Abstract
SummaryBeing far from plate boundaries but covered with seismograph networks, the Fennoscandian Shield features an ideal test laboratory for studies of intraplate seismicity. For this purpose, this study applies 4190 earthquake events from years 2000–2015 with magnitudes ranging from 0.10 to 5.22 in Finnish and Swedish national catalogues. In addition, 223 heat flow determinations from both countries and their immediate vicinity were used to analyse the potential correlation of earthquake focal depths and the spatially interpolated heat flow field. Separate subset analyses were performed for five areas of notable seismic activity: the southern Gulf of Bothnia coast of Sweden (area 1), the northern Gulf of Bothnia coast of Sweden (area 2), the Swedish Norrbotten and western Finnish Lapland (area 3), the Kuusamo region of Finland (area 4) and the southernmost Sweden (area 5). In total, our subsets incorporated 3619 earthquake events. No obvious relation of heat flow and focal depth exists, implying that variations of heat flow are primarily caused by shallow lying heat producing units instead of deeper sources. This allows for construction of generic geotherms for the range of representative palaeoclimatically corrected (steady-state) surface heat flow values (40–60 mW m−2). The 1-D geotherms constructed for a three-layer crust and lithospheric upper mantle are based on mantle heat flow constrained with the aid of mantle xenolith thermobarometry (9–15 mW m−2), upper crustal heat production values (3.3–1.1 μWm−3) and the brittle-ductile transition temperature (350 °C) assigned to the cut-off depth of seismicity (28 ± 4 km). For the middle and lower crust heat production values of 0.6 and 0.2 μWm−3 were assigned, respectively. The models suggest a Moho temperature range of 460–500 °C.
Highlights
The maximum focal depth of earthquakes has been frequently used to model thermal conditions of the lithosphere, especially to approximate the depth below which the ductile transformation dominates over brittle transformation of the crust
Kuusisto et al (2006) modeled the crustal seismic velocity structures in the central part of the Fennoscandian shield using mixtures of different lithologies for the different velocity layers and concluded that quartz-bearing rock types are abundant from the surface down to the depth of 25–30 km
In the intraplate conditions of Fennoscandian Shield, the earthquake mechanisms represent the same rheological properties in different areas, the crustal temperature must be relatively uniform at the cutoff depth
Summary
The maximum focal depth (i.e. cut-off depth) of earthquakes has been frequently used to model thermal conditions of the lithosphere, especially to approximate the depth below which the ductile transformation dominates over brittle transformation of the crust. This depth, useful for the verification of rheological models, has been usually considered a temperature isotherm. The calculated models suggested that the crustal velocity profiles can be simulated with rock-type mixtures where the upper crust consists of felsic gneisses and granitic–granodioritic rocks with a minor contribution of amphibolite and diabase. If the lithospheres were mafic or ultramafic, the brittleductile transition would occur at a greater temperature than the range of 300–400 ◦C assigned for granitic (quartz-bearing) lithology (Blanpied et al 1991)
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