The role of evaporite mobility in modifying subsidence patterns during normal fault growth and linkage, Halten Terrace, Mid-Norway

Well‐calibrated seismic interpretation in the Halten Terrace of Mid‐Norway demonstrates the important role that structural feedback between normal fault growth and evaporite mobility has for depocentre development during syn‐rift deposition of the Jurassic–Early Cretaceous Viking and Fangst Groups. While the main rift phase reactivated pre‐existing structural trends, and initiated new extensional structures, a Triassic evaporite interval decouples the supra‐salt cover strata from the underlying basement, causing the development of two separate fault populations, one in the cover and the other confined to the pre‐salt basement. Detailed displacement–length analyses of both cover and basement fault arrays, combined with mapping of the component parts of the syn‐rift interval, have been used to reveal the spatial and temporal evolution of normal fault segments and sediment depocentres within the Halten Terrace area. Significantly, the results highlight important differences with traditional models of normal fault‐controlled subsidence, including those from parts of the North Sea where salt is absent. It can now be shown that evaporite mobility is intimately linked to the along‐strike displacement variations of these cover and basement faults. The evaporites passively move beneath the cover in response to the extension, such that the evaporite thickness becomes greatest adjacent to regions of high fault displacement. The consequent evaporite swells can become large enough to have pronounced palaeobathymetric relief in hangingwall locations, associated with fault displacement maxima– the exact opposite situation to that predicted by traditional models of normal fault growth. Evaporite movement from previous extension also affects the displacement–length relationships of subsequently nucleated or reactivated faults. Evaporite withdrawal, on the other hand, tends to be a later‐stage feature associated with the high stress regions around the propagating tips of normal faults or their coeval hangingwall release faults. The results indicate the important effect of, and structural feedback caused by, syn‐rift evaporite mobility in heavily modifying subsidence patterns produced by normal fault array evolution. Despite their departure from published models, the results provide a new, generic framework within which to interpret extensional fault and depocentre development and evolution in areas in which mobile evaporites exist.     

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.     

Late syn‐rift evolution of the Vingleia Fault Complex,Halten Terrace,offshore Mid‐Norway; a test of rift basin tectono‐stratigraphic models

Rift basin tectono‐stratigraphic models indicate that normal fault growth controls the sedimentology and stratigraphic architecture of syn‐rift deposits. However, such models have rarely been tested by observations from natural examples and thus remain largely conceptual. In this study we integrate 3D seismic reflection, and biostratigraphically constrained core and wireline log data from the Vingleia Fault Complex, Halten Terrace, offshore Mid‐Norway to test rift basin tectono‐stratigraphic models. The geometry of the basin‐bounding fault and its hangingwall, and the syn‐rift stratal architecture, vary along strike. The fault is planar along a much of its length, bounding a half‐graben containing a faultward‐thickening syn‐rift wedge. Locally, however, the fault has a ramp‐flat‐ramp geometry, with the hangingwall defined by a fault‐parallel anticline‐syncline pair. Here, an unusual bipartite syn‐rift architecture is observed, comprising a lower faultward‐expanding and an upper faultward‐thinning wedge. Fine‐grained basinfloor deposits dominate the syn‐rift succession, although isolated coarse clastics occur. The spatial and temporal distribution of these coarse clastics is complex due to syn‐depositional movement on the Vingleia Fault Complex. High rates of accommodation generation in the fault hangingwall led to aggradational stacking of fan deltas that rapidly (<5 km) pinch out basinward into offshore mudstone. In the south of the basin, rapid strain localization meant that relay ramps were short‐lived and did not represent major, long‐lived sediment entry points. In contrast, in the north, strain localization occurred later in the rift event, thus progradational shorefaces developed and persisted for a relatively long time in relay ramps developed between unlinked fault segments. The footwall of the Vingleia Fault Complex was characterized by relatively low rates of accommodation generation, with relatively thin, progradational hangingwall shorelines developed downdip of the fault block apex, sometime after the onset of sediment supply to the hangingwall. We show that rift basin tectono‐stratigraphic models need modifying to take into account along‐strike variability in fault structure and basin physiography, and the timing and style of syn‐rift sediment dispersal and facies, in both hangingwall and footwall locations.     

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.     

Palaeoseismicity of the Oquirrh fault, Utah from shallow seismic tomography

The magnitude and frequency of normal-fault palaeoearthquakes are usually determined by trenching studies that ascertain the size and number of colluvial wedges along the fault. Such information can be invaluable in predicting the seismic hazard and potential for a future earthquake in that region. Digging trenches across normal faults, however, is environmentally intrusive, expensive and limited in the penetration depth. To overcome these problems we propose the use of 3-D seismic tomography as a means to identify the shapes and sizes of colluvial wedges along normal faults. As an example,2-D and 3-D seismic surveys were conducted across the Oquirrh fault, Utah with the purpose of imaging the normal-fault structure to a depth of about 10  m. Results show that the 3-D tomogram clearly delineates the fault zone and a colluvial wedge, both of which correlate extremely well with the geological cross-section interpreted from an adjacent trench. The thickness of the colluvial wedge image is used in conjunction with a seismic section to compute an estimate of a 6.8 moment magnitude earthquake for the most recent event on this fault, which is in close agreement with the 7.0 estimate based on a nearby trenching study. These tomographic results demonstrate, for the first time, that seismic imaging methods can be used in some cases to estimate unambiguously the shapes of colluvial wedges and the sizes of prehistoric earthquakes. Thus, seismic tomography has the possibility of providing cheaper, deeper and wider, but less resolved, images of fault systems than the intrusive excavation of trenches across faults.     

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.     

Low‐accommodation and backwater effects on sequence stratigraphic surfaces and depositional architecture of fluvio‐deltaic settings (Cretaceous Mesa Rica Sandstone,Dakota Group,USA)

The adequate documentation and interpretation of regional‐scale stratigraphic surfaces is paramount to establish correlations between continental and shallow marine strata. However, this is often challenged by the amalgamated nature of low‐accommodation settings and control of backwater hydraulics on fluvio‐deltaic stratigraphy. Exhumed examples of full‐transect depositional profiles across river‐to‐delta systems are key to improve our understanding about interacting controlling factors and resultant stratigraphy. This study utilizes the ~400 km transect of the Cenomanian Mesa Rica Sandstone (Dakota Group, USA), which allows mapping of down‐dip changes in facies, thickness distribution, fluvial architecture and spatial extent of stratigraphic surfaces. The two sandstone units of the Mesa Rica Sandstone represent contemporaneous fluvio‐deltaic deposition in the Tucumcari sub‐basin (Western Interior Basin) during two regressive phases. Multivalley deposits pass down‐dip into single‐story channel sandstones and eventually into contemporaneous distributary channels and delta‐front strata. Down‐dip changes reflect accommodation decrease towards the paleoshoreline at the Tucumcari basin rim, and subsequent expansion into the basin. Additionally, multi‐storey channel deposits bound by erosional composite scours incise into underlying deltaic deposits. These represent incised‐valley fill deposits, based on their regional occurrence, estimated channel tops below the surrounding topographic surface and coeval downstepping delta‐front geometries. This opposes criteria offered to differentiate incised valleys from flood‐induced backwater scours. As the incised valleys evidence relative sea‐level fall and flood‐induced backwater scours do not, the interpretation of incised valleys impacts sequence stratigraphic interpretations. The erosional composite surface below fluvial strata in the continental realm represents a sequence boundary/regional composite scour (RCS). The RCS’ diachronous nature demonstrates that its down‐dip equivalent disperses into several surfaces in the marine part of the depositional system, which challenges the idea of a single, correlatable surface. Formation of a regional composite scour in the fluvial realm throughout a relative sea‐level cycle highlights that erosion and deposition occur virtually contemporaneously at any point along the depositional profile. This contradicts stratigraphic models that interpret low‐accommodation settings to dominantly promote bypass, especially during forced regressions. Source‐to‐sink analyses should account for this in order to adequately resolve timing and volume of sediment storage in the system throughout a complete relative sea‐level cycle.     

Mesozoic–Cenozoic exhumation events in the eastern North Sea Basin: a multi-disciplinary study based on palaeothermal, palaeoburial, stratigraphic and seismic data

Four Mesozoic–Cenozoic palaeothermal episodes related to deeper burial and subsequent exhumation and one reflecting climate change during the Eocene have been identified in a study of new apatite fission‐track analysis (AFTA^®) and vitrinite reflectance data in eight Danish wells. The study combined thermal‐history reconstruction with exhumation studies based on palaeoburial data (sonic velocities) and stratigraphic and seismic data. Mid‐Jurassic exhumation (ca. 175 Ma) was caused by regional doming of the North Sea area, broadly contemporaneous with deep exhumation in Scandinavia. A palaeogeothermal gradient of 45 °C km^?1 at that time may be related to a mantle plume rising before rifting in the North Sea. Mid‐Cretaceous exhumation affecting the Sorgenfrei–Tornquist Zone is probably related to late Albian tectonic movements (ca. 100 Ma). The Sole Pit axis in the southern North Sea experienced similar inversion and this suggests a plate‐scale response along crustal weakness zones across NW Europe. Mid‐Cenozoic exhumation affected the eastern North Sea Basin and the onset of this event correlates with a latest Oligocene unconformity (ca. 24 Ma), which indicates a major Scandinavian uplift phase. The deeper burial that caused the late Oligocene thermal event recognized in the AFTA data reflect progradation of lower Oligocene wedges derived from the uplifting Scandinavian landmass. The onset of Scandinavian uplift is represented by an earliest Oligocene unconformity (ca. 33 Ma). Late Neogene exhumation affected the eastern (and western) North Sea Basin including Scandinavia. The sedimentation pattern in the central North Sea Basin shows that this phase began in the early Pliocene (ca. 4 Ma), in good agreement with the AFTA data. These three phases of Cenozoic uplift of Scandinavia also affected the NE Atlantic margin, whereas an intra‐Miocene unconformity (ca. 15 Ma) on the NE Atlantic margin reflects tectonic movements of only minor amplitude in that area. The study demonstrates that only by considering episodic exhumation as an inherent aspect of the sedimentary record can the tectonic evolution be accurately reconstructed.     

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.     

Normal fault array evolution above a reactivated rift fabric; a subsurface example from the northern Horda Platform,Norwegian North Sea

The impact of a pre‐existing rift fabric on normal fault array evolution during a subsequent phase of lithospheric extension is investigated using 2‐D and 3‐D seismic reflection, and borehole data from the northern Horda Platform, Norwegian North Sea. Two fault populations are developed: (i) a population comprising relatively tall (>2 km), N‐S‐striking faults, which have >1.5 km of throw. These faults are up to 60 km long, penetrate down into crystalline basement and bound the eastern margins of 6–15 km wide half‐graben, which contain >3 km of pre‐Jurassic, likely Permo–Triassic, but possibly Devonian syn‐rift strata; and (ii) a population comprising vertically restricted (<1 km), NW‐SE‐striking faults, which are more closely spaced (0.5–5 km), have lower displacements (30–100 m) and not as long (2–10 km) as those in the N–S‐striking population. The NW‐SE‐striking population typically occurs between the N‐S‐striking population, and may terminate against or cross‐cut the larger structures. NW–SE‐striking faults do not bound pre‐Jurassic half‐graben and are largely restricted to the Jurassic‐to‐Cretaceous succession. Seismic‐stratigraphic observations, and the stratigraphic position of the fault tips in both fault populations, allow us to reconstruct the Late Jurassic‐to‐Early Cretaceous growth history of the northern Horda Platform fault array. We suggest the large, N‐S‐striking population was active during the Permo–Triassic and possibly earlier (Devonian?), before becoming inactive and buried during the Early and Middle Jurassic. After a period of relative tectonic quiescence, the N‐S‐striking, pre‐Jurassic fault population propagated through the Early‐Middle Jurassic cover and individual fault systems rapidly (within <10 Ma) established their maximum length in response to Late Jurassic extension. These fault systems became the dominant structures in the newly formed fault array and defined the locations of the main, Late Jurassic‐to‐Early Cretaceous, syn‐rift depocentres. Late Jurassic extension was also accommodated by broadly synchronous growth of the NW‐SE‐striking fault population; the eventual death of this population occurred in response to the localization of strain onto the N–S‐striking fault population. Our study demonstrates that the inheritance of a pre‐existing rift fabric can influence the geometry and growth of individual fault systems and the fault array as a whole. On the basis of observations made in this study, we present a conceptual model that highlights the influence of a pre‐existing rift fabric on fault array evolution in polyphase rifts.     

Application of seismic and sequence stratigraphic concepts to a lava‐fed delta system in the Faroe‐Shetland Basin,UK and Faroes

Detailed seismic stratigraphic analysis of 2D seismic data over the Faroe‐Shetland Escarpment has identified 13 seismic reflection units that record lava‐fed delta deposition during discrete periods of volcanism. Deposition was dominated by progradation, during which the time shoreline migrated a maximum distance of ～44 km in an ESE direction. Localised collapse of the delta front followed the end of progradation, as a decrease in volcanic activity left the delta unstable. Comparison with modern lava‐fed delta systems on Hawaii suggests that syn‐volcanic subsidence is a potential mechanism for apparent relative sea level rise and creation of new accommodation space during lava‐fed delta deposition. After the main phase of progradation, retrogradation of the delta occurred during a basinwide syn‐volcanic relative sea level rise where the shoreline migrated a maximum distance of ～75 km in a NNW direction. This rise in relative sea level was of the order of 175–200 m, and was followed by the progradation of smaller, perched lava‐fed deltas into the newly created accommodation space. Active delta deposition and the emplacement of lava flows feeding the delta front lasted ～2600 years, although the total duration of the lava‐fed delta system, including pauses between eruptions, may have been much longer.     

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.     

River response to lateral ground tilting: a synthesis and some implications for the modelling of alluvial architecture in extensional basins

This paper reviews and synthesizes several Holocene field examples of river response to lateral ground tilting. Key aspects of alluvial architecture modelling in extensional basins are addressed, including the nature of gradual lateral migration, the spatial and temporal history of avulsive sequences, and the underlying controls that determine whether a river responds to lateral tilting through gradual migration or avulsion. A new conceptual model for gradual lateral migration is proposed that unifies previously disparate models. Tilt-induced avulsion in several field examples is associated with sequences that move towards and away from the locus of subsidence during active and quiescent tectonic periods, respectively. These avulsion sequences closely correspond to those produced by several 2D and 3D alluvial architecture models. The rate of lateral tilt appears to control the style of channel movement, with gradual migration occurring at low tilt rates, and avulsion at higher rates. This apparent dependence on tilt rate suggests the mode of channel movement, and also the avulsion frequency, may in part be a function of the imposed tectonic regime.     

Evolution of salt structures in the northern Paradox Basin: controls on evaporite deposition,salt wall growth and supra‐salt stratigraphic architecture

The northern Paradox Basin evolved during the Late Pennsylvanian–Permian as an immobile foreland basin, the result of flexural subsidence in the footwall of the growing Uncompahgre Ancestral Rocky Mountain thick‐skinned uplift. During the Atokan‐Desmoinesian (～313–306 Ma) fluctuating glacio‐eustatic sea levels deposited an ～2500 m thick sequence of evaporites (Paradox Formation) in the foreland basin, interfingering with coarse clastics in the foredeep and carbonates around the basin margins. The cyclic deposition of the evaporites produced a repetitive sequence of primarily halite, with minor clastics, organic shales and anhydrite. Sediment loading of the evaporites subsequently produced a series of salt walls and minibasins, through the process of passive diapirism or downbuilding. Faults at the top Mississippian level localised the development of linear salt walls (up to 4500 m high) along a NW–SE trend. A crosscutting NE–SW structural trend was also important in controlling the evaporite facies and the abrupt termination of the salt walls. Seismic, well and field data define the proximal Cutler Group (Permian) as a basinward prograding sequence derived from the growing Uncompahgre uplift that drove salt basinwards (towards the southwest), triggering the growth of the salt walls. Sequential structural restorations indicate that the most proximal salt walls evolved earlier than the more distal ones. The successive development of salt‐withdrawal minibasins associated with each growing salt wall implies that parts of the Cutler Group in one minibasin may have no chronostratigraphic equivalent in other minibasins. Localised changes in along‐strike salt wall growth and evolution were critical in the development of facies and thickness variations in the late Pennsylvanian to Triassic stratigraphic sequences in the flanking minibasins. Salt was probably at or very close to the surface during the downbuilding process leading to localised thinning, deposition of diapir‐derived detritus and rapid facies changes in sequences adjacent to the salt wall structures.     

Relay zone evolution: a history of repeated fault propagation and linkage,central Corinth rift,Greece

Established models indicate that, before being breached, relay zones along rift borders can evolve either by lengthening and rotating during progressive overlap of growing fault segments (isolated fault model), or, by simply rotating without lengthening before breaching (coherent fault model). The spatio‐temporal distribution of vertical motions in a relay zone can thus be used to distinguish fault growth mechanisms. Depositional relay zones that develop at sea level and accommodate both deposition on the ramp itself as well as transfer of sediments from the uplifting footwall into the hangingwall depocentres and provide the most complete record of vertical motions. We examine the development of a depositional relay ramp on the border of the active Corinth rift, Greece to reconstruct fault interaction in time and space using both onshore and offshore (2D seismic lines) data. The Akrata relay zone developed over a period of ca. 0.5 Myr since the Middle Pleistocene between the newly forming East Helike Fault (EHF) that propagated towards the older, more established Derveni Fault (DF). The relay zone captured the Krathis River, which deposited prograding Gilbert‐type deltas on the sub‐horizontal ramp. Successive oblique faults record progressive linkage and basinward migration of accommodation along the ramp axis, whereas marine terraces record diachronous uplift in their footwalls. Although early linkage of the relay zone occurs, continuous propagation and linkage of the EHF onto the static DF is recorded before final beaching. Rotation on forced folds above the upward and laterally propagating normal faults at the borders of the relay zone represents the ramp hinges. The Akrata relay zone cannot be compared directly to a simple fault growth model because (1) the relay zone connects two fault segments of different generations; (2) multiple linkages during propagation was facilitated by the presence of pre‐existing crustal structures, inherited from the Hellenide fold and thrust belt. The linkage of the EHF to the DF contributed to the westward and northward propagation of the southern rift border.     

The linkage between fault throw and footwall scarp erosion patterns: an example from the Bremstein Fault Complex,offshore Mid‐Norway

Studies of normal fault systems in modern extensional regimes (e.g. Basin and Range), and in exhumed, ancient rift basins (e.g. Gulf of Suez Rift) have shown a link between the evolution of fault‐related footwall topography and associated erosional drainage systems. In this study, we use 3D seismic reflection data to image the footwall crest of a gravity‐driven fault system developed during late Middle Jurassic to Early Cretaceous rifting on the Halten Terrace, offshore Mid‐Norway. This 22‐km‐long fault system lacks significant footwall uplift, with hangingwall subsidence accommodating throw accumulation on the fault system. Significant erosion has occurred along the length of the footwall crest and is defined by 96 catchments characterized by erosional channels. These erosional channels consist of small, linear systems up to 750 m long located along the front of the fault footwall. Larger, dendritic channel systems extend further back (up to 3 km normal to fault strike) into the footwall. These channels are up to 7 km long, up to 50 m deep and up to 1 km wide. Fault throw varies along strike, with greatest throw in the centre of the fault decreasing towards the fault tips; localized throw minima are interpreted to represent segment linkage points, which were breached as the fault grew. Comparison of the catchment location to the throw distribution shows that the largest catchments are in the centre of the fault and decrease in size to the fault tips. There is no link between the location of the breached segment linkage points and the location and size of the footwall catchments, suggesting that the first‐order control on footwall erosion patterns is the overall fault‐throw distribution.     

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.