Copyright 1999 by Patrice Rey. All right reserved.
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Eclogite shear zones are one of the geological particularities of Holsnoy, an Island along the coast of South Norway nearby Bergen. 

These shear zones have been formed in Paleozoic time during the subduction of the Baltica margin underneath Laurentia.  They affect mafic rocks, that were metamorphosed and deformed in the granulite facies 1100 million years ago. 

The petrology of these shear zones has been intensively studied by Hakon Austrheim (1985/87).  Metamorphic conditions of the eclogite-facies were approximately 700ºC and 1600 to 2100 MPa.  The conversion fo the granulite-facies rocks into eclogites was facilitated by fluid infiltration, initially along thin veins. 

Strongly deformed eclogites occur in anastomosing subparallel, 30-150 m thick shear zones that are laterally continuous along strike over distances up to several kilometers. The following (enlargeable) pictures show some of their characteristics.  I took these pictures in July 1992 during a field trip during which Dave Fountain, Hakon Austhreim and myself were collecting samples across eclogitic veins to investigate the change in elastic properties across granulite-eclogite transition.


A cold Norwegian sunset in Holsnoy...

A 30m thick eclogite shear zone (dark zone in the valley).

The shear zone is flanked by zones ...

...were granulite blocks (white blocks) are preserved in eclogitic material (dark stuff).

  The shear plane carries a stretching lineation (here stretched garnet/  clinopyroxene  aggregates).

Large garnet/clinopyroxene aggretate showing asymmetric pressure shadows. The picture is perpendicular to foliation and parallel to lineation.

 Asymmetric boudinage of mafic layers. 

Eclogite assemblages develop around veins, overprinting former granulite garnet/clinopyroxene bearing fabric. 

Cross nicols thin-section parallel to the foliation plane showing a aligned zoisite (green/brownish) minerals.  Black minerals are garnet, yellowish minerals are clinopyroxene and kyanite.

All the following micro-photos are parallel to the lineation and perpendicular to the foliation.  This picture shows C/S fabrics.  The S plane (foliation) is parallel to the long axis of the picture.  C planes (shear planes) are running with an angle dipping toward the right.

Detail of a shear band.  Note that the eclogite assemblage is stabe in both S  and C planes.

A pair a locked garnets with asymmetric pressure shadows...

...same under cross-nicols.

Asymmetric pressure shadows around a garnet...

...same under cross-nicols.

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