Significance of strain localization in the lower crust for structural evolution and thermal history of metamorphic core complexes

Abstract

Numerical simulations demonstrate that the dynamic behavior of the lower crust can exert a crucial control on the tectonic denudation of metamorphic core complexes. Our model suggests that the structural architecture and thermal history of terrains exhumed by extensional shear zones can be controlled by whether or not localization of deformation occurs in the lower crust. Predicted architectures range from midcrustal gneiss and granite domes to upper crustal core complexes with brittle deformation in both the upper and lower plates. We simulate continental crust in extension, with a strong, viscous/frictional‐plastic upper crust underlain by a weak, viscoplastic lower crust. Strain is accommodated by plastic shear zones in the upper crust, while viscous flow and plasticity compete in the lower crust. We explore a number of scenarios by varying the viscosity and yield limit of the lower crust. Deformation patterns and thermal architecture change significantly, depending on whether or not localization occurs in the lower crust. For a relatively high viscosity lower crust with a low yield stress limit, deformation is dominated by localization along discrete plastic shear zones. Along these shear zones, increasingly deeper material is exhumed, creating an asymmetrically denuded lower plate with metamorphic grade increasing in the direction of upper plate displacement. In relatively weak, low‐viscosity lower crust, however, the yield limit may never be reached, and strain in the lower crust in this case is accommodated by distributed flow. The denuded lower plate in the distributed flow case resembles a dome‐shaped structure, bounded by opposite facing shear zones.

Crustal versus mantle core complexes

“…Koyi and Skelton, 2001;Tirel et al, 2004;Gessner et al, 2007;Rey et al, 2009b;Huet et al, 2011;Mezri et al, 2015;Wang et al, 2015). However, because…”

“…In models that integrate phase transitions controlled by pressure, temperature and the local amount of free fluid that comes from both external (meteoric) and local (dehydration) sources show a strong positive feedback between the asymmetry of the tectonic structures and the depth of penetration of meteoric fluids. From a general point of view, it must be noted that all models involving some type of strain softening favor asymmetric strain localization and the development of a rolling hinge type detachment, even in models where displacements are applied symmetrically and whatever the type of localizing perturbation (Lavier et al, 1999;2000;Lavier and Buck, 2002;Wijns et al, 2005;Gessner et al, 2007;Rey et al, 2009a;2009b;Tirel et al, 2009;Mezri et al, 2015;Schenker et al, 2012;Wang et al, 2015;Wu et al, 2015;Wu and Lavier, 2015). This suggests that strain softening could play a dominant role in the development of detachments and therefore in the evolution of core complex models.…”

“…Different types of perturbations in the initial conditions are used to localize the deformation at model centre such as: fault in the brittle crust (2), low viscosity anomaly in the ductile crust (4), local cohesion loss (2) or local higher cohesion (1) in the brittle layer, thermal anomaly in the lower crust or in the mantle and wedge shape crust. In models with no perturbation (Wijns et al, 2005;Gessner et al, 2007;Tirel et al, 2009) deformation is more distributed leading to the simultaneous development of several core complexes that can interfere during their development (Tirel et al, 2009). In addition, in 11 models a strain weakening function is used in brittle and/or ductile material.…”

“…Patterns (or modes) of continental extension primarily depend on the rheological layering of the lithosphere (Analytical models: Buck, 1991;Analogue models: Allemand and Brun, 1991;Brun, 1999;Tirel et al, 2006;Numerical models: Lavier et al, 1999;2000;Tirel et al, 2004a;Wijns et al, 2005;Gessner et al, 2007;Gueydan et al, 2008;Rey et al, 2009a;2009b;Huet et al, 2011;Wang et al, 2015;Wu et al, 2015;Wu & Lavier, 2016). For a general appraisal of lithosphere rheology and its effects on large-scale geodynamics see Burov (2007a;2010;.…”

The influence of an initial simple shear deformation on long-wave motion in an elastic layer

“…As examples, we cite the Earth's crust, for which the influence of hydrostatic pressure is significant; see for example [Gessner et al 2007]. Natural mechanisms also exist within layered systems whereby movements over long timescales may cause shear-type deformations; see [Ide et al 2007].…”