Continental extension on sets of parallel faults: observational evidence and theoretical models

Abstract

Abstract Extension of the upper-crustal brittle layer is taken up in many regions on sets of sub-parallel normal faults that have sub-parallel slip vectors and are approximately planar between the earth’s surface and c. 10 km depth. This style of faulting accommodates different deformation senses in the underlying upper mantle lithosphere in different deforming regions. In the western North Anatolian fault zone, southernmost Italy, Tunisia and Libya, simple shear and extensional pure shear are distributed over zones up to c. 200 km wide and are taken up by oblique slip on faults that strike parallel to the simple shear direction. The strain rate tensor in such zones is biaxial, and although vertical vorticity is non-zero, blocks in the brittle layer do not rotate around vertical axes relative to the zone boundaries. In central Italy extension involving uniaxial extensional pure shear is taken up by normal slip on parallel faults. In the northeastern Basin and Range province, central Greece and western Turkey, single sets of parallel faults take up extension in the interiors of deforming regions with dimensions > c. 500 km and appear to rotate around vertical axes relative to the boundaries of the regions. Distributed simple shear, oblique to fault strike, combined with extensional pure shear may a priori account for this deformation sense. However, it requires faults to strike in a unique direction that depends on the ratio of rates of pure and simple shear, and to have slip vector azimuth perpendicular to the simple shear direction. With this geometry, left-lateral slip takes up clockwise rotation and right-lateral slip anticlockwise rotation around vertical axes. To take up finite extension and rotation this unique geometry must persist over time. It is unlikely that this process explains deformation of any region considered. In contrast, uniaxial extension, whether occurring on its own or combined with rotation around a vertical axis at a non-zero rate, does not require any strong geometrical constraints: fault slip vector azimuths follow the direction of extensional pure shear and the sense of any strike-slip depends on the orientation of this direction relative to fault strike. Blocks in the brittle layer generally rotate around vertical axes at the same rate as elements in the underlying plastic deformation, which may equal half the vertical vorticity in the upper mantle lithosphere. This deformation sense, which accounts for the observed deformation in these regions, minimizes local differences in horizontal velocity between the brittle layer and the underlying upper mantle lithosphere.