Estimation of paleostress orientation within deformation zones between two mobile plates

Abstract

The orientation of the principal stress axes within deformation zones between two mobile plates is modeled here analytically, using a thin-plate theory. The simple analytical approach helps to explain why plates cease to move after collision. Orogenic periods last only several tens of million years because the stress associated with a particular constant driving force (causing a constant strain rate) is no longer able to maintain a significant horizontal displacement. In contrast, the uplift rate increases rapidly as the horizontal velocity decreases, and this may explain why the termination of orogenic epochs are usually heralded by the rapid deposition of thick sequences of immature sedimentary rocks or flysch. The analytical model also elaborates the relationship between homogeneous bulk deformations driven by a constant stress orientation and those due to a fixed displacement direction of physical boundaries of the deformation zone. Two major kinematic possibilities are considered: (1) plates converging either orthogonally or obliquely, making the deformation zone an analogue for orogenic collision; and (2) plates diverging either orthogonally or obliquely, so that the deformation zone is dynamically similar to initiating rift basins. The theoretical investigation led to the formulation of the following rules. Deformation zones between converging plates have the major axis of bulk deviatoric stress coinciding with the bisector of the acute angle between the relative plate velocity vector and the normal to the deformation- 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 the 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 sequence of Devonian sandstone and limestone asymmetrically folded adjacent to the dextral Border fault. The steeply plunging Z-folds of the marker beds suggest that the principal deviatoric paleostress, τ 1 , was oriented 37°-44° to the fault trace. The age of the Moroccan Border fault is poorly constrained and may be Variscan or younger. The τ 1 orientation implies a major component of simple shear parallel to the strike-slip fault and a minor component of extension perpendicular to the fault trace. The implied crustal movement is compatible with the modern tectonics of the Eurasian-African collision zone where part of the differential motion between the Eurasian and African plates is accommodated by strike-slip faults.