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Geodynamics of Cenozoic deformation in central Asia
64
Physics of the Earth and Planetary Interiors, 25 (1981) 64—70 Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands
GEODYNAMICS OF CENOZOIC DEFORMATION IN CENTRAL ASIA HAN-SHOU LIU Geodynamics Branch, Goddard Space Flight Center, Greenbelt, MD 20771 (U.S.A.)
(Received April 16, 1980;accepted for publication August 18, 1980)
Liu, H.-S., 1981. Geodynamics of Cenozoic deformation in central Asia. Phys. Earth Planet. Inter., 25: 64—70. This paper presents a study of the tectonic stresses in central Asia based on an interpretation of satellite gravity data for mantle convection and supplemented with published fault plane solutions of earthquakes. Northwest— southeast to north—south compressional stresses exist in the Tien Shan region where reverse faulting dominates. The maximum compressive stress is oriented approximately northeast—southwest in the regions of Altai and southern Mongolia. Farther north, compressive stress gives way to tensional stress which causes normal faulting in the Baikal rift system. It is also shown that all of the tectonic stresses in the Tibetan plateau and Himalayan frontal thrust are related to the convection-generated stress patterns inferred from satellite gravity data. These results suggest that the complex crustal deformation in central Asia can be convincingly described by the deformation of the lithosphere on top of the up- and down-welling asthenospheric material beneath it. This observational fact may not only upset the simple view of the fluid crustal model of the Tibetan plateau, but also provide some useful constraints for the future development of deformation theory of continental crust.
1. Introduction Continental deformation is one of the most puzzling aspects of plate tectonics. The dynamics governing continental deformation are not yet known. At present there are two extreme points of view. One is that the deformation might be described by the motion of a number of tectonic plates moving in a rather complicated way (McKenzie, 1970, 1972; Nowroozi, 1972). The other is that continental regions behave plastically and deformation is distributed over wide regions rather than concentrated on a few large fault systems (Molnar and Tapponnier, 1975; Tapponnier and Molnar, 1979). Neither description is yet sufficiently precise to allow analyses to be carried out with any confidence (McKenzie, 1978) and hence the relationship of plate tectonics to the observed stresses is unclear (Fu et al., 1979; Tseng, 1979). The purpose of this paper is to examine the seismic stresses of central Asia in an attempt to understand the processes now taking place. Some of the seismic zones are the
farthest from the major plate boundary. It has been found that the contemporary tectonics in this vast territory may be influenced by the convection-generated stress systems. The key observation is the recognition, on the satellite-determined patterns, of stress systems that are reconcilable with fault plane solutions of earthquakes. The results of this study indicate that the processes and flows going on in the upper mantle just below the lithosphere of central Asia play an important role in the Cenozoic tectonics.
2. Forces for continental deformation If the momentum involved in continental deformation in central Asia can be neglected, all crustal deformation in this region must be caused by forces now acting. There are three possible forces which could in principle maintain the observed deformation: gravitational force, forces due to plate motion, and forces on the base of the lithosphere. In general,
0031-9201/81/0000—0000/$ 02.50 © 1981 Elsevier Scientific Publishing Company
65
gravitational force must be involved. The similarity between the thrust faults and landslips from LANDSAT imagery indicates that considerable gravitational energy has been stored during the compression. However, it is difficult to believe that this storage maintains the tension in the Tibetan plateau and Lake Baikal region. This argument suggests that the compression is not the direct result of the local storage of gravitational energy, but must be maintained by forces due to plate motion or forces on the base of the.lithosphere which are generated by mantle convection. Forces due to plate motion might be important in the contemporary tectonics in central Asia. However, it is extremely difficult to understand how such collision forces can be transformed into the domal uplift and crustal tension observed in the Lake Baikal region according to the theory of plates and shells (Timoshenko and Woinowsky-Krieger, 1959). Furthermore, active tension behind collision zones is one of the most puzzling aspects in plate tectonics. In a simple view, regional compression is the natural expression of plate convergence. The observational fact that tension takes place just behind the Himalayan frontal thrust upsets this simple view and requires explanation. Although Molnar and Tapponnier (1978) and Tapponnier and Molnar (1979) have compiled geological maps for interpretation, their fluid crust carton (Molnar and Tapponnier, 1978, p. 5373) seems unsuitable to form a basis for analysis of crustal deformation. Thus, there are good reasons to suggest that only forces acting on the base of the lithosphere may account for the observations (Zonn and Florensov, 1979). Because so little is known about these forces from numerical and laboratory experiments on mantle convection systems, the major patterns of such force systems may come from mantle flows as inferred from satellite gravity data (Liu, 1977, 1978, l979a, b, 1980).
3. Convection-generated stress in central Asia Obtaining a global distribution of stresses would be a great achievement of science (Riznichenko, 1972). Unfortunately, stresses in the Earth are elusive variables which are difficult to obtain for even one point in the crust. Stress measurement techniques
often yield conflicting evidence concerning the state of stress in the crust. Because stress measurements are costly, they are scarce. For example, great efforts have failed to achieve the goal of developing a map for at least a small portion of the Earth showing the state of stress (Riecker, 1977). From the theory of mantle convection and the geoid, Runcorn (1964, 1967) has discussed contemporary stresses under the crust exerted by mantle convection flows. Using the mantle flow model as a guide, Liu (1977, 1978, 1979a) has proposed the application of the high degree harmonics of the geopotential from satellite gravity data to calculate the stress distribution in the crust caused by mantle convection. It has been shown that convection-generated stresses are probably the genetic cause for intra-plate volcanism (Liu, 1980) and intra-plate seismicity (Liu, 1979b). The stress components under the crust of central Asia exterted by mantle convection in the eastward and northward directions are determined by (Runcorn, 1967; Uu, 1977)
(~~ E
‘
~
=
Mg(a~\’~’2n + 1 1 n + 1 sin ~ 4ira2 \ a /
n=13 m0
/
m
—
.
—
X P,~(cos ~)[—mCn,m sm(mX) + mSn,m cos(mX)] mn UN(~ X)
,
~
=
n13
~fl1~
~
m=o 4na \ a I
x [P~(cos ~)][~
,~ ‘
2n + 1 n+1
cos(mX) + ~
m
d~ sin(mX)] (2)
where Mis the mass of the Earth, g the acceleration due to the gravity, ae the radius of the Earth, a the radius of the outer spherical surface of the flowing region, ~n,m and S~,n the coefficients of the harmonic terms, A the longitude, ~ the co-latitude and P~(cos~)is the associated Legendre polynomial of degree n, order m and argument cos By applying the methods of stress calculation (Liu, 1978), a stress vector map of central Asia has been produced, as shown in Fig. 1. The stress information in Fig. 1 is extracted from sateffite gravity data obtained by Lerch et al. (1979). This set of gravity data is based on 840000 satellite tracking measurements. The arrows in Fig. 1 are stress vectors and the
66 -: : ;;;;;:~z~’’”~’~’ ~~ II! 11 -. ~-•—--—‘.:~Ii///j////
50 ~
I I
)~::-~
JJ!JJ?~.~
~
~
~
‘~:hT~Il(f I
40
/
\ \‘.~.~‘\\\\\\~
~
~
-~,
~
~~I ~ ~ 20
~///~
‘~~
i iJ~~i ~ -
~
~I’I
I /‘/////I -
~\
-
~
‘ ‘‘‘~‘‘ -
~/,~///j
\\ii//7./ \‘~\II/~~’~/II
i ~\\\
~~~—--
‘‘‘‘
~
-
Fig. 1 are comprised of several consistent stress patterns.Theremarkableradialdivergentfieldoftension intheTibetanplateauisoneofthem.Thenorth— south compression rn the Tien Shan region is another pattern. The resemblance between the compressive stress pattern and the plate collision along the Hima layan frontal thrust seems to be very strong.
4 Assumptions of convection-generated stress At present there is a lack of experimental information on the nature of convection in the upper mantle of the Earth and on the question of whether the lithospheric stress field is accompanied by mantle convection. However, there appear to be a number of
Fig. 1. Convection-generated stresses under central Asia.
geological convection-generated stresses arephenomena postulated in to which take place (Liu, 1977, 1978, 1979a, 1 979b, 1980). For pattern calculations of these stresses we have assumed that the high degree harmonics in the geopotential are related to mantle
magnitudes of the stresses are of the order of 108 dyn cm2 which is the same order of magnitude of stresses associated with the fracture of continents (Liu, 1977). The directions of convection flows in Fig. I are in good agreement with the convection pattems derived from the previous set of gravity data (Wagner et al., 1977; Liu, 1978). The fact that convection patterns derived from different gravity models have essentially the same features appears to reveal the real geophysical dimensions of any convection cells. It can be seen that the stress vectors in
convection patterns or tractions exerted on the base of the lithosphere, using results first given by Runcorn (1964, 1967). This assumption is contentious. Most geophysicists believed that Runcorn’s (1964) suggestion is a serious over-simplification and that Runcom’s theory of mantle convection and the geoid (1967) should be modified. Despite these difficulties it may be useful to apply the theory to central Asia, where the principal axes of stresses from seismic data are well-known. In this way the various simplifying assumptions can be tested or justified.
~
60
70
80
~
90
i:
100
..
LUTUTTTU ~JTTUTJU _
$ iminin
~
•
• uniiiiu tiIllluhi ~Li~’ (~,
Is) DECELERATING: COMPRESSIONAL STRESS
(b) BILATERALLY OPPOSED: COMPRESSIONAL STRESS
4~>
Ic) CONVERGING: COMPRESSIONAL STRESS
Fig. 2. Flow patterns of the asthenosphere and crustal compression. See text for explanation.
67
I
~
I.1.t..t.t.t.t.i.t.tj
+1 t:~W: I I Ti TI Ti ~1
(a) Accelerating:
(b) Bilaterally opposed:
tensional fissures .L
tensional graben
ii~!4~~iI
\.~11.L..t.L;j
~t1ii..t..t.ii.i.i (c) Unequal parallel: feather fissures
(d) Diverging: tensional fissures
Fig. 3. Flow patterns of the asthenosphere and crustal tension. See
II
text for explanation.
5. Stress patterns
lying crust. Thirdly (c), uniform but converging current may likewise cause compressional stresses, which
In principle, three different types of movement in the substratum may cause compression in the overlying crust. These types of movement are illustrated in FIg. 2. The fIrst (a) is a decelerating, horizontally directed current, which is transmitted by friction to the overlying crust and causes compression in the direction of flow. Secondly (b), if a descending current forms bilateral or centripetal branches, these will obviously exert compressional force upon the over-
in some cases will be parallel to the direction of flow. The compressional stresses in the Tien Shan and Himalayan region consist of the three flow patterns shown in Fig. 2. Four different types of movement in the substraturn may cause tension in the overlying crust (Fig. 3). The first (a) is an accelerating, horizontally directed current, which is transmitted by friction to the overlying crust and causes tension in the direction of flow.
68
Secondly(b),ifanascendingcurrentsplitsinto bilateral or centrifugal branches, these will exert tensional force upon the overlying crust. Thirdly (c), horizontal and parallel subcrustal currents of differing velocities will set up torsional forces in the crust, which will give rise to the opening of fractures lying obliquely to the direction of flow. Fourthly (d), a uniform but diverging current may cause tensional fractureswhich in this case wifi be parallel to the direction of flow. The tensional stresses in the Tibetan plateau and Baikal rift region can be explained in terms of the mantle flow patterns shown in Fig. 3. Studies on the state of stress inferred from gravity data and the seismological framework in central Asia may be useful for earthquake research.
50
I
I I
‘/1/I
,‘,‘/
~
Il/Il
1) ~
\
~
,
II
-.
-
~o
7
__________._._—__-.-_\
\
I
F
~
II:
20
——
:
:‘
-‘
I~//// ,_,_,/
1 I
10
6. Interpretation of fault plane solutions Fault plane solutions of earthquakes show that horizontal tectonic stresses are widespread in central Asia (Misharina, 1972;Molnar et al., 1973;Molnar and Tapponnier, l978;Ni, 1978;Ni and York, 1978; Rastogi, 1976; Ritsema, 1961; Sherman, 1978; Shirokova, 1967, 1974;Tapponnier and Molnar, 1979; Yeh et al., 1975). The principal axes of compressional and tensional stresses from fault plane solutions of earthquakes in this region are plotted in Fig. 4. Close examination of Figs. 2, 3 and 4 shows that the stresses as inferred from gravity data are reconcilable with the fault plane solutions of earthquakes. It has been demonstrated that, in plane projection, these stresses outline complex zones differingsharply from each other in the magnitude of horizontal compression, and in some zones compression is replaced by tension, Clearly, the structure of such complex stress fields cannot be investigated by simple fault plane solutions, and indirect geodynamical methods are required to investigate the magnitude and direction of stresses in mantle rocks. From fault plane solutions of earthquakes, a north—south compression system in the Himalayan frontal thrust has been established. Much of the late Cenozoic history of the Tibetan plateau is characterized by east—west tension,but in the Tien Shan and the south Mongolian region there is again north—south compression. North of Mongolia, compressive stress gives way to tensional stress, causing normal faulting
60
70
80
90
100
Fig. 4. Convection-generated stresses in central Asia and the tensional and compressional stresses (heavy arrows) inferred from fault-plane solutions of earthquakes.
in the Baikal rift system. The convection-generated stresses or forces proposed here (Figs. 1 and 4) explain all these fault-plane solutions clearly and simply. In all regions in Figs. 1 or 4, regardless of structure and all other characteristics, systematic horizontal compression and tension is observed. The system of compressive or tensional zones and the persistence of their topographic characteristics are consistent with only one zoning mechanism, namely the upweffing and downwelling of mantle material. In these regions, there exists a clear-cut correlation between the convection-generated stresses and the directions of the principal stresses derived from fault plane solutions of earthquakes. The convectiongenerated stress concentration should occur in narrow fault zones and causes earthquakes (Liu, 1 979b).
7, Is continental collision the only mechanism of Cenozoic tectonics? The hypothesis of New Global Tectonics can hardly explain the nature of intra-continental movement, intra-plate seismicity and intra.plate volcanism (Zorin and Florensov, 1979; Fu et al., 1979, Tseng,
69 1979; Liu, 1979b, 1980). However, based on the analysis of earthquake focal mechanisms, and of the surface structure derived from LANDSAT imagery, Molnar et al. (1973), Molnar and Tapponnier (1975, 1978) and Tapponnier and Molnar (1979) have attempted to explain the nature of Cenozoic tectonics in Asia. They consider the Eurasian plate to be not a monolith, but a body divided into some rigid ancient blocks surrounded by weak zones, which are Paleozoic and Mesozoic fold belts. The collision in the Eocene— Oligocene of the Indian and Eurasian plates has led to formation of the highest mountain ranges, the Himalaya and Pamir, on the plate boundary and to deformation of the Eurasian plate. This hypothesis may be over-simplified because data on the structure of the upper mantle beneath the uplifts of central Asia are neglected by its supporters. For Tibet, based on the absence of the Lg phase in records of earthquake waves, Molnar and Tapponnier (1975) have admitted that they must invoke a separate independent mechanism of thermal processes in the upper mantle to rationalize their models of crustal deformation. Recently, Zorin and Florensov (1979) have shown that the Tibetan plateau, Mongolian—Siberian uplift and the Baikal rift zone must have formed as a result of the initiation of hot spots beneath the lithosphere. As a matter of fact, Liu (1977, 1978, 1 979a) has already demonstrated, from satellite gravity data, that the dynamic effect of mantle upwelling plays a particular role in the formation of uplifts not only in Asia,but also in hfrica and Australia. In Fig. 1 it has been shown that all these uplifts or rift zones are related to tensional stresses resulting from the upweilingmaritle flows. In the creation of the linear frontal thrusting in the Himalaya and Tien Shan region, subcrustal cornpression caused by mantle convection, as shown in Fig. 1, probably played the Main part. The pattern of the downwelling mantle flows under these regions fol lows the outlines of the Tien Shan and Himalayan frontal thrusts. This downwelling of colder mantle material has played a subsidiary role, increasing the weakness of the lithosphere at the locus of the Paleozoic fold belt and shortening the crust through orogenesis in weak zones, The pattern of mantle convection (Fig. 1) inferred from sateffite gravity data does not seem to contradict the hypothesis of plate tectonics as it relates to
convergent plate boundaries. In Tibet the depth of the upwelling mantle convection cell is still less well defined. The mantle here could gain such properties from the accumulation of material from partial melting of the Indian lithospheric plate,which is underthrusting the Eurasian plate. If compression caused by the Indian and Eurasian plates is transferred at such long distances that it influences the structural pattern of the Baikal rift zone, as Molnar and Tapponnier (1975) postulated it must naturally influence all the structures situated between the Siberian plateform and the Himalayas. Therefore, the postulated compressional stresses due to plate collision may be partly responsible only for the creation of second.order structures with northwest- and sublatitudinal-oriented strikes. The exact correspondence of patterns of mantle convection with the main features of the Cenozoic uplifts, and with tectonic stresses inferred from seismic data, indicates that the uplifts are connected with the existence of upwelling mantle flows. For such a mantle flow pattern to exist in the asthenosphere for about 25 My, it must be constantly renewed. The renewal process may result from a constant inflow to the upper asthenosphere of heated material from greater depth, as a result of convection. This constant renewal of material allows the upwelling convection cells in the asthenosphere to remain stable in position.
Conclusion When the fault plane solutions of earthquakes in central Asia are compared with the convectiongenerated stresses calculated from satellite gravity data, several seemingly unrelated phenomena in Asia appear to have a common cause. The simultaneous compression in the Himalayan and Tien Shan regions is related to the convection-generated compressive stress systems. The convection-generated tensional stress patterns also predict tension in the Tibetan plateau adjacent to the plate collision zone and in the Baikal rift zone at a great distance from the plate boundary. The correspondence between the convectiongenerated stress patterns and the fault plane solutions of earthquakes in central Asia seems to support the
70
contention that most of the contemporary tectonics in Asia are influenced by mantle convection, and provides a unffymg explanation for the psieflOmena occurring there and for continental deformation in general.
References Fu, C.Y., Chen, T. and Chen, R., 1979. Research on the physics of the earthquake foci. Acts Geophys. Sin., 22 (4): 315—326. Lerch, Fl., Klosko, S.M., Laubscher, R.E. and Wagner, C.A., 1979. Gravity model improvement using Geos 3 (GEM 9
and 10).J.Geophys.Res., 84: 3897—3916. Liu, H.S., 1977. Convection pattern and stress system under the African plate. Phys. Earth Planet. Inter., 15: 60—68. Liu, H.S., 1978. Mantle convection and subcrustal stress field under Asia. Phys. Earth Planet. Inter., 16: 247256. Liu, H.S., 1979a. Mantle convection and subcrustal stress under Australia. Modern Geol., 7: 29—36. Liu, H.S., 1979b. Convection-generated stress concentration and seismogenic models of the Tangshan earthquake. Phys. Earth Planet. Inter., 19: 307—318. Liu, H.S., 1980. Convection-generated stress field and intraplate volcanism, Tectonophysics, 65: 225—244. McKenzie, D.P., 1970. The plate tectonics of the Mediterranean region. Nature (London), 226: 239—243.
McKenzie, D.P., 1972. Active tectonics of the Mediterranean region. Geophys. J.R. Astron. Soc., 30: 109—185. McKenzie, D.P., 1978. Active tectonics of the Alpine—HimaIayan belt: the Aegean Sea and surrounding regions. Geophys. J.R. Astron. Soc., 55: 217—254. Misharina, L.A., 1972. Stresses in the earthquake foci of the Mongolia—Baikal seismic zone. In: Fields of Elastic Stresses of the Earth and Focal Mechanisms of Earthquakes. Nauka, Moscow, pp. 161—171. Molnar, P. and Tapponnier, P., 1975. Cenozoic tectonics of Asia: effects of a continental collision. Science, 189: 4 19—426.
Molnar, P. and Tapponmer, P., 1978. Active tectonics of Tibet. J. Geophys. Res., 83: 5361—5375. Molnar, P., Fitch, T.J. and Wu, F.T., 1973. Fault plane solu-
tions of shallow earthquakes and contemporary tectonics in Asia. Earth Planet. Sci. Lett., 19: 101—112. Ni, J., 1978. Contemporary tectonics in the Tien Shan region. Earth Planet. Sci. Lett., 41: 347—355.
Ni, J. and York, J.E., 1978. Late Cenozoic tectonics of the Tibetan plateau. J. Geophys. Res., 83: 5377—5384. Nowroozi, A.A., 1972. Focal mechanisms of earthquakes of the Persian plateau Eastern Turkey, Caucasus and the Hindu-Kush regions. Bull. Seismol. Soc. Am., 61:
317—325. Rastogi, B.K., 1976. Source mechanism studies of earthquakes and contemporary tectonics in Himalaya and nearby regions. Bull. Inst. Seismol. Eng., 14: 99—134. Riecker, R.E., 1977. State of stress in the lithosphere. Eos Trans. Am. Geophys. Union, 58: 597—599. Ritsema, A.R., 1961. Some 1951 earthquake mechanisms based on p and PKP data. Geophys. J.R. Astron. Soc., 5: 254—258. Riznichenko, Y.V., 1972. Stress and strain in the Earth and the earthquake foci. Phys. Earth Planet. Inter., 6: 211—
213. Runcorn, S.K., 1964. Satellite gravity measurements and laminar viscous flow model of the Earth’s mantle. J. Geophys. Res., 69: 4389—4394. Runcorn, S.K., 1967. Flow in the mantle inferred from the low degree harmonics of the geopotential. Geophys. J.R.
Astron. Soc., 14: 375—384. Sherman, S.I., 1978. Faults of the Baikal rift zone. Tectonophysics, 45: 31—40. Shirokova, E.J., 1967. General features in the orientation of principal stresses in the Mediterranea—Asian seismic belt. Bull. Aced, Sri. USSR, Earth Phys., 1: 23—26.
Shirokova, El., 1974. Detailed study of the stresses and fault planes at earthquake foci of central Asia. Izv. Aced. Sci. USSR, Phys. Solid Earth, 11: 22—36. Tapponnier, P. and Molnar, P, 1979. Active faulting and Cenozoic tectonics of the Tien Shan, Mongolia and Baikal regions. J. Geophys. Res., 84: 3425—3439. Tiinoshenko, S. and Woinowsky-Krieger, 1959. Theory of Plates and Shells, McGraw-Hill, New York, NY, 580 pp. Tseng, R.S., 1979. A review of crustal and upper mantle research in China (1949—1979), Acta Geophys. Sin., 22 (4): 336—345 Wagner, CA., Lerch, FJ., Brown, J.E. and Richardson, J.A., 1977. Improvements in the geopotential derived from satellite and surface data (GEM 7 and 8). J. Geophys. Res.,
82: 901—914. Yeh, H., Liang, Y.S., Shen, L.Q. and Xiang, H,F,, 1975. The analysis of recent tectonic stress of the Himalayan Moun-
tains Arc and its vicinities. Sci. Geol. Sin., 1: 32—48. Zorin, YA. and Florensov, NA., 1979. On geodynamics of Cenozoic uplifts in central Asia. Tectonophysics, 61: 271—283.
Physics of the Earth and Planetary Interiors, 25 (1981) 64—70 Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands
GEODYNAMICS OF CENOZOIC DEFORMATION IN CENTRAL ASIA HAN-SHOU LIU Geodynamics Branch, Goddard Space Flight Center, Greenbelt, MD 20771 (U.S.A.)
(Received April 16, 1980;accepted for publication August 18, 1980)
Liu, H.-S., 1981. Geodynamics of Cenozoic deformation in central Asia. Phys. Earth Planet. Inter., 25: 64—70. This paper presents a study of the tectonic stresses in central Asia based on an interpretation of satellite gravity data for mantle convection and supplemented with published fault plane solutions of earthquakes. Northwest— southeast to north—south compressional stresses exist in the Tien Shan region where reverse faulting dominates. The maximum compressive stress is oriented approximately northeast—southwest in the regions of Altai and southern Mongolia. Farther north, compressive stress gives way to tensional stress which causes normal faulting in the Baikal rift system. It is also shown that all of the tectonic stresses in the Tibetan plateau and Himalayan frontal thrust are related to the convection-generated stress patterns inferred from satellite gravity data. These results suggest that the complex crustal deformation in central Asia can be convincingly described by the deformation of the lithosphere on top of the up- and down-welling asthenospheric material beneath it. This observational fact may not only upset the simple view of the fluid crustal model of the Tibetan plateau, but also provide some useful constraints for the future development of deformation theory of continental crust.
1. Introduction Continental deformation is one of the most puzzling aspects of plate tectonics. The dynamics governing continental deformation are not yet known. At present there are two extreme points of view. One is that the deformation might be described by the motion of a number of tectonic plates moving in a rather complicated way (McKenzie, 1970, 1972; Nowroozi, 1972). The other is that continental regions behave plastically and deformation is distributed over wide regions rather than concentrated on a few large fault systems (Molnar and Tapponnier, 1975; Tapponnier and Molnar, 1979). Neither description is yet sufficiently precise to allow analyses to be carried out with any confidence (McKenzie, 1978) and hence the relationship of plate tectonics to the observed stresses is unclear (Fu et al., 1979; Tseng, 1979). The purpose of this paper is to examine the seismic stresses of central Asia in an attempt to understand the processes now taking place. Some of the seismic zones are the
farthest from the major plate boundary. It has been found that the contemporary tectonics in this vast territory may be influenced by the convection-generated stress systems. The key observation is the recognition, on the satellite-determined patterns, of stress systems that are reconcilable with fault plane solutions of earthquakes. The results of this study indicate that the processes and flows going on in the upper mantle just below the lithosphere of central Asia play an important role in the Cenozoic tectonics.
2. Forces for continental deformation If the momentum involved in continental deformation in central Asia can be neglected, all crustal deformation in this region must be caused by forces now acting. There are three possible forces which could in principle maintain the observed deformation: gravitational force, forces due to plate motion, and forces on the base of the lithosphere. In general,
0031-9201/81/0000—0000/$ 02.50 © 1981 Elsevier Scientific Publishing Company
65
gravitational force must be involved. The similarity between the thrust faults and landslips from LANDSAT imagery indicates that considerable gravitational energy has been stored during the compression. However, it is difficult to believe that this storage maintains the tension in the Tibetan plateau and Lake Baikal region. This argument suggests that the compression is not the direct result of the local storage of gravitational energy, but must be maintained by forces due to plate motion or forces on the base of the.lithosphere which are generated by mantle convection. Forces due to plate motion might be important in the contemporary tectonics in central Asia. However, it is extremely difficult to understand how such collision forces can be transformed into the domal uplift and crustal tension observed in the Lake Baikal region according to the theory of plates and shells (Timoshenko and Woinowsky-Krieger, 1959). Furthermore, active tension behind collision zones is one of the most puzzling aspects in plate tectonics. In a simple view, regional compression is the natural expression of plate convergence. The observational fact that tension takes place just behind the Himalayan frontal thrust upsets this simple view and requires explanation. Although Molnar and Tapponnier (1978) and Tapponnier and Molnar (1979) have compiled geological maps for interpretation, their fluid crust carton (Molnar and Tapponnier, 1978, p. 5373) seems unsuitable to form a basis for analysis of crustal deformation. Thus, there are good reasons to suggest that only forces acting on the base of the lithosphere may account for the observations (Zonn and Florensov, 1979). Because so little is known about these forces from numerical and laboratory experiments on mantle convection systems, the major patterns of such force systems may come from mantle flows as inferred from satellite gravity data (Liu, 1977, 1978, l979a, b, 1980).
3. Convection-generated stress in central Asia Obtaining a global distribution of stresses would be a great achievement of science (Riznichenko, 1972). Unfortunately, stresses in the Earth are elusive variables which are difficult to obtain for even one point in the crust. Stress measurement techniques
often yield conflicting evidence concerning the state of stress in the crust. Because stress measurements are costly, they are scarce. For example, great efforts have failed to achieve the goal of developing a map for at least a small portion of the Earth showing the state of stress (Riecker, 1977). From the theory of mantle convection and the geoid, Runcorn (1964, 1967) has discussed contemporary stresses under the crust exerted by mantle convection flows. Using the mantle flow model as a guide, Liu (1977, 1978, 1979a) has proposed the application of the high degree harmonics of the geopotential from satellite gravity data to calculate the stress distribution in the crust caused by mantle convection. It has been shown that convection-generated stresses are probably the genetic cause for intra-plate volcanism (Liu, 1980) and intra-plate seismicity (Liu, 1979b). The stress components under the crust of central Asia exterted by mantle convection in the eastward and northward directions are determined by (Runcorn, 1967; Uu, 1977)
(~~ E
‘
~
=
Mg(a~\’~’2n + 1 1 n + 1 sin ~ 4ira2 \ a /
n=13 m0
/
m
—
.
—
X P,~(cos ~)[—mCn,m sm(mX) + mSn,m cos(mX)] mn UN(~ X)
,
~
=
n13
~fl1~
~
m=o 4na \ a I
x [P~(cos ~)][~
,~ ‘
2n + 1 n+1
cos(mX) + ~
m
d~ sin(mX)] (2)
where Mis the mass of the Earth, g the acceleration due to the gravity, ae the radius of the Earth, a the radius of the outer spherical surface of the flowing region, ~n,m and S~,n the coefficients of the harmonic terms, A the longitude, ~ the co-latitude and P~(cos~)is the associated Legendre polynomial of degree n, order m and argument cos By applying the methods of stress calculation (Liu, 1978), a stress vector map of central Asia has been produced, as shown in Fig. 1. The stress information in Fig. 1 is extracted from sateffite gravity data obtained by Lerch et al. (1979). This set of gravity data is based on 840000 satellite tracking measurements. The arrows in Fig. 1 are stress vectors and the
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Fig. 1 are comprised of several consistent stress patterns.Theremarkableradialdivergentfieldoftension intheTibetanplateauisoneofthem.Thenorth— south compression rn the Tien Shan region is another pattern. The resemblance between the compressive stress pattern and the plate collision along the Hima layan frontal thrust seems to be very strong.
4 Assumptions of convection-generated stress At present there is a lack of experimental information on the nature of convection in the upper mantle of the Earth and on the question of whether the lithospheric stress field is accompanied by mantle convection. However, there appear to be a number of
Fig. 1. Convection-generated stresses under central Asia.
geological convection-generated stresses arephenomena postulated in to which take place (Liu, 1977, 1978, 1979a, 1 979b, 1980). For pattern calculations of these stresses we have assumed that the high degree harmonics in the geopotential are related to mantle
magnitudes of the stresses are of the order of 108 dyn cm2 which is the same order of magnitude of stresses associated with the fracture of continents (Liu, 1977). The directions of convection flows in Fig. I are in good agreement with the convection pattems derived from the previous set of gravity data (Wagner et al., 1977; Liu, 1978). The fact that convection patterns derived from different gravity models have essentially the same features appears to reveal the real geophysical dimensions of any convection cells. It can be seen that the stress vectors in
convection patterns or tractions exerted on the base of the lithosphere, using results first given by Runcorn (1964, 1967). This assumption is contentious. Most geophysicists believed that Runcorn’s (1964) suggestion is a serious over-simplification and that Runcom’s theory of mantle convection and the geoid (1967) should be modified. Despite these difficulties it may be useful to apply the theory to central Asia, where the principal axes of stresses from seismic data are well-known. In this way the various simplifying assumptions can be tested or justified.
~
60
70
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$ iminin
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Is) DECELERATING: COMPRESSIONAL STRESS
(b) BILATERALLY OPPOSED: COMPRESSIONAL STRESS
4~>
Ic) CONVERGING: COMPRESSIONAL STRESS
Fig. 2. Flow patterns of the asthenosphere and crustal compression. See text for explanation.
67
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+1 t:~W: I I Ti TI Ti ~1
(a) Accelerating:
(b) Bilaterally opposed:
tensional fissures .L
tensional graben
ii~!4~~iI
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~t1ii..t..t.ii.i.i (c) Unequal parallel: feather fissures
(d) Diverging: tensional fissures
Fig. 3. Flow patterns of the asthenosphere and crustal tension. See
II
text for explanation.
5. Stress patterns
lying crust. Thirdly (c), uniform but converging current may likewise cause compressional stresses, which
In principle, three different types of movement in the substratum may cause compression in the overlying crust. These types of movement are illustrated in FIg. 2. The fIrst (a) is a decelerating, horizontally directed current, which is transmitted by friction to the overlying crust and causes compression in the direction of flow. Secondly (b), if a descending current forms bilateral or centripetal branches, these will obviously exert compressional force upon the over-
in some cases will be parallel to the direction of flow. The compressional stresses in the Tien Shan and Himalayan region consist of the three flow patterns shown in Fig. 2. Four different types of movement in the substraturn may cause tension in the overlying crust (Fig. 3). The first (a) is an accelerating, horizontally directed current, which is transmitted by friction to the overlying crust and causes tension in the direction of flow.
68
Secondly(b),ifanascendingcurrentsplitsinto bilateral or centrifugal branches, these will exert tensional force upon the overlying crust. Thirdly (c), horizontal and parallel subcrustal currents of differing velocities will set up torsional forces in the crust, which will give rise to the opening of fractures lying obliquely to the direction of flow. Fourthly (d), a uniform but diverging current may cause tensional fractureswhich in this case wifi be parallel to the direction of flow. The tensional stresses in the Tibetan plateau and Baikal rift region can be explained in terms of the mantle flow patterns shown in Fig. 3. Studies on the state of stress inferred from gravity data and the seismological framework in central Asia may be useful for earthquake research.
50
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6. Interpretation of fault plane solutions Fault plane solutions of earthquakes show that horizontal tectonic stresses are widespread in central Asia (Misharina, 1972;Molnar et al., 1973;Molnar and Tapponnier, l978;Ni, 1978;Ni and York, 1978; Rastogi, 1976; Ritsema, 1961; Sherman, 1978; Shirokova, 1967, 1974;Tapponnier and Molnar, 1979; Yeh et al., 1975). The principal axes of compressional and tensional stresses from fault plane solutions of earthquakes in this region are plotted in Fig. 4. Close examination of Figs. 2, 3 and 4 shows that the stresses as inferred from gravity data are reconcilable with the fault plane solutions of earthquakes. It has been demonstrated that, in plane projection, these stresses outline complex zones differingsharply from each other in the magnitude of horizontal compression, and in some zones compression is replaced by tension, Clearly, the structure of such complex stress fields cannot be investigated by simple fault plane solutions, and indirect geodynamical methods are required to investigate the magnitude and direction of stresses in mantle rocks. From fault plane solutions of earthquakes, a north—south compression system in the Himalayan frontal thrust has been established. Much of the late Cenozoic history of the Tibetan plateau is characterized by east—west tension,but in the Tien Shan and the south Mongolian region there is again north—south compression. North of Mongolia, compressive stress gives way to tensional stress, causing normal faulting
60
70
80
90
100
Fig. 4. Convection-generated stresses in central Asia and the tensional and compressional stresses (heavy arrows) inferred from fault-plane solutions of earthquakes.
in the Baikal rift system. The convection-generated stresses or forces proposed here (Figs. 1 and 4) explain all these fault-plane solutions clearly and simply. In all regions in Figs. 1 or 4, regardless of structure and all other characteristics, systematic horizontal compression and tension is observed. The system of compressive or tensional zones and the persistence of their topographic characteristics are consistent with only one zoning mechanism, namely the upweffing and downwelling of mantle material. In these regions, there exists a clear-cut correlation between the convection-generated stresses and the directions of the principal stresses derived from fault plane solutions of earthquakes. The convectiongenerated stress concentration should occur in narrow fault zones and causes earthquakes (Liu, 1 979b).
7, Is continental collision the only mechanism of Cenozoic tectonics? The hypothesis of New Global Tectonics can hardly explain the nature of intra-continental movement, intra-plate seismicity and intra.plate volcanism (Zorin and Florensov, 1979; Fu et al., 1979, Tseng,
69 1979; Liu, 1979b, 1980). However, based on the analysis of earthquake focal mechanisms, and of the surface structure derived from LANDSAT imagery, Molnar et al. (1973), Molnar and Tapponnier (1975, 1978) and Tapponnier and Molnar (1979) have attempted to explain the nature of Cenozoic tectonics in Asia. They consider the Eurasian plate to be not a monolith, but a body divided into some rigid ancient blocks surrounded by weak zones, which are Paleozoic and Mesozoic fold belts. The collision in the Eocene— Oligocene of the Indian and Eurasian plates has led to formation of the highest mountain ranges, the Himalaya and Pamir, on the plate boundary and to deformation of the Eurasian plate. This hypothesis may be over-simplified because data on the structure of the upper mantle beneath the uplifts of central Asia are neglected by its supporters. For Tibet, based on the absence of the Lg phase in records of earthquake waves, Molnar and Tapponnier (1975) have admitted that they must invoke a separate independent mechanism of thermal processes in the upper mantle to rationalize their models of crustal deformation. Recently, Zorin and Florensov (1979) have shown that the Tibetan plateau, Mongolian—Siberian uplift and the Baikal rift zone must have formed as a result of the initiation of hot spots beneath the lithosphere. As a matter of fact, Liu (1977, 1978, 1 979a) has already demonstrated, from satellite gravity data, that the dynamic effect of mantle upwelling plays a particular role in the formation of uplifts not only in Asia,but also in hfrica and Australia. In Fig. 1 it has been shown that all these uplifts or rift zones are related to tensional stresses resulting from the upweilingmaritle flows. In the creation of the linear frontal thrusting in the Himalaya and Tien Shan region, subcrustal cornpression caused by mantle convection, as shown in Fig. 1, probably played the Main part. The pattern of the downwelling mantle flows under these regions fol lows the outlines of the Tien Shan and Himalayan frontal thrusts. This downwelling of colder mantle material has played a subsidiary role, increasing the weakness of the lithosphere at the locus of the Paleozoic fold belt and shortening the crust through orogenesis in weak zones, The pattern of mantle convection (Fig. 1) inferred from sateffite gravity data does not seem to contradict the hypothesis of plate tectonics as it relates to
convergent plate boundaries. In Tibet the depth of the upwelling mantle convection cell is still less well defined. The mantle here could gain such properties from the accumulation of material from partial melting of the Indian lithospheric plate,which is underthrusting the Eurasian plate. If compression caused by the Indian and Eurasian plates is transferred at such long distances that it influences the structural pattern of the Baikal rift zone, as Molnar and Tapponnier (1975) postulated it must naturally influence all the structures situated between the Siberian plateform and the Himalayas. Therefore, the postulated compressional stresses due to plate collision may be partly responsible only for the creation of second.order structures with northwest- and sublatitudinal-oriented strikes. The exact correspondence of patterns of mantle convection with the main features of the Cenozoic uplifts, and with tectonic stresses inferred from seismic data, indicates that the uplifts are connected with the existence of upwelling mantle flows. For such a mantle flow pattern to exist in the asthenosphere for about 25 My, it must be constantly renewed. The renewal process may result from a constant inflow to the upper asthenosphere of heated material from greater depth, as a result of convection. This constant renewal of material allows the upwelling convection cells in the asthenosphere to remain stable in position.
Conclusion When the fault plane solutions of earthquakes in central Asia are compared with the convectiongenerated stresses calculated from satellite gravity data, several seemingly unrelated phenomena in Asia appear to have a common cause. The simultaneous compression in the Himalayan and Tien Shan regions is related to the convection-generated compressive stress systems. The convection-generated tensional stress patterns also predict tension in the Tibetan plateau adjacent to the plate collision zone and in the Baikal rift zone at a great distance from the plate boundary. The correspondence between the convectiongenerated stress patterns and the fault plane solutions of earthquakes in central Asia seems to support the
70
contention that most of the contemporary tectonics in Asia are influenced by mantle convection, and provides a unffymg explanation for the psieflOmena occurring there and for continental deformation in general.
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