TECTONIC SHUFFLE ZONES: LITHOSPHERE-SCALE MOVEMENT ZONES THAT REVERSE THEIR SHEAR SENSE DURING OROGENESIS

Tectonic shuffle zones are movement zones in which the complex superimposition of a sequence of ductile shear zones and faults has resulted in the ‘shuffling' of a tectonometamorphic stratigraphy, now preserved in a zone of highly deformed thin slices, exotic lenses, slivers and boudins. By definition tectonic shuffle zones preserve a complex history of movement that reflects the dynamics of orogenesis. The magnitude of the relative movements that must have taken place imply that these are structures that potentially develop on the scale of the lithosphere and/or at the map-scale of the orogen. In this paper we illustrate the characteristics of one such shuffle zone, at Lago di Cignana, NW Italy. The shear zones and faults at Lago di Cignana zone have shuffled the Alpine tectonometamorphic stratigraphy, and juxtaposed rocks metamorphosed deep in the lithosphere against rocks from relatively shallower levels. Coesite-bearing ultra-high-pressure lenses juxtaposed against medium pressure metamorphic rocks in the shuffle zone imply relative translations in excess of 150-170 km.