Taking wax for a spin: microplates in an analog model of plate tectonics

Abstract

W atching the divergence of tectonic plates at a mid-ocean ridge is about as riveting as watching the grass grow; if you had the patience to stare for a year straight, you'd only see move ment of a few centimeters. And, incidentally, with about four kilometers of water between the sea-floor and the closest ship, you'd have to be clever to figure out just how to get a view. Now consider the excitement of watching millions of years of geologic time unfold in seconds, in the comfort of your own laboratory. It's like one of those high-speed movies of clouds skittering across the sky -except that the clock is ticking about 1015 times faster than usual and you're watching dynamic processes that are impossible to observe in their natural setting. We've been whiling away geo logic time just so, and not merely for entertainment.In a paper recently published in the New Journal of Physics [ 1 ] we describe a wax analog model that simulates the divergence of two brittle lithospheric plates above a ductile mantle, exactly as it occurs beneath the sea at a mid-ocean ridge over millions of years (the lithosphere is the mechanical boundary layer at the surface of the Earth where the strength of rock increases dramatically). This work breaks new ground in quantifying the kinematic evolution of a curious tectonic feature of mid-ocean ridges, the microplate. Here we give a summary of that work.In a world where tectonic plates glide slowly from their origin at mid-ocean ridges to where they founder at subduction zones, microplates remain trapped between major plates on a mid-ocean ridge and spin (geologically speaking) about a vertical axis. They grow as they rotate by accreting lithosphere at their edges, leading to a characteristic spiral pattern of pseudofaults, visible to the trained eye through sonar surveys of sea-floor topography. This was geologic theory anyway, derived from the creative minds of marine geophysicists who had never actually watched the sea-floor in motion [2]. We quantified this theory, applied it to detailed fiq .l; europhysics news S E P T E M B E R /o cA rticle available at