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PLATE TECTONICS Last chapter in Davis and Reynolds OUTLINE OF LECTURE • • • • 1. 2. Earth engine Plumes Basic ingredients in plate tectonics Plate kinematics In 2-D On a sphere Review of major questions • Earth layering • The composition of the crust • Rheology of the Earth (lithosphere, asthenosphere) • Types of plate boundaries QuickTime™ and a Cinepak decompressor are needed to see this picture. Magnetic anomalies Spreading at mid-ocean ridges must be compensated by subduction. In addition,there are transform faults in the oceans. Note no volcanism on diagram. QuickTime™ and a Cinepak decompressor are needed to see this picture. What drives plate motion? Mantle drag forces and assembly of supercontinents QuickTime™ and a decompressor are needed to see this picture. Mantle convection • Time scales • Length scales • Plume heads and continental breakup T - scale ~ plate motions Length scales - appear much more complicated than the ridge-trench systems Model linking subduction to plume magmatism QuickTime™ and a Animation decompressor are needed to see this picture. Continental break-up: plumecaused? QuickTime™ and a Video decompressor are needed to see this picture. Sometimes clearly not. Other times, major oceans appear to form during times of major flood basalts -short lived, vigorous plume heads that may have broken the continents apart Plate T throughout Earth history • How far back in the past? • Different in the past? • How much longer will it last? QuickTime™ and a Cinepak decompressor are needed to see this picture. Evidence for PT goes back to the Archean. Faster motions, more melt, smaller continents (the continental nuclei known as cratons or “croutons”) Granite-greenstone belts; old zircons Zircons - as old as 40- 4.2 Ga; evidence for continental crust Continents-succession of orogenic events Future is fairly bright as far as PT goes. But after a while, (4 more Ga?), the Earth’s engine won’t have enough power to drive plate. Convection will stop, so will PT. QuickTime™ and a decompressor are needed to see this picture. Basic kinematic elements • • • • Plate boundaries, triple junctions Absolute plate motion, relative plate motion Euler poles Worked examples Ridges, trenches, transforms Triple junctions, quadruple j’s 3riple junctions are stable; more plates at a point - not stable Absolute plate motions - velocity in an absolute reference frame- say relative to a point outside the Earth. Or an assumed stationary long lived plume…. E.g. Hawaii Otherwise, one uses a relative velocity reference frame. One plate is kept stationary; the velocity of the others relative to the “stationary” plate is monitored. The understanding is that the entire system (including the stationary plate) is actually moving on the globe. In the case of ridges, we use the half spreading rate for velocity calculations. Absolute framework - consider Hawaii a stationary plume (it delivers melts in exactly the same spot over its entire history). We can calculate the velocity vector of the Pacific plate. 75-43 - N20W x cm/yr ; 43-0 Ma N70 W, y cm/yr. There are very few such long lived plume products and it is questionable whether they remain fixed. The common way of tracking plate motions is in a relative framework. Some useful rules: 1. Plate motions are transform parallel; 2. Plate moves away from ridge 3. The sum of relative plate velocities is zero*. *- that is because by definition plates are rigid. Velocity is a vector: magnitude, direction and sense. Examples 2. 1 Worked exercise It’s a right lateral transform boundary Finding the relative velocity of Farallon to North America Complicating a bit- what if the transforms are curved? We then have to admit there’s some rotation involved. Any rotation is achieved around a pole. From geometry, this is called the Euler pole. Transforms form arcs that are segments of circles centered in the Euler pole of a plate. Euler poles Example : Australia and New Zeeland QuickTime™ and a Cinepak decompressor are needed to see this picture. Plate tectonics on a sphere • Angular velocity, linear velocity • Rotations around Euler poles • Projections on stereonets Tectonics on a sphere requires that we use angular velocities v/r and r = R sin g where R is the radius of the Earth. So what? Check out the fig - predicts motion away from Euler pole. In this case - 2 plates with E at N pole Find distances on a sphere; use lat long and g The projections used in 3D plate tectonics are stereonets - equal area - however unlike your usual down view with geo structures, this is a side view. All calculations (angles etc) are similar. What you need to know: • The fundamentals of plate tectonics, driving forces; link to mantle convection; • Differences between present day and past characteristics of PT; • Be able to handle simple 2-3-4-… plate geometry problems in 2D involving only translations. • Calculate velocity vectors for such examples; • Know what the Euler pole is and angular vs. linear velocity. Be able to find one if you have the other. • The lithosphere is divided too into layers: upper mantle, lower crust, upper crust. There are essentially two types of lithosphere: oceanic and continental. You should know the approximate composition and dimensions of each of these. • The lithosphere is broken into about 8 large plates and a number of smaller ones. You should know the geography of these plates; where the boundaries are, what types of boundaries these are, and roughly how the plates are moving with respect to the hot spot • Earthquakes reveal the subsurface geometry of subducting plates (or slabs as they are often called), and show that the configuration of the slab can be quite variable. These seismic zones are called Wadati-Benioff zones. Extensional Tectonics Extension of the lithosphere may occur by several means: by a whole-scale pure shear (in which extension of the whole column occurs, the lower crust and upper mantle homogeneously) or by various asymmetric means (in which extension of the upper crust is laterally offset from the lower crust or upper mantle; extension may also be relatively discrete rather than homogeneous). • Normal faults may be planar or listric, the latter is more commonly shown. Faults in the upper crust are thought to sole into detachment faults that then transfer extension of the lower crust to another location. Faults are often considered passive or inert features where the hanging-wall slides down the fault and does all the deforming. But in fact, the footwalls of normal faults do a lot of deforming, too. Hence, the margins of rift or extensional zones are often lifted to form imposing flanks Convergent Tectonics Thermal Convection and Viscosity of a Fluid Convection in the Earth • Thermal convection is inferred to exist on a large scale in at least two regions in the Earth. The liquid outer core and the upper mantle that behaves as a solid for seismic wave propagation and as a very viscous fluid for long duration geologic processes including convection. Convection Reasons • Original heat from accretion and heat released during radioactive decay of unstable isotopes. • The natural, spontaneous, radioactive decay of unstable isotopes of elements that are distributed throughout the Earth, particularly in the crust and mantle. Viscosity Experiments • Newtonian viscosity is a law of friction for fluids. • Viscosity is defined as the shearing stress divided by the rate of shear for the fluids. • Viscosity can be thought of as resistance of a fluid to flow Possible driving forces for plate tectonics: • • • • • • bottom tractions by convection currents. trench pull. ridge push (sliding off a high) trench suck. global expanding or contracting forces membrane forces on spinning ellipsoid (e.g. variants of polar fleeing forces) • coupled: currents in the mantle raft the overlying plates around. Traction stresses at the base of the plates would be critical. • decoupled: plates move due to internal body forces, and influence the shallow convection current pattern in the mantle. • locally exclusive, but not globally. • Bouyancy driven by gravity acting on density contrasts caused by thermal differences and phase changes. Driving Forces