Plattentektonik

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Plattentektonik

• Institut für Geowissenschaften Universität Potsdam

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Übersicht zur Vorlesung
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Plattentektonik
Ozeane
Kontinente
3 Typen von Plattengrenzen
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(No Transcript)
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Earths Plates
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Divergent boundaries are located mainly along
oceanic ridges
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Divergent plate boundaries

• Oceanic ridges and seafloor spreading
• Seafloor spreading occurs along the oceanic ridge
  system
• Spreading rates and ridge topography
• Ridge systems exhibit topographic differences
• Topographic differences are controlled by
  spreading rates

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Ridge morphology

• Faster spreading ridges are characterized by
• more volcanism
• smoother topography - less faulting
• fewer moderate earthquakes
• Slower spreading ridges are characterized by
• less volcanism
• rough topography - more extension by faulting
• more moderate sized earthquakes

The differences are related to temperature.
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Divergent plate boundaries

• Spreading rates and ridge topography
• Topographic differences are controlled by
  spreading rates
• At slow spreading rates (1-5 centimeters per
  year), a prominent rift valley develops along the
  ridge crest that is wide (30 to 50 km) and deep
  (1500-3000 meters)
• At intermediate spreading rates (5-9 cm per
  year), rift valleys that develop are shallow
  with subdued topography

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Divergent plate boundaries

• Spreading rates and ridge topography
• Topographic differences are controlled by
  spreading rates
• At spreading rates greater than 9 centimeters per
  year no median rift valley develops and these
  areas are usually narrow and extensively faulted
• Continental rifts
• Splits landmasses into two or more smaller
  segments

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Divergent plate boundaries

• Continental rifts
• Examples include the East African rifts valleys
  and the Rhine Valley in northern Europe
• Produced by extensional forces acting on the
  lithospheric plates
• Not all rift valleys develop into full-fledged
  spreading centers

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The East African rift a divergent boundary on
land
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Convergent plate boundaries

• Older portions of oceanic plates are returned to
  the mantle in these destructive plate margins
• Surface expression of the descending plate is an
  ocean trench
• Called subduction zones
• Average angle at which oceanic lithosphere
  descends into the mantle is about 45?

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Convergent plate boundaries

• Although all have the same basic
  charac-teristics, they are highly variable
  features
• Types of convergent boundaries
• Oceanic-continental convergence
• Denser oceanic slab sinks into the asthenosphere

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Convergent plate boundaries

• Types of convergent boundaries
• Oceanic-continental convergence
• As the plate descends, partial melting of mantle
  rock generates magmas having a basaltic or,
  occasionally andesitic composition
• Mountains produced in part by volcanic activity
  associated with subduction of oceanic lithosphere
  are called continental volcanic arcs (Andes and
  Cascades)

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An oceanic-continental convergent plate boundary
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Convergent plate boundaries

• Types of convergent boundaries
• Oceanic-oceanic convergence
• When two oceanic slabs converge, one descends
  beneath the other
• Often forms volcanoes on the ocean floor
• If the volcanoes emerge as islands, a volcanic
  island arc is formed (Japan, Aleutian islands,
  Tonga islands)

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An oceanic-oceanic convergent plate boundary
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Convergent plate boundaries

• Types of convergent boundaries
• Continental-continental convergence
• Continued subduction can bring two continents
  together
• Less dense, buoyant continental lithosphere does
  not subduct
• Result is a collision between two continental
  blocks
• Process produces mountains (Himalayas, Alps,
  Appalachians)

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A continental-continental convergent plate
boundary
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The collision of India and Asia produced the
Himalayas
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Transform fault boundaries

• The third type of plate boundary
• Plates slide past one another and no new
  lithosphere is created or destroyed
• Transform faults
• Most join two segments of a mid-ocean ridge as
  parts of prominent linear breaks in the oceanic
  crust known as fracture zones

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Transform fault boundaries
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East Pacific Rise west of Costa Rica
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Transform fault boundaries

• Transform faults
• A few (the San Andreas fault and the Alpine fault
  of New Zealand) cut through continental crust

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Transform Margin
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Testing the plate tectonics model

• Paleomagnetism
• Ancient magnetism preserved in rocks at the time
  of their formation
• Magnetized minerals in rocks
• Show the direction to Earths magnetic poles
• Provide a means of determining their latitude of
  origin

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• Dip of needle inclination
• When a rock cools below the Curie point, the
  magnetization direction is locked in
• We can determine the paleolatitude
• Also used in archeology

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Paleomagnetism

• The measurement of remnant magnetism can provide
  information important information about where a
  rock may have come from.
• Measuring a paleomagnetic direction
• An individual lava flow may not record an
  average pole (secular variation), so samples
  from a series of flows may be taken
• Oriented (azimuth and dip) rock cores separated
  by up to a few meters are drilled (using
  non-magnetic equipment).
• If the rock has been tilted since its formation,
  this has to be measured.
• The magnetization direction is measured (by
  measuring all three axis of the core) using a
  very sensitive magnetometer.
• The direction, which is relative to the cylinder
  is calculated with respect to north and the
  vertical.
• The magnetization direction is plotted on a
  stereonet.

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Paleomagnetism

• Magnetic inclination varies from vertical in the
  center to horizontal at the circumference.
• Declination is the angle around the circle
  clockwise from north.
• Downward magnetizations (positive inclination)
  are plotted as open circle. Negative
  magnetizations are plotted as solid circles.
• Plot mean direction and 95 confidence interval
  (95 probability of containing the true
  direction).

From Mussett and Khan, 2000
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Magnetostratigraphy

• By measuring the polarity of magnetization of a
  rock of know age (radiometric data, sediment on
  ocean floor above basement) we can build up a
  magnetic polarity timescale.
• At even smaller scales we can examine secular
  variation within a series of lava flow (assuming
  a high resolution series of flows).
• If these flows are historic, we could probably
  date them.
• If they are very old, we could use the pattern of
  secular variation to correlate between outcrops.
• Archeological applications dating ancient
  fireplaces.
• The resultant magnetic timescale can be used to
  date sediments and the seafloor by the
  recognition of distinctive reversal patterns.

From Mussett and Khan, 2000
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Geomagnetic Reversals

• The first comprehensive magnetic study was
  carried out off the Pacific coast of North
  America.
• Researchers discovered alternating strips of
  high- and low- intensity magnetism.
• In 1963 Vine and Matthews demonstrated that
  stripes of high intensity magnetism formed when
  the Earths magnetic field was in the present
  direction, and stripes of low intensity magnetism
  formed when the Earths magnetic field was in the
  reversed direction.

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A scientific revolution begins
From SeaBeam operators manual
From http//www.navsource.org/archives/09/09570302
.jpg

• During the 1950s and 1960s technological strides
  permitted extensive mapping of the ocean floor

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A scientific revolution begins

• An Extensive oceanic ridge system was discovered.
• Part of this system is the Mid-Atlantic Ridge.
• A central valley shows us that tensional forces
  are pulling the ocean crust apart at the ridge
  crest.
• High heat flow.
• Volcanism.

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A scientific revolution begins

• Deep earthquakes showed that tectonic activity
  was taking place beneath the deep trenches.
• Flat topped seamounts were discovered hundreds of
  meters below sea level.
• Dredges of rocks from the seafloor did not
  recover any rocks older than 180 million years
  old.
• Sediment thickness on the seafloor was much less
  than expected (the seafloor being younger than
  expected).

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Testing the plate tectonics model

• Paleomagnetism
• Polar wandering
• The apparent movement of the magnetic poles
  illustrated in magnetized rocks indicates that
  the continents have moved
• Polar wandering curves for North America and
  Europe have similar paths but are separated by
  about 24? of longitude
• Different paths can be reconciled if the
  continents are place next to one another

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Apparent polar-wandering paths for Eurasia and
North America
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Testing the plate tectonics model

• Magnetic reversals and seafloor spreading
• Earth's magnetic field periodically reverses
  polarity the north magnetic pole becomes the
  south magnetic pole, and vice versa
• Dates when the polarity of Earths magnetism
  changed were determined from lava flows

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Testing the plate tectonics model

• Magnetic reversals and seafloor spreading
• Geomagnetic reversals are recorded in the ocean
  crust
• In 1963 the discovery of magnetic stripes in the
  ocean crust near ridge crests was tied to the
  concept of seafloor spreading

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Paleomagnetic reversals recorded by basalt at
mid-ocean ridges
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Inpretation of magnetic anomalies from
ship-track wiggles, (Barckhausen et al. 2001).
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Testing the plate tectonics model

• Magnetic reversals and seafloor spreading
• Paleomagnetism (evidence of past magnetism
  recorded in the rocks) was the most convincing
  evidence set forth to support the concept of
  seafloor spreading
• The Pacific has a faster spreading rate than the
  Atlantic

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Testing the plate tectonics model

• Plate tectonics and earthquakes
• Plate tectonics model accounts for the global
  distribution of earthquakes
• Absence of deep-focus earthquakes along the
  oceanic ridge is consistent with plate tectonics
  theory
• Deep-focus earthquakes are closely associated
  with subduction zones
• The pattern of earthquakes along a trench
  provides a method for tracking the plate's
  descent

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Deep-focus earthquakes occur along convergent
boundaries
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Earthquake foci in the vicinity of the Japan
trench
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Testing the plate tectonics model

• Evidence from ocean drilling
• Some of the most convincing evidence confirming
  seafloor spreading has come from drilling
  directly into ocean-floor sediment
• Age of deepest sediments
• Thickness of ocean-floor sediments verifies
  seafloor spreading

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Testing the plate tectonics model

• Hot spots
• Caused by rising plumes of mantle material
• Volcanoes can form over them (Hawaiian Island
  chain)
• Most mantle plumes are long-lived structures and
  at least some originate at great depth, perhaps
  at the mantle-core boundary

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The Hawaiian Islands have formed over a
stationary hot spot
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Measuring plate motions

• A number of methods have been em-ployed to
  establish the direction and rate of plate motion
• Volcanic chains
• Paleomagnetism
• Very Long Baseline Interferometry (VLBI)
• Global Positioning System (GPS)

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Measuring plate motions

• Calculations show that
• Hawaii is moving in a northwesterly direction and
  approaching Japan at 8.3 centimeters per year
• A site located in Maryland is retreating from one
  in England at a rate of about 1.7 centimeters per
  year

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The driving mechanism

• No one driving mechanism accounts for all major
  facets of plate tectonics
• Several mechanisms generate forces that
  contribute to plate motion
• Ridge push
• Slab pull
• Models
• Layering at 660 kilometers
• Whole-mantle convection
• Deep-layer model

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Deformation

• Deformation is a general term that refers to all
  changes in the original form and/or size of a
  rock body
• Most crustal deformation occurs along plate
  margins
• How rocks deform
• Rocks subjected to stresses greater than their
  own strength begin to deform usually by folding,
  flowing, or fracturing

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Faults

• Faults are fractures in rocks along which
  appreciable displacement has taken place
• Sudden movements along faults are the cause of
  most earthquakes
• Classified by their relative movement which can
  be
• Horizontal, vertical, or oblique

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Faults

• Types of faults
• 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)

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Concept of hanging wall and footwall along a fault
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Faults

• Types of dip-slip faults
• Normal fault
• Hanging wall block moves down relative to the
  footwall block
• Accommodate lengthening or extension of the crust
• Most are small with displacements of a meter or
  so
• Larger scale normal faults are associated with
  structures called fault-block mountains

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A normal fault
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Faults

• Types of dip-slip faults
• Reverse and thrust faults
• Hanging wall block moves up relative to the
  footwall block
• Reverse faults have dips greater than 45o and
  thrust faults have dips less then 45o
• Accommodate shortening of the crust
• Strong compressional forces

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A reverse fault
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A thrust fault
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Faults

• Strike-slip fault
• Dominant displacement is horizontal and parallel
  to the strike of the fault
• Types of strike-slip faults
• Right-lateral as you face the fault, the block
  on the opposite side of the fault moves to the
  right
• Left-lateral as you face the fault, the block
  on the opposite side of the fault moves to the
  left

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A strike-slip fault
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Fault

• Strike-slip fault
• Transform fault
• Large strike-slip fault that cuts through the
  lithosphere
• Accommodates motion between two large crustal
  plates

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The San Andreas fault system is a major
transform fault
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Mountain belts

• Orogenesis the processes that col-lectively
  produce a mountain belt
• Includes folding, thrust faulting, meta-morphism,
  and igneous activity
• Mountain building has occurred during the recent
  geologic past
• Alpine-Himalayan chain
• American Cordillera
• Mountainous terrains of the western Pacific

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Earths major mountain belts
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Mountain belts

• Older Paleozoic- and Precambrian-age mountains
• Appalachians
• Urals in Russia
• Several hypotheses have been proposed for the
  formations of Earths mountain belts

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Mountain building at convergent boundaries

• Plate tectonics provides a model for orogenesis
• Mountain building occurs at convergent plate
  boundaries
• Of particular interest are active subduction
  zones
• Volcanic arcs are typified by the Aleutian
  Islands and the Andean arc of western South
  America

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Mountain building at convergent boundaries

• Aleutian-type 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
• Most are found in the Pacific
• Active island arcs include the Mariana, New
  Hebrides, Tonga, and Aleutian arcs

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Mountain building at convergent boundaries

• Aleutian-type mountain building
• Volcanic island arcs
• Continued development can result in the formation
  of mountainous topography consisting of igneous
  and metamorphic rocks

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Formation of a volcanic island arc
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Mountain building at convergent boundaries

• 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

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Mountain building at convergent boundaries

• Andean-type mountain building
• Stages of development - passive margin
• First stage
• Continental margin is part of the same plate as
  the adjoining oceanic crust
• Deposition of sediment on the continental shelf
  is producing a thick wedge of shallow-water
  sediments
• Turbidity currents are depositing sediment on the
  continental rise and slope

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Mountain building at convergent boundaries

• Andean-type mountain building
• Stages of development active continental
  margins
• Subduction zone forms
• Deformation process begins
• Convergence of the continental block and the
  subducting oceanic plate leads to deformation and
  metamorphism of the continental margin
• Continental volcanic arc develops

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Mountain building at convergent boundaries

• Andean-type mountain building
• Composed of roughly two parallel zones
• Accretionary wedge
• Seaward segment
• Consists of folded, faulted, and meta-morphosed
  sediments and volcanic debris

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Orogenesis along an Andean-type subduction zone
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Orogenesis along an Andean-type subduction zone
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Orogenesis along an Andean-type subduction zone
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Mountain building at convergent boundaries

• Continental collisions
• Two lithospheric plates, both carrying
  continental crust
• The Himalayan Mountains are a youthful mountain
  range formed from the collision of India with the
  Eurasian plate about 45 million years ago

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Mountain building at convergent boundaries

• Continental collisions
• The Himalayan Mountains
• Spreading center that propelled India northward
  is still active
• Similar but older collision occurred when the
  European continent collided with the Asian
  continent to produce the Ural mountains

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Plate relationships prior to the collision of
India with Eurasia
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Position of India in relation to Eurasia at
various times
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Formation of the Himalayas
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Mountain building at convergent boundaries

• Continental accretion and mountain building
• A third mechanism of orogenesis
• Small crustal fragments collide and merge with
  continental margins
• Responsible for many of the mountainous regions
  rimming the Pacific
• Accreted crustal blocks are called terranes

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Mountain building at convergent boundaries

• Continental accretion and mountain building
• Terranes consist of any crustal fragments whose
  geologic history is distinct from that of the
  adjoining terranes
• As oceanic plates move, they carry embedded
  oceanic plateaus, volcanic island arcs and
  microcontinents to an Andean-type subduction zone
  

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Vertical movements of the crust

• In addition to the horizontal movements of
  lithospheric plates, vertical movement also
  occurs along plate margins as well as the
  interiors of continents far from plate boundaries
  

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Vertical movements of the crust

• Isostatic adjustment
• Less dense crust floats on top of the denser and
  deformable rocks of the mantle
• Concept of floating crust in gravitational
  balance is called isostasy
• If weight is added or removed from the crust,
  isostatic adjustment will take place as the crust
  subsides or rebounds

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Der Wilson-Zyklus
Ein Wilson-Zyklus beschreibt die Entstehung, die
Entwicklung und das Verschwinden eines Ozeans.
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Die Stadien eines Wilson-Zyklus
An verschieden weit entwickelter ozeanischer
Kruste kann man einzelne Stadien eines
Wilson-Zyklus beobachten
Bildung eines kontinentalen Grabens
(Ostafrikanischer Graben)
Beginnende Ozeanisierung (Rotes Meer)
Maximale Ausdehnung der ozeanischen Kruste mit
passiven Kontinentalrändern
(Atlantik)
Subduktion der ozeanischen Kruste mit aktiven
Kontinental- rändern (Pazifik)
Restozean (Mittelmeer)
Kontinent Kontinent Kollision (Himalaya)
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1.) Grabenbildung (Rifting)
Beginnt mit einem Tripelpunkt auf kontinentaler
Kruste
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Entwicklung eines kontinentalen Grabens
Evaporite (Salze)
terrestrische Sedimente
Tuffe, vulkanischer Schutt
Lavadecken
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Der Rhein Rhône-Graben
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Profil durch den Rheingraben
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Der Ostafrikanische Graben
Länge 4 000 km Breite 30 70 km Versatz gt 6
000 m
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Merkmale von kontinentalen Gräben
Hohe Seismizität
Hoher Wärmefluß (gt 2.0 HFU)
Alkaliner Magmatismus und Vulkanismus
Negative Schwere-Anomalie (Bouguer-Schwere)
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Schwere-Anomalie
Gemessen wird die Erdbeschleunigung in gal 1 gal
1 cm/sec2 1000 mgal. normal 980 gal
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2.) Stadium
Bildung eines mittelozeanischenRückens
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Entstehung neuer ozeanischer Kruste
Beispiel Rotes Meer, Golf von Aden, Afar-
(Danakil-) Senke
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Unterschiede zu Gräben
Entstehung ozeanischer Kruste
Positive Bouguer-Anomalie
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3. Stadium
Ausbreitung ozeanischer Lithosphäre
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Maximale Öffnung eines Ozeans
nach Press Siever (Spektrum Lehrbuch), 1995
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Profil durch den Atlantik
Schematisches Profil durch den Nordatlantik
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Stadium 4Subduktion ozeanischer Kruste(rezentes
Beispiel Pazifik)
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Subduktion
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paarige metamorphe Gürtel in Japan
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Fossile Subduktionszonen
Eine ehemalige Subduktionszone erkennt man am
Vorhandensein von
Ophiolithen (Ophiolithische Sutur)
magmatischen Gesteinen
Hochdruck-Gesteinen (Blauschiefer)
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Bildung von Randbecken
High Stress Subduktion
Low Stress Subduktion
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Stadium 5Restmeer (Beispiel Mittelmeer)
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Das Mittelmeer und Schwarze Meer als Restmeere
Aus Press Siever, 1995 (Spektrum Lehrbücher)
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Terrankarte des Mittelmeers
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Stadium 6Kontinent-Kontinent-Kollision
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Kontinent-Kontinent-Kollision
Zentral- gürtel
Geosutur
Hinterland
Asthenosphäre
Umgezeichnet nach Eisbacher, 1991
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Kontinent-Kontinent-Kollision
Umgezeichnet nach Press Siever, 1995 (Spektrum)
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Kollision Indiens mit Eurasien
Krustenver- kürzung insgesamt 2000 km in 40 Ma
Aus Press Siever, 1995 (Spektrum Lehrbücher)
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Entstehung des Himalaya
University of Western Australia
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Tektonik im Himalaya-Hinterland
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Zusammenfassung
Die Plattentektonik ist der an der Erdoberfläche
auftretende Ausdruck der Mantelkonvektion im
Erdinneren. Sie beschreibt die Bewegungen der
Lithosphärenplatten und die daraus resultierenden
geologischen Prozesse. Zu diesen zählen u.a. die
Entstehung von Faltengebirgen (Orogenese), von
mittelozeansichen Rücken und von
Transformstörungen. Die großräumigen
Deformationen der Lithosphäre sind wiederum die
Ursache von zahlreichen geophysikalischen
Phänomenen, wie z.B. Vulkanismus oder Seismizität.