Tectonic structures of the southeastern flank of the In’yali-Debin synclinorium (the Verkhoyansk-Kolyma orogenic region)

The paper reports the results of research conducted in the southeastern flank of the In’yaliDebin synclinorium located in the central part of the Verkhoyansk-Kolyma folded area. The main tectonic structure here is the Momontai syncline composed of Middle Jurassic clastic rocks. The syncline is unconformably overlain by the Late Jurassic volcanic and volcaniclastic rocks of the Uyandina-Yasachnaya volcanic arc. A sharp angular unconformity between the intensely deformed Middle Jurassic rocks and Upper Jurassic volcanics is described for the first time in this region. Two deformation stages are recognized. The compressive complex folding of Middle Jurassic clastic rocks has a NW strike, while the superposed large simple open folds of Upper Jurassic volcaniclastic strata are WE-oriented. The first-stage deformation is represented by tight asymmetric, concentric, cylindrical, rarely conical folds. Large, sometimes isoclinal folds overturned to SW are described. Compression and tension axes were calculated using the measurements of the orientation of fault planes and striae on the slickensides. The compression axes determined for tectonic structures of the NE limb of the Momontai syncline are almost normal to the strike of folds of the first deformation stage. Thus, it is established that the structural paragenesis including bedding-plane detachment faults, thrusts, normal faults, and strike slips was formed in the single stress field together with the development of folds of the first deformation stage. Accumulation of the studied Middle Jurassic rocks occurred in near-shore shelfal environments of sedimentation, sometimes changing to deltaic ones. The conglomerates and sandstones contain large, poorly rounded boulders of quartzite and limestone, as well as pebbles of mudstone and volcanic rocks, which is indicative of the proximity of clastics provenances. In the late Middle Jurassic or the early Late Jurassic, this part of the In’yali-Debin synclinorium underwent intense folding of NW strike. Accumulation of Late Jurassic volcanogenic rocks of the Uyandina-Yasachnaya arc and intrusion of subvolcanic rocks took place on/into the already deformed Middle Jurassic strata. Folding of the second deformation stage occurred in post-Late Jurassic time.

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