No Moho uplift below the Baikal Rift Zone: Evidence from a seismic refraction profile across southern Lake Baikal

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114,, doi:10.1029/2008jb005828, 2009 No Moho uplift below the Baikal Rift Zone: Evidence from a seismic refraction profile across southern Lake Baikal Christoffer Nielsen 1 and H. Thybo 1 Received 26 May 2008; revised 6 March 2009; accepted 15 April 2009; published 18 August 2009. [1] The late Cenozoic Baikal Rift Zone (BRZ) in southern Siberia is composed of several individual topographic depressions and half grabens with the deep Lake Baikal at its center. We have modeled the seismic velocity structure of the crust and uppermost mantle along a 360 km long profile of the Baikal Explosion Seismic Transects (BEST) project across the rift zone in the southern part of Lake Baikal. The seismic velocity structure along the profile is determined by tomographic inversion of first arrival times and 2-D ray tracing of first arrivals and reflections. The velocity model shows a gently deepening Moho from the Siberian Platform (41 km depth) into the Sayan-Baikal Fold Belt (46 km depth). We can exclude the presence of any Moho uplift around the 10 km deep sedimentary graben structure of southern Lake Baikal. The lower crust includes a distinct 50 80 km wide high-velocity anomaly (7.4 7.6 ± 0.2 km/s), slightly offset to the northeast from the rift axis. We interpret this feature as resulting from mafic intrusions. Their presence may explain the flat Moho by compensation of lower crustal thinning by intrusion of mafic melts. The Pn wave velocities (8.15 8.2 km/s) are normal for the area and do not show any sign of decompression melting in the sub-moho mantle, such that possible lithosphere thinning has not reached the base of the crust. On the basis of the results of the BEST project, we suggest that the BRZ is formed by passive rifting in the rheologically weak suture between the Siberian Platform and the Amurian plate. Citation: Nielsen, C., and H. Thybo (2009), No Moho uplift below the Baikal Rift Zone: Evidence from a seismic refraction profile across southern Lake Baikal, J. Geophys. Res., 114,, doi:10.1029/2008jb005828. 1. Introduction 1 Department of Geography and Geology, University of Copenhagen, Copenhagen, Denmark. Copyright 2009 by the American Geophysical Union. 0148-0227/09/2008JB005828$09.00 [2] Intracontinental rifting is assumed to be the initial stage of a development, that eventually may lead to breakup and separation of lithospheric plates and formation of new oceanic plates, unless extension ceases and the evolution changes into formation of wide sedimentary depressions or grabens [Turcotte and Emerman, 1983; Olsen and Morgan, 1995]. Because much of the world s hydrocarbon resources are found in extensional regions, there is considerable economic interest in understanding the development of rift structures. The active Baikal Rift Zone (BRZ) is one of the four major Cenozoic continental rift systems, and it reveals most of the characters associated with continental rifting: elongated sedimentary grabens, elevated margins, volcanic provinces, normal faults and high seismic activity. Frequent high-magnitude earthquakes makes the BRZ one of the most seismically active continental rifts in the world [Doser, 1991a, 1991b]. [3] The BRZ is situated in southeast Siberia along the margin of the Siberian Platform (Figure 1). Its location inside the Eurasian continent, more than 2000 km from the nearest active plate boundary, offers an outstanding opportunity to study the early stages of continental breakup. The surface expression of the BRZ is a more than 2000 km long series of depressions or grabens, of which the 660 km long and arch-shaped Lake Baikal covers three main grabens. A maximum water depth of 1634 m makes Lake Baikal the deepest freshwater reservoir in the world [Hutchinson et al., 1992], and it contains roughly more than 20% of the world s available freshwater resources. The rift zone has developed during the last 35 30 Ma at the Sayan-Baikal Fold Belt, a Palaeozoic suture between the Siberian Platform and the Amurian plate. The evolution of the BRZ has been accompanied by remarkably little volcanic activity, mainly observed outside the extensional basins [Kiselev, 1987]. The total volume of volcanic rocks around the BRZ probably does not exceed 6000 km 3, whereas more than 144,000 km 3 of volcanic rocks are estimated around the Kenya Rift [Achauer and Masson, 2002]. This difference in volume indicates a significant dissimilarity in the rifting processes between the Baikal and East African Rift systems. [4] The processes which have initiated and powered the extensional forces in Siberia are poorly understood. In the last decades it has been debated if the BRZ is caused by active processes, which involve a sublithospheric mantle plume centered in southern Siberia and Mongolia [Logatchev and Florensov, 1978; Logatchev and Zorin, 1987, 1992; Windley and Allen, 1993; Gao et al., 1994, 2003; Zorin et al., 2003], or if the rift is caused by passive processes, where the lithosphere is stretched by far field 1of22

Figure 1. Regional map showing the location of the Baikal Rift Zone in southern Siberia and the associated lithosphere plates and microplates that compose the tectonic framework of central Asia. The dashed square marks the study area of the BEST project. After Zonenshain and Savostin [1981] and Moore et al. [1997]. stresses generated at the active plate boundaries of Eurasia [Artemjev and Artyushkov, 1971; Molnar and Tapponnier, 1975; Tapponnier and Molnar, 1979; Zonenshain and Savostin, 1981; Kiselev and Popov, 1992; Petit et al., 1997]. In general, classification of continental rifts as either active or passive is difficult, because many of the same geophysical rift features (e.g., crustal underplating, magmatism, deep lithospheric melting, thermal weakening of the lithosphere and lithospheric thinning) are common in both scenarios [Ruppel, 1995]. In addition, processes such as secondary convection and delamination, caused by density instability in the lithosphere, may blur the picture of the initial rifting process. The discussion on the extensional forces in the Baikal region is to some degree because of different assumptions on the structure of the lower crust and upper mantle beneath the BRZ. [5] Significant crustal thinning below Lake Baikal has been proposed on the basis of Deep Seismic Sounding (DSS) studies [Puzyrev et al., 1973, 1978]. These early results indicate that the crustal thickness is reduced to about 36 km below the rift structure from 37 46 km under the adjacent margins. However, these results are based on experiments with a very coarse distribution of seismographs and sources in an aerial coverage, which makes identification of seismic phases uncertain and provides models with low resolution. Studies of DDS data by Suvorov et al. [2002] in the Baikal region show that significant changes of Moho depth between 35 km and 50 km are present, but there is no pronounced crustal thinning associated with the rift axis. Suvorov et al. [2002] interprets the changes in crustal thickness to be related to accretion entities, including the transition between the Precambrian Siberian Platform and the tectonic collage of the Sayan-Baikal Fold Belt. Thybo et al. [2000] have proposed that intrusion of mantlederived magma into the lower crust may compensate crustal thinning such that the Moho remains relatively flat after the rifting initiated. Lower crustal intrusions (high P wave velocity zones) have been related to rift structures in different tectonic environments, e.g., the active East African Rift system [Birt et al., 1997; Thybo et al., 2000; Mackenzie et al., 2005] and the Dniepr-Donetsk paleorift [DOBREfraction Working Group, 2003; Lyngsie et al., 2007]. A previous seismic study in central Lake Baikal, by Ten Brink and Taylor [2002], reveals a high-velocity zone in the lower crust, which these authors interpret as a relic of the stretched Siberian Platform. However, the data was obtained along the strike of the rift zone, and no information was available across the structure. [6] The Baikal Explosion Seismic Transects (BEST) project is a joint Danish, Polish and Russian enterprise. A 2of22

Figure 2. Detailed map of southern Lake Baikal showing the location of the BEST profile. The dots denote seismic recorders, the stars denote shotpoints, the square shows the location of the Babushkin super vibrator, and the solid line in Lake Baikal shows the location of the air gun shooting line. detailed seismic wide-angle refraction profile of the crust and uppermost mantle has been acquired across southern Lake Baikal (Figure 2). The interpretation of the seismic data, described in this paper, is based on 2-D velocity modeling by tomographic inversion and forward ray tracing modeling. The four main purposes of the BEST project are (1) to outline the overall geometry of the graben structure in the southern Baikal Basin, (2) to determine the depth to the Moho, in order to resolve the amount of crustal thinning, (3) to look for variation in the lower crustal velocity in order to test for possible magmatic intrusions, and (4) to determine variations in the velocity of the uppermost mantle (Pn) in order to test for stretching and heating. By examination of these characteristics and comparison to results from previous studies of the Baikal region, we intend to give new insight to the dynamic processes that have controlled the evolution of the BRZ during the past 35 30 Ma. [7] In this paper, we describe the analyses of the data, the methods, the results of the seismic interpretation, and correlate the velocity models with other published results from the BRZ, with emphasis on DSS studies of the southern Lake Baikal [see also Thybo and Nielsen, 2009]. We discuss the evolution of the rift zone based on the interpreted models and their relation to dynamic processes in the upper mantle and lower crust, together with the issue of active/passive rifting processes. 2. Main Structural Elements and Rift Evolution [8] The more than 2000 km long and generally southwestnortheast striking BRZ consists of a system of elongated Cenozoic depressions or grabens (Figure 3). The central part of the rift system is located at the Caledonian Sayan-Baikal Fold Belt, at the suture between the Precambrian Siberian Platform and the Amurian plate [Logatchev and Florensov, 1978]. The 660 km long and 40 80 km wide Lake Baikal occupies the central third of the entire rift system. The characteristic arch shape of the lake follows the edge of the Siberian Platform. According to Hutchinson et al. [1992] and Delvaux et al. [1997], the Selenga Ridge and the Academician Ridge subdivides Lake Baikal into three independent basins: southern, central and northern Lake Baikal Rift Basin. Seismic measurements indicate the occurrence of 4.4 to 10 12 km thick sedimentary deposits in the three grabens below Lake Baikal [Hutchinson et al., 1992; Suvorov and Mishen kina, 2005], whereas the sedimentary infill in the other grabens in the BRZ are smaller (1.5 2.5 km) [Logatchev and Zorin, 1992]. Location and 3of22

Figure 3. Tectonic map showing the Cenozoic rift grabens in the Baikal Rift Zone. Abbreviations are as follows: Sb, south basin; Cb, central basin; Nb, north basin. After Delvaux et al. [1997]. geometry of the three lake basins are controlled by major (>80 km in length) pre-cenozoic master basement faults along the rigid Siberian Platform [Florensov, 1969; Zamarayev and Ruzhich, 1978; Logatchev and Zorin, 1992; Sherman, 1992; Sherman et al., 2004]. The BRZ is by Logatchev and Florensov [1978], Logatchev and Zorin [1987, 1992], and Zorin et al. [2003] considered to be located within a rift-related updoming structure, referred to as the Sayan-Baikal domal uplift. Delvaux et al. [1997] and Petit et al. [1997] have questioned this domal uplift, since the central part of the BRZ is relatively depressed, and the uplift in the southwestern Baikal region may be related to compressional forces in the Altai-Sayan system, rather than to rift processes. [9] Three primary volcanic provinces of Cenozoic age are associated with the BRZ: Sayan-Khamardaban, Vitim and Udokan. The areas of volcanic activity are in general displaced from the rift basins, except for the Tunka depression where basalt sheets and cones are observed within the sedimentary layers [Logatchev and Zorin, 1987]. According to Logatchev et al. [1983] and Achauer and Masson [2002] the volume of volcanic rocks in the Baikal Rift system does not exceed 6000 km 3, which is considerably less than other Cenozoic rift systems (e.g., 144,000 km 3 in the Kenya Rift). [10] The evolution of the BRZ is closely associated with the pre-cenozoic development of Central Asia, because the main parts of the future rift-controlling master faults developed during the initial closure of the Mongol-Okhotsk Ocean in Late Ordovician-Silurian [Belichenko et al., 1994; Delvaux et al., 1995; Zorin et al., 2003]. In this period an island arc system collided with the Siberian Platform and resulted in large-scale thrust faults, which may have been reactivated and transformed into listric normal faults during the Cenozoic extension [Zorin et al., 2003]. The Cenozoic evolution of the BRZ is by Logatchev and Zorin [1987, 1992] and Mats [1993] subdivided into an initial rift stage (Oligocene-early Pliocene) and a main rift stage (late Pliocene-Recent). In general, the initial stage is referred to as the slow rifting phase and the main stage is referred to as the fast rifting phase. [11] The initial rift stage is characterized by weak or moderate tectonic activity with slow plastic deformation of basement [Logatchev and Florensov, 1978] and a compressional to strike-slip stress regime [Delvaux et al., 1997]. This scenario resulted in prolongation of existing master faults and the development of transverse normal faults and oblique slip faults [Sherman, 1978]. Throughout most of the initial stage, the movements along the rift-controlling faults in the Baikal region were insignificant, and the morphology was characterized by shallow basins surrounded by flat upland and volcanic plateaus [Logatchev and Florensov, 1978; Artyushkov et al., 1990]. The evolution of a lacustrine basin probably started in Oligocene, at the southern part of present Lake Baikal, and was followed by basin formation in central Baikal and the Tunka area [Logatchev and Zorin, 1992]. In the Middle Pliocene, the basin subsidence in both south and central Baikal was intensified, while the north Baikal Basin was initiated [Artyushkov et al., 1990]. Because of slow extension in the initial rift stage, the sedimentation rate was at the same level as the subsidence, and therefore no deep water basins developed during this stage [Logatchev and Florensov, 1978]. According to Kiselev [1987] the largest volume of volcanic rocks erupted during the Miocene-Pliocene, also known as the main volcanic phase. The volcanism was concentrated around the mountainous regions of the Sayan-Khamardaban and the Vitim plateau [Kiselev, 1987; Rasskazov, 1994]. 4of22

Figure 4. Trace normalized seismic sections for the 10 dynamite shots and the super vibrator with the phase correlated seismic arrivals. All sections are filtered by a 2 20 Hz band-pass filter and the travel time is reduced by a velocity of 8 km/s in order to emphasize high velocities in the lower crust and uppermost mantle. The two major gaps in the data are the location of Lake Baikal and Irkutsk Airport. Sections 3, 4, 5, and 6 include the air gun recordings, which can be correlated with the shotpoints. Inserts show zooms to illustrate details of lower crustal and Moho reflectivity for selected sections. Phase notation is as follows: Ps, diving wave in the sedimentary sequence; Pg, diving wave in the upper crust; Pg 0, subphase to Pg; Pi 1, diving wave in the middle crust; Pi 2, diving wave in the lower crust; Pn, diving wave in the upper mantle; PgP, reflection at the base of the sedimentary sequence; Pi 1 P, reflection inside the upper crust; Pi 2 P, reflection at the base of the upper crust; Pi 3 P, reflection at the base of the lower crust; PmP, reflection from the Moho; PIP, reflection from a discontinuity in the uppermost mantle. Notice that the Pi 3 P phases are characterized by strong amplitude and long coda for reflection points beneath Lake Baikal (e.g., shots 4, 5, 6, and 7), and weak amplitude and a short coda outside the rift structure (e.g., shots 1, 2, and 9). Seismograms from within Lake Baikal, in sections 3, 4, 5, and 6, were recorded by seismographs located at the respective shotpoints during air gun shooting in the lake (the principle of reversibility justifies this use of the data). [12] The transition to the main rift stage in the late Pliocene is marked by an increase in the strain rate with a factor of 6 to 10, brittle deformation of the crust and displacement along both normal and reverse wrench faults [Logatchev and Florensov, 1978]. According to Delvaux et al. [1997] the stress regime was pure extension in the area of Lake Baikal and pure strike-slip in the southwestern part the rift zone (Tunka region). A transition from shallow water to deep water sedimentary facies is located between the initial stage and the main rift stage, and it is probably caused by acceleration in flank uplift and basin subsidence [Logatchev and Zorin, 1987; Hutchinson et al., 1992]. The 5of22

Figure 4. (continued) facies transition is marked by a shift from fine-grained, coal-bearing deposits to coarser sands, silts, and argillites [Logatchev and Zorin, 1987, 1992; Mats, 1993; Hutchinson et al., 1992]. An important feature of the main rift stage is that the sedimentation rate for the first time was slower than the subsidence, and deep water basins evolved in the Baikal region [Logatchev and Florensov, 1978; Artyushkov et al., 1990]. The volcanic activity decreased during the Pliocen- Quaternary period, except in the Udokan province where the activity increased [Logatchev and Zorin, 1987]. In general, the main rift stage was not accompanied by any increase in volcanism, and the volume of volcanic products is small compared to the initial stage [Logatchev and Zorin, 1987]. 3. Seismic Data [13] The seismic data was recorded along a 360 km long, wide-angle refraction profile across the Baikal Rift Zone. The profile is covered by 11 seismic record sections (Figure 4), which were recorded in September 2001 and October 2002, as a part of the BEST project (Baikal Explosion Seismic Transects). The seismic sources were 10 dynamite explosions, with total charges of 0.5, 1.5 and 2.5 ton TNT, and one 100 ton stationary super vibrator. The average shot spacing was 35 km and the seismic signals were recorded on 175 portable seismographs (Reftek 125 Texan ) with 2 km of average station spacing. The shot and vibrator sources were supplemented by 234 shots by a 60 l air gun in Lake Baikal, with an average spacing of 0.2 km. The air gun shots were recorded by 26 stations on both sides of Lake Baikal, and it is possible to correlate data from the stations deployed at shotpoint 3, 4, 5 and 6 directly with the explosion sections. [14] All seismic data were recorded with a sample rate of 10 ms and converted to standard SEG-Y format by the PASSCAL and ProMax software. The signal-to-noise ratio is good in all seismic sections recorded with large charges (1.5 and 2.5 ton), and good energy propagation can also be recognized to the ends of the profile for shots with charges of 0.5 ton TNT. Vibrator recordings are of lower quality and especially identification of the onsets of arrivals is uncertain because of the narrow bandwidth and uncertainty regarding the far field waveform. The supplementary air gun data 6of22

provides excellent signals of the first arrivals, but the data are generally influenced by noise on large offsets (>100 km) and the identification of wide-angle reflections is uncertain. [15] Travel times were picked on shot gathers using the interactive plotting and picking program SZplot. During formatting of data to SZplot, the seismic sources were projected onto a best straight line. Phase correlation across the rift zone was primarily confirmed through checking of reciprocal travel times. The two main gaps in the data represent Lake Baikal (50 km) and Irkutsk Airport (25 km), respectively. For some of the seismic phases the crossover point cannot be determined, since the point correlates with the location of either Lake Baikal or Irkutsk Airport in the seismic sections. [16] A seismic phase (Ps) with turning points in the sedimentary layers and an apparent velocity of 5 5.75 km/s is observed along the profile as a first arrival. In the Siberian Platform it is possible to pick a reflection at the base of the sedimentary units (PgP), but such a phase could not be identified in the Sayan-Baikal Fold Belt. The sedimentary phase is followed by a Pg phase with turning points in the upper crystalline crust and an apparent velocity of 6 6.5 km/s. The apparent velocity of the Pg phase increases with offset such that a Pg subphase (Pg 0 ) is observed as a secondary arrival at offsets larger than 175 km. A Pi 1 phase with an apparent velocity of 6.5 km/s and turning points in the middle part of the crust is distinguished as a first arrival at offsets larger than 125 km. At offsets between 150 km and 200 km the middle crustal phase is followed by a diving wave (Pi 2 )in the lower crust with variable apparent velocity. The apparent velocity of the Pi 2 phase with turning points outside the rift axis is about 7 km/s, but inside the rift axis the apparent velocity increases to about 7.5 km/s (shotpoint 1 and 2). A series of intracrustal reflected phases (Pi 1 P, Pi 2 P and Pi 3 P) are correlated on both sides of Lake Baikal. In sections 4, 5, 6, and 7 the Pi 3 P reflector from the transition between middle and lower crust is characterized by a significant reverberant reflectivity or ringing energy at offsets beyond 55 65 km. The duration of the observed Pi 3 P coda is about 400 1000 ms and distributed over 5 6 wavelets. All four sections have reflection bounce points from the lower crust in the vicinity of the rift axis. On the remaining record sections with bounce points outside the rift axis, this reverberant feature is not observed at all. A generally strong reflected phase from the Moho (PmP) is correlated in all record sections, with an average critical offset of 120 km for reflection points below the rift zone and 70 80 km outside the rift zone. The PmP phase is characterized by weak amplitude and short coda (100 200 ms) for reflection points beneath Lake Baikal (shotpoint 4, 6 and 7), and strong amplitude and a long coda (400 700 ms) outside the rift structure (shotpoint 1, 2 and 10). This amplitude distribution indicates a sharp Moho transition outside the rift zone and a gradual transition inside the rift structure. The weak PmP amplitude and its long critical offset under the rift may be explained by the low-velocity contrast across Moho due to the very high velocity of the lower crust. A distinct diving wave from the uppermost mantle (Pn) with an apparent velocity between 8.0 km/s and 8.2 km/s is observed at offsets larger than 200 km in on sections with appropriate offsets, also from below the rift zone. A strong amplitude reflection (PIP) is observed at offsets larger than 200 km. It is presumably a reflection from a discontinuity in the uppermost mantle. 4. Tomographic Inversion [17] We have applied 2-D tomographic inversion [Hole, 1992] to first arrival picks. Raypaths and travel times for the first arrivals are calculated through a velocity model based on a finite difference algorithm [Vidale, 1990]. This algorithm is using the eikonal equation of ray tracing, which relates the gradient of travel times to the velocity field. The model is sampled on a 1 by 1 km grid as a minimum structure velocity model without layer boundaries. The starting model is generated on the basis of a typical velocity structure from previous interpretations of deep seismic sounding measurements [Zorin et al., 2003; Suvorov et al., 2002]. In order to reduce the nonuniqueness of the inverse problem, horizontal and vertical smoothing of the calculated derivatives and velocity perturbations is applied, but the model is never smoothed [Hole, 1992]. The optimal smoothing filter configuration is a large filter at the beginning of each iteration sequence, followed by successive stepwise reduction of the filter size. The maximum considered source receiver offset is successively and stepwise increased during the calculations and the whole set of smoothing filters is applied at each selected offset. By this process the upper parts of the model, are determined before the deeper parts. Fitting of the model is done quantitatively by calculating the misfit between observed and calculated arrival times in the L 2 space [Hole, 1992]. [18] The preferred tomographic velocity model shall explain the largest possible number of observations, have a travel time residual (RMS) close to the data uncertainty, and consist of a continuous and smooth velocity structure. The data used for tomographic inversion includes recordings of P waves from (1) all the dynamite shots, (2) the super vibrator, and (3) the air gun shots that were recorded by stations deployed on the shores of Lake Baikal. We use only the stations closest to the shore, as they represent air gun data with the lowest noise level. Tests with inland stations show that the low velocity of the water body (1.5 km/s), in Lake Baikal, is leaking into the onshore areas characterized by relative high velocities (5.5 km/s) due to the smoothing filters. This results in an unacceptable high misfit of the model and an image with an apparently too wide graben structure. The data uncertainty of the applied data for tomographic inversion is estimated to be 100 ms, but the final model has an RMS misfit of 180 ms because of the strong smoothing in coastal areas where the contact between low and high velocities is sharp (<20 km). Tests have shown that excluding the air gun data leads to models with RMS misfit of 100 ms, which corresponds to a good fit with a c 2 of around 1. 4.1. Tomographic Model [19] The final tomographic inversion model (Figure 5) is reached after 121 iterations, with a RMS travel time residual of 178 ms (the uncertainty of the input data is 100 ms). The model is considered stable because the inversion has been able to ray trace 1579 of the 1587 observed first arrivals and 7of22

Figure 5. Two-dimensional tomographic inversion of seismic first arrival travel times. (a) Onedimensional seismic velocity structure used as a starting model for the inversion. (b) Final velocity model reached after 121 iterations. (c) Ray coverage of the model. The model shows clearly the sedimentary graben structure beneath Lake Baikal and differences in seismic velocity structure of the crust between the Siberian Platform and the Sayan-Baikal Fold Belt. The top of the basement is estimated to be located between the 5.5 and the 6.0 km/s velocity contour and the Moho is probably around the 7.7 km/s contour. Abbreviations are as follows: PRH, Primorsky high; SKH, Sayan-Khamardaban high; LVZ, low-velocity zone. the ray coverage is good. In an overall view the model shows much lateral velocity variation in the upper crustal segment (0 25 km depth), and less lateral velocity variation in the lower crustal and upper mantle segment (25 55 km depth). In the upper crustal section the primary structures are the sedimentary basin beneath Lake Baikal and the adjacent basement highs, Primorsky and Sayan-Khamardaban. The Baikal sedimentary basin is documented to a depth of 11 km if the 5.5 km/s velocity contour is chosen as the transition between sediment and basement. Basement depressions or sedimentary basins can also be recognized in both the Siberian Platform and the Sayan Baikal fold belt, marked by the 6.0 km/s and the 5.5 km/s contour, respectively. The Moho discontinuity is at a depth of about 40 45 km, defined as the 7.7 km/s contour to correct for the inherent smoothing. According to the velocity field of the lower crustal segment, the Moho is slightly deeper under the Sayan Baikal fold belt than the Siberian Platform. An elongated low-velocity zone, bounded by the 6.0 km/s velocity contour, is found in the southeastern part of the model at 15 km depth. 4.2. Resolution Test of the Tomographic Model [20] The reliability of the velocity structures in the final tomographic model is examined by a resolution test. The test is based on an artificial test model, which is interpreted from the final model, by taking the smoothing effects into accounts. Synthetic travel times, calculated by ray tracing in the test model, are used as input to the tomographic inversion routine. Starting model, program parameters and 8of22

Figure 6 9of22

number of iterations are exactly the same as during the inversion of the original data. Comparison between the resulting theoretical model and the final model reveals the well-defined velocity structures, which will have approximately the same location as in the final model. An element that cannot be reproduced or appears without being present in the test model is considered uncertain. [21] In order to test the resolution of the final model, we create the following three artificial models: (1) a model which includes all the velocity structures (Figure 6a), (2) a model without the low-velocity zone in the southeastern part of the model (Figure 6c), (3) a model where only the upper crustal segment is present above a 1-D lower crust with a flat Moho (Figure 6e), and (4) a model with a 5 km uplift in the 7.5 km/s and 7.7 km/s contours below Lake Baikal (Figure 6g). Inversion of travel times, from the first test model, shows that the three main depressions in the upper crust and the flanking ridges of Lake Baikal (Primorsky and Sayan-Khamardaban), are reproduced by the inversion (Figure 6b). Only the relatively low velocities (3 4.5 km/s) that characterize the Baikal sedimentary basin are not fully resolved, probably because of smearing by the smoothing, but they are still acceptable. This indicates that some of the sedimentary layers may have smaller velocity than revealed by the final model. The lower crustal part of the model and the presumed Moho topography around the 7.7 km/s contour, are also reproduced as in the final model, although in smoothed versions. The resulting model of the second test shows that the low-velocity zone appears without being present in the test model (Figure 6d); it is therefore considered as a possible model artifact. The southeastern part of this artifact may have emerged because the relatively low velocities in the overlying basement depression (4 5 km/s) are projected downward into the model by smoothing. The northwestern part of the body may be caused by the lack of crossing ray coverage in that part of the model. On the basis of the strong similarities of the two test models, we assume that the influence of the low-velocity zone on the other structures of the model is insignificant. The third test model (Figure 6f) shows that the velocity structures of the upper crustal segment have considerable influence on the underlying part of the model. The 7.7 km/s contour (Moho) appears affected to the order of ±5 km within areas covered by rays. The fourth test model (Figure 6h) shows that a Moho uplift below Lake Baikal affects the velocity field of the lower crust, but not more than a flat Moho as in model 6f. This result reveals, in combination with test model 6f, that the velocity field of the lower crustal segment is not properly resolved by the inversion of first arrival traveltimes only. The ray coverage is denser in the synthetic models than in the final model, which indicates that the synthetic input models are too simple and smoothed compared to the true earth structures. This suggests that the first arrival tomographic inversion correctly resolve the true earth structure within the limited resolution, and we regard the tomographic result as being representative of the main upper crustal structures in the BRZ. 5. Two-Dimensional Ray Tracing [22] To obtain a more detailed model than by the tomographic inversion approach, we have used 2-D ray tracing on the basis of the combined forward and inverse modeling program Rayinvr [Zelt and Smith, 1992]. The forward part of the program is based on ray tracing, where raypaths are calculated through a velocity model by numerical solution of the eikonal equation in a 2-D medium [Zelt and Smith, 1992]. The inversion part of the program is based on a damped least squares method to solve the linearized problem. Rayinvr allows incorporation of known geological structures in the starting model. The starting model is inspired by the velocity model from the tomographic inversion. Both refracted and reflected arrival times of P waves are modeled and the model includes first-order interfaces. The uncertainty of the P wave arrival times are estimated to ±100 ms. [23] The ray tracing model is developed by fitting theoretical travel times to picked travel times with a top-down strategy. This forward model is later refined by linear inversion to minimize the travel time root mean square residuals (RMS). The criteria for the final velocity model is a RMS value close to the data uncertainty (±100 ms), a normalized chi square value (c 2 ) close to one, and a model that explains the highest possible number of observed arrival times. The data applied for Rayinvr includes recordings of P waves from (1) all the dynamite shots, (2) the super vibrator source, and (3) the air gun shots that were recorded by stations located around some of the dynamite shots (SP 3, 4, 5 and 6). 5.1. Two-Dimensional Ray Tracing Model [24] The ray tracing model interpreted by use of the program Rayinvr (Figure 7) explains 3290 of 3442 observations of refractions and reflections. The travel time misfit (RMS) is 103 ms and the normalized chi 2 (c 2 ) is 1.044 (Table 1). Our documentation of the model (Figure 8) includes the calculated travel times superimposed onto the observed travel times and the ray coverage. On the basis of velocities and reflections we subdivide the model into the following five units: (1) a sedimentary section, (2) an upper crustal section, (3) a middle crustal section, (4) a lower crustal section, and (5) an uppermost mantle section. [25] The most pronounced feature in the upper part of the model is Lake Baikal and the associated rift graben structure, which is estimated to be around 10 km deep. On the basis of bathymetric maps of the southern part of Lake Baikal, the upper 0.8 1 km of the structure is modeled as Figure 6. Resolution test of the final tomographic model. (a) Synthetic model, which includes all the velocity structures and (b) the resulting model of the test. (c) Synthetic model without the low-velocity zone in the southeastern part of the model and (d) the resulting model of the test. (e) Synthetic model where only the upper crustal segment is present above a 1-D lower crust and (f) the resulting model of the test. (g) Synthetic model with a 5 km uplift in the 7.5 and 7.7 km/s contours and (h) the resulting model. Abbreviations are as follows: PRH, Primorsky high; SKH, Sayan-Khamardaban high; LVZ, low-velocity zone. 10 of 22

Figure 7. Final 2-D velocity model for the BEST profile across the BRZ obtained from forward modeling and inversion of seismic travel times. Seismic velocities given in km/s. (a) The model from the surface to the uppermost mantle. (b) Similar plot from the surface to the middle of the upper crust. Thick black lines mark the modeled reflection points. The 10 km deep Baikal Rift depression is marked by relatively low seismic velocities (1.9 5.25 km/s) and appears to include two major sequences. The upper and middle crust are characterized by normal continental crustal velocities. The lower crust has a normal to high velocity for a cratonic area, with a pronounced unusually high-velocity zone (7.4 7.6 km/s) beneath the rift axis. The Moho topography is gentle, slightly shallower in the Siberian Platform than in the Sayan-Baikal Fold Belt, and there is no Moho uplift associated with the rift structure. The mantle velocities are normal (8.15 8.20 km/s) for a platform area and no low-velocity anomaly is observed to the reflector at almost 60 km depth. water (1.48 km/s). The underlying sedimentary sequences can be subdivided into two units on the basis of seismic velocity: a 3.5 4.5 km thick upper unit defined by velocities between 1.9 km/s and 3.6 km/s and a 5 6 km thick lower unit characterized by velocities of 4.5 5.25 km/s. The subdivision of the basin is only a proxy, because it is not supported by reflections, and the refraction data is too coarse to resolve different sedimentary units in details. However, the seismic velocities indicate an overall two layered structure of the basin. The interface between the sedimentary units of Lake Baikal and the crystalline base- Table 1. Phase Notation and Statistical Parameters of the Seismic Modelling a Phase Layers in Model Phase Description Total Number of Picks Number of Modelled Picks t rms (ms) c 2 Ps First arrival in the sedimentary units 166 163 59 0.346 PgP Reflection at base of the sedimentary units 50 50 71 0.513 Pg First arrival in the upper crust 1007 948 b 130 1.692 Pi 1 P Internal reflection in the upper crust 206 202 47 0.220 Pg 0 Secondary refraction in the upper crust 231 197 103 1.058 Pi 2 P Reflection at base of the upper crust 385 357 38 0.148 Pi 1 First arrival in the middle crust 178 161 75 0.571 Pi 3 P Reflection at base of the middle crust 271 271 68 0.462 Pi 2 First arrival in the lower crust 135 135 25 0.065 PmP Reflection at the Moho discontinuity 536 532 61 0.368 Pn First arrival in the uppermost mantle 198 196 49 0.246 PIP Sub-Moho reflection 79 78 75 0.569 Total All phases 3442 3290 103 1.044 a All picks are assigned an uncertainty of 100 ms. b The dissimilarity between number of picks and number of modelled picks is caused by the circumstance that rays from the Pg phases could not be fully ray traced in Lake Baikal at shot 3, 4, 5, and 6. 11 of 22

Figure 8. Fit between calculated (lines) and observed (bars) travel times of the seismic data including the corresponding ray coverage. All plots are based on two-point ray tracing between source and location of picks. The length of the bars indicates the picking uncertainty (100 ms). All travel times are reduced with a velocity of 8 km/s. At shotpoints 4, 5, 6, 7, and vibrator the boundary of the sedimentary basin beneath Lake Baikal has been removed in order to trace rays across the structure. ment is not supported by reflections, but interpreted as the >5.25 km/s velocity contour. [26] The Siberian Platform and the Sayan-Baikal Fold Belt are both covered by a layer with relatively low velocity (5.1 5.8 km/s). The thickness of this layer is 1.2 3.7 km in the platform and 0.4 2 km in the fold belt, and it may correspond to Palaeozoic-Cenozoic sedimentary deposits or, possibly, weathered basement. Below the apparent sedimentary layer the top of the crystalline basement (defined as the >5.95 km/s layer) shows considerable variations toward the rift margins, which corresponds to the Primorsky and Sayan-Khamardaban basement highs. Normal continental crustal velocities of 5.95 6.8 km/s are modeled to 31 km depth, with the highest velocities near the rift axis. In general, the Sayan-Baikal Fold Belt has a more heterogeneous velocity field than the Siberian Platform. An intracrustal reflector observed at 9 km depth beneath the Siberian Platform, deepens slightly toward the fold belt, where it is modeled at 12 km depth. Because of a lack of ray coverage in the rift axis, it is unclear if this reflector represents a continuous boundary across the model. The boundary is modeled as a floating reflector where the velocity contrast across it is insignificantly small (0.02 km/s). [27] A second intracrustal reflector is modeled at 16 km depth, between the upper and middle crustal section, and again it is not possible to determine if this boundary continues across the rift axis because of poor ray coverage. The reflector separating the middle and lower crustal section has a more continuous ray coverage than the latter two mentioned reflectors. It is modeled at 28 km depth beneath the Siberian Platform and at 33 km below the fold belt. The lower crust generally has velocities of 7.2 7.3 km/s in both 12 of 22

Figure 8. (continued) the Siberian Platform and the Sayan-Baikal Fold Belt. A significant feature of the lower crust is a 50 80 km wide and sharply defined zone with anomalously high velocity (7.4 7.6 km/s). This velocity anomaly is located below the rift axis with a slight extension into the Siberian Platform. [28] The Moho discontinuity is well documented by refractions and reflections along the entire profile. It is shallowest beneath the Siberian Platform at 41 km depth and deepest below the Sayan-Baikal Fold Belt at 46 km depth. In the rift axis the Moho is modeled at 43 km depth and no Moho uplift is related to the axis, where there is excellent coverage by wide-angle reflections. The modeled Pn wave velocity is between 8.15 km/s and 8.2 km/s along the whole profile. There is excellent reversed ray coverage (shotpoint 1, 2, 3, 7 and 10) of the sub-moho mantle in model distance 100 250 km, such that any reduction in velocity associated with the rift structure would have been observed in the seismic data. We have no indications of a reduced seismic mantle velocity along any part of the profile. The interpreted Pn velocity corresponds to normal sub-moho velocity of a stable area with low heat flow. A possible mantle reflector at 60 km is modeled in the central part of the profile with relatively large uncertainty, due to lack of velocity information immediately above it. If there is any low-velocity anomaly in the upper mantle associated with the south Lake Baikal Rift Zone, it must be located below this reflector. The mantle reflector may represent the top of a proposed low-velocity anomaly, although such mantle reflectors are often observed in platform areas [e.g., BABEL Working Group, 1993; Grad et al., 2002; Thybo et al., 2003]. 5.2. Resolution Test of the Ray Tracing Model [29] An inversion test of the lower crust and the Moho boundary was performed to test the robustness of the final ray tracing model and the possibility of a Moho uplift below 13 of 22

Figure 9 14 of 22

Lake Baikal. We tested the following three scenarios: (1) inversion of lower crustal velocity and depth to Moho in a model identical to the final model (Figure 9a), (2) inversion of lower crustal velocity and depth to Moho with a starting model for the inversion identical to the final model, but with a 5 km Moho uplift below Lake Baikal (Figure 9c), and (3) inversion of lower crustal velocity and depth to Moho with a starting model with constant lower crustal velocity (7.2 7.3 km/s) and 5 km Moho uplift below Lake Baikal (Figure 9e). The resulting models (Figures 9b, 9d, and 9f), after inversion with lower crustal velocity and Moho depth nodes free to vary, include in all three cases a flat Moho boundary. We therefore conclude that there cannot be any Moho uplift below or in the immediate vicinity of the BRZ. [30] A single parameter test is used to analyze the spatial resolution of those parts of the model that have ray coverage. The concept of the test is to examine the maximum possible changes that can be applied to a chosen model parameter without unacceptable ray coverage, RMS misfit or chi 2 value. For each tested parameter (a single depth or velocity node at a time), the parameter is stepwise changed until subsequent inversion of all other parameters cannot achieve a reasonable fir between observed and calculated traveltimes. In this case, we have selected a tolerance level of 5% in ray coverage, RMS and chi 2. The single parameter test is not absolute because the test assumes that all fixed model parameters are correct. However, the test provides a good indication of the reliability of the model in its different parts. [31] According to our analysis, (Figure 10), the velocity field in the sedimentary section of the model is well defined. The maximum uncertainty of ±0.25 km/s is observed in the Baikal sedimentary basin, and we consider this as acceptable, because the basin is mostly defined by air gun data without recording in the lake. Further, the ray tracing method cannot adequately deal with details of abrupt changes in parameters. The total depth of the Baikal graben structure (10 km) is not fully constrained by the data; it is determined by the seismic >5.25 km/s velocity contour, which can fluctuate with ±0.7 km without loss of resolution. In the Siberian Platform the depth to the crustal basement can be placed 0.1 0.3 km deeper than in the model, but it is still within the assumed uncertainty of the data. The basement interface in the Sayan-Baikal Fold Belt shows a considerably larger depth uncertainty (up to ±0.6 km) due to the lack of a PgP reflection in the area. [32] The velocity field in the upper 16 km of the crustal section is exceptionally well defined with a maximum uncertainty of ±0.1 km/s. This part of the model is primarily based on the Pg phase, which is well correlated and easily recognized as first arrivals in all the seismic sections. The intra crustal boundary at 9 12 km depth is on the whole well constrained with a maximum uncertainty of only ±0.4 km. The intra crustal boundary at 16 km depth, which defines the interface between the upper and middle crustal section, is relatively poorly determined (about ±1.2 km). In the northwestern part of the model this result was unexpected considering the good ray coverage by the reflection Pi 2 P. Compared to the upper part of the model, the velocity field of the middle crustal section has a considerably higher uncertainty (up to ±0.25 km/s). The modeled interface between the middle and lower crustal sections, at 28 33 km depth, is well constrained by the ray coverage. However, the test indicates that, the interface can be placed up to 1 km deeper within the assumed uncertainty of the data. The velocity field of the lower crustal section have a maximum uncertainty near the rift axis of ±0.2 km/s. This result indicates that, the modeled high-velocity zone in the lower crust may have a smaller velocity than actually shown in the model. However, the test results confines the presence of the localized zone with high velocity, which cannot be lower than 7.4 km/s. [33] The Moho discontinuity at 41 to 46 km depth is well determined (about ±1 km), as expected for the PmP reflection which has been correlated in all record sections. The velocity field in the uppermost mantle is generally well defined with a minor uncertainty of only ±0.1 0.2 km/s. The upper mantle boundary at 60 km depth can be modeled 4 km deeper than in the model without significant decrease in resolution. This uncertainty is caused by the lack of accurate velocity control in the mantle interval of the model, due to coarse ray coverage and the lack of turning waves. [34] In general the single parameter test shows that the different model parameters are well defined in areas with high ray coverage, with the possible exception of the 16 km deep boundary (upper to middle crustal interface). In areas with low or no ray coverage the uncertainty is significant, as observed at the margins of the model. The relatively small uncertainty detected by the test, shows that the model can be regarded as representative for the main structures in the southern part of the Baikal Rift Zone. 6. Discussion [35] Interpretation of the BEST data, which transect the southern part of Lake Baikal, has lead to two 2-D velocity models, based on seismic refractions and wide-angle P wave reflections. The modeling procedures applied are first arrival tomographic inversion [Hole, 1992] and ray tracing modeling and inversion [Zelt and Smith, 1992]. Both methods use ray theory for simulation of the propagation of seismic energy through the earth and the two models mutually support each other. [36] The model achieved by the tomographic inversion method is a simple approximation of the velocity field in the BRZ. This model is a very robust representation of the overall smoothed velocity field, because it is based on the first arrivals which can be identified and picked with a high Figure 9. Inversion tests of the final ray tracing model. (a) Starting model, which is identical to the final model and (b) the resulting model after inversion with free nodes of lower crustal velocity and depth to Moho. (c) Starting model similar to the final model but with a 5 km Moho uplift and (d) the resulting model after inversion with free nodes of lower crustal velocity and depth to Moho. (e) Starting model with constant velocity of the lower crust (7.2 7.3 km/s) and 5 km Moho uplift and (f) the resulting model after inversion with free nodes of lower crustal velocity and depth to Moho. 15 of 22

Figure 10. Results of the resolution test superimposed on the final 2-D velocity model. (a) The uncertainty of the layer boundaries in km and (b) the uncertainty of the seismic velocities in km/s. The plus and minus signs illustrate how much a given model parameter (depth or velocity) can be added or subtracted without loss of resolution. The size of the symbols is relative and not a measure of the absolute values. Thick black lines mark the modeled reflection points. degree of reliability and small uncertainty. Layer boundaries are not included and each perturbation is smoothed with different filters. It is important to keep in mind that there is no exact solution in inversion; a model is only one solution out of many possible solutions which can fit the data [Nolet et al., 1999]. The velocity field is better constrained vertically than horizontally by the applied inversion owing to the use of first arrivals, which corresponds to waves which travel horizontally at the turning point where the velocity is determined. Our results demonstrate that the method has problems with zones with large lateral velocity contrast between different geological structures. This problem is significant at the interface between the sedimentary units of Lake Baikal and the surrounding basement, where the velocities abruptly changes from 3.5 km/s to >5.5 km/s. The tomographic inversion has smeared these changes to a width of 25 km. The relatively high velocity of the flanking basement causes the rays to concentrate in those areas, and thereby contribute to a higher velocity instead of propagating the high velocity down into the model. The velocity field in the sedimentary basin is accurately defined, but the velocity field of the flanking basement may be overestimated by up to 0.25 km/s in the tomographic model. However, we still regard the shallow flanking basement as a true geological feature consisting of metamorphic or igneous basement rocks. [37] The model determined by the forward ray tracing method is more detailed than the first arrival result and has significantly higher resolution, owing to the introduction of layer boundaries based on reflections. However, the ray tracing method may be more subjective than the first arrival tomography, because the model parameters are defined by the program operator. An advantage of the ray tracing technique is that a priori information on, e.g., geological structures can be added to the starting model and retained during the modeling, such as the known location and structure of Lake Baikal and its underlying sedimentary graben. The reflection phases generally have higher picking uncertainty than the first arrivals, and seismic sections with the best signal-to-noise ratio are weighted highest in the modeling (shotpoint 1, 2, 6, 7, and 10). [38] Because of constraints in Rayinvr the final model consists of layers that extend across the whole model. This requirement can potentially lead to overinterpretation of the 16 of 22