Crestal fault geometries reveal late halokinesis and collapse of the Samson Dome, Northern Norway: Implications for petroleum systems in the Barents Sea

This paper uses 2D and high-quality 3D seismic reflection data to assess the geometry and kinematics of the Samson Dome, offshore Norway, revising the implications of the new data to hydrocarbon exploration in the Barents Sea. The study area was divided into three (3) zones in terms of fault geometries and predominant strikes. Displacement-length (D-x) and Throw-depth (T-z) plots showed faults to consist of several segments that were later dip-linked. Interpreted faults were categorised into three families, with Type A comprising crestal faults, Type B representing large E-W faults, and Type C consisting of polygonal faults. The Samson Dome was formed in three major stages: a) a first stage recording buckling of the post-salt overburden and generation of radial faults; b) a second stage involving dissolution and collapse of the dome, causing subsidence of the overburden and linkage of initially isolated fault segments; and c) a final stage in which large fault segments were developed. Late Cretaceous faults strike predominantly to the NW, whereas NE-trending faults comprise Triassic structures that were reactivated in a later stage. Our work provides scarce evidence for the escape of hydrocarbons in the Samson Dome. In addition, fault analyses based on present-day stress distributions indicate a tendency for ‘locking’ of faults at depth, with the largest leakage factors occurring close to the surface. The Samson Dome is an analogue to salt structures in the Barents Sea where oil and gas exploration has occurred with varied degrees of success

Long-term slope instability induced by the reactivation of mass transport complexes: An underestimated geohazard on the Norwegian continental margin

Submarine landslides are significant geohazards, capable of displacing large volumes of sediment from continental margins to deposit mass transport complexes (MTCs) and generate offshore tsunamis. However, the reactivation of MTCs after their initial failure has long been overlooked. By analyzing high-quality three-dimensional seismic reflection data and seismic attribute maps, as well as comparing the geometry of different MTCs, we investigate the development of long-term slope instability and its hazardous consequences on the northwest flank of the Storegga Slide on the Norwegian margin. Our results demonstrate that the reactivation of MTCs can deform both their inner structure and overlying strata, promoting the formation of sinuous channels and local slope failures on the seafloor. These findings further reveal the MTCs that are underconsolidated or comprise slide blocks may remain unstable for a long time after their initial failure, particularly when affected by slope undercutting and a corresponding reduction in lateral support. This study shows that MTC-prone sequences are more likely to comprise regions of continental slopes with long-term instability and recurring marine geohazards

Geometric and kinematic analysis of normal faults bordering continental shelves: a 3D seismic case study from the northwest South China Sea

Normal faults border many a continental shelf, but have only been considered to play a subordinate role to basin-controlling faults in previous studies. Their detailed geometries and kinematic histories are still poorly known. In this study, high-quality three dimensional (3D) seismic data are used to investigate distinct families of normal faults bordering the continental shelf of the Qiongdongnan Basin, northwest South China Sea. Sixty-six (66) normal faults are interpreted and found to mostly tip out in the Upper Miocene. Three large-scale faults offsetting Horizon T30 (the base of Pliocene strata), and three other faults terminating beneath this same horizon, were selected and studied in detail. We discriminate between growth and blind faults by analysing upper tip folding geometry and fault displacement distribution using throw-depth (T-z) plots. The dips of the upper parts of faults are almost twice that of their lower parts. A new three-stage model for fault reactivation in Qiongdongnan Basin is therefore proposed based on our results. In the northwest South China Sea, the propagation of normal faults on the continental shelf can be attributed to: (1) the rotation of the South China block and a reversal in the movement of the Ailao Shan-Red River Fault Zone from left-to right-lateral slip, and (2) large-scale slope instability in the Qiongdongnan Basin at ~5.5 Ma. The results presented here outline the structural evolution of the continental shelf of the northwest South China Sea after 5.5 Ma, and how regional-scale structures such as the Ailao Shan-Red River Fault Zone controlled the evolution of normal faults on continental shelves

Three-dimensional (3-D) seismic imaging of conduits and radial faults associated with hydrothermal vent complexes (Vøring Basin, Offshore Norway)

Here, we document a suite of radial faults associated with hydrothermal vent complexes in the Vøring Basin, offshore Norway. These complexes have pyramid-shaped, cylindrical- and conical-shaped conduits, with a dome-, or eye-shaped morphology at their summit, intruding on Paleogene sedimentary rocks. Hydrothermal vents are intimate with the tips of magmatic sills that were emplaced at depths ranging between 1800 and 5800 ms Two Way Travel Time (TWTT). At shallower depths of 1800 to 3000 ms TWTT and intermediate depths of 3000 to 5000 ms TWWT, magmatic sills regularly intersect the lower parts of the vent conduits, which are characterized here as pipes. An important parameter that is used to characterize the morphology of a hydrothermal vent conduit is the width of the conduit, which is defined as the longest axis marking the extent of the vents' conduit within the surrounding host-rock strata. Our findings reveal that radial faults are commonly associated with the summits of hydrothermal vents, implying the existence of local stress fields around the vents, where the maximum compressive stress is radial and minimum stress is circumferential, which overrides the regional stress field and indicate variable stress regimes as opposed to tectonic faults. Importantly, circumferential stretching due to catastrophic plumbing of hydrothermal fluids, differential compaction and intensive fracturing enabled the polygonal faults to realign in a radial pattern resulting in the formation of radial faults at the vent summit. As a corollary of this work, we hypothesize that pyramid-shaped hydrothermal conduits are possibly markers of protracted sill emplacement in sedimentary basins

A 3-dimensional seismic method to assess the provenance of Mass-Transport Deposits (MTDs) on salt-rich continental slopes (Espírito Santo Basin, SE Brazil)

Provenance studies of Mass-Transport Deposits (MTDs) frequently rely on the analysis of kinematic indicators. In this work, we use seismic facies analyses, measurements of preserved blocks, and correlations between MTDs thickness and salt diapir geometry to introduce a new method to estimate the provenance of MTDs on 3D seismic data. Our method, applied to a high-quality 3D seismic volume from the Espírito Santo Basin (SE Brazil), provides information on the location of source areas of MTDs, their bulk composition, and on their directions of transport. Whenever blocks of strata are found, their orthogonal axes are measured to estimate parameters such as Maximum Projection Sphericity Index (MPSI), Oblate Prolate Index (OPI), and ratio of short and intermediate axes (ds/di). These parameters are used to quantify the relative degrees of remobilization and transport of MTDs. Statistical data of importance include the variation of MTDs thickness with the diameter and distance from the centre of growing salt diapirs. In addition, the presence of cubic or equant blocks with c/a>0.4 and c/b>0.65 ratios are typical of proximal areas in MTDs, reflecting small transporting distances. Our work is important because it shows that distinct correlation coefficients exist between the thickness of MTDs and diameter of salt diapirs. Positive coefficients of correlation between diapir diameter and MTD thickness characterise active diapirism during the deposition of MTDs. This quantification allows the interpreters to distinguish MTDs sourced from local diapirs from MTDs sourced from more distal regions on the continental slope. Thus, the new method used in this research provides an alternative technique to kinematic indicators for determining the provenance of mass transport deposits on continental margins

Ramps and flats of mass-transport deposits (MTDs) as markers of seafloor strain on the flanks of rising diapirs (Espírito Santo Basin, SE Brazil)

Ramps and flats reflect variations in the morphology of basal shear surfaces in mass-transport deposits (MTDs), which are often mapped without considering their potential as strain markers. In this work ramps and flats are mapped on the margins of salt diapirs, using a high-quality 3D seismic volume from SE Brazil (Espírito Santo) to investigate how the morphology of mass-transport deposits (MTDs) relates to diapir growth and near-seafloor strain. In parallel, aspect ratios of MTDs are used to estimate their source areas and possible causal mechanisms. Our results show diapir-related MTDs to have length/width ratios ≤ 3, in a region where sediment failure was triggered by faulting and seafloor tilting due to halokinesis. Sections of MTDs that were triggered and later uplifted on the flanks of growing diapirs are termed ‘drag zones’. Ramps within this drag zones are shown as local changes in gradient at the basal shear surfaces, and are linked to promontories and older fault scarps. The results in this paper are important because they show that drag zones are elongated in a NW–SE direction, parallel to the dominant trend of basal ramps and promontories. Basal ramps and promontories in these drag zones constitute markers for seafloor strain around growing salt diapirs, with variations in the internal character of MTDs occurring across these same ramps and promontories. As a result, a spectrum of seismic and depositional facies can occur in specific quadrants of drag zones. In the study area are identified debrites (DBs), slightly deformed blocks (SDBs), coherent or unrotated blocks (CUBs), and rafted blocks (RBs)

Provenance of a Blocky Debris Flow Deposit in Mid-continental Slopes (Espírito Santo Basin, SE Brazil)

The provenance of an Oligocene-Miocene Mass Transport Deposit (MTD) in mid continental slope Espírito Santo Basin, SE Brazil was analysed using seismic facies and measurement of preserved blocks. Our method, applied to a high-quality 3D seismic volume from the Espírito Santo Basin (SE Brazil), provides information on the location of source areas of MTDs, their bulk composition and styles of disaggregation in relation to their transporting distances. Whenever blocks of strata are found, their orthogonal axes are measured to estimate parameters such as Maximum Projection Sphericity Index (MPSI), Oblate Prolate Index (OPI), and ratio of short and intermediate axes (ds/di). Eighty two (82) blocks mapped in the MTD have no preferred orientation: they have compact-bladed, bladed, elongate, very-bladed, very-platy and very-elongate shapes. Average block thickness, area coverage and volume are ~128.82m, 0.802 sq. km and ~0.196 cubic km with mean MPSI, OPI, flatness and elongation ratios of 0.398, 0.991, 0.19989 and 0.59861. In addition, the presence of equant blocks with c/a>0.4 and c/b>0.65 ratios are typical of proximal areas in MTDs, reflecting small transporting distances. Thus, the new method used in this research provides an alternative technique to kinematic indicators for determining the provenance of blocky debris flow deposits

The Significance of Ramps and Flats of Mass Transport Deposits (MTD) as Kinematic Indicators

Ramps and flats of mass transport deposits are commonly studied on seismic profiles as kinematic indicators. In this study, ramps and flats on the margin of salt diapirs are mapped in a high quality 3D seismic volume from SE Brazil (Espírito Santo Basin). Our aim is to test how differently oriented ramps and flats at the basal shear surface of specific MTD interval can be used to infer the direction of mass flow. Statistical analysis of thickness variation of the mass transport deposit within depocentre created by ramps and flats was estimated along slope in order to elucidate the mode of emplacement of the associated MTD. Sliding at the plane of failure is enhanced by seafloor faulting and nearby halokinesis. Ramps significantly contribute to local changes in gradient at the basal shear surfaces, as they are linked to promontories (Type I) and paleo-fault scarps (Type II)