Orogen-parallel extension and oblique tectonics: The relation between stretching lineations and relative plate motions

Abstract

Many collisional orogens exhibit two distinct deformation phases. The first, and relatively early, phase forms within an obliquely subducting continent and is characterized by regionally consistent stretching lineations subparallel to the orogen and to associated isoclinal and sheath folds. Second-phase folds and stretching lineations develop during and after imbrication of the subducting plate into the overriding plate, and they become progressively disharmonic with stretching lineations oriented perpendicular to the orogen. Structural morphology and stretching lineations indicate that a fundamental kinematic transition occurs as material is imbricated from footwall to hanging wall. Kinematic modeling, based on modern oblique convergent boundaries, and considerations of buoyancy forces suggest that only the relatively early structures reflect the relative plate motion, but that the correspondence is not simple. In particular, the orientation of stretching lineations is a function of finite strain, kinematics of deformation, and angle of subduction and convergence, and it will consistently underestimate the angle of oblique convergence. Second-phase structures reflect isostatic emergence of the hanging wall and are independent of relative plate convergence. Examples are drawn from the south-central Canadian Rockies, the Western Alps, the Variscides of Ireland and France, and the Himalaya.