Crustal reheating and mantle upwelling during continental break-up triggered by lithospheric instabilities

Oceanic basins are formed where continents are broken apart. At magma-poor continental margins, this has long been explained by uniform thinning of the lithosphere accompanied by passive upwelling of hot asthenosphere. Non-uniform, depth-dependent thinning has been proposed as an alternative to explain the anomalously shallow environment of deposition along many continental margins. Depth-dependent thinning models predict that the lower crust and sub-continental lithospheric mantle undergo a phase of increased heat flow during thinning of the lithospheric mantle. However, this early syn-rift heating is yet to be clearly documented at magma-poor continental margins and the physical mechanism responsible for initiation of depth- dependent thinning remains enigmatic. Here, we show that the lower crust of the Alpine Tethyan margin experienced reheating during· the break-up phase of continental rifting, consistent with the depth-dependent thinning hypothesis. Diffusion modeling of U-Pb ages and trace elements in rutile from the Ivrea Zone, Southern Alps, shows that conductive heating of the lower crust occurred during the transition from plate stretching to thinning and was followed by advective heating associated with emplacement of asthenospheric melts. Combined with dynamic models of the rifting process, we use these data to show that a mechanical instability forms in the lithosphere during coupling of deformation in the crust and lithospheric mantle. Growth of the instability drives depth-dependent thinning. Our results demonstrate that active mantle upwelling during formation of magma-poor rifted margins can be initiated by lithospheric processes alone and does not require anomalously hot asthenosphere.