TS Must-Read – Mckenzie and Parker (1967) The North Pacific: an example of tectonics on a sphere

The paper describes how large, rigid, aseismic regions can be defined with Euler’s theorem, which describes the geometry of motion on a sphere, i.e. the Earth’s surface. Points on these large and rigid blocks move in relation to other blocks describing small circles set by their rotation pole. The paper describes this as “paving stone theory”, where tectonic plates are “paving stones”, and successfully demonstrates the validity of the theory, which we now call plate tectonics, for the North Pacific Ocean.

Arguably, the biggest strengths of the contribution lay in the link made between the motion of tectonic plates, manifested as the intrinsic kinematics of their boundaries, and several other sets of evidence, such as volcanic centers and earthquakes, and the orientation of vectors of slip in earthquake focal mechanisms. Using the latter, plate boundaries can be defined by the instantaneous motions along specified small circles; ridges, where plates diverge, trenches, where plates converge, and transform faults, where plates pass each other along a small circle.

In a talk at AGU Spring 1967, Jason Morgan entirely changed the topic of his abstract in favor of the description of an approach similar to that in McKenzie’s paper but with magnetic anomalies, not earthquakes. McKenzie read the original abstract but missed the talk, and later that year published the article. This led to a small controversy and is a clear sign that these ideas were settling among Earth scientists. A still larger controversy arose at the time of publication, as many colleagues failed to acknowledge a new theory that questioned their research. Nowadays, the main discrepant points for the theory of plate tectonics perhaps lies in the rigidity of plates and the localization of plate boundaries, or lack thereof, questions that can be seen to a certain extent as a matter of scale.

Commonly considered to be ‘the first article’ of plate tectonics, McKenzie and Parker 1967’s paper was regarded as a paradigm shift in Earth Sciences. Antecedent theories, like those of the shrinking or expanding Earth and that of geosynclines, were eventually put aside in favor of the new views laid by this article, a handful of contributions before, and many, many contributions after, which accounted for, and fitted, observations at a global scale.

Utsav Mannu, Pan Luo, and David Fernández-Blanco participated in the discussion. Utsav Mannu mentioned that the thought analysis and data presented in this paper makes it more like a long discussion, something that we see often in other disciplines (such as particle physics), but not so much in Earth Sciences. For this point, David and Pan explain that the most important contribution in McKenzie (1967)’s paper is to put all pieces together, which looks like building the first order architecture of a house, not to give detailed interpretations. McKenzie’s work in this paper is in this sense more similar to Diezt’s work on “seafloor spreading theory” (Dietz, 1961).

Written by David Fernández-Blanco, Pan Luo, Utsav Mannu and the TS Must-Read Team

References

Dietz, R., (1961). Continent and Ocean Basin Evolution by Spreading of the Sea Floor. Nature 190, 854–857 https://doi.org/10.1038/190854a0