Page 260 - Microtectonics

252 9 · Natural Microgauges
9.3.5 9.3.8
Porphyroblasts Al-Cr Zoning in Spinel

Rosenfeld (1970) and Ghosh (1987) suggested methods Ozawa (1989) showed that many spinel grains in peri-
to determine W from comparison of the actual amount dotite have an asymmetric sector zoning of Al and Cr,
k
of rotation of porphyroblasts and the theoretically with Al concentrated in the direction of the stretching
expected rotation angle for finite strain measured in lineation, and Cr in the direction of the normal to the
the rock. Although this is an attractive method, it foliation in the peridotite. He suggests that Cr concen-
will only work if porphyroblasts are equidimensional, trated in the σ and Al in the σ direction due to un-
1 3
rotated free in the matrix without interference with equal diffusivity of these ions when the spinel grains
other blasts, have perfect coupling with the matrix deformed by solid-state diffusion creep. In fact, the sec-
and if the strain is known for the period of porphy- tor zoning is more likely to lie in the direction of ISA
roblast rotation. Rotation of elongate porphyroblasts and may be useful to determine sense of shear and W if
m
of biotite can also be used to determine the kinematic sector symmetry planes are oblique to the shape fabric
vorticity number of flow if inclusions can show the in the peridotite.
amount of rotation for each blast, or if the orientation
of groups of porphyroblasts can be compared with 9.3.9
some standard pattern (Holcombe and Little 2001). W History and Accuracy
k
However, rotational behaviour of porphyroblasts can
be complex (Sect. 7.6.8; Biermeier et al. 2001) and is A problem with all determination of kinematic vorticity
presently not well enough understood to be used as a in naturally deformed rocks is the large number of as-
vorticity gauge. sumptions in the methods used, and the uncertainties
about progress of deformation. Most of the methods dis-
9.3.6 cussed above are two dimensional for practical reasons,
Tension Gashes and Foliations in Shear Zones using outcrop surfaces or thin sections, while in reality
veins or porphyroclasts have a complex 3D shape. Other
The tips of tension gashes are thought to develop in the problems are assumptions of monoclinic flow, homoge-
direction of the shortening ISA, and in combination neous deformation and invariable flow conditions dur-
with the orientation of the shear zone in which they form, ing progressive deformation, all of which are unrealistic.
they can be a useful vorticity gauge (Sect. 6.2.2). Simi- The assumption of monoclinic flow can be checked by
larly, the deflection of foliations into shear zones can be controlling the geometry of a large number of fabric ele-
used as a vorticity gauge, since the orientation of the fo- ments for symmetry (e.g. Sawaguchi and Ishii 2003). Ho-
liation is a function of strain, W and A of the flow mogeneity of flow can be assured by using data from a
k
k
(Sect. 9.2). If strain is known independently and A can small volume of rock. The possibility of variable flow con-
k
be estimated (e.g. in plane strain volume-constant flow ditions with time can be countered by using a mean value
it is zero), the orientation may be used to determine W of W over time, W . This mean value W , established
m k m m
in the shear zone. with the methods given above, is difficult to interpret but
a possible solution to this problem is to measure W in a
m
9.3.7 rock using several different gauges which re-equilibrate
Oblique Foliations at different rate during the deformation history. For ex-
ample, quartz fabrics are thought to reequilibrate rela-
Theoretically, the angle between an oblique foliation tively quickly, while deformed veins will record the whole
(Sect. 5.6.2; Box 4.2) and the fabric attractor is a func- deformation history if they predate deformation; if both
tion of W , strain rate and recrystallisation rate (and methods give different values, this may indicate the trend
k
therefore probably of temperature). The angle can there- of change in W during the deformation history (Passchier
k
fore be used to determine W and is expected to be maxi- 1988a, 1990a; Wallis 1992a; Law et al. 2004). Reconstruc-
k
mal for simple shear. A problem with this method is tions of deformation paths, including W history, may
k
that several other factors may influence the angle, and also be developed using fibrous veins and fringes (Ramsay
that finite strain should be sufficiently high to rotate and Huber 1983; Ellis 1986).
fabric elements into parallelism with the fabric attractor In conclusion, none of the methods to determine W are
k
(Sect. 2.9; Box 4.2). For example, some oblique foliations as yet very accurate and the best that can be hoped for in
make an angle with the fabric attractor that exceeds 45°; most settings is to show that flow was not simple shear, but
such orientations are difficult to explain with the avail- contained a pure shear component. Great care should be
able theory. taken that vorticity measurements are not over-interpreted.