Mimicking shear zones: An example from Wadi Filk, Jordan

Ductile shear zones can develop in at least two ways: (1) a nucleus can grow laterally by free propagation into undeformed host rock, like most faults or joints; (2) the zone may nucleate and grow on or in a planar discontinuity and mimick its orientation. Most small-scale ductile shear zones are mimicking zones, but large-scale ductile shear zones could be free-propagating. The Wadi Filk mylonite zone in Jordan is a two km long, ten meter wide mylonite zone flanked by ultramylonite zones, developed in undeformed Neoproterozoic porphyritic monzogranite. Since mineral and major element composition of mylonite and monzogranite are identical, the structure seems to have formed by free propagation. Only detailed observations of the microstructure and trace element chemistry of the mylonite indicate that it is mimicking a precursor rhyolitic dyke. The Wadi Filk mylonite zone shows that even km-scale ductile shear zones can be mimicking dykes. Fine-grained chilled margins of dykes can act as a nucleus of ultramylonite formation.