Impact of crustal rheology and inherited mechanical weaknesses on early continental rifting and initial evolution of double graben structural configurations: Insights from 2D numerical models

Abstract

During the early stages of continental rift, two main grabens are often formed in the upper crust. This double graben structural pattern is typically short-lived, only briefly accommodating rift-related extension. Although the formation and evolution of this structural pattern could be related to the existence of different types of (pre-rift) inherited crustal weaknesses, and possibly determined by their different number and spatial disposition, the process that causes the early nucleation of these grabens is still not fully understood. We hence carry out a set of numerical experiments to investigate the influence of lower crust mechanical weaknesses (weak-seeds) in the early rift nucleation of grabens, while simultaneously assuming different rheological configurations for the extending continental crust. Our results show that double graben structural patterns are generally favoured by crustal rheological configurations comprising a weak middle layer sandwiched between an upper brittle crust and a strong lower crust. In models in which this middle layer is relatively thick, two upper crustal main grabens nucleate above one single seed. In numerical experiments with a thinner middle layer this double graben pattern is observed to form even in the absence of any seeds, illustrating the critical role of the rheological configuration in early-rift double graben nucleation. We argue that different assumed crustal rheologies determine different modes of accommodating brittle deformation in the upper crust, resulting in different structural-mechanical grains imprinted in this layer, prior to the formation of the main grabens, and driving the subsequent nucleation of different structural configurations (including double grabens). The transient nature of the double graben configuration is also confirmed by our results, which consistently show that rift-related extensional strain is eventually fully concentrated in only one of these early formed structures.