Localized shear and distributed strain accumulation as competing shear accommodation mechanisms in crustal shear zones: constraining their dictating factors

Abstract

Understanding the underlying mechanisms of strain localization in the Earth’s lithosphere is crucial for explaining the mechanics of tectonic plate boundaries and various failure-assisted geophysical phenomena, such as earthquakes. Geological field observations suggest that shear zones are the most important lithospheric structures demonstrating intense shear localization at plate boundaries, accommodating a major portion of tectonic deformations. Despite extensive studies over the past several decades, the factors governing how shear zones accommodate bulk shear, whether via distributed strain (i.e. the development of macroscopic S (schistosity) foliations normal to the principal shortening strain axis) or via localized shearing (i.e. the formation of shear-parallel C bands, where C refers to the French “cisaillement” (shear)), remain largely unexplored. This study aims to address this gap in knowledge by providing observational evidence of varying S and C development in crustal shear zones from two geological terrains in eastern India. These field observations are complemented by 2D viscoplastic numerical simulations within a strain-softening rheological framework to constrain the factors controlling two competing shear accommodation mechanisms: distributed strain accumulation and shear band formation. The model-based analysis recognizes the bulk shear rate (γ˙b), initial viscosity (ηv), and initial cohesion (Ci) of a shear zone as the most critical factors determining the dominance of one mechanism over the other. For a given Ci value, low γ˙b and ηv values facilitate the formation of S foliation (uniformly distributed strain), which transitions to a C-dominated shear accommodation mechanism as ηv increases. However, increasing γ˙b facilitates shear accommodation through a combination of the two mechanisms, leading to S–C structures. The article finally discusses the conditions under which shear zones can significantly intensify rates of localized shear, producing rapid slip events, such as frictional melting and seismic activities.