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Электронное научное издание Альманах Пространство и Время. Т. 4. Вып. 1 • 2013 Специальный выпуск СИСТЕМА ПЛАНЕТА ЗЕМЛЯ
Кора — мантия — ядро
Crust — Mantle — Core / Krusten — Mantel — Kern
Yu.G. Gatinsky T.V. Prokhorova D.V. Rundquist G.L. Vladova
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Yu.G. Gatinsky
T.V. Prokhorova
D.V. Rundquist
G.L. Vladova
Up-to-Date Block Structure of Central Asia in Geophysical Fields
*Yury G. Gatinsky, D.Sc. (Geology and Mineralogy), Vernadsky State Geological Museum RAS (Moscow), Main Scientific Researcher
E-mail: [email protected], [email protected]
**Tatiana V. Prokhorova, International Institute of Earthquake Prediction and Mathematical Geophysics RAS (Moscow), Scientific Researcher
E-mail: [email protected]
***Dmitry V. Rundquist, D.Sc. (Geology and Mineralogy), Professor, Academician of Russian Academy of Science, Vernadsky State Geological Museum RAS (Moscow), Main Scientific Researcher
E-mail: [email protected]
****Galina L. Vladova, Sc.D (Geology and Mineralogy), International Institute of Earthquake Prediction and Mathematical Geophysics RAS (Moscow), Senior Scientific Researcher
E-mail: [email protected]
During 2004—2009 authors worked out a problem of up-to-date geodynamic heterogeneity of the Eurasian continent with establishing the north Eurasian lithosphere plate and some transit zones between it and neighboring plates. The zones consist of numerous blocks limited by active faults, and what's more the maximal tectonic activity coincides with interblock zones. Since 2009 we fulfilled the closer definition of block boundaries and interblock zones in central Asia. The majority of active faults and epicenters of the strongest earthquakes coincide with them, so their detail investigation and correlation with different geophysical fields are important for establishing the level of the seismic activity in this region. In the seismic energy field the maximal volume of energy release in plate boundaries and interblock zones of the central Asian transit zone. In the field of up-to-date tectonic stress the compression distinctly predominates in this transit zone and changes partly on extension and slipping with extension in the east Asian zone. High positive anomalies of the magnetic field (up to +50...+100 nT) characterize the great part of interblock zones and large faults limited them. The gravitational field of the most part of central Asia in the Bouger reduction is characterized by negative values up to -50.-150 mGal. The distinct extending gravitational lineament crosses in NNE direction the significant part of the continent from the Bacbo Bay to the Okhotsk Sea coast with changing of above mentioned negative values by more positive in the east. This change is connected with the sharp decreasing of the continental crust thickness. Heat-flow values increase up to 80-100 |jW/m2 and more in interblock zones, which are situated in boundaries of Hangay, Amu-rian, Tibet's, and Tarim blocks as well as in some inner continental rifts. Some of heat-flow anomalies can be connected with mantle plumes under Hangay and north part of the Amurian Block. The crust thickness changes in central Asia from 25-30 km in the east up to 50—75 km in the west under Tibet and neighboring blocks. The lithosphere thickness changes in the same direction from 60—80 km up to 120—150 km, but it decreases up to 100 km and less under inner continental rifts coinciding with interblock zones. The direction of the P- and S-waves anisotropy shows within the great part of the region the coupling deformation in the lithosphere upper mantle and crust. Differently directed vectors of horizontal displacement are established in the crust and upper mantle only east of the east Himalayan syntax indicating decoupling these layers under the influence of the Hin-dustan-Asia collision and "a threshold" of the SE China thick lithosphere in its boundary with Tibet.
Keywords: Central Asian transit zone, areas of active faults and tectonic activity, interblock zones, mantle plume, lithosphere thickness, crust thickness, magnetic field anomalies, heat-flow anomalies, gravity anomalies, gravitational lineament.
Gatinsky Yu.G., Prokhorova T.V., Rundquist D.V., Vladova G.L. Up-to-Date Block Structure of Central Asia in Geophysical Fields
Introduction
Central Asia includes a territory situated between the Lake Baikal, upper Enisei, Ob and Irtish in the north, east Kazakhstan, Tienshan and the Pamirs in the west, southwest China and the Himalayas in the south, the middle course of the Amur River and costs of the east and south China seas in the east (Figure 1). The territory is characterized by a complicated geologic structure and history with development of numerous suture zones of different ages. They limit blocks joined to Eurasia (before that to Laurasia) from the late Precambrian to Cenozoic. Boundaries of blocks are formed now as a rule by active faults. The geodynamic heterogeneity of central Asia and whole Eurasia was established also by the analysis of seismicity, which resulted in the attribution a part of the continent to the north American Plate and revelation of the central Asian, Amuri-an, Okhotsk, Indochina and other subplates or blocks after works of [Molnar and Tapponier 1975; Zonenshain and Savostin 1981; Gatinsky 1986; Gatinsky and Rundquist 2004; Rundquist et al. 2005] and many others.
Figure 1. Up-to-date block structure of central Asia and adjacent territories. Red lines - active faults after [Xu and Deng, 1996; Trifonov et al., 2002; Yin, 2010; Sherman et al., 2011], blue lines — rivers. Boundaries: dark blue — lithosphere plates, green — blocks, yellow — interblock zones; violet — boundary between central Asian and east Asian transit zones; light blue — supposed boundaries [Gatinsky et al., 2005, 2009]. Blocks (numerals in figure): (1) Altai, (2) west Mongolian, (3) Ebi Nur, (4) south Gobi, (5) Tienshan, (6) Pamirs, (7) west Kunlun, (8) west Qinlin, (9) Qilian, (10) Taihangshan, (11) south Tibet, (12) Kam Dian, (13) Ryukyu - central Honshu, (14) Andaman's — west Myanmar, (15) north Luzon. Note, active faults emphasize the selection of majority blocks.
During previous investigations some transit zones were established, which divide main lithosphere plates of Eurasia [Gatinsky et al., 2005, 2007, 2009]. In the considering territory they are: the central Asian zone between north Eurasian and Indian plates and the east Asian zone dividing north Eurasian, Philippine and Pacific plates. Each of zones consists of numerous blocks, the independent existence of which is proved by the widespread development of active faults and epicenters of earthquakes ascertaining geodynamic heterogeneity of central Asia. In our paper we'll consider briefly the block structure of central Asia, which was made more precise during last year's, and after that shall try to examine its position, especially interblock zones, in various geophysical fields of this territory. The main purpose of our investigation is to establish the connection of modern seismicity including catastrophic earthquakes with these fields and with deep-seated structures of the Asian lithosphere.
Gatinsky Yu.G., Prokhorova T.V., Rundquist D.V., Vladova G.L. Up-to-Date Block Structure of Central Asia in Geophysical Fields
Block structure and kinematics of central Asia
The main blocks can be distinguished already by analysis of active faults in the considering territory (Figure 1). Among them following units are visible the most clearly: Hangay, Junggar, Tarim, Qaidam, Ordos, Amurian, north Tibet, SE China. By their composition blocks within transit zones differ each from other. Tarim, Junggar, Hangay, north and south Tibet, Ordos, and some others correspond to single tectonic units, as a rule to the old Precambrian massifs. Blocks like Tienshan, Sayan, Altai, south Gobi, Beishan, Qilian, east Kunlun, west Qinlin, and the Himalayas include fragments of some older fold belts. More large Amurian, SE China and Japanese-Korean blocks in the east Asian transit zone have a complex composition including different tectonic units: old massifs, fragments of fold belts, sometimes island arcs and deep water marginal basins. But in all cases the blocks are limited by active faults with rather high seismicity.
Note, that boundaries of some block up-to-now are discussed. So the east boundary of the Amurian Block (Amurian Plate in other interpretation) is drawn in this work along the active Tanlu Fault and its contuniation in the Russia territory (Figure 1). At the same time some geologists suppose, that it passes more east along the Sakhalin Fault and a segment of the Japanese subduction zone [Ashurkov et al. 2011]. But in such case the east part of the south boundary of this structure will cross nearly perpendicularly NNE active faults in the east China Sea and Korean Peninsula.
It is worth to note that the central Asian zone is broken down on relatively smaller blocks in comparison with the east Asian zone. It is more probably connected with the influence of the Hindustan indenter pressure. Besides that some scientists suppose existing here double subduction process: one to the north from the Indian Plate and the other to the south from Tarim and Qaidam [Kao et al. 2001; Chung et al. 2005]. So the Himalayas—Tibet orogenic belt can represent a compression region between two subduction zones. We think that due to such process model velocities of the horizontal displacement sharply diminish north of Tarim (Figure 2).
At the same time model vectors of horizontal displacement form a characteristic divergence with a west deflection (10° NE — 350° NW) near the west syntax of the Himalayas in NW Tibet and Tienshan and east deflection up to 50—70° NE and more near the east syntax in SE Tibet, Qaidam and in the west of the Sichuan Province of China. This divergence confirms Tibet's "crawling off" with rifting its central parts connected perhaps with moving aside of the crust material in front of the Hindustan indenter [Shen et al. 2000, 2005]. This process can be connected also with a possible influence of stress from the relatively rigid Tarim and SE China blocks. Some geologists explain the vectors divergence by a slab tear model, in which the Indian lithosphere has split into two slabs. A northward moving slab subducts steeply beneath the western sector of the Tibet Plateau and Pamirs, and a northeastward moving slab subducts at a low angle beneath the eastern sector of the Plateau and the Three Rivers region [Xiao et al. 2007; Hu et al. 2012; Liang et al. 2012].
The seismic energy field
The areal spreading of seismic energy intensity in central Asia (Figure 2) was calculated by the formula from [Kanamori and Anderson 1975]:
logEs = aMs+b, where a = 1.5, b = 11.8.
The magnitude (Ms) of surface seismic waves was taken from the NEIC Catalog (http://earthquake.usgs.gov/regional/neic) by summarizing all earthquakes in the considering territory for the period of 1966—2011 without exception for aftershocks. In our case, areas confined by 1x1 degree on either side were taken into account. The volume of energy in concrete interblock zones was considered for areas confined to a 50-km distance on either side of the block boundary. Both the total energy within each interblock zone and the specific energy per 1 km of the zone length or, at adopted parameters of the zone width and the hypocenter position, as a rule, not deeper than 40 km, per 4000 km3, were calculated.
The inner parts of lithosphere plates and large blocks in transit zones (Amurian, SE China and some others) are practically aseismic. Inside of transit zones the releasing energy volume changes from 1x10-2-1x105 J (Amurian Block) up to 1x1012-1x1015 J (Tienshan, Pamirs, Himalayas, north Tibet). The most active interblock zones limit the latter blocks and Bayan Har, where the majority of active faults are developed together with epicenters of strong earthquakes, as instrumental measured as historical beginning from VIII Century (Figure 2). A volume of seismic energy releasing along each of mentioned zones reaches > 5x1015 J, while along other boundaries it doesn't exceed 3x1012...2x1015 J. In the east boundary of the Bayan Har block it comes by our calculation to > 9x1016 J after the Wenchuan earthquake in May 2008 [Gatinsky et al. 2011]. The latter figure can be compared with the energy of the most active Pacific subduction zones [Gatinsky and Vladova 2008]. The same interblock zones are characterized by a maximal specific energy par 1 km of their length (> 5.3x1012 J) and by a maximal deviation of GPS vectors from average vector values of main plates. Seismic transects constructed across some of studied interblock zones within subduction structures it decreases up to 100 km and less it decreases up to 100 km and less [Gatinsky et al. 2009] show energy increasing in low horizons of the lithosphere in the case of the sharp slab deeping (west Pamirs, partly the Japanese-Korean Block) and relatively even distribution of the energy, when there is a shallow sloping slab (Himalayas, south and north Tibet). Contrast energy increasing characterizes also boundary faults of other interblock zones (Tienshan, Bayan Har, and north boundary of the Amurian block).
Gatinsky Yu.G., Prokhorova T.V., Rundquist D.V., Vladova G.L. Up-to-Date Block Structure of Central Asia in Geophysical Fields
Figure 2. The block structure of central Asia in the seismic energy field. Each increasing of the color intensity corresponds to increasing seismic energy volume on 1x10-1 or 1x1c1 J. Some energy values are shown in the scheme in jouls. Epicentres: circle — instrumental (NEIC), stars — historical from VIII to XIX century [Xu and Deng 1996]. Earthquakes magnitude: violet — 6.0—6.9, black —7.0—7.9, red — > 8.0. Arrows: red — experimental ITRF vectors of horizontal and blue — of vertical displacement, black — model vectors with respect to stable Eurasia. Boundaries, not signed blocks and names of towns see in Figure 1.
The stress field
The predominance of compression within the central Asian transit zone results in the development of the transpressive regime due to pressure from the Hindustan indenter. Comparison between focal mechanisms in epicenters of earthquakes by CMT data from the Catalog of Global Centroid Moment Tensor Project (http://www.globalcmt.org/CMTfiles.html), model and GPS vectors (http://itrf.ensg.ign.fr/ITRF_solutions/2008 /ITRF2008.php) shows the development in this zone mainly thrusts and slips with compression in the north — NNE direction (Figures 3 and 4). Velocities of the model displacement with respect to stable Eurasia decrease from 30—35 mm/yr near the Hindustan—Eurasia collision zone down to 4—10 mm/yr north in the Sayan Block. Experimental vectors in the ITRF System are directed mainly northeast with velocities from 48 mm/yr in the south of Tibet down to 23—25 mm/yr withdrawn from the collision zone.
Space-geodetic data and stress field on the east Asian transit zone differ noticeably from above-mentioned results. Experimental ITRF vectors are directed mainly 106°—121° SE with velocities 26—35 mm/yr east of the 102-103° E lineament (Figure 4), which approximately coincides with the most part of the boundary between central and east Asian transit zones [Rundquist et al 2004]. A transtension tectonic regime predominates in the East Asian zone with development of numerous rifts in the Baikal System, around the Ordos, inside the Japanese-Korean and SE China blocks.
The change of GPS vector directions can be the most distinct seen west and east of the south edge of the Lake Baikal, where it coincides with the change of the stress regiment (Figure 4). Transpression predominates in the west in Sayan Mountains, where NE thrusts are developed proved by orientation of stress axes. East from there in the Tunka Trough left-lateral strike-slips predominates, which is clearly seen in the displacement of streamlet thalwegs and left-lateral moving along the Main Sayan Fault after earthquake mechanisms. Further east strike-slips are replaced by normal faults in flanks of the Baikal Rift and Barguzin depression, which is also included in this rift system. The latter dislocations already correspond to the transtension tectonic regime [Gatinsky et al. 2009, 2011].
Gatinsky Yu.G., Prokhorova T.V., Rundquist D.V., Vladova G.L. Up-to-Date Block Structure of Central Asia in Geophysical Fields
Figure 3. The correlation between block structure of central Asia and types of the seismic stress after data on mechanisms in earthquake epicenters. For boundaries, town names and not signed blocks see Figure 1. Note the distinct predominance of compression in the central Asian transit zone (besides central part of Tibet, where tension is connected with rifting) and relatively wide development of tension in the east Asian zone.
Figure 4. Stress axes [World Stress Map 2008] and vectors of horizontal displacement in central Asia. Stress axes correspond to: thrust (blue), slip (green), and normal fault (red). Colored arrows show ITRF2008 vectors: GPS (red), DORIS (blue), and SLR (green). Black arrows correspond to model vectors in respect to stable Eurasia. 102—103° E lineament [Rundquist et al., 2004] is shown by the hatched gray stripe. For boundaries, town names and not signed blocks see Figures 1. Note the distinct prevalence thrusts in the central Asian transit zone, at first within the majority of blocks boundaries. The same type of stress prevails in Pacific subduction zones in the SE part of the figure. The noticeable change of ITRF and model vectors can be seen from north-NNE in the central Asian transit zone up to NE and SE in the east Asian zone.
Gatinsky Yu.G., Prokhorova T.V., Rundquist D.V., Vladova G.L. Up-to-Date Block Structure of Central Asia in Geophysical Fields
The mentioned sharp change of vectors and stress axes direction has different explanations: squeezing out some blocks including Amurian one to the east under influence of the collision process and a mantle flow generated by distant influence of the Pacific subduction slab [Barruol et al. 2008; Parfeevets and Sankov 2012], and besides that rising of mantle plumes underneath north Mongolia and the Lake Baikal [Grachev 2000; Gatinsky et al. 2009]. As concern the same changes of vectors in the more south region, some scientists suppose a gravitational rolling down of crust layers from the highly emerged Tibet Plateau [Copley 2008].
The magnetic field
In the magnetic field [World Digital... 2007] the majority of blocks are limited by high positive striped anomalies (up to + 50...+100 nT), which coincide with interblock zones and faults within them (Figure 5). These anomalies characterize some large faults, among them Altun Tagh on the boundary of Tarim, Qaidam and north Tibet, Tanlu between Amurian and Japa-nese-Korean blocks and the most part of the 102—103° E lineament. The same anomalies predominate over blocks Tarim, Junggar, Beishan and in Pacific subduction zones. Interchange of wider northeast strips take place over Ordos, Amurian and north China blocks, where magnetic field values fluctuates between +100.-100 nT. This field corresponds apparently to the strike of early Precambrian basement structures.
80° 90° 100° 110° 120° E
Near-surface data [nT]
-3700 -100 -50 -20 -10 -5 0 5 10 20 50 100 8300
Model data [п"П К
100 -50 -20 -10 -5 0 5 10 20 50 100
Figure 5. The block structure of central Asia in the magnetic field. Boundaries, names of blocks and lithosphere plates see in the Figure 1.
A poorly differentiated magnetic field is established over SE China and Tibet's blocks. Steady intensive negative anomalies (-20.-100 nT) are developed over Tibet and between Himalayas and the Indian plate. Interchange of intensive positive and negative anomalies is developed over Indian and north Eurasian lithosphere plates. Within the latter the Main Sayan Fault of the NW strike is distinctly seen west from the Lake Baikal by the contrast linear change of different sizes anomalies (Figure 5).
Gatinsky Yu.G., Prokhorova T.V., Rundquist D.V., Vladova G.L. Up-to-Date Block Structure of Central Asia in Geophysical Fields
The gravitational field
The gravitational field of the most part of Central Asia [Bonvalot et al. 2012] is characterized by negative values up to -50.-150 mGal (Figure 6). The most intensive negative anomalies coincide with Pamirs, Tienshan, Hangay, east Kunlun, north and south Tibet, and the most part of the Himalayas. High negative values of these anomalies correspond to significant increasing of the crust thickness of above-mentioned blocks.
Figure 6. The block structure of central Asia and adjacent territories in the gravitational field in the Bouger reduction. For boundaries and not signed blocks see Figure 1.
The distinct extending gravitational lineament crosses in NNE direction the great part of the continent from the Bacbo Bay in the south to the Okhotsk Sea coast in the north with changing of above mentioned negative values in the west by more high in the east (-50-0 mGal and up to +50 mGal in shelves). This change is connected with the sharp decreasing of the continental crust thickness in east Asia and in its coastal part. At the same time the connection of interblock zones and the lineament of 102—103° E with gravitational anomalies doesn't establish.
The heat-flow field
Heat-flow values often increase up to 80—100 |jW/m-2 and more under interblock zones, which limit Amurian, Tibet's, Tarim, Ordos, SE China and some other blocks. Maximal values correspond to interblock zones, which are in the tension regime with developing continental rifts, especially under the Lake Baikal (up to 80—120 jW/m-2) and under Ordos north boundaries (80— 160 jW/m-2). Other heat-flow anomalies are known under the boundary of Amurian and Japanese-Korean blocks along the large Tanlu Fault and in its flanks (up to 80—120 jW/m), to the lesser extent under the boundary of Tarim with Kunlun and Qaidam
Gatinsky Yu.G., Prokhorova T.V., Rundquist D.V., Vladova G.L. Up-to-Date Block Structure of Central Asia in Geophysical Fields
along the Altun Tagh Fault, at the boundary of Bayan Har with SE China blocks (up to 90—120 jW/m-2) and under some other interblock zones. At boundaries of south and north Tibet the temperature increasing near the Moho discontinuity up to 800— 1100 o C results in the wide development the Quaternary basalt volcanism [Gornov et al. 2009; Xia et al. 2011]. Heat flow values in rifts of the central part of the Tibet Plateau reach 150—190 jW/m-2.
But besides that we can see increasing of heat-flow values underneath inner part of some blocks: Tienshan (up to 100—140 jW/m-2), Hangay (up to 120—190 jW/m-2), Amurian (up to 80—100 jW/m-2), SE China (up to 120—190 jW/m-2), where they often coincide with areas of S-velocities slowing-down. This increasing supposedly corresponds to the influence of mantle plumes on the lithosphere of central Asia. One of these plumes is selected under Hangay and Sayan blocks at the depth 100— 150 km. Here the thickness of the lithosphere decreases down to 70—50 km, and the temperature at the depth of 50 km can be about 1000—1200° C according to correlation 3He/ 4He isotopes [Lysak 2009; Duchkov et al. 2010]. At the more depth down to 200-300 km this plume moves aside east under the Lake Baikal and Transbaikalia (Figure 7), where intensive Neogene's and Quaternary basalt volcanism can be connected with it [Grachev 2000]. Heat-flow anomalies under Qilian and east Qinlin blocks can be supposedly connected with the existence a low velocity channel of the asthenosphere material at the depth 125—200 km [Zhang et al. 2011].
Figure 7. The block structure of central Asia in the heat-flow and tomography heterogeneities field. Values of heat-flow are given from [Karta teplovogo... 1980; The Global Heat-Flow ... 2011]. Fields of the high heat-flow (> 80 pW/m-2) are limited by red lines on the basis of the values size and after works [Sokolova and Duchkov 2008; Tan and Shen 2008; Gornov et al. 2009; Lysak 2009; Duchkov et al. 2012]. Dotted red line corresponds to the suppose boundary of fields. Thin black lines of different types limit projections of S-waves' velocity slowing-down up to < 4.2—4.25 km/sec-1 on 50, 100, 150, 200, 250, and 300 km depth levels [Kozhevnikov, Yanovskaya 2005]. Note coinciding the majority of the high heat-flow areas with S-waves velocity slowing-down projections. For boundaries and not signed blocks see Figure 1.
Central Asian crust structure
Changes of the crust thickness and relation between its layers in central Asia are shown on the velocity profile passed from Altai up to SE China (Figure 8). One of sharp crust thickness gradients is noticeable at the boundary Altai and Junggar blocks
Gatinsky Yu.G., Prokhorova T.V., Rundquist D.V., Vladova G.L. Up-to-Date Block Structure of Central Asia in Geophysical Fields
with crust decreasing in southeast direction from 55—60 down to 35—40 km, at first due to the lower crust. One another can be seen in the interblock zone between Tarim and Qilian coinciding with the Altun Tagh Fault. There the crust grows in the same direction from 45—50 up to 65—70 km due to lower crust and anomalous low velocity layers. On the boundary Qaidam—Kunlun the crust also increases to SE from 50 up to 65—70 km. In the east coastal part of the Asian continent the crust thickness doesn't exceed 25—30 km [Zhang et al. 2011]. But it increases up to 46—75 km in the west under Ordos, Tibet and neighboring blocks [Hu et al. 2011; Yu et al. 2012]. This increasing is connected with crust doubling in consequence of un-derthrusting the Indian Plate under Asia.
Figure 8. The Earth crust structure profile in northwest and central parts of central Asia [Crustal structure of China... 2001] is composed by P-waves velocity (km/sec. — in the numerator at the profile) and the Poisson's ratio (in the denominator). Colors correspond to the upper (5.9—6.3/0.21—0.26), middle (6.25—6.5/0.24—0.27), and lower (6.7—7.0/0.26—0.29) crust. The not colored horizon indicates an anomalous layer between the middle and lower crust. This layer is developed under north Tibet and north from it and composed by relatively heated material of the low viscosity (5.8—5.9/0.25). 7.0—8.0 P-waves velocities characterize the lithosphere mantle. The vertical exaggeration at the profile is 5:1. The paper authors supplemented the profile by blocks names. Note the visible increasing of the crust thickness in the SE part of the profile under Kunlun and Bayan Har blocks connected with the Indian slab subduction under north Eurasia.
The total 40 km crust thickness remains at both sides of the interblock zone between Bayan Har and SE China blocks, but at the same time the upper crust twice increases there to SE together with noticeable decreasing the middle and especially lower crust (Figure 8). The east boundary of the Bayanhar Block coincides with a global lineament of 102—103° E [Gatinsky et al. 2008, 2011], which passes here along the Longmen Shan fault zone. It is just the same zone, where catastrophic earthquakes occurred in May 2008 and April 2013. A steep step corresponds to this fault in the crust and whole lithosphere [Yuan et al. 2000].
The sharp increasing of the upper crust east of the Longmen Shan Fault in SE China Block is supposedly connected with flowing and uprising there the plastic material, composing the lower and middle crust west under Bayan Har and Tibet [Li et al. 2011; Zhang et al. 2009]. Because of that the upper crust tears away and moves independently east along the more plastic middle and low crust [Flesch et al., 2005; Shen et al. 2005]. This process is proved by existing layers of high electric conductivity in the south Tibet crust at the 20—45 km depth, which supposedly corresponds to the partial melting of the crust material [Li et al. 2003; Solon et al. 2005; Oreshin et al. 2011]. Mentioned data show the direct connection of interblock zones with crust thickness gradients, especially in boundaries of Amurian, Ordos, Bayan Har and Tibet's blocks.
The central Asia lithosphere and its kinematics
As concern the whole lithosphere thickness it comes in the east continental margin of Asia to not more than 60—80 km. That was established by studying upper mantle xenoliths and can be connected with thinning out the lithosphere due to intensive Mesozoic tension processes [Chen 2009;Li and Yang 2011;Zhang et al. 2012]. Tomography data and analysis of the hypocenters depth make it possible to suppose that the Pacific subduction slab gradates at the depth about 600 km along the boundary between upper and lower mantle. After that it continues west approximately up to the 102—103° E lineament at the distance more than 1500 km. That results in the lithosphere heating with development of the inner continental volcanism and formation of tension basins in the east Asian transit zone [Huang and Zhao 2009].
However the lithosphere thickness increases under Taihangshan, Ordos and some other old blocks up to 120—200 km. Gradients of its increasing corresponds to interblock zones, which coincide with rifts, where lithosphere thickness decreases down to 100 km [Bao et al. 2011]. The shortened lithosphere thickness exists under Tienshan (80—90 km against 200 km under Tarim and the north Asian Plate). Low-ohm zones of density losing and plastic deformation coincide with the low crust in Tienshan interblock zones, where the most intensive earthquakes are connected with gradients of electric resistance and S-wave velocities [Batalev et al. 2011].
Gatinsky Yu.G., Prokhorova T.V., Rundquist D.V., Vladova G.L. Up-to-Date Block Structure of Central Asia in Geophysical Fields
Data on anisotropy are the indicator of the strain and solidity for the mantle depths. These data are established by splitting of seismic waves from remote earthquakes. Pn-wave velocities are mainly low in the east part of central Asia and higher in the west (Figure 9). The highest Pn-wave velocities are noted in the upper mantle under old blocks: Tarim, Junggar, and Qaidam, northwest part of SE China and under the great part of north Tibet [Huang et al. 2011; Wang et al. 2013]. Relatively high velocities characterize the 102—103° E lineament, near meridional rifts in Tibet and partly Ordos [Li et al 2011]. The most intensive anisotropy is established under Tarim and in neighboring parts of the central Asian transit zone, where the NNE direction of fast splitting Pn-waves coincides with the direction of the maximal compression [Pei et al. 2007] and GPS vectors. The authors of the work [San'kov et al. 2011] come to the same conclusion for the Baikal, north and central Mongolia region noting the complete conjugation of the strain direction in the crust and lithosphere mantle.
Figure 9. The anisotropy of P-waves in the upper mantle of China and adjacent territories after works [Sol et al. 2007; Pei et al. 2007; Huang, and Zhao 2009]. Colors correspond to fast Pn-wave velocities, thin black lines — to the direction of splitted waves. The length of lines is proportional to the anisotropy amplitude. Note, that the mantle anisotropy direction is mainly WNW within the most part of the Himalayas, Pamirs and south Tibet corresponding to the longwise spreading of the mantle material, which results in the beginning collapse of the collisional orogen [Shen et al. 2000, 2005]. North from there NNE anisotropy directions coincides with GPS vectors indicating conjugation of the strain direction in the crust and lithosphere mantle. Latitudinal and NE anisotropy direction in south China and in the north part of the Indochina Peninsula don't conform with the clock-wise turn of GPS vectors in this region and allow to suppose existing non-conjugation of the crust and lithosphere mantle displacement there.
The direction of the P- and S-waves anisotropy shows coupling deformations in the lithosphere upper mantle and crust within the great part of the region, but different vectors of horizontal displacement in crust and mantle east of the east Himalayan syntax indicate decoupling these layers under the influence of the Hindustan — Asia collision. The thickness of the east Tibet lithosphere is 100—120 km against 130—170 km under the Sichuan Basin of SE China [Zhao et al. 2010; Hu et al. 2012]. The thinner lithosphere may indicate that the lower part of eastern Tibet lithosphere is heated and delaminated by the underlying hot as-thenosphere flow, which is resisted by the cold rigid Sichuan Basin lithosphere, thus branching into northeastward and southeastward parts. The east extrusion of the Tibet lithosphere was at first assumed in the work [Molnar, Tapponnier, 1975].
The Tibet lithosphere delaminating results in its fragmentation during interaction with the rigid and cold SE Asia Block. GPS data show the intensive clock-wise turning of the crust within Bayan Har, Kam Dian, Shan and SE China blocks (Figure 10). The dynamic model for the lithosphere shows that the Yunnan and Indochina crust is moving SE and south with respect to the mantle at velocities as high as ~30 mm/yr, while the mantle is displaced northeast. At the same time beneath the more west part of Tibet both move north (Flesch et al. 2005; Sol et al. 2007).
Gatinsky Yu.G., Prokhorova T.V., Rundquist D.V., Vladova G.L. Up-to-Date Block Structure of Central Asia in Geophysical Fields
Figure 10. The intensive clock-wise turn of GPS vectors in the ITRF system corresponding to the crust material rotation around the east Himalayas Syntax in east Myanmar, west Yunnan and west Sichuan after works [Pei et al. 2007; Zhang and Wang 2009]. Note, that GPS vectors direct in central and west Tibet, Tarim and north of it approximately so as within the Indian Plate.
Conclusion
Studying the connection of the central Asia block structure with different geophysical fields shows that the majority of interblock zones coincide with increased anomalies of the seismic energy releasing, magnetic fields and often with the high heat-flow, as well as with gradients of the crust and mantle thickness. These zones are sometimes situated within areas of the intensive delaminating of the crust and whole lithosphere with differently directed motions of their layers. All these peculiarities bring about the high up-to-date geodynamic activity of interblock zones including development within them epicenters of the most catastrophic earthquakes.
Following interblock zones of central Asia can be mentioned as the most seismically active: at boundaries of the Tienshan Block in Kazakhstan, Kirghizia and NW part of the Xinjiang Province of China (the volume of the specific energy is 2.88— 3.97x1013 J); at boundaries of Hangay and Amurian blocks in the Baikal region in Russia and Mongolia (2.9x1012 J); at north and south boundaries of the Pamirs in Tajikistan and Afghanistan (> 1.44x1013 J); at the boundary between the Himalayas Block and Indian Plate in Tibet and north India (9.51x1012); and at all boundaries of the Bayan Har Block in China (3.99x1013—9.25x1016 J). The majority of catastrophic earthquakes took place within above mentioned interblock zones during some last centuries (Figure 2). This emphasizes the applied significance of the detail studying seismicity and geodynamics of interblock zones with attraction of data from international catalogs of earthquakes, analysis of active faults, geophysical fields, GPS and Glonass vectors, as well as deep structures of territories.
Main causes of the high inner continental seismicity development in interblock zones connect with the deep continuation of collision slabs as it is under Tibet and the Pamirs, the intensive displacement of rheology layered crust horizons along faults under the influence of collision processes as at SE boundary of the Bayan Har Block, existence of the large lithosphere unhomogeneity including mantle plumes as in Sayan, Hangay, and NW part of the Amurian Block. The dependence of the seismic
Gatinsky Yu.G., Prokhorova T.V., Rundquist D.V., Vladova G.L. Up-to-Date Block Structure of Central Asia in Geophysical Fields
activity increasing from the relatively long seismic gap was established for the SE boundary of the Bayan Har Block [Gatinsky et al. 2008, 2009, 2011], where catastrophic earthquakes are repeated now every 5 years after near 40 years interval.
So the detail investigation of interblock zones geological structure, geophysical fields, and seismicity has the great significance for the prediction of catastrophic earthquakes and selection of specific areas of the most possible displaying highly seismic events.
Acknowledgement. This investigation is fulfilled with assistance of the Presidium RAS, Moscow (Program 4, Project "Appraisal and means of decreasing consequences of up-to-date tectonic movements and earthquakes in main mining regions and strategic power-stations in the Asian part of Russia and neighboring foreign countries") and Russian Foundation for Basic Research (Project No. 13—05-00109). Authors are grateful to Dr O.N. Tynyanova for the assistance in publication of this work and useful advices during its preparation.
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Cite MLA 7:
Gatinsky, Yu. G, T. V. Prokhorova, D. V. Rundquist, and G. L. Vladova. "Up-to-Date Block Structure of Central Asia in Geophysical Fields." Elektronnoe nauchnoe izdanie Al'manakh Prostranstvo i Vremya, Spetsialny vypusk Sistema planeta Zemlya [Electronic Scientific Edition Almanac Space and Time. Special Issue 'The Earth Planet System'] 4.1 (2013). Web.
<2227-9490e-aprovr_e-ast4-1.2013.26>.
УДК 550.34:551.24:502/504
СОВРЕМЕННАЯ БЛОКОВАЯ СТРУКТУРА ЦЕНТРАЛЬНОЙ АЗИИ В ГЕОФИЗИЧЕСКИХ ПОЛЯХ
Настоящая работа выполнена при поддержке Программы 4 фундаментальных исследований Президиума РАН («Оценка и средства уменьшения последствий от современных тектонических движений и землетрясений в основных регионах добычи и стратегических электростанций в азиатской части России и странах ближнего зарубежья") и Российского фонда фундаментальных исследований (Проект № 13 -05-00109).
Гатинский Юрий Георгиевич, доктор геолого-минералогических наук, старший научный сотрудник, главный научный сотрудник Государственного геологического музея им. В.И. Вернадского РАН
E-mail: [email protected], [email protected]
Прохорова Татьяна Викторовна, научный сотрудник Международного института теории прогноза землетрясений и математической геофизики РАН
E-mail: [email protected]
Рундквист Дмитрий Васильевич, доктор геолого-минералогических наук, академик РАН, главный научный сотрудник Государственного геологического музея им. В.И. Вернадского РАН
E-mail: [email protected]
Владова Галина Львовна, кандидат геолого-минералогических наук, старший научный сотрудник Международного института теории прогноза землетрясений и математической геофизики РАН
E-mail: [email protected]
В течение 2004—2009 гг. авторы разрабатывали проблему современной геодинамической неоднородности континента Евразия с выделением Северо-Евразийской литосферной плиты и транзитных зон между ней и соседними плитами. Зоны состоят из многочисленных блоков, ограниченных активными разломами, при этом максимальная тектоническая активность совпадает с межблоковыми зонами. Начиная с 2009 г. в Центральной Азии нами выполнено более детальное изучение границ блоков и межблоковых зон. С ними совпадает большинство активных разломов и эпицентров наиболее сильных землетрясений, в связи с чем их изучение и корреляция с различными геофизическими полями важны для установления уровня сейсмической активности в рассматриваемом регионе. В поле сейсмической энергии ее максимальные объемы высвобождаются на границах плит и в межблоковых зонах Центрально Азиатской транзитной зоны. В поле современных тектонических напряжений в этой транзитной зоне преобладает сжатие, сменяющееся частично в Восточно-Азиатской зоне на растяжение и сдвиги с растяжением. Высокие положительные аномалии магнитного поля (до +50...+100 nT) характеризуют большинство межблоковых зон и ограничивающих их крупных разломов. Гравитационное поле в редукции Буге на большей части площади Центральной Азии характеризуются отрицательными значениями до -50.-150 mGal. Отчетливый протяженный гравитационный линеамент пересекает значительную часть континента от залива Бакбо до побережья Охотского моря. Вдоль него происходит смена упомянутых отрицательных гравитационных аномалий на более положительные на востоке. Эта смена связана с резким уменьшением мощности континентальной коры. Значения теплового потока в межблоковых зонах возрастают до 80—100 pW/m-2 и выше на границах таких блоков, как Хангай, Амурский, Тибетские и Тарим, а также в некоторых внутри континентальных риф-
Gatinsky Yu.G., Prokhorova T.V., Rundquist D.V., Vladova G.L. Up-to-Date Block Structure of Central Asia in Geophysical Fields
тах. Под Хангаем и северной частью Амурского блока отдельные аномалии теплового потока могут быть связаны с мантийными плюмами. Мощность континентальной коры в Центральной Азии изменяется от 25—30 км на востоке до 50—75 км на западе под Тибетом и соседними блоками. Мощность литосферы изменяется в том же направлении от 60—80 км до 120—150 км, но под внутриконтинентальными рифтами, совпадающими с межблоковыми зонами, она уменьшается до 100 км и менее. Направление анизотропии P- и S-волн в пределах большей части рассматриваемого региона отвечает совместной деформации литосферной верхней мантии и коры. Различно направленные векторы горизонтального перемещения в коре и верхней мантии установлены только к востоку от Восточно-Гималайского синтаксиса, что указывает на разъединение этих слоев под влиянием Индостано-Азиатской коллизии и влияния «порога» мощной литосферы Юго-Восточного Китая на границе с Тибетом.
Ключевые слова: Центрально Азиатская транзитная зона, зоны активных разломов и тектонической активности, межблоковые зоны, мантийный плюм, мощность континентальной коры, мощность литосферы, аномалии магнитного поля, аномалии теплового потока, гравитационные аномалии, гравитационный линеамент.
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Цитирование по ГОСТ Р 7.0.11—2011:
Gatinsky, Yu. G, Prokhorova, T. V., Rundquist, D. V., Vladova, G. L. Up-to-Date Block Structure of Central Asia in Geophysical Fields [Современная блоковая структура Центральной Азии в геофизических полях] [Электронный ресурс] / Ю.Г. Гатинский, Т.В. Прохорова, Д.В. Рундквист, Г.Л. Владова // Электронное научное издание Альманах Пространство и Время. — 2013. — Т. 4. — Вып. 1: Система планета Земля — Стационарный сетевой адрес: 2227-9490e-aprovr_e-ast4-1.2013.26
