Plate Tectonics Lecture 2: Plate Motions

Transcription

1 Plate Tectonics Lecture 2: Plate Motions By the end of this lecture, you should understand: How to construct plate velocity diagrams How to transfer plates and plate boundaries from map space into velocity space Rules for velocity lines of each plate boundary type in velocity space READING: Mussett & Khan, Chapter 20. Rules of Plate Tectonics The lithosphere is in motion with respect to the underlying asthenosphere. Lithospheric plates move with respect to each other. Plates are rigid that is, all activity is at the plate boundaries. There are three types of plate boundaries: ridges, trenches, and transforms. All motion on a sphere can be represented by rotations around a pole. We draw the three types of boundaries schematically like this: Trench (T) Ridge (R) Transform Fault (F) The teeth on the trench symbol go on the upper plate. The arrows would be reversed on a left-lateral transform fault. 1

2 The type of boundary (R, T, or F) that exists between two plates depends on (1) the orientation of the boundary, and (2) the relative plate motion vector. For example, what kind of boundary would occur in the figure below? (arrow is motion of plate B relative to A) A B Or The type of boundary (R, T, or F) that exists between two plates depends on (1) the orientation of the boundary, and (2) the relative plate motion vector. What about now? A B The type of boundary (R, T, or F) that exists between two plates depends on (1) the orientation of the boundary, and (2) the relative plate motion vector. And now? A B 2

3 The type of plate boundary between two plates can change along the boundary if the orientation of the boundary changes: B A The Pacific-North American plate boundary shows all three types of boundary (ridge, trench, transform fault). Transform Faults vs. Fracture Zones How will these markers on the seafloor move with time? 3

4 Transform Faults vs. Fracture Zones Transform faults show the direction of relative plate motion. Plate Motions Our approach will be to look at the kinematics (description of motion), rather than the dynamics (driving forces) of plate tectonics. Plate velocities are vectors. We can draw the vectors by choosing a reference frame (usually one of the plates in the problem). BV A = The velocity of A with respect to B AV B = The velocity of B with respect to A Note that A V B = - B V A Plate Motions In an absolute reference frame, we can also describe the velocities of plates A and B as: V(A) and V(B) Thus, A V B = V(B) V(A) B V A = V(A) V(B) 4

5 Plate Motions: Spreading Ridge For a symmetrically spreading ridge, the velocity of the ridge (V R ) is at the midpoint of V(A) and V(B). That is, All units are km/my Plate Motions: More than Two Plates Let s add a trench to the previous plate configuration. Does plate B grow or shrink with time? => Depends on spreading rate A V B Plate Motions: More than Two Plates Let s look at this in velocity space. Case 1: Fast spreading ( A V B = 60 km/my) V R What is the velocity of the ridge? Plate B grows because ridge moves west w.r.t plate C. 5

6 Plate Motions: More than Two Plates Let s look at this in velocity space. Case 2: Slow spreading ( A V B = 30 km/my) V R What is the velocity of the ridge? Plate B shrinks because ridge moves east w.r.t plate C. Plate B is doomed to disappear. Plate boundaries move they are not fixed on the Earth or relative to each other. in Velocity Space Linear features (e.g., plate boundaries) in map space map to lines in velocity space. The line in velocity space represents all possible velocities for which a point originally on a boundary remains on the boundary through time. In this example, let s look at a hotspot that is currently situated on a spreading ridge. It will move north through time but REMAIN on the ridge. in Velocity Space Linear features (e.g., plate boundaries) in map space map to lines in velocity space. The line in velocity space represents all possible velocities for which a point originally on a boundary remains on the boundary through time. After some spreading, we ve created new oceanic crust on each side of the ridge, and the hotspot has moved north. Note that there will be seamounts at the previous positions of the hotspot (H 0 ). 6

7 in Velocity Space Linear features (e.g., plate boundaries) in map space map to lines in velocity space. The line in velocity space represents all possible velocities for which a point originally on a boundary remains on the boundary through time. Finally the hotspot has moved further north. There is a line of seamounts that create a hotspot track. What is the velocity of the hotspot in velocity space? in Velocity Space: Ridges Linear features (e.g., plate boundaries) in map space map to lines in velocity space. The line in velocity space represents all possible velocities for which a point originally on a boundary remains on the boundary through time. The line of all possible velocities falling on the ridge is the perpendicular bisector of the line connecting V A and V B. in Velocity Space: Transforms For a transform, the velocity line connects the velocities of A and B: The line of all possible velocities falling on the transform connects the velocities V A and V B (dotted line above). 7

8 in Velocity Space: Trenches For a trench, the velocity line goes through the upper plate: The line of all possible velocities falling on the trench goes through the upper plate velocity and is parallel to the orientation of the trench (dotted line above). 8