Lower crustal flow and subaqueous Archean continents

Potentially higher sea levels do not fully explain why subaqueous flood volcanism on top of continental crust was common throughout the Archean. One possible way of maintaining basaltic piles several kilometers thick below sea level is via gravity-driven lower crustal flow of hot continental crust. In this study, we run numerical experiments to determine the relaxation time of a topographic load emplaced on continental crust as a function of Moho temperature. We use a visco-plastic model in which the viscosity depends on temperature. We apply the results of these models to the Fortescue Group, in the Pilbara craton. Data regarding the erupting time and stratigraphic thickness of the Maddina basalts, together with sedimentary structures pointing to shallow initial and final depth of emplacement, constrain the relaxation time for this section of the Fortescue Continental Flood Basalt to between 0.75 and 2 Myr. According to our modeling results this translates to a Moho temperature range of 610 to 690 °C for continental crust 40 km thick (580 to 665 °C for continental crust 30 km thick). This is significantly higher than the present-day Moho temperature range of 400 to 500 °C for Archean cratons, suggesting that the cooling of the studied continental lithosphere has been of approximately 200 °C over 2.72 Ga. The observation that many Archean Continental Flood Basalts failed to emerge despite their thickness suggests that gravity-driven lower crustal flow of hot continental crust was an efficient process that maintained continents below sea level throughout the Archean.