Crustal Deformation
Why should we study the deformation of the crust…what can it tell us?
What causes the crust to deform?
How do we know when we are looking at deformed crust?
Evidence for deformation
Folded rock
Faulted rock
Submerged or uplifted shorelines
Marine fossils on mountain tops
Deformed crystals
What causes rocks deform?
Force = Stress
Types of Stress
Differential stress: unequal stress in different directions
Compression, tension and shear
Strain: how the rock reacts to stress
Accumulation of energy in rock due to force
Causes deformation
How do rocks deform?
Plastic deformation
Fold or flow
Brittle deformation
Break…i.e. faulting
Factors that control deformation
Amount of stress
Temperature
Rock Composition
Limestone? Sandstone? Shale?
All behave differently
Strain Rate
How much deformation with time
Confining pressure
High: increases strength of rock
Low: decreases strength of rock
Deformation follows a path
Elastic deformation
Primary creep
Remove stress rock returns to original shape
Plastic deformation
Secondary creep
Slow strain rate; rock folds/flows--non-recoverable
Brittle deformation
Tertiary Creep
Accelerated strain rate…rock fails
Folded structures
Compressional stress
Anticlines & synclines
Plunging anticlines and synclines
Domes: tensional stress
Basins: compressional stress
Faults
Represents Brittle deformation
Crust that has broken
Strain exceeds strength of rock
2 groups of faults
Gravity faults
Shear faulting
How do faults and folds relate to mountain building and plate tectonics?
Vocabulary to know:
Orogenesis
Mountain building
Orogeny
a singe episode of mountain building
Craton or Shield
The oldest and central portion of the continents
What are Mountains?
Folded or faulted crust
Continental Volcanics (more in a bit)
Mountains have “roots”
2/3 of a mountain is “pushing” in to the mantle
Similar to iceberg in water
How do we know this?
Discovered using gravity
All mass exerts a gravitational force on all other mass
The bigger the mass the greater the force
Mountains have a high gravitational anomaly and thin parts of the crust will have a low anomaly.
Gravitational Anomalies
Unequal variations in gravity due to changes in mass
Positive anomalies
High gravitational readings
Caused by high amounts of mass
What causes Positive anomalies?
Plate collisions
Causes crust to thicken
Volcanoes
Adds mass through lava
Ice sheets
Thick sedimentation
Negative anomalies
Low gravitational readings
created by less mass
Sudden removal of surface mass (ice sheet)
Impact crater
Gravitational studies in 1700 & 1800’s discovered the roots of mountains
Gravitational mapping across mountain regions expected a high gravitational reading due to large amounts of mass
Smaller reading than expected
Suggested that denser mantle material is displaced by less dense crust pushing down into the mantle creating the lower than expected reading.
What does this imply?
1) Mountains have deep roots based on the gravity readings.
2) Mountains are a low density rock and float on surrounding denser mantle rocks
3) ~10% of mass is above mantle at anytime
Most of a mountain are roots that we don’t see
Principle of Isostasy
Isostosy of mountains:
An equilibrium of the lithosphere with the asthenosphere.
Mountains are crust floating in a gravitational balance
Affects erosional behavior of mountains
Isostatic adjustment: The process of establishing a new equilibrium as tectonic and earth processes disrupt equilibrium
How do mountains form?
Primarily through plate interaction
Subduction
Collision
Faulting
Thrust
reverse
Folding
Passive continental margins
Trailing edge of a continent (east coast
of
No mountain building or volcanism
Erosion dominates
Active Continental Margins
Leading Edge of a continent
Convergent boundary
Examples?
Why are Passive Margins important to the process of mountain building?
Don’t always stay passive
Collect sediments that will later become part of the continent when collision occurs
Adds Mass
Continental to oceanic collisions
Volcanism and plutonic activity
Accretionary wedges
Often become core and root of new mountains due to thrust faulting and folding
Can melt and create plutons
Can uplift, fold and create a parallel range of mountains
Terrane accretion
Adding
micro-continents onto continental margins
2 types:
Short distance travel
Transform faults
Baja and west side of San Andreas
Seafloor or near shore islands
Long distance travel (>3000 miles)
Exotic terranes or suspect terranes
How do we ID them?
How to ID exotic terranes
Abrupt changes
in stratigraphy
in fossils (types and ages)
Paleo magnitism
Age of rocks
Often broken into segments
Elongated and spread out
West Coast of N. America Created this way
Continental to Oceanic collisions can Eventually can lead to:
Continental to Continental collisions
Mountain building through compression and faulting
Thickens ocean sediment on continental shelf
Creates largest mountain ranges
Lots of metamorphism
Creates lots of faults (What type?)
Mountains that form away from plate boundaries.
How would this occur?
Structures?
Basin and Range
Horsts and Grabens
Create by a series of normal faults.
Major
orogenies in
Trans-Hudson
Wopmay
Grenville
Taconic
Caladonian
Acadian
Antler
Nevadan
Sevier
Larimide