Ghoshflow

Ghoshflow©

Simulates the rotation of particles (porphyroclasts) of different aspect ratios within a flowing medium of varying vorticity (pure and simple shear).
Free

What it does

  • Ghoshflow was developed as a tool to explore the statistical orientation distribution of internal foliations within rotated porphyroclasts, but the program evolved as a useful teaching tool to demonstrate flow in materials, and particularly the concept of flow apophyses as' fabric attractors'. The simulation was based on mathematics presented in the paper by Ghosh and Ramberg (1976) and the application is named in honour of S.K.Ghosh and his wonderful textbook on structural geology. 
  • There is no Help file within the application but it should be self-explanatory to academics.  The left hand graph shows the relative orientation of particles of different aspect ratio relative to their orientation within the flow (where the plane of flow (extensional flow apophysis) has a value of 90 degrees). In the undeformed state (shear strain=0), particles of all aspect ration are oriented uniformly in all orientations. Once the vorticity number is set, the shear strain can be increased incrementally or continuously to show what happens to the particles.
  • In particular note that below a critical aspect ratio (that depends on the vorticity) all particles rotate continuously within the flow, although at different rates. Above that critical aspect ratio the particles rotate until they approach the flow apophyses where they  slow down. This produces a clustering of grains oriented parallel to the plane of flow and demonstrates the role of the flow apophysis as a 'fabric attractor' (a name coined by Cees Passhier (see Passchier and Trouw, 2005). Higher aspect ratio grains gradually stop rotating within this plane, but others may gradually break away and then continue to rotate until they meet the flow plane again. If the vorticity is set closer to the flow being a pure shear, grains oriented either side of the flow apophysis can be seen to rotate toward (demonstrating the concept of coaxial flow).
  • The right hand panel allows one of three sets of graphs to be displayed. The first is a histogram of the orientation of the long axes of the grains, and demonstrates the intensity of any fabric that is produced simply by rotation of particles. It is useful to use this to show how quickly a fabric develops. The second and third plots relate to the orientation of the internal foliation preserved within porphyroclasts. The assumption made is that prior to deformation, all porphyroclasts (of all orientation and aspect ratio) had overgrown an early fabric constantly oriented parallel to the shear plane. This internal fabric (Si) rotates as the host grain rotates. The second plot shows a histogram of the Si orientations.  The third plot, which is of most interest, shows the orientation of Si plotted against the orienation of the long axis of that grain, but colour coded by aspect ratio. What becomes clear in this plot is that grains of different aspect ratio show distinct patterns of Si vs long axis distribution that are a function of the vorticity and the shear strain. Holcombe and Little (2001) contains an example of these plots used to estimate shear strain and vorticity from rotated biotite grains adjacent to the Alpine Fault in New Zealand.
References:
  • Ghosh, S.K., Ramberg, H., 1976. Reorientation of inclusions by combination of pure shear and simple shear. Tectonophysics 34, 1-70.
  • Passchier, C.W. & Trouw, R.A.J., 2005. Microtectonics (2nd ed). Springer, 366p.
  • Holcombe,R.J. & Little,T.A. 2001. A sensitive vorticity gauge using rotated porphyroblasts, and its application to rocks adjacent to the Alpine Fault, New Zealand. Journal of Structural Geology 23, 979-989.
Ghoshflow_zip.zip
Zip file (.zip) containing the full set of installation files [~5.2 Mb]

Notes
  • The software is specific to Microsoft Windows. (It may install and run under Windows emulations on Macs.