Page 151 - Microtectonics

140 5 · Shear Zones
ongoing deformation (Passchier and Simpson 1986; porphyroclast only causes a deflection of the displace-
Fig. 5.27). The resulting shape can give information about ment paths (Passchier et al. 1993). In simple shear,
the development of the porphyroclast system as a whole a boundary can be defined between the far field dis-
and bulk deformation (Sect. 5.6.7.2). If bonding is im- placement paths and the elliptical paths, known as a sepa-
perfect and the porphyroclast elongate, it may obtain a ratrix. Experimental data and numerical modelling
stable position in the extensional quadrant of flow and (Passchier et al. 1993; Bons et al. 1997) show that the sepa-
erode to obtain the form of a mineral fish (Fig. B.5.5; ratrix around a spherical porphyroclast with perfect
Sect. 5.6.7.4). If fluid pressure is high, voids will open bonding with the matrix in simple shear flow can have
aside porphyroclasts and be filled with a different mate- an ‘eye-shape’ or a ‘bow-tie shape’ in a section normal to
rial to form strain shadows or fringes. In this case, the the rotation axis of the porphyroclast (Fig. 5.26). Porphy-
rotation behaviour of the central object is impaired, and roclasts of a different shape and slip between porphyro-
this will have influence on the shape of structures in the clast and matrix can give rise to a separatrix with a more
fringes (Chap. 6). All kinds of transitions between these complex shape (Bons et al. 1997; Bose and Marques 2004).
situations can be envisaged, where bonding increases or If a soft mantle exists around a porphyroclast, it will be
decreases, and mantled objects may change into fish and deformed in the flow. The geometry of the deformed
vice versa (Fig. 5.20). mantle depends, for a spherical porphyroclast, on the
thickness of the mantle and the exact shape of the sepa-
5.6.7.2 ratrix (Fig. 5.27). Wide mantles give rise to φ- or σ-type
Development of Mantled Porphyroclasts clasts in eye or bow-tie shaped separatrices respectively
(Figs. 5.23, 5.27c,f, ×Videos 5.27c,f,fs,fss); thinner man-
If the mantle of an equidimensional porphyroclast has tles produce δ-type clasts by wrapping of the wings
the same rheology as the matrix, the following behav- around the rotating central porphyroclast (Figs. 5.22,
iour can be predicted. A porphyroclast in a flowing fine- 5.24, 5.27b,e, ×Videos 5.27b,e,es,ess), and very thin man-
grained matrix will cause a perturbation of the flow field, tles give no wings at all (Θ-type clasts, Fig. 5.27a,d,
as shown in Fig. 5.26 and ×Video 5.26. With progressive ×Videos 5.27a,d; Passchier et al. 1993). Experimental evi-
deformation, particles adjacent to the porphyroclast dence and numerical modelling indicates that the shape
move in ellipses, but further away the presence of the of the separatrix depends on several factors such as the
initial shape and orientation of the porphyroclast, the
change of these factors with time, the bonding between
clast and matrix, the rheology of the matrix, the flow
vorticity in the matrix and finite strain (Passchier 1988a;
Passchier and Sokoutis 1993; Passchier et al. 1993;
Passchier 1994; Bjørnerud and Zhang 1995; Pennacchioni
et al. 2000; Bose and Marques 2004). It probably also de-
pends on an ‘isolation factor’ of the porphyroclast in the
shear zone, which is high when the porphyroclast is iso-
lated in a relatively wide shear zone, and low if it lies in a
relatively narrow shear zone or if porphyroclasts are close
together (Marques and Coelho 2001; Bose and Marques
2004). Only a bow tie shaped separatrix leads to signifi-
cant stair stepping (Passchier 1994). If recrystallisation
rate in the rim of the porphyroclasts is small, a δ-type
clast can form since only part of the recrystallised man-
tle will cross the separatrix. If the porphyroclast is
recrystallising rapidly and syntectonically, the separatrix
will shrink and the mantle will extend over most of the
separatrix. In this case, σ-type clasts will develop. If the
porphyroclast has an elongate shape, the separatrix will
change shape while the clast rotates and secondary wings
may form, resulting in complex clasts (Fig. 5.21); com-
Fig. 5.26. Perturbation of a simple shear flow pattern around a plex clasts may also form where recrystallisation rate is
spherical rigid object in simple shear flow, in the vorticity profile irregular or if the porphyroclast starts recrystallising
plane. Two types have been reported: a eye-shaped and b bow-tie-
shaped flow perturbations. A separatrix surface lies between ellip- after a δ-type clast was formed (Passchier and Simpson
tical displacement paths near the rigid object and open displace- 1986; Passchier 1994). This effect is caused by the fact,
ment paths further away that once a δ-type wing is established, no new material