Evolution of parallel, oblique and triple-junction rifts: Insights from analogue modelling

Abstract

The study of extensional structures such as normal faults, grabens and rift systems using analogue modelling techniques has been widely used to understand the evolution of sedimentary basins. These have been particularly helpful to understand the generation of oil reservoirs and structural traps. The present work was inspired by the discussion on the origins of two sub-parallel grabens located in the intracratonic Parecis Basin in the centre-west of Brazil. Parallel, oblique or intersecting grabens may form by the reactivation of inherited basement weaknesses that can act as velocity discontinuities, with their geometry and proximity controlling the evolution of the resulting grabens. In this work, we use sandbox analogue experiments to investigate the geometric and kinematic evolution of two nearby, sometimes intersecting, grabens. We do this by varying the distance and the geometry of two basal velocity discontinuities (VDs). In all experiments, an extension of 3 cm (scaling to 12 km) was prescribed to affect a 3 cm thick sand pack. The materials were deformed using a plexiglass box with two basal velocity discontinuities attached to two lateral movable walls. In a first set of experiments, we varied systematically the distance between the VDs. In the second set, we varied the angles between the VDs with different configurations, including situations in which the VDs intersect forming triple junctions. The results show that when the VDs are initially close (<2 cm, corresponding to ~8 km in nature) there is the formation of a wide graben composed of two sub-grabens that interfere with each other, while if the spacing between the VDs is >2 cm, the grabens develop independently and do not interfere. Furthermore, it is possible to conclude that whenever (not interfering) VDs are prescribed to the base of a sand pack it always leads to the formation of asymmetric grabens, except when the two VDs are initially close to each other. In the experiments with conjugate triple-junction VDs, the extension was shared between two independent asymmetric grabens where the VDs were at a certain distance from each other, and always led to the formation of a wide graben where the VDs were close or intersect each other.