GEOLOGIC STRUCTURES---CRUSTAL DEFORMATION


This photo shows deformation of rocks resulting in folded structures--internal Earth forces can cause such geologic structures to form--geologic structures can harbor important energy sources and valuable mineral ore deposits


I. Strike and dip

A. Strike

B. Dip

click here to see strike and dip

II. Folded structures

1. Anticline

·         is a series of up-arched strata with side portions (limbs) dipping in opposite directions away from the central portion of fold split by a plane called the axial plane and observed in the top (plan) view as the fold axis--an eroded surface indicates the rocks become progressively younger away from the fold axis

·         click here to see the parts of an anticline ----

·          click here to see a photo of an anticline

2. Syncline

·         is a series of down-arched strata with limbs dipping inwards in opposite directions towards the fold axis--an eroded surface indicates the rocks become progressively older away from the fold axis

·         click here to see a photo of a syncline --- see pages 417-419 in the text for more examples of anticlines and synclines

     B. Specific types of anticlines and synclines---see page 418 in text

1. Symmetrical fold--is a fold with the sides showing a mirror image with respect to the axial plane

2. Asymmetrical fold--a fold without a mirror image in respect to the axial plane

3. Overturned fold--a fold in which the axial plane is tilted and beds may dip in same direction on both sides of the axial plane

click here to see symmetrical, asymmetrical, and overturned folds

4. Recumbent fold--a fold with the axial plane horizontal

·         click here to see a photo of a recumbent fold

5. non-plunging and plunging folds

·         plunging is the tilting of the fold towards the front or back end--all anticlines and synclines have some degree of plunge

·         in non-plunging folds contact lines separating formations shown in the surface view are parallel and straight---the contact lines in one of the two profile views are horizontal and parallel---the contacts in the other profile view are arched up or down--in plunging folds contact lines in the surface view are curved---contact lines in one of the profile views dip in the direction of plunge---contact lines in the other profile view are arched up or down--the surface contact lines between formations are convexed (closed) in the direction of plunge for the anticline and concave (open) in the direction of plunge for the syncline

            6.  formation and location of petroleum and natural gas is associated with anticlines and synclines— see page 529 in text to see this

click here to see the formation contacts of plunging anticlines and synclines

3. Dome

·         is an up-arched series of strata with beds on all sides dipping away from the center throughout 360 degrees--an eroded surface indicates the rocks become progressively younger away from the center of the structure

4. Basin

·         is a down-arched series of strata with beds on all sides dipping in towards the center throughout 360 degrees--an eroded surface indicates the rocks become progressively older away from the center of the structure

click here to see a dome and basin --- see page 420 to see another example of a dome and basin

 

5. Monocline

·         is a bend in the strata resulting in a local steepening in dip of the strata which is almost flat lying on both sides away from the bend-- there is only one direction of dip in the monocline

·         click here to see the monoclinesee another example on page 421 in text

III. Fault structures

A. Definition

B. Types of faults

1. Vertical or dip slip fault

·         is that type with movement along the dip of the fault--the concepts of hanging wall and footwall are important in defining the different types of vertical faults

·         if the angle of dip is not 90 degrees the side of the fault with the portion of land which appears to be hanging over a lower portion before movement is called the hanging wall and that on the bottom, the footwall

·         see below

·        

·         Normal fault--hanging wall moves down in respect to footwall

·         Reverse fault--hanging wall moves up in respect to footwall--a low angled reverse fault is called a thrust fault

click here to see hanging wall-footwall related to normal and reverse faults or see page 422 at bottom in text

click here to see a photo of a normal fault

·         Horst and graben--a wedge of land which moves up (horst) or down (graben) between 2 normal faults--best example is the Rhine River Graben in Europe

·         click here to see a horst and graben see middle of page 423 for another example of a horst and graben

2. Horizontal or strike slip fault

·         is that type with horizontal movement along the cracked rock--movement can be left lateral or right lateral--best example of right lateral strike slip fault is the San Andreas fault in California--looking down the fault line in the front plan view, the right side of the fault appears to have moved towards you if a right lateral fault or left side moves towards you if a left lateral fault

·         click here to see a right lateral strike slip fault --- see page 422 at bottom in text for an example of a right lateral strike slip fault and page 424

 

3. Oblique fault

·         is a fault which has a major dip slip and strike slip component

IV. Joint structures

A. Definition of joints

B. Causes of joints

1. Unloading or sheeting effects (see Weathering)

2. Stresses in a cooling magma

·         can cause columnar jointing in which there are 6-sided fractures comprising the jointing pattern--best example is Devil's Tower, Wyoming

·         click here to see a photo of Devil's Tower --- see another picture of columnar jointing on page 83 of text

V. Unconformity structures

A. Definition

B. Kinds of unconformities

 

1. Disconformity

·         a series of sedimentary rocks form, next a non-deposition or massive erosion event occurs, then a series of more sedimentary rocks are deposited

·         click here and see a photo of a disconformity--blue line defines the non-deposition or erosion surface

2. Angular unconformity

·         a series of sedimentary rocks form, next rocks are folded or tilted, next a non-deposition or massive erosion occurs, then a series of more sedimentary rocks form

·         click here to see a photo of an angular unconformity--blue line defines the non-deposition or erosion surface

3. Nonconformity

·         igneous or metamorphic rock form, next a non-deposition or massive erosion event occurs, then a series of sedimentary rocks form

·         click here to see a photo of a nonconformity

·         click here to see figure on the sequential events defining the kinds of unconformities discussed above

VI. Importance of geologic structures

 



page 357
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page 359
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page 363
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page 362
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page 360
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anticline
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syncline
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recumbent fold
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page 361
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page 365
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page 364
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page 365
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page 368
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page 369
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disconformity
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angular unconformity
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nonconformity
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unconformities
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