MECHANISMS OF ACCRETION AND ORIGIN OF VERTICAL CRUSTAL STRUCTURE OF ACCRETIONARY OROGENIC SYSTEMS: INSIGHT FROM GEOPHYSICAL AND GEOCHEMICAL DATA

The new paleomagnetic and structural geology data from the Mongolia CAOB show that the N-S trending linear accretionary system evolved through Devono-Carboniferous interoceanic accretion and was followed by Permo-Triassic oroclinal bending in between Siberian and North-Chinese cratonic jaws. The post-oroclinal bending structure of CAOB in Mongolia was examined using processed magnetic and gravity grids that reveals E-W trending gravity and magnetic lineaments. The matched-filtering shows good continuity of signal to the depth located along the boundaries of high magnetic anomalies that may imply the presence of deeply rooted tectono-magmatic zones. The axes of high-density bodies in the western and central part of the studied CAOB are characterized by periodic alternations of NW–SE trending high frequency and high amplitude gravity anomalies corresponding to late Permian to Triassic cleavage fronts up to 20 km wide. The matched-filtering analysis shows that the largest deformation zones are deeply rooted down to 20 km depth. The orogenic lower crust underneath most of Siluro-Devonian oceanic accreted system t is entirely missing the geophysical signal and was consequently explored using geochemistry of Late Carboniferous to Permian granites crosscutting the crust. The zircon Hf isotopic data show that the molten crust is juvenile, probably 500 Ma old and its protolith corresponds to amphibolite to graywacks. Such a characteristics are typical for Cambrian arc located in the north of oceanic domain. We therefore propose a model of redistribution of juvenile Cambrian orogenic lower crust underneath the Siluro-Devonian oceanic system due to oroclinal neding effect. The latter is demonstrated by means of analogue modeling.