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GEOLOGIC STRUCTURES

 

 

What is Structural Geology, and Why Study It?

 

Structural geology- Study of how the lithosphere is bent, broken, and deformed during plate tectonics.

 

Structural geology is important for understanding:

 

1) The locations of earthquakes.

2) The formation of mountains.

3) The tectonic history of the earth.

4) How to safely building structures such as bridges and dams.

5) How to locate natural resources like oil and gold.

 

 

 

Mechanical Behavior of the Lithosphere

 

Stress = Force/Area

 

Strain = A change in shape or volume in response to a stress.

 

Stress is the cause, strain is the effect!

 

There are three types of stress:

 

1) Compressive stress – Forces that squeeze or push toward one another from opposite directions (cause shortening or flattening).

2) Tensional stress – Forces that pull away from one another in opposite directions (cause stretching or extension).

3) Shear stress – Forces that are offset from one another and operate in opposite directions (cause a shear strain).

 

The behavior of rock during stress depends on:

 

1) The type of rock.

2) The type and amount of stress.

3) The temperature.

4) The pressure.

5) The rate at which the stress is applied (think about Silly PuttyÔ).

 

Structural geologists do “rock squeezing” experiments to examine how rocks respond to stress.

 

The three types of rock behavior are:

1) Elastic behavior

2) Brittle behavior

3) Ductile/plastic behavior

 

Elastic Behavior – A lightly-deformed material will recover its original shape after the stress is removed.  Elastic behavior occurs at very low stresses.

 

Every material has an elastic limit, the maximum amount of stress a material can feel and still recover to its original shape.

 

The behavior of rock upon reaching the elastic limit depends mostly on the temperature and pressure.

 

Brittle Behavior – Near the Earth’s surface (low T and P), rocks exhibit brittle behavior.  When a stress exceeds the elastic limit, rocks will fracture (faults and joints form).

 

Ductile Behavior – At depth in the Earth (high T and P), rocks exhibit plastic behavior.  When a stress exceeds the elastic limit, rocks will bend and flow (folds and foliations form).

 

 

 

The Development of Folds by Plastic Deformation

 

Rocks that are deeply seated within the Earth exhibit plastic behavior during stress.

 

Fold- A bend or wavelike feature in rocks formed by compressive stresses.

 

There are two main types of folds:

 

1) Anticline- An upward arching fold.

Wikipedia Commons photo.

 

2) Syncline- A downward arching fold.

Rainbow Basin Syncline near Barstow, California. Mark A. Wilson (Department of Geology, The College of Wooster)

 

Synclines and anticlines often occur adjacent to one another as part of a larger fold belt.

 

A syncline-anticline pair, Wikipedia Commons Image.

 

Warning! Anticlines do not necessarily form hills and synclines do not necessarily form valleys on the surface of the earth! 

Folds may be beveled flat at the Earth’s surface due to erosion.

 

How would a syncline look after erosion?  Would the oldest or youngest layers appear in the center of an anticline?

 

How would an anticline look after erosion, and which layer (oldest or youngest) would appear at the center?

 

Other Vocabulary Related to Folds

 

Axial plane- A plane that bisects a fold

 

Fold axis- A line marking the intersection of the axial plane with the folded layers (also called the hinge of the fold).

 

Limbs- The portions of a fold on each side of the fold axis.

 

Symmetrical fold- A fold in which the axial plane is vertical.

 

Asymmetrical fold- A fold in which the axial plane is inclined.

 

Overturned fold- A fold in which one of the limbs has overturned bedding.

 

Recumbent fold- A fold in which limbs are overturned to the extent that the limbs are essentially horizontal.

 

Plunging Fold- Not every fold has a fold axis that is parallel to Earth’s surface.  The fold axes may plunge into the earth.

 

    A plunging anticline will have the oldest layer in the center and will "V" in the direction of the plunge.

 

    A plunging syncline will have the youngest later in the center and will "V" in the direction opposite of the plunge.

 

 

 

Measuring Strike and Dip

 

Tilted Strata- The law of original horizontality says that sedimentary rocks are deposited in horizontal layers.  However, stress may cause the layers to become tilted (or folded) such that they are no longer horizontal.

James D. Dana, New Text-book of Geology (New York: Ivison, Blakeman & Company, 1883)

 

Geologists measure geologic structures and plot their orientations on maps in order to learn about the stresses that caused them (thereby unraveling the tectonic history).

 

The orientation of a planar surface like a bedding plane can be fully defined using strike and dip.

 

Strike- The compass direction of a line marking the intersection of an inclined plane with a horizontal plane (i.e., the Earth’s surface).

 

Dip- The maximum angle between the inclined plane and the horizontal plane.  Dip is measured perpendicular to strike.  Dip has both an angle and a compass direction.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The above images are from James D. Dana, New Text-book of Geology (New York: Ivison, Blakeman & Company, 1883)

 

Anticlines and synclines can be detected by looking at the strike and dip of layering.  The limbs of a syncline dip towards the fold axis, whereas the limbs of an anticline dip away from the fold axis.

 

 

 

 

The Development of Joints and Faults by Brittle Deformation

 

Rocks at Earth's surface are brittle and tend to fracture, or break, due to stress.

 

Joint - A planar feature (a type of fracture) along which there has been no movement of one side of the fracture relative to the other side.  As conduits for fluid flow, joints may be sites of ore deposition and groundwater migration.

 

Primary joint- A joint that forms as a result of non-tectonic stresses,

 

Tectonic joint- A joint that forms by tectonic stresses.

 

Columnar jointing- A type of primary jointing that forms when basalt cools from solidification temperatures.

 

Columnar basalts at Devils Postpile, California, © Bruce Molnia, Terra Photographics.

 

Sheet jointing- A type of primary jointing that forms by the unloading of rock (also known as exfoliation).

 

Sheet jointing in Granodiorite. © Marli Miller, University of Oregon.

 

Joint set- A set of parallel joints (often indicative of tectonic stresses).

 

Joint-controlled streams in central New York flow along two joint sets that are oriented at right angles to each other. Copyright © Bruce Molnia, Terra Photographics.

 

Fault- A planar feature in which there has been movement of rock, called displacement, along the plane.  The displacements may range from centimeters to hundreds of kilometers.

 

Fault breccia- Some faults are characterized by ground up rock fragments that may be cemented together to form a fault breccia.

 

Slickenside- Rocks moving against each other along a fault plane may develop smooth surfaces and striations called slickensides.

 

The three main types of faults (classified by how the rocks on each side move relative to each other):

 

1) Dip Fault- Movement of the rocks is parallel to the dip of the fault plane.

 

    Hanging wall block- The part of the fault above the fault plane.

 

    Footwall block- The part of the fault below the fault plane

 

    Normal fault- A type of dip fault in which the hanging wall moves down relative to footwall.  Normal faults are the products of tensional stress.

 

 

 

    Tension in the Earth’s crust leads to normal faulting and a horst and graben terrain (the Basin & Range topography of the southwestern US).

 

        -Horst- A block of land that is bounded by normal faults and moves upward relative to adjacent blocks.

 

        -Graben- A block of land that is bounded by normal faults and moves down relative to adjacent blocks.

 

    Reverse fault- A type of dip fault in which the hanging wall moves up relative to footwall.  Reverse faults are the result of compressional stress.  A low angle reverse fault is also known as a thrust fault (imagine a subduction zone).

 

 

 

2) Strike-Slip Fault- A type of fault in which rock moves horizontally along the fault plane.

 

 

        -Right-lateral Fault- Looking across the fault, a feature like a stream has been displaced to the right.  The San Andreas Fault in California is a right-lateral strike-slip fault.

 

        -Left-lateral Fault - Looking across the fault, a feature like a stream has been displaced to the left.

 

3) Oblique Fault- A type of fault in which movement is neither pure dip fault nor pure strike-slip fault.  In other words, this type of fault has components of both dip and stike-slip faults.