Preprints
https://doi.org/10.5194/egusphere-2024-1077
https://doi.org/10.5194/egusphere-2024-1077
22 Apr 2024
 | 22 Apr 2024
Status: this preprint is open for discussion.

Localized shear versus distributed strain accumulation as shear-accommodation mechanisms in ductile shear zones: Constraining their dictating factors

Pramit Chatterjee, Arnab Roy, and Nibir Mandal

Abstract. Understanding the underlying mechanisms of strain localization in Earth’s lithosphere is crucial to explain the mechanics of tectonic plate boundaries and various failure-assisted geophysical phenomena, such as earthquakes. Geological observations suggest that ductile shear zones are the most important lithospheric structures of intense shear localization, sharing a major part of tectonic deformations. Despite extensive studies in the past several decades, the factors governing how they accommodate the bulk shear, whether by distributed homogeneous strain (i.e., development of S tectonic foliation normal to the principal shortening strain axis) or by localized shearing (formation of shear-parallel C bands) remain largely unexplored. This article aims to address this gap in knowledge, providing observational evidences of varying S and C development in ductile shear zones from two geological terrains of Eastern India. The field observations are complemented with 2D-viscoplastic numerical simulations within a strain-softening rheological framework to constrain the factors controlling the two competing shear-accommodation mechanisms: homogeneously distributed strain accumulation versus shear band formation. The model based analysis recognizes the bulk shear rate (γb), the bulk viscosity (ηv) and the initial cohesion (Ci) of a shear zone as the most critical factors to determine the dominance of one mechanism over the other. For a given Ci, low γb and ηv facilitate the formation of S foliation (uniformly distributed strain), which transforms to C-dominated shear-accommodation mechanism with increasing ηv. However, increasing γb, facilitates shear accommodation in a combination of the two mechanisms leading to CS- structures. The article finally discusses the conditions in which ductile shear zones can enormously intensify localized shear rates to produce rapid slip events, such as frictional melting and seismic activities.

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Pramit Chatterjee, Arnab Roy, and Nibir Mandal

Status: open (until 08 Jun 2024)

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Pramit Chatterjee, Arnab Roy, and Nibir Mandal
Pramit Chatterjee, Arnab Roy, and Nibir Mandal

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Short summary
Understanding the strain accumulation processes in ductile shear zones is essential to explain the failure mechanisms at great crustal depths. This study explores the rheological and kinematic factors determining the varying modes of shear accommodation in natural shear zones. Numerical simulations suggest that an interplay of the following parameters: initial bulk viscosity, bulk shear rate, and internal cohesion governs the dominance of one accommodation mechanism over the other.