Mud volcanism and mud breccia clasts: how did they reach the surface?

Mud volcanism is a geological phenomenon broadly diffused in active and passive tectonic settings and it is typically associated with petroleum systems. The mechanisms driving this process are the overpressure resulting from the generation of hydrocarbons at depth combined with the gravitative instability (e.g., shale buoyancy and density inversion) of the more buoyant rapidly buried sediments.

Mud volcanoes experience episodic short-lived eruptive events during which the overpressured fluids and sediments are burst at the surface featuring spectacular explosive events. The erupted material is termed “mud breccia” and represents a mixture of all the lithologies that are intersected by the mud volcano conduit. The overpressured fluids driving the growing diapir promote the grinding and fracturing of the buried sedimentary formations throughout the conduit. As a result, the erupted mud breccia is a mixture of fine-grained sediments (i.e. matrix=clay, silt and sand) and larger clasts. Clasts can vary in size from few centimetres up to several cubic meters. So far it remained unclear how large clasts can travel from sedimentary sequences buried at several kilometers depth.

Here we present some numerical modelling experiments to simulate the rise of large blocks moving from the roots of the mud volcano feeder channel and ultimately reaching the surface during the powerful eruptions.

First, we study the slow rise of large clasts within the growing diapir of buoyancy sediment in the deep part of the mud volcano conduit. The geodynamical two-phase (solid + fluid) flow equations of mass balance, force balance and Darcy’s law are solved with Pseudo-transient method. The preliminary results shows that long distance (e.g. several km) transport of clasts within the diapir is possible, under the driven force provided by the buoyancy of the sediment and the upward fluid flow. The actual rise speed is dependent on the viscosity and the permeability of the solid matrix.

Second, we apply a Verlet method model to simulate the eruption phase. We assume that the eruptive events occur when at shallow depth within the conduit is reached an overpressure sufficient to overcome the overburden lithospheric pressure. When these conditions occur, and when the overpressured fluids are expelled at the surface, the fluidization of the sediments is triggered, and large clasts may be transposrted to their final destination at the surface. Preliminary results reveal that during this process large clasts can be transported. This method focuses on the movement of the clasts themselves. The viscosity of the surrounding fluids is taken into account with a linear drag force. Nevertheless, a method considering the movement of the surrounding fluids could be a possible improvement.