On the structure and formation of earthquake sources in the faults located in the subsurface and deep levels of the crust. Part II. Deep level

Abstract

In the part 2 of the study [Ruzhich, Kocharyan, 2017, we aimed at identifying the elements of paleoearthquake sources in the crust, which formed at the hypocentral depths in the exhumed Primorsky segment of the ancient collisional suture. The study area covered the southeastern margin of the Siberian craton (Pribaikalie, East Siberia). Slickensides, pseudo-tachyllite (basaltic glass) and other petrological evidence of intensive tectonic movements were sampled. The structure of the deep segments of the collisional suture were reconstructed from on the data on coseismic ruptures and faults, and the PT parameters were estimated. In the past decades, similar research problems were actively investigated (e.g. [Sibson, 1973;Byerlee, 1978;Morrowetal., 1992;Hodges, 2004;Kirkpatricketal., 2012). In Russia, the interest in studying geological and geophysical features of the deeply denuded areas in ancient faults is still limited [Sherman, 1977;Ruzhich, 1989, 1992, 1997;Savel’eva et al., 2003;Ruzhich et al., 2015; Kocharyan, 2016. The deeply denuded Primorsky segment of the collisional suture of the Siberian Craton underwent the geological evolution of a billion years. In the analysis, we used additional geological data from the petrology studies of the Main Sayan fault zone and other exhumed fault segments, including the seismogenerating faults in the Mongolia-Baikal region [Zamaraev,Ruzhich, 1978;Zamaraev et al., 1979;Ruzhich et al., 2009. From the PT conditions for the occurrence of the slickensides, pseudo-tachylyte, and the Primorsky segment structure, the40Ar/39Ar method estimated the age of the slickensides containing tourmaline at 673±4.8 Ma, which may correspond to the Neoproterozoic stage of the breakdown of the megacontinent Rodinia. Another dating, 415.4±4.1 Ma, obtained for the muscovite sample from a decompressional rupture, refers to the Early Paleozoic stage in the development of the collisional suture, when accretion of the Siberian Craton and the Olkhon terrain took place [Donskaya et al., 2003;Fedorovsky et al., 2010. Based on these ages and other available petrological data, the depths of the heterochronous systems of coseismic ruptures were estimated:18 km in the Neoproterozoic, and12 km in the Middle Paleozoic stage of the seismotectonic evolution of the crust in Pribaikalie. The deep paleoseismological settings need to be further investigated in order to more thoroughly clarify the physical and chemical conditions that contributed to the occurrence of the ancient and recent sources of strong earthquakes in the deep segments of faults in the crust. Such information is a prerequisite for further progress towards resolving the problems of securing seismic safety in various regions.