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

Abstract

Over long enough time scales, an antagonistic balance acting between tectonics (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 implication is that the long-term uplift or down-warp history of ancient continental segments provides direct information on secular changes in the isostasy of the adjacent ocean basins through time. The amount of uplift can be estimated from the regional metamorphic grade. The burial pressures at present exposure of ten undisturbed Archean granite–greenstone segments are estimated to be ∼1.5±0.5 kb. From this, we infer a maximum long-term uplift and erosion of 5±2 km for stable continent since ∼3.0 Ga, implying a mean continental thickness of ∼46 km at the time of crustal stabilization. For a constant mean ocean depth, isostasy requires a palaeo-oceanic crust 14±2 km thick in the middle Archean. Alternatively, if little continent existed then, the oceans would have been shallower and the palaeo-oceanic crust 21±2 km thick. Assuming plate spreading did occur in the Archean, potential temperatures in the shallow mantle at ∼3 Ga were 90±30°C (or 170±20°C for no continent) hotter than those prevailing today. These temperatures translate into `time averaged' cooling rates for the shallow mantle of ∼57°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 against 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 evolution.