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Whole Earth Structure and Plate Tectonics Processes in Structural Geology & Tectonics Ben van der Pluijm © WW Norton+Authors, unless noted otherwise 3/29/2016 2:04 PM We Discuss … • • Earth’s topography Earth’s layers • • • Tenets of plate tectonics • • • • • Today’s plates Plate boundaries Kinematic of plate tectonics • • The crust The mantle Linear and angular velocities Absolute and relative motions Mechanics of plate tectonics Tectonic cycles • • Wilson Cycle Supercontinent Cycle Tectonics ©PSG&T 2 Earth’s Surface - 3D Topography Tectonics ©PSG&T 7 Hypsometric (=cumulative frequency) Elevation Curve Tectonics ©PSG&T 8 Earth’s Seismologic and Rheologic Layering strong weak Tectonics ©PSG&T 9 Crustal Section and Characteristic Rock Types Gabbro Granite 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 Gabbro 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 add to or change continental crust. Oceanic crust all forms at mid-ocean ridges by the process of seafloor spreading. Thickness 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. Heterogeneity 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. Age 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 about 4 Ga). Most oceanic crust, gets carried back into the mantle during subduction, so there is no oceanic crust on Earth older than about 200 Ma, with the exception of the oceanic crust in ophiolites that have been emplaced and preserved on continents. Moho The Moho at the base of the oceanic crust is very sharp, suggesting that the boundary between crust and mantle is sharp. The continental Moho tends to be less distinct. Tectonics ©PSG&T Granite 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 The Kinematics of Plate Tectonics Relative velocity Absolute velocity Mantle hotspots as “fixed” reference frame Tectonics ©PSG&T 21 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 22 Extra: Triple Junctions and Evolution (a) Stable ridge-trench-transform triple junction. (b) With time ridge-trench-transform triple junction location changes (from T to T’), but the geometry stays same. Tectonics ©PSG&T 23 Today’s Plate Motions (Absolute and Relative Velocities) Tectonics ©PSG&T 24 Absolute Motions and Speed Limit? Zahirovic et al., 2015 Tectonics ©PSG&T 25 Mechanics of Plate Tectonics - Driving Forces Gravitational forces (colored): • Ridge push: topographic spreading • Slab pull: negative buoyancy of slab Resistive forces (black) are relatively small Tectonics ©PSG&T 26 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 27 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 28 1 b.y. of Plate Motion (late Precambrian-Future) Scotese, 2004 Tectonics ©PSG&T 29