Paleomagnetism of late Mesozoic rocks from northeastern China: the role of the Tan-Lu fault in the North China Block

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Tectonophysics 262 (1996) 301-319

Paleomagnetism of late Mesozoic rocks from northeastern China: the role of the Tan-Lu fault in the North China Block H. U c h i m u r a

a

M. K o n o b,, H. T s u n a k a w a a G. Kimura c Q. W e i d T. H a o d H. Liu e

a Department of Applied Physics, Tolo'o Institute of Technology, Meguro-ku, Tokyo 152, Japan b Department of Earth and Planeta O, Physics, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan Department of Earth Sciences, CIAS, Universi~' of Osaka Prefecture, Sakai-shi, Osaka 593, Japan a Institute of Geophysics, Academia Sinica, Beijing, China e Tianjin Institute of Geology, Tianjin, China Received 25 April 1994; accepted 5 January 1996

Abstract Paleomagnetic studies were performed on Jurassic, Cretaceous and Tertiary rocks sampled from the Qitaihe area in Heilongjiang and Benxi area in Liaoning Provinces, northeast China. Both locations are near the Tancheng-Lujiang (Tan-Lu) fault system; Benxi is close to but on the eastern side of the fault while Qitahe lies between two major branches of the northwestern extension of this fault. In Mesozoic rocks, secondary magnetization in the present field direction was observed, but it was possible to retrieve the primary components by taking the high-temperature portion of the demagnetizing spectra. The Mesozoic poles thus obtained, especially those for the Cretaceous, deviate from the paleomagnetic poles of similar ages from the central part of the North China Block (NCB), Siberian Block or South China Block (SCB). Although the distances to the poles (flattening) are quite similar, the Benxi pole suggests a small clockwise rotation with respect to the central NCB poles, while the the Qitaihe poles indicate a much larger rotation in the opposite direction. It is shown that the deviation of the Benxi pole is similar to that observed for the Korean Peninsula and Shangdong Province, which all lie to the east of the Tan-Lu fault in the NCB. The Qitaihe pole position is quite different from the poles either west or east of the Tan-Lu fault. From these observations, it is concluded that a left-lateral strike-slip movement at the Tan-Lu fault system since the Cretaceous is the cause of systematic deviation in the position of the poles obtained from east of the fault including the Benxi area, while anomalous direction of Qitaihe rocks may represent a small scale rotation within the Tan-Lu fault system. The estimation of the movement along the Tan-Lu fault depends on which branch of the fault system is considered most active. If the main branch is assumed to be the place of slip, the movement can be represented by an Euler pole which lies to the south of Honshu Island (20°N, 150°E), with an estimated total displacement of 800 km since the Cretaceous.

1. I n t r o d u c t i o n

* Corresponding author.

Since the report of Lin on the preliminary polar wander path from China (Lin, 1984; Lin et al., 1985), a considerable amount of literature was pub-

0040-1951/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 0 4 0 - 1 9 5 1 ( 9 6 ) 0 0 0 1 6 - 9

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lished on the paleomagnetic measurements of Chinese rocks. However, lack of abundant data still limits detailed discussion of the movements of individual blocks which compose continental China today. Although there is agreement among researchers that China has been constructed by a series of collisions and accretions of various blocks since the Paleozoic (e.g., Klimetz, 1983; Zhang et al., 1984), even some of the block boundaries are not well defined. Possible intraplate movements are also speculated by various authors (e.g., Kimura et al., 1990; Enkin et al., 1992). One of the interesting questions in this category is the role played by the TanchengLujiang (Tan-Lu) fault on the tectonic evolution of the North China Block (NCB). Previous paleomag-

netic studies (e.g., McElhinny et al., 1981; Lin, 1984; Lin et al., 1985; Zhao and Coe, 1987; Zhao et al., 1990) treated the T a n - L u fault as an intraplate structure, i.e., the movement along the fault was neglected in considering the plate motion. However, this is probably because Late Mesozoic and Cenozoic data are not abundant enough to distinguish the movement rather than because there is some evidence to support this conjecture. The Tancheng-Lujiang (Tan-Lu) fault spans more than 2000 km in eastern China, with its south end in the Qinling fold belt and extending to the northeast (Fig. 1). The maximum horizontal displacement is estimated to be more than 700 km in the left-lateral sense since its initiation in the Late

Fig. 1. Schematic map of Eastern Asia showing the major tectonic blocks (after Compilation Group of the Geological Map of Asia, 1982) and the studied areas (solid stars). Solid squares indicate the areas where paleomagnetic results of Late Jurassic to Cretaceous ages are available. They are used for comparison with our data.

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H. Uchimura et al. / Tectonophysics 262 (1996) 301-319

Triassic (Xu, 1980). The average rate of displacement was 1-10 cm/yr. Present-day movement on this system, however, appears to be right-lateral from seismic analysis (Wu et al., 1981; Chen and Nfibelek, 1988). Therefore, the sense of movement must have reversed since Late Cretaceous (Klimetz, 1983). As the amount of total displacement was estimated from comparison of the two sides of the fault using geological evidences, the values quoted above may

not be too well constrained. There is also some doubt in assuming that this fault has always been a strikeslip type fault (Zhang et al., 1984). Kimura et al. (1990) suggest that deformation along the Tan-Lu fault occurred with the Yenshenian thrust movement. We have carried out an extensive sampling of phanerozoic rocks in the NCB in 1987 and 1988, as a part of the Japan-China cooperation on paleomagnetism. From these studies, some reliable results

Table 1

Site informations for the samples collected in the Qitaihe area Site

Formation

Locality

Bedding

Latitude (°N)

Longitude (°E)

Strike (o)

Dip (o)

45.738 45.738 45.810 45.745 45.784 45.784 45.784 45.784 45.774 45.774 45.774 45.774 45.774 45.770 45.770 45.770 45.813 45.813 45.813 45.813 45.813 45.813

130.771 130.771 131.229 130.804 130.817 130.817 130.817 130.817 131.108 131.108 131.108 131.108 131.108 131.039 131.039 131.039 13 I. 131 131.131 131.131 131.131 131.131 131.13 l

horizontal horizontal N40W N60W N24W N 15W N 18W N32W N 12E N 6W N 10E N90E N58W N90E N82W N80W N70E N73E N75E N75E N76E N76E

5E 20N 32W 31W 48W 41W 12E 8E 8E 29S 49S 42S 34S 36S 75N 70N 80N 73N 82N 82N

45.775 45.826 45.926 45.895

131.103 130.923 131.319 131.304

horizontal horizontal N40W N 18W

75E 47E

46.031 46.031 46.031 46.031 46.016

131.585 131.585 131.585 131.585 131.299

horizontal horizontal horizontal horizontal horizontal

l_ztte Jurassic" to Early Cretaceous sandstones

QW01 QW02 QZ01 QD01 QD02 QD03 QD04 QD05 QF01 QF02 QF03 QF04 QF05 QQ01 QQ02 QQ03 QG01 QG02 QG03 QG04 QG05 QG06

Houshiguo Fro. Houshiguo Fm. Houshiguo Fm. Muling Fm. Chengzihe Fro. Chengzihe Fm. Chengzihe Fm. Chengzihe Fm. Chengzihe Fro. Chengzihe Fm. Chengzihe Fm. Chengzihe Fm. Chengzihe Fro. Chengzihe Fm. Chengzihe Fro. Chengzihe Fm. Didao Fm. Didao Fm. Didao Fm. Didao Fm. Didao Fm. Didao Fm.

Late Jurassic" to earl 3' Cretaceous sandstones and welded tuff~"

QK01 QH01 QSO1 QB02

undefined Didao/Muling Fm. undefined Dongshan Fm.

Neogene basalts

QN01 QN02 QN03 QN04 QB01

Bedding angle refers to the magnetic north. To convert to geographic direction, the magnetic declination ( - 10.7°) should be added to the strike.

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14. Uchimura et al. / Tectonophysics 262 (1996) 301-319

were obtained from the central part of the NCB (Zheng et al., 1991; Zheng, 1992) and from the northeastern part (this study). As these data correspond to the same age interval, comparison of paleomagnetic poles from both sides of Tan-Lu fault became possible. The present data will also be useful in considering the tectonic evolution of the eastern edge of the Eurasian continent, such as the opening of the Japan Sea in the Miocene, because they provide a reference data on the continental side.

2. Geology and sampling The samples from northeastern China were collected in Qitaihe city in Heilongjiang Province (45.8°N, 131.0°E) and in Benxi city in Liaoning Province (41.3°N, 123.8°E). Locations of these places and the general tectonic setting of China are indicated in Fig. 1. Sampling was carried out using portable engine drills. About twenty cores were drilled at each site, of which half was assigned to the Tokyo laboratory and the other half to the Beijing laboratory. The present paper is based on measurements carried out in the Tokyo laboratory. The orientations of the samples were measured by using the direction of the sun when possible, or by magnetic compass direction as refered to IGRF directions. There were no significant systematic differences between the directions obtained by the two methods when both measurements were available. This is probably because most of the samples were collected from weakly magnetized sedimentary layers. Consequently, orientation errors were estimated to be less than a few degrees even for the ones determined by magnetic compass.

2.1. Qitaihe area In the Qitaihe area, middle to late Mesozoic continental sedimentary rocks and Neogene volcanic rocks are distributed. The Mesozoic formations are called, from older to younger, Didao, Chengzihe, Muling, Donshan and Houshiguo Formations. Paleomagnetic samples were collected from sandstones (22 sites) and welded tufts (4 sites) of Mesozoic formations, mostly from fresh outcrops in quarrys. Because the sampling sites are far apart, and because most of the

area is covered by Quaternary sediments, these outcrops are almost the only exposed rocks and their stratigraphic correlation is not immediately apparent. However, regional correlation is made by widely-distributed coal-bearing strata as well as by plant fossils (Li et al., 1983; Wang et al., 1985; Hao et al., 1986), so that the stratigraphy of these formations (see Table 1) can be taken as well established. Most of the sandstones are yellowish medium to coarse grained, and show considerably tilted bedding planes in some strata. The strike and dip of each layer was measured in the field and summarized in Table 1. In addition, samples were also collected from Neogene basalt flows. Although the base of these flows are not exposed, field observations are not inconsistent with the assumption that they are nearly horizontal. We assume that the bedding of these flows are horizontal and that tilt corrections are not needed for them. Ages of the Mesozoic formations in this area are given based on the correlation of plant fossils. There are some disagreements in the geological literature; Li et al. (1983) placed the ages of these formations between Late Jurassic to Early Cretaceous, whereas Hao et al. (1986) and Wang et al. (1985) concluded that they are all Early Cretaceous formations. Recently, however, K - A r ages of 144.2 and 160.5 Ma were reported from Chenzihe and Didao formations, which were obtained by isotope dilution method from whole-rock samples (Wang and Piao, 1987). Reliability of these ages is unfortunately unknown, since only the ages are quoted. Furthermore, a standard plant fossil (dinoplagates) was found from the Chenzihe formation, which corresponds to the age of just above the Jurassic-Cretaceous boundary (Sun Ge, pers. commun., 1988). This is consistent with the above-mentioned radiometric ages. Therefore, age of the studied formations may be put at late Jurassic to early Cretaceous. As can be seen later, paleomagnetic results also seem to support this age assignment. Neogene basaltic samples were collected from five cooling units from two different sites. Absolute ages are not available for these rocks. The stratigraphic relation among these flows are not known either. Paleomagnetic results show that the four lava flows from one site (QN01-QN04) are of normal polarity while that from the other site (QB01) is

H. Uchimura et al. / Tectonophysics 262 (1996) 301-319

reversed. Unfortunately, there is no satisfactory evidence showing that the eruptions of these units took place well separated in time. It is possible that the number of sites is not enough to average out the secular variation and the mean is deviated from the true axial dipole direction. Still, they may be used for tectonic discussion, since the two sites are widely separated, there are both normal and reversed polarities, and since the magnetiation is quite stable. The sampling locality, formation name and the bedding data for each site are summarized in Table 1. A site map for Qitahe rocks is not given because large-scale maps are not available to foreigners because of the nearness of the sites to the national border. The only geologic map we had access to was at a scale of 1:4,000,000 (Chinese Academy of Geological Science, 1973), which is not good to show the details of geological setting.

of outcrops. We collected samples from Phanerozoic sequences at 38 sites. Among them, we report here the paleomagnetic results from seven younger sites, spanning the ages from Late Permian to Early Triassic and Late Cretaceous, which are of comparable ages with the rocks in the Qitaihe area. Older samples tend to carry some overprints and their magnetizations are much more difficult to interprete. Therefore we restrict ourselves to these younger sites in the present paper. Cretaceous samples were obtained from the Dayu formation. The total thickness of this formation is about 80 m at the sampling site and the rocks are purplish sandstones with medium to coarse grains. We selected fine-grained parts as far as possible, and collected samples from 5 units at intervals of about 5 to 15 m. The Dayu formation is folded moderately, which gives the opportunity to check the results by a fold test. The age of the rocks is suggested to be Late Cretaceous from plant fossils, but the correlation with other areas in the NCB is not very convincing, since the distribution of the Dayu formation is restricted to the eastern part of Liaoning Province (Tianjin Institute of Geology and Mineral Resources, 1984). However, its stratigraphic relation with re-

2.2. B e n x i a r e a

In the Benxi area, sedimentary rocks from latest Proterozoic to Cretaceous are distributed (Fig. 2). These rocks are well exposed and their stratigraphic relations are quite well defined from the examination

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306

14. Uchimura et al. / Tectonophysics 262 (/996) 301-319

spect to Shiqianfeng formation, which we also sampled, is quite well established locally. Thus the age assignment cannot be seriously in error. Shiqianfeng formation is widely distributed in the NCB and its age is considered to be late Permian to early Triassic (Tianjin Institute of Geology and Mineral Resources, 1984). The rocks are purplish medium-grained sandstones with cross lamination. We obtained samples from 2 sites, which are horizons about 5 m apart in a continuous stratum and the bedding is almost the same. Table 2 lists the data for all the sampling sites in the Benxi area.

3. Paleomagnetic measurements and results All the measurements of remanences were carried out by a 2G superconducting magnetometer at the Tokyo Institute of Technology. Seven to nine specimens 25 mm in diameter and 22 mm long were used in paleomagnetic experiments. For magnetic cleaning and for obtaining the primary component of magnetization, every specimen from sedimentary rocks was subjected to stepwise thermal demagnetization up to 680°C. The demagnetization steps were chosen with 100 ° intervals at low temperatures, and with smaller and smaller intervals at higher temperatures, reaching as small as a 10°C interval near Curie points. For Tertiary basalt samples from the Qitaihe area, ther-

mal as well as alternating field demagnetization to 80 mT was applied. Characteristic remanence (ChRM) directions were determined by fitting a straight line to the orthogonal plot of the demagnetization results (Kirschvink, 1980). For some sites, especially sandstones with reversed remanences, combined analysis using straight lines and great circles (McFadden and McE1hinny, 1988) was applied to obtain site-mean directions, because the plot of directions of most of the samples in these sites showed change of direction approximately along a great circle, but a linear decrease to the origin in orthogonal plot was not observed even at the highest demagnetizing step. McFadden and McElhinny's (1988) method with the constraint on the quadrant was effective in obtaining the ChRM direction in these samples. In this section, only the results from Jurassic to Cretaceous rocks are given. Paleomagnetic results from Neogene basalts of Qitahe and from Permian to Triassic rocks of Benxi are summarized in the Appendix, since these data are not strongly related to elucidation of movements along the Tan-Lu fault since the Cretaceous.

3.1. Qitaihe samples In the Late Jurassic to Early Cretaceous sandstones (Didao, Chengzihe, Muling and Housiguo formations), Q Q 0 1 - 0 3 (Chengzihe formation)

Table 2 Site informations for the samples collected in the Benxi area Site

Formation

Locality

Bedding

Latitude (°N)

Longitude (°E)

Strike (°)

Dip (°)

Late Cretaceous sandstones LY01 Dayu Fro. LY02 Dayu Fm. LY03 Dayu Fm. LY04 Dayu Fm. LY05 Dayu Fm.

41.303 41.303 41.303 41.303 41.303

123.803 123.803 123.803 123.803 123.803

N 4E N 18E N 10E N28W N28W

22E 26E 21E 20E 20E

Late Permian to Early Triassic sandstones LL01 Shiqianfeng Fm. LL02 Shiqianfeng Fm.

41.287 41.287

123.711 123.711

N75W N80W

12S 12S

Bedding angle refers to the magnetic north. To convert to geographic direction, the magnetic declination ( - 8.2 °) should be added to the strike.

307

H. Uchimura et al./Tectonophysics 262 (1996) 301-319

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Fig. 3. Typical examples of thermal demagnetization for samples from the Qitaihe area. In this and following orthogonal plots, the dots and circles indicate horizontal and vertical components in geographic coordinates. (a)-(b) Late Jurassic to Early Cretaceous sandstones. (c) Late Jurassic to Early Cretaceous welded tuff.

showed unstable behavior to stepwise thermal demagnetization. Other units gave satisfactory results in spite of the fact that many of them are considerably coarse-grained, which may suggest that most of the stable remanence is carried by fine-grained matrix. The intensity of natural remanent magnetization (NRM) ranged between 3)< 10 2 and l X 10 -3 A / m . In thermal demagnetization, the intensity of magnetization was reduced to less than a few percent of the NRM above about 580°C, and, in demagneti-

zation at higher temperatures, direction of magnetization changed in a erratic way. We conclude that the remanences measured at these temperatures are spurious and that the contribution of hematite to the NRM is small. Some of the typical examples of stepwise demagnetization are shown in Fig. 3 using the orthogonal plot in geographic coordinates (i.e., the coordinate before tilt correction). Fig. 3a and b is for sandstone samples and 3c is for a welded tuff sample. Behav-

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Fig. 4. (a)-(b) Examples of thermal demagnetization results for samples which changes their directions along great circles. (c) The site-mean direction (the cross with the circle of 95% cofidence) of QF02 obtained by the method of McFadden and McEIhinny (1988) plotted in equal-area projection. Demagnetization data of individual samples are also plotted by solid (lower hemisphere) or open (upper hemisphere) symbols with the fitted great circle.

308

H. Uchimura et al. / Tectonophysics 262 (1996) 301-319

iors similar to Fig. 3a and b were observed in more than half of all the sandstone samples. In most cases, they showed the existence of a low-temperature component, which was generally demagnetized at temperatures less than 400°C. High-temperature components was obtained at or above about 450°C. In some samples demagnetization to a temperature higher than 500°C was needed to obtain the hightemperature component. For the samples such as are shown in Fig. 3a and b, directions of the high-temperature components were determined by leastsquares fitting to the linear segment in the orthogohal plot. There were cases such as Fig. 4a and b in which samples did not give a linear segment at the hightemperature portion, but the directions are approximately aligned along a great circle. Such cases were observed in the samples from four sites. All of them seem to have reversed remanence at high temperatures. For these samples, we determined poles of the great circle by least-squares fitting, and determined the site-mean directions of the high-temperature components by the method of McFadden and McEIhinny (1988). Unfortunately, however, some of the samples from these sites did not allow even the determination of the great circles, and have to be excluded from the analysis. Therefore, the number of samples was not so large in these sites to determine very reliable site-mean directions. Fig. 4c shows an example of them, in which we only had great circle data and not a single direction datum was obtained directly from linear portions of orthogonal plots. Therefore, no set points were available in deriving the mean direction. The maximum likelihood estimate of the precision parameter K for this case was obtained as k = ( N - 2 ) / 2 ( N - R), where N is the number of great circles and R is the resultant of the sum of N unit vectors of which position on the great circles were determined iteratively (McFadden and McElhinny, 1988). As discussed by McFadden and McElhinny, the possibility of the bias associated with the use of great circle data can be discarded by the inclusion of sector constraint. The directions for these units were determined with moderate errors because poles of great circles are tightly grouped in all sites (e.g., Fig. 4). The late Jurassic to early Cretaceous welded tufts gave quite satisfactory results. The NRM intensities

N

wI

N

+ s (a) Before tilt correction

s (b) After tilt correction

LOW-TEMP. COMP. N

N

s (c) Before tilt correction

s (d) After tilt correction

HIGH-TEMP. COMP.

Fig. 5. Site-mean directions before and after tilt correction for the low- and high-temperature components of Late Jurassic to Early Cretaceous rocks from Qitaihe area. Dots and circles are in the lower and upper hemispheres in equal-area projection, respectively. Ovals in (c) and (d) are 95% circles of confidence of the mean direction. Low-temperature component (a) before and (b) after tilt correction, which fails in the fold test. High-temperature component (c) before and (d) after tilt correction passes the fold test.

ranged between 1 and 5 × 10 -~ A / r e . Only the samples from QS01 gave purely single component behavior. The other samples showed behaviors similar to Fig. 3c, in which small low-temprature component is removed by thermal demagnetization to about 450°C. This behavior is also similar to the results of demagnetization of sandstones samples. Fig. 5 shows the directions of the low- and hightemperature components for the individual sites. All the low-temperature components have a positive inclination before tilt correction (Fig. 5a), with the average direction of I = 71.3 °, D = 12.2 ° (0%5 = 8.2 °, k = 17.8), which is not significantly different from the present axial dipole direction. After tilt correction (Fig. 5b), they scatter widely and the precision parameter decreases considerably (k = 6.5). Thus they do not pass the fold test (McFadden and

H. Uchimura et al. / Tectonophysics 262 (1996) 301-319

Jones, 1981). We can conclude that the low-temperature component must be the viscous remanence which have been acquired in the Brunhes normal chron. On the other hand, the high-temperature components seem to show the characteristic direction. Before tilt correction (Fig. 5c), the directions scatter considerably and the precision parameter k is only 6.5. After tilt correction (Fig. 5d), they have the mean of I = 65.6 ° and D = - 2 1 . 1 ° and the precision parameter increases significantly (k = 28.6, a95 = 6.2°). These direction sets pass the fold test (McFadden and Jones, 1981) at a 95% confidence level. Furthermore, both normal and reversed polarities are

309

observed and they are almost antipodal. Therefore we can conclude that the high-temperature components are characteristic and of primary origin. All the mean directions are listed in Table 3, but QG04, QG06 and QB02 are excluded from Fig. 5c and d and from later calculations because their 0/.95 are larger than 25 ° (QG04 and QG06) or the direction is anormalous (QB02), which is an obvious single outlier to the Fisher-like distribution of directions formed by all the accepted data. If we average VGPs, the paleomagnetic pole of Late Jurassic to Early Cretaceous in Qitaihe is situated at 74.7°N, 61.9°E with a95 = 9.0°. From the magnetostratigraphic point of

Table 3 Site mean directions of Jurassic to Cretaceous rocks in the Qitaihe area Site

N

Tilt corrected

In situ I

D

Late Jurassic to Earl3, Cretaceous sandstones QW01 9/9 59.9 321.1 QW02 8/9 66.6 337.8 QZ01 9/9 68.2 357.9 QD01 8/9 70.5 22.4 QD02 8/9 52.6 26. I QD03 t 6(6 )/9 - 57.6 184.2 QD04 6/9 51.4 35.8 QD05 9/9 46.6 17.6 QF01 8/9 64.0 301.8 QF02 + 5(5 ) / 8 - 65.1 156.1 QF03 8/9 65.1 342.2 QF04 + 6(6)/8 - 54.9 165.9 QF05 9/9 30.6 355.8 QG01 7/9 45.9 155.6 QG02 9/9 67.0 162.9 QG03 * 5(4 )/7 - 44.9 332.6 QG04 * 8/8 54.7 169.5 QG05 9/9 48.0 149.7 QG06 * 7/7 45.3 161.3 Late Jurassic to Early Cretaceous welded tufts QK01 8/8 61.9 350.7 QH01 7/7 78.0 327.8 QS01 7/7 51.9 223.5 QB02 * 7/7 - 38.0 354.2

1

Dispersion

VGP

D

k

ct 95

59.9 66.6 64.2 50.5 72.7 59.5 70.3 70.0 73.3 64.9 66.3 83.6 67.0 58.7 42.5 55. I 50.5 49.7 52.3

32 I. I 337.8 4.9 20.9 330.8 130.4 294.1 312.9 321.9 173.3 359.8 151.4 310.0 320.9 326.7 156.4 320.5 341.1 328.4

13.7 134.1 13.7 20.0 7.8 t 5.4 24.9 14.1 25.9 70.1 9.3 15.9 26.4 12.2 12.3 10.3 3.4 9.6 5.2

14.4 4.8 14.4 12.7 21.1 17.6 13.7 14.2 11.1 9.2 19.2 17.3 10.2 18.0 15.3 25.0 35.4 17.5 29.2

61.9 78.0 52.9 - 40.1

350.7 327.8 35.0 312.9

8.8 267.1 49.1 500.8

28.7

-

-

-

Mean of the Jurassic-Cretaceous data (dispersion of pole: K = 14.2, A95 = 9.0) J3-KI 20 65.6 338.8

Lat.

Lon.

61.5 74.7 86.6 68.2 68.5 53.9 48.5 59.2 63.8 85.3 87.1 56.5 56.8 60.8 55.9 69.5 56.6 68.8 62.9

45.0 61.6 216.5 255.7 86.3 230.5 77.7 73.7 85.1 236.4 128.4 300.1 66.2 42.5 14.0 199.6 29.3 1.5 23.8

19.8 3.7 8.7 2.7

82.8 62.9 60.7 9.23

22.3 103.7 233.4 54.5

6.2

74.7

62.0

-

-

-

N: Number of samples used/measured (great circle data in parentheses); 1, D: the mean inclination and declination; k, K: Fisher's (1953) precision parameter for field directions and for poles; c%5, A95: radius of circle of 95% confidence for the mean direction or the pole; Lat., Lon.: latitude (north positive) and longitude (east positive) of VGP. All angles are in degrees. Daggers (t) after site names indicate that the method of McFadden and McElhinny (1988) was employed. Atrerisks ( * ) indicate the sites which were not used in the calculation of the

overall mean.

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H. Uchimura et a l . / Tectonophysics 262 (1996) 301 319

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o

E Dn

',o

(b)

"0

o

E Dr,

Fig. 6. Orthogonal polts of typical thermal demagnetization results for samples of late Cretaceous sandstones from Benxi area. Most of the samples behave similar to (a), but some samples showed two distinct components as in (b).

view, Didao and Chenzihe formations are of mixed polarity, and Houshiguo formation is of normal polarity. Although our data does not have good resolution in time, there is good agreement with paleontologically and radiometrically determined ages (Wang et al., 1985; Wang and Piao, 1987). The Didao and Chenzihe formations can be considered to belong to

a Jurassic-Cretaceous mixed polarity zone (Harland

et al., 1990). 3.2. Benxi samples

Examples of stepwise thermal demagnetization for the samples from the late Cretaceous Dayu for-

Table 4 Site mean directions of Cretaceous rocks in the Benxi area Site

N

In situ

Dispersion

Tilt corrected

VGP

1

D

/

D

k

o~,~5

Lat.

Lon.

65.3 67.8 67.9 73.6 75. I

353.1 351.5 349.8 31.8 23.3

58.2 62,2 63.3 54.3 56.3

33.5 49.6 36.9 43.4 40.2

198.0 79.8 207.5 102.2 105.6

4.3 6.8 4.2 6.0 5.9

64.4 53.7 63.0 55.6 58.7

207.9 193.2 193.7 210.2 208.4

Mean of the Cretaceous data (dispersion of pole: K = 163.6, A95 = 6.0) K2 5 59.0 40.7

269.7

4.7

59.3

202.6

Late Cretaceous sandstones LY01 7/7 LY02 7/7 LY03 7/7 LY04 7/7 LY05 7/7

For legend, see Table 3.

H. Uchimura et al./ Tectonophysics 262 (1996) 301-319

mation are shown in Fig. 6. The NRM intensity of the samples ranged from 1 X 10 -3 to 1 × 10 -2 A / m . Fig. 6a shows a typical example in which a linear segment decreasing to the origin was observed above 200°C-300°C demagnetization. In some samples, however, gradual changes of direction were observed and demagnetization above 500°C was necessary to reach such a linear segment (Fig. 6b). Characteristic

311

directions were obtained by least-squares fitting to high-temperature portion of demagnetization diagram. We also tried to obtain the directions of low-temperature components, as done in the Qitaihe area, but for the samples similar to Fig. 6a, the directions are widely scattered even in the same site. Thus the low-temperature portions may represent a spurious component acquired after sampling.

Table 5 Late Mesozoic paleomagnetic results from the North and South China Blocks No.

Location

Age

Site

N

Lat.

Lon.

K2 Kt KI J3 J2 J2 J2

40. I 45.4 42.0 49.5 40.1 36.6 36.7

112.9 107.6 119.2 117.5 112.9 117.9 109.2

K2 K2 K2 K2 K1 KI KI KI KI KI J3 J3 J3 J2 J2

32.0 26.6 30.0 26.0 29.7 27.9 30.0 22.2 26.0 22.7 29.4 29.9 26.2 26.6 27.5

K2 K K1 KI J3 J2 J3-KI

41.3 36.0 35.9 35.9 35.9 38.0 45.8

Pole

A95

Tests

Ref.

5.8 4.9/6.4 5.7 4.5 8.3 6.8/9.5 3.7/5.7

T T,P A,T,P,F A,T,R,F A,T A,T T,P

13 9 12 12 13 7 10

172.6 186.6 241.1 207.2 227.6 196.2 229.0 171.9 221.4 26.4 213.7 235.3 187.6 185.3 187.0

10.3 4.3/6.7 4.1/7.4 5.0 5.5 11.5/18.3 2.7/4.7 10.6 5.4 6.8 12.6 8.7/15.5 6.0/9.5 10.2/15.6 7.7/12.4

T,F T T,P A,T,F A,T,P T T,P,C A,T,P A,F T,F A,T T,P,F T A,T,P T

4 14 2 3 7 14 2 1 11 3 7 2 14 7 14

202.6 195.0 205.1 200.9 225.7 199.3 61.9

6.0 6.4/8.4 5.8 12.5/17.5 6.7/10.3 12.8 9.0

T,F T,F T,P,F A,T,P A,F A,T,F T,P,F

15 8 6 7 7 5 15

Lat.

Lon.

4(29) 3(66) 6(54) 24(176) 3(19) 4(26) 6(37)

79.6 82.9 82.9 67.6 76.2 72.1 74.3

170.1 221.7 249.5 232.4 199.9 202.0 232.8

119.0 102.4 102.9 117.3 120.3 102.3 102.9 114.2 117.3 108.7 120.0 102.9 101.5 106.7 101.8

10(43) 3(20) 16(189) 2(20) 7(62) 2(11) 23(305) 12(69) 5(35) 8(72) 6(34) 5(62) 5(26) 3(18) 5(27)

76.3 78.9 72.8 65. I 77.1 77.4 74.5 78.2 66.9 86.5 73.0 74.6 66.2 67.8 62.2

123.8 128.5 128.5 119.4 119.4 127.0 131.0

5(35) 65(290) 19(78) 5(48) 10(72) 7(53) 20(151)

59.3 64.0 67.6 69.0 71.3 59.5 74.7

dp/dm

North China Block (NCB) 1 2 3 4 5 6 7

Datong Mongolia Inner Mongolia Inner Mongolia Datong Shandong Shaanxi

South China Block (SCB) 8 9 10 11 12 13 14 15 16 17 18 19 20 2l 22

Nanjing Sichuan Sichuan Fujian Zhejiang Sichuan Sichuan Hong Kong Fujian Guanxi Zhejiang Sichuan Sichuan Guizhou Sichuan

East of the Tanlu fault 23 24 25 26 27 28 29

Benxi South Korea South Korea Shandong Shandong South Korea Qitaihe

No.: pole number used in Fig. 12. Age: J = Jurassic, K = Cretaceous. Lat., Lon.: northern latitude and eastern longitude. N: number of sites (samples). A95 , d p / d m : semi-axis of 95% confidence circle or semi-axes of 95% confidence oval. Tests: demagnetization and reliability tests - A = alternating field demagnetization; T = thermal demagnetization; P = two polarities; C = baked contact test; F = positive fold test. Ref.: reference - 1. Chan (1991), 2. Enkin et al. (1991a,b), 3. Gildor et al. (1993), 4. Kent et al. (1986), 5. Kim and Van der Voo (1990), 6. Lee et al. (1987), 7. Lin (1984), 8. Otofuji et al. (1986), 9. Pruner (1987), 10. Yang et al. (1992), 11. Zhai et al. (1992), 12. Zhao et al. (1990), 13, Zheng et al. (1991), 14. Zhu et al. (1988), 15. this study.

312

H. Uchimura et al. / Tectonophysics 262 (1996) 301-319

The average directions of the characteristic component for each site are listed in Table 4. Fig. 7 shows these directions before and after tilt corrections. Although all the directions before tilt correction are to the north and downward, they are significantly different from the present axial dipole direction and they are also separated into two groups. After tilt corrections, they are brought together and the precision parameter increased from 111.7 to 268.8. This direction set passes the fold test (McFadden and Jones, 1981) at a 95% confidence level. Thus, a primary origin is strongly suggested for the characteristic directions. The paleomagnetic pole obtained by averaging the VGPs is (59.3°N, 202.6°E) with c~95 = 6.0°.

4. Paleomagnetic poles and tectonic implications Paleomagnetic poles obtained from Jurassic and Cretacoues rocks in this study are given in Tables 3 and 4, and are also shown in Fig. 8. The Late Jurassic to Early Cretaceous pole from the Qitaihe area and Late Cretaceous pole from the Benxi area seem to be quite reliable because of the positive results of the field test, sufficient numbers of sites and samples, very small confidence circles, and (for Qitahe data) existence of two polarities with nearly antipodal directions. The other data summarized in the Appendix are considered to be less reliable; the Neogene pole from Qitaihe has quite a large circle of confidence, and the Late Permian to Early Triassic

N

W

N

E w

s (a) Beforetiltcorrection

E

s (b) Aftertiltcorrection

Fig. 7. Equal-area projections of site mean directions of the characteristic component of Late Cretaceous sandstones from the Benxi area, (a) before and (b) after tilt correction. The cross in (a) represents the direction expected from the present axial dipole. Circles indicate the 95% confidence limit.

Fig. 8. Paleomagnetic pole posisions with circle of 95% confidence obtained in this study. The solid triangles are from the Qitalhe area and the dot is from the Benxi area. J 3 - K I : Upper Jurassic to Lower Cretaceous; K2: Upper Cretaceous; N: Neogene.

pole from Benxi was obtained from a very small number of samples (2 directions and 5 great circles). As it was not possible to construct a polar wander path for this area, we compare the paleomagnetic poles obtained in this study with those for the same time intervals (Jurassic to Cretaceous) from adjacent areas. These data are summarized in Table 5 and the pole positions are plotted in Fig. 9a (Cretaceous) and b (Jurassic). In compiling Table 5, data were selected which pass acceptance criteria similar to the ones used by Enkin et al. (1992). The only difference to their criteria is that the number of sites should be 2 or more. Most of the data in this Table are also in the Chinese pole list of Enkin et al. (1992). Besides the results from the present study, data are also included from Otofuji et al. (1986), Zhai et al. (1992) and Gildor et al. (1993). Both Qitaihe and Benxi areas are situated near the margin of the NCB and is close to the Tan-Lu fault system which is an important element in considering the tectonic evolution of northeastern Asia. In this comparison, the poles from the South China Block (SCB) are also included because the number of reliable poles from the NCB is limited, and because it is usually thought that the SCB and NCB formed a single block in the upper Jurassic (Zheng et al., 1991; Enkin et al., 1992). The summary of Chinese

H. Uchimura et al./ Tectonophysics 262 (1996) 301-319

poles are listed in Table 6, where Siberian poles (Khramov, 1987) are also included for comparison. For the Cretaceous, there is a very distinct difference between the poles from two sides of the Tan-Lu fault (Fig. 9a). Paleolatitudes of these areas in the Cretaceous are nearly the same as those of the Present (Lee et al., 1987). The poles from the west of the Tan-Lu fault, both from the NCB and SCB, are located in the Arctic Sea, while the four poles from the east of the Tan-Lu fault (including Korean, eastern Shandong and Benxi poles but excluding Qitahe pole) are clustered in the Alaskan Peninsula. The Jurassic-Cretaceous pole from Qitaihe is situated near Novaya Zemlya, very far apart from the poles on either side of the Tan-Lu fault. For Late Jurassic, there are only two poles from the east of the Tan-Lu fault and thus the circle of confidence is very large (~95 = 35.3°). Therefore, it is not possible to make a meaningful comparison of the late Jurassic poles from the two sides of the Tan-Lu fault (Fig. 9b). Further complication arises for this age period because the suturing between the Chinese and Siberian blocks was not yet complete as shown by the difference between the poles of the two blocks (Zhao et al., 1990; Zheng et al., 1991). Thus, we have to concentrate on the Cretaceous poles (Fig. 9a) in the following. If we compare the two sides of the fault in the Cretaceous (Table 6), the mean pole of the east group is 65.0°N, 159.2°W and that for the west group is 78.5°N 151.0°W, and their 95% confidence angles (0/,95) are 4.6 ° and 5.3 °, which do not overlap (Fig. 10a). The rotation (R) and flattening ( F ) (Beck,

313

1980) of the block east of the Tan-Lu fault with respect to the combined north and south China poles are R = 17.9 + 9.8 °, F = 1.4 + 5.7 °, which strongly suggests a clockwise rotation of the east-of-Tan-Lu block in the post-Cretacous time without a significant movement in latitudinal direction (Fig. 10b). The Qitaihe pole is displaced too much to the west from the rest of the Chinese poles, and cannot be included in either of the two groups (Fig. 9). We calculate the rotation of the Qitaihe area with respect to the main part of the Chinese block (west of the Tan-Lu fault but including both the NCB and SCB) and obtain a large value ( R = - 3 7 , 7 + 16.6°) in counterclockwise sense. The flattening, on the other hand, is within uncertainties ( F = - 0.6 + 7.1°). Our interpretation is that the discrepancy between the east and west of the Tan-Lu fault (Fig. 10a) is real and is caused by the movement of two blocks along the Tan-Lu fault (Fig. 10b). On the other hand, the data from Qitaihe represent a local rotation of a small block near the fault system and is not a large-scale movement. Similar local movement has been observed at the western end of North America, where small blocks in the fault zone between North America and the Pacific Ocean are found to experience clockwise rotation (Beck, 1980). In our case, the fault movement is left-lateral and the rotation is counterclockwise (Fig. 10b), while in North America the fault movement is right-lateral and the rotation is clockwise. Thus the sense of the rotation is consistent in the two areas. The present-day Tan-Lu fault reaches to the Qinling Mountains in the south and extends to the NNE

Table 6 Summary of Late Mesozoic paleomagnetic poles Block

Age

N

Pole Lat.

Lon.

Siberia

K J K J3-J2 K J3-J2 K J3-J2 K J3-J2

7 4 3 4 10 5 13 9 4 2

74.0 65.0 83.2 73.2 77.1 69.7 78.5 71.5 65.0 65.9

180.0 148.0 208.0 218.3 209.2 197.8 209.0 206.0 200.8 209.5

North China Block South China Block West of the T a n - L u Fault East of the T a n - L u Fault

k

A95

Data source

41 10 189 154 72 84 81 94 298 52

10.0 22.0 9.0 7.4 5.7 8.4 4.6 5.3 5.3 35.3

Khramov (1987) Khramov (1987) Table 5, 1-3 Table 5, 4 - 7 Table 5, 8-17 Table 5, 18-22 Table 5, 1-3, 8-17 Table 5, 4-7, 18-22 Table 5, 23-26 Table 5, 27-28

314

H. Uchimura et aL /Tectonophysics 262 (1996) 301-319

direction to Sikhote Alin in the north (Fig. 1). We fitted a small circle to this fault in a least-squares sense using the strikes read from a map at 200 km intervals. The Euler pole for the rotation was estimated to lie at 20°N, 150°E in the western Pacific. Fig. 10b shows the small circle corresponding to this Euler rotation in comparison with the main branch of the T a n - L u fault. It can be seen that the fit is very reasonable. When this pole is taken as the center of rotation, the rotation and flattening of the east-ofT a n - L u with respect to Cretaceous China pole is

(a) <

\ ~

2 11,0,

115"

t

,

i'' <2-~S-_ ~;.~-<

,

,

-~

120"

I

~-~.}~ 1) East ~

£"f 125"

130"

135"

140'

145 °

140"

145"

i/ ......"

1

~';N

115"

120"

125"

130"

135"

Fig. 10. (a) Mean pole positions with 95% confidence circles lbr Cretaceous paleomagnetic poles from the east (square) and west of Tan-Lu fault. Difference between the two poles is significant. (b) The sampling sites (open stars), the main branches of Tan-Lu fault (thick lines) and a small circle centered at the Euler pole of T a n - L u movement since Cretaceous (20°N. 150°E) shown by a broken line. Left lateral movement of about 800 km since Cretaceous is interred.

J

,,,\

~

Fig. 9. Paleomagnetic poles obtained from eastern Asia. Dots, solid squares and triangle indicate individual poles obtained from the NCB, SCB and east of the T a n - L u fault, respectively. Open symbols are the mean poles for the three groups and the corresponding 95% confidence circles are also shown. The mean for the east-of-Tan-Lu group excludes the J - K pole from Qitahe (near Novaya Zemlya). Open stars indicates our sampling areas (Qitahe and Benxi). (a) Cretaceous poles, (b) Jurassic (J3-J2) poles.

R = 1 5 . 0 ! 7 . 4 °, F = - 5 . 5 ± 5 . 8 ° , and only the clockwise rotation is significant. The estimated amount of left-lateral movement along the T a n - L u fault is about 800 km. This is in good agreement with the geologically estimated maximum displacement of 700 km (Xu, 1980). An alternative possibility is that the motion along the T a n - L u fault has mainly taken place along another major branch of the fault shown in Figs. 1 and 10b. In this case, the Euler pole lies near 31°N, 135°E and the amount of slip, needed to explain the rotation of paleomagnetic poles, is about 400 kin.

14. Uchimura et aL / Tectonophysics 262 (1996) 301-319

There are several branches of faults running in quasi-parallel directions in the northeastern part. Among them, the eastern branch passing through Shengyang and extending to the direction of Mudanjiang (Fig. 10b) seems to be one of the main fault in the Tan-Lu system. Because Qitaihe is situated between the main and major eastern branch, we propose that the deflection of the Qitahe paleomagnetic direction was caused by the local rotation of the block sandwiched between these two major fault branches. It must be pointed out that the most active movement along the Tan-Lu fault is usually thought to have taken place in the Jurassic to Early or Middle Cretaceous and the maximum displacement is estimated to be about 700 km (Xu, 1980). The present results, however, show that total movement of similar amount has occurred since Cretaceous time. Therefore, some modification of the conventional view of tectonics in northeast China seems to be necessary. Paleogene basalts are distributed exclusively along the eastern branch of the Tan-Lu fault, which may be an indication of tectonic activity in this part of the fault system in the early Tertiary (Wang and Wang, 1986). The reversal of the sense of fault movement may then be a quite recent event. One might wonder why such a large-scale motion along a nearly N - S trending fault does not produce a clear distinction between the flattening of the directions from Benxi and Qitahe. The answer is that both areas are on the same (eastern) side of the Tan-Lu fault so that no intraplate displacement is expected between these two sites. Even if this interpretation is incorrect and the two sites belong to different sides of the fault, our data is still consistent with the inferred 800-km movement along the fault. The reason is that the large uncertainty in the JurassicCretaceous Qitahe data (7. i °) precludes the possibility of detection in the flattening. Even if the fault is exactly N - S , an 800-km displacement (i.e., __+7.2° movement along the meridian) from or to a latitude of 47.8 ° (corresponding to the paleolatitude of Qitahe) changes the inclination only by + 5.1 ° or - 5.9 °. Clearly, this is smaller than the uncertainty of the data, and so detection of fault movement by paleomagnetic data is unfortunately not possible in the present case.

315

5. Conclusions Paleomagnetic directions were obtained from Jurassic/Cretaceous sedimentary rocks and Neogene volcanics in Qitaihe, Heilongjiang Province, and from Cretaceous sedimentary rocks in Benxi, Liaoning Province, northeast China. For sedimentary samples, a fold test gave positive results and the characteristic remanences are concluded to be of primary origin. About the ages of Mesozoic formations in the Qitaihe area, which is controversial, we obtained mixed polarity from the Didao and Chenzihe formations, suggestive of the age belonging to the JurassicCretaceous mixed polarity zone (Harland et al., 1990). This also agrees with the newly obtained paleontology data (Sun Ge, pers. commun., 1988). Jurassic/Cretaceous rocks of the Qitaihe area show anomalous declination suggesting a small scale rotation of the local block close to the fault system: The Cretaceous poles from the Benxi area are consistent with those from other parts to the east of the Tan-Lu fault (Korea, Shandong Province, etc.) and are systematically displaced from the corresponding poles of Siberia-China for the same ages. To explain this difference, left-lateral movement of about 800 km along the Tan-Lu fault is suggested. The Euler pole of rotation lies in the western Pacific at around 20°N, 150°E. Because the amount of displacement is not too large, this movement does not contradict with the assumption that the NCB is tectonically a single block to a first approximation. However, it is necessary to treat the east and west of the Tan-Lu faults as separate blocks if we are interested in clarifying the tectonic evolution of a small to medium sized areas such as the Japan Sea.

Acknowledgements We thank Masaki Takahashi (Ibaraki University), Zhu Xiangyuang and Zhang Wenxi (Institute of Geophysics) for the assistance in sampling and Hidefumi Tanaka (Tokyo Institute of Thechnology) for the help in the laboratory. Tan Minchang and Liang Zhihua (Geological Team of Qitaihe Area) helped us in sampling and geological correlation. Discussion with Zheng Zhong (Tokyo Institute of Technology)

H. Uchimura et al. / Tectonophysics 262 (1996) 301-319

316

was also helpful. We also thank Sun Ge (Nanjing Institute of Geology) who kindly showed his paleontological data. Shinjiro Mizutani (Nagoya University) and Koichiro Fujimoto (Geological Survey of Japan) are thanked for information about available geological maps. Comments and suggestions from Randolf Enkin and anonymous reviewer were very helpful in revising the paper. The expenses for field trips were subsidized by Grant-in-Aid for Overseas Researches (Nos. 6104116, 6204117 and 63044175) from the Ministry of Education, Science and Culture of Japan.

Appendix A In this Appendix, we summarize the results of paleomagnetic measurements of Neogene and Jurassic to Triassic rocks which are not included in the main text because they are not strongly connected with the tectonic interpretations developed in this paper. As data for these ages are not abundant for northeast China, we believe that they are nevertheless important for further study of paleomagnetism and tectonics of this area. Neogene basalts from Qitahe gave results quite

satisfactory in terms of the stability of remanence. Fig. 1 la is a typical example of the results of thermal demagnetization. Alternating field (AF) demagnetization was also applied to at least one specimen for each site, but there was no significant difference between the two demagnetization results. Most of samples have almost single component remanence and free from secondary magnetization. The sitemean directions are illustrated in equal-area projection in Fig. 1 lb. Three of the basalt-flows show very similar mean directions and there is a possibility that they erupted in a very short time. Tilt correction is not applied because the bottom of the flows is not exposed and precise bedding angles are not known. However, it was concluded that the tilt of these layers must be quite small from the observation of flow patterns in the field. The fact that the normal and reverse directions are nearly antipodal supports this deduction and suggests that tilt corrections are unnecessary. The average of VGPs of all the five sites gives the pole (83.3°N, 145.0°E) with oL9s= 23.0 °. On the other hand, it is not easy to obtain characteristic remanence direction from samples of the Late Permian to Early Triassic Shiqianfeng formation collected in the Benxi area. Fig. 12 shows examples of

N qNo3ol O.OxlOo) $1

~-..t-,~ _

I

I

I

N

~ $20

", ,q

W

E

~ 430

Q 300 ~ 200

(a)

' E On

'Ko o

( S

Fig. 11. (a) Typical thermal demagnetization results for a Neogene basalt sample. (b) Equal-area projection of site mean directions of Neogene basalts from the Qitaihe area.

H. Uchimura et a l . / Tectonophysics 262 (1996) 301-319

317

N

.o2 o;, (;+.o:; 1

LLOZOS(S.Oxl 0"3) S

~./

S501,~

I 660 o~w Up

I

+

j.

t m 5SO~

4SQ

I "",o:2

w

" z

~300

"+x q300

~Xo20 o

(a)

',,o

,oo

b

E On

E 011

S Fig. 12. Change of directions in thermal demagnetization for sites LL01 and LL02. (a) An example in which the direction of remanence was determined from the highest temperature portion. (b) An example in which only a great circle was defined. (c) Determination of the site mean direction (cross with circle of 95% cofidence). Double circles indicate directions determined from linear portions. Small solid and open marks indicate directions at various stages of demagnetization from which the great circles were determined.

stepwise thermal demagnetization. Among the samples from two sites, only two samples gave a linear segment in high-temperature portion and the characteristic component could be obtained (Fig. 12a). Five samples did not give such a segment but the great circle could be obtained (Fig. 12b). The number of available data is quite small compared to the similar cases in the Qitaihe area. Since these two sites are only a few meters apart and have similar bedding angles, it seems that there is no large time difference and we can combine the data from them together to

obtain a mean direction. Fig. 12c is the result of analysis using the method of McFadden and McEIhinny (1988). Since the mean direction is reversed and significantly different, even from the reverse direction corresponding to the present axial dipole, this characteristic direction may be primary. Corresponding VGP position after tilt correction is (45.0°N, 28.5°E). However, reliabily of this pole is low considering that no field test is available and the number of samples is small (7). These results are summarized in Table 7.

Table 7 Site mean directions of neogene and Permo-Triassic rocks Site

N

Tilt corrected

In situ 1

Dispersion

D

1

D

k

22.2 16.5 13.1 11.4 151.7

87.6 64.5 65.4 62.9 - 52.6

22.2 16.5 13.1 11.4 151.7

697.1 1017 5856 697.2 65.8

VGP (x95

Lat.

Lon.

2.9 2.4 1.0 2.9 9.5

50.4 78.6 80.9 81.8 -64.9

134.5 213.9 207.1 229.8 200.7

Neogene basalts in the Qitahe area QN01 QN02 QN03 QN04 QB01

5/5 5/5 5/5 5/5 5/5

87.6 64.5 65.4 62.9 - 52.6

Mean of the Qitahe Neogene basalts (dispersion of pole: K = 12.0, A95 = 23.0) N2

5

67.8

1. l

28.0

14.7

83.3

144.7

- 42.5

129.5

18.0

14.6

- 45.0

208.5

Late Permian to Early Triassic sandstones LL01/02

7(5 )/14

For legend, see Table 3.

- 34.9

137.0

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