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38-488: The complex is made up of an imbricate stack in a sequence that is the same both at Narooma and Murruna Point, Batemans Bay. The top layer consists of turbidite sequence from the Early Ordovician. Below this is a high strain zone full of broken fragments. Special textures from the high strain zone include pressure solution , dilational veins and boudinage . Some of the rock appears as mylonite . Underneath

76-421: A Silva compass . Any planar feature can be described by strike and dip, including sedimentary bedding , fractures , faults , joints , cuestas , igneous dikes and sills , metamorphic foliation and fabric , etc. Observations about a structure's orientation can lead to inferences about certain parts of an area's history, such as movement, deformation, or tectonic activity . When measuring or describing

114-414: A cleavage with lenses of chlorite , quartz and white mica . The strike direction of the cleavage is 330°. The interpretation of the mélange is that it is either an olistostrome or an upwelling. The mélange was underplated beneath the chert layer. Pockets of underplated material are expected to form low angle detachments. The inland zone has a chevron folded structure with reverse faults. From

152-480: 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 form at the tip of a thrust fault where propagation along the decollement has ceased, but displacement on the thrust behind the fault tip continues. The formation of an asymmetric anticline-syncline fold pair accommodates

190-425: A feature with a dip of 45° and a dip direction of 75°, the strike and dip can be written as 345/45 NE, 165/45 NE, or 075,45. The compass quadrant direction for the strike can also be used in place of the azimuth, written as S15E or N15W. Strike and dip are measured in the field using a compass and with a clinometer . A compass is used to measure the azimuth of the strike, and the clinometer measures inclination of

228-412: A few conventions geologists use when measuring a feature's azimuth. When using the strike, two directions can be measured at 180° apart, at either clockwise or counterclockwise of north. One common convention is to use the "right-hand rule" (RHR) where the plane dips down towards the right when facing the strike direction, or that the dip direction should be 90° clockwise of the strike direction. However, in

266-470: A number (between 0° and 90°) indicating the angle in degrees below horizontal. It can be accompanied with the rough direction of dip (N, SE, etc) to avoid ambiguity. The direction can sometimes be omitted, as long as the convention used (such as right-hand rule) is known. A feature that is completely flat will have the same dip value over the entire surface. The dip of a curved feature, such as an anticline or syncline , will change at different points along

304-402: 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

342-408: 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. With continued displacement on the thrust, higher stresses are developed in

380-423: A single three-digit number in terms of the angle from true north (for example, N25°E would simply become 025 or 025°). A feature's orientation can also be represented by its dip direction. Rather than the azimuth of a horizontal line on the plane, the azimuth of the steepest line on the plane is used. The direction of dip can be visualized as the direction water would flow if poured onto a plane. While true dip

418-399: 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

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456-421: Is analogous to dip direction and "plunge" is the dip angle. Strike and dip are measured using a compass and a clinometer . A compass is used to measure the feature's strike by holding the compass horizontally against the feature. A clinometer measures the feature's dip by recording the inclination perpendicular to the strike. These can be done separately, or together using a tool such as a Brunton transit or

494-815: Is measured perpendicular to the strike, apparent dip refers to an observed dip which is not perpendicular to the strike line. This can be seen in outcroppings or cross-sections which do not run parallel to the dip direction. Apparent dip is always shallower than the true dip. If the strike is known, the apparent dip or true dip can be calculated using trigonometry: α = arctan ⁡ ( sin ⁡ β × tan ⁡ δ ) {\displaystyle \alpha =\arctan(\sin \beta \times \tan \delta )} δ = arctan ⁡ ( tan ⁡ α ÷ sin ⁡ β ) {\displaystyle \delta =\arctan(\tan \alpha \div \sin \beta )} where δ

532-401: Is the true dip, α is the apparent dip, and β is the angle between the strike direction and the apparent dip direction, all in degrees. The measurement of a linear feature's orientation is similar to strike and dip, though the terminology differs because "strike" and "dip" are reserved for planes. Linear features use trend and plunge instead. Plunge, or angle of plunge, is the inclination of

570-416: Is typically a lozenge-shaped duplex. Most duplexes have only small displacements on the bounding faults between the horses, which dip away from the foreland. Occasionally, the displacement on the individual horses is more significant, such that each horse lies more or less vertically above the other; this is known as an antiformal stack or imbricate stack . If the individual displacements are still greater,

608-611: The Alps , and the Appalachians are prominent examples of compressional orogenies with numerous overthrust faults. Thrust faults occur in the foreland basin , marginal to orogenic belts. Here, compression does not result in appreciable mountain building, which is mostly accommodated by folding and stacking of thrusts. Instead, thrust faults generally cause a thickening of the stratigraphic section . When thrusts are developed in orogens formed in previously rifted margins, inversion of

646-412: The ocean trench margin of subduction zones, where oceanic sediments are scraped off the subducted plate and accumulate. Here, the accretionary wedge must thicken by up to 200%, and this is achieved by stacking thrust fault upon thrust fault in a melange of disrupted rock, often with chaotic folding. Here, ramp flat geometries are not usually observed because the compressional force is at a steep angle to

684-562: The UK, the right-hand rule has sometimes been specified so that the dip direction is instead counterclockwise from the strike. Some geologists prefer to use whichever strike direction is less than 180°. Others prefer to use the "dip-direction, dip" (DDD) convention instead of using the strike direction. Strike and dip are generally written as 'strike/dip' or 'dip direction,dip', with the degree symbol typically omitted. The general alphabetical dip direction (N, SE, etc) can be added to reduce ambiguity. For

722-407: The attitude of an inclined feature, two quantities are needed. The angle the slope descends, or dip, and the direction of descent, which can be represented by strike or dip direction. Dip is the inclination of a given feature, and is measured from the steepest angle of descent of a tilted bed or feature relative to a horizontal plane. True dip is always perpendicular to the strike. It is written as

760-514: The buried paleo-rifts can induce the nucleation of thrust ramps. Foreland basin thrusts also usually observe the ramp-flat geometry, with thrusts propagating within units at very low angle "flats" (at 1–5 degrees) and then moving up-section in steeper ramps (at 5–20 degrees) where they offset stratigraphic units. Thrusts have also been detected in cratonic settings, where "far-foreland" deformation has advanced into intracontinental areas. Thrusts and duplexes are also found in accretionary wedges in

798-498: The coast as the trench is approached. The country here was shortened between middle Silurian to Middle Devonian in the east–west direction, with many folds and thrust faults . Inland the rocks have developed a scaly cleavage . The chert on the coast has developed a dextral shear . Imbricate stack A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. A thrust fault

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836-544: The continental margin and began to include sediments derived from the continent, such as sandstone , siltstone , argillite and shale as well as chert bands. After formation the terrane was accreted to the Lachlan Fold Belt in the early Silurian . The rock was deformed in the Benambran Orogeny in early Silurian. A low angle oblique imbrication formed. Rocks have become more deformed closer to

874-446: The continuing displacement. 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 fault-bending and fault-propagation folds' characteristics. Duplexes occur where two decollement levels are close to each other within

912-407: The dip line on both sides of the strike, and horizontal bedding is denoted by a cross within a circle. Interpretation of strike and dip is a part of creating a cross-section of an area. Strike and dip information recorded on a map can be used to reconstruct various structures, determine the orientation of subsurface features, or detect the presence of anticline or syncline folds. There are

950-549: The dip. Dr. E. Clar first described the modern compass-clinometer in 1954, and some continue to be referred to as Clar compasses. Compasses in use today include the Brunton compass and the Silva compass . Smartphone apps which can make strike and dip measurements are also available, including apps such as GeoTools . These apps can make use of the phone's internal accelerometer to provide orientation measurements. Combined with

988-422: The feature and be flat on any fold axis . Strike is a representation of the orientation of a tilted feature. The strike line of a bed , fault, or other planar feature, is a line representing the intersection of that feature with a horizontal plane. The strike of the feature is the azimuth (compass direction) of the strike line. This can be represented by either a quadrant compass bearing (such as N25°E), or as

1026-407: The feature measured downward relative to horizontal. Trend is the feature's azimuth, measured in the direction of plunge. A horizontal line would have a plunge of 0°, and a vertical line would have a plunge of 90°. A linear feature which lies within a plane can also be measured by its rake (or pitch). Unlike plunge, which is the feature's azimuth, the rake is the angle measured within the plane from

1064-429: 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

1102-472: The high strain is chert from the Late Cambrian to Late Ordovician . The lowest part of the stack is block in mélange. The blocks are mostly turbidite, but also includes chert, and some pillow lava basalt . The blocks are at all different orientations, different sizes all mixed together. The sediments were deformed into these blocks before they turned into stone. Later deformation has developed

1140-505: The horses have a foreland dip. Duplexing is a very efficient mechanism of accommodating the shortening of the crust by thickening the section rather than by folding and deformation. Large overthrust faults occur in areas that have undergone great compressional forces. These conditions exist in the orogenic belts that result from either two continental tectonic collisions or from subduction zone accretion. The resultant compressional forces produce mountain ranges. The Himalayas ,

1178-425: The name of Thrust-planes. They are strictly reversed faults, but with so low a hade that the rocks on their upthrown side have been, as it were, pushed horizontally forward. Strike and dip In geology , strike and dip is a measurement convention used to describe the plane orientation or attitude of a planar geologic feature . A feature's strike is the azimuth of an imagined horizontal line across

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1216-445: The plane, and its dip is the angle of inclination (or depression angle ) measured downward from horizontal. They are used together to measure and document a structure's characteristics for study or for use on a geologic map . A feature's orientation can also be represented by dip and dip direction , using the azimuth of the dip rather than the strike value. Linear features are similarly measured with trend and plunge , where "trend"

1254-890: The sedimentary layering. Thrust faults were unrecognised until the work of Arnold Escher von der Linth , Albert Heim and Marcel Alexandre Bertrand in the Alps working on the Glarus Thrust ; Charles Lapworth , Ben Peach and John Horne working on parts of the Moine Thrust in the Scottish Highlands ; Alfred Elis Törnebohm in the Scandinavian Caledonides and R. G. McConnell in the Canadian Rockies. The realisation that older strata could, via faulting, be found above younger strata

1292-681: The stratigraphic point of view the terrane comprises the Wagonga Group . This consists of the Narooma Chert overlain by the Bogolo Formation . Deep sea chert (Narooma Chert) was deposited on the Pacific Ocean floor over a period of 50 million years from Late Cambrian to Ordovician. Fossils from the chert include the conodonts Paracordylodus gracilis and Acodus cf. A. comptus . The terrane gradually approached

1330-540: The strike line. On geologic maps , strike and dip can be represented by a T symbol with a number next to it. The longer line represents strike, and is in the same orientation as the strike angle. Dip is represented by the shorter line, which is perpendicular to the strike line in the downhill direction. The number gives the dip angle, in degrees, below horizontal, and often does not have the degree symbol. Vertical and horizontal features are not marked with numbers, and instead use their own symbols. Beds dipping vertically have

1368-569: 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 ). If the fault plane terminates before it reaches the Earth's surface, it is called a blind thrust fault. Because of the lack of surface evidence, blind thrust faults are difficult to detect until rupture. The destructive 1994 earthquake in Northridge, Los Angeles, California ,

1406-439: Was arrived at more or less independently by geologists in all these areas during the 1880s. Geikie in 1884 coined the term thrust-plane to describe this special set of faults. He wrote: By a system of reversed faults, a group of strata is made to cover a great breadth of ground and actually to overlie higher members of the same series. The most extraordinary dislocations, however, are those to which for distinction we have given

1444-456: 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. Thrust faults, particularly those involved in thin-skinned style of deformation, have a so-called ramp-flat geometry. Thrusts mainly propagate along zones of weakness within

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