Seismic Experiment Searches for Active Magmatic Source in Deep Lithosphere, Central Europe

Transcription

1 Eos, Vol. 84, No. 4 0, 7 October 2003 Seismic Experiment Searches for Active Magmatic Source in Deep Lithosphere, Central Europe PAGES 4 0 9, The Bohemian Massif (BM) is the largest coherent surface exposure of basement rocks in central Europe. It is a geodynamically active part of the Hercynian orogenic belt representing a collage of magmatic arcs and micro-conti nents caused by the collision of Laurasia (Laurentia-Baltica) and Africa (Gondwana). The general northwest direction of accretion is typical of the northern part of the Hercynian belt. Irregularly-shaped colliding blocks resulted in a very complicated structure of convergence, lithospheric subduction, and crustal shortening, followed by extensional processes and rifting. The western part of the Bohemian Massif is the well-known health and resort landscape of Bohemia, Saxonia, and Bavaria, with Karlovy Vary (Karlsbad) as the flagship of the famous spa towns of the region (Figure 1).Allegedly, the Emperor Charles IV founded the spa in the years at the site, which was already well known for its hot springs. For centuries, 12 springs in Karlovy Vary ranging in temperatures between 42 C and 72 C have b e e n exploited, especially for the treatment of digestive system disorders and metabolic diseases. A multidisciplinary experiment planned for , the BOHEMA project (BOhemian Massif HEterogeneity and Anisotropy),is bringing together geoscientists from 10 institutions in the Czech Republic, Germany, and France for a cooperative study of the structure and dynamics of the lithosphere and asthenosphere in a part of the BM (Figure 1). The overall objective of BOHEMA is to use all available techniques, such as high-resolution tomography, 3-D anisotropy studies based on shear-wave splitting and P-residual spheres, as well as receiver functions down to the 410and 660-km transitions, to image the crust and upper mantle. We propose to develop a geodynamic model of the lithosphere-asthenosphere, which satisfies all available geophysical, geo logical, and petrologic data. Central to the project is a passive seismic tomography experiment which has b e e n oper ating from the fall of 2001 to the spring of An array of three-component, broadband and short-period stations covers a region of about 160 x 300 km elongated perpendicular to the Eger rift (Figure l ). T h e large aperture of the network, which consists of 61 permanent and 84 temporary stations at spacing between 12 and 30 km, will provide a high ray density B Y VLADISLAV BABUSKA,JAROSLAVA PLOMEROVA, AND THE BOHEMA WORKING GROUP refraction and reflection profiles, with the recent refraction experiment CELEBRATION [Guterch et al, 2001].The importance of such a multidisciplinary approach to the research of tectonic evolution of the region c a n b e seen by the results of the deep-drilling program KTB (Figure 2 a ). T h e revealed complexity of crustal structures demonstrated the need to consider 3-D aspects in seismic imaging [Emmermann and Lauterjung, 1997]. coverage, thereby enabling tomographic inver sion down to depths of about 250 km. Enigmatic Tectonic Setting The western part of the BM represents a juncture of three first-order tectonometamorphic units: Saxothuringian (ST),Tepla-Barrandian ( T B U ), a n d Moldanubian (MD, Figure 2 ). The region is of fundamental importance for understanding Hercynian tectonics in general. The Tertiary Eger rift (ER), a 300-km-long, ENEWSW-striking structure characterized by high heat flow and Cenozoic volcanism, separates the ST in the north from the TBU and MD units in the south. Active tectonics is primarily manifested by the periodic o c c u r r e n c e of earthquake swarms, Quaternary volcanism, emanations of C0 -dominated gases of mantle origin, and neotectonic crustal movements. The deep crustal structure of the western part of the BM has b e e n investigated using several 2 12' Short-period Stations Dynamics of the Deep Lithosphere Recent findings of micro-diamonds in metamorphic gneiss lenses within migmatites at the northern flank of BM (ST, Figure 1) provide evi dence for ultra-high-p metamorphism of conti nental material subducted to >150 km depth [Stockhert et al, 2001 ].Thus, the passive seismic tomography of BOHEMA project will also b e a tool for imaging such deep structures as traces of paleosubduction zones. Both the seismological and magnetotelluric estimates of the lithosphere thickness indicate a lithosphere thinning beneath the ER to about 80 km from km in the neighboring units [Plomerovd et al, 1998].The ER belongs to the European Cenozoic rift system, which may have a c o m m o n source of volcanism in the mantle [Granet et al., 1995].The rift also marks a suture between the tectonic units characterized by different orientations of 14* Broad-band or Long-period Stations Fig. 1. Simplified map of the region with BOHEMA network of seismic stations (blue, red, and black symbols denote Czech, French, and German stations, respectively), and major tectonometamorphic units: ST- Saxothuringian, TBU - Tepld-Barrandian Unit, and MD - Moldanubian. SZ and f. are shear zones and faults, respectively. The upper right insert is a map of Europe. The lower right insert is a histori cal engraving depicting the Karlovy Vary (Karlsbad) geyser (1830) and a milieu of this famous spa. The symbol of the town is the geyser with the capacity of2000 liters per minute, gushing into the height of up to 15 meters. During the second half of the 20th century, the region experienced serious environmen tal problems as a consequence of open-pit mining of brown coal in close proximity to the spas. Recent ly, there has also been a problem of increasing exploitation of mineral waters, which are often saturated by natural gas in part of mantle origin. Original color image appears at back of volume.

2 Eos,Vol. 84, No. 4 0, 7 October 2003 seismic anisotropy in the mantle lithosphere [Babuska and Plomerova, 2001 ]. The most c o m m o n method of studying seis mic anisotropy uses the shear-wave splitting, an analog to optical birefringence. Figure 3 illustrates lateral variations of splitting param eters (azimuth of the fast shear wave and a delay time of the slow wave) evaluated from a part of BOHEMA data for two teleseismic events.the observed delay times of 31 highquality observations vary from 0.7s to 1.5s, and c a n b e explained by an olivine-rich anisotropic layer in the upper mantle of 50 to 110 km thickness, assuming a shear-wave anisotropy of 6% calculated for Bohemian mantle xenoliths by Christensen et al. [2001]. Azimuths of the fast shear waves are coherent within the tectonic units, but differ for the two events, with waves arriving from almost oppo site directions. These observations, along with the systematic directional d e p e n d e n c e of teleseismic P-wave residuals [Plomerova et al, 1998],strongly suggest an anisotropic mantle lithosphere with inclined symmetry axes. The Hercynian suture between the ST unit in the north and the TBU/MD units in the south probably created a zone of mechanical weakness leading to development of the Cenozoic ER, as well as a predisposition for earthquake swarms [Fischer and Horalek, ]. Most earthquakes o c c u r within the brittle part of the upper crust (Figures 2b and 2c), above the intersection of this deep-reaching suture with the Marianske Lazne (ML) fault system (Figure 1). Moreover, more than 100 mineral water springs and several hundred gas vents in eight mofette fields are located near this intersection. Current hypotheses assert that s o m e of the free gas phase in the mineral waters and emanations of C 0 and He are supplied from a magmatic reservoir located in the uppermost mantle [Weinlich et al, 1999]. The faults are thus considered to represent a deep-reaching fracture system that enables an ascent of gases from a hypothetical mantle plume (Figure 2 b ). 2 Major Objectives of the BOHEMA Project Among the important objectives of this project is to provide a link between the near-surface structures and the structure of the deep litho sphere. The subcrustal lithosphere obviously plays a dominant role during collisions of lithospheric blocks, and their boundaries or sutures are potential zones of weakness for the o c c u r r e n c e of intraplate earthquakes and volcanic activity. The primary scientific goals of BOHEMA are to: Construct a 3-D tomographic model of the lithosphere/asthenosphere in broader surround ings of the ER; Develop geodynamic models of the tectonic evolution of the western BM; Investigate the relationship of the ER to the ST-TBU/MD suture at depth, and the possible role of inherited structures in recent active tectonics; Investigate why the earthquake swarms originate within the relatively stable part of central Europe; Fig. 2. Schematic diagrams showing the tectonic situation in the western Bohemian Massif, (a) N-S section of the deep structure of the Saxothuringian (ST) - Moldanubian (MD) contact based on observations of seismic anisotropy /Plomerova et al., 1998; Babuska and Plomerova, 2001J. Mesozoic sediments in the German territory are green and those of the Cheb Basin are yellow. KTB - site of the German deep borehole / E m m e r m a n n and Lauterjung, 1999]; (b) Close-up of the rear of the lower diagram shows a hypothetical plume head beneath the western Eger Rift [Granet et al., 1995], and a hypothetical paleosubduction of ST crust beneath the MD and TBU /Tomek et al., 1997]. Granite massifs (red) are also schematically shown; (c) Close-up of the distribution of foci of earthquake swarms in Novy Kostel - Kraslice region /Fischer and Horalek, 2003], as shown in panel (b). Gas flow from the mantle may play an important role in triggering earthquake swarms /Brauer et al., 2003]. Original color image appears at back of volume. Find out whether any plume structure exists in the mantle beneath the western BM, comparable to the French Massif Central or the Eifel in Germany; Identify the source region of the gases in the mineral springs and mofettes; and Evaluate the distribution of fluids near the earthquake hypocenters from the detailed seismic tomography of the crust. Acknowledgments The Czech part of BOHEMA project has been supported by the Ministry of Environment (Grant 128/630/01) and by the Czech Grant Agency (No.205/01/1154).We thank the French and German authorities, as well as the ETH Zurich for making available seismic stations from their national and institutional pools. The German research is supported by DFG Grants KO1068/6-1,10314/15-1, and KL776/3-1. The authors thank U. Achauer, M. Granet, H. Kampf, R. Kind, and R. C. Liebermann for their c o m m e n t s on the manuscript. The BOHEMA Working Group The authors of this article, and L.Vecsey J. Zednik, P Jedlicka,V Vavrcuk, J. Horalek, A. Bouskova,T. Fischer, and B. Ruzek, Inst, of Geophysics, CAS, Prague; M. Broz, and J. Malek, Inst, of Rock Structure and Mechanics, CAS, Prague;V Nehybka, Inst, of Phys. Earth, Masaryk University Brno; 0. Novoty Dept. of Geophysics, Charles University Prague; M. Granet, U. Achauer, and T Piquet, Inst. Phys. du Globe, Univ Strasbourg; M. Korn, S. Wendt, S. Funke, M. Brunner, and D. Rossler, Inst. Geophys., Univ. Leipzig; R. Kind, H. Kampf,W Geissler, and B. Heuer, GeoForschungZentrum Potsdam; K. Klinge, T. Plenefisch, K. Stammler, and M. Lindemann, Seism. ZentralObservatorium Erlangen; K. Brauer, UmweltForschungZentrum, Leipzig-Halle; and PMalischewsky Institut Geowissenschaft, University of J e n a. References Babuska,V and J. Plomerova, Subcrustal lithosphere around the Saxothuringian-Moldanubian Suture

3 Eos, Vol. 84, No. 4 0, 7 October ' I!' 14 S i i j - : I 50! 49 i 11 j SKS I 14 NULL noisy ; j: Fig. 3. Fast shear-wave polarization azimuths and delay times of slow shear-waves are shown for two seismic events of 12 October 2002 from (left) and 12 October 2001 from the Mariana Islands (right), arriving from almost opposite azimuths (large open arrows). The arrows at individual stations point in azimuths of the fast shear-wave polarization vectors evaluated in 3-D. Thin arrows at stations indicate less reliable measurements. Zone - a model derived from anisotropy of seis mic wave velocities, Tectonophys., 332, , Brauer, K., H. Kampf,G. Strauch,and S. M.Weise, Isotopic evidence (3He/4He, 1 3 C C 0 2 ) of fluid-trig gered intraplate seismicity, J. Geophys. Res., 108, 2,doi: /2002JB002077,2003. Christensen, N. I., L. G. Medaris, H. EWang, and E. Jelinek, Depth variation of seismic anisotropy and petrology in central European lithosphere: A tectonothermal synthesis from spinel lherzolite, J. Geophys.Res., 7 0 6, , Emmermann, R. and J. Lauterjung,The German Con tinental Deep Drilling Program KTB: overview and major results,./ Geophys.Res., 102,18,179-18,201, Fischer,T. and J. Horalek, Space-time distribution of earthquake swarms in the principal focal zone of the NW Bohemia/Vogtland seismoactive region: period , J Geodyn., 35, ,2003. Granet, M., M.Wilson, and U A c h a u e r, Imaging a mantle plume beneath the Massif Central (France), Earth Planet. Sci. Lett., 17, ,1995. Guterch, A., A. Grad, and G. R. Keller, Seismologists celebrate the new millennium with an experiment in Central Europe, Eos,Trans.,AGU, 82,534,2001. Plomerova, J.,V Babuska, J.Sileny, and J. Horalek, Seis mic anisotropy and velocity variations in the man tle beneath the Saxothuringicum-Moldanubicum c o n t a c t in central Europe, Pure andappl. Geophys., 7 5 7, , Stockhert, B., B. Dyster, C.Trepmann, and H.-J. Massonne, Microdiamond daughter crystals pre cipitated from supercritical COH + silicate fluids included in garnet, Erzgebirge, Germany Geology, 29, ,2001. Accounts from 19th-century Canadian Arctic Explorers' Logs Reflect Present Climate Conditions PAGES 4 1 0, The widely perceived failure of 19th-century expeditions to find and transit the Northwest Passage in the Canadian Arctic is often attri buted to extraordinary cold climatic conditions associated with the "Little Ice Age" evident in proxy records. However, examination of 4 4 Tomek,C.,V Dvorakova, and S.Vrana, Geological interpretation of the 9HR and 503M seismic profiles in western Bohemia, in Geological Model of Western Bohemia related to the KTB Borehole in Germany, S.Vrana and V Stedra ( e d s. ), Sbor. Geol. V d, 4 7, , Prague, Weinlich, PH., K. Brauer, H. Kampf, G. Strauch, J.TesF.and S.M. Weise, An active subcontinental mantle volatile system in the western Eger rift, Central Europe: Gas flux, isotopic (He, C, and N) and compositional fingerprints, Geochim. et Cosmochim.Acta, 63, ,1999. Author Information Vladislav Babska and Jaroslava Plomerova, Institute of Geophysics, Czech A c a d e m y of Science, Prague; and the BOHEMA Working Group century is frequently s e e n as a vain and tragic failure. Polar exploration during the Victorian era s e e m s to us today to have b e e n a costly exercise in h e r o i c futility, which in many respects it was.this perspective has b e e n reinforced s i n c e the 1970s, w h e n paleoclimate reconstructions b a s e d o n Arctic i c e c o r e stratigraphy appeared to confirm the e x i s t e n c e of exceptionally c o l d c o n d i t i o n s explorers' logs for the western Arctic from consistent with the period glaciologists had 1818 to 1910 reveals that climate indicators termed the "Little I c e Age" (Figure l a ), with such as navigability, the distribution and thick temperatures m o r e than o n e standard devia ness of annual s e a i c e, monthly surface air tion c o l d e r relative to an early 20th-century temperature, and the onset of melt and freeze m e a n [Koerner, were within the present range of variability 1990; Overpeck The quest for the Northwest Passage through B Y K.WOOD AND J.E. OVERLAND Brazil the Canadian archipelago during the 19th 1977; Koerner and Fisher, et al., ]. In r e c e n t years, the view of the Little Ice Age as a synchronous worldwide and prolonged c o l d e p o c h that

4 Eos, Vol. 84, No. 40,7 October " 14' Short-period Stations Broad-band or Long-period Stations Fig. 1. Simplified map of the region with BOHEMA network of seismic stations (blue, red, and black symbols denote Czech, French, and German stations, respectively), and major tectonometamorphic units: ST - Saxothuringian, TBU - Tepld-Barrandian Unit, and MD - Moldanubian. SZ and f. are shear zones and faults, respectively. The upper right insert is a map of Europe. The lower right insert is a historical engraving depicting the Karlovy Vary (Karlsbad) geyser (1830) and a milieu of this famous spa. The symbol of the town is the geyser with the capacity of 2000 liters per minute, gushing into the height of up to 15 meters. During the second half of the 20th century, the region experienced serious environmental problems as a consequence of open-pit mining of brown coal in close proximity to the spas. Recently, there has also been a problem of increasing exploitation of mineral waters, which are often saturated by natural gas in part of mantle origin.

5 Eos, Vol. 84, No. 40,7 October 2003 Fig. 2. Schematic diagrams showing the tectonic situation in the western Bohemian Massif, (a) N-S section of the deep structure of the Saxothuringian (ST) - Moldanubian (MD) contact based on observations of seismic anisotropy /Plomerova et al., 1998; Babuska and Plomerova, 2001]. Mesozoic sediments in the German territory are green and those of the Cheb Basin are yellow. KTB - site of the German deep borehole /Emmermann and Lauterjung, 1999]; (b) Close-up of the rear of the lower diagram shows a hypothetical plume head beneath the western Eger Rift [Granet et al, 1995], and a hypothetical paleosubduction of ST crust beneath the MD and TBU /Tomek et al., 1997]. Granite massifs (red) are also schematically shown; (c) Close-up of the distribution of foci of earthquake swarms in Novy Kostel - Kraslice region /Fischer and Horalek, 2003], as shown in panel (b). Gas flow from the mantle may play an important role in triggering earthquake swarms /Brauer et al., 2003].