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.     

Listric extensional fault systems - results of analogue model experiments

Abstract Analogue models are a powerful tool for investigating progressive deformation in extensional fault systems. This paper presents exciting new insights into the progressive evolution of hanging wall structures in listric extensional terranes. Analogue models, scaled to simulate deformation in a sedimentary sequence, were constructed for simple listric and ramp/flat listric extensional detachments. For each detachment geometry homogeneous sand, sand/mica and sand/clay models were used to simulate respectively, deformation of isotropic sediments, of anisotropic sediments and of sedimentary sequences with competency contrasts. Roll-over anticlines with geometrically necessary crestal collapse graben structures are characteristic of the steepening-upwards segments of listric extensional fault systems in all of our models. With progressive deformation, crestal collapse grabens show hanging wall nucleation of new faults. Variations in graben size, amount of fault rotation and throw, are dependent on detachment curvature and amount of extension. Individual faults and associated fault blocks may significantly change shape during extension. Complex and apparently conjugate fault arrays are the result of superposition of successive crestal collapse grabens. Ramp/flat listric extensional fault systems are characterized by a roll-over anticline and a crestal collapse graben system associated with each steepening-upwards segment of the detachment and a ramp zone consisting of a hanging wall syncline and a complex deformation zone with local reverse faults. The roll-over anticlines and crestal collapse graben are similar in geometry to those formed in simple listric extensional systems. The models demonstrate that the geometry of the detachments exerts a fundamental control on the evolution of hanging wall structures. Analysis of particle displacement paths for these experiments provides new insights into the mechanical development of roll-over anticlines. Two general models for deformation above simple listric and ramp/flat listric extensional detachments have been erected.     

Basin and petroleum system modelling of the East Coast Basin,New Zealand: a test of overpressure scenarios in a convergent margin

In the East Coast Basin (ECB), an active convergent margin of the North Island, New Zealand, the smectite‐rich Eocene Wanstead Formation forms an effective regional seal, creating high overpressure in the underlying Cretaceous through Palaeocene units due to disequilibrium compaction. This study examines the evolution of pore pressure and porosity in Hawke Bay of the ECB based on stepwise structural reconstruction of a stratigraphic and structural framework derived from interpretation of a regional two‐dimensional seismic line. This framework is incorporated into a basin and petroleum system model to predict the generation, distribution, and dissipation of overpressure, and examine the influence of faults, erosion, structural thickening, and seal effectiveness of the Wanstead Formation on pore pressure evolution. We find that natural hydraulic fracturing is likely occurring in sub‐Wanstead source rocks, which makes it a favourable setting for potential shale gas plays. We use poroelastic modelling to investigate the impact of horizontal bulk shortening due to tectonic compression on pore pressure and the relative order of principal stresses. We find that shortening modestly increases pore pressure. When 5% or greater shortening occurs, the horizontal stress may approach and exceed vertical stress in the last 4 Myr of the basin's history. Shortening impacts both the magnitude and relative order of principal stresses through geological time. Due to the overpressured nature of the basin, we suggest that subtle changes in stress regime are responsible for the significant changes in structural deformational styles observed, enabling compressional, extensional, and strike‐slip fault regimes to all occur during the tectonic history and, at times, simultaneously.     

Structure of the footwall of a listric fault system revealed by 3D seismic data from the Niger Delta

We use three‐dimensional (3D) seismic reflection data to analyse the architecture of the footwall of a listric fault, in a gravitationally driven extensional system, in the north‐western Niger Delta. In contrast to conventional listric normal fault models with a single master listric fault plane the level of detachment switches from a deeper to shallower level. The footwall evolves through the generation of new master detachment faults and detachments, which transfers hanging wall rocks into the footwall. New detachments form by branching off pre‐existing detachment levels, cutting‐up through stratigraphy to the next mechanical weakness, separating discrete sections of extended strata. As a consequence a deeper, older array of seaward‐dipping, tilted extensional fault blocks is now located in the footwall beneath the master listric detachment fault. The structural complexity located below the master detachment fault highlights extensional episodes on separate detachment faults that are not captured in conventional listric models. We speculate that changes in the level of the detachment are caused by mechanical weaknesses controlled by lithology, pore pressure and episodes of sediment loading related to deltaic progradation.     

The propagation and linkage of normal faults: insights from the Strathspey–Brent–Statfjord fault array, northern North Sea

Through examination of the scaling relations of faults and the use of seismic stratigraphic techniques, we demonstrate how the temporal and spatial evolution of the fault population in a half-graben basin can be accurately reconstructed. The basin bounded by the ≫62-km-long Strathspey–Brent–Statfjord fault array is located on the western flank of the Late Jurassic northern North Sea rift basin. Along-strike displacement variations, transverse fault-displacement folds and palaeo-fault tips abandoned in the hangingwall all provide evidence that the fault system comprises a hierarchy of linked palaeo-segments. The displacement variations developed while the fault was in a prelinkage, multisegment stage of its growth have not been equilibrated following fault linkage. Using the stratal architecture of synrift sediments, we date the main phase of segment linkage as latest Callovian – middle Oxfordian (10–14 Myr after rift initiation). A dense subpopulation of faults is mapped in the hangingwall to the Strathspey–Brent–Statfjord fault array. The majority of these faults are short, of low displacement and became inactive within 3–4 Myr of the beginning of the extensional event. Subsequently, only the segments of the proto-Strathspey–Brent–Statfjord fault and a conjugate array of antithetic faults located 3.5 km basinward continued to grow to define a graben-like basin geometry. Faults of the antithetic array became inactive ∼11.5 Myr into the rift event, concentrating strain on the linked Strathspey–Brent–Statfjord fault; hence, the basin evolved into a half-graben. As the rift event progressed, strain was localized on a smaller number of active structures with increased rates of displacement. The results of this study suggest that a simple model for the linkage of 2–3 fault segments may not be applicable to a complex multisegment array.     

Extensional fault and fold growth: Impact on accommodation evolution and sedimentary infill

Extensional faults and folds exert a fundamental control on the location, thickness and partitioning of sedimentary deposits on rift basins. The connection between the mode of extensional fault reactivation, resulting fault shape and extensional fold growth is well‐established. The impact of folding on accommodation evolution and growth package architecture, however, has received little attention; particularly the role‐played by fault‐perpendicular (transverse) folding. We study a multiphase rift basin with km‐scale fault displacements using a large high‐quality 3D seismic data set from the Fingerdjupet Subbasin in the southwestern Barents Sea. We link growth package architecture to timing and mode of fault reactivation. Dip linkage of deep and shallow fault segments resulted in ramp‐flat‐ramp fault geometry, above which fault‐parallel fault‐bend folds developed. The folds limited the accommodation near their causal faults, leading to deposition within a fault‐bend synclinal growth basin further into the hangingwall. Continued fold growth led to truncation of strata near the crest of the fault‐bend anticline before shortcut faulting bypassed the ramp‐flat‐ramp structure and ended folding. Accommodation along the fault‐parallel axis is controlled by the transverse folds, the location and size of which depends on the degree of linkage in the fault network and the accumulated displacement on causal faults. We construct transverse fold trajectories by tracing transverse fold hinges through space and time to highlight the positions of maximum and minimum accommodation and potential sediment entry points to hangingwall growth basins. The length and shape of the constructed trajectories relate to the displacement on their parent faults, duration of fault activity, timing of transverse basin infill, fault linkage and strain localization. We emphasize that the considerable wavelength, amplitudes and potential periclinal geometry of extensional folds make them viable targets for CO₂ storage or hydrocarbon exploration in rift basins.     

Analogue modelling of inverted domino‐style basement fault systems

Inversion of pre‐existing extensional fault systems is common in rift systems, back‐arc basins and passive margins. It can significantly influence the development of structural traps in hydrocarbon basins. The analogue models of domino‐style basement fault systems shown in this paper produced, on extension, characteristic hangingwall growth stratal wedges that, when contracted and inverted, formed classic inversion harpoon geometries and asymmetric hangingwall contractional fault‐propagation folds. Segmented footwall shortcut faults formed as the basement faults were progressively back‐rotated and steepened. The pre‐existing extensional fault architectures, basement fault geometries and the relative hangingwall and footwall block rotations exerted fundamental controls on the inversion styles. Digital image correlation (DIC) strain monitoring illustrated complex vertical fault segmentation and linkage during inversion as the major faults were reactivated and strain was progressively transferred onto footwall shortcut faults. Hangingwall deformation during inversion was dominated by significant back‐rotation as the inversion progressed. The mechanical stratigraphy of the cover sequences strongly influenced the fold and fault evolution of the reactivated fault systems. The implications of the experimental results for the interpretation and analysis of inversion structures are discussed and are compared with natural examples of inverted basement‐involved extensional faults observed in seismic datasets.     

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.     

Normal faulting, extension and uplift in the outer thrust belt of the central Apennines (Italy): role of the Caramanico fault

The outer Adriatic zones of the central Apennines (Italy) provide good conditions for analysing geometry and kinematics of the earliest normal faults, superposed onto the thrust belt. During the latest stages of thrusting onto the Adriatic foreland (late Pliocene–early Pleistocene), the outermost imbricates of the thrust belt were subjected to normal faulting, coeval with differential uplift. Crosscutting normal faults get younger towards the foreland, thus the easternmost normal faults record the latest stages of fault propagation and growth. The Caramanico fault, on the western flank of Mt. Maiella, is the largest outcropping normal fault of the outer zones. This high‐angle fault (dip > 70°) has cumulative offsets ≤ €4.2 km, and propagated with slip rates of 2.6 mm/year in a short time interval (≤ 1.6 Ma), concomitant with intense uplift of Mt. Maiella. In contrast with normal faults in a more internal position, the Caramanico fault maintains a high‐angle planar geometry, and does not reach the major basal detachment of the thrust belt. Thus the fault did not cause large extensional displacements; its major role was rather to accommodate ongoing components of vertical uplift of the overthickened thrust wedge. Downfaulting of the thrust belt on the western flank of Mt. Maiella represents the youngest end member of the same processes that have operated since 11 Ma in the Tyrrhenian hinterland, where large extensional strains and crustal thinning of the orogenic belt were achieved by long‐lasting activity of listric normal faults detached at lower crustal depths.     

Fault architecture, basin structure and evolution of the Gulf of Corinth Rift, central Greece

The style of extension and strain distribution during the early stages of intra-continental rifting is important for understanding rift-margin development and can provide constraints for lithospheric deformation mechanisms. The Corinth rift in central Greece is one of the few rifts to have experienced a short extensional history without subsequent overprinting. We synthesise existing seismic reflection data throughout the active offshore Gulf of Corinth Basin to investigate fault activity history and the spatio-temporal evolution of the basin, producing for the first time basement depth and syn-rift sediment isopachs throughout the offshore rift. A major basin-wide unconformity surface with an age estimated from sea-level cycles at ca . 0.4 Ma separates distinct seismic stratigraphic units. Assuming that sedimentation rates are on average consistent, the present rift formed at 1–2 Ma, with no clear evidence for along-strike propagation of the rift axis. The rift has undergone major changes in relative fault activity and basin geometry during its short history. The basement depth is greatest in the central rift (maximum ∼3 km) and decreases to the east and west. In detail however, two separated depocentres 20–50 km long were created controlled by N- and S-dipping faults before 0.4 Ma, while since ca . 0.4 Ma a single depocentre (80 km long) has been controlled by several connected N-dipping faults, with maximum subsidence focused between the two older depocentres. Thus isolated but nearby faults can persist for timescales ca . 1 Ma and form major basins before becoming linked. There is a general evolution towards a dominance of N-dipping faults; however, in the western Gulf strain is distributed across several active N- and S-dipping faults throughout rift history, producing a more complex basin geometry.     

Miocene extensional basin development in the Betic Cordillera, SE Spain revealed through analysis of the Alhama de Murcia and Crevillente Faults

The Alhama de Murcia and Crevillente faults in the Betic Cordillera of southeast Spain form part of a network of prominent faults, bounding several of the late Tertiary and Quaternary intermontane basins. Current tectonic interpretations of these basins vary from late‐orogenic extensional structures to a pull‐apart origin associated with strike–slip movements along these prominent faults. A strike–slip origin of the basins, however, seems at variance both with recent structural studies of the underlying Betic basement and with the overall basin and fault geometry. We studied the structure and kinematics of the Alhama de Murcia and Crevillente faults as well as the internal structure of the late Miocene basin sediments, to elucidate possible relationships between the prominent faults and the adjacent basins. The structural data lead to the inevitable conclusion that the late Miocene basins developed as genuinely extensional basins, presumably associated with the thinning and exhumation of the underlying basement at that time. During the late Miocene, neither the Crevillente fault nor the Alhama de Murcia fault acted as strike–slip faults controlling basin development. Instead, parts of the Alhama de Murcia fault initiated as extensional normal faults, and reactivated as contraction faults during the latest Miocene–early Pliocene in response to continued African–European plate convergence. Both prominent faults presently act as reverse faults with a movement sense towards the southeast, which is clearly at variance with the commonly inferred dextral or sinistral strike–slip motions on these faults. We argue that the prominent faults form part of a larger scale zone of post‐Messinian shortening made up of SSE‐ and NNW‐directed reverse faults and NE to ENE‐trending folds including thrust‐related fault‐bend folds and fault‐propagation folds, transected and displaced by, respectively, WNW‐ and NNE‐trending, dextral and sinistral strike–slip (tear or transfer) faults.     

A multidisciplinary study of a slow-slipping fault for seismic hazard assessment: the example of the Middle Durance Fault (SE France)

Assessing seismic hazard in continental interiors is difficult because these regions are characterized by low strain rates and may be struck by infrequent destructive earthquakes. In this paper, we provide an example showing that interpretations of seismic cross sections combined with other kinds of studies such as analysis of microseismicity allow the whole seismogenic source area to be imaged in this type of region. The Middle Durance Fault (MDF) is an 80-km-long fault system located southeastern France that has a moderate but regular seismicity and some palaeoseismic evidence for larger events. It behaves as an oblique ramp with a left-lateral-reverse fault slip and has a low strain rate. MDF is one of the rare slow active fault system monitored by a dedicated dense velocimetric short period network. This study showed a fault system segmented in map and cross section views which consists of staircase basement faults topped by listric faults ramping off Triassic evaporitic beds. Seismic sections allowed the construction of a 3-D structural model used for accurate location of microseismicity. Southern part of MDF is mainly active in the sedimentary cover. In its northern part and in Alpine foreland, seismicity deeper than 8 km was also recorded meaning active faults within the crust cannot be excluded. Seismogenic potential of MDF was roughly assessed. Resulting source sizes and estimated slip rates imply that the magnitude upper limit ranges from 6.0 to 6.5 with a return period of a few thousand years. The present study shows that the coupling between 3-D fault geometry imaging and accurate location of microseismicity provides a robust approach to analyse active fault sources and consequently a more refined seismic hazard assessment.     

Modelling deformation rates in the western Gulf of Corinth: rheological constraints

The Gulf of Corinth is one of the most active extensional regions in the Mediterranean area characterized by a high rate of seismicity. However, there are still open questions concerning the role and the geometry of the numerous active faults bordering the basin, as well as the mechanisms governing the seismicity. In this paper, we use a 2-D plane strain finite element analysis to constrain the upper crust rheology by modelling the available deformation data (GPS and geomorphology). We consider a SSW–NNE cross-section of the rift cutting the main active normal faults (Aigion, West Eliki and Off-Shore faults). The models run for 650 Kyr assuming an elasto-viscoplastic rheology and 1.3 cm yr⁻¹ horizontal extension as boundary condition (resulting from GPS data). We model the horizontal and vertical deformation rates and the accumulation of plastic strain at depth, and we compare them with GPS data, with long term uplift rates inferred from geomorphology and with the distribution of seismicity, respectively. Our modelling results demonstrate that dislocation on high-angle normal faults in a plastic crustal layer plays a key role in explaining the extremely localized strain within the Gulf of Corinth. Conversely, the contribution of structures such as the antithetic Trizonia fault or the buried hypothetical subhorizontal discontinuity are not necessary to model observed data.     

Evidence of quaternary transtensional tectonics in the Nekor basin (NE Morocco)

The geodynamic processes in the western Mediterranean are driven by both deep (mantle) processes such as slab‐rollback or delamination, oblique plate convergence and inherited structures. The present‐day deformation of the Alboran Sea and in particular the Nekor basin area is linked to these coeval effects. The seismically active Nekor basin is an extensional basin formed in a convergent setting at the eastern part of the Rif Chain whose boundaries extend both onshore and offshore Morocco. We propose a new structural model of the Nekor basin based on high‐resolution offshore data compiled from recent seismic reflection profiles, swath bathymetry acquisitions and industrial seismic reflection profiles. The new data set shows that the northern limit of the basin is oriented N49° with right‐stepping faults from the Bousekkour–Aghbal fault to the sinistral Bokkoya fault zone. This pattern indicates the presence of an inherited left‐lateral basement fault parallel to the major inherited Nekor fault. This fault has been interpreted as a Quaternary active left‐lateral transfer fault localized on weak structural discontinuities inherited from the orogenic period. Onshore and offshore active faults enclose a rhombohedral tectonic Nekor Basin. Normal faults oriented N155° offset the most recent Quaternary deposits in the Nekor basin, and indicate the transtensional behaviour of this basin. The geometry of these faults suggests a likely rollover structure and the presence at depth of a crustal detachment. Inactive Plio‐Quaternary normal faults to the east of the Ras Tarf promontory and geometries of depocentres seem to indicate the migration of deformation from east to west. The local orientations of horizontal stress directions deduced from normal fault orientations are compatible with the extrusion of the Rifian units and coherent with the westward rollback of the Tethyan slab and the localization of the present‐day slab detachment or delamination.     

The structural evolution of the Halten Terrace,offshore Mid‐Norway: extensional fault growth and strain localisation in a multi‐layer brittle–ductile system

Tectonic subsidence in rift basins is often characterised by an initial period of slow subsidence (‘rift initiation’) followed by a period of more rapid subsidence (‘rift climax’). Previous work shows that the transition from rift initiation to rift climax can be explained by interactions between the stress fields of growing faults. Despite the prevalence of evaporites throughout the geological record, and the likelihood that the presence of a regionally extensive evaporite layer will introduce an important, sub‐horizontal rheological heterogeneity into the upper crust, there have been few studies that document the impact of salt on the localisation of extensional strain in rift basins. Here, we use well‐calibrated three‐dimensional seismic reflection data to constrain the distribution and timing of fault activity during Early Jurassic–Earliest Cretaceous rifting in the Åsgard area, Halten Terrace, offshore Mid‐Norway. Permo‐Triassic basement rocks are overlain by a thick sequence of interbedded halite, anhydrite and mudstone. Our results show that rift initiation during the Early Jurassic was characterised by distributed deformation along blind faults within the basement, and by localised deformation along the major Smørbukk and Trestakk faults within the cover. Rift climax and the end of rifting showed continued deformation along the Smørbukk and Trestakk faults, together with initiation of new extensional faults oblique to the main basement trends. We propose that these new faults developed in response to salt movement and/or gravity sliding on the evaporite layer above the tilted basement fault blocks. Rapid strain localisation within the post‐salt cover sequence at the onset of rifting is consistent with previous experimental studies that show strain localisation is favoured by the presence of a weak viscous substrate beneath a brittle overburden.     

Structural evolution of a gravitationally detached normal fault array: analysis of 3D seismic data from the Ceduna Sub‐Basin,Great Australian Bight

The growth, interaction and controls on normal fault systems developed within stacked delta systems at extensional delta‐top settings have not been extensively examined. We aim to analyse the kinematic, spatial and temporal growth of a Cretaceous aged, thin‐skinned, listric fault system in order to further the understanding of how gravity‐driven fault segments and fault systems develop and interact at an extensional delta‐top setting. Furthermore, we aim to explore the influence of a pre‐existing structural framework on the development of gravity‐driven normal faults through the examination of two overlapping, spatially and temporally distinct delta systems. To do this, we use three‐dimensional (3D) seismic reflection data from the central Ceduna Sub‐basin, offshore southern Australia. The seismic reflection data images a Cenomanian‐Santonian fault system, and a post‐Santonian fault system, which are dip‐linked through an intervening Turonian‐early Campanian section. Both of these fault systems contain four hard‐linked strike assemblages oriented NW–SE (127–307), each composed of 13 major fault segments. The Cenomanian‐Santonian fault system detaches at the base of a shale interval of late Albian age, and is characterised by kilometre‐scale growth faults in the Cenomanian‐Sanontian interval. The post‐Santonian fault system nucleated in vertical isolation from the Cenomanian‐Santonian fault system. This is evident through displacement minima observed at Turonian‐early Campanian levels, which is indicative of vertical segmentation and eventual hard dip‐linkage. Our analysis constrains fault growth into four major evolutionary stages: (1) early Cenomanian nucleation and growth of fault segments, resulting from gravitational instability, and with faults detaching on the lower Albian interval; (2) Santonian cessation of growth for the majority of faults; (3) erosional truncation of fault upper tips coincident with the continental breakup of Australia and Antarctica (ca. 83 Ma); (4) Campanian‐Maastrichtian reactivation of the underlying Cenomanian‐Santonian fault system, inducing the nucleation, growth and consequential dip‐linkage of the post‐Santonian fault system with the underlying fault system. Our results highlight the along‐strike linkage of fault segments and the interaction through dip‐linkage and fault reactivation, between two overlapping, spatially and temporally independent delta systems of Cenomanian and late Santonian‐Maastrichtian age in the frontier Ceduna Sub‐Basin. This study has implications regarding the growth of normal fault assemblages, through vertical and lateral segment linkage, for other stacked delta systems (such as the Gulf of Mexico) where upper delta systems develop over a pre‐existing structural framework.     

Gravitational deformation and inherited structural control on slope morphology in the subduction zone of north‐central Chile (ca. 29–33°S)

Subduction zones provide direct insight into plate boundary deformation and by studying these areas we better understand tectonic processes and variability over time. We studied the structure of the offshore subduction zone system of the Pampean flat‐slab segment (ca. 29–33°S) of the Chilean margin using seismic and bathymetric constraints. Here, we related and analysed the structural styles of the offshore and onshore western fore‐arc. Overlying the acoustic top of the continental basement, two syn‐extensional seismic sequences were recognised and correlated with onshore geological units and the Valparaíso Forearc Basin seismic sequences: (SII) Pliocene‐Pleistocene and (SI) Miocene‐Pliocene (Late Cretaceous (?) to Miocene‐Pliocene) syn‐extensional sequences. These sequences are separated by an unconformity (i.e. Valparaíso Unconformity). Seismic reflection data reveal that the eastward dipping extensional system (EI) recognised at the upper slope can be extended to the middle slope and controlled the accumulation of the older seismic package (SI). The westward dipping extensional system (EII) is essentially restricted to the middle slope. Here, EII cuts the eastward dipping extensional system (EI), preferentially parallel to the inclination of the older sequences (SI), and controlled a series of middle slope basins which are filled by the Pliocene‐Pleistocene seismic sequence (SII). At the upper slope and in the western Coastal Cordillera, the SII sequence is controlled by eastward dipping faults (EII) which are the local reactivation of older extensional faults (EI). The tectonic boundary between the middle (eastern outermost forearc block) and upper continental slope (western coastal block) is a prominent system of trenchward dipping normal fault scarps (ca. 1 km offset) that resemble a major trenchward dipping extensional fault system. This prominent structural feature can be readily detected along the Chilean erosive margin as well as the two extensional sets (EI and EII). Evidence of slumping, thrusting, reactivated faults and mass transport deposits, were recognised in the slope domain and locally restricted to some eastern dipping faults. These features could be related to gravitational effects or slope deformation due to coseismic deformation. The regional inclination of the pre‐Pliocene sequences favoured the gravitational collapse of the outermost forearc block. We propose that the structural configuration of the study area is dominantly controlled by tectonic erosion as well as the uplift of the Coastal Cordillera, which is partially controlled by pre‐Pliocene architecture.     

Evolution of basin architecture in an incipient continental rift: the Cenozoic Most Basin, Eger Graben (Central Europe)

The Oligo-Miocene Most Basin is the largest preserved sedimentary basin within the Eger Graben, the easternmost part of the European Cenozoic Rift System (ECRIS). The basin is interpreted as a part of an incipient rift system that underwent two distinct phases of extension. The first phase, characterised by NNE–SSW- to N–S-oriented horizontal extension between the end of Eocene and early Miocene, was oblique to the rift axis and caused evolution of a fault system characterised by en-échelon-arranged E–W (ENE–WSW) faults. These faults defined a number of small, shallow initial depocentres of very small subsidence rates that gradually merged during the growth and linkage of the normal fault segments. The youngest part of the basin fill indicates accelerated subsidence caused probably by the concentration of displacement at several major bounding faults. Major post-depositional faulting and forced folding were related to a change in the extension vector to an orthogonal position with respect to the rift axis and overprinting of the E–W faults by an NE–SW normal fault system. The origin of the palaeostress field of the earlier, oblique, extensional phase remains controversial and can be attributed either to the effects of the Alpine lithospheric root or (perhaps more likely because of the dominant volcanism at the onset of Eger Graben formation) to doming due to thermal perturbation of the lithosphere. The later, orthogonal, extensional phase is explained by stretching along the crest of a growing regional-scale anticlinal feature, which supports the recent hypothesis of lithospheric folding in the Alpine–Carpathian foreland.     

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.     

Polygonal faults-furrows system related to early stages of compaction – upper Miocene to recent sediments of the Lower Congo Basin

A new polygonal fault system has been identified in the Lower Congo Basin. This highly faulted interval (HFI), 700±50 m thick, is characterized by small extensional faults displaying a polygonal pattern in plan view. This kind of fracturing is attributed to volumetric contraction of sediments during early stages of compaction at shallow burial depth. 3‐D seismic data permitted the visualization of the progressive deformation of furrows during burial, leading to real fractures, visible on seismic sections at about 78 m below seafloor. We propose a new geometrical model for volumetrical contraction of mud‐dominated sediments. Compaction starts at the water–sediment interface by horizontal contraction, creating furrows perpendicular to the present day slope. During burial, continued shrinkage evolves to radial contraction, generating hexagonal cells of dewatering at 21 m below seafloor. With increasing contraction, several faults generations are progressively initiated from 78 to 700 m burial depth. Numerous faults of the HFI act as highly permeable pathways for deeper fluids. We point out that pockmarks, which represent the imprint of gas, oil or pore water escape on the seafloor, are consistently located at the triple‐junction of three neighbouring hexagonal cells. This is highly relevant for predictive models of the occurrence of seepage structures on the seafloor and for the sealing capacity of sedimentary cover over deeper petroleum reservoirs.