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Finite element modelling of crustal deformation in the Baikal rift zone: new insights into the active–passive rifting debate
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Tectonophysics 289 (1998) 327–340
Finite element modelling of crustal deformation in the Baikal rift zone: new insights into the active–passive rifting debate O. Lesne a,Ł , E. Calais a , J. Deverche`re b a
UMR 6526 Ge´osciences Azur, CNRS, Universite´ Paris VI, 250 Rue Albert Einstein, Sophia Antipolis, 06560 Valbonne, France b UMR 6526 Ge ´ osciences Azur, CNRS, Observatoire Oce´anologique, B.P. 48, 06230 Villefranche-sur-Mer, France Received 27 January 1997; accepted 11 December 1997
Abstract The origin of the forces responsible for crustal extension in the Baikal rift zone, Siberia, is the object of a debate between ‘passive rifting’ models, where crustal extension is primarily caused by horizontal forces related to the kinematics of Asia (India–Eurasia collision), and ‘active rifting’ models, where crustal extension is primarily caused by a diapiric mantle upwelling. In this work, we used a two-dimensional visco-elastic finite element model in order to determine whether horizontal forces alone can account for the present-day deformation in the Baikal rift zone. We tested a number of kinematic boundary conditions and compared predictions of various models against the observed stress and strain field deduced from seismotectonic data (earthquake focal mechanisms and microtectonic analyses). By adjusting the kinematic boundary conditions and using a three-plate model with a differential displacement between the Mongolian and Amurian plates, we found a best-fit model that correctly accounts for the observed strain and stress field over the entire Baikal rift zone. The fact that our model does not take into account vertical forces but still explains most of the observed deformation suggests that the present-day opening of the Baikal rift is essentially controlled by horizontal forces related to the regional kinematics. These forces could have their origin in the India–Eurasia collision zone further south. This result does not imply that the asthenosphere played no role in the rifting process, in particular before the ‘fast rifting’ stage of the Baikal rift evolution (3–4 Ma), but might suggest a recent (Plio–Quaternary) triggering effect of the India–Asia collision on the deformation in the Baikal rift. 1998 Elsevier Science B.V. All rights reserved. Keywords: Baikal rift; tectonics of Asia; numerical deformation modelling; finite element method
1. Introduction The Baikal rift zone (eastern Siberia) is the largest active continental rift system in Eurasia (Fig. 1). It forms the northern edge of a broad zone of active deformation in Central Asia related to the India–Asia collision (Tapponnier and Molnar, 1979; Tapponnier et al., 1986). The rift extends over a distance of Ł Corresponding author: Tel.: C33 4 9294 2606; Fax: C33 4 9294 2610; E-mail: [email protected]
about 2000 km and follows an S-shaped Palaeozoic suture that separates the Siberian Platform from the Sayan–Baikal mobile belt (Fig. 2; Logatchev and Zorin, 1992). This intracontinental extension is associated with a high seismic activity (Doser, 1991a,b; Deverche`re et al., 1991, 1993), surface deformations (Houdry, 1994; Levi et al., 1995; McCalpin and Khromovskikh, 1995), as well as strong gravity contrasts (Diament and Kogan, 1990; Ruppel et al., 1993; Petit et al., 1997). The origin of the forces responsible for crustal
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Fig. 1. Simplified kinematic framework of Asia (after Tapponnier et al., 1982; Avouac and Tapponnier, 1993). The convergence between India and Eurasia translates into large sinistral strike-slip faults from Tien-Shan to Mongolia and extension in the Baikal rift zone.
extension in the Baikal rift has long been the object of a debate. For ‘passive rifting’ models, crustal extension is primarily caused by horizontal forces related to the kinematics of Asia (India–Eurasia collision). In this hypothesis, the deformation is mainly controlled by the geometry of the major faults. Because of the S-shaped contact between the Siberian Platform and the Sayan–Baikal mobile belt, an eastward displacement of north China relative to Eurasia would cause extension in the Baikal rift zone (Tapponnier and Molnar, 1979). For ‘active rifting’ models, crustal extension is primarily caused by the diapiric upwelling of a mantle asthenolith (Zorin, 1981; Logatchev and Zorin, 1987; Kiselev and Popov, 1992; Windley and Allen, 1993). This hypothesis is based on the existence of broad topographic domes in northern Mongolia (Hangai and Hentai domes) and in the southwestern part of the Baikal rift zone, associated with local elevated heat flow (up to 120 mW=m2 ; Khutorskoy and Yarmoluk, 1989), and Miocene–Quaternary alkaline basaltic volcanism (Rasskazov, 1994). This study is aimed at bringing new insights into the active=passive rifting debate in the Baikal rift zone through a numerical approach. Using a two-
dimensional visco-elastic finite element algorithm, we simulate the present-day stress and strain field in the Baikal rift zone in order to test whether the regional first-order kinematics and active fault geometry are sufficient to explain the present-day stress and strain field, or if other forces or processes, such as caused by an asthenospheric upwelling, need to be invoked. 2. Strain and stress directions in the Baikal rift zone Recent seismotectonic synthesis (Petit et al., 1996; Delvaux et al., 1998) have brought quantitative constraints to the present-day strain and stress field in the Baikal rift zone. These studies, based on the inversion of earthquake focal solutions and microtectonic data, show a nonuniform stress and strain pattern for which three main strain regimes can be distinguished (Figs. 2 and 3): (1) Along Lake Baikal, the stress regime is clearly extensive. The principal axis ¦3 is horizontal, trends around N130E and remains roughly collinear with the T-axis direction of major earthquake focal mechanisms (Petit et al., 1996). The intermediate stress
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Fig. 2. Seismotectonic map of the Baikal rift zone, eastern Siberia, with simplified sketch of major active faults and focal mechanisms of M > 5 earthquakes recorded between 1950 and 1994 (Mercator projection; after Doser, 1991a,b; Solonenko et al., 1993; Petit et al., 1996).
axis ¦2 is extensional. In this area, the major part of the current deformation is concentrated along a single 450 km long normal fault (Primorsky fault, Fig. 2) that follows the western bank of the lake. (2) North and east of Lake Baikal, ¦2 becomes progressively compressional, with a continuous transition from extensional strain along the lake to wrench-extensional strain in the northeast (Petit et al., 1996). The minimal stress axis ¦3 usually shows a clockwise rotation compared to the lake area, and trends about N150–160E. In this area, the main active strain features are several large en-echelon grabens (Muyakan, Tchara, Tsipa, and Eastern rift
basins), trending N50E, aligned along a general E– W direction and limited by a series of about 80-kmlong normal faults (Houdry et al., 1993). (3) South and west of Lake Baikal, the strain regime is wrench-compressional. The principal axes ¦1 and ¦3 are horizontal, with ¦3 trending N32–37E (Petit et al., 1996). In this area, the major active features are the Sayan range and the Tunka basin, bounded to the south by the Tunka left-lateral strikeslip and normal fault (Delvaux et al., 1998). The transition from extension to wrench-compressional strain appears rather abruptly around 104ºE longitude (Fig. 3).
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Fig. 3. (a) Horizontal projections of P (solid lines) and T (shaded lines) axes for M > 5 earthquakes in the century. (b) Horizontal projections of ¦1 (solid lines) and ¦3 (shaded lines) axes (after Petit et al., 1996). (c) M > 5 earthquake distribution from the Russian database (between 1700 and 1990). This figure and the following ones use a cartesian coordinate plane system.
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This segmentation of the strain regime along the Baikal rift zone corresponds to three segments with general structural trends of N90E, N40E, and N80E (from the SW to the NE). This suggests a strong influence of the fault geometry on the strain regime. In the following, our basic observables will be the fault geometry and the stress and strain fields. Using a numerical model, we will test whether it is possible, given a known fault geometry, to find a regional far-field kinematics compatible with the known kinematics of central Asia that satisfies the present-day stress and strain fields described above. 3. Models and constraints Since we are interested in instantaneous and largescale deformation, we have chosen to model the lithosphere as a visco-elastic incompressible Newtonian fluid of uniform rheology. In addition, in order to restrict the problem to two-dimensions, we assumed a condition of plane stress that simulates the behaviour of a plate of much larger horizontal than vertical dimensions (‘thin plate’ hypothesis). The third dimension can however be simulated by computing the rate of vertical deformation žzz (thinning or thickening of the lithosphere) from the normal strain rates in the horizontal plane žxx and ž yy using žzz D .žxx Cž yy /. The values of Young’s modulus and Poisson’s ratio have been respectively chosen equal to 7 ð 1010 Pa and 0.25, which are average values for the lithosphere. The coefficient of viscosity was taken equal to 1021 Pa s. Several values of the viscosity have been tested, in particular a higher value for the Siberian craton with respect to the Sayan–Baikal range. This had no significant influence on the predicted stress and strain directions, which was to be expected given the short time range of our models (106 years), therefore close to fully elastic. This assumption is supported by the fact that the present-day deformation pattern we intend to simulate is related to a recent (Plio–Quaternary) fast rifting stage of the Baikal rift zone evolution (Logatchev and Zorin, 1987). Computations were made with the finite element code TECTON (version 1.51, Melosh and Raefsky, 1981) that solves for nodal displacements and stress and strain rates for each grid element. Our grid consists of 193 nodes and 182 elements of triangular or quadrilateral shape that correspond
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to homogeneous tectonic domains (Fig. 4). The fault traces were deduced from geological maps, satellite image interpretations and field studies (Houdry et al., 1993; Levi et al., 1995) and were simplified in order to match the model discretization. Boundary conditions are modeled as constraints on nodal displacement degrees of freedom. Faults are modeled as free-shear boundaries by inserting an extra degree of freedom for each node on the fault in the direction of the fault slip (‘slippery node’ method, Melosh and Williams, 1989). As friction is not modeled, the accumulated shear stress on the faults is entirely transferred into displacement (zero resolved shear stress condition). Field observations south of the Baikal rift zone down to southern Mongolia show large left-lateral strike-slip faults and a transpressional tectonic regime (Baljinnyam et al., 1993; Schlupp, 1996). These observations imply an eastward displacement of crustal blocks in central Asia at a velocity that is still under discussion (e.g. Avouac and Tapponnier, 1993; Molnar and Gipson, 1996). We took these geological constraints into account in our models by using simple boundary conditions (Fig. 4): (1) we assumed the Eurasian plate as fixed and consequently applied a non-displacement condition along the upper boundary of the models; (2) since displacements on the lateral sides of our models are poorly constrained from geological observations, we left these boundaries free to adjust in either direction (fixing the northwestern boundary of the Eurasia plate had no influence on the results); and, finally, (3) we varied the boundary conditions along the bottom edge of our models in order to simulate southeastward to eastward displacement of the Mongolia and Amur blocks relative to stable Eurasia. 4. Results and discussion 4.1. Test of the far-field kinematics We tried a number of different velocity directions in order to match the resulting stress and strain field as closely as possible with the observed stress and strain field provided by the seismotectonic analysis. Concerning the far-field velocity modulus, no direct measurement at the scale of the entire Baikal rift is currently published. Large-scale velocity so-
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Fig. 4. Finite element grid and boundary conditions of the models (see text for details).
lutions in Asia, e.g. obtained from earthquake moment tensors (Holt et al., 1995) or from geological displacement on active faults (Peltzer and Saucier, 1996) led most authors to minimize the strain rate in the Baikal rift, and are therefore of little use for assuming initial velocity vector modulus and direction. However, palaeoseismicity (McCalpin and Khromovskikh, 1995) and morphotectonic studies of active fault scarps (Houdry, 1994) show fault slip rates on the order of 5 to 10 mm=yr. Triangulation surveys in the Tunka basin and Sayan range show horizontal displacements of 10 to 20 mm=yr (Kesselman et al., 1992). In the Selenga delta region, triangulation surveys show 15 mm=yr of horizontal displacement (over 3 years) and 24 mm=yr (over 35 years) in the Muya region (V. Sankov, pers. commun., 1996). Given the wide range of displacement rates provided by these different methods, we chose
an average value and used a constant far-field velocity modulus of 10 mm=yr for all models. This value is confirmed by preliminary results obtained from GPS measurements in the southern part of the Baikal rift zone (Lesne et al., 1996). We present here two two-plate models (Eurasian and Amurian plates) that permit to test the two most common regional kinematics proposed for the Baikal rift: Model 1: E–W left-lateral shear between the Eurasian and Amurian plates. In this hypothesis, the Baikal rift is considered as a large pull-apart structure located on a releasing bend (Fig. 5c) Model 2: NW–SE divergence between the Eurasian and Amurian plates, perpendicular to the major normal faults of the central Baikal rift zone (Fig. 6c). On the first order, both models show a largely extensional strain and stress field, with a very nar-
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Fig. 5. Model 1: predicted (a) P (solid lines) and T (shaded lines) horizontal strain axes; (b) ¦1 (solid lines) and ¦3 (shaded lines) horizontal stress axes; and (c) velocity field.
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Fig. 6. Model 2: predicted (a) P (solid lines) and T (shaded lines) horizontal strain axes; (b) ¦1 (solid lines) and ¦3 (shaded lines) horizontal stress axes; and (c) velocity field.
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row deformation zone that follows the major active faults. In the central part of the rift, model 2 shows a purely extensional strain regime with ¦3 horizontal and trending about N140E, which is rather consistent with the ¦3 direction deduced from the inversion of fault plane solutions. Model 1 also predicts extension but with an additional component of shear, and a N110E ¦3 direction which does not match the observed stress field (Fig. 3b). In the northeastern part of the rift, both models predict a wrench-extensional strain regime, in agreement with the geological interpretation of the northern basins as en-echelon grabens. Model 2 shows a good fit to the observed stress field, with ¦3 trending N150E to N160E and rotated clockwise compared to the central part of the rift zone. Model 1 predicts a ¦3 direction of N110E, similar to the central part of the rift. Here again, the fit to the observed stress field is better for model 2 than for model 1. Finally in the southwestern part of the rift, both models predict a very similar stress field, with ¦3 horizontal and trending N170E to N180E in the Tunka basin, and ¦3 horizontal and trending N45E to N50E in the Sayan range. All these predictions are in rather good agreement with the observed stress field but do not permit to discriminate the two models. The major difference between them is rather qualitative: model 2 shows an equivalent amount of shear in the Sayan range as in the Tunka basin (Fig. 6a), whereas model 1 predicts a strong partitioning of the strain field between extension in the Tunka basin and compression in the Sayan range (Fig. 5a). This latter result is in better agreement with geological observations which suggest that the Sayan range is a compressive structure thrusting northeastward over the Angara basin, and that the Tunka fault is a left-lateral strike-slip and normal fault (Delvaux et al., 1998). It therefore appears that model 2 matches the observed strain and stress field better than model 1 in the central and northeastern parts of the rift zone, whereas the opposite is true in the southwestern part of the rift. However, none of these two models satisfactorily predicts the current deformation, neither along the whole rift nor outside and south of the rift zone, in the Khamar Daban range, revealed by the significant level of diffuse seismicity in that area.
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4.2. A best fit model The above conclusion led us to test the hypothesis of a three-plate model where the Amurian plate is divided into two sub-plates (Mongolian and Amurian plates), as was originally proposed by Zonenshain and Savostin (1981). According to these authors, the boundary between the Mongolian and Amurian plates does not correspond to any active fault but to a larger area over which the deformation is spread out. In our model, we do not constrain the nodal displacements in that area but apply different kinematic boundary conditions along the southern edge of the Mongolian and Amurian plates in order to simulate their kinematics, using the rotation parameters derived by Zonenshain and Savostin (1981) as starting values (Fig. 7c). We then slightly adjust the boundary conditions until we find the model that fits best the observed stress and strain field. The final boundary conditions are close to the initial ones derived from Zonenshain and Savostin’s rotation parameters and roughly reproduce those of model 1 in the western part of the model and those of model 2 in the central and eastern parts of the model (Fig. 7c). The predicted strain field shows compression in the Sayan range and a combination of left-lateral shear and N–S extension in the Tunka basin, similarly to model 1 and in accordance with geological observations (Delvaux et al., 1998). This partitioning cannot be observed on the horizontal stress direction map (Fig. 3b) because only one stress tensor has been computed by Petit et al. (1996) in the Sayan and Tunka regions, due to a lack of reliable focal solutions. The strain field becomes extensive to strike-slip along Lake Baikal northward, with ¦3 trending N130E to N140E and rotating clockwise in the northeastern part of the rift zone. These results are similar to model 2 for the central and northeastern part of the rift and agree with the observed strain and stress field (Petit et al., 1996; compare Fig. 3a with Fig. 7a, and Fig. 3b with Fig. 7b). South and outside of the rift zone, in the Khamar Daban range, this model predicts a small amount of shear deformation, that accommodates the small velocity differential between the Mongolian and Amurian plates underlined by the diffuse seismicity (Fig. 3c). It matches the seismotectonic data that show strike-slip focal mechanisms in that area (Fig. 2; Doser, 1991a).
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Fig. 7. Best fit model: predicted (a) P (solid lines) and T (shaded lines) horizontal strain axes; (b) ¦1 (solid lines); and (c) ¦3 (shaded lines) horizontal stress axes and velocity field.
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Fig. 8. Best fit model: predicted vertical strain rate (see text for explanations).
Since we have assumed the lithosphere to be incompressible and the horizontal velocity components to be constant with depth, we can compute the rate of thinning or thickening of the lithosphere žzz from the horizontal strain rates žxx and ž yy (see above). Because we do not model isostatic compensation and erosion or sedimentation rates, it is not possible to directly compare žzz with a geological observable such as uplift or subsidence rate, which are anyway not yet available in the Baikal region. However, since isostatic compensation should be fairly uniform regionally, žzz can be used as a crude indication of the regional topographic pattern, with negative values in subsiding basins and positive values in uplifting mountain ranges. As shown in Fig. 8, the predicted rate of vertical deformation matches fairly well the most prominent regional topographic features (Fig. 2). Lake Baikal and the northeastern basins are modelled as subsiding areas with the strongest subsidence predicted to occur in the central part of the lake. This area corresponds to the oldest part of the rift zone (Logatchev, 1984) and the thinnest crust and lithosphere according to seismic studies (Gao et al., 1994) and gravity models (Zorin et al., 1990; Petit et al., 1997). On
both sides of the lake, the model predicts elongated areas of uplift that correspond to rift shoulders. Rift shoulders are well known along the Baikal Lake and seems to be not only the consequence of a flexural isostatic response to extensional unloading of the rift (Van der Beek, 1997) but also of local crustal thickening adjacent to the rift (Burov et al., 1994; Petit et al., 1997). In the southwestern part of the rift zone, the model predicts a strong uplift in the highest mountain range of this region (Sayan range) and subsidence in the Tunka basin, in agreement with geological observations (Delvaux et al., 1998). However, high elevations found in the Stanovoy range (NE of the Baikal rift) are not reproduced at all by our model, suggesting that they might be related to other processes, such as underplating or previous tectonic events for instance. 5. Conclusion and perspectives Using a two-dimensional visco-elastic finite element model, we were able to test the influence of the far-field kinematics on the stress and strain pattern in the Baikal rift zone. We show that the observed stress and strain patterns can be satisfactorily ex-
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plained using a three-plate model that accounts for a differential displacement between the Mongolian and Amurian plates. The fact that our model does not take into account vertical forces but still explains most of the observed deformation suggests that the major forces that drive the present-day opening of the Baikal rift zone are horizontal forces related to the far-field kinematics. These forces probably have their origin in the India–Asia collision zone further south, which could have triggered the Late Pliocene– Quaternary ‘fast-rifting stage’ (Logatchev and Zorin, 1987; Logatchev, 1993), misleadingly called ‘activerift’ stage by Delvaux et al. (1998). Vertical forces cannot be excluded but seem currently to be of several orders of magnitude lower than horizontal forces. It must be remembered that this model is only valid for the present-day deformation regime and does not preclude a major influence of asthenospheric upwelling during earlier phases of the Baikal rift evolution. Indeed, the time and space evolution of the volcanic activity in the Baikal rift (and especially the stepwise westward shift of the east Sayan volcanic field) favours a long-term heat supply, presumably coming from a mantle plume located near the southern edge of the Siberian craton (Rasskazov, 1994). Our best-fit three-plate model simulates the existence of a differential displacement between an Amurian plate and a Mongolian plate. However, the boundary between these two blocks is not necessarily a plate boundary s.str. but could be a ‘diffuse’ transition zone from N–S compression in Mongolia to NW–SE extension in the central and northern Baikal region that vanishes further south. The model presented here is rather simple and could be improved by using more realistic rheological parameters and a more detailed fault geometry, but it only aims at providing a first-order test on the role of kinematic conditions upon the present-day rifting. In particular, the hypothesis of a vertically homogeneous lithosphere is a large simplification: a more accurate model would require the use of a realistic stratification of the lithosphere, especially in the crust with a frictional rheology in its upper part and a transition to dislocation creep in its lower part. Furthermore, such a model does not reflect the contrast in mechanical behaviour between the rigid Siberian craton, the Aldan shield and the Sayan–Baikal range
(Fig. 2; Petit et al., 1996, 1997). Further models should take into account these contrasting rheologies, but data on the structure of the lithosphere in the Baikal region are not yet accurate enough to reach such a level of detail. Finally, Global Positioning System geodetic measurements that are currently carried out in the Baikal rift zone should provide a new and important data set in the near future (Lesne et al., 1996). In particular, GPS results should allow us to fix kinematic boundary conditions and fault slip rates, which, in turn, will permit detailed numerical tests of the dynamics of the current deformation in the Baikal rift zone and of strain partitioning between the Amurian, Mongolian and Siberian tectonic blocks. Acknowledgements This study was carried out in the framework of a cooperation with the Institute of the Earth’s Crust, Siberian Branch of the Russian Academy of Science, Irkutsk. We are particularly grateful to its director, Academician N.A. Logatchev, and its Deputy Director, K.G. Levi, for their efficient support, and to V.A. Sankov for his scientific input. We thank C. Petit for providing us with material in advance of publication. Critical comments by P. Van der Beek and an anonymous reviewer helped to significantly improve the manuscript. This work was supported by the French Ministry for Education and Research (DSPT3) and the French Ministry of Foreign Affairs (‘Enveloppe Echanges Scientifiques 1996’) and is a contribution to IGCP Project 400. Ge´osciences Azur Contribution Nr. 147. References Avouac, J.P., Tapponnier, P., 1993. Kinematic model of deformation in central Asia. Geophys. Res. Lett. 20 (10), 895– 898. Baljinnyam, I., Bayasgalan, A., Borisov, B.A., Cisternas, A., Dem’yanovich, M.G., Ganbaatar, L., Kochetkov, V.M., Kurushin, R.A., Molnar, P., Philip, H., Vashchilov, Y.Y., 1993. Ruptures of major earthquakes and active deformation in Mongolia and its surroundings. Geol. Soc. Am., Mem. 181, 62 pp. Burov, E.B., Houdry, F., Diament, M., Deverche` re, J., 1994. A broken plate beneath the North Baikal rift zone revealed by gravity modelling. Geophys. Res. Lett. 21 (2), 129–132. Delvaux, D., Moeys, R., Stapel, G., Petit, C., Levi, K., Miroshnichenko, A., Ruzhich, V., San’kov, V., 1998. Paleostress re-
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Baikal rift in the context of the Eastern Siberia rift system: a review. Bull. Cent. Rech. Explor. Prod. Elf Aquitaine 17, 353–370. Logatchev, N.A., Zorin, Y.A., 1987. Evidence and causes of the two-stage development of the Baikal rift. Tectonophysics 143, 225–234. Logatchev, N.A., Zorin, Y.A., 1992. Baikal rift zone: structure and geodynamics. Tectonophysics 208, 273–286. McCalpin, J.P., Khromovskikh, V.S., 1995. Holocene paleoseismicity of the Tunka fault, Baikal rift, Russia. Tectonics 143 (3), 594–605. Melosh, H.J., Raefsky, A., 1981. A simple and efficient method for introducing faults into finite element computations. Seismol. Soc. Am. Bull. 71, 1391–1400. Molnar, P., Gipson, J.M., 1996. A bound on the rheology of continental lithosphere using very long baseline interferometry: the velocity of South China with respect to Eurasia. J. Geophys. Res. 101 (B1), 545–553. Melosh, H.J., Williams, C.A., 1989. Mechanics of graben formation in crustal rocks: a finite element analysis. J. Geophys. Res. 94 (B10), 13961–13973. Peltzer, G., Saucier, F., 1996. Present-day kinematics of Asia derived from geologic fault rates. J. Geophys. Res. 101 (B12), 27943–27956. Petit, C., Deverche`re, J., Houdry, F., Sankov, V.A., Melnikova, V.I., Delvaux, D., 1996. Present-day stress field changes along the Baikal rift and tectonic implications. Tectonics 15 (6), 1171–1191. Petit, C., Burov, E.V., Deverche`re, J., 1997. On the structure and mechanical behavior of the extending lithosphere in the Baikal rift from gravity modelling. Earth Planet. Sci. Lett. 149, 29– 42. Rasskazov, S.V., 1994. Magmatism related to the Eastern Siberia Rift System and the geodynamics. Bull. Cent. Rech. Explor. Prod. Elf Aquitaine 18, 437–452. Ruppel, C., Kogan, M.G., McNutt, M.K., 1993. Implications of new gravity data for Baikal rift zone structure. Geophys. Res. Lett. 20, 1635–1638. Schlupp, A., 1996. Ne´otectonique de la Mongolie Occidentale analyse´e a` partir de donne´es de terrain, sismologiques et satellitaires. The`se Univ. Louis Pasteur, Strasbourg, 172 pp. Solonenko, A.V., Solonenko, N.V., Melnikova, V.I., Kuzmin, B.M., Kuchai, O.A., Sukhanova, S.S., 1993. Stresses and fault plane motions of earthquakes in Siberia and Mongolia (in Russian). In: Seismicity and Seismic Zoning of Northern Eurasia. IFE RAS 1, 113–122. Tapponnier, P., Molnar, P., 1979. Active faulting and cenozoic tectonics of the Tien Shan, Mongolia and Baikal regions. J. Geophys. Res. 84, 3425–3459. Tapponnier, P., Peltzer, G., Le Dain, Y., Armijo, R., Cobbold, P., 1982. Propagating extrusion tectonics in Asia: new insights from simple experiments with plasticine. Geology 10, 611– 616. Tapponnier, P., Peltzer, G., Arrnijo, R., 1986. On the mechanics of the collision between India and Asia. In: Collision Tectonics. Geol. Soc. Spec. Publ. 19, 115–157. Van der Beek, P., 1997. Flank uplift topography at the cen-
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tral Baikal Rift (SE Siberia): a test of kinematic models for continental extension. Tectonics 16 (1), 122–136. Windley, B.F., Allen, M.B., 1993. Mongolian plateau: evidence for a late Cenozoic mantle plume under central Asia. Geology 21, 295–298. Zonenshain, L.P., Savostin, L.A., 1981. Geodynamics of the Baikal rift zone and plate tectonics of Asia. Tectonophysics 76, 1–45.
Zorin, Y.A., 1981. The Baikal rift: an example of the intrusion of asthenospheric material into the lithosphere as the cause of disruption of the lithosphere plates. Tectonophysics 73, 91– 104. Zorin, Y.A., Novoselova, M.R., Turutanov, E.K., Kozhevnikov, V.M., 1990. Structure of the lithosphere of the Mongolian– Siberian mountainous province. J. Geodyn. 11, 327–342.
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Finite element modelling of crustal deformation in the Baikal rift zone: new insights into the active–passive rifting debate O. Lesne a,Ł , E. Calais a , J. Deverche`re b a
UMR 6526 Ge´osciences Azur, CNRS, Universite´ Paris VI, 250 Rue Albert Einstein, Sophia Antipolis, 06560 Valbonne, France b UMR 6526 Ge ´ osciences Azur, CNRS, Observatoire Oce´anologique, B.P. 48, 06230 Villefranche-sur-Mer, France Received 27 January 1997; accepted 11 December 1997
Abstract The origin of the forces responsible for crustal extension in the Baikal rift zone, Siberia, is the object of a debate between ‘passive rifting’ models, where crustal extension is primarily caused by horizontal forces related to the kinematics of Asia (India–Eurasia collision), and ‘active rifting’ models, where crustal extension is primarily caused by a diapiric mantle upwelling. In this work, we used a two-dimensional visco-elastic finite element model in order to determine whether horizontal forces alone can account for the present-day deformation in the Baikal rift zone. We tested a number of kinematic boundary conditions and compared predictions of various models against the observed stress and strain field deduced from seismotectonic data (earthquake focal mechanisms and microtectonic analyses). By adjusting the kinematic boundary conditions and using a three-plate model with a differential displacement between the Mongolian and Amurian plates, we found a best-fit model that correctly accounts for the observed strain and stress field over the entire Baikal rift zone. The fact that our model does not take into account vertical forces but still explains most of the observed deformation suggests that the present-day opening of the Baikal rift is essentially controlled by horizontal forces related to the regional kinematics. These forces could have their origin in the India–Eurasia collision zone further south. This result does not imply that the asthenosphere played no role in the rifting process, in particular before the ‘fast rifting’ stage of the Baikal rift evolution (3–4 Ma), but might suggest a recent (Plio–Quaternary) triggering effect of the India–Asia collision on the deformation in the Baikal rift. 1998 Elsevier Science B.V. All rights reserved. Keywords: Baikal rift; tectonics of Asia; numerical deformation modelling; finite element method
1. Introduction The Baikal rift zone (eastern Siberia) is the largest active continental rift system in Eurasia (Fig. 1). It forms the northern edge of a broad zone of active deformation in Central Asia related to the India–Asia collision (Tapponnier and Molnar, 1979; Tapponnier et al., 1986). The rift extends over a distance of Ł Corresponding author: Tel.: C33 4 9294 2606; Fax: C33 4 9294 2610; E-mail: [email protected]
about 2000 km and follows an S-shaped Palaeozoic suture that separates the Siberian Platform from the Sayan–Baikal mobile belt (Fig. 2; Logatchev and Zorin, 1992). This intracontinental extension is associated with a high seismic activity (Doser, 1991a,b; Deverche`re et al., 1991, 1993), surface deformations (Houdry, 1994; Levi et al., 1995; McCalpin and Khromovskikh, 1995), as well as strong gravity contrasts (Diament and Kogan, 1990; Ruppel et al., 1993; Petit et al., 1997). The origin of the forces responsible for crustal
0040-1951/98/$19.00 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 4 0 - 1 9 5 1 ( 9 8 ) 0 0 0 0 4 - 3
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Fig. 1. Simplified kinematic framework of Asia (after Tapponnier et al., 1982; Avouac and Tapponnier, 1993). The convergence between India and Eurasia translates into large sinistral strike-slip faults from Tien-Shan to Mongolia and extension in the Baikal rift zone.
extension in the Baikal rift has long been the object of a debate. For ‘passive rifting’ models, crustal extension is primarily caused by horizontal forces related to the kinematics of Asia (India–Eurasia collision). In this hypothesis, the deformation is mainly controlled by the geometry of the major faults. Because of the S-shaped contact between the Siberian Platform and the Sayan–Baikal mobile belt, an eastward displacement of north China relative to Eurasia would cause extension in the Baikal rift zone (Tapponnier and Molnar, 1979). For ‘active rifting’ models, crustal extension is primarily caused by the diapiric upwelling of a mantle asthenolith (Zorin, 1981; Logatchev and Zorin, 1987; Kiselev and Popov, 1992; Windley and Allen, 1993). This hypothesis is based on the existence of broad topographic domes in northern Mongolia (Hangai and Hentai domes) and in the southwestern part of the Baikal rift zone, associated with local elevated heat flow (up to 120 mW=m2 ; Khutorskoy and Yarmoluk, 1989), and Miocene–Quaternary alkaline basaltic volcanism (Rasskazov, 1994). This study is aimed at bringing new insights into the active=passive rifting debate in the Baikal rift zone through a numerical approach. Using a two-
dimensional visco-elastic finite element algorithm, we simulate the present-day stress and strain field in the Baikal rift zone in order to test whether the regional first-order kinematics and active fault geometry are sufficient to explain the present-day stress and strain field, or if other forces or processes, such as caused by an asthenospheric upwelling, need to be invoked. 2. Strain and stress directions in the Baikal rift zone Recent seismotectonic synthesis (Petit et al., 1996; Delvaux et al., 1998) have brought quantitative constraints to the present-day strain and stress field in the Baikal rift zone. These studies, based on the inversion of earthquake focal solutions and microtectonic data, show a nonuniform stress and strain pattern for which three main strain regimes can be distinguished (Figs. 2 and 3): (1) Along Lake Baikal, the stress regime is clearly extensive. The principal axis ¦3 is horizontal, trends around N130E and remains roughly collinear with the T-axis direction of major earthquake focal mechanisms (Petit et al., 1996). The intermediate stress
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Fig. 2. Seismotectonic map of the Baikal rift zone, eastern Siberia, with simplified sketch of major active faults and focal mechanisms of M > 5 earthquakes recorded between 1950 and 1994 (Mercator projection; after Doser, 1991a,b; Solonenko et al., 1993; Petit et al., 1996).
axis ¦2 is extensional. In this area, the major part of the current deformation is concentrated along a single 450 km long normal fault (Primorsky fault, Fig. 2) that follows the western bank of the lake. (2) North and east of Lake Baikal, ¦2 becomes progressively compressional, with a continuous transition from extensional strain along the lake to wrench-extensional strain in the northeast (Petit et al., 1996). The minimal stress axis ¦3 usually shows a clockwise rotation compared to the lake area, and trends about N150–160E. In this area, the main active strain features are several large en-echelon grabens (Muyakan, Tchara, Tsipa, and Eastern rift
basins), trending N50E, aligned along a general E– W direction and limited by a series of about 80-kmlong normal faults (Houdry et al., 1993). (3) South and west of Lake Baikal, the strain regime is wrench-compressional. The principal axes ¦1 and ¦3 are horizontal, with ¦3 trending N32–37E (Petit et al., 1996). In this area, the major active features are the Sayan range and the Tunka basin, bounded to the south by the Tunka left-lateral strikeslip and normal fault (Delvaux et al., 1998). The transition from extension to wrench-compressional strain appears rather abruptly around 104ºE longitude (Fig. 3).
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Fig. 3. (a) Horizontal projections of P (solid lines) and T (shaded lines) axes for M > 5 earthquakes in the century. (b) Horizontal projections of ¦1 (solid lines) and ¦3 (shaded lines) axes (after Petit et al., 1996). (c) M > 5 earthquake distribution from the Russian database (between 1700 and 1990). This figure and the following ones use a cartesian coordinate plane system.
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This segmentation of the strain regime along the Baikal rift zone corresponds to three segments with general structural trends of N90E, N40E, and N80E (from the SW to the NE). This suggests a strong influence of the fault geometry on the strain regime. In the following, our basic observables will be the fault geometry and the stress and strain fields. Using a numerical model, we will test whether it is possible, given a known fault geometry, to find a regional far-field kinematics compatible with the known kinematics of central Asia that satisfies the present-day stress and strain fields described above. 3. Models and constraints Since we are interested in instantaneous and largescale deformation, we have chosen to model the lithosphere as a visco-elastic incompressible Newtonian fluid of uniform rheology. In addition, in order to restrict the problem to two-dimensions, we assumed a condition of plane stress that simulates the behaviour of a plate of much larger horizontal than vertical dimensions (‘thin plate’ hypothesis). The third dimension can however be simulated by computing the rate of vertical deformation žzz (thinning or thickening of the lithosphere) from the normal strain rates in the horizontal plane žxx and ž yy using žzz D .žxx Cž yy /. The values of Young’s modulus and Poisson’s ratio have been respectively chosen equal to 7 ð 1010 Pa and 0.25, which are average values for the lithosphere. The coefficient of viscosity was taken equal to 1021 Pa s. Several values of the viscosity have been tested, in particular a higher value for the Siberian craton with respect to the Sayan–Baikal range. This had no significant influence on the predicted stress and strain directions, which was to be expected given the short time range of our models (106 years), therefore close to fully elastic. This assumption is supported by the fact that the present-day deformation pattern we intend to simulate is related to a recent (Plio–Quaternary) fast rifting stage of the Baikal rift zone evolution (Logatchev and Zorin, 1987). Computations were made with the finite element code TECTON (version 1.51, Melosh and Raefsky, 1981) that solves for nodal displacements and stress and strain rates for each grid element. Our grid consists of 193 nodes and 182 elements of triangular or quadrilateral shape that correspond
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to homogeneous tectonic domains (Fig. 4). The fault traces were deduced from geological maps, satellite image interpretations and field studies (Houdry et al., 1993; Levi et al., 1995) and were simplified in order to match the model discretization. Boundary conditions are modeled as constraints on nodal displacement degrees of freedom. Faults are modeled as free-shear boundaries by inserting an extra degree of freedom for each node on the fault in the direction of the fault slip (‘slippery node’ method, Melosh and Williams, 1989). As friction is not modeled, the accumulated shear stress on the faults is entirely transferred into displacement (zero resolved shear stress condition). Field observations south of the Baikal rift zone down to southern Mongolia show large left-lateral strike-slip faults and a transpressional tectonic regime (Baljinnyam et al., 1993; Schlupp, 1996). These observations imply an eastward displacement of crustal blocks in central Asia at a velocity that is still under discussion (e.g. Avouac and Tapponnier, 1993; Molnar and Gipson, 1996). We took these geological constraints into account in our models by using simple boundary conditions (Fig. 4): (1) we assumed the Eurasian plate as fixed and consequently applied a non-displacement condition along the upper boundary of the models; (2) since displacements on the lateral sides of our models are poorly constrained from geological observations, we left these boundaries free to adjust in either direction (fixing the northwestern boundary of the Eurasia plate had no influence on the results); and, finally, (3) we varied the boundary conditions along the bottom edge of our models in order to simulate southeastward to eastward displacement of the Mongolia and Amur blocks relative to stable Eurasia. 4. Results and discussion 4.1. Test of the far-field kinematics We tried a number of different velocity directions in order to match the resulting stress and strain field as closely as possible with the observed stress and strain field provided by the seismotectonic analysis. Concerning the far-field velocity modulus, no direct measurement at the scale of the entire Baikal rift is currently published. Large-scale velocity so-
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Fig. 4. Finite element grid and boundary conditions of the models (see text for details).
lutions in Asia, e.g. obtained from earthquake moment tensors (Holt et al., 1995) or from geological displacement on active faults (Peltzer and Saucier, 1996) led most authors to minimize the strain rate in the Baikal rift, and are therefore of little use for assuming initial velocity vector modulus and direction. However, palaeoseismicity (McCalpin and Khromovskikh, 1995) and morphotectonic studies of active fault scarps (Houdry, 1994) show fault slip rates on the order of 5 to 10 mm=yr. Triangulation surveys in the Tunka basin and Sayan range show horizontal displacements of 10 to 20 mm=yr (Kesselman et al., 1992). In the Selenga delta region, triangulation surveys show 15 mm=yr of horizontal displacement (over 3 years) and 24 mm=yr (over 35 years) in the Muya region (V. Sankov, pers. commun., 1996). Given the wide range of displacement rates provided by these different methods, we chose
an average value and used a constant far-field velocity modulus of 10 mm=yr for all models. This value is confirmed by preliminary results obtained from GPS measurements in the southern part of the Baikal rift zone (Lesne et al., 1996). We present here two two-plate models (Eurasian and Amurian plates) that permit to test the two most common regional kinematics proposed for the Baikal rift: Model 1: E–W left-lateral shear between the Eurasian and Amurian plates. In this hypothesis, the Baikal rift is considered as a large pull-apart structure located on a releasing bend (Fig. 5c) Model 2: NW–SE divergence between the Eurasian and Amurian plates, perpendicular to the major normal faults of the central Baikal rift zone (Fig. 6c). On the first order, both models show a largely extensional strain and stress field, with a very nar-
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Fig. 5. Model 1: predicted (a) P (solid lines) and T (shaded lines) horizontal strain axes; (b) ¦1 (solid lines) and ¦3 (shaded lines) horizontal stress axes; and (c) velocity field.
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Fig. 6. Model 2: predicted (a) P (solid lines) and T (shaded lines) horizontal strain axes; (b) ¦1 (solid lines) and ¦3 (shaded lines) horizontal stress axes; and (c) velocity field.
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row deformation zone that follows the major active faults. In the central part of the rift, model 2 shows a purely extensional strain regime with ¦3 horizontal and trending about N140E, which is rather consistent with the ¦3 direction deduced from the inversion of fault plane solutions. Model 1 also predicts extension but with an additional component of shear, and a N110E ¦3 direction which does not match the observed stress field (Fig. 3b). In the northeastern part of the rift, both models predict a wrench-extensional strain regime, in agreement with the geological interpretation of the northern basins as en-echelon grabens. Model 2 shows a good fit to the observed stress field, with ¦3 trending N150E to N160E and rotated clockwise compared to the central part of the rift zone. Model 1 predicts a ¦3 direction of N110E, similar to the central part of the rift. Here again, the fit to the observed stress field is better for model 2 than for model 1. Finally in the southwestern part of the rift, both models predict a very similar stress field, with ¦3 horizontal and trending N170E to N180E in the Tunka basin, and ¦3 horizontal and trending N45E to N50E in the Sayan range. All these predictions are in rather good agreement with the observed stress field but do not permit to discriminate the two models. The major difference between them is rather qualitative: model 2 shows an equivalent amount of shear in the Sayan range as in the Tunka basin (Fig. 6a), whereas model 1 predicts a strong partitioning of the strain field between extension in the Tunka basin and compression in the Sayan range (Fig. 5a). This latter result is in better agreement with geological observations which suggest that the Sayan range is a compressive structure thrusting northeastward over the Angara basin, and that the Tunka fault is a left-lateral strike-slip and normal fault (Delvaux et al., 1998). It therefore appears that model 2 matches the observed strain and stress field better than model 1 in the central and northeastern parts of the rift zone, whereas the opposite is true in the southwestern part of the rift. However, none of these two models satisfactorily predicts the current deformation, neither along the whole rift nor outside and south of the rift zone, in the Khamar Daban range, revealed by the significant level of diffuse seismicity in that area.
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4.2. A best fit model The above conclusion led us to test the hypothesis of a three-plate model where the Amurian plate is divided into two sub-plates (Mongolian and Amurian plates), as was originally proposed by Zonenshain and Savostin (1981). According to these authors, the boundary between the Mongolian and Amurian plates does not correspond to any active fault but to a larger area over which the deformation is spread out. In our model, we do not constrain the nodal displacements in that area but apply different kinematic boundary conditions along the southern edge of the Mongolian and Amurian plates in order to simulate their kinematics, using the rotation parameters derived by Zonenshain and Savostin (1981) as starting values (Fig. 7c). We then slightly adjust the boundary conditions until we find the model that fits best the observed stress and strain field. The final boundary conditions are close to the initial ones derived from Zonenshain and Savostin’s rotation parameters and roughly reproduce those of model 1 in the western part of the model and those of model 2 in the central and eastern parts of the model (Fig. 7c). The predicted strain field shows compression in the Sayan range and a combination of left-lateral shear and N–S extension in the Tunka basin, similarly to model 1 and in accordance with geological observations (Delvaux et al., 1998). This partitioning cannot be observed on the horizontal stress direction map (Fig. 3b) because only one stress tensor has been computed by Petit et al. (1996) in the Sayan and Tunka regions, due to a lack of reliable focal solutions. The strain field becomes extensive to strike-slip along Lake Baikal northward, with ¦3 trending N130E to N140E and rotating clockwise in the northeastern part of the rift zone. These results are similar to model 2 for the central and northeastern part of the rift and agree with the observed strain and stress field (Petit et al., 1996; compare Fig. 3a with Fig. 7a, and Fig. 3b with Fig. 7b). South and outside of the rift zone, in the Khamar Daban range, this model predicts a small amount of shear deformation, that accommodates the small velocity differential between the Mongolian and Amurian plates underlined by the diffuse seismicity (Fig. 3c). It matches the seismotectonic data that show strike-slip focal mechanisms in that area (Fig. 2; Doser, 1991a).
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Fig. 7. Best fit model: predicted (a) P (solid lines) and T (shaded lines) horizontal strain axes; (b) ¦1 (solid lines); and (c) ¦3 (shaded lines) horizontal stress axes and velocity field.
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Fig. 8. Best fit model: predicted vertical strain rate (see text for explanations).
Since we have assumed the lithosphere to be incompressible and the horizontal velocity components to be constant with depth, we can compute the rate of thinning or thickening of the lithosphere žzz from the horizontal strain rates žxx and ž yy (see above). Because we do not model isostatic compensation and erosion or sedimentation rates, it is not possible to directly compare žzz with a geological observable such as uplift or subsidence rate, which are anyway not yet available in the Baikal region. However, since isostatic compensation should be fairly uniform regionally, žzz can be used as a crude indication of the regional topographic pattern, with negative values in subsiding basins and positive values in uplifting mountain ranges. As shown in Fig. 8, the predicted rate of vertical deformation matches fairly well the most prominent regional topographic features (Fig. 2). Lake Baikal and the northeastern basins are modelled as subsiding areas with the strongest subsidence predicted to occur in the central part of the lake. This area corresponds to the oldest part of the rift zone (Logatchev, 1984) and the thinnest crust and lithosphere according to seismic studies (Gao et al., 1994) and gravity models (Zorin et al., 1990; Petit et al., 1997). On
both sides of the lake, the model predicts elongated areas of uplift that correspond to rift shoulders. Rift shoulders are well known along the Baikal Lake and seems to be not only the consequence of a flexural isostatic response to extensional unloading of the rift (Van der Beek, 1997) but also of local crustal thickening adjacent to the rift (Burov et al., 1994; Petit et al., 1997). In the southwestern part of the rift zone, the model predicts a strong uplift in the highest mountain range of this region (Sayan range) and subsidence in the Tunka basin, in agreement with geological observations (Delvaux et al., 1998). However, high elevations found in the Stanovoy range (NE of the Baikal rift) are not reproduced at all by our model, suggesting that they might be related to other processes, such as underplating or previous tectonic events for instance. 5. Conclusion and perspectives Using a two-dimensional visco-elastic finite element model, we were able to test the influence of the far-field kinematics on the stress and strain pattern in the Baikal rift zone. We show that the observed stress and strain patterns can be satisfactorily ex-
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plained using a three-plate model that accounts for a differential displacement between the Mongolian and Amurian plates. The fact that our model does not take into account vertical forces but still explains most of the observed deformation suggests that the major forces that drive the present-day opening of the Baikal rift zone are horizontal forces related to the far-field kinematics. These forces probably have their origin in the India–Asia collision zone further south, which could have triggered the Late Pliocene– Quaternary ‘fast-rifting stage’ (Logatchev and Zorin, 1987; Logatchev, 1993), misleadingly called ‘activerift’ stage by Delvaux et al. (1998). Vertical forces cannot be excluded but seem currently to be of several orders of magnitude lower than horizontal forces. It must be remembered that this model is only valid for the present-day deformation regime and does not preclude a major influence of asthenospheric upwelling during earlier phases of the Baikal rift evolution. Indeed, the time and space evolution of the volcanic activity in the Baikal rift (and especially the stepwise westward shift of the east Sayan volcanic field) favours a long-term heat supply, presumably coming from a mantle plume located near the southern edge of the Siberian craton (Rasskazov, 1994). Our best-fit three-plate model simulates the existence of a differential displacement between an Amurian plate and a Mongolian plate. However, the boundary between these two blocks is not necessarily a plate boundary s.str. but could be a ‘diffuse’ transition zone from N–S compression in Mongolia to NW–SE extension in the central and northern Baikal region that vanishes further south. The model presented here is rather simple and could be improved by using more realistic rheological parameters and a more detailed fault geometry, but it only aims at providing a first-order test on the role of kinematic conditions upon the present-day rifting. In particular, the hypothesis of a vertically homogeneous lithosphere is a large simplification: a more accurate model would require the use of a realistic stratification of the lithosphere, especially in the crust with a frictional rheology in its upper part and a transition to dislocation creep in its lower part. Furthermore, such a model does not reflect the contrast in mechanical behaviour between the rigid Siberian craton, the Aldan shield and the Sayan–Baikal range
(Fig. 2; Petit et al., 1996, 1997). Further models should take into account these contrasting rheologies, but data on the structure of the lithosphere in the Baikal region are not yet accurate enough to reach such a level of detail. Finally, Global Positioning System geodetic measurements that are currently carried out in the Baikal rift zone should provide a new and important data set in the near future (Lesne et al., 1996). In particular, GPS results should allow us to fix kinematic boundary conditions and fault slip rates, which, in turn, will permit detailed numerical tests of the dynamics of the current deformation in the Baikal rift zone and of strain partitioning between the Amurian, Mongolian and Siberian tectonic blocks. Acknowledgements This study was carried out in the framework of a cooperation with the Institute of the Earth’s Crust, Siberian Branch of the Russian Academy of Science, Irkutsk. We are particularly grateful to its director, Academician N.A. Logatchev, and its Deputy Director, K.G. Levi, for their efficient support, and to V.A. Sankov for his scientific input. We thank C. Petit for providing us with material in advance of publication. Critical comments by P. Van der Beek and an anonymous reviewer helped to significantly improve the manuscript. This work was supported by the French Ministry for Education and Research (DSPT3) and the French Ministry of Foreign Affairs (‘Enveloppe Echanges Scientifiques 1996’) and is a contribution to IGCP Project 400. Ge´osciences Azur Contribution Nr. 147. References Avouac, J.P., Tapponnier, P., 1993. Kinematic model of deformation in central Asia. Geophys. Res. Lett. 20 (10), 895– 898. Baljinnyam, I., Bayasgalan, A., Borisov, B.A., Cisternas, A., Dem’yanovich, M.G., Ganbaatar, L., Kochetkov, V.M., Kurushin, R.A., Molnar, P., Philip, H., Vashchilov, Y.Y., 1993. Ruptures of major earthquakes and active deformation in Mongolia and its surroundings. Geol. Soc. Am., Mem. 181, 62 pp. Burov, E.B., Houdry, F., Diament, M., Deverche` re, J., 1994. A broken plate beneath the North Baikal rift zone revealed by gravity modelling. Geophys. Res. Lett. 21 (2), 129–132. Delvaux, D., Moeys, R., Stapel, G., Petit, C., Levi, K., Miroshnichenko, A., Ruzhich, V., San’kov, V., 1998. Paleostress re-
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