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.     

Extension across the Indian–Arabian plate boundary:the Murray Ridge

Seismic reflection profiles from the Murray Ridge in the Gulf of Oman, northwest Indian Ocean, show a significant component of extension across the predominantly strike-slip Indian–Arabian plate boundary. The Murray Ridge lies along the northern section of the plate boundary, where its trend becomes more easterly and thus allows a component of extension. The Dalrymple Trough is a 25 km wide, steep-sided half-graben, bounded by large faults with components of both strike-slip and normal motion. The throw at the seabed of the main fault on the southeastern side of the half-graben reaches 1800 m. The northwest side of the trough is delineated by a series of smaller antithetic normal faults. Wide-angle seismic, gravity and magnetic models show that the Murray Ridge and Dalrymple Trough are underlain by a crystalline crust up to 17 km thick, which may be continental in origin. Any crustal thinning due to extension is limited, and no new crust has been formed.
We favour a plate model in which the Indian–Arabian plate boundary was initially located further west than the Owen Fracture Zone, possibly along the Oman continental margin, and suggest that during the Oligocene–Early Miocene Indian Ocean plate reorganization, the plate boundary moved to the site of the present Owen Fracture Zone and that motion further west ceased. At this time, deformation began along the Murray Ridge, with both the uplift of basement highs, and subsidence in the troughs tilting the lowest sedimentary unit. Qalhat Seamount was formed at this time. Subsequent sediments were deposited unconformably on the tilted lower unit and then faulted to produce the present basement topography. The normal faulting was accompanied by hanging-wall subsidence, footwall uplift, and erosion. Flat-lying recent sediments show that the major vertical movements have ceased, although continuing earthquakes show that some faulting is still active along the plate boundary.     

The structural and sedimentological evolution of early synrift successions: the Middle Jurassic Tarbert Formation, North Sea

ABSTRACT This study addresses the complex relationship between an evolving fault population and patterns of synrift sedimentation during the earliest stages of extension. We have used 3D seismic and well data to examine the early synrift Tarbert Formation from the Middle–Late Jurassic northern North Sea rift basin. The Tarbert Formation is of variable thickness across the study area, and thickness variations define a number of 1- to 5-km-wide depocentres bounded by normal faults. Seismic reflections diverge towards the bounding faults indicating that the faults were active contemporaneous with the deposition of the formation. Many of these faults became inactive during later Heather Formation times. The preservation of the Tarbert Formation in both footwall and hangingwall locations demonstrates that, during the earliest synrift, the rate of deposition balanced the rate of tectonic subsidence. Local space generated by hangingwall subsidence was superimposed upon accommodation generated due to a regional rise in relative sea-level. In basal Tarbert Formation times, transgression across the prerift coastal plain produced lagoons and bays, which became increasingly marine. During continued transgression, barrier islands moved landward across the drowned bays. In the southern part of our study area, shallow marine sediments are erosionally truncated by fluvial deposition. These fluvial systems were constrained by fault growth monoclines, and flowed parallel to the main faults. We illustrate that stratal architecture and facies distribution of early sedimentation is strongly influenced by the active short-lived faults. Local depocentres adjacent to fault displacement maxima focused channel stacking and allowed the aggradation of thick shoreface successions. These depocentres formed early in the rift phase are not necessarily related to Late Jurassic – Early Cretaceous depocentres developed along the major linked normal fault systems.     

Geometry of half-grabens containing a mid-level viscous décollement

In this work, we explore by means of analogue models how different basin-bounding fault geometries and thickness of a viscous layer within the otherwise brittle pre-rift sequence influence the deformation and sedimentary patterns of basins related to extension. The experimental device consists of a rigid wooden basement in the footwall to simulate a listric fault. The hangingwall consists of a sequence of pre-rift deposits, including the shallow interlayered viscous layer, and a syn-rift sequence deposited at constant intervals during extension. Two different geometries exist of listric normal faults, dip at 30 and 60° at surface. This imposes different geometries in the hangingwall anticlines and their associated sedimentary basins. A strong contrast exists between models with and without a viscous layer. With a viscous décollement, areas near the main basement fault show a wide normal drag and the hangingwall basin is gently synclinal, with dips in the fault side progressively shallowing upwards. A secondary roll-over structure appears in some of the models. Other structures are: (1) reverse faults dipping steeply towards the main fault, (2) antithetic faults in the footwall, appearing only in models with the 30° dipping fault and silicone-level thicknesses of 1 and 1.5 cm and (3) listric normal faults linked to the termination of the detachment level opposite to the main fault, with significant thickness changes in the syn-tectonic units. The experiments demonstrate the importance of detachment level in conditioning the geometry of extensional sedimentary basins and the possibility of syncline basin geometries associated with a main basement fault. Comparison with several basins with half-graben geometries containing a mid-level décollement supports the experimental results and constrains their interpretation.     

Syn-sedimentary structural controls on basin deformation in the Gulf of Corinth, Greece

Abstract The Plio-Quaternary history of the Gulf of Corinth Basin has been controlled by dominantly north-south extension. The basin has an asymmetric graben geometry that is, at the present time, controlled by a master fault (the Gulf of Corinth Fault) downthrowing to the north and running offshore from the north Peloponnese coast.
Detailed structural interpretation of single-channel seismic data collected during RRS 'Shackleton' cruise 1/82 combined with onshore structural studies indicates that the basin geometry is not controlled simply by the main Gulf of Corinth Fault. The subsidence history for the uppermost 1 km of sediment can be documented using time-structure contour maps and isochron maps. These indicate that there is a general narrowing in the size of the basin with time, achieved by fault-controlled subsidence switching to antithetic faults concentrated towards the basin centre. It can also be demonstrated that growth of sediments into topographic lows is not only controlled by sea bed rupture but also by more passive sea bed flexure over 'blind' faults at depth.
The main conclusion of this study is that the 3D geometry of the Gulf of Corinth Basin changes not only spatially but also temporally. Active growth faulting and, therefore, the position of depocentres can switch across the basin and the relative importance of synthetic and antithetic faults controls the geometry of the basin, forming grabens, asymmetric grabens and half-grabens throughout the basin history.     

Sedimentation processes and basin-filling depositional architecture in an active asymmetric graben: Strava graben, Gulf of Corinth, Greece

This paper presents data on the sedimentation processes and basin-fill architecture in an incipient submarine intrabasinal graben, the Strava graben. The Strava graben is a relatively small intrabasinal structure about 15 km long and 3 km wide formed some time during the late Pleistocene. It connects the Alkyonidhes basin to the Corinth basin, in the Aegean back arc, which is characterized by fast rates of extension and intensive seismicity. Analysis and interpretation of high-resolution 3.5-kHz and sparker profiles together with sonar imagery have shown that gravity-driven sediment transport, triggered by earthquakes, is the dominant sedimentation process and that this sediment forms the vast bulk of the basin-fill. The sediment deposited in the Strava graben is derived from the uplifted footwall blocks bounding the graben and is transported to the basin initially as liquefied flows, some of which may progressively evolve to turbidity flows. The deposits of the liquefied flows have accumulated in the graben floor as aggradational stacks, consisting of sheet-like, low-relief lobes, forming base of slope aprons that are fed by multiple sediment sources along active faults. In addition to the lateral (footwall-derived) sediment transport there is also a gravity-controlled axial transport. The axial transport has formed a depositional system in the down-dip termination of the Strava graben, where it enters the Corinth basin. The axial depositional system grows outwards and upwards and consists of liquefied flow depositional lobes which are separated by turbidites. The sedimentation transport processes and basin infilling style described for the Strava graben can be used as a predictive model for the early synrift stage of ancient submarine intrabasinal structures, in which the major sediment source area is the bounding fault scarps and not drainage basins in the hinterland.     

Role of fault interactions in controlling synrift sediment dispersal patterns: Miocene, Abu Alaqa Group, Suez Rift, Sinai, Egypt

Although fault growth is an important control on drainage development in modern rifts, such links are difficult to establish in ancient basins. To understand how the growth and interaction of normal fault segments controls stratigraphic patterns, we investigate the response of a coarse-grained delta system to evolution of a fault array in a Miocene half-graben basin, Suez rift. The early Miocene Alaqa delta complex comprises a vertically stacked set of footwall-sourced Gilbert deltas located in the immediate hangingwall of the rift border fault, adjacent to a major intrabasinal relay zone. Sedimentological and stratigraphic studies, in combination with structural analysis of the basin-bounding fault system, permit reconstruction of the architecture, dispersal patterns and evolution of proximal Gilbert delta systems in relation to the growth and interaction of normal fault segments. Structural geometries demonstrate that fault-related folds developed along the basin margin above upward and laterally propagating normal faults during the early stages of extension. Palaeocurrent data indicate that the delta complex formed a point-sourced depositional system developed at the intersection of two normal fault segments. Gilbert deltas prograded transverse into the basin and laterally parallel to faults. Development of the transverse delta complex is proposed to be a function of its location adjacent to an evolving zone of fault overlap, together with focusing of dispersal between adjacent fault segments growing towards each other. Growth strata onlap and converge onto the monoclinal fold limbs indicating that these structures formed evolving structural topography. During fold growth, Gilbert deltas prograded across the deforming fold surface, became progressively rotated and incorporated into fold limbs. Spatial variability of facies architecture is linked to along-strike variation in the style of fault/fold growth, and in particular variation in rates of crestal uplift and fold limb rotation. Our results clearly show that the growth and linkage of fault segments during fault array evolution has a fundamental control on patterns of sediment dispersal in rift basins.     

Architecture and controls on Bathonian–Kimmeridgian shallow-marine synrift wedges of the Oseberg–Brage area, northern North Sea

The Oseberg Fault-Block, situated along the eastern flank of the northern Viking Graben in the North Sea, was affected by Middle–Late Jurassic rifting initiated in Bajocian–Bathonian times. Temporal variations in stretching rates exerted the major control on the depositional infill patterns of the Bathonian–Kimmeridgian Heather Formation and its intercalated Middle Callovian to Early Oxfordian Fensfjord and Late Oxfordian to Kimmeridgian Sognefjord Formations. Three shallow-marine, regressive–transgressive synrift wedges are recognized, and are interpreted in terms of discrete rift phases. The lower, regressive segments of the synrift wedges were deposited during periods of relatively low tectonic activity, whereas the upper, overall transgressive segments correspond to extensional pulses or stages during which significant fault-related subsidence and fault-block rotation occurred. These rotational tilt stages are further subdivided into an early, a climax and a late synrotational substage. The lower, regressive segments consist of stacked, shallowing-upward units, which reflect the advance of wide shallow-marine, rift-marginal shorelines during the tectonically quiescent periods. During the intervening rotational tilt stages renewed basin floor tilting and increased basinal subsidence led to retreat of the rift-marginal depositional systems, renewal of the half-graben topography, formation of intrabasinal sediment sources (footwall islands) and the re-establishment of localized footwall, hangingwall and axial depositional systems. These localized depositional systems generally have an overall forestepping-to-backstepping character superimposed on the larger-scale transgressive trend. There was an associated shift from a wave- and storm-dominated environment during deposition of the lower, regressive segment to a more protected, partly current-(?tidally) influenced environment in the upper, transgressive segment. This reflects a shift from a broad open basin in tectonically quiescent periods to smaller subbasins (embayments or estuaries) during periods with increased rates of rifting. The footwall highs which formed intrabasinal sediment sources were of limited size compared with the volume of the adjacent depositional sinks. As a consequence, complete infilling of individual half-grabens were not achieved during the synrotational stages, leaving the subbasins underfilled at the end of each successive rift phase. Mudstone drapes represent periods with deprivation of clastic material and basinal condensation during the latest synrotational to early tectonic quiescence substages, when footwall islands were small or completely submerged and there was a large distance to the (then progradational) rift-marginal shoreline.     

Tectonic control on the distribution of Palaeocene marine syn‐rift deposits in the Fenris Graben,northwestern Vøring Basin,offshore Norway

The location, shape and stacking pattern of deep‐marine clastic sediments on drifting stage passive continental margins are strongly influenced by the slope and basin floor topography. The tectonic control on sediment routes and dispersal patterns, however, is less understood on rift margins, particularly the impact of subaqueous transfer zones or relay ramps. In this study, an area of the Palaeocene marine syn‐rift succession in the Vøring Basin is mapped in detail to unravel the relationship between fault geometries and sedimentary infill patterns. Using root‐mean‐square (RMS) amplitudes and deposit thicknesses interpreted from seismic data, sedimentary elements in the Fenris Graben and the Gjallar Ridge are related to the fault patterns and the overall basin geometry. Older deposits are found to be aligned parallel to the basin axis, with the greatest sediment thicknesses on the hanging walls and adjacent to rotated faults. The main sediment supply is interpreted to be sourced from the Vøring Marginal High and Greenland, presumably containing a significant proportion of coarser grained material and comprising numerous local depocentres. With continued rifting and decreased fault activity, finer grained deposition draped the previous basin infill and smoothed the basin floor topography. Deposits close to the foot of relay ramps along the Gjallar Ridge, however, suggest that the high may have acted as a local sediment source leading to local depocentres. Transfer zones played a significant role in sediment transport during the early rifting phase, and were able to maintain some influence into the late rifting and early drifting stage. Identification of early‐ and late‐stage transfer zones may therefore help in locating coarser grained depocentres and potential hydrocarbon reservoirs.     

Half-graben basin filling models: new constraints on continental extensional basin development

Three end-member models of half-graben development (detachment fault, domino-style, and fault growth) evolve differently through time and produce different basin-filling patterns. The detachment fault model incorporates a basin-bounding fault that soles into a subhorizontal detachment fault; the change in the rate of increase in the volume of the basin during uniform fault displacement is zero. Younger strata consistently pinch out against older synrift strata rather than pre-rift rocks. Both basin-bounding faults and the intervening fault blocks rotate during extension in the domino fault block model; a consequence of this rotation is that the change in the rate of increase of the volume of the basin is negative during uniform extension. Basin fill commonly forms a fanning wedge during fluvial sedimentation, whereas lacustrine strata tend to pinch out against older synrift strata. In the fault growth models, basins grow both wider and longer through time as the basin-bounding faults lengthen and displacement accumulates; the change in the rate of increase in basin volume is positive. Fluvial strata progressively onlap pre-rift rocks of the hanging wall block, whereas lacustrine strata pinch out against older fluvial strata at the centre of the basin but onlap pre-rift rocks along the lateral edges. These fundamental differences may be useful in discriminating among the three end-member models. The transition from fluvial to lacustrine deposition and hanging wall onlap relationships observed in numerous continental extensional basins are best explained by the fault growth models.     

Fault propagation and climatic control of sedimentation on the Ghoubbet Rift Floor: insights from the Tadjouraden cruise in the western Gulf of Aden

A detailed geophysical survey of the Ghoubbet Al Kharab (Djibouti) clarifies the small-scale morphology of the last submerged rift segment of the propagating Aden ridge before it enters the Afar depression. The bathymetry reveals a system of antithetic normal faults striking N130°E, roughly aligned with those active along the Asal rift. The 3.5 kHz sub-bottom profiler shows how the faults cut distinct layers within the recent, up to 60 m thick, sediment cover on the floor of the basin. A large volcanic structure, in the centre of the basin, the 'Ghoubbet' volcano, separates two sedimentary flats. The organization of volcanism and the planform of faulting, with en echelon subrifts along the entire Asal–Ghoubbet rift, appear to confirm the westward propagation of this segment of the plate boundary. Faults throughout the rift have been active continuously for the last 8400 yr, but certain sediment layers show different offsets. The varying offsets of these layers, dated from cores previously retrieved in the southern basin, imply Holocene vertical slip rates of 0.3–1.4 mm yr⁻¹ and indicate a major decrease in sedimentation rate after about 6000 yr BP, and a redistribution of sediments in the deepest troughs during the period that preceded that change.     

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.     

Two-phase evolution of the Shadow Valley Basin, south-eastern California: a possible record of footwall uplift during extensional detachment faulting

Miocene strata of the Shadow Valley Basin rest unconformably on the upper plate of the Kingston Range - Halloran Hills detachment fault system in the eastern Mojave desert, California. Basin development occurred in two broad phases that we interpret as a response to changes in footwall geometry. In southern portions of the basin, south of the Kingston Range, phase one began with near synchronous initiation of detachment faulting, volcanism and basin sedimentation shortly after 13.4 Ma. Between c. 13.4 and c. 10 Ma, concordantly bedded phase one strata were deposited onto the subsiding hangingwall of the detachment fault as it was translated 5–9 km south-westward with only limited internal deformation. Phase two (c. 10 to 8–5 Ma) is marked by extensional dismemberment of the detachment fault's upper plate along predominantly west-dipping normal faults. Phase two sediments were deposited synchronously with upper-plate normal faulting and unconformably overlie phase one deposits, displaying progressive shallowing in dip and intraformational onlap. Northern portions of the basin, in the Kingston Range, experienced a similar two-phase development compressed into a shorter interval of time. Here, phase one occurred between c. 13.4 and 12.8–12.5 (?) Ma, whereas phase two probably lasted for no more than a few 100000 years immediately prior to c. 12.4 Ma. Differences in the duration of basin development in and south of the Kingston Range apparently relate to position with respect to the detachment fault's breakaway; northern basin exposures overlie the upper plate adjacent to the breakaway (0–15 km) whereas southern basin exposures occur far from the breakaway (20–40 km). We interpret the phase one to phase two transition as recording breakup of the detachment fault's hangingwall during footwall uplift. We propose a model for supradetachment basin evolution in which early, concordantly bedded basin strata are deposited on the hangingwall as it translates intact above a weakly deforming footwall. With continuing extension, tectonic denudation along the detachment fault leads to an increasing flexural isostatic footwall response. We suggest that isostatic footwall uplift may drive internal breakup of the upper plate as the detachment fault is rotated to a shallow dip, mechanically unfavourable for simple upper-plate translation. Additionally, we argue that continuing hangingwall thinning during phase two places geometrical constraints on the timing, amount and, thus, rate of footwall uplift. Kinematically determined footwall uplift rates (0.5–4.5 mm/yr) are comparable with rates determined independently by thermochronological and geobarometric methods.     

The role of inherited extensional fault segmentation and linkage in contractional orogenesis: a reconstruction of Lower Cretaceous inverted rift basins in the Eastern Cordillera of Colombia

Lower Cretaceous early syn‐rift facies along the eastern flank of the Eastern Cordillera of Colombia, their provenance, and structural context, reveal the complex interactions between Cretaceous extension, spatio‐temporal trends in associated sedimentation, and subsequent inversion of the Cretaceous Guatiquía paleo‐rift. South of 4°30′N lat, early syn‐rift alluvial sequences in former extensional footwall areas were contemporaneous with fan‐delta deposits in shallow marine environments in adjacent hanging‐wall areas. In general, footwall erosion was more pronounced in the southern part of the paleorift. In contrast, early syn‐rift sequences in former footwall areas in the northern rift sectors mainly comprise shallow marine supratidal sabkha to intertidal strata, whereas hanging‐wall units display rapid transitions to open‐sea shales. In comparison with the southern paleo‐rift sector, fan‐delta deposits in the north are scarce, and provenance suggests negligible footwall erosion. The southern graben segment had longer, and less numerous normal faults, whereas the northern graben segment was characterized by shorter, rectilinear faults. To the east, the graben system was bounded by major basin‐margin faults with protracted activity and greater throw as compared with intrabasinal faults to the west. Intrabasinal structures grew through segment linkage and probably interacted kinematically with basin‐margin faults. Basin‐margin faults constitute a coherent fault system that was conditioned by pre‐existing basement fabrics. Structural mapping, analysis of present‐day topography, and balanced cross sections indicate that positive inversion of extensional structures was focused along basin‐bounding faults, whereas intrabasinal faults remained unaffected and were passively transported by motion along the basin‐bounding faults. Thus, zones of maximum subsidence in extension accommodated maximum elevation in contraction, and former topographic highs remained as elevated areas. This documents the role of basin‐bounding faults as multiphased, long‐lived features conditioned by basement discontinuities. Inversion of basin‐bounding faults was more efficient in the southern than in the northern graben segment, possibly documenting the inheritance and pivotal role of fault‐displacement gradients. Our observations highlight similarities between inversion features in orogenic belts and intra‐plate basins, emphasizing the importance of the observed phenomena as predictive tools in the spatiotemporal analysis of inversion histories in orogens, as well as in hydrocarbon and mineral deposits exploration.     

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.     

Testing models of rift basins: structure and stratigraphy of an Eocene–Oligocene supradetachment basin, Muddy Creek half graben, south-west Montana

We examine the basin geometry and sedimentary patterns in the Muddy Creek half graben of south-west Montana by integrating geological mapping, structural and basin analysis, ⁴⁰Ar/³⁹Ar geochronology, biostratigraphy and reflection seismic data. The half graben formed in late Middle Eocene to early Oligocene (?) time at the breakaway of a regional, WSW-dipping detachment system. Although the structure of the half graben is that of a supradetachment basin, facies patterns and basin architecture do not conform to a recent model for extensional basins above detachment faults. The border fault, the Muddy Creek fault system, consists of three en echelon, left-stepping normal faults separated by two relay ramps. The fault steepens southward toward each en echelon step, ranges in dip from 8 to 60° near the surface, but flattens at depths between 0 and 3 km. A broad ENE-plunging displacement-gradient syncline defines the central part of the half graben and is flanked by narrow SE-and NE-plunging anticlines to the north and south. Fine-grained deposits of the syntectonic basin-fill are thickest in the central syncline and interfinger with footwall-derived conglomerate near the adjacent anticlines. These facies patterns suggest that folding was coeval with extension and sedimentation in the half graben. Pre-extensional volcanic rocks and interbedded conglomerate filled a major ESE-trending palaeovalley along the future axis of the Muddy Creek half graben. Synextensional sedimentary deposits include lacustrine and paludal shale, mudstone and sandstone ponded in the centre of the half graben, and a narrow (typically <1.5 km wide) fringe of coarse alluvial-fan and fan-delta conglomerate and sandstone derived from the footwall. Angular unconformities and rock-slide deposits occur only locally within the syntectonic sequence. These facies patterns agree well with the half-graben depositional model of Leeder & Gawthorpe but not with a more recent supradetachment basin model of Friedmann & Burbank despite the demonstrably low dip-angle of the basin-bounding normal fault. These data show that it may not be possible to differentiate between supradetachment basins and half graben with steeper border faults using the architecture of the associated basin-fill deposits.     

Quaternary basin evolution and present tectonic regime in the area of the 1997–1998 Umbria–Marche seismic sequence (central Italy)

Geological and geomorphological surveys have been performed in the area affected by the 1997–1998 Umbria–Marche seismic sequence (M_max=6.0) aimed at defining the Quaternary tectonic history and the characteristics of the present tectonic regime. Data have been collected from: (1) the analysis of the remnant landsurfaces by means of aerial photos and field surveys; (2) geological surveys in the Cesi–San Martino basin and in the easternmost sector of the Colfiorito basin in order to identify deformative features affecting the Quaternary deposits; (3) the analysis of boreholes and geo-electrical data (derived from previous surveys performed in the 1960s) in order to reconstruct the top of the pre-Quaternary substratum in the Colfiorito basin. Two different successions of remnant landsurfaces have been identified along the faults bounding the basins to the east, in the hangingwall and the footwall, respectively. The difference accounts for a fault-controlled evolution of the landscape at least during the Upper Pliocene–Early Pleistocene. The deformation affecting the Quaternary deposits and landforms in the investigated basins indicates a decreasing tectonic activity along the master faults since the Middle Pleistocene. Surface deformation due to tectonics is faint and displayed by gentle warping of the landforms during the late Quaternary. As for the basin geometry, subsurface data show that two minor depressions formed in the Colfiorito Basin during the Quaternary, the oldest one close to the fault bounding the basin, while the youngest (and deepest) formed in the inner portion of the basin. Therefore, the present geometry is different from that of other fault-bounded Quaternary depressions of the central Apennines (typically half-graben basins), showing the maximum depth of the substratum in the area close to the master fault. Tectonic history may be summarised as follows: (1) origin of the Quaternary fault-bounded Colfiorito and Cesi–San Martino basins; (2) evolution of the basins with a half-graben style; (3) significant reduction of tectonic activity since the Middle Pleistocene. During the third phase, the evolution of the basins is no longer related to a half-graben style. In the case of the Colfiorito basin, a new depression is superimposed to on the previous half-graben whose evolution is related to the lowering of the inner portion of the basin through warping. Moreover, present activity does not result in fault-related surficial displacements but only in “continuous” deformation spread over the basins. These conclusions have fundamental implications for the seismotectonic framework of the 1997–1998 earthquake sequence. This deformation style is, indeed, in agreement with the coseismic deformation modelled by means of the SAR interferometry analyses carried out by other institutions during the seismic sequence, and with the lack of evident surface faulting related to the mentioned events in the investigated area. This evidence indicates that the evolution of the investigated area since the Middle Pleistocene resulted from the summation of deformative episodes similar to that which occurred during the recent seismic sequence. As a consequence, no earthquakes with magnitude larger than 6 are expected in the area.     

Rift basins and supradetachment basins: intracontinental extensional end-members

Two end-members characterize a continuum of continental extensional tectonism: rift settings and highly extended terrains. These different styles result in and are recorded by different extensional basins. Intracontinental rifts (e.g. East Africa, Lake Baikal) usually occur in thermally equilibrated crust of normal thickness. Rift settings commonly display alkali to tholeiitic magmatism, steeply dipping (45–60°) bounding faults, slip rates <1 mm yr^-1 and low-magnitude extension (10–25%). Total extension typically requires > 25 Myr. The fault and sub-basin geometry which dominates depositional style is a half-graben bounded by a steeply dipping normal fault. Associated basins are deep (6–10 km), and sedimentation is predominantly axial- or hangingwall-derived. Asymmetric subsidence localizes depocentres along the active basin-bounding scarp. Highly extended continental terrains (e.g. Colorado River extensional corridor, the Cyclade Islands) represent a different tectonic end-member. They form in back-arc regions where the crust has undergone dramatic thickening before extension, and usually reactivate recently deformed crust. Volcanism is typically calc-alkalic, and 80–90% of total extension requires much less time (<10 Myr). Bounding faults are commonly active at shallow dips (15–35°); slip rates (commonly > 2 mm yr^-1) and bulk extension (often > 100%) are high. The differences in extension magnitude and rate, volcanism, heat flow, and structural style suggest basin evolution will differ with tectonic setting. Supradetachment basins, or basins formed in highly extended terrains, have predominantly long, transverse drainage networks derived from the breakaway footwall. Depocentres are distal (10–20 km) to the main bounding fault. Basin fill is relatively thin (typically 1–3 km), probably due to rapid uplift of the tectonically and erosionally denuded footwall. Sedimentation rates are high (? 1 m kyr^-1) and interrupted by substantial unconformities. In arid and semi-arid regions, fluvial systems are poorly developed and alluvial fans dominated by mass-wasting (debris-flow, rock-avalanche breccias, glide blocks) represent a significant proportion (30–50%) of basin fill. The key parameters for comparing supradetachment to rift systems are extension rate and amount, which are functions of other factors like crustal thickness, thermal state of the lithosphere and tectonic environment. Changes in these parameters over time appear to result in changes to basin systematics.     

Miocene-Pliocene half-graben evolution, detachment faulting and late-stage core complex uplift from reflection seismic data in south-east Arizona

Reflection seismic data show that the late Cenozoic Safford Basin in the Basin and Range of south-eastern Arizona, is a 4.5-km-deep, NW-trending, SW-dipping half graben composed of middle Miocene to upper Pliocene sediments, separated by a late Miocene sequence boundary into lower and upper basin-fill sequences. Extension during lower basin-fill deposition was accommodated along an E-dipping range-bounding fault comprising a secondary breakaway zone along the north-east flank of the Pinaleño Mountains core complex. This fault was a listric detachment fault, active throughout the mid-Tertiary and late Cenozoic, or a younger fault splay that cut or merged with the detachment fault. Most extension in the basin was accommodated by slip on the range-bounding fault, although episodic movement along antithetic faults temporarily created a symmetric graben. Upper-plate movement over bends in the range-bounding fault created rollover structures in the basin fill and affected deposition within the half graben. Rapid periods of subsidence relative to sedimentation during lower basin-fill deposition created thick, laterally extensive lacustrine or alluvial plain deposits, and restricted proximal alluvian-fan deposits to the basin margins. A period of rapid extension and subsidence relative to sediment influx, or steepening of the upper segment of the range-bounding fault at the start of upper basin-fill deposition resulted in a large downwarp over a major fault bend. Sedimentation was restricted to this downwarp until filled. Episodic subsidence during upper basin-fill deposition caused widespread interbedding of lacustrine and fluvial deposits. Northeastward tilting along the south-western flank of the basin and north-eastward migration of the depocentre during later periods of upper basin-fill deposition suggest decreased extension rates relative to late-stage core complex uplift.     

Dynamic modelling of passive margin salt tectonics: effects of water loading, sediment properties and sedimentation patterns

We investigate the evolution of passive continental margin sedimentary basins that contain salt through two‐dimensional (2D) analytical failure analysis and plane‐strain finite‐element modelling. We expand an earlier analytical failure analysis of a sedimentary basin/salt system at a passive continental margin to include the effects of submarine water loading and pore fluid pressure. Seaward thinning sediments above a weak salt layer produce a pressure gradient that induces Poiseuille flow in the viscous salt. We determine the circumstances under which failure at the head and toe of the frictional–plastic sediment wedge occurs, resulting in translation of the wedge, landward extension and seaward contraction, accompanied by Couette flow in the underlying salt. The effects of water: (i) increase solid and fluid pressures in the sediments; (ii) reduce the head to toe differential pressure in the salt and (iii) act as a buttress to oppose failure and translation of the sediment wedge. The magnitude of the translation velocity upon failure is reduced by the effects of water. The subsequent deformation is investigated using a 2D finite‐element model that includes the effects of the submarine setting and hydrostatic pore pressures. The model quantitatively simulates a 2D approximation of the evolution of natural sedimentary basins on continental margins that are formed above salt. Sediment progradation above a viscous salt layer results in formation of landward extensional basins and listric normal growth faults as well as seaward contraction. At a later stage, an allochthonous salt nappe overthrusts the autochthonous limit of the salt. The nature and distribution of major structures depends on the sediment properties and the sedimentation pattern. Strain weakening of sediment favours landward listric growth faults with formation of asymmetric extensional depocentres. Episodes of low sediment influx, with partial infill of depocentres, produce local pressure gradients in the salt that result in diapirism. Diapirs grow passively during sediment aggradation.