Fault anatomy

Faults drawn on seismic or geologic sections are usually portrayed as single lines of even thickness. In detail, however, faults are rarely simple surfaces or zones of constant thickness. In fact, most faults are complex structures consisting of a number of structural elements that may be hard to predict. Because of the variations in expression along, as well as between, faults, it is not easy to come up with a simple and general description of a fault. In most cases it makes sense to distinguish between the central fault core or slip surface and the surrounding volume of brittlely deformed wallrock known as the fault damage zone, as illustrated in Figure 8.10.

The fault core can vary from a simple slip surface with a less than millimeter-thick cataclastic zone through a zone of several slip surfaces to an intensely sheared zone up to several meters wide where only remnants of the primary rock structures are preserved. In crystalline rocks, the fault core can consist of practically non-cohesivefault gouge, where clay minerals have formed at the expense of feldspar and other primary minerals. In other cases, hard and flintycataclasites constitute the fault core, particularly for faults formed in the lower part of the brittle upper crust. Various types ofbreccias, cohesive or non-cohesive, are also found in fault cores. In extreme cases, friction causes crystalline rocks to melt locally and temporarily, creating a glassy fault rock known aspseudotachylyte. The classification of fault rocks is shown in heading below.

In gentle, sedimentary rocks, fault cores typically encompass non-cohesive smeared-out layers. In a few cases, tender layers which include clay and silt may be smeared out to a continuous membrane which, if continuous in 3 dimensions, might also greatly lessen the ability of ?Uids to cross the fault. In general, the thickness of the fault middle shows a superb boom with fault throw, despite the fact that variations are exceptional even alongside a single fault within the identical lithology.

The damage zone is characterized by a density of brittle deformation structures that is higher than the background level. It envelops the fault core, which means that it is found in the tip zone as well as on each side of the core. Structures that are found in the damage zone include deformation bands, shear fractures, tensile fractures and stylolites, and Figure below shows an example of how such small-scale structures (deformation bands) only occur close to the fault core, in this case defining a footwall damage zone width of around 15 meters.

Damage zone in the footwall to a normal fault with 150–200 m throw. The footwall damage zone is characterized by a frequency diagram with data collected along the profile line. A fault lens is seen in the upper part of the fault. Entrada Sandstone near Moab, Utah.

The width of the damage quarter can vary from layer to layer, but, as with the fault core, there is a superb correlation between fault displacement and damage area thickness (Figure under a). Logarithmic diagrams such as shown in Figure under are widely used in fault analysis, and instantly lines in such diagrams indicate a consistent relation among the two plotted parameters. In precise, for facts that plot alongside one of the instantly lines on this ?Gure, the ratio between fault displacement D and harm region thickness DT is the equal for any fault size, and the gap among adjoining strains on this ?Gure represents one order of significance. Much of the facts in Figure underneath a plot around or above the line D=DT, which means that the fault displacement is close to or extremely large than the harm zone thickness, at the least for faults with displacements as much as 100 meters. We should use this diagram to estimate throw from harm quarter width or vice versa, but the huge spread of statistics (over orders of value) offers a especially signi?Cant uncertainty.

A similar relationship exists between fault core thickness (CT) and fault displacement (Figure below b). This relationship is constrained by the straight lines D=1000CT and D-10CT, meaning that the fault core is statistically around 1/100 of the fault displacement for faults with displacements up to 100 meters.

(a) Damage zone thickness (DT) (one side of the fault) plotted against displacement (D) for faults in siliciclastic sedimentary rocks. (b) Similar plot for fault core thickness (CT). Note logarithmic axes. Data from several sources.

Layers are commonly deflected (folded) around faults, particularly in faulted sedimentary rocks. The classic term for this behavior is drag, which should be used as a purely descriptive or geometric term. The drag zone can be wider or narrower than the damage zone, and can be completely absent. The distinction between the damage zone and the drag zone is that drag is an expression of ductile fault-related strain, while the damage zone is by definition restricted to brittle deformation. They are both part of the total strain zone associated with faults. In general, soft rocks develop more drag than stiff rocks.

Fault rocks

When fault movements regulate the authentic rock suf?Ciently it is became a brittle fault rock. There are numerous sorts of fault rocks, relying on lithology, con?Ning strain (depth), temperature, ?Uid pressure, kinematics and many others. On the time of faulting. It is beneficial to differentiate among different sorts of fault rocks, and to split them from mylonitic rocks shaped within the plastic regime. Sibson (1977) suggested a classi?Cation based on his statement that brittle fault rocks are generally non-foliated, while mylonites are properly foliated. He in addition made a distinction between cohesive and non-cohesive fault rocks. Further subclassi?Cation changed into finished primarily based at the relative amounts of big clasts and ?Ne-grained matrix. Sibson?S classi?Cation is descriptive and works nicely if we additionally upload that cataclastic fault rocks may additionally show a foliation in a few instances. Its courting to microscopic deformation mechanism is likewise clear, for the reason that mylonites, which result from plastic deformation mechanisms, are genuinely separated from cataclastic rocks inside the decrease a part of the diagram.

Fault breccia is an unconsolidated fault rock consisting of less than 30% matrix. If the matrix fragment ratio is higher, the rock is called a fault gouge. A fault gouge is thus a strongly ground down version of the original rock, but the term is sometimes also used for strongly reworked clay or shale in the core of faults in sedimentary sequences. These unconsolidated fault rocks form in the upper part of the brittle crust. They are conduits of fluid flow in non-porous rocks, but contribute to fault sealing in faulted porous rocks.

Pseudotachylyte consists of dark glass or microcrystalline, dense material. It forms by localized melting of the wall rock during frictional sliding. Pseudotachylyte can show injection veins into the sidewall, chilled margins, inclusions of the host rock and glass structures. It typically occurs as mm- to cm-wide zones that make sharp boundaries with the host rock. Pseudotachylytes form in the upper part of the crust, but can form at large crustal depths in dry parts of the lower crust.

Crush breccias are characterised by their large fragments. They all have less than 10% matrix and are cohesive and hard rocks. The fragments are glued together by cement (typically quartz or calcite) and/or by microfragments of mineral that have been crushed during faulting.

Cataclasites are distinguished from crush breccias by their lower fragment–matrix ratio. The matrix consists of crushed and ground-down microfragments that form a cohesive and often flinty rock. It takes a certain temperature for the matrix to end up flinty, and most cataclasites are thought to form at 5km depth or more.

Mylonites, which are not really fault rocks although loosely referred to as such by Sibson, are subdivided based on the amount of large, original grains and recrystallised matrix. Mylonites are well foliated and commonly also lineated and show abundant evidence of plastic deformation mechanisms rather than frictional sliding and grain crushing. They form at greater depths and temperatures than cataclasites and other fault rocks; above 300 C for quartz-rich rocks. The end-member of the mylonite series, blastomylonite, is a mylonite that has recrystallized after the deformation has ceased (postkinematic recrystallization). It therefore shows equant and strain-free grains of approximately equal size under the microscope, with the mylonitic foliation still preserved in hand samples.

Credits: Haakon Fossen (Structural Geology)

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