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 About Structural Geologey

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Geo Ahmad
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ذكر عدد الرسائل : 542
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الموقع : wwww.marmarstars.ahlamontada.net
العمل/الترفيه : واحد ماشي في كلية العلوم
المزاج : عالي اوي وهاي
السٌّمعَة : 2
نقاط : 22
تاريخ التسجيل : 14/08/2008

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مُساهمةموضوع: About Structural Geologey   الأربعاء فبراير 25, 2009 8:36 pm


عزيزي القارئ يوجد صور سوداء برجاء تظليلها عن الوصول اليها لتسهل مشاهدتها
مع مراعاة الصور التي سوف تحتاج الى تظليل ستكون عندها هذه العلامه *********النجوم
Tectonic Structures


Tectonics is the study of crustal deformation and structural behavior
Plate Tectonics is the deformation and structural behavior of crustal plates


Stress

Stress is any force which acts to deform rocks
Compression - a stress that acts to press or squeeze rocks together
Tension - a stress that acts to stretch a rock, or pull a rock apart
Shear - a stress which acts tangential to a plane through a body, causing two contiguous parts to slide past each other

Structural Behavior

As a general rule
Rocks tend to have a relatively high compressive strength
Rocks tend to have a relatively low tensile and shear strength

Strain

When a stress is applied, deformation may occur
Depending on the rate of stress
Depending on the amount of stress
Strain is the change in shape or volume of a body as a result of stress; deformation

Ductile Deformation
Brittle and Ductile deformation

During ductile deformation rocks bend or flow
Folding or bending of material without breaking
Specifically defined as a rock that is able to sustain, under a given set of conditions, 5-10% deformation before fracturing
Folds can be microscopic in size or kilometers in extent


Anticlines

Folds which arch up



.






Synclines

Folds which sink down






Monoclines

Folds in which rock layers on both sides of the fold are horizontal but at different levels







Domes

Folds which are equivalant to anticlines, but are comprised of layers which are shaped like an inverted bowl








Basins

Folds which are equivalant to synclines, but are comprised of layers which are shaped like a bowl







Brittle Deformation

During brittle deformation rocks break or fracture
Two main styles of fracture, joints and fualt
Both are the result of relatively rapid stress
For example: modeling clay will break if stress is applied rapidly, but will bend if stress is applied slowly

Joints

Joints are fracture surfaces along which there has been no displacement
Joints can form from compressional, tensional and shear stress, and can range in size from microscopic to kilometers in length
Joint sets and jointing has a major influence on landform development
Erosion is able to occur at a faster rate along joints



Faults

Faults are fractures along which there has been displacement of the material on either side of the fault
Faults are classified based on
the sense of movement (the direction in which the blocks on either side of the fault move) - this is controlled by the type of stress that is applied
the orientation of the fault surface (the angle of the plane of fracture)

Fault Terminology

Fault Plane - the plane along which the rock or crustal material has fractured
Hanging Wall Block - the rock material which lies above the fault plane
Footwall Block - the rock material which lies below the fault plane

Strike-Slip Faults

Fault plane is generally vertical
Movement is horizontal due to shear stress
Left-Lateral Strike-Slip - displacement is such that the material on the other side of the fault appears to be displaced to the left

Right-Lateral Strike-Slip - displacement is such that the material on the other side of the fault appears to be displaced to the right

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Normal Faults

Fault plane is oriented between 30 and 90 degrees (measured from horizontal)
Movement has both a horizontal and vertical component
Normal faults result from tensional stress and results in the hanging wall moving down relative to the footwall

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Detachment Faults

Fault plane is at less than 30 degrees
Movement is more horizontal than vertical due to the low angle of the fault plane
Develop due to tensional stress





Reverse Faults

Fault plane is oriented between 30 and 90 degrees (measured from horizontal)
Movement has both a horizontal and vertical component
Reverse faults result from compressional stress and results in the hanging wall moving up relative to the footwall

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Thrust Faults

Fault plane is at less than 30 degrees
Movement is more horizontal than vertical due to the low angle of the fault plane
Develop due to compressional stress


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Horsts and Grabens


Horsts are up thrown blocks bounded on either side by parallel normal faults.
Grabens are downthrown blocks bounded on either side by parallel normal faults
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Half-Graben


Half-grabens develop when parallel faults on either side of a block develop, but the block becomes tilted instead of dropping down as in a graben

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Fault Map Symbols




Measurement of Orientation

Strike - compass direction of the outcrop
- the line formed by the intersection of a horizontal plane with the structure
Dip - the angle between the horizontal plane and the planar surface being measured
Dip is always perpendicular to Strike





_________________
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الرجوع الى أعلى الصفحة اذهب الى الأسفل
معاينة صفحة البيانات الشخصي للعضو
Geo Ahmad
رئيس المنتدى
ورئيس المجلس الاعلى للمنتدى
رئيس المنتدى  ورئيس المجلس الاعلى للمنتدى
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ذكر عدد الرسائل : 542
العمر : 28
الموقع : wwww.marmarstars.ahlamontada.net
العمل/الترفيه : واحد ماشي في كلية العلوم
المزاج : عالي اوي وهاي
السٌّمعَة : 2
نقاط : 22
تاريخ التسجيل : 14/08/2008

بطاقة الشخصية
الأنتشار في المكان:
100/100  (100/100)

مُساهمةموضوع: رد: About Structural Geologey   الأربعاء فبراير 25, 2009 9:22 pm

Measurements and Techniques

The most obvious thing to do when trying to decipher the structural history of a formation is to describe it. One way of doing this is to measure the dip and strike. The dip is the amount a bed of rock is tipped from the horizontal. The strike is the direction which is ninety degrees from the dip, i.e. along the horizontal line on the bed. The strike can be in two directions, hence the dip could be in one of two directions also. There is a convention for the strike to be the in the direction you are facing if the rocks are dipping to your right. Some geologists prefer to measure the dip direction, rather than strike, as it is slightly simpler. However, all maps use dip and strike, not dip direction.
This is complicated slightly by apparent dip. This is due to the fact that you are not always looking edge on (perpendicular) to the bed you are measuring. If you are looking at a bed at a slight angle, then you see the apparent dip. The true dip will be greater than the apparent dip, as it is the maximum amount of dip, so the apparent dip can appear to be anything from 0° to the maximum (true) dip






Figure 1: Dip and strike of a bedding plane
In this diagram, the dip is 30°, with a strike north/south (0°/180°), the dip direction is 270°. On a geological map, symbols are used for the dip and strike. The strike is represented by a bar, and the dip by a mark on the strike bar on the downdip direction with the dip written alongside, as shown on the map below left


A geological cross section can be drawn from the map showing the subsurface structure. Obviously, only features which can be seen on the surface can be represented. The cross-section below right is drawn using the values in the map alongside




Figure 2a: A geological map


Folds

Folding of rocks is caused by the compression of rocks. This occurs slowly, over a long period of time. If this happened quickly, the rocks would break, and fault. This is due to the mechanical properties of rocks, namely it's plastic nature. If a rock is stretched slowly, then it will behave in a ductile fashion. If stretched quickly, the rock behaves in a brittle fashion. This behavior can be mimicked by using Blu-Tac. A typical fold is shown below, outlining the terms used in describing folds




Figure 3: Nomenclature used when describing folds
Hinge: Where curvature of the fold is at a maximum Crest & Trough: Where fold surface reaches a minimum and maximum respectively Limb: Beds between two hinges Antiform & Synform: Convex upwards or convex downward folds respectively Anticline & Syncline: Older or younger beds at the core respectively. Can be used in conjunction with antiform and synform, i.e. an antiformal syncline Folds are classified by shape and the chronological order of rocks in them. The shape of a fold is described by the angle between the limbs, which are given the terms: gentle (120-180°), open (70-120°), close (30-70°), tight (5-30°) or isoclinal(0-5°)
The chronological order of the rocks in a fold are described by syncline and anticline, as described above
See if you can identify some features on the picture below




Figure 4: Lulworth cove, Dorset
Faults

Faults are caused by short-term stress on rocks. They occur discontinuously along fault planes, and are the cause of most earthquakes. Faults are classified in terms of the type of force causing the fault which determines the direction of movement. There are many terms used in describing faults. These are shown in the diagram below




Figure 5: Fault nomenclature
The strike is the horizontal distance moved. The throw is the vertical distance and the heave is the distance moved perpendicular to the fault.
The types of fault, with arrows showing motion, are shown below




DiagramNameMode of Formation...Normal FaultExtension









(tension)Reverse Fault (or Thrust)CompressionStrike SlipShear


Faults rarely fall exactly into these categories, as they have some of the other types of motion as well, particularly some Strike Slip and rotational motion. Faults can also be reactivated, meaning a normal fault can be "reused" at a later time in a compressional regime. This produces complex drag folds along the side of a fault. Drag folds occur as the rock is bent due to the movement of the fault

_________________
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About Structural Geologey
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