The Influence of the Relative Orientation of Multi-Phase Rifting on Fault Growth and Interaction

Abstract

It is known that rifted basins experience multiple phases of extension during their evolution where both the orientation and rate of extension vary through time. This has been examined by the application of physical analogue modelling to the evolution of rifting in these environments. The full three-dimensional geometrical relationship between the growth and interaction of the evolving fault network, however, is difficult to fully investigate and constrain from this technique. A 3D numerical model has been developed that addresses the growth and interaction of normal faults in a multi-phase environment. The crust is represented by spherical elements in a two-layer system with both brittle and ductile interactions between elements, where gravitational and isostatic forces are included. Faults are defined through the displacement between particle pairs and their growth, interaction and throw are recorded through time. As a consequence, specific interactions between pairs or groups of faults can be highlighted and investigated. The time at which bonds fail on faults is also recorded which permits investigation of the timing of fault activity relative to the two-phase system. This study examines the relationship between the orientation of Phase I and Phase II extension on the resulting fault population. Phase I faulting is carried out for 15% extension of the basin. Following this, a further 15% extension is carried out at varying orientations between 10 and 80 degrees to Phase I. Faults that develop in Phase I form through the lateral propagation and linkage of smaller faults and appear in conjugate sets perpendicular to the extension direction. At the initiation of Phase II, the primary location of continued fault growth and propagation is focussed on the tips and relays between Phase I faults. Continued Phase II extension results in the rotation of the alignment of faulting into the Phase II orientation where later Phase II faults bisect and connect existing Phase I structures. Some Phase I faults are active or reactivated within Phase II depending on the relative orientation of the two phases. The distribution of faulting exhibits a fabric with depth, where faulting lower in the brittle crust shows a strong link with the Phase I orientation, where short Phase II orientated failure is observed at the fault tips. Rising through the brittle layer results in an increase in the number of faults present in both Phase I and Phase II orientations.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016