Kinematic Indicators

Kinematic Indicators

1. introduction

   • A kinematic indicator is any structure that indicates a sense of shear or transport during deformation.
   • There are three types of porphyroblastic inclusions formed during recrystallization in relation to deformation- pre, syn and post kinematic.
   • To recognise which one of these chronologies is present one must look at critical cross cutting relationships, overgrowths and contrasts in expression and orientation of fabric.
   • Critical chronology is preserved in the pattern and types of mineral present within poikiloblasts relative to the texture of the matrix surrounding them.
     • However, many of these projections do not provide single interpretations. One thin section may not fully portray the three-dimensional geometry of the porphyroblasts relationship to the matrix. This all depends on where the section cuts through the grain.

2. Above diagram shows pre, syn and post tectonic timing of porphyroblast growth relative to matrix foliation development from deformation. Arrows indicate which type of strain is acting on each grain.

3. Pre-Kinematic

   • Si (internal foliation) is inherited from an earlier deformation. Se (external foliation) is compressed around the porphyroblast and a pressure shadow develops.
   • In pre-kinematic fabrics, the grains existed in the rock prior to deformation.
   • Porphyroblasts hosted in the fabrics can either have randomly oriented inclusions or straight inclusions oblique and unrelated to the fabric that surrounds the grain.
     • - Randomly arrayed inclusions occur when the porphyroblast over grew an isotropic ground mass.

4. Syn-Kinematic

   • - Rotational porphyroblast where Si is continuous with Se, suggesting that deformation did not outlast porphyroblast growth.
   • - A common synkinematic poikiloblast-matrix relation develops when mineral growth and co-axial ductile flow are essentially con-current and continuous- at the same rate. The result is a relative rotation between growing porphyroblast and surrounding foliation.
   • Above is a diagram of the formation of a spiral porphyroblast: Rotation of porphyrpblast, from deformation, at the same time as the crystal is growing. - “Snowball Garnet” is an excellent example of this process.
   • most common encountered in rocks with an essentially single cycle metamorphism where deformation and mineral growth are virtually simultaneous

5. Post-Kinematic

   • - Si is identical to, and continuous with, Se.
   • - Post kinematic fabrics are represented in many cases as modification of grain intergrowths in pre-existing anisotropic fabrics.
     • - Mica crystals are planar and favour foliated fabrics as it is a platy mineral. In the diagram below mica is seen to be mimicking and partially taking place of th fabric produced by deformation.

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