User:Ywangg/Extensional fault

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An extensional fault is a fault caused by stretching of the Earth's crust. Stretching reduces the thickness and horizontally extends portions of the crust and/or lithosphere.

In most cases such a fault is also a normal fault, but may create a shallower dip usually associated with a thrust fault. Extensional faults are generally planar. If the stress field is oriented with the maximum stress perpendicular to the Earth's surface, extensional faults will create an initial dip of the associated beds of about 60° from the horizontal. The faults will typically extend down to the base of the seismogenic layer. As crustal stretching continues, the faults will rotate, resulting in steeply-dipping fault blocks between them.

Fault Interaction
Faults in a given fault population can form interaction that depends on a number of factors, such as fault density, fault distribution and spatial arrangement of faults in the population, and the size of stress field around the faults. Once two subparallel fault segments get close enough, they will interact due to the formation of ramps. The spacing at which the two fault tips interact is important to understand the growth history of fault populations. The spacing is related to fault length and fault displacement.



Boundary Condition
The overall arrangement of faults in the fault population is controlled by kinematic boundary conditions which are the external physical characteristics of sediments as well as rheology and mechanical layer properties. The base and the walls are visualized for physical models. Naturally the base of a basin is a detachment with weak preexisting faults where location and arrangement can influence the structure and distribution in the overburden.

The correlation between basement structures and transfer zone has been the cause for several rifts throughout the world. When strain is relayed from one side of the rift to the other, it makes the basement structure influence the first-order transfer zones. The interaction of faults can be observed both in plan and in cross-section.

Relay Ramp
As faults overlap and the relay structure is established, the ramp will deform internally and breaching will occur if strain keeps accumulating. The deformation structures that form within the relay structure depend on the mechanical rock properties. For example, sandstones are more likely to develop deformation bands that lead to hardening in the ramps and eventually faulting comparing to sand.

With sedimentation taking place during fault growth, there is a more mature ramp as faults form which lead to an evolution of breached relay structure. Relay ramps also become steeper downwards as strain increases. The critical amount of curvature in the relay ramp is used for predicting breaching faults from seismic interpretations. Unbreached ramps have a smaller curvature than barely-breach ramps and well-breached ramps have the largest curvature. The stress and strain development in the ramp are also influenced by displacement of two faults during fault interaction.

Curved Fault Segments
Curved geometry is a common feature of many large-scale extensional faults. The arrangement of faults in the population is determined by external causes and controls on the magnitude of strain in the column of sediments as well as layer properties. The preexisting faults in the basement of regions undergoing extension may influence the formation and localization of transfer structures.

Curved faults segments are considered individual faults that link large-scale normal fault system. One factor that can influence curved fault pattern is the preexisting heterogeneities including the reactivation of older faults that formed under different stress. Curved geometry is a common feature of many large-scale extensional faults.

Fault in Sedimentary Basins
Extensional deformation within sedimentary basins developed within the upper crust is characterized by Navier-Coulomb brittle behaviour. During stretching of the Earth's crust, sand layers were added to maintain a constant, horizontal surface and to activate sedimentation that fills the extensional basin. There are curvatures at the ends of the fault traces in the sedimentary basins that are caused by frictional drag along the glass side walls. Faults also propagate due to slightly uneven extension on the basin detachment.

Extensional fault-propagation folds are common when there is a distinct difference between the basement of the fault and its sedimentary basin. The kinematics of linked basement faulting and extensional fault-propagation folding shows that there is an upward widening propagation fold linked to faults at depth. Additionally, the model also predicts that the hanging wall-synclines and footwall-anticlines adjacent to the fault are a breached monocline.