ИСТИНА

A number of issues concerning Precambrian geodynamics still remain unsolved because of uncertainity of many physical (thermal regime, lithosphere thickness, crust thickness, etc.) and chemical (mantle composition, crust composition) parameters, which differed considerably comparing to the present day values. In this work, we show results of numerical supercomputations based on petrological and thermomechanical 2D model, which simulates the process of collision between two continental plates, each 80-160 km thick, with various convergence rates ranging from 5 to 15 cm/year. In the model, the upper mantle temperature is 150-200 0C higher than the modern value, while the continental crust radiogenic heat production is higher than the present value by the factor of 1.5. The present study investigates the dependence of collision style on various continental crust parameters, especially on crust composition. The 3 following archetypal settings of continental crust composition are examined: 1) completely felsic continental crust; 2) basic lower crust and felsic upper crust; 3) thin basic upper crust (<5km) and felsic lower crust (hereinafter referred to as inverted crust). The results has shown that even in the Paleoproterozoic conditions continental subduction is widespread process. The primary parameter, which has the most significant influence on continental subduction style is composition of the continental crust. Continental subduction with the felsic crust is short-termed and lasts less than 5 Myr. Rocks exhume very fast (< 1 Myr). In the case of basic lower crust, a continental subduction is more stable and last over 15 Myr. Continental subduction also occurs in the case of inverted continental crust. However, in the latter case, the exhumation of felsic rocks is blocked by upper basic layer and their subsequent interaction depends on their volume ratio. Thus, if the total inverted crust thickness is about 15 km, felsic rocks cannot be exhumed. If the total thickness is 30 to 40 km and that of the felsic layer is 25 to 35 km, it breaks through the basic layer leading to formation of turbulent structure. These results suggest a new mechanism of continental collision evolution in Precambrian conditions. Computing resourses were provided by MSU supercomputer center.