G. UFIMTSEV, (c)
by Gennady UFIMTSEV, Dr. Sc. (Geol. & Mineral.), Earth Shell Institute, RAS Siberian Branch, Irkutsk
Lake Baikal, a unique natural phenomenon, leaves no scientist cold. Scientists in other countries too.
Of late Baikal studies have become international - say, within the Baikal-Drilling Project involving research scientists of several countries.
Core samples of loose deposits are an object of close attention, as we learn from the literature and mass media (our magazine, by the way, carried a report back in 1995).
Yet a good deal is also done by lone-wolf explorers.
Suffice if we recall such big names as Ivan Chersky (1845- 1892), Academician Vladimir Obruchev (1863-1956), Gleb Vereshchagin (1889-1944) and many others who are behind what we know about Lake Baikal described as the gem of Siberia. Today we are witnessing quite a bit of rivalry between research teams and lone explorers.
Yet it's a fact: large research staffs often feed on ideas supplied by free lances.
ENIGMAS OF BAIKAL GEODYNAMICS
Lake Baikal is a big enigma indeed: the more you study it, the more question marks. I won't mention the main thing-the origin of this lake as a result of deep tectonic settling (trough, rift) filled with Cenozoic deposits (up to 65 million years old) over 6 kilometers thick. Now I would rather tell about the riddles in and around Baikal - that is what we can see on its shores and adjacent tracts of land in the light of new data now available to us. The most intriguing enigma is that of the seismotectonics of the Baikal basin (rift).
Here's what this problem is all about. Numerous traces of young tectonic shifts have been detected at the feet and scarps of the basin's boards: steep walls of rock as high as 12-15 meters, micrograbens (small subsidences) associated with young faults that often engulf the surface clastic product. This is indicative of the tectonic opening (fissuring) of fractured zones where mountain rock blocks have been, and still are shifting. Such kind of fractures of the contemporary surface of the earth are considered to be paleoseismodislocations, that is these are traces of earthquakes that, in our case, must have occurred more than 300 years ago. This is particularly conspicuous near the cape of Sredny Kedrovy on the western shore of middle Baikal.
The young fractures are often associated with the traces of gigantic cavings from steep scarps or else are thought to be a preliminary to such cavings (with narrow rock plates sliding). Such deformations of the relief are common in the zone of the Obruchev fault bounding the lake basin and shores in the west. The amplitude of young tectonic shifts is usually above 3 or 4 kilometers there.
However, the geographic distribution of the foci of contemporary earthquakes, according to instrumental study data, shows a completely different picture. In Baikal's south and further northeast these foci are conjugated with the central part of the basin (that is, at the Selenga delta and then towards the eastern board); and beyond the peninsula of Svyatoi Nos in the middle - with the eastern mountainous rim of the rift. As to the western board, specifically, the Obruchev fault, in the past hundred years this region is found to have been "seismically dull" - it has not generated tectonic tremors. We should also add that a maximum of seismic energy is released under Baikal at a depth of 11 km, immediately above a deconsolidated stratum of enhanced electrical conduction, the waveguide (12 to 17 km deep). It may be regarded as a horizontal stripping - a boundary dividing the lithosphere of the Baikal rift into two tectonic plates with different mechanic properties.
Yes, the situation is puzzling indeed: the large paleoseismodislocations and the extended Obruchev fault are situated on the western board of the basin, but contemporary seismic activity shows itself elsewhere, in the central and eastern parts of the rift. This is considered to be due to the flexural strains of the ancient surface of planation (peneplain). Yet the actual picture is far more complex. Ensembles of tectonic relief blocks of different height are located between the chain of the Khamar- Daban and Ulan-Burgasy ridges and the blocky rise of the Barguzin Range along the eastern coast, and also within the Baikal basin itself (in a strip of up to 30 km wide). Such ensembles occur in different forms-as step-like blocky rises, sunken and uplifted steps, and small faults; these compose the eastern board of the Baikal rift with a structure much more complex than that of the opposite, western board formed by the Obruchev fault.
Of common occurrence in the west are so-called intermediate steps which are the tectonic blocks splitting off from the mountainous rim of the lake basin and submerging. Quite often they enter into the mountainous rim at an inclination, which means they are shifting down the faults flattening out in-depth (geologists call that listric faults).
The latter move slowly into the powerful zone of the horizontal stripping, or waveguide. But on the eastern board of the rift (where listric faults are rare) the main faults of the upper plate abut against the boundary above.
Perhaps these differences in the structure of the western and eastern boards of the Baikal basin hold a key to the puzzle of its seismotectonics? Yet one thing is obvious: the structural characteristics of both account for quite different conditions of energy accumulation and release.
At this point I would like to suggest two equipollent hypotheses on the riddle of Baikal seismicity. The first hypothesis: the eastern board and the central part of the basin generate its background seismicity, while the western board, in the zone of the Obruchev fault, sees catastrophic, albeit rare, earthquakes. The other hypothesis can be formulated thus: the young deformations on the western board come as a result of sluggish shifts, while seismic activity is determined by the geodynamics of the central part of the basin and on the eastern board. But before adopting either of these hypotheses, we should first look into the young tectonics and geodynamics of the eastern board of the Baikal basin.
It has a complex geological structure in which two kinds of forms have been identified, characterized by gradual changes in their morphology; and so morphologically, we can group these forms into genetic series with consecutive transformations - from the young forms to the "relict", or "senile" elements. The first form is represented by large blocky scarps which, at their "mature age", became separated from the mountainous rim of the Baikal rift by submerged steps or small faults (the mountains on the Svyatoi Nos peninsula in the middle of the lake), with the "relict" forms located on the floor (the Posolskaya shoal rise at the Selenga delta). At all the stages of their development these structures of the "Svyatoi Nos" type have changed periodically the direction of their transpositions-from subsidences to rises. It is there, in the region of their location, that the general level of seismic activity increases.
The other kind of structural forms is a combination of a narrow coastal rise (horst) and a small fault behind. Grouped in a genetic series, they ultimately produced a "relict" element of transformation, the bay Proval ("Sink", "Pit") and the
Sakhalin shoal separating it from the lake... Now the reader may wonder: why Sakhalin, an island lying thousands of miles to the east, in the Pacific? But over there, on the coast of the Sea of Okhotsk, you can also find Baikalyonok, or "small Baikal", on one of the summits of the Jugjur range... The forms of the second kind, situated near the northern part of the Selenga delta, appeared suddenly in consequence of a disastrous shock in 1862 and another violent earthquake that hit the mid-Baikal area in 1959.
In-depth studies on the eastern board of the Baikal basin show: the main feature of its geodynamics consists in the restructuring of the upper lithospheric plate in the process of its horizontal (linear) deformation and attenuation. This is a rearrangement of tectonic blocks whose cross section is largely triangular or rhombic (with the largest, trapezoid); such blocks rest on the roof of a horizontal stripping, or waveguide. The interaction of the faults of the upper lithospheric plate with this roof must be playing a significant part in the contemporary seismogeodynamics of the basin. It is not accidental that a maximum of seismic energy in the rift, as we have said, is released under the roof of the waveguide, i.e. under the lake at a depth of 11 km.
RIDDLES OF BAIKAL TERRACES
Perhaps the biggest geomorphological riddle of Baikal is in its small lake-shore (lacustrine) terraces. At any rate, western Baikal is actually devoid of these horizontal or slightly tilted areas. Elevated shore lines are found only within the limits of intermediate steps which kind of "ride up", uplifting thereby some of the sites of the coastal zone. Such are the terraces of the Boguchansky island and the elevated, water-polished rock surfaces near the river Ledyanaya delta in the north. But lacustrine terraces are fairly common north of the Rel river delta, within the bounds of the Tyia- Kotelnikovo step.
These terraces have accumulated, besides beach deposits, also the material carried by inflowing rivers. As seen from their composition, these deposits were formed at a time when the level of the lake over the coastal slope kept rising fast. Here and there we come across the rear seams - what remains of the terraces - and the water-polished rock planes with indentations and embedded pebble.
Low terraces are more common on the eastern shore, between the Selenga delta and the Barguzin estuary. A whole series of such terraces, some of them over 150 meters high, is found on the capes of Valukan (eastern shore of northern Baikal) and Nizhneye Izgolovye (the southern promontory of the peninsula Svyatoi Nos). These are areas of young rises.
But we enter a real kingdom of lacustrine terraces on the Ushkanyi islands in the middle of Baikal. These islands are formed by summits of the subwater Akademichesky Range rising above water and dividing the Baikal rift into two major troughs. Here the terraces dominate the landscape - the Bolshoi Ushkanyi island has a staircase often such planes climbing up to 200 meters.
Baikal lake-shore terraces are local formations of different height. Therefore they cannot be integrated into a single-level chain along the perimeter of the lake. For the most part they are proper to the areas of young rises and are absent on the shores formed by the Obruchev fault, with the exception of the southeastern shore of Olkhon; the larger part of this island is made up by a young blocky rise tilted northwest and bounded in the southeast by a steep bench about 800 m high. Lying at its foot are lacustrine terraces of different height or locally elevated shore lines. The same is true of the position of terraces on the Ushkanyi isles.
WHAT ANCIENT VALLEYS TELL US
These paleovalleys offer yet another geomorphic and paleographic riddle of Baikal, a lake unique in many ways. This riddle is related to the changing direction of the runoff from the lake. We have three different hypothesis to this effect. The first one: formerly some of the Baikal water got into the
Lena river basin down the broad declines crossing the Angara- Lena watershed, and down the valleys of the Primorsky Range on the western coast, along the rivers Bolshaya Goloustnaya, Buguldeika and Anga. According to the second hypothesis, once upon a time there was a runoff into the Angara down the valley of the Irkut; from the Tunka depression that runoff reached the southwestern extremity of the lake but then took an abrupt turn northwards and crossed the Olkhon upland south of present-day Irkutsk. And finally, the third hypothesis: the Irkut might have emptied into Baikal through a deep intermontane depression linking the Baikal and Tunka faults. But that was hardly possible: for otherwise the inclination of the Irkut's bed in the low reaches would have been twice as high as what it is now, and under like conditions the river would have been "chained" to the mighty lake for good.
As far as the Baikal water runoff is concerned, one thing is more or less clear here: the Angara headsource (outflow in this case) is quite young, both geologically and morphologically, and it must have taken body and form in the Late Pleistocene, that is not more than 100 thousand years ago.
All the way back in the 1930s geologists located paleovalleys in the upper reaches of the Bolshaya Goloustnaya, the Buguldeika and the Manzurka; the bottoms of these river valleys are filled with thick deposits of alluvium, or fluvial deposits, later called Manzurka alluvium. The alluvium layer is often more than 50 meters thick. The longest of these valleys stretches from Baikal's western coast, taking its rise near the mouth of the Bolshaya Goloustnaya. Crossing the watershed, it goes further down the Manzurka and enters the Lena river at the village of Kachug. Subsequently Manzur alluvium was found on the scarp of the Obruchev fault, not far from the "head" of this old valley. All that is clear evidence that in the past the Baikal water flowed into the Lena river basin through large valleys rising from what is now the area of present- day Baikal.
But next came new puzzles. First, the complex temporal structure of the Manzur alluvium: while formerly it was dated to the Eopleistocene (absolute age, ca. 1.5 millions years), now it was found to consist of horizons, or levels, related to five ages - from the Middle Pliocene (over 2.5 million years) to the Upper Pleistocene (under 100
thousand years); all these levels, built of loose material similar in composition, belong to the fluvial deposits of large rivers.
This age-related differentiation of the Manzur alluvium suggests the idea of several generations of ancient valleys. One such system of the Middle Quaternary was found near Olkhon, between the lower reaches of the rivers Buguldeika and Anga. This system may include two major formations: one stretching for more than 35 kilometers in the foot of the scarp of the Primorsky fault, and the other coming onto the coast from Baikal, then joining the first formation, with both reaching toward the lake near the Buguldeika mouth.
The thickness of the Manzur alluvium on the beds of the old valleys exceeds ten meters, and its fluvial lamination indicates the presence of large channels here in the past. The pebbly material in the older alluvium is of local origin, similar in composition to the rocks of Baikal beaches. Therefore this second system of ancient valleys must be in no way connected with the Manzurka paleovalleys of the Baikal-Lena interfluve.
But yet another, and older valley system is found here, in the region adjacent to Olkhon. It comes upon Baikal at the cape of Krestovy; and on its bed resting on a leveled surface putatively of neogenic age (about ten million years) there have been found rocks now building the western slope of the Primorsky Range. Which means: the river in this paleovalley had a runoff toward Baikal; it crossed the contemporary Primorsky Range, and its upper reaches were situated on the site of yet another old valley - along the rivers Bolshaya Goloustnaya and Manzurka (Baikal waters drained into the Lena down this valley).
So, next to the western coast of middle Baikal are ancient valleys of at least three generations with differently directed flows of paleorivers: from the edge of the Siberian platform toward Baikal; from it to the Lena basin; and from and toward Baikal.
WAS THERE "GREATER BAIKAL" IN THE PAST?
Now, was Baikal far larger in size should we admit that its water engulfed the rift valleys of Barguzin, Upper Angara and Tunka? Had its water level risen by a mere twenty or thirty meters, the dry beds of the Upper Angara and Barguzin valleys and a significant part of the Selenga river valley would have become giant gulfs. True, the Tunka rift is situated
higher, and thus the water level had to rise by more than 200 meters to turn it into a gulf.
But this is a trilling problem to paleogeographers concerned with the paleoreconstruction of the Baikal region. Usually they point at the complex of Pleistocene sands (about 65 thousand years old) that occur in the Tunka and other rift valleys of the Baikal region. Such sands are deposited on valley slopes, and now and then fill their beds, something that tempts geologists to detect traces of Baikal "floods" in them.
Yet the above-mentioned fluvial lamination is apparent within these sands (were they not redeposited by subsequent wind erosion processes), which is evidence for major channels. These sands also carry shells of ground mollusks, they are elevated to significant heights and riven by numerous faults. Remnants of the fauna of what is known as the Upper Paleolithic complex (mammoth, woolly rhinoceros, northern deer, bison) have been found in the sands of the mid-Selenga mountains. Should we attribute these sands to a "Greater Baikal", then we must propose something quite out of the way: both the mammoth and the woolly rhinoceros were... water animals!
Discussing the "Greater Baikal" problem, one overlooks two important factors. First, Lake Baikal is situated at the periphery of Inner Asia with its sharply continental climate - low precipitation (mostly in summer) and anticyclonic sunny winter. Judging by the composition of deposits, such climatic conditions have been proper to this region for the last ten million years at least. And so, where could one take so much water from to fill a lake many times larger than modern Baikal?
And the second factor boils down to the following. What is important is that Baikal is a tectonic lake filling nearly all of the rift of the same name. In size this lake is comparable to the freshwater lake Tanganyika in east-central Africa. Are not these two lakes finite in dimensions for a given geological situation? That is why in all our constructions - tectonic, geodynamic, paleographic and geomorphological - we proceed from present-day Baikal.
Our work was supported by the Russian Fund of Basic Research (Grant No. 99-05-65638).
Опубликовано на Порталусе 14 сентября 2018 года
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