Heat flow and thermal structure of the lithosphere across the Baltic Shield and northern Tornquist Zone

Abstract

Terrestrial heat-flow data from Archean and Proterozoic Provinces of the Baltic Shield, its southwestern margin, and the tectonic transition to the younger tectonic units of Central Europe (northern segment of the European Geotraverse) are compiled and examined for local and regional variations. A 2-D numerical lithospheric temperature and heat-flux model is presented covering 1800 km of the deep seismic Fennolora profile across the Baltic Shield and 500 km of Eugeno-s/ northern Eugemi seismic lines across the northern Tornquist Zone and the Danish and North German basins. Thermal lithosphere, defined as the outer layer with relatively low temperatures ( T < 1200–1400° C) in which heat transfer is dominantly by conduction, is found to vary markedly in thickness from 85–110 km beneath basins and Tornquist Zone to about 150 km in central shield areas, and 200–250 km in areas of low heat flow along the northern shield profile. These results are in very good agreement with seismological information from surface wave dispersion analysis and the refraction / wide-angle seismic interpretation of Fennolora data. The combined thermal and seismologic evidence indicates that the uppermost mantle region stratified in seismic velocity is restricted to the upper, relatively cold thermal layer, which is interpreted to constitute shield lithosphere. Local and to some extent regional variations in surface heat flow are found to be closely related to variations in near-surface radiogenic heat production. For the southern Baltic Shield a heat-flow / heat-production relationship of q 0 = 32.8 + 7.6 H 0 is derived. Regional increase in heat flow from northern and central shield provinces to the shield margin and the Tornquist Zone is interpreted to be related partly to differences in upper-crustal heat generation, partly, and in southernmost areas mainly, to an increased heat flux from the upper mantle. Upper-mantle heat flux is estimated at 20–25 mW m −2 along the northern and central shield transect, increasing to 30–40 mW m −2 beneath the shield margin, basins, and Tornquist Zone. Lithospheric thickness variations in this region of northern Europe are found to be closely associated with variations in heat flux from the uppermost mantle and perhaps to a large extent controlled by geodynamically determined lateral differences in upper-mantle heat flux. Earthquake focal depth data and information on crustal strength show Baltic Shield seismicity to be limited generally to the upper 20–25 km brittle part of the crust at temperatures less than 300–400°C. Maximum source depth of magnetic anomalies at temperatures near 600°C is in the deepest part of the crust near the crust-mantle boundary in the southern region (about 25 km depth along the Tornquist Zone and about 35–40 km in southern shield areas) and in the uppermost mantle (60–70 km depth) in the northern shield areas. Some petrologic implications are discussed. The present deep crust is supposed to be at lower temperature-pressure conditions than at early stages of formation. However, also the present deepest parts of the shield crust (> 35–40 km) are within the stability field of eclogites. The general absence of significant gravity anomalies in areas of marked crustal thickness variations indicate high-density eclogites to be present in significant amounts in the deep shield crust.