Lecture notes - Bill Engstrom: Instructor GLG 101 – Physical Geology

Geologic Structures & Mountain Building

Many of the sedimentary rocks that we know were deposited horizontally are now tilted. And, some

marine sediments are now at high elevations. How do horizontal ocean sediments end up well above

sea level? They had to be moved there somehow. In this section we’ll examine the structures we see

in rocks as a result of the enormous stresses that occur related to crustal movement and mountain

building.

First, we need to understand the difference between topography and geologic structures

Topography = relief or terrain, the three-dimensional quality of the surface, and the identification

of specific landforms. These can be influenced by structures or erosion. In other words, just

because you see a hill, it doesn’t mean that the rock is folded beneath it. The hill may have been

formed by erosion.

Geologic Structures = the three-dimensional distribution of rock units with respect to their

deformational histories (What’s beneath the surface)

Deformation is a general term that refers to all changes in the original form and/or size of a rock body.

Note: Most crustal deformation occurs along plate margins.

Deformation involves Force—that which tends to put stationary objects in motion or changes the

motions of moving objects

• Stress—force applied to a given area (e.g. with the interaction of Earth’s

plates)

• Types of stress

» Compressional (pushing together) stress shortens a rock body.

» Tensional (pulling apart) stress tends to elongate or pull apart

a rock unit.

» Shear (sliding side to side) stress produces a motion similar to

slippage that occurs between individual playing cards when

the top of the stack is moved relative to the bottom.

In what tectonic environments might these stresses occur?

These are important to remember. Although knowing the types of stress is OK, what’s

really important is understanding where they occur and how it fits into the theory of

plate tectonics.

Compressional Stress occurs at Convergent Boundaries (e.g. Chile

and Japan)

Tensional Stress occurs at Divergent Boundaries (e.g. African rift

and mid-Atlantic ridge)

Shear Stress occurs at Transform Boundaries (e.g. San Andreas

Fault)

Strain—changes in the shape or size of a rock body ….caused by stress

Rocks subjected to stresses greater than their own strength begin to

deform by folding, flowing, or fracturing.

Types of Deformation – Strain can be elastic or inelastic

– Elastic deformation (strain) —The rock returns to nearly its original

size and shape when the stress is removed (e.g. like a rubber band)

– Inelastic deformation (strain) - Inelastic means that the material or

rock does not return to its original state or shape. Once the elastic

limit (strength) of a rock is surpassed, it either:

» flows (ductile deformation) or

» fractures (brittle deformation).

Factors Influencing Strain

Strain rate/time - how fast material is deforming

Material/Rock type

Temperature (& confining pressure) conditions

Cold materials - exhibit brittle behavior under most stresses

Hot materials - behave plastically/ductile.

As an example, think of a chocolate bar. If it’s warm in can easily

flow/bend/fold. If it’s cold it will fracture/break.

Folding (like the warm candy bar) is considered to be inelastic

ductile strain, and Faulting (breaking like the cold candy bar) is

inelastic brittle strain

Folding -Inelastic – ductile deformation under compressive stress

During crustal deformation, rocks are often bent into a series of wave-like undulations

called folds.

Most folds result from compressional stresses that shorten and thicken the crust.

How can we describe folds?

• Parts of a fold

– Limbs refers to the two sides of a fold.

– An axis is a line drawn down the points of maximum curvature of each

layer.

– An axial plane is an imaginary surface that divides a fold

symmetrically.

We need to be able to map these features. How is that done?

Mapping Geologic Structures

When conducting a study of a region, a geologist identifies and describes the dominant rock

structures.

• Usually, only a limited number of outcrops (sites where bedrock is exposed at

the surface) are available.

• Work is aided by advances in aerial photography, satellite imagery, and global

positioning systems (GPSs).

Describing and mapping the orientation or attitude (e.g. strike and dip) of a rock layer or fault

surface involves determining the features.

• Strike (trend)

– The compass direction of the line produced by the intersection of an

inclined rock layer or fault with a horizontal plane

– Generally expressed as an angle relative to north

• Dip (inclination)

– The angle of inclination of the surface of a rock unit or fault measured

from a horizontal plane

– Includes both an of inclination and a direction toward which the rock

is inclined.

Here is an illustration of

strike and dip from the

laboratory manual.

Again……I recommend that you look at the Tutorial on the GCC Geology Home Page – left side

Department News & Info

- Faculty Home Pages & Contacts

- Campus Location

- GeoAssist (for help in geology)

- Geologic Time, Structures & Maps Tutorial ---- THIS ONE

What are the common types of folds?

• Anticline—upfolded or arched rock layers (Oldest rock in center & beds dip

away from axial plane)

• Syncline—downfolds or troughs of rock layers (Youngest rocks in center &

beds dip toward axial plane)

• Anticlines and synclines may be either….

– Symmetrical, asymmetrical, recumbent (overturned fold), or plunging

Here are a couple of ideas to help you remember the difference between Anticlines

and Synclines

» Anticline = resembles letter “A” or “Anthill” – upturned fold

» Syncline = resembles a smile beginning with letter “S”, or “Sink”- downturned

fold

Folds form topography (hills and mountains). However, when the “solar engine” gets

to work, the mountains and hills will erode, eventually to a flat surface). So, we need

to look at what the map patterns are after erosion has occurred.

Map patterns (e.g. after erosion of surface) are a “mirror” symmetry of rock units

(stripes of different units on either side of the fold)

• Anticlines – beds

dip away from axial

plane/fold axis &

oldest rock in

center

• Synclines – beds

dip toward axial

plane/fold axis &

youngest rocks in

center

Here is an illustration of anticlines and

synclines from the laboratory manual

Anticlines (and domes) as Petroleum Traps – Economic Significance

Oil and gas are less dense than the water that is also trapped in the rock. As a

result, oil and gas essentially “float” to the top. Wherever there is an anticline, the

oil and gas can be trapped at the top (crest) of the anticlinal flold. Not all anticlines

can be observed on the surface of the Earth. Most early petroleum fields were

found this way. Now, other methods are used to find subsurface (buried) anticlines

in areas where oil and gas might be present. Typically, sound waves are reflected

off buried layers of rock to create a “picture” of the subsurface structures (seismic

waves). These sound waves are also generated during earthquakes. However, for

mapping, devices that “thump” the ground or explosives are used to create the

sound source. We will talk about earthquakes and seismic waves in a future lesson.

Folds can also be tilted – This is also

called “plunging” (not like a plunger

you use for a toilet). The rules we

covered above remain the same, but

now there is an “S” shaped geometry

to the map pattern.

Plunging folds from the laboratory

manual

» S-Shaped Geometry –

This is the map pattern you would see between a series of folds that

occur together.

» Age rules/relationships still hold true

» Plunge of a fold – Angle the fold axis makes relative to horizontal

• Domes (“bulls-eye” map pattern)

– Upwarped displacements of rocks

– Circular or slightly elongated structures

– Oldest rocks are in

the center;

younger rocks are

on the flanks.

– All beds dip away

from the center

The Black Hills of South Dakota is a

good example of a large dome.

Illustration of domes and basins

from the laboratory manual

Basins (“bulls-eye” map

pattern)

– Circular or slightly elongated structures

– Downwarped displacements of rocks

– Youngest rocks are found near the center; oldest rocks are on the

flanks.

– All beds dip toward the center

The Michigan Basin (all of eastern Michigan) is a good example of a large basin.

Monoclines

• Large, step-like folds in

otherwise horizontal

sedimentary strata.

• Formed over fault blocks

The Black Hills, which is a large dome,

is actually flanked (all around the

outside edge) by monoclines. From Tarbuck & Lutgens/Pearson Education

Folds can form as open or tight folds, and can be overturned. However, there is a limit to how far

they can fold. Eventually, the rocks will break. So, let’s look at the features formed by brittle strain

(faults and joints).

Brittle Structures – Faults & Joints

Joints

• Very common rock structures

• A joint is a fracture with no movement.

• Most occur in roughly parallel groups.

• Significance of joints

• Chemical weathering tends to be concentrated along joints.

• Many important mineral deposits are emplaced along joint systems.

• Highly jointed rocks often represent a risk to construction projects.

Faults

• Faults are fractures in rocks, along which appreciable displacement has taken place

(movement).

• Sudden movements along faults are the cause of most earthquakes.

• Classified by their relative movement, which can be horizontal, vertical, or oblique.

Fault Types

Basic fault

types – from

the laboratory

manual

Dip-slip faults

– Movement is mainly parallel to the dip of the fault surface

– May produce long, low cliffs called fault scarps

– Parts of a dip-slip fault include the hanging wall (rock surface above the fault)

and the footwall (rock surface below the fault).

Hanging Wall & Footwall of a Fault – An important concept (illustration from lab manual)

Types of Dip-Slip Faults

Normal faults (aka gravity faults)

» The hanging wall moves down relative to the footwall.

» Accommodate lengthening or extension of the crust

» Most are small with displacements of 1 meter or so.

» Larger scale normal faults are associated with structures called

fault-block mountains.

Important note: Normal faults are formed by tensional stress AND tensional

stress is common in divergent plate environments- net effect is to lengthen

and thin lithosphere.

Horst and Grabens from Normal Faults

Horst (aka range) – This is the upthrown

(higher) block between two normal faults.

Graben (aka basin) – This is the downthrown

(lower) block between two normal faults.

Horsts and Grabens = Basin & Range in the Western

United Satates. In Phoenix, we live in what’s called

the Basin and Range Province. The mountains around

Phoenix are the ranges or horsts, and the valleys are

basins or horsts.

Detachment Fault - A normal fault that dips less than 45 degrees. Horst & Half Grabens are also

created from a series of detachment faults.

Reverse and Thrust faults

» The hanging wall block moves up relative to the footwall

block.

» Reverse faults have dips greater than 45 degrees and thrust

faults have dips less then 45 degrees.

» Accommodate shortening of the crust

» Strong compressional forces

Important note: Reverse faults are formed by compressional stress

AND compressional stress is common in convergent plate

environments- net effect is to shorten and thicken lithosphere.

Thrust Fault = Low Angle Reverse Fault. This is a reverse fault dipping less

than 45°

Strike-Slip Faults

• Dominant displacement is horizontal and parallel to the strike of the fault

• Types of strike-slip faults

– Right-lateral (dextral)—As you face the fault, the opposite side of the

fault moves to the right.

– Left-lateral (sinistral)—As you face the fault, the opposite side of the

fault moves to the left.

Transform faults – a special type of strike slip fault

– Large strike-slip faults that cut through the lithosphere

– Accommodate motion between two large crustal plates

Important note: strike-slip faults are formed by shear stress AND shear stress

is common along transform faults. There is no net shortening or lengthening

of the lithosphere.

The San Andreas Fault System is a good example of a right lateral transform fault system.

Strike slip movements happen over a long period of time….. In short “jerks” causing earthquakes.

In between these movements there is little movement while strain builds.

Oblique-slip faults – combination of strike slip and dip slip

• Most faults have some oblique component

• Most faults tend to be dominantly dip or strike- slip

• We simply classify most faults as if they were completely one way or the other

Special Note: Remember…..Fault motion causes earthquakes- NOT vice versa.

Now that we have covered the basic structures that are found in mountains and valleys, it’s time to

look at how mountains are built.

Several hypotheses have been proposed for the formations of Earth’s mountain belts.

• With the development of plate tectonics, it appears that most mountain

building occurs at convergent plate boundaries.

Mountain building has occurred during the recent geologic past. Here are some examples.

• American Cordillera—the western margin of the Americas from Cape Horn to

Alaska, which includes the Andes and Rocky Mountains

• The Alpine–Himalaya chain

• The mountainous terrains of the western Pacific

Here are some examples of older Paleozoic and Precambrian-age mountains

• The Appalachians

• The Urals in Russia

Orogenesis is the processes that collectively produces a mountain belt.

• Includes folding, thrust faulting, metamorphism, and igneous activity

Mountains created by Continental Collisions

Two lithospheric plates, both carrying continental crust collide

• Continental collisions result in the development of compressional mountains

that are characterized by shortened and thickened crust.

• Most compressional mountains exhibit a region of intense folding and thrust

faulting called a fold-and-thrust belt.

• The zone where the two continents collide is called the suture.

The Appalachian Mountains – older mountain belt

• Formed long ago and substantially lowered by erosion

• Resulted from a collision among North America, Europe, and northern Africa

• Final orogeny occurred about 250 million to 300 million years ago.

The Himalayan Mountains – younger mountain belt

• Collision began about 45 million years ago.

• India collided with Eurasian plate.

• Similar but older collision occurred when the European continent collided with

the Asian continent to produce the Ural Mountains.

Fault Block Mountains

Formed by Tensional Stress

Fault-Block Mountains – formed by tensional stress

Continental rifting causes the formation of fault-block mountains.

• Fault-block mountains are bounded by high-angle normal faults. These faults

flatten with depth.

• Examples include the Grand Tetons of Wyoming and Basin and Range

“province” of the western United States.

Basin and Range Province

• One of the largest regions of fault-block mountains on Earth

• Tilting of these faulted structures has produced nearly parallel mountain

ranges that average 80 kilometers in length.

• Extension beginning 20 million years ago has stretched the crust twice

its original width.

• High heat flow and several episodes of volcanism provide evidence that

mantle upwelling caused doming of the crust and subsequent

extension.

Mountain Building created by Plate Convergence and Subducting Plates

Major features of subduction zones

• Deep-ocean trench—a region where subducting oceanic lithosphere bends and

descends into the asthenosphere

• Volcanic arc—built upon the overlying plate

– Island arc if on the ocean floor or

– Continental volcanic arc if oceanic lithosphere is subducted beneath a

continental block

Subduction and Mountain Building

Island arc mountain building

• Where two ocean plates converge and one is subducted beneath the

other

• Volcanic island arcs result from the steady subduction of oceanic

lithosphere.

– Continued development can result in the formation of mountainous

topography consisting of igneous and metamorphic rocks.

Mountain Belts- Volcanic on Continents

Andean-type mountain building

• Mountain building along continental margins

• Involves the convergence of an oceanic plate and a plate whose

leading edge contains continental crust

– Exemplified by the Andes Mountains (best example)

• Building a volcanic arc – Andean Type (brief explanation of how

volcanoes form in this setting)

– Subduction and partial melting of mantle rock generate primary

magmas.

– Magma is less dense than surrounding rock, so it begins to

buoyantly rise.

– Differentiation of magma produces andesitic volcanism

dominated by pyroclastics and lavas.

• Emplacement of plutons – Andean Type (brief explanation of how

magma creates igneous rock bodies at depth (plutons) and and erosion

forms batholiths in this setting)

– Thick continental crust impedes the ascent of magma.

– A large percentage of the magma never reaches the surface and

is emplaced as plutons.

– Uplift and erosion exposes these massive structures called

batholiths (i.e., the Sierra Nevada in California and the Peruvian

Andes)

– Batholiths are typically intermediate to felsic compositions.

• Development of an accretionary wedge – Andean Type (brief

explanation of the formation of a wedge in this setting through the

accumulation of rocks carried in and deformed by the subducting plate)

– An accretionary wedge is a chaotic accumulation of deformed

and thrust-faulted sediments and scraps of oceanic crust.

– Prolonged subduction may thicken an accretionary wedge

enough so that it protrudes above sea level.

– Descending sediments are metamorphosed into a suite of high-

pressure, low-temperature minerals.

• Forearc basin – Andean Type (brief explanation of basin formation in

this setting)

– The growing accretionary wedge acts as a barrier to sediment

movement from the arc to the trench.

– This region of relatively undeformed layers of sediment and

sedimentary rock is called a forearc basin.

The Sierra Nevada and the Coast Ranges Example – Andean Type

• One of the best examples of an active Andean-type orogenic belt

• Subduction of the Pacific basin under the western edge of the North American

plate

• The Sierra Nevada batholith is a remnant of a portion of the continental volcanic

arc.

• The Franciscan Formation of California’s Coast Ranges constitutes the

accretionary wedge.

Terranes and Mountain Building

Another mechanism of orogenesis - The nature of terranes (different from Terrain-relief or

topography)

• Small crustal fragments collide and merge with continental margins.

• Accreted crustal blocks are called terranes (any crustal fragments whose

geologic history is distinct from that of the adjoining terranes).

• Prior to accretion, some of the fragments may have been microcontinents.

• Others may have been island arcs, submerged crustal fragments, extinct

volcanic islands, or submerged oceanic plateaus.

Accretion and orogenesis

• As oceanic plates move, they carry embedded oceanic plateaus, island arcs, and

microcontinents to Andean-type subduction zones.

• Thick oceanic plates carrying oceanic plateaus or “lighter” igneous rocks of

island arcs may be too buoyant to subduct.

Accretion in Western U.S.

• With the breakup of Pangea

• East Pacific basin (Farallon Plate) subducted under North America

• Results in addition of crustal fragments to western margin of U.S.

Erosional Remnants- Sedona AZ = example

In addition to structural mountains, there are also mountains which are formed through

erosion. In Sedona, there are numerous flat topped hills/mountains which are formed through

the erosion of horizontal sedimentary layers.

8/2011