Tectonic Structures in the Southeastern Flank of the Junction Zone Between the In’yali-Debin Synclinorium and the Omulevka Terrane (Kolyma-Omolon Microcontinent, NE Asia)

Abstract

The study area is located in the zone of junction of the In’yali-Debin synclinorium and the Omulevka terrane (Kolyma-Omolon microcontinent of the Verkhoyansk-Kolyma fold belt). The main tectonic structure is the Momontai syncline made of Middle Jurassic clastic rocks overlain by Upper Jurassic volcanic and volcaniclastic rocks of the Uyandina-Yasachnaya volcanic arc. The Au and Au-Ag occurrences known here are confined to NE-trending faults and hosted in subvolcanic and volcanogenic rocks, mainly rhyolites and dacites. The studied Middle Jurassic rocks are found to be characteristic of near-shore shelfal environments changing, in certain time intervals, to deltaic conditions of sedimentation. The presence in the conglomerates and sandstones of large poorly rounded quartzite and carbonate fragments and of mudstone and volcanite pebbles indicates a proximal provenance of the clastics. For the first time we established a sharp angular unconformity between the intensely deformed Middle Jurassic and Upper Jurassic rocks in the region. Relations between the Late Jurassic subvolcanic deposits and the host rocks were studied. The carried out structural and tectonophysical investigations showed that folding occurred in two deformation stages. During the first stage, the Middle Jurassic clastic rocks were draped into small recumbent to isoclinal folds, cylindrical and parallel-type, with a NW strike. Cleavage is rare. The structural paragenesis of bedding-plane detachment faults, thrusts, normal faults, and strike slip faults is found to have formed in a single stress field together with the development of folds of the first deformation stage. The intensity of the first-stage folding increases from west to east. Restored are axes of paleostresses responsible for the formation of fold-and-thrust structures of the first deformation stage. The Upper Jurassic volcanogenic-sedimentary strata were deformed into superposed large, simple, open folds of the second stage, which exhibit sublatitudinal orientation. They associate with small-scale thrusts and, rarely, strike-slip faults. It is recognized that in the late Middle Jurassic or the early Late Jurassic the region was affected by intense folding which produced tectonic structures of NW strike. Accumulation of the Late Jurassic volcanogenic rocks and intrusion of subvolcanic deposits occurred on/into the earlier deformed Middle Jurassic rocks.