Download Essentials of Geology, 3rd edition

Crags,
Cracks, and
Crumples
Mountain Belts

Mountains frequently occur in elongate, linear belts.

Mountains are constructed by tectonic plate
interactions in a process called orogenesis.
Mountains

Mountain building involves…

Deformation.

Jointing.

Faulting.

Folding.

Partial melting.

Foliation.

Metamorphism.

Glaciation.

Erosion.

Sedimentation.

Constructive processes build mountains up…

Destructive processes tear them back down again.
Orogenic Belts

Mountains are born and have a finite lifespan.
 Young
mountains are high, steep and growing upward.
 Middle-aged
 Old-age
mountains are dissected by erosion.
mountains are deeply eroded and often buried.
Deformation

Orogenesis causes deformation, consisting of…

Bending.

Breaking.

Tilting.

Squashing.

Stretching.

Shearing.

Deformation is a force applied to the rocks.

Change in shape via deformation is called strain.

Structural geology is the study of rock deformation.
Deformation

Deformation strain creates geologic structures.
 Joints
– Fractures that have no offset.
 Folds
– Layers that are bent by slow plastic flow.
 Faults
– Fractures that are offset.
 Foliation
– Planar metamorphic fabric.
Deformation

Deformed terrane passes into undeformed terrane.
 Undeformed
(unstrained).
Horizontal beds, spherical sand grains, no folds or faults.
 Deformed
(strained).
Tilted beds, metamorphic alteration, folding and faulting.
Deformation

Deformation results in one or all of the following...
 Translation
 Rotation
– Change in orientation.
 Distortion

– Change in location.
– Change in shape.
Deformation is often easy to see.
Strain

Changes in shape caused by
deformation.
 Stretching
– Pulling apart.
 Shortening
– Squeezing
together.
 Shear
– Sliding past.
Deformation Types

Two major deformation types: Brittle
and ductile.
 Deformation
type depends on
temperature and pressure.
 Brittle
deformation – Rocks break by
fracturing.
Deformation Types

Ductile deformation – Rocks
deform by flow and folding.
Ductile deformation occurs
in the deeper crust.

A transition between the two
occurs at 10 to 15 km depth.
Causes of Deformation

Strain is the result of deformation, but what causes it?
 Caused

by force acting on rock; known as stress.
Stress is the force applied per unit area across an area.
A
large force per area results in much deformation.
A
small force per area results in little deformation.
Causes of Deformation

Types of stress:
 Compressional
 Tensional
 Shear

– Squeezing.
– Pulling apart.
– Sliding past.
Tectonic collision produces
horizontal compression.
 Large
 Most
scale.
common type of
deformation.
Stress

Compression – Squeezing (greater stress in one
direction).
 Tends
to thicken material.
Stress

Extension – Pull-apart (greater stress in one
direction).
 Tends
to thin material.
Stress

Shear – Blocks of rock sliding past one another.
 Crust
is neither thickened or thinned.
Stress

Pressure – An object feels the same stress on all
sides.
Geologic Structures


Geometric features created by
deformation.

Folds, faults, joints, etc.

Often preserve information about
stress fields.
3-D structural orientation is
described by strike and dip.

Strike – Horizontal intersection
with a tilted surface.

Dip – Angle of surface down from
the horizontal.
Measuring Structures

Dip is always…
 Perpendicular
 Measured

to strike.
downslope.
Linear structures measure similar
properties.
 Bearing
 Plunge
– compass direction.
– Angle from the
horizontal.
Joints

Planar rock fractures without offset.

Result from tensional tectonic stresses.

Systematic joints occur in parallel sets.

Minerals can fill joints to form veins.

Joints control weathering of rock.
Faults

Planar fractures offset by movement across the break.

Faults are abundant and occur at a variety of scales.

Faults may be active or inactive.

Sudden movements along faults cause earthquakes.

Faults vary by type of stress and crustal level.

Compression.

Tension.

Shear.

Brittle (shallow).

Ductile (deep).
Faults

Faults may offset large blocks of Earth.

The amount of offset is a measure called displacement.

The San Andreas (below) – Displacement of hundreds of
kilometers.
 The
recently developed
stream is offset ~100m.
Fault Movement


The direction of relative block motion…

Reflects the dominant type of
crustal stress.

Defines the type of fault.
All motion is relative. To help visualize
fault motion…

Imagine that one block is stationary
(fixed in place).

Then, imagine that faulting moves
the other block.
Fault Orientation

On a dipping fault, the blocks are
classified as the…
 Hanging
wall block (above the
fault), and the...
 Footwall
fault).
block (below the
Fault Classification

Fault geometry varies – Vertical, horizontal, dipping.

Relative motion of the offset blocks.
 Dip-slip
– Blocks move parallel to fault-plane dip.
 Strike-slip
– Blocks move parallel to fault-plane strike.
 Oblique-slip
– Combination.
Dip Slip Faults

Sliding is parallel to fault-plane dip.

Thus, blocks move up or down the slope of the fault.

Two kinds of dip-slip fault depend on relative motion.

Reverse Fault – Hanging wall moves up.
Thrust fault (a special type of reverse fault).

Normal fault – Hanging wall moves down.
Normal Fault

Hanging wall moves down relative to the footwall.

Accommodate crustal extension (pulling apart).

The fault below shows displacement and drag-folding.
Reverse and Thrust Faults

Hanging wall moves up the footwall.

Reverse faults – Fault dip is steeper than 35o.

Thrust faults – Fault dip is less than 35o.

Accommodate crustal shortening (compression).
Strike-Slip Faults

Fault motion is parallel to the strike of the fault.

Classified by the relative sense of motion. To find this…


Right lateral – Opposite block moves to observer’s right.

Left lateral – Opposite block moves to observer’s left.
Large strike-slip faults may slice the entire lithosphere.
Fault Recognition

Continuous features are displaced across a fault.

Faults may juxtapose different kinds of rock.

Landscape of human features (streams, fences, etc.).

Scarps may form where faults intersect the surface.

Fault friction motion may bend rocks into drag folds.
Fault Recognition

Shattered fault breccias are preferentially eroded.

Fault motion creates slickensides lineations.

Minerals may grow on fault surfaces due to fluid flow.
Folds

Layered rocks may be deformed into curves called folds.

Folds occur in a variety of shapes, sizes, and geometries.

A special terminology is used to describe folds.

Hinge – Portion of maximum curvature on a fold.

Limb – Less curved “sides” of a fold

Axial plane – Imaginary surface defined by connecting hinges of
successively nested folds.
Folds

Folds often occur in a series.

Folding may result in extremely complex geometries.

Orogenic settings produce large volumes of folded rock.

Deformed rock often experiences multiple events.
Fold Identification

Anticline – Arch-like fold; limbs dip away from the hinge.

Syncline – Trough-like fold; limbs dip toward the hinge.

Anticlines and synclines frequently alternate in series.
Fold Identification

Monocline – A fold like a carpet draped over a stairstep.
 Generated
 These
by blind faults in the basement rock.
faults do not cut through to the surface.
 Instead,
displacement folds overlying sedimentary cover.
Fold Identification

Folds are described by hinge geometry.
 Plunging
fold – Has a hinge that is tilted.
 Non-plunging
fold – Has a horizontal hinge.
Fold Identification

Sheep Mountain, Wyoming, is a plunging fold.
 Resistant
 Weaker
sandstones form ridges.
shales erode away.
Fold Identification

Folds are described by their three-dimensional shape.

Dome – Fold with appearance of an overturned bowl.
Erode to expose old rocks in center and younger rocks outside.

Basin – Fold shaped like a bowl.
Erode to expose young rocks in center and older outside.

Domes and basins result from vertical crustal motions.
Forming Folds

Folds develop in two ways.
 Flexural
folds – Layers slip as stratified rocks are bent.
 Analogous
to shear as a deck of cards is bent.
Forming Folds

Folds develop in two ways.
 Flow
rock.
folds – Form by ductile flow of hot, soft
Forming Folds

Horizontal compression causes rocks to buckle.

Shear causes rocks to smear out.
Forming Folds

When horizontal layers move over step faults, they
fold.

Deep faulting may create a monocline in overlying
beds.
Tectonic Foliation

Foliation develops via compressional deformation.

Flattening – Develops perpendicular to shortening strain.

Sand grains flatten and elongate; clays reorient.

Foliation parallels fold axial planes.
Uplift

Construction of mountains requires substantial uplift.
 Mt.
Everest (8.85 km above sea level).
 Comprised

of marine sediments (formed below sea level).
Lofty mountains are supported by a thickened crust.
Crustal Roots

High mountains are supported by thickened lithosphere.

Thickening is caused by collisional orogenesis.


Average continental crust – 35 to 40 km thick.

Beneath orogenic belts – 50 to 70 km thick.
This thickened crust helps buoy the mountains upward.
Isostasy

Surface elevation represents a balance between two forces:
Gravity and Boyancy

The term isostatic equilibrium describes this balance.

Isostasy is compensated after a disturbance.
 Adding
weight pushes the lithosphere down.
 Removing

weight causes isostatic rebound.
Compensation is slow, requiring asthenospheric flow.
Erosional Sculpting

Mountains reflect a balance between uplift and erosion.

Mountains are steep and jagged due to erosion.

Rock characteristics control erosion.
 Resistant
 Easily
layers form cliffs.
eroded rocks form slopes.
Orogenic Collapse

The Himalayas are the maximum height possible. Why?

There is an upper limit to mountain heights.

Erosion accelerates with height.

Weight of high mountains overwhelms rock strength.
Deep, hot rocks eventually flow out from beneath mountains.
The mountains then collapse downward like soft cheese.

Uplift, erosion, and collapse exhume deep crustal rocks.
Causes of Orogenesis


Mountain building is driven by plate tectonics.

Convergent plate boundaries.

Continental collisions.

Rifting.
Orogenic phases may
last several hundred Ma.

Ancient mountains are
deeply dissected by
erosion.
Causes of Orogenesis

Convergent tectonic boundaries create mountains.

Subduction-related volcanic arcs grow on overriding plate.

Accretionary prisms (off-scraped sediment) grow upward.

Thrust faults stack up on the far side of the orogen.
Causes of Orogenesis

Convergent boundaries.
 Island
fragments of continental
lithosphere can enter trench.
 These
are too buoyant to subduct.
Added to the overriding plate.
Called exotic terranes –
Geologic history differs from
surroundings.
Causes of Orogenesis

Continental collisions.

Oceanic lithosphere can completely subduct.
This closes the preexisting ocean basin.
Brings two blocks of continental crust together.

Buoyant continental crust will not subduct.

Instead, subduction is extinguished.
Causes of Orogenesis

Continent – continent collision…

Creates a broad welt of crustal thickening.
Thickening due to thrust faulting and flow folding.
Center of belt consists of high-grade metamorphic rocks.

Fold and thrust belts extend outward on either side.

The resulting high mountains may eventually collapse.
Causes of Orogenesis

Continental rifting.

Continental crust is uplifted in rift settings.
Thinned crust is less heavy; mantle responds isostatically.
Decompressional melting adds asthenospheric magma.
Increased heat flow from magma expands and uplifts rocks.
Rifting creates linear-fault block mountains and linear basins.
Cratons

Two cratonic provinces.
Shields – Outcropping
Pre-C igneous and
metamorphic rocks.
 Platforms – Shields
covered by layers of
Phanerozoic strata.


A craton is crust that
hasn’t been deformed
in 1 Ga.

Low geothermal gradient; cool, strong, and stable crust.
Cratonic Platforms

Sedimentary rocks covering
Precambrian basement.

Exhibit domes and basins.
 Vertical
crustal adjustment.
 Stresses
transmitted from
active margin to interior.
Modern Orogenesis
Horizontal compression.
 Vertical uplift.


Modern
instrumentation can
measure mountain
growth.

Global positioning
systems (GPS)
measure rates of…