Resolving the problem of shallow magnetizations of

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. B8, PAGES 17,715-17,734, AUGUST 10, 1999 Resolving the problem of shallow magnetizations of Tertiary age in Asia: insights from paleomagnetic data from the Qiangtang, Kunlun, and Qaidam blocks (Tibet, China), and a new hypothesis J.P. Cogn and N. Halim Laboratoire de Pa16omagn6tisme, Institut de Physique du Globe de Paris and Universit6 Denis Diderot Y. Chen Laboratoire de G ologie Structurale, D partement des Sciences de la Terre, Universit d'orl ans V. Courtillot Laboratoire de Pa16omagn6tisme, Institut de Physique du Globe de Paris and Universit Denis Diderot Abstract. We present new paleomagnetic results obtained at 39 sampling sites from five sections of Tertiary red bed formations: two Eocene formations from the Qiangtang block of Tibet (Xialaxiu locality; 32.8øN, 96.6øE) and the Xining basin of Qaidam (Xining locality; 36.5øN, 102.0øE) and three Neogene formations from the Xining basin (Jungong locality; 34.7øN, 100.7øE) and the Kunlun block (Tuoluo lake and West Yushu localities; 35.3øN, 98.6øE and 33.2øN, 96.7øE, respectively). Thermal demagnetization of the rocks isolated a high-temperature component that we interpret as the primary magnetization in four localities. The paleopoles lie at 52.6øN/352øE (dp/dm=6.0ø/10.7 ø) for Xialaxiu, 61.6øN/211.3øE (dp/dm=9.7ø/16.1 ø) for Xining, 66.0øN/228.6øE (dp/dm=3.6ø/6.9 ø) for Jungong, and 53.9øN/205.4øE (dp/dm=5.6ø/10.o ø) for West Yushu. As in previous studies of Tertiary formations from Asia, the inclinations we obtained are shallower (by 18 ø to 26 ø) than the magnetic field computed from the Eurasian apparent polar wander path (APWP) at 10 and 20 Ma for Neogene rocks and at 40 and 60 Ma for Eocene rocks. On the basis of a compilation of Eocene data from the South China Block, Tibet, central Asia and Kyrgyzstan, we conclude that this inclination anomaly reflects erroneous predictions of positions of the Siberian craton when based on the APWP of Eurasia. The main reason for this discrepancy might be nonrigid behavior of the Eurasian plate in the Tertiary. Combination of this with intracontinental shortening of Asia under the penetration of India provides a full explanation for the anomaly. Verification of this new interpretation of the "inclination anomaly" will require new geologic and paleomagnetic data from the northern parts of these remote regions in Mongolia and Siberia. 1. Introduction For over two decades, Asia has been regarded as an extraordinary natural laboratory for studying tectonic processes involved in geodynamics of intracontinental collision and mountain building. Considerable efforts have been made to decipher the complex history of Asian construction by successive accretion of lithospheric microblocks and modification of this assemblage since the onset of collision of the last of them, the Indian plate. Paleomagnetism has provided valuable contributions to this history. Because all blocks or microplates forming the Asian mosaic (Figure 1), except India, are thoughto have been accreted to the major Eurasian plate by the Cretaceous, any discrepancy between paleomagnetic data from Cretaceous rock formations from these blocks may be interpreted in terms of Tertiary modification of Copyright 1999 by the American Geophysical Union. Paper number 1999JB /99/1999JB this assemblage. This is a reason why (another being that Cretaceous continental red bed formations are widely distributed over Central Asia, Tibet and China) the last decade has seen a dramatic increase in paleomagnetic data of Cretaceous rocks in Asia, providing a rather satisfactory coverage of paleomagnetic data of Cretaceous age in these regions. Analyses and interpretations of this database [e.g. Enkin et al., 1992; Yang et al., 1992; Yang and Besse, 1993; Chen et al., 1993a; Halim et al., 1998a] showed an overall consistency that allowed proposal of precollisional paleogeographic reconstructions of blocks, and thus provided some insights on tectonics in Asia during the Tertiary due to the collision of India. The main conclusions reached from the Cretaceous results can be summarized as follows. (1) The South China (SCB; Figure 1), North China (NCB), and Mongolia blocks were accreted to Siberia by the Cretaceous and have suffered no relative latitudinal changes since then. (2) Central Asian blocks (Kazakhstan, Junggar, and Tarim), together with northern Tibetan blocks (Kunlun and Qaidam), have experienced km of northward convergence with Siberia. This convergence is absorbed partly through intracontinental shortening within 17,715

2 17,716 COGNI2 ET AL.' RESOLVING SHALLOW TERTIARY MAGNETIZATIONS IN ASIA 60 ø 120 ø 140 ø MON 40 ø 40 ø IND 20 ø Suture Fault I 80oE 100oE 120øE Figure 1. Simplified map of Southeast Asia showing the main sutures and faults (KF, Karakorum fault; MBT, Main Boundary Thrust). The major blocks are AFG, Afganistan; EUR, Europe; INC, Indochina; IND, India; JUN, Junggar; KAZ, Kazakhstan; KUN, Kunlun; MON, Mongolia; NCB, North China Block; QA, Qaidam; QI, Qiangtang; SIB, Siberia; SCB, South China Block; ST, Shan Tai; and TAR, Tarim. Shadedots indicate location of paleomagnetic sites in Cretaceous rocks; open stars indicate location of Paleogene-Neogene sites. Square indicates area of Figure 2. mountain ranges (Tien Shan, Altai, and Qilian Shan) and probably partly through eastward (lateral) extrusion of the SCB- NCB-Mongolia assemblage [e.g. Halira et al., 1998a]. The relative importance of these two mechanisms not yet fully Cretaceous and today. As a second step in deciphering the history of Asian tectonics, it is desirable to obtain Tertiary data from these regions in order to describe, in the light of the Cretaceous reconstructions, their evolution with time under the effect of established. (3) The south Tibetan blocks (Lhasa and Qiangtang) Indian penetration into Asia. However, in contrast to Cretaceous show about km of northward movement with respect to central Asian blocks (i.e., 1700 km with respect to Siberia), data, the Tertiary paleomagnetic results from these regions are far less numerous; moreover, the few available studies all raise a which are believed to be accommodated by the southeastward major problem, known as the "low inclination anomaly" problem extrusion of Indochina in their eastern part [Peltzer and Tapponnier, 1988; Yang et al., 1995; Halim et al., 1998a] and intracontinental shortening within the Kunlun mountain range in their western part [Chen et al., 1993a]. These interpretations are based on comparisons of the paleopoles from different mobile blocks, and the movements of these blocks with respect to Siberia are derived from a comparison of these poles with the "reference" apparent polar [e.g., see Westphal, 1993; Chauvin et al., 1996]. In effect, paleomagnetic data from Tertiary rocks from the Tadjik basin in Kyrgyzstan [Thomas et al., 1993], the Tarim basin [Gilder et al., 1996], and the SCB [Gilder et al., 1993; Zhao et al., 1994] are of high quality, with robust stability tests, yet systematically display paleomagnetic inclinations that are shallower than inclinations deduced from the reference APWP by 15ø-30 ø. In terms of intracontinental shortening between these regions and Siberia, wander path (APWP) of Besse and Courtillot [1991] for Eurasia. these shallow inclinations would suggest convergence We point out here that this APWP, constructed the basis of the magnitudes at leastwice as large as those based on Cretaceous worldwide paleomagnetic database and kinematic parameters data. This conclusion appears to be in conflict with all geological deduced from oceanic magnetic anomalies, holds for Siberia only if Eurasia behaved as a rigid plate since the time of its amalgamation, i.e., since the formation of the Ural mountains the Paleozoic. Cretaceous data, as mentioned above, allow a description of the total movement of the blocks between the end of the evidence. Proposed explanations include magnetic field anomalies, nondipolar fields, and inclination shallowing due to sediment compaction [e.g., Westphal, 1993; Chauvin et al., 1996]. In order to try and contribute to a solution of this problem, we present new paleomagnetic results from five sections of Tertiary

3 COGNI ET AL.' RESOLVING SHALLOW TERTIARY MAGNETIZATIONS IN ASIA 17,717 red bed formations (Figure 2): two Eocene formations from the Qiangtang block of Tibet (Xialaxiu section) and the Xining basin of Qaidam (Xining section) and three Neogene formations from the Xining basin (Jungong section) and the Kunlun block (Tuoluo lake and West Yushu sections). These new data also display anomalously low paleomagnetic inclinations. Consequently, we next review the low inclination anomaly problem and propose a new interpretation for this discrepancy, in which a previously disregarded tectonic model seems to provide a satisfactory explanation. 2. Geology and Sampling detailed work had previously been done. These red sandstones are interbedded with muddy limestones, gypsum layers, and a few volcanic layers. Unfortunately, no fossils have been reported in the literature nor were encountered in the field, but on the basis of lithologic correlations an Eocene age has been assigned to the rocks that are unconformably overlying Triassic limestones. Average local magnetic declination computed after magnetic and Sun azimuths measured in the field is 0.2 ø, a value which compares well with the International Geomagnetic Reference Field (IGRF) declination of-0.4 ø, computed from IGRF 1990 coefficients for July Kunlun Block Sampling of these formations was made during two field trips Two sections were sampled around the border of the Songpanof the Sino-French Kunlun cooperation program in the Summers Ganze accretionary complex in the east of the Kunlun block. of 1993 and For information on the age and geology of the West Yushu section (N2). The sampled formation sampled formations, we refer to the geological map of Qinghai consists of brick-red sandstone interbedded with rare Province published by the Bureau of Geology and Mineral conglomerate layers. Some fossils, such as Valvata sp., and Resources of Qinghai Province (BGMRQP) [1991]. The location pollen provide a Neogene (N2) age for the rocks. Eleven sites of sampling sites is shown in Figure 2. Eight to ten cores were have been drilled in the sandstones from this formation, west of drilled at each site using a gasoline-poweredrill and were Yushu at 33.2øN, 96.7øE. The total thickness exceeds 1000 m. oriented in situ using magnetic and, whenever possible, Sun Measured local magnetic declination is -0.1 o (IGRF 0.4ø). compasses in order to check and correct orientations for local magnetic field declination Tuoluo lake section (N1). Four sites were sampled in a section on the northern border of lake Tuoluo (35.3øN, 98.6øE) Qiangtang Block: Xialaxiu section (E) According to BGMRQP [1991], the sampled section is lithologically correlated to the "type section" from Cumaer River Ten sites were sampled near the town of Xialaxiu (located at basin. Unconformably overlying Permian limestones, this type 32.8øN, 96.6øE), in thick red sandstone beds on which little section is composed from bottom to top of conglomerates with 9O' 108' 40' 92' 94' 96', :0' 102' 104' 106' 40'... :'a::-;,.... -,' a:..u,... "..-:.-:,... i..:i... a;' :: I'... :: : U: ½ $:a::f :z::..j X :: ES z ½ $ f1:p&' { 8[ ' fl - * -:-- [i :E : :: :; s.:;½.,:'::... L' : :[. '... '-.,..."'"':'*:':: :: :: :f::fk..:&:/;;::... ' "- '::.: '"*..': '"x. '.',...,:',..' ",. '...,*'-- a,<ea,,::r :::,,...:.'":':::::: : :½:..'...::: 't. :'....' g ½. ':.'... * :: :' :?; ':... '... ß... ::". :; ;.; : :: : : :s ; :': ::½ :; ' -... '"'.. :.-;;;;:( :;;:,:?. '... :½J :, :: :.:.,.:2--..._,... ß? -..,... ":.:: :... '?:½:: : f.-- -Z :,,%..} :.,.:'.:...:..; :: ::; :; :,... ::L..: I ß.:..."..-...:;[4 *:... '* }**"' ; X :': "...,;,.:,.. L:.: *:: *.,..E,... '... f:.::,. :.:.:::.:..:,:..:: ::...:.:,8 ::::.."';...,;...:. -:[... ':'.....?'2 :'"::;.:: f::.:-. :.: '"' ½::s; f :::: f.;.., : : :f :: :f(: c::i... "' '.... -: :. ß. <: : ::<::'.::.:, :::::.:.:..: ;..:...?: :.:..'n... 90' ' '" " ' ' "" "*... *:... ": :""';"'::.:'.7%... P' ;:';; ::z... ß...1'... %-:.,-... :.'.' '*;;' '-':"'""'" ' 92' 94' 96' 98'! 00' 102' 04' 06' 08' 500 m rn 3000 m m Figure 2. Simplified topographic map with superimposition of major faults. Squares indicate town locations, and open stars are the Tertiary paleomagnetic sampling localities.

4 17,718 COGNI ET AL.: RESOLVING SHALLOW TERTIARY MAGNETIZATIONS IN ASIA interbedded gypsum layers, followed by thick red sandstone beds, overlain by grey limestones of variable thickness. Gastropod, ostracod, and charophyte fossils give a Neogene age (N1) for the formation. In the Tuoluo lake locality, the sampling involved only coarse-grained red sandstones which a lower Neogene age (N1) has been attributed on the base of lithologic correlations with the Cumaer section. No fossils were encountered in the section we sampled. Local magnetic declination is -0.4 ø (IGRF- 0.6ø) Xining Basin Jungong section (N2). Nine sites were sampled in red sandstones near Jungong (34.7øN, 100.7øE). The sampled formation is mainly composed of grey-red mudstones and sandstones, occasionally interbedded with thin gypsum layers, and conglomerates on the top and the bottom. It shows an angular unconformity with the underlying Eocene formation. The total thickness of this formation is m. Pollen fossils assign a Neogene (N2) age to this formation. Local magnetic declination is -0.8 ø (IGRF o) Xining section (E2h). Five sites have been collected in the east of Xining town (36.5øN, 102.0øE) in the Honggou formation [BGMRQP, 1991]. This formation is composed of dark red mudstones on the bottom, interbedded dark and brick-red mudstone and gypsum in the middle part, and yellow muddy limestones interbedded with grey to green gypsum on the top. A paleontologically based Eocene (E2h) age is attributed to this formation. The thickness varies from about 100 to 450 m. Because of cloudy weather, no local magnetic declination could be determined, and instead, we used the IGRF declination of field. Indeed, saturation is not reached by 1.2 T, suggesting the presence of magnetic minerals such as pyrrhotite, goethite, and/or hematite. Apart from a very soft component eliminated by 125øC, orthogonal vector plots of thermal demagnetization (Figure 3b) exhibit two magnetization components. Between 125øC and 550øC a first lower-temperature component (LTC) is resolved. A second, high-temperature component (HTC) is isolated between 550øC and 680øC. This component converges toward the origin during demagnetization. The high unblocking temperatures of this component suggesthat it is carried by hematite. HTC site-mean directions (Figure 3c and Table 1) do not cluster upon unfolding. This is confirmed by an inconclusive fold test [McElhinny, 1964]. The ratio of precision parameters after (ks) and before (kg) tilt correction is ks/kg=l.15 for n=10. This value is well below the 99% and 95% limit values of 3.13 and 2.22 needed for the fold test to be positive. The failure of the fold test is due to the fact that bedding attitude is similar for all sites; moreover, the bedding strikes approximately in a NNW direction, very close to the average paleomagnetic declinations. The single site (site 28) with a reverse polarity is not sufficient to allow a reversal test. However, both the in situ and the tilt-corrected formation-mean directions are significantly different from the present-day dipole field direction, and we assume, clearly without any strong test, that this is the primary Eocene magnetization direction. The tilt-corrected formation-mean inclination ( ø, Table 1) is shallower than the expected inclinations computed after the reference APWP of Besse and Courtillot [1991] of Eurasia for 60 and 40 Ma by ø and ø, respectively. The significance of this inclination flattening will be discussed in section 4. The corresponding paleopole lies at 52.6øN, 352.0øE (dp/dm=6.0ø/10.7ø) Kunlun Block 3. Paleomagnetic Results West Yushu locality (N2). As for Xialaxiu samples, The standard minicores of 2.5 cm diameter were cut into 2.2- IRM acquisition experiments performed on the samples from cm-long specimens in the laboratory. Thermal demagnetization Yushu (Figure 4a) show a continuous increase of magnetization of natural remanent magnetization (NRM) and all magnetic intensity up to an applied field of 1.2 T, where saturation is not measurements were performed in the magnetically shielded room reached. Thermal demagnetization in orthogonal vector plots of the Paleomagnetic Laboratory at the Institut de Physique du (Figure 4b) clearly exhibits two magnetization components. A Globe de Paris / Universitd de Paris 7. Specimens were thermally LTC is first unblocked between 100øC and 450øC with a demagnetized by steps up to 690øC within laboratory-built northerly downward direction. It is followed by a HTC, which is furnaces. Remanent magnetization was measured with a threeprogressively demagnetized between øC and 690øC and axis CTF cryogenic magnetometer, and magnetic mineral converges toward the origin of the diagram. In most sites (9 out transformations during thermal demagnetization were monitored of 11) this component has south directed upward directions by measuring bulk susceptibility after each heating step using a (Figure 4b). These high unblocking temperatures, together with Molspin susceptibility meter. To help in the identification of the the high saturation field, suggest that this component is carried by magnetic carriers, a few selected specimens from each locality hematite. were used to perform an analysis of acquired isothermal reinanent It is clear that site-mean directions of HTC (Figure 4c and magnetization (IRM). For analysis, demagnetization results were Table 1) cluster upon unfolding. The fold test [McElhinny, 1964] plotted as orthogonal vector diagrams [Zijderveld, 1967] and performed on the whole population is positive at the 99% equal-area projections. Paleomagnetic directions were determined probability level, with a ratio of the precision parameters using principal component analysis [Kirschvink, 1980] or using ks/kg=6.41 for n=l 1 sites. This HTC is therefore prefolding the remagnetization circles technique [Halls, 1978]. Site-mean age and is assumed to be the primary magnetization direction of directions were determined using the statistics of Fisher [1953] this formation. The corresponding (normal polarity) paleopole or, in the case of combined directional data and remagnetization lies at 53.9øN, 205.4øE (dp/dm=5.6ø/10.oø). We note, however, circles, McFadden and McElhinny [1988] statistics. All that with only two sites with normal polarity (and nine sites of interpretations and data processing have been made using the reverse polarity) the reversal test [McFadden and McElhinny, PaleoMac application developped at Institut de Physique du 1990] is negative at the 95% confidence level. However, although Globe de Paris (IPGP) by the first author. the normal directions (average Ds=34.2 ø, Is=52.0 ø, k=60.6, c 95=32.6 ø, n=2) are steeper than the reversed ones (average 3.1. Qiangtang Block: Xialaxiu locality (E) Ds=215.8 ø, Is=-26.0 ø, k=42.3, 0t95=8.0 ø, n=9), thermal cleaning IRM acquisition of Xialaxiu locality samples (Figure 3a) (Figure 4b) appears to result in a good separation of LTC and indicates that the main magnetic carriers have high saturation HTC in samples with reversed polarity. We therefore assume that

5 COGNI ET AL.' RESOLVING SHALLOW TERTIARY MAGNETIZATIONS IN ASIA 17, M/Mmax.e-. 10 '3 A/m NRM A C),... Magnetizing field in mt (a) e 67/ 680 ' 6 (b) o B-TC Xialaxiu I E Down o Figure 3. Results of rock magnetic and thermal demagnetization experiments for Xialaxiu locality samples. (a) Isothermal reinanent magnetization (IRM) acquisition in fields up to 1.2 T showing the presence of high-coercivity magnetic mineral. (b) Typical orthogonal vector plot [Zijderveld, 1967], in tilt-corrected coordinates; solid (open) symbols are projection onto the horizontal (vertical) plane. Temperatures are indicated øc. (c) Equal-area projection of site-mean directions (with their c 95 cones of confidence) of the high-temperature component (left) before (IS for In-Situ), and (right) after (TC, for Tilt-corrected) bedding correction. Solid (open) symbols are positive, downward (negative, upward) inclinations. Star indicates the overall-mean direction. the difference in inclination between normal and reverse magnetizations is a consequence of the too low number of normal sites and does not arise from separation of components in sites with reversed polarities. According to the Neogene age of this formation we may compare our tilt-corrected paleomagnetic direction with the 20 and 10 Ma dipole field directions deduced from the Eurasian APWP [Besse and Courtillot, 1991], which would represent the magnetic field direction if the Kunlun block remained fixed with respect to Eurasia since that time. As above, our mean direction (Is=30.7 ø, whole population in normal polarity) is shallower than the reference field directions. This difference amounts to 26.5o+9.3 ø when compared to the 20 Ma reference field and 25.4ø+9.1 o for the 10 Ma reference field Tuoluo lake locality (N1). Contrary to the previous cases, IRM acquisition experiments (Figure 5a) show a rapid increase magnetization intensity at low applied fields. This rapid increase continues until 0.2 T, when the slope changes and becomes less steep, though it continues to increase until 1.2 T, at which saturation is not reached. This suggests that the samples contain two magneticarriers, one with a low saturation field (< 0.2 T), which could be magnetite, and a second one with a saturation field > 1.2 T, which is probably hematite. Apart from a very low (viscous?) unblocking temperature component, which is removed by 125øC, thermal demagnetization in orthogonal plots (Figure 5b) allows resolution of only one magnetization component, which has been considered as a characteristic reinanent magnetization component (ChRM).

6 17,720 COGNI ET AL.' RESOLVING SHALLOW TERTIARY MAGNETIZATIONS IN ASIA Table 1. Site-Mean Paleomagnetic Directions for Characteristic or High Temperature Components of the Studied Regions. Site Bedding n D g Ig D s Is k ct95 Strike/Dip Xialaxiu / / / /40' / / / /36* /38' / Overall Mean Yushu 12 84/ /5* / / / /5* /78* I / / /25' / Overall Mean 11 7 uoluo Lake / / / / Overall Mean Jungong / / / / / /36* / / / Overall Mean 9 - Xining /25* / / / / Overall Mean , For bedding planes, dip is in the direction of strike+90ø; n is number of specimens' D, lg (Ds, Is) are mean directions in geographic, in situ (stratigraphic, tilt-corrected) coordinates; k, ct95 are parameters of Fisher [1953] statistics. * Average bedding strikes an dips in sites with variable bedding attitude.

7 COGNIS, ET AL.' RESOLVING SHALLOW TERTIARY MAGNETIZATIONS IN ASIA 17,721 I:::: 1.0. M/Mmax w Up._N E 33O 10 '3 Nm ,, A ooo 1200 Magnetizing field in mt S , NRM, N (a) (b) B -TC E Down O - ' o + (- ++++t Ig=-3.8 ø ' /... Ds=215.6 Is ) a ø /,,':::?:-. -::'::'., + a ': k = 4.2 / (- ;:, :: 4:( k = 26.9 (c) Figure 4. Same as, for Yushu locality. This ChRM is completely demagnetized by 680øC and is directed northward and dips downward in both in situ and tilt-corrected contains the present-day north pole. For these reasons, this particular result will be given only very small weight in the coordinates. Owing to quite high noise in the demagnetization following discussion. path of specimens from site 02, we failed to resolve the ChRM in this site, which has very large within-site scatter (k=2.5, 1z95=65.2 ø, Table 1). Average site-mean directions are shown in the equal-area projections of Figure 5c (left) before and Figure 5c 3.3. Xining Basin Jungong locality (N2). The IRM curve of a sample from Jungong locality (Figure 6a) is similar to that for Yushu (right) after tilt-correction and are given in Table 1. Accounting locality. Below 0.1 T no significant magnetization is acquired. for its very large within-site scatter, the site-mean direction from site 02 has been excluded from the computation of the overall mean. Because of the monoclinal dip of the beds, between-site dispersion (Figure 5c and Table 1) does not change after bedding tilt correction, giving an inconclusive fold test [McElhinny, 1964]. Although k has a reasonable value (21.8 before and after tilt correction), the small number of sites results in a very large ot95 of the overall mean which contains both the present Earth field (PEF) and the geocentric axial dipole (GAD) directions either in situ or after tilt correction. Indeed, the paleopole Magnetization intensity increases slightly but continuously up to a field of 1.2 T without reaching saturation. In heating (Figure 6b), the magnetization shows a regular decrease in intensity up to 680øC. These two diagrams indicate that the main magnetic carrier of NRM is probably hematite. Two typical examples of thermal demagnetization in orthogonal projection (Figures 6c and 6d) represent the two common kinds of behavior. In the first one (specimen A, Figure 6c) the demagnetization path converges directly toward the origin defining a single characteristicomponent (ChRM). In computed from this direction (71.2øN, 207.4øE, the second type (specimen A, Figure 6d) a first LTC with dp/dm=22.2ø/34.7 ø) has a very large ellipse of confidence which normal polarity (down and northward) is isolated between the

8 17,722 COGN ET AL.' RESOLVING SHALLOW TERTIARY MAGNETIZATIONS IN ASIA 1.0 M/Mmax w up o-4 ^'mi N (a) 01-02A ' Magnetizing field in mt (b) E Down TC NRM o Tuoluo lake o , + q- +_+,-.+_++ ++ ½ {- +_ / IF, + Dg = 19.0 ø / / TC + Ds = 20.1 o / - Ig = 51.1ø /,- - + Is = 46.2ø /, + c, s = 27.1 o / + (Z95 = 27.1 o / ++ k;21.8 / k_;21.8 / Figure 5. Same as Figure 3, for Tuoluo lake locality. Solid diamond (shaded star) in Figure 5b' present-day field, after IGRF 1990 for July 1993 (geocentric axial dipole field). NRM and 300øC steps, followed by a HTC of reversed polarity (up and southward) defined by the rectilinear path composed between 500øC and øC which, although quite noisy, converges toward the origin. The ChRM and HTC site-mean directions (Figure 6e and Table 1) obviously cluster upon unfolding. The ks/kg ratio of the precision parameters is 3.45; the fold test [McElhinny, 1964] is thus positive at the 99% level of confidence. The whole population of ChRM and HTC passes the reversal test [McFadden and Lowes, 1981]. However, following the classification of McFadden and McElhinny (1990), this is a class "C" test with a critical angle T,- higher than 10 ø (%=12.8ø). We therefore conclude that these magnetizations are prefolding in age and assume that they representhe primary magnetization of this Neogene formation. The formation-mean direction allows the computation of a paleopole which lies at 66.0øN, 228.6øE (dp/dm=3.9ø/6.9ø). As above, we compare our tilt-corrected paleomagnetic direction with the 20 and l0 Ma field directions deduced from the Eurasian APWP [Besse and Courtillot, 1991], and again, our paleomagnetic inclination is shallower than these reference field directions. This difference amounts to ø when compared to the 20 Ma reference field and ø for the 10 Ma reference field Xining locality (E2h). IRM acquisition curves of Xining samples (Figure 7a) show a rapid increase in magnetization intensity in the range T. Above 0.2 T, the slope decreases but magnetization acquisition continues until the maximum applied field of 1.2 T, where saturation is not reached. This suggests the presence of two magnetic carriers. The first has a low saturation field (< 0.2 T) and could be magnetite, and the second one, which saturates above 1.2 T, might be hematite. The orthogonal vector plots of thermal demagnetization (Figure 7b) clearly exhibit two components of magnetization. A

9 ,, COGN ET AL.: RESOLVING SHALLOW TERTIARY MAGNETIZATIONS IN ASIA 17,723 M/Mo M/Max E (a) 0.5 (b) Magnetizing field in mt X A 0 1;0 2;0 3; o C NRM 50 N I ß o own O 2 / 10 '4 A/m Up (c) N / NRM_/ 640% I I I W E aoo S Down 10-3 A/m S Up A -TC A -TC o Jungong o 270 -I (: ++ Ds=19.8 o / + Is=33.5 ø + 0(95=6.0 ø / ' + k= Figure 6. Results of rock magnetic and thermal demagnetization experiments for Jungong locality samples. (a) As in Figure 3a. (b) Normalized NRM decay curve. (c) and (d) As in Figure 3b. (e) As in Figure 3c.

10 17,724 COGNI ET AL.' RESOLVING SHALLOW TERTIARY MAGNETIZATIONS IN ASIA.o M/Mmax N 300 ß up 1 _ 10-3 Nm 0.5 NRM A , Magnetizing field in mt (a) w B -TC S Down (b) o, xining I o + +,+s , _+5+_ ++ + Dg =24'8ø //."'... '", Ds=29'3ø / + Ig=11.2 ø /,,,' ',, ' Is=40.8 o (z95=13.2 o /,,. o... _':_. _ (z95=13.2 ø,/ (' '"., k=34.8 / ",, /""..,";".,, k=34.4 /.. o,...-',, N=5 / (C) '-',,--' o +.,,: N=5 / Figure 7. Same as Figure 3, for Xining locality. LTC is demagnetized by 300ø-350øC, followed by a HTC that Tertiary. We therefore assume that the HTC in tilt-corrected has an unblocking temperature range of 350ø-680øC. This HTC coordinates is probably the primary magnetization of this has been resolved in the five sites of this section and shows either formation. The corresponding paleopole lies at 61.6øN, 211.3øE normal or reverse polarity. Because of too slight differences in (dp/dm=9.7ø/16.1 o) bedding attitude between the different sampling sites, the We note that the data appear to be more scattered in dispersion of the HTC mean-site directions (Figure 7c) is the declination than in inclination (Figure 7c). This may be due to same before and after tilt correction, and the fold test is small local rotations between sampling sites, and because we are inconclusive. However, we note the presence of two polarities, primarily interested paleolatitudinal information from this with three sites having normal polarity and two having reversed series, we may compute the average inclination using the ones. Although the reversal test of McFadden and Lowes [1981] statistics of McFadden and Reid [1982] in order to reduce the is positive at the 95% probability level, the classification of uncertainty paleolatitude. This mean inclination, after tilt McFadden and McElhinny [1990] leads to an inconclusive correction, is Is=39.8ø+7.0 ø. Once again, this inclination is reversal test, with Tc=32.6 ø. We note, however, that in situ shallower than the reference field direction deduced from the directions would give the region a subequatorial paleoposition Eurasian APWP by 17.9ø+7.5 ø for 60 Ma and ø for 40 Ma. (Ig=l 1.2ø+13.2ø), which could not have occurred since the early

11 COGNI ET AL.' RESOLVING SHALLOW TERTIARY MAGNETIZATIONS IN ASIA 17, Discussion The sampled formations from the different studied areas display either a stable HTC or ChRM. Owing to the small number of sites and very large uncertainty, we reject the paleopole from Tuoluo lake from further discussion, as discussed in section For the Neogene formations of Yushu and Jungong localities the magnetizations pass positive fold test and are therefore prefolding magnetizations that we consider primary magnetizations for these rocks. Unfortunately, the monoclinal attitude of sampled beds did not allow us to obtain a fold test in the two Eocene localities of Xining and Xialaxiu. Although some reversed magnetizations do exist in both localities, no reversal test [McFadden and McElhinny, 1990] is positive. However, as discussed in sections 3.1. and 3.3.2, for both formations, in situ directions do not conform to a recent or present-day dipole field direction. We therefore assume that the Xining and Xialaxiu magnetizations are also primary for these formations Possible Causes for Low Inclinations The most striking feature common to these data is their significantly shallow inclination. For all four localities the observed inclination is shallower than expected from the reference APWP for Eurasia for the appropriate ages (40-60 Ma and Ma) by ø. It is not the first time that such low inclinations have been reported in Tertiary formations from the Tethyan realm [e.g., Huang and Opdyke, 1992; Westphal, 1993; Thomas et al., 1993; Gilder et al., 1993, 1996], as well as in Cretaceous formations from China, Tibet, and central Asia [e.g., Chen et al., 1993a, b; Thomas et al., 1994: Frost et al., 1995; Halim et al., 1998a]. Several mechanisms have been proposed to account for these low inclinations, which deserve special attention. Four main causes possibly leading to low inclinations may be identified: (1) a magnetic field anomaly, (2) a compaction-induced shallowing of magnetization, (3) errors in the age determination of studied formations, and (4) tectonic movements between Eurasia and mobile parts of the Asian mosaic Non-dipolar magnetic field. For the Tertiary of Eurasia the most commonly invoked anomaly is a long lasting nondipole field due either to a rapid global shift of the dipole [Westphal, 1993] or to local field anomalies [Chauvin et al., 1996]. Chauvin et al. [1996] propose a map of this anomaly, where the difference between observed and expected inclinations progressively increases from 0-5 ø in Europe to 20~30 ø in central Asia, more or less following the orogenic zones of the Tethyan domain. However, studies of the global magnetic field for the last 5 Myr [Constable and Parker, 1988; Quidelleur et al., 1994; Johnson and Constable, 1996; Carlut and Courtillot, 1998] and longer periods [Coupland and Van der Voo, 1980; Schneider and Kent, 1990; Besse and Courtillot, 1991] show that the main departure from an axial dipole is an axial quadrupole and that this persistent quadrupole amounts to no more than 3-5% of the total magnetic field. The quadrupolar term results in a far-sided effect on paleopole computations. In a new compilation of paleomagnetic data from major plates, data from Deep Sea Drilling Project (DSDP) cores and skewness analysis of marine magnetic anomalies from the Indian Ocean over the last 130 Myr, Torcq [1997] and J. Besse and F. Torcq (in preparation, 1999) conclude that the error in paleoreconstruction due to this far-sided effect hardly exceeds 2 ø in latitude. This would correspond to a maximum inclination error of-3 ø for intermediate latitudes, far smaller than the 20o-25 ø of shallowing observed in central Asia [Thomas et al., 1993; this paper]. We conclude that there is at present no robust evidence for very large, persistent nondipolar global field effects that could account for the large inclination differences that we observe in Asia Compaction-induced shallowing. Because most Cretaceous and Tertiary paleomagnetic results from China and Asia have been obtained on sedimentary rocks (red beds), one can raise the problem of the effect of compaction on the paleomagnetic inclination carried by these sediments. In effect, because it may result in the rotation of elongated particles toward the bedding (flattening) plane, compaction may induce a preferred orientation of these particles. This fabric, which is reflected by the development of an anisotropy of magnetic susceptibility (AMS), may lead the detrital (DRM) or post detrital (PDRM) remanent magnetization to be acquired by the sediment with an inclination lower than that of the magnetic field. This mechanism has been shown to hold in natural sediments [e.g., Stamatakos et al., 1989; Arason and Levi, 1990a; Deamer and Kodama, 1990] and in experiments [Blow and Hamilton, 1978; Arason and Levi, 1990b; Tan and Kodama, 1996]. However, these studies show that compaction-induced inclination shallowing is only significant for clay-rich sediments (up to 15 ø- 20 ø for an initial intermediate inclination) and becomes negligible for quartz-rich and clay-poor sediments, where compaction is known to be small [Tan and Kodama, 1996]. Most red beds that we, and others, have sampled in China, Tibet, and central Asia are continental quartz-rich sandstones rather than pelitic red beds. The isotropic distribution of magnetic grains within the beds is very seldom demonstrated. However, the few available AMS studies [e.g., Thomas et al., 1993] show only very weak anisotropy degrees, of the order of 3-5%, with a random distribution of AMS ellipsoid axes, thus confirming that the effect of magnetic fabric in these beds can generally be neglected. We obtained similar results (unpublished) in at least four different areas: Fenghuoshan and Xining-Lanzhou [Halira et al., 1998a] and Huatougou and Honggoutzi in the Qaidam block (work in progress). A second and stronger argument against a dominant effect of compaction in the Asian data is also provided by Thomas et al. [1993]. They observed a significant inclination difference of 15ø+7 ø in the Toru-Aygur Eocene basalt flows from the Issyk- Kul basin in west Tien-Shan, thus the same order of magnitude as the one observed in sedimentary beds from this basin (16ø+5ø). Of course, compaction processes cannot be invoked in lava flows. A third argument is given by the latitudinal distribution of "inclination errors" in Cretaceous formations going from the Lhasa block to Mongolia through central Asian blocks (we discuss here the Cretaceous and not the Tertiary data because the Asian Cretaceous database is particularly large and far more complete than the Tertiary one). It is generally accepted that compaction-induced inclination flattening AI follows an equation of the form tan (I-AI) = (1 -f) tan L (1) where I is the local magnetic field inclination, AI is the difference between I and the measured paleomagnetic inclination, and f is an empirical factor depending on sediment composition, porosity loss during compaction, etc. For example, Anson and Kodama [1987] give an expression in the form f=aav, where A V is the relative volume loss during compaction and a is an empirical coefficient. If we consider a porosity, hence a volume

12 17,726 COGNI ET AL.' RESOLVING SHALLOW TERTIARY MAGNETIZATIONS IN ASIA loss, in sandstones of [Baldwin and Butler, 1985; Lemge and Gugguen, 1996] and an a coefficient of 1.3 [e.g., Arason and Levi, 1990a], values off-0.3 to 0.5 seem to give an upper limit to compaction-induced AI. The ZX/curve obtained for two values off (0.3 and 0.5) as a function of I (Figure 8) shows that maximum flattening is reached at ø magnetic field inclination (corresponding to ø in latitude) and vanishes at I=0 ø (equator) and 1=90 ø (pole). This clearly reflects the latitudinal dependence of this compaction-induced inclination shallowing. Also on Figure 8, we have reported points corresponding to Cretaceous palcomagnetic results obtained in different Asian blocks. We have recomputed I and ZX/from poles compiled by Halim et al. [1998a, Table 5], where I is the inclination of the magnetic field deduced from the Cretaceous part of the Eurasian APWP and ZX/is the difference of this inclination with the one deduced from the poles listed by Halim et al. [1998a]. Namely, these blocks are from south to north: south Lhasa (1 in Figure 8), north Lhasa (2), Qiangtang (3), Kunlun (4), Tadzhik basin (5), Xining-Lanzhou (6), Tarim (7), and Junggar (8). Indeed, data from these blocks fail to follow the kind of trend suggested by equation (1) for any value of f: southern blocks (Lhasa, Qiangtang) show a larger inclination shallowing than the northern ones (Tarim, Xining, Junggar), whereas this trend is opposite for the theoretical curves, for values of I below the maximum M. Finally, we note that Cretaceous results from the Ordos basin [Fang et al., 1991], the North China Block [Pruner, 1988; Zhao et al., 1990; Zheng et al., 1991; Fang et al., 1991;Ma et al., 1993; Wu et al., 1993; Gilder and Courtillot, 1997], and the South China Block [Kent et al., 1987; Zhu et al., 1988; Enkin et al., 1991b] provide palcopoles that are fully consistent with the Cretaceous part of the Eurasian APWP. Therefore there is no inclination shallowing in these regions. Altogether, we conclude that compaction-induced inclination shallowing is very unlikely to be the mechanism responsible for the observed low inclinations in these Cretaceous rocks Age uncertainties. As mentioned in section 4.1.2, most Mesozoic and Tertiary palcomagnetic results from Asia have been obtained on continental red sandstones, which occur all over central Asia and China from at least the Late Jurassic to the Tertiary. These red bed formations are, in principle, dated using paleontological constraints; however, actual fossils are very rare, 25 ø c 20 ø "15 10½, 4 5 o 5 8 f=0.3 0 i i i 0 o 30 ø 60 ø 90 ø Inclination Figure 8. Plot of inclination shallowing (AI) as a function of initial inclination (I). Thin lines after equation (1) (see text) for f-0.3 and f-0.5. Points with error bars are Cretaceous data from south Lhasa (1), north Lhasa (2), Qiangtang (3), Kunlun (4), Tadzhik basin (5), Xining-Lanzhou (6), Tarim (7), and Junggar (8), recalculated from Table 5 of Halim et al. [1998a]. and mapping of these formations is often made using lateral facies correlations, which are known to be quite risky. Moreover, an actual, direct determination of the age of the paleomagnetic sites themselves is very seldom available. For Cretaceous studies of Asia this problem is not critical because, as demonstrated by Enkin et al. [1991a, b], or Besse and Courtillot [1991], there is a standstill in the Eurasian APWP during the entire Ma period. Therefore, as advocated by Chen et al. [1993a] or Halira et al. [1998a], data from different stages in the Cretaceous may be compared one to the other, neglecting uncertainties in age determination within the Cretaceous. However, the situation is different for the Tertiary, because between 70 and 50 Ma, the Eurasian synthetic paleopoles drift rapidly, by >10 ø (Figure 9). This rapid change results in a rapid paleolatitudinal drift of the region due to the "along-track" position of Asia with respec to APWP drift. Therefore incorrect age determinations of Asian sedimentary formations may result in large uncertainties paleolatitude of the sampled blocks and thus lead to erroneous tectonic interpretations. This problem will be considered (together with its maximum effect) in sections 4.2 and Tectonics of Asia. Finally, the last possible cause we can imagine for observed low inclinations in lithospheric blocks is a significant component of northward motion of these blocks with respect to the reference since the time of magnetization acquisition. Regarding Cretaceous data of Asian blocks, our previous compilations and analyses [Enkin et al., 1991a, b; Fang and Besse, 1993; Chen et al., 1993a; Halim et al., 1998a] have all shown an overall consistency in the paleomagnetic database, which could be interpreted in the frame of a major post- Cretaceous tectonic modification, most likely due to the effect of Cenozoic collision of India with Asia. Because paleopoles from the North and South China Blocks and Mongolia are concordant with the Cretaceous part of the reference Eurasian APWP [Besse and Courtillot, 1991], these blocks are assumed to have been accreted to Siberia by that time and to have suffered no relative (north-south) movement afterward. In contrast, data from central Asia (from Tibet to Junggar through Qiangtang, Kunlun, Qaidam, and Tarira) systematically show lower inclinations than those expected from this Eurasian reference, which we have interpreted to be due to relative northward convergence of these blocks with respect to Siberia. Because all these blocks were accreted by the Lower Cretaceous, this convergence is assumed to have been absorbed in part by intracontinental shortening within several mountain belts (Himalaya, Kunlun, Tien Shan, and Altay [e.g., Chen et al., 1993a]) and in part by lateral extrusion of rigid lithospheric blocks such as Indochina [Peltzer and Tapponnier, 1988; Fang and Besse, 1993; Fang et al., 1995] and Mongolia [Halfin et al., 1998a]. The magnitudes of post-cretaceous convergence absorbed north of the Himalayas, with respect to Siberia, are of the order of 1700 km for the south Tibet and Qiangtang blocks, km for the Kunlun, Tarira, and Qaidam blocks, and 650 km for the Junggar block. For the Cretaceous we therefore concluded that tectonics had been the main cause for low inclinations observed all over central Asia and Tibet and that other causes such as compaction-induced shallowing or nondipolar field geometry were of minor importance. For the Tertiary, however, the situation drastically changes and new problems arise. In sections 4.2 and 4.3 we discuss successively results from Paleocene-Eocene and Neogene formations.

13 ß,,. COGNI ET AL.' RESOLVING SHALLOW TERTIARY MAGNETIZATIONS IN ASIA 17, Central Asia SOMa +2.6 ø! 40Ma yichang - ". ',- Hotan... Xining Feng. Turfan Nanning Junggar \ Kunming ' / Lun x / l Huili Uengyang x.' Lhasa -. +, '!-,,I 7.8ø +4.6ø Youjian 12.2 ø & Tibet Xialaxiu 60Ma? 40Ma ø_+4.6,, 20.8 ø + + o Kyrgyzstan Toru-Aygur 60Ma x 40Ma Naryn Chaktal +", Ala-Buka/' 14.5ø_+7.4o I 20.70_ Figure 9. Equal-area projections of Eocene paleopoles (solid dots with ellipses of confidence) divided in three regions: South China Block (SCB), central Asia and Tibet, and Kyrgyzstan. Stars are site locations. Small dots are reference APWP (1 point every 10 Myr) for Eurasia [after Besse and Courtillot, 1991]. Shaded areas are the small circles with confidence limits passing through the paleopoles and centered on the average site locations (shaded squares). Small circles centered on the same points and running through the 40 and 60 Ma poles of the APWP are drawn, and angular differences with experimental small circles are indicated. shaded ellipses are the results from the present study (Xialaxiu and Xining). Shaded dots (Yichang and Turfan) are poles not included in small circle computations Eocene Formations (Xialaxiu and Xining Localities) In effect, if we follow the same analysis as in section 4.1 and translate directly paleomagnetic directions into paleopole positions to be compared to the reference APWP for Eurasia, we obtain amounts of convergence with Eurasia that are at least twice as large as the ones obtained from Cretaceous beds. For example, post-cretaceous northward convergence of the Xining- Lanzhou basin with respect to Siberia was estimated at ø by Halim et al. [1998a], whereas post-eocene convergence would amount to 15.7ø+5.1 ø up to ø, based on the present study (see Table 2), depending on the age of the reference pole, 60 or 40 Ma. Another example is the Tarim basin for which Cretaceous data [Chen et al., 1992; Cogne et al., 1995] reveal 10.2ø+3.1 ø of northward movement with respect to Siberia, whereas Eocene results [Gilder et al., 1996] would be translated as a convergence of 18.0ø+10.6 ø at 60 Ma or 24.0ø+10.4 ø at 40 Ma. Assuming the hypothesis that the magnetic field was dipolar, even accepting a minor effect of compaction, as discussed above, we may invoke three main explanations for this dilemma: (1) The ages are erroneous. Indeed, if the formations are not Eocene but Cretaceous, the differences vanish, and we obtain convergence amounts consistent with previous Cretaceous results. (2) There has been a relative southward motion of (at least) central Asian blocks with respect to Siberia between the Late Cretaceous and the Eocene. (3) The reference APWP for Eurasia is not valid for Siberia in the Tertiary, leading to erroneous determinations of paleolatitude differences Compilation of Paleocene-Eocene data from Asia. In order to try and solve this problem, we have compiled available

14 17,728 COGNI ET AL.: RESOLVING SHALLOW TERTIARY MAGNETIZATIONS IN ASIA >. c o o.,. o.,.,. o., o

15 COGNI ET AL.' RESOLVING SHALLOW TERTIARY MAGNETIZATIONS IN ASIA 17,729 Paleocene-Eocene paleomagnetic results from Asia. This compilation, together with the new results from the present study the Qiangtang block may reflect the nonrigid behavior of this Tibetan block, as was proposed by Chen et al. [1993b] from (Xining and Xialaxiu localities), is given in Table 2. Because of Cretaceous results in Lhasa and Domar. the rapid drift of poles in the Late Cretaceous to Early Tertiary Far-sided poles and the SCB discrepancy. Still part of the APWP, and of possible age uncertainties as discussed assuming the hypothesis of a dipolar paleofield, the fidelity of in section 4.1.3, data are compared to the Eurasia APWP of Besse and Courtillot [1991] in terms of expected paleolatitudes and paleolatitude differences computed from both 60 and 40 Ma poles, which should provide lower and upper estimates of relative differences. These poles (Figure 9) are separated into three groups (South China Block (SCB), central Asia and Tibet, and paleomagnetic recordings and the accurate dating of the formations, the small circles in Figure 9 (shaded areas) represent the possible loci of Paleocene-Eocene poles from the areas considered. In each stereonet these small circles are compared with those running through the 60 and 40 Ma poles of the reference APWP. Indeed, all of the data, without exception, are Kyrgyzstan) in order to take into accountheir wide longitudinal far-sided with respec to the Eurasian APWP. Interpreted in terms separation. Four out of the six poles for SCB, namely, Kunmin and Lunan [Liang et al., 1986] and Yichang and Hengyang [Li and Ye, 1965] come from the Chinese literature, as cited by Yang [1992], and of tectonic movement, this would reflect a net post-eocene convergence of central Asian and Tibetan blocks, taken as a whole, with respect to Siberia of 15ø-20 ø. The same magnitude is observed for Kyrgyzstan formations. This cannot be reconciled the quality of data is difficult to evaluate. However, apart from with the values estimated from Cretaceous formations of these the Yichang pole, which has a very large uncertainty, these poles align on a well-defined small circle centered on the average site coordinates, which also contains the more recent results of Zhao et al. [1994] on the Nanning basin and Gilder et al. [1993] on the Youjiang basin. In central Asia we have selected the Eocene and Paleogene poles of Thomas et al. [1993] from Kyrgyzstan. In the same way, we selected only the lower Eocene results from Chen et al. [1991] for the Junggar basin. The Tarim basin is well defined by regions (60+4 ø for Junggar, 10ø+3 ø for Tarim, 70+4 ø for Xining, 70+4 ø for Kunlun, and 11.5 ø to 14ø+3 ø for Tibetan blocks [see Halim et al., 1998a]). Even more puzzling is the discrepancy of ø to 12.2ø+4.4 ø of the poles from the South China Block. Indeed, the SCB had been accreted to the North China Block (based on Cretaceous paleomagnetic data) and suffered no further significant relative movement with respect to it [Enkin et al., 1991b; Yang et al., 1992]. Both blocks (SCB and NCB) had been a recent study of Gilder et al. [ 1996], to which we tried to add the Eocene sites of Cognd et al. [1995] from the Turfan basin. However, there are only three sites from this study, and this results in a poorly defined pole for this area. For the Tibet blocks, apart from Xining and Xialaxiu (present study), Fenghuoshan [Lin and Watts, 1988; Halim et al., 1998a], and south Lhasa [Achache et al., 1984] areas, we have also considered the Leidashu formation at Huili from Huang and Op vke [1992] as being part of one of the Tibetan blocks (Lhasa block?). In effect, as noted by Yang and Besse [1993], the Huili area is situated west of the Xian Shui He fault system that is the western boundary of the Indochina block. Therefore this formation is not part of the SCB but could have belonged to the eastern end of the Lhasa block, which is known to have had a much lower paleolatitude than SCB in the Cretaceous Tectonic rotations. Indeed, the first obvious feature in Figure 9 is that poles from each region are quite scattered. However, this scatter defines small circles that are centered on the average site locations in each case (shaded zones in stereonets). This denotes the importance of post-eocene local or block rotations around vertical axes, which can indeed be related to the recent tectonics of Asia, dominated by indentation of India. We will not discuss these rotations in detail. However, we note that SCB poles are all rotated counterclockwise with respecto the APWP reference poles, which could be related to left-lateral movement along the Red River fault, which separates the SCB and Indochina. The poles from Kyrgyzstan [Thomas et al., 1993] are rotated counterclockwise as well. This has been interpreted by the authors as local rotation of the small basins sampled due to a general left-lateral movement along a line joining the Pamir syntaxis to the Baikal rift through central Asia [e.g. see Thomas et al., 1993; Halim et al., 1998a]. For the other areas from central Asia and Tibet, poles are rotated either clockwise (e.g., Hotan from the Tarim block, Xining basin, Fenghuoshan from the Qiangtang) or counterclockwise (Xialaxiu from the Qiangtang block). The large relative rotation of Fenghuoshan and Xialaxiu localities within accreted to Mongolia, to the north of which the Mongol Okhostk ocean had been closed since the end of the Jurassic [Pruner, 1988; Halim et al., 1998b] Convergence estimates from Eocene data. Before attempting to explain this discrepancy, we perform the following analysis: if we remove the paleolatitude difference between the SCB and Eurasian APWP (7.8 ø-12.2 ø) from the same difference pertaining to central Asian and Tibetan blocks (that is, if we take the SCB small circle as a reference), the latter reduces to 7.5 ø and ø. The same operation gives 6.7 ø and ø for Kyrgyzstan sites. These values therefore representhe maximum paleolatitudinal differences between poles from the South China Block and central Asian and Tibetan blocks. Translated into relative paleolatitudinal movement of blocks, this reflects a northward relative movement of about 8 ø (with a large uncertainty of the order of 4o-7 ø ) of mobile blocks with respecto south China, very similar to the relative movements of these blocks based on Cretaceous data. We therefore propose that at least a part of the low Eocene inclinations is due to post-eocene tectonic movements due to continued indentation of Asia by the Indian plate since 60 Ma. In order to estimate convergence between central Asia and Tibet on one hand and Eurasia on the other hand, on the basis of Eocene data we have substracted the 7.8 ø and 12.2 ø of SCB offset from the 40 and 60 Ma paleolatitude differences quoted in Table 2. Following the above analysis, we assume that these differences should provide estimates of relative N-S motion with respect to SCB, which, if SCB remained stable with respect to Siberia, would be directly comparable to convergences with Siberia, determined in Cretaceous formations. This is illustrated in Figure 10, where these estimates are compared to Cretaceous convergences from Halim et al. [1998a]. Removal of SCB offset from the paleolatitude differences with Eurasia results in convergence values for the Eocene that are in excellent agreement with those deduced from Cretaceous formations. A further interesting point may be observed from Figure10, concerning the Tibetan blocks. As evidenced by Halim et al.

16 17,730 COGN15. ET AL.' RESOLVING SHALLOW TERTIARY MAGNETIZATIONS IN ASIA 20 ø 30 ø 35 ø 40 ø 45oN 15 ø 10 ø 5 o 0 o Present-day site latitudes Figure 10. Estimated convergences with Eurasia along a traverse situated at 90øE, running from the Lhasa block to the Junggar block, as a function of latitude. Stars and dark shaded areas are Cretaceous estimates. Solid squares.and open dots with light shaded area are Tertary estimates from comparisons with SCB and Eurasia APWP at 40 and 60 Ma, respectively. [1998a], the two southernmost blocks of Tibet, Lhasa and Qiangtang, display high values of post-cretaceous convergence, of the order of 15 ø, whereas the northern one, the Kunlun block, suffered a lower value of 7 ø, comparable to those of Xining and Tarim basins. This has been interpreted as due to the presence of the Indochina blocks with rather high paleolatitudes, higher than those from Lhas and Qiangtang. In Figure 10, data from Two different possibilities may be advocated in order to explain the misfit between the Eurasian APWP and Siberia: either a magnetic field anomaly in Siberia or some unknown tectonic effects. Again following the lines of the above discussion, we consider that a long-lasting nondip01e component or a strong regional magnetic anohaaly has no strong observational or theoretical bases. We therefore assume that the dipole-field Qiangtang and Lhasa blocks show systematically lower Eocene convergence amounts than Cretaceous ones and display Eocene convergence values of---10 ø, very similar to values from northern central Asian blocks. This could be related to the beginning of intracontinental shortening due to the collision of India and might denote an early age for the beginning of lndochina extrusion A non-rigid Eurasia plate during the Tertiary? The last, but not least, problem concerning these Eocene data is the 8ø-12 ø offset of the SCB small circle with respect to Eocene poles of the E'urasian APWP. On the other hand, Figure 9 clearly shows that this small circle would be fully consistent with all Cretaceous poles from the reference APWP. Because the Cretaceous poles from the SCB are also consistent with the APWP, this would imply a constant paleolatitude for the SCB from the Cretaceous to the Eocene. However, the' reference APWP for Eurasia between 100 and 50 Ma undoubtedly implies a 10 ø northward motion of Siberia during this time if, of course, Siberia remained rigidly attached to Europe, as is illustrated in Figure 11 (left). The paleoposition of Eurasia at 100 Ma is shown hypothesis still holds in the Tertiary. To advocate a tectonic cause, we recall that the reference APWP has been constructed using paleomagnetic data from major plates, transferred in a common reference frame using rotation parameters from oceanic magnetic anomalies. This allowed Besse and Courtillot [1991] to construct a "master APWP" which, transferred to any convenient plate, should describe the movements of that plate. This holds, of course, only if specific plates were indeed rigid over the time considered. Concerning Eurasia, we note that the only high-quality Tertiary data yet available come from western Eurasia (i.e., the British Tertiary Igneous Province [Van der Voo, 1993]) and that the continent is not constrained by any paleomagnetic data from Siberia itself. Moreover, examination of Figures 9 and 11(left) shows that between 100 and 50 Ma, the 'shape of the Eurasian APWP, as seen from Europe, implies a slight latitudinal shift of western Europe and a slight counterclockwise rotation. Owing to the large size of Eurasia, which covers more than 180 ø in longitude, as a whole, these small movements in western Europe in light shading and at 50 Ma in dark shading, according to the translate into at least 10 ø of absolute northward motion of the reference APWP of Besse and Courtillot [ 1991 ]. So, to explain the paleolatitude stability of the SCB while Siberia was drifting northward, we have only two alternatives: either there has been some 1000 km of N-S extension, north of the SCB, and southern margin of Eurasia at the longitude of Siberia. We therefore propose that based on paleomagnetic data from the SCB and central Asia, the southerh margin of Mongolia, and thus Siberia, remained stable in latitude between the Cretaceous (according to the above discussion) probably north of mobile and at least the Eocene, the differential movement between blocks of central Asia, or the reference APWP for Eurasia does not properly describe the paleoposition of Siberia. Indeed, such large Cenozoic extension has never been reported in Asia, where most structures reveal compressive tectonics due to India indentation. At this stage of the discussion we must therefore conclude that the Eurasian APWP does not apply to Siberia. western Europe and Siberia being absorbed by some diffuse, and so far not yet recognized, tectonic movement, somewhere in between. A tentative solution is proposed in Figure 1 l(right), where we have "cut" the Eurasian plate into three parts (western Europe, central Europe, and eastern Eurasia), separated by two zones, the Tornquist-Tesseyre line (the contact zone between the

17 COGNE ET AL.: RESOLVING SHALLOW TERTIARY MAGNETIZATIONS IN ASIA 17,731 OMa...,..,......,...-**, 100 M a rigid... Non-rigid ': ß < a. <g.,,.,. -.. :-. ***< t < :, &,g ,-,,.,+, {,, 0 ß. i ½gg' * g? a: :* ' *< :T -:d;} ; ::, , '.'.'... -'.m ß :+,,-- Z. '** ;. i.,:'**::,.%...**:.*?..%...'"-... :.;:;,, --:-x:';: Rigid Eurasia from 100 to 50Ma Rigid vs. Non-rigid at 50Ma Figure 11. (left) Paleoreconstruction of the Eurasian pla. te at 100 Ma (light shading) and 50 Ma (dark shading) based on Eurasian APWP of Besse and Courtillot [1991] under the hypothesis of a rigid plate. The northward displacement of the south Mongolian margin is indicated by the shorthick arrow. (right) Reconstruction at 50Ma of a rigid Eurasia plate (light shading) and of a Eurasia divided into three subplates along the Tornquist-Tesseyre line and the Urals mountain range (dark shading). In both configurations, western Europe is reconstructed according to the 50 Ma reference pole for Eurasia. Zones of overlap are underlined in darker shading. Fennoscandian-East Europe Precambrian Craton with the Paieozoic Platform of central Europe [e.g,. see Pegrum, 1984; Hippolyte et al., 1996]) and the Ural mountain belts, where some tectonic deformation could have taken place in the Tertiary. The compatibility (e.g., compressive versus extensive tectonics) of the movements across these areas, required by our suggestion to keep Siberia fixed in latitude while western Europe is rotated, has not been tested. However, it is important to note that the proposed limits do not need to be very large: the width of overlapping zones or gaps as proposed on the schematic reconstruction of Figure 11(right) do not exceed km. Moreover, if deformation across central and western Europe is not concentrated in these two zones but is of a more diffuse nature, these values could be overestimated. Finally, we point out that new high-quality paleomagnetic data from the eastern Eurasian plate itself are definitely needed to solve this problem of low Eocene inclinations in Asia. impossible for the Xining-Lanzhou basin and the Kunlun block to have been located at a paleolatitude of 15ø-20øN by the Neogene. This is illustrated in Figure 12, where we have drawn the paleolatitudes of a point that would presently be located at Yushu (33:2øN, 96.7øE), following the Eurasian (squares) and Indian (dots) APWPs, from 80 Ma to the Present. This represents the theoretical paleolatitudes of Yushu locality had it been rigidly attached with either Eurasia, or India. The light shaded area enclosed between the two curves represents the possible paleolatitudes of the Kunlun block. If we assume a dipole field, insignificant compaction shallowing of inclinations, and no later 33.2øN, 96.7øE ø 4.3. Neogene ormations (Yushu and Jungong Localities) As was the case for the Eocene formations discussed in section 4.2, the Neogene formations from Yushu and Jungong localities display low inclinations and yield low paleolatitudes of 16.5ø+5.6 ø and 18.3ø+3.9 ø, respectively. Notwithstanding the above discussion of a possible discrepancy between the actual position of Siberia and that predicted from the reference Eurasian APWP in Eocene times, we face an additional problem involving the paleoposition of India itself. Following the reconstructions proposed by Patfiat and Achache [ 1984] for India in the absolute hotspot reference frame, the Yarlung Zangbo Suture Zone, taken to be the northern margin of India, is located at 24 ø to 30øN at 10 Ma, and 19 ø to 25øN at 20 Ma. Similar results are obtained with the Indian APWP of Besse and Courtillot [1991]. It is therefore Age (Ma) Figure 12. Theoretical paleolatitudes for Yushu locality (33.2øN, 96.7øE) following the reference APWPs of India (open dots) and Eurasia (solid squares) from 0 to 80 Ma. The light shaded area underlines the possible paleolatitude range of the Kunlun block. Heavy line and dark area area: actual paleolatitude and confidence interval determined from Neogene (?) red beds at Yushu locality.

18 17,732 COGNI ET AL.: RESOLVING SHALLOW TERTIARY MAGNETIZATIONS IN ASIA relative N-S movements between the Kunlun block and India, the paleolatitude deduced from the Yushu data (dark shaded in Figure 12) implies an age no younger than 30 to 40 Ma. To a few degrees, the same illustration and interpretation hold for Jungong area as well. Indeed, if these formations are Eocene rather than Neogene, they would match the above interpretation. However, without further elements in hand, we cannot claim studies of Neogene formations and actual (hopefully accurate) age determination on the sampled sites themselves could remove these uncertainties. The problem of "anomalous" low inclinations from Tertiary red beds in Asia has been nagging paleomagnetists for more than a decade. The problem is compounded by the fact that it has been found, during the last 10 years, that on the contrary, (very to have a solid interpretation. An error in age determination, similar) Cretaceous red beds provided results that could be resulting from mapping using lateral lithological correlations, interpreted in a geologically realistic framework of accretion and appears to us to be quite possible, among other possible causes deformation of the Asian mosaic following the indentation of for low inclinations and corresponding low paleolatitudes. India. The discrepancy between Cretaceous and Tertiary results, obtained on the same kinds of formations with the same 5. Conclusions We have presented an analysis of paleomagnetic results obtained at two localities of Eocene red beds situated in the anomalous global field behavior, shallowing due to sediment Xining-Lanzhou basin and the Qiangtang block (Xialaxiu locality) and three Neogene red bed formations located in the Xining-Lanzhou basin (Jungong locality) and the Kunlun block (Yushu and Tuoluo lake localities). Except in the Tuoluo lake locality, we have been able to isolate a ChRM or an HTC, which pass positive fold tests in the two Neogene formations of Yushu and Jungong and which we have also interpreted as primary in the two Eocene formations of Xining and Xialaxiu, where the monoclinal dip of beds did not allow to perform a fold test. As in compaction, undetected tectonic effects, age uncertainties, and errors in determination of reference curves. Our analysis leads us to reject the first two causes. The first one is very "expensive", though it has been used, a bit too rapidly we believe, by several authors in the past. Failure of the axial dipole hypothesis by very large amounts (10ø-20 ø ) over very large areas ( ,000 km) would be a major problem for all paleomagnetists and would cast doubt on much of the plate tectonic work done by them over the last decades. We previously published Tertiary results from Asia, the characteristic find that there is no strong evidence in any place and at any time paleomagnetic inclinations we find appear to be >20 ø shallower than the magnetic field predicted for these areas at the relevant ages from the reference APWP for Eurasia. For the Eocene data, based on a discussion of possible causes for low inclinations and on the internal consistency of the paleomagnetic Eocene database, we interpret these low for such behavior, including Asia in the Cenozoic. The second hypothesis, compaction, would also be very worrying to paleomagnetists. Although it clearly occurs in certain types of sediments, it is unlikely to have affected all red sandstones of Cenozoic age in Asia by such large amounts (again ---20ø). Moreover, identical sanstones of Cretaceous age would inclinations as due to the combined effects of an 8ø-12 ø error in not have been affected. This hypothesis can only be invoked in the paleolatitude of the southern margin of Siberia when it is deduced from the Eurasian APWP and a remaining 10 ø of northward convergence of the central Asian and Tibetan blocks with respecto Siberia, consistent with previous estimates based on Cretaceous data from Asia [e.g., Chen et al., 1993a; Halim et specific and limited cases and cannot offer a general explanation for our Asian paradox. The third, tectonic, hypothesis is clearly valid in many cases. Indeed, this is the reason for all of our previous paleomagnetic work in central Asia and that of most other paleomagnetic al., 1998a]. We propose that the -10 ø misfit between the probable laboratories. Having selected an area where there was no paleolatitude of the southern margin of Siberia and the one predicted from the APWP could be due to a nonrigid behavior of the Eurasian plate during the Tertiary. The exact location of tectonic movements responsible for this misfit remains to be remaining oceanic subduction trapped between the continental blocks and fragments of the Asian mosaic after 60 Ma, the aim was to account for paleomagnetic results in terms of (major) intracontinental deformation. This line of reasoning has been sandstones similar to older ones, it appears impossible for the Kunlun block to have been at such paleolatitudes in the Neogene because at that time the northern margin of India was already located at 20øN latitude. We therefore suspecthat the sampled formations are erroneously mapped as Neogene and propose that they cannot be younger than 40 Ma. Only new paleomagnetic techniques (and confirmed by a number of different laboratories) were particularly annoying. We have reviewed what we believe are all potential contributors to anomalous inclinations: clarified. A first very rough suggestion places these deformations quite successful for Cretaceous data. However, it has met with its either within the Tornquist-Tesseyre zone or along the Urals or limits with Tertiary data, for which a pure tectonic interpretation both. Minor displacements (minor for a paleomagnetist, we clearly leads to impossibilities. Our discussion leads us to acknowledge, not for a field geologist) of the order of 100 km or emphasize the last two hypotheses. less could lead to displacements of the predicted location of the The fourth hypothesis, i.e., errors in paleontologic age southern margin of Siberia by 1000 km, solving much of the assignment, has been a worry for many if not most authors since inclination anomaly problem. For the Jungong and Yushu the early 1980s. Widespread use, for lack of fossils, of lateral localities, reputed to be Neogene, however, the same facies correlation is likely to have led to a number of errors in age demonstration cannot hold. Because of the lack of other Neogene assignment. This is, for instance, the only reasonablexplanation data which could allow testing of the consistency of the observed we are left with in the case of the Neogene results. low inclinations, we cannot ascertain the very low paleolatitude The last, particularly interesting idea is that of errors in the ( ø for Yushu and ø for Jungong) deduced from reference APWPs we use for the major blocks, in the present case these data. We only point out that if no compaction-induced Siberia, which forms the northern "buttress" of the tectonic zone inclination shallowing occurs in these beds, which are red we are interested in. It has been assumed by all authors, including us, that Siberia had been perfectly rigidly attached to the rest of "stable" Eurasia, all the way to north western Europe, since the early Mesozoic and in any case for all the period of interes to us here (Cretaceous and Cenozoic). Yet our database for this large plate is very poor for the Tertiary. If a pole for all of Eurasia is actually based on a single (or few) observations, say in Europe, a

19 COGNI ET AL.: RESOLVING SHALLOW TERTIARY MAGNETIZATIONS IN ASIA 17,733 minor uncertainty will be transformed in a large one in China through a simple lever arm effect. This would be compounded by occurrence of minor deformation within the plate. We argue here, and this may be the more novel and controversial part of this work, that such deformation may indeed have taken place. Therefore the apparently complex and long lasting inclination problem in Tertiary Asia, though certainly complex and with possibly multiple origins, has most likely a solution in terms of principally age uncertainties and errors in reference data, at least for the Eocene. New Tertiary paleomagnetic data are needed from Siberia; they would provide valuable information on its early Tertiary paleoposition and would further advance our understanding of Asian dynamics. Acknowledgments. We thank R. Coe, J. Geissman, and M. McElhinny for their kind and valuable comments on an early draft of this manuscript. 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Press, New York, (Received December 2, 1998; revised April 12, 1999; accepted April 21, 1999.)