EXTENSIONAL LAYER-PARALLEL SHEAR AND NORMAL FAULTING

An extensional fault system in Pare 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 inters ections. Layer-parallel shear along discrete bedding planes locally of fsets normal faults, and shear distributed within layers reorients blo ck-bounding normal faults. In simple rigid block models of extension a ccommodated by normal faults above a low-angle detachment or decolleme nt, extension causes faults to rotate to progressively shallower dips, while originally horizontal beds rotate to steeper dips. These rotati ons reorient Faults out of originally optimum conditions for slip into orientations of a lower slip tendency, whereas bedding rotates to ste eper dips with progressively higher slip tendency. The timing or amoun t 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 norm al faults may cause faults to lock as extension increases. Alternative ly, bedding and faults may become simultaneously active, progressively lowering dips of faults and bedding until neither is well oriented fo r slip, at which point new faults are required to accommodate addition al extension. At Bare Mountain, early extension within the fault syste m was accomplished by fault slip and associated block rotation. Contin ued extension took place by slip along bedding within fault blocks. (C ) 1998 Published by Elsevier Science Ltd. All rights reserved.