Conductive and convective heat flow in the bottom of Lake Baikal and in the surrounding mountains.

Up until now heat flow through the bottom of Lake Baikal has been determine d in more than 700 sites. Mean measured flux is 73.5 +/- 24 mW/m(2); correc ted for topography and sedimentation it is 77.8 +/- 24 mW/m(2). Heat flow v alues vary greatly in a range from 0 to 20 000 mW/m(2). The zero values wer e obtained in localities of recent landslides, while the extreme highs are attributed to sites of hydrothermal discharge. In the northern Baikal, narr ow positive heat flow anomalies are associated with near-shore faults exten ding along bottom foothills on both sides of the basin. An intense heat flo w anomaly, about 8 km wide, stretches along the southeastern side of Centra l Baikal. Its position coincides with a fault detected by seismic multichan nel reflection. The most prominent positive anomaly is the one over 30 km wide, found in So uth Baikal. An attempt was made earlier to explain this anomaly by injectio n of mantle material into the upper crust. Estimations performed with regar d to hydrothermal outflow of heat show that such an intrusion, even if it w ere 10 to 12 km wide, would be able to sustain the anomaly in South Baikal for less than 1 My. If the anomaly had existed for a longer time it must ha ve been fed by renewable sources of heat. Ground waters may provide such a source. They penetrate along faults to depths of 3 to 6 km and deeper benea th the rift shoulders where they heat-up in the regional thermal field, the n move towards the Baikal basin and ascend through the faulted bottom of La ke Baikal already as thermal waters. Modelling shows that such redistributi on of deep heat by moving ground waters is also responsible for low heat fl ows of less than 40 mW/m(2)observed in shallow boreholes in the rift should ers.