Layer-Parallel Shear above Curved Normal Faults
David A. Ferrill, Alan P. Morris, D. Brent Henderson, and
A common geometric/kinematic model used in the construction of balanced cross sections assumes constant bed length and bed thickness, and no through-going, bed-parallel shear in the hangingwall ("general shear"). This approach is effectively limited to fault dips of 30° or less. Various geometrical construction methods use the same assumptions and provide "balanced" finite state solutions, but these are not kinematically viable because they cannot be developed by continuous forward-modeling. Relaxation of the constraint of no general shear permits formulation of a wider range of kinematically viable solutions, including those for fault dips greater than 30°. Steep listric (downward flattening) normal faults require general hangingwall shear with top towards the fault surface (i.e. down dip in a typical rollover structure), whereas antilistric (downward steepening) normal faults require the opposite sense of general shear in the hangingwall. Evidence for down-dip shear at mesoscopic to microscopic scales is apparent in the hangingwalls of high-angle listric normal faults developed in the Paleozoic carbonate sequence of Bare Mountain, Nevada. The faults are Mesozoic to Cenozoic in age and the faulted rocks were undoubtedly fully lithified and unmetamorphosed at the time of deformation. Bedding is the primary mechanical anisotropy in the faulted sequence. Conservation of bed length and thickness were probably the most important constraints on hangingwall deformation. The observed pattern of early synthetic normal faults cut by bedding-parallel down-dip shear is consistent with increasing general shear as fault displacement accumulated.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California