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Traces of volcanic deposits and evidence of magmatic intrusions are often found within complex impact craters (~10 to 100 km in radius) at the surface of terrestrial planets. Relying on these observations on the Moon as well as on mechanical models of magma ascent, Michaut and Pinel (2018) proposed that the surface unloading due to a crater could provide a driving pressure to the magma stalling at depth, allowing its ascent through the crust despite its negative buoyancy. This effect increases with the impact crater size. On Venus, RADAR observations of the surface from the MAGELLAN NASA mission suggests two categories of impact craters: bright-floored and dark-floored craters (Figure 1), dark-floored ones representing a third of the crater population (Herrick et al, 1997). Both categories are found in the high plateaus as well as in the low volcanic plains. Dark-floored craters are interpreted as craters having smooth floors, hence appearing dark on radar images, because of partial filling by lava after their formation (Sharpton, 1994). Bright-floored craters would therefore represent a non-modified stage for craters. Dark-floored craters indeed appear shallower than bright-floored ones in rim-to-floor depth measurements. Since the smooth surface is localized within the crater and is in general surrounded by a bright aureole, these observations suggest that the magma could indeed ascend through the crust because of the crater unloading. Here, we use mechanical models of magma ascent in the crust combined with quantitative observations on Venusian craters to constrain the magma buoyancy and crust density on Venus that have allowed magma to reach the crater interior. We focus our study on a selection of craters in two main high plateaus of Venus: Ishtar Terra and Aphrodite Terra. Indeed, emissivity observations point to a low-density crust in the tesserae of the high plateaus (Gilmore et al, 2017), therefore suggesting that the magma may be negatively buoyant within the crust in these particular regions. The filling thickness of dark-floored craters is estimated from the depth difference between bright and dark-floored craters. We calculate the stress and pressure fields generated by a surface unloading on top of a semi-infinite half-space in a cylindrical coordinate system using an analytical model or using COMSOL Multiphysics for a surface unloading on top of an elastic lithosphere of finite thickness. From analytical models of magma ascent at the axis of a crater, we calculate the most probable crust to magma density ratio as well as magma storage depth that would fit the observed filling thickness. Results point to a magma slightly denser than the crust. Since the stress field due to a surface unloading also tends to horizontalize magma flow at shallow depth, we also use 2D numerical models of magma ascent to study magma trajectory below the crater (Maccaferri et al, 2011) and to constrain the necessary ingredients for magma ascent up to the crater interior. ACKNOWLEDGMENT This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement No. 101001689). REFERENCES [1] Michaut, C. & Pinel, V. (2018). Magma ascent and eruption triggered by cratering on the Moon. Geophysical Research Letters, 45, 6408–6416. [2] Herrick, R., Sharpton, V., Malin, M., Lyons, S., Reely, K. (1997) "Morphology and Morphometry of Impact Craters. University of Arizona Press, eds. S. W. Bougher, D. M. Hunten, and R. J. Phillips, pp. 1015-1046. [3] Sharpton, V. (1994). Evidence from Magellan for unexpectedly deep complex craters on Venus. Geological Society of America, Special Paper 293 [4] Gilmore, M., Treiman, A., Helbert, J., Smreker, S. (2017). Venus surface composition constrained by observation and experiment. Space Science Reviews [5] Maccaferri,F., Bonafede, M., Rivalta, E. (2011). A quantitative study of the mechanisms governing dike propagation, dike arrest and sill formation. Journal of Volcanology and Geothermal Research 208 (2011) 39–50 Figure 1. Examples of bright-floored crater (left) and dark-floored crater (right) in Aphrodite and Ishtar Terra (images obtained using JMars software). The craters represented here are Magnani and Piscopia. |
