DEFORMATION DURING MULTIPLE PHASES OF EXTENSION IN EXPERIMENTAL MODELS: KINEMATICALLY SIMPLE, MECHANICALLY COMPLEX

We use scaled clay models to study the geometry and evolution of faults formed during two phases of non-coaxial extension. Although the resultant strain states and particle-displacement paths are simple, the fault patterns are complex. Pre-existing normal faults create mechanical heterogeneities, affecting the number and geometry of subsequent faults. The exact influence of the pre-existing normal faults depends on the magnitude of the first-phase extension and the angle between the two extension directions. As the magnitude of the first-phase extension increases, fewer and shorter normal faults form during the second phase. The strike of the second-phase normal faults is oriented 70-80° (rather than 90º) from the direction of the second-phase extension. As the angle between the extension directions for the two phases decreases, fewer and shorter normal faults form during the second-phase extension. Also, the reactivation of pre-existing first-phase normal faults increases. Pre-existing faults reactivate with both dip-slip and strike-slip components, and reactivation occurs even when the angle between the extension directions is large (i.e., 45° or greater). The interactions between the pre-existing faults and new faults vary considerably. Newly formed normal faults cut and offset pre-existing normal faults, they emanate from the tips of pre-existing faults, and they terminate against the surfaces of pre-existing faults. The modeling results suggest that, using fault patterns alone, it is difficult to infer the relative timing, directions, and intensities of multiple extensional episodes. Fault patterns in the models resemble fault patterns determined from 3D seismic-reflection surveys on the eastern flank of the Viking Graben in the North Sea and the Jeanne d'Arc basin, Grand Banks.