LOW-ANGLE NORMAL FAULT (LANF) MECHANICS: PRESENTLY INADEQUATE, VERY IMPORTANT, AND POISED FOR A MAJOR BREAKTHROUGH

Understanding LANF mechanics is key to fault mechanics in general. In turn, models of crustal strength depend on fault strength (e.g., Byerlee friction), so LANF mechanics are key to crustal strength too. The problems and proposed solutions are similar for steep reverse faults and for the weak(?) San Andreas fault. However, LANFs are mechanically more difficult because rocks are weak in extension, the maximum principal stress is at a high angle to LANFs, and its magnitude is limited by upper plate thickness (which is also an advantage: magnitude is relatively unconstrained in other faulting regimes). Solving the LANF paradox will likely solve most existing fault mechanical problems.

Models that "explain" LANF formation rely on preexisting anisotropy (not present in some cases) or ad hoc boundary conditions and rock strength values; none are truly adequate. Models for LANF slip use lab-based rock strengths and failure criteria. Strong rocks (large differential stress) surround weaker LANFs, allowing high shear traction. These can explain LANF slip at depth < 6-7 km, dip of ~20-30°, and with hydrostatic pore pressure (Pp), and imply that crustal strength is seriously underestimated by popular models (Byerlee friction on optimally oriented faults). However, they fail to explain deeper and/or gentler LANFs even with high Pp. In other models, high Pp or weak materials allow LANF slip and/or cause stress rotation in the fault core. However, known weak minerals are generally absent along LANFs and elevated paleo-fluid pressure and stress rotation in LANF cores are not (yet?) documented.

Regardless, LANFs offer a unique opportunity for major advances in fault mechanics: unlike strike-slip and thrust faults, they deliver key fault rocks directly to the surface. Most current models ignore potentially very important factors in LANF evolution: e.g., fault-zone chemistry, alteration/mineralization history, large slip magnitude, long fault life, varying slip rate, hydrolytic weakening, fluid wetting characteristics, the seismic cycle, and earthquake dynamics. Geologists, particularly with modern microanalytical methods, are well poised to solve the LANF mechanical paradox through careful study of the setting and evolution of LANFs and of the textures, microstructures, chemistry, and strength of LANF fault rocks.