ES 1022 AW5Th Lec9 Deformationwebpdf PDF

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2018‐10‐09

Earth Sciences 1022A: Earth Rocks!

Lecture 9:Deformation

Deformation Rock seems to be a pretty sturdy material. However…every body of rock, no matter how strong, has the potential to be deformed. Deformation ( de = out, forma = form) refers to all changes in the original form of a rock body subjected to stress. Most deformation of Earth’s crust occurs along or near margins of plates lithospheric _______ where movement occurs.

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Stress versus strain Stress: The amount of force applied to a given material. Two main forms: 1. Uniform stress: stress applied to material equally in all directions. 2. Differential stress: Stress applied to material unequally. stress Strain: The observed result of ______ (as revealed by structures produced by deformation)

Types of Differential Stress Undeformed rock body.

Compressional stress (squeezing) : causes rock bodies to shorten and thicken vertically

Tensional stress (stretching) causes rock bodies to lengthen and thin vertically.

Shear stress (slicing motion) change causes rock bodies to ______ shape laterally.

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Stress and Plate Movement Each of these three types of stress predominates at each of the three main types of plate boundaries.

Convergent plate boundary: Lithospheric plates move toward one another (compressive stress) Divergent plate boundary: Lithospheric plates move toward one another (tensional stress) Transform boundary: Lithospheric Plates move parallel opposite one another (shear _________ stress)

How Rocks Deform

Rocks tend to deform in either of two ways: 1. Brittle Deformation: Occurs when rocks behave like a brittle solid and break (producing fractures). 2. Ductile deformation: Occurs when rocks can “____” internally and behave in a more plastic manner (tending to bend and fold instead of breaking).

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Describing the orientation of geologic structures Rock bodies (especially sedimentary strata) that have undergone deformation can be tilted or bent. Also, fractures (e.g. faults are often inclined) When mapping geologic structures, it is important to note the orientation of strata (and other planar features). An important method of describing the orientation of strata is to measure their dip and strike.

Strike and Dip

In this case: We can recognize a dipping plane The intersection of the dipping plane with a horizontal plane defines a line of strike The dip angle is 30O (from the horizontal)

30O

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Adding compass directions The dip direction (compass direction) is 90O (due east) The strike direction (compass direction) is 180O (due south) West (270O)

Note that strike has two possible directions.

East (90O)

30O

As a convention, the strike is read 90O to the left (counterclockwise) of the dip direction. So the strike in this case is 90O - 90 O = 0O

Strike and Dip Symbol All of this information (for this particular example) can be portrayed on a map this way: 30o

Meaning that the surface of this layer:

30o

Dips toward the East (i.e., 90O from North) …at an angle of 30o (relative to the horizontal) …striking North-South (i.e. with a strike direction of 0O North)

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Geologic Structures Now we can consider some common geologic structures formed by deformation: Structures formed by brittle deformation (e.g joints, faults)

Structures formed by ductile deformation (e.g folds)

Brittle Deformation Features: Joints Joints are fractures along which no significant displacement has occurred.

Joints are commonly produced as tension “_______ cracks” by relatively gentle warping of the crust (e.g. dolostone in Niagara Escarpment).

Joints can also develop as cooling features (shrinkage cracks) in lava flows and other igneous rock bodies(e.g. basalt of the Giant’s Causeway, Ireland)

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Brittle Deformation Features: Faults Faults are fractures along which significant displacement has occurred. Two main types of faults: Dip slip faults, involving primarily vertical (but some lateral) movement along dipping fault planes.

Strike-slip faults, involving lateral primarily _______ movement.

Dip-Slip Faults: Some Terminology

The terminology for the “blocks” of rock above and below a dipping fault plane has been handed down by miners (who commonly focussed on mineral deposits associated with fault zones). The “block” of rock above the dipping fault plane is called the “hanging wall” (as it would appear to be “hanging over” the miner. under the dipping The “block” of rock _____ fault plane is called the “footwall” (as it would be under the miner’s feet).

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Main Types of Dip-Slip Faults:

A normal fault is produced when the hanging wall moves down relative to the footwall.

A reverse fault is produced when the hanging wall moves up relative to the footwall.

Normal faults: common at divergent plate margins.

tensional stress predominates, At divergent plate margins where _________ normal faults commonly develop. Bounded by normal faults are downthrown blocks called grabens and upthrown blocks called horsts

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Reverse faults: common at convergent plate margins. compressive stress At convergent plate margins where _____________ predominates, reverse faults commonly develop. In the Canadian Rockies, low-angle reverse faults (called thrust faults) formed in response to compressive stress applied from the west (more on this later).

Strike-slip faults: at transform plate margins.

At transform plate margins, shear stress where _____ predominates, strike-slip faults predominate. The San Andreas Fault in California is a classic example of a strike-slip fault.

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Strike-Slip Faults: For a strike-slip fault, we will assume (for now) that the fault is vertical (so there is no hanging wall or footwall. The blocks move laterally relative to one another. Strike-slip faults are said to be rightlateral or left-lateral depending on the relative movement of the blocks. This example is a right lateral strike slip fault. If you were to stand on one block and look across the fault, the other block would appear to be moving to the right.

Ductile Deformation Features: Folds Rock bodies that experience ductile deformation tend to warp, fold or crinkle. In few cases will folds be directly observed in crosssection. But, by considering the dip and strike patterns (and the relative ages) of strata exposed at the surface, we _______ can deduce their structure under the surface.

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Anatomy of a fold Features of a simple symmetrical fold include: • An axial plane that divides the fold into mirror-image “limbs.” • A fold axis that defines the direction of the crest of the fold. In a symmetrical fold, the axis is horizontal. A symmetrical fold whose fold axis is tilted plunge is said to “______.” As in “dip,” the plunge of a fold has an plunge angle (relative to the horizontal), and a direction (compass direction)

plunge angle

Main Types of Folds If relative ages of strata are known*, we can identify synclines and anticlines. In a syncline (a trough-like structure), strata dip toward the centre of the fold. The youngest strata are in the centre. In an anticline (an arch-like structure), strata dip away from centre of the fold. The oldest strata are in the centre.

anticline syncline

*If relative ages are not known, the terms “synform” and “antiform” are used.

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Plunging Folds

In plunging folds, the same principle of ages apply. In a syncline, the youngest strata are in the centre. In an anticline, the oldest strata are in the centre. When exposed in a horizontal erosion surface, arrowhead-shaped patterns emerge. In a syncline, the arrowhead pattern point away from the direction of plunge. In an anticline, the arrowhead pattern points toward the direction of plunge.

Larger-Scale Features: Basins and Domes Deformation can also produce large circular features • Basins are bowlshaped (with youngest strata in ________ the centre)

Basin

• Domes are hump-shaped oldest (with the ______ strata in the centre) Dome

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Larger-Scale Features: Examples

Michigan Basin The ________ (note that we lie in the northeast portion of this basin)

The Black Hills of South Dakota (note that Mt. Rushmore is formed of old granite in the centre of the dome)

End of Lecture

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