Extensional layer-parallel shear and normal faulting

An extensional fault system in Bare Mountain, Nevada, U.S.A., contains abundant evidence of layer-parallel shear deformation contemporaneous with faulting. Layer-parallel shear is manifest by deformation of pre-faulting fabrics and cleavage at low angles to bedding that indicate shear in the down-dip direction, perpendicular to fault-bedding intersections. Layer-parallel shear along discrete bedding planes locally offsets normal faults, and shear distributed within layers reorients block-bounding normal faults. In simple rigid block models of extension accommodated by normal faults above a low-angle detachment or décollement, extension causes faults to rotate to progressively shallower dips, while originally horizontal beds rotate to steeper dips. These rotations reorient faults out of originally optimum conditions for slip into orientations of a lower slip tendency, whereas bedding rotates to steeper dips with progressively higher slip tendency. The timing or amount of rotation before the initiation of layer-parallel shear depends on the frictional resistance to sliding or resistance to shearing within layering in fault blocks. Offset or deflection of block-bounding normal faults may cause faults to lock as extension increases. Alternatively, bedding and faults may become simultaneously active, progressively lowering dips of faults and bedding until neither is well oriented for slip, at which point new faults are required to accommodate additional extension. At Bare Mountain, early extension within the fault system was accomplished by fault slip and associated block rotation. Continued extension took place by slip along bedding within fault blocks.