Physical Modeling of Gravity Spreading Induced by Fluid Overpressure Varying in Space and Time

Mourgues, Regis, Bruno Vendeville, Université des Sciences et Technologies de Lille I, Villeneuve d’Ascq Cedex, France

Many deep-sea fans are underlain by a mechanically weak décollement layer or detach­ment made of salt or overpressured shale. Loading of the margin by clastic sediments caus­es gravity spreading of the overburden characterized by proximal extension and distal short­ening. Traditionally, analog modelers have assumed that shale-detached systems behave similarly to salt systems and thus have used viscous polymers to simulate overpressured shale. There are, however, some fundamental differences between the two. Rocksalt’s low viscosity is due to its mineralogical properties, whereas shale’s strength depends on the pore-fluid pressure and therefore may vary through time and space. As a margin progrades, the front of pore-fluid pressure advances seaward and parts of the overburden that were previously fixed may start to deform. Likewise, lateral (i. e., parallel to the margin) shifts of regional depocenters can cause shifts in the area of active gravity tectonics. We present results from a series of analogue experiments based on a new technical design that allowed us to simulate changes in pore-fluid pressure in space and time during syndepositional gravity spreading of a deep-sea fan.