Emplacement of giant mud volcanoes in the South Caspian Basin: 3D seismic reflection imaging of their root zones

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.