Geodynamics of the Baikal-Stanovoy seismic belt

Physics of the Earth and Planetary Interiors, 31 (1983) 77-82

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Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

Geodynamics of the Baikal-Stanovoy seismic belt Han-Shou Liu Geodynamics Branch, Goddard Space Flight Center, Greenbelt, MD 20771 (U.S.A.) (Received May 6, 1982; revision accepted July 21, 1982)

Liu, H.-S., 1983. Geodynamics of the Baikal-Stanovoy seismic belt. Phys. Earth Planet. Inter., 31: 77-82. The convection generated tensional stress field in the Earth, as inferred from satellite gravity data, reveals an anomalous lens of upwelling mantle rocks under the Baikal rift zone. The point of no strain at 56°N 116°E forms a seismic gap along the Baikal-Stanovoy seismic belt. East of this point, the stress field changes from extension to compression. Therefore, the position of no strain at the eastern termination of the rift accounts for the dying-out of the rift zone and for the appearance of a compressive structure in the Stanovoy Range.

1. Introduction

time. Most significantly there exists a continuous belt of seismicity that cuts Asia almost exactly through the middle, from the Pamir Range southeast of Lake Baikal to the Stanovoy Range in the northeast. As shown in Fig. 1, the earthquake epicenters in Asia are generally scattered, and only

Although the Baikal uplands and the Stanovoy Range in the eastern Eurasian plate is generally thought to be an intraplate region, a number of major earthquakes have occurred there in historic 601

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within the Baikal rift zone and the Stanovoy Range are the epicenters concentrated into a narrow, linear seismic zone called the Baikal-Stanovoy seismic belt. An understanding of the tectonic processes responsible for this seismic belt is needed to place the assessment of seismic activity throughout eastern Asia on a firm geological and geophysical basis. Liu (1979) has shown that intraplate seismicity arises at the margin of convection-generated stress concentration, and that the direction of stresses in the earthquake foci may be related to the up- and down-welling mantle flows. The present article presents recent progress in understanding the convection-generated stress field, as inferred from satellite gravity data, under eastern Asia and the interaction of this stress field with specific geological structures. In keeping with the intended purpose of the application of the satellite gravity data to the development of mantle convection, an attempt is made to discuss areas where research can be focused to accelerate progress in understanding intraplate seismicity and tectonism.

2. The state of stress under the Baikal-Stanovoy region The stresses under the Baikal-Stanovoy region can be inferred from satellite gravity data (Liu, 1978). By applying the methods of stress calculation (Runcorn, 1967; Liu, 1977), a subcrustal stress m a p has been produced of the Baikal-Stanovoy region (Fig. 2). The stress vectors in Fig. 2 are comprised of several consistent stress patterns including the remarkable tensional stress field under the Baikal rift zone and the north-south compression in the Stanovoy Range region. Satellite stress fields should be analyzed in conjunction with other seismological structures. Thus the state of stress as inferred from satellite gravity data and the seismological framework in the Baikal-Stanovoy region may provide explanations for the puzzling seismic gap along the Baikal-Stanovoy seismic belt.

3. Seismicity and geological structure The region of diffused seismicity of central Asia terminates at the southern tip of the Baikal rift

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79 where it is replaced by a narrow belt called the narrow Baikal-Stanovoy seismic belt. All the strong earthquakes of the region are concentrated within this belt (Golonetsky, 1976; Kondorskaya and Shebalin, 1977; Sotonenko, 1978), which includes the Baikal rift zone and extends eastward as far as the sea of Okhotsk (Fig. 1). Around 118-120°E there is a small seismic gap where earthquake foci are absent. The principal topographic relief and neotectonic structures are significantly different for the western and eastern segments of the Baikal-Stanovoy seismic belt. The western segment coincides with the Baikal rift zone. Structures here are under extensional condition. The eastern segment of the belt is located between 120°E and the Okhotsk Sea coast. Within its limits, Baikal-type grabens disappear completely and the uplifted topography here points to compression rather than extension. The stress orientation of earthquake foci (Misharina, 1972; Misharina et al., 1975, 1977; Misharina and Solonenko, 1977; Zonenshain and Savostin, 1981) are also different in the western and eastern segments of the Baikal-Stanovoy seismic belt. The regional pattern of tectonic stresses in the western segment revealed by the focal mechanisms, for strong as well as groups of weak earthquakes, is amazingly consistent along almost the entire length of the Baikal rift zone. The regional stress pattern is characterized by the horizontal orientation of maximum extensional axes across the strike of the entire rift zone: In the eastern segment of the Baikal-Stanovoy seismic belt, at the Stanovoy Range, the focal mechanisms have not been studied as precisely as those in the western segment. Several focal mechanism solutions for the seismic events in the 121°E area show a dextral strike-slip displacement. One focal mechanism solution for the earthquake of 1972, with an epicenter 56.2°N 123.6°E (Das and Filson, 1975), indicates compression across the strike of the seismic belt which is in marked contrast to the extension observed in the Baikal segment of the seismic belt. These data, although sparse, show the pattern of tectonic stresses in the eastern segment to be significantly different to that in the western segment. In agreement with and in support of other results, the data indicate

that the eastern segment is in a state of compression. The pattern of convection-generated stresses (Fig. 2), as inferred from satellite gravity data, seems to agree with these seismological features. The most likely point of no strain along the Baikal-Stanovoy seismic belt is calculated to lie at 56°N 116°E (Fig. 2), which lies approximately at the position of the afore-mentioned seismic "gap". The computed stress patterns also adequately account for the principal neotectonic features of this region. The disappearance .of Baikal-type grabens in the eastern continuation of the Baikal zone at the Stanovoy Range is in full agreement with the fact that west of the point of no strain the Eurasian and Amurian plates are moving apart due to upweUing mantle flows and east of it they must be moving towards one another because of the downwelling mantle rocks. The stress pattern in the east side of the point of no strain indicates a state of compression, rather than extension, which agrees with the orientation of stresses in earthquake foci. Therefore, the position of no strain at the eastern termination of the Baikal rift accounts for the dying-out of the rift zone and for the appearance of the compressive structures in the Stanovoy Range. For the Baikal opening and the interaction between the Eurasian and Amurian plates, the computed stress pattern, as inferred from satellite gravity data, is perhaps a more reasonable explanation than the speculated distribution of stresses due to Eurasia-India plate collision. It is observed that the computed position of the pole of rotation between the Eurasian and Amurian plates at 56.95°N 117.45°E (Zonenshain and Savostin, 1981) coincides almost exactly with the point of no strain (56°N l16°E), or seismic gap, along the Baikal-Stanovoy seismic belt. This result suggests that mantle convection may be the main cause for seismotectonic block movements. Further studies on this subject are needed. 4. Mantle anomalies

The entire system of grabens in the Baikal rift zone was created in late Cenozoic time, primarily by extension rather than vertical arch-forming

80 movements (Zonenshain and Savostin, 1981). Zorin (1971) and Sherman (1977) have interpreted the characteristics of the Baikal rift zone as a precise reflection of development along a former zone of weakness. The Baikal rift zone is associated with major anomalies in the deep crustal structure. In this region, the crust is thinned to 35-37 km, compared to 38-40 km in the Siberian plate, but the contrast is more pronounced between this zone and the adjacent mountain ranges where the crustal thickness is as much as 45 km (Zorin, 1971; Puzyrev and Krylov, 1977). The isostatic equilibrium of the basins, and the crustal thinning beneath them, infers the presence of an excess of mass, or "antiroot" (Zorin, 1971; 1977). Such an antiroot may have been formed by the intrusion into the crust of basic and ultrabasic material from the mantle. Seismic sounding has shown that the mantle beneath Baikal has low seismic velocities of about 7.75 km s -1 (Puzyrev and Krylov, 1977), indicating that the asthenospheric layer closely approaches the base of the crust. Rogozhina (1977) outlined the region of the mantle anomaly which corresponds approximately to the area of the Baikal rift zone. A model of the deep structure of the Baikal rift zone has been proposed by Puzyrev and Krylov (1977). Their results suggest the presence, at the foot of the crust beneath the rift zone, of a lens-like anomalous body with low seismic velocities. Independent evidence for the existence of a deep structural anomaly under Baikal is given by results of magnetotelluric probing (Vanyan and Kharin, 1967; Popov, 1977) that show a high conductivity layer at 35-40 km depth, coincident with the base of the crust, which is underlain by a low-conductivity layer, and outlines the lens-like shape of the high-conductivity body. The convection stress pattern in Fig. 2 seems to agree with the deep structure and the tectonic development of the Baikal rift zone. Figure 2 shows an anomalous lens of upwelling mantle rocks under the rift zone which can be confidently interpreted as an asthenospheric protrusion.

5. Mechanism for Baikal rift system

Two conditions need to be fulfilled to enable a continental region to rift apart, (1) horizontal deviatory tension, (2) a copious supply of basaltic magma in the upper mantle to initiate and continue the intrusion. Therefore, two main problems of continental rifting are to understand how the tension and the magma source arise. The convection stress pattern in Fig. 2 indicates that the tension may occur in response to the development of a hot magma saturated region in the underlying upper mantle. Here, the hot upper mantle has probably been formed by convective upwelling from deep in the mantle, allowing the escape of heat from the Earth's interior. This causes volcanism and isostatic uplift of the heated and thinned lithosphere. It is improbable that the crustal tension causing the Baikal rifting was originated by either the suction force acting on overriding plates at trenches (Elsasser, 1971; Forsythe and Uyeda, 1975) or the collision force between the Indian and Eurasian plate motions along the Himalayan frontal thrust (Molnar and Tapponnier, 1975). The convection stress patterns in this region, as inferred from satellite gravity data, may provide another example of tectonic processes for continental rifting. Is stress-generated crustal rifting, caused by the thermally generated upwelling mantle rocks, associated with doming and volcanism? One possibility is that doming occured first, followed by rifting. Alternatively, tensional cracking of the lithosphere may occur first and the hot spots may be a consequent development of upwelling into the cracks (Oxburgh and Turcotte, 1974). The geological evidence, from the order of events (Kiselev et al., 1978) in this region, seems to show that early volcanism precedes doming and that doming precedes rifting. Thus, the thermally generated upwelling rocks under the Baikal region, as shown in Fig. 2, appear to be the primary feature, with subsequent rifting. I have described the essential dynamics of rifting, the forcing of mantle flow on the base of the crust and its stress patterns in the Baikal region. Illustrations of the dynamic principles were drawn from satellite gravity data. The subcrustal stress

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fields are consistent with the Structure of the mantle anomaly. Geodynamics seeks to ascertain the causes of crustal rifting and its fundamental connection with geophysical and seismological features. This study has developed a genetic model which may account for the Baikal rift system and the associated crustal compression of the Stanovoy Range. Nature has presented major continental rift systems on the Earth, which show a wide variety of morphologies. Nevertheless, I have shown that the same geodynamic principles govern the structure of the eastern African rifts (Liu, 1981). It is suggested here that rift evolution in continents is driven by the inexorable dissipation of thermal energy from the mantle and the concomitant upward transport of the asthenosphere.

6. Discussion

The knowledge provided by the tectonic structure of the Baikal-Stanovoy region is profound. It provides geological and geophysical facts to test whether the solutions of the focal mechanism for earthquakes, the distribution of orogenic activity at any point in Cenozoic time, recent crustal movements and upper mantle structure can be integrated within a single space geodynamics program. Some time ago, detailed calculations on the subcrustal stresses from satellite gravity data were carried out by the author. Since the original calculation, a number of applications of the theory have become plausible, and the recent improvements of the satellite gravity data enables a re-evaluation of the applicability of the subcrustal stress patterns. In this paper, analysis by the Goddard Gravity Field Model (Lerch et al., 1979) is used in developing a method for determining the direction and horizontal projections of subcrustal stresses in the Baikal-Stanovoy region. This study involves a search for mutual agreement over large areas of the geopotential field with respect to seismic stresses. The results determined a seismic gap and recognized two types of stress regime in the structure of the Baikal-Stanovoy seismic belt. The method and program developed for identifying

stress zones from satellite gravity data make it possible to direct the search for the mechanisms of earthquakes and rifts toward these stress structures. The interdisciplinary investigations of the tensional and compressional tectonic regimes validate and support the subcrustal stress pattern as the most probable model for the anomalous mantle beneath the Baikal rift region. Satellite measurements of the Earth's gravity offer a variety of models to interpret the interior of the Earth. Among the various interpretations, the mantle convection patterns appear to provide an intraplate parameter which corresponds to the state of stress, Therefore, the mantle convection patterns inferred from satellite gravity data are of general applicability for geodynamics. However, it is important to take various precautions in the patterning, such as consideration of the block areas involved and the mode of pattern production. For application of convection patterns to different regions, it is particularly important that the geodynamic features of their construction should be considered.

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