Contrasting geometric adjustment styles of drifting continents and spreading sea floors

Abstract

The geometry of moving oceanic lithospheric plates is modeled easily by combining plate subduction and destruction with an application of Euler's theorem to the Euclidean sphere of the earth's surface. However, continental masses must respond to drift motion in a different manner because they cannot sink into the denser mantle. A theoretical geometric analysis, tested by data from the eastern Pacific margin, is presented to illustrate the unique response to drift of large continental masses. If they are large enough to comprise a significant fraction of the earth's circumference, an alignment of continental masses should undergo extension until the frontier between them and the ocean over which they encroach approximates a global great circle. This extension should entail secondary sea-floor spreading transverse to primary spreading directions, localized where continental masses along the frontier are narrow and therefore weakest, and ductile stretching and fracturing of the narrow continental spans. Continued sea-floor spreading and continental drift beyond the stage at which the continent-ocean frontier approximates great circle configuration presents a new problem. The maximized frontier length must now adjust to a decreased area of encroached ocean. Adjacent continents must now approach each other along the frontier direction, and formerly stretched and disrupted continental spans must be ‘bent’ to account for the excess frontier length. Recent reconstructions of post-Paleozoic continental drift indicate that the American and Antarctic continents and intervennig Caribbean and Scotia seas and island arcs may have undergone such a history.