STRUCTURAL GEOMETRIES AND SECONDARY FAULT PATTERNS ASSOCIATED WITH EXTENSIONAL FAULT-PROPAGATION FOLDS

Experimental models of extensional fault-propagation (drape) folds were used to study the evolution of the deformation zones and secondary fault patterns for single faults and trapdoor structures associated with two intersecting faults. Two-layer clay models, consisting of stiff and soft clay were used to model the deformation zones in the basement and sedimentary cover, respectively. Extension on single basement faults normal to the direction of extension results in the development of a deformation zone with expanding width, in which the fault density initially increases and then remains approximately constant. Extension on terminating faults results in conical drape folds with faults oblique to the fold axis along the propagating tip. Basement faults oblique to the direction of extension are characterized by oblique secondary faults along the boundaries of the deformation zone, which curve into a direction normal to the extension direction in the center. Trapdoor fault geometries result in a triangular uplift with maximum relief at the apex. The difference in relief between the flanks and the apex is greatest in the case of propagating basement faults. En echelon secondary faults initiate on the flanks and extend into the apex area, curving into orientations that are closer to normal to the regional direction of extension. The width of the fault zone and the total fault length increase with increasing extension. The fault density initially increases and then remains approximately constant with increasing extension.