On the production of two inclined cleavages during a single folding event; Stirling Range, S.W. Australia

Abstract

In any one area of the Stirling Range Proterozoic low-grade fold-foreland, the first phase of folding to be associated with cleavage development has generated two inclined tectonic fabrics each of which is closely related in geometry to the folds. The most likely fold history has been determined by comparing predictions of theoretical fold mechanisms against the observed field relations and strain states seen in an arenite and minor mudrock multilayer. In an initial phase of folding dominated by layer-parallel shortening, a well-spaced mica-band cleavage was, initiated, intensified, and able to maintain a near axial plane relationship, until body rotation of limbs took over at a fold dihedral angle of about 140°. The resultant 70° angle between solution cleavage and bedding on the fold limbs was preserved by flexural slip until the fold had tightened to about 100° when, for mechanical reasons, flattening rapidly became important. During this phase, a mica-film cleavage, with grainscale spacing, developed approximately axial planar and the solution cleavage/bedding angle on the limbs was reduced to 55°.