Geomechanical perspectives and reviews on the development and evolution of cross-scale discontinuities in the Earth's crust: Patterns, mechanisms and models

Knowledge concerning the development and evolution of cross-scale discontinuities, such as crevasses, joints, dikes, and faults, caused by crust movement and resultant stress, is essential for geoscience and geo-resources engineering communities. These discontinuities serve as significant conduits for mass and heat transfer, and are associated with magma, meltwater, hydrocarbons, or other ore-forming fluids, which to some extent determine landforms and landscapes, the fate of ice shelves, and the geometry of ore bodies or hydrocarbon reservoirs. Massive endeavors have been made over the last 100 years or even longer in understanding the onset, propagation, and mechanical interaction of cross-scale discontinuities under evolving stress and flow environments. Especially in the past 20 years, various advancements have emerged in elucidating the mechanisms of evolving multi-scale discontinuities. Proposing a proper mechanical model would benefit geoscientists in achieving a deeper understanding of the sequence of existing structures and be valuable for industrial engineers to access the potential of underground resources, which entails complementary studies from geosciences and industries. In this paper, we review state-of-the-art technologies in order to study the evolution of cross-scale discontinuities in: (1) observations from cores and outcrops; (2) in-situ monitoring or geophysical surveys; (3) analogue experiments on scale dependence; and (4) cross-scale numerical models for mechanical analyses. The driving forces, evolving patterns, and geological and engineering importance of cross-scale discontinuities are also discussed in relation to both natural and industrial fluid-driven fracturing processes. This broad review intends to bridge the understanding of the evolution of discontinuities from both Earth science and industrial communities.