STRUCTURE AND THERMOCHRONOLOGY OF THE SOUTH VERKHOYANSK THRUST BELT, EASTERN SIBERIA

The Verkhoyansk fold and thrust belt of eastern Siberia extends for 1500-km from the Laptev Sea to the Sea of Okhotsk. It involves more than 7 km of Late Precambrian and Early Paleozoic shelfal strata, and a great thickness of intensely deformed Ordovician to Triassic basinal strata deposited along the paleo-Pacific margin of the North Asian craton. This margin developed as a consequence of rifting events in the Late Precambrian and in the Late Devonian to early Mississippian. Shortening in the fold and thrust belt took place during the Late Jurassic to Early Cretaceous. This deformation was accompanied by low-grade metamorphism in the internal parts of the belt, the emplacement of high-level granitic bodies, and foredeep subsidence in front of the north Verkhoyansk only. Deformation was likely due to collision with island arcs and small cratonal fragments.

Along the Allakh'-Yun River the southern Verkhoyansk is characterized by three distinct structural domains: 1) The frontal thrusts (Kyllakh domain) composed of four major west-vergent thrust sheets accounting for 18 km of displacement (70% shortening). The rocks involved are Late Proterozoic to Early Paleozoic platformal units. 2) A structural fan (Sette-Daban domain) within intensely deformed greenschist grade Cambrian to Silurian rocks of basinal origin. A poor quality151±1 Ma ⁴⁰Ar/³⁹Ar age from white mica offers the first constrain on the timing of metamorphism in the Sette-Daban. 3) An east-vergent zone (Allakh-Yun' domain) dominated by a thick and monotonous succession of Carboniferous and Permian turbidites metamorphosed to low-greenschist grade. Layering is generally tranposed by the west-dipping cleavage. Granodioritic plutons intrude in the core of the range and are associated with extensive gold-bearing quartz-vein systems. Cross-cutting relationships near that margin of the Tarbagganakh pluton (120 ± 1 Ma, ⁴⁰Ar/³⁹Ar, biotite) indicate that the pluton was emplaced during the waning stages of ductile deformation.

The overall structure of the South Verkhoyansk, with the oldest rocks exposed in the foreland and progressively younger, but more intensely metamorphosed units in the hinterland is determined by the inversion of the thick upper Paleozoic clastic wedge that lay a adjacent to the ancient Siberian margin prior to the onset of deformation.