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7.7: Geologic Structures - Geosciences

7.7: Geologic Structures - Geosciences


INTRODUCTION

Geologic structures such as faults and folds are the architecture of the earth’s crust. Geologic structures influence the shape of the landscape, determine the degree of landslide hazard, bring old rocks to the surface, bury young rocks, trap petroleum and natural gas, shift during earthquakes, and channel fluids that create economic deposits of metals such as gold and silver.

Folds, faults, and other geologic structures accommodate large forces such as the stress of tectonic plates jostling against each other, and smaller forces such as the stress of gravity pulling on a steep mountainside. An understanding of the structures that shape the earth’s crust can help you see when and where the crust was subjected to pushing or pulling, terrane accretion or crustal rifting.

PHYSICAL BEHAVIOR OF ROCKS: STRESS AND GEOLOGIC STRUCTURES

Before exploring geologic structures, we need to look at how rocks respond to the forces that create the structures. Stress refers to the physical forces that cause rocks to deform. There are three basic types of stress that deform rocks:

  • compression (pushing together)
  • tension (pulling apart)
  • shear (twisting or rotating)

In response to stress, rocks will undergo some form of bending or breaking, or both. The bending or breaking of rock is called deformation or strain.

If rocks tend to break, they are said to be brittle. If a rock breaks, it is said to undergo brittle behavior. If rocks tend to bend without breaking, they are said to be ductile. If a rock bends but is able to return to its original position when the stress is released, it is said to undergo elastic behavior. If a rock bends and stays bent after stress is released, it is said to undergo plastic behavior.

A combination of elastic and brittle behavior causes earthquakes. Rocks get bent in an elastic fashion until they reach their limit, then they break in brittle fashion. The rocks on either side of a break act like rubber bands and snap back into their original shape. The snap is an earthquake. The break along which the rocks slide back to their original shape is a fault.

Earthquakes and faults occur in the shallow crust, where rocks are relatively cold and therefore brittle. In the deep crust and in the earth’s mantle, rocks are very hot and subject to high pressure caused by the weight of the overlying rock. The heat and pressure cause deep crustal and mantle rocks to be ductile. In fact, rocks deep in the continental crust and upper mantle can be so hot and soft that they behave almost like a slow-moving liquid, even though they are actually solid. They “flow,” or bend in a plastic manner, at a geological pace.

Now let us look at the specific types of geologic structures, the breaks and bends that deform rock in response to stress.

FOLDS

Ductile rocks behave plastically and become folded in response to stress. Even in the shallow crust where rocks are cool and relatively brittle, folding can occur if the stress is slow and steady and gives the rock enough time to gradually bend. If the stress is applied too quickly, rocks in the shallow crust will behave as brittle solids and break. Deeper in the crust, where the rocks are more ductile, folding happens more readily, even when the stress and strain occurs rapidly.

Anticlines and Synclines

The most basic types of folds are anticlines and synclines. Imagine a rug, the sides of which have been pushed toward each other forming ridges and valleys – the ridges are “up” folds and the valleys are “down” folds. In terms of geologic structures, the up folds are called anticlines and the down folds are called synclines.

In block diagrams like those shown below, the top of the block is the horizontal surface of the earth, the map view. The other two visible sides of the box are cross-sections, vertical slices through the crust. The colored layers represent stratified geologic formations that were originally horizontal, such as sedimentary beds or lava flows. Use the block diagrams to visualize the three-dimensional shapes of the geologic structures. Keep in mind that erosion has stripped away the upper parts of these structures so that map view reveals the interior of these structures.


In map view, an anticline appears as parallel beds of the same rock type that dip away from the center of the fold. In an anticline, the oldest beds, the ones that were originally underneath the other beds, are at the center, along the axis of the fold. The axis is an imaginary line that marks the center of the fold on the map.

In map view, a syncline appears as a set of parallel beds that dip toward the center. In a syncline the youngest beds, the ones that were originally on top of the rest of the beds, are at the center, along the axis of the fold.

Anticlines and synclines form in sections of the crust that are undergoing compression, places where the crust is being pushed together.

Plunging Anticlines and Synclines

A plunging anticline or a plunging syncline is one that has its axis tilted from the horizontal so that the fold is plunging into the earth along its length. Plunge direction is the direction in which the axis of the fold tilts down into the earth.


In map view, a plunging anticline makes a U-shaped or V-shaped pattern that points, or closes, in the direction of plunge. A cross-section at a right angle to the axis of a plunging anticline looks the same as an anticline.

In map view, a plunging syncline makes a U-shaped or V-shaped pattern that opens in the direction of plunge.

Basins and Domes

A basin is a bowl-like depression in the strata (layers of rock). A basin is similar to a syncline, but instead of an axis it has a single point at the center. The strata all dip toward the center point and the youngest rock is at the center. In map view, the strata form concentric circles – a bull’s eye pattern – around the center point.

A dome is an bulge in strata. A dome is similar to an anticline, but instead of an axis it has a single point at the center. The strata all dip away from the center point and the oldest rock is at the center. In map view, the strata form concentric circles – a bull’s eye pattern – around the center point.

FAULTS

A fault is a planar surface within the earth, along which rocks have broken and slid. Faults are caused by elastic strain that culminates in brittle failure. The rocks on either side of a fault have shifted in opposite directions, called the offset directions. If a fault is not vertical, there are rocks above the fault and rocks beneath the fault.

The rocks above a fault are called the hanging wall.

The rocks beneath a fault are called the footwall.

Normal and Detachment Faults

In a normal fault, the hanging wall has moved down relative to the footwall.


A detachment fault is a particular kind of normal fault that generally dips at a low angle. It separates rocks that were deep in the crust and ductile (granite and gneiss) from rocks of the upper crust (sedimentary or volcanic) that were brittle. Detachment faults occur along the boundaries of metamorphic core complexes (see below).

Normal and detachment faults form in sections of the crust that are undergoing tension, places where the crust is being stretched apart. A divergent plate boundary is a zone of large normal faults. Normal faults also occur in other zones of crustal tension, such as in the Basin and Range landscape region of the western United States.

Reverse and Thrust Faults

In a reverse or thrust fault, the hanging wall has moved up relative to the footwall. The difference between a reverse fault and a thrust fault is that a reverse fault has a steeper dip, more than 30°.


Reverse and thrust faults form in sections of the crust that are undergoing compression. A convergent plate boundary is a zone of major reverse and thrust faults. In fact, subduction zones are sometimes referred to as mega-thrust faults. Reverse and thrust faults also occur in other settings where the crust is being compressed, such as the Transverse Mountain Ranges, just north of Los Angeles.

Strike-Slip Faults

Strike-slip faults are steep or vertical faults along which the rocks on either side have moved horizontally in opposite directions. A transform plate boundary is a zone of large strike-slip faults. The San Andreas fault is an example of a major strike-slip fault at a transform boundary. Strike-slip faults also occur in other settings.

REFLECTION QUESTIONS

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Structural geology

Structural geology deals with the geometric relationships of rocks and geologic features in general. The scope of structural geology is vast, ranging in size from submicroscopic lattice defects in crystals to mountain belts and plate boundaries.

Structures may be divided into two broad classes: the primary structures that were acquired in the genesis of a rock mass and the secondary structures that result from later deformation of the primary structures. Most layered rocks (sedimentary rocks, some lava flows, and pyroclastic deposits) were deposited initially as nearly horizontal layers. Rocks that were initially horizontal may be deformed later by folding and may be displaced along fractures. If displacement has occurred and the rocks on the two sides of the fracture have moved in opposite directions from each other, the fracture is termed a fault if displacement has not occurred, the fracture is called a joint. It is clear that faults and joints are secondary structures i.e., their relative age is younger than the rocks that they intersect, but their age may be only slightly younger. Many joints in igneous rocks, for example, were produced by contraction when the rocks cooled. On the other hand, some fractures in rocks, including igneous rocks, are related to weathering processes and expansion associated with removal of overlying load. These will have been produced long after the rocks were formed. The faults and joints referred to above are brittle structures that form as discrete fractures within otherwise undeformed rocks in cool upper levels of the crust. In contrast, ductile structures result from permanent changes throughout a wide body of deformed rock at higher temperatures and pressures in deeper crustal levels. Such structures include folds and cleavage in slate belts, foliation in gneisses, and mineral lineation in metamorphic rocks.

The methods of structural geology are diverse. At the smallest scale, lattice defects and dislocations in crystals can be studied in images enlarged several thousand times with transmission electron microscopes. Many structures can be examined microscopically, using the same general techniques employed in petrology, in which sections of rock mounted on glass slides are ground very thin and are then examined by transmitted light with polarizing microscopes. Of course, some structures can be studied in hand specimens, which were preferably oriented when collected in the field.

On a large scale, the techniques of field geology are employed. These include the preparation of geologic maps that show the areal distribution of geologic units selected for representation on the map. They also include the plotting of the orientation of such structural features as faults, joints, cleavage, small folds, and the attitude of beds with respect to three-dimensional space. A common objective is to interpret the structure at some depth below the surface. It is possible to infer with some degree of accuracy the structure beneath the surface by using information available at the surface. If geologic information from drill holes or mine openings is available, however, the configuration of rocks in the subsurface commonly may be interpreted with much greater assurance as compared with interpretations involving projection to depth based largely on information obtained at the surface. Vertical graphic sections are widely used to show the configuration of rocks beneath the surface. Balancing cross sections is an important technique in thrust belts. The lengths of individual thrust slices are added up and the total restored length is compared with the present length of the section and thus the percentage of shortening across the thrust belt can be calculated. In addition, contour maps that portray the elevation of particular layers with respect to sea level or some other datum are widely used, as are contour maps that represent thickness variations.

Strain analysis is another important technique of structural geology. Strain is change in shape for example, by measuring the elliptical shape of deformed ooliths or concretions that must originally have been circular, it is possible to make a quantitative analysis of the strain patterns in deformed sediments. Other useful kinds of strain markers are deformed fossils, conglomerate pebbles, and vesicles. A long-term aim of such analysis is to determine the strain variations across entire segments of mountain belts. This information is expected to help geologists understand the mechanisms involved in the formation of such belts.

A combination of structural and geophysical methods are generally used to conduct field studies of the large-scale tectonic features mentioned below. Field work enables the mapping of the structures at the surface, and geophysical methods involving the study of seismic activity, magnetism, and gravity make possible the determination of the subsurface structures.

The processes that affect geologic structures rarely can be observed directly. The nature of the deforming forces and the manner in which the Earth’s materials deform under stress can be studied experimentally and theoretically, however, thus providing insight into the forces of nature. One form of laboratory experimentation involves the deformation of small, cylindrical specimens of rocks under very high pressures. Other experimental methods include the use of scale models of folds and faults consisting of soft, layered materials, in which the objective is to simulate the behaviour of real strata that have undergone deformation on a larger scale over much longer time.

Some experiments measure the main physical variables that control rock deformation—namely, temperature, pressure, deformation rate, and the presence of fluids such as water. These variables are responsible for changing the rheology of rocks from rigid and brittle at or near the Earth’s surface to weak and ductile at great depths. Thus, experimental studies aim to define the conditions under which deformation occurs throughout the Earth’s crust.


Chapter 7: Section 5 - Geologic Time

In this section you will find materials that support the implementation of EarthComm, Section 5: Geologic Time.

Learning Outcomes

  • Develop a model of geologic time using a number of major events in Earth's history.
  • Carry out an investigation to explore the geologic time scale and the use of the biosphere to divide geologic time.
  • Use a model to explain how radioactive decay can be used to determine the age of a rock.

Inquiring Further

  1. To learn more about the development of the geologic time scale, visit the following web sites:

The Geologic Time Scale in Historical Perspective, University of California Museum of Paleontology
Provides a basic history of the contributions made to the geologic time scale.

Geochronology, USGS
Provides an overview of selected methods used by USGS scientists for dating and determining the time sequence of events in the rock record.

Absolute Age, Brooklyn College
Describes methods for making quantitative estimates of the number of years ago an event occurred.

Resources

To learn more about this topic, visit the following web sites:

The Geologic Time Scale

Geologic History, The Virtual Fossil Museum
Looks at the geologic time scale and major events that occurred in different periods.

Geologic Time, USGS
Overview of geologic time, the geologic time scale, and how geologic events are dated.

What is Geologic Time?, National Park Service
Examines subdivisions of geologic time. Includes time scales that are drawn to scale in order to compare the relative lengths of geologic time divisions.

Geologic Timeline, San Diego Natural History Museum
In-depth descriptions of common life forms present in the subdivisions of geologic time.

Dating Rocks Using Radioactive Decay

Radiometric Time Scale, USGS
Looks at the discovery and research of radioactive decay and describes how the process works.

How do geologists date rocks? Radiometric dating!, USGS
Examines radioactive decay as a technique for determining the age of rocks and geological events and processes.

How Do Geologists Know How Old a Rock Is?, Utah Geological Survey
Looks at relative dating and absolute dating techniques, including radioactive decay.

Physics of Radiometric Dating, Tulane University
Principles of absolute dating techniques using radioactive decay. Includes examples of isotope systems used to date geologic materials.


Photos :

Arsenic crystals with Loellingite St. Andreasberg, Harz Mountains, Germany Miniature, 4.7 x 3.1 x 3.0 cm © irocks Löllingite Crovino mine, Susa valley, Piedmont, Italy Specimen weight:30 gr. Crystal size:2 mm Overall size: 50mm x 32 mm x 28 mm © minservice Löllingite Huanggang nr. 1 mine – Chifeng – Inner Mongolia – China Specimen weight:314 gr. Crystal size:mm. 24 Overall size: 72mm x 55 mm x 35 mm © minservice Carlés Mine, Carlés, Salas, Asturias, Spain © JRGL

Stratabound Massive Sulphide Mineralization

Stratabound Massive Sulphide (“SMS”) mineralization occurs in the lower portion of the Main Zone and is crosscut by the Main Quartz Vein.

SMS mineralization consists of banded to semi-massive fine-grained sphalerite, coarse-grained galena and disseminated to massive pyrite. SMS contains only half as much galena, but substantially more iron sulphide/pyrite than typical Main Quartz Vein material. Silver is contained in solid solution within galena.


Reviews

Reviewed by Henry Agbogun, Assistant Professor, Fort Hays State University on 5/3/21

This books covers basic topics in Structural Geology with very good examples of material and processes to enhance comprehension. The book is missing a section focused solely on the behaviors of rocks when subjected to stress as a function of. read more

Reviewed by Henry Agbogun, Assistant Professor, Fort Hays State University on 5/3/21

Comprehensiveness rating: 3 see less

This books covers basic topics in Structural Geology with very good examples of material and processes to enhance comprehension. The book is missing a section focused solely on the behaviors of rocks when subjected to stress as a function of various environments factors. It would have been great to see a capstone chapter discussing the tectonic history a selected region in details. This type of chapter will provide readers with an opportunity to see the overall incorporation of separately discussed topics into one theme to buttress the interrelationships between the various topics.

Content Accuracy rating: 5

Subjects in this book have been accurately presented and discussed without preferences and preconceptions.

Relevance/Longevity rating: 4

This book starts with basic concepts, processes, and materials which represent the building block to understanding the behaviors of rocks when subjected to stress. These fundamental concepts are universal and applicable in advance structural geology. While online resources in the area of stereographic projection is mentioned, it would have been more beneficial if it had been elaborated on in greater details. This is because students are increasingly having to learn a great deal virtually, and are more dependent on various types of apps for learning activities.

Concepts, processes and materials have been succinctly presented and discussed throughout this book.

The terminologies, conventions and units used in this books are consistent and are maintained throughout.

The subsections within each section are not listed in the table of content, neither have they been listed at the beginning of sections. This presents a challenge for readers as they are unable to plan what to read without reading through the whole book or section. The subsections have also not been numbered, thereby presenting a difficulty to readers in organizing how and where to read or in locating particular subsections in the book.

Organization/Structure/Flow rating: 3

The sections and subsections in this book follow a logical progression with each being very concise. However, a section discussing the responses of rocks to stress should have been included and discussed before the discussion on Folds.

The table of content is not elaborate and detailed enough to include subsections. The sections and subsections are not labeled appropriately, this presents navigational issues in locating topics without going through the whole book. A couple of images are also not very clear either due to the quality or resolution of the pictures used.

Grammatical Errors rating: 4

This is an easy to read book with the use of simple sentences and appropriate grammar.

Cultural Relevance rating: 5

This book is not culturally offensive and examples of geological formations and features have been presented from different parts of the world.

This is a well written book missing two important sections already pointed out. The table of content will benefit from the inclusion of subsections that are appropriately and sequentially labeled or numbered to enable easy navigation by readers. The laboratory exercises in this book of particular very useful in introducing students to techniques in structural geology.


7.7: Geologic Structures - Geosciences

All articles published by MDPI are made immediately available worldwide under an open access license. No special permission is required to reuse all or part of the article published by MDPI, including figures and tables. For articles published under an open access Creative Common CC BY license, any part of the article may be reused without permission provided that the original article is clearly cited.

Feature Papers represent the most advanced research with significant potential for high impact in the field. Feature Papers are submitted upon individual invitation or recommendation by the scientific editors and undergo peer review prior to publication.

The Feature Paper can be either an original research article, a substantial novel research study that often involves several techniques or approaches, or a comprehensive review paper with concise and precise updates on the latest progress in the field that systematically reviews the most exciting advances in scientific literature. This type of paper provides an outlook on future directions of research or possible applications.

Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to authors, or important in this field. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.


Sample pages

The present volume provides a comprehensive overview of the geology of the Antarctic continent. It is principally based on the meagre 1% of ice-free area of Antarctica and geophysical data. The only previous, fairly comprehensive treatment of Antarctic geology was published more than 25 year ago. Since then, our knowledge about the geology of the continent has increased immensely despite the km-thick ice shield, which covers large parts of Antarcticas geology, particularly at its center.

An outstanding feature of this book is that it combines the present knowledge in just one single volume.

Following an introduction with a geographic outline and a general synopsis of Antarctic geology, individual chapters describe the regional geology of the seven main physiographic regions of Antarctica in detail:
-the Antarctic Peninsula,
-West Antarctica (Marie Byrd Land and Enderby Land),
-Transantarctic Mountains,
-the Shackleton Range and its surroundings (including the Bertrab, Littlewood and Moltke Nunataks),
-Dronning Maud Land,
-Lambert Glacier and the area surrounding it,
-East Antarctica from Kaiser-Wilhelm-II.-Land to George V Land/Terre Adélie.

All seven chapters were written by acknowledged specialists in their field. Each chapter contains a topographic, historical and geological overview, a description of the respective geological units, their stratigraphy and related data and the tectonic structure of the respective region. Mostly, the findings are placed in a continent-wide/plate tectonic/geological context. The book closes with chapters on the mineral resources and the palaeontological record of the Antarctic continent.

Even if the Antarctic Treaty System prohibits prospection and any exploitation of mineral resources in Antarctica at this time, occurrences of iron, other metal and coal deposits are known, and many more do probably exist. The mineral resources chapter does not just deal with known and presumed deposits, but also highlights environmental problems and relevant international treaties, economic issues and practical or general problems. The Antarctic ice is the largest fresh water resource on Earth.

All nine authors are or have been active field geologists in the corresponding Antarctic regions and are affiliated with American, British, French and German research institutions.

The book addresses researchers, students of geosciences, geologists and all other scientists interested in Antarctic science in general.


Abstract

Interpretation of faults in seismic images is central to the creation of geological models of the subsurface. The use of prior knowledge acquired through learning allows interpreters to move from singular observations to reasoned interpretations based on the conceptual models available to them. The amount and variety of fault examples available in textbooks, articles and training exercises is therefore likely to be a determinant factor in the interpreters' ability to interpret realistic fault geometries in seismic data. We analysed the differences in fault type and geometry interpreted in seismic data by students before and after completing a masters module in structural geology, and compared them to the characteristics of faults represented in the module and textbooks. We propose that the observed over-representation of normal-planar faults in early teaching materials influences the interpretation of data, making this fault type and geometry dominant in the pre-module interpretations. However, when the students were exposed to a greater range in fault models in the module, the range of fault type and geometry increased. This work explores the role of model availability in interpretation and advocates for the use of realistic fault models in training materials.


Spring Term, 2013

Instructions for the Field Site Reports will be given in class on 3/15/13. Sites will be assigned in class on March 15, 2013. Papers are due 4/5/13, and late work is unacceptable . Information on Compressing Large Digital Images is here (thanks to Dr. Hindle).

Examination

Examination Key - Class average 84.4% - Class average advanced students -82.7% Class average First year students - 85.7%. A very severe case of SENIORITIS for some students. Grades are posted on Blackboard. For those students in petrology exams will be returned Wednesday. Everyone else please stop by my office when you bring your field gear in.

Virtual Field Trips

Additional Resources

The background of students in this course is quite varied. The resources listed below may be useful to students who have not yet had certain courses.

Background Geology Information

Pamela J. W. Gore, (March 20, 2002). Crustal Deformation and Folds, http://facstaff.gpc.edu/

pgore/geology/geo101/crustaldeform.php, (last seen January 7, 2013). Dr. Gore is a professor at Georgia Perimeter College, Clarkston, GA, and has prepared some of the best instructional geology web pages available on the Internet.

Pamela J. W. Gore, (July 17, 1999 ). Faults, http://facstaff.gpc.edu/

pgore/geology/geo101/faults.htm, (last seen January 7, 2013).

Erwin J. Mantei, (August 31, 2005). Geologic Structures, http://courses.missouristate.edu/EMantei/creative/glg110/GeoStruct.html, (last modified November 15, 2006). An excellent overview of structural geology terminology, with many diagrams and links to other useful sites. If you haven't had structural geology yet, you should visit this site. In particular, you should review the idea of faulting, especially thrust faulting, (last seen January 10, 2007).

Ray Sterner, (November 9, 2005). Color Landform Atlas of the United States, http://fermi.jhuapl.edu/states/states.html, (last seen January 11, 2005). Links to various images of all fifty states. Image types include satellite images, and various types of maps, (last seen January 13, 2006).

Birth of the Mountains USGS publication describing the formation of the Southern Appalachian Mountains, by Sandra H.B. Clark (December 1, 2001). (last seen January 13, 2006).

References to the Geology of Stone Mountain, Georgia

Geologic Map of Georgia (May 16, 1998) Large scale map of Georgia showing various geomorphic provinces (last seen January 7, 2013).

Geologic Terms relevant to Stone Mountain (April 23, 2002) Short list of definitions pertinent to Stone Mountain (last seen January 13, 2006).

A Stone Mountain Photo Album, Bill Witherspoon, (April 23, 2002). (last seen January 13, 2006).

Geology of Stone Mountain, Georgia Written by Professor Pamela Gore, this is an online version of a guidebook for the Southeastern Section, Geological Society of America Field Trip Athens, GA September 6, 2004 (last seen January 7, 2013).

Stone Mountain Field Trip (August 18, 2004) Photos from a field trip to Stone Mountain, (last seen January 13, 2006).

References to the Geology of Eastern Tennessee.

Cades Cove Geologic Mapping Project , Scott Southworth, (June 7, 2005) This USGS site has links to a number of useful references, including geologic maps and photos (with descriptions) keyed to the maps.(last seen January 10, 2007).

Geologic Map of the Great Smoky Mountains National Park Region, Tennessee and North Carolina, USGS, (September 21, 2005) Links to the map file, report text, and an explanation of the map units (last seen January 10, 2007).

Geology of the Ocoee Whitewater Center, Cherokee National Forest (August 17, 1999) USGS description of the local geology - They have a short geology tour, with map and descriptions, here (last seen January 13, 2006).

Copper Basin Reclamation Project by Patricia Clark, University of Tennessee (April 19, 2005)- documents efforts to restore Polk County, TN (last seen January 13, 2006).

MINERALS AND MINING OF THE COPPER BASIN" by Kim Cochran , (April 8, 2002) A good overview of the minerals found in the Copper Basin, the geologic history, and the mining history of the region. There is also a very good summary of environmental damage and remediation. (last seen January 13, 2006).

Ocoee Dams - (April 26, 2005) Information from the TVA about each dam (last seen January 13, 2006).


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