THE LIMITS OF UNIFORMITARIANISM: REASONS TO CONSIDER ALTERNATIVE TECTONIC STYLES ON ARCHEAN EARTH

The deep-seated geologic instinct for uniformitarianism makes it tempting to imagine that present-day plate tectonic processes operated as long ago as the time of formation of the oldest preserved rocks. There are, however, simple but compelling reasons that this should not be the default assumption. First, the absence of evidence for Earth-like tectonics on Mercury, Venus, or Mars indicates that developing a plate tectonic system is not an inevitable destiny for a rocky planet but instead requires a specific combination of thermal conditions and volatile inventory. While these planets have basaltic crusts that could be broadly equated with ocean crust on Earth, there is no evidence for efficient crustal recycling via deep subduction, which is arguably the defining characteristic of plate tectonics. Second, subduction on Earth today is driven by the strong slab pull force that arises largely from conversion of mafic oceanic lithosphere to eclogite, but eclogite formation may have been suppressed by higher geothermal gradients in Archean time. Field studies of partly eclogitized mafic granulites in western Norway indicate that eclogite-forming reactions are inhibited in the absence of hydrous fluids, a condition that may have been common when underthrust rocks became dehydrated at shallower depths on a hotter Earth. Third, the dimensions of most modern plates and plate boundaries are an order of magnitude larger than those of Archean terranes. Even if these could be considered microplates, the boundary conditions and forces on scores of tiny plates would be very different from those in a system dominated by a dozen large plates. Fourth, if Archean greenstone belts are equivalent to modern ocean crust and can be considered to be ophiolites, they point to a time when ophiolite formation – essentially failed subduction -- was far more common than it is today. The supposition that the tectonic system has always operated as it does at present implicitly discounts the possibility that various components of the system may have emerged at different times. It seems likely that Earth’s tectonic system evolved through several overlapping stages, with crustal production initially outstripping recycling (forming long-lived cratons) but becoming more balanced by Neoarchean time as true subduction became an important global process.