Two Roles of Pore-Fluid Pressures In the Mechanics of Thrust Belts and Accretionary Wedges

Since the classic 1959 paper of Hubbert & Rubey, pore-fluid pressures in excess of hydrostatic have been considered to play a dominant role in thrust mechanics by weakening both the thrust sheet and its basal detachment, with the weakening of the detachment dominating. We examine these two classic roles of pore-fluid pressure in thrust mechanics. [1] Recent advances in critical-taper wedge mechanics confirm that many basal detachments of thrust belts and accretionary wedges are indeed very weak (μ<0.1). However, in some of these cases, including the western Taiwan thrust belt and Nankai Trough Japan, the observed ambient pore-fluid pressures are either hydrostatic or only moderately elevated surrounding the faults showing that the classic Hubbert-Rubey mechanism is quantitatively unimportant in these cases. Other processes appear to dominate the weak fault problem, for example dynamical processes that activate in large earthquakes. Nevertheless there are some cases of fluid pressures dominated by disequilibrium compaction in which elevated ambient pore-fluid pressure may be adequate to the Hubbert-Rubey hypothesis, especially those in which the depth of detachment Z>5Z_FRD where Z_FRD is the fluid-retention depth below which shales do not compact. [2] In contrast, recent advances in wedge mechanics suggest that fold-and-thrust belts and accretionary wedges may be strong, with the horizontal stress σ _Hmax 1.6 to 2 times the vertical load ρgz, even though the thrust ramps and basal detachment are very weak. Western Taiwan is a case in point. The likely locus of wedge strength is at fault bends where the interiors of thrust sheets must continually deform new rock. In this case it is likely that pore pressures dominate the strength. Under the disequilibrium compaction mechanism, it is predicted that wedge strength σ_Hmax–ρgz should be constant below the fluid-retention depth Z_FRD, which agrees with observations in an extensional wedge from offshore Texas.