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Deep structure of the baikal and other continental rift zones from seismic data
Tectonophysics, 45 (1978) 15-22 0 Elsevier Scientific Publishing Company,
15 Amsterdam
- Printed
in The Netherlands
DEEP STRUCTURE OF THE BAIKAL AND OTHER CONTINENTAL RIFT ZONES FROM SEISMIC DATA
N.N. PUZYREV l, M.M. MANDELBAUM G.V. PETRIK l, and G.V. KRUPSKAYA
2, S.V. KRYLOV
‘, B.P. MISHENKIN
I,
2
1 Institute of Geology and Geophysics, Novosibirsk (U.S.S.R.) 2 Eastern Geophysical Trust, Irkutsk (U.S.S.R.) (Submitted
April 20, 1977; received
May 10, 1977)
ABSTRACT Puzyrev, N.N., Mandelbaum, M.M., Krylov, S.V., Mishenkin, B.P., Petrik, G.V. and Krupskaya, G.V., 1978. Deep structure of the Baikal and other continental rift zones from seismic data. In: N.A. Logatchev and P.A. Mohr (editors), Geodynamics of the Baikal Rift Zone. Tectonophysics, 45: 15-22. Abyssal variations beneath the Baikal rift zone are revealed in an irregular seismic stratification of the crust, the presence of an intracrust waveguide and by the vast (>200,000 km2) underlying area of anomalously low velocity (P, = 7.6-7.6 km/set) uppermost mantle. In its abyssal structure the Baikal rift is heterogeneous along the strike, with sharp changes in crustal thickness (35-50 km). Comparison of first-arrival seismic-velocity curves and also the respective velocity columns reveals the essential similarity of upper-mantle seismic cross-sections for all continental rift zones. The anomalous upper layer of the mantle (ca. 7.7 km/set) is relatively thin (15-13 km) and can be linked with the mantle waveguide only locally.
INTRODUCTION
The first systematic deep seismic investigations of continental rifts were carried out in the early sixties in western U.S.A. by the point and piecewise profiling method (Pakiser, 1963). Also, a specific nuclear explosion experiment was performed in Nevada with wave recordings at distances of up to 1000 km and more, to provide characteristics both of the crust and upper mantle. In 1966-67 large-scale explosion seismology was begun around rift zones in Europe (the Rhine and the Liman graben). Since that time, profiles have been made to virtually encompass these zones (Illies and Fuchs, 1974; Mueller et al., 1969). Point observations provided individual travel-time curves. The East African rift system has been less well studied by explosion seismology methods. In 1967-68 observations at ten recording stations between Lakes Rudolf and Hannington were made with explosion points in these
16
lakes (Griffiths, 1972). Lately, extensive seismic refraction done in the Afar region (Berckhemer et al., 1975). EXPLORATION
work has been
METHODS
The Baikal rift zone has been studied since 1968 by deep seismic sounding (DSS) techniques as a modification of point (differentiated) soundings (Puzyrev et al., 1975), work carried out by the Eastern Geophysical Trust under the Ministry of Geology and the Institute of Geology and Geophysics, Siberian Branch of the U.S.S.R. Academy of Sciences (Puzyrev et al., 1974). Thus far, an area of 400,000 km2 with a total length of profiles of some 4000 km has been covered, primarily within the central part of the zone. Study of the terminal segments of the Baikal rift zone has recently been started, and though the work is not yet complete, the Baikal zone is perhaps more thoroughly studied than any other continental rift system. Lately the depths reached by explosion seismic studies have been increased, in work performed around the northeastern periphery of the Iceland rift zones (Zverev et al., 1975). A series of overtaking travel-time curves for a 500 km length was obtained from land and sea recording stations by the technique of movable marine explosion sites, which yielded cross-section depths down to about 60 km. The seismic work done in the Baikal and Icelandic rift zones is not yet sufficiently detailed to detect other than the gross features of the deepest structures. Study of local and remote natural earthquakes has greatly supplemented the study of the upper mantle beneath rift zones. The use of portable telecontrolled “Taiga” equipment has made it possible to do field work in almost inaccessible regions. REVIEW
OF RESULTS
We consider the main results obtained from the Baikal rift and compare them with seismic data for other continental rifts, to seek common abyssal features of these zones. The DSS-observations in the Baikal region were aimed at obtaining new data to lay the basis for a more reliable interpretation of other geophysical parameters, which otherwise left considerable ambiguity concerning even the general features of the Baikal rift deep structure. Taking into account the well-known peculiarities of the deep structure of rift zones, the application of DSS-techniques proceeded from a consideration of the following factors. (1) Rifting is the result of mantle processes. Therefore, it is essential to obtain as much information about the structure and properties of the upper parts of the mantle as possible. Contributing to this work, the records of natural and artificial earthquakes have been interpreted in conjunction (Krylov et al., 1974). (2) It is not always possible to approximate the complicated deep struc-
17
tures typical of rifts by two-dimensional seismic models even on a regional scale. For this reason areal seismic sounding schemes were used on the more complicated rift sectors, concentrating on the Moho and its local variations in depth. (3) Rift zones are characterized both by a complicated internal geometry and an anomalous physical state of the deep material. Therefore, special importance is attached to the study of seismic velocities in that material to detect and map low-velocity layers in three dimensions. The reliability of information on velocities and geometry of the anomalous mantle was enhanced by combined interpretation of waves of differing types: reflected, quasileading and refracted. Only the most pronounced and clearly distinguishable waves were recorded for interpretation, at the optimum distances from the shotpoints. Interpretation based on the use of special time fields permitted us to take into account large horizontal inhomogeneities in the upper mantle. The standard error of individual velocities is estimated to be 10.1--0.2 km/set and that of depths +l-2 km. The main results of deep seismic investigations in the Baikal rift zone are as follows. The areal and profile observations reveal the pattern of crustal thickness in the central and eastern parts of the rift zone, as well as in the adjacent parts of the Siberian platform and Zabaikalye area. The Moho map (Fig. 1) shows that in the central sector of the Baikal rift the crustal thickness is subject to greater variations than in the neighbouring less active areas. Depths vary from 34 to 48 km in the rift zone, from 37 to 39 km in the Siberian platform and from 39 to 42 km in Zabaikalye. An essential difference in crustal thickness was observed under the southwestern (deeper water) and northwestern parts of Lake Baikal (34-36 and 40-44 km, respectively). The conjunction of these sharply differing crustal regimes occurs near Olkhon Island and Svyatoy Nos Peninsula where a large deep fault zone is considered to exist. Let us examine the Moho along different cross-sections of the Baikal depression (Fig. 1). In the extreme northern cross-section, the base of the crust occurs at nearly the same level (42-44 km) under the eastern margin of Baikal, under the Barguzin depression and under the Barguzin range separating them. Further to the south, at the conjunction of the northern and southern sub-basins of the lake, the Moho is depressed except in its axial part where it rises steeply, i.e. there is a peculiar combination of a root and a superimposed antiroot, with a change of crustal thickness from 44 to 35 km. Two sections across the southern Baikal sub-basin in the regions of Olkhon Island and the Selenga River delta show a similar root-antiroot Moho profile, but here the antiroot is observed under the northwestern flank of the Baikal depression with an amplitude of 3-6 km. The eastern continuation of the Baikal rift zone is crossed by the ChitaLensk profile in the Muya depression area, and shows a wide sag of the Moho
1x
35 o-d_-
100
200k-cm
__.
Lnh ---
4.5km
2 300000 I
I d&++o
/ /
/
.oo
0
00°
Fig. 1. Schematic diagram of the upper-mantle structure in the Baikal region. I = depth scale to the mantle surface; 2 = limits of the region with anomalously low velocity (upper surface); 3 = limits of the rift zone from geological data; 4 = DSS-profiles.
with an amplitude of about 5 km and a maximum depth of 43 km. Thus, the data obtained by DSS-method on Moho morphology beneath the Baikal rift zone characterize it as a complicated surface, and irregular along the rift-zone strike. Discrepancies with earlier assumptions concerning crustal thickness values and gradients are rather great. A feature of the interior structure of the Earth’s crust in the Baikal region is the varying degree of stratification of crustal elasticity in different parts of the region, and in velocity inversion at depth. Regular seismic-velocity strati-
19
fication within the crust, with some distinct and extensive subhorizontal boundaries, has been established in the south of the Siberian platform and in the Chita region. The intracrustal seismic boundaries in the rest of the area, including the rift zone and the adjacent parts of Zabaikalye, can be continuously traced over only short horizontal distances. Is is of great importance to delineate low-velocity layers in the Earth’s crust since rock material with lower rigidity, which plays a major role in controlling deep geodynamics and the distribution of earthquake foci, may correspond to these layers under certain conditions. With this in mind, special observations of refracted and reflected seismic waves were carried out at two sites on the southeastern shore of Lake Baikal, some 150 km apart and taking into account the previously established horizontal inhomogeneities of the lithosphere. A low-velocity layer with a velocity differential of minus 0.2-0.3 km/set relative to the containing medium has been detected under both sites. Other geophysical investigations show that the low-velocity crustal layer is composed of rocks with high electric conductivity, and the major mass of magnetic rocks and the level containing most earthquake foci overlie this layer. Most likely the low-velocity layer results from increased heating and partial melting. The upper-mantle cross-section in the Baikal zone, as compiled from the data of explosion and earthquake seismology (Alekseyev et al., 1971; Krylov et al., 1974; Krylov et al., 1975), is characterized as follows (Fig. 2): The
I
h.hm
Fig. 2. Columns of P-wave velocities for continental rift zones: I = the Baikal zone; II = northern part of the East African zone; III = northern part of the Rhine graben; IV = North American Basin and Range Province. 1 = the crust (the dashed portion is an interval with a low P-wave velocity); 2 = upper-mantle layer with anomalous low velocity (7.7 km/set); 3 = mantle with normal velocity (8.0-8.2 km/set); 4 = low-velocity zone;M = Moho discontinuity.
20
uppermost mantle is formed by a layer of anomalously low velocity (7.67.8 km/see). This anomalous layer has a relatively small thickness averaging only 17 km over the entire region. The anomalous layer is separated from the Gutenberg waveguide by a rock mass some tens of kilometres thick with normal mantle P-wave velocity of 8.0-8.1 km/see. A narrow vertical connection between the anomalous layer and the deeper low-velocity layer is most likely located under the strip of the Baikal-Vitim gravity minimum, i.e. along the rift zone. The area of the anomalous upper mantle has been mapped over an area of 200,000 km2 (Fig. 1). Outside this area the P,-velocity has normal values of 8.1-8.2 km/set, increasing to 8.6 km/set in the Lensk region. The boundaries of the anomalous mantle occupy a discordant position with regard to the ancient basement structures of the crust, though they do coincide along some short sectors, The area of the anomalous mantle is 2-3 times wider than the Baikal rift zone proper as defined from surface geology. The central part of the rift zone (the Baikal depression) lies over the northwestern fringe of the anomalous mantle area and the eastern prolongation of the zone lies over the central part of the area. We now briefly compare results of seismic studies of the Baikal region with data on other continental rifts. Limiting the comparison to general characteristics, to avoid influence from differences in investigative techniques: crustal thinn~g occurs beneath certain rift zones, for example the Rhine graben (Mueller et al., 1969) and the Basin and Range province (Belyaevsky, 1970), but is not common to all continental rifts, and in the instance of Baikal is absent. Intracrustal low-velocity layers appear to be typical of continental rifts. Apart from Baikal, the existence of such layers is noted under the Rhine graben (Mueller et al., 1969) and in the western part of North America (Prodehl, 1970) (Fig. 2). In some parts of the World Rift System, for example Iceland and the Red Sea (Belyaevsky, 1970), high velocity values explicable in terms of axial intrusion of dense mantle rocks are found in the upper parts of the crust. But no such feature has been detected along the axis of the BaikaI rift zone. To make an objective comparison of seismic ch~ae~~stics of the upper mantle under rifts, summ~ travel-time curves of Pa-waves from explosions (nuclear and chemical) and local earthquakes recorded at several hundreds of kilometres from the shotpoints were compiled for all the continental rifts including Baikal (Krylov et al., 1975). Published values for the Eastern Rift of Africa (Griffiths, 1972; Searle and Gouin, 1971), the northern part of the Rhine graben (Mueller et al., 1969) and the North American Basin and Range province (Pakiser and Hill, 1963; Ryal and Stuart, 1963) served as the initial data. Summary P, travel-time curves are generally remarkably similar for all these regions. At distances of 120-180 km from the shotpoint, the refracted wave with a velocity of about 7.7 km/see is the first to be recorded every-
21
where, and can be detected out to as far as 300 km. Farther from the shotpoint the travel-time curve inflects, and the head-wave has a mean velocity, of 8.0-8.2 km/set. Figure 2 shows schematic velocity columns for world rift zones compiled from seismic data on the Gutenberg waveguide (Valyuss et al., 1969; Jonson, 1967; Knopoff and Schlue, 1972). An anomalously lowvelocity upper-mantle layer, VP = 7.7 km/set, is of regional extent. The thickness of this layer in different regions averages from 15 to 30 km. Velocity values in this anomalous layer and in the mantle Gutenberg waveguide are practically the same, but these objects do not represent a single structure as they are separated by a high velocity, 8-8.2 km/set, level. CONCLUSIONS
Comparison of the Baikal rift with other continental rifts establishes an essentially similar seismic layering down through the crust, and especially the upper mantle as deep as the Gutenberg layer. It is likely that recent deep processes and the physical state of the upper mantle are the same beneath all continental rifts of the world. The wellknown velocity decrease in the uppermost part of the mantle is peculiar in that it characterizes a relatively thin layer, and can be linked with the asthenospheric waveguide only locally. REFERENCES Alekseyev, A.S., Lavrentyev, M.M., Mukhometov, R.G., Nersesov, I.L. and Romanov, V.G., 1971. Digital method of determining the Earth’s upper-mantle structure. In: Matemat. problemy geofiiiki. VC AN SSSR, Novosibirsk, pp. 143-165. Belyaevsky, N.A. (Editor), 1970. Systems of the Earth’s rifts. Mir, 280 p. Berckhemer, H., Baier, B., Bartelsen, H., Behle, A., Burkhardt, H., Gebrande, H., Makris, J., Menzel, H., Miller, H. and Vees, R., 1975. Deep seismic soundings in the Afar region and on the highland of Ethiopia. In: A. Pilger and A. RGsler (Editors), Afar Depression of Ethiopia. Schweizerbart, Stuttgart, pp. 89-107. Griffiths, D.H., 1972. Some comments on the seismic refraction experiment in the Kenya rift. Tectonophysics, 15: 151-156. Illies, J.H. and Fuchs, K. (Editors), 1974. Approaches to Taphrogenesis. Schweizerbart, Stuttgart, 315 pp. Jonson, L.R., 1967. Array measurements of P-velocitites in the upper mantle. J. Geophys. Res., 72: 6309-6325. Knopoff, L. and Schlue, J.W., 1972. Rayleigh wave phase velocities for the path Addis Ababa-Nairobi. Tectonophysics, 15: 157-163. Krylov, S.V., Golenetsky, S.I. and Petrik, G.V., 1974. Agreement of seismology and DSSdata on the uppermost mantle structure of the Baikal rift zone. Geol. i geofiz. No. 12: 61-65. Krylov, S.V., Mishenkin, B.P., Mishenkina, Z.R., Petrik, G.V. and Seleznev, V.S., 1975. Seismic cross-section of the lithosphere in the Baikal rift zone. Geol. i geofiz. NO. 3: 72-83. Mueller, St., Peterschmitt, E. and Ansorge, J., 1969. Crustal structure beneath the Rhine graben from seismic refraction and reflection measurements. Tectonophysics, 8: 529542.
22 Pakiser, L., 1963. Structure of the crust and upper mantle in the western United States. J. Geophys. Res., 68: 5747-5756. Pakiser, L.C. and Hill, D.P., 1963. Crustal structure in Nevada and southern Idaho from nuclear explosions. J. Geophys. Res., 68: 5757-5766. Prodehl, C., 1970. Seismic refraction study of crustal structure in the western United States. Geol. Sot. Am. Bull., 81: 2629-2646. Puzyrev, N.N., Mandelbaum, M.M., Krylov, S.V., Mishenkin, B.P., Krupskaya, G.V. and Petrik, G.V., 1974. Deep structure of the Baikal rift from explosion seismology data. Geol. i geofiz, No. 5: 155-167. Puzyrev, N.N., Krylov, S.V. and Mishenkin, B.P., 1975. Techniques of reconnaissance deep seismic investigations. Nauka, Moscow, 158 pp. Ryal, A. and Stuart, D.J., 1963. Travel times of some local earthquake phases originating from nuclear explosions. Nevada Test Site to Ordway, Colorado. J. Geophys. Res., 68: 5821-5835. Searle, R.C. and Gouin, P., 1971. Analysis of some local earthquake phases originating near the Afar triple junction, Bull. Seismol. Sot. Am., 61: 1061-1067. Valyuss, V.P., Keilis-Borock, B.I. and Levshin, A.L., 1969. Determinations of the velocity cross-section of the upper mantle of Europe. Dokl. AN SSSR, 185, No. 3: 564-567. Zverev, S.M., Kosminskaya, I.P., Krasilshchikova, G.A. and Mikhota, G.F., 1975. Deep structure of Iceland and the Iceland-Faeroes-Scotland region from the results of seismic studies, Bull. MQIP, otd. geol., vyp. 3: 99-I 15.
15 Amsterdam
- Printed
in The Netherlands
DEEP STRUCTURE OF THE BAIKAL AND OTHER CONTINENTAL RIFT ZONES FROM SEISMIC DATA
N.N. PUZYREV l, M.M. MANDELBAUM G.V. PETRIK l, and G.V. KRUPSKAYA
2, S.V. KRYLOV
‘, B.P. MISHENKIN
I,
2
1 Institute of Geology and Geophysics, Novosibirsk (U.S.S.R.) 2 Eastern Geophysical Trust, Irkutsk (U.S.S.R.) (Submitted
April 20, 1977; received
May 10, 1977)
ABSTRACT Puzyrev, N.N., Mandelbaum, M.M., Krylov, S.V., Mishenkin, B.P., Petrik, G.V. and Krupskaya, G.V., 1978. Deep structure of the Baikal and other continental rift zones from seismic data. In: N.A. Logatchev and P.A. Mohr (editors), Geodynamics of the Baikal Rift Zone. Tectonophysics, 45: 15-22. Abyssal variations beneath the Baikal rift zone are revealed in an irregular seismic stratification of the crust, the presence of an intracrust waveguide and by the vast (>200,000 km2) underlying area of anomalously low velocity (P, = 7.6-7.6 km/set) uppermost mantle. In its abyssal structure the Baikal rift is heterogeneous along the strike, with sharp changes in crustal thickness (35-50 km). Comparison of first-arrival seismic-velocity curves and also the respective velocity columns reveals the essential similarity of upper-mantle seismic cross-sections for all continental rift zones. The anomalous upper layer of the mantle (ca. 7.7 km/set) is relatively thin (15-13 km) and can be linked with the mantle waveguide only locally.
INTRODUCTION
The first systematic deep seismic investigations of continental rifts were carried out in the early sixties in western U.S.A. by the point and piecewise profiling method (Pakiser, 1963). Also, a specific nuclear explosion experiment was performed in Nevada with wave recordings at distances of up to 1000 km and more, to provide characteristics both of the crust and upper mantle. In 1966-67 large-scale explosion seismology was begun around rift zones in Europe (the Rhine and the Liman graben). Since that time, profiles have been made to virtually encompass these zones (Illies and Fuchs, 1974; Mueller et al., 1969). Point observations provided individual travel-time curves. The East African rift system has been less well studied by explosion seismology methods. In 1967-68 observations at ten recording stations between Lakes Rudolf and Hannington were made with explosion points in these
16
lakes (Griffiths, 1972). Lately, extensive seismic refraction done in the Afar region (Berckhemer et al., 1975). EXPLORATION
work has been
METHODS
The Baikal rift zone has been studied since 1968 by deep seismic sounding (DSS) techniques as a modification of point (differentiated) soundings (Puzyrev et al., 1975), work carried out by the Eastern Geophysical Trust under the Ministry of Geology and the Institute of Geology and Geophysics, Siberian Branch of the U.S.S.R. Academy of Sciences (Puzyrev et al., 1974). Thus far, an area of 400,000 km2 with a total length of profiles of some 4000 km has been covered, primarily within the central part of the zone. Study of the terminal segments of the Baikal rift zone has recently been started, and though the work is not yet complete, the Baikal zone is perhaps more thoroughly studied than any other continental rift system. Lately the depths reached by explosion seismic studies have been increased, in work performed around the northeastern periphery of the Iceland rift zones (Zverev et al., 1975). A series of overtaking travel-time curves for a 500 km length was obtained from land and sea recording stations by the technique of movable marine explosion sites, which yielded cross-section depths down to about 60 km. The seismic work done in the Baikal and Icelandic rift zones is not yet sufficiently detailed to detect other than the gross features of the deepest structures. Study of local and remote natural earthquakes has greatly supplemented the study of the upper mantle beneath rift zones. The use of portable telecontrolled “Taiga” equipment has made it possible to do field work in almost inaccessible regions. REVIEW
OF RESULTS
We consider the main results obtained from the Baikal rift and compare them with seismic data for other continental rifts, to seek common abyssal features of these zones. The DSS-observations in the Baikal region were aimed at obtaining new data to lay the basis for a more reliable interpretation of other geophysical parameters, which otherwise left considerable ambiguity concerning even the general features of the Baikal rift deep structure. Taking into account the well-known peculiarities of the deep structure of rift zones, the application of DSS-techniques proceeded from a consideration of the following factors. (1) Rifting is the result of mantle processes. Therefore, it is essential to obtain as much information about the structure and properties of the upper parts of the mantle as possible. Contributing to this work, the records of natural and artificial earthquakes have been interpreted in conjunction (Krylov et al., 1974). (2) It is not always possible to approximate the complicated deep struc-
17
tures typical of rifts by two-dimensional seismic models even on a regional scale. For this reason areal seismic sounding schemes were used on the more complicated rift sectors, concentrating on the Moho and its local variations in depth. (3) Rift zones are characterized both by a complicated internal geometry and an anomalous physical state of the deep material. Therefore, special importance is attached to the study of seismic velocities in that material to detect and map low-velocity layers in three dimensions. The reliability of information on velocities and geometry of the anomalous mantle was enhanced by combined interpretation of waves of differing types: reflected, quasileading and refracted. Only the most pronounced and clearly distinguishable waves were recorded for interpretation, at the optimum distances from the shotpoints. Interpretation based on the use of special time fields permitted us to take into account large horizontal inhomogeneities in the upper mantle. The standard error of individual velocities is estimated to be 10.1--0.2 km/set and that of depths +l-2 km. The main results of deep seismic investigations in the Baikal rift zone are as follows. The areal and profile observations reveal the pattern of crustal thickness in the central and eastern parts of the rift zone, as well as in the adjacent parts of the Siberian platform and Zabaikalye area. The Moho map (Fig. 1) shows that in the central sector of the Baikal rift the crustal thickness is subject to greater variations than in the neighbouring less active areas. Depths vary from 34 to 48 km in the rift zone, from 37 to 39 km in the Siberian platform and from 39 to 42 km in Zabaikalye. An essential difference in crustal thickness was observed under the southwestern (deeper water) and northwestern parts of Lake Baikal (34-36 and 40-44 km, respectively). The conjunction of these sharply differing crustal regimes occurs near Olkhon Island and Svyatoy Nos Peninsula where a large deep fault zone is considered to exist. Let us examine the Moho along different cross-sections of the Baikal depression (Fig. 1). In the extreme northern cross-section, the base of the crust occurs at nearly the same level (42-44 km) under the eastern margin of Baikal, under the Barguzin depression and under the Barguzin range separating them. Further to the south, at the conjunction of the northern and southern sub-basins of the lake, the Moho is depressed except in its axial part where it rises steeply, i.e. there is a peculiar combination of a root and a superimposed antiroot, with a change of crustal thickness from 44 to 35 km. Two sections across the southern Baikal sub-basin in the regions of Olkhon Island and the Selenga River delta show a similar root-antiroot Moho profile, but here the antiroot is observed under the northwestern flank of the Baikal depression with an amplitude of 3-6 km. The eastern continuation of the Baikal rift zone is crossed by the ChitaLensk profile in the Muya depression area, and shows a wide sag of the Moho
1x
35 o-d_-
100
200k-cm
__.
Lnh ---
4.5km
2 300000 I
I d&++o
/ /
/
.oo
0
00°
Fig. 1. Schematic diagram of the upper-mantle structure in the Baikal region. I = depth scale to the mantle surface; 2 = limits of the region with anomalously low velocity (upper surface); 3 = limits of the rift zone from geological data; 4 = DSS-profiles.
with an amplitude of about 5 km and a maximum depth of 43 km. Thus, the data obtained by DSS-method on Moho morphology beneath the Baikal rift zone characterize it as a complicated surface, and irregular along the rift-zone strike. Discrepancies with earlier assumptions concerning crustal thickness values and gradients are rather great. A feature of the interior structure of the Earth’s crust in the Baikal region is the varying degree of stratification of crustal elasticity in different parts of the region, and in velocity inversion at depth. Regular seismic-velocity strati-
19
fication within the crust, with some distinct and extensive subhorizontal boundaries, has been established in the south of the Siberian platform and in the Chita region. The intracrustal seismic boundaries in the rest of the area, including the rift zone and the adjacent parts of Zabaikalye, can be continuously traced over only short horizontal distances. Is is of great importance to delineate low-velocity layers in the Earth’s crust since rock material with lower rigidity, which plays a major role in controlling deep geodynamics and the distribution of earthquake foci, may correspond to these layers under certain conditions. With this in mind, special observations of refracted and reflected seismic waves were carried out at two sites on the southeastern shore of Lake Baikal, some 150 km apart and taking into account the previously established horizontal inhomogeneities of the lithosphere. A low-velocity layer with a velocity differential of minus 0.2-0.3 km/set relative to the containing medium has been detected under both sites. Other geophysical investigations show that the low-velocity crustal layer is composed of rocks with high electric conductivity, and the major mass of magnetic rocks and the level containing most earthquake foci overlie this layer. Most likely the low-velocity layer results from increased heating and partial melting. The upper-mantle cross-section in the Baikal zone, as compiled from the data of explosion and earthquake seismology (Alekseyev et al., 1971; Krylov et al., 1974; Krylov et al., 1975), is characterized as follows (Fig. 2): The
I
h.hm
Fig. 2. Columns of P-wave velocities for continental rift zones: I = the Baikal zone; II = northern part of the East African zone; III = northern part of the Rhine graben; IV = North American Basin and Range Province. 1 = the crust (the dashed portion is an interval with a low P-wave velocity); 2 = upper-mantle layer with anomalous low velocity (7.7 km/set); 3 = mantle with normal velocity (8.0-8.2 km/set); 4 = low-velocity zone;M = Moho discontinuity.
20
uppermost mantle is formed by a layer of anomalously low velocity (7.67.8 km/see). This anomalous layer has a relatively small thickness averaging only 17 km over the entire region. The anomalous layer is separated from the Gutenberg waveguide by a rock mass some tens of kilometres thick with normal mantle P-wave velocity of 8.0-8.1 km/see. A narrow vertical connection between the anomalous layer and the deeper low-velocity layer is most likely located under the strip of the Baikal-Vitim gravity minimum, i.e. along the rift zone. The area of the anomalous upper mantle has been mapped over an area of 200,000 km2 (Fig. 1). Outside this area the P,-velocity has normal values of 8.1-8.2 km/set, increasing to 8.6 km/set in the Lensk region. The boundaries of the anomalous mantle occupy a discordant position with regard to the ancient basement structures of the crust, though they do coincide along some short sectors, The area of the anomalous mantle is 2-3 times wider than the Baikal rift zone proper as defined from surface geology. The central part of the rift zone (the Baikal depression) lies over the northwestern fringe of the anomalous mantle area and the eastern prolongation of the zone lies over the central part of the area. We now briefly compare results of seismic studies of the Baikal region with data on other continental rifts. Limiting the comparison to general characteristics, to avoid influence from differences in investigative techniques: crustal thinn~g occurs beneath certain rift zones, for example the Rhine graben (Mueller et al., 1969) and the Basin and Range province (Belyaevsky, 1970), but is not common to all continental rifts, and in the instance of Baikal is absent. Intracrustal low-velocity layers appear to be typical of continental rifts. Apart from Baikal, the existence of such layers is noted under the Rhine graben (Mueller et al., 1969) and in the western part of North America (Prodehl, 1970) (Fig. 2). In some parts of the World Rift System, for example Iceland and the Red Sea (Belyaevsky, 1970), high velocity values explicable in terms of axial intrusion of dense mantle rocks are found in the upper parts of the crust. But no such feature has been detected along the axis of the BaikaI rift zone. To make an objective comparison of seismic ch~ae~~stics of the upper mantle under rifts, summ~ travel-time curves of Pa-waves from explosions (nuclear and chemical) and local earthquakes recorded at several hundreds of kilometres from the shotpoints were compiled for all the continental rifts including Baikal (Krylov et al., 1975). Published values for the Eastern Rift of Africa (Griffiths, 1972; Searle and Gouin, 1971), the northern part of the Rhine graben (Mueller et al., 1969) and the North American Basin and Range province (Pakiser and Hill, 1963; Ryal and Stuart, 1963) served as the initial data. Summary P, travel-time curves are generally remarkably similar for all these regions. At distances of 120-180 km from the shotpoint, the refracted wave with a velocity of about 7.7 km/see is the first to be recorded every-
21
where, and can be detected out to as far as 300 km. Farther from the shotpoint the travel-time curve inflects, and the head-wave has a mean velocity, of 8.0-8.2 km/set. Figure 2 shows schematic velocity columns for world rift zones compiled from seismic data on the Gutenberg waveguide (Valyuss et al., 1969; Jonson, 1967; Knopoff and Schlue, 1972). An anomalously lowvelocity upper-mantle layer, VP = 7.7 km/set, is of regional extent. The thickness of this layer in different regions averages from 15 to 30 km. Velocity values in this anomalous layer and in the mantle Gutenberg waveguide are practically the same, but these objects do not represent a single structure as they are separated by a high velocity, 8-8.2 km/set, level. CONCLUSIONS
Comparison of the Baikal rift with other continental rifts establishes an essentially similar seismic layering down through the crust, and especially the upper mantle as deep as the Gutenberg layer. It is likely that recent deep processes and the physical state of the upper mantle are the same beneath all continental rifts of the world. The wellknown velocity decrease in the uppermost part of the mantle is peculiar in that it characterizes a relatively thin layer, and can be linked with the asthenospheric waveguide only locally. REFERENCES Alekseyev, A.S., Lavrentyev, M.M., Mukhometov, R.G., Nersesov, I.L. and Romanov, V.G., 1971. Digital method of determining the Earth’s upper-mantle structure. In: Matemat. problemy geofiiiki. VC AN SSSR, Novosibirsk, pp. 143-165. Belyaevsky, N.A. (Editor), 1970. Systems of the Earth’s rifts. Mir, 280 p. Berckhemer, H., Baier, B., Bartelsen, H., Behle, A., Burkhardt, H., Gebrande, H., Makris, J., Menzel, H., Miller, H. and Vees, R., 1975. Deep seismic soundings in the Afar region and on the highland of Ethiopia. In: A. Pilger and A. RGsler (Editors), Afar Depression of Ethiopia. Schweizerbart, Stuttgart, pp. 89-107. Griffiths, D.H., 1972. Some comments on the seismic refraction experiment in the Kenya rift. Tectonophysics, 15: 151-156. Illies, J.H. and Fuchs, K. (Editors), 1974. Approaches to Taphrogenesis. Schweizerbart, Stuttgart, 315 pp. Jonson, L.R., 1967. Array measurements of P-velocitites in the upper mantle. J. Geophys. Res., 72: 6309-6325. Knopoff, L. and Schlue, J.W., 1972. Rayleigh wave phase velocities for the path Addis Ababa-Nairobi. Tectonophysics, 15: 157-163. Krylov, S.V., Golenetsky, S.I. and Petrik, G.V., 1974. Agreement of seismology and DSSdata on the uppermost mantle structure of the Baikal rift zone. Geol. i geofiz. No. 12: 61-65. Krylov, S.V., Mishenkin, B.P., Mishenkina, Z.R., Petrik, G.V. and Seleznev, V.S., 1975. Seismic cross-section of the lithosphere in the Baikal rift zone. Geol. i geofiz. NO. 3: 72-83. Mueller, St., Peterschmitt, E. and Ansorge, J., 1969. Crustal structure beneath the Rhine graben from seismic refraction and reflection measurements. Tectonophysics, 8: 529542.
22 Pakiser, L., 1963. Structure of the crust and upper mantle in the western United States. J. Geophys. Res., 68: 5747-5756. Pakiser, L.C. and Hill, D.P., 1963. Crustal structure in Nevada and southern Idaho from nuclear explosions. J. Geophys. Res., 68: 5757-5766. Prodehl, C., 1970. Seismic refraction study of crustal structure in the western United States. Geol. Sot. Am. Bull., 81: 2629-2646. Puzyrev, N.N., Mandelbaum, M.M., Krylov, S.V., Mishenkin, B.P., Krupskaya, G.V. and Petrik, G.V., 1974. Deep structure of the Baikal rift from explosion seismology data. Geol. i geofiz, No. 5: 155-167. Puzyrev, N.N., Krylov, S.V. and Mishenkin, B.P., 1975. Techniques of reconnaissance deep seismic investigations. Nauka, Moscow, 158 pp. Ryal, A. and Stuart, D.J., 1963. Travel times of some local earthquake phases originating from nuclear explosions. Nevada Test Site to Ordway, Colorado. J. Geophys. Res., 68: 5821-5835. Searle, R.C. and Gouin, P., 1971. Analysis of some local earthquake phases originating near the Afar triple junction, Bull. Seismol. Sot. Am., 61: 1061-1067. Valyuss, V.P., Keilis-Borock, B.I. and Levshin, A.L., 1969. Determinations of the velocity cross-section of the upper mantle of Europe. Dokl. AN SSSR, 185, No. 3: 564-567. Zverev, S.M., Kosminskaya, I.P., Krasilshchikova, G.A. and Mikhota, G.F., 1975. Deep structure of Iceland and the Iceland-Faeroes-Scotland region from the results of seismic studies, Bull. MQIP, otd. geol., vyp. 3: 99-I 15.