Oblate strains are often observed in meso-scale ductile shear zones and this is generally taken to indicate narrowing across the shear zone during formation. Volume loss is one mechanism that could produce shear zone narrowing. However, not all shear zones display characteristics consistent with volume loss, and in such cases, the narrowing must be accomplished by the extrusion of material from within the shear zone. To explore the relationship between shear zone geometry, volume loss, and extrusion, shear zones were mathematically modeled. Results demonstrate the important influence of pure shear and volume loss on controlling the geometry, displacement, and vorticity of ductile shear zones. Further, volume loss does not preclude extrusion unless, for a given volume loss, the strain is of a specific geometry. Extrusion is a likely mechanism important in shear zone development, even if volume loss occurs. Extrusion presents strain compatibility problems because, unlike crustal-scale shear zones, meso-scale ductile shear zones do not possess a free surface. If extrusion occurs, bulk strain compatibility can be maintained if shear zones interlink in anastomosing arrays or change in thickness, though not all shear zone systems display such characteristics. Modeling results elucidate the deformation style of shear zone in the northwest Adirondacks in NY and in the Kebnekaise region in northern Sweden.
Bibliographical noteFunding Information:
The Tarfala Research Station of Stockholm University is thanked for very comfortable accommodations during field investigations of the shear zones in the area. UMN Department of Geology and Geophysics Summer Research Grants partially supported the fieldwork in Tarfala. GBB thanks Keith A. Brugger for valuable field assistance in Sweden. Fieldwork in the Adirondacks was partially supported by a Geological Society of America Graduate Student Research Grant (7327-03) to GBB. Much of the modeling work was accomplished while GBB was funded by the Graduate Assistance in Areas of National Need Fellowship provided by the U.S. Department of Education. Annia Fayon is thanked for her review of an early draft of this paper. Haakon Fossen and Scott Giorgis are thanked for their careful and insightful reviews that greatly improved the manuscript.
- 3D strain
- Ductile shear zones
- Strain compatibility
- Volume loss