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.

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.

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-skinned' 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 collapse' 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.

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