The dynamics of fluidisation during mud volcano eruptions

Mud volcanoes erupt sediment sourced from subsurface, consolidated mudstones via a conductive flow pathway (conduit). A 3-D seismic survey of mud volcanoes in the Eastern Mediterranean shows localised thinning of the source unit in zones at the base of each conduit, interpreted to result from mud depletion [1]. These depletion zones are typically bowl-shaped, suggesting that they grow radially outward from the base of the conduit. Fluidisation, whereby consolidated sediments can be mobilised by migrating pore fluids of a sufficient velocity, has previously been proposed as a mechanism to explain mud volcano formation [2,3]. However, the dynamics of fluidisation during eruptions are poorly understood due to limited subsurface observations. We hypothesise that the sudden opening of the conduit initiates rapid fluid expulsion, inducing porous flow through and fluidisation of the source rock. This is in contrast to previous modelling work, which attributes the flow of mud to plastic failure [4]. We present a novel theoretical model of flow-driven fluidisation, capturing the dynamic interface between the solid and fluidised regions. The solid region is modelled as a poroelastic material and the fluidised region is modelled as a viscous fluid. Our results indicate that fluidisation initiates at the conduit and spreads radially. We demonstrate that fluidisation amplifies the rate of fluid flow and vice versa, leading to nonlinear growth of the fluidised region. We explore the mechanisms that regulate this growth to produce a depletion zone with a characteristic size.

[1] Kirkham, Chris, et al. "The spatial, temporal and volumetric analysis of a large mud volcano province within the Eastern Mediterranean." Marine and Petroleum Geology, https://doi.org/10.1016/j.marpetgeo.2016.12.026

[2] Brown, Kevin M. "The nature and hydrogeologic significance of mud diapirs and diatremes for accretionary systems." Journal of Geophysical Research: Solid Earth, https://doi.org/10.1029/JB095iB06p08969

[3] Nermoen, Anders, et al. "Experimental and analytic modeling of piercement structures." Journal of Geophysical Research: Solid Earth, https://doi.org/10.1029/2010JB007583

[4] Mazzini, Adriano, et al. "Strike-slip faulting as a trigger mechanism for overpressure release through piercement structures. Implications for the Lusi mud volcano, Indonesia." Marine and Petroleum Geology, https://doi.org/10.1016/j.marpetgeo.2009.03.001