CONTINENTAL BREAKUP
Geometric aspects of continental breakup: How do continents break ? Pure shear vs simple shear ...
Divergence of continental landmasses leads to surface extension and therefore thinning of the continental lithosphere. The geometry of extension can follow a variety of modes. The sketches on the right show two end-members mode of extension. The finite uniform extension model (McKenzie, 1978; Jarvis and McKenzie, 1980) and depth dependent extension model (Royden and Keen, 1980) are so-called pure-shear models, as in these models large-scale lithospheric extension is symmetric and does not involve rotation (right figure, top). The simple shear extension model by Wernicke (1985) involves a low-angle shear zone which extends through the entire lithosphere (right figure, bottom). In Wernicke’s (1981) model, faulting occurs along a detachment surface that extends through the entire lithosphere. This results in differential stretching in the upper and lower lithosphere to form a plate detachment, resulting in a “lower plate” and “upper plate” margin.
A number of geometries are possible from pure shear (McKenzie, 1978) to simple shear models (Wernicke, 1981).