We discuss and develop further the methods of surface wave tomography in the frame of the geometric ray approximation. The general approach for determining the lateral phase or group velocity distribution, which is a standard 2-D tomography problem, involves linearization, representation of the unknown function as a series in some basis functions, and evaluation of the coefficients by the methods of linear algebra. If the wave paths cover the area under investigation non-uniformly, the basis functions should not be chosen a priori, but constructed proceeding from the pattern of paths. Different criteria for constructing the basis functions are compared, and a relation between them is considered.A more preferable approach is joint interpretation of phase and group velocity data for different periods, because it allows the information about phase velocity variations to be enlarged due to the use of the group velocity data. Both the phase and group traveltimes are represented as linear functionals of the unknown phase slowness corrections. A specific form of the data kernels allows the basis functions to be represented as a product of two functions, one depending on the horizontal coordinates, and the other on frequency.

Geological and geodynamic models of the mantle often rely on joint interpretations of published seismic tomography images and petrological/geochemical data. This approach tends to neglect the fundamental limitations of, and uncertainties in, seismic tomography results. These limitations and uncertainties involve theory, correcting for the crust, the lack of rays throughout much of the mantle, the difficulty in obtaining the true strength of anomalies, choice of what background model to subtract to reveal anomalies, and what cross‐sections to select for publication. The aim of this review is to provide a relatively non‐technical summary of the most important of these problems, collected together in a single paper, and presented in a form accessible to non‐seismologists. Appreciation of these issues is essential if final geodynamic models are to be robust, and required by the scientific observations.

Abstract:The article reviews three typical concepts concerning the age of the Baikal rift (BR) which development is still underway: 5 Ma (the BR development start in the Late Pliocene), 30 Ma (Miocene or Oligocene), and 60-70 Ma (the Late Cretaceous).Under the concept of the young BR age (Pliocene-Quaternary) [Artyushkov, 1993;Nikolaev et al., 1985; Buslov, 2012], according to E.V. Artyushkov, BR is not a rift, but a graben due to the fact that the pre-Pliocene structure of BR does not contain any elements that would be indicative of tensile stresses. However, field studies reported in [Lamakin, 1968;Ufimtsev, 1993;Zonenshain et al., 1995;Mats, 1993Mats, , 2012Mats et al., 2001] have revealed that extension structures, such as tilted blocks and listric faults, are abundant in the Baikal basin (BB), and thus do not support E.V. Artyushkov's argumentation.The opinion that BR is young is shared by M.M. Buslov [2012]; he refers to studies of Central Asia and states that only the Pliocene-Quaternary structure of BB is a rift, while the oldest Cenozoic structures (Upper CretaceousMiocene) are just fragments of the large Cenozoic Predbaikalsky submontane trough (PBT) which are not related to the rift. However, the coeval Cenozoic lithological compositions, thicknesses of sediment layers and types of tectonic structures in PBT and BB have nothing in common. Across the area separating PBT and BB, there are no sediments or structures to justify a concept that BR and PBT may be viewed as composing a single region with uniform structures and formations. The idea of the Pliocene-Quaternary age of BR should be rejected as it contradicts with the latest geological and geophysical data. Seismic profiling in BB has revealed the syn-rift sedimentary bed which thickness exceeds 7.5 km. Results of drilling through the 600-metre sedimentary sequence of Lake Baikal suggest the age of 8.4 Ma [Horiuchi et al., 2004], but M.M. Buslov believes that it took only about 5 Ma to form the entire syn-rift sequence of Lake Baikal.In [Bazarov, 1986;Rasskazov et al., 2014;Mashchuk, Akulov, 2012;Hutchinson et al., 1993;Zonenshain et al., 1995;Kaz'min et al., 1995], the BR age is determined as the Miocene (Oligocene-Miocene) according to the age of the Tankhoi suite (Miocene or Oligocene-Miocene) and the correlation between the lower seismostratigraphic complex (SSC-1) and the Tankhoi suite [Hutchinson et al., 1993;Zonenshain et al., 1995]. The Tankhoi suite lies directly on the crystalline basement of the rift and is believed to mark the start of the Baikal syn-rift profile. However, this concept does not take into account the main specific feature of the profile, i.e. a developing rift. As shown in Fig. 6, the most ancient elements in the syn-rift profile are inside the deep part of the rift. At the day surface, the basement is overlaid by the younger elements of the sedimentary wedge due to the 'expansion non-conformity effect' (as termed in [Khain, Mikhailov, 1985]). In our opinion, it is incorrect to correlate SSC-1 and the Tankhoi suires -th...

Devonian dikes of the Urik-Belaya and Shagayte-Gol-Urik zones and Miocene lavas of the Urik volcanic field are spatially associated with each other at the structural junction between the Neoproterozoic Tuva-Mongolian massif and Siberian craton. The former dike belt is represented by basalts and basaltic andesites of tholeiitic series and the latter one by trachybasalts, trachyandesitic basalts of moderately alkaline series and trachybasalts, phonotephrites of highly alkaline one. The Urik volcanic field is composed of trachybasalts and trachyandesitic basalts of moderately alkaline series. A partial similarity between magmatic series of different age is found in terms of major oxides, trace elements, and Sr, Pb isotopes. The common component corrected for age was defined through its converging mixing trends with those of the lithospheric mantle and crust. The component identification was a basis for deciphering the nature of isotopic and geochemical heterogeneity of evolved magmatic sources. It was inferred that the common component characterizes either a modified (depleted) reservoir of the lower mantle or, more likely, a local region of the convecting asthenospheric mantle that underlies the Tuva-Mongolian massif. The latter interpretation assumes the formation of a locally convecting asthenosphere in the middle Neoproterozoic, along with the development of the Oka zone at the massif, and puts constrains on later sufficient processing of the asthenosphere due to rising plumes or subducting slabs.Key words: Eastern Sayans, Devonian, Miocene, dikes, basalts, lithosphere, asthenosphere. Recommended by E.V. Sklyarov 24 April 2012Citation: Rasskazov S.V., Il'yasova A.M., Yasnygina T.A., Fefelov N.N., 2012 Аннотация: В структурном сочленении неопротерозойского Тувино-Монгольского массива с Сибирским кратоном пространственно совмещены между собой девонские дайки Урик-Бельского и Шагайтэ-Гол-Урикского поясов и миоценовые лавы Урикского вулканического поля. Первый дайковый пояс представлен базальтами-андезибазаль-тами толеитовой серии, второй -трахибазальтами-трахиандезибазальтами умереннощелочной серии с локальным