Back-arc spreading in the proterozoic — a theoretical approach

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Abstract The early Proterozoic Earth probably had a hotter mantle and a lower residence time for oceanic plates at the surface than the modern one. The hotter mantle may have led to thinner oceanic plates. Young ocean floor has low negative buoyancy and therefore exerts low slab-pull forces in subduction zones. This effect would have been greater if the plates were thinner, since they would have warmed up more quickly as they descended into the mantle. Thus the negative buoyancy of slabs and the slab-pull forces were probably substantially lower in the early Proterozoic. Calculations show that this applies even if subduction rates were higher and the crust was thicker. Lower slab-pull leads to lower rates of ‘rollback’ in subduction zones and lower rates of retreat of upper plates from trenches. The limited age-range of oceanic crust and the more rapid heating of descending slabs in subduction zones probably leads to a smaller range of slab-profiles, since this range is controlled by thermal properties of the slab, and therefore to more stability in subduction zones. Thus, many of the proposed mechanisms for back-arc spreading may have been less effective, and backarc spreading may have been less common, in early Proterozoic subduction zones. This may explain the apparent absence of ophiolite complexes of that age.