Model of the transient temperature field of the Baikal Rift lithosphere

Abstract

In order to coordinate available geophysical data on the deep-seated structure with observed heat flow, it has to be borne in mind that the thermal field of the Baikal rift zone is not in a steady-state. A two-dimensional model of the transient temperature field of the lithosphere was constructed by a numerical method for the solution of the energy conservation equation. The authors suggest that asthenospheric upwelling, 250 km wide, was formed on account of roof block detachment. Such a mechanical replacement of the lithosphère by the asthenosphere was simulated by a positive (compulsory) upward movement of the 1200°C isotherm with constant velocity. The duration of this movement was taken as long as 30 m.y., which corresponds to the duration of the first (slow) rifting stage. As soon as the asthenosphere had reached the Mohorovičić discontinuity, its ascending movement on a wide scale ceased, because the density of the crust was less than that of the asthenosphere. Temperature calculations were carried out in the lithosphère above the 1200°C isotherm. This model permits us to reproduce theoretically the present value of the regional heat flow and the Curie depth, when the time passed from the cessation of the upwelling top movement became equal to 3.5 m.y. The last figure corresponds to the duration of the second (fast) stage of rifting. On the regional scale the crust of the rift zone (Sayan-Baikal domal uplift) continues to be heated. Cooling of the crust can only occur at the segments corresponding to the local heat flow anomalies, which are suggested as being produced by dikes. The intrusion time of the dike beneath the southern Baikal is estimated to be 3 m.y.