Interior layered deposits of Valles Marineris, Mars: analogous subice volcanism related to Baikal Rifting, Southern Siberia

摘要

The relatively unknown volcanism of the Azas Plateau in the Tuva Republic is linked with the Baikal Rift system and it presents many excellently preserved tuyas that are volcanic edifices formed by subice and underwater eruptions. These tuyas were formed in the Quaternary, when eruptions characterized by trachybasalt and basanite compositions coincided with extensive glaciation in southern Siberia. Their stratigraphy is typically composed of very thick, weak and often layered hyaloclastite sequences, and near horizontal lava cap layers produced by subaerial eruptions. The interior layered deposits (ILDs) in Valles Marineris of Mars, in general, overlie unconformably canyon floors, canyon walls, and in some cases, chaotic terrain with onlap geometry. Many of these ILD mounds have either flat-topped or domal profiles. Thin layers of a few to tens of meters in thickness are extensively observed with relatively limited post-depositional deformations, stacking up to form hundreds to possibly thousands of meters-thick sequences. One example shows possible cross-beds, but bedforms in the layers are rare within the studied MOC images. These characteristics are indicative of low-energy depositional environments such as lacustrine basins or air-falls of dust or volcanic ash rather than sediments deposited in strong currents. Volcanic sheet flows and distal facies of turbidites, however, could be also similar to some thin layers of the ILDs. The ILD mounds are also associated with possible volcanic landforms such as feeding dikes, volcanic necks and vents. Cap units, if layered, are in general near horizontal whereas lower strata could have an angular unconformity relationship with the cap layers, which can be interpreted as lava-fed deltas. These observations support the hypothesis of subice and/or underwater volcanism for the formation of the ILDs. However, not all ILDs have ideal tuya morphology and stratigraphy, implying more complex history of the ILDs in Valles Marineris. The complex interactions between rift formation, associated volcanism, ice coverage due to climate change and floods are observed in the Baikal Rift system of southern Siberia and these could be direct analogues for the Valles Marineris region of Mars.