On the Evolution of Planets: From Convective Bi-stability to Volcanic Edifice Instability
Doctor of Philosophy
The Eastern Olympus Mons Basal Scarp (EOMBS) is conditionally stable when the edifice contains pore fluid, and critically stable, or in failure, when the edifice contains a dipping-overpressured-confined aquifer and mechanical sublayer at depth. Failure of the fault bounded portion of the flank results in estimated volumes of material ranging from 5600–6900km3, or 32–39% of the estimated volume of the “East” Olympus Mons aureole lobe. We suggest that the EOMBS faults may be an expression of early stage flank collapse and aureole lobe formation. Ages of deformed volcano adjacent plains indicate that this portion of the edifice may have been tectonically active at <50Ma, and may be coeval with estimated ages of adjacent outflow channels, 25–40Ma. This observation suggests that conditions that favor flank failure, such as water at depth below the edifice, existed in the relatively recent past and potentially could drive deformation to the present day. Coupled 3D mantle convection and planetary tectonics models are used to explore the links between tectonic-regimes, the level of internal heating (Q) within the mantle, planetary surface-temperature, and planetary lithospheric-strength. At high and low values of Q, for moderate to high yield, hot and cold single-plate planets prevail. For intermediate Q, multiple stable tectonic-states exist. In this parameter space, the specific evolutionary path of the system has a dominant role in determining its tectonic state. For low to moderate lithospheric yield strength, mobile-lid behavior (a plate tectonic-like mode of convection) is attainable for high degrees of internal heating (i.e., early in a planet’s thermal evolution). This state is sensitive to climate driven changes in surface-temperatures. Relatively small increases in surface-temperature can be sufficient to usher in a transition from a mobile- to a stagnant-lid regime. Once stagnant, a return to mobile-lid is not attainable by a reduction of surface-temperatures alone. For lower levels of Q, the tectonic regime becomes less sensitive to surface-temperature changes. These results indicate that terrestrial planets can alternate between multiple tectonic-states over giga-year timescales. Within parameter space regions that allow for bi-stable behavior, any model-based prediction as to the current mode of tectonics is inherently non-unique in the absence of constraints on the geologic and climatic histories of a planet.
Planetary evolution; Geodynamics; volcanic evolution