Выпуск: 4 , Том: 12 , Год издания: 2000

In the Kokchetav ultrahigh-pressure (UHP) massif in northern Kazakhstan, diamond-bearing UHP rocks occur exclusively in a western, rhomb-shaped domain, that differs from an eastern transpressional domain with coesite-bearing remnants indicating highest UHP conditions. Different mechanisms may have contributed to the early ascent of the UHP Kokchetav massif. The geometry and structure of the diamond-bearing domain are interpreted as a sheath-like fold, coeval with early stage melting. In contrast, the coesite-bearing domain has a sheet-like geometry. At mid-crustal level this early difference in the ascending UHP wedge is reflected in a western rhomb-horst and an eastern transpression structure, respectively. The latter extends to the east (Borovoye) where the uppermost sequences of the UHP massif were defined by others. Sheath folding is postulated as a suitable mechanism contributing to the early buoyancy-driven ascent of the subducted rocks, and explains the selective spatial distrib ution of diamond-and coesite-bearing sequences preserved in the wedge.