Cyclical summit collapse events at Ascraeus Mons, Mars

The sequential development of multiple summit calderas of shield volcanoes gives clues about how the spatial location of shallow magma chambers evolve s with time and how the magma supply into these chambers evolves. We re-int erpret the sequence in which caldera collapse events took place at the Mart ian shield volcano Ascraeus Mons and argue that the formation of the centra l caldera, usually accepted to be the most recent, was followed by two furt her collapse events. We also present structural evidence for an early, larg e central caldera and, therefore, also an early large magma reservoir. This challenges the current hypothesis for Ascraeus Mons that successive magma reservoirs have increased in size as this volcano matured and that a reserv oir occupied a central location within the volcano only at the end of its a ctive lifespan. We propose that there has been a cyclical oscillation in th e site of shallow storage of magma at Ascraeus Mons between central and per ipheral magma reservoirs. The earliest reservoir formed centrally, but magm a was subsequently redirected into peripheral reservoirs, over which the pe ripheral calderas formed. Later, the central location was utilized again, o nly to be followed once more by a development of peripheral storage reservo irs. This alternating pattern implies substantial variations with position and time in both the magma supply rate from the Martian mantle and the dist ribution of stresses within the volcanic edifice.