Analogue modelling of multiphase rift systems

Abstract

Analogue models of multiphase rift systems show that structures produced by the first phase of extension exertsignificant control on the geometries of the later-phase extensional faults. Two experimental series, Series 1 and Series 2, used different modelling media subjected to two phases of extension. Series 1 models used a simple, homogeneous sandpack, whereas Series 2 models used a sand-polymer multilayer. In each series kinematic scenarios included orthogonal extension followed by oblique extension, or vice versa. Extension directions for oblique rift kinematics varied from 75° to 30° to the axis of rifting. In all models, where oblique rifting superseded earlier orthogonal rift structures, the second deformation phase produced segmented fault systems and reorientation of fault tips into high angles with respect to the extension vector. Higher angles of obliquity resulted in increased fault segmentation. Orthogonal extension superposed on earlier oblique rifting leads to coalescence of segmented faults, although remnants of the inherited initial rift structure remained, even for low angles of obliquity. The sand-polymer multilayer models produced more complex fault patterns than those in the simple, homogeneous sandpack, showing the possible complexities that can be generated during polyphase rifting. Persistent first-phase rift faults caused boundary fault segmentation, and salients and embayments along the boundary faults during the second deformation phase. The natural occurrence of similar structural styles, particularly in map view, may indicate that previously unrecognised multiphase rifting has occurred in these natural examples.