The structure and formation of mud volcano summit calderas

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 theology 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.