INSTANTANEOUS VERSUS FINITE MOTION OF A LITHOSPHERIC PLATE RELATIVE TO ANOTHER PLATE

Bullard, Everett and Smith published their analysis of the best fit of the continents around the Atlantic Ocean in 1965. They introduced the idea of a "center of rotation," citing Euler's fixed-point theorem (1776). They wrote, "any displacement of a ... continent may be considered as a rigid rotation about a vertical axis through some point on the surface of the Earth." This was a very productive idea.

If we assume Earth is spherical to simplify modeling, the instantaneous relative motion of one rigid lithospheric plate as observed from another plate can properly be described as an instantaneous rotation around an axis through Earth's center, intersecting Earth's surface at two poles. And the displacement of a rigid plate from its position at one time to another time, viewed from a reference frame external to that plate, can also be properly described as a finite rotation around a stage pole. The "center of rotation" used by Bullard, Everett and Smith is a type of stage pole.

The idea that plate motion could be described as a rigid-body rotation led other geoscientists to propose kinematic explanations of transform faults, the shape of oceanic fracture zones, and the geometric stability of triple junctions in a series of influential papers published between 1965 and the early 1970s. These initial attempts were typically based on what Allan Cox called his second postulate of plate tectonics: "The pole of relative motion between a pair of plates remains fixed relative to the two plates for long periods of time" (Cox, 1973, p. 40-42). In describing the three-plate problem of plate tectonics, Cox recognized that his second postulate was not generally true in any plate system containing three or more plates (Cox, 1973, p. 408). Although the first-generation explanation of transform faults, the shape of oceanic fracture zones, and the geometric stability of triple junctions is still the norm whenever plate kinematics is discussed in geoscience textbooks published today, these ideas have been known to be fundamentally inaccurate for more than four decades. Transform faults, oceanic fracture zones and triple junctions are all features that result from the finite motion of plates relative to each other, which is not circular for most, if not all, plate pairs (bearspace.baylor.edu/Vince_Cronin/www/PlateKinematics/).