extensional diapir that was subsequently squeezed. We argue that this multiphase style of deformation occurs as a consequence of base-salt geometry and relief creating local variations of salt flow that localize extension at the top and along the ramps, and contraction at the base. Translation and extension of salt and its overburden across major base-salt ramps resulted in three ramp syncline basins northeast of the study area, partially bounded by salt-detached listric faults. The temporal and spatial distribution and evolution of these and other key salt and overburden structures, and their relationship to base-salt relief, suggest 30 to 60 km of horizontal gravity-driven translation of salt and overburden.

How to cite: B. Amarante, F., A-L. Jackson, C., M. Pichel, L., M. S. Scherer, C., and Kuchle, J.: Pre-salt rift morphology controls salt tectonics in the Campos Basin, offshore SE Brazil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3263, https://doi.org/10.5194/egusphere-egu21-3263, 2021.

Abstract

The North Sea is a complex rift system that has undergone a polyphase evolutionary history from the Palaeozoic to Recent, including the deposition, and subsequent mobilisation of Upper Permian Zechstein salt. This halokinesis has played an integral role in the geologic evolution of the North Sea, controlling the present-day structural style. The driving mechanisms and kinematics of salt deformation have gained widespread interest partly due to the potential role of salt in hydrocarbon systems, and also due to its potential uses for nuclear waste disposal. However, the primary driving mechanism for salt-related deformation in the North Sea is debated. Here, we focus on the Mid-North Sea High (MNSH), an area of the North Sea in which salt-related deformation is widespread. We interpret open access data made available by United Kingdom Oil and Gas Authority (OGA) including 2D seismic reflection, gravity, magnetic and well data in Petrel, followed by forward modeling and restoration in the MOVE software. The results show that, the style of salt-related deformation in the MNSH region is highly variable, with the influence of local stratigraphy, as well as basement structures, also contributing to the deformation style.

Keywords: Salt tectonics, Halokinesis, North Sea, Mid North Sea High

How to cite: Nnadi, C. and Peace, A.: Reconstruction of Salt Tectonics: Insights from the Mid North Sea High, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8999, https://doi.org/10.5194/egusphere-egu21-8999, 2021.

The Sahel basin in eastern Tunisia has been subject for hydrocarbon exploration since the early fifties. Despite the presence of a working petroleum system in the area, most of the drilled wells were dry or encountered oil shows that failed to give commercial flow rates. A better understanding of the tectono-sedimentary evolution of the Sahel basin is of great importance for future hydrocarbon prospectivity. In this contribution, we present integration of 2D seismic reflection profiles, exploration wells and new acquired gravity data. These subsurface data reveal that the Sahel basin developed as a passive margin during Jurassic-Early Cretaceous times and was later inverted during the Cenozoic Alpine orogeny. The occurrence of Triassic age evaporites and shales deposited during the Pangea breakup played a fundamental role in the structural style and tectono-sedimentary evolution of the study area. Seismic and gravity data revealed jointly important deep-seated extensional faults, almost along E-W and few along NNE–SSW and NW-SE directions, delimiting horsts and grabens structures. These syn-rift extensional faults controlled deposition, facies distribution and thicknesses of the Jurassic and Early cretaceous series. Most of these inherited deep-seated normal and transform faults are ornamented by different types of salt-related structures. The first phase of salt rising was initiated mainly along these syn-extensional faults in the Late Jurassic forming salt domes and continued into the Early and Late Cretaceous leading to salt-related diapir structures. During this period, the salt diapirism was accompanied by the development of salt withdrawal minibasins, characterized important growth strata due the differential subsidence. These areas represent important immediate kitchen areas to the salt-related structures. The later Late Cretaceous - Cenozoic shortening phases induced preferential rejuvenation of the diapiric structures and led to the inversion of former graben/half-graben structures and ultimately to vertical salt welds along salt ridges. These salt structures represent key elements that remains largely undrilled in the Sahel basin. Our results improve the understanding of salt growth in eastern Tunisia and consequently greatly impact the hydrocarbon prospectivity in the area.

How to cite: Belkhiria, W., Boussiga, H., Hamdi Nasr, I., Amiri, A., and Inoubli, M. H.: Interplay between tectonic inheritance and salt tectonics in the tectono-sedimentary evolution of the Sahel basin (eastern Tunisia): implications for hydrocarbon prospectivity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13593, https://doi.org/10.5194/egusphere-egu21-13593, 2021.

Salt tectonics at salt-bearing margins is often interpreted as the combination of gravity spreading and gravity gliding, mainly driven by differential sedimentary loading and margin tilting, respectively. Nevertheless, in the Western Mediterranean Sea, the classical salt tectonics models are incoherent with its morpho-structural setting: the Messinian salt was deposited in a closed system, formed several Ma before the deposition, horizontally in the entire deep basins, above a homogenous multi-kilometre pre-Messinian thickness. The subsidence is purely vertical in the deep basin, implying a regional constant initial salt thickness, the post-salt overburden is homogenous and the distal salt deformation occurred before the mid-lower slope normal faults activation. Instead, the compilation of MCS and wide-angle seismic data highlighted a clear coincidence between crustal segmentation and salt morphology domains. The geometrical variation of salt structures seems to be related to the underlying crustal nature segmentation. Regional thermal anomalies and/or fluid escapes, associated with the exhumation phase, or the mantle heat segmentation, could therefore play a role in adding a further component on the already known salt tectonics mechanisms. The compilation of crustal segmentation and salt morphologies in different salt-bearing margins, such as the Santos, Angolan, Gulf of Mexico and Morocco-Nova Scotia margins, seems to depict the same coincidence. In view of what is observed in Western Mediterranean Sea, the heat segmentation influence in the passive margins should not be overlooked and deserves further investigation.

How to cite: Bellucci, M., Aslanian, D., Moulin, M., Rabineau, M., Leroux, E., Pellen, R., Poort, J., Del Ben, A., Gorini, C., and Camerlenghi, A.: Salt morphologies and crustal segmentation relationship: new insights from Western Mediterranean Sea and other salt passive margins, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4182, https://doi.org/10.5194/egusphere-egu21-4182, 2021.

This study presents the interpretation of reprocessed seismic data covering the southwestern Balearic promontory and the central Algerian basin. The new depth processing of 2D seismic lines dataset allows for the first time a good resolution on salt structures in the deep basin. Most of the salt structures result from active diapirism. In the deep basin, sedimentary loads and regional shortening are proposed to be the dominant driving forces, showing an overall contractional salt system. The north Algerian margin tectonic reactivation could have provoked a regional shortening of the salt structures and overburden. Identified unconformities suggest that this process probably started shortly after salt deposition and is still active nowadays. It is expressed by salt sheets, pinched diapirs and a décollement level. The African convergence and the narrowness of the western Algerian basin could be the explanation of an overall greater salt deformation intensity compared to the eastern Algerian basin. This demonstrates how in tectonic and sedimentary components appear to be dominant in salt deformation in the central Algerian basin compared to gravitational gliding, only localized in the proximal parts of the margin.

How to cite: Blondel, S., Camerlenghi, A., Del Ben, A., and Bellucci, M.: Late Miocene to present-day tectonostratigraphy of the northern central Algerian Basin: Evidence of a contractional salt system from reprocessed seismic data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9640, https://doi.org/10.5194/egusphere-egu21-9640, 2021.

The Algerian margin, located in the Western Mediterranean basin, is reactivated in compression since 8 My due to the convergence between Africa and Eurasia, and is nowadays subjected to a N45W compression of several mm/y (Jolivet et al., 1995; Noquet et Calais, 2004). While the reactivation is attested by GPS measurements and destructive seismic events, such as the earthquake of Boumerdes in 2003 (M 6.8), the visualization in the seismic data of the deep structures is made difficult by the presence of a thick Messinian salt layer. The seismic reflection profiles acquired on the Algerian margin during the “Maradja I” oceanographic survey (2003) highlighted the presence of north-verging thrusts offshore Algiers (Déverchère et al., 2005; Domzig et al., 2006), as well as the peculiar geometry of the Messinian salt layer (Lofi et al., 2011, Obone Zue Obame, 2011).

Between 2 and 4° East, the margin presents particularly complex salt structures, partly associated to the uplift of the plateau as a consequence of the crustal convergence (Déverchère et al., 2005; Domzig et al., 2006). One of the consequences of the uplift of the plateau is the dipping of the base salt horizon towards W to NNW. Moreover, from the analysis of the seismic reflection profiles, the presence of early (syn-UU) salt movement in the profiles parallel to the margin is clear, while the profiles perpendicular to the margin show compressional features mostly active during the Pliocene to Quaternary period.

From the observation of the natural example, and from the comparison with different analogue models, we conclude that offshore Algiers we find the major salt structures and minibasins formed through salt spreading, while the area offshore Boumerdès is characterized by gravity gliding due to the uplifted plateau. Although from this point of view the N-S compressional tectonics favors gravity gliding through the plateau uplift, on the other hand it influences the salt structure development direction, which present a mainly E-W development and a minor and delayed N-S one. A partial influence of the sedimentary body from Algerian rivers on the position of the major salt structures is inferred.

How to cite: Travan, G., Gaullier, V., Vendeville, B., Déverchère, J., Raad, F., and Lofi, J.: Gravity gliding and spreading in a compressional setting: the example of the Algerian margin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11948, https://doi.org/10.5194/egusphere-egu21-11948, 2021.