Crustal-scale structure of the southern Rhinegraben from ECORS ...

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tures mainly show strongly attenuated continen- tal crust (e.g., Atlantic margins—Peddy et al.,. 1989; Viking graben—Klemperer, 1988) or ex- tension of crust ...

Crustal-scale structure of the southern Rhinegraben from ECORS-DEKORP seismic reflection data J. P. Brun Laboratoire de tectonique, Université de Rennes, 35042 Rennes, France

F. Wenzel Geophysikalisches Institut, Universität Fridericiana Karlsruhe, Hertzstrasse 16, D-7500 Karlsruhe 21, Germany

ECORS-DEKORP team R. Blum, C. Bois, J.-P. Burg, B. Colletta, B. Damotte, H. Durbaum, H. Durst, K. Fuchs, N, Grohmann, M. A. Gutscher, M. Hübner, T. Karcher, G. Kessler, M. Klöckner, F. Lucazeau, E. Liischen, J. M. Marthelot, L. Meier, M. Ravat, C. Reichert, S. Vernassa, T. Villemin, G. Wittlinger

ABSTRACT Two seismic lines with a total length of 207 km were shot in 1988 by the French and German deep reflection seismic groups (ECORS and DEKORP) across the Rhinegraben. The southern profile shows the following characteristics. (1) The Moho discontinuity is marked by a strong reflector, except beneath the eastern part of the graben and at the western end of the profile. Its depth is only 8 s (two-way traveltime) below the graben and increases progressively to 10 s below the Lorraine basin. (2) The layered lower crust reveals strong variations in the seismic signature and apparent thickness. (3) East-dipping reflectors in the middle crust may be attributed to Variscan features. (4) The Lorraine basin is characterized by Carboniferous and Permian-Triassic strata onlapping the Vosges crystalline basement. (5) The Rhinegraben is markedly asymmetric, possibly owing to extensional movement along a listric shear zone, which appears to merge at depth with a flat-lying detachment in the ductile lower crust. The Oligocene sedimentary infill was controlled by an east-dipping normal fault whose vertical throw is about 3 km. INTRODUCTION Available deep seismic images of rift structures mainly show strongly attenuated continental crust (e.g., Atlantic margins—Peddy et al., 1989; Viking graben—Klemperer, 1988) or extension of crust previously thickened during compressional orogenesis (e.g., Basin and Range province—Allmendinger et al., 1983; Rio Grande Rift—De Voogd et al., 1988). The French Etude de la croûte Continentale et Océanique par Réflexion et Réfraction Sismique (ECORS) and the German Deutsches ¡Continentales Reflexionsseismisches Program (DEKORP) deep seismic reflection programs therefore combined to shoot a deep seismic profile across the Rhinegraben to illustrate the crustal-scale structure of a continental rift resulting from the moderate extension of a nonthickened crust. Two seismic lines were shot because geologic data indicate significant differences in the evolution and structure of the northern and southern segments of both the graben and its shoulders. The northern line has been described by Wenzel et al. (1991). A comparison between the two lines and complete interpretation will be done in the near future. GEOLOGIC SETTING The 300-km-long Rhinegraben is one of the most conspicuous and best documented basins 758

in the West European rift system (Fig. 1). It trends roughly north-northeast from Basel to Frankfurt, varying in width from 30 to 40 km. The main characteristics of the crust in this area result from the Variscan collisional orogeny (ca. 400 to ca. 325 Ma; e.g., Ziegler, 1986) followed by postorogenic extensional tectonics (ca. 320 to ca. 290 Ma; Eisbacher et al., 1989). The Lalaye- Lubine-Baden Baden shear zone (Fig. 1) is a major fault that separates the Moldanubian zone to the south from the SaxoThuringian zone to the north. It crops out as a steep, southward-dipping shear zone and shows evidence of a multiphase movement history (Wickert and Eisbacher, 1988). Basement exposures in the Black Forest and the Vosges Mountains indicate that the Moldanubian crust is composed mostly of high-grade (amphibolite to granulite facies) polymetamorphic gneisses intruded by voluminous late Carboniferous-age granitoids. The Saxo-Thuringian zone contains a thick sequence of Precambrian(?) to Visean sedimentary and volcanic rocks, which underwent Variscan folding and thrusting with a mostly northwestward vergence (e.g., Weber, 1981). Post-Variscan sedimentation was continuous from Permian to Late Jurassic time. Cretaceous and early Cenozoic-age strata are not known, possibly because of a large-scale Late Cretaceous uplift of the Rhinegraben area related to

the early Alpine compressive event (lilies, 1975). The Rhinegraben area began to subside at the end of the Eocene. Subsidence due to east-west extension was active all along the trough during Oligocene and Aquitanian time. Later, subsidence was confined to the northern part of the graben, while the southern part, along with the Vosges and Black Forest shoulders, was uplifted and eroded (Villemin et al., 1986). Considerable seismic refraction investigation has shown that the Moho shallows broadly beneath the Rhinegraben, the apex of which is located in the southern part of the rift at a depth of 24 km (Edel et al., 1975). The present-day thickness of the crust is about 30 km outside this bulge and away from the graben region. Comprehensive information on the geology, petrology, and geophysics of the Rhinegraben can be found in Rothé and Sauer (1967), lilies and Mueller (1970), and lilies and Fuchs (1974). SEISMIC LINE The southern seismic line is 139 km long and trends approximately west-northwest. Its eastern end is located in the Black Forest, where it is linked to the previous KTB 84-03 deep seismic profile (Liischen et al., 1987), 11 km east of the master fault bounding the Rhinegraben. The line has an east-west trend across the Rhinegraben and the Vosges Mountains, but then turns northwest to be nearly perpendicular to Variscan structures at its termination in Lorraine. Data Acquisition Data were acquired in September-November 1988 by Compagnie Générale de Géophysique. The 15.76 km symmetric split spread included 192 groups (eighteen 10 Hz geophones per group) with 80 m spacing. The seismic source consisted of five 13.5 t vibrators equipped with automatic force control units. Four 9-39 Hz 30 s upsweeps were carried out every 40 m. Vibration points every 80 m have been obtained by stacking two of these recordings. To prevent a GEOLOGY, v. 19, p. 758-762, July 1991

Figure 1. A: General outline of West European rift system. B: Geological and structural sketch of Rhinegraben showing location of ECORS-DEKORP lines. LL-BB = LalayeLubine-Baden Baden shear zone.

possible Vibroseis record failure beneath the graben sedimentary rock between the Black Forest and the Vosges Mountains, five explosive shots (with blasting charges of 75 and 150 kg) at a distance of 15 km were performed to obtain twofold coverage with a spread of double length. Both vibrators and dynamite shots were recorded for 40 s with a 4 ms sampling rate (SN 348, Sercel Laboratory). Neither stacking nor correlation was carried out in the field. Correlation was subsequently processed in order to obtain a 20 s record length. GEOLOGY, July 1991

Processing Processing was done by Compagnie Générale de Géophysique. Statics were computed from both Vibroseis records and refracted arrivals of the dynamite shots. Ninety-six-fold sections were compiled with a 4 ms sampling rate and 20 m between common midpoints (CMPs) down to 5 s. The whole section was processed down to 20 s with a 48-fold coverage, an 8 ms sampling rate, and 40 m between CMPs. Explosive shots were also processed with twofold coverage and were added to the Vibroseis section. These un-

dershot explosive data have greatly supplemented the Vibroseis information at depth beneath the graben, where information is particularly poor. Main Features The southern profile reveals the following features (Fig. 2). (1) The Moho is sharply defined at the bottom of a layered lower crust. It is clear under the Black Forest and is particularly bright below the Vosges Mountains. Its image is weak beneath the eastern part of the Rhinegraben 759

(CMPs 500 to 1000) and in Lorraine (CMPs 2300 to 2700 and beyond C M P 3000). Its depth changes from 8.9 s beneath the Black Forest to 8 s below the Rhinegraben and then progressively increases northwestward to 10 s at the northwestern end of the profile. Overall, the reflection Moho shows a regional and large-wavelength rise, which virtually coincides with the refraction Moho as mapped by Edel et al. (1975). However, its shape beneath the Rhinegraben shows a conspicuous asymmetry, with a possible stepwise offset in the east (Fig. 3). (2) A highly

LORRAINE 3000

reflective lower crust is well defined along the northwestern part of the profile (although it is not imaged between CMPs 2350 and 2650, and visibility is poor beyond C M P 3000) and underneath the Black Forest. Its 3-s (two-way traveltime [TWT]) mean thickness decreases progressively to less than 2 s below the graben, with some asymmetry in shape. It displays flat-lying and gently northwest-dipping short reflectors. Reflections in the lower crust are not traceable below the eastern part of the rift. (3) The middle and upper crust are comparatively transparent

VOSGES 2000

BRUCHE

VOSGES 1500

but show some southeast-dipping reflection sets. (4) In the western part of the profile, the upper part of the crust is overlain by well-defined and almost continuous reflections that correspond to upper Carboniferous, Permian, and Mesozoic sedimentary strata. Their thickness increases northwestward from 0 s ( T W T ) at the boundary of the crystalline Vosges, to 3 s at the end of the profile. Within the Rhinegraben, less continuous reflections correspond mainly to the Cenozoic sedimentary infilling and the Mesozoic prerift sequences. Tilted blocks are well defined to the

ALSACE 1000

RHIN

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-i?

- f - ^ i ^ 10

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s TWT 120 k m

Figure 2. Southern Rhinegraben profile. A: Seismic data. Section includes information coming from explosive shots beneath graben. Pull-down effect of graben's Tertiary sediments has been removed by megastatic correction, accounting for inversion of topographic surface. B: Interpretative hand-made line drawing. C = Carboniferous, P = Permian, TR = Triassic, J = Jurassic. Dotted area in Rhinegraben indicates Tertiary sedimentary infill. Heavy line at bottom of laminated lower crust shows Moho. C: Plot of coherence measured on stacked section. For each sample, coherence is computed for 19 values of dip within range of ±0.25 s/km (±45° for velocity of 6 km/s) across window of 15 seismic traces. Samples having coherence value larger than 0.5 for any value of dip are represented by black pixel. 760

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west (Fig. 4), and to the east. (5) A tions occur within eastern part of the

a rollover anticline is present few southeast-dipping reflecthe upper mantle beneath the rift.

INTERPRETATION CrustaJ Structures Between CMPs 2000 and 2500, the midcrustal reflectors, with an apparent southeast dip, are truncated at the top of the basement. They can be related to Variscan features with some confidence. In effect, they indicate an overall southeast dip of lithological a n d / o r thrust boundaries, consistent with the bulk vergence of the Variscan belt in this area (e.g., Ziegler, 1986; Eisbacher et al., 1989). Gabbroic plutons, supposedly responsible for large positive magnetic and gravity anomalies in

Lorraine (Edel, 1982), should be roughly southeast-dipping bodies. The Lalaye-LubineBaden Baden shear zone, which we expected would be present in the middle crust beneath the rift, is difficult to correlate to any particular reflector. Lorraine Basin From the western Vosges boundary to the northwestern termination of the profile, the Lorraine basin consists of gently west-dipping, upper Carboniferous, Permian, and Triassic rocks with monoclinal structures. Carboniferous deposits wedge out east of C M P 2000, whereas Permian-Triassic series onlap the Variscan basement eastward. The thickness of the Carboniferous series increases rapidly west of C M P 3100. At the very northwest end of the profile, the

upper Carboniferous and Lower Permian deposits are cut by a steep, west-dipping reverse fault. Its vertical throw is about 0.8 s ( T W T ) in the Carboniferous. This fault, which was active until the end of Early Permian time, is covered by Upper Permian strata. Between CMPs 2700 and 2000 and below the main set of gently west-dipping strata, discontinuous but rather strong reflectors (at about 2 s T W T ) may also be attributed to sedimentary strata of early Carboniferous age. Rhinegraben The profile cuts obliquely (Fig. 1) across the southern end of the Saverne fault system (A in Fig. 5). Normal faults cutting Triassic strata at the surface are probably connected with the southeast-dipping reflectors of the middle crust.

Figure 3. Enlargement of Rhinegraben part ot profile (see location in Fig. 2B). Moho offset, from 8.5 s below Black Forest to 8 s below axial part of graben, occurs in east between CMPs 900 and 500.

1300

I

1250

l

1£00

i

11 SO

l

1100

1

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I

-0.0

Figure 4. Enlargement of western part of profile showing tilted faulted blocks (see location in Fig. 2B).

2.0-

-

2.0

s TWT

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Figure 5. Interpretative cross section of Rhinegraben. See explanation in text. Ruled area indicates prerift sedimentary series.

The Rhinegraben shows a general rollover structure controlled by a steep master fault in the west. Vertical throw along this fault can be estimated at about 3 km, taking into account the erosion of the western shoulder. This fault appears to be connected with a low-angle dipping fault (possibly inherited from Variscan structures) at the bottom of the middle crust. The resulting listric master fault most probably crosscuts (B in Fig. 5) the Saverne fault system at depth. The western one-third of the graben is characterized by synthetic faults and tilted blocks (C in Fig. 5). The rollover anticline in the eastern part is accommodated by a system of conjugate normal faults ( D in Fig. 5) that are comparable to those produced by listric geometry in sandbox experiments (e.g., McClay and Ellis, 1987). The 0.5 s ( T W T ) offset of the Moho observed in the eastern part of the graben (Fig. 3) coincides geometrically with the continuation at depth of the western master fault. However, the absence of clear dipping reflections within the bottom of the crust (E in Fig. 5) makes it difficult to delineate a continuous listric fault from the surface down into the mantle (Wernicke, 1985). This fact, added to the asymmetrical thinning of the lower crust, suggests ductile behavior of the lower crust. If the relative two-way traveltime thicknesses of the upper (5 s) and lower (4 s) crusts of the Vosges and Black Forest are taken as references, there is an apparent 1.5 s thickening of the upper crust beneath the rift (F in Fig. 5) and the Saverne area (E in Fig. 5). This may suggest that the thinning of the laminated lower crust results not only from stretching but also from a loss of reflectivity of the top of the lower crust. If we assume a volume conservation of the crust, the large-wavelength rise of the Moho (Fig. 2) and the associated rift structure imply a 12 km extension. This crustal thinning does not seem to be entirely a result of rifting, because the crustal extension deduced from the restoration of the graben structure is only 4 - 5 km. This discrepancy suggests that the Moho geometry 762

results at least in part from an event distinct from the rift extension or that crustal stretching and thinning are not directly related. W e will examine this problem after further processing of the seismic data. Our observations suggest that the fault pattern defined in the upper and middle brittle crust is accommodated by more diffuse deformation in the lower ductile crust. From a kinematic point of view, the Rhinegraben is mainly controlled by a listric movement zone resulting from the connection between a steeply dipping normal fault in the upper crust and a flat-lying detachment zone within the lower ductile crust. CONCLUSIONS 1. The Moho discontinuity and layered lower crust are clearly imaged along most of the southern Rhinegraben E C O R S - D E K O R P profile. Although there is a lack of reflections at depth beneath the eastern half of the rift, the data indicate a clear asymmetry in the overall structure. This asymmetry is associated with a steeply dipping western master fault, which merges at depth with possibly preexisting Variscan lowangle, east-dipping shear zones. 2. The surface asymmetry of the southern Rhinegraben reflects the asymmetry of structures and deformations at deeper crustal levels. Although the data indicate a 0.5 s stepwise offset of the Moho in the eastern part of the graben, the lack of clear reflections within the lower crust does not allow us to delineate a continuous listric fault from the surface to the mantle. 3. Part of the large-wavelength rise of the Moho results from the rifting process. 4. Reactivation of old crustal structures by extensional deformation probably controlled the geometry of the rifting process. REFERENCES CITED Allmendinger, R.W., Sharp, J.W., Vontish, D., Serpa, L., Kaufman, S., Oliver, J., and Smith, R.B., 1983, Cenozoic and Mesozoic structure of the eastern Basin and Range province, Utah, from COCORP seismic reflection data: Geology, v. 11, p. 532-536. Printed in U.S.A.

De Voogd, B., Serpa, L., and Brown, L., 1988, Crustal extension and magmatic processes: COCORP profiles from Death Valley and Rio Grande rift: Geological Society of America Bulletin, v. 100, p. 1550-1567. Edel, J.B., 1982, Le socle varisque de l'Europe moyenne: Apports du magnétisme et de la gravimétrie: Bulletin des Sciences Géologiques, v. 35, p. 207-224. Edel, J.B., Fuchs, K., Gelbke, C., and Prodehl, C., 1975, Deep structure of the Rhinegraben area from seismic refraction investigations: Journal of Geophysics, v. 41, p. 333-356. Eisbacher, G.H., Luschen, E., and Wickert, F., 1989, Crustal-scale thrusting and extension in the Hercynian Schwarzwald and Vosges, Central Europe: Tectonics, v. 8, p. 1-21. lilies, J.H., 1975, Recent and paleo-intraplate tectonics in stable Europe and the Rhinegraben Rift system: Tectonophysics, v. 29, p. 251-264. lilies, J.H., and Fuchs, K., editors, 1974, Approaches to taphrogenesis: Stuttgart, Germany, Schweizerbart, 460 p. lilies, J.H., and Mueller, St., editors, 1970, Graben problems: Stuttgart, Germany, Schweizerbart, 316 p. Klemperer, S.L., 1988, Crustal thinning and nature of extension in the northern North Sea from deep seismic profiling: Tectonics, v. 7, p. 803-821. Lüschen, E., Wenzel, F., Sandmeier, K.J., Menges, D., Rühl, T., Stiller, M., Janoth, W., Keller, F., Söllner, W., Thomas, R., Krohe, A., Stenger, R., Fuchs, K., Wilhelm, H., and Eisbacher, G., 1987, Near-vertical and wide-angle seismic surveys in the Black Forest, SW Germany: Journal of Geophysics, v. 62, p. 1-30. McClay, K.R., and Ellis, P.G., 1987, Geometries of extensional fault systems developed in model experiments: Geology, v. 15, p. 341-344. Peddy, C., Pinet, B., Masson, D„ Scrutton, R., Sibuet, J.C., Warner, M.R., Lefort, J.-P., and Schroeder, I.J. (BIRPS and ECORS), 1989, Crustal structure of the Goban Spur continental margin, Northeast Atlantic, from deep seismic reflection profiling: Geological Society of London Journal, v. 146, p. 427-437. Rothé, J.P., and Sauer, K., 1967, The Rhinegraben progress report: Landsamtes in Baden-Württemberg, Abhandlungen des geologischen, v. 6, 146 p. Villemin, T., Alvarez, F., and Angelier, J., 1986, The Rhinegraben: Extension, subsidence and shoulder uplift: Tectonophysics, v. 128, p. 47-59. Weber, K., 1981, The structural development of the Rheinisches Schiefergebirge: Geologie en Mijnbouw, v. 60, p. 149-159. Wenzel, F., Brun, J.P., and DEKORP-ECORS team, 1991, A deep reflection seismic line across the northern Rhinegraben: Earth and Planetary Science Letters (in press). Wernicke, B., 1985, Uniform sense normal simple shear of continental rifting: Tectonophysics, v. 94, p. 91-108. Wickert, F., and Eisbacher, G.H., 1988, Two-sided Variscan thrust tectonics in the Vosges Mountains, northeastern France: Geodinamica Acta, v. 2, p. 101-120. Ziegler, P.A., 1986, Geodynamic model for the Paleozoic crustal consolidation of western and central Europe: Tectonophysics, v. 126, p. 303-328. Manuscript received November 8, 1990 Revised manuscript received March 13, 1991 Manuscript accepted March 22, 1991

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