6 Faults and shear zones
6.7 Geometry of shear zones Shear zones can form conjugate
Shear zones are narrow, sub-parallel- sided zones of strong non-coaxial deformation. They occur on all scales from crustal size to microscopic and
range from brittle to ductile in character—in fact, many fault zones can be treated as shear zones. Brittle shear zones form in the upper 5 km of the crust, whereas ductile shear zones generally form below 5-10 km in the crust. Ductile shear zones are common in deformed crystalline basement rock. They are charac
terised by high shear strains, strong foliation development and large dis
placements (relative to their width).
Typically they form in homogeneous isotropic rocks, but once formed, deformation is concentrated within the shear zone.
6.7 Geometry of shear zones Shear zones can form conjugate arrays, and these, or the individual shear zones, can be analysed to deter
mine strain displacements and palaeostress directions. Ramsay (1982) tabulated the properties of shear zones in the crust (Table 6.5).
6.7.1 Types of shear zones
The geometries of simple brittle-to- ductile shear zones are shown in Figs. 6.19—6.21. In each case simple shear (Ramsay, 1967) is assumed, and the shear zone boundaries are at 45° to the principal compressive stress σ1. Brittle shear zone Three sets of frac
tures may develop in the shear (fault) zone. R1 principal Reidel shears; R2
conjugate Riedel shears (generally subordinate); and P synthetic shears, whose directions are imposed by boundary conditions and may or may
F i g . 6.18 Fault rocks and their textures: (a) G o u g e and breccia fragments developed in a fault in siltstones. N o t e the very angular frag
ments. (b) Pseudotachylyte vein in faulted gneisses. Note the rounded gneiss fragments (probably due to thermal fragmentation). (c) Crush rock breccia developed in mylonitic gneiss. Angular fragments with n o preferred orientation. (d) Fine-grained quartz mylonite with intrafolial folds. (e) Quartz-eye phyllonite comprising strongly sheared quartz veins in a mica-rich phyllonitic matrix.
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not develop. Stress orientations and sense of shear orientations can be deduced from the pattern of Riedel shears and from the fabric in the fault gouge (Fig. 6.19a). An example of a brittle shear zone is shown in Fig.
6.19b.
Semi-brittle shear zone {en-echelon tension gashes) Here the deformation is partly ductile with the development
Fig. 6.19 Brittle shear zones: (a) Brittle shear zone showing the development of a gouge
fabric, R1 and R2 Riedel shears and a low-angle
P shear. The stress systems for the Riedel shear systems and for the through-going shear zone are also shown. (b) Brittle dextral shear zone in massive volcanic breccias. Note the develop
ment of R1 Riedel shears.
of pressure-solution cleavage and partly brittle with extensional veins developed (total volume change = 0).
The tension gashes have their tips oriented parallel with σ1 and are gen
erally infilled with fibrous minerals that grow incrementally in the σ3
direction (Fig. 6.20a). The pressure- solution cleavage (if developed) forms 90° to σ1 and the vein tips, but becomes rotated towards parallelism with the shear zone walls in the cen
tral part of the shear zone. An exam
ple of a semi-brittle shear zone with en-echelon tension gash veins is shown in Fig. 6.20b.
Ductile shear zone Here the defor
mation is entirely ductile and prod
uces a strong schistosity which orig
inates at 45° to the shear zone (and perpendicular to σ1). As deformation proceeds the schistosity is rotated towards the shear zone plane until, at large strains, it is nearly parallel to the shear zone boundaries (Fig.
6.21a). An example of a ductile shear zone is shown in Fig. 6.21b.
The total shear strain and dis
placement within ductile and brittle- ductile simple shear zones are easily analysed using the methods of Ram
say and Graham (1970) but require detailed grid mapping and/or pho
tography so that all the structural elements can be recorded across the shear zone.
Conjugate shear zones Shear zones may develop in conjugate arrays (Fig. 6.22) and, as such, may be ana
lysed to determine principal stress orientations (Table 6.5).
Fig. 6.20 Semi-brittle to semi-ductile shear zones: (a) Semi-brittle shear zone showing the development of en-echelon fibrous tension gashes. The fibre orientations reflect the
incremental orientations of the σ3 stress axis as
the tension gash grows. Inside the shear zone a pressure-solution cleavage may be locally
developed. The ideal orientation of the σ1 and
of the σ3 stresses outside the shear zone are also
shown. (b) Semi-brittle dextral shear zone in greywackes. Several sets of en-echelon quartz tension gashes are developed.
Note: It is important to remember that a shear 2one must start and end.
At the ends of a shear zone, complex foliation and strain patterns occur, so that the simple geometries described
Fig. 6.21 Ductile shear zones: (a) Ductile shear zone showing the development of a foli
ation at 45• to the shear zone margin (and 90°
to σ1) and rotation of this foliation into the
shear zone. (b) Ductile sinistral shear zone in a tonalite. Note the development of a schistosity at the shear zone margins, and the rotation of this schistosity into parallelism with the shear zone.
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T a b l e 6.5 Geometric properties of shear zones in the crust.
Approxim ate depth
> 1 0 k m ductile shear zones
5-10 km ductile, brittle-ductile shear zones.
0 - 5 km brittle shear zones
Metamorphic facies
granulite, amphibolite blueschist greenschist, zeolite
Anchimetamor- phism, no metamorphism
Structural features of shear zones
ductile flow, strong sigmoidal schistosity in zones.
ductile to semi-brittle;
localised schistosity; en- echelon vein arrays;
pressure-solution features.
Brittle; fault breccia and clay gouge; some pressure-solution features.
20, angle between conjugate shear zones
120°-90°
90°-60°
60°
above only apply in the central part of a shear zone undergoing simple shear deformation.
F i g . 6.22 A conjugate array of semi-brittle shear zones as seen on a bedding plane in deformed sandstones. N o t e the en-echelon quartz tension gashes and pressure-solution seams at 90° to the tension gashes. Field of view 2 m.
6.8 Structures in shear zones