Lower crustal flow kept Archean continental flood basalts at sea level

Large basaltic provinces as much as 15 km thick are common in Archean cratons. Many of these flood basalts erupted through continental crust but remained at sea level. Although common in the Archean record, subaqueous continental flood basalts (CFBs) are rare to absent in the post-Archean. Here we show that gravity-driven lower crustal flow may have contributed to maintaining Archean CFBs close to sea level. Our numerical experiments reveal that the characteristic time to remove the thickness anomaly associated with a CFB decreases with increasing Moho temperature (TM), from 500 m.y. for TM ≈ 320 °C to 1 m.y. for TM ≈ 900 °C. This strong dependency offers the opportunity to assess, from the subsidence history of CFBs, whether continental geotherms were significantly hotter in the Archean. In particular, we show that the subsidence history of the ca. 2.7 Ga upper Fortescue Group in the East Pilbara Craton, Western Australia, requires Moho temperatures >>700 °C. Applied to eight other unambiguous subaqueous Archean CFBs, our results indicate Moho temperatures >>650 °C at the time of eruption. We suggest that the decrease in the relative abundance of subaqueous CFBs over Earth's history could reflect the secular cooling of the continental lithosphere due to the decrease in radiogenic heat production.