Abstract

The classic work of Hubbert and Rubey on the role of fluid pressure in the mechanics of overthrust faulting concentrated almost exclusively on the special case of a subaerial block of water-saturated sediments. More general expressions are required because almost all the known sediments with pore-fluid pressures approaching the overburden pressure are below sea level and also because marine seismic investigations are revealing submarine slides on the continental margins.

Hubbert and Rubey's terms (1 − λ) in their expressions for subaerial sliding and (1 – λa) in those for submarine sliding can be replaced with greater generality by δ = (1 – λ)/(l – λe), where λ is the ratio of pore-fluid pressure to total normal stress, and λe, is the ratio of ambient fluid pressure (air or water) to total normal stress, both at the sliding surface. The critical slope 𝛉 down which subaerial and submarine sliding can take place is given by tan 𝛉 = (τ0/σe) + δ tan ϕ, where τ0 is the initial shear strength of the material at the sliding surface, σe is the normal component of the loading or “active” stress, and tan ϕ is the coefficient of sliding friction for the materials of the sliding surface.

For identical water-saturated blocks, the critical angle of submarine sliding is always steeper than the subaerial. A water-saturated subaerial block sliding at low velocity down a constant slope that is equal to its critical subaerial slope cannot slide far into the sea because the resistance to sliding increases relatively. If a block exceeds some critical length (for which approximate expressions are developed) when its sliding is impeded by changes in the sliding parameters, thrusts or overthrusts may develop.

Because almost all known sediments with pore pressures high enough for low-angle sliding are below sea level, the relatively favorable mechanics of subaerial sliding is unlikely to be common. Some form of lubrication seems required for extensive sliding. This will commonly be provided by abnormally pressured shales of low equivalent viscosity.

For evaluating the mechanism of submarine slides revealed by marine seismic surveys, the following expression may be useful when the initial shear strength of the material can be regarded as negligibly small compared to σe: δ = tan 𝛉/tan ϕ. From the value of δ an estimate of the pore-fluid pressures at the sliding surface at the time of sliding can be obtained.

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