Download Structures, Mountains and Continents

Geologic Structures
Mountains
and
Formation of
Continents
Fig. 13-22c, p. 412
Summary
Geologic structures are the forms that rock layers take when a
stress is applied. They include folds, fractures and faults
Folds are bends; fractures are breaks and faults are breaks
that have moved
Folds, fractures and faults can occur at any spatial scale, from
very small to very big
The spatial orientation of planes such as rock layers is
described using Strike and Dip
Types of faults include normal (extensional), reverse
(compressional) and strike-slip (side to side)
Folds produce structures in the rocks such as monoclines
(simple folds), anticlines (shaped like an A-frame), synclines
(shaped like a sink), domes (shaped like a dome) and basins
(shaped like a bowl)
In Anticlines and Domes the oldest rocks are in the middle
In Synclines and Basins the youngest rocks are in the middle
Mountains and mountain belts are usually formed at plate
margins due to collision
Mountains can be formed due to fold-and-thrust, block uplift,
and igneous and volcanic activity
Continents have grown by continual addition of
microcontinents and island arcs formed in subduction zones
Ancient mountains that form the core of our continent are
exposed in shield areas, generally covered with sedimentary
rocks in the platform, and being added to at the margins in the
mobile/mountain belts
Geologic Structures
• Changes in the shape and/or orientation of rocks in
response to applied stress
Figure 15.19
Can be as big as a „breadbox‟
Or much bigger than a „breadbox‟
Three basic types
Fractures >>> The rocks break but don‟t move
Faults
>>> The rocks break and move
Folds
>>> Rocks don‟t break, but deform
in a ductile way
What forces are
involved?
Stress and Strain
• Stress is force per unit area
– The three basic types of stress are compressive,
tensional and shear
• Strain is a change in size or shape in response to
stress
– Geologic structures are indicative of the type of stress
and its rate of application, as well the physical
properties of the rocks or sediments
Rock Deformation
• Stress is the pressure or force applied to
rocks that cause deformation to occur
• Uniform (confining) stress is equal in all
directions (hydrostatic)
– Rocks are confined by the rock around them
• Differential stress is not equal in all
directions (directional)
– This is what deforms rocks
Rock Deformation
• Three types of differential stress
– Tensional - pulling apart
– Compressional - squeezing together
– Shear - slipping, twisting, or wrenching
• Strain is the change in size and/or
shape of a solid resulting from
applying a stress
Tension and compression
Shear stress
Rock Deformation
• Strain produces a spectrum of
deformation
– Elastic deformation
• Rocks return to original shape
– Ductile deformation
• Irreversible change in size and/or shape
• Volume and density may change
– Brittle deformation - Fracture
• Stress exceeds the ductile limit
• Irreversible break
How Rocks Respond to Stress
• Rocks behave as elastic, ductile or brittle
materials depending on:
– amount and rate of stress application
– type of rock
– temperature and pressure
• If deformed materials return to original shape
after stress removal, they are behaving elastically
• However, once the stress exceeds the elastic limit
of a rock, it deforms permanently
– ductile deformation involves bending plastically
– brittle deformation involves fracturing
Orientation of Geologic Structures
• Geologic structures are most obvious
in deformed sedimentary rocks
• Tilted beds, joints, and faults are planar
features whose orientation is described
by their strike and dip
– Strike is the compass direction of a line
formed by the intersection of an inclined
plane with a horizontal plane
– Dip is the direction and angle from
horizontal in which a plane is oriented
How do we
describe rock
relationships in
nature?
Geometry of Rock Structures
• Structures may be defined by the
orientation of planes
– Dip – the angle of inclination downward
from a horizontal plane
– Strike – the compass bearing of a
horizontal line where the inclined plane
intersects an imaginary horizontal plane
Figure 15.7
Fig. 7.5.
Strike & Dip
Structures and Geologic Maps
• Rock structures are determined
on the ground by geologists
observing rock outcrops
– Outcrops are places where bedrock
is exposed at the surface
• Geologic maps use standardized
symbols and patterns to represent
rock types and geologic structures,
such as tilted beds, joints, faults
and folds
Figure 15.8
Figure 15.9
Geologic Structures:
Fractures and
Faults
Joints
• Fractures created by tension in brittle
rocks
– No displacement has occurred
– Often form as overburden is removed,
confining stress reduced
– Often form during cooling of igneous rocks
– Often occur in sets
Joint
systems
Faults
• Fractures that have been displaced
– Most faults are inclined at some angle
measured from horizontal
• The dip angle of the fault
• Two blocks are defined, one on either side of
the fault: footwall and hanging wall
Fig. 13-15a, p. 401
Fig. 13-15b, p. 401
Fig. 13-15d, p. 401
Fig. 13-15c, p. 401
Fig. 13-16, p. 402
Fig. 13-16b, p. 402
Fig. 13-16a, p. 402
Fault Types
• Faults may be divided into three categories
– Normal faults
• Hanging wall moves down relative to foot
wall
• Block slides down the dip angle
– Reverse faults
• Hanging wall moves up relative to foot wall
• Block moves in the reverse direction to what
seems normal
Fault Types
– Strike slip faults
• Displacement to sideways in a horizontal
direction
• Strike is the direction of the line formed by
the intersection of the fault plane with the
Earth‟s surface
• Movement is parallel to the strike of the fault
plane
Major types of faults
Normal Faults
• Normal faults are created by tensional
forces, i.e. pulling apart
– Rifts are created by parallel normal faults
dipping toward each other
• The block in the center which drops down is
a graben
• The Rio Grande valley in New Mexico is a
rift graben
p. 420
A special type of normal fault
– Fault blocks, bounded by normal
faults, that drop down or are
uplifted are known as grabens and
horsts, respectively
• Grabens associated with divergent
plate boundaries are called rifts
Normal faults produce grabens & horsts
Extensional (Normal) Faults
reviewed
Reverse Faults
• Compressional stress usually causes
reverse faults to form
– Reverse faults are common at convergent
plate boundaries
– Reverse faults cause a thickening of the
crust as rocks are piled up
– Older rocks may be found above younger
rocks
Reverse Faults
• Thrust faults are a special kind of
reverse fault
– Shallow dip angle, > 45o
– Common in large mountain ranges
– Horizontal displacement may be many
tens of kilometers
– Evidence of thrust faults in sedimentary
rocks is seen when a sequence of the
same rocks are repeated
Strike-Slip Faults
• Strike-Slip faults
– Principle movement is horizontal
• Left or Right Lateral
• Little or no vertical movement
– Caused by shear stress
– Indicated by abrupt changes in drainage
patterns
Concept Art, p. 411
Strike-slip faults offset drainage
Strike-slip faults
Types of Faults
• Strike-slip faults have movement that is
predominantly horizontal and parallel to
the strike of the fault plane
– A viewer looking across to the other side of a
right-lateral strike-slip fault would observe it
to be offset to their right
– A viewer looking across to the other side of a
left-lateral strike-slip fault would observe it
to be offset to their left
• Oblique-slip faults have movement with
both vertical and horizontal components
Right-lateral San Andreas Fault
F aulitng.exe
Movement Along Faults
• Rarely exceeds a few meters in a single
event
• Small movements, cm scale, may
occur on a regular basis
– Tectonic creep
• Total displacement may be km, but
does not occur in a single event
Geologic Structures:
Folds
Folds
• Folds are wavelike bends in layered rock
– Represent rock strained in a ductile manner,
usually under compression
• The axial plane divides a fold into its
two limbs
– The surface trace of an axial plane is called
the hinge line (or axis) of the fold
• Anticlines are upward-arching folds, and
synclines are downward-arching folds
Folds
• Warps in rock strata due to ductile
deformation
– 3-D structures of wide ranging scale
– Generally indicate horizontal compression
– Multiple generations of folding may exist
Folds
• Three simple fold forms exist
– Synclines warp downward
– Anticlines warp upward
– Monoclines dip in one direction
Folds
• Folds are described by:
– The strike of their hinge line
• The hinge line is the intersection of the hinge
plane with the folded layer
• Hinge lines may be inclined in a plunging
fold
– The angle of dip of their limbs
Fold geometry
Types of folds
Anticlines & Synclines
• The sequence of ages of strata indicate
the geologic structure in folds
– Anticlines have the oldest layers exposed at
the center of the fold along the axial plane
– Synclines have the youngest strata exposed
at the center along the axial plane
A series of anticlines & synclines
Fig. 13-9, p. 396
Fig. 13-7, p. 395
Fold Belts
• Orogenic belts consist of long linear
series of folds
– Fold geometry is not overly complex
– Pattern of outcrops may appear complex
– Complex folds may develop as folds are:
• Re-folded
• Cut by thrust faults
Orogenic belt with complex folding
Complex Folds
• Folds may be very complex
– Application of shear stress
– Multiple folding events
– Complex forms are created
A plunging anticline
Types of Folds
• Plunging folds are folds in which
the hinge line is not horizontal
– Where surfaces have been leveled by erosion,
plunging folds form V- or horseshoe-shaped
patterns of exposed rock layers (beds)
• Open folds have limbs that dip gently, whereas isoclinal folds
have parallel limbs
• Overturned folds have limbs that dip in the same directions, and
recumbent folds are overturned to the point of being horizontal
A monocline
Fig. 13-6b, p. 394
F olds.exe
Fig. 13-1, p. 390
Fig. 13-11d, p. 398
Complex Folds
• Domes & Basins
– Generally occur in continental interiors
– Broadly warped regions
– Roughly circular pattern of outcrops
Structural Domes and Basins
• Domes are structures in
which the beds dip away
from a central point
– Sometimes called doubly
plunging anticlines
• Basins are structures in
which the beds dip toward
a central point
– Sometimes called doubly
plunging synclines
Fig. 13-13b, p. 400
Fig. 13-13a, p. 400
A small dome
Box 15.1 Figure 1a
Fig. 13-13c, p. 400
Black Hills Uplift
The Michigan Basin
Complex Folds
• Diapirs
– Less dense salt layers may rise up
– Some overlying strata may be pierced
– Salt diapir has an inverted teardrop shape
– Strata above diapir are domed upward
Complex Folds
• Plunging folds occur when the folds axis is
dipping or plunging
• Limbs of Asymmetrical folds are not the
same, one dips more steeply than the other
• Overturned and Recumbent folds occur
when folding is so extreme that beds are
turned upside-down
Fig. 13-12, p. 399
Fig. 13-12a, p. 399
Fig. 13-12b, p. 399
Fig. 13-12c, p. 399
Making Mountains
Mountain Belts
• Mountain belts are chains of
mountain ranges that are
1000s of km long
– Commonly located at or near the
edges of continental landmasses
– Usually formed by tectonic and
volcanic processes associated
with plate collision
– The tectonic processes include
folding and faulting
– The igneous processes included
volcanic eruptions and
emplacement of granitic
batholiths
Characteristics of Mountain Belts
• Mountain belts are very long compared to their width
– The North American Cordillera runs from southwestern
Alaska down to Panama
• Older mountain ranges (Appalachians) tend to be lower
than younger ones (Himalayas) due to erosion
– Young mountain belts are tens of millions of years old, whereas
older ones may be hundreds of millions of years old
• Ancient mountain belts (billions of years old) have
eroded nearly flat to form the stable cores (cratons) of
the continents
– Shields - areas of cratons laid bare by erosion
Fig. 13-19, p. 407
6
6
7 5
2
1
2
?
10
9
8
34
Rocky Mountain Fold and
Thrust Belt
A
B
C
The Plains
Central Rocky Mountain Foreland Province
(Uplifted Blocks)
A
B
Fold and Thrust
B
C
Igneous Intrusions and
Eruptions
Mt/ID
Border
A
Northern Montana E-W
cross section
B
MT/ND
Border
C
Northern Wyoming E-W cross
Idaho/Wyoming Border
Black Hills,
section
S.D.
Central Montana N-S cross
Northern MT
Northern WY
section
Fig. 13-26, p. 417
Fig. 13-26a, p. 417
Fig. 13-26b, p. 417
Fig. 13-26c, p. 417
Mesozoic
Batholiths
Tectonic Events in Western U.S.
• Sundance Sea
– Retreated as it filled
with sediments
• Morrison Formation
• Reddish river
sediments. Famous for
the dinosaur fossils
Oregon/Washington
Cascades/Olympics
Idaho
Rockies
Structure of western NA
Montana
Rock Patterns in Mountain Belts
• Mountain belts typically contain thick
sequences of folded and faulted
sedimentary rocks, often of marine origin
– May also contain great thicknesses of
volcanic rock
• Fold and thrust belts (composed of many
folds and reverse faults) indicate crustal
shortening (and thickening) produced by
compression
– Common at convergent boundaries
– Typically contain large amounts of
metamorphic rock
Orogeny
Caused by:
1. Collision - Fold and Thrust Mountains
Orogeny
Caused by:
2. Subduction - Volcanic (Arc) Mountains
Fig. 13-22a, p. 412
Fig. 13-22b (1), p. 412
Fig. 13-22b (2), p. 412
Fig. 13-22c, p. 412
Rock Patterns in Mountain Belts
• Erosion-resistant batholiths and rock layers can form
hogbacks and mountains
• Localized tension in uplifting mountain belts can
result in normal faulting
– Horsts and grabens can produce mountains and valleys
• Earthquakes common along faults in mountain ranges
Northern Rocky Mountains
Young folded and faulted mountains eventually become …
Become old folded and faulted mountains/hills
Evolution of Mountain Belts
• Rocks (sedimentary and volcanic)
that will later be uplifted into
mountains are deposited during
accumulation stage
– Typically occurs in marine environment,
at opening ocean basin or convergent
plate boundary
• Mountains are uplifted at convergent
boundaries during the orogenic stage
– Result of ocean-continent, arc-continent, or
continent-continent convergence
– Subsequent gravitational collapse and
spreading may bring deep-seated rocks to the
surface
Growth of Continents
• Continents grow larger as mountain
belts evolve along their margins
– Accumulation and igneous activity add
new continental crust
• New accreted terranes can be added
with each episode of convergence
– Western North America (especially
Alaska) contains many such terranes
– Numerous terranes, of gradually
decreasing age, surround older cratons
that form the cores of the continents
North
American
Craton
Shield
Western North
American
Mobile Belt
Platform
Mobile
Belt
Eastern North
American
Mobile Belt
Orogenic
(Mobile) Belt
Basement Rock
Anatomy of a Continent
Heat Generated by Radioactive Decay
Hadean
Earth was
much hotter!
Convection
was more
intense
Continental Evolution
Up to 60% by the Proterozoic
Precambrian Cratons:
Shields, Platforms and Mobile Belts
As Plates Move - Islands “Accrete”
to Form Larger Islands
pillow lavas
http://www.pmel.noaa.gov/vents/nemo/explorer/concepts/pillow_lava.html
Tonalites
(Granites)
Felsic intrusions, slow
recycling of sediments
creates less dense
materials that stabilize
the continental crust
Greenstones
Layered volcanics (pillow basalts; MORBs),
conglomerates and greywackes (turbidites,
mudstones and shales formed from volcanic
sediments in deep water. More felsic at the top.
Green due to chlorite formed during
metamorphism.
Ocean-Continent convergence
Fig. 13-20a, p. 408
Fig. 13-20b, p. 408
Fig. 13-20c, p. 408
Fig. 13-20d, p. 408
Accreted terranes
along convergent
margin
Fig. 13-23, p. 413
Early Formation of ‘Continental’ Crust
Fig. 13-24a, p. 414
Early Formation of ‘Continental’ Crust
Fig. 13-24b, p. 414
Early Formation of ‘Continental’ Crust
Fig. 13-24c, p. 414
Accumulation of Island Arcs to form
first continents
Fig. 13-24d, p. 415
Accumulation of Island Arcs to form
first continents
Fig. 13-24e, p. 415
Accumulation of Island Arcs to form
first continents
Fig. 13-24f, p. 415
Growth of North American Continent
A-2
Basement
Provinces
of North
America
GF
“The United Plates
of America”
Archean Basement
(>2.5 billion years)
Proterozoic Basement
(2.5-0.5 billion years)
Mobile Belts
A-2
Northwest
Basement
Wyoming Provenance
2.8-3.5 Gyrs old
Includes rocks found in
Beartooth Mtns and
Grand Teton National Park
Hearne Provenance
2.8-3.5 Gyrs old
Collision Zone:
Great Falls Tectonic
Zone (GF)
2.8 Gyrs ago
GF
The Beartooth valley: Precambrian rocks uplifted during the late
Cretaceous, glacially eroded in the Holocene, still eroding today
Fig. 13-25a, p. 416
Fig. 13-25b, p. 416
Fig. 13-25c, p. 416
Evolution of Shields
• Shields are the eroded remnants of
folded mountain belts
– Isostatic adjustment and erosion have
nearly reached equilibrium
– Local relief is usually < 100 m
Canadian Shield
• The Canadian Shield is typical of
shields worldwide
– Covers 1/4 of NA, over 3 million km2
– Basic structural features well exposed
• Only relief is resistant rocks up to 100 m
above adjacent surface
– Evidence shows the core of several
different mountain belts
Where Rocks of the Proterozoic are
exposed
Uplifted Exposed Precambrian Rocks
Basement
Canadian Shield