The Formation History of Olympus Mons from Paleo-topography

Introduction: The Olympus Mons shield volcano on the northwestern edge of Mars’ Tharsis rise is the largest volcano in the solar system. Its volcanic history is intimately tied with the volcanic and geodynamic history of Tharsis and of Mars as a whole. Previous studies used crater counting to estimate the age of the flanks of Olympus, with typical values of ~200 Myr [1]. However, establishing the formation history for Olympus Mons is complicated by the fact that each volcanic eruption resurfaces the flanks, erasing the previous crater record. Here we place constraints on the bulk formation history of Olympus Mons by reconstructing the paleo-flexural history of the surrounding region. Crater retention ages from lava flows on the flexural trough surrounding the edifice that pre-date the trough are used to constrain the onset of volcanic loading. Crater retention ages from the aureole deposits are used to constrain the age at which a significant fraction of the edifice was in place. These two ages bracket the main constructional period of Olympus. Paleo-topography of the Olympus Mons flexural trough. The concept of paleo-topography is used in terrestrial geodynamics to reconstruct the vertical motions of the lithosphere [2], but has seen less use in planetary applications [3]. Our paleo-topographic reconstructions are predicated on the fact that fluids (e.g., lava) flow along the path of steepest descent. Olympus Mons is surrounded by a large flexural trough that has been partially infilled with volcanic eruptions (Figure 1a). We surveyed the outer inwardsfacing flanks of the trough in MOLA topography using ENVI and ArcGIS to identify topographically discordant flows whose paths deviated significantly from the down-slope direction. Lava flows follow sinuous paths down-slope, with the flow vectors oriented within ±10° of the path of steepest descent. Several topographically discordant flows were identified on the slopes of the flexural trough that deviated from the downslope direction by more than this natural variability. In the northeast quadrant of the Olympus Mons flexural trough, one lava flow was observed to be flowing 78° away from the current down-slope direction, as determined by fitting a plane to the local topography (Figure 1b). The implication is that this lava flow formed before the flexural trough altered the region’s topography. To test this, we used thin-shell spherical harmonic models of the flexural response to volcanic loading at Olympus Mons [4]. The load was iterated to match the approximate topography of Olympus Mons, including Figure 1. (a) MOLA topography of Olympus Mons and the surrounding flexural trough. (b) MOLA topography and contour map showing the topographically discordant lava flow (arrows).