View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Bern Open Repository and Information System (BORIS) 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 + +++++ + +++ 0 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 prob- lem because no borehole data are available in the Alps. Several dif- ferent approaches were used to nar- row down the possible range of velo- cities; 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 sedi- ments. 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 sec- tions 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 ............•.;•::,:, .•'..."'"'""•'•½•-:'..:,':::::":::.?.-.:":"•'. ...:...:%..•?::;::;• ...%....-. :• %----,•'7::-..- ..... ,•. ::;::: • '"...',,:½-::•::----.•::,•¾:::;:::.•:....:.,,..:"-:":....'.:: ........ ....... ,3;{:-:.,'e •,.:,-,,,-•*- ,... :..•..,., / ß" ., ;•, ½ ½; . ,•. '•:• •:..,..; "::......... ....'•.:•... ....... :'•:=•' : ,..-:•½•;'",:•4'.:........... -.:'4: /" ¾'" ...,. .. •.•'".,"...::• - .,.•.. •-::•::4. ....... , . .......................:'"" ::::.:. St•uble et al.- Three-Dimensional Seismic Modeling, Helvetic Alps 9]5 The values ultimately chosen