TRANSTENSION: KINEMATICS AND SCALE

In transtension (TT), the trans-zone slip vector (TD) is the attractor towards which all lines, planes and finite elongation axis rotate. The ZB-orthogonal, coaxial, extensional, component yields all the vertical crustal thinning and part of the horizontal extension. The ZB-parallel, non-coaxial, component yields the vorticity, all the horizontal shortening and part of the horizontal extension. The instantaneous stretching direction (Xi) is the bisector of the acute angle between TD and the ZB orthogonal, attractor. One of the two, spatially-constant, lines of no infinitesimal strain (LNIS) is parallel with ZB, the other is normal to TD. One line of no finite strain (LNFS) is spatially-constant and parallel with ZB, the other rotates, with vorticity, towards TD. The critical point about strain in TT is that any horizontal rhombohedron, in the plane containing ZB and TD, increases in area but ZB-parallel length (LNIS) is constant and, therefore, rotating lines must shorten or lengthen because velocity field lines do not cross TD-parallel lines. The difference between large (regional to plate boundary) and small (outcrop) scale TT zones is profound; kinematics and dynamics cannot be scaled up and down. To maintain compatibility, small-scale TT zones must be biaxial and dilational because they have no free face and isostasy, at this scale, is irrelevant. Only fluids, can be drawn into the deformation zone in the ZB plane. Volume increase occurs by tension gashes at <45° and decrease by cleavage at >45° to ZB; the former must exceed the latter. In contrast, bulk strains in large-scale TT zones must be triaxial because TD is sub-horizontal and isostasy causes crustal thinning. The combination of coaxial and non-coaxial strain components generates roughly volume-constant constriction in TT. Theoretical models have been suggested in which TT zones shorten, parallel with the length of the deforming zone by material being drawn into the zone in addition to the thickening or thinning of the crust. This would involve line length changes parallel with ZB and either major zone boundary slip or massive horizontal shear strain gradients between zero-slip ZB's, which is dynamically and kinematically unrealistic.