Faults

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Undergraduate Geology - Part 1 (Geological Structures) Note on Faults, created by siobhan.quirk on 05/13/2013.

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Faults can be described by: fault plane: plane of ffracture, along which rocks have been displaced throw: the vertical displacement of rocks along the fault plane fault dip: the maximum inclination of the fault plane as measured from the horizontal hanging wall: side that lies above the fault plane footwall: side that lies below the fault plane upthrow: the side that has moved up (in relation to the other side) downthrow: side of fault that has moved down (in relation to the other side) Fault Types: largely vertical movement: dip-slip faults largely horizontal movement: strike-slip faults

Dip-Slip FaultsWhere the movement along the fault plane is parallel to the dip of the fault plane. Two main types: normal faults (result of tension) and reverse faults (result of compression). These faults often have a steep dip and their outcrop pattern is usually straight.Normal FaultsThe Earth's crust is being stretched and close to the surface, where the rocks are cold and brittle, they fracture to form a normal fault. In a normal fault the hanging wall is the downthrow side. Extension of the crust is the result of the tensional forces. This causes a lengthening of the crust and in cross-section there is clearly a gap created in the formerly continuous beds of rock.The downthrow side of the fault sinks uinder the influence of gravity. The principal stress direction forming these faults is vertical forces due to the weight of the overlying rocks, with the minimum stress being horizontal tensional forces.Graben and HorstWhere two normal faults face each other , that is dip towards each other, then a graben or rift valley is formed. The graben is the area that forms the downthrow between the two faults. Rifts occur at the centre of mid-oceanic ridges and form the East African Rift Valley system.Where two normal faults face away from each other, that it dip away from each other, a horst structure is formed. The horst is the uplifted block between the two faults.

Reverse FaultsFormed by compressional forces, which causes a shortening of the Earth's crust. In a reverse fault the hanging wall is the upthrow side. Reverse faults can be recognised because there is an overlap of the strata created by the fault movement, causing a repetition of the formerly continous bed. The principal stress direction forming these faults is horizontal compressional forces with the minimum stress being vertical. This suggests that these faults form relatively close to the Earth's surface where there is less overlying rock.Thrust FaultsA type of reverse fault, where the fault dip is less than 45 degrees. They are formed by compressional forces. These structures are often associated with major fold mountain system and have displacements which can be measured in kilometers. In northwest Scotland the Moine Thrust brings Precambrian rocks over 500 Ma to rest above the Cambrian rocks less than 560 Ma. The fault formed in the Caledonian orogeny. Thrusts can result in inverted strata, particularly when they form on the limb of a fold.

Strike-Slip FaultsWhere the fault plane is vertical and the movement along the fault is horizontal - parallel to the strike of the fault plane.These faults are often large-scale structures, with large displacements. There are two main types: tear faults and transform faultsTear FaultsResult of shearing forces. Example is Great Glen Faultin the Highlands of Scotland. This fault is thought to have a displacement of around 100km and was produced as part of the formation of the Caledonian mountains. One piece of evidence used to recognise tjos fault is the existence of two granite bodies on either side, which are similar in composition, age and structure. These granites are now located at either end of the Great Glen on opposite sides of the fault.When describing a tear fault, the terms dextral and sinistral are used. These terms reflect the sense of movement along a fault plane. If the block on the opposite side of the fault has moved to the right it is dextral and if it has moved to the left it is sinistral. Transform FaultsAssociated with plate margins and often described as conservative plate margins. Common at right angles to a MOR. They can be thousands of kilometers long. They are the result of different rates of movement within a plate and allow the rigid plates to adjust for these differences in the rate of movement. Superficially they resemble tear faults but when the movements along the fault plane are considered, they are clearly a different type of fault.

SlickenslidesScratch marks found on fault planes. They are left by the grinding of pieces of rock along the fault plane are the fault moves. They are best seen were the fault plane is coated in a mineral precipated from a fluid that moved along the fault plane. The two surfaces are polished with linear grooves and ridges parallel to the direction of movement. Therefore the striations show the direction of the last movement along the fault plane.Fault BrecciaThis rock is found along fault planes and consists of fragments broken from the rocks on either side of the fault plane. The fragments are large and angular and made of hard, competent rock. They may be cemented by minerals precipitated in the fault zone at a later stage.

Largely Vertical Movement

Largely Horizontal Movement

Features Associated with Fault Planes

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