Abstract
The high-temperature rheology of heterogeneous anorthite — diopside aggregates has been investigated numerically, in support to experimental data obtained by triaxial torsion tests, performed at high pressure (400 MPa) and temperature (1150° C). The mechanical data exhibited linear viscous flow. Accordingly, scanning electron microscopy revealed grain sliding mechanisms, but also crystal slip plasticity, recrystallization and micro-fracturing. Finite element computations at the aggregate scale aimed at the understanding of the sequence of active mechanisms and their link to the macroscopic behavior. For instance, we show that the presence of coarser and stronger diopside inclusions in weaker and fine grained anorthite matrix results in very heterogeneous local stress fields, allowing for the activation of multiple deformation mechanisms. Our study indicates that shear zones in the lower crust should be dominated by Newtonian rheology in relation with grain sliding mechanisms, even though complementary accommodation mechanisms such as crystal plasticity and damage may be necessary at the local scale, due to the heterogeneous microstructures.
Highlights
Large-scale geomechanical processes in the upper crust, such as formation and evolution of local stress fields, which govern the choice of a mineral extraction technology, in many ways depend on the globe-scale geodynamics of the lithosphere and astenosphere
We considered typical representative elementary volumes (REV, Fig. 3) constituted of anorthite matrix containing irregular diopside inclusions, which assembly is embedded in an effective medium presenting the homogenized bulk properties of a composite with diopside
The results show a mixed type of behavior, combining areas dominated respectively by grain boundary sliding (GBS) or by crystal slip plasticity (CSP), which corresponds to our experimental observations
Summary
Large-scale geomechanical processes in the upper crust, such as formation and evolution of local stress fields, which govern the choice of a mineral extraction technology, in many ways depend on the globe-scale geodynamics of the lithosphere and astenosphere. In a recent experimental work [8] have clearly characterized the stress – strain rate conditions at which either CSP or GBS dominates the rheological behaviour of heterogeneous two-phase gabbroic (plagioclase – clinopyroxene) samples representative of lower crustal rocks [9]. They have shown the existence of a transition zone where both mechanisms contribute and where the overall rheology is characterized by a composite type of flow law. This is why, considering the same kind of material, we have performed an additional experimental investigation based on torsion deformation allowing for high shear strain and a finite element numerical analysis with specific emphasis on the identification/quantification of the locally active mechanisms in respect with the global mechanical response
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