Origin of regional, rooted low‐angle normal faults: A mechanical model and its tectonic implications

Abstract

Rooted listric low‐angle normal faults (< 20°) of regional extent have been recognized widely in the past few years in the North American Cordillera and elsewhere. The low‐angle geometry of these crustal‐scale normal faults conflicts with Anderson's [1942] classic theory of faulting. In that theory the orientations of principal stresses are assumed to be vertical and horizontal; the predicted dip angle of normal faults is about 60° rather than 20° or less. Recent geological and geophysical studies in the mid‐Tertiary extensional terrane of southeastern California and western Arizona suggest that thick mylonitic gneisses in the lower plates of low‐angle detachment faults may represent unidirectionally sheared laminar flow in and below the midcrust. Directed ductile flow, possibly related to the gravitational spreading of thickened lower crust, may induce a shearing traction on the horizontal or subhorizontal base of the brittle upper crust. Thus the orientations of the principal stresses can no longer be vertical and horizontal at this interface. A simple elastic model incorporates the effect of basal shearing due to gravitational spreading on stress distributions in an elastic upper crust. This model shows that parallel belts of compression and extension can be produced if a shearing traction acting on the base of the elastic upper crust is considered. In particular, appropriate stress conditions for the formation of regional low‐angle normal faults (< 20°) can be produced by the superposition of two stress fields: a basal shear stress field induced by the basal shear traction and a contractional stress field in which the horizontal deviatoric stress is compressional and the vertical gradient of the horizontal normal stress component is constant. This superposed stress field may represent a tectonic setting where a stress field with compressional deviatoric stress induced by plate subduction or convergence is superposed on a basal shear stress field induced by gravitational spreading of thickened lower crust. These results may explain both puzzling parallel belts of extension and compression and the occurrence of major low‐angle normal faults in some orogenic systems.