The Laptev Sea in the Siberian Arctic represents a unique tectonic junction of an active spreading ridge, the Gakkel Ridge in the Eurasian oceanic basin, with the Siberian Arctic continental margin. New long-offset seismic profiles acquired in recent years provide a reliable basis for deciphering the structural and seismic stratigraphic characteristics of the Laptev Rift System. The tectonic development of the Laptev Shelf represents a sequence of four phases controlled by relative plate movements: (1) intense brittle normal faulting (an initial rifting or stretching phase) affected the entire shelf in the Late Cretaceous(?)–Paleocene(?); (2) a thinning/exhumation phase resulted in exhumation of the lower continental crust and probably upper mantle in the western part of the rift system – this phase is inferred to have occurred during the Late Paleocene to Early Eocene, preceding and accompanying continental break-up in the Eurasia Basin; (3) a stalled rift phase characterized by either a dramatically reduced rate of extension, or a non-extension/compression regime controlled by major reorganization of the plate movements – the onset of this fourth phase is inferred to coincide with the initiation of seafloor spreading in the southern Eurasia Basin at around 53–50 Ma; and (4) reactivation of the rifting in the mid-Miocene (a second rift phase).

Consolidated crust in the North Barents basin with sediments 16–18 km thick is attenuated approximately by two times. The normal faults in the basin basement ensure only 10–15% stretching, which caused the deposition of 2–3 km sediments during the early evolution of the basin. The overlying 16 km of sediments have accumulated since the Late Devonian. Judging by the undisturbed reflectors to a depth of 8 s, crustal subsidence was not accompanied by any significant stretching throughout that time. Dramatic subsidence under such conditions required considerable contraction of lithospheric rocks. The contraction was mainly due to high-grade metamorphism in mafic rocks in the lower crust. The metamorphism was favored by increasing pressure and temperature in the lower crust with the accumulation of a thick layer of sediments. According to gravity data, the Moho in the basin is underlain by large masses of high-velocity eclogites, which are denser than mantle peridotites. The same is typical of some other ultradeep basins: North Caspian, South Caspian, North Chukchi, and Gulf of Mexico basins. From Late Devonian to Late Jurassic, several episodes of rapid crustal subsidence took place in the North Barents basin, which is typical of large petroleum basins. The subsidence was due to metamorphism in the lower crust, when it was infiltrated by mantle-source fluids in several episodes. The metamorphic contraction in the lower crust gave rise to deep-water basins with sediments with a high content of unoxidized organic matter. Along with numerous structural and nonstructural traps in the cover of the North Barents basin, this is strong evidence that the North Barents basin is a large hydrocarbon basin.

The Laptev Shelf in the eastern Siberian Arctic represents a rare tectonic setting 12 where an active oceanic spreading centre, the Gakkel Ridge, intersects a continental margin. 13 The North America-Eurasia plate boundary follows the Gakkel Ridge and passes into a 14 continental shelf; this has resulted in the development of a wide rift system that has been 15 active since the Late Cretaceous. The new long-offset seismic profiles provide a reliable basis 16 for deciphering the structural characteristics of this rift system. We use two new seismic 17 profiles, along with one acquired in the 1990s, to examine the crustal architecture of the rift 18 system. Our approach combines seismic interpretation, time to depth conversion of seismic 19 profiles and 2D gravity forward modelling. The obtained results indicate the presence of 20 hyperextended continental crust beneath the Ust' Lena Rift Basin and exhumed continental 21 mantle at the base of the syn-rift succession along the rift axis. The upper crust was removed 22 by brittle stretching, while the lower crust experienced extreme ductile thinning. Our results 23 show that continental crust can be eliminated in the course of rifting without a considerable 24 heat input from asthenospheric mantle.

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