HR: 08:00h
AN: T41F-01    [Abstracts]
TI: Pore-Fluid Pressures and Crustal Strength
AU: * Suppe, J
EM: suppe@princeton.edu
AF: Deapartment Geosicences National Taiwan University, No. 1, Sec. 4 Roosevelt Road, Taipei, 10617, Taiwan
AU: Yue, L
EM: lifanyue@gmail.com
AF: Chevron Energy Technology Company, 14141 Southwest Freeway, Sugar Land, TX 77478, United States
AB: The classic graph of crustal strength as a function of depth shows linearly increasing brittle strength above the brittle-plastic transition. This linear increase is a consequence not only of the pressure dependence of brittle strength but also an assumption that the depth-normalized pore-fluid pressure λ = P_f/ ρ_r gz is constant, which is perhaps only plausible in the case of hydrostatic pore-fluid pressures (λ ≈0.4). Much deep bore-hole stress data agrees with this assumption, showing the predicted linear increase in strength together with stress magnitudes that are consistent with hydrostatic pore-fluid pressures. In contrast, observed pore-fluid pressures in deeper parts of deforming clastic sedimentary basins and active plate-boundary mountain belts are commonly in excess of hydrostatic. These deforming sedimentary basins typically have pore-fluid pressures that are dominated by disequilibrium compaction, showing fully compacted sediments with hydrostatic fluid pressures at shallow depths until the fluid-retention depth z_FRD is reached, below which sediments are increasingly undercompacted and overpressured. For this disequilibrium- compaction mechanism, the fractional brittle weakening (1-λ) below the fluid-retention depth is a simple function of depth (1-λ)≈0.6(z_FRD/z), which directly leads to a predicted crustal-strength profile that is radically different from the classic hydrostatic profile. The brittle strength below the fluid-retention depth is predicted to be constant and equal to the strength at the fluid-retention depth. Some stress measurements in deeper parts of sedimentary basins appear consistent with this constant-strength prediction. Furthermore, observations from western Taiwan show that z_FRD is fixed relative to the land surface during active uplift and erosion, therefore crustal strength should be approximately unchanged by exhumation except for cohesive effects. The full limits of the disequilibrium-compaction regime are not well know. However it is only as non- hydrostatic pore-fluid pressures decay that deformed sedimentary mountain belts are expected to show crustal strengths similar to the classic graph.
DE: 8045 Role of fluids
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 8164 Stresses: crust and lithosphere
DE: 8169 Sedimentary basin processes
SC: Tectonophysics [T]
MN: 2007 Fall Meeting