Australia: The Land Where Time Began

A biography of the Australian continent 

Normal Faults

The tectonic contexts in which normal faults form involve horizontal extension of the crust. The larger principal stress is due to the vertical load, based on Anderson's theory, so the remaining axes must be of a lower magnitude of compression. Normal faults form in a number of geologic settings, in both continental and oceanic environments, the most important of which are plate margins that are divergent and so subject to extension. According to the authors¹ the main areas are rifting zones on continents and extensional provinces, mid-ocean ridges, back-arc spreading areas, and examples of a more local type are magmatic and salt intrusions - diapirs and calderas, the fronts of deltas, as well as other areas with unstable slopes such as cliffs that are vulnerable to gravitational collapse.

Horizontal extension by rigid block rotation in brittle domains is accommodated by normal faults, the resulting deformation producing horizontal lengthening and vertical thinning of the crust. A succession of horsts and grabens or half grabens are produced by combined movement of conjugate normal faults. Elevated foothill blocks of 2 or more conjugate faults form horsts, which are topographic high areas, and grabens and half grabens are the low basin-like areas that form between horsts. "Grabens are symmetrical structures with both opposite-dipping conjugate faults developed equally, whereas half graben structures are asymmetric, being formed by a main fault and a set of minor synthetic and antithetic faults belonging to one or both conjugate sets"¹. The combined movements of faults that are related and tectonic structures that have been observed that formed in extensional settings can be explained by several models for normal faulting, most of which depend on the initial geometry of the fault - flat, listric or stepped. The authors¹ note that the addition of normal faults in progressive faulting cannot have the result of unwanted gaps along the fault surfaces, which will result if both the faults cut each other, simultaneously forming an X configuration, with the central block being displaced downwards. The domino model, a simple model for blocks bounded by flat surfaces, involving the rigid rotation of several blocks to accommodate an extension. This occurs in a similar manner to what occurs in a bookshelf of books that are tightly packed, the books lean to one side of the bookshelf when several books are removed to introduce a horizontal gap in the packed books. A shear movement is formed along the fault surfaces that are initially formed between the individual blocks, as a result of rotations of blocks, with the fault surfaces undergoing a dip angle decrease, as the horizontal space occupied by the inclined blocks becomes larger, and a decrease in the vertical thickness. Rotation of the blocks over a listric and detachment fault is involved in a most sophisticated version of the domino model. Triangular gaps are formed in the lower boundary with the detachment surface as the result of a geometric problem in both situations, as when the blocks are rotated they stand on 1 of their corners. A number of deformations have been invoked to solve this inconvenience, such as ductile flow, the gaps being filled by intrusions, as well as other deformations. It is very often shown by seismic lines that the geometry involves the blocks being flattened at the bottom to adjust to the detachment surface, though it is shown in small-scale examples that rotated blocks have an intact rectangular shape. The authors¹ suggest that further shearing and fracturing of the corners of the blocks can achieve this deformation.

Several displacements have been proposed for deformation of blocks in listric faults. As rigid rotation or translation of the hangingwall block causes gaps between the blocks it is not allowed. Distortion by internal rotation of the hangingwall block to form a rollover anticline is involved in different models, as the blocks must keep in touch along the entire fault surface. The formation of additional synthetic faulting in the hangingwall block in more rigid environments can accommodate the extension, thereby dividing the block into smaller blocks that rotate in a similar manner to the domino model. There can also be formation of a set of imbricate synthetic listric faults that rotate like small rock slides down the fault surface. Block subsidence is increased by sliding, and when it reaches the subsided area this gives way to flattening of the block, though bedding and other layering that was initially horizontal becomes progressively steeper. Back-faulting is progressive fault formation, the faults being of decreasing age towards the footwall. In the hangingwall a combination of synthetic and antithetic listric faulting can be produced, with the adjustment of the holes between the blocks being provided by ductile deformation or minor fracturing.

Special deformation structures that involve distinctive kinematics can be developed by stepped faults with flat and ramp geometries. If the rocks are sufficiently ductile anticlines and synclines can be caused by deformation of the hangingwall block over the steps. Areas where shear deformation is increased can cause the formation by bending of the flanks to flat- or ramp-related folds, preferred sites for secondary faulting of the hangingwall block. As extension progresses by the cutting of sigmoidal rock slices, called horses, from the footwall block ramps change position. A duplex structure, that is bounded in the upper part by a roof fault and a floor fault at the bottom if formed by the combination of all the horses. As the floor fault (which is active) is part of the main fault, experiencing shear displacements along the surface, the roof fault playing a secondary role as it active only when the corresponding horse forms.

Sources & Further reading

1. Leeder, Mike, Perez-Arlucea, Marta, 2006, Physical Processes in Earth and Environmental Sciences, Blackwell Publishing Ltd.