What role did Tharsis formation have on the geomorphological history of Mars

On Earth, the characteristics of fluvial erosion (drainage direction, discharge, drainage area, and channel complexity) depends on climate (rain fall) and tectonic development (uplift rate, slope, and relief). Mars is a planet that experienced erosion driven by liquid water in its history, but its tectonics are vastly different from Earths. The formation of the Tharsis dome, a vast volcanic province represents a major magmato-tectonic upheaval for the planet. This event occurred during the early history of Mars, when it had an active water cycle, evidenced by the formation of fluvial valleys that have been preserved until present. The timing of the rise of the Tharsis province and the carving of valley network are debated (Phillips et al., 2001; Bouley et al., 2016). If they simultaneously occurred, it is likely we would see the tectonic change in Mars surface due to Tharsis affect the characteristics of fluvial erosion as listed above. Conversely, if both events are separate in time then the valley networks will have formed on a strictly Pre-Tharsis Mars, or a Post-Tharsis Mars. Nonetheless it is important to settle this debate in order to determine precisely the surface conditions prevailing during a period of time when the red planet could have seen the development of life.This study is the first to model the influence of the growth of Tharsis on the formation of the Valley networks. We present results from numerical simulations performed using a Stream Power Law (SPL) algorithm on Mars during the emplacement of Tharsis to assess how the patterns of drainage are affected by the tectonic development of the planet, in an attempt to reproduce the observed distribution of valley networks and their geometry. We find that the tectonic change due to Tharsis measurably affects drainage systems across the planet and adjust the timing until we reach a close match between our modelled network and the observed one. As Tharsis emplaces and regions uplift and subside in response, their local slope and relief is altered, having the effect of dispersing networks, lowering discharge, drainage area and erosion rate in uplifting regions and converging networks, increasing discharge, coalescing drainage areas, and increasing erosion rate in subsiding regions. This causes a preservation of older networks in the uplifting regions and a replacement of networks in subsiding regions, making it appear that certain valley networks are older and others are younger (Fig. 1). This suggests that not only is Tharsis emplacement necessary to produce the relief needed to form the observed networks on Mars today, but that a prolonged (and likely intermittent) period of fluvial activity on Mars must have occurred concurrently with the emplacement of Tharsis. In conclusion, Mars offers the opportunity to understand how landscape may evolve under a wider range of tectonic regimes, compared to Earth.Fig 1. A map of Mars withtotal topographic changehypothesised to have been caused by Tharsis (Matsumaya and Manga 2010), overlain with SPL model results divided into 15x15 bins and coloured based on when in the model time the valley networks best match with observations when the two are compared using a Discrete Fourier Transform (DFT) signal comparison. Where the observational data best matches model dataPre-Tharsis, at 3.8 Ga model time is marked in black, Mid-Tharsis at 3.65 Ga in dark gray, and Post-Tharsis at 3.5 Ga in light gray.Crosses on the map denote valley networks dated by Hoke Hynek (2009) with their respective ages.ReferencesBouley, S., Baratoux, D., Matsuyama, I., Forget, F., Sjourn, A., Turbet, M., and Costard, F.: Late Tharsis formation and implications for early Mars, Nature, 531, 344347, https://doi.org/10.1038/nature17171, 2016.Hoke, M. R. T. and Hynek, B. M.: Roaming zones of precipitation on ancient Mars as recorded in valley networks, Journal of Geophysical Research: Planets, 114, https://doi.org/10.1029/2008JE003247, 2009.Matsuyama, I. and Manga, M.: Mars without the equilibrium rotational figure, Tharsis, and the remnant rotational figure, Journal of Geophysical Research: Planets, 115, https://doi.org/10.1029/2010JE003686, 2010.Phillips, R. J., Zuber, M. T., Solomon, S. C., Golombek, M. P., Jakosky, B. M., Banerdt, W. B., Smith, D. E., Williams, R. M. E., Hynek, B. M., Aharonson, O., and Hauck II, S. A.: Ancient Geodynamics and Global-Scale Hydrology on Mars, Science, 291, 25872591, https://doi.org/10.1126/science.1058701, 2001.