EVOLUTION OF THE OLYMPUS MONS CALDERA, MARS

Synoptic images of the Martian volcano Olympus Mons are of a quality and quantity that are unique for Mars and, somewhat surprisingly, are appreciably better than image data that exist for many volcanoes on Earth. Useful information about the evolution of shield volcanoes on Earth can thus be derived from the investigation of this extraterrestrial example. We have used shadow-length measurements and photoclinometrically derived profiles to supplement and refine the topographic map of the Olympus Mons caldera. As much as 2.5 km of collapse took place within the 80 x 65 km diameter caldera and the elevation of the caldera rim varies by almost 2.0 km (low around the oldest collapse events, high around the youngest). An eightstage evolutionary sequence for the caldera of Olympus Mons is identified which shows that caldera subsidence was a longterm process rather than the near-instantaneous event that has been interpreted from comparable terrestrial examples. Tectonic features on the caldera floor indicate a transition from an extensional environment (graben formation) around the perimeter of the caldera to compression (ridge formation) towards the caldera center. This transition from a compressional to extensional environment is surprisingly sudden, occurs at a radial distance of approximately 17 km from the caldera center, and is import because it can be used to infer that the magma chamber was relatively shallow (thought to be at a depth of < approximately 16 km beneath the caldera floor; Zuber and Mouginis-Mark 1990). Ample evidence is also found within the Olympus Mons caldera for solidified lava lakes more than 30 km in width, and for the localized overturning and/or withdrawal of lava within these lakes.