Paleomagnetic evidence for post-Early Cretaceous tectonic rotation of the Sikhote-Alin Superterrane, Far East Russia

Journal of Asian Earth Sciences 111 (2015) 88–99

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Paleomagnetic evidence for post-Early Cretaceous tectonic rotation of the Sikhote-Alin Superterrane, Far East Russia Ryutaro J. Ichihashi a, Haider Zaman b, Yutaka Wada c, Yoshiaki Sugamori d, Yohei Kajikawa a, Hyeon-Seon Ahn a, Koji Uno e, Petr S. Zimin f, Vladimir G. Sakhno g, Yo-ichiro Otofuji a,⇑ a

Department of Earth and Planetary Sciences, Faculty of Science, Kobe University, Kobe, Japan Department of Geology, Faculty of Science, Taibah University, Madinah 41477, Saudi Arabia Department of Earth Sciences, Nara University of Education, Nara 630-8528, Japan d Department of Geosciences, Faculty of Science, Osaka City University, Sumiyoshi, Osaka, Japan e Graduate School of Education, Okayama University, 3-1-1 Tsushimanaka, Kita-ku, Okayama, Japan f Russian Academy of Science, Pacific Oceanological Institute, Vladivostok, Russia g Russian Academy of Science, Far East Geological Institute, Vladivostok, Russia b c

a r t i c l e

i n f o

Article history: Received 2 February 2015 Received in revised form 28 July 2015 Accepted 5 August 2015 Available online 5 August 2015 Keywords: Tectonics Paleomagnetism Early Cretaceous Asian Continent Zhuravlevka-Amur Terrane Far East Russia

a b s t r a c t We present new Early Cretaceous paleomagnetic results from the Zhuravlevka-Amur Terrane of the Eurasian Continent, Far East Russia. Out of 34 total sites, 14 were collected from Komsomolsk-onAmur area (50.6°N, 137.2°E) and 20 from Vaninsky area (49.1°N, 139.2°E). Thermal demagnetization reveals the presence of two interpretable magnetization components in 19 sites, with laboratory unblocking temperatures of 350 °C and/or 500–580 °C. The remanent directions of the lowtemperature component are either parallel or anti-parallel to those obtained from the hightemperature component. Results of fold tests show that both components are secondary. Rock magnetic and reflected light microscopic observations indicate a chemical origin for both of these components, as evident from the presence of secondary pyrrhotite and magnetite. The Komsomolsk-on-Amur area provides an in-situ formation mean direction of D = 127.5°, I = 66.7° (k = 28.2, a95 = 9.3°, N = 10 sites). When combined with the reported paleomagnetic data from Early to Middle Cretaceous accretionary wedge rocks of the Kiselevka-Manoma Terrane and the Early Cretaceous Western Sakhalin turbidite basin rocks (D = 94.2° and D = 57.1°, respectively), large magnitude of clockwise rotations of 66–118° is demonstrated for the eastern part of the Sikhote-Alin Superterrane with respect to Eurasia. In addition, these three landmasses maintained their E–W elongated orientations before the start of rotation, implying southward directed subduction of the oceanic plates beneath northern margins. These reconstructions of the Sikhote-Alin Superterrane provide clues on the tectonic evolution of Panthalassa Ocean. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The present-day Eurasian Continent is a collage of many continental blocks, crustal fragments, ancient island arcs and accretionary prisms/wedges, formed as a result of sequential collisions and suturing. An eastern part of this mega continent is shared by Siberian and Amurian blocks (Fig. 1a). The Mongol-Okhotsk Ocean separated the Siberian and Amurian Blocks in the Late Jurassic (Zonenshain et al., 1990; Kravchinsky et al., 2002; Cogné et al., 2005; Kelty et al., 2008). The Amurian Block in turn itself is a collage of several massifs and terranes, including the Paleozoic massifs (e.g., Bureya, Jiamusi and Khanka massifs) and the ⇑ Corresponding author. E-mail address: [email protected] (Y.-i. Otofuji). http://dx.doi.org/10.1016/j.jseaes.2015.08.008 1367-9120/Ó 2015 Elsevier Ltd. All rights reserved.

Jurassic–Cretaceous sub-terranes of the Sikhote-Alin Superterrane (Khanchuk, 2001; Kemkin and Filippov, 2001; Kojima et al., 2008; Kemkin, 2012). The eastern part of the Amurian Block is occupied by the Sikhote-Alin Superterrane, in which about eight Jurassic to Cretaceous age sub-terranes with NNE–SSW elongated slivers are distributed (Fig. 1b) (Kemkin and Filippov, 2001). In general, Jurassic to Early Cretaceous accretionary wedges, Lower Cretaceous turbidites, and Early Cretaceous island arc systems are sequentially arranged from west to east. According to the interpretation of Khanchuk et al. (2004), this region is characterized by complicated distribution of coeval terranes and juxtaposition of different age units. Other researchers (Khanchuk, 2001; Malinovsky et al., 2008; Abrajevitch et al., 2012; Didenko et al., 2014) have postulated large scale northward displacement of the

R.J. Ichihashi et al. / Journal of Asian Earth Sciences 111 (2015) 88–99

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Fig. 1. Regional tectonic setting of the study areas. (a) Schematic map showing an eastern part of the Asian Continent. (b) Tectono-geological map of Sikhote-Alin and adjoining areas (after Khanchuk (2006), Kemkin (2012)). The present study areas are shown by red solid circles, where K is for Komsomolsk-on-Amur, B for Ozer Bolon Lake, V for Vaninsky and KS for Kiselevka. ZH-A BD, ZH-A ND and ZH-A SM: the Zhuravlevka-Amur Terrane (ZH-A) in the Badzhal (BD), Nadankhada, Sarmarka (SM) paired zones, respectively. BD: the Badzhal Wedge, ND: Nadankhada wedge, KM: the Kiselevka-Manoma Terrane, KE: the Kema Terrane, TU: Taukha Terrane. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Jurassic–Cretaceous accretionary prisms and the Early Cretaceous island arc systems along the Asian Continent. Didenko et al. (2014) have reported east deflected paleomagnetic data (D = 83.6°) from Lower Cretaceous volcanosedimentary rocks of the Kiselevka-Manoma Terrane (51.4°N, 139.0°E) (Fig. 2), implying that the present day NNE–SSW stretching Kiselevka-Manoma Terrane trended E–W during the Cretaceous. To shed further light on the tectonic evolution of the Sikhote-Alin Superterrane, particularly arising of the east deflected declinations in the eastern part of Amurian Block, a paleomagnetic study of Early Cretaceous rocks in the Zhuravlevka-Amur Terrane was conducted.

paired strip zone with the Jurassic accretionary wedges. Three geological paired zones are named as the Badzhal (western), Nadankhada (central) and Sarmarka (eastern) paired zones. The Nadankhada paired zone is the smallest among them, in which the Cretaceous terrane is arranged west of the Jurassic wedge. These three paired zones present a unique ‘b’ (reverse of letter N) shaped structural pattern in the area. The large left-lateral strike-slip Central Sikhote-Alin (CSA) fault crosses through the Sikhote-Alin Superterrane in a Northeast– Southwest orientation, dividing the Sarmarka Terrane into two parts. A displacement of 150–250 km has been estimated along this fault that is of pre-Early Cretaceous (Natal’in, 1993; Faure et al., 1995; Utkin, 2012).

2. Geologic setting 3. Sampling Jurassic accretionary wedges are distributed along the eastern edge of the Paleozoic massifs in the eastern part of Amurian Block. From north to south, they are named the Badzhal, Nadankhada and Sarmarka wedges. East of these wedges, Early Cretaceous turbidites (Zhuravlevka-Amur Terrane), Early Cretaceous accretionary wedges (Kiselevka-Manoma Terrane), and Early Cretaceous island arc systems (Kema Terrane) are distributed (Khanchuk, 2001; Zyabrev and Anoikin, 2013). The Zhuravlevka-Amur Terrane consists of Lower Cretaceous turbiditic shelf sedimentary rocks (with molluscan fossils) and Valanginian age alkaline flows of picritic basalts (Khanchuk, 2001). These rock units were deposited along the base of a continental slope. Almost all stratigraphic units in this terrane are intensely folded and faulted. Because the Zhuravlevka-Amur Terrane is juxtaposed to the west by Badzhal and Sarmarka wedges and to the east by Nadankhada wedge, this terrane forms a geological

As shown in Figs. 1b and 2, sedimentary rocks in the Zhuravlevka-Amur Terrane of the Badzhal and Sarmarka paired zones were sampled for the current paleomagnetic study. Detailed description of the sampled rocks in these two areas is given as follows:

3.1. The Zhuravlevka-Amur Terrane in the Badzhal paired zone According to s the recent geologic studies (Kirillova, 2002; Zyabrev, 2011), Late Jurassic to Early Cretaceous fore-arc and back-arc marine terrigenous deposits and turbidites (Komsomol’sk Group) are exposed on the right bank of River Amur in Komsomolsk-on-Amur area (137.2°E, 50.6°N) and on the northwestern shore of Lake Ozer Bolon (136.3°E, 49.9°N) (Fig. 1b).

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Fig. 2. Map showing trends of declinations in the study areas from Early Cretaceous. Red arrows indicate declinations from the present study. K: Komsomolsk-on-Amur area, B: Ozer Bolon Lake area and V: Vaninsky area of the Zhuravlevka-Amur Terrane (ZH). Blue arrow shows a reported declination from Kiselevka area (KS) of the KiselevkaManoma Terrane (KM), while black arrow indicates declination at the Naiba River area (N) of West Sakhalin basin (WS). Declinations are presented in-situ. Color symbols for geologic terranes are the same as in Fig. 1b. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

A sequence of sandstone and mudstone layers forms part of the exposures on the bank of Amur River. Maximum thicknesses of these beds are
0.5 m and
0.3 m, respectively. These alternating layers have an E–W directed strikes and south directed high angle dips (30–70°). In general, the sandstone beds are characterized by cross lamination and grading and the mudstones layers by welldeveloped shear fractures. Assuming that each sandstone bed represents a spot reading of the field, each bed is regarded as a site. Fifty oriented block samples were collected at 10 sites (BK21 to BK30) from sandstone beds along the River Amur in the range of 7.5 km. In the Ozer Bolon Lake area, sandstone and mudstone are the main sedimentary facies. Maximum thickness of these layers in this area is
0.2 m, which strike about N30°E to N40°E and dip SE of 30–50°. Cross lamination, grading and sand pipes are the obvious signatures of sandstone layers, while shear fractures generally characterize the mudstone layers. Features like kink folding have been observed in the area around site BK31. Twenty samples (5 samples per site) were collected at 4 sandstone sites in the range of 4.6 km from this locality. Alternation of sandstone and mudstone beds is predominant in sites BK31 and BK34, and mudstone is exposed at sites BK32 and BK33.

3.2. The Zhuravlevka-Amur Terrane in the Sarmarka paired zone Upper Jurassic to Lower Cretaceous turbidites are exposed in the Vaninsky area (49.1°N, 139.2°E) on the road from Komsomolsk-on-Amur to Sovetskaya Gavan (Fig. 1b). Paleomagnetic samples were collected at 20 sites from three localities; the western (BK43
BK46), central (BK52
BK64) and eastern (BK49
BK51) localities. Outcrops at all sampling sites are composed of sandstone and mudstone beds, which generally strike in a NE–SW direction. However, the dipping angle changes significantly between western and eastern areas, i.e., from 18° to 94° toward west. Strata in the central part are intensely folded and bedding attitude varies from vertical to overturned with 72–158° dips to the west or east. The colors of mudstone and sandstone in the western and central parts are generally black and dark gray, respectively. However, the color of mudstone changes to deep purple and that of sandstone to white in all eastern sites (BK43
46) and in several central sites (BK56
BK61), suggesting possible thermal alteration of the sampled rocks. 203 samples were collected at 20 sandstones sites in the range of 16.8 km from this area. A magnetic compass was used for the orientation of paleomagnetic samples. The present day declination value at each sampling

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locality was evaluated from the International Geomagnetic Reference Field (International Association of Geomagnetism and Aeronomy Working Group V-MOD, 2010). 4. Paleomagnetism 4.1. Laboratory procedure Core samples of 2.5 cm in diameter were drilled from every hand sample and then cut into 2.2 cm long specimens in the laboratory. Natural remanent magnetizations (NRMs) of the representative specimens were measured in the magnetically shielded laboratories of Kobe and Kyoto universities using three-axis cryogenic magnetometers of 2-G Enterprises (Pacific Grove, CA, USA). All selected specimens were subjected to progressive thermal demagnetization (THD) using Natsuhara TDS-1 thermal demagnetizer. Pilot specimens from each site were thermally demagnetized in 13 temperature steps up to a maximum level of 590 °C. Demagnetization results were plotted on orthogonal vector diagrams (Zijderveld, 1967) and on equal-area projections. Paleomagnetic directions were determined by principal component analysis (Kirschvink, 1980). Site-mean and formation-mean directions were calculated using Fisherian Statistics (Fisher, 1953).

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4.2. Paleomagnetic results from the Badzhal paired zone The NRMs of 50 specimens from Komsomolsk-on-Amur area and 19 specimens from Ozer Bolon Lake area reveal an intensity range of 1.0  104 to 14.3  104 A/m. Samples from Komsomolsk-on-Amur area have in general higher NRM intensities than those from Ozer Bolon Lake area; 8.2 ± 0.4  104 A/m and 3.9 ± 0.8  104 A/m, respectively. The highest intensity value (12.2 ± 2.2  104 A/m) is associated with samples from site BK27 from Komsomolsk-on-Amur area. The lowest value of 2.3 ± 0.2  104 A/m in site BK34 from Ozer Bolon Lake area. Based on NRM’s behavior in progressive thermal demagnetization, samples from Komsomolsk-on-Amur area are categorized (Fig. 3) into single (41 samples) and two components magnetizations (9 samples). As shown in Fig. 3a, d, sable endpoint trends are observed up to 450–500 °C in the samples with single component behavior, although complete unblocking is reached at higher temperature (450–590 °C). The samples with two magnetic components show over 50% of the intensity unblocked by 350 °C and then a gradual decrease by 530–590 °C (Fig. 3b, c). However, well-defined magnetization is isolated at higher temperature between 350 °C and 450–500 °C. Samples from the Ozer Bolon Lake area samples also show a mixture of single or two component

Fig. 3. Representative thermal demagnetization plots in geographic coordinates for 9 sites of the Zhuravlevka-Amur Terrane. (a)–(d), the Komsomolsk-on-Amur area; (e) and (f), the Ozer Bolon Lake area; (g)–(i), the Vaninsky area. The associated intensity plots, showing normalized intensity versus temperature (in degrees Celsius) are also given. Open (solid) symbols show projection onto the vertical (horizontal) plane.

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behavior but in contrast to Komsomolsk-on-Amur area the NRM in most samples is unblocked by 350 °C (Fig. 3e, f). Four samples from Ozer Bolon Lake area show unblocking temperatures of about 250 °C. The low-temperature component (LTC), which is generally unblocked by 250 °C, is identified in nine samples (BK216, 220, 240, 243, 250, 262, 272, 286 and 301) from Komsomolsk-onAmur area and in four samples (BK336, 339, 341 and 347) from Ozer Bolon Lake area. An in-situ mean direction of D = 9.3°, I = 61.0° (k = 13.8, a95 = 11.6°) is obtained from these 13 samples, which is sub-parallel to the present geomagnetic field direction (D = 12°, I = 65°) or the axial dipole field direction (D = 0°, I = 67°) in the study area. This component can, therefore, be ascribed to a VRM acquired during the Brunhes chron. The high-temperature component (HTC) is identified in all specimens from Komsomolsk-on-Amur area (Fig. 4). The NRM directions for each site are well clustered as indicated by a precision parameter values between 17.7 and 710.7 (Table 1). The formation-mean directions calculated for 10 sites are D = 127.5°, I = 66.7° (k = 28.2, a95 = 9.3°) in geographic coordinates and D = 160.2°, I = 26.1° (k = 9.1, a95 = 17.0°) in stratigraphic coordinates (Fig. 4). A classical fold test of McElhinny (1964) is negative at 95% confidence limit. Because the Upper Jurassic to Lower Cretaceous strata forms a monoclinal structure in the area, a second definition (n2) of the McFadden’s fold test (1990) is applied as well, indicating a negative fold test at 99 per cent confidence levels. In the case of Ozer Bolon Lake samples, directions of the magnetic component unblocked by 350 °C are only taken into accounts. However, due to a dramatic drop in the NRM intensity between 300 °C and 350 °C, the temperature level of 300 °C is considered as a blanket datum for mean direction calculation. Clustered mean directions are obtained from 3 out of 4 sites (i.e., BK31, BK33 and

BK34), revealing the precision parameter values >24. The in-situ formation-mean direction obtained from 3 sites is D = 135.2°, I = 55.0° (k = 19.6, a95 = 28.5°), which is sub-parallel to the in-situ direction obtained from Komsomolsk-on-Amur area. 4.3. Paleomagnetic results from the Sarmarka paired zone (the Vaninsky area) The NRMs of 183 specimens from this area show a significant variation in intensity values, i.e., from 0.6  104 to 1069  104 A/m. Samples from western (sites BK43
46) and eastern (sites BK49
51) parts have high intensity values (107 ± 229  104 A/m) compared to those from sites BK52
64 in the central part (8.0 ± 25.3  104 A/m). In most samples an abrupt decrease in the intensity of NRM is observed between 300 °C and 350 °C (Fig. 3h, i). Higher laboratory unblocking temperatures (i.e., between 450 °C and 590 °C) are observed in the samples from sites BK43, BK45, BK54 and BK55. Vector end points are clearly defined only in seven sites from western and eastern parts (BK43
BK46, BK49
BK51) (Table 2). After the removal of LTC at 250 °C, a medium-temperature component (MTC) of both reverse and normal polarities is unblocked by 350 °C. A similar NRM direction is observed in the HTC (500– 560 °C) of seven samples from sites BK45 and BK46. However, in 17 samples from four sites (BK43, BK45, BK46 and BK49) an opposite polarity NRM direction appears in the HTC between 500 and 590 °C (Fig. 3g). Samples of the remaining13 sites from central part show spurious demagnetization behavior and thus give no clear NRM direction. The LTC (unblocked by 250 °C) is identified in 26 specimens, giving an in-situ mean direction of D = 14.6°, I = 62.1° (k = 8.7, a95 = 10.2°), which is almost parallel to present geomagnetic field direction (D = 12°, I = 65°) or an axial dipole field direction (D = 0°, I = 67°) in the study area. A classical fold test of McElhinny (1964) proved negative at 95% confidence limit. Similar to the Badzhal area, the LTC here is also ascribable to a VRM acquired during the Brunhes chron. As shown in Fig. 5, the MTC (unblocked between 300 and 350 °C) is identified in 35 samples of seven sites (BK43
46 and BK49
51). Among them, four samples from sites BK44 and BK45 show normal polarity directions, while most (31) samples show reverse polarity. An in-situ mean direction (D = 222.0°, I = 70.2°, k = 34.7, a95 = 4.2°, n = 35) for this component is calculated after inverting the normal polarity directions into reverse. A high precision parameter (k) is observed before tilt-correction. The HTC of normal polarity is identified in 24 samples of sites BK43
46 and BK49 (5 sites) (Fig. 5). An in-situ mean direction of D = 39.5°, I = 68.3° (k = 22.5, a95 = 6.4°) is calculated on the basis of 24 samples and the highest k value is obtained at 5% unfolding. After applying the classical fold test of McElhinny (1964) to sample directions as well as site-mean directions, negative test are obtained at 95% confidence limit for both the MTC and HTC. However, when both these options were tested under the McFadden’s (1990) second definition (n2) fold test, negative results at 99% confidence levels are obtained for MTC and HTC. These results indicate that magnetizations related to MTC and HTC were most likely acquired after folding. 5. Magnetic mineralogy investigations

Fig. 4. Equal-area projections of the site mean directions for Komsomolsk-on-Amur area. Black circles indicate in-situ directions and green squares the tilt corrected directions. The formation mean directions are shown by large solid circle and square with 95% confidence circles. The present geocentric axial dipole field direction is depicted by red star. Solid symbols show positive downward inclinations. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

5.1. Rock magnetism Progressive acquisition of the isothermal remanent magnetization (IRM) was performed up to a maximum field of 2.7 T using a 2G-pulse magnetizer. Thermal demagnetization of the composite

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R.J. Ichihashi et al. / Journal of Asian Earth Sciences 111 (2015) 88–99 Table 1 Paleomagnetic results of the Early Cretaceous Zhuravlevka-Amur Terrane (Komsomolsk-on-Amur and Ozer Bolon lake areas). Sampling site

Longitude (E)

Latitude (N)

Lithology

Strike

Dip

Intensity 10

Komsomolsk-on-Amur BK21 BK22 BK23 BK24 BK25 BK26 BK27 BK28 BK29 BK30

137°11.90 137°11.70 137°11.50 137°11.00 137°10.50 137°10.30 137°08.40 137°08.30 137°07.80 137°07.50

50°37.10 50°36.90 50°36.80 50°36.40 50°36.00 50°35.70 50°34.20 50°34.10 50°33.90 50°33.70

s.s. s.s. s.s. s.s. s.s. s.s. s.s. s.s. s.s. s.s.

96.1 92.6 95.6 67.6 77.6 59.6 126.6 92.6 92.6 102.6

51 64 58 66 28 23 37 48 45 60

4

A/m

136°18.60 136°19.90 136°21.80 136°21.90

49°51.40 49°51.60 49°52.70 49°52.80

s.s. s.s. s.s. s.s.

42.8 220.3 42.3 27.8

52.5 47 34 48

Tilt corrected

k

a95 (°)

Inc. (°)

Dec. (°)

Inc. (°)

5 5 5 5 5 5 5 5 5 5

138.2 109.1 121.8 111.1 133.2 98.2 118.2 169.8 198.1 110.9

58.2 68.9 56.7 48.6 57.9 62.5 76.5 57.8 80.2 79.6

162.2 161.2 155.1 128.8 143.6 119.1 194.1 175.7 185.8 180.9

15.0 18.5 13.7 6.3 32.0 44.7 52.8 10.4 35.5 28.0

17.7 63.2 91.2 29.9 79.9 74.2 710.7 52.1 46.8 121.0

18.7 9.7 8.1 14.2 8.6 8.9 2.9 10.7 11.3 7.0

10 10

127.5

66.7 160.2

26.1

28.0 9.1

9.3 17.0

5 5 5 5

99.1 – 137.7 155.5

62.8 – 50.1 45.8

117.6 – 136 143.1

13.9 – 16.2 4.0

60.5 0.9 24.8 36.7

9.9 – 15.7 12.8

3 3

135.2

55.0 132.4

11.6

19.6 31.6

28.5 22.3

2.9 ± 1.6 7.7 ± 5.3 6.0 ± 2.0 8.9 ± 2.6

Mean (in situ) Mean (tilt corrected)

IRMs (2.7 T, 0.4 T and 0.12 T along z, y and x axes, respectively) was carried out to detect unblocking temperature spectra in the selected specimens (Lowrie, 1990). Thermomagnetic analyses were conducted on several samples in air atmosphere using an automatic recording magnetic balance of Kyoto University (with a magnetic field of 0.85 T). Four samples from Komsomolsk-on-Amur area of the Badzhal paired zone were chosen for rock magnetic investigations (Fig. 6a, b). For all samples, IRM is almost saturated at 500 mT but shows a gradual increase to 2500 mT. This behavior not only indicates the presence of magnetite (which is typically saturated by 300 mT) but also the possible presence of hematite or goethite. The presence of pyrrhotite is clearly inferred by thermal demagnetization of the composite IRM (Fig. 6a, b). The medium and hard coercivity (Hc) components of the composite IRMs are unblocked at 340 °C for all specimens of the Komsomolsk-on-Amur area. This behavior differs from greigite-bearing samples, which show considerable IRM decrease at 200 °C in soft and medium Hc components and complete demagnetization by 320 °C (Torii et al., 1996). An unblocking temperature of about 590 °C is shown in the medium and hard Hc components of the specimens, indicating the presence of magnetite (Fig. 6a, b). Thermomagnetic analyses show reversible heating and cooling curves but without any prominent Curie temperature points (Fig. 7a, b). The presence of goethite is not supported by thermal demagnetization of the composite IRM or thermomagnetic analyses. Similar to Badzhal Terrane samples, 11 samples from the Sarmarka Terrane also indicate no saturation up to 500 mT but a gradual increase to 2500 mT (Fig. 6c, d). The presence of pyrrhotite is indicated by an unblocking temperature of 340 °C in all three IRM components (Fig. 6c, d). The presence of magnetite is also confirmed by an unblocking temperature of
590 °C in the soft Hc component. Thermomagnetic analyses indicate either a reversible mode during heating and cooling or an increase in magnetic intensity during cooling. The later behavior implies production of magnetite as a result of pyrrhotite breakdown (Fig. 7d).

In situ Dec. (°)

5.0 ± 1.1 5.3 ± 0.8 6.0 ± 2.0 8.9 ± 2.6 10.8 ± 1.4 9.1 ± 1.2 12.2 ± 2.2 7.0 ± 2.3 9.6 ± 2.5 8.1 ± 2.0

Mean (in situ) Mean (tilt corrected) The Ozer Bolon lake BK31 BK32 BK33 BK34

n

ied with a Nikon optical microscope to identify the occurrence and texture of iron oxides. In the reflected light, iron sulfide grains, which are distinctly yellowish in color, are uniformly distributed throughout the rocks (Fig. 8). Most sulfide minerals are pale yellow in color and do not show pleochroism, suggesting pyrite grains. Pyrite occasionally occurs in veins (Fig. 8a; BK21-3) and also as framboidal aggregates (Fig. 8b; BK44-4). Pyrrhotite, which shows pinkish1 yellow in color and noticeable pleochroism, is of less abundance (Fig. 8c; BK27-1, BK27-3, BK34-4, BK43-7). Magnetite in the samples is identified by rock magnetic data (as evident from unblocking temperature level of 590 °C), but the small grain sizes make identification difficult in optical microscopy (BK46-8).

6. Discussion 6.1. Secondary remanent magnetization in the Early Cretaceous Zhuravlevka-Amur Terrane

5.2. Microscopic observation

On the basis of fold test results, a post-folding origin is interpreted for the ChRM from the Komsomolsk-on-Amur and Vaninsky areas of the Zhuravlevka-Amur Terrane (Figs. 4 and 5). As evident from the above mentioned results, primary NRMs are completely overprinted by a secondary magnetization in the rocks studied. Based on rock magnetic investigations, these secondary magnetizations reside in pyrrhotite and magnetite (Figs. 6 and 7). The presence of the two minerals along with pyrite is confirmed by optical microscopic observations (Fig. 8). Iron sulfide minerals (including pyrrhotite, greigite and pyrite) are often found in organic rich terrigeneous sediments deposited along continental margins (e.g., Karlin and Levi, 1983; Kodama, 2012). Active marginal basins are host to rapidly deposited terrigenous sediments in reducing diagenetic environments (Rowan and Roberts, 2006). One of the diagenetic factors is the suboxic (reducing) environment during turbidite formation (Robinson, 2000). The shelf deposits and turbidites of the Zhuravlevka-Amur Terrane was likely suitable hosts for the growth of authigenic iron sulfide minerals. Abundant sulfide minerals have been observed (Weaver et al., 2002) in the Miocene age mudstone of west Sakhalin fore-arc basin.

Polished thin sections of 7 representative samples (BK21-3, BK27-1, BK27-3, BK34-4, BK43-7, BK44-4 and BK46-8) were stud-

1 For interpretation of color in Fig. 8, the reader is referred to the web version of this article.

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Table 2 Paleomagnetic results of the Early Cretaceous Zhuravlevka-Amur Terrane (Vaninsky area) (in situ). Sampling site

Latitude

Longitude

Lithology

Strike

Dip

NRM intensity

HTC (350 °C  560 °C)

104 A/m

Dec.

Inc.

n/k/a95

Dec.

Inc.

n/k/a95

Dec.

Inc.

n/k/a95

22.6 – – –

74.1 – – –

8/118.6/5.1 – – –

– 55.1⁄ 47.5 34.1

– 37.2⁄ 69.8 68.8

– 2/2.2/– 5/56.4/10.3 3/113.4/1.6

221.8 257.5 – –

59.7 66.3 – –

8/48.1/8.1 4/7.9/35.0 – –

– – – – – – – – – – – – –

– – – – – – – – – – – – –

– – – – – – – – – – – – –

– – – – – – – – – – – – –

– – – – – – – – – – – – –

– – – – – – – – – – – – –

– – – – – – – – – – – – –

– – – – – – – – – – – – –

– – – – – – – – – – – – –

– – –

– – –

– – –

216.0 221.7 220.6

72.9 75.9 76.4

9/173.3/3.9 6/210.1/4.6 5/202.9/5.4

228.9 222.0

70.3 70.2

5/77.2/8.8 35/34.7/4.2

Komsomolsk-on-Amur Western locality BK43 138°52.60 BK44 138°52.60 BK45 138°54.40 BK46 138°54.60

49°09.00 49°09.00 49°09.50 49°09.50

s.s. s.s. s.s. s.s.

38.6 216.6 219.6 242.6

88 94 47 18

12.8 ± 11.8 88.5 ± 87.0 309.1 ± 431.0 116.3 ± 210.5

Central locality BK52 139°12.90 BK53 139°12.70 BK54 139°12.70 BK55 139°12.70 BK56 139°12.40 BK57 139°12.40 BK58 139°12.40 BK59 139°12.40 BK60 139°12.40 BK61 139°12.40 BK62 139°12.40 BK63 139°12.30 BK64 139°12.40

49°08.00 49°07.90 49°07.90 49°07.90 49°08.20 49°08.20 49°08.20 49°08.20 49°08.20 49°08.20 49°08.20 49°08.80 49°09.00

s.s. s.s. s.s. s.s. s.s. s.s. s.s. s.s. s.s. s.s. s.s. s.s. s.s.

255.6 203.6 325.6 174.6 177.6 197.6 197.6 175.6 192.6 192.6 179.6 178.6 198.6

158 115 128 120 103 100 100 110 113 105 107 72 74

19.2 ± 30.5 14.6 ± 24.6 3.8 ± 2.1 3.1 ± 1.4 6.1 ± 4.3 3.6 ± 1.6 5.3 ± 2.0 5.3 ± 4.9 5.3 ± 4.5 1.6 ± 0.9 1.6 ± 1.0 32.7 ± 81.0 6.2 ± 5.2

Eastern locality BK49 139°16.60 BK50 139°16.90 BK51 139°17.10

49°06.80 49°06.80 49°06.70

s.s. s.s. s.s.

165.6 161.6 187.6

68 75 88

34.8 ± 14.9 142.9 ± 246.7 20.1 ± 12.4

43.5 – –

65.4 – –

6/59.7/8.7 – –

Site Sample

38.0 39.5

69.0 68.3

4/252.7/5.8 24/22.5/6.4

Mean

MTC + HTC (200 ° C  560 °C)

MTC (200 °C  350 °C)

Site mean direction of BK44 (shown by asterisk⁄) is not used for the formation mean calculation because of small number of samples.

The formation of pyrrhotite as an iron sulfide mineral in the Zhuravlevka-Amur Terrane could be linked to post-depositional diagenetic processes. The presence of sulfide minerals has been associated with top column of the sedimentary layer (e.g. Berner, 1984; Roberts and Weaver, 2005; Rowan et al., 2009). Some workers argue that pyrrhotite formation can occur any time after deposition because its growth depends on changes in pore water chemistry as a result of some external forces (e.g. Robinson, 2000; Rowan and Roberts, 2006). Laboratory experiments (Schoonen and Barnes, 1991; Lennie et al., 1995) show that formation of pyrrhotite is fairly rapid at 180 °C temperature under appropriate diagenetic conditions. Geologic observation of the Zhuravlevka-Amur Terrane indicates that sedimentary strata in the Badzhal and Sarmarka paired zones experienced moderate burial temperature. A lenticular texture of shale observed in the Badzhal paired zone suggests a deeply buried metamorphic environment for turbiditic sediments. The colors of mudstones and sandstones in Sarmarka paired zone (e.g., sites BK43
46 and BK56
BK61) also imply thermal alteration of the sedimentary strata in the Zhuravlevka-Amur Terrane, likely associated with pyrrhotite formation under an appropriate authigenic condition. Magnetite is also expected in sedimentary rocks where abundant of iron sulfide are present (Brothers et al., 1996; Rowan and Roberts, 2006). Formation of coexisting magnetite and pyrrhotite is a common diagenetic phenomenon as reported by Woods et al. (2000) from Jurassic sediments in Scotland. Laboratory experiments by Brothers et al. (1996) suggest that pyrite is replaced by magnetite. We conclude that authigenic magnetite is the carrier of secondary remanent magnetization in the rocks of the Zhuravlevka-Amur Terrane. The pyrrhotite and magnetite carry secondary remanent magnetizations in these rocks are attributed to a chemical remanent magnetization (CRM). Although the timing of pyrrhotite and magnetite formation is post-folding, the age of folding is uncertain (Kirillova and Anoikin, 2011; Zyabrev, 2011). The Early Cretaceous to Quaternary is the time interval assigned

Fig. 5. Equal-area projections of the site mean directions from Vaninsky area: (a) the medium temperature component (MTC) and (b) the high temperature component (HTC). Mean directions for both these components are shown by green and red stars with 95% confidence circle. Solid (open) symbols indicate positive downward (negative upward) inclinations. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

to the acquisition of secondary remanent magnetization in the Zhuravlevka-Amur Terrane.

6.2. Anomalous paleomagnetic direction from the Zhuravlevka-Amur Terrane in the Badzhal paired zone An anomalous direction (D = 127.5°) of magnetization is observed in rocks at the Komsomolsk-on-Amur area of the Zhuravlevka-Amur Terrane (Figs. 2 and 4). However, the mean in-situ inclination of this secondary NRM (67.6 ± 9.3°) is consistent with that of the present geomagnetic field (I = 65.6°) as well as the

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95

Fig. 6. The IRM acquisition and thermal demagnetization of the composite IRMs for the selected samples from the Zhuravlevka-Amur Terrane. (a) The Komsomolsk-on-Amur area, and (b) the Vaninsky area. The composite IRMs are imparted by DC fields of 2.7 T, 0.4 T and 0.12 T along three perpendicular axes.

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Fig. 7. Strong field thermo-magnetic analysis of the selected samples in air condition. (a) and (b) represent the Komsomolsk-on-Amur area, while (c) and (d) the Vaninsky area. Arrows indicates the heating (red) and cooling (blue) curves. Ms(T)/Mso is normalized magnetization. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 8. Photographs of reflected-light microscopy for representative samples. (a) Pyrite occurring in veins (BK44-4), (b) pyrite forming framboidal aggregates (BK21-3), and (c) pyrrhotite (BK43-7).

present axial dipole field (I = 67.0°) in the study area. This mean inclination is also, notably, not different from the inclination (I = 71.1°) expected from the Early Cretaceous (130 Ma) East Asian Paleomagnetic pole (Cogné et al., 2013) (Fig. 7). This paleomagnetic direction could either be attributed to instantaneous changes in geomagnetic field behavior (i.e., geomagnetic polarity change and/or excursion) or to tectonic rotation.

Because CRM in pyrrhotite, in some circumstances, occurs within 1000 year during the diagenesis (Kodama, 2012), the CRM provides a snapshot of the geomagnetic field. As reported by Laj et al. (1991) and Love (1998), paleomagnetic poles during the geomagnetic field excursion or polarity inversion generally pass through the preferred bimodal longitudinal belts of East Asia (120 ± 28°E) and America (300° ± 28°E). However, as shown in Fig. 9, a pole position

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Fig. 9. Projection of a paleopole (red solid circle) with 95% confidence circle for Komsomolsk-on-Amur area (star) of the Zhuravlevka-Amur Terrane. The Early Cretaceous paleomagnetic poles (130 Ma) from Europe (Besse and Courtillot, 2002) and East Asia (Cogné et al., 2013) are shown by blue and green squares, respectively. Two longitudinal belts through East Asia (120° ± 28°E) and America (300° ± 28°E), shown by dotted pattern, are the preferred bimodal for paleomagnetic poles during polarity change and excursion. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

of the present anomalous direction from Komsomolsk-on-Amur area is located at 21.2°N, 170.7°E (A95 = 14.2°), which clearly falls off the preferred paths. The observed discrepancy between the pole position from the study area and the preferred longitudinal belts cannot be ascribed to geomagnetic field variation, rather we claim clockwise tectonic rotation about the vertical axis. The magnitude of clockwise rotation is estimated here by comparing the observed declination (D = 127.5°) with that expected from the East Asian APWP (Cogné et al., 2013) for a period between 130 Ma and the present. A maximum amount of 127.5° ± 7.4° clockwise rotation is obtained when compared with expected direction calculated from the present rotation axis, while a minimum amount of 117.6° ± 8.3° is obtained when compared with expected direction calculated from 130 Ma pole position, implying that the Komsomolsk-on-Amur area, after acquisition of the secondary remanence, experienced a clockwise tectonic rotation of over 90°. Large degree clockwise deflected declination (D = 135.2°) is also obtained from the remagnetized rocks at the Ozer Bolon Lake area in the Zhuravlevka-Amur Terrane, located about 70 km from Komsomolsk-on-Amur area. We assert that the large clockwise tectonic rotation is characteristic of a fairly large area in the Zhuravlevka-Amur Terrane (i.e., between the Komsomolsk-on-Amur and Bolon Lake areas). 6.3. Tectonic model to explain observed large clockwise rotation Recently, two paleomagnetic data sets of Early Cretaceous age have been reported from the Kiselevka-Manoma Terrane in the Sikhote-Alin Superterrane (Didenko et al., 2014) and the West Sakhalin Basin (Abrajevitch et al., 2012). Low paleolatitudes of 18°N and 28°N, respectively, have been determined from these origin data sets which are based on primary magnetizations. To justify such low paleolatitudes, significant northward displacement of these terranes is required since the Early Cretaceous. Several reconstruction models are postulated to explain the northward

displacement of the Jurassic–Cretaceous accretionary complexes in the Amurian Continent (Khanchuk, 2001; Kato and Saka, 2006; Malinovsky et al., 2008; Abrajevitch et al., 2012; Didenko et al., 2014). The left lateral Central Sikhote-Alin strike slip fault and other parallel faults in the area have been noted for their role in displacing the terranes northward. However, these models do not involve large clockwise tectonic rotation. We discuss two possible models (Fig. 10) to explain the observed clockwise rotation in the Zhuravlevka-Amur Terrane involving either; (1) internal block rotation or (2) rotation of the entire terrane. In the former case, the terrane would be divided into several small blocks (ball bearing model, bookshelf structure) separated local faults and then each block rotated in a clockwise sense with dextral strike slip faulting during northward displacement (e.g. Garfunkel and Ron, 1985; Nur et al., 1986; Ron et al., 1986; Schreurs, 1994). However, in the previously reported models (Khanchuk, 2001; Malinovsky et al., 2008), northward displacement was attributed to sinistral movement along the Central Sikhote-Alin Fault. The former model involving dextral faulting clearly contradicts with sinistral fault movement in the SikhoteAlin Superterrane. Clockwise rotation as a single coherent terrane is considered the most plausible tectonic model for the Zhuravlevka-Amur Terrane. Post-Early Cretaceous clockwise rotation is observed in the Kiselevka-Manoma Terrane (Didenko et al., 2014) and Western Sakhalin Basin (Abrajevitch et al., 2012) as well as the Zhuravlevka-Amur Terrane of the Badzhal paired zone (Fig. 2). A large east deflected declination (D = 83.6°) is reported by Didenko et al. (2014) from Lower Cretaceous volcanic and sedimentary rocks of Kiselevka area (51.4°N, 139.0°E) (KS in Fig. 2), the Kiselevka-Manoma Terrane. Clockwise rotations of 66.3° and 58.1° are reported by Abrajevitch et al. (2012) from the Early to Late Albian sedimentary rocks of the Naiba River area (N in Fig. 2), Western Sakhalin Basin. Thus, these data suggest that large clockwise rotations (40–128°) are predicted for the tectonic blocks

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Fig. 10. Schematic reconstructions of the Zhuravlevka-Amur Terrane, the Kiselevka-Manoma Terrane and the East Sakhalin accretionary wedge in Late Jurassic to Early Cretaceous. (a) Paleopositions and paleo-attitudes of the Zhuravlevka-Amur Terrane, the Kiselevka-Manoma Terrane and the East Sakhalin accretionary wedge during Early Cretaceous. Paleolatitudes of 18° ± 5°N and 28° ± 5°N are estimated for the Kiselevka-Manoma Terrane (KS) and West Sakhalin basin (A) using the data sets of Didenko et al. (2014) and Abrajevitch et al. (2012), respectively. These three terranes were extended in east–west direction during Early Cretaceous, while their declinations (arrows depicted as in Fig. 2) were directed toward north. Reconstruction of Asian continents is after Kravchinsky et al. (2002). The Zhuravlevka-Amur Terrane forms a paired zone with the Badzhal Jurassic accretionary wedge. Shaded area between the Siberian and Amurian continents was possibly a dry land in Early Cretaceous. Sib: Siberian Continent, Am: Amurian Continent, NCB: North China block, SCB: South China block, IDC: Indochina block. (b) A series of subduction zones, including the E–W extended belts, were probably located in the Panthalassa Ocean 200 Ma ago (van der Meer et al., 2012).

located along the eastern margin of the Amurian Continent later than Early Cretaceous. The steep inclination (I = 67.6° ± 9.3°) observed in the secondary magnetization of the Komsomolsk-on-Amur area (the Zhuravlevka-Amur Terrane) indicates that post remagnetization clockwise rotation took place at or very near to the present day latitude. Almost similar trend of inclinations (I = 70.2° and 68.3°) along with east deflected declinations (D = 222.0° and 39.5°) are observed in reverse + normal polarity secondary magnetizations in the Zhuravlevka-Amur Terrane of the Sarmarka paired zone. We conclude that the Zhuravlevka-Amur Terrane, the KiselevkaManoma Terrane and the Western Sakhalin Basin experienced clockwise rotation after their arrival at present day locations from low latitudes (Abrajevitch et al., 2012; Didenko et al., 2014). Our new data provide fresh insights into how the tectonic terranes are distributed along the eastern margin of the Amurian Continent. In the present day geographic framework, the ZhuravlevkaAmur Terrane, the Kiselevka-Manoma Terrane and the East Sakhalin accretionary wedge are distributed in a north–south direction with eastward opening to the Pacific Ocean (Fig. 1b). Reconstructed back to their Early Cretaceous orientations by 90° counterclockwise rotation, these terranes assume an east–west extended arrangement (Fig. 10a), and the northern margin of these accretionary complexes opened to an ocean. Oceanic plates were subducted southward along the northern margin of these terranes. This reconstruction thus calls for the subduction of the MongolOkhotsk or Panthalassa oceanic plates rather than the Izanagi oceanic plate. As inferred by Van der Meer et al. (2012) from seismic tomography, an east–west and north–south trending belts of intra-oceanic subduction zones separated the Panthalassa Ocean from Mongol-Okhotsk Ocean during the Jurassic (Fig. 10b). Tectonic situation of Fig. 10a is an analogy of an oceanic island arctrench system, such as, the Japan Arc-Japan Trench, Izu/Bonin Arc-Izu/Ogasawara Trench or Mariana Arc-Mariana Trench systems. These subduction zones may have caused the paired zone of turbiditic shelf sediments and accretionary wedges like the Zhuravlevka-Amur Terrane and Nadankhada wedge paired zone. After paired zones are rotated and accreted to the eastern margin of the Amurian Continent, the Sikhote-Alin volcanic series were formed from 80 to 50 Ma (Zonenshain et al., 1990; Otofuji et al., 1995; Matsuda et al., 1998; Alenicheva and Sakhno, 2008). Large clockwise rotation of the terranes within the Sikhote-Alin Supert-

errane is an important component of the tectonic evolution of East Asia and Panthalassa oceanic plate.

7. Conclusion The Lower Cretaceous shelf deposits and turbidites from Zhuravlevka-Amur allocthonous Terrane are Paleomagnetically studied to investigate post-Early Cretaceous tectonism of the eastern margin of the Amurian Continent. No primary magnetizations are identified in the rocks of the Zhuravlevka-Amur Terrane in the Badzhal and Sarmarka paired zones. Instead, pyrrhotite and magnetite carry secondary magnetizations that have with large east deflected declinations. The formation mean direction from the sedimentary rocks in the Komsomolsk-on-Amur area of the Badzhal paired zone is D = 127.5°, I = 66.7° (k = 28.2, a95 = 9.3). A large clockwise deflected declination (D = 135.2°) is also observed in the rocks of Ozer Bolon Lake area, which is located about 70 km from Komsomolsk-on-Amur area. Combined with large clockwise deflected declinations reported from Lower Cretaceous rocks of the Kiselevka-Manoma Terrane (D = 83.6°) and the Western Sakhalin Basin (D = 57.1°), a clockwise rotation of more than 60° is predicted for Jurassic to Cretaceous terranes in the eastern part of Amurian Continent Prior to clockwise rotation, the KiselevkaManoma and Zhuravlevka-Amur Terranes maintained an east– west orientation accompanied with southward directed oceanic plates subduction under their northern margins. Largemagnitude clockwise rotations discovered from the ZhuravlevkaAmur Terrane in this study should be incorporated into tectonic reconstruction models for the eastern margin of the Amurian continent.

Acknowledgements Comments from Rob Van der Voo, John Geissman, Baochun Huang and anonymous reviewers have improved this manuscript. We thank H. Inokuchi, K. Takemoto, K. Takaba and S. Sato for their help during the sampling trip. This work is supported by ‘The 21st Century COE Program of Origin and Evolution of Planetary Systems’ through the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT). In addition, this research is partly

R.J. Ichihashi et al. / Journal of Asian Earth Sciences 111 (2015) 88–99

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