Shear zone initiation and fractures

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Subhorizontal fracture (vein) with some ductile bending of the granulitic foliation. Greenish zone (20 cm wide) represents eclogitized granulite. Holsnøy near Bergen, Norwegian Caledonides. This is the early stage of shear zone formation at 50 km depth in these dry rocks. The central vein is usually erased in more advanced stages of shearing.

Ductile shear zones are the results of strain localization in the crust, mostly the middle and lower crust where plastic deformation mechanisms govern. They typically nucleate on preexisting structures such as foliations, veins and fractures, or they can initiate on a new fracture that develops into a plastic shear zone as shearing progresses. Fluid infiltration along these early fractures is important, and softens the rock around the fracture. This development has been described from the Alps, Cap de Creus (Spain), SW Norway and several other places. Here I show an example of a shear zone nucleating on fracture formed under eclogite facies conditions, and a shear zone involving a sheared central epidote vein that guided the strain localization.

jotun-nappe-sheared-vein

Shear zone from the Caledonian Jotun Nappe (S Norway) showing a central ultramylonitic core that represents an early or preexisting vein on which the shear zone nucleated.

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About Haakon Fossen

Professor of structural geology, University of Bergen. Author of book Structural Geology, published at Cambridge University Press
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2 Responses to Shear zone initiation and fractures

  1. Thom Davis says:

    Great! But how do the zones grow into a crustal scale structure? And are new zones forming all the time?

  2. Good question! They grow by in-plane propagation to some extent, where displacement increases as they grow longer and usually thicker. However, the more efficient way of growing into a larger shear zone structure is by linkage of initially isolated shear zone structures to form longer and usually more composite networks. Large shear zones always consist of internal high-strain zones that may be related to this process. However, much more research is needed on this.

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