Fluid overpressures and strength of the sedimentary upper crust

Abstract

a b s t r a c tThe classic crustal strength-depth profile based on rock mechanics predicts a brittle strength s 1 À s 3 ¼ kðrgz À P f Þ that increases linearly with depth as a consequence of [1] the intrinsic brittle pressure dependence k plus [2] an assumption of hydrostatic pore-fluid pressure, P f ¼ r w gz. Many deep borehole stress data agree with a critical state of failure of this form. In contrast, fluid pressures greater than hydrostatic rgz > P f > r w gz are normally observed in clastic continental margins and shale-rich mountain belts. Therefore we explore the predicted shapes of strength-depth profiles using data from overpressured regions, especially those dominated by the widespread disequilibrium-compaction mechanism, in which fluid pressures are hydrostatic above the fluid-retention depth z FRD and overpressured below, increasing parallel to the lithostatic gradient rgz. Both brittle crustal strength and frictional fault strength below the z FRD must be constant with depth because effective stress ðrgz À P f Þ is constant, in contrast with the classic linearly increasing profile. Borehole stress and fluid-pressure measurements in several overpressured deforming continental margins agree with this constant-strength prediction, with the same pressure-dependence k as the overlying hydrostatic strata. The role of z FRD in critical-taper wedge mechanics and jointing is illustrated. The constant-strength approximation is more appropriate for overpressured crust than classic linearly increasing models.

Deeper penetration of large earthquakes on seismically quiescent faults

“…S11). The effective compressive normal stress in the interseismic period, when the shear heating effects on pore pressure are negligible, is 3 MPa near the surface, increases to 50 MPa at 3 km depth, and stays constant over the deeper regions; this distribution of σ is appropriate for an over-pressurized crust at depth (88). Circular VW patches at the transitional depth are used to illuminate the effect of the larger-scale model behavior on microseismicity.…”

Reappraisal of gravity instability conditions for offshore wedges: consequences for fluid overpressures in the Niger Delta

“…This coefficient could be larger (0.4) for gouge materials depending on the organic content and of course on the rate of straining (Kohli & Zoback 2013). In situ measurements by leak-off tests for the Gulf of Mexico point to a bulk friction coefficient in the range 0.25-0.3 (Suppe 2014). The same author suggests that the fault in shales should have a smaller coefficient, ∼0.2, a value which is adopted here.…”

“…Although density inversion should not be disregarded, this information confirms the idea that the Niger Delta could be interpreted to first order as a frictional wedge. The detachment has certainly a complex structure within the Akata Formation and, following and Suppe (2014), we propose to represent this complex zone by a single, planar detachment from the extensional domain to the thrusting domain. This detachment is dipping at β ∼ 1.5…”

“…There has been some applications of the CCW theory to fluid-saturated wedges Suppe 2007Suppe , 2014 on the Niger Delta frontal compressive region, assuming a frictional behaviour of the sediments and the detachment. It was found that the pore-fluid pressure within the detachment is 90 per cent of the lithostatic pressure (Hubbert-Rubey fluid-pressure ratio λ ∼ 0.9).…”

“…It applies to the extensional upslope (active) and to the compressive downslope (passive) domains but does not propose any connection between the two. The extent of the intermediate region can also be estimated following the same argument of Crans et al (1980), for an inclined layer, as presented by Mourgues et al (2009).There has been some applications of the CCW theory to fluid-saturated wedges Suppe 2007Suppe , 2014 on the Niger Delta frontal compressive region, assuming a frictional behaviour of the sediments and the detachment. It was found that the pore-fluid pressure within the detachment is 90 per cent of the lithostatic pressure (Hubbert-Rubey fluid-pressure ratio λ ∼ 0.9).…”