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Transcript
10/11/2009 Plate Tectonic Theory Lithosphere and Asthenosphere • Not continental ‘drift’ !! • Importance: – The unifying theory for all other theories in the Earth sciences – Has resulted in a more detailed understanding of Earth history – Has enabled geologists to more precisely (and cheaply) discover new oil, natural gas, and other subsurface natural resource deposits Important Definitions • Lithospheric Plate – a large segment of Earth’s crust and upper parts of mantle; extends to top of the low velocity zone • Lithospheric Plate Boundary (Margin) – Divergent - plates move away from one another – Convergent – plates move toward one another – Transform – plates move horizontally past one another Alfred Lothar Wegener (1880-1930) Part I Development of Plate Tectonic Theory 1 10/11/2009 Pangaea (Greek, meaning ‘all lands’) Wegener’s Evidence for Pangaea: • ‘Fit of the Continents’ – Continental margins fit together like pieces of a jigsaw puzzle • Paleontological – Similar fossils on opposite sides of ocean basins • Geological – Similar rock types on opposite sides of ocean basins • Climatological – Evidence of glaciation in tropical climates – Fossils of tropical plant species in polar climates Positions of the continents approximately 200 million years ago Fit of the Continents Paleontological Evidence Locations of Mesosaurus fossils Glossopteris 2 10/11/2009 Geological Evidence Glossopteris Geological Evidence Similarity of rock types and ages in the Appalachian, British Isles and Caledonian mountain systems Scottish Highlands Reconstruction of the Appalachian mountain system at the time it formed, approximately 300 million years ago Highlands, North Carolina Climatological Evidence 3 10/11/2009 Glacial Grooves Why was Wegener’s idea rejected? Wegener (left) and an Innuit guide on his last expedition in Greenland. This is one of the last photographs taken of Wegener, taken shortly before his death. Discovery of The Mid-Ocean Ridge System Mid-Ocean Ridge System Paleomagnetism – the study of Earth’s magnetic field as recorded in ancient rocks Magnetic minerals ‘point’ in the direction of the magnetic north pole. 4 10/11/2009 Apparent Polar Wandering: Two North Poles? Use of Magnetic Minerals to Determine Paleolatitude Apparent Polar Wandering: Single North Pole Step #2: Determine Paleolatitude From Magnetic Dip Step 1: Determine Magnetic Dip: a) 90o (NH) b) 63o (NH) c) 37o (NH) d) 60o (SH) e) 6o (SH) Cox and Dallrymple: Magnetic Reversals from all continents record changes in Earth’s magnetic field at the same time a)90o N (North Pole) b)42o N c) 18o N d)38o S e)3o S Cox and Dallrymple: The Magnetic Time Scale 5 10/11/2009 Harry Hess: The mid-ocean ridge is a site of sea floor spreading Vine and Matthews: Magnetic sea floor stripes record magnetic reversals Normal Polarity – exhibited by rocks with magnetic minerals which ‘point’ towards today’s position of the magnetic north pole Reversed Polarity – exhibited by rocks with minerals which ‘point’ towards today’s position of magnetic south As the sea floor spreads, repeated ‘stripes’ of normal and reversed polarity ocean floor basalts are erupted along the MOR Problem: What did the theory of sea floor spreading imply about the size of the Earth? 6 10/11/2009 Distribution of shallow-, intermediate-, and deep-foci earthquakes: Solution to Problem: Benioff Zones Benioff Zones Part II The Theory Types of Lithospheric Plate Boundaries Types of Lithospheric Plate Boundaries • Convergent Plate Boundaries • Transform Plate Boundaries – Characterized by compressive tectonic stress – Oceanic – continental convent boundary – Oceanic-oceanic convergent boundary – Continental-continental convergent boundary • Divergent Plate Boundaries – Characterized by shear tectonic stress – Continental-continental transform plate boundary – Oceanic-oceanic transform plate boundary – Characterized by extensional tectonic stress – Oceanic-oceanic divergent boundary – Continental-continental divergent boundary 7 10/11/2009 Oceanic-Continental Convergent Plate Boundary Formation of a subduction zone and continental margin volcanic arc • Subduction of higher density (mafic) oceanic lithosphere beneath lower density (felsic) continental lithosphere – Formation of magma in the mantle wedge – Formation of a continental margin volcanic arc on the overriding continental lithosphere • Examples: Cascade Range, Andean Range Cascade Range Oceanic-Oceanic Convergent Plate Boundary Andes Range Formation of a subduction zone and volcanic island arc • Subduction of (mafic) oceanic lithosphere beneath (mafic) oceanic lithosphere – Formation of magma in the mantle wedge – Formation of a volcanic island arc on the overriding oceanic lithosphere • Examples: Lesser Antilles, western Aleutians, Japanese islands 8 10/11/2009 Lesser Antilles Japanese Islands Aleutian Islands Continental-Continental Convergent Plate Boundary • Low density (felsic) continental rocks pushed upward by compressive tectonic forces – Medium to high grade metamorphism – Earth’s highest mountain ranges • Examples: Himalayas, Appalachians Appalachians Himalayas 9 10/11/2009 Formation of a mountain range along a continental-continental convergent plate boundary The On-Going Collision of India and Asia Approximately 45 million years ago Paleolatitudes of India (71 million years to present) Modern-Day Tectonic Setting of India Mount Everest, Himalayas Oceanic Lithosphere-Oceanic Lithosphere Divergent Plate Boundary • Results in partial melting of underlying ultramafic mantle rock, producing a mafic (basaltic) magma • Mafic lavas are erupted on ocean floor, forming new ocean floor basalts • Lithospheric plates on either side of the plate boundary mive in opposite directions • Off-set segments of the ridge axis are connected by transform faults 10 10/11/2009 Oceanic Lithosphere-Oceanic Lithosphere Divergent Plate Boundary Continental Lithosphere-Continental Lithosphere Divergent Plate Boundary Continental Rifting • Stretching, fracturing and sinking of continental lithosphere forms a continental rift (rift valley) – Example: East African Rift • Eruption of basaltic lavas and filling with seawater forms a linear sea – Examples: Red Sea, Gulf of Aden • Widening of linear sea forms an ocean basin East African Rift East African Rift Volcanoes 11 10/11/2009 San Andreas Fault Transform Boundaries • Continental lithospheric plates move laterally past one another – Transform fault – a fault which connects two off-set segments of a mid-ocean ridge • Characterized by frequent earthquake activity – Example: San Andreas Fault System Plate Tectonic Boundaries Western North America 50 ma 12 10/11/2009 Present Cascade Range Sierra Nevada Range and Coast Range Basin and Range Rocky Mountains 13 10/11/2009 Terrane Accretion – Formation of Coast Range Terrain vs. Terrane • Terrain – the natural features of an area of land; the lay of the land’ • Terrane – A crustal fragment whose geologic history is distinct from that of adjoining terranes – Accreted Terrane - a terrane accreted to adjoining terranes by tectonic forces Accreted Terranes in Western N. America Formation of Rocky Mountains and Basin and Range Provinces • Rocky Mountains – Major intraplate mountain system – Exhibits evidence of compressive stresses, typical of a convergent plate margin • Basin and Range – Major intraplate mountain system – Exhibits evidence of extensional stresses, typical of a divergent plate margin Rocky Mountains – formed due to ‘shallow’ subduction of oceanic lithosphere beneath western N. America. 14 10/11/2009 Rocky Mountains Basin and Range Basin and Range – formed due to ‘sinking’ of the subducted lithosphere, allowing for upwelling of hot mantle rock beneath the crust. Rio Grande Rift – part of the Basin and Range Rio Grande Rift Rio Grande Rift 15 10/11/2009 Mantle Plumes – Intraplate Volcanism Pacific Ocean Hot Spot Tracks The Driving Force of Plate Tectonics • Mantle Convection – Heat transfer from Earth’s core drives convection currents within the mantle • Slab-Pull/Slab-Push – Gravity ‘pushes’ oceanic lithosphere (slabs) away from mid-ocean ridge axes, and ‘pulls’ slabs into subduction zones • Hot Plumes – Hot rising plumes of mantle rock drive mantle convection currents Mantle Convection Slab-Pull/Slab-Push 16 10/11/2009 Hot Plumes Pangaea – 200 million years ago The Breakup of Pangaea The Breakup of Pangaea The Breakup of Pangaea The Breakup of Pangaea 17 10/11/2009 Present-day Tectonic Plate Rates and Directions of Motion Tectonic Setting of Northern Sumatra 18