EXPERIMENTAL MODELS OF TRANSFER ZONES IN RIFT SYSTEMS: 1. CONVERGENT AND DIVERGENT TRANSFER ZONES

Transfer zones are common features of rift basins where deformation between adjacent normal faults is accommodated by the formation of oblique structures and complex secondary fault systems. The main faults dip towards each other in convergent transfer zones and away from each other in divergent transfer zones. Experimental models were conducted to determine the geometry, evolution and fault patterns associated with convergent and divergent transfer zones with initially approaching, laterally offset and overlapping fault geometries. The models consisted of two layers, with stiff clay representing basement and soft clay representing the sedimentary cover. Laser scanning and three-dimensional surface modeling were used to determine the map geometry. The experimental models showed many similarities with conceptual models, but also showed more detail and a few significant differences. Divergent transfer zones are narrower than convergent transfer zones for the same initial spacing between basement faults. The differences between the different initial fault configurations (approaching, laterally offset, or overlapping) are the degree of interaction of the secondary faults, the amount of overlap between the fault zones, and in some cases, the width of the transfer zone. The main faults propagate laterally and upward and curve in the direction of dip of the faults, so that the faults curve towards each other in convergent transfer zones and away from each other in divergent transfer zones. A primary difference with schematic models is the significant component of extensional fault propagation or drape folding, accompanied by secondary faulting within the sedimentary cover, especially in the early stags of fault propagation. The folding plays an important role in determining the shape and width of the transfer zones. The hanging walls of the faults are marked by a gentle flexure or rollover into the fault, the amount of flexure increasing with fault throw away from the fault tip. The geometries and fault patterns of the experimental structures compared well with their natural counterparts in the East African and Gulf of Suez rift systems.