Rolling hinges as a mechanism for buckle fold growth: evidence from the Haut Giffre, French Alps.

It is controversial whether migration of fold hinges plays a significant roll in the development of buckle folds. Geometric models of buckle fold evolution predict this behaviour in order to maintain an area-balance at all stages of development.

This contribution presents new field data from the external French-Swiss Alps (Helvetics) of the Haut Giffre, where vein and fabric relationships within kilometre-scale buckle folds suggest a mechanism of fold development in which the migration of bed-length through fold hinges is an important factor. Detailed structural observations can identify the point within the bed at which the fold nucleated.

Vein sets perpendicular to bedding, representing a longitudinal extensional strain, are largely confined to the overturned limbs of fold pairs. It is proposed that these veins originate through a mechanism of outer-arc-stretch at the instantaneous hinge, as material is passed through the rolling hinge on to the overturned limb. Therefore, with respect to a given hinge, the outer layer surface should contain a greater proportion of vein material than the inner. Such veins owe their preservation to the tensional regime within the overturned limb.

Since rolling of the antiform and synform will create similar effects on opposite surfaces of the layer it is possible to deduce the bed lengths which have migrated through the antiform and synform respectively. Hence the nucleation point of the fold within that layer can be determined. Such information provides an additional constraint on kinematic models of structural evolution, such as in the Alpine fold-thrust belt.

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