Guest blogger: Marcella Jurotich
Life Cycles. Lakes tend to be relatively short lived in geological time scales. The same water that flows into lakes, bringing water and nutrients, also brings sediments some of which become trapped, eventually filling the lake. So, lakes have life cycles from formation by such events as glacial melt, river flooding, or damming by ice, landslide, or lava. As they slowly fill with sediments their nature changes from being deep and clear to being shallow and cloudy. Eventually some lakes become wetlands before disappearing completely. Exceptionally, some lakes are long lived, ancient and large. Two such lakes in Asia are Lake Baikal in Russia and Lake Hövsgöl in Mongolia. Lake Baikal is by far the larger, incredibly old, storing over 20% of the world’s fresh water and is estimated to be over 20 million years old. Lake Hövsgöl is part of the Lake Baikal watershed. It is smaller but also over a million years old. Both of these ancient lakes are rift zone lakes, formed when land subsides between two rift zones, then the subsidence area becomes filled with water.
Rift Zones. The Baikal rift zone is a ~2000 km long rift system that extends from the Stanovoy Mountains through Lake Hövsgöl and encompasses most of the eastern edge of the Amurian-Eurasian plate boundary (Fig. 10). The rift is ringed by the Siberian craton to the north and Sayan-Baikal fold belt in the southeast (Fig. 10; Zhao et al., 2006; Yang et al., 2018). Normal faults and half-grabens that developed in the Late Cenozoic dominate the rift system and are parallel to the rift axis (Fig. 10; Zorin et al., 2003; Zhao et al., 2006). Lake Baikal, for which the rift is named, has been filled with water for ~8.4 ma, and is the deepest lake in the world (Ivanov and Demonterova, 2009).
Origins. The origin of the Baikal rift dates back to the Eocene-Oligocene (Tapponnier and Molnar, 1979; Zonenshain and Savostin, 1981; Ivanov and Demonterova, 2009). However, the exact reason for the rifting remains contentious. While Arzhannikova et al. (2018), Zonenshain and Savostin (1981), and many others assert that deformation radiating outward from the Indo-Eurasian collision initiated rifting, Zorin et al. (2003), Zhao et al. (2006), and others argue that rifting in this region began due to an upwelling of the upper mantle.
Earthquakes. The Baikal rift zone is seismically active with 13 earthquakes over Mw 6.5 in the past 280 years (Zhao et al., 2006). There is little magmatism in the rift basin (Zorin et al., 2003; Yang et al., 2018), and volcanism that does occur is offset from the rift axis. Rather, volcanism is concentrated on Tuva-Mongolia massif and on the Amurian plate. Rift-related volcanism took place through in the Miocene-Oligocene and intensified into the Quarternary during a period of rift development and increased rates of rifting (Zorin et al., 2003; Arzhannikova et al., 2018).
Biodiversity. Lake Baikal and Lake Hövsgöl are ultra-oligotrophic, very clear with few nutrients. Oligotrophic lakes tend to have low diversity due to the low concentrations of nutrients at the base of the food web. However, the deep, ancient, tectonically active nature of these lakes gives rise to complex lake structure or morphometry that supports diverse communities of invertebrates, microbes, and fish, particularly in Lake Baikal. Ancient rift lakes are fascinating in their origins and in their characteristic biodiversity and will be an ongoing feature of Tethysphere.
Arzhannikova, A., Arzhannikov, S., Braucher, R., Jolivet, M., Aumaître, G., Bourles, D., and Keddadouche, K., 2018, Morphotectonic analysis and 10 Be dating of the Kyngarga river terraces (southwestern flank of the Baikal rift system, South Siberia): Geomorphology, p. 94-105.
Ivanov, A.V., and Demonterova, E.I., 2009, Tectonics of the Baikal rift deduced from volcanism and sedimentation: a review oriented to the Baikal and Hövsgöl Lake systems: Biosilica in Evolution, Morphogenesis, and Nanobiotechnology, p. 27-54.
Tapponnier, P., and Molnar, P., 1979, Active faulting and Cenozoic tectonics of the Tien Shan, Mongolia, and Baykal regions: Journal of Geophysical Research: Solid Earth, no. B7, p. 3425-3459.
Yang, H., Chemia, Z., Artemieva, I. M., and Thybo, H., 2018, Control on off-rift magmatism: A case study of the Baikal Rift Zone: Earth and Planetary Science Letters, p. 501-509.
Zhao, D., Lei, J., Inoue, T., Yamada, A., and Gao, S. S., 2006, Deep structure and origin of the Baikal rift zone. Earth and Planetary Science Letters, no. 3-4, p. 681-691.
Zonenshain, L. P., and Savostin, L. A., 1981, Geodynamics of the Baikal rift zone and plate tectonics of Asia. Tectonophysics, no. 1-2, p. 1-45.
Zorin, Y. A., Turutanov, E. K., Mordvinova, V. V., Kozhevnikov, V. M., Yanovskaya, T. B., and Treussov, A. V., 2003, The Baikal rift zone: the effect of mantle plumes on older structure: Tectonophysics, no. 1-4, p. 153-173.