Low geothermal heat flow of the Urals fold belt - Implication of low heat production, fluid circulation or palaeoclimate?

The Urals are characterized by extremely low heat flow density (HFD). We present a discussion on the relevant factors which may contribute to the observed distribution of heat flow values. The available heat flow data in the Urals and surrounding East European and West Siberian platforms are based on borehole measurements at about 300 sites which range from the Arctic Sea coast to the Kazakshtan Plain. Along the Urals fold belt heat flow density is 30 mW m(-2) or less, whereas the platform areas are characterized by 20-40 mW m(-2) higher values. The low heat flow density zone is 50-100 km wide. Its extension to the north is not exactly known, but the minimum extends at least to the latitude 61 degrees N. We present new results of heat flow and heat production measurements, Peclet number analyses on advective heat transfer by groundwater flow, as well as numerical conductive models of heat transfer in the lithosphere in the Troitsk DSS transect. The most important factor contributing to the low heat flow density in the Urals seems to be the low level of radiogenic heat production in the crust in the Tagil-Magnitogorsk Zone. The minimum is apparently enhanced by the palaeoclimatically induced vertical variation in HFD produced by the periglacial climatic conditions during the latest glaciation epoch 70,000-10,000 years B.P. The boreholes used for HFD measurements are shallower (500-1500 m) in the Tagil-Magnitogorsk Zone than in the adjoining areas in the west (1000-3000 m) or in the east (300-3000 m), and therefore the palaeoclimatic disturbance is more pronounced in these boreholes.