The sea of Okhotsk Geotraverse: Tectonomagmatic evolution of Cenozoic extension structures and implication for their deep structure

Abstract

Issues of the origin of marginal seas and genesis of their igneous rocks remain controversial because of the insufficient amount of documentary information (small number of drilled holes, rare network of seismic profiles, and so on). Therefore, data on seismic profiles carried out within the framework of the Geotraverse International Project [1, 2] are of great significance. The geotraverse extends from the margin of Asia (Sikhote Alin) to the northwestern periphery of the Pacific plate (Fig. 1) and includes large Cenozoic extension structures, such as the Tatar continental rift and the South Kuril marginal basin. Comprehensive analysis of seismic data on deep zones of this profile coupled with the available information about the regional tectonic setting and magmatism have made it possible to detect a correlation between the Cenozoic geodynamic mode (correspondingly, tectonics and magmatism) at the crustal and deeper (lithospheric mantle and asthenosphere) levels. Ultimately, these data made it possible to refine the evolution model of marginal basins in the study region and provided new insights into the issue of passive and active rifting. The Tatar continental rift located in the western part of the geotraverse (Fig. 2) represents the northern termination of the Japan basin. Both structures are bounded by a convergent system of submeridional dextral strike-slip faults partly transformed into normal faults. The Tatar rift basement incorporates a thin continental crust with the velocity of seismic waves (hereafter, V ) ranging from 5.8 to 7.2 km/s [4, 7]. The continental crust began to break down after its disintegration in the terminal Paleocene‐Eocene into a system of narrow (5‐10 km) horsts and grabens [2, 4] and the accumulation of terrigenous sediments (up to 1.5 km thick) at the initial rifting stage. The next (Oligocene‐middle Miocene) stage of maximum extension of the Tatar rift is marked by the unconformably overlying transgressive sequence of deep-water clayey and siliceous‐clayey rocks (up to 4 km). The next (middle Miocene) regressive sequence is composed of coastal-marine sediments. In contrast to the almost undeformed cover of middle Miocene‐Holocene terrigenous rocks (1.5‐5 km thick) deposited after the rifting (hereafter, postrift complex), rocks deposited during rifting (rift complex) are characterized by significant dislocations. All of the rift and postrift complexes include basaltoid fields in some places. As will be shown below, these rock complexes have specific isotope‐geochemical parameters. The structure beneath the Tatar rift is characterized by drastic reduction of the continental crust (up to 25 km) and ascent of the asthenospheric layer up to the level of 50 km (Fig. 2). Therefore, this region is marked by high thermal flux (123‐132 mW/m 2 ). The temperature in the upper zone of the asthenospheric diapir is estimated at 1100 ° C [2‐4]. The diapir is bounded by narrow gradient stages [4] that extend to higher lithospheric levels and generally correspond to strike-slip faults, which serve as boundaries of the Tatar and Japan basins. The faults presumably extend to the mantle and asthenosphere.