Page 187 - Microtectonics

176 6 · Dilatation Sites – Veins, Strain Shadows, Fringes and Boudins
Antitaxial fringe structures usually consist of quartz, for grains along the fringe-core object contact (Köhn et al.
calcite and locally also chlorite (Mügge 1928; Pabst 1931; 2003). Previously, it was thought that fibre growth in
Williams 1972b). In some cases, the fibres are deformed fringes could be either entirely displacement-controlled
and tend to lose information in the distal, oldest and most (Figs. 6.20, 6.22; Choukroune 1971; Ramsay and Huber
strongly deformed parts. Rigid strain fringes have been 1983; Etchecopar and Malavieille 1987) or face-control-
studied for spherical, rectangular and irregular core ob- led (Figs. 6.21, 6.22) and that displacement controlled
jects (Durney and Ramsay 1973; Ramsay and Huber 1983; grains grow in the direction of extensional ISA. How-
Ellis 1986; Etchecopar and Malavieille 1987; Aerden 1996; ever, fringe objects are much more complicated than
Köhn et al. 2001a,b, 2003). In general, the following be- veins, and therefore cannot be treated in such a simple
haviour is seen: If a rigid object in non-coaxial flow de- way. The tracking or non-tracking behaviour of indi-
velops fringes at its side, fringes and core object will ro- vidual grains in fringes depends on a complex interplay
tate with respect to kinematic reference axes at different of opening velocity of a crack between the growing fringe
and changing rates, while the fringes are pulled away and the core object, and growth competition between
from the core object. The external and internal geom- grains in the fringe. Computer and analogue experiments
etry of fringes depends at least on the bulk shape of the have shown that just as in veins, grains can track the
core object, the roughness of the core object surface, its opening direction of a fringe but only if grain growth
initial orientation with respect to kinematic axes, the flow keeps pace with opening, and if the core-object surface
regime in the surrounding matrix and growth conditions is rough. If fibres are tracking, their growth direction
Fig. 6.20a.
Strain fringes of quartz adja-
cent to a spherical pyrite fram-
boid in chert. The shape of the
fringes and fibres resembles that
modelled for simple shear pro-
gressive deformation (Fig. 6.24).
Dextral shear sense. Yilgarn
Craton, Western Australia. Width
of view 17 mm. CPL (Photograph
courtesy Dirk Wiersma)