Mantle convection pattern and subcrustal stress field under Asia

Physics of the Earth and Planetary Interiors, 16 (1978) 247—256 © Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands

247

MANTLE CONVECTIONPATTERN AND SUBCRUSTAL STRESS FIELD UNDER ASIA HAN-SHOU LIU Geodynamics Branch, Goddard Space Flight Center, Greenbelt, Md. (U.S.A.)

(Received April 4, 1977; revised and accepted June 21, 1977)

Liu, H.S., 1978. Mantle convection pattern and subcrustal stress field under Asia. Phys. Earth Planet. Inter., 16: 247—256. Gravitational field models derived from satellite tracking and surface gravity data have been used to derive the forces in the earth’s mantle under Asia. Based on studies of tectonic forces from these models, a subcrustal stress field under China has been obtained. The stresses are due to mantle convection. According to the stress patterns, the east and west China blocks and five seismic zones are identifed. The tensional stresses exerted by the upwelling mantle convection flows under the crust of Tibet seem to be related to the Tibetan uplift. The compressional orogenic region from the southern tip of Lake Baikal, through Tien Shan, Hindu Kush and the Himalayas to northern Burma appears to be connected with the downwelling mantle convection flows. It is found that the directions of the subcrustal stresses under China are disposed perpendicularly to the major fault systems and seismic belts. The results of stress calculations show that the crust of north China should be in compression and that stresses within it should be sufficient to form the Shansi Graben and Linfen Basin Systems and fracture the lithosphere. This gives a possible explanation of why strong earthquakes occurred in north China which is an isolated narrow region of highest seismjcity far from plate boundaries. The tensional stress fields, caused by the upwelling mantle convection flows, are found to be regions of structural kinship characterized by major concentrations of mineral and metal deposits in China.

1. Introduction The theory of lithospheric plates or shells has been successful in explaining tectonic phenomena associated with plate boundaries. However, some seismicity and crustal deformation occur within plates and there is no obvious relationship to events at their boundaries. To a good approximation the subcrustal stresses exerted by mantle convection can be calculated from the hannonics of the geopotential (Runcorn, 1967; Liu et a!., 1976). By utilizing the satellite and gravity measurements of the potential field of the earth, Liu (1977) has shown that intraplate tectonics may result from mantle convection. The compressional and tensional stresses under the crust caused by mantle convection are of the order of 108 dyn cm2. Under appropriate configurations the strains to be expected from them may be localized in fractures of

the lithosphere and in some cases may lead to deformation of the crust. We may conjecture that the rock of the earth’s mantle, the deep plastic region below the earth’s elastic crust, must be churning slowly in vast convection cells under Asia. In this paper, it is proposed to apply these ideas to the metallogenic provinces and the tectonic and seismic systems in China and show how many of their features may be explained by crustal deformation associated with stresses due to mantle convection. North China is an isolated narrow region of highest seismicity far from plate boundaries. Historic large earthquakes, including the 1976 Peiking—Tangshan earthquake, occurred in this region. What forces caused these earthquakes and where did they originate? Sateffite detection of forces in the earth’s mantle should furnish some clue. it is shown that the significant compressional stresses in the entire litho-

248

sphere of north China exerted by the downwelling mantle convection could account for some of the seemingly puzzling features associated with seismic activities in this region. 2. Subcrustal stresses exerted by mantle convection The crust above the flowing mantle can be assumed as an elastic shell. The shear-stress components exerted by the convection flows on the crust in the eastward and northward directions are determined by (Runcorn, 1967): 2W \ OE(~,X) = sin ~ ~d /d r=a —-~—

=

~ E

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n=2 m~ 4ira

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(l)

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of the resultant stresses. The magnitudes and directions of the resultant stresses in the crust associated

=

A

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ra

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+

1964, 1967). The high degree harmonics for n ~ 13 may result from a short-wavelength convection system (Liu et a!., 1976; Liu, 1977). The seismic belts and fault systems in China are probably related to the small-scale subcrustal stress field which corresponds to the high-degree harmonics. By introducing the high-degree harmonic coefficients of the geopotential obtained by Wagner et al. (1977) into the stress eqs. 1 and 2, data can be processed by a computer to produce the results of the magnitudes and directions

ra +

Stress eqs. 1 and 2 are derived from a laminar viscous mantle flow model with a Newtonian viscosity developed by Runcorn (1967). Liu et al. (1976) and flu (1977) have shown that these equations are approximately valid for computation of stress patterns in the lower crust. The successively higher degree harmonics which have been placed in the model of the earth’s gravity field have been one of the most impressive series of measurements in the space program. The harmonic coefficients Cn,m and Sn,m have been determined by satellite and gravity measurements of the geopotential (Gaposchkin, 1974; Smith et al., 1976; Wagner et al., 1977). aThe low-degree harmonics for n (Runcorn, ~ 12 may reflect large-scale mantle flow system

sin(mA)}

(2)

with mantle convection currents can be expressed

where M g p a a

=

mass of the earth gravitational acceleration coefficient of viscosity radlus of earth radius of outer spherical surface of the flowing region associated Legendre polynomial of degree n, order m and argument cos the longitude the co-latitude

=

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3~Subcrustal stress field in Asia

The numerical methods of stress calculations have

C 2,0 C4 0

m Sn m Cn,m = Cn,m for n rr 2 or 4 and m = 0 C~,oand C~,o= coefficients characterizing the ellipof revolution . W = soid a poloidal function of convection cur-

rents

been developed by Liu eta!. (1976). For a more detailed computation of the magnitudes and directions of stresses exerted by the convection currents on the crust in Asia, the following recursive Legendre functions applied (Hobson, 1955): ~) 2(coswere ~) —2(m + 1)cot ~F~’(cos F~ (n m)(n + m + I) F~(cos ~) (5) —

—

249

~F2(cos ~

~)

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n +rn +1 —

n

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d ä-~[F~2(cos~)]= —2(rn —

—

+

(6)

The coefficients of Cn,m and Sn,m (Wagner et a!., 1977) were introduced into eqs. 1 and 2 and pro-

(7)

cessed by a computer components °E(~,A) and to compute ~ A).the Results resulting fromstress eqs. 3 and 4 were obtained at 4000 points having coordinates 100 ~ ~ ~ 60°and 60°~ A ~ 140°.A map of the subcrustal stresses exerted by mantle convection under Asia taken over one-degree grid and synthesized for 13 ~ n ~ 25 is shown in fig. 1. Divergent arrows indicate tensional stresses in the lithosphere caused

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by the upwelling mantle flows. Convergent arrows represent compressional stresses in the lithosphere caused by the downwelling mantle flows. Fig. 1 shows that the major upweffing mantle convection

(8)

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flow associated with tensional stresses under China diverges to the western region under the lithosphere

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of Tibet and the major downwelling m~antleconvection flow associated with compressional stresses converges to the eastern region under the lithosphere of north China. The stress environment of China, as shown in Fig. 1, illustrates that the region from the southern tip of Lake Baikal, through Tien Shan, Hindu Kush and the Himalayas to northern Burma is a region of compression exerted by the downwelling mantle convection. This stress pattern is in agreement with the stresses obtained by Fitch (1970), Das and Filson (1975) and by Yeh eta!. (1975) from faultplane solutions. The uplift of Tibet may result from thermal convection processes and magmatic activity involving the lower crust and upper mantle as probably is the case in the African plate (Liu, 1977). The abundant evidence for volcanism supports this contention (Dewey and Burke, 1973), and the absence of the seismic phase L 5 on seismograms of paths crossing Tibet (Molnar and Tapponnier, 1975; Ruzaikin et a!., 1977) may be explained by high attenuation due to high temperature in the Tibetan crust resulting

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from the hot upwelling convection in the upper mantle under this region. Florensov (1969) has considered the Baikal rift as a region of spreading. The tensional stresses due to the upwelling mantle convection flow in the interior of Asia under the Baikal rift zone seem to be responsible for the spreading of this region. The compressional stresses due to a major downwelling mantle flow under southeast Asia appear to be related to the location of the Philippine Basin. The magnitudes of the tensional and compressional stresses due to mantle convection are in the order of 108 dyn cm2 which is the same order of magnitude of stresses associated with fracturing of continents (Sbar and Sykes, 1973; flu, 1977).

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Fig. 4. Subcrustal stress pattern and seismic belts in China.

The N—S and E—W Seismic Zoning Systems of China have been proposed recently (Wang et a!., 1976; Wu et a!., 1976). The N—S Seismic Zone of China extends nearly along the longitude 104°E,all the way from north to south. This seismic zone divides the crust of China into east and west parts. The crust of the east and west China blocks has been classified into five seismic zones as shown in Fig. 5. The seismicity in China is conspicuous in zonation (Z.Y. Li eta!., 1974; Wang et a!., 1976). Six large earthquakes including the 1976 Peiking—Tangshan earthquake occurred in the North China Zone. The Shansi Graben and Linfen Basin systems are located in this region. Fig. 5 shows that the convection currents in the entire upper mantle under the east China block and the N—S Seismic China Zone converge to (1 10°E,36°N)where the Shansi Graben and Linfen Basin Systems were formed. Significant compressional stresses exerted by the downwelling mantle convection exist in the entire lithosphere of the North

China Zone. It is no wonder that nine disastrous large earthquakes (M> 8) since 1303 including the 1976 Peiking—Tangshan earthquake (M 7.8) occurred in this zone and its adjacent regions. it is noted that the most disastrous earthquake in the world history occurred in 1556 in this zone which killed about 800,000 people according to the Chinese documents. The upwelling mantle flow systems under the Northeast and Southeast China Zones which converge to the North China Zone seem to support the conclusions made by Z.Y. Li et a!. (1974) concerning the variation of the physical properties of rocks and plastic and elastic deformation of lithosphere in these seismological regions. According to the stress patterns, the boundary between the east and west China blocks may be a zone of shearing. This zone seems to be divided into two sections at about the latitude 33°N.They are in good agreement with the limits and features of the N—S Seismic Zone of China investigated by Wang et al.

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(1976). The epicenters of earthquakes have been migrating from the north section toward the south section (Li and Yuan, 1974). This indicates that the two sections in this zone may be alternatively locked.

6. Hypothesis of the Indian—Eurasian collision One of the spectacular aspects of current tectonics of Asia is the coffision of the northward-moving Indian plate with the Eurasian plate (Molnar and Tapponnier, 1975). The coffision mechanism and reconstruction of the evolution of the Himalayas are still very imperfectly understood. There was a period of crustal consumption preceding the collision. What happened to the crust? The convection pattern in Fig. 1 may furnish some clue. This pattern implies that there is a sinking mantle block under the entire Himalayas and the adjacent Pamir and Hindu Kush which could generate intermediate focus earthquakes

due to the phase transformation. This pattern also indicates that the compressional horizontal stresses resulting from the collision between the Indian and Eurasian plates can be transmitted from the’Hima!ayas to further north across the Tibetan crust without deformation. This is plausible because the initial tensional stresses in the crust of the central Tibet exerted by mantle flow are sufficient to compensate the compressiona! stresses due to the Indian— Eurasian plate collision. Furthermore, the E—W flows of material in the lower crust and upper mantle beneath the eastern and western Tibet, as shown in Fig. 1, would also compensate the pressure imposed by the Indian—Eurasian collision. If one argues about the correlation between the Indian—Eurasian collision and the deformation of the lithosphere of North China, the lack of a definite physical mode! becomes very apparent.The high seismic activity and the formation of the fault and graben systems in north China are much more than

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Fig. 6. The distribution of mineral and metal deposits in relation to the tensional subcrustal stress system in China.

would be expected from the collision hypothesis. Speculations in the effect of the Indian—Eurasian collision on tectonics of north China would not be profitable because a simple mode! of coffision derived from the theory of plates and shells would contradict seismological features in north China. 7. Mineral and metal mining in China China is now a significant world producer of several mineral products. Sb and W reserves are believed to be the largest in the world and reserves of aluminous ores, magnesite, Pb, Mn, Mo, Hg and Sn are substantial. When the deposits of the ferrous metals and ferroalloy minerals and non-ferrous metals and minerals in China are considered in relation to the mantle convection patterns, a clear relationship emerges defining the distinct type of tensional—metallogenic domains characterized by China’s important concentrations of

minerals. The specific and more localized provinces of mineral and metal concentration within the tensional stress fields of the crust exerted by mantle convection are based on published production figures and ore deposits in the MineralMap of the People’sRepublic of China (November, 1971). The distributions of the principal deposits of Fe ore, Mn, Mo, W, Al, Sb, Cu, Pb and Zn, Hg, Sn and magnesite, in relation to the subcrustal stresses under China are shown in Fig. 6. Fig. 6 shows that the distribution of these ore products in China seem to overlie the upwelling mantle convection associated with tensional stresses in the crust. One of the fundamental concepts to emerge from research on mineral and metal deposits is that heated aqueous fluids are the critical factor in the transport and ~depositionof minerals and metals in the large majority of ore deposit types (Zeng and Yang, 1975). Metal deposits are local accumulations or concentrations of rocks and metals that can be recovered at a profit. The accumulation or concentration of metals may involve movement of fluids associated with

255

metamorphic process. Accumulation or concentration of the old minerals in the tensional stress regimes may be caused by the forces from the hotter upwelling mantle flows which set the fluids, fluids in rocks, in motion because of changes in temperature and stresses (Clifford, 1971; Zeng and Yang, 1975). Recognition of the full spectrum of ore deposits in China that may be associated with tensional stress regimes clue to the hot upwelling convection flows should prove to be an aid in exploration. Probing the earth by space scientists to generate gravity data is generally outside the domain of mineral exploration activity, but the availability of such data will enable us to calculate the tensional stress regimes in the crust in which tension-related ore concentration may have operated. It is well known that the extensive and successful geological explorations in China since 1949 were largely guided by the classical principles of geological mechanics. The dynamic mantle convection patterns in Fig. 6, derived from satellite data, may have applications in resource survey. In this regard, serious consideration has been given to the possibility that this tens~onal—metallogenicrelationship is fortuitous, particularly in view of the large areas of the western China have not been adequately prospected; that possibility may, however, be discarded in the light of the close correlation between the tensional subcrustal stress pattern and the distribution of the major known ore deposits in the eastern part of China as shown in Fig. 6. It is stressed that the present discussion is directedtowards a better understanding of the broad pattern of mineral deposits in China. Even now we do not fully understand the fundamental origin of many mineral deposits. However, we can with a fair degree of confidence state that economic concentrations of minerals and metals are most likely to occur in specif~cregions of the crust of China according to the subcrustal stress pattern under China. The development ~f the subcrustal stress patterns applies not only to China but to other continents such as Africa (Liu, 1977).

geopotentia! and the geoid have developed a subcrustal stress field under Asia. This stress field is due to mantle convection. According to the stress patterns, the east and west China blocks and five seismic zones are identified and a clear relationship defining the distinct type of tensional—metallogenic domains characterized by China’s important concentrations of minerals and metals is revealed. All major fault systems and 23 seismic belts in China appear to be disposed perpendicularly to the directions of the subcrustal stresses. It is proposed that tectonic features and seismic activities in China are influenced by the tensional and compressiona! stresses in the crust due to mantle convection. This suggested mode! provides a possible explanation of why historic strong earthquakes occurredin north China which is an isolated narrow region of highest seismicity far from plate boundaries. Acknowledgment The author thanks Edward S. Chang of the staff of the Wolf Research Development Group for computational assistance. References Clifford,T.N., 1971. Location of mineral deposits. In: I.G. Gass, P.J. Smith and R.C.L. Wilson (Editors), Understanding the Earth, the MIT Press, Cambridge, Mass., Das,315—325. S. and Filson, J., 1975. On the Tectonics of Asia. Earth

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