TECTONICS, VOL. 15, NO. 5, PAGES 1036-1064, OCTOBER 1996 Geophysical-geological transect and tectonic evolution of the Swiss-Italian Alps S. M. Schmid,10. A. Pfiffner,2 N. Froitzheim,1 G. Sch6nborn,3 and E. Kissling4 Abstract. A complete Alpine cross section inte- relatively strong lower crust and detachmentbetween grates numerous seismicreflection and refraction pro- upper and lower crust. files, across and along strike, with published and new field data. The deepest parts of the profile are con- strained by geophysicaldata only, while structural fea- Introduction tures at intermediate levelsare largely depicted accord- ing to the resultsof three-dimensionalmodels making Plate 1 integrates geophysicaland geologicaldata useof seismicand field geologicaldata. The geometryof into one singlecross section acrossthe easternCentral the highest structural levels is constrainedby classical Alps from the Molasse foredeep to the South Alpine along-strikeprojections of field data parallel to the pro- thrust belt. The N-S section follows grid line 755 of nounced easterly axial dip of all tectonic units. Because the Swisstopographic map (exceptfor a southernmost the transect is placed closeto the westernerosional mar- part near Milano, see Figure i and inset of Plate 1). gin of the Austroalpinenappes of the Eastern Alps, it As drawn, there is a marked difference between the tec- contains all the major tectonic units of the Alps. A tonic style of the shallower levels and that of the lower model for the tectonic evolution along the transect is crustal levels. This difference in style is only partly real. proposedin the form of scaledand area-balancedprofile The wedgingof the lower crust stronglycontrasts with sketches.Shortening within the Austroalpinenappes is the piling up and refolding of thin flakesof mostly up- testimonyof a separateCretaceous-age orogenic event. per crustal material (the Alpine nappes),particularly West directed thrusting in theseunits is related to west- in the central portion of the profile. This contrast is ward propagationof a thrust wedgeresulting from con- probably the most spectacular and unforeseenresult of tinental collision along the Meliata-Hallstatt Ocean fur- recent seismicinvestigations in the frameworkof the Eu- ther to the east. Considerable amounts of oceanic and ropeanGeotraverse (EGT) and the National Research continentalcrustal material were subducted during Ter- Program on the Deep Structure of Switzerland(NFP tiary orogeny,which involved some500 km of N-S con- 20). Partly, however,this differencein stylereflects the vergencebetween Europe and Apulia. Consequently, different types of data usedfor compilingthis integrated only a very small percentage of this crustal material crosssection. Upper crustal levels have been drawn on is preserved within the nappes depicted in the tran- the basis of projected surface information, locally con- sect. Postcollisionalshortening is characterizedby the strainedby the resultsof geophysicalmodeling (parts simultaneousactivity of gently dipping north directed of the northernforeland and the Penninicnappes). The detachmentsand steeply inclined south directed detach- geometryof the lower crustal levels,on the other hand, ments,both detachmentsnucleating at the interfacebe- relies entirely on the results of deep seismicsoundings tweenlower and uppercrust. Largescale wedging of the which have a different scale of resolution. As a result, Adriatic (or Apulian) lowercrust into a gapopening be- lower and upper crustal levels may look more different tween the subducedEuropean lower crust and the pile in style and therefore less related than they probably of thin uppercrustal flakes (Alpine nappes) indicates a are. Deformation certainly was not plane strain within •Geologisch-Pal•iontologischesInstitut, Universit•it Basel, this N-S-section. Shortening,extension, and displace- Basel, Switzerland. ments repeatedly occurred in and out of the section. 2GeologischesInstitut, Universit•itBern, Bern, Switzerland. 3Institut de G•ologie, Universit• de Neuch&tel, Neuch&tel, Faults with a strike-slip component,such as, for exam- Switzerland. ple, the Periadriatic(Insubric) line, currently juxtapose 4 Institut ffir Geophysik, EidgenSssischeTechnische Hoch- crustal segments,the internal structuresof which may schule-HSnggerberg, Zfirich, Switzerland. have developed somewhereelse. Becausethis contribu- tion is primarily aimed at a discussionof Plate 1, it will Copyright 1996 by the American Geophysical Union. strongly focuson the cross-sectionalview. However,the Paper number 96TC00433. three-dimensionalproblem [Laubscher, 1988, 1991] will 0278-7407 / 96/96TC-00433 $12.00 be repeatedly addressed. 1036 SCHMID ET AL.- GEOPHYSICAL-GEOLOGICAL TRANSECT OF THE ALPS 1037 7øE 10ø E I I Austroalpine .•-• Molassebasin nappes Southern .•-•-•massifsexternal Alps .[,,, ß 47øN 46 ø N 50 km Figure 1. Network of seismiclines usedfor constrainingthe profile of Plate 1. Solid lines are refractionprofiles; broken lines are reflectionprofiles. The transect of plate i closely follows the EGT pro- Geophysical Data file and line E1 of the NFP 20 project(Figure 1, insetof Plate 1). The area aroundthis transectprobably rep- The solid lines labeled "crustal model along EGT" resentsthe best-investigatedpart of a collisionalorogen (Plate 1) denotethe positionof the upper and lower worldwide, both from a geophysicaland geologicalpoint boundary of the lower crust which is generally charac- of view. Geologically, its position is ideal: it follows terized by a significant increasein the P wave velocity closelythe N-S running western erosionalmargin of the and often high reflectivity. In the profile of Plate 1 the Austroalpine units of the Eastern Alps, which are miss- position of these interfaces of the lower crust is drawn ing further to the west and which almost completely after the resultsof refractionwork lye, 1992; Buness, coverlower structural units further to the east (Figure 1992] and an integratedinterpretation of both refrac- 1, inset of Plate 1). This allowsfor the projectionof tion and reflectionseismic data [Holliger and Kissling, the Austroalpine units into the profile. This procedure 1992]. This allowsthe reader to assessthe degreeof enlarges considerablythe crosssection in a vertical di- compatibility with the results obtained by the reflec- rection: in the southern Penninic units, downward ex- tion method, also displayed in Plate 1. The positions trapolation using geophysicaldata to depths of about of these interfaces depicted in Plate i do not differ sig- 60 km can be complemented by an upward projection nificantlyfrom thosegiven by Valasek[1992]. Positions reaching 20 km above sea level. where the interfaces are not well constrained by the data are indicated by broken lines. Only well-constrained seismicvelocities from Ye [1992]are indicatedin Plate Methods and Data Used for the i where they do not spatially overlap with the draw- Construction of the Integrated Cross ing of geologicalfeatures. Velocitiesof around 6.5 to Section 6.6 km s-• typicalfor the lowercrust (with a notable exeptionfor the lower crust in the northernforeland) The different parts of the crosssection have been ob- (accordingto Ye [1992]),contrast with valuesbetween tained by a variety of different construction and projec- 6.0 and 6.2 km s- • in the lowerparts of the uppercrust. tion methods. These methods, as well as the nature and Layers characterizedby lower velocitiesthan those in- quality of the geologicaland geophysicaldata, need to dicated in Plate 1, including velocity inversions,are ob- be outlined briefly for a better appreciation of the as- servedat shallowerdepths [Y e, 1992]. Low-velocitylay- sumptions underlying the construction and the nature ers(about 5.8 km s-•) arefound beneath the external of the data sources. Molassebasin, within the lower Penninic nappes, and 1038 SCHMID ET AL.: GEOPHYSICAL-GEOLOGICALTRANSECT OF THE ALPS underneaththe SouthernAlps [Mueller, 1977; Mueller agreement between the position of the lower crust of et al., 1980; Ye, 1992]at a depth of about 10 km. The the northern European foreland derived from refraction solid lines denoting the interfacesof the lower crust are work (solidlines in Plate 1) and the zoneof high re- constrainedby migrated wide-anglereflections (modi- flectivity. The reasonfor this gap in the near-vertical fied after Holli#er and Kissling[1992] and Ye [1992]). reflectionprofile is not clear, but it is unlikely to rep- The position of these reflectionsis very strongly con- resent a gap in the European Moho as postulated by trolled by seven refraction profiles oriented parallel to Laubscher[1994]. Wide-angle data from the EGT pro- the strikeof the chain (Figure 1) (seealso Holli#er and file and from severalorogen-parallel profiles (Figure 1) Kissling[1991, 1992]and Baumann[1994]). All these showstrong seismic phases from the Moho in this region profiles intersect the profile of Plate 1 and the EGT re- [Kissling,1993; Ye et al., 1995],whose position is indi- fraction profile lye, 1992];hence there is considerable catedby a solidline in Figure2a [Holligerand Kissling, three-dimensional control on the position of the reflec- 1991].By applyinga normalmove out (NMO) correc- tors. The depth migration procedure within the EGT tion, Valaseket al. [1991]displayed these wide-angle profile (almostidentical with the crosssection of Plate Moho reflections along the EGT profile in a manner 1) integratesnew data by Ye [1992]and is outlinedby commonlyused for near-verticalreflection