Triassic Evaporites Research Articles

Modes of Deformation in Sedimentary Multilayers Interacting With Sub‐Décollement Structures: Insights From 3D Seismic Interpretation in the Eastern Jura Fold and Thrust Belt

AbstractThe Jura fold and thrust belt is characterized by dominant thin‐skinned thrusting and variable interaction with inherited structures and deep‐seated (i.e., sub‐décollement) faults. Using 3D seismic data, the styles of deformation and the relationships with inherited structures at the eastern termination of the fold and thrust belt are documented in an area located NW of the city of Zürich. The investigated seismic volume includes two major, ENE trending fold‐thrust zones involving the Triassic‐Jurassic epicontinental platform succession and the stratigraphically overlying Cenozoic Molasse Basin deposits. The location of both fold‐thrust zones appears to be controlled by variable interaction between the décollement level hosted in the Triassic evaporites of the Muschelkalk Group and faults linked to an underlying Permian‐Carboniferous graben. Different modes of interaction of the Mesozoic multilayer (including the weak evaporite level close to its base) and underlying upper Paleozoic basin fill with inherited faults produced a marked contrast in structural style (pop‐up and triangle zone geometry) between the N and S fold‐thrust zones. We suggest that the different types of structures provide an unprecedented record of the transition from thin‐skinned thrusting (dominant in the N fold‐thrust zone) to deeply rooted deformation controlled by the inherited structural architecture of the upper Paleozoic basin. Such a transition was characterized by focusing of strain from the upper Paleozoic section to a narrow belt of Mesozoic strata, triggering coeval buckling and reverse faulting (manifested in the S fold‐thrust zone). Propagation of sub‐décollement faults and segment linkage eventually led to breaching of the regional detachment.

Coupling of X-ray fluorescence and strontium isotopes to track the influence of brine in continental deposits: study of Oligocene sediments in the Digne area (SE basin, France)

It is often difficult to decipher past salt tectonics events in orogenic domains. The southwestern Alpine foreland in France presents a long and extensive salt tectonics history in the Mesozoic, which was later inverted during the Cenozoic Alpine Orogeny. Synorogenic Cenozoic salt-related deformations are difficult to identify due to the contemporaneous shortening experienced by the foreland. This study is based on sedimentological and geochemical (Sr concentrations and 87 Sr/ 86 Sr ratios) analyses of the Oligocene non-marine succession of St-Geniez (Digne region, France). Oligocene salt influences are highlighted by (1) high Sr concentrations (>1000 ppm) in most of the series, (2) the occurrence of halophytic gastropods and (3) the deposition of two gypsum beds. The 87 Sr/ 86 Sr ratios measured in the limestones and the gypsum beds confirm a Triassic origin for the Sr, originating from the Sorine diapir located to the SW, for the first part of the series and from the Authon thrust, located to the north, for the upper part of the series. The geochemistry also suggests the occurrence of two Triassic evaporite levels: one already known and attributed to the Carnian–Norian (Late Triassic) and one attributed to the Olenekian–Anisian (Early to Mid-Triassic). These results show that the combined use of field geology and geochemistry can provide information about previously erased salt tectonics events.

Groundwater–rock interactions and mixing in fault–controlled karstic aquifers: A structural, hydrogeochemical and multi-isotopic review of the Pontina Plain (Central Italy)

Karstic aquifers represent crucial water resources and are categorized as either stratigraphically or fault-controlled. This study investigates groundwater-rock interactions and mixing processes within one of the largest fault-controlled karstic aquifers in Central Italy, adjacent to the Pontina plain, which is a highly populated area where agricultural activities and climate change challenge the groundwater assessment of a complex aquifer. We conducted structural, hydrogeochemical, and multi-isotopic screening of ten selected springs with different degrees of mineralization (ranging from Ca-HCO3 to Na-Cl hydrofacies), incorporating new analyses and modeling of δ34S(SO4), δ18O(SO4), 87Sr/86Sr, and δ11B. Additionally, the reinterpretation of a seismic section provides a more detailed framework extending to depths of approximately 5-7km that allows the identification of the geometry of normal faults, which act as pathways for upwelling fluids. Our findings reveal that hydrogeochemical compositions result from multiple interactions between karstic water and deeper fluids that have interacted with different rocks. Concentration (Na/Li) and isotope (SO4-H2O) geothermometers, coupled with geochemical modeling and trace element analysis, enabled the estimation of a water temperature equilibrium of approximately 95.5°C, with Triassic evaporites generally corresponding to a depth of approximately 3km and a temperature of 40°C with magmatic rocks at approximately 1km depth, which is likely associated with ongoing tectonics and the Quaternary tectonically controlled Volsci Volcanic Field. To obtain the latter estimate, we used a new geothermometer activity based on the equilibrium between analcime and pollucite. Furthermore, this multidisciplinary approach enhances the understanding of groundwater behavior in fault-controlled karstic aquifers, where mantle-derived CO2 dissolved in groundwater is the driving force behind water-rock interactions. Given the potential for further variations in mixing, which may worsen water quality and increase aquifer vulnerability, periodic monitoring of these processes is essential in a human-impacted environment amidst ongoing climate change.

Analogue modelling of a salt ridge growth in M'Rhila-Labaied-Trozza fault relay zone, central Tunisia

Analogue modelling of a salt ridge growth in M'Rhila-Labaied-Trozza fault relay zone, central Tunisia

Salt Distribution in the South Pyrenean Central Salient: Insights From Gravity Anomalies

AbstractTriassic evaporites represent the regional décollement of the Pyrenees and form two salt provinces north and south of the South Pyrenean Central Salient (SPCS). We present an updated Bouguer and residual Bouguer anomaly map built upon the homogenization of available gravity data of the SPCS together with four new and representative cross‐sections, constrained by geological data acquired in the field, seismic, well, and gravity data (gravity forward modeling). Gravity anomaly maps and cross‐sections are used to characterize the present‐day uneven distribution of Triassic evaporites. Outcropping Triassic evaporites is not necessarily associated with an underlying evaporite accumulation and the absence of it at surface does not involves its non‐existence at depth. Northwest of the salient, a major accumulation of Triassic evaporites floors a thick syn‐orogenic Upper Cretaceous basin. South of it, Triassic rocks core salt‐detached anticlines related to the Pyrenean orogeny. Along the southernmost (and youngest) thrust sheet of the salient, diapirs, and evaporite accumulations are associated with a salt‐inflated area.

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

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

Influence of multiple detachments on structural vergence and evolution of the thin-skinned fold-and-thrust belt in the eastern Sichuan Basin: Insights from numerical modeling

Influence of multiple detachments on structural vergence and evolution of the thin-skinned fold-and-thrust belt in the eastern Sichuan Basin: Insights from numerical modeling

Contrasting Styles of Salt‐Tectonic Processes in the Ionian Zone (Greece and Albania): Integrating Surface Geology, Subsurface Data, and Experimental Models

AbstractThe Ionian Zone (IZ) is one of the key elements of the fold and thrust belt (FTB) of the Albanian and Hellenides orogen and contains large outcrops of Triassic evaporites. The IZ consists of various thrust sheets with a general westward vergence, stacking over the Apulian and Pre‐Apulian zones, and repeating a thick carbonate sequence of Upper Triassic to Eocene age. Thrusting becomes younger toward the west with a piggyback sequence, starting during the latest Oligocene Epoch in the Internal Ionian and ending in the Pliocene in the External Ionian. We have studied the IZ in southern Albania and northwestern Greece using field observations and borehole data and by fully interpreting a recently acquired 2D seismic data set. Our objectives are to establish the geometry and nature of the contacts associated with the major Triassic outcrops, to unravel precursor salt diapirs, and to assess their role during the Alpine contraction. Salt structures include gentle salt pillows, isolated salt plugs and diapirs, thrust welds, and salt walls. Combining these observations with experimental modeling results, we show how these structures control the geometry and kinematics of the Alpine thrusts or the location and kinematics of recent strike‐slip faults. Salt minibasins have also been identified, demonstrating salt mobility conditioned Mesozoic sedimentation in the Ionian Basin. The use of salt‐tectonics principles to evaluate the structural style and evolution of the IZ FTB also opens new directions for interpreting the subsurface structure and evolution of the region.

Dolomitization of early-post rift Lower Jurassic carbonate platforms along the Moroccan Atlantic Margin: Origin and significance

Dolomitization of early-post rift Lower Jurassic carbonate platforms along the Moroccan Atlantic Margin: Origin and significance

Temporal constraints on hydrothermal circulation associated with strike-slip faulting in the Permian Maokou carbonates, central Sichuan Basin (SW China)

Temporal constraints on hydrothermal circulation associated with strike-slip faulting in the Permian Maokou carbonates, central Sichuan Basin (SW China)

The Bougrine Zn-Pb deposit, the largest salt diapir-related Mississippi Valley-type deposit in the Eastern Maghreb salt diapir province, Tunisia

The Bougrine Zn-Pb deposit, the largest salt diapir-related Mississippi Valley-type deposit in the Eastern Maghreb salt diapir province, Tunisia

Evidence of new diapiric structures in the southern Adria Plate (Eastern Margin of Tethyan Ocean): Implications for Triassic paleogeography and evaporites remobilization during subduction/collision (Northern Ionian Sea, Central Mediterranean)

Evidence of new diapiric structures in the southern Adria Plate (Eastern Margin of Tethyan Ocean): Implications for Triassic paleogeography and evaporites remobilization during subduction/collision (Northern Ionian Sea, Central Mediterranean)

Tectonics of the Western Internal Jura fold-and-thrust belt: 2D kinematic forward modelling

The balancing technique, called 2D kinematic forward modelling, is a powerful tool to understand the kinematic evolution of fold-and-thrust belts. This study presents a new 2D kinematic forward model for the westernmost Internal Jura fold-and-thrust belt (FTB), situated immediately adjacent to the Geneva Basin. The technique used not only provides a new valid balanced cross-section but also offers new insights regarding the kinematic evolution of the Western Internal Jura FTB. Our model proposes a pure thin-skinned style dominated by forward stepping deformation accompanied by minor back-stepping thrust sequences. A first deformation step is attributed to the thrusting of the Crêt de la Neige Anticline, followed by the Crêt Chalam Thrust and its imbrications. This is followed by thrusting along the Tacon and the Bienne thrusts. Imbricate fault-bend folding explains the steep southern limb of the Crêt de la Neige and the Bellecombe anticlines. 2D kinematic forward modelling yields a total amount of shortening by 23.6 km for the Western Internal Jura FTB. In addition to the primary décollement located at the base of the Keuper Group evaporites, three other décollements are found within the marly layers of the Aalenian “faciès de transition” units, the Oxfordian “Couches d’Effingen-Geissberg” members and the Berriasian Goldberg formation. The multiple thrust horizon approach is supported by new precise seismic interpretations. Our model provides a valid alternative to previous models that either propose local thickening of the Triassic evaporites or inversion of normal faults in the basement. This fully explains the elevated position of the Mesozoic cover in the Jura FTB.

Reconstructing the Iberian Salt‐Bearing Rifted Margin of the Southern Pyrenees: Insights From the Organyà Basin

AbstractReinterpretation of the Organyà Basin, based on new detailed field observations and subsurface data, emphasizes the key contribution of Upper Triassic evaporites in the tectono‐sedimentary evolution of the South‐Central Pyrenees. Results are integrated in a 65‐km long restored cross‐section through the Serres Marginals, Montsec and eastern Organyà salt‐related depocenters. The reconstructed part of the Jurassic–Cretaceous northern Iberian salt‐rich rifted margin shows a template characterized by inherited Permo‐Triassic basement normal faults and an initial salt thickness of 0.7 km to the south and 1.5 km to the north. The Organyà Basin is part of the South Pyrenean Diapiric Province, a large system of salt related depocenters and minibasins, that is limited to the north by the more than 120‐km long Senterada salt wall complex separating the supra‐salt and sub‐salt domains in the Southern Pyrenees. Three main stages of diapiric activity are recognized along the northern Iberian margin from Asturias to the Eastern Pyrenees: a Jurassic early salt mobilization; a latest Jurassic–middle Albian main diapiric evolution associated with rifting; and a Campanian–Miocene diapiric reactivation during basin inversion that produced salt welds and thrust welds and translated the salt province some 60 km to the south.

Groundwater–rock interactions in crystalline rocks: evidence from SIMS oxygen isotope data

AbstractThe diffusive exchange of dissolved material between fluid flowing in a fracture and the enclosing wallrocks (rock matrix diffusion) has been proposed as a mechanism by which radionuclides derived from a radioactive waste repository may be removed from groundwater and incorporated into the geosphere. To test the effectiveness of diffusive exchange in igneous and metamorphic rocks, we have carried out an investigation of veins formed at low temperatures (<100°C), comparing the oxygen isotopic composition of vein calcite with that of secondary calcite in the wallrocks. Two examples of veins from the Borrowdale Volcanic Group, Cumbria, and one from the Mountsorrel Granodiorite, Leicestershire, UK, have remarkably similar vein calcite compositions, ca. +20‰(SMOW) or greater, substantially heavier than the probable compositions of the host rocks, and these vein calcite compositions are inferred to reflect the infiltrating fluid and the temperature of vein formation. Calcites from the wallrocks are similar to those in veins, with little evidence for exchange with the wallrocks. The results support existing models for this type of vein which suggest low-temperature growth from formation brines originally linked to Permian or Triassic evaporites. The results are consistent with flow through fractures being attenuated through a damage zone adjacent to the fracture and provide no evidence of diffusional exchange with pore waters from wallrocks.

Triassic gypsum layers in Keban Metamorphics, eastern Tauride Belt, Southeastern Turkey: New data about Triassic rifting in northern Tethys margin

ABSTRACT A gypsum interlayer with 100 m thick was identified within the Early-Middle Triassic shelf type clastic and carbonates of the Keban Metamorphics, along the shore of the Fırat River, ~500 m west of Keban district (Elazığ, southeast Turkey). Gypsum layers have thickness of 15–50 cm, and exhibit the original sedimentary bedding intercalated with dolomitic carbonate rocks. They were subjected to hydrothermal alteration related to late Cretaceous magmatic intrusions. Hydrothermal activity caused the dissolution and recrystallization of gypsum, as well as occurrences of secondary gypsum, elemental sulphur and quartz (amethyst). The geochemical (trace element and sulphur and oxygen isotope) compositions of gypsum and dolomites indicate typical marine evaporite compositions. The δ34S and δ18O values of the Keban gypsum correspond to Early Triassic evaporites, based on Early-Middle Triassic seawater sulphate composition. Keban gypsum represents Triassic rifting along the northeastern margin of Gondwana and exhibits similar age (Early-Middle Triassic) and palaeogeographic setting to Kemer (Antalya, Turkey) gypsum, associated with opening of the Neotethyan Ocean between Gondwana (Pangea) and the Cimmerian microcontinent. However, the Keban gypsum is different from the late Triassic Honaz-Denizli and Nusaybin-Gaziantep gypsum representing the southern margin of the Paleo-Tethys Ocean (Tauride Belt) and southern margin of the Neo-Tethys (Arabian Plate). The presence of gypsum is observed on both passive continental margins of Neo-Tethys means that their deposition took place before the subduction of the Neo-Tethys Ocean during Late Triassic-Jurassic period.

Structural styles at the junction between the Eastern Atlas Fold and Thrust Belt and the northern Sahel-Gulf of Hammamet foreland basin (Tunisia)

The Tunisian Eastern Atlas Fold and Thrust Belt is characterized by NE-SW fold-thrust structures derived from intracontinental basin inversion during the convergence between Africa and Eurasia. In this paper, we study the transition from this folding-thrusting domain to its foreland basins by discussing a regional structural transect based on field investigations, well data correlations and, seismic lines interpretation. Well correlations highlight significant thickness and facies variations particularly affecting the Cretaceous units, deposited during the Tethyan rifting. Successive compressional events during the Eocene and the upper Miocene-Quaternary have controlled the inversion process, thus shaping the overall present-day structure. Along the Atlas, we observed several large anticlines controlled by deep NE-SW thrusts that root down into a regional-scale major décollement that corresponds to Triassic evaporites. In the field, contractional structures such as fault propagation folds and pop-ups are identified. Unlike the Atlas Fold and Thrust Belt, the Sahel Mesozoic and Paleogene series are buried under thick Neogene to Quaternary syntectonic deposits with sediments supplied from the erosion of the uplifted Atlas structures, recording a high subsidence rate. The Sahel foreland is characterized by buried fold and thrust structures, which define an outer deformation front. Besides contraction, the Sahel Foreland basin is also affected by extension related to the Strait of Sicily rift. Seismic profile interpretation highlights the presence of a large scale rollover developed over a listric fault controlled by a deeper extensional system affecting the Paleozoic series.

Metamorphic Domes in Northern Tunisia: Exhuming the Roots of Nappe Belts by Widespread Post‐Subduction Delamination in the Western Mediterranean

AbstractCenozoic extension in the Western Mediterranean has been related to the dynamics of back‐arc domains. Although, in most of its orogenic belts extension propagated into the fore‐arc nappe domains. Here we revisit the structure, metamorphism and radiometric ages of the Tunisian Tell, where HP/LT rocks (350°C at 0.8 GPa), were exhumed by the sequential activity of extensional detachments after heating and decompression (410°C–440°C at 0.6–0.3 GPa) in a plate convergent setting. Normal faults thinning the Tunisian Tell detached at two different crustal levels. The shallower one cuts down into the Atlas Mesozoic sequence, involving Tellian Triassic evaporites in the hanging‐wall forming halokinetic structures in the Mejerda basin late Miocene. The deeper‐detachment bounds metamorphic domes formed by marbles and metapsammites from the Atlas domain. Illite crystallinity on Triassic rocks shows epizonal to anchizonal values, at deep and intermediate structural depths of the Tell‐Atlas nappe belt, respectively. New U‐Pb 49.78 ± 1.28 Ma rutile ages from Tellian metabasites, together with existing phlogopite 23–17 Ma K‐Ar ages in Atlas marbles from the footwall of the deepest detachment, indicate a polymetamorphic evolution. The Tell rocks underthrusted the Kabylian flysch in the early Eocene. Further, early Miocene shortening thrusted the metabasites over lower‐grade sediments, producing HP/LT metamorphism and ductile stretching at the base of the Atlas belt. The exhumation of midcrustal roots of Western Mediterranean nappe belts after tectonic shortening is a common feature related to tearing at the edges of the subduction systems and inboard delamination of their subcontinental lithospheric mantle.

Constrained 3D gravity modelling of a multilayered aquifer system in an arid region: Case of Sebkhat El Bhira basin (Central Tunisia)

In arid regions, the significant and increasing demand for water has caused intensive exploitation of groundwater resources. The Sebkhat El Bhira basin, shared between Kairouan and Sidi Bouzid governorates in Central Tunisia, presents a multi-layer aquifer system composed of four main reservoirs: the sandy Mio-Plio-Quaternary (MPQ), the Paleogene, the Upper Cretaceous limestone and the Lower Cretaceous limestone, sand and dolomite reservoirs. The intensive exploitation of the shallower aquifer has led to a drawdown of the water table and a deterioration of water quality. The purpose of this work is to characterize the geometry of these potential reservoirs for sustainable water management, by applying geophysical methods. We used land gravity data covering 1387 Km2, twelve seismic reflection profiles with a total length of approximately 308 Km, two petroleum wells and 39 water wells. The processing of the gravity data shows the subdivision of Sebkhat El Bhira basin into four sub-basins and the identification of principal and minor lineaments oriented N-S, NE-SW, NNW-SSE, and E-W. The interpretation of reflection seismic lines allowed the identification of five main horizons: the bottom of MPQ, the bottom of Paleogene, the bottom of Upper Cretaceous, the bottom of Lower Cretaceous and the bottom of Jurassic (Top Triassic). For a better understanding of the subsurface structuring, we built twelve two-dimensional (2D) forward models of density. These latter are constrained in the area inside the plain by the converted time-to-depth seismic reflection data and wells. The interpolation of the 2D models allowed us to visualize the lateral and vertical thickness variations of each of the chrono-stratigraphic units and to produce a three-dimensional (3D) density model. The geophysical modelling allowed us to highlight the post-Jurassic tectonic and the Triassic halokinesis activities responsible for the actual subsurface geometry. The Triassic evaporites, in the study area, flow and rise through the fault's subvertical branches. It outcrops locally at the North-South Axis mountains range (NOSA). Inside the plain, the evaporitic uprising shows different schemes like diapir, wall and pillow. This Triassic halokinesis, with the tectonics, has shaped the deep morphology of the basin and managed the available space for the deposition of the overlying stratigraphic formations. The general structure of the potential aquifers is characterized by a depth which increases from the East to the West and towards the central part of the plain. Inside the basin, the structure of these aquifers is complex.

Parametric mechanical analysis of thin- versus thick-skinned tectonics applied to the Jura belt

Field observations and seismic interpretations testify that the front of the Jura fold-and-thrust belt is still submitted to compressive deformation, but whether the basement is deforming (thick-skinned) or not (thin-skinned) is an active question. We propose a mechanical point of view using the Kinematic approach of the Limit Analysis theory (KLA). We first draw cross-sections containing a major shallow décollement level in the Triassic evaporites, including the Alps up to the topographic maximum and including the whole crust. We submit the cross-sections to a compressive force at their southern end, and the KLA determines the location and geometry of the incipient ruptures by optimisation of the associated compressive force, accounting for force balance and for the rock strength (Coulomb criterion). Five cross-sections span the whole Jura from west to east, allowing us to explore the lateral variations. From the analysis of 500 simulations (100 for each cross-section), varying the friction angles of the Triassic décollement and of the lower crust between 1° and 10°, we have identified five types of tectonics at the Jura front depending on the emergence of a basement thrust beyond the Jura front (type 1), at the Jura front (type 2) with simultaneous activation of the shallow décollement (type 3), or south of the Jura front (type 5), with activation of the shallow décollement at the Jura front (type 4). The analysis allows us to draw two conclusions. First, the transitions between the various tectonic styles occur abruptly upon continuous changes in the friction parameters, revealing a threshold behaviour that we interpret as an extension of the concept of wedge criticality in the Critical Coulomb Wedge theory: at criticality, several tectonic types may occur within a narrow, critical range of parameter values. Second, the critical range evolves systematically between cross-sections, in such a way that the front of the thick-skinned deformation crosses laterally the Jura belt. The two most western cross-sections exhibit only thin-skinned or no tectonics at the Jura front (types 1, 4 and 5), the central one hosts all five styles, and the two Eastern ones show thick-skinned solutions (types 1, 2 and 3), for all values tested. We also show that a thick-skinned tectonic style can be accompanied by a simultaneous activation of the shallow Triassic décollement (type 4), complicating the interpretation of apparent thin-skinned field structures. Overall, our simulations yield tectonic styles compatible with natural seismicity and GPS data for values of the lower-crust friction angle below 3 to 7°. Modifications of our cross-sections to explore the effect of a bumpy upper/lower crust interface, or of a major décollement at the upper/lower crust interface, or of higher cohesion values, show that the numerical outcomes are rather robust. They only slightly modify the critical ranges at which the tectonic changes occur. These findings may serve as guides, or first order questions, for more sophisticated mechanical approaches including temperature and rate-dependent rheologies and the three dimensions of space that are necessary to capture the competition between compressive and strike-slip tectonic modes.