New aspects of rhomb structures

The acute angles of 48 rhombic, trapezoidal and triangular grabens, horsts and upthrusts throughout the world were determined. The results indicate an angular spread of 7—73°. In most structures the two acute angles differ considerably. Most rhomb structures are grabens which show affinities to seismic faults, and only a few are horsts and upthrusts. In this connection modes of static crustal fracture propagation are compared to dynamic propagation. Rhomb structures occur along strike-slip faults and in rifts. These results question the universality of the pull-apart model as a mechanism for rhomb structure development. The two mechanisms that can lead to the formation of rhomb structures are (1) interaction of en échelon or non-coplanar fractures by shear and extensional modes and (2) fracture bifurcation in an extensional mode. The two mechanisms may result in faults of different shapes characterized by curved and straight boundaries, respectively. The grabens and horsts in extensional regions are the consequence of early fracture and later vertical displacements.