Metamorphism, denudation and sea level in the Archean and cooling of the Earth

Over long enough time scales, an antagonistic balance acting between tecton ics (uplift and rifting) and the sedimentary cycle (erosion and deposition) probably keeps continental surfaces close to sea level We argue that this balance constitutes the primary mechanism for keeping long-term continental freeboard constant by regulating the continental thickness. An important i mplication is that the long-term uplift or down-warp history of ancient con tinental segments provides direct information on secular changes in the iso stasy of the adjacent ocean basins through time. The amount of uplift can b e estimated from the regional metamorphic grade. The burial pressures at pr esent exposure of ten undisturbed Archean granite-greenstone segments are e stimated to be similar to 1.5 +/- 0.5 kb. From this, we infer a maximum lon g-term uplift and erosion of 5 +/- 2 km for stable continent since similar to 3.0 Ga, implying a mean continental thickness of similar to 46 km at the time of crustal stabilization. For a constant mean ocean depth, isostasy r equires a palaeo-oceanic crust 14+/-2km thick in the middle Archean. Altern atively, if little continent existed then, the oceans would have been shall ower and the palaeo-oceanic crust 21+/-2 km thick. Assuming plate spreading did occur in the Archean, potential temperatures in the shallow mantle at similar to 3 Ga were 90 +/- 30 degrees C (or 170+/-20 degrees C for no cont inent) hotter than those prevailing today. These temperatures translate int o 'time averaged' cooling rates for the shallow mantle of similar to 57 deg rees C Ga-1 and are comparable to estimates of the present-day cooling rate for the whole Earth. The low burial pressures are a strong argument agains t the existence of catastrophic mantle flush events in the geological past which have been predicted by some mantle convection models. Taken together, these constraints suggest a plume-related geological setting (i.e. oceanic plateau-like, flood basalt) for the initial stage of greenstone belt evolu tion. (C) 1998 Elsevier Science B.V. All rights reserved.