Constrictional Strain and Linear Fabrics as a Result of Deformation Partitioning: A Multiscale Modeling Investigation and Tectonic Significance

摘要

Rocks with a well-developed lineation but weak or no foliation (L-tectonites) commonly occur as isolated volumes dispersed in other tectonites. We consider L-tectonites that reflect constrictional finite strains here and use a multiscale approach to investigate the conditions for constrictional strain fields. The approach combines the strength of kinematic and mechanical analyses in large strain three-dimensional deformations. Our modeling shows that, in simple shearing and thinning zone progressive deformations, constrictional strains develop only in rheological heterogeneities that are moderately stronger than the bulk material as a whole. Stronger elements never accumulate enough internal strains for any fabric to develop. Inclusions weaker than the bulk material will develop flattening strains. L-tectonites are most likely developed in macroscale simple shearing, simple-shearing-dominated plane-strain general shearing, or simple-shearing-dominated Sanderson and Marchini transpression. The lineations of the L-tectonites are always nearly parallel to the lineations in the bulk material. Where the lineations are nearly 90 degrees from the vorticity axis, the macroscale flow is close to a plane-strain general shearing. Where the lineations are oblique to the vorticity axis or more variable, a simple-shearing-dominated triclinic thinning zone with mainly uniaxial boundary stretching is likely. The concept of homogeneous transtension deformation, combining a homogeneous pure shearing and a transcurrent simple shearing, is unsupported by fabric evidence and is likely unrealistic. Under an oblique divergence boundary condition, the upper lithosphere deforms by folding and fracturing and the ductile lithosphere develops simple-shearing-dominated detachment shear zones. Constrictional strains (hence L-tectonites) can develop in these detachment zones due to flow partitioning.