Lecture 13: Global Tectonics This starts the final phase of this class; Where we draw upon everthing weÕve learned to explain Earth observations and how the Earth works. Tectonics provides the unifying theory for geology of the Earth it is a paradigm. Paradigm -- a unifying concept upon which explains a very large amount of data and upon which new ideas are based and tested. --Virtually nobody questions plate tectonics today. It is like atomic theory. Evidence for Plate Tectonics --Continents fit together to form Pangaea (Òall landsÓ); -- broken up into Gondwanaland in the south, and Laurasia in the north. -- Has all of the problems we talked about before (--no mechanism). Paleomagnatism provides some of the most compelling evidence for horizontal rock movement. --The study of paleo-magnetic direction, deduced from the orientation (inclination) of magnetite grains. --In either igneous or sedimentary rocks [draw picture of settling magnetite grains.] Inclination changes with latitude because of magnetic field [draw] --Can only get latitude info, not longitude info. On continents, magnetite grains in old rocks give notion of apparent polar wandering. Of course, it is the rocks that have wandered, not the pole. The pole has, however, randomly switched from north to south during Earth history. These switches have been carefully studied and dated, so that they form a time scale. Normal and reversed poarity --Note randomness is impt. Soon discovered that seafloor is ÒstripedÓ with respect to paleomagnetic orientation. This discovery showed that seafloor increases in age away from the ridges. Therefore, seafloor must form at the ridges, hence the name spreading ridges. Also, the oldest seafloor (Jurassic in the western Pacific) is located near a trench. Therefore, seafloor must be destroyed at convergent-margin trenches. Plate Velocity We like to know how low long things take to happen and the rates of geological processes. --Velocity is a vector, -- involves direction and rate. --Relative Velocity (or motion) vs Absolute Velocity (or motion). Relative velocity is what one would see if he were moving also; with respect to another plate For example, magnetic lineations on either side of a spreading ridge. Absolute velocity is with respect to the mantle. On Earth, recorded by hot-spot volcanism. Characteristics of Plate Margins -- Characteristics of plate margins so that we can apply knowledge to old rocks in order to deduce EarthÕs tectonic history. All the action takes place at plate margins, while regions away from plate margins are pretty stable. Plate margins, however, can be fairly wide; ranging from Santa Cruz to the Sierras, or even Utah. i.e., the strain is distributed over a wide geographical area. --Plate margins divided by rock type and plate motion Oceanic vs continental divergent, convergent, transform Know characteristics of various kinds of plate margins. Divergent Margins Oceanic Topography -- always a large ridge Formation of oceanic crust here: lots of basaltic volcanism -- Sometimes a rift in the middle (Tensional environment), depends on fast- or slow-spreading Elevation decreases away from ridge crest because of interplay between temperature, isostasy, and density At ridge, lithosphere is very thin, allows asthenosphere to inflate upwards. Away from ridge, asthenosphere cools to form lithosphere (like ÒfreezingÓ, but both are solids). Gradual cooling makes a denser lithosphere, which ÒfloatsÓ lower on the asthenosphere. Thus, depths increase away from ridge because of simple (i.e., we understand) conductive cooling. Continental here it is called a rift Ex: Red Sea Rift, Rio Grande Rift formation of normal faults, horsts and grabens. Basaltic and rhyolitic (crustal melting) volcanism Actually, rifts evolve into spreading centers if tension and extension continue long enough (millions of years). Example, mid-Atlantic spreading must have started in middle of Pangea continent. It is an evolutionary thing, see fig 16.20 Basin and Range is another type of expression of tensional tectonics. Could also evolve into spreading. Convergent Margins Deep-sea trenches and volcanic-arc volcanism. Trenches form where plates subduct. Very deep EQs (up to 700 km) record the minimum depths to which these EQs subduct. Subducting slab is cold (has been at surface) and cools region around margin, making cool geotherm. Vol.canic arc = magmatic arc = island arc (oceanic) = continental voclanic arc (continental) = line of volcanoes in a line parallel to the trench. Characterized by large strato-volcanoes. Marianas, Japan (oceanic), Andes, Cascades (continental). Subducted slab releases fluids, lowers solidus of overlying mantle wedge, which melts. Produces basaltic magma that differentiates to andesite plus. Probably involved in continental crust formation. At least forms bouyant material that can later be accreted. Accretionary Wedge Material scraped off the downgoing plate just above the trench. Margins can be accretionary or erosional Melange very deformed material formed within the subduction zone; usually matrix is easily deformed muds, or serpentine, blocks often oceanic seamounts, chert, etc... Fold and Thrust Belt -- The kind of deformation that occurs in accretinary prisms where the rocks are more coherent. Fore-arc ridge and basin Names for topographical regions on top of the accretionary wedge between the arc and trench. Continent-continent convergent margin is a collision, like the Himalayas or Papua New Guinea. Collisions characterized by Shallow to moderate EQs over a broad region. Little volcanism; granitic intrusion instead. Intense metamorphism from crustal thickening. Young mountain ranges, i.e., high and jagged More on this later, under mountain building. Transform Fault Margins -Where two plates slide past one another. -If there is some tension or separation, a little volcanism can occur, like on the San Andreas --oceanic is basaltic; continental may be more differentiated. - Minor mountain building can also occur if there is a small amount of compression along the fault. In oceans, there may be large differential in elevation on either side of the fault because of difference in crustal age. San Andreas may be a good example. Triple Junctions -- Where three plate boundaries come together. Because of Geometry and plate motion, triple junctions can move [draw Mendocino Triple Junction] Plate Interiors Hot Spots --Oceanic islands, or volcanic regions on land, formed by volcanism above hot spots Hot plumes of material rise to surface to produce volcanoes. Hawaiian volcanoes (for example) get older away from Big island of Hawaii. Like hand moving over cigarette. These provide estimates of absolute plate motion. Formed by plumes [draw plume with plume head] Plume heads cause flood basalts or oceanic plateaus Examples include the Columbia River Flood Basalts or the Ontong Java Plateau. Continental Shields --assemblage of ancient rocks that have come to tectonic and isostatic equilibrium; comprised of cratons and oregens. Craton -- the oldest rocks at the core of the continent. Could be the oldest orogen. Orogen -- groups of rocks surrounding the craton that were all deformed duing a single deformational or mountain building event. Usually elongate in shape like mountains. They are the deep roots of ancient mountain belts. -Note distinction craton vs orogen on fig 16.19; three pages later fig 16.26 all is called craton These together form continental shields. Bed rock identified by high grade metamorphism, granitic plutonism, and ductile deformation. Shields can have sediments (often undeformed) deposited on top of the basement rock. Passive Continental Margin Why is this in Plate Interiors? Because the edge of a continent (like the East Coast) can be in the middle of a plate (like the North American Plate. Great accumulation of sediments cause subsidence. These huge pile of sediments deform when the margin finally enters asubduction zone. Before plate tectonics, these huge piles of deformed oceanic sediments were called geosynclines. Colllisions and Mountain Building Anything thick and bouyant enough to resist subduction will cause a collision. These can be continents, island arcs, or oceanic plateaus. Compressional deformation common: fold and thrust mountain belts. The Suture is the main fault between the two former land masses that have collided. Ophiolites common in these areas. Ophiolites: fossil Òoceanic crustÓ, but most often with chemistry of volcanic arcs. Òsupra-subduction zone setting; something to do with subduction and convergent margins. Folds and Thrusts (crustal thickening) common, Metamorphism and isoclinal folding closer to the core and suture zone. Note examples of Appalachians, Alps, and Canadian Rockies in book. Accreted Terranes Classic theory did not explain western Cordillera very well, thus, terrane theory. Terrane -- a fault-bounded sequence of rocks that cannot be unequivocally related to the surrounding rocks on the other side of the faults. Rocks can be different ages, stratigraphy, deformational style, depositional environment, paleomagnetic character. Suspect Terrane -- is used if it is not clear where the terrane came from. Accreted Terrane -- refers to the addition of new material to the continent. This may be a primary way of continent growth. --Much developmental work happenned on the west coast and in Alaska, where it was very difficult to explain the rocks any other way. Plate Tectonic Engine Not sure what makes plate tectonics tick. Know that convection occurs, but not easily matchable with tectonics. Know that the plates themselves exert some influence. Greatest correlation is between plate motion and subduction motion, or at least percentage of plate actively subducting. Not neccessarily expected. If mantle movement controlled everything, spreading ridges would not subduct. Also, transform faults, overlaps, etc... difficult to explain if upwelling at ridges drives motion. Slab Pull is most accepted today, although does not explain everything. 1) Slab Pull -- blueschist and eclogite metamorphism make lithosphere denser than asthenosphere, so the slab sinks. 2) Ridge Push -- eidge is higher than surrounding abyssal plain 3) Basal Drag -- friction between lithosphere and asthenosphere: this may be positive or negative 4) Friction along transform faults 5) Friction between plates at convergent margins -- not all subduction zones thought to be highly frictional. Plates not accelerating -- therefore forces are balanced Problem with slab pull -- North American Plate.