Influence of fluid overpressure on sliding with or without frontal buttress. Insights from analytical and experimental modeling
Abstract
Hydrocarbon cracking can generate gases that migrate upward and may be trapped beneath low-permeability strata. The resulting fluid overpressure reduces shear strength, allowing gravitational sliding of the overlying cover. The driving force is the slope-parallel component of the weight of the cover, whereas the resisting forces are the friction at the base of the cover and the buttressing resistance to shortening downslope. Typically, a slide is bounded by normal faults upslope and thrusts downslope. But, sometimes, the slide is bounded downslope by a creek incision, and no compressional structures are found. We show how these two types of slides markedly differ in terms of mechanics, geometry, and kinematics using both analytical and experimental models. Mourgues et al. (2009) proposed an analytical model for gravity sliding of a laterally continuous sedimentary pile overlying an overpressured horizon. There, sliding can occur only if the driving force can overcome the buttressing resistance downslope, i.e., if the slide has a minimum required length, which depends on the thickness and rheological properties of the cover, and fluid pressure. The predicted length of the slide decreases with increasing pore pressure and decreasing cover thickness (Fig.1A). We ran the same calculation for a set up in which the base of the slope is incised, hence there is no downslope buttress. Unlike the first set up, the sliding sheet length increases with increasing fluid pressure (Fig.1B). We also tested the influence of varying slope angles, cover thicknesses and permeabilities of the décollement layer. The fluid pressure required to trigger sliding decreases where the basal slope and the cover thickness increase. Changes in décollement permeability have only a minor influence. We undertook a series of analogue experiments to check the evolution predicted by the analytical models. Fluids were simulated by compressed air applied at the base of models made of sand and low-permeability glass microbeads. We measured the sliding sheet length while monitoring the applied air pressure. Results confirm the analytical predictions. The slide’s length increases exponentially with increasing pressure coefficient. The fluid pressure required to trigger sliding was lower for greater cover thicknesses and/or slope angles. Another important difference deals with geometry and kinematics. Buttressed slides consist of one large slope-parallel mass rigidly translated and bounded by thrusts and normal faults. By contrast, a non-buttressed slide shows internal strain: deformation starts with normal faults forming near the incision, then younger normal faults propagate upslope throughout the slide’s evolution.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMNH41C1268L
- Keywords:
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- 5415 PLANETARY SCIENCES: SOLID SURFACE PLANETS / Erosion and weathering