Page 114 - Microtectonics

102 4 · Foliations, Lineations and Lattice Preferred Orientation
4.4 refer to the intersection of a foliation and a flat outcrop 4.4
surface as a lineation (Fig. 4.3); this could be referred to Lattice-Preferred Orientation (LPO)
as a foliation track. By analogy, intersections of a flat out-
crop surface with an object lineation give rise to a linea- 4.4.1
tion track (Fig. 4.3). Introduction

4.3.2 In many deformed rocks, the lattice orientation of crys-
Development of Lineations tals is not randomly distributed, but arranged in a sys-
tematic way. Such rocks have a lattice-preferred orienta-
The development of lineations is complex and treated only tion (LPO) for a specific mineral. In the case of crystals
briefly in this book. with a planar or elongate shape in a particular crystallo-
Aggregate lineations can form by stretching of equidimen- graphic direction such as micas and amphiboles, an LPO
sional aggregates of grains to a linear shape, or by similar is easy to recognise as a foliation or lineation. However,
deformation of large single crystals and subsequent recrys- for minerals such as quartz and calcite this is more diffi-
tallisation (Box 4.2). Grain lineations either form by defor- cult. In the case of quartz, the presence of an LPO can be
mation of equidimensional grains without recrystallisation checked by inserting a gypsum plate under crossed polars;
or by dissolution and growth (similar to Fig. 4.16(2, 3); when the microscope table is turned, a dominant blue or
Box 4.2). Minerals with a typical elongate shape such as sil- yellow colour for a quartz aggregate in different orien-
limanite, tourmaline and amphiboles may either rotate or tations is an indication for an LPO. In other minerals with
grow in a preferred direction creating a grain (or mineral) higher birefringence, special techniques are required to
lineation. As a result, most aggregate and grain lineations determine if an LPO is present. Several processes can con-
are parallel to the X direction of the finite strain ellipsoid. tribute to development of an LPO (Skrotzki 1994). LPO
However, it has to be investigated with care whether these patterns and LPO development in quartz is treated in
lineations really represent a direction of extension, or some detail as an example.
whether they formed by boudinage or pressure solution.
Trace lineations form by polyphase deformation, or by 4.4.2
changes in the deformation regime as e.g. in the case of Origin of Lattice-Preferred Orientation
shear band cleavage development (Sect. 5.6.3). These
structures do not have a simple relationship with strain LPO can be formed by the processes mentioned in
axes or directions of tectonic transport, although they are Sect. 4.2.7, but for minerals with equant grain shape, dis-
commonly oblique to object lineations of the same age location creep seems to be the most important mecha-
(Fig. 4.3). Trace lineations are commonly parallel to buckle nism (Sect. 3.4). Dislocation creep changes the shape of a
fold axes, and at a high angle to the shortening direction crystal and the interaction with neighbouring crystals may
in a volume of rock. result in its rotation with respect to the instantaneous
There is no simple relationship between types of line- stretching axes (ISA) of bulk flow (Fig. 4.39). Deforma-
ations and metamorphic conditions. As for foliations, the tion twinning has a similar rotation effect. The effect can
nature of minerals that constitute aggregate of grain line- be visualised by a pile of books sliding on a shelf; the books
ations gives information on metamorphic conditions dur- change orientation with respect to the shelf and their
ing development of the lineation. normal rotates towards the direction of gravity. If defor-
Aggregate and grain lineations that represent the long- mation starts in a crystalline aggregate with random ini-
est strain axis (X) can be used to find the “direction of tec- tial orientation, e.g. in a sandstone, the result after some
tonic transport” if they form in ductile shear zones with deformation will be a preferred orientation. As an ex-
approximately simple shear flow (Passchier 1998). Trace ample, Fig. 4.39b,c shows how an LPO pattern may de-
lineations can be used to find the orientation of fold axes in velop in a deforming crystal aggregate with a single slip
buckle folds, and to obtain information on the nature of system in coaxial flattening progressive deformation.
polyphase deformation; in principle each intersection line- When several slip systems are active, the rotational be-
ation represents a deformation phase, but for instance a third haviour of grains and the resulting LPO patterns will be
deformation phase, D , may produce different intersection more complex.
3
lineations between S and S , S and S and S and S . The type of slip systems or deformation twinning that
3 2 3 1 3 0
Object lineations are commonly overprinted by trace will be active in a crystal depends on their critical re-
lineations of different age, or even of the same age. It is solved shear stress (CRSS) and therefore indirectly on
also common that foliation planes contain more than one metamorphic and deformation conditions (Sect. 3.4).
generation of trace lineations, each representing a defor- Usually, more than one slip system can operate in a min-
mation phase. Overprinting object lineations of different eral and the CRSS of each slip system changes with tem-
age on one foliation plane are less common, but do occur. perature and chemical activity of certain components,