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Shortening styles in salt-influenced basins can vary markedly, with the volume and distribution of salt prior to shortening being a key control. Here we use a suite of physical models to examine styles of thin-skinned regional shortening in settings where the pre-shortening structure comprised minibasins surrounded by salt (‘isolated-minibasin’ provinces). Our models show that the high volume of mechanically-weak salt localizes lateral regional shortening, with shortening inducing salt flow towards the foreland that subsequently contributes to three key processes - translation, tilting and rotation of minibasins. First, we demonstrate that the flowing salt pushes against minibasins, propelling them in the regional shortening direction. Minibasin translation is enhanced by fast-flowing salt streams and impeded by basal friction due to welding and base-salt buttresses. Second, we show how minibasin tilt directions and magnitudes vary spatially and temporally during regional shortening. Minibasins tilt away from zones of pressurized salt, the locations of which may shift due to changes in salt flow regimes. Tilt directions may also change as minibasins pivot on primary welds, or due to forces associated with minibasin collision. Third, minibasins can rotate around sub-vertical axes during regional shortening. We speculate that this rotation is caused by a combination of: i) traction imparted on the minibasin boundary by differential horizontal flow of adjacent salt; and ii) pivoting on primary and secondary welds. We synthesize our results in a series of 3-D conceptual models, before we compare and contrast regional shortening styles and processes in salt-influenced basins with different pre-shortening salt configurations. Our findings contribute to the understanding of the geometry and kinematics of shortened salt basins, as well as a deeper understanding of the tectono-stratigraphic evolution of minibasins.

[Abstract Shortening styles in salt‐influenced basins can vary markedly, with the volume and distribution of salt prior to shortening being a key control. Here, we use a suite of physical models to examine styles of thin‐skinned regional shortening in settings where the preshortening structure comprised minibasins surrounded by salt (‘isolated‐minibasin’ provinces). Our models show that the high volume of mechanically weak salt localizes lateral regional shortening, with shortening inducing salt flow towards the foreland that subsequently contributes to three key processes – translation, tilting and rotation of minibasins. First, we demonstrate that the flowing salt pushes against minibasins, propelling them in the regional shortening direction. Minibasin translation is enhanced by fast‐flowing salt streams and impeded by basal friction due to welding and base‐salt buttresses. Second, we show how minibasin tilt directions and magnitudes vary spatially and temporally during regional shortening. Minibasins tilt away from zones of pressurized salt, the locations of which may shift due to changes in salt flow regimes. Tilt directions may also change as minibasins pivot on primary welds, or due to forces associated with minibasin collision. Third, minibasins can rotate around sub‐vertical axes during regional shortening. We speculate that this rotation is caused by a combination of: (a) traction imparted on the minibasin boundary by differential horizontal flow of adjacent salt; and (b) pivoting on primary and secondary welds. We synthesize our results in a series of 3‐D conceptual models, before we compare and contrast regional shortening styles and processes in salt‐influenced basins with different preshortening salt configurations. Our findings contribute to the understanding of the geometry and kinematics of shortened salt basins, as well as a deeper understanding of the tectono‐stratigraphic evolution of minibasins., 3‐D conceptual block model synthesising the structural styles and processes that occur in isolated‐minibasin provinces at low regional shortening strain. ]

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.

Stratal geometries of salt-floored minibasins provide a record of the interplay between minibasin subsidence and sedimentation. Minibasin subsidence and resulting stratal geometries are frequently interpreted by considering the minibasins in isolation and implicitly assuming that internal geometries are the result of purely vertical halokinetic processes. However, minibasins rarely form in isolation and may record complex subsidence histories even in the absence of tectonic forces. In this study we use numerical models to investigate how minibasins subside in response to density-driven downbuilding. We show that minibasins subsiding in isolation result in simple symmetrical minibasins with relatively simple internal stratigraphic patterns. In contrast, where minibasins form in closely spaced arrays and subside at different rates, minibasins can kinematically interact due to complex patterns of flow in the encasing salt, even during simple density-driven subsidence. More specifically, we show that minibasins can: (1) prevent nearby minibasins from subsiding; (2) induce lateral translation of nearby minibasins; and (3) induce tilting and asymmetrical subsidence of nearby minibasins. We conclude that even in areas where no regional or dominant salt flow regime exists, minibasins can still be genetically related and the minibasin subsidence histories cannot be fully understood if considered in isolation. Thematic collection: This article is part of the Mechanics of salt systems: state of the field in numerical methods collection available at: https://www.lyellcollection.org/cc/mechanics-of-salt-systems

This paper has benefited greatly by the thorough and helpful comments and reviews by Chris Morley, Peter Connolly, and an anonymous reviewer. Andrea Billi is also thanked for his editorial support. We thank Nancy Cottington for initial figure drafting and Cari Breton for creating the map for Fig. 1. The project was funded by the Applied Geodynamics Laboratory (AGL) Industrial Associates program, comprising the following companies: Anadarko, Aramco Services, BHP Billiton, BP, CGG, Chevron, Condor, Ecopetrol, EMGS, ENI, ExxonMobil, Hess, Ion-GXT, Midland Valley, Murphy, Nexen USA, Noble, Petrobras, Petronas, PGS, Repsol, Rockfield, Shell, Spectrum, Equinor, Stone Energy, TGS, Total, WesternGeco, and Woodside (http://www.beg.utexas.edu/agl/sponsors).The ideas and content of this contribution have been strongly benefited from various fruitful discussions with colleagues such as Peter Flemings, Doug Jerolmack, Maria Nikolinakou, and Demian Saffer. Publication authorized by the Director, Bureau of Economic Geology, The University of Texas at Austin., Structural systems involving mobile shale represent one of the most difficult challenges for geoscientists dedicated to exploring the subsurface structure of continental margins. Mobile-shale structures range from surficial mud volcanoes to deeply buried shale diapirs and shale-cored folds. Where mobile shales occur, seismic imaging is typically poor, drilling is hazardous, and established principles to guide interpretation are few. The central problem leading to these issues is the poor understanding of the mechanical behaviour of mobile shales. Here we propose that mobile shales are at critical state, thus we define mobile shales as “bodies of clay-rich sediment or sedimentary rock undergoing penetrative, (visco-) plastic deformation at the critical state”. We discuss how this proposition can explain key observations associated with mobile shales. The critical-state model can explain the occurrence of both fluidized (no grain contact) shales (e.g., in mud volcanoes) and more viscous shales flowing with grain-to-grain contact (e.g., in shale diapirs), mobilization of cemented and compacted shales, and the role of overpressure in shale mobility. Our model offers new avenues for understanding complex and fascinating mobile-shale structures., Applied Geodynamics Laboratory (AGL) Industrial Associates program

We present an overview of the crustal architecture of the continental margins of the oceanic Algerian Basin in the westernmost Mediterranean Sea. During the Cenozoic, and with a variable oblique convergence between the African and Eurasian plates during the Cenozoic, the Western Mediterranean Sea has experienced thinning and extension behind a tight orogenic arc formed by the Betics, Rif, and Tell Cordilleras. This study is focused on the structural style affecting the Messinian salt layer, which is mostly restricted to the deep domains of the Algerian Basin, where it is floored by a thin oceanic crust of probable Miocene age. Using deep-penetrating seismic profiles and wells from offshore western Algeria to southeastern Spain, we have analyzed the crustal structures affecting the domains close to the oceanic-continent transition on the three margins of the western Algerian Basin. Since the Early Miocene, active shortening in the Tell-Atlas domain has accommodated most of the plate convergence in the basin, whereas the Alboran margin in the west and the Iberian margin in the north experienced eastward and southward crustal extension and thinning, respectively, accompanied by volcanism. The Algerian margin in the south shows incipient thrusting of African continental crust over oceanic crust. This shortening occurred since at least the Late Miocene, also promoting decoupling and contraction of the deep, sub-horizontal Messinian salt layer. The salt exhibits diapir squeezing and suprasalt folding, whereas the presalt sequence preserves partially-inverted half-grabens. Salt tectonic processes along the northern and western margins of the Western Mediterranean Basin show contrasting structural styles formed by narrow extensional and transtensional domains with gentle salt anticlines. This region shows therefore a somewhat unusual salt-tectonic style, departing from the gravity-driven model typical of continental margins that contain an initial continuous, gently-dipping salt layer. In the Algerian Basin, salt is mostly restricted to deep water domain floored by oceanic crust, so it does not participate in significant gravity-driven deformation. Instead, Messinian salt and the suprasalt sequences underwent significant shortening along the southern margin, simultaneous with thick-skinned extension involving the Messinian evaporites in the northern and eastern margins.

We use a 2D forward finite-element model to explore how a laterally continuous permeable bed impacts the geological evolution and the geomechanical properties of a salt basin. We show that a permeable bed tilted by rise of a salt diapir substantially increases pore pressure in sediments near the diapir through hydraulically connecting these sediments to deep, high-overpressure sediments far from the diapir. The pore-pressure increase near the diapir has the following significant consequences: it causes a faster rise of the diapir; brings sediments near the diapir close to shear failure in situ; causes unloading of sediments around the crest of the permeable bed; and reduces the margin of appropriate mud weights for drilling near the diapir. The rise of the salt diapir induces concentrated lateral deformation and thereby overpressure in mudrocks encasing the permeable bed in an area near the bottom of the basin. This anomalously high overpressure is in marked contrast with the overpressure in the permeable bed, resulting in a large pore-pressure gradient between the permeable bed and encasing mudrocks. Our study provides insight into the importance of permeable beds to the structural evolution of a salt basin and to the exploration and production of hydrocarbon near salt diapirs.

We use evolutionary poromechanical models to study stress and deformation in sediments that are deposited above a salt sheet, then are upturned in front of salt and eventually buried below salt. Sediments in our model represent rocks that are able to deform plastically. We find that high differential stresses develop, leading to shear failure of sediments as they are upturned in front of salt. However, we show that strength and failure evolve together with the salt system because the mode of loading from the advancing salt changes as sediments fold below salt. We also show that the path of salt flow is affected by the continuous changes in sediment strength. We discuss how the present-day geometry of the salt base may provide evidence of the level of shear below salt. We show that our forward modeling approach can help evaluate mechanical trap integrity by identifying areas that experienced elevated shear during earlier stages of the evolutionary process. Overall, our poromechanical models allow us to correlate kinematics of salt emplacement with the stress history of sediments and identify potential drilling hazards.

The Isthmian salt basin in the southern Gulf of Mexico can be divided into the Yucatan and Campeche subbasins, separated by a base-salt high near the nose of the Yucatan platform. Despite their proximity, these two subbasins experienced radically different histories in the period immediately following salt deposition. Portions of the Yucatan subbasin are characterized by large-scale (locally as much as 60 km [37 mi]) downdip translation of salt and suprasalt sediments during the Late Jurassic. This translation produced a major detached extensional province at the updip end of the basin, which is not compensated by observed shortening downdip. We interpret this history to be a result of unconfined seaward flow of salt and its cover during basin opening, a process mirrored on the conjugate Florida margin. The Campeche subbasin, in contrast, shows no evidence of significant Late Jurassic translation detached on salt. No large-scale extensional or contractional provinces of Mesozoic age are evident, although some minor translation did occur. We suggest that salt in the Campeche subbasin was confined at its seaward end, which prevented the seaward salt flow experienced in the Yucatan subbasin. Furthermore, salt at the seaward end of the Campeche subbasin lies 2–3 km (1–2 mi) above oceanic crust, in contrast to salt lying on crust whose top sits at or below the level of oceanic crust at the seaward ends of the Tamaulipas, Yucatan, and Florida margins. The Campeche subbasin thus appears to have been perched relative to other parts of the Gulf of Mexico.

Passive-margin salt basins tend to be much more deformed than their nonsalt equivalents, but they are by no means all the same. We used seismic data to study the Salina del Bravo region, northeast Mexico, to investigate the ways in which margin configuration and postsalt uplift history can influence passive-margin salt tectonics. The Salina del Bravo area contains four main structural systems, all of which trend NNE across the entire region. These structures are the Bravo trough, Sigsbee salt canopy, Perdido fold-and-thrust belt, and BAHA high. Gravity-driven deformation did not begin until more than 130 m.y. after salt deposition, because of buttressing against the BAHA high. We suggest that deformation was ultimately triggered in the Cenozoic by Cordilleran uplift that tilted the margin seaward and created a major sediment source terrane. Sediments shed from the uplift expelled salt seaward to form the Sigsbee canopy. At the same time, tilted and loaded sediments were translated seaward on the Louann salt until they were buttressed against the BAHA high, forming the Perdido fold-and-thrust belt. A physical model was built to test this hypothesis. The model was able to reproduce most of the major structures in the region, suggesting that the hypothesis is reasonable. The Salina del Bravo region shows how a downdip buttress can inhibit gravity-driven salt deformation in passive-margin salt basins. Furthermore, the area also shows the importance of postsalt uplift, which can destabilize a margin through a combination of tilting and sedimentation., We would like to thank CGG and TGS for permission to use seismic data. Nancy Cottington drafted the diagrams. Cari Breton constructed the geographic information system database. Eva Silverfine Ott edited the text. The paper was greatly improved by reviews from Chris Jackson and an anonymous reviewer. The project was funded by the Applied Geodynamics Laboratory consortium, consisting of the following companies: Anadarko, BHP Billiton, BP, CGG, Chevron, Condor, Ecopetrol, EMGS, Eni, Equinor, ExxonMobil, Fieldwood, Hess, ION Geophysical, Midland Valley, Murphy, Nexen, Noble, Petrobras, Petronas, PGS, Repsol, Rockfield, Saudi Aramco, Shell, Spectrum Geo, Stone, Talos, TGS, Total, West-ernGeco, and Woodside. Additional funding came from the Jackson School of Geosciences. Juan I. Soto also acknowledges the financial support provided by the Ministry of Education in Spain with a “Salvador de Madariaga” fellowship. Publication was authorized by the director of the Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin.

In salt-detached gravity-gliding/spreading systems the detachment geometry is a key control on the downslope mobility of the supra-canopy (supra-salt) sequence. As supra-canopy minibasins translate downslope, they also subside into salt. If the base of salt has high relief, minibasins may weld and stop from further free translation downslope. The degree of minibasin obstruction controls both the kinematics of the individual basins, and the more regional pattern of supra-canopy strain. Here, we use 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 with high relief.At a regional scale, we distinguish two structural domains in the study area: a highly obstructed or locked domain and a highly mobile domain. Large-scale translation of the supra-canopy sequence is recorded in the mobile domain by two different structures (a far-travelled minibasin and a ramp syncline basin). Although identifying the deformation area between the two regional domains is challenging due to its diffusive nature, characterizing domains according to base-of-salt geometry and supra-canopy minibasin configuration is helpful in identifying structural domains that may share similar subsidence and downslope translation histories.At minibasin scale, minibasins that become obstructed modify the local strain field, typically developing a zone of shortening immediately updip of it and an extensional breakaway zone immediately downdip. Seismic attribute analysis performed in a cluster of minibasins in the study area illustrates a long-lived sediment transport system affected by the complex strain patterns associated with minibasin obstruction. At an early stage, a submarine channel system is captured and subsequently rerouted in response to the updip shortening associated with minibasin obstruction. At a later stage, a mass-transport complex (MTC) is steered by the topographic barrier created by the downdip extensional breakaway associated with minibasin obstruction.Our work illustrates how salt-tectonic processes related to minibasin obstruction can affect the canopy dynamics at both regional and minibasin scale. Furthermore, we show that minibasin obstruction processes can modify the seafloor and subsequently control deepwater sediment dispersal, which, ultimately can affect hydrocarbon reservoir distribution on salt-influenced slopes

We used physical models to investigate the structural evolution of segmented extensional rifts containing syn-rift evaporites and their subsequent inversion. An early stage of extension generated structural topography consisting of a series of en-échelon graben. Our salt analog filled these graben and the surroundings before continued extension and, finally, inversion. During post-salt extension, deformation in the subsalt section remained focused on the graben-bounding fault systems whereas deformation in suprasalt sediments was mostly detached, forming a sigmoidal extensional minibasin system across the original segmented graben array. Little brittle deformation was observed in the post-salt section. Sedimentary loading from the minibasins drove salt up onto the footwalls of the subsalt faults, forming diapirs and salt-ridge networks on the intra-rift high blocks. Salt remobilization and expulsion from beneath the extensional minibasins was enhanced along and up the major relay/transfer zones that separated the original sub-salt grabens, forming major diapirs in these locations. Inversion of this salt-bearing rift system produced strongly decoupled shortening belts in basement and suprasalt sequences. Suprasalt deformation geometries and orientations are strongly controlled by the salt diapir and ridge network produced during extension and subsequent downbuilding. Thrusts are typically localized at minibasin margins where the overburden was thinnest and salt had risen diapirically on the horst blocks. In the subsalt section, shortening strongly inverted sub-salt grabens, which uplifted the suprasalt minibasins. New popup structures also formed in the subsalt section. Primary welds formed as suprasalt minibasins touched down onto inverted graben. Model geometries compare favorably to natural examples such as those in the Moroccan High Atlas.

Lateral shortening is expressed in unique ways in salt basins, especially if pre-shortening diapirs are present. We present an overview of previously-published and new physical models and present new 3-D conceptual models that capture the evolution of shortening structures in salt provinces dominated by precursor isolated diapirs (termed isolated-diapir provinces ). In such provinces, isolated diapirs form only a minor volumetric component of a sedimentary basin, however, due to the relative weakness of rock salt and their ability to localize strain, during shortening they have a disproportionately large influence on structural development. We find three key mechanical principles govern the processes and structural styles developed during shortening of isolated-diapir provinces. First, salt diapirs shorten before surrounding sedimentary rocks due to their relative weakness, and so form salients in the thrust front during early shortening. Second, diapirs tend to nucleate folds and faults, which radiate out from the diapirs. Third, as diapir walls converge, the roof must shorten. Extrusive salt sheets are expelled through thin roofs, but thicker roofs resist piercement and so tend to undergo complex folding and faulting. As a result of these principles, the first-order controls on the structural styles expressed across a shortened isolated-diapir province are the configuration the diapir array prior to shortening, the connectivity of these diapirs prior to shortening, total strain magnitude, and diapir roof thickness. Second-order controls include the initial cross-sectional and map-view geometry of diapirs, diapir size, and diapir orientation with respect to the shortening direction.

We simulate the evolution of stress and pore pressure in sediments bounding salt systems. Our evolutionary geomechanical models couple deformation with sedimentation and porous fluid flow. We find that high differential stresses develop near rising diapirs and below salt. Salt emplacement induces significant excess pressures that are comparable to the weight of the salt sheet. In addition, we show that the shear-induced component of the excess pressures is significant. We also find that low effective stresses result in low strength, which enables salt growth. We model salt as a solid viscoplastic and sediments as poro-elastoplastic materials, and calibrate the consolidation properties based on experimental testing on smectite-rich mudrocks typical of those in the Gulf of Mexico. Our approach can be applied to design stable well bores and provide insight into macroscale geological processes. Overall, we show that transient evolutionary models can provide estimates of stress and pore pressure in many geologic systems where large strains, pore fluids, and sedimentation interact. We close with a discussion of the need to better understand material behavior at geologic stress and timescales.

Recent subsalt petroleum discoveries associated with rifted‐margin salt basins have piqued interest in the presalt geology of the Gulf of Mexico margin. Available subsurface data does not uniquely constrain the subsalt geometry, so creating an interpretation of the crustal architecture requires the application of geological models for crustal extension and breakup. However, published interpretations of the nature of the transition from continental rifting to seafloor spreading range from magma‐rich to magma‐poor. To address this uncertainty, we present 2D forward kinematic models for crustal configurations generated by diverse models (symmetric extension, depth‐dependent extension, and volcanic extension). Through a series of conceptual balanced cross‐sections grounded in a ~600 km 2D seismic line from the NE Gulf of Mexico, we demonstrate the implications of each model for the limit of oceanic crust, basement morphology, crustal architecture, and hydrocarbon prospectivity. We discuss evidence for the dominant crustal processes, including geodynamic factors and structural and stratigraphic observations. Based on our observations and the geologic history, we favour an asymmetric, magma‐poor to ‐intermediate margin interpretation for the NE Gulf of Mexico, but suggest that the degree of volcanic input and width of the ocean‐continent transition zone may vary along strike. The along‐strike variability highlights the importance of understanding all potential presalt crustal configurations, their key features, and their implications. With increased data availability on the presalt geology in the Gulf of Mexico the relevance of these scenarios can be assessed, allowing development of comprehensive geodynamic and tectonic models of the margin and consideration of petroleum system elements in the presalt sequence.

We predict pore pressure and stress in an evolving salt basin (salt rising as a diapir and developing into a sheet) using a transient, large-strain, evolutionary geomechanical model. This model simulates mudrocks with a poro-elastoplastic material and couples sedimentation with salt deformation and porous-fluid flow. We show that pore pressures near salt are higher than predicted by assuming vertical uniaxial deformation. During salt-sheet emplacement, subsalt pore pressures equal the weight of salt, resulting in low effective stresses and very low sediment strength. We find that a dissipation zone develops parallel to the base of salt, allowing excess pore pressure to decrease with time by lateral drainage. In addition, we show that welding of the source layer and pedestal subsidence affect pore pressure in the deeper parts of the basin. We discuss how changes in both pore pressure and least principal stress lead to a narrow drilling window subsalt. We translate our stress field into an equivalent P-wave velocity field and find very low seismic velocities below salt. Finally, we compare our geomechanical pore-pressure prediction with that of porosity–effective stress workflows. We demonstrate the role of mean total stress in pressure prediction. In addition, we show that the prediction accuracy of porosity-based workflows depends on the relative level of shear (q/σ′m) in their calibration dataset, and how this ratio compares to the field (q/σ′m)field. Overall, our transient evolutionary model provides an estimate of the full stress tensor and pore pressure over time, and can help identify potential hazardous areas below salt. Our study illustrates the relative contribution of stress-tensor invariants to pore pressure and errors resulting from their omission. Furthermore, it advances our fundamental understanding of the interaction between fluid pressure, stress, and deformation in salt basins.

Salt boundary interpretation is a crucial step for velocity-model building in seismic migration, but it remains a highly labor-intensive task for manual interpretation and a big challenge for automatic methods. We have developed a semiautomatic method to efficiently and accurately extract 2D and 3D complicated salt boundaries from a seismic attribute image that highlights salt boundaries. In 2D salt boundary extraction, we first pick a few points to interpolate an initial curve that is close to the true salt boundary. These points are picked near the salt boundary but are not required to be exactly on the boundary, which makes human interactions convenient and efficient. We then resample the salt boundary attribute image in a band area centered at the initial curve to obtain a new image in which the true salt boundary is an open curve extending laterally. We then extract the salt boundary in the new image using an optimal-path picking algorithm, which is robust enough to track a stable salt boundary from highly discontinuous attribute values by solving a global maximization problem. We finally map the picked path back to the original image to obtain a final salt boundary. In 3D salt boundary extraction, we apply the 2D method to recursively pick 2D salt boundaries in a sequence of inline or crossline slices and then use these 2D boundaries to fit an implicit (level-set) surface of the 3D salt boundary. In our recursive picking, human interactions are greatly reduced by using a salt boundary picked in the previous slice as an initial curve for picking in a followed slice. The effectiveness of our method is confirmed with 2D and 3D real seismic images.

We use a forward finite-element model to explore pore pressure, stresses, and deformation near a vertical weld. Dipping layers of mudrocks interbedded with a mixture of basin-center and basin-wide sand layers are deposited sequentially over a salt layer. After a salt wall rises at the downdip end of the model and emerges at the basin surface, shortening is imposed on the system until the salt wall almost welds where the wall feeder is narrowest (waist). Welding causes a significant increase in horizontal stress, which results in increased pore pressure near the weld. The pore-pressure increase adversely affects hydrocarbon accumulation and drilling near the weld because it: 1) causes remigration of pore water downdip along sand beds cresting near the weld, 2) decreases the hydrocarbon-column capacity of these beds, and 3) narrows the mud-weight window for drilling wellbores near the weld. The least principal stress near the weld is almost vertical and equal to the overburden stress in mudrocks, but horizontal and significantly lower in sand beds. Welding of the salt wall causes steepening of flanking strata and shortening of the weld wall rocks. The impacts of welding begin at a waist width of 1.35 km, long before the salt wall welds.

Seismic interpretation of mobile shales is challenging, mostly because of their unclear seismic expression. Imaging of mobile shales is difficult because of their low seismic-impedance contrast with many sedimentary rocks, spatial variation of their seismic properties, complex geometries of mobile-shales structures, and their complex internal structures. Furthermore, their seismic properties depend strongly on both overpressure and fluid content, which are difficult to predict. To unravel this problem, we reviewed the physical properties of shales, merging data from in situ determinations of density and sonic velocities with experimental data and modeling results. We analyzed how diagenetic transformations during shale burial modify their physical properties and seismic characteristics. We reviewed conditions for smectite-illite transformation and gas generation (mostly methane) by oil cracking to evaluate how thermal gradient, shale composition, and hydrocarbon content modify the densities, sonic velocities, and seismic expressions of mobile shales. We then incorporated the amount and type of fluids released in shales during diagenesis into a study of seismic reflectivity of mobile shales. Results derived theoretically for various types of mobile-shale contacts are compared with high-quality seismic examples, including mud volcanoes and a variety of complex shale diapirs. Observed reflectivity and seismic fabrics are discussed to infer clay composition, fluid content and type, and temperature.

Salt depositional configuration exerted a fundamental control on both extensional and contractional deformation in the south-central Pyrenees. We divide the area into three domains and suggest that these domains are inherited from the original configuration of the Triassic Keuper salt basin. The Northern and Southern Domains are salt-diapir provinces with complex Mesozoic growth histories, whereas the Central Domain separating them has no diapirs and simpler Mesozoic isopachs. We use these patterns to interpret the Northern and Southern Domains as transported salt basins that were originally separated by the Central Horst, on which salt was thinner. During Dogger to mid-Cenomanian rifting, the Southern Salt Basin was an interior basin, isolated from deformation in the rift axis, whereas the Northern Salt Basin hosted an extensional breakaway at the updip end of the axial rift. During Pyrenean shortening, regions with preexisting salt structures were preferentially reactivated, whereas the stronger Central Horst remained relatively rigid. Eventually the sedimentary cover of the Southern Pyrenees was translated southwards and the connection with the original basement structure was lost. We tested our hypotheses using physical models. We confirmed that shortening a network of preexisting salt pillows could generate structures like the dome-and-basin pattern observed in the Southern Domain, and that it is possible to shorten salt basins on either side of a central horst while leaving sediments above the horst relatively undeformed.

We develop a two-dimensional plane strain large-deformation coupled poroelastoplastic finite element model to simulate initiation and rise of a salt wall from a flat salt body during sedimentation. We run transient analyses with high-permeability and low-permeability sediments in the model to simulate drained conditions and overpressure (or pore fluid overpressure). We investigate deformation, stress, and overpressure in the evolving suprasalt basin. Model results show that horizontal stress increases even higher than vertical stress at the flank of the salt wall and in the minibasin due to horizontal pushing out of the rising salt wall and that orientations of principal stresses rotate in the minibasin relative to far-field stress state. Model predicts that compared with far-field overpressure, the overpressure near the salt wall and within the minibasin is largely perturbed by the salt body. We find that perturbations of pore pressure (or pore fluid pressure) near the salt wall and within the minibasin cannot be resolved by traditional pore pressure prediction methods such as normal compaction trend approach and mean stress model approach. In order to predict pore pressure more accurately, especially in the regions near salt bodies or where stresses are perturbed, we need to apply a method that includes both effects of mean effective stress and deviatoric stress on pore pressure and compaction, for example, the Modified Cam Clay model approach, to pore pressure prediction. Our results provide geoscientists insights into evolution of salt basins, near-salt deformation, stress, overpressure, and overpressure prediction methods and have implications for near-salt wellbore drilling programs.

Passive margins underlain by a salt detachment are typically interpreted as kinematically linked zones of updip extension and downdip contraction separated by a zone of translation above a smoothly dipping base of salt. However, salt flow is affected by the base-of-salt geometry across which it flows, and early-stage gravity gliding induced by basin tilt may be complicated by the presence of salt-thickness changes caused by the pre-existing base-salt relief. We investigate these effects using physical models. Dip-parallel steps generate strike-slip fault zones separating domains of differential downslope translation and structural styles, provided the overburden is thin enough. If the overburden is thicker, it resists breakup, but a change in the structural trend occurs across the step. Steps with mild obliquity to the dip direction produce transtensional and transpressional faults in the cover separating structural domains. Deformation complexity in the overburden increases where base-salt steps strike at a high angle to salt flow, and it is especially dependent on the ratio between the thick ([Formula: see text]) and thin ([Formula: see text]) salt across the step at the base of salt. Where the salt-thickness ratio ([Formula: see text]) is high, basal drag generates major flux mismatches, resulting in a contractional thickening of the salt and associated overburden shortening in thin salt above a base-salt high block. Shortening is transient and superseded by extension as the salt thickening allows the flow velocity to increase. When transitioning off a base-salt high block into a low block, the greater flux within the thick salt results in a monocline with extensional and contractional hinges. Structures are further deformed as they translate through these hinge zones. Our physical models demonstrate that extensional diapirs and compressional fold belts can be initiated anywhere on a slope as the salt accelerates and decelerates across base-salt relief. A fold belt from the Campos Basin, offshore Brazil, is used to illustrate these processes.

In the eastern Gulf of Mexico, the pattern of early stage salt flow is complicated by basement topography consisting of a series of plunging arches that trend obliquely to the early flow direction. Seismic lines downdip of the Florida Middle Ground Arch reveal a puzzling array of structures. Sections trending roughly north–south (parallel to the regional dip) document predominantly extensional structures; however, east–west sections reveal shortening structures. Both sets of structures occur well updip of the downdip salt pinchout. We designed a physical-modeling study to investigate these puzzling relationships. Models presented in Part 1 of this paper indicate that localized shortening and extension can occur as salt passes over simple base-salt steps. Physical models were run with complex salt isopachs featuring plunging arches oblique to dip and salt flow. Models reveal the formation of shortening belts as the salt and its thin prekinematic overburden are translated across the arches. The complex salt isopachs deflect salt flow to produce convergent and divergent flow, which, along with flow-velocity gradients, results in the rotation of early formed thrust belts. Rotations of up to 70° were recorded in the most complex model, resulting in transported fold belts with trends that were close to dip parallel, similar to those observed on seismic data from the eastern Gulf of Mexico. Additional zones of shortening are found in and around complex salt pinchouts in the updip zones of the gravity-gliding system. The dynamic nature of these salt-related tectonic systems can result in the downdip translation of fold belts far from the basement topography over which they were created.

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. 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 findings are applicable to other systems detached on allochthonous salt sheets (e.g. Gulf of Mexico; Scotian Margin, offshore eastern Canada), as well as systems where the salt is autochthonous but has significant local basal relief (e.g. Santos Basin, offshore Brazil; Kwanza Basin, Angola).

Salt diapir shapes are influenced by the rate of salt supply into the base of the diapir, the rate of sedimentation, regional slope, local deformation around the salt body, and regional deformation. Prior graphical solutions to this complex problem consider only the salt supply and sediment aggradation rate; but there are major problems with these existing plots. Precise computation of diapir shapes using a new, fast analytical approach reveals that the shapes shown in previous publications were incorrect. “Rise rate”, a proxy used for salt supply in previous models, was inconsistently defined. The fundamental controlling factors used in this study to predict salt diapir shapes are the volumetric rate of salt supply to the base of the diapir, and the rate of sediment aggradation. These are combined to create predictive charts of salt shapes for both radial and linear geometries. We discuss the strengths and limitations intrinsic to our fast analytical models, suggesting geological conditions where they may be appropriate. Simple downbuilding models may not be directly applicable to geological settings that involve subsequent deformation of the sediments or of the salt body. However, the modified salt downbuilding concept as elaborated in our study can explain many real-world situations where the salt body flares or tapers upwards.

The crustal structure of the continent-ocean transition zone in the South Atlantic salt basins is poorly understood. Current interpretations place the limits of oceanic crust at the distal salt limits, with sub-salt crust consisting of rifted continental crust and, in some versions, varying amounts of exhumed mantle. Plate reconstructions that map these limits of oceanic crust onto appropriate-age restorations show poor geometric fits, with unexplained gaps and overlaps. One possible reason for the poor fits is that the distal salt limits are not the real limits of oceanic crust. In this paper we investigate this option by mapping rift basins and seaward-dipping reflectors whose seaward edges mark significant structural boundaries as much as 300 km inboard of the distal salt limits. We interpret these boundaries, which match geometrically in a salt-age (Aptian) plate reconstruction, to be the limits of oceanic crust. We suggest that salt was deposited as seafloor spreading commenced and that, as the South Atlantic opened, salt flowed over the ridge axis, sealing off the extrusive component of oceanic crust, resulting in formation of intrusive oceanic crust. Seafloor spreading eventually broke through the thinning salt, forming breakthrough volcanoes preserved today as basement ramps at the distal salt limits. These ramps formed diachronously, so the distal salt limits are not isochrons, explaining the poor fit of these features in plate reconstructions.

We model the stress–strain response of shale wall rocks to large deformations associated with the emplacement of salt bodies. We further identify the implications of these stress changes for hydrocarbon exploration. We model the mudrocks as porous elastoplastic materials. We employ both static and evolutionary approach for the modeling of salt systems and show that while the static one can model actual geologic geometries, only the evolutionary approach can provide a detailed description of the stress changes associated with the emplacement of salt. Hence, the evolutionary approach can register the overall stress history of the shale wall rocks, which is essential for predicting the present-day state of stress, porosity, and pore pressure. More generally, the evolutionary approach can provide useful insights for understanding Earth processes related to salt-hydrocarbon systems.

The 2-D concept of diapiric downbuilding is expanded to the third dimension. Our advanced approach shows which range of salt fluxes and sedimentation rates (time dependency included) can explain the formation of upward narrowing diapirs (previously thought of as R˙/A˙ 1), and vertical cylindrical diapirs (~ R˙/A˙ = 1). In addition to being a nonrigorous measure, R˙/A˙ does not account for the effect of a regional slope on diapir shape during downbuilding. When a regional topographic slope is absent and lateral changes in sedimentation rate are negligible, our models confirm downbuilding diapirs always develop axisymmetric shapes. However, when the salt source lies on a regional slope, asymmetric diapirs will form. The rate of salt supply from the source and the superposed gravity flow downslope control the flow lines and shape of the advancing salt sheet. In the 2-D gravity spreading plane, the ratio of source flow to gravity flow strengths can be characterized by a Rankine number. Obstruction of lateral spreading of salt issued from the feeder sources due to sedimentation lifts the salt from its 2-D spreading plane, which our model simulates by superposing a velocity component normal to the sedimentation surface. We next apply the 3-D analytical model to an offshore salt sheet in the Gulf of Mexico and analyze which conditions controlled its flow kinematics down the continental slope. Stratal cutoffs at the base of salt combined with the analytical gravity spreading model provide estimates for the dimensional flow rates and the effective viscosity of the Louann Salt.

Jackson et al. (this volume) used 3D seismic reflection data to describe intrasalt deformation in salt walls in the Santos Basin. They focused on the origin of enigmatic allochthonous salt sheets of older evaporites (A1 unit) emplaced above overlying stratified evaporites (A2–A4 units). Their kinematic model incorporates: (i) initial inward flow and thickening of A1 salt within the rising wall, and arching of A2–A4 overburden; (ii) breaching of the arched overburden, ascent of mobile A1 evaporites along single or multiple feeders, and emplacement of upper-wall sheets or canopies; and (iii) a component of regional shortening within the salt. This companion paper uses physical modelling to explain how and why these structures occur and proposes a mechanical basis for the kinematic model. Our first two models simulated salt having uniform internal density, with walls growing by (i) initially symmetric differential loading and (ii) initially symmetric differential loading plus shortening. These models reproduced anticlines and injection folds seen in the simpler deformed walls in the Santos Basin. However, neither model reproduced the most complex structures within the Santos evaporites, which are: (i) allochthonous intrusions, (ii) steep feeders, and (iii) recumbent synclines. Thus, differential loading and shortening alone are insufficient to generate these complex structures. In our third model, a less-dense lower evaporite (A1) was overlain by denser upper evaporites (A2–A4), similar to the density structure found by Santos Basin wellbores. The wall rose solely by differential loading. In this model, A1 breached the overlying evaporites to form vertical diapirs feeding salt sheets and salt wings in the upper part of the salt wall. Breakthrough of A1 folded A2–A4 evaporites into recumbent synclines. Model sections closely resemble Santos seismic examples, suggesting that the key to forming these complex intrasalt structures is a density inversion within the evaporite sequence that creates a Rayleigh–Taylor instability.

Thin-skinned gravitational gliding and spreading drive deformation on salt-bearing passive margins. Such margins typically have an updip extensional domain kinematically linked to a downdip contractional domain. However, calculating magnitudes of extension and shortening in salt-bearing margins is difficult because the initial widths of diapirs are uncertain. Extension and shortening may be cryptic, being hidden in widening or shortening of diapirs. This uncertainty can lead to controversy in regional analysis. The Santos Basin, offshore Brazil, contains a prime example of this uncertainty in the form of an enigmatic structure known as the “Albian Gap,” a zone up to 75 km wide within which the Albian section is missing. The Albian Gap has been variably interpreted as the product of post-Albian extensional faulting (the extension model) or as an Albian salt structure evacuated in response to loading by post-Albian sediments (the expulsion model). We evaluate these two models by: (1) structurally restoring a regional seismic-reflection profile across the Albian Gap using both models; (2) quantitatively analyzing the geometry of the Upper Cretaceous rollover overlying the Albian Gap; and (3) synthesizing and critically evaluating arguments previously advanced in support of extension or expulsion. We propose a revised model for the evolution of the Albian Gap that invokes Albian thin-skinned extension and post-Albian salt expulsion. Our approach shows that critical analysis of geological observations from borehole-constrained seismic-reflection data can be used to assess the relative roles of the key processes in the deformation of salt-bearing passive margins.

Critical-state wedges, ice glaciers, and salt sheets have many geometric and mechanical similarities. Each has a tapering geometry and moves along a basal detachment. Their motions result from the combined effects of internal deformation and basal sliding. Wedge deformation and geometry, basal conditions, and overpressure (pore fluid pressure less hydrostatic pore fluid pressure) development within the substrate interact with each other in this mechanically coupled system. However, the nature of this interaction is poorly understood. In order to investigate this coupled system, we have developed two-dimensional poromechanical finite-element models with porous fluid flow in sediments. We have simulated the advance of a salt sheet wedge across poroelastic sediments in this study. We emphasize that our results have applications beyond salt wedges to both critical-state wedges and ice glaciers. Overpressure develops within the substrate over time during the advance of the wedge. The magnitude of the overpressure influences the wedge geometry and the wedge advance rate. Lower overpressure results in a thicker and steeper wedge geometry, and a slower advance rate, while higher overpressure favors a thinner, wider, and more flattened wedge geometry and a faster advance rate. This study provides key insights into the links between wedge geometry, basal shear stress, and overpressure in substrates.

We compare an evolutionary with a static approach for modeling stress and deformation around a salt diapir; we show that the two approaches predict different stress histories and very different strains within adjacent wall rocks. Near the base of a rising salt diapir, significantly higher shear stresses develop when the evolutionary analysis is used. In addition, the static approach is not able to capture the decrease in the hoop stress caused by the circumferential diapir expansion, nor the increase in the horizontal stress caused by the rise of the diapir. Hence, only the evolutionary approach is able to predict a sudden decrease in the fracture gradient and identify areas of borehole instability near salt. Furthermore, the evolutionary model predicts strains an order of magnitude higher than the strains within the static model. More importantly, the evolutionary model shows significant shearing in the horizontal plane as a result of radial shortening accompanied by an almost-equivalent hoop extension. The evolutionary analysis is performed with ELFEN, and the static analysis with ABAQUS. We model the sediments using a poro-elastoplastic model. Overall, our results highlight the ability of forward evolutionary modeling to capture the stress history of mudrocks close to salt diapirs, which is essential for estimating the present strength and anisotropic characteristics of these sediments.

Salt boundary interpretation is a crucial step for velocity-model building in seismic migration, but remains a highly labor-intensive task for manual interpretation and a big challenge for automatic methods. We propose a semi-automatic method to efficiently and accurately extract 2D and 3D complicated salt boundaries from a seismic attribute image that highlights salt boundaries. In 2D salt boundary extraction, we first pick a few points to interpolate an initial curve that is close to the true salt boundary. These points are picked near the salt boundary but are not required to be exactly on the boundary, which makes human interactions convenient and efficient. We then resample the salt boundary attribute image in a band area centered at the initial curve to obtain a new image where the true salt boundary is an open curve extending laterally. We then extract the salt boundary in the new image using an optimal-path picking algorithm, which is robust to track a highly discontinuous salt boundary following ...