HR: 15:25h
AN: T43C-07    [Abstracts]
TI: The Hubbert-Rubey Weakening Mechanism in Light of Absolute Fault Strengths
AU: * Suppe, J
EM: suppe@princeton.edu
AF: Princeton University, Department of Geosciences, Princeton, NJ 08544 United States
AB: The existence of thin intact thrust sheets as long as 200 km implies very weak detachments relative to their internal strength, which is the classic thrust-fault problem addressed by Hubbert & Rubey (1959). Low-taper critical-taper wedges also require weak detachments relative to wedge strength (Davis et al. 1983). The causes and absolute magnitudes of such apparent fault weakness remain controversial and this controversy extends to wrench faults such as the San Andreas and to low-angle normal faults. A central problem is that we have limited knowledge of in-situ conditions and theories contain difficult to observe parameters. In the face of such difficulties, it is remarkable that in the special case of homogenous critical-taper wedges of variable detachment dip β we can determine absolute detachment strength F and wedge strength W directly from the associated linear variation in topographic slope α = -s β + α_(β=0), where s and α_(β=0) are the slope and intercept of a homogeneous set of data. The normalized basal shear traction F = τ_b / ρ g H in this special case is a simple function of the regression slope and intercept F ~ α_(β=0)/ (1-s), which is essentially independent of assumptions about the actual strength-controlling mechanisms. The corresponding normalized wedge strength W = (σ_xx-σ_zz) / ρ g H is only a function of the regression slope W ≈ s/(1-s). The ratio of basal resistence to wedge strength is then just F/W ~ α_(β=0)/s. We apply these results to several active wedges to explore the possible quantitative importance of the Hubbert & Rubey weakening mechanism in these instances. There is strong evidence that the toes of thin accretionary wedges such as Nankai (H ~ 1km) are mechanically heterogeneous, therefore we restrict our analysis to thick lithified wedges (H ≥ 5km) of Taiwan and the Niger delta, which are more likely homogeneous. A set of taper data across the active Taiwan mountain belt yield a basal resistance F = 0.09, a wedge strength W = 0.6 and W/F = 0.013 in the depth range H = 8-15km (Carena et al. 2002). Data for the toe of the Niger delta (Bilotti & Shaw in press) yield a basal resistance F = 0.04, a wedge strength W = 0.7 and W/F = 0.058 in the depth range H = 5-9km. Therefore these two active wedges are indeed weak, having absolute fault and wedge strengths that are very small relative to the cohesionless high-strength limit of Byerlee's law (μ = μ_b= 0.85-0.6) for hydrostatic fluid pressures of F_max = 0.5-0.35, W_max = 2.2-1.3 and (F/W)_max = 0.6. Fluid-pressure data in the Niger delta wedge indicate λ ≈ 0.55 which predicts a moderate cohesionless internal friction coefficient μ = 0.45 and tenuous indications of a basal fluid pressure of λ_b ≈ 0.9 suggest a similar relatively strong basal friction coefficient μ_b ≈ 0.4. Therefore it seems possible that the Hubbert & Rubey weakening mechanism plays a major role in the Niger delta. In contrast, regional fluid-pressure data in the toe of the Taiwan wedge indicate λ = λ_b =0.4 (Yue & Suppe 2005) in the region of the M7.6 Chi-Chi earthquake where F ≈ 0.1, which indicates μ_b ≈ 0.1-0.2. Therefore the classic Hubbert & Rubey mechanism involving static excess fluid pressures is not the cause of extreme fault weakening in this western Taiwan example. We must look to other mechanisms of large-scale fault weakening, many of which are difficult to test.
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8045 Role of fluids
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8164 Stresses: crust and lithosphere
SC: Tectonophysics [T]
MN: Fall Meeting 2005