EXPERIMENTAL MODELS OF TRANSFER ZONES IN RIFT SYSTEMS: 2. SYNTHETIC TRANSFER ZONES

Synthetic transfer zones and associated relay ramps result from two major normal faults dipping in the same direction. Experimental models were conducted to determine the geometry, evolution, and fault patterns associated with synthetic 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 to compare the models with natural structures. The results of the experimental models show that for a given fault spacing the degree of interaction of the secondary faults, the amount of overlap between the fault zones, and the width of the transfer zone are influenced by the initial fault configurations (approaching, laterally offset, or overlapping). The main faults propagate laterally and upward and curve in the direction of dip of the faults, so that both faults curve in the same direction in synthetic transfer zone systems. The transfer zone is marked by the development of an oblique ramp that connects the undeformed parts of the hanging wall of each fault. The ramp shows a reversal in slope and is generally narrower in the lowest point, forming a structural saddle. Significant extensional fault-propagation or drape folding results in a progressive change in the slope of the ramp, rather than a constant ramp slope. The hanging walls of the major faults return to their regional position at the fault tips. In contrast, most conceptual models and natural examples of synthetic transfer zones show a downward drop of the blocks in the dip direction. This results if there is a progressive downward drop of structures into the basin, or if there is a regional slope that is in the same direction as the dip of the faults. The geometries and fault patterns in the experimental models match some of the observations in natural structures, and provide predictive analogs for the interpretation of surface and subsurface structures in transfer zones.