The Baikal rift: Pliocene (Miocene) - Quaternary episode or product of extended development since the Late Cretaceous under various tectonic factors. A review

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, 1993, 2012; Mats 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 Cretaceous - Miocene) 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 - the representative seismic profile (Fig. 5) shows that SSC-1 falls out of the profile before reaching the day surface and leans against the rising slope of the basement, while SSC-2 correlates with the Tankhoi suite. Besides, correlating SSC-1 with the Tankhoi suite is contradicting to the data of structural studies reported in [San'kov et al., 1997; Delvaux et al., 1997]. SSC-1 originated before the time when the Lake Baikal region was impacted by the Indo-Eurasian collision and formed under the influence of pure expansion when tensile stresses were oriented from NW to SE across the strike of the rift along the SE 145-150° azimuth [Zonenshain et al., 1995]. By the time of the SSC-2 formation, the stress vector turned counterclockwise towards the NE-SW direction at an acute angle to the rift strike. The Baikal rift structure was changed as the single-sided basin was replaced by the SW-NE stretching dual-sided graben; it included SSC-2 and was bordered by listric faults [Zonenshain et al., 1995]. Results of the structural studies conducted on the Lake Baikal shores [San'kov et al., 1997; Delvaux et al., 1997; Parfeevets, San'kov, 2006] suggest that during the Tankhoi period, the rift developed in conditions of transpression and transtension under the influence of stresses oriented subparallel to the strike of the rift and related to the Indo-Eurasian collision. This means that SSC-2 (but not SSC-1) correlates with the Tankhoi suite, and the age of the Tankhoi suite is not indicative of the BR age, and the concept of the Miocene (Oligocene-Miocene) age of BR is thus discarded. The concept of the Late Cretaceous-Paleogenic age of BR [Logachev, 1974, 2003; Mats, 1987, 1993, 2012; Mats et al., 2001; Mats, Perepelova, 2011] is most fully supported by the available geological and geophysical data. This age is evidenced by the Paleogenic (Eocene) palinspectra detected in core samples from deep wells drilled in the Selenga river delta, Southern Baikal basin [Faizulina, Kozlova, 1966]. Besides, the Paleocene-Eocene pre-Tankhoi sediments are discovered at the Khamar-Daban shore of the Southern Baikal basin (the Polovinka river valley) [Mats, 2013]. The sediments of the BB weathering crust [Mats, 2013] correlate with the paleontologically dated Paleogenic sediments of PBT [Pavlov et al., 1976; Popova, 1981]. The BR ancient age is also confirmed by studies reported in [Nikolaev, 1989; Galazii et al., 1999; Kontorovich et al., 2007; Jolivet et al., 2009]. Our review of the BR age concepts gives grounds to conclude that the Pliocene-Quaternary and Oligocene-Miocene ("Tankhoi") ages of BR should be discarded as not supported by the geological and geophysical data collected in the recent studies. Based on the comprehensive studies of the Baikal rift and taking into account an extension of the BR evolution by 60 to 70 Ma, we propose a new concept of the BR development and introduce a three-stage model (Fig. 7) (as a replacement of the well-known two-stage model [Logachev, 2003]) and an impactogenic model as a supplement to the passive and active rifting models [Mats, 2012; Mats, Perepelova, 2011]. In our model, the first stage of the BR development is the Late Cretaceous-Early Oligocene (70-30 Ma): in conditions of the general extension of the lithosphere, BR forms as a slot-type (the term proposed by E.E. Milanovsky) rift and develops, as shown by the passive rifting model, at the background of the original peneplain until the time when the Baikal region is impacted by stresses resulting from the Indo-Eurasian collision; the rift structure is a single-sided basin that comprises the seismically transparent seismostratigraphic complex (SSC-1); it is bordered at NW by the zone of listric faults. The second stage is the Late Oligocene-Early Pliocene (30-5 Ma): BR develops under the impact of stresses resulting from the Indo-Eurasian collision; the dual-sided graben is formed; it comprises SSC-2 that is stratified and deformed. The third stage is the Late Pliocene - Quarter (5 Ma till present): BR develops under the impact of stresses generated by local deep sources, as shown by the active rifting model [Logachev, Zorin, 1987; Zorin et al., 2003]; another single-sided graben is formed; it is bordered by listric faults from the NW and comprises SSC-3 that is stratified but not deformed.