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The Quaternary to late Pliocene sedimentary succession along the margin of the South Caspian Basin contains numerous kilometre- scale submarine slope failures, which were sourced along the basin slope and from the inclined flanks of contemporaneous anticlines. This study uses three-dimensional (3D) seismic reflection data to visualise the internal structure of 27 mass transport deposits and catalogues the syndepositional structures contained within them. These are used to interpret emplacement processes occurring during submarine slope failure. The deposits consist of three linked structural domains: extensional, translational and compressive, each containing characteristic structures. Novel features are present within the mass transport deposits: (1) a diverging retrogression of the headwall scarp; (2) the absence of a conventional headwall scarp around growth stratal pinch outs; (3) restraining bends in the lateral margin; (4) a downslope increase in the throw of thrust faults. The results of this study shed light on the deformation that occurred during submarine slope failure, and highlight an important geological process in the evolution of the South Caspian Basin margin.

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

We use three-dimensional (3D) seismic reflection and magnetic data to interpret and describe the 3D geometry of igneous dykes in the southern North Sea. The dykes were emplaced into Paleozoic and Mesozoic sediments and have a common upper termination in Early Tertiary sediments. We interpret the dykes to be part of the British Tertiary volcanic province and estimate the age of the dykes to be 58 Ma. The dykes are characterized by a narrow 0.5–2 km wide vertical disturbance of seismic reflections that have linear plan view geometry. Negative magnetic anomalies directly align with the vertical seismic disturbance zones and indicate the presence of igneous material. Linear coalesced collapse craters are found above the dykes. The collapse craters have been defined and visualized in 3D. Collapse craters have formed above the dyke due to the release of volatiles at the dyke tip and resulting volume loss. Larger craters have potentially formed due to explosive phreatomagmatic interaction between magma and pore water. The collapse craters are a new Earth analogue to Martian pit chain craters.

Subsurface sediment remobilization and fluid flow processes and their products are increasingly being recognized as significant dynamic components of sedimentary basins. The geological structures formed by these processes have traditionally been grouped into mudvolcano systems, fluid flow pipes and sandstone intrusion complexes. But the boundaries between these groups are not always distinct because there can be similarities in their geometries and the causal geological processes. For instance, the process model for both mud and sand remobilization and injection involves a source of fluid that can be separate from the source of sediment, and diapirism is now largely discarded as a deformation mechanism for both lithologies. Both mud and sand form dykes and sills in the subsurface and extrusive edifices when intersecting the sediment surface, although the relative proportions of intrusive and extrusive components are very different, with mud volcano systems being largely extrusive and sand injectite systems being mainly intrusive. Focused fluid flow pipes may transfer fluids over hundreds of metres of vertical section for millions of years and may develop into mud volcano feeder systems under conditions of sufficiently voluminous and rapid fluid ascent associated with deeper focus points and overpressured aquifers. Both mud and sand remobilization is facilitated by overpressure and generally will be activated by an external trigger such as an earthquake, although some mud volcano systems may be driven by the recharge dynamics of their fluid source. Future research should aim to provide spatio-temporal \\\\\\\'injectite\\\\\\\' stratigraphies to help constrain sediment remobilization processes in their basinal context and identify and study outcrop analogues of mud volcano feeders and pipes, which are virtually unknown at present. Further data-driven research would be significantly boosted by numerical and analogue process modelling to constrain the mechanics of deep subsurface sediment remobilization as these processes can not be readily observed, unlike many conventional sediment transport phenomena.

We report the first structural field mapping of exhumed mud volcano feeder complexes. Three mud volcanoes outcropping onshore in Azerbaijan were selected on the basis of outcrop quality and scale. These examples are all located within 1 km of the axes of NW–SE-trending folds associated with the southern margin of the Greater Caucasus mountain belt. The mapping shows that the intrusive complexes are 200–800 m wide and roughly circular. These feeder complexes consist of a megabreccia of country rock blocks at a scale of tens of metres, enclosed in a matrix of intrusive mud. Minor structures include grid like fractures sets, sinuous fractures, mud plugs and breccia pipes. The country rock blocks are deformed and rotated relative to surrounding sedimentary strata. Alternative mechanisms to explain the strain history of these large blocks in the feeder complexes are: a. stoping, b. flow rotation and c. caldera collapse. Our mapping indicates that the most likely mechanism involves stoping processes, similar to those identified in igneous systems. This study provides a basis for reservoir distribution in commercial geological models that contain the feeder complexes of mud volcano systems, and also constrains conduit geometry for modelling studies of evolution and flow dynamics.

We propose a new mechanism for the formation of some polygonal fault arrays. Seismically imaged opal-A (biogenic silica) to opal-CT (cristobalite and tridymite) diagenetic boundaries from two regions offshore of Norway have developed regular wavelength patterns. The pattern consists of cell-shaped elevations that are 200–2600 m wide and up to 200 m high, separated by troughs. The cells represent regions that undergo diagenesis at shallower burial depths, earlier than adjacent areas. The chemical change leads to mechanical compaction and porosity reduction; therefore subsidence occurs above the cells in the overburden. Roughly circular depressions form above the cells, and a network of folds form above inter-cell areas. Networks of normal faults form on the crests and margins of the folds as a result of flexure during the folding. The progressive lateral growth of the cells causes the depressions to widen and intervening folds to narrow resulting in new differential compaction-induced faults to form with variable strike orientations. Lateral and vertical growth of cells leads to cells conjoining and the re-establishment of a uniform planar reaction boundary. This novel but simple mechanism can explain some polygonal fault arrays that form above opal-A to opal-CT reaction boundaries and in these settings the mechanism should be considered in addition to syneresis, density inversion or low coefficients of residual friction which are the most commonly cited drivers for polygonal fault systems.

We describe the internal structure of a multi-kilometre scale mud volcano edifice from the South Caspian Sea using three-dimensional (3D) seismic reflection data leading to a reconstruction of the volcano system's eruptive history. By adapting elements of classic seismic stratigraphy to the study of this volcano, we have found its edifice to consist of a series of stacked mud cones. This internal architecture is most likely to have formed as a result of repeated episodes of expulsion of a fluid-mud mix. Underlying the stack of cones is an asymmetric fault-bounded caldera measuring approximately 2 km in diameter. This caldera shows close structural similarity to the trapdoor type of magmatic caldera. Based on the geometrical relationships of individual mud cones to this caldera, we conclude that caldera-like collapse of the edifice floor initiated following the deposition of the first mud cone (the pioneer cone). Growth of the caldera continued until the later stages of edifice evolution when it eventually abated. This eruptive history shows strong similarities to recent models for magmatic caldera eruption cycles. The study therefore highlights the potential analogue value of mud volcano systems to the study of igneous volcanism. Furthermore, it identifies 3D seismic data as a potentially useful tool in reconstructing the history of mud volcanic eruption and fluid and sediment expulsion from sedimentary basins.

We interpret recently acquired two-dimensional (2D) and 3D seismic data from the contractional domain of the Tertiary deepwater west Niger Delta, which is an area of current hydrocarbon exploration and development to show that during its gravitational collapse, multiple detachments were active. Detachments are located within (1) what we herein refer to as the ‘Dahomey unit’, (2) the transition between the Agbada and Akata formations (Top Akata) and (3) the Akata formation. Seismic interpretation and quantitative measurements of fault displacements show that the utilisation of different detachments results in contrasting styles of thrust propagation and fold growth. Two geographical zones are defined. In zone A (NW sector of the study area), the stratigraphically shallowest Dahomey detachment is dominant and is associated with thrust truncated folds. In zone B (SE sector of the study area), a stratigraphically lower detachment approximately at the Agbada-Akata formation boundary is associated with thrust propagation folds. A third detachment, within the Akata formation, is locally developed and is also associated with thrust propagation folds. The different deformational histories are probably related to the mechanical stratigraphy and the pore-pressure characteristics of the succession.

Rock samples – collected from a recent deep-water exploration well drilled in the Faeroe-Shetland Channel, northwest of the UK – confirm that a distinctive high-amplitude seismic reflector that cross-cuts the Upper Palaeogene and Neogene succession and covers an area of 10 000 km2 is an example of a fossilized Opal A to Opal C/T (Cristobalite/Tridymite) transition. Analysis of these rock fragments tied to an extensive two-dimensional and three-dimensional seismic database constrains the time at which the boundary was fossilized and in addition reveals the unusual geometrical characteristics of a relict bottom-simulating reflector. The diagenetic transformation of biogenic silica (Opal A) to Opal C/T is predominantly temperature-controlled and requires sediments that contain biogenic silica. The reflector (termed as Horizon E) probably initially represented a biosiliceous ooze or a siltstone that contained a component of biogenic silica that underwent transformation as the diagenetic front migrated upsection during burial. The parallelism it shows with a shallower early Pliocene reflector and its apparent upsection migration during a compressional episode in the basin indicate that it was active during the middle and late Miocene and ceased activity during the early Pliocene when there was between 200 and 400 m of overburden. The present-day burial depth of the boundary is ca. 700 m and the temperature at the inactive diagenetic front at the well location is 24 °C. Given these temperature and depth constraints, we hypothesize that even if this is an example of a relatively low-temperature Opal A to Opal C/T transformation, a temporarily elevated geothermal gradient of ca. 60 °C km−1 would have been required to initiate and arrest upsection migration of the boundary during the middle and late Miocene. Factors such as climatic deterioration and the onset of cold deep-water circulation are likely to only have had a contributory role in arresting the upward migration of the boundary.

Schoolof Earth, OceanandAtmospheric Sciences,TheUniversityofManchester, OxfordRoad,ManchesterM13 9PL, England wShell International Exploration andTechnology, Kessler Park 1, 2288GSRijswijk,The Netherlands zDepartment of Earth Science & Engineering, Imperial College, Prince Consort Road, London SW7 2BP, England ‰Jackson School of Geosciences,The University of Texas at Austin,1University Station, C9000, Austin,TX 78712, USA zDepartment of Earth Sciences, Centre for Research into Earth Energy Systems (CeREES), University of Durham, DurhamDH13LE, England kBPExploration, ChertseyRoad, Sunbury-on-Thames, MiddlesexTW16 7LN, England