Low-angle normal faulting is precluded by elementary rock mechanics arguments. However, the existence of many low-angle normal faults in regions such as western North America has long suggested that this familiar theory is incomplete. Initiation of a low-angle normal fault requires the stress tensor within the brittle upper crust to have inclined principal axes, which necessitates a substantial shear stress within the vertical plane. Nonetheless, no-one has previously identified any specific extensional stress held which is suitably oriented to allow shear failure of previously unfractured rock 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 previous claims to the contrary, this study shows for the first time that appropriately oriented stress fields can exist. However, they require the shear stress 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 extending region. It is suggested that a shear stress of this order can develop due to the combined effects of lower-crustal flow (which imparts a horizontal 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 subducting slabs beneath an extending region. Regional patterns of low-angle normal faulting in western North America are thus interpreted in terms of changes to the geometry of subduction of the Farallon plate. (C) 1999 Elsevier Science B.V. All rights reserved.
