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Salt structures and their surroundings can play an important role in the energy transition related to a number of storage and energy applications. Thus, it is important to assess the current and future stability of salt bodies in their specific geological settings. We investigate the influence of ice sheet loading and unloading on subsurface salt structures using physical models based on the geological setting of northern Germany, which was repeatedly glaciated by the Scandinavian Ice Sheet during the Pleistocene. Apparent spatial correlations between subsurface salt structures in northern Germany and Weichselian ice marginal positions have been observed before, and the topic is a matter of ongoing debate. Recently described geomorphological features – termed surface cracks – have been interpreted as a direct result of ice-sheet-induced salt movement resulting in surface expansion. The spatial clustering and orientation of these surface cracks has not been well understood so far, owing to only a limited number of available studies dealing with the related salt tectonic processes. Thus, we use four increasingly complex physical models to test the basic loading and unloading principle, to analyze flow patterns within the salt source layer and within salt structures, and to examine the influence of the shape and orientation of the salt structures with respect to a lobate ice margin in a three-dimensional laboratory environment. Three salt structures of the northern German basin were selected as examples that were replicated in the laboratory. Salt structures were initially grown by differential loading and buried before loading. The ice load was simulated by a weight that was temporarily placed on a portion of the surface of the models. The replicated salt structures were either completely covered by the load, partly covered by the load, or situated outside the load extent. In all scenarios, a dynamic response of the system to the load could be observed; while the load was applied, the structures outside the load margin started to rise, with a decreasing tendency with distance from the load margin, and, at the same time, the structures under the load subsided. After the load was removed, a flow reversal set in, and previously loaded structures started to rise, whereas the structures outside the former load margin began to subside. The vertical displacements during the unloading stage were not as strong as during the load stage, and thus the system did not return to its pre-glaciation status. Modeled salt domes that were located at distance from the load margin showed a comparably weak reaction. A more extreme response was shown by modeled salt pillows whose margins varied from sub-parallel to sub-perpendicular to the load margin and were partly covered by the load. Under these conditions, the structures showed a strong reaction in terms of strain and vertical displacement. The observed strain patterns at the surface were influenced by the shape of the load margin and the shape of the salt structure at depth, resulting in complex deformation patterns. These physical modeling results provide more evidence for a possible interplay between ice sheets and subsurface salt structures, highlighting the significance of three-dimensional effects in dynamic geological settings. Our results lead to a better understanding of spatial patterns of the surface cracks that were mapped at the surface above salt structures and offer further room for interpretation of the influence of salt movements on the present-day landscape. [ABSTRACT FROM AUTHOR]

Mud volcanoes are fed largely by fracture systems, but fracture systems are common in fold-and-thrust belts (FTBs), and not all of them extrude mud volcanoes. One popular hypothesis is that mud volcanoes extrude up normal faults and fractures in the crests of thrust-related anticlines. Using a high-resolution depth-migrated seismic cube in the northwestern Gulf of Mexico, we demonstrate the occurrence of buried mud volcanoes (shale sheets) that appear to have been fed by fluids moving up the thrust faults themselves. Mobile shales in this deepwater FTB detach a mostly welded allochthonous salt sheet. The main phase of shortening was during the Miocene, accompanying the sedimentation. Shortening generated multiple detachment levels, stacked duplexes, thrusts, and folds affecting the Oligocene mobile shales. Above these detachments, syn-sedimentary shortening initially generated detachment folds that progressively evolved into upward-propagating listric thrusts and fault-propagation anticlines. Shale sheets formed near the central thrust domains, associated with higher fault dips and larger fault displacements. We argue that their feeder systems were probably thrust-related shear zones that widened upward and comprised networks of anastomosing fractures. Finally, we propose that the position of mud volcanoes with respect to the underlying thrust systems depends on the path of least resistance to the surface. Although overpressured muds may vertically reach the surface via extensional fractures in the crests of thrust-related anticlines, we have shown to the contrary that when thrusts are emergent in a deepwater FTB, fluids continue up the inclined thrusts and adjacent fractures, forming mud volcanoes at the thrust tips. [ABSTRACT FROM AUTHOR]

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. Keywords: diapirism, Gulf of Mexico, salt tectonics, sedimentation, subsidence.

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 theology 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. Keywords: diapirism; orogenic belts; passive margins; review; salt tectonics

The Kwanza Basin, Angola, is divided into the Inner and Outer Kwanza salt basins, separated by a chain of synrift basement highs on which Aptian (112-122 Ma) salt is thin or absent. North- to northwest-trending basement structures in the Inner Kwanza Basin have repeatedly been reactivated since Neocomian (144-127 Ma) rifting. Reactivation formed three northwest-striking fold-and-thrust belts near basement uplifts. The thrust belts are bounded by northeast-striking rift-related transfer-fault zones that were apparently reactivated during subsequent shortening. Three episodes of postrift, basement-involved shortening are documented in the Inner Kwanza Basin: (1) Albian-early Cenomanian (112-96 Ma), (2) Senonian (89-65 Ma), and (3) Oligocene-Holocene (34-0 Ma). We relate the Albian-early Cenomanian event to ridge push, the Senonian event to global-plate reorganization, and Oligocene-Holocene shortening to uplift of the African superswell. Structural segmentation of the Inner Kwanza Basin controlled the evolution of salt structures. Adjacent to basement uplifts, diapirs were initiated as buckle folds. Some anticlines were unroofed by erosion and evolved into passive salt walls. Elsewhere, broad salt walls were triggered by either detached extension or basement-block uplift. These walls grew until they exhausted their supply of salt. Thereafter, dissolution rates exceeded rates of salt inflow, so the walls began to subside. Withdrawal of salt from the walls produced the elongate sedimentary troughs for which the basin is famous. Trough fill ranges in age from Cenomanian to Pliocene, and this age varies greatly from trough to trough and along strike within troughs. Keywords: Angola, basement tectonics, Kwanza Basin, passive margins, salt tectonics, segmentation.