Drilling the Eger Rift in Central Europe

T he workshop was aimed at discussing the scientif ic goals that would justif y deep drilling in the western part of the Eger Rift area (Fig. 1). The Eger Rift, located in the northwestern part of the Bohemian Massif, is a key site that has attracted the international geoscience community for many decades. The rift is the result of young, deep-seated geodynamic processes manifested by episodic Cenozoic volcanism (the youngest at 0.2–0.5 Ma), repeated earthquake swarms, numerous mineral springs, CO2 emissions with high 3He content, and abundant mofettes. T he crust– mantle boundary (27 km) and lithosphere–asthenosphere boundary (80–90 km) are shallow compared to the conditions below the Bohemian Massif. Also, surface heat flow (60–80 mW m -2 ) within the rift zone is higher than within

T he workshop was aimed at discussing the scientific goals that would justify deep drilling in the western part of the Eger Rift area (Fig. 1).The Eger Rift, located in the northwestern part of the Bohemian Massif, is a key site that has attracted the international geoscience community for many decades.The rift is the result of young, deep-seated geodynamic processes manifested by episodic Cenozoic volcanism (the youngest at 0.2-0.5 Ma), repeated earthquake swarms, numerous mineral springs, CO 2 emissions with high 3He content, and abundant mofettes.T he crustmantle boundary (27 km) and lithosphere-asthenosphere boundary (80-90 km) are shallow compared to the conditions below the Bohemian Massif.Also, surface heat flow (60-80 mW m -2 ) within the rift zone is higher than within the massif.
Owing to the diversity of available data stretching over various fields of geoscience, the area is an ideal place to foster studies on the interaction between active mantle, crust, and deep biosphere processes.The workshop thus formed a platform on which the state of the art in investigating the rift was summarized and new research targets addressed by drilling were defined.
A ccord i ng to t he i nteract i ng resea rch f ields, t he workshop was streamed into six topical blocks.The first block focused on geology, tectonics, and fluids covering the tectono-sedimentary evolution of the Eger Rift and Cenozoic alkaline volcanic series, as well as meteoric, magmatic, and fossil components of deep fluids.The session "Evolution of the Lithosphere" stressed the receiver function studies from the recent international BOHEMA passive seismic experiment, the analyses of lower crustal and mantle xenoliths, and the necessity to investigate the geothermal and rheological conditions of the lithosphere.Contributions in the session on "Earthquake Swarm Processes" dealt both with space-time distribution of swarm earthquakes and related geophysical and geochemical parameters ,and with models of earthquake swarms.Within the block "Fluid-Related Seismic Processes", such processes were not only analyzed in general, but also the results from two ICDP projects were presented: German Continental Deep Drilling Programm (KTB) and Long Valley Caldera, Calif., U.S.A.The "Deep Biosphere" session thoroughly introduced this interdisciplinary field of science and its dependency on deep drilling and provided two examples of such studies from mineral springs of Bad Brambach (Germany).A session on issues of "Drilling and Borehole Monitoring" covered the technical aspects of drilling, the possibilities of multi-parameter continuous monitoring, and how a deep borehole could be turned to a multi-sensor laboratory.
The workshop concluded that the two manifestations of anomalous mantle-crust interaction (the occurrence of earthquake swarms at shallow depth and the occurrence of CO2-and 3He-rich fluids) are worth investigating by deep drilling and by an extensive monitoring program.The research conducted hitherto into earthquake swarms in the Eger Rift (Fig. 2) focuses on the role of fluids in this process.Thus, one principal scientific objective of drilling the Eger Rift is to improve the understanding of triggering mechanisms that generate swarm-earthquake activity.In most cases, fluid-triggered earthquakes are generated through fluid-injection from the surface.A deep drill hole in the Eger Rift would allow investigating f luid-injection-triggered seismicity from either  M i ner a l spr i ngs a re i mpor t a nt for t he cont i nued prosperity of the surrounding areas (e.g., in Karlovy Vary).The results from the borehole laboratory would help us to understand better the mechanisms driving the spas, for instance, whether the deep fluid circulation is related to shallow fluids feeding the springs in the spas.This is clearly of great socio-environmental relevance.
At the end of the workshop, a field trip introduced the participants to key sites in western Bohemia including the thermal springs at Karlovy Vary Spa, the potential drilling site in the northern part of the Cheb sedimentary basin (in the vicinity of the seismically dominant Nový Kostel earthquake focal zone), the Bublák mofette (surrounded by seismic stations of the WEBNET (Academy of Sciences, Prague) and KR ASNET (Brno University) networks), the CO2 exhalations at Soos, and the Železná Hůrka volcano and its tephra deposits.

Figure 2 .
Figure 2. Schematic 3-D section of the Nový Kostel area.The main epicentral cluster indicated by red surface occurs at depths of 6-11 km below the eastern margin of the Cheb Basin.Triangles indicate seismic stations of WEBNET.EG and MLF denote the main tectonic structures of Eger Rift and Mariánské Lázně Fault.

by Aleš Špičák, Andrea Förster, and Brian Horsfield
v Praha Figure 1.The Eger Rift as part of the European Cenozoic Rift System.Suggested location for drilling is shown.