Minibasin depocentre migration during diachronous salt welding,offshore Angola

Salt tectonics is an important part of the geological evolution of many continental margins, yet the four-dimensional evolution of the minibasins, the fundamental building block of these and many other salt basins, remains poorly understood. Using high-quality 3D seismic data from the Lower Congo Basin, offshore Angola we document the long-term (>70 Myr) dynamics of minibasin subsidence. We show that, during the Albian, a broadly tabular layer of carbonate was deposited prior to substantial salt flow, diapirism, and minibasin formation. We identify four subsequent stages of salt-tectonics and related minibasin evolution: (i) thin-skinned extension (Cenomanian to Coniacian) driven by basinward tilting of the salt layer, resulting in the formation of low-displacement normal faults and related salt rollers. During this stage, local salt welding led to the along-strike migration of fault-bound depocentres; (ii) salt welding below the eastern part of the minibasin (Santonian to Paleocene), causing a westward shift in depocentre location; (iii) welding below the minibasin centre (Eocene to Oligocene), resulting in the formation of a turtle and an abrupt shift of depocentres towards the flanks of the bounding salt walls; and (iv) an eastward shift in depocentre location due to regional tilting, contraction, and diapir squeezing (Miocene to Holocene). Our study shows that salt welding and subsequent contraction are key controls on minibasin geometry, subsidence and stratigraphic patterns. In particular, we show how salt welding is a protracted process, spanning > 70 Myr of the salt-tectonic history of this, and likely other salt-rich basins. The progressive migration of minibasin depocentres, and the associated stratigraphic architecture, record weld dynamics. Our study has implications for the tectono-stratigraphic evolution of minibasins.     

Influence of minibasin obstruction on canopy dynamics in the northern Gulf of Mexico

In salt‐detached gravity‐gliding/spreading systems the detachment geometry is a key control on the downslope mobility of the supra‐salt sequence. Here, we used regional 3D seismic data to examine a salt‐stock canopy in the northern Gulf of Mexico slope, in an area where supra‐canopy minibasins subsided vertically and translated downslope above a complex base‐of‐salt. If thick enough, minibasins can interact with, and weld to, the base‐of‐salt and be obstructed from translating downslope. Based on the regional maps of the base of allochthonous salt and the base of the supra‐canopy sequence, the key controls on minibasin obstruction, we distinguished two structural domains in the study area: a highly obstructed domain and a highly mobile domain. Large‐scale translation of the supra‐canopy sequence is recorded in the mobile domain by a far‐travelled minibasin and a ramp syncline basin. These two structures suggest downslope translation on the order of 40 km from Plio‐Pleistocene to Present. In contrast, translation was impeded in the obstructed domain due to supra‐canopy bucket minibasins subsiding into feeders during the Pleistocene. As a result, we infer that differential translation occurred between the two domains and argue that a deformation area between two differentially translating supra‐canopy minibasin domains is difficult to recognize. However, characterizing domains according to base‐of‐salt geometry and supra‐canopy minibasin configuration can be helpful in identifying domains that may share similar subsidence and downslope translation histories.     

Evolution of salt structures in the northern Paradox Basin: controls on evaporite deposition,salt wall growth and supra‐salt stratigraphic architecture

The northern Paradox Basin evolved during the Late Pennsylvanian–Permian as an immobile foreland basin, the result of flexural subsidence in the footwall of the growing Uncompahgre Ancestral Rocky Mountain thick‐skinned uplift. During the Atokan‐Desmoinesian (～313–306 Ma) fluctuating glacio‐eustatic sea levels deposited an ～2500 m thick sequence of evaporites (Paradox Formation) in the foreland basin, interfingering with coarse clastics in the foredeep and carbonates around the basin margins. The cyclic deposition of the evaporites produced a repetitive sequence of primarily halite, with minor clastics, organic shales and anhydrite. Sediment loading of the evaporites subsequently produced a series of salt walls and minibasins, through the process of passive diapirism or downbuilding. Faults at the top Mississippian level localised the development of linear salt walls (up to 4500 m high) along a NW–SE trend. A crosscutting NE–SW structural trend was also important in controlling the evaporite facies and the abrupt termination of the salt walls. Seismic, well and field data define the proximal Cutler Group (Permian) as a basinward prograding sequence derived from the growing Uncompahgre uplift that drove salt basinwards (towards the southwest), triggering the growth of the salt walls. Sequential structural restorations indicate that the most proximal salt walls evolved earlier than the more distal ones. The successive development of salt‐withdrawal minibasins associated with each growing salt wall implies that parts of the Cutler Group in one minibasin may have no chronostratigraphic equivalent in other minibasins. Localised changes in along‐strike salt wall growth and evolution were critical in the development of facies and thickness variations in the late Pennsylvanian to Triassic stratigraphic sequences in the flanking minibasins. Salt was probably at or very close to the surface during the downbuilding process leading to localised thinning, deposition of diapir‐derived detritus and rapid facies changes in sequences adjacent to the salt wall structures.     

Impact of normal faulting and pre‐rift salt tectonics on the structural style of salt‐influenced rifts: the Late Jurassic Norwegian Central Graben,North Sea

Studies of salt‐influenced rift basins have focused on individual or basin‐scale fault system and/or salt‐related structure. In contrast, the large‐scale rift structure, namely rift segments and rift accommodation zones and the role of pre‐rift tectonics in controlling structural style and syn‐rift basin evolution have received less attention. The Norwegian Central Graben, comprises a complex network of sub‐salt normal faults and pre‐rift salt‐related structures that together influenced the structural style and evolution of the Late Jurassic rift. Beneath the halite‐rich, Permian Zechstein Supergroup, the rift can be divided into two major rift segments, each comprising rift margin and rift axis domains, separated by a rift‐wide accommodation zone – the Steinbit Accommodation Zone. Sub‐salt normal faults in the rift segments are generally larger, in terms of fault throw, length and spacing, than those in the accommodation zone. The pre‐rift structure varies laterally from sheet‐like units, with limited salt tectonics, through domains characterised by isolated salt diapirs, to a network of elongate salt walls with intervening minibasins. Analysis of the interactions between the sub‐salt normal fault network and the pre‐rift salt‐related structures reveals six types of syn‐rift depocentres. Increasing the throw and spacing of sub‐salt normal faults from rift segment to rift accommodation zone generally leads to simpler half‐graben geometries and an increase in the size and thickness of syn‐rift depocentres. In contrast, more complex pre‐rift salt tectonics increases the mechanical heterogeneity of the pre‐rift, leading to increased complexity of structural style. Along the rift margin, syn‐rift depocentres occur as interpods above salt walls and are generally unrelated to the relatively minor sub‐salt normal faults in this structural domain. Along the rift axis, deformation associated with large sub‐salt normal faults created coupled and decoupled supra‐salt faults. Tilting of the hanging wall associated with growth of the large normal faults along the rift axis also promoted a thin‐skinned, gravity‐driven deformation leading to a range of extensional and compressional structures affecting the syn‐rift interval. The Steinbit Accommodation Zone contains rift‐related structural styles that encompass elements seen along both the rift margin and axis. The wide variability in structural style and evolution of syn‐rift depocentres recognised in this study has implications for the geomorphological evolution of rifts, sediment routing systems and stratigraphic evolution in rifts that contain pre‐rift salt units.     

Salt tectonics in a double salt‐source layer setting (Eastern Persian Gulf,Iran): Insights from interpretation of seismic profiles and sequential cross‐section restoration

Salt tectonics in the Eastern Persian Gulf (Iran) is linked to a unique salt‐bearing system involving two overlapping ‘autochthonous’ mobile source layers, the Ediacaran–Early Cambrian Hormuz Salt and the Late Oligocene–Early Miocene Fars Salt. Interpretations of reflection seismic profiles and sequential cross‐section restorations are presented to decipher the evolution of salt structures from the two source layers and their kinematic interaction on the style of salt flow. Seismic interpretations illustrate that the Hormuz and Fars salts started flowing in the Early Palaeozoic (likely Cambrian) and Early Miocene, respectively, shortly after their deposition. Differential sedimentary loading (downbuilding) and subsalt basement faults initiated and localized the flow of the Hormuz Salt and the related salt structures. The resultant diapirs grew by passive diapirism until Late Cretaceous, whereas the pillows became inactive during the Mesozoic after a progressive decline of growth in the Late Palaeozoic. The diapirs and pillows were then subjected to a Palaeocene–Eocene contractional deformation event, which squeezed the diapirs. The consequence was significant salt extrusion, leading to the development of allochthonous salt sheets and wings. Subsequent rise of the Hormuz Salt occurred in wider salt stocks and secondary salt walls by coeval passive diapirism and tectonic shortening since Late Oligocene. Evacuation and diapirism of the Fars Salt was driven mainly by differential sedimentary loading in annular and elongate minibasins overlying the salt and locally by downslope gliding around pre‐existing stocks of the Hormuz Salt. At earlier stages, the Fars Salt flowed not only towards the pre‐existing Hormuz stocks but also away from them to initiate ring‐like salt walls and anticlines around some of the stocks. Subsequently, once primary welds developed around these stocks, the Fars Salt flowed outwards to source the peripheral salt walls. Our results reveal that evolving pre‐existing salt structures from an older source layer have triggered the flow of a younger salt layer and controlled the resulting salt structures. This interaction complicates the flow direction of the younger salt layer, the geometry and spatial distribution of its structures, as well as minibasin depocentre migration through time. Even though dealing with a unique case of two ‘autochthonous’ mobile salt layers, this work may also provide constraints on our understanding of the kinematics of salt flow and diapirism in other salt basins having significant ‘allochthonous’ salt that is coevally affected by deformation of the deeper autochthonous salt layer and related structures.     

Respective contributions of tectonic and gravity‐driven processes on the structural pattern in the Eastern Nile deep‐sea fan: insights from physical experiments

In the Nile deep‐sea fan, thin‐skinned deformation detaching on a layer of Messinian salt has generated an upslope to downslope progression from growth faults, to polygonal minibasins bounded by salt ridges, to buckle folds. Such progression is common in salt‐bearing passive margins, where gravity spreading of the salt–sediment system causes proximal thin‐skinned extension on the shelf and upper slope, and distal contraction along and in front of the lower slope. In the Eastern Nile deep‐sea fan, this structural progression seems to be restricted to a corridor bounded by NW–SE‐trending lineaments more than 200 km in length. These are associated with salt ridges and record strike–slip movements. In the absence of a large grid of deep‐penetrating seismic data accurately imaging the basement, different likely hypotheses have been advanced about the origin of this corridor: (1) it may result from possible deep‐seated tectonics related to the Rift of Suez, combined with salt‐related deformation or, (2) by complex interaction between the overburden's gravity spreading and pre‐existing pre‐Messinian paleo‐topographic features, particularly the possible buttressing effect of a seamount located North of the eastern Nile deep‐sea fan. In order to understand how this corridor could have been generated, we used a series of physical experiments to test the effect on three‐dimensional spreading of a sediment lobe of the following parameters: (1) active, crustal, oblique extension, (2) a dormant subsalt graben, (3) a passive buttress, such as a seamount and (4) complex paleo‐topographic features along the Egyptian margin affecting initial salt distribution. These experiments show that the presence of a distal buttress, combined with a complex Messinian topography best explain the complex deformational pattern observed in the eastern Nile deep‐sea fan.     

Base-salt relief controls salt-tectonic structural style,São Paulo Plateau,Santos Basin,Brazil

Base-salt relief influences salt flow, producing three-dimensionally complex strains and multiphase deformation within the salt and its overburden. Understanding how base-salt relief influences salt-related deformation is important to correctly interpret salt basin kinematics and distribution of structural domains, which have important implications to understand the development of key petroleum system elements. The São Paulo Plateau, Santos Basin, Brazil is characterized by a >2 km thick, mechanically layered Aptian salt layer deposited above prominent base-salt relief. We use 3D seismic reflection data, and physical and conceptual kinematic models to investigate how gravity-driven translation above thick salt, underlain by complex base-salt relief, generated a complex framework of salt structures and minibasins. We show that ramp-syncline basins developed above and downdip of the main pre-salt highs record c. 30 km of Late Cretaceous-Paleocene basinward translation. As salt and overburden translated downdip, salt flux variations caused by the base-salt relief resulted in non-uniform motion of the cover, and the simultaneous development of extensional and contractional structures. Contraction preferentially occurred where salt flow locally decelerated, above landward-dipping base-salt and downdip of basinward-dipping ramps. Extension occurred at the top of basinward-dipping ramps and base-salt plateaus, where salt flow locally accelerated. Where the base of the salt layer was broadly flat, structures evolved primarily by load-driven passive diapirism. At the edge of or around smaller base-salt highs, salt structures were affected by plan-view rotation, shearing and divergent flow. The magnitude of translation (c. 30 km) and the style of salt-related deformation observed on the São Paulo Plateau afford an improved kinematic model for the enigmatic Albian Gap, suggesting this structure formed by a combination of basinward salt expulsion and regional extension. These observations contribute to the long-lived debate regarding the mechanisms of salt tectonics on the São Paulo Plateau, ultimately improving our general understanding of the effects of base-salt relief on salt tectonics in other basins.     

The stratigraphic record of minibasin subsidence,Precaspian Basin,Kazakhstan

Minibasins are fundamental components of many salt-bearing sedimentary basins, where they may host large volumes of hydrocarbons. Although we understand the basic mechanics governing their subsidence, we know surprisingly little of how minibasins subside in three-dimensions over geological timescales, or what controls such variability. Such knowledge would improve our ability to constrain initial salt volumes in sedimentary basins, the timing of salt welding and the distribution and likely charging histories of suprasalt hydrocarbon reservoirs. We use 3D seismic reflection data from the Precaspian Basin, onshore Kazakhstan to reveal the subsidence histories of 16, Upper Permian-to-Triassic, suprasalt minibasins. These minibasins subsided into a Lower-to-Middle Permian salt layer that contained numerous relatively strong, clastic-dominated minibasins encased during an earlier, latest Permian phase of diapirism; because of this, the salt varied in thickness. Suprasalt minibasins contain a stratigraphic record of symmetric (bowl-shaped units) and then asymmetric (wedge-shaped units) subsidence, with this change in style seemingly occurring at different times in different minibasins, and most likely prior to welding. We complement our observations from natural minibasins in the Precaspian Basin with results arising from new physical sandbox models; this allows us to explore the potential controls on minibasin subsidence patterns, before assessing which of these might be applicable to our natural example. We conclude that due to uncertainties in the original spatial relationships between encased and suprasalt minibasins, and the timing of changes in style of subsidence between individual minibasins, it is unclear why such complex temporal and spatial variations in subsidence occur in the Precaspian Basin. Regardless of what controls the observed variability, we argue that vertical changes in minibasin stratigraphic architecture may not record the initial (depositional) thickness of underlying salt or the timing of salt welding; this latter point is critical when attempting to constrain the timing of potential hydraulic communication between sub-salt source rocks and suprasalt reservoirs. Furthermore, temporal changes in minibasin subsidence style will likely control suprasalt reservoir distribution and trapping style.     

Structural style and evolution of a salt‐influenced rift basin margin; the impact of variations in salt composition and the role of polyphase extension

Because salt can decouple sub‐ and supra‐salt deformation, the structural style and evolution of salt‐influenced rifts differs from those developed in megoscopically homogenous and brittle crust. Our understanding of the structural style and evolution of salt‐influenced rifts comes from scaled physical models, or subsurface‐based studies that have utilised moderate‐quality 2D seismic reflection data. Relatively few studies have used high‐quality 3D seismic reflection data, constrained by borehole data, to explicitly focus on the role that along‐strike displacement variations on sub‐salt fault systems, or changes in salt composition and thickness, play in controlling the four‐dimensional evolution of supra‐salt structural styles. In this study, we use 3D seismic reflection and borehole data from the Sele High Fault System (SHFS), offshore Norway to determine how rift‐related relief controlled the thickness and lithology of an Upper Permian salt‐bearing layer (Zechstein Supergroup), and how the associated variations in the mechanical properties of this unit influenced the degree of coupling between sub‐ and supra‐salt deformation during subsequent extension. Seismic and borehole data indicate that the Zechstein Supergroup is thin, carbonate‐dominated and immobile at the footwall apex, but thick, halite‐dominated and relatively mobile in high accommodation areas, such as near the lateral fault tips and in the immediate hangingwall of the fault system. We infer that these variations reflect bathymetric changes related to either syn‐depositional (i.e. Late Permian) growth of the SHFS or underfilled, fault scarp‐related relief inherited from a preceding (i.e. Early Permian) rift phase. After a period of tectonic quiescence in the Early Triassic, regional extension during the Late Triassic triggered halokinesis and growth of a fault‐parallel salt wall, which was followed by mild extension in the Jurassic and forced folding of Triassic overburden above the fault systems upper tip. During the Early Cretaceous, basement‐involved extension resulted in noncoaxial tilting of the footwall, and the development of an supra‐salt normal fault array, which was restricted to footwall areas underlain by relatively thick mobile salt; in contrast, at the footwall apex, no deformation occurred because salt was thin and immobile. The results of our study demonstrate close coupling between tectonics, salt deposition and the style of overburden deformation for >180 Myr of the rift history. Furthermore, we show that rift basin tectono‐stratigraphic models based on relatively megascopically homogeneous and brittle crust do not appropriately describe the range of structural styles that occur in salt‐influenced rifts.     

Geology and tectonics of Neoproterozoic salt diapirs and salt sheets in the eastern Willouran Ranges,South Australia

Allochthonous salt structures and associated primary and secondary minibasins are exposed in Neoproterozoic strata of the eastern Willouran Ranges, South Australia. Detailed geologic mapping using high‐quality airborne hyperspectral remote‐sensing data and satellite imagery, combined with a qualitative structural restoration, are used to elucidate the evolution of this complex, long‐lived (>250 Myr) salt system. Field observations and interpretations at a resolution unobtainable from seismic or well data provide a means to test published models of allochthonous salt emplacement and associated salt‐sediment interaction derived from subsurface data in the northern Gulf of Mexico. Salt diapirs and sheets are represented by megabreccias of nonevaporite lithologies that were originally interbedded with evaporites that have been dissolved and/or altered. Passive diapirism began shortly after deposition of the Callanna Group layered evaporite sequence. A primary basin containing an expulsion‐rollover structure and megaflap is flanked by two vertical diapirs. Salt flowed laterally from the diapirs to form a complex, multi‐level canopy, now partly welded, containing an encapsulated minibasin and capped by suprasalt basins. Salt and minibasin geometries were modified during the Late Cambrian–Ordovician Delamerian Orogeny (ca. 500 Ma). Small‐scale structures such as subsalt shear zones, fractured or mixed ‘rubble zones’ and thrust imbricates are absent beneath allochthonous salt and welds in the eastern Willouran Ranges. Instead, either undeformed strata or halokinetic drape folds that include preserved diapir roof strata are found directly below the transition from steep diapirs to salt sheets. Allochthonous salt first broke through the diapir roofs and then flowed laterally, resulting in variable preservation of the subsalt drape folds. Lateral salt emplacement was presumably on roof‐edge thrusts or, because of the shallow depositional environment, via open‐toed advance or extrusive advance, but without associated subsalt deformation.     

Complex interplay of faulting,glacioeustatic variations and halokinesis during deposition of upper Viséan units over thick salt in the western Cumberland Basin of Atlantic Canada

The late Palaeozoic Cumberland Basin of Nova Scotia and New Brunswick (eastern Canada) developed as a strike‐slip basin in the aftermath of the Middle Devonian Acadian Orogeny. Following deposition of thick salt during the middle Viséan (middle Mississippian), this basin mainly accommodated fault‐controlled continental deposits during the late Viséan, which generated halokinesis from clastic loading. The Mississippian halokinetic history of this basin is cryptic, as it was severely distorted by subsequent tectonic and halokinetic overprints. After minor structural restoration, the study of upper Viséan minibasin units in wide coastal sections and deep wells allowed a fairly detailed reconstruction of the Mississippian halokinetic setting to be made. Paleoenvironments and depositional settings in the western part of the basin include sectors that were proximal to three fault‐bounded source areas and characterized by alluvial fan systems transitioning laterally into gravelly to sandy braidplain environments. More central areas of the basin were characterized by tidal flats transitioning laterally into shallow marine environments. Because of halokinesis, the marine body was eventually forced to subdivide into three separate salt expulsion minibasins. Although late Viséan marine incursions were short‐lived in the rest of eastern Canada due to ongoing glacioeustatic variations, there are sedimentologic and stratigraphic lines of evidence for the long‐lasting entrapment of restricted marine bodies in salt expulsion minibasins of the western Cumberland Basin. In one minibasin that was characterized by especially high accommodation rates, NE of Hopewell Cape (New Brunswick), the proximal conglomerates and marine carbonates of a fan‐delta setting transition laterally into thick sulphate over a short distance, away from freshwater inputs from the source area. The vertical continuity of the latter sulphate succession suggests that this entrapped evaporitic basin was cut‐off from significant marine influxes, even at times of glacioeustatic highstands. This is in contrast with salt expulsion minibasins in open marine shelf settings, which always remain open to global marine transgressions and regressions.     

Obstructed minibasins on a salt-detached slope: An example from above the Sigsbee canopy,northern Gulf of Mexico

Salt-detached gravity gliding/spreading systems having a rugose base-of-salt display complex strain patterns. However, little was previously known about how welding of supra-salt minibasins to the sub-salt may influence both the downslope translation of minibasins on salt-detached slopes and the regional pattern of supra-salt strain. Using a regional 3D seismic reflection data set, we examine a large salt-stock canopy system with a rugose base on the northern Gulf of Mexico slope, on which minibasins both subside and translate downslope. Some minibasins are welded at their bases and others are not. We suggest that basal welds obstruct downslope translation of minibasins and control regional patterns of supra-canopy strain. The distribution of strain above the canopy is complex and variable. Each minibasin that becomes obstructed modifies the local strain field, typically developing a zone of shortening immediately updip and an extensional breakaway zone immediately downdip of the obstructed minibasin. This finding is corroborated by observations from a physical sandbox model of minibasin obstruction. We also find in our natural example that minibasins can be obstructed to different degrees, ranging from severe (e.g., caught in a feeder) to mild (e.g., welded to a flat or gently dipping base-of-salt). By mapping both the presence of obstructed minibasins and the relative degree of minibasin obstruction, we provide an explanation for the origin of complex 3-D strain fields on a salt-detached slope and, potentially, a mechanism that explains differential downslope translation of minibasins. In minibasin-rich salt-detached slope settings, our results may aid: i) structural restorations and regional strain analyses; ii) prediction of subsalt relief in areas of poor seismic imaging; and iii) prediction of stress fields and borehole stability. Our example is detached on allochthonous salt and where the base-of-salt is rugose, with the findings applicable to other such systems worldwide (e.g., Gulf of Mexico; Scotian Margin, offshore eastern Canada). However, our findings are also applicable to systems where the salt is autochthonous but has significant local basal relief (e.g., Santos Basin, Brazil; Kwanza Basin, Angola).     

Strain analysis of a seismically imaged mass‐transport complex,offshore Uruguay

Strain style, magnitude and distribution within mass‐transport complexes (MTCs) are important for understanding the process evolution of submarine mass flows and for estimating their runout distances. Structural restoration and quantification of strain in gravitationally driven passive margins have been shown to approximately balance between updip extensional and downdip contractional domains; such an exercise has not yet been attempted for MTCs. We here interpret and structurally restore a shallowly buried (c. 1,500 mbsf) and well‐imaged MTC, offshore Uruguay using a high‐resolution (12.5 m vertical and 15 × 12.5 m horizontal resolution) three‐dimensional seismic‐reflection survey. This allows us to characterise and quantify vertical and lateral strain distribution within the deposit. Detailed seismic mapping and attribute analysis shows that the MTC is characterised by a complicated array of kinematic indicators, which vary spatially in style and concentration. Seismic‐attribute extractions reveal several previously undocumented fabrics preserved in the MTC, including internal shearing in the form of sub‐orthogonal shear zones, and fold‐thrust systems within the basal shear zone beneath rafted‐blocks. These features suggest multiple transport directions and phases of flow during emplacement. The MTC is characterised by a broadly tripartite strain distribution, with extensional (e.g. normal faults), translational and contractional (e.g. folds and thrusts) domains, along with a radial frontally emergent zone. We also show how strain is preferentially concentrated around intra‐MTC rafted‐blocks due to their kinematic interactions with the underlying basal shear zone. Overall, and even when volume loss within the frontally emergent zone is included, a strain difference between extension (1.6–1.9 km) and contraction (6.7–7.3 km) is calculated. We attribute this to a combination of distributed, sub‐seismic, ‘cryptic’ strain, likely related to de‐watering, grain‐scale deformation and related changes in bulk sediment volume. This work has implications for assessing MTCs strain distribution and provides a practical approach for evaluating structural interpretations within such deposits.     

Syn‐depositional thin‐ and thick‐skinned extensional tectonics in the mid‐Cretaceous Lasarte sub‐basin,western Pyrenees

In the mid‐Cretaceous Lasarte sub‐basin (LSB) [northeastern Basque‐Cantabrian Basin (BCB)] contemporaneous and syn‐depositional thin‐ and thick‐skinned extensional tectonics occur due to the presence of a ductile detachment layer that decoupled the extension. Despite the interest in extension modes of rift basins bearing intra‐stratal detachment layers, complex cases remain poorly understood. In the LSB, field results based on mapping, stratigraphic, sedimentological and structural data show the relationship between growth strata and tectonic structures. Syn‐depositional extensional listric faults and associated folds and faults have been identified in the supra‐detachment thin‐skinned system. But stratigraphic data also indicate the activation of sub‐detachment thick‐skinned extensional faults coeval with the development of the thin‐skinned system. The tectono‐sedimentary evolution of the LSB, since the Late Aptian until the earliest Late Albian, has been interpreted based on thin‐ and thick‐skinned extensional growth structures, which are fossilized by post‐extensional strata. The development of the thin‐skinned system is attributed to the presence of a ductile detachment layer (Upper Triassic Keuper facies) which decoupled the extension from deeper sub‐detachment basement‐involved faulting under a regional extensional/transtensional regime.     

Base-salt relief controls salt-related deformation in the Outer Kwanza Basin,offshore Angola

Salt-influenced passive margins are widespread and commonly hydrocarbon-rich. However, they can be structurally complex, with their kinematic development being poorly understood. Classic models of salt tectonics divide such margins into updip extensional, mid-slope translational and downdip contractional kinematic domains. Furthermore the faults, folds, and salt walls associated with each kinematic domain are typically assumed to form perpendicular to the maximum principal stress, which in gravitationally driven systems means broadly perpendicular to base-salt dip. We use high-resolution 3D seismic reflection data from the Outer Kwanza Basin, offshore Angola to show that these models cannot explain the diversity of salt structures developing on passive margins, especially those defined by considerable relief on the base-of-salt surface. Overburden seismic-stratigraphic patterns record the basinward translation and rotation, allowing us to reconstruct the origin and evolution of the salt structures. We show structures in the transitional domain of the Outer Kwanza Basin display three dominant trends, each characterised by different structural styles: (a) salt walls perpendicular to the overall base-salt dip, (b) salt walls parallel to the base-salt dip and (c) salt walls oblique to the base-salt dip. We show that each set of walls has a unique history, with synchronous phases of extension and compression occurring in adjacent structures despite their close spatial relationship. Our analysis suggests that, in the Outer Kwanza Basin, the structural evolution of the salt and overburden is predominantly controlled by translation over relief on the base-salt surface formed above fault scarps associated with a preceding phase of rifting. Changes in the downdip volumetric flux and velocity of the salt over topographic features can cause local extension or contraction of the salt and its overburden, associated with local acceleration or deceleration of the salt, respectively. This interaction with base-salt relief creates locally variable stress fields that deform the salt and its overburden, overprinting the broader, margin-scale salt tectonics typically associated with gravity gliding and spreading.     

The role of evaporite mobility in modifying subsidence patterns during normal fault growth and linkage, Halten Terrace, Mid-Norway

Well‐calibrated seismic interpretation in the Halten Terrace of Mid‐Norway demonstrates the important role that structural feedback between normal fault growth and evaporite mobility has for depocentre development during syn‐rift deposition of the Jurassic–Early Cretaceous Viking and Fangst Groups. While the main rift phase reactivated pre‐existing structural trends, and initiated new extensional structures, a Triassic evaporite interval decouples the supra‐salt cover strata from the underlying basement, causing the development of two separate fault populations, one in the cover and the other confined to the pre‐salt basement. Detailed displacement–length analyses of both cover and basement fault arrays, combined with mapping of the component parts of the syn‐rift interval, have been used to reveal the spatial and temporal evolution of normal fault segments and sediment depocentres within the Halten Terrace area. Significantly, the results highlight important differences with traditional models of normal fault‐controlled subsidence, including those from parts of the North Sea where salt is absent. It can now be shown that evaporite mobility is intimately linked to the along‐strike displacement variations of these cover and basement faults. The evaporites passively move beneath the cover in response to the extension, such that the evaporite thickness becomes greatest adjacent to regions of high fault displacement. The consequent evaporite swells can become large enough to have pronounced palaeobathymetric relief in hangingwall locations, associated with fault displacement maxima– the exact opposite situation to that predicted by traditional models of normal fault growth. Evaporite movement from previous extension also affects the displacement–length relationships of subsequently nucleated or reactivated faults. Evaporite withdrawal, on the other hand, tends to be a later‐stage feature associated with the high stress regions around the propagating tips of normal faults or their coeval hangingwall release faults. The results indicate the important effect of, and structural feedback caused by, syn‐rift evaporite mobility in heavily modifying subsidence patterns produced by normal fault array evolution. Despite their departure from published models, the results provide a new, generic framework within which to interpret extensional fault and depocentre development and evolution in areas in which mobile evaporites exist.     

Syn-depositional halokinesis in the Zechstein Supergroup (Lopingian) controls Triassic minibasin genesis and location

Salt tectonics is typically caused by the flow of mobile evaporites in response to post-depositional gravity gliding and/or differential loading by overburden sediments. This situation is considerably more complex near the margins of salt basins, where carbonate and clastic rocks may be deposited at the same time as and be interbedded with more mobile, evaporitic strata. In these cases, syn-depositional salt flow may occur due to density differences in the deposited lithologies, although our understanding of this and related processes is relatively poor. We here use 3D seismic reflection and borehole data from the Devil's Hole Horst, West Central Shelf, offshore UK to understand the genesis, geometry, and kinematic evolution of intra-Zechstein Supergroup (Lopingian) minibasins and their effect on post-depositional salt deformation. We show that immobile, pinnacle-to-barrier-like, carbonate build-ups and anhydrite are largely restricted to intra-basin highs, whereas mobile halite, which flowed to form large diapirs, dominates in the deep basin. At the transition between the intra-basin highs and the deep basin, a belt of intra-Zechstein minibasins occurs, forming due to the subsidence of relatively dense anhydrite into underlying halite. Depending on primary halite thickness, these intra-Zechstein minibasins created topographic lows, dictating where Triassic minibasins subsequently nucleated and down-built. Our study refines the original depositional model for the Zechstein Supergroup in the Central North Sea, with the results also helping us better understand the style and distribution of syn-depositional salt flow within other layered evaporitic sequences and the role intra-salt heterogeneity and related deformation may have in the associated petroleum plays.     

The synrift phase of the early Domeyko Basin (Triassic,northern Chile): Sedimentary,volcanic, and tectonic interplay in the evolution of an ancient subduction‐related rift basin

The geodynamic setting along the SW Gondwana margin during its early breakup (Triassic) remains poorly understood. Recent models calling for an uninterrupted subduction since Late Palaeozoic only slightly consider the geotectonic significance of coeval basins. The Domeyko Basin initiated as a rift basin during the Triassic being filled by sedimentary and volcanic deposits. Stratigraphic, sedimentological, and geochronological analyses are presented in order to determine the tectonostratigraphic evolution of this basin and to propose a tectonic model suitable for other SW Gondwana‐margin rift basins. The Domeyko Basin recorded two synrift stages. The Synrift I (~240–225 Ma) initiated the Sierra Exploradora sub‐basin, whereas the Synrift II (~217–200 Ma) reactivated this sub‐basin and originated small depocentres grouped in the Sierra de Varas sub‐basin. During the rift evolution, the sedimentary systems developed were largely controlled by the interplay between tectonics and volcanism through the accommodation/sediment supply ratio (A/S). High‐volcaniclastic depocentres record a net dominance of the syn‐eruptive period lacking rift‐climax sequences, whereas low‐volcaniclastic depocentres of the Sierra de Varas sub‐basin developed a complete rift cycle during the Synrift II stage. The architecture of the Domeyko Basin suggests a transtensional kinematic where N‐S master faults interacted with ~NW‐SE basement structures producing highly asymmetric releasing bends. We suggest that the early Domeyko Basin was a continental subduction‐related rift basin likely developed under an oblique convergence in a back‐arc setting. Subduction would have acted as a primary driving mechanism for the extension along the Gondwanan margin, unlike inland rift basins. Slab‐induced dynamic can strongly influence the tectonostratigraphic evolution of subduction‐related rift basins through controls in the localization and style of magmatism and faulting, settling the interplay between tectonics, volcanism, and sedimentation during the rifting.     

Mass-transport complexes (MTCs) document subsidence patterns in a northern Gulf of Mexico salt minibasin

Mass-transport complexes (MTCs) dominate the stratigraphic record of many salt-influenced sedimentary basins. Commonly in such settings, halokinesis is invoked as a primary trigger for MTC emplacement, although the link between specific phases of salt movement, and related minibasin dynamics, remains unclear. Here, we use high-quality 3D seismic reflection and well data to constrain the composition, geometry and distribution (in time and space) of six MTCs preserved in a salt-confined, supra-canopy minibasin in the northern Gulf of Mexico, and to assess how their emplacement relate to regional and local controls. We define three main tectono-sedimentary phases in the development of the minibasin: (a) initial minibasin subsidence and passive diapirism, during which time deposition was dominated by relatively large-volume MTCs (c. 25 km³) derived from the shelf-edge or upper slope; (b) minibasin margin uplift and steepening, during which time small-volume MTCs (c. 20 km³) derived from the shelf-edge or upper slope were emplaced; and (c) active diapirism, during which time very small volume MTCs (c. 1 km³) were emplaced, locally derived from the diapir flanks or roofs. We present a generic model that emphasizes the dynamic nature of minibasin evolution, and how MTC emplacement relates to halokinetic sequence development. Although based on a single data-rich case study, our model may be applicable to other MTC-rich, salt-influenced sedimentary basins.