21.3 TECTONICS OF THE ALTAIDS: AN EXAMPLE OF A TURKIC …

21.3.1 IntroductionWhat would happen, if one were to double the entireNorth American Cordillera back onto itself? Or if onewere to collide it with the eastern margin of Asia? Orif the North American Cordillera collided with an oro-genic belt similar to itself? In all three cases, onewould get a wide orogenic belt, with an extremelycomplicated interior consisting dominantly of rocktypes and structures similar to those encountered todayalong the convergent continental margins of NorthAmerica and Asia and also along the continentalstrike-slip margins such as those in California, BritishColumbia, and the Komandorsky (Bering) Islands.Such an interior would thus be dominated by turbiditesdeposited in abyssal basins, trenches, and marginalbasins; less abundant pelagic deposits (ribbon cherts,pelagic mudstones, some pelagic limestones); local,unconformable shallow water deposits, possiblyincluding coral reefs, lagoonal deposits, local coarseclastic sedimentary rocks; and much volcanogenicmaterial. All of these would appear multiply andhighly deformed, tectonically intercalated with diverserock types making up the oceanic crust and upper man-tle (pillow basalts, massive diabases, gabbros, cumu-late and/or massive tectonized ultramafic rocks form-ing the members of the ophiolite suite), and intrudedby calc-alkalic plutons ranging from gabbros to gran-odiorites. Why would this be so? Much of the ensem-ble described above would be covered by the volcanicequivalents of the calc-alkalic plutonic rocks, fromtholeiitic basalts to andesites, and even to local rhyo-lites as well as wide blankets of welded tuffs. Thestructural picture of the ensemble would be bewilder-ingly complex and would include many episodes of

folding, thrusting with inconsistent (but mainly ocean-ward) vergence, orogen-parallel to subparallel strike-slip faulting, and even some normal faulting, alternat-ing in time and space. Both the rock types and thestructures within the internal part of such an orogenwould display longitudinal discontinuity over tens tohundreds of kilometers. By contrast, the outer marginsof the orogen would be marked by regular, laterallypersistent (on scales of hundreds to thousands of kilo-meters) fold-and-thrust belts made up dominantly ofshelf and epicontinental strata with dominant vergenceaway from the orogen.

How does such an orogen differ from other colli-sional orogens such as the Alps and the Himalaya?Simply in the collective width of the scrunched-upoceanic remnants between the colliding continentaljaws, the amount of subduction-related magmaticrocks, and the magnitude and frequency of incidenceof orogen parallel to subparallel strike-slip motion dur-ing orogeny. Both in the Alps and in the Himalaya, thewidth of oceanic offscrapings never exceeds a fewkilometers at most; generally they are much narrower(a few hundreds of meters), in places reducing to noth-ing, where the original opposing continental jaws comeinto contact. Associated magmatic arcs usually displaya single magmatic axis whose wandering across thestrike in time hardly exceeds a few tens of kilometers.In some small collisional orogens, such as the Alps, themagmatic arcs are so very poorly developed as to invitesuspicion of whether they ever existed.

The Altaids were named by the Austrian geologistEduard Suess, after the Altay Mountains in CentralAsia, shared by Russia, Mongolia, and China. Thelarge, mainly Paleozoic orogenic complex dominatesmuch of central Asia and extends to the Arctic in thewest and to the Pacific in the east. Its tectonic unitsare homologous to those occupying across-strikewidths of a few hundred meters in the Himalaya and

5352 1 . 3 T H E A L T A I D S

21. 3 T ECTONI C S OF T H E A LTA IDS : A N E X A M PLE OF A T U R K I C -T Y PEORO GE N — An essay by A. M. Cêlal Sengör1 and Boris A. Natal’in2

21.3.1 Introduction 53521.3.2 The Present Structure of the Altaids 53821.3.3 Evolution of the Altaids 539

21.3.4 Implications for Continental Growth 54521.3.5 Closing Remarks 545

Additional Reading 545

1Istanbul Technical University, Istanbul, Turkey.2Russian Academy of Sciences, Khabarovsk, Russia.

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536 E A S T E R N H E M I S P H E R E

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F I G U R E 2 1 . 3 . 1 (a) Major tectonic subdivisions of Asia, showing the position of the Altaidswithin the structure of the continent. (b) Tectonic map of the Altaids and surrounding large-scaletectonic entities, showing their first-order tectonic units. The Baykalide and the pre-Uralideorogenic systems are not further subdivided. Numbers on the Altaid units correspond with thenumbers cited for those units in the text.

the Alps but occupy widths of 1500 to 2000 kilome-ters (Figure 21.3.1). In the North AmericanCordillera, the edge of the Precambrian crystallinebasement is taken to be roughly along the Sri = 0.704line. This line delimits a band, about a 500 km wideand parallel to the coast, north of central California.If one juxtaposes the North American Cordilleraagainst its mirror image, one obtains a band of “off-scraped oceanic material” about 1000 kilometerswide. This is comparable to the width of the internalparts of the Altaids, when one considers that theAltaid evolution lasted some 50 to 100 my longerthan the Cordilleran evolution. Similarly, both in theAltaids and in the North American Cordillera, onefinds that magmatic axes wandered over distances on

the order of 1000 km. In fact, if we bend the lattercollisional orogen approximately 90° about a verticalaxis, the picture we obtain is remarkably similar tothat of the Altaids.

The term Turkic-type has been applied to collisionalorogens resulting from the collision of continents thatbegan with very wide subduction-accretion material,long-lasting subduction and rich sedimentary materialfed into trenches. These orogens are named after thedominant ethnic group that has populated, in much ofknown history, the area of development of their bestexample, namely the Altaids. The purpose of this chap-ter is to present a synopsis of both the present structureand the history of evolution of the Altaids, which arean example of a Turkic-type development.

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21.3.2 The Present Structure of the Altaids

In Figure 21.3.1a we see the Altaids within the contextof the structure of Asia, while Figure 21.3.1b is a tec-tonic map of the Altaids. Only four main types ofgenetic entities are displayed in Figure 21.3.1. Theyare (1) units made up of continental crust that hadalready formed before the Altaid evolution com-menced, (2) subduction-accretion complexes formedduring the Altaid evolution, (3) ensimatic magmaticarc massifs formed during the Altaid evolution, and(4) continental crust (of any age) stretched and thinnedas a consequence of Altaid evolution. These geneticentities are grouped into 44 main tectonic units definedon the basis of their function during the Altaid evolu-tion, and include units such as arcs, cratons, passivecontinental margins, and the like. In the following listwe briefly characterize their function; more detailedcharacterization and a summary of the rock contentcan be found in the readings at the end of this essay.The numbers of the units in the list correspond with thenumbers shown in Figure 21.3.1.

1. Valerianov-Chatkal unit: Pre-Altaid continentalcrust, Paleozoic accretionary complex and mag-matic arc.

2. Turgay unit: Pre-Altaid continental crust, Altaidaccretionary complex and magmatic arc; allburied under later sedimentary cover.

3. Baykonur-Talas unit: Pre-Altaid continentalcrust, Early Paleozoic magmatic and arc accre-tionary complex.

4.1. Djezkazgan-Kirgiz unit: Pre-Altaid continen-tal crust, Paleozoic accretionary complex andmagmatic arc.

4.2. Jalair-Nayman unit: Pre-Altaid continentalcrust, Early Paleozoic marginal basin remnants,Early Paleozoic magmatic arc and accretionarycomplex.

4.3. Borotala unit: Pre-Altaid continental crust,Early Paleozoic magmatic arc and accretionarycomplex.

5. Sarysu unit: Paleozoic accretionary complex andmagmatic arc.

6. Atasu-Mointy unit: Pre-Altaid continental crust,Early Paleozoic (including the Silurian) mag-matic arc and accretionary complex.

7. Tengiz unit: Pre-Altaid continental crust,Vendian-Early Paleozoic magmatic arc andaccretionary complex.

8. Kalmyk Köl-Kökchetav unit: Pre-Altaid conti-nental crust, Vendian-Early Paleozoic magmaticarc and accretionary complex.

9. Ishim-Stepnyak unit: Pre-Altaid continentalcrust, Vendian-Early Paleozoic magmatic arcand accretionary complex.

10. Ishkeolmes unit: Early Paleozoic ensimatic mag-matic arc and accretionary complex.

11. Selety unit: Early Paleozoic ensimatic magmaticarc and accretionary complex, with questionablepre-Altaid continental basement in fault contact.

12. Akdym unit: Vendian(?) and Early Paleozoic ensi-matic magmatic arc and accretionary complex.

13. Boshchekul-Tarbagatay unit: Early Paleozoic(including the Silurian) ensimatic magmatic arcand accretionary complex.

14. Tekturmas unit: Ordovician-Middle Paleozoicaccretionary complex, Middle Devonian-EarlyCarboniferous magmatic arc.

15. and 16. Junggar-Balkhash unit: Early (ensi-matic) through Late (ensialic) Paleozoic mag-matic arc, Middle through Late Paleozoic accre-tionary complex.

17. Tar-Muromtsev unit: Early Paleozoic ensimaticmagmatic arc and accretionary complex.

18. Zharma-Saur unit: Early to Late Paleozoic ensi-matic magmatic arc, Early Palaeozoic accre-tionary complex.

19. Ob-Zaisan-Surgut unit: Late Devonian-EarlyCarboniferous accretionary complex, strike-slipfault-bounded fragments of the Late Devonian-Early Carboniferous arc, Late Paleozoic mag-matic arc.

20. Kolyvan-Rudny Altay unit: Early and Middle-Late Paleozoic magmatic arc.

21. Gorny Altay unit: Early Paleozoic magmatic arc and accretionary wedge, superimposed byMiddle Palaeozoic magmatic arc; farther west inthe “South Altay,” Middle Paleozoic accre-tionary complex with forearc basin.

22. Charysh-Chuya-Barnaul unit: Pre-Altaid conti-nental crust, Early Paleozoic magmatic arc andaccretionary complex, and Middle Paleozoicmagmatic arc and forearc basin.

23. Salair-Kuzbas unit: Pre-Altaid continental crust,Vendian-Early Paleozoic magmatic arc andaccretionary complex, Ordovician-Silurian fore-arc basin, Devonian pull-apart basin, LatePalaeozoic foredeep.

24. Anuy-Chuya unit: Early Paleozoic magmatic arcand accretionary complex.


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25. Eastern Altay unit: Pre-Altaid continental crust,Early Paleozoic magmatic arc and accretionarycomplex.

26. Kozykhov unit: Early Paleozoic magmatic arcand accretionary complex.

27. Kuznetskii Alatau unit: Pre-Altaid continentalcrust, Early Paleozoic magmatic arc and accre-tionary complex.

28. Belyk unit: Vendian-Middle Cambrian magmaticarc and accretionary complex.

29. Kizir-Kazyr unit: Vendian-Middle Cambrianmagmatic arc and accretionary complex.

30. North Sayan unit: Vendian-Early Paleozoicmagmatic arc and accretionary complex.

31. Utkhum-Ota unit: Pre-Altaid continental crust,Early Paleozoic magmatic arc and accretionarycomplex.

32. Ulugoi unit: Vendian-Early Cambrian magmaticarc and accretionary complex.

33. Gargan unit: Pre-Altaid continental crust, EarlyPaleozoic magmatic arc, Vendian-Early Paleo-zoic accretionary complex.

34. Kitoy unit: Early Paleozoic magmatic arc.35. Dzida unit: Early Paleozoic magmatic arc and

accretionary complex.36. Darkhat unit: Pre-Baikalide continental crust,

Riphean magmatic arc and accretionary complex.37. Sangilen unit: Baikalide microcontinent that

collided in the Riphean with the Darkhat unit(unit 36) and the Tuva-Mongol Massif (see unit43.1) and experienced dextral strike-slip dis-placement during the early Altaid evolution.

38. Eastern Tannuola unit: Early Paleozoic mag-matic arc and accretionary complex.

39. Western Sayan unit: Early Paleozoic magmaticarc and accretionary complex.

40. Kobdin unit: Early and Middle Paleozoic mag-matic arc and accretionary complex.

41. Ozernaya unit: Vendian-Early Cambrian mag-matic arc and accretionary complex.

42. Han-Taishir unit: Pre-Altaid continental crust,Vendian-Early Cambrian magmatic arc andaccretionary complex.

43. Tuva-Mongol unit:43.1. Tuva-Mongol arc massif: Pre-Altaid con-

tinental crust and Vendian-Permian mag-matic arc.

43.2. Khangay-Khantey unit: Vendian-Triassicaccretionary complex.

43.3. South Mongolian unit: Ordovician to EarlyCarboniferous accretionary complex.

44. South Gobi unit: Pre-Altaid continental crust,Paleozoic magmatic arc, Early and Late Paleo-zoic accretionary complex.

21.3.3 Evolution of the AltaidsM E T H O D O F R E C O N S T R U C T I O N The problem with theAltaids has long been how to establish the trend-line ofthe orogen amidst the abundance of the variably orien-tated tectonic units listed in the preceding section.Since the late 1800s, the trend-lines of orogenic beltshave been depicted as the direction, in any given crosssection, of the average strike, particularly in the later-ally persistent external fold-and-thrust belts. Thatapproach worked well for narrow, linear, and/or arcu-ate orogens such as the Alpine system and even theCordillera, but the strange map shape of the Altaidsmakes such an approach suspect. The median line ofan orogen along its internal parts, consisting of medianmasses (Zwischengebirge) and the so-called scar-lines(Narbe), has also been used to follow the orogenictrend-line, but if one tried that method on the Altaids,the trend-line obtained from the externides and thetrend of the median line would give two different, con-tradictory results. Try to confirm this for yourselves byusing Figure 21.3.1b. So, that approach will not be sat-isfactory either.

The difficulty of identifying the trend-line of the oro-genic edifice in the Altaids has led to proposals thatthey might consist of more than one independent oro-genic belt. However, the great similarity of the rockmaterial involved in their architecture, the significantuniformity of style of their internal structure, the broadcorrespondences between their disparate sectors in tim-ing of tectonic evolution, and the difficulty of finding“junctures” where one orogen would join another one,make it unlikely that there are a number of independentorogenic belts tucked away within the Altaid realm.

It has been proposed that magmatic fronts of arcsconstitute convenient markers to follow the orogenictrend owing to their easy identification, lateral persis-tence, and indication of facing; they are very sharp onthe ocean side (i.e., the side toward which they are saidto “face”), but more diffuse on the backarc side. Thisidea was applied to the entire Altaids to trace the first-order trend-lines of the orogen, and it was possible toshow that the chaotic internal structure could be inter-preted in terms of the deformation of formerly simplerarc geometries.

In Figure 21.3.2 we see how this idea is applied tothe western and central sector of the Altaids (the

5392 1 . 3 T H E A L T A I D S

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F I G U R E 2 1 . 3 . 2 (a) Carboniferous magmatic fronts (I) and their schematic palinspasticinterpretation (II). Magmatic fronts are shown with thrust symbols, and face in the oppositedirection from the apices of the triangles. The numbers in I correspond to the unit identifications inFigure 21.3.1b and in the reconstructions displayed in Figure 21.3.4. In II, the numbers 1, 2, and 3correspond to the Valerianov/Tien Shan, the central Kazakhstan, and the Zharma-Saur fronts,respectively. In II, the figure on top is a highly schematized version of the present geometry of theCarboniferous magmatic fronts, as shown in I; the bottom figure is an interpretation of the topconfiguration in terms of a former single arc. (b) Silurian magmatic fronts (I) and their schematicpalinspastic interpretation (II). The numbers in I correspond to the unit identifications in Figure21.3.1b and in the reconstructions displayed in Figure 21.3.4. In II, the numbers 1, 2, 3, and 4correspond to the Valerianov/Tien Shan, the Atasu-Mointy, the central Kazakhstan, and theZharma-Saur fronts, respectively. In II, the figure on top is a highly schematized version of thepresent geometry of the Silurian magmatic fronts, as shown in I; the bottom figure is aninterpretation of the top configuration in terms of a former single arc. (c) Ordovician magmaticfronts (I) and their schematic palinspastic interpretation (II). The numbers in I correspond to theunit identifications in Figure 21.3.1 and in the reconstructions displayed in Figure 21.3.4. In II, themagmatic fronts can be grouped into three domains: fronts 1–6, the Tien Shan–southwestKazakhstan domain; fronts 7–13, north-central Kazakhstan; and fronts 17 and 18, the Zharma-Saur domain. Here the enumeration of the magmatic fronts corresponds with the unit numbers inFigure 21.3.1b. In II, the figure on top is a highly schematized version of the present geometry ofthe Ordovician magmatic fronts, as shown in I. The bottom figure is an interpretation of the topconfiguration in terms of a former single arc.

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Kazakhstan-Tien Shan sector). In Figure 21.3.2a, thelocations of the Carboniferous magmatic fronts areshown (I). Note that, with the exception of the centralKazakhstan front, they exist only on the outer periph-ery of the ensemble of tectonic units and pass fromone unit to another. This indicates that, by the Car-boniferous, the units making up the western and cen-tral sector of the Altaids had already come together(because the magmatic front “stitches” the outer unitstogether). Panel II shows how this picture may beinterpreted in terms of a single arc. At the top ofPanel II a simplified trend-line pattern is shown,which corresponds with the actual geometry in PanelI. At the bottom of Panel II, this is interpreted in termsof strike-slip disruption of a formerly continuousfront. Figure 21.3.2b shows the magmatic fronts of theSilurian. In general they are not dissimilar to those inthe Carboniferous, but they show a marked differencein detail. The fronts have migrated inwards along theouter periphery, whereas in central Kazakhstan theymoved outward, and the front along units 5 and 6 haschanged its polarity. Note how these changes affectthe interpretation. Figure 21.3.2c I exhibits a muchgreater complication and we no longer see the neatpicture of the earlier frames. As seen in Panel II, the

interpretation is correspondingly more complicated.(Can you tell wherein lies the complication?)

The magmatic fronts in the Altaids may thus beinterpreted in terms of a single arc. But was it really so?In other words, does this agree with the rest of theirgeology? In order to check that we must find tie-points(i.e., points that are now far away from one another, butthat used to be adjacent to one another) on adjacentunits that would allow us to bring them back to theirprefaulting positions. This is not a straightforward exer-cise, because many geologic features can be used as tie-points. This analysis uses the arc massif–accretionary complex contacts as one set of tie-points.Faulted and displaced forearc basin parts, segments ofbackarc basins, and metamorphic complexes are amongother features that have been used as features providingtie-point sets for reconstructions in the Altaids.

In Figure 21.3.3 we see the disassembled units inthe Kazakhstan-Tien Shan sector of the Altaids, withan emphasis on the arc massif–accretionary complexcontacts. The reconstructed single arc at the bottom ofthe figure is generated by bringing into juxtapositionthe tie-points on adjacent units. Thus, much of theAltaid edifice could be interpreted as the deformedremnants of a single magmatic arc, called the Kipchak

arc (after the dominant ethnic groupinhabiting the area where its frag-ments are now found), now disruptedand squeezed between the two cra-tons of Russia and Angara. Altaidsextending eastwards into Mongoliaevolved from a second arc (Tuva-Mongol) as shown in the reconstruc-tions in Figure 21.3.4. (Can you findthis second arc in Figure 21.3.1?)

The location of the two arcs withrespect to the two continental nucleiof Russia and Angara in the geologicpast is established by finding a partof the arc that has remained attachedto Angara (in the case of the Kipchakarc, in the vicinity of the southern tipof Lake Baykal; in the case of theTuva-Mongol Arc, in the Stanovoy

Mountains along the Stanovoy Fault;see Figure 21.3.1), and then compar-ing the geology to that of the marginsof the Angara and the Russian cratons.Ideally one would like to support thisprocedure with paleomagnetic data tocheck paleolatitudes and paleo-orientations of the individual Altaidunits, but, in the case of the Altaids,

5412 1 . 3 T H E A L T A I D S

Ki

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Kazakhstan-Tien Shantectonic unitsreconstructed

F I G U R E 2 1 . 3 . 3 Method used in the reconstruction of the Kipchak arc. At thetop is a “disassembled” version of the Kazakhstan–Tien Shan sector of the Altaidorogenic collage. These units were then reconstructed into the Kipchak arc by usingtie-points. These tie-points represent points on correlative accretionarycomplex–backstop contacts. The reconstruction is then checked against thosemade by correlating magmatic fronts (shown in Figure 21.3.2). While doing ourreconstructions, we also used other sorts of tie-points, correlating such features asforearc basins, metamorphic complexes, and backarc basin sutures, but they are notshown here to maintain legibility.

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reliable paleomagnetic data are very scarce. Only thepositions of the two major cratons are known with anydegree of confidence (Russia being better known thanAngara). Figure 21.3.4 shows the result of such geologiccomparisons.

First-Order Tectonic Evolution of the AltaidsIn Figure 21.3.4a we show the picture obtained byreconstructing the positions of the two major cratonsof Angara and Russia in the Vendian (∼630–530 Ma),and placing the Kipchak and the Tuva-Mongol arcsonto the resulting large continent. Rifting in the Ven-dian left fields of dykes and normal-fault-boundedbasins both to the north of Angara and Russia. As thetwo began to separate from each other, the Kipchak arcalso detached itself (as shown in Figure 21.3.4b; EarlyCambrian, ∼547–530 Ma) in front of an opening mar-ginal basin (the future Khanty-Mansy Ocean3) thatmay have been as large as the present Philippine Sea orthe Tasman Sea, both of which also opened as backarcbasins behind migratory arcs.

Units 10–18 have been formed by ensimatic arcsthat may have nucleated along a long transform faultthat connected the Kipchak subduction zone with theTuva-Mongol subduction zone. That transform faultmust have met the Kipchak arc at a triple junction as shown, otherwise the kinematics does not makesense. Can you guess why? By Late Cambrian time(∼514 Ma) the triple junction migrated along theKipchak arc, and the former transform fault turned intoa subduction zone nucleating magmatic arcs alongitself, thus lengthening the Kipchak arc. West-dippingsubduction also started below the future Urals alongthe eastern margin of the present-day Mugodzhar unit,an ensialic magmatic arc remnant in the southern andcentral Urals.

Transpression along the outer Tuva-Mongol sub-duction zone had begun slicing up the active marginand transporting arc and accretionary complex frag-ments towards the Angara craton. By contrast, alongthe inner margin, facing the Khangai-Khantey Ocean,the geometry of subduction was much simpler. It isnow impossible to reconstruct palinspastically theTuva-Mongol fragment itself.

In the Middle Ordovician (∼458 Ma) the same pic-ture as in the Late Cambrian seems to persist (Fig-ure 21.3.4c), except that in two places along the

Kipchak arc, marginal basins (Boschekul-Tarbagatayand Djezkazgan-Kirgiz-Jalair-Nayman) resemblingthe present-day West Mariana Basin in position, butthe Japan Sea in tectonic character, began to open.(Can you guess why they were similar to the WestMariana Basin in position, but to the Japan Sea in tec-tonic character?) Also rifting began tearing away astrip of land from Russia that eventually formed theMugodzhar microcontinental arc in front of the open-ing marginal basin of Sakmara-Magnitogorsk in thesouthern and central Urals.

By Late Ordovician time, the two major cratons ofAngara and Russia had rotated sufficiently towardseach other and began squeezing the oceanic spacebetween them, spanned by the Kipchak arc. Thisresulted in the collision of the tip of the arc with theMugodzhar arc, similar to the collision of the Izu-Bonin arc with the Japan arc today. Further shorteningled to the internal deformation of the Kipchak arc thatwas expressed by cutting up the arc by strike-slip faults(perhaps similar to the Philippines Fault cutting thePhilippine island arc system today) and stacking itspieces beside each other along the strike-slip faults.The resulting geometry resembles thrust stacks tippedon their side.

In the Middle Silurian (about 433 Ma), the Kipchakarc broke along a left-lateral transform fault system,bounding units 5, 6 and 7, 8. In the Early Devonian(∼390 Ma; Figure 21.3.4d), the transform fault becamelengthened and its southern parts turned compres-sional. A substantial microcontinent had thus becomeassembled in the middle parts of the arc by the previ-ous stacking. In the north, the eastern part of the Tuva-Mongol fragment collapsed onto the Sayan Mountains.From now on, strike-slip transfer of units along theTuva-Mongol fragment passed directly onto the futuresite of the Altay Mountains.

The Late Devonian (∼363 Ma) witnessed much thesame sort of evolution as in the Early Devonian, exceptthat at this time, the Angara craton began a right-lateralshear motion with respect to the Russian craton. Vari-ous backarc and pull-apart basins opened in the con-tinuously deforming stacked Kipchak arc ensembleand this deformation began tightening the Kazakhstanorocline. Newer units were continuously being fed intothe Altay from the coastwise transport along the Tuva-Mongol fragment (not unlike the Cordilleran System,especially in Canada and Alaska in the Mesozoic andCenozoic). This further narrowed the deep gulf withinthe Kazakhstan orocline into which rich turbiditedeposits were being fed from the surrounding moun-tainous frame. This was also the time in which the Sakmara-Magnitogorsk marginal basin reached its


3The name Khanty-Mansy is another name derived from the aboriginallocal populations living in the northern part of the Western Siberian Lowlands.

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5432 1 . 3 T H E A L T A I D S

F I G U R E 2 1 . 3 . 4 Reconstructions of the Altaids. (a) Vendian reconstruction (∼630–530 Ma). The legend shown here applies toall reconstructions shown in Figure 21.3.4. (b) Early Cambrian reconstruction (∼547–530 Ma). (c) Middle Ordovician reconstruction(∼458 Ma). (d) Early Devonian reconstruction (∼390 Ma). (e) Early Carboniferous reconstruction (∼342 Ma). (f) Early Permianreconstruction (∼270 Ma). The text describes the evolution during intervening times; additional maps and more detaileddescriptions are found in the reading list.

20°N

30°N

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Angaracraton

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Fu

t ure

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ch

ak

arc

T u v a - M o n g o l

a

r c

craton

Balkaides and Pre-Uralides

Precambriamcontinental crustin the Altaids

arc and accretionarycomplex

subduction zone

strike-slip fault

K i p

c ha

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Khangai-KhanteyOcean


North Caspian Basinclosed back

Viluyi riftclosed back

Rift of the futureKipchak arc

Angaracraton

Russiancraton


Khanty-MansiOcean

Viluyi Riftclosed back

Patom shorteningrestored

Patom shorteningrestored

Djezkazgan-Kirgiz-Jalair-Naiman

double arc

Boschekul-Tarbagataydouble arc

Opening of theSakmara-Magnitogorsk

marginal sea

Khanty-MansiOcean

Angaracraton

Russiancraton


Viluyi Riftclosed back

Sakmara-Magnitogorskmarginal sea

Kazakhstanorocline

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orocline


Hidden subduction

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opening

Stacking ofQilian Shan


SolonkerOcean

(a) Vendian (b) Early Cambrian

(c) Middle Ordovician (d) Early Devonian

(e) Early Carboniferous (f) Early Permian

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apogee and began closing by northward-directed sub-duction (Late Devonian geographic orientation) underthe Mugodzhar arc. It was during the Late Devonianthat the North Caspian Basin (pre-Caspian depressionin some publications) began opening as a riftedembayment very similar to the Jurassic opening of theGulf of Mexico. In fact, the present-day Gulf of Mex-ico is the best analog for the North Caspian Basin.

In the Early Carboniferous (∼342 Ma; Fig-ure 21.3.4e), the Sakmara-Magnitogorsk marginalbasin was almost completely closed, but because of thecontinuation of the arc-related magmatism on theMugodzhar arc, some subduction probably lasted tillthe end of the Mississippian. To the south of the still-growing Altay Mountains (Early Carboniferous geo-graphic orientation), units 19 (Ob-Zaisan-Surgut) and20 (Kolyvan-Rudny Altay) were emplaced along thegiant Irtysh shear zone into their present locations withrespect to the Altay-Sayan Mountain complex. Thisfurther narrowed the large embayment within theKazakhstan orocline, which was rapidly being filledwith clastics eroded from the surrounding Altaid units.The right-lateral shear of Angara with respect to theRussian craton continued throughout the Early Car-boniferous. In the Late Carboniferous (∼306 Ma), thegulf within the Kazakhstan orocline was obliterated bythe continued convergence of the Angara craton andthe Russian craton. Any further accretion to the rem-nants of the Kipchak arc was made impossible by thecollision, along the present Tien Shan Mountains, ofthe Tarim block with the assembled Altaid collagebetween the Angara and the Russian cratons. Thenature of the crust underlying the Tarim block isunknown owing to lack of outcrop and paucity of geo-physical data. What little is known indicates that it is astrong, old (Late Proterozoic) crust, which may be atrapped oceanic plateau similar to the present-dayOntong-Java now colliding with the Solomon Islandsin the southwest Pacific Ocean.

The right-lateral motion of the Angaran craton withrespect to the Russian craton and the Kazakh part ofthe Altaid collage continued in the Early Permian(∼270 Ma; Figure 21.3.4f). This motion was beingaccommodated mostly along two major shear zones,namely the Irtysh and the Gornostaev. The deep NurolBasin within the basement of the West Siberian Low-lands, just north of the Kolyvan Ranges north of theAltay Mountains began forming in the Early Permianas a pull-apart structure along the Irtysh shear zone.This was the earliest harbinger of the beginning exten-sion in the West Siberian Lowlands that lasted until the

Middle Jurassic in different places and under differentstrain regimes. The earlier episodes of this extensionespecially are directly related to the Altaid evolutionand constitute one spectacular example of extensionalbasins forming atop former orogenic belts, such as theEocambrian Hormuz salt basins in Arabia followingthe latest Precambrian Pan-African Orogeny, or theMiddle to Late Cenozoic Western Mediterranean andthe Late Cenozoic Aegean Basins following the AlpineOrogeny. (Can you think of similar post-orogenicbasins within the United States? How do you thinkthey formed?)

In the Late Permian (∼250–255 Ma), the right-lateral motion of the Angara craton with respect to theRussian craton and the Kazakh sector of the Altaid col-lage reversed. The resulting left-lateral motion waslargely accommodated along a broad swath of shearzones south of (Late Permian geographic orientation),and including, the Gornostaev shear zone, creating abroad keirogen (a deformed belt dominated by strike-slip motion). The late Permian extensional basins ofNadym, Alakol, Junggar, and Turfan opened along thiskeirogen as pull-apart structures, involving internalcounterclockwise rotations exceeding 90° about verti-cal axes.

In the Kazakhstan-Tien Shan sector, the Late Per-mian saw the end of the Altaid orogenic evolution,although extension in the West Siberian Lowlandscontinued until the Early Jurassic, possibly resultingfrom continued limited jostling of the Angara and theRussian cratons along the large Gornostaevkeirogen. Farther east (present geographic orienta-tion), Altaid orogenic evolution continued in theMongolian-Far Eastern sector by the ongoing clo-sure of the Khangai-Khantey Ocean as a conse-quence of the collision of the North China block withthe South Gobi units (unit 44). The progressive nar-rowing of the Khangai-Khantey Ocean lasted untilthe Jurassic. During this narrowing, a part of theaccretionary fill, formed by the Kangai-Khanteyaccretionary complex (unit 43.2), was extrudednorthwestwards. (Can you guess why?) By Jurassictime, the Mongolia/east Russia/northeast Chinaregions acquired more-or-less their present-dayarchitecture; there was some shortening in theextreme far eastern Russian Altaids continuing intothe early Cretaceous, but by then it was under theinfluence of the Nipponide evolution (i.e., orogenyresulting from the interaction of oceanic plates in thePacific Ocean with the eastern margin of Asia) andnot an integral part of the Altaid development.


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21.3.4 Implications for Continental GrowthThe time slices in Figure 21.3.4 show how significantvolumes of new continental crust were added to theAltaid edifice as it developed from Vendian timesonward. It is estimated that some 1012 km3 of materialmay have been added to the bulk of Asia between theVendian and the Late Permian (not taking into accountthe Khangei-Khantey accretionary complex), whichaccounts for some 40% of the total Paleozoic crustalgrowth rate. Clearly, the Altaids represent a major tec-tonic system during Earth’s history in the Paleozoic.This accretionary aspect of Turkic-type orogeny mayoffer insights into the formation of Earth’s earliest con-tinental crust, as discussed in the following section.

21.3.5 Closing RemarksThe Altaids are the most spectacular example of a Turkic-type collisional orogen in the Phanerozoic. Turkic-type orogens form by collision of very large,subcontinent-size subduction-accretion complexesfringing two (or more) converging continents and/orisland arc systems. Three main processes contribute tothe consolidation of such large subduction-accretioncomplexes into respectable continental basement:

1. Invasion of former forearc regions by arc plutonsthrough the trenchward migration of the magmatic axis as the trench recedes and the subduction-accretion complex becomes wider.

2. Continuing bulk shortening of the subduction-accretion complex.

3. Metamorphism of the accretionary complex upto high-amphibolite grade, either by ridge sub-duction or by the exposure of its bottom to hotasthenosphere by steepening of the subductionangle.

4. Further thickening of subduction-accretion com-plexes and melting of their bottoms as a conse-quence of the convective removal of the litho-spheric mantle.

Recognition of Turkic-style orogeny has long beenhampered by emphasis on Alpine- and Himalayan-type collisional orogens, which has been conditionedby the familiarity of the world geologic communitywith the Alps, the Himalaya, the European Hercynides,and the Caledonian/Appalachian mountain ranges. Thestudy of the Altaids and their comparison with theNorth American Cordillera have begun to uncover the

main features of their architecture and the rules of theirdevelopment. Many of the familiar features guidingthe geologist in Alpine- and Himalayan-type colli-sional orogens lose their significance in the Turkic-type orogenic systems. Although abundant ophioliticslivers, nappes, and flakes exist in vast areas occupiedby Turkic-type orogens, these do not necessarily marksites of sutures. Intra-accretionary wedge-thrust faultsand, especially, large strike-slip faults, commonly jux-tapose assemblages formed in distant regions, anddeformed and metamorphosed at different structurallevels. Such faults are likely to mislead geologists intothinking that they are sutures, bounding different,originally independent microcontinental “terranes.”The recognition of the Turkic-type orogeny has thusmade necessary not only detailed and careful fieldmapping and description, in terms of genetic labels,but also detailed geochemical sampling to see howmuch of the accreted material is juvenile, and howmuch is recycled.

It is proposed that Turkic-type collisional orogenywas very widespread in the Proterozoic, possiblylargely dominated the Archean development, and con-tributed significantly to the growth of the continentalcrust through time.

A DDITION A L R E A DING

Allen, M. B., Sengör, A. M. C., and Natal’in, B. A.1995. Junggar, Turfan, and Alakol basins as LatePermian to ?Early Triassic sinistral shear structuresin the Altaid orogenic collage, Central Asia. Journalof the Geological Society of London, 152, 327–338.

Burke, K., 1977. Aulacogens and continental breakup.Annual Review of Earth and Planetary Sciences, 5,371–396.

Kober, L. 1921. Der Bau der Erde. Gebrüder Born-traeger: Berlin.

Sengör, A. M. C., 1990. Plate tectonics and orogenicresearch after 25 years: a Tethyan perspective. EarthScience Reviews, 27, 1–201.

Sengör, A. M. C., 1991. Plate tectonics and orogenicresearch after 25 years: synopsis of a Tethyan per-spective. Tectonophysics, 187, 315–344.

Sengör, A. M. C., and Natal’in, B. A., 1996a. Palaeo-tectonics of Asia: fragments of a synthesis. In Yin, A., and Harrison, M., eds., The tectonic evolu-tion of Asia, Rubey Colloquium. Cambridge Uni-versity Press: Cambridge.

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Sengör, A. M. C., and Natal’in, B. A., 1996b. Turkic-type orogeny and its role in the making of the con-tinental crust. Annual Review of the Earth and Plan-etary Sciences, 24, 263–337.

Sengör, A. M. C., and Okurogullari, A. H., 1991. Therole of accretionary wedges in the growth of conti-nents: Asiatic examples from Argand to plate tec-tonics. Eclogae Geologicae Helvetiae, 84, 535–597.

Sengör, A. M. C., Natal’in, B. A., and Burtman, V. S.,1993. Evolution of the Altaid tectonic collage andPalaeozoic crustal growth in Eurasia. Nature, 364,299–307.

Stille, H., 1928. Der Stammbaum der Gebirge und Vorlaender: XIVe Congrès Géologique International(1926, Espagne), 4. Fscl., 6. Partie, Sujet XI(Divers), Graficas Reunida S. A., Madrid,1749–1770.

Suess, E., 1901[1908], The face of the Earth (DasAntlitz der Erde), v. 3, translated by H. B. C. Sollasunder the direction of W. J. Sollas. Clarendon Press:Oxford.


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21.3 TECTONICS OF THE ALTAIDS: AN EXAMPLE OF A TURKIC …