Tectonics on a sphere: the geometry of plate tectonics - The Solid Earth - page 40

11. Figure 2.29 depicts a mythical planet whose tectonic features comprise three plates
A, B and C with boundaries as shown. The plate boundary A-B is a fault with sense
of motion as shown and relative velocity of 4 cm yr − 1 , and that between B and C is
also a fault, with velocity 3 cm yr − 1 .
(a) Using angular velocity vectors, find the pole of relative motion between plates A
and C, giving its geographic coordinates. Calculate the relative angular velocity
in degrees per year (the radius of the mythical planet is 6000 km).
(b) Describe the tectonic nature of the boundary between plates A and C. Give details
of places where magnetic lineations might be found (the mythical planet has a
reversing magnetic field like the Earth's), what the maximum spreading rate might
be expected to be, and any other geological and geophysical information you can
deduce.

Greenwich
meridian

Equator

Plate C

Figure 2.29. A mythical
planet has three plates.

(c) Consider the triple junction where plates A, B and C all meet on the visible side of
the mythical planet. What will happen to the triple junction if this instantaneous
velocity description continues for several million years? In particular, think about
the way ridges spread symmetrically and trenches consume on only one side. Is
the junction likely to persist as drawn? What about the junction on the other side
of the mythical planet? Is it similar?

References and bibliography

Altman, S. L. 1986. Rotations, Quaternions and Double Groups .New York: Oxford
University Press.
Argus, D. and Gordon, R. 1991. No-net rotation model of current plate velocities
incorporating plate motion model NUVEL.1. Geophys. Res. Lett. , 18 , 2039-42.
Barazangi, M. and Dorman, J. 1969. World seismicity maps compiled from ESSA Coast and
Geodetic Survey, epicenter data, 1961-1967. Bull. Seism. Soc. Am. , 59 , 369-80.
Bath, M. 1979. Introduction to Seismology , 2nd edn. Boston: Birkhauser.
Bullard, E. C., Everett, J. E. and Smith, A. G. 1965. Fit of continents around the Atlantic. In
P. M. S. Blackett, E. C. Bullard and S. K. Runcorn, eds., A Symposium on Continental
Drift , Roy. Soc. London, Phil. Trans. Ser. A , 258 , 41-75.
Carter, W. E. and Robertson, D. S. 1986. Studying the Earth by very-long-baseline-
interferometry. Sci. Am. , 255 , 46-54.
Chase, C. G. 1978. Plate kinematics: the Americas, East Africa and the rest of the world.
Earth Planet. Sci. Lett. , 37 , 355-68.
Chu, D. H. and Gordon, R. G. 1999. Evidence for motion between Nubia and Somalia along
the southwest Indian ridge. Nature , 398 , 64-7.
Coode, A. M. 1965. A note on oceanic transcurrent faults. Can. J. Earth Sci. , 2 , 400-1.
Cox, A., ed. 1973. Plate Tectonics and Geomagnetic Reversals . San Fransisco, California:
Freeman.
Cox, A. and Hart, R. B. 1986. Plate Tectonics: How it Works . Oxford: Blackwell Scientific.
Dalrymple, G. B., Silver, E. A. and Jackson, E. D. 1973. Origin of the Hawaiian islands. Am.
Scientist , 61 , 294-308.
DeMets, C., Gordon, R. G., Argus, D. F. and Stein, S. 1990. Current plate motions. Geophys.
J. Int. , 101 , 425-78.