Kinematics of recent tectonic motions in the east of the Mongol-Okhotsk Fold Belt

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ScienceDirect Russian Geology and Geophysics 57 (2016) 1688–1695 www.elsevier.com/locate/rgg

Kinematics of recent tectonic motions in the east of the Mongol–Okhotsk Fold Belt V.S. Zhizherin *, M.A. Serov Institute of Geology and Natural Management, Far Eastern Branch of the Russian Academy of Sciences, Relochnyi per. 1, Blagoveshchensk, 675000, Russia Received 3 March 2015; received in revised form 8 April 2016; accepted 26 April 2016

Abstract We present the first data of GPS measurements on recent motions in the east of the Mongol–Okhotsk Fold Belt. Processing of GPS data yielded a vector field of the velocities of observation point displacements in the geodynamic network of the Upper Amur region. Comprehensive analysis of geological and geophysical data and the estimated velocities of site displacements in this network have shown kinematic heterogeneity of the present-day Mongol–Okhotsk Fold Belt and an intricate deformation pattern there. The tectonic regime within the belt permits considering it a buffer or transit zone, where tectonic stresses arise as a result of the different kinematics of the surrounding tectonic structures. © 2016, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: modern geodynamics; GPS geodesy; crustal deformations; Mongol–Okhotsk Fold Belt

Introduction Study of recent large tectonic structures of East Asia by GPS measurements was and remains one of the top-priority methods of geological exploration of this region (Antonovich, 2005; Ashurkov et al., 2011; Gatinskii et al., 2008; Genike and Pobedinskii, 2004; Gerasimenko, 2000; Imaev et al., 2005; Lukhnev et al., 2013; Miroshnichenko et al., 2008; Timofeev et al., 2014; Voitenko et al., 2007; Wang et al., 2009; Zonenshain et al., 1990). The general regularities of motions of major tectonic blocks have been established, whereas the kinematics of certain crustal sites, especially those located at the junction of large structures, is still debatable. The zone of junction of the Eurasian and Amurian Plates is poorly studied in terms of geodynamics. It is a wide (up to 400 km) band of activity of endogenous processes (Gatinskii et al., 2008; Malyshev et al., 2007; Shevchenko and Kaplun, 2007). According to modern concepts, this band, stretching from Lake Baikal in the west to Uda Bay of the Sea of Okhotsk in the east, is bounded by the Olekma–Stanovoi seismic zone in the north and by the Tukuringra–Dzhagdy seismic zone in the south. The established velocity of the * Corresponding author. E-mail address: [email protected] (V.S. Zhizherin)

Amurian Plate displacement relative to the Eurasian Plate is low, 1–3 mm/year (Timofeev et al., 2011). Though their kinematic parameters are virtually identical, the zone of their junction is clearly expressed in the deformation field gradients (Kreemer et al., 2003), fault density (State..., 2009), and seismicity. The main geologic structure, which can be called a “suture” structure (i.e., marking the site of collision of continental blocks in the zone of junction of the Eurasian and Amurian Plates), is the Mongol–Okhotsk Fold Belt. Data on present-day crust motions within this belt were absent until our research. Extrapolation of GPS velocities obtained for other zones of the Amurian Plate (Ashurkov et al., 2011; Miroshnichenko et al., 2008; Timofeev et al., 2011) cannot yield a clear and detailed deformation pattern of the study area. In this work we present the first data on present-day tectonic motions in the east of the Mongol–Okhotsk Fold Belt, which were obtained by GPS measurements in the geodynamic network of the Upper Amur region.

An overview of the structure of the Mongol–Okhotsk Fold Belt The studied site of the Mongol–Okhotsk Fold Belt is located between two most active faults of the Upper Amur

1068-7971/$ - see front matter D 201 6, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.201 + 6.04.008

V.S. Zhizherin and M.A. Serov / Russian Geology and Geophysics 57 (2016) 1688–1695

region, North Tukuringra and South Tukuringra. By kinematics, these are sinistral and reverse faults (Nikolaev et al., 1979). The faults separate the study site from the southern framing of the North Asian craton and from the Argun massif, respectively. The width of the belt varies slightly along its strike: from 5–10 km on the western flank to 20 km on the eastern one. The Mongol–Okhotsk Fold Belt is one of the largest structures in East Asia. At present, it is regarded as a relic of the Mongol–Okhotsk Paleoocean (Parfenov et al., 1999, 2003). In present-day structural plan the belt is an E–W striking suture zone composed of mostly volcanoterrigenous and terrigenous rock complexes of terranes of the accretionary wedge (Parfenov et al., 1999, 2003; Sorokin, 2001). The age of these complexes is debatable (see the review in (Parfenov et al., 1999)); on present-day geological maps (State..., 2009) they are assigned to the Middle and Upper Paleozoic. The presence of basaltoids of different geochemical types, fragments of ophiolites (Sorokin and Dril’, 2002), and gabbro-granitoid complexes of different age (from Ordovician to Late Permian) (Sorokin et al., 2003, 2007) in the belt testifies to the long intricate history of its formation. This is confirmed by results of paleomagnetic studies (Kravchinsky and Sorokin, 2001; Kravchinsky et al., 2002; Metelkin et al., 2007), showing the existence of an area between the southern margin of Siberia and the continental massifs on the southern framing of the Mongol–Okhotsk Fold Belt in the Paleozoic. The time of the closure of the Mongol–Okhotsk Paleoocean and, correspondingly, the final formation of the Mongol–Okhotsk Fold Belt is the subject of discussions. Analysis of the structure of Mesozoic sedimentary basins and the chronology of magmatism on the belt framing shows that the eastern branch of the ocean was closed no later than the Late Jurassic (Parfenov et al., 1999, 2003; Smirnova et al., 2014; Sorokin and Kudryashov, 2013; Sorokin et al., 2004). If we relate the Late Mesozoic metamorphism events in the belt framing to the continuing formation of the folded structure of the belt, then the time of collision processes should be prolonged to the end of the Neocomian (Kotov et al., 2013, 2014; Larin et al., 2006, 2014; Sal’nikova et al., 2006). The neotectonic stage of evolution of the Upper Amur region was due to the continuing convergence of the Eurasian and Amurian Plates at an acute angle. The recent tectonic activity, which started in the Neogene after the prolonged Late Cretaceous–Paleogene peneplanation (Imaeva et al., 2012), resulted in systems of uplifts and intermontane troughs of different strikes. The evolution of these systems cut by numerous faults that are still seismically active governs the present-day topography of the study region. Interplate and interblock motions gave rise to present-day fault–block structures, which evolve on earlier tectonic units. Large active systems of faults (South Tukuringra, North Tukuringra, Dzheltulak, and Stanovoi) favored the further development of intermontane and piedmont troughs. Geophysical data (Malyshev et al., 2007; Shevchenko and Kaplun, 2007) revealed evidence for recent horizontal displacements

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of lithospheric blocks and spatial superposition of compression and extension zones in the area of the fault influence. Geomorphological research within the Tukuringra–Dzhagdy zone (Nikolaev et al., 1979) showed that the South Tukuringra fault, along which the Mongol–Okhotsk Fold Belt borders upon the Argun massif, shifts the Pleistocene–Holocene deposits in the headwaters of the northern tributaries of the Urkan River westward (i.e., is a sinistral fault). With the maximum amplitude of displacements equal to 0.8 km, their average velocity is evaluated at 5–6 mm/year. Results of geophysical research evidence that the calculated average crust thickness within the Mongol–Okhotsk Fold Belt is 40–42 km, i.e., is typical of continental crust (Didenko et al., 2010). The depth of occurrence of the lithosphere base gradually increases from 85 km in the east to 100 km in the west (Malyshev et al., 2007). The crust within the belt is composed of more compact rocks as compared with the Argun massif, 2.842 g/cm3, reaching 2.862 g/cm3 on the northern framing. The density of the lithospheric mantle is persisted in the strike, 3.289 g/cm3; close values were also obtained for other tectonic units making contact with the Mongol–Okhotsk Fold Belt (Didenko et al., 2010). In the geoelectrical fields (Kaplun, 2006), the Mongol–Okhotsk Fold Belt is expressed as a number of electroconductive layers. The 20 km thick upper-crust layer with a resistivity of 1250 Ohm⋅m is underlain by the 15–20 km thick lower-crust layer with a resistivity of 570 Ohm⋅m, which, in turn, is underlain by a thin (5–10 km) layer with an extremely low resistivity, 25–35 Ohm⋅m. The lithosphere base bordering upon the 50–70 km thick lithospheric mantle with a resistivity of 1600–1800 Ohm⋅m is sharply bent. Thus, the above parameters of geoelectrical section indicate tectonic layering of the lithosphere and suggest different levels of block motion. This is indirectly evidenced by the absence of recorded earthquakes with the depth of the hypocenter location of more than 40 km. The Mongol–Okhotsk Fold Belt is clearly visible on a scheme of the gravity field (State..., 2009), where it is expressed as a belt of positive anomalies reaching 65 mGal. A negligible negative anomaly on the scheme corresponds to the central part of the belt. The northern and southern framings of the belt have E–W striking zones of medium positive anomalies. The belt is characterized by an unsteady thermal regime (Gornov et al., 2009). Heat flow reaches a maximum, 82 mW/m2, in its central area and slightly weakens south- and northward. The belt lies in a highly seismic zone; the highest seismic-energy density is detected on its eastern and western framings. The above data on the structure of the Mongol–Okhotsk Fold Belt (heterogeneous geologic structure, layered lithosphere, high heat flow, and high seismicity) indicate the existence of zones where stresses caused by the interaction of conjugate tectonic structures are highly probable. These zones, or sites with low mechanical strength, should be areas with the maximum amplitudes of horizontal and vertical motions.

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Fig. 1 (to be continued).

V.S. Zhizherin and M.A. Serov / Russian Geology and Geophysics 57 (2016) 1688–1695

Fig. 1 (to be continued).

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Fig. 1. Time series for the points located in the study area of the Mongol–Okhotsk Fold Belt and for the nearby points (a–j).

Methods of study The present-day kinematics within the Mongol–Okhotsk Fold Belt was studied by calculation of the observation point displacements in the Upper Amur geodynamic network founded by us together with the colleagues from the Institute of the Earth’s Crust (Irkutsk) in 2007. Four observation points (TAHT, SOLO, SOSN, and PIKA) have been put into action by now (Fig. 2). These are special reference objects (rods made of alloy steel) fixed in the bedrock or concrete base. The sites for them were chosen so that the influence of slope, cryogenic, and landslide processes is eliminated, rather far from any human-activity objects. Measurements at the reference points were usually performed once a year (in the period from 2007 to 2013), using Ashtech UZ-12 satellite receivers equipped with choke ring antennas. Information was recorded by receivers with 30 s intervals for at least 36 h, which is the world standard for field works (Antonovich, 2005; Herring et al., 2010; Voitenko et al., 2007). To minimize the impact of seasonal variations on the positioning accuracy, all field works were carried out since August till September. The obtained GPS data were processed using the GAMIT/GLOBK software package (Herring et al., 2010).

During the processing, we used navigation files, precise satellite orbits, and RINEX files of the IGS stations, downloaded by the software package from the NASA server [cddis.nasa.gov], in order to establish the true coordinates of the points. To determine the reference system, we used data on the location and velocities of displacement of at least 25 IGS stations involved in the ITRF2008 reference system.

Results and conclusions Processing of GPS measurement data yielded time series for each observation point. Below we present the time series for the observation points located in the study area of the Mongol–Okhotsk Fold Belt and for the nearby points (Fig. 1). We also calculated a vector field of the velocities of observation point displacements in the Upper Amur geodynamic network (Table 1). A comprehensive analysis of the velocities and geomorphological and geophysical data shows a predominance of deformations caused by the crust extension on the western flank of the study area (Fig. 2), whereas its central part and eastern flank are zones of tectonic compression with a velocity of few mm/year. Based on the distribution of earthquake focal mechanisms, Imaev et al. (2005) and

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Fig. 2. Schematic map of structural zoning of the Upper Amur region (State..., 2009). Major tectonic structures of the region: 1, Argun continental massif; 2, 3, structures of the Selenga–Stanovoi superterrane, blocks: 2, Urkan, 3, Mogocha; 4–7, structures of the Dzhugdur–Stanovoi superterrane, blocks: 4, Larba, 5, Bryanta, 6, Ilikan, 7, Dambuka; 8, 9, suture zones: 8, Dzheltulak suture zone, 9, Mongolo–Okhotsk Fold Belt; 10, faults of different ranks, the most active ones: ST, South Tukuringra, NT, North Tukuringra, DZh, Dzheltulak. Vectors (arrows) of the velocity of the observation point displacements relative to the SKOR point are shown by 95% confidence interval ellipses.

Imaeva et al. (2012) reported a similar westward change of tectonic regimes in the area of transition from the northeastern flank of the Baikal Rift system to the Stanovoi zone. The calculated vectors of observation point displacements within the Mongol–Okhotsk Fold Belt differ considerably from each other both in direction and in amplitude, which indicates an intricate deformation pattern. Thus, the tectonic regime within the Mongol–Okhotsk Fold Belt and its specific geophysical structure permit it to be considered a site of a buffer or transit zone of tectonic stress caused by the difference in the kinematics of surrounding tectonic structures. The structure of transit zones and their role in the geodynamics of continental lithosphere are described in detail by Gatinskii et al. (2008). Publications concerned with measurements of present-day motions focus major attention on the assessment of the influence of strong earthquakes on the field of the motion velocities. The 14 October 2011 Skovorodinskoe earthquake (Khanchuk et al., 2012) is among the strongest ones in the Upper Amur region throughout the history of instrumental

seismological observations. This is an anomalous earthquake, because, as seen from the macroseismic-field data, its magnitude at the epicenter disagrees with the distance of recording of noticeable vibrations and the epicenter is shifted relative to the mapped exposed segments of the South Tukuringra fault (Khanchuk et al., 2012). The above time series for the SOLO point Fig. 1f) located at ~50 km to the northeast from the earthquake epicenter partly confirms the conclusions on the earthquake focal mechanism drawn by Khanchuk et al. (2012), namely, an E–W striking sinistral shear, and also shows the contribution of a N–S striking shear. In general, the deviation of the SOLO point displacements from the trend values in the period of measurements from August 2011 to August 2012 reaches 20 mm in the northern direction and 20 mm in the western one. However, the time series for the SKOR point located at 22 km to the southeast from the earthquake epicenter (Fig. 1e) does not show any deviations from the trend displacements during the measurements made before and after the earthquake. This, in turn, testifies to the high mobility of the Mongol–Okhotsk

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Table 1. Horizontal velocities of GPS point displacements in the geodynamic network of the Upper Amur region (ITRF2008) Coordinates of points, deg.

Components of displacement velocities, mm/year

Errors of determination of velocity components, mm/year

N

E

east

north

east

north

127.43

53.77

25.44

–15.66

0.89

1.25

Point

Observation duration, years

PIKA

3

127.28

53.75

24.55

–16.88

0.39

0.48

ZEYA

3

125.80

53.46

24.44

–13.54

0.30

0.39

MAGD

5

125.50

55.27

24.40

–14.64

1.20

1.69

JIVO

3

124.94

55.51

26.38

–11.21

1.21

1.53

MOGO

4

124.90

54.19

25.47

–10.87

0.86

1.24

SOSN

3

124.89

54.03

22.06

–11.01

0.89

1.21

BUGO

3

124.75

55.15

20.30

–13.88

0.37

0.48

TIND

5

124.64

54.53

22.31

–12.39

0.55

0.69

DJEL

5

124.55

53.75

22.64

–12.30

0.26

0.34

TALD

5

124.46

54.29

30.57

–19.03

0.80

0.95

SOLO

4

124.20

55.21

29.67

–14.50

0.29

0.38

KUVI

5

124.11

53.97

24.79

–16.27

0.27

0.35

SKOR

6

123.80

54.56

27.19

–13.40

1.23

1.83

ANOS

3

123.78

54.26

23.49

–15.95

0.93

1.21

TAHT

3

123.20

55.35

23.13

–9.98

0.62

0.83

URKI

4

122.91

54.03

24.82

–12.55

0.32

0.42

URUH

5

121.96

53.99

22.24

–13.53

0.65

0.83

EROF

4

Note. The errors of velocity determination are given in the 95% confidence interval.

Fold Belt and the relative stability of the northern part of the Argun continental massif. This work was supported by grant 13-05-00190 from the Russian Foundation for Basic Research.

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Editorial responsibility: A.D. Duchkov