Evidence for Triassic salt domes in the Tunisian Atlas from gravity and geological data

Detailed gravity data were analyzed to constrain two controversial geological models of evaporitic structures within the Triassic diapiric zone (Triassic massifs of Jebel Debadib and Ben Gasseur) of the northern Tunisian Atlas. Based on surface observations, two geological models have been used to explain the origin of the Triassic evaporitic bodies: (1) salt dome/diapiric structure or (2) a “salt glacier”. The gravity analysis included the construction of a complete Bouguer gravity anomaly map, horizontal gravity gradient (HGG) map and two and a half-dimensional (2.5D) forward models. The complete Bouguer gravity anomaly map shows a prominent negative anomaly over the Triassic evaporite outcrops. The HGG map showed the location of the lateral density changes along northeast structural trends caused by Triassic/Cretaceous lithological differences. The modeling of the complete Bouguer gravity anomaly data favored the diapiric structure as the origin of the evaporitic bodies. The final gravity model constructed over Jebel Debadib indicates that the Triassic evaporitic bodies are thick and deeply rooted involving a dome/diapiric structure and that the Triassic material has pulled upward the younger sediment cover by halokinesis. Taking in account kinematic models and the regional tectonic events affecting the northern margin of Africa, the above diapirs formed during the reactive to active to passive stages of continental margin evolution with development of sinks. Otherwise, this study shows that modeling of detailed gravity data adds useful constraints on the evolution of salt structures that may have an important impact on petroleum exploration models.     

Geodynamic evolution of the Salinas de Añana diapir in the Basque-Cantabrian Basin,Western Pyrenees

The Salinas de Añana diapir is located in the Basque-Cantabrian basin part of the great evaporite basin, along with the Gulf of Mexico and the Central European basin, when the fragmentation of Pangea started. The evolution of these basins can only be achieved by understanding the control of salt in the sedimentary and tectonic evolution of these basins.Sedimentation began with clastic Buntsandstein sediments and minor Muschelkalk limestones. Subsequent Keuper evaporites are the bottom of sedimentary cover constituted by Jurassic limestones and marls, a clastic Lower Cretaceous and an alternant limestone and marl Upper Cretaceous, whose deposition has been conditioned by salt tectonics. The emplacement of salt extends from the Aptian until now, favored by the duplication of the salt thickness associated with the thrust of Sierra Cantabria, so it is an excellent example to study changes in the regime of intrusion along the time. The geodynamic evolution of the Salinas de Añana diapir was determined through the interpretation of 45 reprocessed seismic lines, along with information from three wells. Migration of the salt in this diapir, conditioned by N120E and N30E pre-Alpine basement lineations, was determined using time isopach maps of the various rock layers. Vertical evolution of the diapir was determined through the reconstruction of a north-south section at various geologic times by flattening the respective seismic horizons. A minimum of salt flow into the diapir coincides with a minimum rate of sedimentation during the Turonian. Similarly, maximum flows of salt into the diapir occurred during the Coniacian and Lower Santonian and again from the end of the Lower Miocene to the present, coinciding with maximum rates of sedimentation during these times. In the Tertiary, probably during the Oligocene, the diapir was displaced to the south by the Sierra Cantabria thrust, maintaining the contact between the evaporites of diapir and the same evaporites of the lower block. Since the Oligocene, the salts of the lower block migrated towards and into the diapir, deforming the trace of the overthrust.     

The structure of Jebel El Mourra,Tunisia: a diapiric structure causing a positive gravity anomaly

Detailed gravity data integrated with geological data and mining well data are analysed to constrain the shape of the Triassic evaporitic body at Jebel El Mourra (northern Tunisian Atlas) and the mechanism of its emplacement at the surface. The gravity data analysis included the construction of a gravity anomaly maps, and synthetic and forward 2.5D gravity models. The complete Bouguer and residual gravity anomaly maps indicate a positive amplitude gravity anomaly over the Triassic evaporitic outcrops and prominent NE–SW‐trending features associated with the boundary of the Triassic rocks and surrounded layers. A NW–SE‐trending gravity model that crosses the Triassic evaporitic outcrop at Jebel El Mourra shows that the positive gravity anomaly can be explained by a deep‐rooted salt diapir. Conventional models of salt dome formation suggest that they produce negative gravity anomalies; however, this study shows that this model is not universal. The studied area is an example of a diapir expressed by positive gravity anomaly and this result is supported by synthetic gravity models at different stages of salt piercing.     

Three-dimensional forward and backward modelling of diapirism: numerical approach and its applicability to the evolution of salt structures in the Pricaspian basin

Numerical studies of ductile deformations induced by salt movements have, until now, been restricted to two-dimensional (2D) modelling of diapirism. This paper suggests a numerical approach to model the evolution of three-dimensional (3D) salt structures toward increasing maturity. This approach is also used here to restore the evolution of salt structures through successive earlier stages. The numerical methodology is applied to study several model examples. We analyse a model of salt diapirs that develop from an initial random perturbation of the interface between salt and its overburden and restore the evolved salt diapirs to their initial stages. We show that the average restoration errors are less than 1%. An evolutionary model of a 2D salt wall loaded by a 2D pile of sediments predicts a decomposition of the salt wall into 3D diapiric structures when the overburden of salt is supplied by 3D synkinematic wedge of sediments. We model salt extrusion feeding a gravity current over the depositional surface and estimate an average rate of extrusion and horizontal velocity of salt spreading. Faulting of the overburden to salt overhangs initiates new secondary diapirs, and we analyse the growth of these secondary diapirs. We also study how lateral flow effects the evolution of salt diapirs. The shape of a salt diapir can be very different if the rate of horizontal flow is much greater than the initial rate of diapiric growth solely due to gravity. We discuss the applicability of the results of the models to the evolution of Late Permian salt structures in the Pricaspian basin (Russia and Kazakhstan). These structures are distinguishable into a variety of styles representing different stages of growth: salt pillows, diapirs, giant salt massifs, 2D diapiric walls and 3D stocks complicated by large overhangs. The different sizes, shapes and maturities of salt structures in different parts of the Pricaspian basin reflect areal differences in salt thickness and loading history. Our results suggest that the numerical methodology can be employed to analyse the evolution of all salt structures that have upbuilt through younger ductile overburdens.     

Geometry and structural evolution of Lorbeus diapir,northwestern Tunisia: polyphase diapirism of the North African inverted passive margin

Detailed geologic mapping, structural analysis, field cross-sections, new dating based on planktonic foraminifera, in addition to gravity signature of Lorbeus diapir, are used to characterize polyphase salt diapirism. This study highlights the role of inherited faulting, which controls and influences the piercement efficiency and the style and geometry of the diapir; and also the localization of evaporite early ascent displaying diapiric growth during extension. Salt was extruded along the graben axis developed within extensional regional early Cretaceous tectonic associated with the North African passive margin evolution. Geologic data highlight reactive diapirism during Albian time (most extreme extension period) and passive diapirism during the late Cretaceous post-rift stage. Northeastern Maghreb salt province gives evidences that contractional deformations are not associated with significant diapirism. During shortening, the initial major graben deforms as complex anticlines where diapirs are squeezed and pinched from their feeding.     

美国Paradox盆地盐构造新生代的拉伸活化

美国科罗拉多高原Paradox盆地中的裂口盐墙和盐背斜,主要是在古生界和中生界沉积物的差异负载作用下发育起来的。盐构造顶部的平缓褶皱,通常被认为是晚白垩纪至古新世拉腊米挤压运动的产物,而其脊顶地堑与山谷,则被认为是挤压后的松弛拉伸及盐溶作用所造成的。基于野外调查、物理模拟以及现代盐构造学理论,研究认为这些平缓褶皱和脊顶地堑,主要是新生代始新世至渐新世北北东向区域拉伸作用的产物。因为岩盐较其围岩软弱,拉伸形变主要集中在早先形成的盐墙、盐背斜及其较薄的顶板中,使盐构造发生活化并形成新的拉伸构造。顶板中的拉伸构造主要包括正断层、地堑、断层滑移(断滑)褶皱以及滚动褶皱。由于区域拉伸方向斜交盐构造走向,断层分布多呈羽列状。在母盐层较厚地区,拉伸使盐刺穿在正断层之下次动上升;而在母盐层较薄地区,拉伸使盐刺穿下降。在初始盐刺穿较高地区,盐构造的侧翼下降;而在初始盐刺穿较低地区,盐构造整体下降。拉伸使盐刺穿变宽,脊顶地堑下沉,形成拉伸型下降盐刺穿特征性的尖角盐体构造。局部地区,盐体从破碎的顶板间溢出,覆盖了下沉的脊顶地堑。     

Fluvial sedimentation in a salt‐controlled mini‐basin: stratal patterns and facies assemblages,Sivas Basin,Turkey

The Sivas Basin, located on the Central Anatolian Plateau in Turkey, is an elongate Oligo‐Miocene basin that contains numerous salt‐walled mini‐basins. Through field analysis, including stratigraphic section logging, facies analysis and geological mapping, a detailed tectono‐stratigraphic study of the Emirhan mini‐basin and its 2·6 km thick sediment fill has been undertaken. Three main palaeoenvironments are recognized – playa‐lake, braided stream and lacustrine – each corresponds to a relatively long‐lived depositional episode within a system that was dominated overall by the development of a distributive fluvial system. At local scale, this affects the geometry of the succession and influences facies distributions within preserved sequences. Sequences affected by wedge geometries are characterized by localized channelized sandstone bodies in the area of maximum subsidence and these pass laterally to floodplain mudstone towards the diaper; several internal unconformities are recognized. By contrast, sequences affected by hook geometries display narrow and steep drape‐fold geometries with no evidence of lateral facies change and apparent conformity in the preserved succession. The sediment fill of the Emirhan mini‐basin records the remobilization of diapir‐derived detritus and the presence of evaporitic bodies interbedded within the mini‐basin, implying the growth of salt walls expressed at the surface as palaeo‐topographic highs. The mini‐basin also records the signature of a regional change in stratigraphic assemblage, passing from playa‐lake facies to large‐scale highly amalgamated fluvial facies that represent progradation of the fluvial system. The initiation and evolution of this mini‐basin involves a variety of local and regional controls. Local factors include: (i) salt withdrawal, which influenced the rate and style of subsidence and consequently temporal and spatial variation in the stratigraphic assemblage and the stratal response related to halokinesis; and (ii) salt inflation, which influenced the topographic expression of the diapirs and consequently the occurrence of diapir‐derived detritus intercalated within the otherwise clastic‐dominated succession.     

A study of erosion rates on salt diapir surfaces in the Zagros Mountains, SE Iran

Salt exposures and weathering residuum on several salt diapirs in different geographic/climatic settings were studied. Anhydrite, gypsum, hematite, calcite, dolomite, quartz, and clay minerals are the main constituents of the weathering residuum covering the salt diapirs in various thicknesses. Erosion rates of residuum as well as of rock salt exposures were measured at selected sites for a period of 5 years by plastic pegs as benchmarks. Recorded data were standardized to a horizontal surface and to long-term mean precipitation. For the rock salt exposures the following long-term denudation rates were determined of 30–40 mm a⁻¹ for coastal diapirs and up to 120 mm a⁻¹ for mountain salt diapirs. Long-term mean superficial denudation rate measured on weathering residuum of low thickness reached 3.5 mm a⁻¹ on coastal diapirs. The total denudation rate estimated for the thin residuum is close to 4–7 mm a⁻¹ based on apparent correlation with the uplift rate on Hormoz and Namakdan diapirs. Denudation of rock salt exposures is much faster compared to parts of diapirs covered by weathering residuum. The extent of salt exposures is an important factor in the morphological evolution of salt diapirs as it can inhibit further expansion of the diapir. Salt exposures produce huge amounts of dissolved and clastic load, thus affecting the surrounding of the diapir.     

Salt redistribution during extension and inversion inferred from 3D backstripping

A 3D backstripping approach considering salt flow as a consequence of spatially changing overburden load distribution, isostatic rebound and sedimentary compaction for each backstripping step is used to reconstruct the subsidence history in the Northeast German Basin. The method allows to determine basin subsidence and the salt-related deformation during Late Cretaceous–Early Cenozoic inversion and during Late Triassic–Jurassic extension. In the Northeast German Basin, the deformation is thin-skinned in the basinal part, but thick-skinned at the basin margins. The salt cover is deformed due to Late Triassic–Jurassic extension and Late Cretaceous–Early Cenozoic inversion whereas the salt basement remained largely stable in the basin area. In contrast, the basin margins suffered strong deformation especially during Late Cretaceous–Early Cenozoic inversion. As a main question, we address the role of salt during the thin-skinned extension and inversion of the basin. In our modelling approach, we assume that the salt behaves like a viscous fluid on the geological time-scale, that salt and overburden are in hydrostatical near-equilibrium at all times, and that the volume of salt is constant. Because the basement of the salt is not deformed due to decoupling in the basin area, we consider the base of the salt as a reference surface, where the load pressure must be equilibrated. Our results indicate that major salt movements took place during Late Triassic to Jurassic E–W directed extension and during Late Cretaceous–Early Cenozoic NNE–SSW directed compression. Moreover, the study outcome suggests that horizontal strain propagation in the salt cover could have triggered passive salt movements which balanced the cover deformation by viscous flow. In the Late Triassic, strain transfer from the large graben systems in West Central Europe to the east could have caused the subsidence of the Rheinsberg Trough above the salt layer. In this context, the effective regional stress did not exceed the yield strength of the basement below the Rheinsberg Trough, but was high enough to provoke deformation of the viscous salt layer and its cover. During the Late Cretaceous–Early Cenozoic phase of inversion, horizontal strain propagation from the southern basin margin into the basin can explain the intensive thin-skinned compressive deformation of the salt cover in the basin. The thick-skinned compressive deformation along the southern basin margin may have propagated into the salt cover of the basin where the resulting folding again was balanced by viscous salt flow into the anticlines of folds. The huge vertical offset of the pre-Zechstein basement along the southern basin margin and the amount of shortening in the folded salt cover of the basin indicate that the tectonic forces responsible for this inversion event have been of a considerable magnitude.     

Salt movements in the Northeast German Basin and its relation to major post-Permian tectonic phases—results from 3D structural modelling, backstripping and reflection seismic data

The NW–SE-striking Northeast German Basin (NEGB) forms part of the Southern Permian Basin and contains up to 8 km of Permian to Cenozoic deposits. During its polyphase evolution, mobilization of the Zechstein salt layer resulted in a complex structural configuration with thin-skinned deformation in the basin and thick-skinned deformation at the basin margins. We investigated the role of salt as a decoupling horizon between its substratum and its cover during the Mesozoic deformation by integration of 3D structural modelling, backstripping and seismic interpretation. Our results suggest that periods of Mesozoic salt movement correlate temporally with changes of the regional stress field structures. Post-depositional salt mobilisation was weakest in the area of highest initial salt thickness and thickest overburden. This also indicates that regional tectonics is responsible for the initiation of salt movements rather than stratigraphic density inversion.Salt movement mainly took place in post-Muschelkalk times. The onset of salt diapirism with the formation of N–S-oriented rim synclines in Late Triassic was synchronous with the development of the NNE–SSW-striking Rheinsberg Trough due to regional E–W extension. In the Middle and Late Jurassic, uplift affected the northern part of the basin and may have induced south-directed gravity gliding in the salt layer. In the southern part, deposition continued in the Early Cretaceous. However, rotation of salt rim synclines axes to NW–SE as well as accelerated rim syncline subsidence near the NW–SE-striking Gardelegen Fault at the southern basin margin indicates a change from E–W extension to a tectonic regime favoring the activation of NW–SE-oriented structural elements. During the Late Cretaceous–Earliest Cenozoic, diapirism was associated with regional N–S compression and progressed further north and west. The Mesozoic interval was folded with the formation of WNW-trending salt-cored anticlines parallel to inversion structures and to differentially uplifted blocks. Late Cretaceous–Early Cenozoic compression caused partial inversion of older rim synclines and reverse reactivation of some Late Triassic to Jurassic normal faults in the salt cover. Subsequent uplift and erosion affected the pre-Cenozoic layers in the entire basin. In the Cenozoic, a last phase of salt tectonic deformation was associated with regional subsidence of the basin. Diapirism of the maturest pre-Cenozoic salt structures continued with some Cenozoic rim synclines overstepping older structures. The difference between the structural wavelength of the tighter folded Mesozoic interval and the wider Cenozoic structures indicates different tectonic regimes in Late Cretaceous and Cenozoic.We suggest that horizontal strain propagation in the brittle salt cover was accommodated by viscous flow in the decoupling salt layer and thus salt motion passively balanced Late Triassic extension as well as parts of Late Cretaceous–Early Tertiary compression.     

The control of salt supply on entrainment of an anhydrite layer within a salt diapir

The influence of four parameters (sedimentation rate, viscosity of salt, stratigraphic location of the anhydrite layer within the salt layer, and the perturbation width) on salt supply to down-built diapirs and its entrainment capacity are studied systematically in numerical models. Model results show that these four parameters affect salt supply, and the evolution history of a salt diapir. As such, these parameters strongly influence the style and the amount of entrainment of dense inclusions into a diapir. In active diapirs (i.e. unburied diapirs), salt supply increases with increasing sedimentation rate whereas it decreases with an increase in salt viscosity. Diapirs initiating from wide perturbation provide more salt supply to feed the diapir. Presence and initial stratigraphic location of any denser layer (e.g. an anhydrite layer) within a salt layer also affects salt supply. When lateral forces are negligible, salt supply into a diapir depends on these four parameters, which directly control the entrainment of any embedded anhydrite layer into the diapir.     

下刚果盆地白垩系盐构造的形成演化

下刚果盆地具有巨大的油气资源和潜力,盐构造控制着油气成藏和分布。其盐岩流动的不规律性,增加了盐构造的形成演化和演化期次分析的难度,前人主要通过平衡剖面恢复的方法对断裂等构造形态进行定量恢复,但对盐构造形成演化过程的认识及变形期次刻画的准确性明显不足。本文基于数值模拟手段,通过沉积地层的分时期加载方法,正演了下刚果盆地的构造演化过程,探讨并明确了盐构造的演化阶段。数值模拟结果表明,下刚果盆地盐构造的形成主要分为3个阶段：盐岩初始流动阶段、盐构造形成阶段和构造稳定阶段。形成底辟构造的主要时期是古新世-中新世。其中,古新世-渐新世开始形成底辟构造,中新世多个大规模底辟构造形成,且该时期是盐构造活动最强烈时期。在此基础上进一步完成了下刚果盆地盐构造活动变形与重力作用流动、上覆沉积地层厚度不均一以及沉积扇体的进积作用关系分析。     

Potash in a salt mushroom at Hormoz Island, Hormoz Strait, Iran

Increasing volumes of potash are currently being discovered in a cluster of diapirs of Hormoz (formerly Hormuz) salt near Bandar Abbas, Iran. Most of the potash beds studied so far occur in complex recumbent folds in a salt mountain that would be difficult to exploit safely. However, Holocene marine erosion removed any salt mountains from a sub-group of near-shore Zagros diapirs and exposed their deeper structural levels. Even though these diapirs are still active, their potash deposits are likely more tractable to safe exploitation than in a salt mountain — as we make clear here for Hormoz Island.Geochemical surveys on Hormoz Island reveal two separate potash anomalies that are valuable pseudo-stratigraphic markers. Integrating field measurements of the attitudes of bedding with lineaments on air photos suggests that Hormoz Island consists of a mature bell- or plume-shaped mushroom diapir with potash beds wound around a toroidal axis of rotation near current exposure levels.2D numerical models simulate the salt mushroom on Hormoz Island and its internal circulation. They also suggest that the diapir has a wide overhand above a narrow stem in this gas-rich region. We use the mushroom diapir model to outline a regional exploration strategy that has the potential of influencing the world potash market thereafter.     

The Glueckstadt Graben, a sedimentary record between the North and Baltic Sea in north Central Europe

The Glueckstadt Graben is one of the deepest post-Permian structures within the Central European Basin system and is located right at its “heart” at the transition from the North Sea to the Baltic Sea and from the Lower Saxony Basin to the Rynkoebing–Fyn High.The Mesozoic to recent evolution is investigated by use of selected seismic lines, seismic flattening and a 3D structural model. A major tectonic event in the latest Middle–Late Triassic (Keuper) was accompanied by strong salt tectonics within the Glueckstadt Graben. At that time, a rapid subsidence took place within the central part, which provides the “core” of the Glueckstadt Graben. The post-Triassic tectonic evolution of the area does not follow the typical scheme of thermal subsidence. In contrast, it seems that there is a slow progressive activation of salt movements triggered by the initial Triassic event. Starting with the Jurassic, the subsidence centre partitioned into two parts located adjacent to the Triassic “core.” In comparison with other areas of the Central European Basin system, the Glueckstadt Graben was not strongly affected by additional Jurassic and Cretaceous events. During the late Jurassic to Early Cretaceous, the area around the Glueckstadt Graben was affected by relative uplift with regional erosion of the elevated relief. However, subsidence was reactivated and accelerated during the Cenozoic when a strong subsidence centre developed in the North Sea. During Paleogene and Quaternary–Neogene, the two centres of sedimentation moved gradually towards the flanks of the basin.The data indeed point toward a control of post-Permian evolution by gradual withdrawal of salt triggered by the initial exhaustion along the Triassic subsidence centre. In this sense, the Glueckstadt Graben was formed at least partially as “basin scale rim syncline” during post-Permian times. The present day Hamburger, East and Westholstein Troughs are the actual final state of this long-term process which still may continue and may play a role in terms of young processes and, e.g., for coastal protection.     

The Central Andean gravity high, a relic of an old subduction complex?

The Central Andean gravity high (CAGH) is a positive anomaly in isostatic residual gravity with its center located at the western flank of the Central Andes at about 24°S. The gravity was analyzed by various methods to draw quantitative conclusions about the sources of this anomaly and their process of formation. Methods include the analysis of the gravity gradients, power spectrum, wavelength filters, and Euler deconvolution.Numerical investigations of gravity field in the area of the CAGH indicate the presence of a dense body of nearly 400 km length and about 100–140 km width, that masses lie at varying depths between 10 and 38 km. A correlation between the location of the residual anomalies and the topographic lows in the area between the Salars de Atacama and Pipanaco is observed, which indicates the strong influence of the anomalous-causing rocks of the CAGH within the formation process of the Andean orogen. An influence of these causing bodies of rock on the trend of Holocene volcanic arc is likely. Genesis of the anomalous dense formations of rock could be traced back to Ordovician–Silurian time when a pre-Andean subduction zone is postulated in the region of northern Chile with its corresponding volcanic arc in the region of the CAGH.

Zusammenfassung

El campo de gravedad alto de los Andes Centrales (CAGH) consiste en una pronunciada anomalía positiva de la gravedad isostática, cuyo centro se encuentra en el borde oeste de los Andes Centrales a los 24°S. En este estudio se analizó el campo de gravedad mediante distintos métodos, de manera de poder establecer conclusiones cuantitativas sobre el causante de esta anomalía y el proceso de formación de este causante.La investigación numérica de las anomalías gravimétricas del CAGH indica la presencia de un cuerpo de alta densidad con aproximadamente 400 km de largo y 100–140 km de ancho, que se encuentra a profundidades variables entre 10 y 38 km. Se observa una correlación entre la posición de la anomalía residual y los bajos topográficos en los areas de Salares de Atacama, Arizaro, Antofalla y Pipanaco, la cual indica una fuerte influencia de rocas productoras de la anomalía en el CAGH, dentro del proceso de formación del orógeno andino. Es probable que estos cuerpos de rocas causantes de la anomalía tengan incluso influencia en el alineamiento del arco volcánico holocénico. La generación de cuerpos de rocas con una densidad anómala puede remontarse al Ordovícico–Silúrico, tiempo para el que postula una subducción pre-Andina en la región del norte de Chile y que corresponde con el arco volcánico en la región del CAGH.     

四川盐源盐矿成盐成钾分析预测

四川盐源盆地盐矿成盐于早、中三叠世时期,区域构造复杂,经历多期次的构造变动与演化,在成盐后经历了种种的基底反复变动后形成盐矿底辟构造。以前在该区域做过多方面的研究工作,其中包括三叠纪岩相古地理及生物古地理、三叠系地层、区域古构造演化及水化学分析、物探、钻探等勘探工程。通过这些工作可以证实,盐源盆地在早、中三叠世,经历了蒸发岩盆地的发育阶段。盆地不仅发育有石膏夹层,而且保存有石盐层及含盐卤水层,但有盐无钾。通过对上覆的陆相古近纪和新近纪盆地中出现的三叠系岩盐层和盐丘的研究发现,它们与基底为含盐的海相蒸发岩有一定的内在联系,尽管还不能确定其盐、钾资源前景,但可以确定该区域有成钾盐矿藏的条件和可能,因此,该盆地有可能成为深入研究成盐找钾的重点地区。     

Paleostress pattern and salt tectonics within a developing foreland basin (north-western Subhercynian Basin, northern Germany)

Analysing the paleostress field in sedimentary basins is important for understanding tectonic processes and the planning of drilling campaigns. The Subhercynian Basin of northern Germany is a perfect natural laboratory to study the paleostress field in a developing foreland basin. The simple layer-cake geometry of the basin-fill is dominated by several piercing and non-piercing salt structures. We derived the paleostress field from the orientation of fracture sets, faults, slickensides and stylolites. On a regional scale, the basin-fill is characterized by a horizontal compressional paleostress vector that is mainly NNE-SSW-oriented, which reflects the Late Cretaceous inversion phase in Central Europe. We show that the local paleostress field is distinctly perturbated due to the salt structures. Along the edge of the salt pillows, the maximum horizontal paleostress vector is deflected by up to 90° from the regional trend. In the case of the Elm salt pillow, it forms a radial pattern. Restoration of balanced cross-sections demonstrates at least 9 % of the shortening of the north-western part of the Subhercynian Basin was achieved by folding. The salt structures in the north-western Subhercynian Basin are the result of varying stress conditions. Initial extension in the Triassic caused first salt movements that prevailed during the Jurassic and Early Cretaceous. Most important is the Late Cretaceous contractional phase that shortened the diapirs and led to the formation of the salt pillows between diapirs due to detachment folding. We derive four main controlling factors for such salt-dominated contractional basins: (1) the wedge-shape basin-fill is the product of the dynamic load at the southern margin of the basin, (2) a basal salt layer fed the diapirs and acted as a detachment horizon during the later shortening, (3) detachment folding was the dominating deformation mechanism during contraction, and (4) the pre-existing diapirs controlled the position of the detachment folds.     

莺歌海盆地泥-流体底辟树型输导系统及运移模式

莺歌海盆地发育多期泥.流体底辟.底辟活动的多期性造成底辟区断裂和裂隙不断开启,输导天然气从深部向浅部垂向运移,充注底辟两侧及周缘的砂体,在剖面上构成树形样式,称之为底辟树型输导系统.底辟树型输导系统连通了浅层和中一深层的构造和岩性圈闭,是莺歌海盆地中央凹陷带构造及岩性天然气藏成藏的主要输导通道和样式,其相应运移方式分为能量保持型和能量释放型两种.本文提出了莺歌海盆地中央凹陷带发育底辟树型输导系统及其运移的地质模型,泥-流体底辟活动控制下的底辟树型输导系统及其运移模型是对伸展-转换盆地油气成藏理论的发展和补充.     

Filtering of the gravimetric anomalies to the study of the geological structures of Oued Zarga (Septentrional Tunisia): structural implications

Gravity data were analysed in the Northern Tunisian Atlas (case study of Oued Zarga area) to better understand the organization of its underlying structures. The gravity data analysis included the construction of a gravity anomaly maps and two and a half dimensional gravity model. The qualitative analysis of gravimetric maps served for the foundation of a new structural map of the study region that constitutes a new contribution of the gravimetry in this present work. In addition, the complete Bouguer and residual gravity anomaly maps indicate a negative gravity anomaly over the Triassic evaporitic outcrops of Jebel Guerouaou and prominent NE?CSW-trending features associated with the boundary of the Triassic rocks and surrounded layers. A NW?CSE-trending gravity model that crosses the Triassic evaporitic outcrop at Jebel Guerouaou can be explained by a deep-rooted salt diapir.