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 1012 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. © 2011 Geological Society of London.