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what is a thrust fault

what is a thrust fault

what is a thrust fault

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Thrust fault

A thrust fault is a break in the Earth’s crust, across which older rocks are pushed above younger rocks.

Thrust geometry and nomenclature

Reverse faults

A thrust fault is a type of reverse fault that has a dip of 45 degrees or less.

If the angle of the fault plane is lower (often less than 15 degrees from the horizontal) and the displacement of the overlying block is large (often in the kilometer range) the fault is called an overthrust or overthrust fault. Erosion can remove part of the overlying block, creating a fenster (or window) – when the underlying block is exposed only in a relatively small area. When erosion removes most of the overlying block, leaving island-like remnants resting on the lower block, the remnants are called klippen (singular klippe).

Blind thrust faults

If the fault plane terminates before it reaches the Earth’s surface, it is referred to as a blind thrust fault. Because of the lack of surface evidence, blind thrust faults are difficult to detect until they rupture. The destructive 1994 earthquake in Northridge, Los Angeles, California, was caused by a previously undiscovered blind thrust fault.

Because of their low dip, thrusts are also difficult to appreciate in mapping, where lithological offsets are generally subtle and stratigraphic repetition is difficult to detect, especially in peneplain areas.

what is a thrust fault
what is a thrust fault

Fault-bend folds

Thrust faults, particularly those involved in thin-skinned style of deformation, have a so-called ramp-flat geometry. Thrusts mostly propagate along zones of weakness within a sedimentary sequence, such as mudstones or halite layers, these parts of the thrust are called decollements. If the effectiveness of the decollement becomes reduced, the thrust will tend to cut up the section to a higher stratigraphic level until it reaches another effective decollement where it can continue as bedding parallel flat. The part of the thrust linking the two flats is known as a ramp and typically forms at an angle of about 15°–30° to the bedding. Continued displacement on a thrust over a ramp produces a characteristic fold geometry known as a ramp anticline or, more generally, as a fault-bend fold.

Fault-propagation folds

Fault-propagation folds form at the tip of a thrust fault where propagation along the decollement has ceased but displacement on the thrust behind the fault tip is continuing. The continuing displacement is accommodated by formation of an asymmetric anticline-syncline fold pair. As displacement continues the thrust tip starts to propagate along the axis of the syncline. Such structures are also known as tip-line folds. Eventually the propagating thrust tip may reach another effective decollement layer and a composite fold structure will develop with characteristics of both fault-bend and fault-propagation folds.

Thrust duplex

Duplexes occur where there are two decollement levels close to each other within a sedimentary sequence, such as the top and base of a relatively strong sandstone layer bounded by two relatively weak mudstone layers. When a thrust that has propagated along the lower detachment, known as the floor thrust, cuts up to the upper detachment, known as the roof thrust, it forms a ramp within the stronger layer.

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With continued displacement on the thrust, higher stresses are developed in the footwall of the ramp due to the bend on the fault. This may cause renewed propagation along the floor thrust until it again cuts up to join the roof thrust. Further displacement then takes place via the newly created ramp. This process may repeat many times, forming a series of fault bounded thrust slices known as imbricates or horses, each with the geometry of a fault-bend fold of small displacement. The final result is typically a lozenge shaped duplex.

What is a fault and what are the different types?

A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. This movement may occur rapidly, in the form of an earthquake – or may occur slowly, in the form of creep. Faults may range in length from a few millimeters to thousands of kilometers. Most faults produce repeated displacements over geologic time. During an earthquake, the rock on one side of the fault suddenly slips with respect to the other. The fault surface can be horizontal or vertical or some arbitrary angle in between.

Earth scientists use the angle of the fault with respect to the surface (known as the dip) and the direction of slip along the fault to classify faults. Faults which move along the direction of the dip plane are dip-slip faults and described as either normal or reverse (thrust), depending on their motion. They which move horizontally are known as strike-slip faults and are classified as either right-lateral or left-lateral. Faults which show both dip-slip and strike-slip motion are known as oblique-slip faults.

The following definitions are adapted from The Earth by Press and Siever.

normal fault – a dip-slip fault in which the block above the fault has moved downward relative to the block below. This type of faulting occurs in response to extension and is often observed in the Western United States Basin and Range Province and along oceanic ridge systems.

Normal Fault Animation

thrust fault – a dip-slip fault in which the upper block, above the fault plane, moves up and over the lower block. This type of faulting is common in areas of compression, such as regions where one plate is being subducted under another as in Japan. When the dip angle is shallow, a reverse fault is often described as a thrust fault.

Thrust Fault Animation

Blind Thrust Fault Animation

strike-slip fault – a fault on which the two blocks slide past one another. The San Andreas Fault is an example of a right lateral fault.

what is a thrust fault
what is a thrust fault

Strike-slip Fault Animation

A left-lateral strike-slip fault is one on which the displacement of the far block is to the left when viewed from either side.

A right-lateral strike-slip fault is one on which the displacement of the far block is to the right when viewed from either side.

THRUST FAULT AND REVERSE FAULTS

Compressed land creates thrust faults
Thrust faults occur when one section of land slips over another at a low angle when the land is compressed.  Thrust faults do not usually show on the surface of the Earth.

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Angle is steeper in a reverse fault
A reverse fault forms when two landmasses are being compressed together like a thrust fault. The difference between the two faults is the angle of the fault. The angle of the fault plane in a reverse fault is greater than 45 degrees  The hanging wall on one side of the fault moves upward and is usually visible on the surface of the Earth.

WHERE FAULTS FORM

Coast ranges in subduction zones
Subduction zones often have multiple faults where land on the ocean floor is scraped off the oceanic plate.The faults pile up on the continent. The coast range Oregon, Washington and parts of California formed in this manner.

Formation thrust fault
Thrust faults form when the angle is less than 45 degrees. The angle allows one section of the land to go over the top of the other block of land.

Crust thickens
This type of fault movement thickens and shortens the crust. A normal fault develops where land is pulling apart and one block of land drops down. Blocks of land that move up or down in normal and reverse faults have steeply inclined planes.

BLIND THRUST FAULT

Type of reverse fault
A blind fault occurs in areas a shallow-dipping reverse fault terminates before it reaches the Earth’s surface. Tension in the crust will cause the rocks to fracture but the fractures does not have any surface features. Many unknown thrust faults are suspected in California. Some faults are not discovered until a major earthquake occurs.

NORTHRIDGE EARTHQUAKE

1994 Northridge earthquake
The 1994 Northridge earthquake was caused by a blind fault that scientists did not know about until it occurred. The earthquake killed 61 people and approximately $40 billion in damage. The fault line is between 12 miles at its southern edge and 3 miles below the Earth’s surface at its northern edge. About 10 to 12 miles of land broke along the fault plane during the earthquake.

what is a thrust fault
what is a thrust fault

Break along fault line
The actual break only took about 8 seconds but the shaking in the area due to sediments lasted between 20 and 30 seconds. The movement along the fault was approximately 13 feet.

No foreshocks
Everyone in the region included seismologists were taken by surprise. There were no foreshocks and no strain was seen in the rocks at any time before the earthquake occurred.

Abstract

Thrust faulting is a fundamental mode of crustal deformation, yet many of the key geometrical attributes of thrust faults and the controls on fault rupture, growth, and linkage remain poorly resolved. Numerous surface-rupturing thrust faults cut through upper Quaternary glacial outwash terraces within the Ostler Fault zone, an active thrust system in the Southern Alps, New Zealand. We use these deformed marker surfaces to define the three-dimensional deformation field associated with their surface expression and to map displacement and length on ∼40 fault segments. Displacement transfer across two fault segment arrays occurs in distinctly different styles. In one, displacement is transferred between en échelon fault segments to produce a smooth, linear displacement gradient.

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In the other, large-scale folding and a population of small faults transfer displacement between two non-overlapping fault segments, with a residual displacement minimum within the transfer zone. Size distribution of fault-segment length and maximum displacement follow a power-law scaling relationship. Maximum displacement (Dmax) scales linearly with and represents ∼1% of segment trace length (L). Dmax/L data from the segments of the Ostler Fault zone display similar scaling to a global fault dataset. This similarity is surprising, given that the observed displacements along the Ostler Fault represent only 20 ky of slip on fault segments that are likely to be many times older.

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