Analogue modelling of orthogonal and oblique rifting

Abstract

The three-dimensional progressive development of orthogonal and oblique rift systems has been simulated using scaled, analogue sandbox models. The orientation, α, of the zone of rifting (where α = angle between the rift axis and the extension direction) was varied systematically from α = 90° (orthogonal rifting) to α = 75°, 60° and 45° (oblique rifting). Completed models were impregnated and sectioned both vertically and horizontally. Orthogonal rift models are characterized by long, straight rift border faults and intra-rift faults perpendicular to the extension direction. Oblique rift models are characterized by shorter, segmented rift border faults and intrarift faults. In the oblique models the rift border faults formed en echelon arrays parallel to the inderlying zone of basement stretching, whereas the intra-rift faults formed at a high angle to the extension vector. Increased extension in the moderately oblique and oblique rift systems (α = 60° and 45°) caused the intra-rift faults to rotate towards parallelism with the rift border faults. No strike-slip or oblique-slip transfer faults were found in any experiments. Rift border faults and intra-rift faults in the models are segmented along-strike with relay ramp structures formed between overlapping fault tips. Along the strike of the rift variations in half-graben polarities and offsets of depocentres are accomplished by accommodation zones formed by interlocking conjugate fault arrays. The experimental fault patterns compare well with the fault architectures found in those modern or relatively young rift systems where well defined, basement controlled strike-slip or oblique-slip transfer faults are absent. An accommodation zone model consisting of interlocking conjugate extension faults is proposed for the along-strike switching of half-graben asymmetries and offsetting of depocentres in rift systems. The fault geometries in the analogue models can be used as templates for recognition of fault styles in both orthogonal and in oblique rift environments.