ESTIMATION OF PALEOSTRESS ORIENTATION WITHIN DEFORMATION ZONES BETWEEN 2 MOBILE PLATES

The orientation of the principal stress axes within deformation zones between two mobile plates is modeled here analytically, using a thin-p late theory. The simple analytical approach helps to explain why plate s cease to move after collision. Orogenic periods last only several te ns of million years because the stress associated with a particular co nstant driving force (causing a constant strain rate) is no longer abl e to maintain a significant horizontal displacement. In contrast, the uplift rate increases rapidly as the horizontal velocity decreases, an d this may explain why the termination of orogenic epochs are usually heralded by the rapid deposition of thick sequences of immature sedime ntary rocks or flysch. The analytical model also elaborates the relati onship between homogeneous bulk deformations driven by a constant stre ss orientation and those due to a fixed displacement direction of phys ical boundaries of the deformation zone. Two major kinematic possibili ties are considered: (1) plates converging either orthogonally or obli quely, making the deformation zone an analogue for orogenic collision; and (2) plates diverging either orthogonally or obliquely, so that th e deformation zone is dynamically similar to initiating rift basins. T he theoretical investigation led to the formulation of the following r ules. Deformation zones between converging plates have the major axis of bulk deviatoric stress coinciding with the bisector of the acute an gle between the relative plate velocity vector and the normal to the d eformation-zone boundary. In case of extension within a deformation or rift zone separating diverging plates, the bisector will outline the minor axis of the bulk deviatoric stress. The deformation tensor of th e analytical model yields a new method for estimating the orientation of paleostress in natural examples, here applied to the deformed wall rock of the Moroccan Border fault. The marker used is a competent sequ ence of Devonian sandstone and limestone asymmetrically folded adjacen t to the dextral Border fault. The steeply plunging Z-folds of the mar ker beds suggest that the principal deviatoric paleostress, tau1, was oriented 37-44-degrees to the fault trace. The age of the Moroccan Bor der fault is poorly constrained and may be Variscan or younger. The ta u1 orientation implies a major component of simple shear parallel to t he strike-slip fault and a minor component of extension perpendicular to the fault trace. The implied crustal movement is compatible with th e modern tectonics of the Eurasian-African collision zone where part o f the differential motion between the Eurasian and African plates is a ccommodated by strike-slip faults.