Lecture-6 1 Lecture #06- Plate Motions

Lecture-6 2 Tectonic Plates are Rigid “Caps” not Flat, Planar Sheets

Lecture-6 3 Plate Motions on a Sphere F Mathematically speaking, the most general way to describe the movement of a rigid plate (cap) on a sphere is through rotation about a fixed pole F The is known as Euler’s Fixed Point Theorem

Lecture-6 4 Plate Motions on a Sphere The pole of rotation for plate motion is an abstract mathematical concept, it is not a physical feature. The point representing the pole is known as an Euler pole.

Lecture-6 5 Euler Poles Euler poles for plate motions are different than the geographic North and South Poles. The Euler pole for the motion of a plate is often “located” on another plate

Lecture-6 6 Example of Euler pole describing the relative motion of Africa from a fixed, North American reference frame.

Lecture-6 7 Euler Poles and Ridges

Lecture-6 8 Euler Poles and Ridges/Transforms Ridge Segments “point” towards the Euler pole Corresponding transform faults are arcs of circles having the Euler pole as a center.

Lecture-6 9 GPS Derived Plate Motions Where is the Euler Pole ?

Lecture-6 10 Relative Plate Motions F It is important to remember that plate motions are generally given relative to a reference frame. F Example: The half-spreading rate vs. full spreading rate at a MOR.

Lecture-6 11 Relative Plate Motions Hypothetical three plate example: We know the relative velocity of B and C; we also know the relative velocity of A and B. What is the relative velocity of A and C???

Lecture-6 12 Relative Plate Motions F To solve this problem we note the basic law relating relative plate velocities: a V c = a V b + b V c F The velocity of plate C relative to plate A equals the velocity of plate B relative to plate A plus the velocity of plate C relative to plate B.

Lecture-6 13 Remember That Velocities are Vector Quantities

Lecture-6 14 Remember That Velocities are Vector Quantities (This is an example of adding two vectors graphically.)

Lecture-6 15 Solution to Problem The velocity of plate C relative to plate A has a direction that is perfectly horizontal, and a magnitude of (3)(1.414) cm/yr.

Lecture-6 16 Plate Motion Summary F Tectonic plates are spherical caps F The motion of tectonic plates is best described as rotation about an Euler pole F Plate motion is generally generally given with respect to a fixed reference frame F The velocities of plate are vectors, they have a magnitude and direction