An Introduction to Structural Geology and Tectonics
12. Ductile Shear Zones, Textures and Transposition
Imagine a cold and wet day in northern Scotland, which may not be a far stretch of the imagination if you've ever visited the area. As you are mapping a part of the Scottish Highlands you are struck by the presence of highly deformed rocks that overlie relatively undeformed, flat-lying, fossiliferous sediments. This relationship is even more odd because the overlying unit has experienced much higher grade metamorphism than the underlying sediments, and it contains no fossils. When you arrive at the contact between these two characteristic rock suites, you notice that they are separated by a distinctive layer of particularly fine-grained rock. The regional relationships of these two suites and their superposition already suggest that the contact is a low-angle reverse (i.e., thrust) fault. So, what is the distinctive fine-grained rock at the contact? In your mind you envision the incredible forces associated with the emplacement of one unit over the other and you surmise that the rock at the contact was crushed and milled, like what happens when you rub two bricks against each other. Using your classes in ancient Greek you decide coin the name mylonite for this fine-grained rock unit, because 'mylos' is Greek for milling. Something like this happened over a hundred years ago in Scotland where the late Precambrian Moine Series ('crystalline basement') overlie a Cambro-Ordovician quartzite and limestone ('platform') sequence along a Middle Paleozoic low-angle reverse fault zone, called the Moine Thrust. This area was mapped by Sir Charles Lapworth of the British Geological Survey in the late 19th century. Anecdote has it that Lapworth became convinced that the Moine Thrust was an active fault and that it would ultimately destroy his nearby cottage and maybe take his life; his life ended in great emotional distress. In many areas you will find similar zones in which the deformation is markedly concentrated. The deformation in these zones is manifested by a variety of structures, which may include isoclinal folds, disrupted layering, well-developed foliations and lineations, and other deformation features. These zones, called ductile shear zones, may contain some of the most important information about the deformation history of an area, so let us begin this chapter with their definition. A ductile shear zone is a tabular band of definable width in which there is considerably higher strain than in the surrounding rock. The total strain within a shear zone typically has a large component of simple shear, and as a consequence, rocks on one side of the zone are displaced relative to those on the other side. In its most ideal form, a shear zone is bounded by two parallel boundaries outside of which there is no strain. In real examples, however, shear zone boundaries are typically gradational. The adjective ductile is used because the strain accumulated by ductile processes that may range from cataclastic flow to crystal-plastic processes and diffusional flow (Chapter 9).
So, a shear zone is like a fault in the sense that it accumulates relative displacement of rock bodies, but unlike a fault, displacement in a ductile shear zone occurs by ductile deformation mechanisms and no throughgoing fracture is formed. The absence of a single fracture is largely a consequence of movement under relatively high temperature conditions or low strain rates. Consider a major discontinuity that cuts through the crust and breaches the surface. In the first few km, brittle processes will occur along the discontinuity, which result in earthquakes if the frictional resistance on discreet fracture planes is overcome abruptly. Displacement may also occur by the movement on many small fractures, a ductile process called cataclastic flow (Chapter 9). In either case, frictional processes dominate the deformation at the higher crustal levels of the discontinuity and this crustal segment is therefore called the frictional regime. With depth, crystal plastic and diffusional processes, such as recrystallization and superplastic creep, become increasing important mainly because temperature increases. Where these mechanisms are dominant, typically below 10-15 km depth for normal geothermal gradients (20-30 degrees C/km) in quartz-dominated rocks, we say that displacement on the discontinuity occurs in the plastic regime. Not surprisingly, the transitional zone between a dominantly frictional and dominantly plastic regime is called the frictional-plastic transition, but more commonly we call this the brittle-plastic transition . Note that technically it is not correct to call this the brittle-ductile transition, because ductile processes (such as cataclasis) may occur in the frictional regime. So, a crustal discontinuity that is a brittle fault at the surface, is a ductile shear zone with depth. Associated with this contrast in deformation processes, we predict that the discontinuity changes from a relatively narrow fault zone to a broader ductile shear zone with increasing depth because the host rock weakens (i.e., a reduction in differential stress).
Mylonites are dominated by the activity of crystal-plastic processes,
which produces yet another characteristic element of deformed rocks: crystallographic-preferred
fabrics or textures. The topic of textures will both be introduced and
applied in this chapter, although some of the theory logically follows
the material presented in Chapter
9. Secondly, rocks within ductile shear zones typically are intensely
folded and the original layering is transposed into a tectonic foliation.
Transposition will close the chapter, but we emphasize that it is common
but not unique to ductile shear zones. In our chapter you will see that
shear zones may have more than one foliation, a strong lineaton and that
shear zone rocks commonly contain rotated fabric elements, grain-shape
fabrics, and in particular a grain size that is less than that of the host
rock. Arguably, ductile shear zones are the most varied structural feature,
and perhaps the most informative.