Emplacement of lava flows on eroded terrain, part I: The case of the Tiretaine valley (Chaîne des Puys, France)

Lava flows often erupt onto eroded landscapes, flowing across plateaus, into valleys and basins, and down escarpments. With the Tiretaine lava flow system in the Chaîne des Puys (Central France), lava flowed over the complex topography of an ancient rift margin. Erupted 41.3 ± 1.3 ka ago onto the granite plateau of the Massif Central, flow initially advanced over a surface that sloped 2°E. On reaching a fault scarp that delimits the plateau, flows descended the 6° slopes of a ravine that cuts the scarp E-W. In part 1 of this work we thus consider the case where lava is emplaced onto relatively flat terrain, then passes over a break-in-slope onto steeper slopes. To do this, we carried out mapping and facies analysis down this ∼6-km-long system, collecting 24 samples on which we completed textural and geochemical analyses. Results reveal three lava facies:(i)a 3 × 2 km proximal zone of inflated pāheohoe with a thickness of 25–50 m;(ii)a channelized medial zone with simple rubble levees nested in the ravine; and(iii)a valley-ponded distal zone with tubes and well-formed columnar jointing.