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

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

The tectono‐sedimentary evolution of a supra‐detachment rift basin at a deep‐water magma‐poor rifted margin: the example of the Samedan Basin preserved in the Err nappe in SE Switzerland

We describe the tectono‐sedimentary evolution of a Middle Jurassic, rift‐related supra‐detachment basin of the ancient Alpine Tethys margin exposed in the Central Alps (SE Switzerland). Based on pre‐Alpine restoration, we demonstrate that the rift basin developed over a detachment system that is traced over more than 40 km from thinned continental crust to exhumed mantle. The detachment faults are overlain by extensional allochthons consisting of upper crustal rocks and pre‐rift sediments up to several kilometres long and several hundreds of metres thick, compartmentalizing the distal margin into sub‐basins. We mapped and restored one of these sub‐basins, the Samedan Basin. It consists of a V‐shape geometry in map view, which is confined by extensional allochthons and floored by a detachment fault. It can be restored over a minimum distance of 11 km along and about 4 km perpendicular to the basin axis. Its sedimentary infill can be subdivided into basal (initial), intermediate (widening) and top (post‐tectonic) facies tracts. These tracts document (1) formation of the basin initially bounded by high‐angle faults and developing into low‐angle detachment faults, (2) widening of the basin and (3) migration of deformation further outboard. The basal facies tract is made of locally derived, poorly sorted gravity flow deposits that show a progressive change from hangingwall to footwall‐derived lithologies. Upsection the sediments develop into turbidity current deposits that show retrogradation (intermediate facies tract) and starvation of the sedimentary system (post‐tectonic facies tract). On the scale of the distal margin, the syn‐tectonic record documents a thinning‐ and fining‐upward sequence related to the back stepping of the tectonically derived sediment source, progressive starvation of the sedimentary system and migration of deformation resulting in exhumation and progressive delamination of the thinned crust during final rifting. This study provides valuable insights into the tectono‐sedimentary evolution and stratigraphic architecture of a supra‐detachment basin formed over hyper‐extended crust.     

Along‐strike evolution of folding,stretching and breaching of supra‐salt strata in the Plataforma Burgalesa extensional forced fold system (northern Spain)

The Plataforma Burgalesa is a partly exposed extensional forced fold system with an intermediate salt layer, which has developed along the southern portion of the Basque‐Cantabrian Basin from Malm to Early Cretaceous as part of the Bay of Biscay‐Pyrenean rift system. Relationships between syn‐ and pre‐rift strata of the supra‐salt cover sequence and distribution of intra‐cover second‐order faults are observed both along seismic sections and at the surface. These relationships indicate an along‐strike variability of the extensional structural style. After a short period of salt mobilization and forced folding, high slip rates in the central portion of the major basement faults have rapidly promoted brittle behaviour of the salt layer, preventing further salt mobilization and facilitating the propagation of the fault across the salt layer. In contrast, at the tip regions of basement faults, slower slip rates have facilitated ductile salt behaviour, ensuring its further evaporite evacuation, preventing fault propagation across the salt layer and, in essence, allowing for a long‐living forced folding process. Our results indicate the important effect of along‐strike variation in displacement and displacement rates in controlling evaporite behaviour in extensional basins. Amount of displacement and displacement rates are key factors controlling the propagation of basement faults across evaporite layers. In addition, growth strata patterns are recognized as a powerful tool for constraining the up‐dip propagation history of basement faults in extensional fault‐related fold systems with intermediate décollement levels.     

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

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

Tectonics of the late Mesozoic wide extensional basin system in the China–Mongolia border region

The China–Mongolia border region contains many late Mesozoic extensional basins that together constitute a regionally extensive basin system. Individual basins within the system are internally composed of a family of sub‐basins filled with relatively thin sedimentary piles mostly less than 5 km in thickness. There are two types of sub‐basins within the basins, failed and combined, respectively. The failed sub‐basins are those that failed to continue developing with time. In contrast, the combined ones are those that succeeded in growing by coalescing adjacent previously isolated sub‐basins. Thus, a combined sub‐basin is bounded by a linked through‐going normal fault that usually displays a corrugated trace on map view and a shallower dip on cross‐section. Along‐strike existence of discrete depocenters and alternation of sedimentary wedges of different types validate the linkage origin of combined sub‐basins. Localized high‐strain extension resulted in large‐amount displacement on linked faults, but contemporaneously brought about the cessation of some isolated fault segments and the formation of corresponding failed sub‐basins in intervening areas between active linked faults. Some combined sub‐basins might have evolved into supradetachment basins through time, concurrent with rapid denudation of footwall rocks and formation of metamorphic core complexes in places. A tectonic scenario of the broad basin system can be envisioned as an evolution from early‐stage distributed isolated sub‐basins to late‐stage focused combined or/and supradetachment sub‐basins bounded by linked faults, accompanied by synchronous cessation of some early‐formed sub‐basins. Initiation of the late Mesozoic extension is believed to result from gravitational collapse of the crust that had been overthickened shortly prior to the extension. Compression, arising from collision of Siberia and the amalgamated North China–Mongolia block along the Mongol–Okhotsk suture in the time interval from the Middle to Late Jurassic, led to significant shortening and thickening over a broad area and subsequent extensional collapse. Pre‐ and syn‐extensional voluminous magmatism must have considerably reduced the viscosity of the overthickened crust, thereby not only facilitating the gravitational collapse but enabling the lower‐middle crust to flow as well. Flow of a thicker crustal layer is assumed to have occurred coevally with upper‐crustal stretching so as to diminish the potential contrast of crustal thickness by repositioning materials from less extended to highly extending regions. Lateral middle‐ and lower‐crustal flow and its resultant upward push upon the upper crust provide a satisfying explanation for a number of unusual phenomena, such as supracrustal activity of the extension, absence or negligibleness of postrift subsidence of the basin system, less reduction of crustal thickness after extension, and non‐compression‐induced basin inversion, all of which have been paradoxical in the previous study of the late Mesozoic basin tectonics in the China–Mongolia border region.     

Tectono‐sedimentary development of early syn‐rift deposits: the Abura Graben,Suez Rift,Egypt

The thickness and distribution of early syn‐rift deposits record the evolution of structures accommodating the earliest phases of continental extension. However, our understanding of the detailed tectono‐sedimentary evolution of these deposits is poor, because in the subsurface, they are often deeply buried and below seismic resolution and sparsely sampled by borehole data. Furthermore, early syn‐rift deposits are typically poorly exposed in the field, being buried beneath thick, late syn‐rift and post‐rift deposits. To improve our understanding of the tectono‐sedimentary development of early syn‐rift strata during the initial stages of rifting, we examined quasi‐3D exposures in the Abura Graben, Suez Rift, Egypt. During the earliest stage of extension, forced folding above blind normal fault segments, rather than half‐graben formation adjacent to surface‐breaking faults, controlled rift physiography, accommodation development and the stratigraphic architecture of non‐marine, early syn‐rift deposits. Fluvial systems incised into underlying pre‐rift deposits and were structurally focused in the axis of the embryonic depocentre, which, at this time, was characterized by a fold‐bound syncline rather than a fault‐bound half graben. During this earliest phase of extension, sediment was sourced from the rift shoulder some 3 km to the NE of the depocentre, rather than from the crests of the flanking, intra‐basin extensional forced folds. Fault‐driven subsidence, perhaps augmented by a eustatic sea‐level rise, resulted in basin deepening and the deposition of a series of fluvial‐dominated mouth bars, which, like the preceding fluvial systems, were structurally pinned within the axis of the growing depocentre, which was still bound by extensional forced folds rather than faults. The extensional forced folds were eventually locally breached by surface‐breaking faults, resulting in the establishment of a half graben, basin deepening and the deposition of shallow marine sandstone and fan‐delta conglomerates. Because growth folding and faulting were coeval along‐strike, syn‐rift stratal units deposited at this time show a highly variable along‐strike stratigraphic architecture, locally thinning towards the growth fold but, only a few kilometres along‐strike, thickening towards the surface‐breaking fault. Despite displaying the classic early syn‐rift stratigraphic motif recording net upward‐deepening, extensional forced folding rather than surface faulting played a key role in controlling basin physiography, accommodation development, and syn‐rift stratal architecture and facies development during the early stages of extension. This structural and stratigraphic observations required to make this interpretation are relatively subtle and may go unrecognized in low‐resolution subsurface data sets.     

Architecture,deformation style and petrophysical properties of growth fault systems: the Late Triassic deltaic succession of southern Edgeøya (East Svalbard)

The Late Triassic outcrops on southern Edgeøya, East Svalbard, allow a multiscale study of syn‐sedimentary listric growth faults located in the prodelta region of a regional prograding system. At least three hierarchical orders of growth faults have been recognized, each showing different deformation mechanisms, styles and stratigraphic locations of the associated detachment interval. The faults, characterized by mutually influencing deformation envelopes over space‐time, generally show SW‐ to SE‐dipping directions, indicating a counter‐regional trend with respect to the inferred W‐NW directed progradation of the associated delta system. The down‐dip movement is accommodated by polyphase deformation, with the different fault architectural elements recording a time‐dependent transition from fluidal‐hydroplastic to ductile‐brittle deformation, which is also conceptually scale‐dependent, from the smaller‐ (3rd order) to the larger‐scale (1st order) end‐member faults respectively. A shift from distributed strain to strain localization towards the fault cores is observed at the meso to microscale (<1 mm), and in the variation in petrophysical parameters of the litho‐structural facies across and along the fault envelope, with bulk porosity, density, pore size and microcrack intensity varying accordingly to deformation and reworking intensity of inherited structural fabrics. The second‐ and third‐order listric fault nucleation points appear to be located above blind fault tip‐related monoclines involving cemented organic shales. Close to planar, through‐going, first‐order faults cut across this boundary, eventually connecting with other favourable lower‐hierarchy fault to create seismic‐scale fault zones similar to those imaged in the nearby offshore areas. The inferred large‐scale driving mechanisms for the first‐order faults are related to the combined effect of tectonic reactivation of deeper Palaeozoic structures in a far field stress regime due to the Uralide orogeny, and differential compaction associated with increased sand sedimentary input in a fine‐grained, water‐saturated, low‐accommodation, prodeltaic depositional environment. In synergy to this large‐scale picture, small‐scale causative factors favouring second‐ and third‐order faulting seem to be related to mechanical‐rheological instabilities related to localized shallow diagenesis and liquidization fronts.     

The timing of salt structure growth in the Southern Permian Basin (Central Europe) and implications for basin dynamics

In this paper, a literature‐based compilation of the timing and history of salt tectonics in the Southern Permian Basin (Central Europe) is presented. The tectono‐stratigraphic evolution of the Southern Permian Basin is influenced by salt movement and the structural development of various types of salt structures. The compilation presented here was used to characterize the following syndepositional growth stages of the salt structures: (a) “phase of initiation”; (b) phase of fastest growth (“main activity”); and (c) phase of burial’. We have also mapped the spatial pattern of potential mechanisms that triggered the initiation of salt structures over the area studied and summarized them for distinct regions (sub‐basins, platforms, etc.). The data base compiled and the set of maps produced from it provide a detailed overview of the spatial and temporal distribution of salt tectonic activity enabling the correlation of tectonic phases between specific regions of the entire Southern Permian Basin. Accordingly, salt movements were initiated in deeply subsided graben structures and fault zones during the Early and Middle Triassic. In these areas, salt structures reached their phase of main activity already during the Late Triassic or the Jurassic and were mostly buried during the Early Cretaceous. Salt structures in less subsided sub‐basins and platform regions of the Southern Permian Basin mostly started to grow during the Late Triassic. The subsequent phase of main activity of these salt structures took place from the Late Cretaceous to the Cenozoic. The analysis of the trigger mechanisms revealed that most salt structures were initiated by large‐offset normal faults in the sub‐salt basement in the large graben structures and minor normal faulting associated with thin‐skinned extension in the less subsided basin parts.     

Structural controls on the stratigraphic architecture of net‐transgressive shallow‐marine strata in a salt‐influenced rift basin: Middle‐to‐Upper Jurassic Egersund Basin,Norwegian North Sea

In this study, we integrate 3D seismic reflection, wireline log, biostratigraphic and core data from the Egersund Basin, Norwegian North Sea to determine the impact of syn‐depositional salt movement and associated growth faulting on the sedimentology and stratigraphic architecture of the Middle‐to‐Upper Jurassic, net‐transgressive, syn‐rift succession. Borehole data indicate that Middle‐to‐Upper Jurassic strata consist of low‐energy, wave‐dominated offshore and shoreface deposits and coal‐bearing coastal‐plain deposits. These deposits are arranged in four parasequences that are aggradationally to retrogradationally stacked to form a net‐transgressive succession that is up to 150‐m thick, at least 20 km in depositional strike (SW‐NE) extent, and >70 km in depositional dip (NW‐SE) extent. In this rift‐margin location, changes in thickness but not facies are noted across active salt structures. Abrupt facies changes, from shoreface sandstones to offshore mudstones, only occur across large displacement, basement‐involved normal faults. Comparisons to other tectonically active salt‐influenced basins suggest that facies changes across syn‐depositional salt structures are observed only where expansion indices are >2. Subsidence between salt walls resulted in local preservation of coastal‐plain deposits that cap shoreface parasequences, which were locally removed by transgressive erosion in adjacent areas of lower subsidence. The depositional dip that characterizes the Egersund Basin is unusual and likely resulted from its marginal location within the evolving North Sea rift and an extra‐basinal sediment supply from the Norwegian mainland.     

Post-Jurassic tectono-sedimentary evolution of the Northern Lusitanian Basin (Western Iberian margin)

Seismic reflection and well stratigraphic data are used to investigate the post‐Jurassic evolution of the Northern Lusitanian Basin, offshore west Iberia. Stratigraphic correlations between 11 exploration wells were attained in order to characterize the variations in depositional facies associated with salt tectonics. Latest Triassic–Hettangian salt, which generated multiple salt pillows during the Jurassic rifting, was reactivated after the early Aptian in two main phases. The first phase stretches from the late Turonian to the Maastrichtian. The second relates to Miocene tectonic inversion. The compression of the post‐salt overburden caused the amplification of Jurassic detachment folds, forming barriers to the westward progradation of sediment into distinct salt‐withdrawal sub‐basins. Particularly during the Miocene, thin‐skinned overburden shortening was accommodated by growing salt structures that suffered thrusting and extrusion. This structural style contrasts with that of salt‐scarce areas where a simple westerly tilted, fault‐bounded monocline was generated.     

Tectonics and sedimentation in the hangingwall of a major extensional detachment: the Devonian Kvamshesten Basin, western Norway

The Middle Devonian Kvamshesten Basin in western Norway is a late-orogenic basin situated in the hangingwall of the regional extensional Nordfjord–Sogn Detachment Zone. The basin is folded into a syncline with the axis subparallel to the ductile lineations in the detachment zone. The structural and stratigraphic development of the Kvamshesten Basin indicates that the basin history is more complex than hitherto recognized. The parallelism stated by previous workers between mylonitic lineation below the basin and intrabasinal fold axes is only partly reflected in the configuration of sedimentary units and in the time-relations between deposits on opposing basin margins. The basin shows a pronounced asymmetry in the organization and timing of sedimentary facies units. The present northern basin margin was characterized by bypass or erosion at the earliest stage of basin formation, but was subsequently onlapped and eventually overlain by fanglomerates and sandstones organized in well-defined coarsening-upwards successions. The oldest and thickest depositional units are situated along the present southern basin margin. This as well as onlap relations towards basement at low stratigraphic level indicates a significant component of southwards tilt of the basin floor during the earliest stages of deposition. The inferred south-eastwards tilt was most likely produced by north-westwards extension during early stages of basin formation. Synsedimentary intrabasinal faults show that at high stratigraphic levels, the basin was extending in an E–W as well as a N–S direction. Thus, the basin records an anticlockwise rotation of the syndepositional strain field. In addition, our observations indicate that shortening normal to the extension direction cannot have been both syndepositional and continuous, as suggested by previous authors. Through most of its history, the basin was controlled by a listric, ramp-flat low-angle fault that developed into a scoop shape or was flanked by transfer faults. The basin-controlling fault was rooted in the extensional mylonite zone. Sedimentation was accompanied by formation of a NE- to N-trending extensional rollover fold pair, evidenced by thickness variations in the marginal fan complexes, onlap relations towards basement and the fanning wedge geometry displayed by the Devonian strata. Further E–W extension was accompanied by N–S shortening, resulting in extension-parallel folds and thrusts that mainly post-date the preserved basin stratigraphy. During shortening, conjugate extensional faults were rotated to steeper dips on the flanks of a basin-wide syncline and re-activated as strike-slip faults. The present scoop-shaped, low-angle Dalsfjord fault cross-cut the folded basin and juxtaposed it against the extensional mylonites in the footwall of the Nordfjord–Sogn detachment. Much of this juxtaposition may post-date sedimentation in the preserved parts of the basin. Basinal asymmetry as well as variations in this asymmetry on a regional scale may be explained by the Kvamshesten and other Devonian basins in western Norway developing in a strain regime affected by large-scale sinistral strike-slip subparallel to the Caledonian orogen.     

Interaction of tectonics, eustasy, climate and carbonate production on the sedimentary evolution of an early/middle Jurassic extensional basin (Southern Provence Sub-basin, SE France)

This paper describes the evolution of an extensional basin in regard to the nature and sequence stratigraphic arrangement of its carbonate deposits. The purpose of this study is to evaluate the respective effects of tectonism, eustasy, climate and oceanography on a carbonate sedimentary record. The case study is the early to mid‐Jurassic age carbonate succession of the Southern Provence Sub‐basin (SE France), located within the southern part of the extensional Western European Tethyan Margin. This work is based on sedimentologic, biostratigraphic (using ammonites and brachiopods) and sequence stratigraphic analysis of the carbonate facies of the Cherty Reddish Limestone Formation (late Sinemurian to earliest Bajocian). These strata were deposited in shoreface to lower offshore depositional environments. The succession of the various environments together with the recognition of key stratigraphic surfaces allow us to define four second‐order depositional sequences; of late Sinemurian to earliest Pliensbachian, early Pliensbachian to late Pliensbachian, earliest Toarcian to middle Aalenian and late Aalenian to early Bathonian ages. The architecture of the depositional sequences (thickness and facies variations within the systems tracts, wedge‐shaped geometries) reflects a strong tectonic control. The sub‐basin was structured by extensional faults (oriented approximately 070–090/250–270). Sea‐level variations, fluctuations in carbonate production and preservation, and environmental changes were also significant controlling factors of the carbonate deposition. The interplay of the tectonic control with the other factors resulted in five main phases in the sedimentary evolution of the sub‐basin: (1) dominant tectonic control during the initial rifting stage (late Sinemurian to early Pliensbachian); (2) increasing extensional tectonics (mid‐Pliensbachian); (3) global climato‐eustatic sea‐level fall (latest Pliensbachian) and global climato‐eustatic sea‐level rise plus hypoxia/anoxia (early Toarcian); (4) relative sea‐level fall linked to tectonic uplift related to the ‘Mid‐Cimmerian phase’ (mid‐Aalenian) and (5) oceanographic events (upwelling) and reduction in carbonate production (hypoxia/anoxia) plus tectonic downwarping (late Aalenian/earliest Bajocian).     

The Miocene tectono-sedimentary evolution of the southern Tyrrhenian Sea: stratigraphy, structural and palaeomagnetic data from the on-shore Amantea basin (Calabrian Arc, Italy)

We report on new stratigraphic, palaeomagnetic and anisotropy of magnetic susceptibility (AMS) results from the Amantea basin, located on‐shore along the Tyrrhenian coast of the Calabrian Arc (Italy). The Miocene Amantea Basin formed on the top of a brittlely extended upper plate, separated from a blueschist lower plate by a low‐angle top‐to‐the‐west extensional detachment fault. The stratigraphic architecture of the basin is mainly controlled by the geometry of the detachment fault and is organized in several depositional sequences, separated by major unconformities. The first sequence (DS1) directly overlaps the basement units, and is constituted by Serravallian coarse‐grained conglomerates and sandstones. The upper boundary of this sequence is a major angular unconformity locally marked by a thick palaeosol (type 1 sequence boundary). The second depositional sequence DS2 (middle Tortonian‐early Messinian) is mainly formed by conglomerates, passing upwards to calcarenites, sandstones, claystones and diatomites. Finally, Messinian limestones and evaporites form the third depositional sequence (DS3). Our new biostratigraphic data on the Neogene deposits of the Amantea basin indicate a hiatus of 3 Ma separating sequences DS1 and DS2. The structural architecture of the basin is characterized by faulted homoclines, generally westward dipping, dissected by eastward dipping normal faults. Strike‐slip faults are also present along the margins of the intrabasinal structural highs. Several episodes of syn‐depositional tectonic activity are marked by well‐exposed progressive unconformities, folds and capped normal faults. Three main stages of extensional tectonics affected the area during Neogene‐Quaternary times: (1) Serravallian low‐angle normal faulting; (2) middle Tortonian high‐angle syn‐sedimentary normal faulting; (3) Messinian‐Quaternary high‐angle normal faulting. Extensional tectonics controlled the exhumation of high‐P/low‐T metamorphic rocks and later the foundering of the Amantea basin, with a constant WNW‐ESE stretching direction (present‐day coordinates), defined by means of structural analyses and by AMS data. Palaeomagnetic analyses performed mainly on the claystone deposits of DS1 show a post‐Serravallian clockwise rotation of the Amantea basin. The data presented in this paper constrain better the overall timing, structure and kinematics of the early stages of extensional tectonics of the southern Tyrrhenian Sea. In particular, extensional basins in the southern Tyrrhenian Sea opened during Serravallian and evolved during late Miocene. These data confirm that, at that time, the Amantea basin represented the conjugate extensional margin of the Sardinian border, and that it later drifted south‐eastward and rotated clockwise as a part of the Calabria‐Peloritani terrane.     

Deformed Messinian markers in the Cyprus Arc: tectonic and/or Messinian Salinity Crisis indicators?

This paper focuses on Messinian Salinity Crisis (MSC) evaporites in the Cyprus Arc (eastern Mediterranean) using high‐resolution reflection seismic and multi‐beam data. The results shed new light on the Miocene to Present tectonic evolution of this area and contribute to our general knowledge of the MSC in a deep basin setting. The evaporites and overlying formations show a complex deformation pattern due to a combination of thick‐skinned plate‐tectonic convergence and thin‐skinned disharmonic deformation related to the mobile evaporite‐bearing unit. Several MSC markers are identified and precisely mapped: the base of the MSC unit is a ‘decollement’ level, whereas the top is clearly identified as a toplap surface. Intra‐MSC markers and two MSC subunits are identified and mapped over the entire study area. The geometry of MSC markers shows that the lower MSC subunit was deposited in a relatively quiet tectonic setting. The nature of the anisopachous upper unit indicates a syn‐depositional phase of large‐scale plate‐tectonic activity. A thin‐skinned phase of compressional deformation during the Late Miocene affected the entire MSC unit, overlain by undeformed Pliocene–Quaternary layers. A second thin‐skinned phase, well expressed in the bathymetry, occurred from the Pliocene to Recent, resulting in extensional gravity‐gliding within the evaporites and the Pliocene–Quaternary sequence. We show that the MSC had a dramatic impact on the regional structure. For instance, the erosive nature of the top of the MSC unit is linked to the desiccation episode rather than to the cessation of tectonic activity. This particularly strong and short‐lived erosion may have been enhanced by the regional effects of the MSC, owing to differential uplift/subsidence caused by the drawdown. The evaporites are essential markers for constraining the tectonic framework, provided that active deformation can be distinguished from passive gliding associated with extensional/contractional deformation.     

Rift-phase extensional fabrics of the back-arc Drummond Basin, eastern Australia

The base of the Late Devonian–Early Carboniferous Drummond Basin, a major backarc extensional feature in eastern Australia which formed in response to detachment faulting, is extensively exposed in central Queensland. Here a crystalline basin floor is overlain by the Silver Hills Volcanics, a synrift sequence of predominantly silicic ash flow tuffs and lavas ranging to over 2 km in thickness. Detailed mapping of faults and stratigraphic logging of thickness changes within the Silver Hills Volcanics have allowed the rift-phase structural architecture that accompanied initial subsidence near the basin margin to be resolved. A complex mosaic of block faults with throws of up to 1 km is indicated. Locally developed mosaics may conform to, or depart from, the configuration predicted by the detachment faulting model. Structural fabric of the basement was a critical determinant of the extensional geometry. Distributed shear along pre-existing penetrative planar fabrics is considered to have accommodated hangingwall extension at lower strain rates whereas the propagation of tension fractures and the development of block faults by failure on pre-existing, brittle, basement dislocations facilitated extension at higher strain rates. The detachment fault inferred to lie beneath the extended hangingwall carapace has not been mapped at the surface and is thought to dissipate into a broad zone of distributed shear within basement to the east of the basin. Volcanism coincided with the initiation of extensional movements at which time deep crustal repositories for evolved magma were tapped by extensional fractures. The main extensional faults cutting the basinal succession were not used as conduits for magmatic products which were sourced from the basin margin and from extended hinterland to the east.     

Subsurface evidence for late Mesozoic extension in western Mongolia: tectonic and petroleum systems implications

New seismic reflection profiles from the Tugrug basin in the Gobi‐Altai region of western Mongolia demonstrate the existence of preserved Mesozoic extensional basins by imaging listric normal faults, extensional growth strata, and partially inverted grabens. A core hole from this region recovered ca. 1600 continuous meters of Upper Jurassic – Lower Cretaceous (Kimmeridgian–Berriasian) strata overlying Late Triassic volcanic basement. The cored succession is dominated by lacustrine and marginal lacustrine deposits ranging from stratified lacustrine, to subaqueous fan and delta, to subaerial alluvial‐fluvial environments. Multiple unconformities are encountered, and these represent distinct phases in basin evolution including syn‐extensional deposition and basin inversion. Prospective petroleum source and reservoir intervals occur, and both fluid inclusions and oil staining in the core provide evidence of hydrocarbon migration. Ties to correlative outcrop sections underscore that, in general, this basin appears to share a similar tectono‐stratigraphic evolution with petroliferous rift basins in eastern Mongolia and China. Nevertheless, some interesting contrasts to these other basins are noted, including distinct sandstone provenance, less overburden, and younger (Neogene) inversion structures. The Tugrug basin occupies an important but perplexing paleogeographic position between late Mesozoic contractile and extensional provinces. Its formation may record a rapid temporal shift from orogenic crustal thickening to extensional collapse in the Late Jurassic, and/or an accommodation zone with a Mesozoic strike‐slip component.     

Structural evolution of sheared margin basins: the role of strain partitioning. Sørvestsnaget Basin,Norwegian Barents Sea

Spatio‐temporal analysis of basins formed along sheared margins has received much less attention than those formed along orthogonally extended margins. Knowledge about the structural evolution of such basins is important for petroleum exploration but there has been a lack of studies that document these based on 3D seismic reflection data. In this study, we demonstrate how partitioning of strain during deformation of the central and southern part of the Sørvestsnaget Basin along the Senja Shear Margin, Norwegian Barents Sea, facilitated coeval shortening and extension. This is achieved through quantitative analysis of syn‐kinematic growth strata using 3D seismic data. Our results show that during Cenozoic extensional faulting, folds and thrusts developed coevally and orthogonal to sub‐orthogonal to normal faults. We attribute this strain partitioning to be a result of the right‐lateral oblique plate motions along the margin. Rotation of fold hinge‐lines and indications of hinge‐parallel extension indicate that the dominating deformation mechanism in the central and southern Sørvestsnaget Basin during opening along the Senja Shear Margin was transtensional. We also argue that interpretation of shortening structures attributed to inversion along the margin should consider that partitioning of strain may result in shortening structures that are coeval with extensional faults and not a result of a separate compressional phase.     

Neogene tectonostratigraphic history of the southern Neuquén basin (39°–40°30′S,Argentina): implications for foreland basin evolution

Although the Neuquén basin in Argentina forms a key transitional domain between the south‐central Andes and the Patagonian Andes, its Cenozoic history is poorly documented. We focus on the sedimentologic and tectonic evolution of the southern part of this basin, at 39–40°30′S, based on study of 14 sedimentary sections. We provide evidence that this basin underwent alternating erosion and deposition of reworked volcaniclastic material in continental and fluvial settings during the Neogene. In particular, basement uplift of the Sañico Massif, due to Late Miocene–Pliocene intensification of tectonic activity, led to sediment partitioning in the basin. During this interval, sedimentation was restricted to the internal domain and the Collon Cura basin evolved towards an endorheic intermontane basin. From stratigraphic interpretation, this basin remained isolated 7–11 Myr. Nevertheless, ephemeral gateways seem to have existed, because we observe a thin succession downstream of the Sañico Massif contemporaneous with the Collon Cura basin‐fill sequence. Comparisons of stratigraphic, paleoenvironmental and tectonic features of the southern Neuquén basin with other foreland basins of South America allow us to classify it as a broken foreland with the development of an intermontane basin from Late Miocene to Late Pliocene. This implies a thick‐skinned structural style for this basin, with reactivation of basement faults responsible for exhumation of the Sañico Massif. Comparison of several broken forelands of South America allows us to propose two categories of intermontane basins according to their structural setting: subsiding or uplifted basins, which has strong implications on their excavation histories.     

Tectonic implications of transtensional supradetachment basin development in an extension‐parallel transfer zone: the Kocaçay Basin,western Anatolia,Turkey

The Kocaçay Basin (KÇB) is a key area in western Anatolia – a well‐known extended terrane where regional segmentation has received limited attention – for investigating strike‐slip faults kinematically linked to detachment faults. In this paper, we present results of an integrated sedimentologic, stratigraphic, and structural study of Miocene alluvial fan/fan‐delta/lacustrine deposits that accumulated in the KÇB, a NE‐trending basin with connections to the Menderes Metamorphic Core Complex (MCC). We mapped and evaluated most of the key faults in the KÇB, many for the first time, and recognised different deformation events in the study area near the E margin of the MCC. We also present field evidence for kinematic connections between low‐angle normal and strike‐slip faults which were developed in an intermittently active basement‐involved transfer zone in western Anatolia. We find that the KÇB contains a detailed record of Miocene transtensional sedimentation and volcanism that accompanied exhumation of the MCC. Structural data reveal that the basin was initially formed by transtension (D1 phase) and subsequently uplifted and deformed, probably as a result of early Pliocene wrench‐ to extension‐dominated deformation (D2 phase) overprinted by Plio‐Quaternary extensional tectonics (D3 phase). These results are consistent with progressive deformation wherein the axis of maximum extension remained in the horizontal plane but the intermediate and maximum shortening axes switched position in the vertical plane. Combining our results with published studies, we propose a new working hypothesis that the KÇB was a transtensional supradetachment basin during the Miocene. The hypothesis could provide new insights into intermittently active extension‐parallel zone of weakness in western Anatolia.These results also suggest that the termination of low‐angle normal fault systems within an extension parallel transfer zone may have resulted in a transtensional depressions which are different from classical supradetachment basins with respect to the sedimentation and deformational pattern of the basin infills.     

Intraplate subsidence and basin filling adjacent to an oceanic arc–continent collision: a case from the southern Caribbean‐South America plate margin

The upper Campanian–Lower Eocene synorogenic sedimentary wedge of the Ranchería Basin was deposited in an intraplate basin resting on a tilted continental crustal block that was deformed by collision and subsequent subduction of the Caribbean Plate. Upper Cretaceous–Lower Eocene strata rest unconformably upon Jurassic igneous rocks of the Santa Marta Massif, with no major thrust faults separating the Santa Marta Massif from the Ranchería Basin. The upper Campanian–Lower Eocene succession includes, from base to top: foraminifera‐rich calcareous mudstone, mixed carbonate–siliciclastic strata and mudstone, coal and immature fluvial sandstone beds. Diachronous collision and eastward tilting of the plate margin (Santa Marta Massif and Central Cordillera) favoured the generation of accommodation space in a continuous intraplate basin (Ranchería, Cesar and western Maracaibo) during the Maastrichtian to Late Palaeocene. Terrigenous detritus from the distal colliding margin filled the western segments of the continuous intraplate basin (Ranchería and Cesar Basins); in the Late Paleocene, continental depositional systems migrated eastwards as far as the western Maracaibo Basin. In Early Eocene time, reactivation of former extensional structures fragmented the intraplate basin into the Ranchería‐Cesar Basins to the west, and the western Maracaibo Basin and Palmar High to the East. This scenario of continent–oceanic arc collision, crustal‐scale tilting, intraplate basin generation and fault reactivation may apply for Upper Cretaceous–Palaeogene syntectonic basins in western Colombia and Ecuador, and should be considered in other settings where arc–continent collision is followed by subduction.