Folds and thrust faults formed by layer-parallel shortening coaxial with extensional structures such as normal dip-slip faults and ductile necking structures with orthorhombic fabric symmetry are usual, but little-recognised structures formed within normal dip-slip shear zones bounding rifts. They are generated because of the shear distribution in a zone of progressive deformation and may be later extended and disrupted depending on which part of the strain ellipsoid they may be located.

The essential difference in the formation between conjugate brittle shear fractures and ductile shear zones is that the intersection angle of conjugate faults in the contractional quadrants in the former is acute (usually w60
) and obtuse (usually 110
) in the latter. The Mohr-Coulomb failure criterion is an experimentally validated empirical relationship, which structural geologists use to interpret the stress directions based on the orientation of the brittle shear fractures. However, a simple application of this criterion assuming that the principal stresses are vertical or horizontal throughout the crust fails to explain crustal scale low-angle normal faults, high-angle reverse faults and certain types of conjugate strike-slip faults that have intersection angles in the compressional quadrants greater than 90
.

This book provides an introduction into the mechanics of faulting in the brittle crust of the Earth. It developed from my annual two-semester course on tectonomechanics for graduate students of engineering geology and of rock engineering at the Technical University of Graz (Austria). 

“Structural geology is the study of the structural (as opposed to compositional) features of rocks, of the geographic distribution of the features and their causes. A structural feature is one produced in rock by movements after deposition and commonly after consolidation of the rock.”# These definitions are drawn independently of the size of the structural features considered, and the range of sizes involved is great.

A fracture model for quartz ribbons in straight gneisses
Origin and incremental evolution of brittle/ductile shear zones in granitic rocks: natural examples from the Southern Abitibi Belt, Canada
Stretching lineations in transpressional shear zones: an example from the Sierra Nevada Batholith, California
Shear zones as a result of intraplate tectonics in oceanic crust: the example of the Basal Complex of Fuerteventura (Canary Islands)
Significance of crustal-scale shear zones and synkinematic mafic dykes in the Nagssugtoqidian orogen, SW Greenland: a re-examination
Evidence for synchronous thin-skinned and basement deformation in the Cordilleran fold-thrust belt: the Tendoy Mountains, southwestern Montana
The effect of cover composition on extensional faulting above re-activated basement faults: results from analogue modelling
A note comparing parameters controlling low-angle normal and thrust movement

This book introduces the principles of structural geology for undergraduate students of geology. It also provides the basic knowledge of structural geology needed for professionals in related disciplines of science and engineering, and is designed to be equally suitable as a text for classroom instruction and for self study. Prior knowledge of elementary geology, physics, and algebra is assumed. This book succeeds my earlier Structural Geology (Ronald Press 1972, later John Wiley & Sons),

Broadly interrelated assemblages of geologic structures constitute the fundamental structural styles of petroleum provinces. These assemblages generally are repeated in regions of similar deformation, and their associated hydrocarbon traps can be anticipated prior to exploration. Styles are differentiated on the basis of basement involvement or detachment of sedimentary cover. Basement-involved styles include wrench-fault structural assemblages, compressive fault blocks and basement thrusts, extensional fault blocks, and warps. Detached styles are decollement thrust-fold assemblages, detached normal faults (‘growth faults’’ and others), salt structures, and shale structures.

In the preparation of this new edition I have followed the general policy adopted in the previous editions. As before, the emphasis has been placed on the principles of structural geology. Some consideration was given to the inclusion of several chapters on regional structure and the fascinating subject of mountain ranges and continents. But to treat these * subjects adequately would take so many pages that the book would either be greatly lengthened or some of the fundamental principles would be less fully treated.

The usual problem of proper balance has arisen. A definition of the _ exact limits of structural geology is an academic question of little practical importance. Nevertheless, an author must make arbitrary decisions on this matter. He must decide, for example, which subjects lie within the fields of stratigraphy and sedimentation and which are primarily the concern of the structural geologist. I suggest that the origin of those features related to the processes of sedimentation are not part of structural geology. Thus the origins of bedding and of primary structures (ripple marks, cross-bedding, etc.) are the concern of the sedimentationist. The structural geologist, however, is interested in these features as tools for working out the major structure or as features that may be confused with tectonic structures. <...>