Assignments

Terms: stress, strain, deformation, tension, compression, shear, anticline, syncline, joint, fault, types of faults (dip-slip, strike-slip fault, normal, reverse, right and left lateral lateral), unconformities (disconformity, nonconformity, angular)

Structural geology is the study of the three-dimenional arrangement of rocks, fractures, and deformational fabrics in rocks.  Rocks that have been compressed and folded have a different structure than those that have not.  Flat-lying sedimentary rocks have a horizontal structure.  Rocks that have been deformed by compression, extension or shearing during tectonic activity will be folded and fractured.   Folds may be gentle, tight or overlapping depending on the extent of deformation.  Fractures, such as faults, many be large and displace the rocks on opposite sides, or they may be small and numerous like joints that displace nothing. Discontinuities in rocks called unconformities formed by erosion are another type of structure.  Structures record geologic events.  They also but also effect rocks erode and the the grain of a landscape.  A landscape underlain by horizontal sedimentary rocks  (i.e. Colorado Plateau) will have a distinctly different topography than one underlain by folded sedimentary rocks (ie.Appalachian Highlands) or faulted rocks (i.e. Basin and Range).

Review of stress and deformation

Simple classification of joints:

1. Shear joints: form at an angle to the direction of maximum compressive stress
2. Tension joints: form parallel to the direction of maximum compressive stress and pependicular to the direction of maximum extension
   

Classification of faults:

Dip slip fault: Motion is parallel to the dip of the fault plane. The classification is based on the relative motion of the hanging and footwall blocks. Dip slip faults typically the result of horizontal extension or compression.

1. Normal faulting occurs with the hanging wall above the fault moves down relative to the footwall below the fault.  Normal fault are formed in response to stretching of the crust, such as along a divergent boundary. 
2. Reverse faulting is when the hanging wall moves up.  Reverse fault are formed by compressing, such as along a convergent boundary.
3. Thrust faults are low angle reverse fault.  They accommodate large amounts of crustal shortening during compression.
   

Dip slip faults in cross section

1. Normal Faults	2. Reverse Fault	3. Thrust Fault

Strike-slip fault: Motion is largely parallel to the strike of the fault plane. Classification is based on the relative displacement of the block across the fault from the observer. Strike slip faults typically result from horizontal shear, particularly that occurring along a transform boundary, such as the San Andreas fault in California.

1. left lateral
2. right lateral

Oblique (diagonal) slip

related terms: hanging wall, foot wall, horst, graben, relative vs. absolute motion

Question: What tectonic environments are these faults associated with? Give examples.

Slide show of recent faults from the NOAA National Data Center

Unconformities - Erosional structures

Unconformities are geologic structures created by erosion that are often difficult to distinguish from faults. An unconformity is an erosional surface between two or more bodies of rock. An unconformity is defined by the nature of the rocks that are juxtaposed.

Type	Description and Significance	Illustration
disconformity	A subtile unconformity between parallel beds of sedimentary rock. Significance: The surface may represent only a few thousand or million years of erosion, such as what might accompany a tempory lowering of sea level.
angular unconformity	Flat lying strata overlie dipping or folded strata. Significance: The underlying strata were deformed and eroded before the overlying beds were deposited. The type of unconformity records a much longer period of erosion in addition to a deformational event.
nonconformity	Metamorphic or plutonic igneous rocks are overlain by sedimentary strata. Significance: Represents a very long period of erosion, perhaps several hundreds of millions of years, during which several kilometers of rocks were removed. Typically an entire mountain belt was created and deeply eroded prior to the depositon of the overlying strata.

Types of mountains

Examples: Hawaiian Islands, Kilimanjaro

Formed in areas of extensional and compressional tectonic activity.Examples:

• The mountains of the Basin and Range region of the United State are bound by normal faults created by crustal extension.
• The Rocky Mountains are composed of fault blocks bound by reverse faults formed by compression.
  

Formed along collision boundaries: Generally composed of both deformed, metamorphosed sedimentary rocks and igneous rocks. Thrust faulting prevalent.

• oceanic-oceanic (Island arc)
• continental-oceanic (Cordilleran -Type orogen)
• continental-continental (Alpine-Type orogen)

Major zones of active orogenic formation

• Alpine-Himalayan Belt (Alps, Himalayas)
• Circum-Pacific Orogenic Belt (Cacades, Aleutian Islands, Japanese Island, etc.)

Mountain belts

Cordilleran-type orogen

Alpine-type orogen - Formation of the Himalayas and Tibetan Plateau (from Understanding Plate Motion - USGS)

Puzzles and Quizzes Interactive crossword puzzle: Deformation and Geologic Structures

Multiple Choice: Test your ability to identify geologic structures in cross section. Matching: Relative age dating and identification of geologic structures and events