Abstract

The geometric features of the Helvetic nappes of the Alps show intense internal deformation set up predominantly by simple shear during nappe transport. The nappes are bounded by thrusts that parallel incompetent layers and ramp upward stratigraphically across competent units in the direction of tectonic transport; stratal layering is oblique to nappe boundaries. Thrust surfaces formed after ductile shear zones developed in underlying pre-Triassic gneissic basement. The broad zone of shearing narrowed down as deformation increased to form zones of intense shear and ultimately to form thrust faults. Contraction along competent layers obliquely inclined to shear surfaces produced buckle folds. The geometry of these folds was modified by further shearing as their normal limbs became aligned in the extension field of the strain ellipse for simple shear, producing normal faults.

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