Correlation between cumulative horizontal extension and strike-slip displacement in releasing bends: Discrete element analysis

Abstract

Releasing bends are structural features formed by localized extension along strike-slip fault systems. Synclines, pull-apart basins, and extensional strike-slip duplexes develop mainly at releasing bends. Exploring the correlation between strike-slip displacement and cumulative horizontal extension displacement of transtensional structures could be useful for estimating strike-slip displacement when horizontal extension displacement (horizontal fault offsets of en echelon normal faults) is known. The formation and evolution of the releasing bend can be effectively simulated by 2D particle discrete element method. The microscopic parameters of the particles, such as effective modulus and stiffness ratio, are calibrated to model Young's modulus and Poisson's ratio. Six setups of simple strike-slip geometry and releasing bend geometries with different fault separations (the distances between adjacent faults) are then designed. The modelling results reveal a quantitative correlation between the strike-slip displacement L, the cumulative horizontal extension displacement $\sum _1^n\frac{T_i}{\sin {\theta }_i}$, and the fracture propagation angle θ: $\sum _1^n\frac{T_i}{\sin {\theta }_i}=k\ast L$. The k value serves as a comprehensive coefficient to take complex factors into account. In our models, the k values range from 0.9 to 1.6 and are greater for soft rocks than for hard rocks. This quantitative correlation shows a good fit with several natural examples in similar strike-slip or pull-apart tectonic settings, effectively supporting our conclusion.