1. (Chapter 10 in D & R) Geometry and Kinematics: Plates

2. Today's lecture 1) Building Blocks of the Earth: What's getting deformed? 2) Geometry + Kinematics = Plate Tectonics 3) Kinematic evolution

3. Earth Structure: defined by seismic discontinuities; concentric shells of rock or liquid with contrasting chemical and physical properties Core: iron alloy Mantle: ultramafic Crust: mafic to felsic

4. Note the figure that is to scale! Boundary between mantle and crust is called the Moho, short for Mohorovicic discontinuity

5. Oceanic Crust - forms at mid-ocean ridges by partial melting of mantle - basaltic (mafic) in composition - igneous extrusion and intrusion - 5 to 10 km-thick - Oceanic crust is no older than ~200 Ma. Why??

6. Continental Crust - 5 to 10 times thicker than oceanic crust- 40 km avg. - This is a simplified sketch! Continental crust is very heterogeneous - Long and complex deformation history. Majority of continental crust formed during the Precambrian (before ~570 Ma) The oldest known rock is ~4 Ga! Why so much older than oceanic crust??

7. Contours of crustal thickness

8. -Most of the continental surface lies within 1 km of sea level. -Most of ocean floor lies at depths ~4 km below sea level.

10. Rheology (behavior during deformation) of the Earth Lithosphere: "lithos" = rock, implying strength. It exhibits a component of elastic strength over geological time scales. Includes crust + uppermost mantle! Varies in thickness. Moves as a plate- exceptions are orogens. Asthenosphere: Weak. It is solid, but behaves like a viscous fluid (convective flow) over geological time scales.

11. Plates of the Earth

12. A plate can include both oceanic and continental lithosphere oceanic vs. continental lithosphere can be distinguished with bathymetry (oceanic equivalent of topography)- WHY?

13. How do we know that plates move? - Earthquakes localized along plate boundaries

14. Most folks were convinced when spreading was demonstrated at mid-ocean ridges

16. What is it? Geometry: Sketch Kinematic models 1) 2) Predictions? Tectonic significance?

17. Transform faults: more evidence for plate motion Not observed! Observed!

18. Oceanic transform faults: Spreading occurs perpendicular to the ridge axis and any curvature is taken up by transform faults. Different rates of spreading.

19. GPS (Global Positioning System) Satellite-based geodesy: A new revolution in Earth Sciences! (more near end of semester!) InSAR Interferometric Synthetic Aperture Radar

20. Continents also break- to form new oceans

21. So far: Opening of oceans and creation of oceanic crust Is the Earth expanding?

22. Spreading at mid-ocean ridges must be compensated by subduction

23. Ocean-Continent convergent margins

25. Continent-Continent convergent margins

26. An example: The Indo-Asian Collision

27. Digital elevation map (DEM) of the Himalayan- Tibetan orogen Warmer colors represent higher elevations

29. Transform plate boundaries An example: San Andreas

30. Velocity vector diagrams

32. On a sphere, plate movement is described by a rotation at a specified angular velocity (w) around an imaginary axis (rotation pole, also known as Euler pole). Not related to N or S or magnetic poles!

33. Oceanic plates move perpendicular to ridges and parallel to transform faults. Euler poles can be determined for oceanic plates by drawing lines perpendicular to transform faults (and fracture zones).

34. Thought experiment: How much time did it take to produce the present-day area of oceanic crust? The surface area of the Earth is = 4  R 2. With R = 6,378 km, area = 5.1x10 8 km 2. 55% of Earth surface is underlain by oceanic crust (0.55 x 5.1x10 8 km 2 = 2.8x10 8 km 2 ). On average, crust is created at mid-ocean ridges at a rate of 2 to 3 cm/year. The length of ridges is 60,000 km. Thus, oceanic crust is created at a rate of 2 to 3 cm/year x 60,000 km = 1.2 to 1.8 km 2 /year. The time required to produce the present-day area of oceanic lithosphere is 2.8x10 8 km 2 / (1.2 to 1.8 km 2 /year) = 155 to 230 Ma. What does this imply about the plate configuration of Earth ~200 m.y.a?

35. Pangea supercontinent and Tethys Ocean http://www.scotese.com/earth.htm

36. Concept of the supercontinent cycle Rodinia- Pangea mantle convection Wilson cycle: opening and closing of oceans at roughly similar positions

37. Extraterrestrial plate tectonics? DEM of Aleutian trench DEM of Artemis Corona on Venus; same scale as above

38. Plates are rigid! Kinematics described in terms of translations and rotations Next: Geometry and Kinematics of Deformation Starting with Fractures (Read D&R, p. 38-50; 204-226)

39. Important terminology/concepts - Crust: continental vs. oceanic - Mantle - Moho - Rheology: Lithosphere vs. Asthenosphere - Earth's plates - Bathymetry - Evidence for plate motion? - Mid-oceanic ridges - Transform faults and fracture zones - Ocean-continent convergent margins - trench, accretionary wedge, forearc - Continent-continent convergent margins - Plate kinematics - Euler Pole - Velocity vector diagrams - Kinematic history - Supercontinent cycle