Institut für Geowissenschaften Universität Potsdam Plattentektonik Institut für Geowissenschaften Universität Potsdam

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3 Typen von Plattengrenzen Plattentektonik Ozeane Kontinente 3 Typen von Plattengrenzen

Earth’s Plates

Divergent boundaries are located mainly along oceanic ridges

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

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.

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

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

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

The East African rift – a divergent boundary on land

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

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

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)

An oceanic-continental convergent plate boundary

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)

An oceanic-oceanic convergent plate boundary

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)

A continental-continental convergent plate boundary

The collision of India and Asia produced the Himalayas

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

Transform fault boundaries

East Pacific Rise west of Costa Rica

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

Transform Margin

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

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 A compass needle would become aligned with these lines of force. The angle of the dip needle relative to the Earth is called the inclination. If a rock is heated above the Curie point (585C for magnetite) it looses its magnetization. As a rock cools below the Curie point, iron rich grains in the rock gradually become aligned with the Earth’s magnetic field. Once the rock has cooled, the magnetization direction is locked in. If the rock is then moved, it will retain the magnetization of its original location. We know the latitude at which a rock formed, and how far from the pole it formed. This can also be used over shorter time periods for artifacts, e.g. clay ovens.

The Earth’s magnetic field acts like a bar magnet at a slight angle to the axis of rotation. The present north magnetic pole is in N. Canada. Invisible lines of force pass through the Earth from one pole to the other.

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.

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

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

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. High intensity stripes are due to normally magnetized rocks reinforcing the Earth’s magnetic field. Low intensity stripes are due to reversely magnetized rocks weakening the Earth’s magnetic field.

A scientific revolution begins From SeaBeam operators manual From http://www.navsource.org/archives/09/09570302.jpg R/V Robert D. Conrad. Echo sounding. During the 1950s and 1960s technological strides permitted extensive mapping of the ocean floor

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.

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). Flat topped seamounts are believed to be evidence of erosion at sea level followed by subsidence.

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

Apparent polar-wandering paths for Eurasia and North America

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

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

Paleomagnetic reversals recorded by basalt at mid-ocean ridges

Inpretation of magnetic anomalies from ship-track wiggles, (Barckhausen et al. 2001).

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

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

Deep-focus earthquakes occur along convergent boundaries

Earthquake foci in the vicinity of the Japan trench

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

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

The Hawaiian Islands have formed over a stationary hot spot

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)

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

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

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

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

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)

Concept of hanging wall and footwall along a fault

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

A normal fault

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

A reverse fault

A thrust fault

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

A strike-slip fault

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

The San Andreas fault system is a major transform fault

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

Earth’s major mountain belts

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

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

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

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

Formation of a volcanic island arc

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

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

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

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

Orogenesis along an Andean-type subduction zone

Orogenesis along an Andean-type subduction zone

Orogenesis along an Andean-type subduction zone

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

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

Plate relationships prior to the collision of India with Eurasia

Position of India in relation to Eurasia at various times

Formation of the Himalayas

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

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

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

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

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

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)

1.) Grabenbildung (Rifting) Beginnt mit einem Tripelpunkt auf kontinentaler Kruste Kreide Tripelpunkt Rezent Rotes Meer Golf von Aden Benue-Trog (Aulakogen) Afar-Senke

Entwicklung eines kontinentalen Grabens Evaporite (Salze) terrestrische Sedimente Tuffe, vulkanischer Schutt Lavadecken aufdringendes basaltisches Magma

Der Rhein Rhône-Graben Tiefe der Kruste- Mantel-Grenze

Profil durch den Rheingraben Kontinentale Kruste Asthenolith (Mantelkissen, Manteldiapir) Oberer Mantel

Der Ostafrikanische Graben Nairobi Länge 4 000 km Breite 30 – 70 km Versatz > 6 000 m

Merkmale von kontinentalen Gräben Hohe Seismizität Hoher Wärmefluß (> 2.0 HFU) Alkaliner Magmatismus und Vulkanismus Negative Schwere-Anomalie (Bouguer-Schwere)

Gemessen wird die Erdbeschleunigung in gal Schwere-Anomalie Gemessen wird die Erdbeschleunigung in gal 1 gal = 1 cm/sec2 = 1000 mgal. normal: 980 gal leichte Sedimente hochliegende Moho Bouguer- Schwere

Bildung eines mittelozeanischen Rückens 2.) Stadium Bildung eines mittelozeanischen Rückens

Entstehung neuer ozeanischer Kruste Beispiel: Rotes Meer, Golf von Aden, Afar- (Danakil-) Senke Rotes Meer Golf von Aden Afar - Dreieck

Unterschiede zu Gräben Entstehung ozeanischer Kruste Positive Bouguer-Anomalie

Ausbreitung ozeanischer Lithosphäre 3. Stadium Ausbreitung ozeanischer Lithosphäre

Maximale Öffnung eines Ozeans Beispiel Atlantik Tiefseebecken passive Kontinentalränder nach Press & Siever (Spektrum Lehrbuch), 1995

Profil durch den Atlantik Schematisches Profil durch den Nordatlantik

Stadium 4 Subduktion ozeanischer Kruste (rezentes Beispiel: Pazifik)

Subduktion 3. Magmatischer Bogen 4. Seismizität an der Wadati- Benioff-Zone 5. paarige meta- morphe Gürtel Hochtemperatur- Niedrigdruck-Metam. Hochdruck-Niedrig- temperatur-Metam.

paarige metamorphe Gürtel in Japan

Fossile Subduktionszonen: Eine ehemalige Subduktionszone erkennt man am Vorhandensein von: Ophiolithen (Ophiolithische Sutur) magmatischen Gesteinen Hochdruck-Gesteinen (Blauschiefer)

Bildung von Randbecken High Stress Subduktion steiler Eintauch- winkel Low Stress Subduktion

Stadium 5 Restmeer (Beispiel: Mittelmeer)

Das Mittelmeer und Schwarze Meer als Restmeere Aus Press & Siever, 1995 (Spektrum Lehrbücher)

Terrankarte des Mittelmeers

Stadium 6 Kontinent-Kontinent-Kollision

Kontinent-Kontinent-Kollision Akkretions- keil Vorland- becken Zentral- gürtel Geosutur Hinterland Litho- sphäre Regionale Metamorphose und Anatexis Asthenosphäre Mantel- delamination Umgezeichnet nach Eisbacher, 1991

Kontinent-Kontinent-Kollision Sutur- zone Mélange kontinentale Kruste Ophiolithe Über- schiebungen Underplating Slab- breakoff Umgezeichnet nach Press & Siever, 1995 (Spektrum)

Kollision Indiens mit Eurasien Krustenver- kürzung insgesamt 2000 km in 40 Ma Aus Press & Siever, 1995 (Spektrum Lehrbücher)

Entstehung des Himalaya University of Western Australia

Tektonik im Himalaya-Hinterland Dehnung Konvergenz Escape-Tektonik

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.