- Email: [email protected]
Detection of active crustal structures in the Upper Rhine Graben using local earthquake tomography, gravimetry and reflection seismics
ARTICLE IN PRESS
Quaternary Science Reviews 24 (2005) 339–346
Detection of active crustal structures in the Upper Rhine Graben using local earthquake tomography, gravimetry and reflection seismics G.G.O. Lopes Cardozo, J.B. Edel, M. Granet Institut de Physique du Globe Strasbourg UMR 7516 CNRS/ULP, 5. Rue Rene´ Descartes, 67100 Strasbourg, France Received 12 December 2002; accepted 24 March 2004
Abstract This paper presents a case study, in which three different geophysical methods are combined to link the large-scale seismically active deformation of the basement to the deformation of the sedimentary cover in the seismically active southern end of the Upper Rhine Graben. Local earthquake tomography shows velocity contrasts related to large-scale faulting in the basement. The observed faults coincide with the discontinuities in the Bouguer anomaly map for the upper part of the basement and the sedimentary cover. Reflection seismic profiles show distinct faulting in the sedimentary cover at the position of the structures observed in the local earthquake tomography and the Bouguer anomaly maps. They provide the link of the seismogenic structures at depth and the nearsurface geology. It is shown that the structure of the southern end of the Rhine Graben is dominated by faults that have a NNESSW trend, slightly oblique to the graben axis. The presence of earthquake hypocenters nearby the faults indicate their present-day activity. r 2004 Elsevier Ltd. All rights reserved.
1. Introduction Neotectonic activity in the Upper Rhine Graben (see Fig. 1) is evident from the occurrence of earthquakes in the region and has been studied with the use of trenching and geomorphological indicators. However, the link between seismogenic structures at depth and near surface tectonics remains unclear. Historical seismicity in the graben proves the potential for large earthquakes such as the destructive 1356 Basel earthquake with an estimated magnitude of 6.5. More recent events include the Sierentz earthquake in 1980 (Ml. 4.9). Meghraoui et al. (2001) identified an 8 km long fault just south of Basel, trenching showed recent activity. Nivie`re and Corresponding author. Current address: Shell International Exploration and Production B.V. Kesslerpark 1, 2288 GS Rijswijk (ZH), The Netherlands. E-mail address: [email protected] (G.G.O. Lopes Cardozo).
0277-3791/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2004.03.016
Winter (2000) present a study that integrates the use of seismic profiles, and geomorphology. They conclude that the deformation of the sedimentary cover in the southern end of the Rhine Graben is decoupled from the basement tectonics. An analysis of speleotherms made by Lemeille et al. (1999) shows that the growth of stalagmites in the Jura Mountains south of Basel was influenced by recent seismic activity. This study is aimed at the combined interpretation of a relatively detailed P wave velocity model from local earthquake tomography (Lopes Cardozo and Granet, 2003), Bouguer gravity anomaly maps (Edel et al., 2002), and seismic reflection profiles in order to study the link between faults in the upper basement and in the sedimentary cover in the region. The Rhine Graben forms the central part of the European Cenozoic rift system (ECRIS) (Ziegler, 1992). This system crosses the European continent from the Mediterranean Sea to the North Sea. Ziegler (1992) gives a complete overview of the tectonic development
ARTICLE IN PRESS 340
G.G.O. Lopes Cardozo et al. / Quaternary Science Reviews 24 (2005) 339–346
Fig. 1. Geological map of the Upper Rhine Graben. In the southern part of the graben, Paleogene, and Neogene sediments outcrop in the Mulhouse Horst (MH). In the southern end of the study area, the Jurassic outcrops of the Jura Mountains. DB=Dannemarie Basin. AB=Allschwil Basin. BBF and L-L are the Baden Baden, and Lalaye Lubine faults that are part of the separation between the Saxothuringhian (to the NW), and the Moldanubian zones (to the SE). S-M= Ste. Marie-aux-Mines fault, which is described as a Late Variscan shear zone by Edel and Weber (1995).
of the rift system, and its relation to the collision between the Eurasian and the African plates. Sissingh (1998) discusses the Tertiary sedimentary development of the rift system. In order to fully understand the development of the Rhine Graben, it is important to go back to the Variscan structural grain of Central Europe. The pre-rift history of the Rhine Graben area is closely related to the development of Variscan massifs such as the Massif Central, the Rhenish Massif and the Bohemian Massif. The development of the Rhine Graben started in the Middle to Late Eocene with graben subsidence along inherited structures under E–W orientated extension in the foreland of the Alpine orogenic belt. Thickness variations of Late Eocene sediments show a strong relationship with the Early Permian structure of the crust (Schumacher, 2002). This relationship is an indication that the initial development of
the rift was influenced by reactivation of the Variscan structures. The rifting reached a climax in the Early Oligocene under WNW–ESE orientated extension (Illies, 1978; Bergerat, 1985; Schumacher, 2002). During the Miocene, after a change in direction of the Africa-Europe convergence, the stress field in the foreland rotated towards an overall NW–SE orientated compression (Schmitt, 1981; Delouis et al., 1993; Edel et al., 2001; Schumacher, 2002; Mueller et al., 2003). With this change in stress direction, the rifting in the southern part of the Rhine Graben stopped. In the northern part, subsidence continued in pull-apart basins that were formed by this sinistral reactivation of the Graben (Illies, 1978). In the Late Miocene, the southern end of the graben was overthrusted by the Jura fold-and-thrust belt. Today, the boundary faults of the graben are reactivated in a sinistral strike slip sense under the stable NW–SE
ARTICLE IN PRESS G.G.O. Lopes Cardozo et al. / Quaternary Science Reviews 24 (2005) 339–346
compressive stress regime in northern Europe (Illies and Greiner, 1979; Bonjer et al., 1984; Bonjer, 1997; Plenefisch and Bonjer, 1997). A large scale geological map of the Upper Rhine Graben is shown in Fig. 1.
2. Geophysical techniques and observations 2.1. Local earthquake tomography In local earthquake tomography the travel time residuals of local earthquakes are inverted to obtain a 3D velocity image of the upper crust (Thurber, 1983, 1993; Eberhart-Phillips, 1986, 1990, 1993; Dorbath and Granet, 1996). This method is used to obtain relatively detailed images of the velocity structure in the upper 10 km of the crust. Local earthquakes are recorded in the Upper Rhine Graben by the French, Swiss and German permanent networks. For a detailed LET, however, a denser network of seismic stations is required. The data used for the tomography presented in this paper come from three separate campaigns, in which mobile seismic stations were used. The data set contains a total of 133 seismic events with a moment magnitude ranging from 1 to 3 (see Fig. 2). Lopes
341
Cardozo and Granet (2003) describe the used techniques and data. Fig. 3 shows the absolute P-wave velocity map, resulting from the tomographic inversion, at 2 km depth. At this depth, the velocity structure reflects mainly the difference between dense crystalline basement rocks that have a high P-wave velocity, and less dense porous sediments with low P-wave velocities. A high-velocity body (velocities up to 6 km/s) is found underneath the cities of Mulhouse and Altkirch. This feature in the velocity structure is interpreted to be related to the crystalline rocks of the Mulhouse Horst. The high-velocity body is bounded towards the west, north and east by low-velocity bodies. These low velocities are interpreted to be the signatures of the Dannemarie Basin, the Rhine Graben and Allschwil Basin, respectively, on the velocity structure. The separation between the bodies with contrasting velocities, and thus between rocks with different physical or chemical properties, is rather linear and could be related to faulting. The northwestern corner of the map shows the high velocities corresponding to the basement of the Vosges Mountains. This high-velocity body is bounded on the eastern side by the Rhine Graben western boundary faults. High velocities related to the crystalline basement of the Black Forest are found on the eastern side of the map.
Fig. 2. Digital elevation model and seismicity map of the study area. The red dots mark the instrumentally recorded seismicity for the period 1987–2001 (599 events), blue dots mark the events used in the tomographic inversion. White lines mark the boundaries of the Tertiary outcrops in the Rhine Graben, the Dele´mont basin and the northern rim of the Swiss molasse basin. The network covers the southern end of the graben, and the northern Jura, compare with Fig. 1 for the geology.
ARTICLE IN PRESS 342
G.G.O. Lopes Cardozo et al. / Quaternary Science Reviews 24 (2005) 339–346
Fig. 3. Absolute P-wave velocity map at 2 km depth. Black stars show the surface events used in this study. Note the presence of a high-velocity body underneath the Mulhouse Horst. The adjacent low-velocity anomalies are related to the Dannemarie Basin (west) end the Allschwil Basin (east). Thin black lines show the position of seismic profiles, arrows show the position of the faults on the seismic profiles. Profiles 87-08, and 85-15 are shown in Figs. 5 and 6. Mul.=Mulhouse, Alt.=Altkirch, Dan.= Dannemarie, Bas.=Basel. See text for discussion.
2.2. Bouguer gravity anomaly Gravimetric Bouguer anomaly maps represent the gravitational anomaly of the earth with respect to a reference ellipsoid and are corrected for nongeological sources of gravity variation. The corrected gravimetric anomalies reflect thickness variations of the sedimentary cover and the density contrasts due to various petrographic units in the basement. Gravimetric techniques have been used in the Upper Rhine Graben mainly to detect diapirs for salt and potassium mining. For this paper, the sediment corrected map of the first-order derivative of the gravimetric Bouguer anomaly is used to derive the structure of the basement in the region (Fig. 4). The gravimetric data used in this study are a compilation of three data sets: (1) a very dense data set made for potassium mining in Alsace (MDPA), (2) a set of French and German data collected by Rousset et al. (1993) in the framework of the ECORS project, and (3) data collected by the German SEISMOS company. Edel and Weber (1995), Edel et al. (2002) and Rousset et al. (1993) explain the used gravimetric methods and Edel et al. (2002) describe the gravimetric data set and the processing. The rough interpretation of the Bouguer gravity anomaly map presented in Fig. 4, shows a general
NNE–SSW orientation of the structures in the southern end of the graben. The NW corner of the map in Fig. 4 shows a gravimetric contrast that is interpreted to be related to the eastern border of the Vosges Mountains. Two parallel trending faults are shown to the west, and to the east of the line Mulhouse–Altkirch. Another fault is shown west of the city of Basel. Edel et al. (2002) state that these faults were active as sinistral strike–slip sense during the Variscan convergence. Parallel wrench faults can be observed in outcrops in the Vosges mountains (Ste. Marie-auxMines Fault in Fig. 1). These faults played a major role during the Tertiary tectonics and sedimentation (Edel et al., 2002). 2.3. Reflection seismic sections Reflection seismic profiles are widely used for exploration in the hydrocarbon industry. The Rhine Graben was thought to have hydrocarbon potential and therefore high-quality industry seismic lines are available, which image the upper 3 km of the crust. Fig. 3 shows the P-wave velocity image at 2 km depth, with the positions of the seismic lines superposed. The reflection seismic profile shown in Fig. 5 crosses the western side of the Mulhouse Horst (see Fig. 3 for position). The faulting that is shown in the profile forms
ARTICLE IN PRESS G.G.O. Lopes Cardozo et al. / Quaternary Science Reviews 24 (2005) 339–346
343
Fig. 4. Sediment corrected map of the first order derivative of the gravimetric Bouguer anomaly, which images the structures in the sedimentary cover and the upper basement. The basement faults are derived from Edel et al. (2002). Note that the positions of the structures interpreted by Edel et al. (2002) coincide with the zones of faulting observed in the tomography at 2 km depth (Fig. 3).
the separation between the Mulhouse Horst and the Dannemarie Basin. This fault zone, known as the Illfurth fault, was known from previous studies (Nivie`re and Winter, 2000) and is thought to have been reactivated recently. The Dannemarie Basin is observed on the western side of the line and the Mulhouse Horst on the eastern side. A steep westward dipping normal fault is observed. Differential movement on the minor faults within the fault zone suggests a strike–slip activation of the fault. The folding of the sediments could be an indication of a later compressional phase. Fig. 6 shows a seismic profile that crosses the eastern border of the Mulhouse Horst (Fig. 3). The profile shows faulting that is located on the velocity contrast observed in tomography. The steep faults form the western boundary of an eastward dipping tilted block. On top of this block a sedimentary basin, known as the Allschwil Basin, has developed. The tilting of the block is thought to have resulted from the movement on the eastern Rhine Graben boundary fault, situated to the east. The observed faulting helps to accommodate the tilting of the block in an oblique strike–slip setting and does not have a strong normal component as observed on the Illfurth fault (Fig. 5).
3. Discussion Comparison of the map view images of the LET and the presented Bouguer anomaly map show a general coincidence of the position of the zones of increased gravimetric gradient, interpreted in Fig. 4, and the sharp velocity contrasts in the tomography at 2 km depth. The N35 orientated structures, west and east of the line Mulhouse-Altkirch, correspond to the Illfurth and Sierentz faults as interpreted in the tomography. The orientation of these structures differs from the overall N10 orientation of the axis of the Upper Rhine Graben. Edel and Weber (1995) describe structures with similar orientations in the Vosges as Late Variscan wrench faults. The velocity model and the gravimetry map show sharp contrasts in velocity and gravimetry, respectively, that are interpreted to be related to faulting. Both methods, however, do not show the exact position of the faults, since there is not one unique faultrelated value for both properties. The exact position of the faults can be constrained with the use of the seismic profiles. The seismic profiles presented in Figs. 5 and 6 show clearly that the discontinuities in the velocity structure at 2 km depth coincide with faulting (Fig. 3). The projection of faults interpreted on four seismic lines,
ARTICLE IN PRESS 344
G.G.O. Lopes Cardozo et al. / Quaternary Science Reviews 24 (2005) 339–346
Fig. 5. Seismic profile (number 87-08) situated west of the line Mulhouse-Altkirch upper image shows the interpreted profile. Clear normal faulting is observed, this fault is described as the Illfurth fault by Nivie`re and Winter (2000). This normal faulting separates the sedimentary infill of the Dannemarie Basin in the west and the Mulhouse Horst in the east. This fault zone is known as the Illfurth fault. Interpreted horizons: green (J) the base of the Jurassic, in blue (R) the Middle Rupelian. See Fig. 3 for position.
onto the velocity image for a depth of 2 km shows that the orientation of the faults observed in the seismic profiles is similar to the orientation of structure interpreted on in the velocity model, and gravimetry. To show the depth extent of the structure derived from Fig. 3 and the seismic profiles, a cross section through the velocity model is combined with the two seismic lines in Fig. 7. The cross section shows low P-wave velocity bodies that are related to the sedimentary infill and fractured rocks of the Dannemarie and Allschwil Basins. The two low-velocity bodies are separated by a high-velocity body that is interpreted to be associated to the crystalline rocks of the Mulhouse Horst. It can be seen that earthquake hypocenters are situated on the sharp velocity contrast between the Mulhouse Horst and the Allschwil Basin. These seismic events are the recorded aftershocks of the Sierentz earthquake in 1980. The focal mechanism of the main event showed sinistral strike slip movement on a NNE–SSW orientated plane (Rouland et al., 1980). This is an indication that this fault is seismically active.
4. Conclusions The combination of different geophysical tools can be used to study the configuration of large-scale faults in the upper basement and sedimentary cover of the Upper Rhine Graben. Although the geophysical methods are divers in precision and scale, a coherent image of the structural geology can be derived. The combination of the observed structures and the location of seismic events show that some of the observed structures are active today. The interpretation of the velocity images obtained by local earthquake tomography shows the same orientation of structures as the interpreted Bouguer anomaly map. The presence of faults and their orientations is confirmed independently by the observations on the seismic profiles. Also in the vertical plane, the combination of the velocity model and the seismic profiles show similarity in structure. This study shows that the internal faults of the southern end of the Upper Rhine Graben have an orientation oblique to the axis of the graben. The internal structure of the southern end of the Upper Rhine Graben can be described as two
ARTICLE IN PRESS G.G.O. Lopes Cardozo et al. / Quaternary Science Reviews 24 (2005) 339–346
345
Fig. 6. Seismic profile (number 85-15) situated east of the city of Altkirch, upper image shows the interpreted profile. The strike-slip fault zone (described as the Sierentz Fault in this study) on the western side of the profile has steep faults, and the blocks in the upper part seem to differentially offset along the different faults. Towards the east, an eastward dipping basement block is seen, on top of which the Allschwil Basin has developed. Interpreted horizons: green (J) the base of the Jurassic, in blue (R) the Middle Rupelian. See Fig. 3 for position.
Fig. 7. A comparison between the two seismic lines presented in Figs. 5 and 6, and a profile through the tomography at the same position as the seismic lines. The positions of the faults in the seismic profiles correspond with the sharp velocity contrasts between the velocities in the Mulhouse Horst (in the middle of the profile) and the low velocities of the adjacent Dannemarie (west) and Allschwil (east) basins. Note the seismic events (black stars), aftershocks of the Sierentz earthquake, on the Sierentz Fault.
ARTICLE IN PRESS 346
G.G.O. Lopes Cardozo et al. / Quaternary Science Reviews 24 (2005) 339–346
sedimentary basins (the Dannemarie and Allschwil Basin), separated by a crystalline basement elevation (the Mulhouse Horst). This horst is bounded on two sides by faults. Post sedimentary compressional structures prove distinct activity of the observed faults after the opening of the graben and small earthquakes on the Sierentz Fault show its present-day activity.
Acknowledgements Gideon Lopes Cardozo and Michel Granet are part of the ENTEC network, funded by the EU, HPRN-CT2000-00053. Thanks are due to Marc Schaming and Enterprise Oil for the seismic sections presented in this study. The earthquake recording and the processing of the gravimetric data was conducted in the framework of the GeoFrance3D project. Marcus Schumacher, and Charles Gumiaux are kindly thanked for their concise review of the manuscript. References Bergerat, F., 1985. De´formations cassantes et champs de contrainte Tertiaires dans la plate forme Europe´enne. Ph.D. Thesis, Universite´ Pierre et Marie Currie Paris. Bonjer, K.-P., 1997. Seismicity pattern and style of seismic faulting at the Eastern borderfault of the Southern Rhine Graben. Tectonophysics 275, 41–69. Bonjer, K.P., Gelbke, C., Gilg, B., Rouland, D., Mayer-Rosa, D., Massinon, B., 1984. Seismicity and dynamics of the Upper Rhine Graben. Journal of Geophysics 55, 1–12. Delouis, B., Haessler, H., Cisternas, A., Rivera, L., 1993. Stress tensor determination in France and neighbouring regions. Tectonophysics 221, 413–437. Dorbath, C., Granet, M., 1996. Local earthquake tomography of the Altiplano and the Eastern Cordillera of northern Bolivia. Tectonophysics 259, 117–136. Eberhart-Phillips, D., 1986. Three-dimensional velocity structure in northern California Coast Ranges from inversion of local earthquake arrival times. Bulletin of the Seismological Society of America 76 (4), 1025–1052. Eberhart-Phillips, D., 1990. Three-dimensional P and S velocity structure in the Coalinga Region, California. Journal of Geophysical Research 95 (B10), 15,343–15,363. Eberhart-Phillips, D., 1993. Local earthquake tomography: source regions. In: Iyer, H.M., Hirahara, K. (Eds.), Seismic Tomography. Chapman & Hall, London, pp. 613–642 (Chapter 22). Edel, J.B., Weber, K., 1995. Cadomian terranes, wrench faulting and thrusting in the central Europe Variscides: geophysical and geological evidence. Geologische Rundschau 84, 412–432.
Edel, J.B., Dubois, D., Marchant, R., Hernandez, J., Cosca, M., 2001. La rotation mioce`ne infe´rieur du block corso-sarde. Nouvelles contraintes pale´omagne´tiques sur la fin du movement. Bulletin de la Societe Ge´olgique de France 172, 275–283. Edel, J.B., Lutz, H., Elsass, P., 2002. Le socle Varisque et les tectoniques Rhe´nanes dans le Fosse´ Rhe´nan meridional; traitement et interpre´tation de la carte gravime´trique du fosse´ a` partir du leve´ haute densite´ de MDPA. Geologie de la France 3, 43–59. Illies, J., 1978. Neotektonik, geothermale Anomalie und Seismizita¨t im Vorfeld der Alpen. Oberrheinische Geologische Abhandlungen 27 (1/2), 11–31. Illies, J.H., Greiner, G., 1979. Holocene movements and state of stress in the Rhinegraben rift system. Tectonophysics 52, 349–359. Lemeille, F., Cushing, M., Carbon, D., Grellet, B., Bitterli, T., Flehoc, C., Innocent, C., 1999. Co-seismic ruptures and deformations recorded by speleotherms in the epicentral zone of the Basel earthquake. Geodinamica Acta 12 (3–4), 179–191. Lopes Cardozo, G.G.O., Granet, M., 2003. New insight in the tectonics of the Southern Rhine Graben-Jura region using local earthquake seismology. Tectonics 22 (6), 1078. Meghraoui, M., Delouis, B., Ferry, M., Giardini, D., Huggenberger, P., Spotke, I., Granet, M., 2001. Active normal faulting in the Upper Rhine Graben and paleoseismic identification of the 1356 Basel earthquake. Science 293, 2070–2073. Mueller, B., Reinecker, J., Heidbach, O., Fuchs, K., 2003. The 2003 release of the World Stress Map. Available online at www.worldstress-map.org. Nivie`re, B., Winter, T., 2000. Pleistocene northwards fold propagation of the Jura within the southern Upper Rhine Graben; seismotectonic implications. Global and Planetary Change 27, 263–288. Plenefisch, T., Bonjer, K.P., 1997. The stress field in the Rhine Graben area inferred from earthquake focal mechanisms and estimation of frictional parameters. Tectonophysics 275, 71–97. Rouland, D., Haessler, H., Bonjer, K.P., Gilg, B., Mayer-Rosa, D., Pavoni, N., 1980. The Sierentz Southern Rhinegraben earthquake of July 15, 1980. Preliminiary results. In: Proceedings of the 17th Assembly of the European Seismological Commission Budapest, pp. 441–446. Rousset, D., Bayer, R., Guillon, D., Edel, J.B., 1993. Structure of the southern Rhine Graben from gravity and reflection seismic data (ECORS-DECORP program). Tectonophysics 221, 125–135. Schmitt, T., 1981. The West European stress field: new data and interpretation. Journal of Structural Geology 3 (3), 309–315. Schumacher, M.E., 2002. Upper Rhine Graben: role of preexisting structures during rift evolution. Tectonics 21 (1), 1–17. Sissingh, W., 1998. Comparative Tertiary stratigraphy of the Rhine Graben, Bresse Graben, and Molasse Basin: correlation of Alpine foreland events. Tectonophysics 300, 249–284. Thurber, C.H., 1983. Earthquake locations and three-dimensional crustal structure in the Coyote Lake area, central California. JGR 88, 8226–8236. Thurber, C.H., 1993. Local earthquake tomography: velocities and Vp/Vs—theory. In: Iyer, H.M., Hirahara, K. (Eds.), Seismic Tomography. Chapman & Hall, London, pp. 563–580 (Chapter 20). Ziegler, P.A., 1992. North sea rift system. Tectonophysics 208, 55–75.
Quaternary Science Reviews 24 (2005) 339–346
Detection of active crustal structures in the Upper Rhine Graben using local earthquake tomography, gravimetry and reflection seismics G.G.O. Lopes Cardozo, J.B. Edel, M. Granet Institut de Physique du Globe Strasbourg UMR 7516 CNRS/ULP, 5. Rue Rene´ Descartes, 67100 Strasbourg, France Received 12 December 2002; accepted 24 March 2004
Abstract This paper presents a case study, in which three different geophysical methods are combined to link the large-scale seismically active deformation of the basement to the deformation of the sedimentary cover in the seismically active southern end of the Upper Rhine Graben. Local earthquake tomography shows velocity contrasts related to large-scale faulting in the basement. The observed faults coincide with the discontinuities in the Bouguer anomaly map for the upper part of the basement and the sedimentary cover. Reflection seismic profiles show distinct faulting in the sedimentary cover at the position of the structures observed in the local earthquake tomography and the Bouguer anomaly maps. They provide the link of the seismogenic structures at depth and the nearsurface geology. It is shown that the structure of the southern end of the Rhine Graben is dominated by faults that have a NNESSW trend, slightly oblique to the graben axis. The presence of earthquake hypocenters nearby the faults indicate their present-day activity. r 2004 Elsevier Ltd. All rights reserved.
1. Introduction Neotectonic activity in the Upper Rhine Graben (see Fig. 1) is evident from the occurrence of earthquakes in the region and has been studied with the use of trenching and geomorphological indicators. However, the link between seismogenic structures at depth and near surface tectonics remains unclear. Historical seismicity in the graben proves the potential for large earthquakes such as the destructive 1356 Basel earthquake with an estimated magnitude of 6.5. More recent events include the Sierentz earthquake in 1980 (Ml. 4.9). Meghraoui et al. (2001) identified an 8 km long fault just south of Basel, trenching showed recent activity. Nivie`re and Corresponding author. Current address: Shell International Exploration and Production B.V. Kesslerpark 1, 2288 GS Rijswijk (ZH), The Netherlands. E-mail address: [email protected] (G.G.O. Lopes Cardozo).
0277-3791/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2004.03.016
Winter (2000) present a study that integrates the use of seismic profiles, and geomorphology. They conclude that the deformation of the sedimentary cover in the southern end of the Rhine Graben is decoupled from the basement tectonics. An analysis of speleotherms made by Lemeille et al. (1999) shows that the growth of stalagmites in the Jura Mountains south of Basel was influenced by recent seismic activity. This study is aimed at the combined interpretation of a relatively detailed P wave velocity model from local earthquake tomography (Lopes Cardozo and Granet, 2003), Bouguer gravity anomaly maps (Edel et al., 2002), and seismic reflection profiles in order to study the link between faults in the upper basement and in the sedimentary cover in the region. The Rhine Graben forms the central part of the European Cenozoic rift system (ECRIS) (Ziegler, 1992). This system crosses the European continent from the Mediterranean Sea to the North Sea. Ziegler (1992) gives a complete overview of the tectonic development
ARTICLE IN PRESS 340
G.G.O. Lopes Cardozo et al. / Quaternary Science Reviews 24 (2005) 339–346
Fig. 1. Geological map of the Upper Rhine Graben. In the southern part of the graben, Paleogene, and Neogene sediments outcrop in the Mulhouse Horst (MH). In the southern end of the study area, the Jurassic outcrops of the Jura Mountains. DB=Dannemarie Basin. AB=Allschwil Basin. BBF and L-L are the Baden Baden, and Lalaye Lubine faults that are part of the separation between the Saxothuringhian (to the NW), and the Moldanubian zones (to the SE). S-M= Ste. Marie-aux-Mines fault, which is described as a Late Variscan shear zone by Edel and Weber (1995).
of the rift system, and its relation to the collision between the Eurasian and the African plates. Sissingh (1998) discusses the Tertiary sedimentary development of the rift system. In order to fully understand the development of the Rhine Graben, it is important to go back to the Variscan structural grain of Central Europe. The pre-rift history of the Rhine Graben area is closely related to the development of Variscan massifs such as the Massif Central, the Rhenish Massif and the Bohemian Massif. The development of the Rhine Graben started in the Middle to Late Eocene with graben subsidence along inherited structures under E–W orientated extension in the foreland of the Alpine orogenic belt. Thickness variations of Late Eocene sediments show a strong relationship with the Early Permian structure of the crust (Schumacher, 2002). This relationship is an indication that the initial development of
the rift was influenced by reactivation of the Variscan structures. The rifting reached a climax in the Early Oligocene under WNW–ESE orientated extension (Illies, 1978; Bergerat, 1985; Schumacher, 2002). During the Miocene, after a change in direction of the Africa-Europe convergence, the stress field in the foreland rotated towards an overall NW–SE orientated compression (Schmitt, 1981; Delouis et al., 1993; Edel et al., 2001; Schumacher, 2002; Mueller et al., 2003). With this change in stress direction, the rifting in the southern part of the Rhine Graben stopped. In the northern part, subsidence continued in pull-apart basins that were formed by this sinistral reactivation of the Graben (Illies, 1978). In the Late Miocene, the southern end of the graben was overthrusted by the Jura fold-and-thrust belt. Today, the boundary faults of the graben are reactivated in a sinistral strike slip sense under the stable NW–SE
ARTICLE IN PRESS G.G.O. Lopes Cardozo et al. / Quaternary Science Reviews 24 (2005) 339–346
compressive stress regime in northern Europe (Illies and Greiner, 1979; Bonjer et al., 1984; Bonjer, 1997; Plenefisch and Bonjer, 1997). A large scale geological map of the Upper Rhine Graben is shown in Fig. 1.
2. Geophysical techniques and observations 2.1. Local earthquake tomography In local earthquake tomography the travel time residuals of local earthquakes are inverted to obtain a 3D velocity image of the upper crust (Thurber, 1983, 1993; Eberhart-Phillips, 1986, 1990, 1993; Dorbath and Granet, 1996). This method is used to obtain relatively detailed images of the velocity structure in the upper 10 km of the crust. Local earthquakes are recorded in the Upper Rhine Graben by the French, Swiss and German permanent networks. For a detailed LET, however, a denser network of seismic stations is required. The data used for the tomography presented in this paper come from three separate campaigns, in which mobile seismic stations were used. The data set contains a total of 133 seismic events with a moment magnitude ranging from 1 to 3 (see Fig. 2). Lopes
341
Cardozo and Granet (2003) describe the used techniques and data. Fig. 3 shows the absolute P-wave velocity map, resulting from the tomographic inversion, at 2 km depth. At this depth, the velocity structure reflects mainly the difference between dense crystalline basement rocks that have a high P-wave velocity, and less dense porous sediments with low P-wave velocities. A high-velocity body (velocities up to 6 km/s) is found underneath the cities of Mulhouse and Altkirch. This feature in the velocity structure is interpreted to be related to the crystalline rocks of the Mulhouse Horst. The high-velocity body is bounded towards the west, north and east by low-velocity bodies. These low velocities are interpreted to be the signatures of the Dannemarie Basin, the Rhine Graben and Allschwil Basin, respectively, on the velocity structure. The separation between the bodies with contrasting velocities, and thus between rocks with different physical or chemical properties, is rather linear and could be related to faulting. The northwestern corner of the map shows the high velocities corresponding to the basement of the Vosges Mountains. This high-velocity body is bounded on the eastern side by the Rhine Graben western boundary faults. High velocities related to the crystalline basement of the Black Forest are found on the eastern side of the map.
Fig. 2. Digital elevation model and seismicity map of the study area. The red dots mark the instrumentally recorded seismicity for the period 1987–2001 (599 events), blue dots mark the events used in the tomographic inversion. White lines mark the boundaries of the Tertiary outcrops in the Rhine Graben, the Dele´mont basin and the northern rim of the Swiss molasse basin. The network covers the southern end of the graben, and the northern Jura, compare with Fig. 1 for the geology.
ARTICLE IN PRESS 342
G.G.O. Lopes Cardozo et al. / Quaternary Science Reviews 24 (2005) 339–346
Fig. 3. Absolute P-wave velocity map at 2 km depth. Black stars show the surface events used in this study. Note the presence of a high-velocity body underneath the Mulhouse Horst. The adjacent low-velocity anomalies are related to the Dannemarie Basin (west) end the Allschwil Basin (east). Thin black lines show the position of seismic profiles, arrows show the position of the faults on the seismic profiles. Profiles 87-08, and 85-15 are shown in Figs. 5 and 6. Mul.=Mulhouse, Alt.=Altkirch, Dan.= Dannemarie, Bas.=Basel. See text for discussion.
2.2. Bouguer gravity anomaly Gravimetric Bouguer anomaly maps represent the gravitational anomaly of the earth with respect to a reference ellipsoid and are corrected for nongeological sources of gravity variation. The corrected gravimetric anomalies reflect thickness variations of the sedimentary cover and the density contrasts due to various petrographic units in the basement. Gravimetric techniques have been used in the Upper Rhine Graben mainly to detect diapirs for salt and potassium mining. For this paper, the sediment corrected map of the first-order derivative of the gravimetric Bouguer anomaly is used to derive the structure of the basement in the region (Fig. 4). The gravimetric data used in this study are a compilation of three data sets: (1) a very dense data set made for potassium mining in Alsace (MDPA), (2) a set of French and German data collected by Rousset et al. (1993) in the framework of the ECORS project, and (3) data collected by the German SEISMOS company. Edel and Weber (1995), Edel et al. (2002) and Rousset et al. (1993) explain the used gravimetric methods and Edel et al. (2002) describe the gravimetric data set and the processing. The rough interpretation of the Bouguer gravity anomaly map presented in Fig. 4, shows a general
NNE–SSW orientation of the structures in the southern end of the graben. The NW corner of the map in Fig. 4 shows a gravimetric contrast that is interpreted to be related to the eastern border of the Vosges Mountains. Two parallel trending faults are shown to the west, and to the east of the line Mulhouse–Altkirch. Another fault is shown west of the city of Basel. Edel et al. (2002) state that these faults were active as sinistral strike–slip sense during the Variscan convergence. Parallel wrench faults can be observed in outcrops in the Vosges mountains (Ste. Marie-auxMines Fault in Fig. 1). These faults played a major role during the Tertiary tectonics and sedimentation (Edel et al., 2002). 2.3. Reflection seismic sections Reflection seismic profiles are widely used for exploration in the hydrocarbon industry. The Rhine Graben was thought to have hydrocarbon potential and therefore high-quality industry seismic lines are available, which image the upper 3 km of the crust. Fig. 3 shows the P-wave velocity image at 2 km depth, with the positions of the seismic lines superposed. The reflection seismic profile shown in Fig. 5 crosses the western side of the Mulhouse Horst (see Fig. 3 for position). The faulting that is shown in the profile forms
ARTICLE IN PRESS G.G.O. Lopes Cardozo et al. / Quaternary Science Reviews 24 (2005) 339–346
343
Fig. 4. Sediment corrected map of the first order derivative of the gravimetric Bouguer anomaly, which images the structures in the sedimentary cover and the upper basement. The basement faults are derived from Edel et al. (2002). Note that the positions of the structures interpreted by Edel et al. (2002) coincide with the zones of faulting observed in the tomography at 2 km depth (Fig. 3).
the separation between the Mulhouse Horst and the Dannemarie Basin. This fault zone, known as the Illfurth fault, was known from previous studies (Nivie`re and Winter, 2000) and is thought to have been reactivated recently. The Dannemarie Basin is observed on the western side of the line and the Mulhouse Horst on the eastern side. A steep westward dipping normal fault is observed. Differential movement on the minor faults within the fault zone suggests a strike–slip activation of the fault. The folding of the sediments could be an indication of a later compressional phase. Fig. 6 shows a seismic profile that crosses the eastern border of the Mulhouse Horst (Fig. 3). The profile shows faulting that is located on the velocity contrast observed in tomography. The steep faults form the western boundary of an eastward dipping tilted block. On top of this block a sedimentary basin, known as the Allschwil Basin, has developed. The tilting of the block is thought to have resulted from the movement on the eastern Rhine Graben boundary fault, situated to the east. The observed faulting helps to accommodate the tilting of the block in an oblique strike–slip setting and does not have a strong normal component as observed on the Illfurth fault (Fig. 5).
3. Discussion Comparison of the map view images of the LET and the presented Bouguer anomaly map show a general coincidence of the position of the zones of increased gravimetric gradient, interpreted in Fig. 4, and the sharp velocity contrasts in the tomography at 2 km depth. The N35 orientated structures, west and east of the line Mulhouse-Altkirch, correspond to the Illfurth and Sierentz faults as interpreted in the tomography. The orientation of these structures differs from the overall N10 orientation of the axis of the Upper Rhine Graben. Edel and Weber (1995) describe structures with similar orientations in the Vosges as Late Variscan wrench faults. The velocity model and the gravimetry map show sharp contrasts in velocity and gravimetry, respectively, that are interpreted to be related to faulting. Both methods, however, do not show the exact position of the faults, since there is not one unique faultrelated value for both properties. The exact position of the faults can be constrained with the use of the seismic profiles. The seismic profiles presented in Figs. 5 and 6 show clearly that the discontinuities in the velocity structure at 2 km depth coincide with faulting (Fig. 3). The projection of faults interpreted on four seismic lines,
ARTICLE IN PRESS 344
G.G.O. Lopes Cardozo et al. / Quaternary Science Reviews 24 (2005) 339–346
Fig. 5. Seismic profile (number 87-08) situated west of the line Mulhouse-Altkirch upper image shows the interpreted profile. Clear normal faulting is observed, this fault is described as the Illfurth fault by Nivie`re and Winter (2000). This normal faulting separates the sedimentary infill of the Dannemarie Basin in the west and the Mulhouse Horst in the east. This fault zone is known as the Illfurth fault. Interpreted horizons: green (J) the base of the Jurassic, in blue (R) the Middle Rupelian. See Fig. 3 for position.
onto the velocity image for a depth of 2 km shows that the orientation of the faults observed in the seismic profiles is similar to the orientation of structure interpreted on in the velocity model, and gravimetry. To show the depth extent of the structure derived from Fig. 3 and the seismic profiles, a cross section through the velocity model is combined with the two seismic lines in Fig. 7. The cross section shows low P-wave velocity bodies that are related to the sedimentary infill and fractured rocks of the Dannemarie and Allschwil Basins. The two low-velocity bodies are separated by a high-velocity body that is interpreted to be associated to the crystalline rocks of the Mulhouse Horst. It can be seen that earthquake hypocenters are situated on the sharp velocity contrast between the Mulhouse Horst and the Allschwil Basin. These seismic events are the recorded aftershocks of the Sierentz earthquake in 1980. The focal mechanism of the main event showed sinistral strike slip movement on a NNE–SSW orientated plane (Rouland et al., 1980). This is an indication that this fault is seismically active.
4. Conclusions The combination of different geophysical tools can be used to study the configuration of large-scale faults in the upper basement and sedimentary cover of the Upper Rhine Graben. Although the geophysical methods are divers in precision and scale, a coherent image of the structural geology can be derived. The combination of the observed structures and the location of seismic events show that some of the observed structures are active today. The interpretation of the velocity images obtained by local earthquake tomography shows the same orientation of structures as the interpreted Bouguer anomaly map. The presence of faults and their orientations is confirmed independently by the observations on the seismic profiles. Also in the vertical plane, the combination of the velocity model and the seismic profiles show similarity in structure. This study shows that the internal faults of the southern end of the Upper Rhine Graben have an orientation oblique to the axis of the graben. The internal structure of the southern end of the Upper Rhine Graben can be described as two
ARTICLE IN PRESS G.G.O. Lopes Cardozo et al. / Quaternary Science Reviews 24 (2005) 339–346
345
Fig. 6. Seismic profile (number 85-15) situated east of the city of Altkirch, upper image shows the interpreted profile. The strike-slip fault zone (described as the Sierentz Fault in this study) on the western side of the profile has steep faults, and the blocks in the upper part seem to differentially offset along the different faults. Towards the east, an eastward dipping basement block is seen, on top of which the Allschwil Basin has developed. Interpreted horizons: green (J) the base of the Jurassic, in blue (R) the Middle Rupelian. See Fig. 3 for position.
Fig. 7. A comparison between the two seismic lines presented in Figs. 5 and 6, and a profile through the tomography at the same position as the seismic lines. The positions of the faults in the seismic profiles correspond with the sharp velocity contrasts between the velocities in the Mulhouse Horst (in the middle of the profile) and the low velocities of the adjacent Dannemarie (west) and Allschwil (east) basins. Note the seismic events (black stars), aftershocks of the Sierentz earthquake, on the Sierentz Fault.
ARTICLE IN PRESS 346
G.G.O. Lopes Cardozo et al. / Quaternary Science Reviews 24 (2005) 339–346
sedimentary basins (the Dannemarie and Allschwil Basin), separated by a crystalline basement elevation (the Mulhouse Horst). This horst is bounded on two sides by faults. Post sedimentary compressional structures prove distinct activity of the observed faults after the opening of the graben and small earthquakes on the Sierentz Fault show its present-day activity.
Acknowledgements Gideon Lopes Cardozo and Michel Granet are part of the ENTEC network, funded by the EU, HPRN-CT2000-00053. Thanks are due to Marc Schaming and Enterprise Oil for the seismic sections presented in this study. The earthquake recording and the processing of the gravimetric data was conducted in the framework of the GeoFrance3D project. Marcus Schumacher, and Charles Gumiaux are kindly thanked for their concise review of the manuscript. References Bergerat, F., 1985. De´formations cassantes et champs de contrainte Tertiaires dans la plate forme Europe´enne. Ph.D. Thesis, Universite´ Pierre et Marie Currie Paris. Bonjer, K.-P., 1997. Seismicity pattern and style of seismic faulting at the Eastern borderfault of the Southern Rhine Graben. Tectonophysics 275, 41–69. Bonjer, K.P., Gelbke, C., Gilg, B., Rouland, D., Mayer-Rosa, D., Massinon, B., 1984. Seismicity and dynamics of the Upper Rhine Graben. Journal of Geophysics 55, 1–12. Delouis, B., Haessler, H., Cisternas, A., Rivera, L., 1993. Stress tensor determination in France and neighbouring regions. Tectonophysics 221, 413–437. Dorbath, C., Granet, M., 1996. Local earthquake tomography of the Altiplano and the Eastern Cordillera of northern Bolivia. Tectonophysics 259, 117–136. Eberhart-Phillips, D., 1986. Three-dimensional velocity structure in northern California Coast Ranges from inversion of local earthquake arrival times. Bulletin of the Seismological Society of America 76 (4), 1025–1052. Eberhart-Phillips, D., 1990. Three-dimensional P and S velocity structure in the Coalinga Region, California. Journal of Geophysical Research 95 (B10), 15,343–15,363. Eberhart-Phillips, D., 1993. Local earthquake tomography: source regions. In: Iyer, H.M., Hirahara, K. (Eds.), Seismic Tomography. Chapman & Hall, London, pp. 613–642 (Chapter 22). Edel, J.B., Weber, K., 1995. Cadomian terranes, wrench faulting and thrusting in the central Europe Variscides: geophysical and geological evidence. Geologische Rundschau 84, 412–432.
Edel, J.B., Dubois, D., Marchant, R., Hernandez, J., Cosca, M., 2001. La rotation mioce`ne infe´rieur du block corso-sarde. Nouvelles contraintes pale´omagne´tiques sur la fin du movement. Bulletin de la Societe Ge´olgique de France 172, 275–283. Edel, J.B., Lutz, H., Elsass, P., 2002. Le socle Varisque et les tectoniques Rhe´nanes dans le Fosse´ Rhe´nan meridional; traitement et interpre´tation de la carte gravime´trique du fosse´ a` partir du leve´ haute densite´ de MDPA. Geologie de la France 3, 43–59. Illies, J., 1978. Neotektonik, geothermale Anomalie und Seismizita¨t im Vorfeld der Alpen. Oberrheinische Geologische Abhandlungen 27 (1/2), 11–31. Illies, J.H., Greiner, G., 1979. Holocene movements and state of stress in the Rhinegraben rift system. Tectonophysics 52, 349–359. Lemeille, F., Cushing, M., Carbon, D., Grellet, B., Bitterli, T., Flehoc, C., Innocent, C., 1999. Co-seismic ruptures and deformations recorded by speleotherms in the epicentral zone of the Basel earthquake. Geodinamica Acta 12 (3–4), 179–191. Lopes Cardozo, G.G.O., Granet, M., 2003. New insight in the tectonics of the Southern Rhine Graben-Jura region using local earthquake seismology. Tectonics 22 (6), 1078. Meghraoui, M., Delouis, B., Ferry, M., Giardini, D., Huggenberger, P., Spotke, I., Granet, M., 2001. Active normal faulting in the Upper Rhine Graben and paleoseismic identification of the 1356 Basel earthquake. Science 293, 2070–2073. Mueller, B., Reinecker, J., Heidbach, O., Fuchs, K., 2003. The 2003 release of the World Stress Map. Available online at www.worldstress-map.org. Nivie`re, B., Winter, T., 2000. Pleistocene northwards fold propagation of the Jura within the southern Upper Rhine Graben; seismotectonic implications. Global and Planetary Change 27, 263–288. Plenefisch, T., Bonjer, K.P., 1997. The stress field in the Rhine Graben area inferred from earthquake focal mechanisms and estimation of frictional parameters. Tectonophysics 275, 71–97. Rouland, D., Haessler, H., Bonjer, K.P., Gilg, B., Mayer-Rosa, D., Pavoni, N., 1980. The Sierentz Southern Rhinegraben earthquake of July 15, 1980. Preliminiary results. In: Proceedings of the 17th Assembly of the European Seismological Commission Budapest, pp. 441–446. Rousset, D., Bayer, R., Guillon, D., Edel, J.B., 1993. Structure of the southern Rhine Graben from gravity and reflection seismic data (ECORS-DECORP program). Tectonophysics 221, 125–135. Schmitt, T., 1981. The West European stress field: new data and interpretation. Journal of Structural Geology 3 (3), 309–315. Schumacher, M.E., 2002. Upper Rhine Graben: role of preexisting structures during rift evolution. Tectonics 21 (1), 1–17. Sissingh, W., 1998. Comparative Tertiary stratigraphy of the Rhine Graben, Bresse Graben, and Molasse Basin: correlation of Alpine foreland events. Tectonophysics 300, 249–284. Thurber, C.H., 1983. Earthquake locations and three-dimensional crustal structure in the Coyote Lake area, central California. JGR 88, 8226–8236. Thurber, C.H., 1993. Local earthquake tomography: velocities and Vp/Vs—theory. In: Iyer, H.M., Hirahara, K. (Eds.), Seismic Tomography. Chapman & Hall, London, pp. 563–580 (Chapter 20). Ziegler, P.A., 1992. North sea rift system. Tectonophysics 208, 55–75.