10 Search and Development

PLUTONIC GEODYNAMICS AS THE MECHANISM OF EARTH EVOLUTION

by Acad. Mikhail KUZMIN, Vinogradov Institute of Geochemistry, RAS Siberian Branch (Irkutsk); Acad. Vladimir YARMOLYUK, Institute of the Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences, Moscow; Professor Vadim KRAVCHINSKY, University of Alberta (Edmonton, Canada)

The last fifty years have seen great changes in geological knowledge. Owing to achievements of seismic tomography, two vast regions of hot matter have been detected in the bowels of the earth reaching as far down as the central core. Remarkably, their projections to the planet’s surface actually concur with hot fields of the mantle identified by Soviet geologists thirty years ago by indirect evidence. These and related discoveries underlie the global geodynamics theory that has made it possible to link plutonic processes within the mantle to global surface geology. Search and Development 11

Depth α Olivine Crust (6-70 km) (km) Wadsleyite Lithosphere (~100-200 km) Ringwoodite Upper mantle Transitional zone PEROVSKITE + MAGNESIOWÜSTITE

Lower mantle Postperovskite D” layer (150-350 km) Liquid iron Outer core

Internal structure of the earth. Upper/lower mantle interface, Solid 660 km. iron Inner core Partly molten matter contained in the upper mantle often contacts the asthenosphere.

INTERNAL STRUCTURE vent to the hot matter of the mantle* capable of reaching OF THE EARTH out to the global surface. First, some background information. In his major work Lying underneath the lithosphere is the asthenosphere; on fundamentals of geology (published in 1830-1833 in it extends from the base of the lithosphere down to about three volumes) the British geologist Charles Lyell formu- 700 km; this is a comparatively weak layer that read- lated the principles of what he termed actualism (con- ily deforms by creep. The asthenosphere contains partly temporary observations make it possible to postulate geo- molten matter responsible for convective (heat-transfer- logical processes of the past) and uniformism (all natural ring) flows. In mid-ocean ridges (MOR) the astheno- changes notwithstanding, the laws responsible for them sphere comes up to the earth surface; it causes a fusion remain immutable). Later on two American geologists, of MOR basalts with the low content of lithophilic ele- James Hall and James Dana (in 1858 elected honor- ments.** The asthenosphere “lost” most of them 1.8 to ary member of the St. Petersburg Academy of Sciences) 2 bln years ago to the earth’s crust being formed then. The came up with the concept of geosynclines (with respect to lower mantle was not implicated in these events since its mobile zones of the earth) explaining the origins of folded composition is more akin to the primary mantle of the mountain masses. And the Russian earth scientist Acad. earth. We should note here that nearly half of the glob- Alexander Karpinsky identified stable regions on earth, al mass is composed of Mg-perovskite, stable in a wide the platforms (1887, 1894). The works of these eminent range of pressure. This is the basic mineral of the lower scientists became a groundwork for the geological para- mantle. digm of the late eighteen-hundreds and the nineteen- The D” layer discovered in the 1980s is of major signifi- hundreds up to the 1960s when the plate tectonics theory cance for processes of plutonic geodynamics. It lies on came to the fore. the foot of the mantle. As much as 150 to 350 km thick, it Our planet is composed of a number of spheres (shells) is noted for a high temperature gradient: about 4,000 oC that vary in thickness and that are different in their min- on the bottom and 3,000 oC toward the upper boundary. eral and chemical composition. Their seismic boundaries It is through this layer that the mantle is interacting with are clear-cut. The uppermost shell, the lithosphere (up to the core. 10 km thick in the oceans and to 200 km and more on the * The layer of the earth’s interior between the crust and the core reaching continents). It is superposed by the earth crust, 6 to 70 down 2,900 km ; we distinguish between the upper (down to 660 km) and km thick. The lithosphere is rather brittle—this is where the lower mantle extending to the core.—Ed. ** A group of chemical elements (as many as 53) making up the bulk of earthquakes take place causing splitoffs and rents giving minerals in the crust.—Ed. 12 Search and Development

CA

P By surface manifestations of intraplate magmatism A over the last 15 mln years as many as 47 hot spots have been identified. They group into four extensive Tas (up to 10 thous. km across) but compact zones called “hot fields of the earth mantle”; African, Pacific, Central Asian and Tasmanian (Sonenschein, Kuzmin, 1983).

The discovery of postperovskite in 2002 to 2004 was ing in horizontal shifts of the oceanic lithosphere sidewise a great achievement of experimental mineralogy. It has from the mid-ocean ridges. Thereupon, in 1968, Jayson the same chemical composition as perovskite, though its Morgan of the United States and other research scientists density is 1.2 percent higher. As shown by experimental pointed to essential differences between the abysmal geo- data, the present temperatures in the earth’s interior are physical structures of the ridges and the zones of festoon conducive to this mineral’s formation to a depth of 2,600- islands (island arcs). The latter are characterized by un- 2,900 km, i.e. within the D” layer. Our estimates of the derthrusts, or the sinking of the oceanic lithosphere into thermal evolution of the globe’s interior invite the con- the mantle to a depth of 600 km. This is the process of clusion that postperovskite started forming after a consid- subduction (“pushing under”). erable cooling of the earth around 2.3 bln years ago. From As soon as plate tectonics fundamentals had been de- that time on the continents began to grow faster (nearly vised in full, the plate tectonics theory explaining the pre- twice as fast), that is plate tectonics confined to the upper sent-day global dynamics gained worldwide recognition. mantle happened to be at work. Its principles are rather easy to grasp. The lithosphere and the asthenosphere, the two outer PLATE TECTONICS THEORY envelopes of our planet, are interactive. The astheno- Looking into a bathymetric map of the ocean floor in sphere’s matter is capable of flowing and, consequently, 1961, Robert Dietz, a British geophysicist, and Harry of causing convective flows sustained by the energy of Hess, a U.S. earth scientist, came to the conclusion that the inner envelopes. The lithosphere, a hard rocky shell extended mountain ridges towering above abyssal (deep- encasing the earth, reacts but passively to processes in water) valleys to 1-2 km are confined to the central parts the asthenosphere. It is dissected by two narrow belts or of the oceans. They demonstrated that a new crust was zones noted for high tectonic (seismic in particular) and being formed within the rifted structures of these ridges. magmatic activity: these are the rifted mid-ocean ridges The two scientists called the process of sea floor extension and subduction zones. Such zones as well as the transfor- “sea floor spreading”. mation faults break the lithosphere into hard solid plates In 1963 two British geophysicists, F.S. Vine and D.H. that, acted upon by viscid friction forces brought about Matthews, confirmed the phenomenon of spreading by convective (or other) flows in the asthenosphere, are through the presence of banded (striped) magnetic in motion with respect to one another. anomalies formed via magnetization of oceanic core rocks in line with the polarity of the global magnetic HOT SPOTS field changing periodically from direct (present situa- As far back as 1963, when the plate tectonics theory was tion) to reverse. still in its swaddling clothes, John Wilson called attention Then, in 1965, John Wilson of Canada singled out a to active volcanoes within oceanic plates; such volcanoes special type of fractures—transformation faults—appear- formed chains oriented opposite the vector of plate move- Search and Development 13

CAHF

The “hot fields” PACIFIC boundaries concur roughly LLSVP with LLSVP contours AFRICAN (LLSVP, large low shear velocity LLSVP provinces known also as superplumes). LLSVP relation to the present manifestations of volcanicity is confirmed by the localization of 49 hot spots known to date on the terrestrial surface; the mantle provinces determined by the method of seismotomography (Sonenschein, 1991; Burke, Torsvik, 2004).

ments. These chains, he proposed, were linked to hot spots icochemical parameters of the mantle make it but little of the mantle burning through the lithosphere above. probable that such columns could ever be, and they pro- Many researchers accepted the hypothesis of hot spots pounded different hypotheses to explain the origins of hot of the mantle. It was suggested (and then confirmed) that spots. But all these hypotheses did not allow for the pres- such hot spots are actually geochemical anomalies, for their ence of any plutonic mantle structures capable of acting magmatic rocks are enriched (if compared with MOR) with on geological processes in the upper shells of the earth. many rare lithophilic elements, and this is not typical of upper mantle melts. Such rocks are connected with spots of HOT FIELDS OF THE MANTLE the heated asthenosphere which, while immobile, are sus- By 1980 intraplate magmatic activity had been proved tained by mantle plumes or streams coming up from lower both for the oceans (volcanic islands and plateaux) and mantle depths, possibly, from the core/mantle interface. In for the continents alike. Yet there were no works on how oceans the intraplate magmatism is represented mostly by hot spots (as plutonic formations) and surface geological basalts of plateaux and islands set off into a peculiar geo- structures were interconnected. To make up for this gap chemical type, OIB (ocean island basalt). Similar in com- Leo Sonenschein (1929-1993), an eminent Soviet earth position are basalts of trap* provinces, in Siberia for one. scientist and full member of the Russian Academy of Sci- The notions of hot spots made for the presence of nar- ences, suggested that one of the authors of the present row (about 50 km across) mantle jets (columns) piercing article, Mikhail Kuzmin, address this problem. The ap- all through the mantle but remaining immobile (compared proach was quite simple—find purely geographical dis- with lithospheric plates for tens of millions of years). A tribution patterns for intraplate magmatism products on volcano is formed within a plate like that traveling above the terrestrial surface. Taken into consideration were to be a “spot” of heated mantle; when the plate moves with re- only relatively young objects, 0 to 15 mln years old, so as spect to the hot spot yet another volcano is born; as a re- to exclude errors due to possible continental drifts. sult today we see a chain of dead volcanoes actually trac- As it follows from the distribution map thus obtained, there ing the trail burned through by the hot spot. Conspicuous are four regions of contemporary intraplate magmatism: two in this regard is the Hawaiian hot spot implicated in the large ones, the Pacific and the African, and two smaller ones, appearance of the Hawaiian Emperor Ridge in the Pacific the Central Asian and the Tasmanian. The largest (African that has been in existence for nearly 100 mln years. and Pacific) measure 10,000 km across, and are comparable Some earth scientists, however (like Keit Runcorn in size to the main lithospheric plates, though their bounda- for instance), maintained that the geological and phys- ries do not concur with the contours of the regions. * Trap, a continental plateau composed of diabases, dolerites, basalts, The results of these studies are summed up in the article gabbro-diabases and gabbro.—Ed. title “Intraplate Magmatism and Its Significance for an 14 Search and Development

Area belt Superplume oceanic continental lithosphere lithosphere km Upper mantle Subduction layer С

Plume uplift Lower mantle layer ”

outer core А

km Lower mantle

layer D ”

ULVZ The lithospheric (oceanic) plate sinks into the mantle (A). The greater part of the plate stays in the C layer, but some of the lithospheric outer core B material submerges down to the layer D” where a superplume originates (B).

Understanding of Processes in the Mantle of the Earth” tectonics, the detection of hot fields pointed to the more (L. Sonenschein, M. Kuzmin, Geotectonics, 1983); the plutonic nature of convective flows. That is to say, pro- two authors designated the intraplate magmatism regions ceeding from the new evidence it became now possible to as hot fields of the earth. These were said to coincide with speak of an interconnection of processes in the lower and major positive anomalies in relief and also with positive upper mantle. The narrow mantle jets related to the hot aberrations of the geoid’s form. Judging by the geochemi- spots could be in the form of plumes* traveling from the cal characteristics of constituent rocks, material compo- lower/upper mantle interface to meet the heated matter sition anomalies—possibly linked to the lower mantle— of the lower mantle. It is these jets that give rise to inter- correspond to these regions. polate magmatism and create the system of hot spots. We Thus, the identified hot fields of the earth mantle could should note that these conclusions had been formulated be visualized as upwelling regions for the substance and before seismotomography entered the stage in the 1980s; energy of the lower mantle, while the cold fields in be- its methods enabled earth scientists to get a better under- tween, as zones where the substance sinks into the man- standing of the mantle’s internal structure. Japanese and tle. In total a coordinated system of convective mantle American scientists, for one, were able to detect large flows stood out. volumes of matter within the mantle related to higher and If processes associated with the upper earth shells could * Hot mantle jets traveling independently from convective flows in the be described within the framework of lithospheric plate mantle.—Ed. Search and Development 15

А Kenorland Columbia Rodinia Pangaea Supercontinents At least four supercontinents are thought to have been in existence in the geological history of the earth. B The time intervals of their origination and breaking under the effect of superplumes (A) have been determined. events and cycles Numbers of plume These events occur in cycles of their occurrence and correlate with plume activity (B) Age (Ma) (Li, Zhong, 2009; Torsvik, 2004). lower velocities of seismic waves. The principal conclu- continents became centrifugal. It is supposed that at least sions of these studies boil down to the following: there four supercontinents have been in existence in the geo- are two large low shear velocity provinces (LLSVP), the logical history of the earth (Kenorland, Columbia, Rod- African and the Pacific (also called superplumes today). inia and Pangaea). Seismography data indicate the upwelling of D” plutonic matter. It is to be noted that the projections of LLSVP to INTRAPLATE MAGMATISM OF SIBERIA the terrestrial surface coincide with the hot mantle fields Siberia was a component part of the Rodinian conti- established earlier. nent which came into being about a billion years ago but, In contrast to the low velocity provinces those of high ve- about 250 mln years thereafter, started breaking apart un- locity are confined to subduction zones where lithospheric der the action of the underlying Rodinian superplume. plates plunge into the mantle. The subducted (absorbed) Simultaneously with the Rodinian superplume an antip- lithosphere stays in part at the upper/lower mantle bound- odal plume was thought to be in existence. With Rodin- ary, while yet another portion of it submerges down to the ia’s breakup the component continents, Siberia included, core/mantle interface. The lithosphere substance getting traveled into the region of the Late Rifean ocean. into the D” layer, acted upon by heat emanating from the Siberia was not large in those days, it comprised the Si- core, form partly molten masses begetting hot plumes ris- berian Craton (platform) alone. Thereupon it took in fold- ing to the earth surface. Such upswelling is conducive to ed orogenic belts, the Central Asian in the first place, and expansion of the volume with the transition of postperovs- then the Kazakhstan and China continents; the closure of kite from the D” layer in the lower mantle to perovskite; it the Paleo-Urals ocean led to the creation of the vast Eu- likewise activates the entry of volatile chemical elements, roasian continent. All these processes were associated with C, S, O, and H above all. As it follows from the differentials plate tectonics setting the stage for small and large conti- of the density of matter in the inner and outer core, these nental masses being joined to Siberia. This matter is con- elements are contained in the outer core and are carried sidered in greater detail in the monograph authored by Leo into the mantle where they are implicated in the forma- Sonenschein, Mikhail Kuzmin and Lev Natapov—Lith- tion of plumes. Thus, the notion of low and high velocity ospheric Plate Tectonics in the Territory of the USSR (1990). mantle provinces helps to link together two streams (jets): Siberia is remarkable for many intraplate magmatic the plunging of cold matter into the lower mantle and the complexes. Large rock masses similar to OIB have been upwelling of hot jets to the terrestrial surface. discovered out there. This means that about 600 mln The conjugation of these flows in the mantle points to years ago the ocean washing the Siberian continent had a tight connection of plutonic geodynamics with plume islands formed by hot spots. Such spots acted upon the and plate tectonics. The interconnection of formative continent as well to engender regions of intraplate mag- processes of supercontinents and superplumes in single netism. In practical terms all through the Phanerozoic*, cycles argues well for that. Today we know that supercon- up to the latest time (>25 mln years) the continent and its tinents emerged in the process of earth evolution; these * A geologic epoch still on over these last 540 mln years, also known as supercontinents bring actually all continental masses the time of “visible” life (Mesozoic (252-66 mln years ago); Cenozoic (66 mln years ago— stroyed by superplumes, while the movements of separate Holocene, or recent period).—Ed. 16 Search and Development

Tarim South China Siberia India North China Australia

Antarctica Laurassia Kalahari Arabia

Nubia Congo Sahara Baltic Amazonia

South Africa

Restoration Oceans Projections of mantle hot fields of the Rodinian plume responsible for Rodinia’s breakup Ancient cratons Rift zones (Li, Zhong, 2009). The antipodal superplume Continental shelf Subduction zones shown in the opposite sector of the earth.

ambient oceanic environment had been acted upon by a (Khangai); its formation is also associated—like that of hot mantle field, the superplume (Yarmolyuk et al., 2006; Angara-Vitim—with the melting of the crust above the Kuzmin et al., 2010). In the Early and Middle Phanero- mantle plume. The development of the rift system ended zoic (~540-360 mln years ago), after the disintegration of with the formation of the zonal Mongolian-Transbai- Rodinia, this led to the formation of two large magmatic kal magmatic area in the Early Mesozoic (~200 mln provinces—the Altai-Sayan and the Vilyui. Later on (310- years ago). 190 mln years) several other intraplate magmatic provinc- About 190 mln years ago the intraplate activity slowed es were formed. One, the Barguzin, over 2x105 km2 large, down dramatically. However, in the Late Mesozoic the ac- is of zonal structure, with rift zones at the periphery, and tion of mantle plumes on the lithosphere of the Siberian the giant Angara-Vitim granitoid batholith in the center, continent resumed, with several rift regions emerging with- over 0.5 mln km3 large and formed due to an extensive in the limits of the Central Asian area of the Siberian plat- melting of the crust under the thermal effect of the man- form. The climax of tectonic and magmatic activity con- tle plume. curred with the onset of the Cretaceous period (145 mln A great event took place at the close of the Late Paleo- years ago). In subsequent periods this activity fell off. zoic within a geologically short span of time (3 mln years) Yet another “outburst” of such activity occurred in the in the North Asian continent: a giant magmatic province Late Cenozoic (<25 mln years ago) covering the territory was formed there. It embraced the trap region of the Si- of Central and Eastern Asia. New volcanic areas took berian platform and the rift system of Western Siberia; body and form (South Baikal, Udokan, Vitim, among stretching over 1,500 km, it goes through the base of the others) in the wake of the birth of hot spots (Kuzmin et West Siberian Lowland. al., 2010). The same period also witnessed intraplate magmatism In order to understand the scenario of Siberia’s inter- taking in the southern folded fringe of Siberia. Formed action with hot spots, and how these are linked to the there were Tarim traps and the conjugated system of earth’s plutonic structures we need “absolute” paleore- subparallel rift zones within Mongolia: those of Gobi- constructions in line with present-day geographical co- Tien-Shan and the Main Mongolian Lineasment (frac- ordinates. We attempted such kind of reconstruction in ture). Two other rift systems—the Gobi-Altai and North our article published in the Earth-Science Reviews, 102 Mongolian—correspond to the onward movement of the (2010). Our results made it possible to answer certain centers of plume magmatic activity deep into the Siberian questions related to the role of plumes in the geologi- continent. Simultaneously with the latter two rift zones, cal history of our planet and in particular, that of the hot there appeared in between them a granitoid batholith spots of the mantle. Search and Development 17

Within-p-late magmatic areas Products of oceanic type magmatism (Age, Ma) persist in the structures of the Siberian paleocontinent < 25 thus indicating the activity of the African superplume. 160-25 Large eruptive provinces: 220-190 Early-Middle Paleozoic: I—Altai-Sayan; 290-240 II—Vilyui, Late Paleozoic; 410-390 III—Barguzin-Vitim; IV—Central Asian, Permotriassic; Batholiths V—Siberian traps, Early Mesozoic; VI—West Siberian rift system, Traps Late Mesozoic-Cenozoic rift systems; VII—East-Mongolian transBaikal system; Rift zones VIII(1)—South-Khanga (Gobi-Altai), Geological structures VIII(2) East Mongolian, VIII(3)—West-transBaikal, Precambrian platforms VIII(4)—Central Aldan (Yarmolyuk, 2000, 2003, 2006, modified). Fold belts West-Siberian lowland Faults

180o

Barguzin-Vitim magmatic area

285-320 210 290

210 o 250 o 270 100 130 180 90 90 Gobi-Tian-Shan 150 magmatic area 80 70 Siberian 60 traps Migration of the Icelandic 70 hot spot in the Arctic basin 50 40 in the present geographical coordinates: 50 1—not later than o 130 mln years ago, 2—from 150 to 250 mln years ago, 40 o 3—Siberian traps, 4—mid-ocean ridges, 5—magmatism age in millions of years, 6—migration of magmatism, connected with Siberian plume, o 0 7—migration of magmatism, connected with Hangai plume (Kuzmin, 2010, 1 2 3 4 250 5 6 7 Kharin, 2000 et al.).

Science in Russia, No.6, 2013 18 Search and Development and Search not gobeyond theselimitsfrom theEarlyPhanerozoic mately between 70 Like today theAfricanhotfieldwas situatedapproxi- even thoughitspaleolatitudinal positionwas changing. longitudinal positiondidnot seeanydramaticchanges, Permian/Triassic period.WhichmeansthatSiberia’s at leastfrom theepochofEarlyPaleozoic uptothe Siberian continentwas drifting over thehotmantlefield plate magmaticactivityinSiberiademonstratethatthe throughout thePhanerozoic.Signsofextensive intra- vik etal.,2008),we infer thatSiberiaremainedtherein the staticcoordinatesover theselast300 mlnyears (Tors- African hotmantlefield.Sincethisfieldhaspersistedin hot spotwas situatedabove thenorthernextremityof any rateallthroughthePermian andtheTriassic thisvery 250 mlnyears. grating continentalblocksoftheArcticbasinforlast hot spottracesareimpressedinthelithosphereofmi- are summedupinourwork (Kuzminetal.,2010)).So, seen intheNorthAtlanticandGreenland(thesedata spot intheLateMesozoicandEarlyCenozoicare (more than250mlnyears ago). gions intheLatePaleozoic andintheEarlyMesozoic must knowthepaleographicpositionofmagmaticre- carry out“absolute”paleoreconstructionsofSiberiawe the Cretaceous,orforaslong490-470mlnyears. To of amantlehotfield,atleastfrom theOrdovician onto berian continentshows ittobelocatedwithinthebounds A DRIFT570MLNYEARSLONG cal termsthesameasthatofIcelandtoday (62 during itsformation(250mlnyears ago)was inpracti- 342 The coordinatesoftheIcelandichotspot:65 The tracesofthemagmaticactivityIcelandichot The continualintraplatemagmaticactivityontheSi- o E. ThepaleolatitudeoftheSiberiantrapprovince o and330

GONDWANA LAURUSSIA hot spot Icelandic o E, thatisthis continent did

15° Siberian Platform

30° aahtnTarim Kazakhstan Siberia Western

45° Paleothesis Paleothesis

o 60° ±7 Ocean o

N and o Mongol-Okhotsk

). At Ocean et al.,2010)designatedtheold(paleo)latitude30 southeastern boundaryorientednorthward. We (Kuzmin situated neartheequator, withitspresentsouthernand past. DuringtheVendian-Early Cambrianperiodsitwas geographical coordinatesoftheSiberiancontinentin dic mantleplumecoincide,we areabletodeterminethe sition ofSiberiaandthepresentpositionIcelan- prior tothePermian andtheTriassic. Sincethethenpo- Amuria in theSouthernHemisphere(30 nental drift. nent atvelocities higherthanthoseofthepresentconti- this fieldandexcludedramaticmigrationsoftheconti- as tominimizetheshiftsofSiberiancontinentwithin Siberian CambrianwithintheAfricanhotmantlefieldso in Siberia,and thePripet-DonetsriftinEurope). of theDevonian occurred onbothplatforms(Viluian rift the Africanhotfieldsince dramaticmagmaticevents authors puttheRussianand theSiberianplatformsabove ian, i.e.thelongitudinalmigration didnotchangegreatly. means Siberiawas moving latitudinallyalong themerid- yr. Thiscontinental driftvelocity isratherhigh,which at themeanrateoflatitudinaltranspositionabout7.3cm/ netic evidenceintheEarlyPaleozoic Siberiadriftednorth beria was moving over theAfricanhotfield.Bypaleomag- ago) ontothePermotriassic (250mlnyears ago)whenSi- nent were connectedfrom theCambrian(510mln years any significantchangesinlongitudinaltranspositions. present continentalplates),andthustherecouldnotbe 512 to480mlnyears agowas 5cm/yr(finitevelocity for as theequatoriallatitudes.Therateofitslatitudinaldrift to theEarly/MidOrdovician itdriftednorthward asfar in themiddleofthatperiod(520-505mlnyears ago)up In theEarlyCambrian(~537mlnyears ago)Siberialay In theirpaleoreconstruction (Kuzminetal.,2010)the The intraplatemagmaticevents intheSiberianconti- Block North China Asia South-Eastern Block South China of geologicaltime. rather stableoveralongstretch regions isconsidered The layofthebasicLLSVP cold matter. the sinkingofrelatively of hotmantlematter, blue— Bright red,theupswelling to thecore/mantleinterface. of LLSVPclose is thepropagation the continentalcontours 250 mlnyears.Shownbeneath and continentsover of theSiberianplatform Reconstruction

o S, 20 o E). Beginning o forthe Search and Development 19

Present

Paleoconstruction of the migration of the Siberian continent over the African mantle province within the last 570 mln years (Kuzmin, 2010).

After the Devonian Siberia migrated north, simultane- The tie-in of superplumes with processes implicated ously turning 60o clockwise (360-250 mln years ago), i.e. in the formation and breaking up of supercontinents is before it reached the Icelandic hot spot. The latitudinal now universally accepted. Yet the latest evidence allows migration velocity averaged about 4 cm/yr, which is con- us to say that the fragments of supercontinents upon their sistent with the present rates of the continental drift. In destruction by the killer superplume are migrating into the time stretch of 250 to 200 mln years ago Siberia de- regions controlled by the antipodal plume to form a new parted from the Icelandic hot spot. This departure was supercontinental agglomeration. This dual role of the caused by the opening of the North Atlantic. In the Late plumes must be reflecting their antiphasic activity asso- Mesozoic intraplate magmatic activity moved to Central ciated probably with different shows of responding con- and Eastern Asia, and it shrunk considerably by the end vective processes. As demonstrated by Acad. Vyacheslav of the Cretaceous. In these last 250 mln years the Sibe- Kovalenko and colleagues, the thermostatic effect could rian continent migrated to its present position, drifting be one of the causes of this phenomenon. latitudinally across the North Pole at 1.7 cm/yr. Now about the future research prospects. Separate Summing up, let us stress this point: the earth is a self- continents (Siberia) passing above the hot spots of a par- organizing system whose further evolution is tied in with ticular hot field of the mantle and involved in a super- the interaction of its internal shells, namely with convec- continent’s formation, say of Pangaea or Eurasia, keep tion processes and mantle plumes. Now we know that races of these hot spots. This fact allows us to suppose: the upwelling mantle jets are mainly concentrated in two already in the near future the available methods of stud- areas set off as superplumes, the Pacific and the African. ies into magmatic rocks will help to evaluate the evolu- Their role in the formation of the lithospheric structure tion of mantle sources both for individual plumes and for can hardly be overestimated. Hence the pertinent ques- superplumes at large. Ultimately this will contribute to a tion: when were superplumes born and why? How long is further understanding of the general laws of the evolution the life span, and how stable is their action on the litho- of the earth. sphere? The authors of the present article have made a certain The studies were supported by Program No. 4 of RAS Pre- contribution in clearing these matters. To begin with they sidium, Program No. 10 of RAS Earth Sciences Branch, demonstrated that the shows of intraplate activity on the Integration Project No. 87 of RAS SB, and RFFI grants Siberian continent throughout the Phanerozoic came as 13-05-12043 and 13-05-12026. a followup of its migration over the hot field; such mani- festations are comparable to the present African super- plume. This superplume has been alive for no less than 570 mln years. What with the Rhodinean superplume that smashed Rodinia being compared to the Pacific plume (Yuen et al., 2002), both should be considered to be the most long-living structure of the earth. Illustrations supplied by the authors