02- Die Plattentektonik

Transcription

1 02- Die Plattentektonik

2 Geological evidences for continental drift G r e enlan d Baltic shield Russian platform The fit of the continents around the North Atlantic, after Bullard et al. (1965), and the trends of the Appalachian-Caledonian and Variscan (early and late Paleozoic) fold belts (orange and green respectively). African Foreland Correlation of cratons and younger mobile belts across the closed southern Atlantic Ocean

3 Geological evidences for continental drift Correlation of Permo-Carboniferous glacial deposits, Mesozoic dolerites, and Precambrian anorthosites between the reconstructed continents of Gondwana (after Smith & Hallam, 1970)

4 Geological evidences for continental drift Correlation of stratigraphy between Gondwana continents (from Hurley, 1968)

5 Paleoclimatological evidences for continental drift Present distributions of Pangean flora and fauna (from Tarling & Tarling,1971)

6 Paleoclimatological evidences for continental drift Use of paleoclimatic data to control and confirm continental reconstructions (from Tarling & Tarling, 1971)

7 Continental drift reconstruction The first mathematical reassembly of continents based solely on geometric criteria was performed by Bullard et al. (1965), who fitted together the continents on either side of the Atlantic. This was accomplished by sequentially fitting pairs of continents after determining their best fitting poles of rotation. The only rotation involving parts of the same landmass is that of the Iberian penin- sular with respect to the rest of Europe. This is justified because of the known presence of oceanic lithosphere in the Bay of Biscay which is closed by this rotation. Geologic evidence and information provided by magnetic lineations in the Atlantic indicate that the reconstruction represents the continental configuration during late Triassic/early Jurassic times approximately 200 Ma ago.

8 Hot-spots Wilson, 1963 Sketch of the Pacific ocean. Heavy arrow show nine linear chains of islands and seamounts which increase in age in direction of arrow. Single-headed arrows show direction of motion, where known, along large transcurrents faults. Small arrows show postulated direction of flow away from median ridges. Some possible patterns of convection, showing that, if active volcanos form overrrising vertical currents, chains of extinct volcanoes might be formed by the horizontal flow or the currents. The shaded areas represent stable cores of cells

9 Hot-spots World map of the main traps (or flood basalts). Some have been linked to the currently active hot spot volcanoes, whose birth may be the cause of the traps. Active hot spot volcanoes not related to traps

10 Hot-spots 50 Monts sous marins Meiji 63 Ma 56 Ma Iles Aléoutiennes Océan Pacifique 54 Ma 40 Monts sous-marins de l'empereur 43 Ma 30 Ma Moving of the Pacific plate Midway 20 Ma 12 Ma Chaîne de Hawaii 10 Ma < 2 Ma Hawaii W 160 W 140 W 20 Hot spot B Moving of the plate Lithosphere Plume

11 N Cold basin + + D Hawaii Pacific superplume EPR Tibet Cold downwelling Melt (ULVZ) Melt (ULVZ) Inner core Outer core + + S. America African superplume D Lower mantle Upper mantle + Mid-Atlantic Ridge S

12 Sea-floor spreading The difference between transcurrent and transform faults, (a) In a transcurrent (or strike-slip) fault, the direction of movement can be determined from the offset of a feature intersecting the fault. If the feature is moved to the left, it is a left-lateral fault, as shown here. The north side of the fault has moved to the left (west), the south side of the fault has moved to the right (east), and the fault may continue indefinitely, (b) In a ridge-to-ridge transform fault, a section of the mid-ocean ridge is fractured perpendicular to its length. In this case, the right side of the ridge is moving to the right (east), the left side is moving to the left (west), and the sense of motion is opposite of that illustrated in (a). Note also that the fault does not extend indefinitely, but terminates against the north-south running ridge segments. Wilson, 1965

13 Geometry on a sphere Morgan, 1967 Fig. 4. On a sphere, the motion of block 2 elative to block I must be a rotation about some pole. All faults on the boundary between I and 2 must be small circles concentric about the pole A.

14 The Euler s theorem states that the movement of a portion of a sphere across its surface is uniquely defined by a single angular rotation about a pole of rotation. The pole of rotation, and its antipodal point on the opposite diameter of the sphere, are the only two points which remain in a fixed position relative to the moving portion. Consequently, the movement of a continent across the surface of the Earth to its predrift position can be described by its pole and angle of rotation

15 Axe de rotation de la Terre Axe de rotation de la plaque P Axe de rotation de la Terre Q R S T Plaque 1 Plan équatorial de la plaque en rotation Plaque 2

16 P Plate B is moving counter-clockwise relative to plate A. The motion is defined by the angular velocity ω about the pole of rotation P. B A Double lines are ridge segments, and arrows denote direction of motion on transform fault

17 Bücher (a) c A b (b) c A b B a C B a C sina a sinb b sinc c a 2 b 2 c 2 2bc cos A sina cos a sinb cos b sinc cos c cos a cos b cos c sin b sin c cos A

18 a) A a a sin a a P O u rel = ωa sin where a is the radius of the Earth and is the angle subtended at the center of the Earth by the pole of rotation P and the point A on the plate boundary

19 b) Geographic North A O s P Pole of Rotation Equator The angle is related to the colatitude Θ and east longitude Ψ of the rotation pole and the colatitude Ψ and east longitude Θ of the point on the plate boundary A cos = cos cos + sin sin cos

20 Die Plattentektonik, heutige Model Eurasia Pacific Philippines Juan de Fuca Cocos North America Caribbea Africa Eurasia Arabian Indian Australian Nazca South America Scotia Divergent Plattenränder 4,1 Rate in cm/a Ozean Konvergent

21 Earth s magnetism

22 Earth s magnetism Inclination (magnetic): The angle between the magnetic field direction (or the direction of magnetization in a rock) and the local h o r i z o n t a l p l a n e ( c o u n t e d p o s i t i v e downwards, negative upwards). Inclination is zero at the magnetic equator and ±90 (vertical) at the magnetic poles. A simple formula helps to derive magnetic latitudes from magnetic inclination in the case where the field is that of a dipole (which is roughly the case for the Earth) declination inclination NORTH (geographic) NORTH (magnetic EAST (geographic) Magnetic field (vector) Declination (magnetic): The angle between magnetic north (as given by the compass needle)and geographic north (in the horizontal plane, counted positive eastward) Vertical (downward)

23 Earth s magnetism Magnetic north Geographic north N i S D N i=0 S Magnetic equator N i S i: inclination D: declination

24 Earth s magnetism 60 N (a) Inclination N N N N S S W E S 30 S 60 S 180

25 Earth s magnetism 60 N (b) Total intensity N 30 N N S S 60 S S W E 180

26 Earth s magnetism The internal structure of the Earth (left side) and the vertical component of the geomagnetic field represented at the surface (top right side) and at the coremantle boundary (bottom right side), for the epoch (based on the Olsen et al. (2006) model). The core magnetic field is mainly dipolar but the field is modulated by smaller scales. The structure of the vertical component depends on the depth at which the magnetic field is represented, the smaller scales being more apparent at the core-mantle boundary than at the Earth s surface. For dynamo modelling, the magnetic field is represented at the core-mantle boundary

27 Magnetism on the Earth Magnetism recorded on land or on sea is showing different patterns: - fine and regular in the oceans - irregular and coarse on the continent

28 Paleomagnetism Recording of the Earth s magnetism within rocks Igneous rocks Sedimentary rocks geomagnetic field direction Magnetization magnetite grains ins Curie point Temperature wwater a ferromagnetism paramagnetism matrix mineral field direction magnetite grain magnetization direction sediment

29 Paleomagnetism (a) Dipole axis horizontal I tan I = 2 tan dipole field line (a) The geocentric axial dipole hypothesis predicts the relationship tan I =2tan λ between the inclination I of a dipole field and the magnetic latitude λ. r Equator (b) Site mean inclination, I (b) The inclinations measured in modern deep- sea sediment cores agree well with the theoretical curve (based on data from Schneider and Kent, 1990) Latitude, tan I = 2 tan

30 Paleomagnetism 180 Paleomagnetic pole positions for rocks of Plio-Pleistocene to Recent age (after McElhinny, 1973) 50 N 90 W 9 0 E 0 Plio-Pleistocene to Recent paleomagnetic poles (younger than 5 M a) Present-day geomagnetic pole The direction of the Earth s magnetic field for two declination series, Paris and London.

31 Paleomagnetism North American APW E European APW W 0 B Rifting A Single continent A Divergence B Convergence A + B Sutured continent

32 Paleomagnetism Ordovician 000 E E 443 E quator Cambrian Silurian Devonian Carboniferous 30 S 290 Permian 60 S Trias sic Jura s 142 C retaceous 248

33 Paleomagnetism W 90 E 75 N 60 N 45 N 0 _European paleomagnetic poles: Pliocene and Pleistocene Permian

34 Paleomagnetic scale Polarity interpretation VGP latitude 90 S 0 90 N normal polarity chron excursion polarity subchron reverse polarity chron polarity transitions Polarity: normal reverse transitional

35 Paleomagnetic scale & 0 Cox et al., 1963 Cox et al., 1964 Doell & Dalrymple, 1966 McDougall Chamalaun, 1966 Cox et al., 1968 Opdyke, 1972 Events Epochs Brunhes normal Jaramillo Olduvail Matuyama reversed 1 2 Age (Ma) normal polarity reverse polarity Kaena Mammoth Cochiti Nunivak Sidufjall Thvera Gauss normal Gilbert reversed Epoch 5 normal Age (Ma)

36 Paleomagnetism nt Age (Ma) Gilbert inverse Gauss normal Matuyama inverse Distance (km) Brunhes normal Matuyama inverse Gauss normal Gilbert normal Eau de mer Sédiments Basaltes et gabbros océaniques Lithosphère Asthénosphère

37 Paleomagnetism vs. age in oceans

38 Paleomagnetism & plate movement

39 Paleomagnetism Paleomagnetic Apparent Polar Wander True Polar Wander Hotspot Apparent Polar Wander (a) (c) (b) 60

40 Vor 200 Millionen Jahren 01-Geodynamik und Tektonik Superkontinent Pangea

41 Vor 70 Millionen Jahren 01-Geodynamik und Tektonik

42 Vor 50 Millionen Jahren 01-Geodynamik und Tektonik

43 Vor 20 Millionen Jahren 01-Geodynamik und Tektonik

44 Heute 01-Geodynamik und Tektonik

45 Die Plattentektonik, heutige Model Eurasia Pacific Philippines Juan de Fuca Cocos North America Caribbea Africa Eurasia Arabian Indian Australian Nazca South America Scotia Divergent Plattenränder 4,1 Rate in cm/a Ozean Konvergent