Longitude: Linking Earthʹs ancient surface to its deep interior

Earth scientists have had no direct way of calculating longitudes for times before those of the oldest hotspot track eruption sites in the Cretaceous (~ 130 Myr ago). For earlier times palaeomagnetic data constrain only ancient latitudes and continental rotations. We have recently devised a hybrid plate motion reference frame that permits the calculation of longitude back to Pangean assembly at ~ 320 Ma. This reference frame, here corrected for True Polar Wander (TPW), places most reconstructed Large Igneous Provinces (LIPs) of the past 300 Myr radially above the edges of the Large Low Shear wave Velocity Provinces (LLSVPs) in Earthʹs lowermost mantle. This remarkable correlation between surface and deep mantle features, which is also discernible for all hotspots with a deep-plume origin, provides a new way of reconstructing the original positions of LIP sites, and therefore the position of continents whose longitudes have hitherto been unknown. We place the 258 Ma Emeishan LIP eruption of South China at 4°N and 140°E, in that way constraining the width and the geometry of the Palaeotethys Ocean during the Late Permian. If LLSVPs have remained stable for even longer and TPW has been small, we can, under these assumptions, also restore Siberia and Gondwana longitudinally for Late Devonian (~ 360 Ma) and Late Cambrian (~ 510 Ma) times.