Murton and Biggs 1 Marine Geology 14/6/01 NUMERICAL MODELING OF MUD VOLCANOES AND THEIR FLOWS USING CONSTRAINTS FROM THE GULF OF CADIZ

Mud volcanoes, ranging in size between 50cm and 800m high, are found both on land and in submarine environments. Their variation in height and shape reflects the driving forces building them and the physical properties of their materials. The driving force behind the construction of mud volcanoes is overpressure of fluidised mud at depth within the sediment column. Although some component of overpressure may derive from volatile generation in a mud reservoir by dehydration of clay minerals and regional tectonic compressive stresses, a major parameter is the thickness of the sediment column. The maximum height theoretically attained by mud volcanoes is, therefore, largely a function of the density contrast between the fluidised mud and its sedimentary overburden, and the depth of the mud source. From a model of isostatic compensation between the mud column and the sediment column, we are able to predict the depth of fluidised mud reservoirs beneath mud volcanoes. Once erupted, the general behaviour of mud flows is also predictable. Using a viscous-gravity current model to describe the eruption of individual mud flows, we argue that conical-shaped mud volcanoes comprise multiple, superimposed radial flows. The model demonstrates that the thickness, eruption rate and speed of mud flows is strongly dependant on the viscosity, density and over-pressure of the fluidised mud. Assuming continuity in these parameters, the model predicts the lowermost flows will be the oldest, thickest and have the greatest length while the uppermost flows will be the youngest and thinnest and shortest. This model is in contrast to more traditional models of stratiform mud volcano construction in which younger flows progressively bury older ones and travel furthest from the summit. Murton and Biggs 2 Marine Geology 14/6/01 Our model predictions are tested against observations of submarine mud volcanoes in the Gulf of Cadiz and the Moroccan Margin. The surface morphology of two prominent circular mud volcanoes, their bathymetric profiles and evidence from gravity cores taken from their summits are consistent with the volcanoes having reached their maximum height, and being formed from a series of superimposed radial flows in which the outer ones are oldest. Applying our model to the two studied mud volcanoes, we derive quantitative estimates of flow rates, exit and flow velocities, eruption durations, heights and conduit radii. For example, with an average kinematic viscosity of 1.5ms for the fluidised mud breccia, a density of 1.8 g/cc, and individual flow heights of between 1.3m and 1.6m, we calculate eruption rates of between 10 and 20ms. At these rates, the mud flows require between 5 and 12hrs to reach the flanks of the volcanoes. For a flow rate of 10ms, the volcano has an average conduit radius of 1.4m, giving an exit velocity for the fluidised mud breccia of about 1.6ms. Also, using our model of isostatic compensation, we predict the depth of the mud reservoir to be about 4600m. To construct conical-shaped mud volcanoes of ~250m height, like the two studied in the Gulf of Cadiz, we estimate a total of ~234 flows taking a minimum of 117 days of eruption activity. 469 words