Morphotectonic inversion in the Tunka rift basin (southwestern Baikal region)

Abstract

The general basement subsidence trend in the Tunka rift is locally interrupted by uplift (basin inversion). The inversion uplift causes deformation to basin sediments and shows up in the surface topography as morphostructures of two types. Inversion in the area is either part of rifting, when the subsidence-to-uplift change is driven by the rifting mechanism, or perturbs the rifting trend as superposed Gobi-type mountain growth, but is never associated with change from continental rifting to other tectonic setting. The presence of buried erosion cutouts in the rift valley floor indicates that wave-like vertical motions, with erosion during uplift and deposition in the erosion cutouts during subsidence, superpose on differentiated (orogenic) motions. The latest phase of basin inversion acted in the Tunka rift in the second half of the Late Pleistocene-Holocene, and the amount of uplift varied from a few tens to a few hundreds of meters. The highest 300 m uplift was in the Tor rift basin, as estimated from relative elevation of its ~55 kyr sediments. In general, inversion uplift occurred over 40% of the Tunka basin area (872 km2 of 2240 km2), and about 450 km2 of this uplift (49% of the uplifted area or 20% of the rift valley floor) grew by the Gobi-type mechanism. Quaternary sediments lie with a hiatus upon the Neogene strata in almost all sedimentary sections of basin margins, thus indicating that the deposition area reduced for a long period in the rift history and reached the former extent only in the earliest Late Pleistocene.