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Experimental Investigation on the Role of Pore-Fluid Pressure during Active Extensional Shale Tectonics

Lucie Baudouy1, Régis Mourgues2, and Bruno Vendeville3
1 University College Dublin, Dublin, Ireland
2 Université du Maine, Le Mans, France
3 Universite des Sciences et Technologies de Lille I, Villeneuve d'Ascq Cedex, France

Since the pioneering work of Terzaghi (1923) it is well known that the presence of high pore-fluid pressure decreases the effective strength of sedimentary rocks, thus allowing them to glide spontaneously down a basin's slope under the sole effect of gravity forces. In addition, in an actively tectonic setting, a high pore-fluid pressure also affects how deeply-rooted faults propagate upward in overpressured sediments. Using a systematic set of analogue experiments (with a basal velocity discontinuity), we conducted a sensitivity analysis on how the fault pattern can change in the same extensional setting when varying the slope (with or without), the pore-fluid pressure (with or without), and the stratigraphy (isotropic or anisotropic in terms of permeability, hence pore-fluid pressure). Results indicate that the primary parameter is the pore-fluid pressure. Without overpressure, results of all experiments were practically identical, even when the other parameters varied. With no overpressure, one synthetic permanent fault and a few transient antithetic faults formed. The resulting topographic relief was sharp. However, when a high pore-fluid pressure was applied, the other parameters (slope and layering) had a significant effect. The deformation was more pervasive, and much less relief was created. With a slope and a stratigraphically uniform model, overpressure-related seepage forces caused a marked change in fault dips, with synthetic faults dipping more gently. With combined slope and anisotropic layering, a secondary detachment level formed in the model's cover, changing altogether the overall deformation style.