Emplacement of monogenetic lava flows on eroded terrain, Part II: The case of the Artière valley (Grave Noire, France)

• Published in: Journal of volcanology and geothermal research Vol. 438; p. 107812
• Main Authors: Harris, Andrew, Latutrie, Benjamin, Van Wyk de Vries, Benjamin, Saubin, Élodie, Foucher, Marine, Gurioli, Lucia, Zanella, Elena, Médard, Etienne, Nauret, François
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.06.2023; Elsevier
• Subjects: Break-in-slope; Earth Sciences; Effusion rate; Lava flow dynamics; Perched pond; Petrography; Sciences of the Universe; Seeps; Volcanology; ʻaʻā sheet flow; Perched pond; Effusion rate; Break-in-slope; Lava flow dynamics; ʻaʻā sheet flow; Seeps
• ISSN: 0377-0273; 1872-6097
• DOI: 10.1016/j.jvolgeores.2023.107812

At the Grave Noire on the eastern edge of the Chaîne des Puys (Central France) we have a well-exposed case of lava erupted directly onto steep slopes to pass, over an abrupt break-in-slope, onto into a flat-floored basin. Erupted 60 ka ago at the head of a fault scarp, flows descended the 25° slopes of the scarp to spread into the basin at its foot. We carried out mapping and facies analysis down the main (Artière-Cézeaux) lava flow, collecting 29 samples down its 6-km length and on which we completed textural, geochemical, and Anisotropy of Magnetic Susceptibility (AMS) analyses. Results reveal four lava flow facies:i)a near-vent zone of thin (≤ 0.6 m) ʻaʻā sheet flow on the steep (25°) slopes of the fault scarp;ii)a > 26 m thick lobate body of outgassed lava at the foot of the fault scarp;iii)a zone of lava-scoria breccia and marl intruded by dense, outgassed lava; and,iv)a 20–30 m thick distal flow of outgassed lava on slopes of 0.5–0.8°. Down flow trends in vesicularity and crystal content, as well as the AMS data, mass balance calculations and analogy with systems displaying similar facies, reveal high effusion rate (40–60 m3/s) fountain-fed flows fed an outlet channel on the steep slopes of the fault scarp. At the break-in-slope at the foot of the scarp, flows stalled, spread and dug into the country rock (marl) to become ponded behind a self-constructed barrier of lava breccia and cone material. Lava resident in this perched pond outgassed, intruded the base of the barrier and surrounding marl, and fed seeps in the breccia barrier. Seeps fed a slow moving (0.001 m/s) plug-dominated flow of dense lava that moved a further 3.6 km, digging down into the marl substrate by at-least 15 m and carrying a mixture of lava breccia, air fall scoria, debris flow material and cone rafts all the way to the flow front. As at the Tiretaine lava flow (Part 1, Latutrie et al., 2023) we find that breaks-in-slope aid in creating a system of reservoirs and transfers. For Grave Noire, rapidly emplaced flows on steep slopes fed a zone of storage (a perched pond) at the break-in-slope at the base of a fault scarp. The pond, in turn, fed slow moving flow across the flat floor of a basin. Interaction with an easily eroded substrate led to mechanical erosion, as well as 30–40 m of inversion of topography in the 60 ky since the emplacement of the flow system. Thus, the slope and type basement on which a monogenetic lava flow field is emplaced plays a fundamental role in lava flow dynamics, emplacement style and system architecture, as well as subsequent evolution of the topography. •The Grave Noire lava flows extended from steep onto flat of a UNESCO World Heritage site.•Facies analysis allows association of topographic effects and flow field morphology.•At the break in slope at the base of a fault scarp, a perched pond was formed.•The pond slowly leaked to feed slow moving, outgassed flow that excavated its substrate.•Slope & type basement play fundamental roles in system architecture at monogenetic fields.