Geology
Geological and tectonical development of the Lake Baikal Region,
Siberia
Lake Baikal is one of the most impressing geological features
on earth. It is the central part of the Baikal-Vitim rift zone with a total
length of circa 1,800 km.
[geological overview of the Baikal-Vitim rift, Brink & Taylor (2001)]
The whole rift-system consists of 15 large basins, from which
the lake covers the central three, and the small but deep Selenga basin. Southwest
of the lake is the Tunka basin, which we also visited on the field trip. The
rift is situated on the southeastern border of the Siberian craton and lays
within the Sayan – Baikal fold and thrust belt, which has its origin in
Precambrian and Palaeozoic collisions of terranes and continental blocks with
the Siberian craton.
[geological map of Siberia, Dolginow & Kropetshjow (1994)]
These fold and thrust belts can be subdivided into the Sayan
- Jenissej fold zone southwest of the lake with the Tunka basin and the 1,500km
long Baikal fold zone around and north of the lake. This fold zone is very narrow
in the south around the lake with a width of circa 150 km, but it widens to
the north to up to 600 km. The rocks of this fold zone have mainly Precambrian
age.
[geology of the Lake Baikal region]
There are some relicts of Archean greenstone crustal cores
and granite – gneiss – domes, which show a polyfacial and polycyclic
metamorphism of granulite and amphibolite facies, but most rocks are metamagmatic
and metasedimentary rocks from the lower Proterozoic stage. Around the lake,
these are marbles, granulites, amphibolites, shists, gneisses and granitoids.
Also within the fold - zone are relicts of oceanic
crust and island arcs with ultrabasic
rocks and metamorphosed basic magmatics. The island arcs were once in front
of the southern shore of the Siberian craton, before it collided with Laurentia
and the supercontinent Rhodinia was formed. Rhodinia broke up in the Upper Proterozoic
(~700 Ma) at the place where today the rift is. The Primorskij fault, which
we visited in the Chernorud region, is a relict of this break-up.
The youngest rocks of the fold zone are from the Lower Cambrian. They were deformed
by the last step of folding in the region during early Caledonian stage. Some
Vendian and Cambrian small granite intrusions strike through the fold complex.
Some geologists think that the development of the Baikal fold zone in the Palaeozoic
is an early stage of tectonic magmatic activation, which is related to the development
of the Baikal – Vitim rift system. However, many of these old structures
of the fold and thrust belt were reactivated in the Cenozoic and are now major
tectonic structures of the rift.
The three major basins of Lake Baikal are the Northern, the
Central and the Southern Basin.
[major tectonic elements of the Lake Baikal region, Brink & Taylor (2001)]
The Central and the Southern basin are the deepest ones. The basins reach
depths of over 9 km, from which the most are filled with sediments.
[Model of the Central Basin underground, Brink & Taylor (2001)]
The maximum water depth is reached in the Central basin with
1,650 m below lake level (1,190 m below sea level). The border between the Northern
and Central basin is the “Academician Ridge”, a complex tectonic
block between Olkhon island and the Svyatoy Nos peninsula, mostly covered by
water today. The only part of the block that is not under water is the Ushkany
island famous for its seals.
[tectonical structures of the Central basin with the Academician Ridge, Mats
et al. (1999)]
The border between the Central and the Southern basin is approximately
at the height of the Selenga Delta.
The main tectonic structures are the faults on the western side of the basins.
They are on the border between the western mountain ridges and the rift-basins.
They can be easy seen as major topographical elements. The major faults of the
rift at the part we visited are from north to south the Baijkalsky fault at
the northern basin, the Morskiy fault at the Central Basin, dividing the Olkhon
block from the basin, Primorskij fault at the Maloye More Strait, Obrushevsky
fault at the southern basins and the Tunka fault, dividing the Sayan mountains
from the Tunka basin. These are deep listric faults reaching the earth´s
mantle, dipping to the east / southeast, so large blocks are tilted on the faults,
giving the basins a half graben like character. A good example for this is the
Olkhon block, which is tilted on its western border on the Primorskij fault,
so that the western part of this block is under water forming the Maloye More
Strait, and the eastern part at Olkhon island reaching heights up to 1,200 m.
As already said, these faults have their origin in the Proterozoic, but were
reactivated in the Cenozoic/Mesozoic.
The rift is dominated by extensional tectonic with a strike-slip component,
resulting from the south-eastward movement of the Amur-Plate relative to the
stable Siberian platform. Measurements by GPS showed, that the rifting rate
is 4.5 mm/a in a WNW-ESE direction at the lake, and 6.3 mm/a in a 125 ±
15° direction between Irkutsk and Ulan-Bataar (Mongolia). In a distance
of circa 50 years, earthquakes with strengths of over 6.5 on the Richter-scale
happened. These major earthquakes occurred at the Morskij and the Obruchevskij
fault.
[earthquakes at the Baikal Rift, Déverchère et al. (2000)]
The Moho below the rift is in a depth of 39 to 42.5 km, above
this is a 8 km thick, lateral high velocity zone (7.05 – 7.4 km/s). It
is not completely clear, if this zone is a magmatic addition to the lower crust
or a relict of the Pre-rift-lower crust. There are some evidence, that this
high velocity zone is a crust relict, which means, that the rifting is just
related to the middle and upper crust. Such a rifting could be achieved when
the major faults soling into a midcrustal detachment.
[cross-section of the crust under the Baikal Rift, Brink & Taylor (2001)]
There is a second type of faults we´ve seen, crossing
the main faults on a right angle. For example, the Sarma Fault crosses the Primorskiy
fault north of Chernorud, or the Kingirga fault crosses the Tunka fault at Archan.
These faults have some hydrogeological importance, because many rivers use these
faults, for example the Angara. On the crossing points between this two faults
types, thermal springs can occur, as we´ve seen them a lot, for example
at Archan or the Kotelnikovsky spring.
The responsible force for the rifting and the development of the rift is not
completely understood yet and there are different theories about this. One of
these theories says that the rift has its origin in the collision of the Indian
sub-plate with Eurasia. That means, that the rift is a kind of intracontinental
back arc basin. After a second theory, the rifting has its origin in an abnormal
mantle below the rift zone.
The model for the development of the rift written below is from Mats et al.
(1999), but there are also other scenarios.
[development of Lake Baikal, Mats et al. (1999)]
The development started with the so called “Pre-rift
stage”, without any tectonic movement, but intensive basic volcanism,
hydrothermal activity and deep-crustal circulation of surface waters.. The oldest
basalts have an age of 72 Ma and can be found in the Tunka basin, and this stage
ended with the plateau basalts of the Khamar-Daban ridge southwest of lake Baikal
at 27 Ma. At this time the area was a flat peneplane, only locally altitudes
of 300 to 400 m were reached. Large lakes with some 10 m depth filled the initial
depression of the Baikal. Rifting itself started in the late Oligocene at 27
Ma. It can be subdivided into an Early Rift Stage (27 - 3.5 Ma) and a Late Rift
Stage (3.5 Ma – present) after Mats et al. (1999) or into the more or
less equivalent Fast and Slow Rift Stage after Logachev and Zorin. These substages
can be further subdivided.
In the Central and Southern basin a deep water lake formed in the Late Oligocene
/ Early Pliocene. The shoreline was formed by the eastern border of the Olkhon
block, so the Maloye More Strait was not under water. It is not clear, when
the Northern basin filled with water. According to Moore et al. (1997) it should
be at the same time as the other two basins, but other scientists have good
field data to propose that the Northern Basin must be significantly younger.
At this time, the lake reached a depth of 400 m.
During the middle Miocene, the border between the Northern and the Central basin,
the Academician Ridge, broke into several sub-blocks, which submerged one by
one. So the shoreline of the Central basin moved to the northwest.
Because of tectonical processes the Buguldeyka-Maloye More Block started to
submerge, and half-graben like structures formed at the Maloye-More Strait.
The Strait was flooded in the late Miocene, when extensional processes at the
Central basin intensified. At this time a stable connection was built between
the Northern and Central basin. The subsidence of the Northern basin continued
in the late Miocene to early Pliocene. During the Pliocene, the Olkhon block
was uplifted, which caused a regression in the Maloye More Strait. In the late
Pliocene there was a tectonic pulse, which uplifted the Baikal rift shoulders
while the basins continued deepening. This caused the ultra-deep-water basins
the Baikal is famous for. At the end of the Pliocene or beginning of Quaternary,
the uplifting stopped causing the backfilling of the Pre-Manzurka valley, the
single outlet of the lake at this time.
This valley remained the outlet of the lake till another uplift during Pleistocene
cut it, which resulted in a transgression of the lake until it could flow in
the Irkut River valley to the Yenisej River system.
There is evidence that the lake level had fallen again later in the Pleistocene,
maybe as a result of climatic changes. In the Late Pleistocene, the last phase
of strong tectonic activity started. This involved the further subsidence of
the rift basins and the uplifting of the rift shoulders. This activity lasted
until the Holocene.
With the subsidence of the Listvianka block, the lake got a new outlet through
the Angara River. We visited this location on our last day of the field trip.
It is estimated that this outlet has an age of 60 to 40 ka.
The uplifting of the ridges and the subsidence of the basins generated the environment
for the formation of aeolic, alluvial and lake sediments. Especially in the
Tunka – valley, but also at Cuzhir on Olkhon Island, we could see large
sand dunes, covered by woods. Weathering of carbonates produced typical karst
structures. We could see typical conical formed karst mountains on the western
coast of the Maloye More Strait and carst forms also an important ground water
aquifer.
[karst-mountains at the Maloye More Strait]
References
- Lake Baikal Geografiteckii Atlas, Federal´naja slushbageodezii
i kartografii Rossii 1995
- Déverchère et al. „Depth dirstribution of earthquakes
in the Baikal rift system and its implications fort the rheology of the lithosphere“
Geophys. J.Int. (2001) 146, page 714 – 730
- Brink, Taylor “Crustal structure of central Lake Baikal: Insights into
intracontinental rifting” Journal of Geophysical Research, Vol. 107, No.
B7 (2002)
- Mats VD et al. « Evolution of the Academician Accomodation Zone
in the central part of the Baikal Rift, from high-resolution reflection seismic
profiling ad geological field investigations » Int J Earth Sci (2000),
page 229 – 250
- Dolginow, Kroptschjow “Abriß der Geologie Rußlands und angrenzender
Staaten“ E.Schweizbart`sche Verlagsbuchhandlung 1994
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