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Structural mapping, nearest neighbour and two-point azimuth statistical analysis of mud volcano vent distributions from nine examples in Azerbaijan and the Lusi mud volcano in east Java are described. Distributions are non-random, forming alignments subparallel to faults within anticlines, ring faults, conjugate faults and detachment faults; this finding confirms a spatial relationship and supports a model for subsurface flow along these features as well as showing fractionation at depth. As fracture and fault orientations are related to structures such as anticlines and the in situ stress state they are therefore predictable. We use vent distributions in Azerbaijan, where the structural geology is well constrained, to propose what controls the distribution of 169 vents at the Lusi mud volcano. This mud volcano system shows evidence for initial eruptions along a NE-SW trend, parallel to the Watukosek fault, changing to eruptions that follow east-west trends, subparallel to regional fold axes. Our analysis indicates that regions east and west of the Lusi mud volcano are more likely to be affected by new vents than those to the north and south, owing to probable onset of elongate caldera collapse within a 10 km diameter of the central vent.

A new method for estimating the duration of a mud volcano eruption is applied to the LUSI mud volcano in East Java. The estimate is based upon carbonates at depths in the range 2500–3500 m being the water source, with an estimated area of 100–600 km 2 , thickness of 0.2–1.0 km, porosity of 0.15–0.25, an initial pressure between 13.9 and 17.6 MPa, and a separate, shallower source of mud ( c . 1200–1800 m depth). The resulting 50 percentile for the time it takes for flow to decline to −1 is 26 years. By analogy with natural mud volcanoes it can be expected to continue to flow at lower rates for thousands of years. Assuming subsidence rates of between 1 and 5 cm day −1 , land surface subsidence of between c . 95 and c . 475 m can be expected to develop within the 26 year time period.

Field data collected from mud volcanoes in Azerbaijan are used to describe a process in mud volcano development that involves portions of the constructional edifices collapsing outwards in ‘thin-skinned9 slides. These events create kilometre-scale scarps that are tens of metres in height, arcuate in plan view, elongate and facing downdip. Similar morphological features occur on igneous volcanoes and have been described as ‘sector collapse9 structures. The largest sector collapses in igneous volcanoes involve some 10 12 tons of mobilized material; equivalent structures in mud volcanoes are several orders of magnitude smaller. We employ a shape parameter that can be utilized in field and satellite-based mapping, to distinguish between slope failure and eruptive deposits. Three mud volcanoes with kilometre-scale sector collapses are described and controlling mechanisms are reviewed. The updip domains of these collapses are characterized by fluid escape, showing that there is also linkage to deeper mud volcano structure. The observations are reconciled in a model consisting of a deflating mud chamber that triggers thin-skinned sector collapse. The updip domain of the sector collapse is localized above a deep-seated zone of volume loss and the downdip domain of the collapse runs down the edifice flank onto the surrounding plain.

Three-dimensional seismic data from the NE Atlantic margin have revealed opal-A to opal-CT reaction zones hosted in Oligo-Pliocene strata as bright reflection events that cross-cut stratigraphic reflections. In some areas, the topographic relief at the top of the reaction zone has a remarkable geometry comprising roughly circular flattish regions, spaced 1–3 km apart, separated by networks of steep-sided troughs or ridges with a relief of up to 200 m. Here we show that these topographic structures formed around the upper tips of polygonal faults, and that remnants exist at deeper stratigraphic levels below the opal-A to opal-CT cross-cutting reflection. Furthermore, we deduce that the chemistry of formation water expelled transiently through the polygonal faults must have played a key role in their formation. Within silica-bearing successions, the transformation of opal-A to opal-CT is followed by the transformation of opal-CT to quartz at greater depth. The opal-CT to quartz transformation releases magnesium, which is known to promote opal-A to opal-CT transformation at lower temperatures. Consequently, we suggest that when the opal-A to opal-CT reaction zone nears the top of the polygonal fault system, it advances more rapidly through strata flushed by pore water that is richer in magnesium.

Circular depressions bounded by inward-dipping faults found at the upper terminations of large mud volcano systems (>500 m diameter) are termed ‘mud volcano summit calderas’. From new mapping and comparison with previously identified examples we describe a series of common structural and morphological features found at a number of calderas and propose a mechanism for caldera formation. A typical example consists of concentric deformational and volcanic zones including an outermost topographic rim, inward-dipping circular fault system, ‘moat’ and raised central ‘pedestal’ of freshly extruded mud volcanic sediment. This distinctive ‘moat and pedestal’ morphology can be explained in terms of the quantity and rheology of extruded mud, and appears characteristic of calderas from the South Caspian Basin and elsewhere. The association of fresh mud volcanic deposits with the calderas in combination with extensive literature on caldera modelling leads us to conclude that the most likely causal mechanism is subsidence as a result of evacuation of fluids and sediment from shallow structural levels during eruptions. Summit calderas mapped in this study characterize dormant periods of mud volcanism in subaerial and submarine settings, and appear to be a common structural element in the extrusive domain of mud volcano systems.

Three-dimensional seismic analysis is used to describe the detailed morphology of the Silverpit crater, a proposed marine impact crater located in the southern North Sea, UK. We examine and constrain for the first time the geometry, extent and age of the post-crater sediment units that define the crater fill. A circular bowl-shaped depression has been mapped and interpreted to represent the upper limit of impact deformation and the geometry of the crater at the end of cratering processes. Reflections exhibit a parallel onlap fill onto the bowl-shaped crater floor, indicating that accommodation space was created instantaneously without any contemporaneous sedimentation. Using regional lithostratigraphic markers constrained by microfossil and calcareous nannofossil analysis, we have dated the first onlapping sediments onto the crater. The age of the onlapping sediments allows us to constrain the crater formation to be Middle Eocene in age. This date is c. 10–15 Ma younger than the previous ascribed age for the crater formation and rules out the Silverpit crater as a K–T boundary impact. The age of crater formation considerably predates regional folding and accompanying salt flowage, effectively ruling out a salt withdrawal origin for the Silverpit crater.

Two-dimensional (2D) seismic reflection data reveal two silica diagenetic boundaries located in the North Sakhalin Basin, offshore Sakhalin Island. These boundaries form distinctive high-amplitude seismic reflections that predominantly cross-cut Miocene stratigraphy. The boundaries represent the opal-A to opal-CT (cristobalite and tridymite) transition and the subsequent deeper opal-CT to quartz transition and cover an area of c . 107 000 km 2 . The reflections represent diagenetic reaction fronts that advance through silica-rich host sediments during burial. The oldest strata within which the diagenetic fronts have been identified are of Miocene age and the fronts commonly show a parallel relationship to a Late Miocene unconformity. These diagenetic fronts develop a variety of morphological forms. In the North Sakhalin Basin these front geometries are probably the result of the interference of the diagenetic fronts with inclined and folded stratigraphy together with lateral changes in the composition and, therefore, depth of transformation of the sediment. We define criteria for the recognition of these 2D cross-sectional morphological elements and by so doing provide the basis for the objective description and categorization of diagenetic front geometry.

Exceptional quality 3D data for the largest mud volcano yet described provide the first detailed imaging of the plumbing architecture that connects a major volcanic edifice to its source layer at depth. The volcano is in the South Caspian Basin and consists of an extruded submarine mud bicone, 10 km wide and 1.4 km thick, overlying an oval caldera 1.2–1.6 km in width and 0.5 km in depth. The caldera narrows downwards into a zone of collapsed country rock forming a downward tapering cone, 1 km in height, the vertex of which is located close to the top of the mud source layer. The imaged structural elements lead to an evolutionary model. A narrow, steep fluidization pipe fed the oldest, ‘pioneer’ cone. We propose that numerous additional fluidization pipes injected the country rock, forming a densely intruded, cylindrical zone, similar to ‘gryphon’ swarms observed at outcrop onshore. Wall-rock erosion and compaction of the intruded zone led to collapse of the downward tapering cone that linked upwards into ring faults that define the caldera margins. Later mud flowage focused on the conical sheared margins. Volumetric contraction of the extruded volcanic cone led to an unusual concentric system of minor, outward-facing normal faults. This model has many similarities to syntheses of igneous maar–diatreme–caldera systems, for which it may be analogous.

We welcome the opportunity to further clarify aspects of our paper and challenge key elements of Owen's discussion and the inferences he makes regarding the rigour our interpretations and those of previous workers on the origin of the Silverpit structure. We deal with each of his comments in the order that he raises them. Owen refers to the interpretation of the Silverpit structure as an impact crater as a ‘speculative hypothesis'. This is consistent with an earlier statement he has made: ‘This feature, termed the Silverpit Crater, has earlier been interpreted as a meteor impact structure (Stewart & Allen 2002), without a shred of scientific justification' (Thomson et al . 2005). The inference that the hypothesis lacks any observational basis disregards a wealth of 3D seismic analysis by Stewart & Allen (2002, 2005). The statement is at best disingenuous, as Owen chooses to ignore the thorough work of earlier workers. Although all the observations for and against a impact origin are based upon 3D seismic data and there are as yet no other independent supportive lines of evidence we would highlight the vast body of work on impact cratering and the numerous impact craters on planetary bodies that have been identified purely from their surface morphology (Melosh 1989); in other words, the absence of shocked quartz and other impact criteria does not invalidate the impact crater interpretation. We have been very careful to term the Silverpit structure a ‘probable impact crater' and will continue to do so until such unequivocal evidence is discovered. In our paper we identify a key reflection that we term CF …