Passive Diapirism Research Articles

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

AbstractBase‐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 seismic evidence of passively evolving Messinian Salt in the Offshore Syria

Accurate interpretation and analysis of good quality 2D-seismic data has contributed to understanding evolution of salt diapirism in the Syrian Offshore located in the easternmost Mediterranean. Salt diapirs in the study area are mainly characterized by sub-annular shape structures resultant of flow and withdrawal of the Messinian Salt as passive diapirs during the Plio-Pleistocene-Recent times. Analysis and interpretation of seismic data in the Syrian Offshore indicates that the Messinian Salt diapirs have evolved and rose up passively by downbuilding during and after deposition of Plio-Pleistocene Formation and persisted to rise until the present time. The deposition and the compressional tectonic forces had a significant role in triggering Messinian Salt and creating the sub-annular shape structures. The accelerated deposition of the lower Plio-Pleistocene Formation created utmost subsidence of the Latakia Basin and passive diapirism of the Messinian Salt. Additional proofs in the Plio-Pleistocene sediments, such as thickness variations constrain the passive diapirism of the Syrian Offshore. This article offers models for the subsurface evolution of Messinian Salt diapirs in the study area based on analysis and interpretation of 2D-seismic data sets. The relevance between the evolution of Messinian Salt and deposition processes is also explained and an ideal evolutionary model offered as to their passive origins.

Deformation of intrasalt competent layers in different modes of salt tectonics

Abstract. Layered evaporite sequences (LESs) comprise interbedded weak layers (halite and, commonly, bittern salts) and strong layers (anhydrite and usually non-evaporite rocks such as carbonates and siliciclastics). This results in a strong rheological stratification, with a range of effective viscosity up to a factor of 105. We focus here on the deformation of competent intrasalt beds in different endmember modes of salt tectonics, even though combinations are common in nature, using a combination of conceptual, numerical, and analog models, and seismic data. In bedding-parallel extension, boudinage of the strong layers forms ruptured stringers, within a halite matrix, that become more isolated with increasing strain. In bedding-parallel shortening, competent layers tend to maintain coherency while forming harmonic, disharmonic, and polyharmonic folds, with the rheological stratification leading to buckling and fold growth by bedding-parallel shear. In differential loading, extension and the resultant stringers dominate beneath suprasalt depocenters, while folded competent beds characterize salt pillows. Finally, in passive diapirs, stringers generated by intrasalt extension are rotated to near vertical and encased in complex folds during upward flow of salt. In all cases, strong layers are progressively removed from areas of salt thinning and increasingly disrupted and folded in areas of salt growth as deformation intensifies. The varying styles of intrasalt deformation impact seismic imaging of LES and associated interpretations. Ruptured stringers are often visible where they have low dips, as in slightly extended salt layers or beneath depocenters, but are poorly imaged in passive diapirs due to steep dips. In contrast, areas of slightly to moderately shortened salt typically have well-imaged, mostly continuous intrasalt reflectors, although seismic coherency decreases as deformation intensifies. Similarly, wells are most likely to penetrate strong layers in contractional structures and salt pillows, less likely in extended salt because they might drill between stringers, and unlikely in tall passive diapirs because the stringers are near vertical. Thus, both seismic and well data may be interpreted to suggest that diapirs and other areas of more intense intrasalt deformation are more halite rich than is actually the case.

Temporal evolution of the Darmadan salt diapir, eastern Fars region, Iran

A wide range of diapirs crop out along the eastern Fars region. They are constituted by the Pre-Cambrian to Early Cambrian Hormuz Fm. and located in the transition from the frontal structures of the Zagros fold-and-thrust belt and the Oman Ranges. While, in different previous works about the Persian Gulf, off-shore diapirs the deformation history has been determined on the basis of subsurface data, onshore evidence for salt tectonics activity is often limited to post-Oligocene stratigraphic units and is significantly overprinted by shortening. The Darmadan diapir is one of the few salt structures where is possible to observe halokinetic sequences involving the pre-collision Mesozoic succession in the Fars region of the Zagros fold-and-thrust belt. The objective of our study was to characterize on the base of field data, the stratigraphic and structural relationships between the Hormuz salt and its overburden in order to establish its temporal evolution. The Darmadan diapir is interpreted as a salt stock connected to a salt wall structure at depth evolving to a passive diapir, at least since Late Jurassic-Early Cretaceous times. Expansional geometries toward the diapir reveal that primary welding was already occurred at the Albian time. The onset of contractional deformation in the internal part of the Zagros and Oman orogenic systems is recorded by the reactivation of the Darmadan diapir during the Late Cretaceous. The Darmadan diapir continued to be exposed during the Paleogene and was squeezed during the Neogene to recent Zagros deformation. Second-order contractional features and related growth geometries constrain the timing of the secondary welding during the early stages of the late Miocene Darmadan anticline development. The explained structure remarks that the present structural trends of the Zagros fold-and-thrust belt in the eastern Fars region are the result of the reactivation of preexistent salt structures.

Controls on minibasin infill in the Nordkapp Basin: Evidence of complex Triassic synsedimentary deposition influenced by salt tectonics

The Nordkapp Basin is a northeast-trending upper Paleozoic rift basin located in the western Barents Sea. The presence of salt structures makes this basin of interest for hydrocarbon exploration. Nevertheless, no commercial discoveries have been made so far, and the basin remains poorly understood in terms of distribution of reservoirs and source rocks within its salt minibasins. In this study, a three-dimensional data set together with well data in a small area located in the southwestern subbasin of the Nordkapp Basin has been studied, with the main purpose of obtaining a better understanding of salt kinematics and minibasin infill, with special focus on the Triassic. The minibasin infill of the Nordkapp Basin has been subdivided into five second-order sequences, which mark the main regional events recorded in the Barents Sea. Further subdivision into third-order sequences has been made in the Triassic in response to the local halokinetic movements of the basin. In terms of salt kinematics, the Nordkapp minibasins have been affected by the following: (1) late Permian–Early Triassic minor salt mobilization, (2) latest Early Triassic reactive diapirism, (3) Early to Middle Triassic transition between reactive–passive diapirism, (4) Middle Triassic–Jurassic passive diapirism, (5) Late Jurassic–Early Cretaceous gravity gliding, and (6) Cenozoic regional contraction. The rapid changes in depositional environment observed in well 7228/7-1A, together with the large variability of seismic responses and reflection terminations, provide evidence of a complex Triassic paleogeography controlled by salt tectonics. Early to Middle Triassic differential loading of minibasins into the underlying salt formed broad diapir-induced highs, which might have acted as local sources of sediment for submarine debris flows and fan deltas developed adjacent to salt structures. During the Middle Triassic–Jurassic last stage of passive growth, salt diapirs formed local topographic highs covered by ephemeral roofs and caused narrow areas of deformation widths less than 1 km (3281 ft), developing composite halokinetic sequences. In minibasins, salt evacuations and associated subsidence controlled the spatial transgressive–regressive trends of fluviodeltaic systems coming from the southeast of the Barents Shelf. Rapidly subsiding minibasins were flooded by marine conditions, and intervals of organic-rich source rocks might have been deposited in salt withdrawal-related depressions. As subsidence decreased, newly developed regressive sequences of northwest-trending fluviodeltaic systems sourced from the southeast covered previous marine environments, possibly depositing potential fluviodeltaic reservoirs within the salt minibasins.

The impact of syn- and post-extension prograding sedimentation on the development of salt-related rift basins and their inversion: Clues from analogue modelling

Various studies have demonstrated the intrinsic interrelationship between tectonics and sedimentation in salt-related rift basins during extension as well as during their inversion by compression. Here, we present seven brittle–ductile analogue models to show that the longitudinal or transverse progradation of sediment filling an elongate extensional basin has a substantial impact on the growth of diapirs and their lateral geometrical variations. We use five extensional models to reveal how these prograding systems triggered diapir growth variations, from proximal to distal areas, relative to the sedimentary source. In the models, continuous passive diapir walls developed, after a short period of reactive–active diapiric activity, during syn-extensional homogeneous deposition. In contrast, non-rectilinear diapir walls grew during longitudinal prograding sedimentation. Both longitudinal and transverse post-extensional progradation triggered well-developed passive diapirs in the proximal domains, whereas incipient reactive–active diapirs, incipient roller-like diapirs, or poorly developed diapirs were generated in the distal domains, depending on the modelled sedimentary pattern. Two models included final phases of 6% and 10% shortening associated with basin inversion by compression, respectively, to discriminate compressional from purely extensional geometries. With the applied shortening, the outward flanks of existing diapir walls steepened their dips from 8°–17° to 30°–50°. Likewise, 6% of shortening narrowed the diapir walls by 32%–72%, with their fully closing (salt welds) with 10% of shortening. We compare our results with the distribution of salt walls and minibasins of the Central High Atlas diapiric basin in Morocco, which was infilled with a longitudinally prograding mixed siliciclastic and carbonatic depositional sequence during the Early–Middle Jurassic with a minimum thicknesses of 2.5–4.0 km.

Diapiric growth within an Early Jurassic rift basin: The Tazoult salt wall (central High Atlas, Morocco)

The Central High Atlas (Morocco) constitutes a diapiric province that hosts a complex array of elongated diapirs and minibasins that formed during the Lower Jurassic rift of the Atlas Basin. This paper aims to study the structure and growth evolution of the Tazoult diapiric wall, located in the Central High Atlas, by means of structural and sedimentological fieldwork integrated with remote sensing mapping. The Tazoult salt wall is a 20 km long x 3 km wide NE-SW trending ridge that exposes Upper Triassic red beds and basalts along its core. The succession flanking the salt wall ranges from Hettangian to Bajocian ages displaying spectacular sedimentary wedges in the SE and NW flanks. The Hettangian-early Sinemurian carbonates mainly crop out as blocks embedded in the core rocks. The ~1-km thick Pliensbachian platform carbonates display large subvertical flap structures along the flanks of the Tazoult salt wall with unconformities bounding tapered composite halokinetic sequences. In contrast, the ~2.5-km thick late Pliensbachian-Aalenian mixed deposits form tabular composite halokinetic sequences displaying small-scale hook halokinetic sequences. Passive diapirism resulted in the lateral extrusion of the evaporite-bearing rocks to form an allochthonous salt sheet towards the adjacent SE Amezrai minibasin. The Bajocian platform carbonates partially fossilized the Tazoult salt wall and thus constitute a key horizon to constrain the timing of diapir growth and discriminate diapirism from Alpine shortening. The Pliensbachian carbonate platform evolved as a long flap structure during the early growth of the Tazoult salt wall, well before the onset of the Alpine shortening.

Megaflaps adjacent to salt diapirs

Megaflaps are steep stratal panels that extend far up the sides of diapirs or their equivalent welds. They have multiple-kilometer fold widths and structural relief and are thus distinct from smaller-scale composite halokinetic sequences. Maximum dips range from near-vertical to completely overturned. Although overturned megaflaps are associated with flaring salt, there is no direct link between megaflap formation and the initiation of salt sheets. Strata within a megaflap are usually convergent, and the lower boundary is typically concordant with the top salt. The upper boundary ranges between a prominent onlap surface and a more diffuse zone of gradual rotation and thinning, and growth strata likewise display both onlap and stacked wedge geometries. We use quantitative cross-section restoration to elucidate the origin and development of megaflaps. Megaflaps typically represent the relatively thin roofs of early salt structures that include single-flap active diapirs, passive diapirs, salt pillows, and salt sheets. They develop during halokinetic drape folding as the minibasin sinks, during contractional squeezing of the diapir and its roof, or during some combination of the two. The kinematics are dominated by either limb rotation or kink-band migration, in which roof strata move through a fold hinge into a lengthening steep megaflap. Both restoration results and direct field evidence suggest that internal strain is minor, with little bed lengthening and thinning. Recognition and understanding of megaflaps are critical to successful petroleum exploration of three-way truncation traps against salt. Megaflaps also have implications for the lateral seal of stratigraphic traps and fluid pressures in minibasins.

Supra-salt normal fault growth during the rise and fall of a diapir: Perspectives from 3D seismic reflection data, Norwegian North Sea

Normal faulting and the deep subsurface flow of salt are key processes controlling the structural development of many salt-bearing sedimentary basins. However, our detailed understanding of the spatial and temporal relationship between normal faulting and salt movement is poor due to a lack of natural examples constraining their geometric and kinematic relationship in three-dimensions. To improve our understanding of these processes, we here use 3D seismic reflection and borehole data from the Egersund Basin, offshore Norway, to determine the structure and growth of a normal fault array formed during the birth, growth and decay of an array of salt structures. We show that the fault array and salt structures developed in response to: (i) Late Triassic-to-Middle Jurassic extension, which involved thick-skinned, sub-salt and thin-skinned supra-salt faulting with the latter driving reactive diapirism; (ii) Early Cretaceous extensional collapse of the walls; and (iii) Jurassic-to-Neogene, active and passive diapirism, which was at least partly coeval with and occurred along-strike from areas of reactive diapirism and wall collapse. Our study supports physical model predictions, showcasing a three-dimensional example of how protracted, multiphase salt diapirism can influence the structure and growth of normal fault arrays.

Geology of the Cotiella thrust sheet, southern Pyrenees (Spain)

ABSTRACTThis paper presents a geological map and cross-sections of the Cotiella thrust sheet at 1:25,000 and 1:50,000 scale, respectively. The map covers an area of 225 km2 in the southern Pyrenees. The structure of the studied area is dominated by an upper Santonian – lower Miocene thrust system, which transported a middle Coniacian-lower Santonian extensional basin, the Cotiella Basin, 10 s of kilometres southwards during the Pyrenean orogeny. The map focuses on the internal structure of the Cotiella Basin, its partially inverted extensional faults and its up to 6.5 km of upper Albian – lower Santonian sediments that unconformably overlie Upper Triassic salts. The map and cross-sections, together with outcrop-scale observations, have led us to interpret the rising of passive diapirs in the area. This newly recognized salt tectonic event in the Cotiella Basin is important because of the insights it can provide for the paleogeographic reconstruction of the Upper Cretaceous paleomargin of the northern parts of the Iberian Peninsula.

Evidence of early passive diapirism and tectonic evolution of salt structures in the western Kuqa depression (Quele area), southern Tianshan (NW China)

High-resolution remote-sensing imagery, seismic, well-log, magnetotelluric data, and field observations are integrated to investigate the Cenozoic salt-related structures in the western Kuqa depression (Quele area), southern Tianshan, NW China. Field observations of Oligocene to Miocene halokinetic sequences suggest early passive diapirism was developed in the Kuqa depression. More evidence also indicate that the progradation of the Paleogene Quele alluvial fan and basement steps have important controlling roles on the formation of the early salt structures. Rock salt was driven by the progradation wedge to flow basinward, then basement steps would provide resistance and hold the positions of early salt structures. On the basis of the kinematic model, it is suggested that the early Quele salt wall, which acts as a stress concentration zone, is deformed to the Quele salt nappe during the subsequent regional shortening, and controls the changes of along-strike geometries in the Quele area.

Experimental studies of the controls of the geometry and evolution of salt diapirs

Experimental models were conducted to study the controls of the evolution and geometry of salt diapirs, and the transition from passive to active diapirism. Both constant and variable sedimentation rate experiments were conducted. Constant sedimentation rate experiments show that the evolution of diapirs is strongly dependent on the rates of sand deposition and the thickness of the source layer. Low rates of sedimentation result in high ratios of salt flow to aggradation, leading to cylindrical diapirs which subsequently develop flared shapes with overhangs, whereas high sediment rates result in low ratios of salt flow to aggradation, resulting in tapered shapes and eventual eclipse and occlusion of the diapirs. Thick source layers result in higher salt flow rates than thin layers. Variable sedimentation rates result in changing shapes of diapirs over time. A small increase in sedimentation rates for flared or cylindrical diapirs result in an initial eclipse followed by tapering and a transition to active diapirism, enabling the diapir to pierce the overlying sediments. The narrower diapir continues to grow by passive diapirism. A large increase in rates results in a permanent eclipse, because the tapered diapir is unable to penetrate the greater thickness of the overburden. Finally, thicker source layers generally result in wider diapirs during both phases of deformation. Tapering and flaring are mechanisms of maintaining equilibrium between the rate of sedimentation and net salt flow rate. The resulting models are directly applicable to the analysis of poorly or partially imaged salt diapirs in natural subsurface examples.

3D geometric reconstruction of Upper Cretaceous passive diapirs and salt withdrawal basins in the Cotiella Basin (southern Pyrenees)

High-quality images provided by 3D seismic surveys have transformed the interpretations of salt geometries in passive margins. Complementary field studies are needed to expand the ideas derived from seismic image interpretation. The Cotiella Basin exposes middle Coniacian–early Santonian basins developed by post-rift gravity-driven extension and passive diapirism in the proto-Atlantic Ocean. Salt-related sedimentary and structural features were preserved during the subsequent Pyrenean Orogeny, but the diapirs were squeezed to such a degree that they were completely destroyed. As a result, their influence on the development of the Cotiella Basin remains unclear. This paper presents a 3D geometric reconstruction of the extensional faults, the stratigraphic surfaces and the associated salt structures with the main objective of discussing the role of extension versus salt withdrawal. Our results indicate that the main Cotiella Basin was dominated by extensional faulting combined with along-strike salt migration, whereas the smaller Armeña, Mediodía and Seira basins were mainly dominated by salt withdrawal, involving the growth of passive diapirs, salt pillows and transfer faults. This is relevant because the described structures constitute an almost unique example to study in outcrop the structural style and kinematics of structures usually found offshore.

Structural style and evolution of the Pyrenean-Provence thrust belt, SE France

AbstractThe Pyrenean-Provence fold-thrust belt is characterized by a geological complexity arising from superimposed tectonic history and the propagation of the deformation through a heterogeneous mechanical substratum inherited from Paleozoic and Mesozoic times. The construction of a regional balanced cross section together with field data show that the thrust system of the southeastern Provence region is characterized by a mixed thick- and thin-skinned tectonic style related to the inversion of deep-seated late Paleozoic-Triassic extensional structures and the décollement of the Mesozoic-Cenozoic sedimentary cover above Triassic series. Earliest Cenomanian restoration state highlights the northward pinched-out of the Lower Cretaceous sedimentary series above the main long-wavelength Durance High uplift. Latest Santonian restoration state indicates a southward tilting of ~2° of the basin attributed to the initial growth of the Pyrenean-Provence prism controlling the external flexure of the foreland. Thrusts propagation in the northern part of the Pyrenean-Provence fold-thrust belt was recorded to be synchronous during latest Cretaceous to Eocene time and produced a ~7° southward basin tilting. This major tilting is attributed to the tectonic inversion and basement thrust stacking of the Cap Sicié-Sainte Baume units. Cross section balancing shows a total horizontal basement shortening of 40 km (~35 %) across the Pyrenean-Provence foreland. The main part of this shortening (~37 km) was accommodated by thick-skinned thrusts involving basement south of the Arc syncline. ~5 km of shortening were accommodated northward by the Arc syncline and eastern Sainte-Victoire thin-skin structures, resulting from slip transferred from the deep thick-skinned intercutaneous thrust wedge. Finally we interpret salt tectonic structures of the southeastern Provence as passive diapirism growth during Jurassic to late Cretaceous time, and then reactivated during Pyrenean-Provence compression. Late normal faulting related to hypothetical reactive diapirism during the Oligocene extension episode was predominantly localized above inherited salt structures and probably controlled by inherited basement faults.

Extensional salt tectonics in the partially inverted Cotiella post-rift basin (south-central Pyrenees): structure and evolution

The Cotiella Massif in the south-central Pyrenees hosts upper Cretaceous gravity-driven extensional faults which were developed in the Bay of Biscay–Pyrenean paleorift margin of the Atlantic Ocean. They accommodate up to 6 km of post-rift carbonates above relict upper Triassic salt. Subsequent Pyrenean contractional deformation preserved the main extensional features, but most of the upper Triassic salt was expulsed and then dissolved, leaving little indications of the original salt volume. Nonetheless, several distinctive salt-related features are still recognizable both at outcrop and at basin scale, providing an exposed analogue for salt-floored extensional basins developed on passive margins. Based on field research, we re-interpret the tectonic evolution of the area and suggest that passive diapirs were coeval with gravity-driven extension during the development of the Cotiella basin. The given interpretations are supported with detailed geological maps, original structural data, cross sections and outcrop photographs. The discovery of previously unknown post-rift salt structures in the Cotiella Massif is an extra element to consider in the paleogeographic reconstructions of the upper Cretaceous passive margin of the Bay of Biscay–Pyrenean realm and consequently helps in our understanding of the evolution of current Atlantic-type margins.

The Szamotuły salt diapir and Mid-Polish Trough: Decoupling during both Triassic-Jurassic rifting and Alpine inversion

The Szamotuły diapir is located on the southwestern shoulder of the Mid-Polish Trough in west-central Poland. The area underwent crustal-scale extension during the Triassic-Jurassic and Alpine-related inversion during the Late Cretaceous to Paleogene. The diapir is sourced entirely from the Permian Zechstein salt, but there are also thin evaporites within the Triassic. A regional 2D depth-migrated seismic profile, an array of 2D time-migrated data, and quantitative structural restorations are used to illustrate that extensional and contractional deformation were almost completely decoupled by the Zechstein salt. Beneath the salt, interpreted Carboniferous half-grabens were reactivated during the Triassic, offsetting the base salt but not the top salt and causing regional thickening of the Triassic-Jurassic overburden. Inversion was accommodated by reverse movements on the deep faults and uplift of the Triassic-Jurassic strata to form the broad anticlinorium of the Mid-Polish Swell. Cover extension and contraction were concentrated around the Szamotuły Diapir. A linear reactive diapir formed during the Early to Middle Triassic and broke through to become a passive diapir during the Late Triassic that subsequently widened into the Jurassic. Along strike, coeval extension was recorded by ongoing reactive diapirism. Alpine contraction caused squeezing of the passive diapir and the correlative reactive diapir, folding of flanking and overlying strata, and inversion of some of the reactive normal faults. However, shortening was accommodated differently above and below the Upper Triassic Keuper salt. Lower and Middle Triassic strata moved laterally into salt, whether into the passive diapir or into the reactive diapir along strike. Younger strata were folded and thrusted, with delamination at the Keuper evaporites that were depositionally thicker adjacent to the reactive diapir. Zechstein salt squeezed from deeper levels flowed passively into the space created by delamination, producing an allochthonous salt wing in the subsurface.

Sedimentary and structural record of the Albian growth of the Bakio salt diapir (the Basque Country, northern Spain)

AbstractHowever salt has a viscous rheology, overburden rocks adjacent to salt diapirs have a brittle rheology. Evidence of deformation within the overburden has been described from diapirs worldwide. Gravity‐driven deposits are also present along the flanks of several diapirs. The well‐known example from the La Popa Basin in northern Mexico shows that such deposits may be organized into halokinetic sequences. This leads to several questions: (i) How does diapir growth contribute to overburden deformation? (ii) Are halokinetic sequence models valid for other areas beyond the La Popa Basin. The Bakio diapir and its well‐exposed overburden in Basque Country, Spain provides key elements to address these questions. The Bakio diapir consists of Triassic red clays and gypsum and is flanked by synkinematic middle to upper Albian units that thin towards the diapir. The elongate diapir parallels the Gaztelugatxe normal fault to the NE: both strike NE–SW and probably formed together during the middle Albian, as synkinematic units onlap the fault scarp. The diapir is interpreted as a reactive diapir in response to middle Albian motion on the Gaztelugatxe fault. The rate of salt rise is estimated to be about 500 m Myr−1 during this passive stage. During Late Albian, the diapir evolved passively as the Gaztelugatxe fault became inactive. Synkinematic units thinning towards the diapir, major unconformities, slumps and other gravity‐driven deposits demonstrate that most deformation related to diapir growth occurred at the sea floor. Halokinetic sequences composed of alternating breccias and fine‐grained turbidites recorded cyclic episodes of diapir flank destabilization. This work provides insights into drape fold and halokinetic sequence models and offers a new simple method for estimating rates of diapir growth. This method may be useful for outcrop studies where biostratigraphical data are available and for other passive diapirs worldwide.

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

AbstractAllochthonous 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.

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

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

Salt kinematics and regional tectonics across a Permian gas field: a case study from East Frisia, NW Germany

This study presents a reconstruction of the tectonic history of an Upper Rotliegend tight gas field in Northern Germany. Tectonism of the greater study area was influenced by multiple phases of salt movement, which produced a variety of salt-related structural features such as salt walls, salt diapirs as well as salt glaciers (namakiers). A sequential 2D retro-deformation and stratal backstripping methodology was used to differentiate mechanisms inducing salt movement and to discuss their relation to regional tectonics. The quantitative geometric restoration included sedimentary balancing, decompaction, fault-related deformation, salt movement, thermal subsidence, and isostasy to unravel the post-depositional tectonic overprint of the Rotliegend reservoir rock. The results of this study indicate that reactive salt diapirism started during an Early Triassic interval of thin-skinned extensional tectonics, followed by an active diapirism stage with an overburden salt piercement in the Late Triassic, and finally a period of intensive salt surface extrusion and the formation of salt glaciers (namakiers) in Late Triassic and Jurassic times. Since the Early Cretaceous, salt in the study area has been rising by passive diapirism.