Relationship between recent heat flow and seismic properties: Some notes from crustal research in Germany

Deep seismic reflection profiling has been used extensively during the last 25 years in order to unravel crustal structure and evolution, complemented by deep drilling in crystalline rocks. Examples are presented from the Variscan/Hercynian crust in Germany showing moderate-to-high surface heat flow densities which can be studied for relationships with seismic reflections and velocities. Four main seismic features are identified which emphasise that heat transfer through the crust is a strong three-dimensional problem similar to that of sedimentary basins: (1) The top and thickness of the highly reflective lower crust as a specific dominant crustal entity correlate with high surface heat flow. The creation of lower crustal reflections is commonly attributed to a post-collisional extensional regime. The rheological character of top and bottom (crust-mantle boundary) of the lower crust indicates decoupling horizons at these levels. (2) Upper crustal reflectivity is frequently attributed to fault and fracture zones. These may delineate pathways for heat transport through fluid circulation due to higher (crack) porosity and possibly higher permeability. (3) Seismic anisotropy of large crustal volumes has been confirmed as a common feature and is particularly strong in gneisses due to their strain-related preferred orientation of the texture. Gneisses are also known for their anisotropy of heat conductivity having the same magnitude and preferred orientation. (4) Seismic in-situ velocities are always lower than laboratory-derived or composition-derived velocities. In consequence, crustal composition models inverted from seismic velocity models tend to be biased towards felsic composition. The reason for this is most likely the porosity of crystalline rocks including some hydrothermal activity which may also explain the occurrence of low-velocity zones often observed in the middle crust. Modelling of crustal temperatures, heat transfer and heat production should account for these phenomena.