What initiates necking? An approach to link natural microstructures with elasto-visco-plastic numerical modeling of boudinage

Layered rocks deformed under viscous deformation often show boudinage, a process which results from differences in effective viscosity between the layers involved (Goscombe et al, 2004 and references therein). Such information and quantification of the effective viscosities are crucial to obtain rheological constraints directly from nature. In the past, various numerical modeling studies of boudinage in powerlaw layers attempted to describe the development of pinch-and-swell structures under different physical deformation conditions (low to high T; e.g. Schmalholz & Maeder, 2012). However, there exists rather limited knowledge about both the origin of necking instabilities as well as the relation between boudinage and flow regime. The finite element modeling software ABAQUS in combination with user-defined subroutines UMAT (Karrech et al., 2011a) was applied to investigate aforementioned tasks. We implemented thermomechanical coupling between elastic, viscous and plastic deformation of pure Qtz or Cc aggregates and Qtz+Cc mixtures. In contrast to previous studies, our rheology in the layers evolves increasing shear strain starting with initial elastic behavior, adding then transient Ramberg-Osgood strain hardening on the way to composite viscous flow (Herwegh et al., in review). Composite flow on the base of dislocation and diffusion creep for both mineral phases is involved. Finite elements, each representing a population density function of a number of individual grains, are arranged in a mesh of 30x10 elements, undergoing plane strain coaxial deformation. In terms of geometry, the pure shear box is built up by 3 layers, consisting of a central layer of coarse-grained populations, surrounded by finer grained populations on bottom and top. Compared to nature, this model setup corresponds to a secondary phase-dominated host rock (pinned and fine-grained matrix grains) around a coarse-grained vein, in which grains can dominantly deform in a plastic manner (dynamical recrystallization). While the small grain sizes in top and bottom layers are defined to be strain invariant, they are allowed to adapt the physical deformation conditions by grain growth or grain size reduction following an energy optimization procedure which is based on the Paleowattmeter approach of Austin and Evans (2007; 2009). Note that the chosen model set up corresponds to situations in natural high strain shear zones such as thrust and detachment faults, where highly strained and fine grained poly-mineralic (ultra-) mylonites are subjected to synkinematic veining. We use natural examples for the Helvetic Alps and the Penninic front to check and discuss the relevance of our numerical results. Finally, we discuss the importance of the grain size evolution and initial heterogeneities, which both potentially result in necking.