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Understanding intrasalt structure may elucidate the fundamental kinematics and, ultimately, the mechanics of diapir growth. However, there have been relatively few studies of the internal structure of salt diapirs outside the mining industry because their cores are only partly exposed in the field and poorly imaged on seismic reflection data. This study uses 3D seismic reflection and borehole data from the São Paulo Plateau, Santos Basin, offshore Brazil to document the variability in intrasalt structural style in natural salt diapirs. We document a range of intrasalt structures that record: (i) initial diapir rise; (ii) rise of lower mobile halite through an arched and thinned roof of denser, layered evaporites, and emplacement of an intrasalt sheet or canopy; (iii) formation of synclinal flaps kinematically linked to emplacement of the intrasalt allochthonous bodies; and (iv) diapir squeezing. Most salt walls contain simple internal anticlines. Only a few salt walls contain allochthonous bodies and breakout-related flaps. The latter occur in an area having a density inversion within the autochthonous salt layer, such that upper, anhydrite-rich, layered evaporites are denser than lower, more halite-rich evaporites. We thus interpret that most diapirs rose through simple fold amplification of internal salt stratigraphy but that locally, where a density inversion existed in the autochthonous salt, Rayleigh–Taylor overturn within the growing diapir resulted in the ascent of less dense evaporites into the diapir crest by breaching of the internal anticline. This resulted in the formation of steep salt-ascension zones or feeders and the emplacement of high-level intrasalt allocthonous sheets underlain by breakout-related flaps. Although regional shortening undoubtedly occurred on the São Paulo Plateau during the Late Cretaceous, we suggest this was only partly responsible for the complex intrasalt deformation. We suggest that, although based on the Santos Basin, our kinematic model may be more generally applicable to other salt-bearing sedimentary basins. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: Salt can be partially removed by viscous flow from between wall rocks to form a salt weld. Welds in autochthonous and allochthonous salt can form significant structures in evaporite basins, where petroleum and mineral discovery can hinge on whether salt welds act as seals or windows for migrating hydrocarbons or brines containing dissolved metals. Despite the importance of welds, little is known about salt evacuation during welding. We investigate viscous flow during welding using analytical and numerical models, based on exact solutions to the Navier–Stokes equations for idealized geometries and boundary conditions. We explore two questions: how does salt thin during evacuation, and what are the limits of viscous flow during salt welding? Hydraulic-gradient and displacement boundary conditions are shown to drive salt evacuation, which is rate-limited by drag along the boundaries of a salt layer. Where salt flow is restricted, for example beneath a broad, prograding sediment wedge, up to ~50m salt can remain in an incomplete weld. Where salt flow is unrestricted, for example beneath a subsiding minibasin, viscous flow can remove all but a vanishingly thin (<<1m) salt layer. In both cases, any remaining salt must be dissolved to leave a weld containing no remnant salt. Evacuation rate increases with increasing differential stress and decreasing flow length and dynamic viscosity of the salt. Translation of wall rock parallel to bedding may result in a fault weld but may also inhibit evacuation if the displacement counteracts flow driven by a hydraulic gradient. Evacuation of multilayered evaporites is controlled by the distribution of layer thickness and viscosity. Multilayered evaporites can be compositionally modified during evacuation. [Copyright &y& Elsevier]

Abstract: Gravity spreading of a buried salt canopy produces thrust faults around the canopy rim. These thrusts may form by shear along the base of the salt, by frontal rolling of the canopy, by expulsion of underlying sediments, or by submarine landsliding off the canopy''s frontal escarpment. Each mechanism has a characteristic structural style, providing clues to processes of shortening in canopy-margin thrust systems. Canopy-margin thrust systems along the Sigsbee Escarpment in the deepwater Gulf of Mexico may have three parts. First, nearly all canopy-margin systems include a roof-edge thrust, which cuts upsection from the tip of the salt canopy and separates condensed strata in the canopy roof from thicker sections on the adjoining peripheral plain. Second, roughly one-third of roof-edge thrusts overlie an imbricate wedge, which telescopes sediments on the peripheral plain above a bedding-plane décollement. Third, in a few areas, salt-roof thrusts shorten the roof of the salt sheet. Because roof-edge thrusts and imbricate wedges both form by shear along the base of the salt allochthon, their abundance along the Sigsbee Escarpment implies that basal shear is the primary mode of advance there. Despite the dominance of basal shear, however, the Sigsbee Escarpment exhibits a wide range of structural styles. We speculate that this range reflects variations in relief on the canopy margin, thickness of the roof above the canopy toe, strength of the base-salt zone, and strength of sediments on the peripheral plain. [Copyright &y& Elsevier]

Why do salt-floored minibasins subside? An almost universal explanation is that salt is forced from beneath the sinking basin by the weight of its sedimentary fill. This explanation is valid if the average density of the basin fill exceeds that of salt, which in the Gulf of Mexico needs at least 2300 m of siliciclastic fill to ensure enough compaction. However, most minibasins start sinking when they are much thinner than this. Some mechanism other than density inversion must explain the early history of these minibasins. Conventional understanding of minibasin subsidence is thus incomplete. Here, we identify five alternatives to density-driven subsidence of minibasins. (1) During diapir shortening, the squeezed diapirs inflate, leaving the intervening minibasins as bathymetric depressions. (2) In extensional diapir fall, stretching of a diapir causes it to sag, producing a minibasin above its subsiding crest. (3) During decay of salt topography, a dynamic salt bulge subsides as upward flow of salt slows, which lowers the salt surface below the regional sediment surface. (4) During sedimentary topographic loading, sediments accumulate as a bathymetric high above salt. (5) Finally, subsalt deformation affecting the base of salt may produce relief at the top of salt. Each mechanism (including density-driven subsidence) produces a different bathymetry, which interacts with sediment transport to produce different facies patterns in each type of minibasin. The particular mechanism for minibasin subsidence depends on the tectonic environment, regional bathymetry, and sedimentation rate. Their spatial variation on a continental margin creates provinces in which a given minibasin style is dominant. An appreciation of subsidence mechanisms should thus improve our understanding of minibasin fill patterns and allow genetic comparisons between minibasins. The mechanics of a minibasin sinking into fluid salt is in many ways analogous to a crustal basin sinking into a fluid asthenosphere. However, minibasins lack the complex rheologies, thermal histories, and compositional variations that make study of crustal basins so challenging. Minibasins are thus natural analogs and have the potential to elucidate fundamentals of subsidence mechanics. [ABSTRACT FROM AUTHOR]

Abstract: Following common usage, we broaden the term “salt” to include all rock bodies composed primarily of halite (NaCl). Salt is mechanically weak and flows like a fluid, even at geologically rapid strain rates. Salt is also relatively incompressible so is less dense than most carbonates and all moderately to fully compacted siliciclastic rocks. Salt''s fluid rheology and incompressibility make it inherently unstable under a wide range of geologic conditions. The primary driving force for salt tectonics is differential loading, which may be induced by gravitational forces, by forced displacement of one boundary of a salt body relative to another, or by a thermal gradient. Buoyancy, long considered a key driver for salt tectonics, is of secondary importance in many settings. Two factors resist salt flow: strength of the overburden and boundary drag along the edges of the salt body. Salt will move only if driving forces exceed the resistance to flow. In order for a salt diapir to be emplaced into its overburden, any rock previously occupying that space must be removed or displaced. Emplacement may occur by extension, erosion, or uplift of the overburden or by overthrusting of the salt. Once salt reaches the surface, it can continue to rise by passive diapirism, in which the diapir grows as sediments accumulate around it. A rapidly rising passive diapir may spread over the sediment surface to form an allochthonous salt sheet. A variety of salt-sheet lineages are possible, depending on the geometry of the feeder and the tectonic setting. Because salt is weak, its tectonism is closely tied to regional deformation. In extension or transtension, diapirs rise up graben axes, taking advantage of the space created by thinning and separation of fault blocks. Later, once the salt source layer is exhausted, diapirs may fall as they continue to widen. In addition, salt typically acts as a detachment in both gravity-driven and basement-involved extension. In compression or transpression, preexisting diapirs are rejuvenated as salt is displaced upward by lateral shortening. This rise is enhanced by buckling and disruption of the diapir roof. In the absence of precursor structures, salt''s primary role in compression is to act as a detachment. Some salt sheets may be emplaced in the hanging walls of thrust faults. [Copyright &y& Elsevier]

Abstract: Ramp syncline basins filled with synkinematic sediments record the incremental translation history and translation rates of strata transported over basement ramps. On passive margins, movement of strata over a stratabound salt detachment is stratigraphically recorded as an isopach thick on the seaward side of a monoclinal ramp. This growth syncline gradually moves seaward down and then off the monocline, as if on a conveyor belt. This shift creates accommodation space for a new isopach thick to form just landward of the older one. Repetition of this cycle creates a shingled series of isopach thicks within the ramp syncline basin. Kinematic forward models and restored cross sections from the Kwanza Basin, Angola, show that the stratigraphic pattern of ramp syncline basins is most influenced by three factors. (1) The relative rates of translation and aggradation control the curvature of the axial trace of the growth synclines. (2) Bathymetric scarps above basement ramps can create spectacular seaward-dipping onlap surfaces >30km long downdip; adjoining ramps generate stacked, interfering onlap surfaces. (3) Salt diapirs or anticlines are commonly shortened at the top of the ramp, especially where this coincides with the base of the continental slope. Diapir shortening provides a buffer to absorb sliding from its landward side and impede sliding on its seaward side. This buffering can have a major control on the entire gravity-spreading system containing the diapirs. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Intrasalt sediment packages containing siliciclastic sediments, carbonate sediments, or non-halite evaporites such as gypsum or anhydrite are common within most salt sequences. Intrasalt sediment packages may have been deposited before, during, or after salt deposition and be incorporated into the salt by various processes. Understanding the origin and evolution of intrasalt sediment packages may yield important insights into the tectonic and geodynamic history of the basin, and also into the understanding of salt tectonics. Despite the importance of intrasalt sediment packages, currently there is no systematic description of their possible origins and their distinguishing criteria. This work is divided into three parts. The first part outlines the possible origins of intrasalt sediment packages, as well as criteria to determine if they originated as subsalt, suprasalt or intrasalt sequences. The second part examines how sediment packages that originated on top of salt, such as minibasins, can be encased within salt. Four key encasement processes are proposed: a) salt expulsion from beneath a minibasin experiencing density-driven subsidence; b) salt expulsion from beneath adjacent subsiding minibasins; c) salt expulsion associated with lateral shortening; d) override of minibasins by a salt sheet sourced from elsewhere. The third part of the paper presents a case study from the SE Precaspian Basin, Kazakhstan, where, using a borehole-constrained 3D seismic reflection dataset, the proposed criteria are applied to an area with abundant, newly discovered sediment packages within salt. [ABSTRACT FROM AUTHOR]

Minibasin provinces are widespread and can be found in all types of salt tectonic settings, many of which are prone to shortening. Previous studies of how minibasin provinces shorten assume that the salt between the minibasins is homogeneous and that the base of salt is flat or of low relief, such that minibasins are free to move laterally. Here we investigate how minibasin provinces respond to shortening when the lateral mobility of the minibasins is restricted by intra-salt sediment bodies, in order to gain a greater understanding of the controls on the structural styles and modes of tectono-stratigraphic evolution exhibited in minibasin provinces. We examine a borehole-constrained, 3D seismic reflection dataset from the SE Precaspian Basin (onshore western Kazakhstan). The study area is characterised by large, supra-salt minibasins and an array of smaller intra-salt sediment packages distributed between these larger minibasins. We first outline the evidence of episodic shortening between the Late Triassic and present, after the onset of supra-salt minibasin subsidence. Next, we document spatial variations in shortening style, showing how these relate to the concentration of intra-salt sediment packages. Finally, we develop synoptic models showing how intra-salt sediment packages influence both the lateral mobility of minibasins during shortening and the resultant structural style, and we compare and contrast our findings with existing models and other natural examples of shortened minibasin provinces. We conclude that minibasin provinces may have different degrees of lateral mobility depending on the presence, or absence, of intrasalt barriers, and that these variations provide a first-order control on basin-shortening style and tectono-stratigraphic evolution. [ABSTRACT FROM AUTHOR]