PLATE TECTONICS Last chapter in Davis and Reynolds

OUTLINE OF LECTURE Earth engine Plumes Basic ingredients in plate tectonics Plate kinematics 1.In 2-D 2.On a sphere

Review of major questions Earth layering The composition of the crust Rheology of the Earth (lithosphere, asthenosphere) Types of plate boundaries

Heat engine- very efficient Earth differentiation- primarily by magmatism Mantle convection- Mostly solid state Melting shallow by adiabatic decompression Lithosphere- the cold lid at the top

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.

What drives plate motion? Most people agree that plates are intimately related to mantle convection; Slab pull? Ridge push? Mantle drag

Mantle drag forces and assembly of supercontinents

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

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 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.

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 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*. Velocity is a vector: magnitude, direction and sense. *- that is because by definition plates are rigid.

Examples 1 2.

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

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  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 

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 … 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.