Along-strike shearing instead of orthogonal compression: A different viewpoint on Orogeny and Regional Metamorphism

Abstract

Orogeny has been regarded for over a century as a compressional phenomenon due either to contraction or to lithospheric collision. However, generation of linear structures like orogens can hardly be explained by variation over large areas of stress gradients, as is to be expected in the rigid-plate convergence assumption. Lateral escape - implied by newer plate-tectonics interpretations to overcome this difficulty - might apply to some degree, but should by no means be more important than up and outward escape, i.e. in the direction of minimum principal stress. Movement indicators in regional metamorphic rocks have shown that tectonic transport along the orogens is a matter beyond question but also that this transport begins at a very early stage, a situation that contradicts the lateral escape model. Therefore it is assumed that the lateral movement is not due to compression, but to transcurrency, which has both the maximum and minimum principal stresses in the horizontal plane. This agrees particularly well with the observed belt-like distribution of deformation. Two models are advanced, the corridor and the gliding-blocks model, which are not mutually exclusive even if each of them relates to a specific geotectonic setting.