Tectonic evolution of the Karakoram metamorphic complex (NW Himalayas) reflected in the 3D structures of spiral garnets: Insights from X-ray computed micro-tomography

M.Sayab, D.Aerden, J.Kuva, W.U.Hassan - Geological Survey of Finland / Departamento de Geodinámica and IACT-CSIC, Universidad de Granada / National Centre of Excellence in Geology, University of Peshawa
  • Virtual petrography, 3D textures and quantitative analysis of spiral garnets.
  • Micro-fold axis determination in spiral garnets using X-ray computed tomography.
  • 3D spiral patterns unlock the tectonic history of the Karakoram Himalaya.

Spiral garnet porphyroblasts are known to record lengthy periods of deformation and metamorphism by preserving single or multiple FIAs (Foliation Intersection Axis) formed normal to tectonic shortening directions. Thanks to technological advances in X-ray computed micro-tomography (XCMT), FIAs can now be readily determined in relatively large samples in contrast to previous methods that require the preparation of a set of radial vertical and horizontal thin sections of samples. XCMT scanning not only alleviates tedious thin section based procedures but also illuminates the complete internal architecture of a rock sample allowing three-dimensional (3D) quantitative shape analysis of an individual porphyroblast as well as precise measurement of FIAs.

How PerGeos is used

After reconstructing the XCMT data, we used the PerGeos software from ThermoFisher Scientific for visualization, segmentation, and quantitative analyses. […] We quantified all the garnet porphyroblasts using advanced recipes of ThermoFisher PerGeos software […] Individual garnet porphyroblasts were separated from the rock matrix using the algorithm ‘marker-based watershed’ segmentation built-in PerGeos software. Volumes were calculated as the number of voxels in the outer layer of each garnet porphyroblast.  […] Using PerGeos, the rock matrix was rendered transparent to inspect the visual distribution of garnet porphyroblasts within the sample. 3D volume rendered image showed more or less even population of garnet porphyroblasts of different sizes