Page 186 - Microtectonics

6.3 · Fringe Structures 175
Cox (1987) and Jessell et al. (1994) suggested an alter- Because the presence of the rigid core object influences
native mechanism for the formation of bedding veins, in the geometry of the flow around a developing fringe struc-
which the different orientation of inclusion bands and ture, its shape and internal structure differ considerably
trails is due to a changing opening direction of the vein, from that of a vein. Fringe structures carry information
or even partly to dissolution in the vein. Mawer (1987) on flow and deformation history in both their internal and
reported veins with two different types of inclusion bands, external shape, and are therefore useful as kinematic in-
but lacking inclusion trails. dicators. Fringe structures can be syntaxial, antitaxial or
Bedding veins form by highly oblique opening of a vein complex. Notice that in fringe nomenclature, syntaxial
and unitaxial, antitaxial, or even composite growth of quartz fringes have the growth surface between the fringe and the
and mica. Crack-sealing may contribute to formation of the wall rock, and antitaxial fringes between the fringe and
inclusion bands and trails. Vein opening at a relatively high the core object (Fig. 6.19). This can be confusing since it
rate normally precluded tracking behaviour of crystals in may seem to be contrary to the terminology used for veins.
the vein, and led to development of elongate grains instead Syntaxial fringes have been observed around crinoid
of fibres. These veins can be used as shear sense indicators stem fragments as core object in limestones (crinoid-type
from the arrangement of inclusion trains and elongate fringes; Ramsay and Huber 1983) but these are relatively
crystals. If opening of the vein is parallel to the inclusion rare. Complex fringes around pyrite crystals, with cal-
trails (Cox and Etheridge 1983; Gaviglio 1986; Cox 1987; cite and a quartz rim, have been reported, but most
Köhn and Passchier 2000), shear sense is indicated by the fringes are antitaxial (pyrite-type fringes; Ramsay and
angle between the inclusion band and the trails (Fig. 6.18, Huber 1983), probably because the core object is usually
×Video 6.18a,b,c). If grain shape and delicate deforma- a mineral with contrasting properties from the matrix,
tion bands have been destroyed by static recrystallisation, such as pyrite in a pelitic, carbonatic or quartz-feldspar
the shear sense can still be determined from the angle be- rock. The remaining part of this section deals with these
tween inclusion trails and the wall rock, or from the shape most common antitaxial fringes.
of jogs in the wall rock. If the opening direction is paral-
lel to inclusion bands, however (Jessell et al. 1994), the in-
ferred sense would be opposite to that discussed above.
Bedding veins can form by fault motion along bedding
planes, as in thrust tectonics, but can also be associated
with large scale folding; in many cases, the lineation in
the veins is normal to fold axes. However, bedding veins
are usually folded and commonly cut by stylolites associ-
ated with folds, and this suggests that they play a role at
initial stages of fold development rather than during
flexural slip in maturing folds (Jessell et al. 1994; Köhn
and Passchier 2000).
6.3 6.3
Fringe Structures

6.3.1
Introduction

Rigid objects in a ductilely deforming rock cause local
perturbations of the stress field and flow pattern. In the
case of low temperature deformation and high fluid pres-
sure, increased pressure solution may occur adjacent to
the rigid object on the side of the shortening ISA, while
extensional gashes may open on the contact of the object
and the matrix on the side of the extensional ISA. New
crystalline material may grow in these gashes and form
strain shadows on both sides of the rigid core object
(Figs. 6.1, 6.3). If the crystals in such strain shadows are
elongate or fibrous, these are known as strain fringes. The Fig. 6.19. Schematic drawing of three types of strain fringes that
occur in nature. The core object of the syntaxial fringes is a crinoid
combination of core object and fringes is called a fringe stem fragment with deformation twins. Arrows indicate the posi-
structure (Figs. 6.1, 6.3; Köhn et al. 2001a). tion of growth surfaces