Valles Marineris tectonic and volcanic history inferred from dikes in eastern Coprates Chasma

摘要

Magmatic dikes have been proposed to have weakened and fractured the crust, allowing the formation of Valles Marineris. Hence, dikes were studied in the region of eastern Coprates Chasma in an area that includes a major transition between Hesperian-aged volcanic deposits in the western walls and pristine Noachian crust in the eastern walls. Over a hundred dikes were identified. Dike widths are 13 m on average. Estimation of magma eruption rates are comparable with previous estimates for Hesperian lava flows on Mars (10~5 to 10~6 m~3 s~(-1)). Dikes dips range from 55° to 90°; orientations record two distinct main tectonic stress fields (90° and 70°) different from the Chasmata. Dikes striking 90° are only observed at elevations below 1500 m. Dikes striking 70° are observed at elevation below 0 m and are therefore considered older. However, linear features are also observed on the late Noachian/early Hesperian surrounding plateaus and could be related to the 70° dike group. In the western part of our study area, dikes (~10% of the total amount of dikes mapped) strike 110, subparallel to Valles Marineris, and suggest a relationship between dike emplacement and graben formation. The presence of preexisting faults in the two directions (90° and 70°) could explain the shape of eastern Valles Marineris and chaotic terrains, which have a different general orientation than the Valles Marineris main rift. Our results suggest a complex relationship between dike emplacement and the formation of Valles Marineris.