• Introduction
• How do rocks respond to stress?
• What are the types of folds?
• Anticlines and Synclines
• Plunging Anticlines and Synclines
• Basins and Domes
• Table of Geologic Folds (Opens in separate web page)
• What are the types of faults?
• Normal and Detachment Faults
• Reverse and Thrust Faults
• Strike-Slip Faults
• Oblique-Slip Faults
• Table of Geologic Faults (Opens in separate web page)
• What are some other geologic structures?
• Grabens and Horsts
• Joints
• Metamorphic Core Complexes
• Glossary Terms

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 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 and 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 deeper, in the earth's mantle, rocks are very hot and subject to high pressure caused by the weight of the overlying rock. This heat and pressure causes 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, or stretch, 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 commonly become folded in response to stress. Folding can happen in the shallow crust 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. However, deeper in the crust, where the rocks are more ductile, folding happens more readily.

Refer to this table of folds and how they are symbolized on a geologic map.

Anticlines and Synclines

The most basic types of folds are anticlines and synclines.

Anticlines are "up" folds; synclines are "down" folds. In box diagrams like these, the top of the box 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 layered 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 the map views reveal horizontal slices through the structures.

Anticline	Syncline

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.

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 most commonly form in sections of the crust that are undergoing compression, places where the crust is being pushed together. Crustal compression is commonly the response to stress from more than one direction, which causes tilting as well as folding. In crust that has been tilted, folds "plunge" into the earth's surface.

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 dips into the earth.

Plunging Anticline		Plunging Syncline

In map view, a plunging anticline makes a U-shaped or V-shaped pattern that points in the direction of plunge. (The plunge direction of the fold is the direction in which the axis of the fold dips downward into the earth).

A cross-section across the axis of a plunging anticline looks the same as a cross-section of a non-plunging anticline. However, a cross-section parallel to the axis, such as the right-hand sides of the two block diagrams above, shows the beds dipping downward in the direction the fold is plunging.

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

Basins and Domes

A basin is a downward bulge in the rock layers, like a syncline without an axis. The beds all dip toward the center and the youngest rock is at the center.

A dome is an upward bulge in the rock layers, like an anticline without an axis. The beds all dip away from the center and the oldest rock is at the center.

Follow this link to a table that summarizes folds, including the map symbol for each type of fold.

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Faults

A fault is a planar (relatively flat) surface within the earth, along which rocks have broken and slid. If a fault is not vertical, there are rocks on top of the fault and rocks beneath the fault.

Refer to this table of faults and how they are symbolized on geologic maps.

The rocks on top of 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 slid down while the footwall has slid up along the fault.

Normal Fault

A detachment fault is a particular kind of normal fault with a low dip angle. It separates rocks that were deep in the crust (usually granite and gneiss), which were hot and underwent ductile deformation, from rocks of the upper crust (sedimentary or volcanic) that were cold and brittle. Detachment faults occur along the borders of metamorphic core complexes (see below).

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

Reverse and Thrust Faults

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

Reverse Fault	Thrust Fault

Reverse and thrust faults form in sections of the crust that are undergoing compression. A convergent plate boundary is generally a zone of major reverse and thrust faults, but reverse and thrust faults also occur in other settings where the crust is being compressed.

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 generally a strike-slip fault or zone of strike-slip faults, but strike-slip faults also occur in other settings.

Strike-Slip Fault

Oblique-Slip Faults

An oblique-slip fault is one where the rocks on either side have moved up, down and sideways, rather than simply up and down or simply sideways.

Follow this link to a table that summarizes faults, including the map symbol for each type of fault.

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Other geologic structures

Grabens and Horsts

A section of the crust that has dropped down along two parallel normal faults is called a graben. A block of crust that has been uplifted between two parallel normal faults is called a horst.

Joints

Joints are the most commonly observed structure at the earth's surface. Near and at the earth's surface, rocks break and crack. Most often the rocks on either side of the crack do not slide or shift their position, so the cracks do not qualify as faults. Such cracks or fracture surfaces are called joints.

Joints are present in nearly every outcrop of bedrock at the earth's surface. Because it is common for joints to be relatively straight and flat they may be mistaken for sedimentary beds.

Metamorphic Core Complexes

A metamorphic core complex is a deep section of the crust that has uplifted along detachment faults and become partly exposed at the earth's surface. The rocks in a metamorphic core complex consist largely of metamorphic rock and granite. Metamorphic core complexes form in areas where the crust is undergoing extension, or being pulled apart, all the way down to the deep crust. This extension is occurring at the same time as the deep crust is undergoing metamorphism and igneous intrusion.

Glossary terms that appear on this page: fault; fold; stress; tectonic plate; terrane; compression; tension; shear; strain; brittle; ductile; elastic; plastic; anticline; syncline; cross-section; dip; axis; basin; dome; hanging wall; footwall; normal fault; detachment fault; metamorphic core complex; divergent plate boundary; reverse fault; thrust fault; convergent plate boundary; strike-slip fault; transform plate boundary; oblique-slip fault; graben; horst; joint; metamorphic rock; granite; intrusion

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