Tectonite

Tectonites are metamorphic or tectonically deformed rocks whose fabric reflects the history of their deformation, or rocks with fabric that clearly displays coordinated geometric features that indicate continuous solid (ductile) flow during formation. Planar foliation results from a parallel orientation of platey mineral phases such as the phyllosilicates or graphite. Slender prismatic crystals such as amphibole produce a lineation in which these prisms or columnar crystals become aligned.

L-S tectonite viewed in the plane of the S fabric

L-S tectonite viewed perpendicular to the plane of the S fabric

Classification

• S-tectonites (from the German, Schiefer for schist) have a dominant planar fabric and may indicate a flattening type of strain. This may also be due to a lack of minerals capable of giving a lineation e.g. in a phyllonite.
• L-tectonites have a dominant linear fabric and generally indicate a constrictional type of strain. This may be due to a lack of platey phases.
• L-S tectonites have equally developed linear and planar fabric elements and may indicate a plane strain deformation. Many mylonites are L-S tectonites consistent with a simple shear deformation.