Fluid Pressure and Mineralization in Low-Angle Normal Fault Mechanics

Low-angle (<30 ° dip) normal faults (LANFs) are now well documented in many localities, including active, seismogenic examples. However, these faults apparently slip while the maximum principal stress is at a high angle to the fault plane. This orientation is inferred from Andersonian principles and from orientations of syn-LANF veins and minor conjugate faults found within meters to hundreds of meters of LANFs. Thus LANFs appear to slip under low resolved shear stress. Among mechanical explanations for LANF slip is the effect of pore fluid pressure. If tensile strength of rocks is taken into account, then shallow LANFs (upper few km) can slip with friction of ~0.6 and hydrostatic pore pressure levels, but slip with friction of 0.85 is problematic. However, the deepest brittle LANFs are not explained unless fluid pressure is >90% of the lithostatic load. This seems unlikely in extensional settings unless the stress field in the vicinity of the fault is modified to prevent hydrofracture (e.g., Rice, 1992). Rotation of the stress field is predicted in such models but existing structural data from LANF surroundings precludes such stress rotations unless they are within a few meters of the fault, where data are not presently available. Geochemical effects of fault-zone fluids on LANF mechanics may be important. Rocks surrounding most LANFs record long histories of high fluid flux, hydrothermal alteration, and mineralization. For example, the 'microbreccia ledge' below the Whipple detachment is composed largely of foam-textured microcrystalline quartz (Phillips, 1982) that was likely hydrothermal in origin (surrounding rocks are feldspathic). The texture was tentatively interpreted to reflect superplasticity of the quartz but temperature probably was not high enough for this to be viable. Alternatively, amorphous silica may have precipitated hydrothermally, lowered dynamic friction (e.g., Di Toro et al, 2004) and the foam texture may record subsequent crystallization and annealing. Such an evolution may explain why pseudotachylyte is relatively common on small faults in the upper footwall but is rare along the LANF itself. However, high static friction at the onset of slip events remains problematic.