Crustal Structure and Reflectivity of the Swiss Alps from Threeв

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TECTONICS, VOL. 12, NO. 4, PAGES 911-924, AUGUST 1993

CRUSTAL STRUCTURE AND INTRODUCTION REFLECTIVITY OF THE SWISS ALPS FROM THREE-DI•NS IONAL In 1 98 6, the Swiss Nat ional SEISMIC MODELING Research Program 20 (NFP 20) i . HELVETIC NAPPES recorded approximately 120 km of multifold seismic data across the eastern Swiss Alps (Figure 1) with M. Stfiuble 1 and O. A. Pfiffner the aim of imaging structures ob- Geological Institute University of served at the surface which are Bern, Bern, Switzerland, likely to have a subsurface continu- ation and dip toward the seismic lines [Pfiffner et al. 1988, 1990a, S. B. Smithson b] . Because the plunging structures Department of Geology and have been mapped in detail, these Geophysics, University of Wyoming, data can serve as calibration for Laramie crustal reflection surveys in a relatively young orogenic belt. One additional line, crossing the main Abstract. Forward modeling based NFP 20 profile at its northern end on combined geologic surface data, (Figure 2) was recorded in the mid- borehole and laboratory data on ve- seventies by the Schweizerische locities, and seismic reflection Erd61 AG for exploration purposes data aid in understanding the seis- and was reprocessed for this study mic response in a transect through by Stfiuble. complex Alpine nappe structures. The Interpretation of the seismic synthetic seismic response from the data recorded in the Swiss Alps model compares well with the ob- represents a three-dimensional served data. The results suggest, problem because of the complex shape however, that strong three-dimen- and plunge of the various tectonic sional effects result in out-of- units thrust northward over the plane reflections and diffractions, Northalpine foreland. The tectonic as well as defocussing and scatter- style is characterized by fold-and- ing of seismic rays, thus rendering thrust structures, multifold imbri- the use of two-dimensional migration cations, and considerable axial techniques questionable. The model plunges attaining up to 30 ø (see shows an unexpected rapid change in also companion paper by Litak et al. internal structure of the Helvetic [this issue]) . nappes between the seismic line and The purpose of this paper is to outcrops located 6-8 km farther give a three-dimensional interpreta- west. Moreover, the basement-cover tion of the reflection profiles interface on the northern flank of which cross the Helvetic nappes and the Aar Massif, a basement uplift its underlying units, using results with a heave of 8 km, does not rise from three-dimensional ray tracing regularly in the hanging wall of a and modeling software of SIERRA single major thrust fault, but rises Geophysical Inc. in a series of steps due to several thrusts and folds. The northernmost structures appear to be fault-relat- GEOLOGIC FRAMEWORK ed open folds, whereas tight fold structures appear to dominate in the south. The Helvetic Alps of eastern Switzerland have long been the aim of detailed studies. Here only a 1Also at Institute of Geophysics, general outline of the geology is ETH ZUrich, Switzerland. given; for detailed information, the reader is referred to Oberholzer [1933] , Tr•mpy [1969] , Schmid Copyright 1 9 93 by the [1975], Pfiffner [1978, 1981, 1986], American Geophysical Funk et al. [1983], and Pfiffner et Union. al. [1990a, b] . The study area Paper number 93TC00378. (Figure 1) lies within the Helvetic 0278-7407/93/93TC00378510.00 zone and is south of the Molasse 912 St•uble et al.' Three-Dimensional Seismic Modeling, Helvetic Alps

......

...... ß +

+++++

+ +++

NAPPES NAPPES NAPPES MASSIFS

1- -',- INTRUSIONS MOLASSE BASIN ALPS

Fig. 1. Tectonic map of Switzerland showing the position of the seismic lines and the study area.

Basin. The major geological units Alpine nappes [ see Stauble and covered are the Infrahelvetic com- Pfiffner, 1991, and references plex, the Subalpine Molasse, and the therein] . These clastics were Helvetic Nappes (Figure 2). deposited in a peripheral foreland The Infrahelvetic complex [Milnes basin, the Molasse Basin, at the and Pfiffner, 1977] is the lowermost close of the Alpine collision. The unit and consists essentially of study area covers the buried tran- crystalline basement rocks sition between the Flysch and (including the Aar Massif basement Molasse sediments. uplift ) and its Mesozoic to The Helvetic Nappes overly the Oligocene cover sediments. The cover Infrahelvetic complex to the south contains thick carbonate sequences and the Subalpine Molasse to the (notably Triassic dolomites and Late north. In the cross section pre- Jurassic-Cretaceous limestones) in- sented here, the Santis thrust sepa- terlayered with shaly and sandy rates the Helvetic Nappes into two clastics. The youngest sediments nappe complexes [Pfiffner, 1981]: belong to an Eocene-Oligocene the Lower Glarus nappe complex and Flysch. The general structure of the the Upper Glarus nappe complex, Infrahelvetic complex comprises a which is also called Santis Nappe. combination of folding and thrusting The Lower Glarus nappe complex con- at the interface between basement sists of a series of imbricate and cover rocks [Pfiffner et al. thrust sheets in the northern part 1990a, b, and references therein]; of the study area (Figure 3a) . thrust faults with considerable dis- Toward the south this structural placements and ductile folding at style gives way to folds (Figure smaller scale are particularly con- 3b). The sedimentary rocks involved spicuous within the sedimentary are of Triassic and Jurassic age cover [Pfiffner, 1978, 1985]. with approximately 600 m of thick To the north the Subalpine competent Upper Jurassic limestones Molasse consists of Oligocene to forming the mechanically stiff Miocene clastics which were layer. Secondary decoupling between imbricated and overridden by the these limestones and Triassic St•uble et al.' Three-Dimensional Seismic Modeling, Helvetic Alps 9]3

MODELING BASE S'•ntis"•' i For the case studied here we have the rare possibility to observe structures at outcrop adjacent to the profile (Figure 3) which are very likely to continue into the

ß.,Santis nappe seismic line and thus can be sampled ß ß .o by the seismic experiment. The starting model is derived from previously published cross sections and maps, two of which are Walensee given in Figure 4 . These cross sections are based on downdip projections and surface structural LowerGlarus data and were completed by structure nappe complex contour maps. The determination of the velocity structure presented a serious problem because no borehole data are available in the Alps. Several different approaches were used to nar- row down the possible range of velocities; they will now be discussed 13 in more detail. 1. Borehole data from exploration wells drilled into the Molasse Basin

Infrahelvetic complex northeast of the study area were used to determine the velocity of 200 the Jurassic and the Tertiary sediments. Lithologies can be traced confidently over large distances, and studies by Lohr [1967, 1978] indicate a clear regional trend of velocities increasing from the Fig. 2. Detailed geologic map and the location of the seismic lines. Molasse Basin toward the Alps due to diagenetic lithification and in- Key' G1, Glarus; S•, S•ntis thrust; creased overburden. For these rea- and AA and BB, traces of cross sections given in Figure 4ß Arrowsare sons,1 are theconsidered velocities to givenrepresent in Tablea major fold axes (with plunges) lower limit of a possible range of seismic velocities. 2. Seismic velocities were mea- sured in the laboratory from repre- dolomites occurs in the mechanically sentative rock samples from the weak shales and sandstones of the study area [Sellami et al., 1990] . Early and Middle Jurassic. The over- Despite the influence of porosity, lying S•ntis Nappe consists of a fluid content, pressure, tempera- folded, 1500-m-thick sequence of ture, and the high frequencies used, Cretaceous limestones (Figure 3a) . these velocities are considered to They were detached from the underly- be more reliable than the values ob- ing Jurassic limestones along the tained from the other methods. lowermost Cretaceous Palfris shales. 3. Interval velocities can be This detachment, the S•ntis thrust derived from stacking velocities, (Figure 3), produced a structural but care has to be applied in com- discontinuity between these two p!ex structures. Stacking velocities units. Particularly important for seem to increase toward the south. the seismic modeling is the 10ø-30 ø This trend, plus the average of the east-northeasterly plunge of the stacking velocities over the whole fold axes in the Helvetic Nappes area, was used as an additional (Figure 2) . constraint...... •.. :.-'• .:.;'' ':,..'.,. .•.,v ,,,,"•-::';..-"•:.:.¾' : :: :.:..•,::;:::'.' .-:--•,•,•.::...::.:.:. I

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The values ultimately chosen for A $ the modeling purpose are listed in the last column of Table 1. Whenever _ - possible, they were picked from what is the most reliable method in the particular situation. The laboratory

. . •i•!.:_•/•u,•,Ju,•,c measurements,observations, arebeing considered nearly directto be 2km / TnaSSlCL.... J...... to the best, the stacking velocities second. Using the preferred velocities, the time sections were first trans- formed to depth. In a second step NW • SE the reflections were correlated to

,•. lithologic contacts (cf. Pfiffner et al. [1990b] for details). In a third step the geologic interpretations of Crystallinebasement the two crossing seismic lines were combined with surface data. The re- Mesozoic sulting three-dimensional surfaces were contoured, digitized, and sub- Fig. 4. Previously published geo- sequently gridded into the model. logic profiles form a basis for the This method could be used for the starting model. Traces of profiles upper part of the model, the are given in Figure 2. (a) Structure Helvetic Nappes with their prominent of the Lower Glarus nappe complex; cross dip. For the lower part of redrawn and completed from TrQmpy the model cross dipping information [1969]. was not available, and construction (b) Structure of the Infrahelvetic had to rely on velocity information complex; redrawn from Pfiffner and on extrapolation of surface [1985]. data of the north dipping basement- cover contact (see Figure 4) . The complete model consists of 16 4. A seismic refraction line was layers. Figure 5 shows two slices recorded in 1986 as part of the through the model. Within the Lower European Geotraverse EGT, which fol- Glarus nappe complex reflections lowed the NFP 20 profile. In 1987, from the bottom of the imbricates as an accompanying experiment to the are likely to stem from the base and NFP 20, a refraction profile was the top of the Late Jurassic (Malm) recorded in the Helvetic zone along limestones which are characterized the northern Swiss Alps [Ye et al., by high seismic velocities of 5.9 1990; Maurer, 1989] . Despite the km/s. The Glarus Thrust is not a limited resolution of these refrac- visible reflection in the seismic tion profiles and the effects of section and is used here as a model- anisotropy, the refraction data were ing tool only to define the base of included as an additional constraint the Lower Glarus nappe complex. The on the velocity spectrum. lowermost surface of the model is

Fig. 3. Structure of the Helvetic nappes along the transect of the NFP 20- EAST profile. The seismic line passes along the flank of the mountains behind the peaks visible in the photo- graphs. Key- MJ, Middle Jurassic; UJ, Upper Jurassic; Cr, Cretaceous; and S•, S•ntis thrust. (a) The Upper Jurassic limestones beneath the S•ntis thrust form an imbricate stack of thrust sheets. The thrust fault with offset T-T' is the same as given in the cross sections in Figures 4a and 10. The Cretaceous limestones above the S•ntis thrust are folded. (b) The Upper Jurassic limestones show a transition from imbricate thrusting in the NNW to folding in the SSE. The Cretaceous limestones above the S•ntis thrust are not affected by these structures. I I I I I

ß ß , ß ß

•o StRuble et al.' Three-Dimensional Seismic Modeling, Helvetic Alps 9]7

observed seismic lines NFP 20-EAST N INDUSTRY LINE S and the INDUSTRY line (Figure 2) . A I A' The reflection coefficient series ....: -..•.• :...... -...... obtained from ray tracing were con- volved with a zero-phase Ricker -5 wavelet (a good approximation to mimic the stacked section) with a

-10 r• • • center frequency of 30 Hz as found in the data. The resulting synthetic 752/240 75•205 sections were then compared with the field-recorded data. As an aid to determine from where the energy B (i.e. the ray path) originates, two NFP-20 EAST E ; B' maps were plotted: (1) Contour maps of each layer with the depth points and the arrays (Figure 6b) and (2) three-dimensional models of the re- spective layers with the rays and the common-depth-point (CDP) line -10 • • • ' (Figure 6a) .

7221228 75W228 Whenever discrepancies between Velocity (m/s) model and observed seismic data ne- cessitated changes, a new contour map was made, digitized, gridded, Fig. 5. Cross sections through the input into the model and tested final model with the individual lay- again in the manner described above. ers shaded according to the veloci- Only the position and shape of the ties. Traces of the cross sections layers were changed, the velocities are given in Figure 6. remained as originally defined as they are not well enough constrained. Starting at the top of the the boundary between the Mesozoic model, this process was repeated carbonates and the overlying with each layer until a satisfactory Tertiary Flysch (in the south) and match was achieved between the model Molasse (in the north). The base- response and the observed data. For ment-cover boundary is not modeled. this reason a complete starting The display of an intelligible model was never compiled, as every model in three dimensions proved to change in the top layer influenced be a problem because the layers of the travel time of the rays. The the Lower Glarus nappe complex starting model thus consisted of one result in an obscure mixture of layer. Only after a satisfactory lines at this scale. For this reason match of the reflection from top the display of the model was primar- layer and the observed data was ily done in two-dimensional cross achieved, was the next lower layer sections (Figure 5). In Figure 6a, added to the model. the model is displayed in three dimensions, but stripped of the top layers (i.e., the Helvetic nappes) DISCUSSION AND CONCLUSIONS in order to fully show the shape of the lowermost layer, that is, the top of the Mesozoic. Ray Tracing Results

FORWARD MODELING WITH RAY A comparison between the observed TRACING sections and the synthetic sections is given in Figures 7 and 8. Although there is a close overall Three-dimensional normal correspondence, a detailed compari- incidence ray tracing was used to son shows several minor discrepan- test the validity of the model. The cies where position and shape of the surface receiver arrays used for the synthetic reflector do not match ray tracing were those defined by perfectly with the observed data. the crooked line processing of the Several limiting factors must be 918

^1 744 748 752 756 226

222

.6.5

218

214

210

. Ragaz Fig. 6. (a) Three-dimensional display of the final model with the rays to the lowermost, that is, "Top Mesozoic" reflector. For better visibility the model is stripped of the top layers, that is, the Helvetic units. (b) Contour map of the same "Top Mesozoic" reflector with the ray endpoints (indicated by plus signs) showing the large scattering of the CDPs. Contours are in kilometers, structural highs and lows are indicated by (large) plus and minus signs. A-A' and B-B' are traces of the cross sections of Figure 5. Numbers show height in kilometers and Swiss national kilometer-grid system. StRuble et al.- Three-Dimensional Seismic Modeling, Helvetic Alps 919

N. A S COP•S 75 1L:wS17S 2L:wS27S 325 37S 4•S 47S S25 S75 625 675 7•S • 8•5• 87S, 92S• 97-q• 0

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CDPL•S SO 75 100 12S 1S0 175 200 22S 2S0 27Si 300i 32Si 350; 375; 4(X) 42Si 4S0i 47S. SO••1

5 km

Fig. 7. (a) Unmigrated stack of seismic line NFP 20-EAST, (b) Synthetic seismogram of the NFP 20 profile using three-dimensional normal-incidence ray tracing.

taken into account to evaluate-these +/-10% amounts to a variation in mismatches- depth of the order of +/- 250 m. 1. The model only contains 16 3. With this uncertainty about layers, which is obviously a the velocity, the dip of the simplification of the real contoured surfaces are situation. In view of the particular approximations only, too. However, a geologic structure and the change in dip would also result in resolution of the seismic data, we sampling the layer at different consider this to be a sufficient positions. For instance, for a approximation. reflector outcropping 7 km laterally 2. The velocity functions used in from the seismic section, a change the model are approximations. Thus, in the depth by 100 m would result for example, for a reflector at 1 s in a change in cross dip of 0.75 ø two-way time (TWT), modeled with a and an updip shift of the sampling velocity of 5 km/s, a large error of point of 30 m. 920 St•uble et al.- Three-Dimensional Seismic Modeling, Helvetic Alps

A NFP-2OEAST E CDP 0 ,ii:.i'•8L)S I . 'I 250 .I.!. .:875...... 300 3•S ..13•.37S 400 4•5 450 47S 500 S25 SSO S7S •30 G•5 GSO 675 . '•i•' '•..•.i.• ',• •,•.,.'.'• •.,..-k•.,'•..•.....•...... , ...... , • • ...... •- • • •.• -'•. •.. •...,•,•. • ...... •.•.,...... ?•..,...... • ...... • ...... • ...... '"••, ,. •.•_.•..,,...• .,.•.•...... ,•, ,.• ,•?•...•,..•+•, ....,.:• ...... •.••.• ...... ,..•,,,•;-•• ..... •..•.•,...... ,•. .•.•:..•,•...... :•"•...,• ....;:!•...... •...... •_.,•,• • ...... ,.•..... • ......

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CDP0 ;;,•.... 25i 50I . 75i 100i i•SI i50I 175i 200i 2•i •S0i 275i 300, 325i 350I 3715 400I 42S. I 450I 475i 0 CDP

5 km

Fig. 8. (a) Unmigrated stack of E-W INDUSTRY line, b) Synthetic seismogram of the E-W profile using three-dimensional normal- incidence ray tracing.

4. When processing seismic data as small structural differences can- the one-dimensional assumption that not always be modeled in detail. In a common-mid-point (CMP) is equal to Figure 6a the rays fan out of the a CDP is made. However, in a three- two-dimensional plane, showing that dimensional situation this assump- the CMP=CDP assumption is not ful- tion is not always fulfilled, and as filled. Moreover, normal-incidence a consequence, energy originating ray tracing considers only rays with from a broad area is summed into a coincident shot and receiver points single CDP. This results in a con- and therefore does not necessarily siderable smearing (or loss in reso- sample exactly the same areas as the lution) of the reflection. field experiment. Incidentally, this "smearing" works Ray path analysis shows a strong in favor of the modeling technique, three-dimensional effect resulting St•uble et al.. Three-Dimensional Seismic Modeling, Helvetic Alps 921

in out-of-plane reflections and Furthermore, out-of-plane diffrac- diffractions. The arcuate reflection tions with a small curvature could between 2.5 and 3.5 s TWT in Figure be mistakenly treated as reflec- 7b (indicated by an arrow) appears tions. The three-dimensional ray as a single strong event. Detailed tracing modeling technique used here analysis of the rays contributing to yields good control of reflector this reflection (arrow in Figure geometry and avoids many of the 6b), however, shows that it is a problems inherent to two-dimensional composite of several out-of-plane migration. reflections. Two-dimensional ray path analysis also suggests that defocussing effects from steep anticlines and GeologicModel diffractions from faults confuse the seismic image considerably [St•uble The geologic section and Pfiffner, 1991]. As an example a corresponding to the three- ray plot obtained from two-dimen- dimensional final model with the sional offset ray tracing is given best matching synthetic seismogram in Figure 9 showing the extreme ray is shown in Figure 10. It shows all scattering and defocussing for shot of the structures typical of the point Ragaz. Alpine fold-and-thrust belt. The top In addition, recording of seismic two features of the model, the data is often oblique to the geo- S•ntis Nappe and the S•ntis thrust logic structures. Consequently, con- appear as in any geologic profile. ventional migration techniques repo- The imbricates within the Lower sitioning reflections in a two-di- Glarus nappe complex are cut off in mensional plane, result in a wrong the model by the Glarus Thrust, position for reflections like the which is due to modeling limitations ones discussed above and thus result of the SIERRA program. In reality in an erroneous interpretation. the imbricate thrusts are

N RAGAZ S l

-2

-4

-5

-6

-8

-101

Fig. 9. Results from two-dimensional offset ray tracing on the northern flank of the Aar Massif (adapted from St•uble and Pfiffner [1991]). The rays are shown from shotpoint RAGAZ (see Figure 2 for location) and show the importance of defocussing and scattering in this complex structural environment. 922 St•uble et al.: Three-Dimensional Seismic Modeling, Helvetic Alps

N S

u.urassic .• Flysch -5 - Molasse

• - ' , '• ,_ :kcrystalline basement-••-•' • 3' -10 ' I ' I I 240 230 220 210 km Fig. 10. Geologic interpretation of a N-S section from the three-dimensional model (Figure 5). The trace of the cross section is given in Figure 6. The seismic line covers the southern 2/3 of the cross section (S of km 230). The steep bulge of the Mesozoic (marked with an arrow) is interpreted as a compressional feature (ramp anticline) . Note the large displacement (about 7.5 km) of the thrust fault marked T-T' (compared to less than 1 km in Figure 4a).

constrained to the Upper Jurassic points to a decrease in displacement limestones and do not extend all the along the S•ntis thrust. Such a way down to the Glarus Thrust change was expected but further [Pfiffner, 1981] . The following new eastward. East of the Rhine river geologic features emerge from this the Upper Jurassic and Cretaceous study. limestones are folded more or less The displacement along the Sennis harmonically without any sign of thrust fault (marked T-T' in thrust separation between the two Figures. 3, 4a, and 10) increases units. The results obtained from from less than 1 km as observed at this modeling suggest that the dis- outcrop 5-7 km west of the study placement along the S•ntis thrust area, to 7-8 km as obtained from vanishes laterally much sooner than modeling. Comparing the initial and anticipated. final geologic profiles (Figures 4a The basement-cover contact on the and 10), it follows that the inter- northern flank of the Aar Massif is nal structure of the imbricate at a depth of about -7.5 km beneath thrust sheets within the Lower the central part of the study area. Glarus nappe complex changes going It then rises southward in a series east. In particular, an increase in of steps due to thrusting and fold- nappe internal shortening by more ing and is exposed in the V•ttis in- extensive imbrications results in a lier situated 5 km south of the greater thickness of the whole Lower section discussed here (Figure 2). Glarus nappe complex. Since the In the model the northernmost of upper bound of this nappe complex these steps appears as a ramp anti- (the S•ntis thrust) is well con- cline (marked with an arrow in strained, it must be assumed that Figure 10), which was unexpected its lower bound, the Glarus Thrust, from the seismic field data or from must be located at a depth of -3.5 geological observations. km rather than at -2.5 km as assumed A basal thrust as shown in Figure in the initial model. This result, 4b, putting basement onto a slab of derived from three-dimensional seis- Mesozoic cover rocks, has been pro- mic modeling, places serious con- posed for this transect [Pfiffner, straints on downdip projections over 1985] as well as for a transect even moderate distances in the through the Western Alps [Guellec et Helvetic Nappes. al., 1990; Butler 1986] If present, The increase in shortening in the such a slab of Mesozoic would have footwall of the S•ntis thrust also been recorded. If, on the other St•uble et al.: Three-Dimensional Seismic Modeling, Helvetic Alps 923 hand, as suggested by Guellec et al. retained. This structural style is [1990] for the Western Alps, a low- in fact observed at the surface some angle thrust fault putting basement 50 km farther west [Pfiffner et al. onto basement is postulated, this 1990a] . contact would likely be associated with a mylonite zone. The absence of reflections in the NFP0 20-EAST profile can be explained in several Acknowledgments. The work was ways' (1) There is no such thrust funded by the Swiss National fault; (2) the thrust fault is asso- Research Program NFP 20. P. Valasek ciated with a very thin mylonite provided valuable help as well as zone (beyond the resolution of the critically reading the manuscript. seismic experiment) and therefore We thank S. MUller for continuous possibly being of little importance support. The paper benefitted (i.e., having a small displacement greatly by critical review of L. only); or (3) the thrust fault and Brown and D. Snyder. We are grateful the associated mylonite zone dip to the BEB, Hannover (D) and the steeply (and therefore not imaged Swisspetrol Holding AG for the in- seismically) and the displacement is dustry line granted in exchange with relatively small. the NFP 20 data. Thanks are extended In the geologic interpretation to A. Werthemann, I. Blaser, and W. (Figure 10) the last possibility is Schaad for technical assistance.

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