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Plattentektonik
Institut für Geowissenschaften
Universität Potsdam
21.01.2009
VL Geodynamik & Tektonik, WS 0809
Übersicht zur Vorlesung
VL Geodynamik & Tektonik, WS 0809
Plattentektonik
Ozeane
Kontinente
3 Typen von
Plattengrenzen
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VL Geodynamik & Tektonik, WS 0809
Earth’s 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.
VL Geodynamik & Tektonik, WS 0809
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
VL Geodynamik & Tektonik, WS 0809
Convergent plate boundaries

Although all have the same basic characteristics, they are highly variable features
 Types of convergent boundaries

Oceanic-continental convergence
– Denser oceanic slab sinks into the asthenosphere
VL Geodynamik & Tektonik, WS 0809
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
VL Geodynamik & Tektonik, WS 0809
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)
VL Geodynamik & Tektonik, WS 0809
An oceanic-oceanic convergent
plate boundary
VL Geodynamik & Tektonik, WS 0809
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 Earth’s 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
VL Geodynamik & Tektonik, WS 0809
VL Geodynamik & Tektonik, WS 0809
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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•
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
Paleomagnetism
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Magnetostratigraphy
•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.
From Mussett and Khan, 2000
•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.
•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.
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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 Earth’s
magnetic field was in the
present direction, and stripes
of low intensity magnetism
formed when the Earth’s
magnetic field was in the
reversed direction.
VL Geodynamik & Tektonik, WS 0809
From SeaBeam operators manual
From http://www.navsource.org/archives/09/09570302.jpg
A scientific revolution begins

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



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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).
VL Geodynamik & Tektonik, WS 0809
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
VL Geodynamik & Tektonik, WS 0809
Apparent polar-wandering paths for
Eurasia and North America
VL Geodynamik & Tektonik, WS 0809
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 Earth’s magnetism
changed were determined from lava flows
VL Geodynamik & Tektonik, WS 0809
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).
VL Geodynamik & Tektonik, WS 0809
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
VL Geodynamik & Tektonik, WS 0809
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
VL Geodynamik & Tektonik, WS 0809
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
VL Geodynamik & Tektonik, WS 0809
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 employed to establish the direction and rate
of plate motion
Volcanic chains
 Paleomagnetism
 Very Long Baseline Interferometry (VLBI)
 Global Positioning System (GPS)

VL Geodynamik & Tektonik, WS 0809
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

VL Geodynamik & Tektonik, WS 0809
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
VL Geodynamik & Tektonik, WS 0809
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
VL Geodynamik & Tektonik, WS 0809
A strike-slip fault
VL Geodynamik & Tektonik, WS 0809
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 collectively produce a mountain belt


Includes folding, thrust faulting, metamorphism, 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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Earth’s major mountain belts
VL Geodynamik & Tektonik, WS 0809
Mountain belts

Older Paleozoic- and Precambrian-age
mountains



Appalachians
Urals in Russia
Several hypotheses have been proposed for
the formations of Earth’s mountain belts
VL Geodynamik & Tektonik, WS 0809
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
VL Geodynamik & Tektonik, WS 0809
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
VL Geodynamik & Tektonik, WS 0809
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
VL Geodynamik & Tektonik, WS 0809
Formation of a volcanic island arc
VL Geodynamik & Tektonik, WS 0809
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
VL Geodynamik & Tektonik, WS 0809
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
VL Geodynamik & Tektonik, WS 0809
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
VL Geodynamik & Tektonik, WS 0809
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
VL Geodynamik & Tektonik, WS 0809
Orogenesis along an Andean-type
subduction zone
VL Geodynamik & Tektonik, WS 0809
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
VL Geodynamik & Tektonik, WS 0809
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
VL Geodynamik & Tektonik, WS 0809
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.
Die einzelnen Stadien sind:
Zerbrechen kontinentaler Kruste,
Entstehung einer ozeanischen Spreizungszone,
maximale Ausdehnung der Ozeanischen Kruste,
Subduktion,
Verschwinden der ozeanischenKruste,
Kontinent – Kontinent - Kollision
VL Geodynamik & Tektonik, WS 0809
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 Kontinentalrändern (Pazifik)
Restozean (Mittelmeer)
Kontinent – Kontinent – Kollision (Himalaya)
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1.) Grabenbildung (Rifting)
Beginnt mit einem Tripelpunkt auf kontinentaler Kruste
Äquator
SüdAmerika
Kreide
Afrika
Tripelpunkt
Rotes Meer
Äquator
Golf von Aden
Afar-Senke
Rezent
Benue-Trog
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(Aulakogen)
Entwicklung eines kontinentalen
Grabens
Evaporite (Salze)
terrestrische Sedimente
Tuffe, vulkanischer Schutt
Lavadecken
aufdringendes basaltisches Magma
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Der Rhein Rhône-Graben
Tiefe der KrusteMantel-Grenze
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Profil durch den Rheingraben
Kontinentale
Kruste
Asthenolith
(Mantelkissen,
Manteldiapir)
Oberer Mantel
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Der Ostafrikanische Graben
W
Western
Rift
Gregory
Rift
E
Nairobi
Länge 4 000 km
Breite 30 – 70 km
Versatz
> 6 000& m
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Tektonik, WS 0809
Merkmale von kontinentalen Gräben
Hohe Seismizität
Hoher Wärmefluß (> 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
leichte
Sedimente
hochliegende
Moho
Bouguer-
>8.0
+
-
~7.0
+
Schwere
>8.0
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2.) Stadium
Bildung eines
mittelozeanischen
Rückens
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Entstehung neuer ozeanischer Kruste
Beispiel: Rotes Meer,
Golf von Aden,
Afar- (Danakil-) Senke
Rotes Meer
Golf von Aden
Afar - Dreieck
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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
Beispiel Atlantik
Tiefseebecken
passive
Kontinentalränder
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nach Press & Siever (Spektrum Lehrbuch), 1995
Profil durch den Atlantik
KontinentalTiefseerand
Becken
Mittelozeanischer
Rücken
Tiefsee- Kontinentalrand
Becken
2000m
4000m
Schematisches Profil durch den Nordatlantik
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Stadium 4
Subduktion ozeanischer
Kruste
(rezentes Beispiel: Pazifik)
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Subduktion
3. Magmatischer
Bogen
4. Seismizität an
der WadatiBenioff-Zone
5. paarige metamorphe Gürtel
Hochdruck-Niedrigtemperatur-Metam.
HochtemperaturNiedrigdruck-Metam.
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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
Flacher Eintauchwinkel
steiler
Eintauchwinkel
High Stress
Subduktion
Low Stress
Subduktion
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Stadium 5
Restmeer
(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 6
Kontinent-Kontinent-Kollision
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Vorlandbecken
Akkretionskeil
Geosutur
Kontinent-Kontinent-Kollision
Zentralgürtel
Hinterland
Lithosphäre
Asthenosphäre
Regionale Metamorphose
und Anatexis
Manteldelamination
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Umgezeichnet nach Eisbacher, 1991
Kontinent-Kontinent-Kollision
Suturzone
Mélange
kontinentale
Kruste
Ophiolithe
kontinentale
Kruste
Überschiebungen
Underplating
Slabbreakoff
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Umgezeichnet nach Press & Siever, 1995 (Spektrum)
Kollision Indiens mit Eurasien
Krustenverkürzung
insgesamt
2000 km
in 40 Ma
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Aus Press & Siever, 1995 (Spektrum Lehrbücher)
Entstehung des Himalaya
University of Western Australia
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Tektonik im Himalaya-Hinterland
Dehnung
Escape-Tektonik
Konvergenz
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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.
21.01.2009
VL Geodynamik & Tektonik, WS 0809