Geodynamics and some Structural Geology

1. Geodynamics and some Structural Geology Rob Bialas

2. Geodynamics • The branch of geophysics that studies the processes leading to deformation of planetary mantle and crust and the related earthquakes and volcanism that shape the structure of the Earth and other planets.

3. FORCE: (Wikipedia) an influence that may cause a mass to accelerate • F = m a • Newton:A newton is the amount of force required to accelerate a mass of one kg at a rate of one m per second squared Kg m/ s2

4. FORCES 2 types • BODY FORCES • No physical contact between 2 objects • Acts through the volume of a body (eg. gravity) • SHEAR FORCES • Contact between 2 objects • Surface forces

5. STRESS and STRAIN • STRESS: force per unit area. In geology we never see stress. We only see the results of stress as it deforms materials. • STRAIN: the amount of deformation an object experiences compared to its original size and shape. ε = ΔL/L

6. Ways Material Can Behave • Elastic • Plastic • Viscous • Combinations of those

7. Elasticity • Deformation is recoverable! (σ=Eε) • Example (rubber band snaps back)

8. Plasticity • Deformation is pushed beyond elastic limit and is no longer recoverable • Example, you stretch the rubber band until it breaks. The part where it breaks is irrecoverable (plastic)

9. Elastic vs. Plastic

10. Viscous behavior • Definition viscosity: τ =μ du/dy • Relates velocity to shear stress viscosity μ is in units Pa s = kg m-1 s-1 • Water running over streambed – low viscosity – low shear stress • Mantle flowing under plates – high viscosity (10^18 to 10^20) – high shear stress

11. Viscosity Examples

12. Viscosity Example: Asthenospheric Counterflow

13. Things that happen on Earth that all the above is used to describe • Plate Flexure • Mantle Convection • Faulting • Mountain Building • Extension • Subduction • etc. • (yes, some of these overlap)

14. Plate Flexure

15. Flexure: Elastic vs Plastic

16. Some real examples: Hawaii

17. Flexure and Earthquakes at Subduction Zones Nelson 1996

18. Strength: “the Christmas Tree Model”

19. Faulting • 3 major kinds • Thrust faults • Normal Faults • Strike Slip Faults Got to: http://www.tinynet.com/faults.html for cheesy cartoons of normal, thrust, and strike slip. (Reverse fault is a thrust fault at a high angle)

20. Strike Slip Faults • Eg. San Andreas, N. Anotolian Fault • Connection of segments, you can stress up one section and then another fires off • Is the San Andreas Week? – big debate • We don’t see enough heat at the SAF so • 1. hydrothermal fluids distribute heat efficiently • 2. the fault has low friction and is weak

21. Compressional Regime: example: Coloumb Wedge

26. Folds • In the lower crust, where things can flow, folds can form under compression • Anticlines – A shaped – oldest in middle • Synclines – U shaped – youngest in middle Small scale folding

27. Folding large scale: Sheep Mountain Anticline

28. Narrow Rifts: (Red Sea, East African, Rio Grande Rift (not active)) Wide Rifts: (Northern Basin and Range, parts Ageaen) Core Complexes: (Southern Basin and Range, parts Ageaen) Extensional Systems

29. Low Angle Normal Fault Paradox • Andersonian Fault theory predicts faults go inactive at 30o,but we observe faults with a lower dip in the field • No earthquake with a low angle mechanism has been observed (according to 99.9% of researchers • There are 1. active at a low angle, 2. inactive and cut by active, high angle faults 3. They form at a high angle and due to strain weakening can be active at lower angles, but eventually get cut off.

30. Brittle Ductile Transition • Top part – cold – is brittle • Lower crust is hot, ductile, it can flow. • The below picture is suspect (do faults really cut into the ductile crust and mantle this easily

31. LANF 2 • Roll over model: (Lavier 1999) • People at lamont say inactive or roll over. NCB and Mark Anders are huge in the anti-LANF camp.

32. Subduction Zone Summary FR force of viscous resistance, FSP slab pull force, FSA sea anchor force, FSU suction force

33. Subduction Accretion vs. Erosion accreting margin non-accreting margin

34. Rollback (which we don’t entirely understand)

35. Example Scotia arc falling back into the Atlantic

36. Mantle Convection • Down flux and recycling of material at subduction zones • Upward movement at plumes • Huge area of research • 1 or 2 layer convection? - someone else chime in here cool movies of convection at: http://www.gps.caltech.edu/~gurnis/Movies/movies-more.html

37. Geodynamics of Other Planets: Mars • Early Mars had plate tectonics but Martian crust is too light and thick to subduct • Subduction ceased, convection in mantle and core cease, and dynomo ceased • Volcanoes are evidence of past plate tectonics. Still active • There was also water but it is mostly gone now (small ice caps, some subsurface ice, some water lost to space, some tied up in sediments)

38. MARS

39. Venusaltimeter topography

40. Venus – latest theories • No impact of body that made moon on earth happened to Venus, so it kept early atmosphere. Green House effect ran-away evaporated ocean, which inhibited subduction and no volatiles were recycled. This kept the Venus mantle strong and there is no convection. Occasionally a singularity developes and huge amounts of volcanics are expelled on the surface.

41. Ice covered, but we don’t know how much Long rifts whole length of planet but no obvious compressional features Europa Io

42. Io • Tons of volcanoes due to tidal affects of sun, jupiter, and other moons. • Predicted and published by 3 sweet dudes a few months before Voyager arrived.