STRAIN, VOLUME LOSS, AND STRAIN COMPATIBILITY IN MESO-SCALE DUCTILE SHEAR ZONES

Modeling meso-scale ductile shear zones, given a finite strain and independently obtained volume loss, provides insight to the factors influencing shear zone displacement, vorticity, and geometry. We focus on the common situation of shear zones displaying oblate strains, which indicates a component of shortening across the shear zone. In this situation, if deformation is isochoric then extrusion is required to accommodate the shortening. The models demonstrate that extrusion can have different geometries and that, even with volume loss, some extrusion is likely.

Meso-scale ductile shear zones are fundamentally different from crustal-scale strike-slip shear zones, which possess a free surface and thus can readily accommodate transpression. By contrast, extrusion in meso-scale ductile shear zones places severe limits on strain compatibility. Compatibility can be maintained by the linking together of shear zones into anastomosing arrays or by varying the thickness of the shear zone. Meso-scale ductile shear zones in the Kebnekaise region of northern Sweden and in the Diana Syenite of the northwest Adirondacks (New York) are different in character, but both are believed to be isochoric and both appear to have recorded flattening strains. The Adirondack shear zones, apart from local complications, are straight, solitary, and have parallel sides, indicating deformation must only have involved simple shear. The flattening strains in this case reflect a commonly encountered problem with strain determination and are in fact inaccurate. The Tarfala Valley shear zones, on the other hand, interlink, curve and have non-parallel sides, consistent with extrusion, and therefore, true flattening strains.