INVESTIGATION OF CONSTRICTION IN SHEAR ZONES WITH TRANSPORT-PARALLEL LINEAR ASPERITIES USING NUMERICAL MODELING

Domains of pure constrictional strain are found locally in high-strain zones not subjected to bulk constriction, including zones with linear asperities in their margins parallel with the transport direction. Additionally, analogue models indicate constriction can occur during bulk flattening deformation in zones with linear asperities in one boundary if the asperity is taller than it is wide. To examine the phenomenon further, we use three-dimensional finite-element numerical simulations built with Gale. The model shear zones all include transport-parallel linear asperities. Thus far, we have examined zones with unconfined walls subjected to a bulk flattening strain. Models subjected to bulk plane strain are in progress. Strain rates, viscosity ratios, and boundary geometries are manipulated to test the effects of different variables. The models indicate constriction can occur within asperities during bulk flattening. Infinitesimal shortening axes inclined 50° to 80° to the boundaries of the model high-strain zones all produce constriction within the asperity which transitions to plane strain and then flattening with increasing finite strain. Constriction is more apparent when the simple shear strain rate is faster than the pure shear strain rate. Varying the viscosity contrast between the shear zone and wall rocks from 5:1 to 50:1 creates no noticeable effect on deformation style within the asperity. The asperity must be deeper than it is wide to produce a significant component of constriction. The model high-strain zones indicate that constriction is possible in both symmetrical double asperities, as well as asperities on only one boundary of the shear zone.