Plate Tectonics Lecture 3: Plate Motions, part 2

Transcription

1 Plate Tectonics Lecture 3: Plate Motions, part 2 By the end of this lecture, you should understand: How to determine stability and velocities of triple junctions. The evolution of the Pacific-North American plate boundary over the Cenozoic. The forces that drive plate tectonics (and how we know which ones are important). READING: Mussett & Khan, Chapter 20. Triple Junctions Triple junctions are places where three plates meet, and also where three plate boundaries meet. Note that the triple junction does not remain on all three plates, just on the boundaries. (Plates generally move with respect to triple junctions.) Rather, the defining characteristic of a stable triple junction is that it always remains on all three plate boundaries. Jimm Myers, UWyo Triple Junctions We name the type of triple junction by the type of plate boundaries that meet there: ridge (R), trench (T), or transform (F). The example below shows an RRR triple junction. 1

2 Stability of Triple Junctions Not all triple junctions that we could imagine or draw are stable. An unstable TJ would be one that can only exist for a geological instant, after which the plate configuration must change. An example is shown at left. The A-B spreading rate is greater than the A-C spreading rate. What happens to the triple junction through time? Stability of Triple Junctions To assess the stability of a triple junction we exploit the fact that it must always remain on all three plate boundaries. First we go to velocity space and draw for each plate boundary the alien submarine lines we drew in the last lecture which show all possible velocities for a point that starts on a given plate boundary to stay on the plate boundary. If the TJ is stable, those three lines must intersect at a single point. The point then shows the velocity of the TJ. Stability of Triple Junctions Since these three dashed lines don t intersect at a point, the TJ is unstable. 2

3 Stability of Triple Junctions: RRR Let s look at a ridge-ridge-ridge (RRR) TJ a type very common on Earth. We can construct a velocity diagram using the relative plate motions vectors (below right). Note that these must sum to zero and thus make a triangular plate circuit. Stability of Triple Junctions: RRR Stability of Triple Junctions: RRR A ridge-ridge-ridge TJ, in contrast, is always stable, because the perpendicular bisectors of the sides of a triangle always intersect at a point. 3

4 Stability of Triple Junctions: FFF A transform-transform-transform (FFF) TJ is always unstable. Stability of Triple Junctions: RTF A ridge-trench-transform (FFF) TJ is only stable if the transform is aligned with the trench (or, in a very special case, if the a-b line just happens to go through point C). Applying plate tectonic principles helped scientists (led by Tanya Atwater) deduce the evolution of the plate boundary at the western edge of N. America. 4

5 There must have been a continuous plate subducting off N. America prior to initiation of the San Andreas. What happens when the P-F spreading ridge intersects the F-N subduction zone?? Because the P-N velocity vectors happen to be parallel to the orientation of the N. American trench, a new transform boundary develops. Let s work through this to find the nature and velocities of the two new TJ s. TJ P P F F Because the P-N velocity vectors happen to be parallel to the orientation of the N. American trench, a new transform boundary develops. TJ The San Andreas is growing with time. Let s work through this to find the nature and velocities of the two new TJ s. 5

6 The San Andreas is growing with time. On a flat planet, plate motions can be described as simple translations and rotations: But on a sphere, we describe plate motions as rotations around a fictional Euler pole (pole of rotation): 6

7 The rotation means that relative plate velocities increase away from the pole: This increase of plate velocity away from the pole affects, for example, the temperature structure and thus the maximum depth of subduction earthquakes at some subduction zones: We can find the pole of rotation by projecting all the ridges (or perpendiculars to the transforms) and finding the point of intersection. 7

8 Possible plate driving forces including basal drive, ridge push, and slab pull. How can we tell which of these might be important? There are also some possible resisting forces, like basal drag and friction at slabs (F C ) or transforms (F TP ) How can we tell which of these might be important? We will compile absolute plate velocities in terms of plates that have lots of effective ridge length, effective subduction zone length. How can we tell which of these might be important? 8

9 We will compile absolute plate velocities in terms of plates that have lots of effective ridge length, effective subduction zone length, and large area. Plate velocities correlate very strongly with length of subduction zones, slightly with length of ridges, and very poorly with plate area. Conclusion: Slab pull is the most important driving force; ridge push also contributes; and basal drive is not very important. 9