Deep seismic reflection profiling in the Swiss Alps

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Deep seismic reflection profiling in the Swiss Alps: Explosion seismology results for line NFP 20-EAST 0 . A. Pfiffner,* W. Frei, P. Finckh, P. Valasek Institut für Geophysik, ETH-Hönggerberg, CH-8092 Zürich, Switzerland

ABSTRACT In September 1986, a 120-km-long seismic line was recorded through the Swiss Alps. The line traverses major units involved in thin- and thick-skinned Alpine tectonics. Our preliminary interpretation indicates that (1) lithologic boundaries such as basement-cover contacts, although severely deformed during the Alpine orogeny, can be identified on the seismic sections; (2) the top of the Aar massif, an external basement massif, extends deep into the Alpine orogen; (3) the upper and lower parts of the crust are more or less transparent, but are separated by a swarm of reflectors at mid-crustal level; (4) these mid-crustal reflectors might be related to trapped fluids from Alpine metamorphism; and (5) the Moho appears as a bright reflection that steepens from the north toward the south and terminates abruptly near the center of the Alpine chain, perhaps because of Alpine deformation. INTRODUCTION The Swiss National Research Program 20 (NFP 20) includes three seismic reflection profiles with a total length of about 350 km. In the summer of 1986, a 120 km line, NFP 20-EAST, was recorded along a north-south transect across the Alps of eastern Switzerland (Fig. 1). For most of the survey, both explosions and Vibroseis were used as sources. In this paper, results from the explosion part of the experiment are presented. The Alps of eastern Switzerland consist of a pile of thrust sheets comprising Variscan basement rocks and Mesozoic and Tertiary sedimentary cover rocks. The Variscan basement of polymetamorphic pre-

Variscan crystalline rocks intruded by late Variscan granitoids is partly overlain by Carboniferous and Permian sediments (Labhart, 1977). Cover rocks belong to two mega-units, the Helvetic and the Penninic zones (Fig. 1), which form the ancient continental margin of Europe. Carbonate platform rocks are the predominant unit of the Helvetic zone (Funk et al., 1983). In the Penninic zone, NFP 20-EAST crosses thick, deep-water clastic sequences (Biindnerschiefer; Figs. 2 and 3) and platform carbonates of a former structural high (fragmented ridge or crustal block of moderate thinning; Funk et al., 1983). The Biindnerschiefer also contain lenses of ultrabasic rocks, indicating volcanic activity during basin subsidence. Basin formation occurred in an extensional regime that coincided with the opening of the Alpine Tethys ocean (now preserved in tectonic units not crossed by NFP 20-EAST). Mesozoic crustal stretching was followed by Tertiary compressional tectonics. Large portions of the sedimentary cover, including the Helvetic nappes and the Biindnerschiefer, were detached from their basement and thrust northward in this process. Basement rocks were included in the deformation at a somewhat later stage (Milnes and Schmutz, 1978; Milnes and Pfiffner, 1977, 1980) and now form a stack of basement wedges (?Aar, Adula, Tambo, and Suretta in Fig. 2). Within the Helvetic zone,

* Present address: Geologisches Institut der Universität Bern, Baltzerstrasse 1, CH-3012 Bern, Switzerland.

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Figure 1. Simplified geologic map of Switzerland showing trace of NFP 20-EAST (heavy line). Vertical crosses = Variscan basement; diagonal crosses = Tertiary Intrusives; circles = Tertiary sediments of Molasse basin and Rhine graben; dots = Helvetic zone; vertical rule = Penninic and Austroalpine zones and southern Alps; white = Mesozoic of Jura Mountains and tabular Jura. GEOLOGY, v. 16, p. 987-990, November 1988

Figure 2. Detailed map of NFP 20-EAST (dashed line) with shot points. Sawteeth on major thrust faults are on upper plate. Stipple = Variscan basement. 987


compression probably started in the late Eocene and continued into Miocene time (Pfiffner, 1986). This Tertiary phase of Alpine deformation corresponded to a continent-continent collision. It led to crustal thickening and deep burial of some units. Subsequent uplift, continuing today at 1 - 2 m m / y r (Gubler, 1976), has brought greenschist- to amphibolite-grade rocks to the surface (e.g., Frey et al., 1974). One of the exciting points of N F P 20 is that the crustal structure of a young, maybe still active orogen is explored. Moreover, N F P 20-EAST provides a north-south cross section of the major features of the Alpine edifice. As such, it enables a comparison of the major crustal-scale thrust faults as seen on seismic sections and in the field. The location of the line coincides with the European Geotraverse (EGT), adding to the volume of multidisciplinary data being collected in this region.

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Because of high ambient noise levels, dynamite shots were fired in addition to Vibroseis to provide sufficient energy for the recording of reflections from great depths, such as from the lower crust and the Moho. These dynamite shots were fired with charges ranging from 70 to 400 kg. In order to obtain a twofold coverage, shots were fired at 18 points along the line (Fig. 2); the average distance between the shotpoints was approximately 6 km. RESULTS The composite line drawing and model shown in Figure 3 are based on unmigrated time sections. Names on the horizontal axis identify in-line shotpoints. Shotpoint Santis lies 4 km northwest of the northern end of the section. In Figure 3, horizontal reflections can be traced over several shots.

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Figure 3. Top: Composite line drawing derived from individual shot gathers of NFP 20-EAST. TWT = two-way traveltime. Bottom: Preliminary geologic interpretation. 988

GEOLOGY, November 1988


In the north a series of reflections at 1 - 2 s two-way traveltime (TWT) extends up to shotpoint Sevelen, and then becomes shallower farther south. These reflections are shown in detail in Figure 4 and are interpreted as the base of the Mesozoic carbonates of the Helvetic nappes overlying Tertiary shales and sandstones of the North Helvetic flysch or Lower Marine molasse. A second series of reflections, also shown in Figure 4, can be followed from the north (at 2.5-3 s T W T ) to shotpoint Sevelen. These reflections are interpreted as autochthonous Mesozoic carbonates overlying basement rocks. South of Sevelen the series of reflections bifurcate; one branch becomes shallower at 1 s beneath Ragaz and Valens, and a second branch continues to 3 s beneath Ragaz and Valens. The upper branch of reflectors is interpreted as autochthonous Mesozoic carbonates overlying basement rocks. This contact is known to dip north between shotpoints Azmoos and Valens and south beneath shotpoint Tamins. It reaches the surface between Valens and Kunkels, forming the Vattis inlier of the Aar massif. The fact that the basement-cover contact forms a dome, rising from a depth of about 7 km (2.3 s T W T ) at Azmoos to 1 km above sea level at Kunkels, requires significant shortening within the basement (see also Pfiffner, 1985). Such shortening might be indicated by the lower branch of reflectors that continues at 3 s beneath Ragaz and Valens and that might stem from an overridden slab of Mesozoic carbonates attached to autochthonous basement, or from a basement-on-basement fault. A similar structure reflecting basement shortening seems indicated at 1 s T W T beneath Ragaz (cf. Fig. 3), but other prominent reflectors within the Aar massif cannot be identified. A dipping line of reflectors can be traced from south of Tamins ( I s ) to Zillis (5 s) and can be weakly followed to Ausserferrera (6.5 s). They are tentatively interpreted as marking the interface between the top of the southern extension of the Aar massif and the basal Penninic thrust fault. Shallow subhorizontal reflections can be traced across most of the southern half of the section (Fig. 3). They probably delineate interfaces between the various crystalline basement wedges and the cover sediments. In the Adula thrust sheet, the basement top can be observed at the surface about 25 km west of the line because of axial plunge. It forms a contact zone sliced up and imbricated between Mesozoic carbonates and a thick shaly-sandy sequence of cover sediments (Biindnerschiefer of the Misox

zone) above and amphibolite-grade gneisses and schists below (Nabholz, 1945). South of Ausserferrera, thin layers of carbonates are sandwiched between the Adula, Tambo, and Suretta basement cores. An enlargement of the Sovrana shot gather provides a detailed view of these reflectors (Fig. 5). Deep reflections include a prominent band 1 - 2 s thick that can be traced from 11 to 13 s T W T in the north to 14 to 16 s beneath shotpoint Canova, where it ends abruptly. This high-amplitude band is identified as the reflection Moho. It coincides with the refraction Moho, which is interpreted to be at this depth and to dip gently south along this segment of the profile (Miiller et al., 1980). The multi-reflector nature of the Moho is illustrated in the Tamins shot gather (Fig. 6). The discontinuation of the M o h o south of Canova is somewhat at odds with the refraction data,

Figure 5. Shot gather of Sovrana shot. Reflections at 0.7 and 2 s twoway traveltime (TWT) indicate Mesozoic carbonates between crystalline basement cores. Reflections at 3.5-5 s TWT are from amphibolitegrade crystalline basement. Coherent signal dipping north at 2 s TWT near center and extending over most of figure represents shear waves.

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Figure 4. Shot gather of Sevelen shot. All shots were recorded with 19.120 m spread length, 240 traces, 80 m geophone group spacing, and 24 geophones per group. Processing sequence applied to singleshot records included field-static and normal-moveout corrections. Reflections at 1 s two-way traveltime (TWT) indicate base of Helvetic nappes that put Mesozoic carbonates over Tertiary shales. Reflections at 2.5 s TWT indicate autochthonous Mesozoic carbonates underlying Tertiary shales and overlying crystalline basement. GEOLOGY, November 1988

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Figure 6. Detail of shot gather of Tamins shot. Band of reflectors between 12.5 and 14 s two-way traveltime dipping gently south represents Moho. 989


which require a Moho on a transverse line between Canova and Thusis at a depth of about 52 km, with a velocity discontinuity from 6.6 to 8.1 km/s. A continuation of the Moho is indicated at the southernmost end of the section by the reflectors at 14 and 15 s. The dipping reflectors at 8 - 2 0 s TWT between Crot and Sovrana would move out of the section after migration. They might, in part, originate from the Insubric fault (see Fig. 3, bottom), a north-dipping mylonite belt separating the central Alps from the southern Alps (Heitzmann, 1987). Swarms of reflectors can be recognized at mid-crustal levels (~7 s TWT) on various shot gathers, but are conspicuously absent in the central part of the line. Individual reflectors are horizontal but laterally discontinuous (Fig. 3). The crust above and below these reflectors is relatively transparent. A concentration of mid-crustal horizontal and north-dipping reflectors at 3 - 5 s TWT is prominent in the southern part of the section. Correlation of the horizontal reflectors with surface data is not straightforward, requiring extrapolation and projection over 60-90 km along strike of the Alpine chain. Projections over such distances are not reliable. However, one possibility is that these reflectors are thin slivers of carbonates and metapelites that are between amphibolite- and granulite-grade crystalline basement of the lowermost Penninic units. DISCUSSION A N D CONCLUSIONS Seismic reflection data across the Alps reveal that upper crustal lithologic boundaries such as contacts between crystalline basement and cover rocks have sufficient impedance contrast to provide traceable reflections. One of the main results is that the top of the external basement massifs extends deeper into the center of the Alpine orogen than previously thought. The top of this Variscan basement forms a large-scale dome, the Aar massif. This doming could be caused by a fault-bend fold (Suppe, 1985) on its northern flank, with a fault linking shortening within the molasse of the northern Alpine foreland to basement shortening. It could still be active because it coincides with the site of present-day uplift (Gubler, 1976). Prominent mid-crustal reflectors are observed to be deeper north of Thusis than to the south. The southern region represents an exhumed section of high-grade crustal rocks that were brought nearer to the surface by upthrusting on the southern flank of the Aar massif, bringing the mid-crustal reflectors to shallower depths compared to the north. The depth of these reflectors corresponds to where Alpine metamorphism reached upper amphibolite grade. A comparison of the reflection data with models derived from coincident refraction data (see Miiller et al., 1980, for a review) reveals that the top of the mid-crustal reflectors coincides with a drop in velocity from 6.7 to 5.9 km/s. It seems, though, that within the possibility of the resolution of refraction seismology the low-velocity zone of 5.9 km/s does extend below the mid-crustal reflectors to include the transparent part of the lower crust. Hall (1986) discussed a model whereby fluids released during high-grade metamorphism can be trapped on subsequent cooling and uplift. If we assume that water accumulated due to a particular metamorphic reaction (e.g., the devolatilization of micas) and that only small crack porosities (0.5%) are needed for a 10% variation in VP (Hall and Ali, 1985), fluid accumulations combined with lithologic layering could explain the reflectivity. The gradual disappearance of the mid-crustal reflectors in the north of NFP 20-EAST and the shallower occurrence of these reflectors in the south would reflect the degree of Alpine metamorphism undergone by these rocks. The reflection Moho can be followed continuously from the European foreland into the Alpine orogen, and then its trace is lost. Truncation of the reflection Moho coincides with the area where severe Alpine deformation is expected at the base of the crust (e.g., Pfiffner, 1985). That refraction data indicate a southern continuation of the Moho may be evidence that the physical character of the Moho changes to render it 990

transparent to reflection seismic techniques. Braile and Chiang (1986) discussed models for a Moho consisting of a layered transition zone, which produces almost no reflections in the case of a linear gradient or a smoothly varying transition in velocities. Their laterally heterogeneous transition-zone model resembles our reflection Moho with a continuous band of 1-2 s duration, consisting of laterally discontinuous individual reflectors. A layered group of mafic rocks about 2 km thick, resembling such a transition-zone model, forms the base of the crust in the Ivrea zone in the southern Alps (Zingg, 1983). Although it is not yet clear in detail how Alpine deformation changes the physical character of the Moho, our data seem to suggest that the Moho is a pre-Alpine structure and did not reestablish itself within the past 30 m.y. REFERENCES CITED Braile, L.W., and Chiang, C.S., 1986, The continental Mohoroviiic discontinuity; results from near-vertical and wide-angle seismic reflection studies, in Barazangi, M., and Brown, L., eds., Reflection seismology; a global perspective: American Geophysical Union Geodynamics Series, v. 13, p. 257-272. Frey, M., Hunziker, J.C., Frank, W., Bocquet, J., Dal Piaz, G.V., Jäger, E., and Niggli, E., 1974, Alpine metamorphism of the Alps: A review: Schweizerische Mineralogische und Petrographische Mitteilungen, v. 54, p. 247-290. Funk, H., Labhart, T., Milnes, A.G., Pfiffner, O.A., Schaltegger, U., Schindler, C., Schmid, S.M., and Trümpy, R., 1983, Bericht über die Jubiläumsexkursion "Mechanismus der Gebirgsbildung" der Schweizerischen Geologischen Gesellschaft in das ost- und zentralschweizerische Helvetikum und in das Nördliche Aarmassiv vom 12. bis 17. September 1982: Eclogae Geologicae Helvetiae, v. 76, p. 91-123. Gubler, E., 1976, Beitrag des Landesnivellements zur Bestimmung vertikaler Krustenbewegung in der Gotthard-Region: Schweizerische Mineralogische und Petrographische Mitteilungen, v. 56, p. 675-678. Hall, J., 1986, The physcial properties of layered rocks in deep continental crust, in Dawson, J.B., Carswell, D.A., Hall, J., and Wedepohl, K.H., eds., The nature of the lower continental crust: Geological Society of London Special Publication 24, p. 51-62. Hall, J., and Ali, M., 1985, Shear waves in a seismic survey of Lewisian basement: An extra control on lithological variation and porosity: Geological Society of London Journal, v. 142, p. 149-155. Heitzmann, P., 1987, Evidence of late Oligocene/early Miocene backthrusting in the central alpine "root zone": Geodinamica Acta, 1/3, p. 183-192. Labhart, T.P., 1977, Aarmassiv und Gotthardmassiv, in Sammlung geologischer Führer: Berlin, Gebrueder Borntraeger, v. 63, 173 p. Milnes, A.G., and Pfiffner, O.A., 1977, Structural development of the Infrahelvetic complex, eastern Switzerland: Eclogae Geologicae Helvetiae, v. 70, p. 83-95. 1980, Tectonic evolution of the Central Alps in the cross section St. GallenComo: Eclogae Geologicae Helvetiae, v. 73, p. 619-633. Milnes, A.G., and Schmutz, H.-U., 1978, Structure and history of the Suretta nappe (Pennine zone), Central Alps—A field study: Eclogae Geologicae Helvetiae, v. 71, p. 19-33. Müller, S., Ansorge, J., Egloff, R., and Kissling, E., 1980, A crustal cross section along the Swiss Geotraverse from the Rhinegraben to the Po Plain: Eclogae Geologicae Helvetiae, v. 73, p. 463-483. Nabholz, W.K., 1945, Geologie der Bündnerschiefergebirge zwischen Rheinwald, Valser- und Safiental: Eclogae Geologicae Helvetiae, v. 38, p. 1-119. Pfiffner, O.A., 1985, Displacements on thrust faults: Eclogae Geologicae Helvetiae, v. 78, p. 313-333. 1986, Evolution of the north Alpine foreland basin in the Central Alps, in Allen, P.A., and Homewood, P., eds., Foreland basins: International Association of Sedimentologists Special Publication 8, p. 219-228. Suppe, J., 1985, Principles of structural geology: Englewood Cliffs, New Jersey, Prentice-Hall, 537 p. Zingg, A., 1983, The Ivrea and Strona-Ceneri Zones (southern Alps, Ticino and N-Italy)—A review: Schweizerische Mineralogische und Petrographische Mitteilungen, v. 63, p. 361-392. ACKNOWLEDGMENTS The Swiss National Research Program NFP 20 is financed by the Schweizerischer Nationalfonds. The data were recorded by Prakla-Seismos and processed at ETH-Zürich. We thank Scott Smithson, Roy Johnson, and Chris Humphreys for help and advice in data acquisition and processing. Manuscript received December 21, 1987 Revised manuscript received July 5, 1988 Manuscript accepted July 2 0 , 1 9 8 8

Printed in U.S.A.

GEOLOGY, November 1988


Geology Deep seismic reflection profiling in the Swiss Alps: Explosion seismology results for line NFP 20-East O. A. Pfiffner, W. Frei, P. Finckh and P. Valasek Geology 1988;16;987-990 doi: 10.1130/0091-7613(1988)0162.3.CO;2

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