Kinematics of fibrous aggregates

• Introduction

FIELD AREAS:

• Veins in the Orobic Alps in northern Italy
• Veins in the Ligurian Units in nothern Italy
• Strain fringes from the Yilgarn Craton, Australia
• Strain fringes and veins from Lourdes, France
 

COMPUTER MODELLING AND PROGRAMMING:

• Vein Growth
• Fringe Growth
• Modelling with FLAC

 

EXPERIMENTS

• Wax fringes in PDMS
• Growth with Salts

FIELD AREAS:

 

Veins in the Orobic Alps in northern Italy

In the Orobic Alps of Italy our study focuses on layer parallel laminated crack-seal veins. They developed in the Permian Collio-Sequence during bedding parallel slip. Elongated crystals in the veins are not tracking the opening trajectory. The important microstructures in the veins are displacement parallel inclusion bands and crack-seal inclusion bands. They can be used to determine the sense of shear and the opening trajectory. paper back to the top of the page

Image of crack-seal inclusion bands

 

Veins in the Ligurian Units in northern Italy

Shales in the Ligurian Units of northern Italy feature a number of interesting vein types. Our study area is closed to Sestri Levante. The shales include bedding parallel veins, slickenfibers and bedding perpendicular fibrous veins. These fibrous calcite veins are restricted to certain stratigraphic units depending on the rheology of the sediments. Interesting features are inclusion bands around the median surface of fibrous veins. Our work in the Ligurian Units was supported by Giancarlo Molli of the University of Pisa. back to the top of the page

Image of a calcite vein

 

Strain fringes from the Yilgarn Craton, Australia

Strain fringes from the Yilgarn Craton, Australia developed around round pyrites with a rough morphology. The form of the fringes shows simple shear and pure shear deformation. A strong curvature of the quartz fibres in the fringes that developed under simple shear conditions suggests a rotation of the round pyrites. We try to model the growth of these fibres with the computer model Fringe Growth. It might give us suggestions on the amount of object rotation during simple shear deformation.  paper back to the top of the page

Image of a strain fringe from Australia

 

Strain fringes and veins from Lourdes, France

Strain fringes from Lourdes, France developed around pyrites of variable shapes with a rough morphology. The strain fringes lie in a sequence of shales between two relatively rigid carbonate units. The form of the fringes suggests non-coaxial deformation. We modelled the fibre growth in these fringes numerically with the program "Fringe Growth", in collaboration with Domingo Aerden of the University of Salamanca.  paper back to the top of the page

Image of a strain fringe from Lourdes

COMPUTER MODELLING AND PROGRAMMING:

 

Vein Growth

We try to model natural examples of veins with the 2d computer model Vein Growth. It was written by Paul Bons (Monash University, Melbourne, Australia) in C for the Macintosh. The crystals and the wall-rock are defined by a number of nodes that are connected with each other forming Polygones. Parameters that can be changed in the program are the anisotropy of the crystals, size of the nuclii, orientation of the crystal lattice, morphology of the wall-rock surface, the opening direction and the amount of opening of the vein, the growth velocity and resolution of the crystals and the velocity of the opening. We try to model the tracking ability of crystals in crack-seal veins (anisotropic growth) and in tension veins (isotropic growth).  paper back to the top of the page

view movie

 

Fringe Growth

The computer model Vein Growth written by Paul Bons (Monash University, Melbourne, Australia) has been developed further with Paul Bons to be able to simulate strain fringes. Fringe Growth is written in C for the Power Macintosh using Metrowerks Code Warrior. The model is able to simulate the growth of fringes around round, cubic or random sized objects (drawn with image and saved as TEXT). The strain rate and the vorticity number of the deformation can be choosen as well as the rotation of the central object. Modelling shows that face controlled fringes develop around smooth objects and displacement controlled fringes around objects with a rough morphology. The tracking ability of the fibrous growing crystals is not only a function of the strain rate but also of the object morphology. The model can also dissolve crystals again once the object "moves" over them.  FringeGrowth Homepage paper back to the top of the page

view movie

 

FLAC

We will study the distribution of stress around strain fringes. We also want to study the progressive rigid body rotation of the fringes and the core-object with respect to an external reference frame and with respect to each other. FLAC is a finite difference program that can calculate the stress field around rigid objects.  back to the top of the page

 

Wax fringes in PDMS

We have undertaken experiments under simple shear conditions to create strain fringes and to study their rigid body rotation with respect to each other and with respect to the shear-zone boundary. The matrix consists of PDMS and the rigid objects are out of wood. If the matrix is sheared it is pulled away from the wooden object and a hole opens that is filled with hot paraffin wax. The wax cooles down and the matrix is sheared again. The old fringes are now pulled away from the wooden object and the free space is filled with different coloured wax. Thus complex strain fringes develop with complex rigid body rotations.  abstract back to the top of the page

view waxfringe

 

Growth with Salts

We try to grow crystal fibres using salts, mainly KDP. Small samples of salt are under pressure or constant load and are being deformed. In these samples rigid glass objects act as objects in strain fringes and influence the stress distribution in the salt matrix.  back to the top of the page