Crustal Structures - Faults
Crust of the Earth
All of the solid earth above Moho - crust
Two distinct types of crust
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Crustal Structures - Faults
Hanging wall - mass of rocks that lie above the shear plane
Normal fault -
Hanging wall goes down
in relation to footwall
Crust is lengthened
Rift zones
Reverse fault
Hanging wall goes up
in relation to shear plane
Crust is shortened
Converging (compression)
zones
Thrust faults
Very low angle shear
plane
Oblique normal fault
Rotation movement
Lateral (strike-slip) fault
Sliding blocks, sideways
movement
Transform plate boundary
Crustal Structures - Folds
Bending of earth strata
Anticlines
Uparching of earth strata
Oldest material in the
center
Synclines
Downward folded rocks
Youngest material is
in the center
Monocline
Single bend to the fold
Domes and basins
Similar to anticlines
and synclines
Elliptical to roughly
circular in shape
"Continental Drift"
Sir Francis Bacon - 1620 - first maps of
the Atlantic Ocean
Noticed parallelism of
opposite shores
Alfred Wegener - 1915
"The Origin of Oceans
and Continents"
Super-Continenent of
Pangea
breaks apart 200,000,000 B.P.
Ocean filling gaps
Wegener's terminology
Pangea - original supercontinent
Panthalassa - large ocean surrounding supercontinent
Laurasia - Europe and North America joined together
Gondwanaland - southern hemisphere portion of supercontinent
Wegener's Evidence
Paleontological - fossils
South America and Africa
Mesosaurus
- freshwater reptile in stratigraphy in both areas
Glossopteris
- plant leaf in both South America and Africa
Rock Types and Structures
Match - Africa and South
America
Wegener matched at shorelines, later Bullard gets better match at edge
of
continental shelf
Virginia and Morocco
Rocks and structure match
Faults, folds, deformation - Appalachians with Scottish highlands
Paleoclimatic evidence
Gondwanaland glaciations
Permian evaporite deposits
Later Evidence
Tectonics
Association of
Active mountain ranges - edge of continents
Earthquakes and volcanic zones
Island arcs
Deep ocean trench
Mid-ocean ridge
Magnetic Studies
Oldest Youngest Oldest
Trench Ridge
Trench
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Each side of the ridge duplicate
Reversals recorded
*Sea floor spreading
Paleomagnetic reversals apply to ocean rocks
Apparent polar wandering - proof that
the continents have moved
North Pole appears to
have changed position
Rocks from N. America
and Eurasia show different North Poles at times in the past
Point to different poles
Impossible!
*North Pole hasn't moved, continents have
changed position
Ocean Floor Sediments
Mid-Ocean Ridge to Deep Ocean Trench
Youngest sediments at
ridge, get progressively older as move towards trench
Thinnest sediments at
ridge, thickest sediments at margins of trench
PLATE BOUNDARIES
1. Converging - Ocean to Ocean plate
Benioff zone outlines subducting plate
Deep focus earthquake is occurring at depth
which should be plastic
Evidence of solid plate
2. Converging Plate Boundary - Continentent to Ocean
3. Converging - Continent to Continent
Suturing, shallow focus earthquakes, granitic
mountain range
Himalayan Mtns. - India/Australian plate
collides with Asian plate - creates Himalayan Mtns.
Destructive earthquakes
in China - 2,000 miles from plate boundary
Diverging (Rifting) Plates
1. Continent-Continent Rifting (Diverging)
2. Ocean - Ocean Divergence (Rifting)
Transform Fault
Sliding plates
One plate grinds past another - friction
Shallow focus earthquakes
Only contact without volcanic activity
San Andreas Fault, Anatolian Fault, Motagua
Fault
Convection - Heat Flow
Heat (convection) cells in mantle/asthenosphere
Lithosphere rests on
asthenosphere
Rising heat cells - plates separate
Sinking heat cells - plates pushed down
into mantle
Measured heat flow
Highest at ridge
Lowest at trench
Radiogenic heat vs. deep heat
Radiogenic heat from
uranium in continental crust (granite) - 30% measured value
Low with oceanic crust
Not involved in moving plates
Deep heat - from convection,
highest under ocean crust
Other thermal plumes
Afar Triangle - numerous
thermal plumes intra African plate
Yellowstone
Pacific Plate contains
numerous thermal plumes
Reunion Islands - similar to Hawaii
Rate of Plate Movement
San Andreas Fault - 5.5 cm/yr
Mid-Atlantic Ridge
Iceland - 1.8 cm/yr;
South Atlantic (Ascension Island) - 3.9 cm/yr
East Pacific Rise - off South America
Most rapid movement -
17.1 cm/yr
Gravity Anomalies
Negative anomalies (less than average
pull of gravity) over margins of deep ocean trench
Less dense material
Negative anomalies also over large mountain
ranges - less dense material
Positive anomalies - stronger than
normal pull of gravity
Mid-Ocean Ridge - dense
rocks of mantle closer to surface
Lost Continents and Alien Terranes
Micro-continent
Seychelles Bank in Indian
Ocean
Granitic crust "floating" over oceanic crust
Broke away from Africa, has not subducted
Beaches of Seychelles Is. - pink sand - indicative of granite
Allochtonous terranes - continental
crust of non-local origins
Indicated by magnetic
studies
Terranes
present in the Cordillera of Western North America
Micro-continents have
collided with the North American continents
Non-oceanic in origin, do not subduct
Added on to continent
Cycles of Plate Tectonics
Numerous cycles of breakup and collision
have preceded Wegener's Pangea
Late Precambrian - continents together in
one land-mass
Break apart during Cambrian
and Ordovician, come back together Devonian through
Permian - reassemble
Pangea
Form Appalachian Mtns.
Cycles of breakup and collision have influence
on biological evolution
Breakup/rifting - continents separate
Milder climate, separation
of forms - genetic drift
Diversity of species
Collisions - continents reassembled
More extreme climate
- land masses together
Species brought together
- competition
Continents reassembled
- times of extinction