The nature and development of shear zone junctions

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Ductile shear zones are usually imagined as straight disc-shaped high strain domains in the rock with simple gradients of vorticity and strain. Most shear zones, however, have a more complex geometry and are part of a network of interconnected zones of variable orientation that branch in a complex manner. Much is known about branching and interconnection of brittle faults, mainly due to 3D information obtained by the minerals industry. However, little is known about the branching systems of ductile shear zones and the way in which they form and develop, what the ductile flow conditions are in such environments and how they affect the strength of the crust. Transient brittle deformation and opening of veins may possible in some shear zone junctions, even in low porosity rocks such as granites. Shear zone junctions can act as fluid pathways in the crust, which can lead to accumulation of mineral deposits and has consequences for nuclear waste deposition sites in granites. Junctions of the largest, crustal scale shear zones may play an important role in deciding how crustal fragments interact along strike slip shear zones, and how the continental crust is assembled in collision and dismembered in extension. Furthermore, mineral deposits are commonly bounded to large scale shear zones. Careful study of different types of junctions in selected shear zones can help to find tools to recognize them in all tectonic settings. We plan to study a number of exceptionally well preserved examples of shear zone junctions in the Grimsel granite, Switzerland, and characterize their 3D field geometry, microstructure, and association with brittle deformation. The study is supported by an ongoing computer modelling study of shear zone junctions.