Extension in the Baikal Rift: Present-Day Kinematics of Passive Rifting

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1 ISSN 8-33X, Doklady Earth Sciences, 9, Vol. 5, No., pp Pleiades Publishing, Ltd., 9. Original Russian Text V.A. San kov, A.V. Lukhnev, A.I. Miroshnichenko, S.V. Ashurkov, L.M. Byzov, M.G. Dembelov, E. Calais, J. Deverchère, 9, published in Doklady Akademii Nauk, 9, Vol., No. 5, pp GEOLOGY Extension in the Baikal Rift: Present-Day Kinematics of Passive Rifting V. A. San kov a, A. V. Lukhnev a, A. I. Miroshnichenko a, S. V. Ashurkov a, L. M. Byzov a, M. G. Dembelov b, E. Calais c, and J. Deverchère d Presented by Academician Yu.G. Leonov January, 8 DOI:.3/S833X956 Received February, 8 Despite the fact that the pattern and deep structure of the Baikal rift system is well understood, there is no general agreement as to the model of its formation. Consistently suggested were mechanisms of passive [3, 8, 3, etc.] and active [] rifting, a model combining both mechanisms [], and then again a model with predominating active rifting under the effect of a local mantle source [] or gravitational instability of sediments []. This work offers additional arguments for substantiating the passive mechanism of extension in the Baikal rift at the present stage of evolution, which were adduced on the basis of the analysis of long-term measurements by the method of GPS geodesy. The Baikal geodynamic polygon network, which was laid in 99, consists of over 5 stations. It covers the southern and central parts of the rift system. Serving for the network as reference points are stations of permanent measurements in Irkutsk (IRKU since 99 and IRKT since 996) and Ulan-Ude (ULAN since 99 and ULAZ since 999). Data on field stations were obtained through annual measurements by two-frequency GPS receivers Ashtech Z and Ashtech ZXtreme with antennas Geodetic II, Geodetic III, and Choke Ring, with recording every 3 seconds for 3 hours over days. The GAMIT program packet was used for the analysis and calculation of pseudoranges and phase records a Institute of the Earth s Crust, Siberian Branch, Russian Academy of Sciences, ul. Lermontova 8, Irkutsk, 6633 Russia; sankov@crust.irk.ru b Buryat Research Center, Siberian Branch, Russian Academy of Sciences, ul. Sakh yanovoi 6, Ulan-Ude, 677 Russia c Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN , USA d UMR 6538 Université de Bretagne Occidentale (UBO), Technopole Brest-Iroise, Place Nicolas Copernic, F-98 Plouzane, France for every day of measurement []. Applying the double-difference method, we obtained coordinates of regional stations and their increments, satellite motion parameters, and 3 zenithal delays for each station and day of measurements, as well as phase ambiguities. To get a solution, we used final orbits obtained by the International GPS Service (IGS), orientation parameters of the Earth s rotation (IERS), tables of parameters of the Sun and the Moon, as well as all the materials recommended by the IGS. To adjust the Russian GPS network to the International reference frame (ITRF), we used 7 global stations of the IGS (ARTU, CHUM, FAIR, GRAZ, IRKT, KIT3, KOKB, NRIL, NVSK, ONSA, POL, SELE, TIDB, TSKB, ULAB, URUM, and USUD). Correction factors of station positions and orbit parameters independently calculated for each day of measurements, as well as their variance covariance matrixes, were combined with the results obtained for global stations by the Institute of Oceanography (SIO) and then processed by Kalman filter (GLOBK) [9]. On adjusting the regional GPS network with the ITRF network, the orientation, translation, and scale transformation of the network were calculated using data of IGS stations. Figure displays vectors of motion rates for each station of the Baikal polygon calculated relative to the Siberian Platform representing part of stable North Eurasia. The platform is represented by three stations permanant IRKT (city of Irkutsk) and field BAYA (Settlement of Bayandai) and LNSK (Settlement of Verlholensk), which are located in the southern part of the Irkutsk amphitheater. Previously [5, 7, 8], calculations were made relative to one IRKT station or North Eurasia as a whole. In the first case, the results of calculation involved the network rotation element relative to the reference station. This has a considerable effect on velocity values in remote stations (the values are always heightened), as well as on vector directions. In the second case, the horizontal component error is rather substantial when adjusting the stations located 5

2 6 SAN KOV et al. N 56 a b Siberian Platform 3 SHAM 6. 5 III LNSK.6 BCHV.9 ULCH 3.7 UZUR. MONK 3.5 ALGU.6 II BAYA.8 ANGA.8 Lake Baikal TURK.5 5 I IRKT. LIST. ULAZ 3. HORN 3. III UDUN 3. II KIAT 3.8 Transbaikalian block I E Fig.. The field of velocities for present-day horizontal movements of the Baikal depression by data of measurements on the Baikal GPS polygon for () Stations of permanent measurements; () field stations: (a) for long-term measurements, (b) for measurements lasting or fewer years. Designated near the stations are abbreviations of their names and motion velocities, mm/year. Velocity vectors for station displacements relative to the Siberian Platform are shown with ellipses of a 95% confidence interval. ( ) Faults: () Obruchev, () Morskoi, (3) North-Baikal, () Barguzin. (I I, II II, III III) Profiles shown in Fig.. on different tectonic blocks within North Eurasia, despite the expected stability of the territory. Having chosen, as a reference frame, the stations located inside a stable craton block situated at a small distance from the deformation zone, we anticipate to get a geologically justified pattern of displacements along the edge of the Siberian Platform. A peculiarity of the horizontal displacement pattern (Fig. ) is that vectors located within the Transbaikalian block representing part of the Amur Plate are in good DOKLADY EARTH SCIENCES Vol. 5 No. 9

3 EXTENSION IN THE BAIKAL RIFT: PRESENT-DAY KINEMATICS OF PASSIVE RIFTING 7 ε, year V hor, mm/yr Transbaikalian block Siberian Platform km m Lake Baikal km Fig.. Diagrams for velocities of horizontal movements (V hor, firm line) relative to the Siberian Platform by the azimuth of 3 and velocities of elongation ( ε, dashed line) on the Baikal plate boundary. () V hor values with their errors for stations of long-term measurements in the southern and central parts of the Baikal basin, () the same, for stations of the Barguzin geodynamic polygon. Given below are superposed sections of the relief by profiles I I, II II, III III through the Baikal basin (see Fig. ). See text for details. agreement. Stations in the southern part of the block, observations on which were carried out for a long time, are characterized by the most consistent direction of movements. They all shift southeastwards at an average azimuth of 3. The direction of vectors varies in a restricted range not exceeding. The maximum errors in the velocity of horizontal movements (V hor ) make up.59 mm/yr for the latitudinal component and.5 mm/yr for the longitudinal component. Stations located within the rift, on the western slope of the Baikal depression (LIST, ANGA, and UZUR), are characterized by a latitudinal displacement with low velocities. In general, the velocity of southeastern displacement of stations increases from the Siberian Platform to Transbaikalia (Fig. ). In the diagram, the distance was calculated relative to the direction of a seismogenic fault with the maximal amplitude of vertical movements, in which most of the relative motions of blocks is realized. This is the Obruchev fault in the southern part of the Baikal depression, the Morskoi fault in the central part, and the Northern Baikal fault in the northern part. The extension velocity grows slowly within the marginal part of the Siberian Platform block, rapidly within the Baikal depression, and again relatively slowly in Transbaikalia. At the maximal velocity 3.8 mm/yr (KYAT station), the velocity jump in the Baikal depression is about..5 mm/yr. Accordingly, the rate of relative elongation ( ε ) is maximal (up to. 8 year ) in the Baikal depression and decreases to both sides across the rift strike. The divergence rate for blocks of the Siberian Platform and Transbaikalia at the azimuth of 3 makes up 3. ±.7 mm/year. The first data on the present-day horizontal movements in the Barguzin depression have been obtained. Stations of the geodetic network are displaced relative to the Siberian Platform in the latitudinal and southeast- DOKLADY EARTH SCIENCES Vol. 5 No. 9

4 8 SAN KOV et al. ern directions. The average azimuth makes up. Variations in the azimuth values are twice as high as that for southern stations. Maximal errors in V hor reach.9 mm/year for the latitudinal component and.85 mm/year for the longitudinal component. This is related to the rather short period of measurements for stations of the Barguzin polygon. The average rate of displacement of all the polygon stations relative to the Siberian Platform block makes up. mm/year. As noted for the Baikal depression, there is a tendency to its increase when moving southeastward from the platform (see Fig. ). According to preliminary results of measurements by the method of GPS geodesy, the extension velocity for blocks in the South-Baikal depression made up.5 ±. mm/year for [7] and about 3 mm/year for 99 [5]. According to the diagram presented in [] and plotted using data from [8], the divergence rate for the Amur and Eurasian plates makes up 3. mm/year. Data we have obtained on velocities and extension vectors correspond to parameters of the long-term constituent of tectonic movements in the rift system. Calculated by data on Holocene coseismic displacements in zones of seismic dislocations, the minimal total extension velocity in the northern part of the rift system made up 3. ±.5 mm/year at an average 3 southeasterly direction []. The rate of present seismotectonic strains for the horizontal component makes up.95 9 year in the South-Baikal depression, the extension axis striking at [6]. Differences by an order of magnitude in strain values are in agreement with the idea that seismic deformations, which were developed over a period shorter than the seismic cycle duration, constitute only a small part of the general tectonic deformation. The proof of the nonrigid behavior of continental lithospheric plates on the divergent boundary represents one of the most important aspects in obtaining data on the present extension in the Baikal rift. It follows from the results of measurements (Fig. ) that subjected to extension were both the marginal part of the platform and the western part of the Transbaikalian block. The zone of the dynamic effect of the intraplate boundary is as wide as km in the studied region. Assuming the realization of the active rifting model, the maximal velocity of the relative motion of plates was to appear within the Baikal depression, and velocity was to decrease toward the Transbaikalian block right up to appearance of compression conditions []. This distribution of velocities of relative motions follows from the model, according to which the lithosphere extends above a local anomaly at the level of the lithospheric mantle (an asthenospheric projection) due to its gravitational instability []. The same distribution pattern also follows from the model of extension due to gravitational instability of sediments along the boundary of the Siberian Platform []. According to data presented above, results of measurements on the Mongolian polygon [8], as well as according to data of Chinese land-surveyors [], the motion velocity remains constant for stations located at a distance of 5 km to the southeast, in inner parts of the Amur Plate, in the direction normal to the Baikal rift. From the above discussion, it appears that the kinematics of the Baikal rift central part corresponds to the model of rifting with a remote tectonic source, i.e., passive rifting [3, 3]. The relative motion of the Amur Plate can be caused by its detachment from Eurasia on the long branch of the convective flow at the braking of the latter or by the effect of subduction processes in the western Pacific [8]. The additional effect of collision between Hindustan and Eurasia is also not in doubt. The effect is noticed in the southern rim of the Siberian Platform at the late stage of rifting when the main depressions of the Baikal rift system were already formed. Hence, on the basis of long-term measurements on the Baikal geodynamic GPS polygon, the velocity of the Siberian and Transbaikalian block divergence in the direction of 3 has been refined. It makes up 3. ±.7 mm/year at a distance comparable with the width of the zone of the plate boundary dynamic effect. This is consistent with parameters of a long-term constituent of the extension velocity, which are established based on geological data, and with the extension direction determined by earthquake focal mechanisms data. The velocity distribution across the rift basin with its gradual growth from one block to another suggests the nonrigid behavior of continental lithospheric plates on the divergent boundary. About 3% (..5 mm/year) of the total growth of the velocity falls on the Baikal depression. The rate of strain there reaches. 8 year and gradually decreases into both sides across the rift. The data on extension in the Barguzin depression have been obtained for the first time. The distribution of the velocity of horizontal movements on the Baikal divergent boundary between the North Eurasian and Amur plates corresponds to the model of passive rifting. ACKNOWLEDGMENTS This work was partially supported by the Russian Foundation for Basic Research, project nos r_siberia, r_siberia_a, and the Siberian Branch, Russian Academy of Sciences, IP no. 87. REFERENCES. S. V. Gol din, V. D. Suvorov, P. V. Makarov, et al., Geol. Geofiz. 7 (), 9 (6).. Yu. A. Zorin and E. Kh. Turutanov, Geol. Geofiz. 6 (7), 685 (5). 3. Yu. G. Leonov, Geotectonics 35 (), 8 () [Geotektonika, No., 3 ()]. DOKLADY EARTH SCIENCES Vol. 5 No. 9

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