Lake Baikal, the Gem of Siberia drawing tourists by the thousand from around the world, has been and remains in the focus of attention of scientists. Scholars want to unravel one of the lake's major puzzles: how was its geological trough formed? The Baikal puzzle has been occupying the minds of scholars at various research centers in this and other countries. No consensus has, however, emerged to this day. A new theory explaining the formation of the Baikal trough has been developed at the Geology Institute of the Academy's Siberian Branch under Ivan Kulakov, Cand. Sc. (Geol. & Miner.). The theory, in his own words, runs as follows:
Despite its remoteness from major junctions of lithospheric plates, the Baikal region shows a high tectonic activity. Geological observations have established that the terrestrial crust in the region is drifting apart at a rate of 4 to 5 mm a year. Actually, the lake trough was formed as a result of this process. What, indeed, is behind this high level of activity at the center of the largest continent? There are several views attempting to answer this question.
First, the active rifting hypothesis, under which the crust is forced apart under the effect of the vast hot blob, or diapir, under the crust. Having risen from the underlying magma, the blob ran against the crust base and is now spreading flat underneath it and pulling the overlaying strata apart. This hypothesis is largely reinforced by gravitational and thermal field observations and some seismic data.
Another explanation attributes the phenomenon to passive rifting, which relates crust expansion to interaction between the lithospheric plates and their fragments. The Eurasian continent is not a solid land mass, but consists of large and small chunks frozen together. They certainly coalesced long ago and generally form a calm system. As soon as they are given a push, however, the continental fabric begins to yield at the seams. According to the proponents of this theory, crust expansion may be caused by stresses generated by the plates colliding at the periphery of the Eurasian continent (in particular, the Hindustan plate, which is still drifting northward, and the Pacific plate subducting under the Eurasian continent in the Far East).
Both theories are supported with powerful scientific arguments, but neither offers an exhaustive explanation of why the earth's crust is sprawling in the Baikal region.
Perhaps, the only way to look deep into the planet's interior is by taking seismic tomographs. The necessary signals can be generated by earthquakes whose waves penetrate through the crust, picking up data about any nonconformities across their path. It is possible, by registering the parameters of many waves, to decode their message by a technique based on the same principle as tomography, which is used, for example, in medicine to X-ray human internal organs.
Computers are used today to design complex algorithms capable of providing detailed images of the Earth's interior. Initial data create a problem, however: if they are scarce and of low quality, the results would be pitiful, no matter how perfect the algorithm and powerful the computer. This problem is particularly discouraging in the Baikal region, where a large number of seismic stations (critical for successfully applying the tomographic technique) can only be built at very high cost for a number of reasons.
To bypass it, my workmates and myself set out to look for alternative schemes. The greatest effect could be expected from the so-called Inverse Teleseismic Scheme (ITS). It is based on a very simple idea: the mantle under Lake Baikal can, surprisingly, be studied from data gathered by stations situated in other parts of the globe, provided, of course, that they can register earthquakes in the Baikal region. That means that we can cooperate with the world's leading stations whose data are stored in electronic form in the server of the World Seismological Center and are available to virtually any user. The quantity and quality of the data collected under this scheme is much more superior and, a further important consideration, allows any seismically active areas of the world to be examined quickly. To date, ITS has been used in Tibet, the Mediterranean, the
Altai-Sayani region, and Mongolia. And now came Baikal's turn.
Our studies, covering the area west of the lake as far as the Altai Mountains, have shown that the mantle under the southern mountainous fringe of Siberia has a distinctly cellular structure of alternating hot and cold anomalies. Probably, this reflects convective currents active in its upper part (at depths between 100 and 700 km). It is hard to say definitely whether or not this structure affects crust formation. There is, however, a certain discrepancy between the surface structures and the direction of flows within the mantle. For example, the Altai and Sayan mountainous systems conform to the descending flows in the mantle, while the depression of the Great Mongolian Lakes accords with the rising flows.
The results obtained for the area under Baikal itself have come as a complete surprise. Instead of confirmation of a hot body lying under the lake, we saw that the mantle under the Baikal fold region was more cool than otherwise. Our expectations were fulfilled elsewhere, however: contrasting hot anomalies have been detected at a depth of 200-700 km in the Upper Lena uplift area, under the Siberian Plate.
By combining these findings with other geophysical data we could develop a model explaining the mechanism influencing the evolution of the Baikal region. In particular, the negative seismic anomaly under the Siberian platform is the consequence of a hot stream (plume) welling up from the mantle. On reaching the base of the strong ancient Siberian Plate, the plume material spread beneath it. The plate edge being nearby, some hot material rushed toward it, only to be trapped in the junction area with another plate. Once in a new place, the hot material heated the lithosphere, saturating it with its compounds and altering its chemical composition. The strength of the lithosphere was significantly weakened, so much that the regional tensile forces generated by the interacting plates breached the crust exactly in this area, stimulating the formation and development of the Baikal trough. Based on actual facts, our hypothesis, therefore, combines elements of both the active and passive rifting mechanisms.
Although the final verdict on the situation deep under Lake Baikal is yet to be pronounced, we can definitely say that the results of seismic tomography have helped us immensely toward understanding the mechanisms behind the Baikal phenomenon.
Опубликовано 08 сентября 2018 года
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