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PLATE TECTONICS. Last chapter in Davis and Reynolds. OUTLINE OF LECTURE. 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
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PLATE TECTONICS Last chapter in Davis and Reynolds
OUTLINE OF LECTURE • 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
Spreading at mid-ocean ridges must be compensated by subduction. In addition,there are transform faults in the oceans. Note no volcanism on diagram.
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
Continental break-up: plume-caused? 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?
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
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.
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
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.
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
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
