| The classic crustal strength-depth profile, based on experimental rock mechanics, predicts a
brittle strength (Ï1 - Ï3) = κ(¯Ï gz - Pf) that increases linearly with depth z, which is a
consequence of [1] the intrinsic pressure dependence κ of brittle frictional strength plus
[2] an assumption that pore-fluid pressure is hydrostatic, Pf = Ïwgz. Much deep
borehole stress data agree with a critical state of failure of this form. In contrast, fluid
pressures greater than hydrostatic ϯ gz > Pf > Ïwgz are normally observed in
deeper parts of thick clastic sedimentary basins on continental margins and active
shale-rich plate-boundary mountain belts. We explore the predicted shape of the crustal
strength-depth profile in such overpressured regions, particularly those dominated
by the widespread disequilibrium compaction mechanism, which displays fully
compacted sediments with hydrostatic fluid pressures at shallow depths until the
fluid-retention depth zFRD is reached, below which sediments are increasingly
undercompacted for their depth and overpressured. We show that the brittle strength at
depths greater than zFRD is predicted to be approximately constant, leading to a
crustal strength profile that is radically different from the classic linearly increasing
hydrostatic profile. We present borehole stress and fluid-pressure measurements in several
overpressured deforming continental margins that agree with this prediction and with a
critical state of failure controlled by the same pressure-dependence κ as the overlying
hydrostatic strata. A first-order extrapolation of observed and theoretical overpressured
strength-depth profiles to much greater depths suggests the possibility of a prolonged
transition at approximately constant strength from brittle to linear-viscous behavior
in deforming shale-rich volumes undergoing the transition to slate, phyllite and
schist.
Our examples of fluid pressures and stresses come from the Yinggehai basin offshore
south China, western Taiwan thrust belt, offshore Texas, Brunei delta and Scotia Shelf
offshore Canada. |