JOURNAL OF CEOPHYSICAL RESEARCH, VOL. 98, NO. B12, PAGES 21,927-21,941, DECEMBER IO. 1993

The Configuration of Asia Prior to the Collision of India:Cretaceous Paleomagnetic Constraints

yaN clm{, I VINcENT @LJRTILIrIr, JEA}.I-PAScAL cbG}.IÉ,JEAI.I BESSE, UIEI,IYU YÆ,lC, AND RAI{DY EtnOf2

Laboratoirc de Paléonagnltlnru ct Gédyumiau, DéNnutcnt de Géoougnétisræ ct Paléorugtrétisnv (URACNRS n9),Itstitut de PhysQru dttGlobe , Parls

Palcomagncric data frm Ccntrrl Arir rhow thu l7mltrlO lrn of rhoncning of rcuthcm Asia sinccCrcoccous timc hevc bccn rbcorbcd by diruibutcd dcformatio bawccn rcûhcm Tibct md thc Sibcrircnron. Thir rcsulr ir bqscd on e compil*im of Crcrrccanr pola frm thc Jrmgger, Trrin, Tibct, Indochina,Soutb Chinr, North Chine, md Mcrgolie blockr, ccnplcurcnting tbc rcccnt onpilrtion of Enkin ct d.(199à). Wc propoec e palcogcognphic recoctruction of Arir in thc Crctrccour, in whidr thc poririoo ofSiberir ir dcrivcd frqn thc rynthetic epprrcnr poler wurder prth of Begæ rnd Courtillc. (l9l). 1}c rcrultingmep, which likcly reprerenu Arie rs it rcrneined throughour the Creteccouc until the collirim sith Indiebegan, fcaturcs en "unbcof Tibcq wilh rn a$t-wcst trending Andean margin rt trcpdcel hinrdcs rnd e rdhcrcmtinuour belt of cqrtinentrl r€d bed besinr cxænding frorn Sic.huan ro Trrim ùmugh Tïbet- Thc mep allowsone to cstimeæ contincntal shoncning ud rqarior bawecn thc blocks, which erc eributed to rhe collisionDcspiæ largc unccrtaintics, thesc havc ûtrouots rnd ccoses which rrc in dl cascs compatiblc with rorrcEccnt kinemrtic modcls such u that of Avonc (1991).

INTRODUCTION

For the last decade, Asia has been the target ofurryrecedentcdactivity by paleomagneticians. Cooperation between Clrineseand principally French and U.S. laboratories has led to aremarkable growth of the corresponding database. The purposeof these studies has been to describe and quantify thegeographical and tectonic evolution of the lithosphericblocks that have accreted to form the Asian mosaic (Figue 1).As time has passd the sampling in both space and time hasincrease{ and resolution is now reaching s stage at whichother geological and geophysical field observations can becompared to and incorporaæd in lhe reconsEuctions proposedby paleomagneticians.

h*in et al. 1I992aJ have proposed a critical evaluation ofthe available data and a set of paleogeographic reconstructionsgoing back to the Permian. Emphasis was placed on the majorblocks, principally North and South China (NCB and SCB,respectively) in their relationship with Siberia. Eurasian polarwander was assessed indirectly by ransfening data from othcrplates or by using data from remote areas of Eurasia itselflBesse and Cotstilbt, 198E, 199U. [.css emphasis was placedon central Asian blocks. Yet these smaller blocks have beenparticularly strongly affecæd by the Paleocene collision ofIndia srd their mode of deformation has been the subject of anongoing controversy for the last decade. End-member modelsinvolve crustal or lithospheric doubling, homogeneous crustalor lithospheric thickening le.g., Englart anù Houseman,19861, inhomogencous strain localized along a few majormountain ranges and sutures, major strike-slip faultingallowing for lateral extrusion [Pekzer and Tqpornicr, 1988].

New paleornagretic data have been collecrcd and publishedfor these blocks, particularly the Jrurggar block in XinjiangProvince lClun et al., l99ll, the Tarim block fChen et al.,19921, and Tibet farther south [Claz et al., 19931. A generalprescntation of thesc data has bec,n proposed by Chen [1992].

T.r., "t Dép"rl".ent des Sciences de la Terrc, Université d'Orléens,

Orléans. France.2Now at Pacific Geoscience Ccnter, Sidney, Britistr Columbia, Canada.

Copyright 1993 by thc Amcricrn Gcophysical Union.

Prper number 93t802075.

0 | 48 -0227 | 93 | 9318-02075 $05 .00

The period for which rhe largest and best qualiry data areavailable happens to be the Cretaceous, the periodimmediaæly preceding the Indian collision. The present paperfocuses on a synthesis of these data and proposes aCretaceous, precollisional reconstnrction of Asia. This can beused to infer the atnounts and modes of deformation that haveled to the present geometry. Although this paper concentrateson blocks prcsently locaæd to thc wcst of, say, 100oElongiurde, reference will be madc to ongoing companion workby Yang IL992l, including new data from Thailand [Yang andBesse, 19931 and from the NCB lYang et al., 1991, L992; Maet aL, L993l and SCB lFl*in et al., l992bl.

GEOI.OGICAL BACKGROTJI{D

Jhe dptailed geometry of thc blocks that comprise some l0 x10o kmz of China is still debatcd but tlre outlines of the mainblocks are now generally agreed upon (Figure l). The exrct ageand nature of thc block boundaries, which havc often had along history md have sometimes becn severcly affected as aconsequence of the India-Asia collision" rtc not alwaysestablished by freld obserrratiorrs. To thc south and wcst of Ùrclarger Siberia, NCB and SCB, we encounter the followingseries of blocks, going roughly from north to south.

The Jrmggar block (JIJN) forms the southeastern niangularærmination of the larger Kazakhstan block (KAZ), butMesozoic or Cenozoic relative motion of the two is notexcluded. Feng et al. [1989] believe that amdgamation ofSiberia, Kazakhstan, and Terim begur in the middleCarboniferous. Carbonifcrous ûo Tcrtiary sediments crop outalong the folded foothills of Tien Shan (Figue 1). TheCretaceous rocks conformably overlie the Jurassic strata"starting with fine dark red sandstone and conglomeratesgrading r4rward to variegeted rnd light green sandstones.Ostracods of probable Brrrcmian agc, r typical AsianPsitaccosaunrs of probablc Albo-Aptim agc, and Hadrosaurs ofSantonian to Marstrichtian rge have been fourd in theseCretaceous formations lChen, l9E3; Hao a al., 19861. Thelower Tertiary is composed of brick-red g5rpsifcrous mudstqnesand ssndstones with thin layers of bioclastic limesones [Pangand Zhang, l9E9l and conformably overlies the Crctaceou.

The Tien Shan marts the boundary betwecn ttre Tûim (IAR)and Junggar blocks. This east-west trcnding mormtain beltextends over 25(X) km h Central Asia; it is e Paleozoic rangethat has bcen reactivat€d durhc the Ccnozoic, probably in the

2r,927

,,tIIII

2t,92E

middle Miocene lMolrcr and Tappornier, 1975;Trypotnieratd Mohar, 1979; Avotuc et al., 199.31.

Tarim is the largest basin in China. During the earlyCretaceous. a series of red clastic rocks of delta facies andargillaceous limestone of lake fscies w88 deposited in thewestern part of the basin. During the late Cretoceous,transgression of thc Tcthys Sea led to the deposition of aseries of littoral sediments. These sediments have beenaffected, with increasing intensity toward the periphery, bylate Paleozoic to Present tectonics fe.g., Molnar andTapponnier t975; Tappomier atù Molnar, 1979; Huang etaI., 1980; Tian a al., 1989; P. Tapponnier et al., msnuscriPtin preparation, 19921.

É.niro South the iiu"t* plateau extends for 1.5 x 106 tm2with an average altitude of 5000 m. It comprises three majort€ctonic subdivisions: the Krurlun-Qaidam, the Qiangtang (orNorth Tibetl and the Lhasa (or South Tibet) blocks which were

GIEùIETAL: @NFIGURAIIONæ AsI Pru8TOTIIE @LusION oFINUA

80"E 100"8 120"E

Fig. l. Schenrrtic block mep of Cllim md Sortheasr Asie rhowing thc main suhrres md feults (KF. Kerakonrm Feulq MBT.Main Boundary Thrurt). Thc melx blockr rrc AFG. Afghanfutan: EuR, Europe; INC, Indochina: IND, Indie; JIJN, JunggaçKAZ, Kazekstrn: KuN, Kunluq MON, Mqrgolir NCB, Nonh Chinr Block; QA, Qaidam; QI, Qiangteng; SIB, Sibcrie; SCB,South Orina Bloch ST, Shu Thri md TAR, Terim. Alro indicarcd rrc rhc main locetiqrr whcre Crcteccour pelcantegncticdata ere rvailablc. Declinrtiqr it indicûcd by rnowr (e rhin linc indicatcr tnæ north notc that duc ro thc projcctian sclccrcd, ithas bÊcn prefcnrr' to ûrcc thcrc rhin refcrenæ liner vcnicrl nrhcr rhrn rctuelly locelly perellel o thc gcri{ien). Opcttlrrows rrr for ercrr ruspcct d of mrjor locd rcdc dcformrtion or rurrclirblc palcomagnctic dete (scc tcxt). Solid tnows ltrùought b bc rep,rcrcntarivc of tlæ individuel blockc rr r brgcr rcelc.

successiyely sccreted onto Asia. Each one has its own,distinctive, sedimentary rnd tcctonic hisory.

The Kunlu block (KUN) is covered with a thick sequenoegoing from Cambrian to Triassic in age. The strongdeformation of Pernro-Triassic formatione is atributed to theuppcr Triassic collision of Krmlu with Qiangtang [Chang etal., 1986; Bourjot, l99ll.

Outcropping formations in Qiangtmg (QA) include basalsand Jurassic limestones [Matte et af., 1990], unconformablyoverlain by red Cretaceous sandstones. These Mesozoicformations are not affected by either strong metamorphism ordeformation-

The Qiangtang is separated from the Lhasa block by theBangong-Nujiang suturc zone [e.g., Girardeay et aI., 1984.;Allègre et al., l98/; Chug ct aL, 1986; Pan and Wang, l99l1'Bourjot, 19911. Ophiolitic melanges haye yielded upperJurassic radiolaria lPan and Wang, l99ll, compuiblc with

CHB{ ET AL: OiIFTGIJRIJIIC{ (T ASA hI(X 10 TIIE @LUSION OF II|IDIA 2t.929

130 M.y.ages obtained by Harris c, al. [198t] forgrurodioriæs md dioritic plulons in tlre east of tlrc plateeu.Collision is rupposcd !o hryc ocsurrcd in ûc uppcr Jurrssic.

Creteccous formations in the Lhere block (LH) occur lsAlbian-Aptian limcstoncs in thc north rnd west of the blockrnd as fluvial md dcltaic rcd rurdstsrce with somc volcmicint€rcalarions to the south and cast Most formarions locatedNorth of the Yarlrurg-Zangbo suure ue strongly dcformed withfolds sometimes ovcrturned. The closcr the suûrre, the strongerthe deformation which is attributed to the collision of India[e.9., Jaeger a al., l9E9l.

The Markam area is sandwiched between theJinshajiang and the Bangong-Nujiang sunues (Figure 1). Athick red bed sequence of Jurassic to Cretaceous ateconformably ovcrlics thc Triasric lequcncêE md is in turrdisconformably overlain by Eocene-Oligocene red molassicdeposits or Pliocene trachytcs rnd contincntal deposia. TheJurassic-Cretaceous red bed sequence has both great thicknessand wide areal distribution qound the Markam eretfRan et al.,19801.

Srurdaland includes Indochina, the Srmda shelf, PeniruularMalaysia and Sumatra" SW Borneo, part of Burma, part ofYurman south of the Red River fault, urd, possibly, para of SETibet (Figure 1). Its presenr lirnits E e marked by urcient sururezones of possible upper Triassic age, the Nm-Uttaradit suurreto the northwest and the Song Ma sutwe b the northeast; thewide range in the age of the associated granites (from Triassicto upper Jurassic) rurderlines our poor understanding of thegeologic configuration in ttre early/middle Mesozoic. Tertiaryactive faults, the Red River fault to the northeast md the WangChao / Threc Pagodas fault systems to thc southwcst, mark itslaæral limis. Throughout the Wmg Chao and Three Pagodasfaults zones, a N-S trending belt of granitcs represenrc theprobable continuation of the Uuaradit sunue zone. Althoughthe ages are not well constraind f&eckinsalc et al.. 1979iLiew and Page.19851, these granires are easily recognized urdmark the limits of Indochina with respect to the Shan-Thai-Malay block. After e major pre-up,per Triassic tectonic phaseknown as the Indosinian orogeny, the Khorat basin was filledby a thick continental succession of molasses (mainly redbeds) ranging in age from upper Triassic (Norian) to upperCretaceous. [n the region of the wesærn Khorat basin, theIndosinian orogeny is marked by the unconformity of theMesozoic series over a strongly folded marine limestoneseries of upper Permian age. The same Albo-AptiurPsittæosaurus farma forurd in Jrurggar end parts of Tibet is alsofound in the Khorat fBufetatt atd Suteethora 19911. Largccontinental basins of the same age can also be fourd in c€ntralYrmnan and Sichuan

Although the amount, rnode, and geometry of thedeformation are all subject to large uncertainties, there isgeneral agreement that by Cretaceous time all of these blockshad become part of Asia with essentially no major area ofoceanic crust intenrening between 8ny two of them [e.g.,Besse atù Courtillot, 1988; En&ir a al., l992sl. A Crctaceousmap of tlresc blocks would thcrefore provide a view of thcAsian continent as it probably remained from late Jurassic tothe end of ùre Cretaceous. Such a reconstruction is essentialfor the qurntifying and understanding of the inuacontinentaldeformation th* occuned sincc tlre onset of collision.

This collision induced major new faula within the Asianlithosphere and reactivated oldcr tectonic features (faults andsuturcs). Aftcr a first contact (astimaad to be as old as 60M.y.by Jaeger et al. U9891), India continued !o progressnorthward at Eome 5 qn/yr lMolnar and Tappomier, 1975;Puriat ard Aclwchc, 19841. The collision generrred rheHimalayan range but also reactivated and/or createdinfacontinental ranges farther north and northeast. such 8sthe Tien Shrn, Kunlun, Qùùing Shan (i.e.. suture berween the

NCB md S8 IPeltzzr a 4r., 19891), rnd Thrce Rivers arce[Tappoanbr ct al., I99Ol. A major, still debatod oonscqucneeof the collision is the southeastward crtrusion of theIndochina peninsula relative to Eurasia [e.g., Pcltzcr andTappornicr, 19881.

PALEOMAGNMC CbMTTIAIIION

Further paleomagnetic sampling has been obtained rcccntlyby ow group, with m cmphasis on Cretsceous siæs: (l) in theJunggar block to the north of the Tien Shan range we.st ofUrumqi fChen et a1., 199U, (2) in the Tarim block at itswestem ærmination to the west and north of Kashgar [Clvn etal., L9921, (3) along a trsverse through west€rrr Tibet, rc.rossthe Qiangtang and Lhasa blocks [Ctraz ct al., 199.31. (4) in theIndochina block on the Khorar plareBu [Yang and Besse,1993;Yang, 19921, (5) in the Sichuan basin of rtre SCB lE;rl*in ctal., l99la, bl md (6) in the Ordos part of the NSB lMa et al.,19931. These new data have been compiled wirh all thosepublished earlier by our group and by others.

This compilation (evailable on microfiche) follows thecriæria and format of that of Erùin et al. [1992a] which itcomplements for the blocks under consideration. For a pole tobe selectd it must meet the following qiteria: (l) rhe studymust hbe based on rnore than l0 specimens, (2) the 957oconlidence circle about the mean must have a radius less than15o, (3) results must be based on complete demagnetizatiorl(4) there must be some evidence for the absence ofremagnetizatiorL (5) the formation sge must be defined at thestage level and (6) sufficient information must be available toassess the qu8lity of the polc (Freld urd stability tess). Câez[1992] compilcs a completc lisr of polcs, which includesCarboniferous, Permian, and Jurassic poles for Junggar,Devoniart Carbonifcrous, Pernriarl Triassic, Jwassic, Eocenc,and Neogene poles for Tarim and Carboniferous, Permian,Triassic, Jurassic, Eocene and Neogene poles for Tibet,complementing the China pole list of Er*in et al. [199221.Altogether, 13, 49, end 59 poles, respectively are listed foreach block. All resulting dsts rf,e given in the appendix,available orr microfichel, with thosc rejecæd marked by urast€risk. Altogether, out of E2 upper ud lower Cretaceouspoles listed in the compilatiog only 16 passed the selectonprocess (two for Junggar, one for the Fergana part ofKazakstan, five for Tuim, and eight for Tibet). They arediscussed in more detail below.

Jtnggar block. "I\ree poles are available for this block(Fignre 2a md Tablc l). Two polcs ftom Clen et af. [199U forthe lower and upper CrerÂceous west of Urumqi pass fold andreversal tests and Ere not significantly different from eachother. A pole ûom Li et aI. [199U has only been published ina rneeting sbstract and cannot be properly assessed. Theselected poles are shown with their rurcertainties in Figrue 2b(following the format used by Eùin a aI. 1L992^l), rogetherwith the rpErent polar wander path (APWP) for Eruasiaderived by Eesse att CowtilIot [199U. The Cretaceous meanfor Jnnggar is at 73.4\ XB.6"E (A95 = l.l).Fergatu basin Bazhctot [193] has sampled 15 localities in

the lower Cretaceous sandstones of the Fergana basin, at ttresouthernrnost part of the Kazakstan block. Seven of theselocelities yield r primary megnetization" Although Bazhenovmentions the possibility that tho data Ere not Fisher-distrihrtrd, which hc rttribubt to indcntation of thc Pamirbelt, a Fisher averagc can be calculatcd leading b m average

I Appcndix ir rvrilablc with entirc pspcr qt microfichc. Ordcr fromAmcricen Geophyricrl Union. 2O00 Floridr Avcnuc. N.W.,Sleshington, DC 20009. Docnrncnt 893-007; $2"50. Paymcnr murtrccomprny ordcr.

2r,930 CTIEI{ ET AI.: GINFTTTUX.E--*TICÈ{ OF ASTA h'IOR TO TIIE COLL$ION OF I}IDIA

<Kl.J.2> --___-.-...--È r

, I+

1800

<K1.J1>----...-È 6f-->

tl\-''" 'i d

4<2.J.1>+ + * * O *

<Kl .F .1>

Pole ôt 7?3'N, n6.o"E (dP=5'4", dttt=E'6"; Figure 2 and Tablei). ttte"e is no evidcnce for sigrificant internal deformation ofthe basin.Tarim blæh Thirteen poles of Cretrceous age ee evaileble

for this block (Figurc 3a rnd Table l). Thcrc is some

ff;';'Htu:ffi ffi Ë'SHffiâff#',n*;i#S"-r.61$HiË'ËftËifr;i"leH;ô;: s.-dù;rr- iËl iiqiù,il. d.r.-r"-'oniA; in sËum m rhc mri. Iacntilicrtio 9f gqdr-pdc-run1 with rn indcx in

ffi. r*f,*'*,ifffoH,{Hm#gl#:lJ;Hii'*::}Ë"#,iffBiïiËli;ooco dorr cvcrv l0 rrr., o"iiiî- di.*qrrp"t"i inia rs mertcd wilh rclid aou (tlo o 70 m.y.) rnd for which rhcri'crrll cavaoli of 95.f, cmfiicnæ intcrvdr ir rbo*l ia rhrdcd prucrc

indication that thc polcs arc ctrcatcd dong a rmall circlcccnæred on Tsim, implying that eomc of the sempling siæsmay havc suffered somc lmorml of rotetion about locallyvertical axes. Thc çrdity of thc publislred rcsults is not rlwayeessy to ass€ss. For instance, Li et aI. U9891 did perform

1 80'

<Kl.J.1>

<K1.F.1 > Fergana, Bazhenov

GE{ ET AI.: @I.{FIGT'RAIIOTII æ ASIA hIOR TO TIIE @LLI$ON OF INilA

TABLE l. Sclcacd Crcrrær Pdcorgncdc Dû of Cbitrr

2l ,93 r

Locdity

cNrB)Agc FolÊ Ags N Tcrt Rcfcrcncc Codc

hllildc,'!{ lmplrdc, % Qldn

Æ.lnlLg Ku45.41191.6 Kl4LOnt92 Kl35.ry118.0 KlAvcnge K

3LUll9.O Ku3O.OlltLg Ku266llUL4 Ku261llgL4 Ku22U114.2 Ku25.U1164 Kun.9lrtL3 Kl29.7n20.3 Kl30.utm9 Kl26.urtl5 KlAvcngc KuAvcngc KlAvcngc K

4l.6183-5 Ku38.5t16.4 Ku39.5n5.O Ku3E.5n6.4 Kl39.5n5.O KlAvcragc KuAvengc KlAvengc K

76372.878.9il57E.267.9n.4n.l74.5@.on.374.9765

6.271.070.870.46.3@.46E5@.1

NorrhchitaBld79.6 170.1 5.t ,l R ?.hcag a eI.$9911P,9 nl.l 4.9b4 3 R Pruur |9t71&9 U9.5 5.7 6 F 7,lao ct al.1l9D0l75.8 2n8.7 75 l0 R Mactall l93|il3 208.3 6.9 4

<I(2-N.2><Kl.N.l><KlN.2><KlN.6>

<K2.T2.1><K2.T2.3><K2.T2.4><Kl.T2.l><KI.T2.2>

<K2J.l><Kl.J. l>

Aclsclp ct aI. 119841 <Kl.Ti.l l>Pozzi ct ol.ll982l <Kl.Ti.l2>Wcstplul ct al.ll983l <Kl.Ti.l3>(slne riær rr ebove)Liaaùlltaas$98,81 d<2-Ti.4>Acluclu ct al.[984] d(iTi.l(}>CIun ct al.|9D�31 <Kl.Ti.l4>oaluji cr al.Il9Blol <Kl.Ti.l6>Huang ct aLÛ9921 <Kl.Ti.ls>Hua^g ct ol.ll992l d0.Ti.l2>

SouthChitaBlæLl7L6 10.3 l0 F Kcat ct al.Il987l <K2.S2>Ul.l 4.1n.4 16 R hthctal.ll9lbl d(2.S.6>1866 13/67 3 - Zluctalll9Bl <K2.S.6>2,20.9 7.1 lt F Hung and O$ytt al992l d(2.S.4>171.9 10.6 12 R Chaa ll99ll <K2.S.t>Itfrz 92 m R HuctaL|l990l d9.S.l4>196.2 ll5/18.3 2 - Zluet al.[l9E8l <Kl.S.6>?27.6 55 7 R? Liâ llgt/.l dfl.s.3>229.0 L714.7 23 R Eakiactal.Il9lbl <Kl.S.4>2M.6 4.3 7 - Hrung and OQytz 119921 <Kl.S.7>195.9 6.9 6213.8 6.5 42t3.t 4.4 l0

TarimBlock

Sourhem Lhasa 30Bl KSouthem Lhase 29.7191.2 KSouthem Lhasa 29.9191.0 K

SouthemLhasa 29.9191.2Nonhem Lhasa 3lD2lfestem Plateau 33.7/t0.2Markam 29.719t.6Markam 29.7198.4Markam 29.7198.7Southem Lhesa AvcngcN+SLhasa Avcngc

K 6 E 2 n 3 5 & 9 8 FK 635 325.4 65 6 FK 62 U5.0 5.1 14 RK 4E5 175.9 95 5Kl '().6 170.5 13.0 12 FKnm 5E.7 173.2 ll.l l0 FK @.r 2t3.5 L8t5.4 16K e.6 gt.s 3.0t5.6 30

d(l.Tltl> rnd <19.1.4><Kl.Till>, do.Ti12>lnd <IO.Ti4><Kz.Tilb. do.Til5>ud <Kl.Til6>

<lil.F.l>

Markam Avenge K 4t.6 173.5 6.0 n

44.?JE6.O KÆ 75.44.2186.0 IK 72.3Avcngc K 73.4

7r.26Eg

45n2.5 K 74.3

92-65 ti^ 76.t130-9t Ma 752

223.3 6.1/10.34 R Lictal. l l9tt l234.0 6.8/11.6 6 F Chen ct aI. $992122L6 5.48.9 ll RF Chcnctal.Il92l2 l L l 6 . f l l 0 . t 3 R C l u n c t a l . l l D 2 l226.6 9.0/15.9 7 R Clunetol.ll92ln6,4 5.3 3no.o - 2n3.8 3.5 5

tuggar Block.2,253 6l/E.9 9IR Clunctal. l l99l lU3 4.Ell.2 13 R' Clunctal.ll9Dlln3.6 4.3 2,

Tiht Block288.4 79 E F3 û r 0 7 F3,fE 5.619.5 6 F

Fergau Blockn6O 5.48.6 7 R Bazhcwv ll993l

EwzsiaBlækt99.7 2:t 19Nl.s 4.0 l0

B cssc and C ast iI lot Il99 llB cssc aû C ourtillot ll99ll

13O-65 Me 76.3 WL6 22 29 Bcssc andCa nillotll9gll

Cretaceous, middle Cretaceous, lower Cretaceous, md all Cretaceors); pole (latitude, longitude), Ag5, @pldn), coordinat€s ofpaleomagnetic poles with rnc€rtainty at he 95Ço probability level; N numb€r of sûrdies, siûes or poles for thc averages; test (R,F), freld æsts (reversal" fold); and code, index of poles coresponding to the appendix on microfichc.

stePwise tlrrlnal or altcrnuing field dernagnetizations but did same s&mplint ard rit€s. Results qe similr but Zrmg ct al.not detcmine charactcristic directions by vecor malysis. add one sits with only rwo epecirnens md r rath€r outlierRather, they use a particulc Btrp in thc dcrnagnetization. Datr dircction. lVc therefore rctain only ùrc dau fuon Li a aI.from Lt .t ar. [l9E8] agdldZlung ct aI. IL989l corrcspond to thc 0988]. Results from Uyra\ close b the wesrem Kunlrm fault

2t,932 QIEN ET AL: @T{FIGUxIT-TIOÈ{ OF ASIA hIOR TO TIIE @IIISION OF [.IUA

4(2.Ta.b

< K 1 . T a . 1 > , I +<K1.Ta.3>

A 1<l€.Ta.5>1ggo

Fig, 3. Equel-rrcr prcldm of Crctrcou pdcr fm Tuim" ScG crtrion of FiSBE Z

IChar et al., 199218re too dispersed to Provide E relfuble pole'

Fitr"Uy, six poles meet the selection criterie (Figure 3b). CheVct al.-tl992i note thst the lowcr Crctaceous pole of Li ct al'

[1988] collectcd nerr Kuchc is romcwhat r€motc Ëom thcôtlrcr poles urd po'poee Ûû ûi8 msy be due to e major f-9ldstnrctrrc with plunging fold axis, prtly rcen on satellit€phoographs. Including this polc does- not churgc the ovcralli"aage very significantly, but we pefer not to use it Finally'

thc Cretaceoru mesr for five surdies stûds at 69.loN, 223.8"8(495 = 3.5o).Tibetaa blochç. As could bc cxpectd tlrs riaration is far

more complex (md thc data more numcrous) in Tibet thm inthe more stabb funggar rnd Trrim blocks. Thirty-one poleeare availablg but as many as 23 cnd up being rejectcd from thefinal selection (fable 1). Rejcctcd resulu include tlnseby 7Àuct al. 11977,19EU which correspond to hernatiæ bearfury rcd

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CEEI{ ET AI-: @NFKut/\flAn OF ASIA PEIOR TO TgE @IISION OF htf,I^ 2l,933

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sandstones which scre dcmagnctized only by the dternatingfield (ef) teclmique, which ir known to be ineffrcient in thiscsse. Thc dabbEs€ is disan$Gd in dctdl W Cr,at a al. ll993l.Notc thrt thc polcs t€nd !o bc etrcakcd rlong e small circlccentsrod on Tibct, dthough lomc polcs rrc quitc clcarlyremoe from thil circlc (Figurc /tr). Thfu includcs r group offour poles sclætcd from erstemmost Tibct, nerr Markan,locaæd about 40 !o 65"N, 170 ro lt0"E, urd rcme polesobtained close to the Zengbo sunrrc (rnostly by Otofuji àt aI.

[19891; w Chen a al. 194.3, Figure 11b]) which arc clearlyaffected by strong shear parallel to thÊ suture. Chen et.l. elsodi6cus6 in dctail thc lcgc rmount of d.ta from the Lhasa rrea incentrd erst€f,n Tibet, particularly on the Trkena red sandstoneformation (ôt!i.l:ep{an) rnd rhe rndesitic Lingzizongformuion (60-50 Ma). Data from Achætu cr af. U9841 uràLin and lTars [1988] reveâl ûû the block has undergoneint€mEl local deformation in thc Lhasa rrea with relitiverotations berween r South Lhasa end e Norrh Lhasa block.

2r,9v

Eight polcs that pass the rclection critcriE (Figure 4b) are stillquia Aispaeed. The data Êom ccntral rnd westcrn Tibet yieldcompatiUte pdeoluitudes but imply deformation at all scales(from a few tc,ns of kilometcrs (c.g., North lhasa vcrsus SouthLhrsa) to hundreds of kilometers (e.g., Lhasa versus WestTibe0). On the other hanû the data from easærn Tibet nearMarkam totofuji ct al.,1989, 1990; Huang et al.' 19921' someof which pass the fold and reversal tess, imply both a verylarge clockwise rotation with respect to the rest and a muchhigher palcolatitude, implying an origin from a differentblock. This is discussed further below.Panir-Punjab. Bazherot and Burtnun [19E6] have publishedCretaccous data for the Pamir-Putjab. However, most sampleswere not demagrretized above 4(X)oC, and fold tests 8re notdocumcnted. Also means are calculatcd from samples ratherthan siæs. As e consequence, these resulB cannot be used withconfidence. They display a large scalter in declination whichis likely due !o the proximiry of thrusts md shear zones, aswas noted above to be the case for the data of Otofuji ct al.

[1989] taken close to the Zangbo sutue (see oP€n rrrows onFigure 1).

irraæn;r- btochYang atû Besse [1993] have sampled some20 localities in and around the Khorat plaæau with well-constrained up'per Jurassic and lower Cretaceous age. Despite alack of fold test, due to the flat lying strata, the sedimentsovedie upper Triassic and lower Jurassic formations for whichboth folà-and reversd tests 8re evailable. Yang and Bessederive a mean pole at 63.8oN, 175.6oE (495=1.7').

DISCUSSION

A remarkable feature of the Cretaceous data from the Asianblocks is the consistency between upper and low€r Cretaceousresults (Iable 1). It seems that in this area the Cretaceous was atime of little relative motion or displacement with respect totlre pole, as notod by Clvn et al. Il99Il for the Jrurggar blockandby Clæn et al. Il992l for the Tarim. The saflre spPears tohold for the NCB and SCB (*e Er*in ct aI. 11992-a'1, updatedby Yang [1992]) and for the Eruasian API#P as a whole this isthe "hairpin loop" or standstill of Bessc and Courtillot,U99U. It is therefore possible to av€rage the lower and upperCretaceous results, leading to Cretsceous poles shown inFigure 5 and listed in Table 2.

Despiæ still rather large uncertainties, the poles of NCB endSCB are compatible with those from Eurasia, implying thatwithin paleomagnetic unccrtaintics, thc main Chinesc blocksand Siberia had come to occupy their present respectivepositions by the late Jurassic fEnkin et al.,1992\ Yang,19921. It is found that ttre Cretaceous poles for Eurasia,Jrmggar, Trrinr, lVestern Tibet, and India which has been addedfrom Eesse and Conrtillot [99U for complete,ness are roug]rlyaligned along a great circle that intersects centrel Asia in aNNE direction (d$hed circle on Figure 5). This means thaq tofirst order, thesc blocks have moved towards cach other in thudirection with littlc rotation.

More precisely, going from north to south, the angulardistance between the Eurasian and Jrurggar poles (6.2't4.8') isstatistically signifrcant and corresponds to 650 krLt530 kmof shoræning in a N-S direction (i.e.. a paleolatiurde differenceof 5.9oi4.8") and an insignificant counterclockwise rotationof 2.4ot5.8o of Jrurggar with respect to Siberia, for a referencepoint u 44oN/86oE.

The angular differencc between thc Junggar and Ferganapoles lcads to a negligible lrtinrdinal difference of 0.3"16.9oand a sigrifrcant rotation of 15.7ot10.0o at a reference siælocared near Fergana (405'N 72.5'E). This is consistent witltthe idea that these two formed parts of Kazakst8n but that theFergana basin has undergone significant counterclockwiserotation related o rightJaterd motion on the Talasso-Fergana

CHEN ET AL: @NFTG:TJRATION G ASIA PruOT TO TIIE æLUSION OF bDTA

Fie. 5. Equrl-uer projectim of Crctrceour mean poles for AsitnU;fr. TË formrr iirs-in Firurcr 2 ro 4. Polcr fq Eunsie, NCB' SCB'Junggar. Terim. lVcsr Tiba (rnd India) lic rougNy or a-(dashcd) greetcirclc (oaleqneridiin) dcmmltruinc thc rcletivc quasi-N-S shoncninglbetwcà ûcrc bloctr, with litÙc rqrrion. Pdcs frorn Tibcr, o thc wcst of95"8 longiurdc, lic m r (doccd) mrell circlc ccntcred-qt t[c platgautindictini rorrdmr wihout htirudind churgc. Polcr from Tibcr (andIndocbina), ro thc crrt of 95"8 langiude, lic o e diffcrtnr (doacd) mallcirclc indicrting mejor rorrdonr rnd htitrdind rhift with rcspca ro thcrerr of Tiba (rcc æxr for furrhcr cxplrntticrs).

fluùt lBazhenov, 19931. Baztrenov argues that this motionoccurred in the Neogene possibly as rece,ntly as the last l0m.y.

The distance between the Junggar and Tarim poles(4.3o15.5') is not sigrrificant at the 957o confidence level, butindividual Tarim poles are systematically farther from theEurasian pole than the individual Junggar poles.Corresponding shortening of 420 km1605 km (paleolatitudedifference of 3.8o15.5o) and rotation of 2.10*6.30 are obtainedfor a reference siæ at 40oNl/7"E.

As noted above, the Tibetan data cannot be reduced to asingle pole. The dua from wcstem and central Tibet lie on asmall circle implying a similar paleolatiude but some amountof relative rotatiorL whereas the dau from eastem Tibet implyboth signifrcant rotations and paleolatitude differences. TheTarim and Wesærn Tibet poles differ by 8.5016.4o, but thepaleolatitude difference is not signifrcant (5.?ot6.2o); thusthese data would imply a (statistically insignificant) N-Sshorte,ning of 630 kmt680 km (i.e., a paleolatitude differenceof 5.7o16.2") and a significant relative rotation of 7.1ot6.4o,for a refcrence point at 34"N/E0"E.

Paleomagnetic work south of the Yarlug-Zangbo suture,hence on the Indian plaæ, near Dingri has led Besse et al.U9841 and Sessc [986] to proposê that some 450 tn*500lm of intracontinental shortening occurred since the onset ofthe India-Asia collision asoss tho Main Boundary and MainCcnual Thnrsts, south of Dingri, ùd thEt 550 kmf650 km ofeho'rtening took place betwec,lr Dingfi md the Lhasa block

Bccausc no othcr majc æctoric cvcnt occurred sincc thetirne of deposition and magnetization of the sediments, it isbelieved thu dl values of shortcnfury and rotation dcducedÊom paleomagrretic data reflect ùre inægral of deformation dueto the collision. Shoræning vdues ere indicated on epaleogeographic reconstruction of Asie in the Cretaceousbased on thesc paleomagnctic dara (Figure 8), which updaæsEr*in et al's [f99à] Figure 27.

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OIEN ET AI.: oI|�TCURATIION OF ASIA I,RIOR TO THE @LIJSION OF DTTIIA 2 | ,93s

Occanic pleæ kinernatics allow cstimuion of ùrc otalconvclg€ncc bctwccn India md Eurasia rt 2600 tni900 tm inthe last 50 m.y. IPuriat and Aclwclu, l9&4; Bessc ardCovrtillot, 19881, i.c.. rougily sincc the onset of thccollisioru dthough origind contrct mey be rome l0 m.y.earlier [sec Jæga ct aI., 19891. lVc notc that thÊ individual(block to block) rhorænings obtained from paleornagncticdeta, csl@latcd neer E0"E longinrde, are 650 km (betwcenSiberia and Jrmggar, mostly in ùre Altri), 420 km (betweenJunggar and Tarirn, mostly in thc Tien Shan), 630 km(bcrween Tarim and Tibet, mostly in the Krmlrm and AltynTagh), and 1000 km in the Himalayss. This adds up a 2700km, in very good agreement with the ocean kinematicpredictions. Howevcr, the rurcertainty on this figure is notcomputable in a etraightforward fashion: total shoræningcalculatcd from thc Eurasirn rnd Westcm Tibet poles amounsto En angular diffcrcncc of 16.4"t5.5o (i.e., 17fi)t610 km in aN-S direction), but as far rs shortening across the Himalayas isconcerned, the Indian pole at 60 m.y.[Besse and Cowtillot,l99U is not ind€pendent from the Eurasian one. since the twoare synthetic poles combining peleomegnetic daùr fromsev€rol plaæs and ocean kinematic models for several ooesns,and the unccrtainty is not casily dcrived. Noæ that shoræningbeæween Eurasie and Ccntal Tibet wcs cstimatpd by Acluchcet al.1l9E4l to be 190ûjE50 km, based on their Lhasa blockpoles and a pole for Ewasia deærmined by Achache et aI.l l983l .

Individual shortening values can be compared o geologicalestimaæs based on field work or topographic and geographicaldEtE that constrEin crustal stnrcture. This comparison is basedon thc work of Avouac [199U. Assuming an initial thicknessof 30 krn for the crust in thc Altsi rnd Sayan Tuva, with apresent ayeragc ùinrd€ of 2000 m, a width of ?fi) krn, and acrustsl thictness of 50 km lPatton, 19801 would imply some300km of crustal Bhortening. This is most likelyrmderestimatcd because (l) initiat cflstsl thictness could havebecn thirurcr (some authors, for instance, Futg et 4J. U9891,rrgue for rn oceanic crust), (2) a large &rction of materid hasccrtrinly bcen rcmovod by erosio4 rs cvidcnccd by Terti.rymd Quaærnary s€dimcnts in thc basiq md (3) displaccmcnton NW trending dcxtral suikc rlip faula in the Altai wasncglected. Therefore, shoræning bascd on this reasoning islikely o have excceded 400 km. to bc compared with thepaleomagnetic value of 65ût530 km.

Cha a at. U99U md Avotrc a aI. ll993l have discussedthe compatibility of tectonic rnd paleomegnetic data relaæd torhortcning in thc Ticn Shan. Rcconsidcration of eorlicrpdcomegretic dau from the Tarim (the Kuchc pole discussedrbove) by Cten a al. ll992l hrs led to sonrc revisioru. Thecetimatod ehortcning proposcd by Avourc et al. for the TienShan (a ùe longitude of Kashgu) is 180 km. Again, thisrssumcs constant crustal volume and is likely anunderestimate. Another estimate cen bc dcrived from akinematic model of rctive deformuion in Asia propoeed byAvottæ Il99Il nd Avouæ a al.Il9.3l (see Fi$ne 6). Avouacoù Tqpomia [19931 hevc sclectcd ! rystem of four rigidrotrting blocke (Lutir, Tibet, Trrim lrd Jurggrr, which theyrasrmrc to bc cqnrccted to Sibcrie fq lack of quurtitativc datain thc Altai). Constraints sc the NIrVELI rngular vclocirydoscribing thc movcnrcnt of India with rcspcct to Eurasia srdtlrc position of ths Tsim vcrsu!. Jungga rotation polc. Inputdatr se cstimrtcd rrrcs of slprt€niru rcÎ�oss thê Ticn Shm rndHimelayrs ud slip rsl€s along thÊ Altyn TsSh urd Kratorumfauls. Invc$ion yiclds thc arnourts end scnses of motionshowa in Figrre 6. Compuison with ftriæ deformrtion endagc deærminations luggcst thet tlro currdrt kincmaticc haveremaincd rbout the same rincc rbout 15 m.y. (i.e., lcss thanone third of the timc rincc thc onset of tbc collision).Although thc rigid block hlryothcsis csurot hold for such a

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G|El{ TT AL: @NFIG.URATION (F ASIA PRION, TO THE COLUSION OF II{DIA

Fir. 6. Kinernrtic modcl of dcformetiqr in Tiber md Ccotnl Arie. Strilc-slip feulting rnd dock rurtio in cutcm Tibarrîcform northwrrd cqnprc*ion of Indir into routheastwrd crtrusiqr of South Qrini. Dashcd lincr rrc thc boundrricr ofdcformins cesrcm fiba- Éulcr polcr dcrcribins rhc motian of Tibct urd Tarim with respect ro Sibcrir urd of Trba wilh rcrpca19 Terirr

-ere etro rhown. Thc m-odcl is thought-o epply bect o 15 m.y. [frun Avoruc,l99li Avotuc ct a1.,19931.

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represents motion ovcr the last 15 m.y. leads to anotherestimate of shortening of 315 km. Convergence in theHimalayas over the same time int€n,d is 270 km.

Altogether, localiæd erain in a few well-identified rangesaccormts for 12fl) km of intracontinentd shortening betweenIndia and Sibcria since 15 m.y. This i8 1500 km less than theotal paleomagnctic cstimatc of convergence since the onsetof the collision (and 500 km less than the minimum value atthe 959o confidence level). This missing amount is eitherrelated o further inærnal deformation of the blocks since 15m.y., or more likely to t€ctonic events prior o 15 Ma Avouac[199U argues that most of the deformation in the Kunlun,Altyn Tagb and Tien Shrn occurred since 20 m.y. Thereforeprevious deformation might havc occurrcd cither within theblocks thernsclves, between thc blocks as oceanic orcontinental eubduction, for irutance, in the Himalayas orTarim, md possibly tluough latcral cxtusion of one or severalblocks.

The data frrom Martqn stand at odds from the rest of Tibet.Concurring resuls fron distinct shrdies lHuang et al., 199�2:Otofuji et al., 19901 rnd a positive fold æst leave no doubtthat a characteristic prcfolding magnetization has beenrecovcred. Therc is no rerson to susp€ct thc age of theformetions, which has been established by correlation withSouth Yunnrn stratigraphy. Athough, rs cen be feared inothcr cascs, particularly with rcd beds, a prcfoldingrcnragnetization always rcnrainr a possibility fe.g., Achachect al., l984.l, we believe that tbc magnetization is indeed a tnreCretaceous, primry one, implying that ttre Markam arca wasnot E part of the Tibetsn blocks, or st least wrs in a verydifferent relative position with respect to them prior tocollision.

long timc and geometrical adjustments must have rqkenplace, Iirst order values can be obtained from Figure 6 (raæsrnultiplied by 15 m.y.). For Jrmggar versus Tarim motionacross the Tien Shrn, this would imply some 200 km ofshortcning and 10o of rotation. lTese arc on thc low urd highsides, respectively, compared to the paleomagnetic estimatcs,but with the same senses.

The Tibetan data demonstrate internal deformation at scalesranging from I to 1000 km lChen a a'1., 19931, correspondingo superimposed block rotations and oroclinal bending. Figure5 shôws the well-lnown totation between the southern andnorthern parts of the Lhasa block [e.g., Achacle et al., 19841'amonnting a 17.7o!82o. Pdeolatinrdes betwcen central Tibet(North Lhasa) and western Tibct (Domar) arc similu (thedifference is m unsignifrcant 1.0o15.8"), but the declinationdifference is lcge reaching 28.5otE.0". This is the safire orderof magriode as the azimuthal differcnce betwecn thc easternand wisærn terminations of the Bangong-Nujiang Jruassicsuture zone (about 25). This may be consistent with the factthat both the tuurrc'rnd the southern Eurssittl margin wereroughly linear prior to collisio,n and that they werc thcn bent(see the similar oroclinal bcnding proposed by Klootwijk etaI. tl9E5l for tlre sunrrc betwccn thc Lhase block urd India).

. Bourjot and Avotuc [f990] cstimate thu shortccring acrossthe Kunlu amounted to 10ùt15 km. But a morc eignificantprft could have been absorbed along the left lateral Alryn Taghieult. Offsets by 500 rnd up to 700 km are advocued byTapponniq ct t. U9861 ard-Pcltzer uù Tappomiet [19881'implying r N-S compone'nt of convergence between 200 and300 km-. This is ûo bc comparcd to thc unfortrmatcly ill-deftrcd) paleomagnetic cstimate of 57Ot680 km. Using themodel of Avouac [1991] under the assumption that it

Rcccat rcsults ftom lndochinr lYang ard Bcssc, L9931indicræ that rs is thc carc for Markam, thir block etoodripificmtly north of Tibct rather than routh rs is the crsc !tP,rctqrt urd that it hrs bccn totrted clockwisc in verysignificurt lmormts sincc collbisl titrtc.

ttc have ploced ur Figure 7 inclinuion md declination"anomdies", defhcd as the differenccs betwecn observedCretaceoru vdues md thoee thet would havc been cxpected atthe srnrc dtes if tlrcsc had bocn rigidly arached to Eurasir, esa frnction of prescnt longiurdc (which is not too different froma plot as a frmction of Cretaccous paleolonginrdc, in rclativctermE). This plot (o be compared with the paleodeclinationmap of Figurc t rnd thc rccorutnrction of Figurc 8) clearlyoutlines that with referencc to thc eastcrnrnost part of Asia(NCB. SCB) which itself is compatible with Eurasian(Siberian) values, Tibet, Tarim urd Junggar have been pushed

2t,937

more thsn lmo km to thc North without much rotation(cxccpt in thc lhase ercr). Lrdochina md Markam hevc bccnrotstcd rcspcctivcly ronrc l5o ùrd 35" clockrrisc, Indochinabeing prshod comc 10fl) km rcuùr with rcspcct o Eurrsie urdChiru end mqe thm 20m km with rcspccr to cenùd Asimblocks.

In Figure 8 wc pro,pose a paleogcographic reconsmrction ofAsia in the Creteceous. Each block hss be€n placed in latiurdemd orientetion in egreernent with the paleomagrretic deta ofTable 2 and edjucted. well within the 95% uncerrainties, rocnsurc closest packing. Mongolia, NCB, and SCB arcsupposed to have been csscntially assembled in their presentconfiguration, although some convetgence in the Qinling andminor cxtnrsion of SCB after the collision are likely (but bothwell below paleomagneric rurcertainty). Indochina must thenhave lied immediately to the west of the SCB. We have

CAEN ET AI.: @NF|(I'I'ATKIII OF ASIA RI(x' TO TIIE 6I.LIIilON OF INIXA

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assumed in this reconstruction thet Markam was nestedbetween Indochina and the SCB. The first effect of thecollision must thereforc havc bcen the southeastwardextrusion of Indochina, with the Markam area squeezedbetwee,lr some of the major NNW trending strike slip faultsthat ængregate in the Ttuee Rivers area.

The recorutnrction outlines a brosd }VÎ.[V-ESE trending zoneof crustal deformation between the Mongolia and North Chinablocks to the north and Junggar, Trrim, Qaidam, andIndochina to the south. This broad zone comprises to the eastthc thick scdimc.ns that have subsequently been deformed andextruded o yield rhe present Songpan-Gurzc triangle. We haveassumed that Markam w8s one of several slivers that have beensevcred from this zone. The western part of the zone must havebeen deformed to yield the Sayan-Tuva-Alui ranges.

The Tibetan blocks have been rurbent in Figure E and resoredto their original topical latiurdes, with Tarim and the Qaidamon one hand and Jrurggar on the other hand placed farthernorth. Stippling undetlines !te!s of continental crust thathave been destroyed (for instance, by continental subduction),shortened, or displaced. Of course, shoræning within theblocks (assumed to be rigid) has not been tsken into account(if for only one reason, because this allows betteridentification of the blocks in the reconstruction) and is infact included in the stip'pled areas.

TÏe reconstruction emphrsizes the rather simple, E-Wùending ehapc of thc routhern margin of the Eurasia Plstc nearl0oN latitude. It also restores a fairly continuous series ofcontinental basins largely filled by red srndstones from theSichuan basin, through Markam, to the Khorat plaæau andthen the Tibetan series. to west€tn Tûim. One of the moststriking feahrres is the present offset of the Khorat withrespect to the Sichuan basin, which it used to form a nahrrdcontinuation.

The Shan Thai block is not yet propcrly constrained bypaleomagnetism and its cxact ancicnt locatio'n, to the east ofthe Tibetan blocks or possibly benrcen the Qaidam and

QIEN ET AI.: @NFIGUX,TTION OF ASIA PRIOR TO THE COLUSION OF INUA

Fs. E: Schqnalic Cretaccous paleogcographic reconstruction of Asia with indicationr of cmstd shoncaing bctrcco thc blækr.CËtaccogs basins filled wiù iurdn€ntal iediments ùrat prcvided many of the pdconrgnctic data ræ thown in drûcr rhrdcdpucm. Thc mrin pelcomrgraic rampling læationr rrt sho*rr ar blacù dots (scc Egurc I for blæh nrmcr).

Kunlun blocks, will further constrsin the part of continentalshortening north of Tibet that must be Ettributed to extrusion.

In conclusion, despite still rather large uncertainties,Cretaceous paleomagnetic data of Asian blocks are nowsufficient to restore a picture that is dmost everywhere inqualitative and even quantitative agr€€ment with the amount ofshoræning and rotation predicted by the tectonic models ofextrusion le.g., Tappornier et aI., l9E6l and deformationwithin Tibet and Tarirn lArmijo et aI., l9E6;' Avouac, 199U. Itis noteworthy that the Egreement with mean paleomagneticvalues appears to be better than could be expected from thecalculated uncertainties. It renrains to be rurderstood why theseuncertainties might be overestimated.

Nue addzd h proof. V/c refer the reader to the recent pap€r byHtung and Opdyke [993] on rotations in southeasærn Asiawhich also summarizes available Cretaceous paleomagneticdata from East Asia and overlaps with and to some exlentcomplenrents the present paper, which it quoæs in preprint asreference 51 (with a misprint repeating reference 50).

Acktowlcdgttunrs. l\te rrc prnioilerly gntcful ro J.P. Avourc fornumcrout dircusrionr, for cmrmcoû o e fint dreft. of rhir papcr, rndfor making r plpcr rvrihblc prior to publicatio. \\lc elso ùlDk P.Trp'ponnicr, M. Mrurucr, M. BrzÀeaov, R. Co.. ud N. Opdyke forcûnm€ntr end/or for meking p.pen rvrihHc priorto publicetim md P.Molner, M. McWillirms, ùd G. Sdrubcrt for thoughdul Éviewr. This isIPGP conrribution l26E end CNRS-INSU-DBT conrriburion 616(dynamiquc globdc).

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