Whole Earth Structure and Plate Tectonics Processes in Structural Geology & Tectonics Ben van der Pluijm WW Norton+Authors, unless noted otherwise 4/5/2017 14:45
We Discuss Whole Earth Structure and Plate Tectonics Earth s topography Earth s layers The crust The mantle Tenets of plate tectonics Today s plates Plate boundaries Kinematic of plate tectonics Linear and angular velocities Absolute and relative motions Mechanics of plate tectonics Tectonic cycles Wilson Cycle Supercontinent Cycle Tectonics PSG&T 2
Today s Plates and Plate Boundaries USGS Tectonics PSG&T 6
Earth s Surface - 3D Topography Tectonics PSG&T 7
Hypsometric (=cumulative frequency) Elevation Curve Tectonics PSG&T 8
Earth s Seismologic and Rheologic Layering strongosphere weakosphere Tectonics PSG&T 9
Crustal Section and Characteristic Rock Types Granite Gabbro Tectonics PSG&T 10
Crustal Thickness http://earthquake.usgs.gov/data/crust/ Tectonics PSG&T 12
The Crust and Geologic Provinces Tectonics PSG&T 13
Age of the Crust Tectonics PSG&T 14
Oceanic vs. Continental Crust Composition Continental crust has a mean composition that is less mafic than that of oceanic crust. Formation mode Continental crust is an amalgamation of rock that originally formed at volcanic arcs or hot spots, and then subsequently passes through the rock cycle. Mountain building, erosion and sedimentation, and continuedvolcanism addto or changecontinental crust.oceaniccrustall formsatmid-ocean ridges by the process of seafloor spreading. Thickness Heterogeneity Age Moho Continental crust ranges between 25 km and 70 km in thickness. Most oceanic crust is between 6 km and 10 km thick. Thus, continental crust is thicker than oceanic crust. Oceanic crust can all be subdivided into the same distinct layers, worldwide. Continental crust is very heterogeneous, reflecting its complex history and the fact that different regions of continental crust formed in different ways. Continental crust is buoyant relative to the upper mantle, and thus cannot be subducted. Thus, portions of the continental crust are very old (the oldest known crust is about4 Ga). Most oceanic crust, gets carried back into the mantle during subduction, so there is no oceanic crust on Earth older than about200 Ma, with the exception of the oceanic crust in ophiolites that have been emplaced and preserved on continents. The Moho at the base of the oceaniccrust is very sharp, suggesting that the boundary between crust and mantle is sharp. The continental Moho tends to be less distinct. Gabbro Granite Tectonics PSG&T 15
The Mantle - Tomography and Plates SW California SE North Carolina Red is slow, is hot Blue is fast, is cold P-waves S-waves Ritsema Grand et al., 1997 Tectonics PSG&T 16
The Tenets of Plate Tectonics http://denali.gsfc.nasa.gov/dtam/ Tectonics PSG&T 17
Types of Plate Boundaries Tectonics PSG&T 18
Examples of Plate Boundaries Convergent: Japan Divergent: Red Sea Transform: New Zealand Tectonics PSG&T 19
Insights from Earthquakes Tectonics PSG&T 20
USGS: https://goo.gl/aomekh IRIS: https://goo.gl/6l7pwq Global Earthquake Browser (IRIS) Tectonics PSG&T 21
USGS: https://goo.gl/aomekh IRIS: https://goo.gl/6l7pwq Global Earthquake Browser (USGS) Tectonics PSG&T 22
The Kinematics of Plate Tectonics Relative velocity Absolute velocity Mantle hotspots as fixed reference frame Tectonics PSG&T 23
Plate Kinematics on a Sphere Rotation on Euler pole (=rotation axis): a) Displacement follows small circles b) Transforms parallel small circle segments c) Same angular velocity (w) between plates Different linear velocity (v) as function of distance from Euler pole Tectonics PSG&T 24
Extra: Triple Junctions and Evolution (a) Stable ridge-trenchtransform triple junction. (b) With time ridge-trenchtransform triple junction location changes (from T to T ), but the geometry stays same. Tectonics PSG&T 25
Today s Plate Motions (Absolute and Relative Velocities) Tectonics PSG&T 26
Absolute Motions and Speed Limit? Zahirovic et al., 2015 Tectonics PSG&T 27
Mechanics of Plate Tectonics - Driving Forces and Plate Mineralogy Gravitational forces (colored): Ridge push: topographic spreading Slab pull: negative buoyancy of slab Resistive forces (black) are relatively small Tectonics PSG&T 28
Plate Tectonic Cycles: The Wilson Cycle a) -b) Continent rifts, such that crust stretches, faults and subsides. c) Seafloor spreading begins, forming a new ocean basin. d) The ocean widens and flanked by passive margins. e) Subduction of oceanic lithosphere begins on one passive margins, closing ocean basin. f) -g) Ocean basin is destroyed by continental collision. Tectonics PSG&T 29
Plate Tectonics Cycles: The Supercontinent Cycle (a) Continents gradually aggregate over a mantle downwelling zone. (b) While supercontinent exists, large-scale convection in the mantle reorganizes. (c) Upwelling begins beneath supercontinent and weakens it, leading to breakup. Tectonics PSG&T 30
1 b.y. of Plate Motion (late Precambrian-Future) Scotese, 2004 Tectonics PSG&T 31