Structure and development of an anastomosing network of ductile shear zones: an example from the Rainy Lake zone (Archean Western Superior Province, Canada)

Jordi Carreras1, Dyanna M. Czeck2, Elena Druguet1 and Peter Hudleston3

1Departament de Geologia, Universitat Autònoma de Barcelona, Spain; 2 Department of Geosciences University of Wisconsin-Milwaukee WI, USA; 3 Department of Geology and Geophysics University of Minnesota, Minneapolis MN, USA.

Ductile shear zones and shear fractures are common structures that accommodate deformation in a wide range of rocks. Although individual shear zones are defined as planar bands of deformation surrounded by less deformed or undeformed rocks, they may also be non planar and often link together in a complicated pattern of anastomosing networks surrounding less deformed lozenges. Whereas the formation and development of networks of brittle shear fractures are well documented in the literature, there is less information on the progressive development of ductile or brittle-ductile shear zones and the interaction between shear zone strands

This contribution is a process-oriented study based on a single outcrop detailed analysis of an anastomosing shear zone network developed in metagabbroic rocks. This case study shows that the shear zone network developed from two conjugate sets of shears, but also from differently oriented shear zones belonging to the same kinematical set. The bulk mean structure consists of dominant dextral shears and subordinate sinistral ones. Together, both sets form an obtuse angle facing the mean shortening direction.

Structural relationships indicate that although all shear zones are pene-contemporaneous, not all of them were simultaneously active. A discrimination of relative age of all shear zones is impossible, however the earliest shears can be distinguished from the latest ones based on geometrical and structural criteria. This analysis shows that the observed geometrical arrangement of the obtuse angle facing the shortening direction is not an original feature, but achieved by progressive rotation of shears. This structural progression involves internal deformation of the lozenges with new shears forming inside, but also new shears nucleating at the shear zone boundaries and propagating towards the lozenge interior.

The unequal development and evolution of both sets (the dextral set largely prevails over the sinistral one) is coherent with a deformation regime evolving from a pure shear-dominated, low vorticity transpression to a dextral transpression of higher vorticity.

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