Plate Tectonics Basic Concepts

1. Plate TectonicsBasic Concepts • Earth’s lithosphere is composed of rigid plates • “Float” upon a hot, plastic layer of the upper mantle • Convection cells in mantle best current theory for driving force of plate movement • 7 major plates • Numerous smaller ones

2. Earth’s Tectonic Plates

3. Basic Concepts(continued) • Plates move relative to each other in three ways • Divergent – tensional stress at Mid-Ocean Ridges (MOR) • Convergent – compressional stress at subduction zones • Transform – shear stress at strike-slip faults or mid-ocean ridges • Divergent plate boundaries • New oceanic crust • ~6-11km thick • Mostly gabbros and basalts • Convergent plate boundaries can involve • Oceanic to oceanic subduction (mostly Pacific) • Oceanic to continental subduction (Western S.A.) • Continent to continent collision (Himalayas) • Tranform plate boundaries • Connect spreading center segments at MOR’s • Most seismic activity, volcanism, and mountain building takes place along plate boundaries • Pacific “Rim of Fire” • The centers of plates are mostly geologically stable

4. Pacific Ring of Fire

5. 3 Types of Plate Boundaries • Divergent • tensional • Convergent • compressional • Transform • shear

6. 3 Types of Plate Boundaries

7. Pangaea (all land)Alfred Wegener’s proposed “supercontinent”

9. Rifting and the Origin of Ocean Basins • Intercontinental upwelling of a convection cell causes crust to thin – tensional stress- and break creating rift zone • Red Sea • Gulf of California • Three arm rift zone • Two continue to form mid-ocean ridge system • One fails to form aulacogen • Rio-Grande, Mississippi valley

10. Rifting in Africa

11. Mid-Ocean Ridges • Production of new oceanic crust • Upwelling of ultra-mafic magma along linear or sinuous ridge systems • Hotter at ridge spreading centers • > cooling at > distance from rift • Topographic high along spreading centers • Ophiolite Suite – • Siliceous or carbonate sediments • Pillow basalts • Gabbros • Ultra-mafic peridotites • Alteration through heated seawater penetration along cracks and fissures

12. Mid-Atlantic Ridge Complex Divergent plate boundary Normal faulting graben structures

13. Transform Boundaries • Offsets perpendicular to ridge complexes • Rocks moving in same direction at distance from ridge complexes • San Andreas Fault system • Zig-zag ocean ridge system • Overridden by subduction zone • Transform boundary extends as subduction zone moves beneath continental plate • Additional motion is taken up in lateral movement

14. San Andreas Fault Zone • Transform fault • Right-lateral SS • Connects spreading segments • 1,300 km long • In places 10’s km wide • Pacific Plate grinding horizontally past North American Plate • 10 million yrs, • Average rate of about 5 cm/yr. • W side of fault zone moving NW relative to E side

15. San Andreas Fault • The Blanco, Mendocino, Murray, and Molokai fracture zones are some of the many fracture zones (transform faults) that scar the ocean floor and offset ridges. The San Andreas is one of the few transform faults exposed on land.

16. Subduction Zones • Oceanic Trenches • Deep, narrow depressions as subducting plate bends downward • Marianas Trench (>36,000ft deep) • Convergent Plate Boundaries • Reverse and thrust faults • Folded and faulted mtn ranges • Benioff zone – concentration of seismic events tracing a subducting plate • Volcanic Arcs • Typically on overriding plate • Partial melting of descending plate results in silicic volcanism on overriding plate

17. Subduction & the Benioff Zone • Variation of EQ foci depth • Deepen beneath the over-riding plate • Shallower, off-shore EQ’s can cause tsunami • Trace out the descending slab

18. 3 Types of Subduction Scenarios

19. Convergent Boundaries • Collision zones • Form large complex mountain chains • Alps, Himalayas, Appalachians, Urals • Suture Zone – where the ocean once existed between continents

20. Cenozoic tectonic settingof the SW U.S.

21. Continental Collision

22. Theoretical Driving Forces for Plate Motion • Ridge Push • From active mid-ocean ridges • Rising and extruding magma pushes plates apart • Compressional forces at ridges not seen • Slab Pull • Colder, denser plate subducting pulls plates apart • Tensional forces at ridge complexes • Plate Sliding • Cooling plate “slides” downslope from elevated hot ridge complex • Tensional forces at ridges • Convection cells • Thermal differences create upwelling and downwelling cells that provide the energy for plate movement

23. Convection cells as a driving force for plate movement