Abstract

The dominant fabric element of the Somport slates of the west-central Pyrenees is a pervasive slaty cleavage of moderate to steep northward dip, attributed to the main phase of Alpine deformation. The slates are locally kinked to lesser dips around sub-horizontal, east-trending kink-fold axes. The morphology and angular relationships of the kink bands attest to an origin by the rotation mechanism, in which cleavage segments of invariant length are externally rotated between the kink-band boundaries.

Pervasive slip on the slaty cleavage surfaces predated formation of the kink bands; this slip ended as kinking destroyed the planarity of the cleavage sheets. Additional strain was accommodated by progressive rotation of cleavage segments to lesser dips within kink bands, and by the continued production of new kinks. Later strain increments led to minor rotation of the long cleavage segments between adjacent kink bands (previously a part of the unkinked domain), resulting in interkink dilation of the cleavage laminae and precipitation of calcite in the voids so formed. The formation of microscopic calcite-filled tension cracks spanned the entire deformation.

Kinking by the rotation mechanism implies a volume expansion, for the kinked laminae in ideal rotation kinks dilate upon folding. Evidence for such dilation is common in the Somport slates and is normally taken as an indication of deformation at very shallow crustal levels. Bulk strain effects of this late deformation may be summarized as a vertical contraction, a horizontal, north-south expansion, and an overall dilation of the rock mass in which the kinks formed. It is suggested that these structures developed in response to postorogenic uplift and erosional dissection of the Pyrenees, for only then would confining pressures have decreased sufficiently to permit a net dilation during kinking.

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