The mechanical feasibility of low-angle normal faulting

Low-angle normal faulting is precluded by elementary rock mechanics argumen ts. However, the existence of many low-angle normal faults in regions such as western North America has long suggested that this familiar theory is in complete. Initiation of a low-angle normal fault requires the stress tensor within the brittle upper crust to have inclined principal axes, which nece ssitates a substantial shear stress within the vertical plane. Nonetheless, no-one has previously identified any specific extensional stress held whic h is suitably oriented to allow shear failure of previously unfractured roc k at a low-angle dip of similar to 30 degrees in preference to the steeper dip of similar to 50-60 degrees predicted by standard theory. Despite previ ous claims to the contrary, this study shows for the first time that approp riately oriented stress fields can exist. However, they require the shear s tress to locally reach similar to 100 MPa near the base of a similar to 10- km-thick brittle layer. This is only possible under extreme conditions, as it requires dramatic lateral variations in the state of stress across the e xtending region. It is suggested that a shear stress of this order can deve lop due to the combined effects of lower-crustal flow (which imparts a hori zontal shear traction at the base of the brittle layer) and loading (which imparts shear traction in the vertical plane) associated with the isostatic response to changes in heat how caused by changes to the geometry of subdu cting slabs beneath an extending region. Regional patterns of low-angle nor mal faulting in western North America are thus interpreted in terms of chan ges to the geometry of subduction of the Farallon plate. (C) 1999 Elsevier Science B.V. All rights reserved.