SEGMENT LINKAGE DURING EVOLUTION OF INTRACONTINENTAL RIFT SYSTEMS: INSIGHTS FROM ANALOGUE MODELLING

Abstract

On the basis of the effective scale-independence of brittle structures, from microcracks to regional fault systems, we have used analogue centrifuge models to provide insights into the initiation and evolution of continental active rifts, with a single dilational fracture segment representing a prototype rift segment. In the models, which are physically and dynamically scaled, a semi-brittle compound material, with flexural rigidity, was devised to simulate the lithosphere as a single layer. This is justified by the fact that, at the largest scale, the lithosphere behaves as a single viscoelastic, flexurally rigid unit. Silicon polymers of two different viscosities and densities represent the asthenosphere. A parallelepiped of lower viscosity and lower density material is embedded within and near the base of the second polymer that fills up most of the model box. The former is activated as a plume-like diapir that rises, spreads laterally and ultimately generates extensional stress in the semi-brittle layer on top. In terms of model fracture distributions, both narrow and wide failure modes were achieved, analogous to narrow and wide modes of rifting. The processes of fracture initiation, propagation and coalescence are the same for each mode. During the early stages of model runs, fracture initiation is more important than the propagation of existing segments in relieving stress, although the latter dominates during later stages. The key parameters of overlap, offset, obliquity and propagation angle, defined and illustrated, are used to distinguish different ways in which pairs of fractures coalesce. We recognize three distinct types of coalescence (type 1, type 2 and type 3) involving both tip-to-tip and tip-to-sidewall linkages. These are described and both graphically and statistically discriminated. Whether narrow or wide mode, an intracontinental rift system consists of a number of discrete fault-bound rift segments. Similar types of interactions to those in the models have been identified between pairs of rift segments from the Cenozoic Baikal and East African rift systems, and the Mesozoic to early Tertiary Central African rift system. The factors that control the type of coalescence between natural rift segments would appear to be precisely those that govern linkages of fracture segments in the models.