Physical experiments of vertical transpression with localized nonvertical extrusion

Previously proposed models of vertical-walled transpression implicitly assume that material extrudes upwards during deformation. This assumption is not necessarily valid at all scales given that: (a) in areas of diverse lithologies, local zones of relatively rigid materials may cause extruding material to deflect around those zones, and (b) ductile strain often forms anastomosing geometries of shear zones. Therefore, it is possible that a local extension direction in otherwise classic transpression may be nonvertical for rocks deforming at depth. Using an analogue experiment, we have modeled a transpression zone with localized nonvertical extrusion. This extrusion is accomplished by the addition of a side 'leak' that allows sideways extrusion in addition to vertical extrusion. The net extension direction depends on the material's position within the deforming zone, resulting in a significant range of lineation orientations with deformation. The strain patterns produced by transpression with localized nonvertical extrusion may explain the wide array of lineation orientations found in some natural ductile transpression zones. (C) 2003 Elsevier Ltd. All rights reserved.