The Baikal Rift Zone is a series of

continent A continent is any of several large landmasses. Generally identified by convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continents. Ordered from largest in area to smallest, these seven ...

al rifts centered beneath

Lake Baikal Lake Baikal (, russian: Oзеро Байкал, Ozero Baykal ); mn, Байгал нуур, Baigal nuur) is a rift lake in Russia. It is situated in southern Siberia, between the federal subjects of Irkutsk Oblast to the northwest and the ...

in southeastern

Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-ei ...

. Current strain in the rifts tends to be extending with some shear movement. A series of

basins Basin may refer to: Geography and geology * Depression (geology) ** Back-arc basin, a submarine feature associated with island arcs and subduction zones ** Debris basin, designed to prevent damage from debris flow ** Drainage basin (hydrology), a ...

form along the zone for more than , creating a rift valley. The rifts form between the

Eurasian Plate The Eurasian Plate is a tectonic plate that includes most of the continent of Eurasia (a landmass consisting of the traditional continents of Europe and Asia), with the notable exceptions of the Indian subcontinent, the Arabian subcontinent an ...

to the west and the

Amur Plate The Amurian Plate (or Amur Plate; also occasionally referred to as the China Plate, not to be confused with the South China Subplate) is a minor tectonic plate in the northern and eastern hemispheres. It covers Manchuria, the Korean Peninsul ...

to the east.

Structures

To the northwest of the rifts is the Siberian Craton, on the Eurasian Plate. The Sayan-Baikal and Mongolia-Okhotsk mobile belts are formations to the southeast of the rifts; beyond the Sayan-Baikal fold belt is the Amur Plate. Basins form along the rift. There are three basins in the area, the South Basin, Central Basin, and North Basin. In the North, half-grabens form smaller basins, such as the North Baikal Basin, and the Chara-Tokka Basin. The Central Basin is the deepest in the rift system. The largest fault it contains is the Morskiy Fault; however, another fault, the Primorsky is becoming the primary fault in the rift. The basins also display evidence of crustal thinning below the rift zone. The thickness of the crust beneath the rift is disputed, as the structures of the rift deep beneath the surface are unknown. The difference in thickness of the crust, between the crust under the rift and that under the surrounding areas, has been bounded to be less than . Although some seismic data is evidence for a rise in the Lithosphere-

Asthenosphere The asthenosphere () is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~ below the surface, and extends as deep as . However, the lower boundary of the asthenosphere is ...

boundary, other researchers have claimed that there are deep structures which influence seismic activity, and that the lower crust is intruded by

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks in ...

sills. They interpret the extension as a pure shear process.

Volcanism

Volcanism is generally associated with the rift.

Hot springs A hot spring, hydrothermal spring, or geothermal spring is a spring produced by the emergence of geothermally heated groundwater onto the surface of the Earth. The groundwater is heated either by shallow bodies of magma (molten rock) or by circ ...

are present both on land and under Lake Baikal, although thus far, no evidence of actual

volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...

has been found in the immediate vicinity of the lake. Despite this,

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configu ...

volcanism has occurred nearby and is probably associated with the Baikal Rift Zone. Early volcanism has been confirmed to exist from the early

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

, although it is believed that

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but t ...

Volcanism exists. The recognized volcanic centers are the Udokan Plateau, located about 400 km ENE of the northern tip of Lake Baikal, the Oka Plateau, located about 200 km WNW of the southwest tip of Lake Baikal, the Vitim Plateau, around 200 km east of the rift, and the Azas Plateau.

Geological history

The area was originally characterized by

Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of th ...

and

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ...

northeast-southwest fold and thrust belts. The Primorsky fault zone of the central basin was present at this time also. Volcanism began in the late

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

in limited areas, but is mostly limited to the

. The late Cretaceous is also the age of sedimentary rocks in some basins, and the same series lasted into the

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', ...

. Rifting resumed beginning in the

, and is commonly held to have increased since the middle

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configu ...

. Magmatic activity and rifting may also be independent events. Outside of the

graben In geology, a graben () is a depressed block of the crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German, meaning 'ditch' or 'trench'. The word was first used in the geologic conte ...

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

volcanics erupted from either end of the rift system during the uplift. The grabens mostly spread without releasing magma, except the Tunkin Depression. Most basin deposits are from the late

, except in the north where basin deposits began in the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

, is the 'middle rift' unit of Pliocene coarse grained

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicat ...

s and

conglomerate Conglomerate or conglomeration may refer to: * Conglomerate (company) * Conglomerate (geology) * Conglomerate (mathematics) In popular culture: * The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes ** ...

s. Finally there is the modern rift unit of fluvial, glacial, and deltaic sediments. Examining Pliocene and younger

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...

s reveals

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class ...

s, argillites, and

silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel ...

s, indicating lacustrine deposition.

Seismic monitoring

The first seismic station in the region was opened in Irkutsk in 1901, which began instrumental observations. New instruments were put in place in 1912, and the system was expanded in the 1950s and 1960s. The monitoring network is still in operation, although the minimum spacing has been criticized for exceeding in the minimum distance between stations. Studies from foreign experiments also provide new information about the rift system.

Tectonic stress and strain

Along the boundary both normal and shear strains have occurred in the rift. Plates are diverging along the boundary at per year, although this varies along the system. In addition to divergent movement, left lateral

strike-slip In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

movement also occurs in the fault system, such as along the Sayan Fault. The rate of slip has been estimated at per year in the north by looking at offsets of morphological features, although this estimate does not agree with current models. The total

throw Throwing is an action which consists in accelerating a projectile and then releasing it so that it follows a ballistic trajectory, usually with the aim of impacting a remote target. This action is best characterized for animals with prehensile ...

that has occurred has been estimated at in extension and in the vertical direction. Due to the distance from active

plate boundaries Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...

, the driving forces of the rift are unknown; however, possibilities include the

subduction Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, ...

of the

Pacific Plate The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and I ...

and the collision of the

Indian subcontinent The Indian subcontinent is a physiographical region in Southern Asia. It is situated on the Indian Plate, projecting southwards into the Indian Ocean from the Himalayas. Geopolitically, it includes the countries of Bangladesh, Bhutan, In ...

with Eurasia. Locally, there may be a

mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...

up-welling driving the extension. This latter theory is held by most Russian scientists.

References