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Home , Austroalpine nappes, Calanda (mountain), Engadine Line, Glarus Alps, Glarus thrust, Helvetic (geology)

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 constrained by geophysicaldata only, while structural fea- Introduction tures at intermediate levelsare largely depicted according 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, differenttypes 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 contribution 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

.[,,,

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 data (Figure Holliger [1991]and Holliger and Kissling[1991, 1992]. 2b). Hence Figure 2 clearly documentsthe continuity We choseto superimposethe resultsof reflectionseis- of the Moho beneath the northern foreland. The Euro- mic work directly onto the features arrived at by refrac- pean lower crust may safely be extended as far south as tion seismicsin order to graphically visualizethe degree beneath the northern rim of the SouthernAlps whereits of compatibility between these two data sets. Major presencehas also been recordedalong seismiclines pro- deepreflections from the El, S1, S3, and S5 lines (Fig- vided by the Italian ConsiglioNazionale delle Ricerche ure 1) havebeen converted into digitizedline drawings (CROP-Alpi Centrali) [Cernoboriand Nicolich, 1994; by a procedureoutlined by Holliger[1991]. These dig- Montrasioet al., 1994]. itized line drawings have been projected into the EGT Plate 1 alsodepicts a wedgeof Adriatic lower crust at line before migration in the N-S section. The eastward a depth of 22 to 48 km beneath the Penninic nappesand projection of data from the Si., S3, and S5 lines was nec- above the European lower crust. The solid lines denot- essary in order to complete the profile along the eastern ing the crustal model along the EGT traverse suggest transect(El), whichterminates well north of the Insub- that this wedge is continuous with the Adriatic lower tic line. The chosenprocedure for eastward projection crust beneath the Southern Alps. This simple geome- is describedby Holliger[1991] and Holligerand Kissling try may represent an oversimplificationcaused by the [1991, 1992],who arguedthat the geometryof middle low resolution of the geophysicaldata at this depth. to lower crustal material is approximately constrained First, the northern tip of this Adriatic wedge is ill- con- by Bouguergravity data (correctedfor the effectsof strained(broken line in Plate 1). Second,its internal the Ivrea body) [Kissling,1980, 1982]. This projection structure is likely to be more complicated due to imbri- procedureguided by gravity data then ultimately (i.e., cations within the wedge. The northward thickeningof after migration) leadsto the configurationdepicted in the Adriatic lower crust within this wedge cannot re- Plate 1. The procedurechosen is supportedby indepen- flect a preorogenicfeature sincethe northern extension dent evidence for the existence of a large lower crustal of the crust beneath the Southern Alps is likely to have wedgefrom the refraction work carried out along the been attenuated during passivecontinental margin for- EGT line lye, 1992]and, additionally,the compatibil- mation. According to the interpretations by Cernobori ity of the refraction-basedmodel of Ye [1992]with the and Nicolich[1994], Marson et al. [1994],and Montra- projected and migrated line drawingsof the major re- sio et al. [1994]the Moho of the Adriatic lithosphere flectors as seen in Plate 1. is wedged beneath the northern part of the Southern In a secondstep, the projected digitized line draw- Alps, rather than being continuousas depicted in Plate ings were migrated accordingto a velocity model which 1. Holliger and I(issling[1992] propose a mixture of needsno projection sinceit is based on strike-parallel predominantly Adriatic lower crust and oceanic crust profiles[Holliger, 1991]. This velocitymodel (Figure 4 within the Adriatic wedge, having a density slightly of Holligerand Kissling[1992]) is the sameas that used higher than that of "normal" lower crust. The reflec- for the migration of the wide-anglereflections discussed tions from the lower crustal Adriatic wedge shown in earlier. The picture emergingfrom this procedureshows Plate 1 crosseach other in many places. This may indi- excellent consistencybetween refraction and reflection cate discontinuitieswithin the wedge,or, alternatively, data and one geologicalfeature which can be traced to it representsartifacts causedby the projectionand/or great depth: the Insubric line. migration procedure. In view of all these uncertainties Except for a reflectivity gap beneath the internal Aar regarding the internal structure of the Adriatic lower massif and the Gotthard "massif", there is excellent crustal wedge, Plate 1 merely depicts its outlines in a. SCHMID ET AL' GEOPHYSICAL-GEOLOGICAL TRANSECT OF THE ALPS 1039

Figure 2. Summary of seismicallydetermined crustal structure and Moho depth along the transectof Plate 1. Horizontaland verticalscale are the samein both panels.(a) Migratednear- vertical reflections along the eastern traverse and generalizedseismic crustal structure derived from orogen-parallelrefraction profiles [Holliger and Kissling,1992]. Solidline indicatesposition of Moho, derived from orogen-parallelrefraction profiles; wiggly line indicates top lower crust; dotted line indicates base of Penninic and Helvetic nappes; thin solid line is the Insubric line; RRL is the Rhine-Rhoneline. (b) Normal-incidencerepresentation of the wide-angleMoho reflections in the EGT (EuropeanGeotraverse) refraction profile perpendicular to the orogenand acrossthe easternSwiss Alps [l/hlaseket al., 1991].

schematicway. It is clear from the compilation on Plate the Insubricmylonite belt [Schmidet al., 1989]. Hence 1, however,that the Adriatic wedgerepresents a zone of these reflectionsalso document a flattening of the Insub- high reflectivity, largely contained within the outlined ric mylonite belt frmn the inclination of 700 measured shape of this wedge except for some gently north dip- at the surface[Schmid et al., 1987, 1989] to about 450 ping reflectionsat a depth of 20-25 kin, slightly above at some 20 km depth. the upper boundary of the wedge as defined by refrac- Helvetic Nappes and Northern Foreland tion work (solidline belowgrid line 140 in Plate 1). Reflectionsrecorded along line S1, dipping with about The top of basement along the profile of Plate 1 is 450 to the north after migration, are related to the In- only accessibleto surfaceobservation in the V&ttis win- subtic line [Bernoulli et al., 1990; Holliger, 1991; Hol- dow (Aar massif). The geometrychosen for the struc- liger and I½issling,1991, 1992]. In Plate i thesereflec- ture of the top of basementis that of model ], discussed tions project into a surfacelocation 5-10 km north of the by StSubleand Pfiffner [1991b]. These authorsevalu- Insubric line within the northern part of the southern ated the seismicresponses of four alternative geometries steep belt, near the axia.1trace of the Cressimantiform. (models1-4) generatedby 2-D normal-incidenceand off- This southern steep belt is parallel to and related to set ray tracing with the reflectionseismic data. They 1040 SCHMID ET AL.: GEOPHYSICAL-GEOLOGICAL TRANSECT OF THE ALPS

producedthe best-matchingevents with this particular change in structural style in respect to profiles further model 1. Thrusts and folds in the Subalpine Molasseare to the west letter, 1987; Probst, 1980]. There (Luk- constructed on the basis of surface data and projected manier area) a major back fold (Chiera-synform,Fig- information obtained along a seismic profile, recorded ure4) (D3 of Etter [1987])formed south of the Gotthard for hydrocarbon exploration, situated immediately west "massif". Such back folding is even more pronounced of the EGT traverse(profile M in Figure1) [StSubleand in the southern part of the external massifsin western Pfiffner, 1991a]. Switzerland[Escher et al., 1988].This severeoverprint The structure of the Helvetic nappesis constrainedby by back folding apparently dies out eastward. the extrapolation of surfaceinformation obtained along The Gotthard "massif" is considered as a lower- the profile trace and by the results of 3-D seismicmod- most Penninic, or more exactly a "Subpenninic" nappe eling [StSubleet al., 1993]. The higher Penninicand [Milnes, 1974], in the structural sense. These Sub- Austroalpine units overlying the Helvetic nappes are penninic units also include the Lucomagno-Leventina only exposed east of the transect and have been pro- and Simano nappes, whose geometry will be discussed jected onto the profile parallel to a N 700 E azimuth later. There is a serious problem with the use of terms by using profiles published by Allcmann and Schwizcr like "Helvetic" and "Penninic" in that, for historical [1979]and NSnny [1948]. Updomingof the baseof the reasonsgoing back to Ar#and [1916],they usuallyre- Austroalpine nappes above the Aar massif corresponds fer to paleogeographicdomains and/or structuralunits. to the Priittigau half windowin map view (Figure 3). Whereasthe Lucomagno-Leventinaand Simanonappes Its geometry was obtained by the stacking of a series and, accordingto our interpretation, also the Got- of profiles acrossthe Priittigau half window [NSnny, thard "massif" may be describedas Penninic in terms 1948]. Stackingdid not use a fixed axial plungebut of structure and metamorphism, it is very likely that instead resulted from lateral correlation between indi- some of this crystalline basement representsbasement vidual profiles. The geometrythus obtained(Plate 1) to the Helvetic and Ultrahelvetic cover nappes. This is results in an average plunge of 150 to the east for the supported directly by the facies of the overturned al- culmination of the base of the Austroalpine units, in ac- lochthonous cover of the southern Gotthard "massif" cordance with the seismicallyconstrained axial plunge letter, 1987, and referencestherein], which has close of the Aar massif[Hitz and Pfiffner, 1994]. affinities with the Helvetic sediments. Use of the term "Subpenninic" helps to resolvethis dilemma. The Gotthard "Massif" and the Transition To the west of our transectthe Tavetschmassif (a into the Lower Penninit Nappes small external massifsouth of the Aar massif)repre- Very strong reflections dipping southward from 2.5 sentsthe substratum of the Helvetic Axen nappe, while to 4.0 s two-waytravel time (TWT) alongline E1 be- the Gotthard "massif" represents the substratum of tween Canova and Thusis (reflector D in Plate 4 of the higher S•intis-Drusbergnappe accordingto Pfiffner Pfiffner et al. [1990b])have beeninterpreted in Plate [1985]and Wyss[1986]. Within the transectof Plate 1, 1 (betweengrid lines175 and 190)to be due to the al- the structural separation amongst individual Helvetic lochthonous cover of the southern Gotthard "massif" nappes above the Glarus is less severe. In the profile [Etter, 1987]. The Penninicbasal thrust is placedim- consideredhere, the S•intis thrust separatesthe Juras- mediately above the inferred allochthonouscover of the sic strata of the Lower Glarus nappe complex from the Gotthard "massif" (labeled"Triassic, Lower, and Mid- Cretaceous strata of the Upper Glarus nappe complex dle Jurassiccover slices" in Plate 1) accordingto the [Pfiffner,1981]. The S/intisthrust actedas a structural modelof the Penninicunits given by Litak et al. [1993]. discontinuity separating different styles of shortening Earlier interpretations basedon north dipping reflectors within the Jurassic and Cretaceous strata. Displace- visibleunder the Gotthard "massif"(northern termina- ment acrossthe Santis thrust decreasessteadily south- tion of reflectorE in Plate 4 of Pfiffneret at. [1990b]), ward and eastward due to imbrications in the Jurassic advocatingback thrusting and/or back folding of the strata [StSubleet al., 1993]. Bed lengthmeasured in the Gotthard massif(model C of Pfiffner et at. [1990b]), UpperJurassic limestone (38 kin) is similarto that mea- are abandoned. suredin the CretaceousSchrattenkalk (33 km). Hence According to the geologicalinterpretation given in both stratigraphic levels must be assignedto the same Plate 1, the basal thrust of the Gotthard "massiv", sit- basement, contrarily to the findings further west. uated in the Urseren-Garvera zone, is steeply inclined This leads to the question if the entire Glarus nappe and would therefore not be imaged seismically. The complexhas to be rootedin the Tavetschmassif[Trfim- Penninic basal thrust is not shown to be strongly back py, 1969]or, alternatively,within the Subpenninicnap- folded but merely steepenedin Plate 1, based on the pes. We prefer the secondoption in view of the consider- geometryconstrained by 3-D seismicmodeling (Figure able difficultiesof finding appropriatevolumes of upper 3a of Litak et al. [1993]). This indicatesa substantial crustal basement material in the very small Tavetsch SCHMID ET AL.' GEOPHYSICAL-GEOLOGICAL TRANSECT OF THE ALPS 1041

Upper Austroalpine, South Alpine: I!!ii!:._jsedimentary cover (A. Dol.: Arosa dolomites) I •' •'1 basement Lower Austroalpine: •TTT• Berninasystem (Az: Albula zone, ble: blezzaun) I t Err system (Cv: Corvatsch, blu: blurtirOl)

blargna-Sella system:

Piemont-Ligurian ophiolites: 1•• blalenco-Forno- Lizun system •-•...... Plattanappe Aro= ngua•rd (m•lange)

Deeper Penninic and Helvetic units: 'x,"x, '•"• m p o'x,'x, 'x,, Tertiary intrusions: 11111

TF: Turba normal fault -I' Berge II + + (Ch): Chur IIIII. (Da): Davos (Sc): Scuol (St): St. bloritz •So•ut•he/n•A,•s •' •' •'

50 km I , , , ' 1

III Illl II IllJ Ill:::::::::::::::::::::: • I I I I I

I • • E Brianc,onnais i Malenco',Margna , Platta ,• Err Ii• Bernina Ela , Ortler

I Forno• Sella

Figure 3. Tectonic map of easternSwitzerland, modified after $chmid et al. [1990]. Circled numbersin insetrefer to ophiolite-bearingunits (solidareas) derived from the SouthPenninic or Piemont-LiguriaOcean (units labeled "1") and from the North Penninicor ValaisOcean (units labeled"2"). Numbersindicate the following:la, Arosa; lb, Platta; lc, Lizun and Avers; ld, Malenco; 2a, Chiavenna; 2b, Misox zone; 2c, ophiolites within North Penninic Bfindnerschiefer; 2d, Areue-Bruschghorn;2e, Martegnas. Profile trace refers to the the profile of Plate 1. massif. In map view, the Tavetsch massif pinches out This excessvolume is caused by some 27 km of crustal eastward and is unlikely to be encounteredin the tran- shortening postdating the detachment of the Helvetic sect of line El. The excess volume of upper crust nappes and related to imbrications in the Subalpine provided by the updoming of the Aar massif is ruled Molasse[Burkhard, 1990; Pfiffner, 1986;Pfiffner et al., out from the search for appropriate basement material. 1990b]. In order to accommodatethe 38 km bed length 1042 SCHMID ET AL- GEOPHYSICAL-GEOLOGICAL TRANSECT OF THE ALPS

SEDIMENTATION ......

DEFORMATION PHASES __ 83.0

MAIN CLEAVAGE OTHER DEFOR- FORMATION MATION PHASES 86.6

PETROLOGY

COOLING AGES DOMINATED ECLOGITE FACIES METAMORPHISM METAMORPHISM • MAGMATISM

Figure 4. Correlationtable showing an attemptto datedeformation phases and metamorphism alongthe transectof Plate 1. Timescaleis accordingto Hatlandet al. [1989].The abbreviations UMM, USM, OMM, and OSM denotethe LowerMarine Molasse,Lower Freshwater Molasse, Upper Marine Molasse,and Upper FreshwaterMolasse, respectively. See text for further expla- nations. For an extensivediscussion, see $chmid et al. [1996]. of Helvetic nappes (assumingplane strain conditions N 700 E, constructed on the basis of structure contour and an upper crustal thicknessof 15 km), an area of maps,allowed for projectionswith variableplunge (100 - about 570 km2 of upper crustalmaterial has to be lo- 35ø). Units wereprojected into the sectionalong these cated somewherein Plate 1. This suggeststhat both strike-parallelsections by assumingthat their thickness the crystalline basement of the Gotthard "massif" and doesnot changealong strike. Geologicaldetails within the Lucmnagno-Leventinanappe (occupying about 540 projected units are drawn accordingto the geometries km2 in our section)may representthis uppercrustal foundwhere these units are exposed(for a recentcom- basementof the Helvetic and Ultrahelvetic nappes. pilationof field data, see$chmid et al. [1996]). The Northern and Central Parts of the Penninic In a secondstep this part of the profile was adjusted to conform to the 3-D model based on seismic informa- Zone tion [Litak et al., 1993]. Theseadjustments were rela- In a first step, all major tectonicboundaries (Fig- tively minor at shallowerdepths and above the Adula ure 3) have been projectedstrictly parallel to a N 700 nappe. The most important modification concernsthe E direction up and down plunge. This direction ap- Misox zone, which has a considerable thickness and proximatesbest the azimuth of most large-scalefold which is shownto be continuoustoward the south,join- structuresin this region. A seriesof sectionsparallel to ing up with the Chiavennaophiolites (Figure 3) exposed SCHMID ET AL.: GEOPHYSICAL-GEOLOGICAL TRANSECT OF THE ALPS 1043 at the surface. This is in contrast to surfacegeology ex- in progress)is interpretedto connectthese lithologies posed west of the profile, where the Misox zone is cut at the base of the pluton with the Chiavenna ophio- out between the Adula and Tambo nappes due to top- lites overlying the Gruf complex along a steeply north east movementsalong the Forcolanormal fault (Figure dipping faulted contact [Schmutz,1976]. Hence this 3). In Plate I the Chiavennaophiolite is portrayedas band of ultramafics and metasediments found on top a long continuousslab, about i km thick, which caused of the Gruf complex is interpreted to be the southern high-amplitudereflections [Litak et al., 1993]. continuation of the Chiavenna ophiolite and the Misox The overallgeometry of the Adula and Simanonappes zone representingthe suture zoneof the north-Penninic followsthat givenin Figure3a of Litak et al. [1993].Or- ocean (Figure 3). Recent mappingrevealed that the namentation in the Adula nappe is based on the data granodioritediscordantly cuts through the remnantsof of LSw [1987]. A considerableamount of speculation both Tambo and Suretta nappes situated south of the led to the depicted geometryof the top of the Gotthard Engadine line. This results in the geometry shown in "massif" and the Lucomagno-Leventinaunits. While Plate 1: In profile view the Bergell pluton occupiesthe their overall position below the Penninic basal thrust structural position of the Tambo and Suretta nappes. is constrainedby the model of Litak et al. [1993],the Magmatic, submagmatic,and solid state deformational portrayed structural details are based on surface infor- fabricsin tonMite, granodiorite,and countryrocks show mation a long way west of the transect. This informa- that the Bergell pluton wasemplaced and solidifieddur- tion, whichwas taken from profilesby Etter [1987],LSw ing a regional tectonic event as originally suggestedby [1987], and Probst [1980], had to be modifiedsignifi- Wenk[1973]. This synmagmaticdeformation first pro- cantly in order to conform to the constraintsimposed by duceda very strong(locally mylonitic) fabric foundat the geometryof the Penninicbasal thrust. As seenfrom the baseof the pluton and subsequentlyled to the large- Plate 1, there is no room for additional Subpenninic scalefolds that shortenedthis contact(Plate 1). Oneof thrust sheetsbetween the Lucomagno-Leventinanappe these folds can be traced directly into the Cressim an- and the Adriatic lower crustal wedge. However, such tiform (plate 1)[Hafner, 1994;Davidson et al., 1996]. lowerunits do in fact exist alongthe S3 line [Bernoulli Initial stagesof vertical movementsalong the Insubric et al., 1990],but they are interpretedto wedgeout east- mylonite belt affected the tonalitic tail of the southern ward. Bergell pluton and were coeval with deformation in the presenceof melts [Rosenberget al., 1994]. This shows Southern Penninic Zone, Bergell Pluton, and Insubric Line that final intrusion, back folding, and initial stagesof back thrusting along the Insubric line are contempora- In the areaof the Bergell(Bregaglia) pluton [Tromms- neousand related to ongoingN-S shortening.Only the dorff and Nievergelt, 1983] the profile is basedon re- vertical,brittle Tonalefault [Fumasoli,1974] is related cent work by Rosenberget al. [1994, 1995], Berger to later strike-slip movements under brittle conditions et ,l. [1996], and Davidsonet ,l. [1996]. In its along the Insubric line. northernpart the Bergellpluton has beensynmagmat- Considerablevertical extrapolation is possiblethanks ically thrust onto the upper amphiboliteto granulite to the pronouncedaxial plungeof the Bergell pluton in- grade migmatitic rocks of the so-calledGruf complex dicatedby structural [Rosenberget al., 1994, 1995]and (Figure3) [Bucher-Nurminenand Droop,1983; Droop petrological[Reusser, 1987] data. In a first step, the and Bucher-Nurminen,1984]. The Gruf complexfinds Tambo and Suretta nappes were projected southward its direct continuationin the migmatitesforming the and upwardby usingstructure contour maps [Pfiffner et southernmostpart of the Adula nappe[Hafner, 1994], al., 1990a],partly modifiedby newfield data. At a point back folded aroundthe Cressimantiform (Plate 1) situatednear Vicosoprano, east of the transect(project- [Heitzmann,1975]. Thereforethe Gruf complex,including well abovesea level in Plate 1), the boundariesof ing a smallwindow below the Bergellpluton (Figure 3), Tambo, Suretta, and overlying nappes were displaced has to be consideredpart of the Adula nappe. vertically acrossthe Engadine line by 4 km, in accor- The quartzo-feldspaticgneisses predominating within dance with the kinematic model for this line proposed the Gruf complex are overlain by a variety of other by Schmidand Froitzheim[1993]. With this procedure lithologiesconsisting of ultramafics,amphibolites, calc- the position of the Tambo and structurally higher tec- silicates, and alumo-silicates,concentrated in an almost tonic units was anchored to their position immediately continuousband concordantlyfollowing the tonalitic south of the Engadine line. base of the Bergell pluton exposedalong the western The position of the base of the Bergell intrusion was margin[ Wenkand Cornelius,1977; Diethelm, 1989]. A evaluatedby projecting auxiliary profileslocated east of narrow antiform within the Gruf complex immediately Plate 1. This projection used structure contour maps southof the Engadineline and north of the Bergellplu- [Davidsonet al., 1996] of the base of the pluton, de- ton (Figure 3, indicatedin Plate I accordingto work formed by NE-SW striking folds. The roof of the intru- 1044 SCHMIDET AL.: GEOPHYSICAL-GEOLOGICALTRANSECT OF THE ALPS

sion was placed at the structural level presentlyexposed Summary of the Tectonic Evolution at the eastern margin of the pluton. This is a minimum altitude, since the eastern contact representsthe side rather than the roof of this pluton [Spillmann,1993; Paleotectonic Structuration Rosenberget al., 1995;Berger and Gier•, 1995]. The ge- Becausethe paleotectonicstructuration strongly in- ometry of the "Ultrapenninic" (in the senseof Tv•impy fluences the later orogenic evolution, a brief discus- [1992]) or AustroalpineMargna and Sella nappes,as sion of our current workinghypothesis is needed(see well as the continuation of the southward outwedging Froitzheimet al. [1996] for a more extensivediscus- Platta ophiolitesand the Corvatsch-Berninanappes, is sion). The paleotectonicrestoration in Figure 5 fol- drawnafter Linigev[1992] and Spillmann[1993] in Plate lowsthe traditional approachguided by stratigraphical 1. analysis and retrodeformationof nappe stacks, taking into account effects of postnappe refolding. In most Southern Alps cases[e.g., Frisch, 1979; Trdmpy, 1980; Platt, 1986]this classicalapproach leads to the postulate for the former The Southern Alps part of the sectionwas taken with- existenceof more than one paleogeographicdomain of out modificationfrom $chSnbovn[1992, crosssection sedimentsdeposited on oceaniccrust and/or exhumed B of the enclosure]except for the northernmostpart mantle (looselyreferred to as "oceanic"in this contri- where compatibility with the shape of the Insubric fault bution). However,recently, an alternativeview, inter- necessitated very minor adjustments. This section of preting the internal zonesof the Alps in terms of an $chSnbovn[1992] almost exactly coincides with N-S grid orogenicwedge formed by subductionerosion and ac- line 755, departing from a N-S orientation only south of cretion[e.g., Polino et al., 1990;Hunziker et al., 1989], E-W grid line 60 in order to incorporateborehole data has been expressed. According to this hypothesis,in- publishedby Pievi and Gvoppi[1981]. terleavingof continentaland oceaniccrustal flakes is in- The profile is balanced and retrodeformability was es- variably due to tectonic complications,only one ocean tablished at all stagesby forward modeling. The deeper beingsubducted within onetrench between Europe and parts of the profile were kept as simple as possibleand Apulia since Cretaceous times. drawn according to geometrical rules of ramp and fiat Accordingto our reconstruction(Figure 5), three geometry indicated for basementand cover by the sur- oceanicbasins did open and closeat different times in face data. The mass balance within the basement, the the Alps and the Western Carpathians: the Meliata- top of whichis constrainedby boreholedata (Plate 1) in Hallstatt, the Piemont-Liguria(or South Penninicin its undeformed portion in front of the Milan thrust belt the Swiss-AustrianAlps) and the Valais(or North Pen- and by the CROP-Alpi Centrali seismicprofile [Mon- ninic) Oceans.While remnantsof two of theseoceanic tvasioet al., 1994],is unaffectedby geometricaldetails. domains,the Piemont-Liguriaand ValaisOceans (map- The total amountof shortening(80 kin) within the sed- pedin Figure3), are foundin the formof ophioliticsliv- iments necessarilyleads to the postulate that parts of ers alongthe crosssection of Plate 1, remnantsof the the upper crustal and all of the lower crustal excessvol- Meliata-Hallstatt Ocean are only found further to the ume must now occur within the Adriatic wedgesituated east(Eastern Alps of Austria,Carpathians). However, below the Penninic nappes and the Insubric line. The becausethis oceanplayed an importantrole during Cre- volume of crustal material available south of the Insub- taceousorogeny, it needsto be brieflydiscussed. ric line is insufficient[$chSnbovn, 1992; Pfiffnev, 1992]. The Meliata-Hallstatt Oceanopened during the Mid- In our view, substantial thinning of the Adriatic lower dle Triassicin a positionsoutheast of the Austroalpine crustduring the final stagesof Jurassicrifting and conti- realm[Nozur, 1992], and it mayhave been connected to nental margin formationcannot be held responsiblefor the Vardar Ocean of the Dinarides and Hellenides. Its this volume deficit since we infer a lower plate margin suture is indicated in the sketch of Figure 5. Triassic situationfor the Apulianmargin (see later discussion). sedimentsof the Austroalpine units record the history In order to allow for a change in structural style of the shelf and passivemargin of Apulia that faced within the deeperbasement [Milano et al., 1991],duc- this ocean[Lein, 1987]. Rifting that led to the opening tile shear zoneshave been schematicallydrawn at depth. of this Triassic ocean is spatially unrelated to the Late These shear zonesare expected to merge with a major Triassicto Early Jurassicrifting leadingto the opening detachment zone situated at the interface between up- of the Piemont-Liguria Ocean which will form at the per and lower Adriatic crust. This major detachment northwesternmargin of the Apulian microplate(west- allows for the northward indentation of the Adriatic ern part of the Austroalpinenappes, Southern Alps). lower crust or, conversely,for southward transport of The remnants of the Meliata-Hallstatt Ocean did not the Insubric line, togetherwith the Central Alps, over reach the area of the profile of Plate 1. However, Cre- the Adriatic lower crust. taceousorogeny resulting from continentalcollision fol- $CHMID ET AL' GEOPHYSICAL-GEOLOGICAL TRANSECT OF THE ALPS 1045

oZ o/ Jurassic-Cretaceous N

eG trace of future Valais ocean Eu ro pe boundary (opening during EarlyCretaceous)• "' ';•i•i•!... ••.•. for(present-day Europe)

•"• • Hallstattocean Liguriaiemont- '•?•*•'• T SA•• %• (closedLate Jurassic)during ocean • B

Addatic S 500 km

Figure 5. Paleogeographicreconstruction for the Jurassic-Cretaceousboundary, after Dercourt et al. [1986],Stampfli [1993], and Schmidet al. [1996],showing the oceanicsuture of the former Meliata-HallstattOcean, the Piemont-Liguria(South-Penninic) Ocean, and the trace along whichthe Valais (North-Penninic)Ocean will open. Abbreviationsare SE, Sesia-Dentblanche extensionalallochthon; MG, Margna extensionalallochthon; SA, passivecontinental margin of Southern Alps; LA, Lower Austroalpine realm; UA, Upper Austroalpine realm; NCA, Northern CalcareousAlps. Geographicalreference points are S, Sardinia; C, Corsica; M, Marseille; G, Geneva; Z, Ziirich; I, Innsbruck; T, Torino; and B, Bologna.

lowing the closure of the Meliata-Hallstatt Ocean dur- We suggestthat the present Margna-Sella nappe sys- ing the Early Cretaceousalso affected the Austroalpine tem occupied a special position near the passive conti- and South Penninic units in our transect. This collision nental margin at the northwesternedge of the Apulian wasfollowed by westwardpropagation of a thrust wedge microplate (Figure 6). FollowingTr•'mpy [1992], we [ThSni and Jagoutz,1993; Neubauer,1994; Froitzheim separated these "Ultrapenninic" units from the lower et al., 1994]toward the marginbetween Apulia and the Austroalpine nappes with the Corvatsch-Bernina units South Penninic Ocean situated in the area of our tran- at their base in the profile of Plate 1. According sect. to Froitzheimand Manatschal[1996] the Margna-Sella Structures of the passivecontinental margin to the nappes in Graubiinden and the Dent Blanche-Sesiaunits northwest of the Apulian microplate are locally well pre- of the Western Alps represent extensional allochthons servedin spite of crustal shorteningin the Austroalpine that became separated from the Apulian margin by nappesof Eastern Switzerland[Froitzheim and Eberli, a narrow intervening zone of denuded mantle rocks 1990; Uonti et al., 1994] and in the SouthernAlps (Platta unit in Plate 1) beforethe formationof a mid- [Bertotti, 1991; Bertotti et al., 1993]. During the fi- oceanicridge west of this extensionalallochthon (Fig- nal rifting phase, related to the opening of the Cen- ures 5 and 6). The presentstructural positionof the tral Atlantic (Toarcianto Middle Jurassic),a system Margna-Sella nappes below the Platta ophiolites and of west dipping detachmentsformed [Froitzheimand abovethe Forno-Malencoophiolites (Figure 7) [Lini#er, Manatschal,1996]. The passivemargin preservedin 1992; Spillmann,1993] is most readily explainedwith the Austroalpinenappes of Graubiinden(Figures 5 and this hypothesis which does not call for an additional 6) is amazinglysimilar to that preservedin the South- ocean or microcontinent. ern Alps [Bernoulliet al., 1993],and both areasexhibit The Brian5onnais domain or terrane according to featurestypical for a lower plate margin [Lemoineet $tampfli[1993] is representedby the Tamboand Suretta al., 1987; Froitzheim and Eberli, 1990]; however,see nappesand detached sedimentary slivers (Schams, Sulz- Marchant[1993] and Trommsdorffet al. [19931for a fluh, and Falknisnappes, Figure 3, Plate 1). According differing view. to many authors a last oceanic domain north of the 1046 SCHMID ET AL.- GEOPHYSICAL-GEOLOGICAL TRANSECT OF THE ALPS

• 9ø30'

Inset' • Austroalplne&Southern Alps m Ophlollte-bearlngunits • S-PennlnlcB,indnerschiefer • MiddlePennlnlc sediments +•--•Tambo-Su retta :.../..•ß:'...' N-Pen ninicsediments [--[-•'-•L.... Penninicbasement • Helveticforeland

Main map:

EL Engadine line IL Insubric line TF Turba normal fault FF Forcola ....

• SouthernAlps

Austroalpineand Margna- Sellanappes

Ophiolites and Ophiolite- bearing units

• S-PenninicBfindnerschieferAvers

,• AIIochthonoussediments Middle-Penninic

• Mesozoicbasement covernappesof Penninic

• FlyschN-Penninic (Cretaceous)Bfindnerschiefer and ')".'•N-Penninic Flysch(Tertiary) --• Tambo-Suretta • LowerPenninic basementnappes Helvetic foreland, including

• massIfover ofsoulhem Gotthard profile(plate 1)--j• 9ø30'

Figure 6. ReconstructedE-W section through the passive margin preserved in the Austroalpine unitsof Graubfinden(bottom) (figure from Froitzheim et al. [1994])and map of the tectonic unitsin the westernmostAustroalpine realm, representing westerly dislocated fragments of this passivecontinental margin. Small triangles along tectonic boundaries point in the directionof thestructurally higher unit, irrespective of the nature of theboundary (thrust or normalfault) (fordetails, see Froitzheim et al. [1994]).A-A' andB-B' denote the profile traces of Figure7.

Brianqonnaisterrane, the Valais Ocean,opened in the by Froitzheimet al. [1996]).As depictedin Figure5, earliestCretaceous [Frisch, 1979; Florineth and Froitz- no extensionof the Brianqonnaisinto the EasternAlps heim, 1994;$tampfli, 1993; Steinmann, 1994] due to is expected.The reconstructionin Figure8a stipulates gradualopening of the North Atlantic.The tracealong an upper plate positionof the Brianqonnaisin respect whichthis new oceanic domain forms is depictedin Fig- to the Piemont-Liguriaas well as to the Valais Ocean. ure 5. In a westward direction the Valais Ocean is This minimizes the volume of continental crust under- linkedto openingin the Bay of Biscayand rifting in lyingthe Brian(;onnaisfacies domain. However, given the areaof the futurePyrenees [$tampfli, 1993]. The the predominanceof sinistralstrike-slip motion between eastwardcontinuation of this oceanis probablyfound Europeand the Brian(;onnaisterrane [$tampfli, 1993; within or near the northernmargin of the preexisting R•'ck,1995; $chmid et al., 1990],opening of the Valais SouthPenninic Ocean (Rhenodanubian fiysch and Up- Oceandoes not necessarilyneed to be asymmetric. per Schieferhfilleof the Tauern window, see discussion The proposedpaleogeographic situation of the Adula

SCHMID ET AL.' GEOPHYSICAL-GEOLOGICAL TRANSECT OF THE ALPS 1049

Landwasser PizzaNalra valley G•pshorn En river ValMdschauns ,, •, , Varusch La d L ], WNWA d•o•lltas', w..... Silvretta! :! . g.....P .... delgno, Ferro ESE i '• • O- D,..... (D\e,•\•.':,' Quatt.... Is ",',

(km, '%"•"•'•::zo••*'+••;'••'d •+•-- -- L• Austroalplne__-- • --• __½• crrLowe r '•.•+:•: :%+% :+: • .• Penninicnappes i • • • • Languardpo

Ldbb•a Orden north of Forcola (Val Bregagha) (Val Forno) Val Fex Morteratsch Plz AIv di L•wgno Austroalpine nappes: i i wsw ! ENE W ,, ', E Cretaceousflysch ,, ' P•zChalchagn i '1 Lowerto MiddleJurassic, • ', ValRoseg • •'-.i I + Languard+ + '• Rhaetian •• Platta , -.. , ---__•___• __•___+ *•.•::.:,::.•,.. Hauptdolomit(NorJan) Platta...•..'•.•.,.,•,, •_ "•-•_,,•.c• • Corvats•h", • i x•x • •%•x ' RaiblGroup (Carnian) ' + + + MiddlecarbonatesTriassic LowerTriassic clastlcs • ...... :.:.:.:.:•:::.:::•..:.:.•::.:.:,:•.:.:.:.:.•:.a enc•:•:•:•::•::•:•3:•3:::•:•:•:•:•:•:•3•:•:•:•&::::•2•3:•:•L: '...... + + +

volcanoclasticPermianvolcanic rocksand • •Suretta ......

basementAustroalpine)(Upper

basementAustroalpina)(Lower

Margnanappe: B 5 km Mesozoiccover i • i i I i

basement Cretaceous faults: Tertiary faults: Penninic nappes: '.'.."..-.•Piemont-Ligurian ophioiltes Tertiaryflysch DNF Ducan normal fault TF Turba normal fault Av.: Avers Bilndnerschiefer TNF Trupchun.... EL Engadine line •:• Brianc,onnais basement & cover CNF Corvatsch ....

Figure 7. Twotectonic profiles through the AustroMpine nappes in EasternSwitzerland. Profile traces are indicated in Figure 6. nappeat the distalmargin of stableEurope [$chmid et et al., 1988; Liniger, 1992; Spillmann,1993]. During al., 1990, 1996]and north of the VMaisocean has far- Tertiary orogenya basalthrust displacedall thesestruc- reachingconsequences regarding the timing of eclogite turally highestunits, whichwere previouslyaffected by faciesmetamorphism in the Adula nappe (a Tertiary Cretaceousorogeny, to the north by at least 75 km rather than Cretaceousage is the corollary),the width [Froitzheimet al., 1994].This orogeniclid ILaubscher, of the Europeandistal marginthat hasbeen subducted, 1983]overrode the present-dayEngadine window and andthe (in thiscase very high) rates of subductionand the Pr•ttigau half window(Figure 6) only after Creta- subsequentexhumation of the Adula eclogites.In order ceousorogeny. Sedimentation in the Brian5onnaisand to minimize amount and rate of Tertiary subduction, eastern Valais domains through to the Early Tertiary width of only 50 km was assumedfor that part of the precludesCretaceous orogeny within theselower struc- North Penninic B/indnerschiefer basin that was origi- tural units. nMly underlainby oceaniccrust (Figure We emphasizethe existenceof two orogenies(Creta- ceousand Tertiary) for the followingreasons: Cretaceous Orogeny 1.The kinematics(top to the westto WNW imbrica- Figures6 and 7 illustrate how the AustroMpinenappe tion associatedwith orogen-parallelstrike- slip move- pile in eastern Switzerland was assembledby oblique ments) of Cretaceousorogeny are distinctlydifferent east-over-westimbrication of the NW passivemargin of from the top to the north or NNW movementscharac- the Apulian microplate[Froitzheim et al., 1994;Handy teristic for Tertiary orogeny. et al., 1993; $chmid and Haas, 1989]. Becausethe 2. A Late Cretaceousperiod of extension to be dis- N-S orientation of the profile in Plate i is not suited cussedbelow separatesthe two orogenies. for discussingCretaceous orogeny, auxiliary WNW-ESE 3. Subduction associatedwith eclogite facies meta- orientedprofiles (Figure 7) have been prepared. The morphism and subsequentexhumation very probably associateddeformation (Trupchun phase in Figure 4) took place twice in the Alps: first during the Creta- [Froitzheimet al., 1994]also affected structurMly lower ceousand then during the Tertiary (seeFroitzheim et units such as the Arosa-Platta ophiolites, the "Ultra- at. I1996]for an extensivediscussion). penninic" Margna-Sella nappes and the Lizun-Forno- Nappe imbrication during the Trupchun phase, the MMencoophiolites underlying the Margna nappe[Ring principle phase related to Cretaceouscrustal shorten- 1050 SCHMID ET AL.: GEOPHYSICAL-GEOLOGICAL TRANSECT OF THE ALPS

N a) Early Paleocene 65Ma

10 ,AN.CONNAIS

50 '50 km N of Insubric line: b) Early Eocene 50 Ma

upperApulian crustmargin of Aa: Austroalpine nappes

PI: Platta-Arosa ophiolites Av:Avers Bi•ndnerschiefer Brianc.onnais upper crust Su: Suretta-

Ta: Tambo- nappes \ Sch: Schams- 50 50

Valais oceanic crust / and subcontinental mantle

Vo: Valais ophiolites NorthPenninic Bi•ndnerschiefer (NPB)

+•=• uppercrust ofEuropean margin 100 Ad &Gr: Adula -Gruf- km Si: Simano- nappes Lu: Lucomag no - Go. Gotthard-

Aar: external massif He: Helvetic nappes c) Late Eocene 40 Ma • lowercrust ofEuropean margin future •-•

0 • • •.• 0

Bergellintrusion _

50 50

d) Earliest Oligocene 35 Ma ,rI • ..... a -100 10 50km 100 km

50 v•

Figure 8. Scaled and area-balancedsketches of the kinematic evolution of the eastern Central Alpsfrom (a-b) early Tertiary convergence and subduction to (c) collisionand (d-g) postcollisional shortening.

ing, cannot have started before about 90 Ma at the (Figure 6) is constrainedto have initiated earlier, at westernmargin of the Upper Austroalpinenappe sys- around 100 Ma, by radiometric ages of synkinematic tem (Ortler unit, Figure 6) becauseof ongoingpelagic temperature-dominatedmetamorphism [ThSni, 1986; sedimentation(Figure 4) up to the Cenomanianor early Schmidand Haas, 1989]. This westwardmigration of Turonian [Caron ½t al., 1982]. Thrustingfurther to Cretaceousorogeny is also seenon a much larger scale the east and along the basal thrust of the Oetztal unit all acrossAustria, e.g., from the depositionof synoro- SCHMID ET AL.- GEOPHYSICAL-GEOLOGICAL TRANSECT OF THE ALPS 1051

N e) Oligocene 32 Ma S

__ Triassico -lO

+ +,.•-+•-.•-++++++u2,

+ •. +

sedimentarycover._c / _ uppercrustal basement .E • •o•e.• •oo Lowercrust lithosphericmantle 150 km

f) Early Miocene 19 Ma

• Aa Av •' U.Triassic Gonfoli..•teLomb. 0 Pl • • • •-" •• lO

t - 50

g) Present

Engadine ,,I line11111 I•'-.• Ill ' Insubricline

o o

lO lO

5o 50 km

i i i i i i

Figure 8. (continued) 1052 SCHMIDET AL' GEOPHYSICAL-GEOLOGICALTRANSECT OF THE ALPS

50 km

European Moho

iatic

a¾ 600 7•0 800

Figure 9. Map outlining the contoursof the top of the Adriatic lower crustal wedgeand its relation to the top of the European lower crust and the Insubric line and, additionally, the Adriatic and EuropeanMoho (easternand southernpart of area coveredby Figure 9). Contour lines are given in 2 km intervals. The contoursof the Insubricline are constrainedby field data [Schmidet al., 1987],migrated reflection lines S1 and C2 [Valasek;1992], and 3-D gravityprofiles G1, G2 and G3 obtainedfrom 3- D modelingby Kissling[1980]. The migratedseismic line W4 is also indicated but was not used for constrainingthe dip of the Insubric line. Contours of the well-constrainedtop of the Europeanlower crust and Moho are taken from Valasek[1992]. The position of the Adriatic Moho is not well constrainedand schematicallycontoured after data compiledby Kissling [1993]and after a seismicrefraction profile by Deichmannci al. [1986]. Line a-a is the profile trace of Figure 10.

genic chromite-bearingRossfeldschichten in the early in plate 1 are of pre-Adamello age. This pre-Adamello Cretaceous[Faupl and Tollmann, 1979] and from the N-S compressionled to more than 25 km shortening. pre-90-Ma age of Alpine eclogitefacies metamorphism Activity along a precursorof the Insubricline during in the Saualpe[ThSni and Jagoutz,1993]. This supports or immediately after Cretaceous orogeny is indicated the postulate that continental collision along the for- from three lines of evidence: mer Meliata-Hallstatt Ocean initiating during the Early 1. Kinematics and structural style of Cretaceousde- Cretaceous in the eastern parts of the Austroalpine formation are totally different within the Austroalpine units of Austria and the Western Carpathians[Kozur nappesnorth of the Insubricline (top-westimbrication, and Mostler, 1992] was followedby westwardpropaga- collision, and penetrative deformation associatedwith tion of a thrust wedgeinto our area of interest by Ceno- metamorphism)and in the SouthernAlps (top-south manian to early Turonian times [Thdiniand Jagoutz, thrusting,foreland deformation). 1993;Ncubauer, 1994; Froitzh½im ½t al., 1994]. 2. The Insubric line marks the limit between Creta- In Figure 4, Cretaceous orogeny is also shown to ceous-agedmetamorphic units to the north (eclogitic have affectedthe SouthernAlps (pre-Adamellophase) and Barrowian type metamorphism[Hunziker ½t al., [Brack,1981; Doglioniand Bos½llini,1987]. The exact 1989; ThSni, 1986; ThSni and Jagoutz,1993]) and the timing of this deformation phase is ill-constrainedbut Southern Alps lacking such an overprint. These units certainlypredates 43 Ma (oldestparts of the Adamello must have been at least partially exhumedby or during intrusion). Assumingthat fiyschsedimentation in the the Late Cretaceous[Dal Piaz ½tal., 1972]and juxta- Lombardian basin is contemporaneouswith this defor- posed with the Southern Alps along this precursorof mation phase and based on radiometric dating of pre- the Insubric line. Adamellodykes crosscutting certain thrusts[Zanchi et 3. The profile of Plate I directly showsa fundamental al., 1990],a Late Cretaceousage is inferred. All the top- difference between Austroalpine nappes and Southern south displacementsalong the Orobic thrust and parts Alps: The former were emplaced as thin allochthonous of the displacementalong the Coltignone thrust shown flakes onto Penninic ophiolites in Cretaceous times, SCHMID ET AL.: GEOPHYSICAL-GEOLOGICAL TRANSECT OF THE ALPS 1053

NNW SSE Penninic nappes Meiringen Q• •-• f,• •/ Var.ese

O- -o

-lO x '•. i ...... •• • • Adria•cupper

• Adriatic • •• •

----• r crU •

50- ....,r.v,,,.o.., 5o km km

Figure 10. Profilewest of the transectof Plate 1 (seeFigure 9 for location)constructed using the contoursgiven in Figure9. The positionof the EuropeanMoho is takenfrom Valasek[1992]. The Ivrea gravity modelcorresponds to the modelof Kissling[1980, profile III]. The top of the Adriatic lowercrust is takenfrom Valasek[1992]. The exactposition of reflector"M" is unknown; the givenapproximate position is taken from Ansorge[1968].

while the latter remained attached to the lower crust for the subsequentorogenic evolution during the Ter- and upper mantle of the Apulian microcontinent. tiary. The Austroalpine units will largely remain un- Late Cretaceous Extension deformedand will act as a rigid block (orogeniclid) [Laubscher,1983], characterizedby friction-controlled The westwardmigrating Cretaceousorogenic wedge Coulomb behavior floating on viscouslydeforming Pen- underwentsevere extension during the Ducan-Ela phase ninic units [Merle and Guillier, 1989]. (Figure4) [Froitzheimet al., 1994],resulting in a series Early Tertiary Convergence and Subduction of normalfaults at highertectonic levels (normal faults (65-50 Ma) depicted in Figure 7, such as the Ducan, Trupchun, and Corvatschnormal faults) and foldingwith horizon- The tectonic evolution during Tertiary orogeny is tal axial planes at a lower tectonic level in the Aus- summarizedin the sketchesof Figure 8 (see also the troalpineunits of easternSwitzerland [Froitzheim, 1992; pioneering work along the same transect by Milnes Froitzheimet al., 1994]. Normal faulting disruptsand [1978, his Figure 31). The earliestpossible onset of thins the earlier formed nappe stack and locally leads to thrusting in the units below the Austroalpine nappes omissionof nappes, e.g., in the eastern part of section and the Platta-Arosa ophiolites(Figures 8a and 8b) A-A' wherethe Upper Austroalpinebasement (Campo- is locally constrained by ongoing sedimentation in the Languard-Silvretta)is cut out by the Trupchunnormal Briangonnaisdomain (Paleocenein the northernmost fault (TNF). Extensionis alsoobserved further to the unit of the Briangonnais,the Falknisnappe) [see Alle- east in Austria where it is related to the formation of mann, 1957]and in the North PenninicBiindnerschiefer the Gosaubasins [Ratschbacher et al., 1989;Neubauer, (early Eocenein the Arblatsch and Prgttigau fiysch) 1994] between90 and 60 Ma. In our area of interest [seeEiermann, 1988;Na'nng, 1948; Ziegler, 1956]. this extensional phase is not well dated. We place the The sketch of Figure 8a representsthe onset of sub- Ducan-Elaphase somewhere between 80 Ma (end of the duction of the Briangonnais domain due to complete Trupchunphase) and 67 Ma (lowerage bracket of a ra- closure of the South Penninic Ocean. The formation of diometricage determination)[Tietz et al., 1993]. This an accretionarywedge within the Avers Biindnerschiefer extensionalphase is viewed as being causedby gravi- (Piemont-LiguriaOcean) and northwardthrusting of tational collapseof an overthickenedorogenic wedge in this wedgeonto the future Suretta nappe (southern- the senseof Platt [1986]. most Briangonnaisdomain) is tentatively placed into Exhumation and cooling of the Austroalpine units the early Paleocene(Avers phase, Figure 4). This al- during this Ducan-Ela phase have severe implications lows for continued sedimentation within most of the 105;4 SCHMID ET AL.: GEOPHYSICAL-GEOLOGICAL TRANSECT OF THE ALPS

Table1. N-S ConvergenceDerived From the Profiles of Figure8

Time Interval Amountof ConvergenceAcross the Alps ConvergenceRate Plate Tectonic Reconstruction [Deweyet al., 1989]

Early Paleoceneto 200 km inferredfrom relative displacement early Eocene betweenpoints a andb in Figure8. 116km of 1.33cm yr -1 0.22cm yr-1, 65 to 55 Ma (65-50 Ma) thinnedcontinental crust of theBrianqonnais 0.4cm yr -1, 55 to 51 Ma domain and the Valais Ocean enter the subducfion zone.

Early to late Eocene 150km inferredfrom relative displacement (50-40 Ma) betweenpoints a andb in Figure8. 150km of 1.5crn yr-1 1.2cm yr -1, 51 to 38 Ma distalEuropean margin situated between the southernedge of the Helvetic domainand the southerntip of stableEurope enter the subduction zone.

Late Eocene to 45 km inferredfrom relativedisplacement atleast 0.55 cm yr-1 Oligocene(40-32 Ma) betweenpoints a and c in Figure8. Detachment of the Helvetic sediments and deformationwithin the Subpenninicnappes; unknownamount of shorteningin the vicinity of the Insubricline and in the SouthernAlps: 45 km representa minimumestimate only.

Oligoceneto early Miocene (32-19 Ma) atotaldisplacement of 58 km betweenconsisting points of33 a kmand fromc in Figurerelative 0.45 cm yr -1 0.94cm yr-1, 38 to 19 Ma 8 (including6 km out of a total of 21 km shorteningin the Aar massif), 15 km from back thrustingalong the Insubricline, and 10 km out of a totalof 56 km post-Adamellophase shorteningin the SouthernAlps.

Early Miocene to a totalof 61 km consistingof 15 km from Recent(19-0 Ma) shorteningin the Aar massifand 46 km from 0.3cm yr-1 (0.5 cm yr -1 if 0.3cm yr-1, 19 to 9 Ma shorteningin theSouthem Alps deformationstopped at 7 Ma) 0.43cm yr -1, 9 to0 Ma

Totaltime span morethan 514 km (65-0 Ma) morethan 0.79 cm yr -1 average:0.72 cm yr -1 (amount of convergence,481 km)

Briangonnaisduring the Paleocene.Onset of southern- the Tamboand Suretta nappes. This significantly pre- most Briangonnaisdomain subduction later than 65 Ma datesthe onsetof the Ferreraphase deformation in the would require unrealisticallyhigh convergencerates of North PenninicBiindnerschiefer (Figure 4). This in- morethan 1.5 cm (seecomparison with plate movement terpretationis in accordancewith the generaltrend of velocitiesshown in Table 1 and discussedlater). northwardyounging for the onsetof penetrativecleav- By the end of the early Eocene(Figure 8b) the en- age formationshown in Figure4. The northernparts tire width of the Briangonnaisdomain had been sub- of the Brianqonnaisbasement are inferred to have been ducted, togetherwith large parts of the North Penninic permanentlysubducted ("subducted Briangonnais" in Biindnerschieferor Valais basin. Sedimentationin parts Figure8b). At about 50 Ma, the southerntip of sta- of the North Penninic Biindnerschiefer basin continued ble Europe,represented by the Adula nappe,is about up to this time [Eiermann, 1988; Nh'nny,1948; Ziegler, to enter the subductionzone. Figure 8b representsa 1956]. The Tambo and Suretta nappes,representing snapshotof the onsetof final collisioncaused by the the southern parts of the Brian(jonnaisdomain conti- complete closure of the North Penninic Bfindnerschiefer nental basement,reached their peak depth correspond- realm. ing to the peak pressuresof 10-13 kbar [Baudin and The convergencerate resultingfrom the relativedis- Marquer,1993] by this time (about50 Ma) and were placement(200 kin) of points"a" (northernedge of the subsequentlyheated to peak temperaturesat 40-35 Ma orogeniclid representedby the Austroalpinenappes) [Hurfordet al., 1989]. Becausethe onsetof penetra- and "b" (southerntip of stableEurope, i.e., the Adula tive cleavageformation (Ferrera phase of Figure4) is nappe)in Figures8a and 8b is 1.3 cm peryear (Ta- associatedwith this metamorphism, it is assumedto ble 1). It is noteworthythat the reconstructionof Fig- have started near the Paleocene-Eoceneboundary in ure 8 impliesthat the northerntip of the Austroalpine SCHMID ET AL.' GEOPHYSICAL-GEOLOGICAL TRANSECT OF THE ALPS 1055 nappesmarked by "a" in Figure 8a (and correspond- der prograde conditions. This phase was followed by ing to the front of the Northern CalcareousAlps in a the Zapport phase [Lb'w,1987], which was character- present-dayprofile, see Figure 8g) hasmoved northward ized by extremely penetrative deformation, initially un- by a total of some 450 km relative to a point attached der eclogitefacies conditions, followed by lower pressure to stable Europe presently situated below the northern metamorphic conditions arising from near isothermal edge of the Northern CalcareousAlps. This 450 km of decompression. convergence(out of a total of 514 km Tertiary N-S con- During Tertiary collision the Penninic nappes thrust vergenceacross the Alps, Table 1) was accommodated north onto the foreland forcing the detachment of sedi- by subduction and shorteningwithin the northern fore- ments that later formed the Helvetic nappes from their land. Relative to stable Europe, point "a" in the sketch original substratum, i.e., the present Gotthard "mas- of Figure 8a would have to have been located near Pisa sif" (D1 in Figure4) at the endof the collisionalstage. in northern Italy at the onset of Tertiary convergence. The ageof the Tertiary coverbasal unconformity [Herb, This illustrates well that Cretaceousorogeny took place 1988; Lihou, 1995] in the Helvetic nappesand the in- far from the present-day position of the Austroalpine ternal Aar massif(Infrahelvetic units) decreasestoward nappes at the northern front of the Alps. the foreland(Figure 4). This decreasemay be interpreted in terms of the northwestward migration of a Tertiary Collision (50-35 Ma) peripheral bulge within the subductingEuropean plate Collision of stable Europe with the orogeniclid led to across the Helvetic realm during the Eocene. Dur- the situation in late Eocenetimes depicted in Figure 8c, ing later stagesof orogeny this bulge migrated into its characterized by the subduction of the southern tip of present-daylocation north of the Molassebasin (Black stableEurope (Adula nappe). Eclogitefacies overprint forest). The northwardthrusting of the Penninicunits in the Adula nappe was immediately followed by rapid also led to a substitution of the cover of the southern exhumation of these eclogitesand the establishmentof Gotthard "massif" by Subpenniniccover slices ("Tri- the stack of the higher Penninic nappes by the early assic, Lower, and Middle Jurassic cover slices" in the Oligocene(Figure 8d). legendon Plate 1). Northwardpropagation of the basal The amount of convergencebetween the stagesof Fig- Penninic thrust is held responsiblefor the detachment ures 8b and 8c (Table 1) is determinedby the peak of North Penninicor Ultrahelvetic(e.g., Sardonafly- depth of the Adula nappe (correspondingto 27 kbar sch) and SouthHelvetic (e.g., Blattengratunit) slivers in the Cima Lungaarea) [Heinrich,1986] and the cho- in its footwall. These slivers were dragged over several sen subduction angle. Other important constraintson tens of kilometers and are now found above the Hel- the convergencerate are offered by the interpretation vetic nappes and, additionally, above the northern Aar of the Adula nappe as representing the southern tip of massifcover [Trfimpy, 1969]. Emplacementof theseex- the European foreland and the Tertiary age of eclog- otic strip sheetsoccurred during the Pizol phase(Fig- ite facies metamorphismin the Adula nappe (Figure ure 4) [Milnesand Pfiffner, 1977]. This phasepostdates 4) [Froitzheimet al., 1996]inferred from geochronolog-the youngestsediments in the respectivefootwalls (late ical [Becker,1993; Gebaueret al., 1996]and structural Eocene in the Helvetic nappes, early Oligocene in the [Partzschet al., 1994]data. On the basisof thesecon- coverof the Aar massif). straints the convergenceamounts to 150 km, or 1.5 cm The early phasesof exhumation of the Adula nappe yr-•. The additional45 km of shorteninginferred to brought the frontal parts of this compositeunit to more have been produced between the stages illustrated in moderate pressuresaround 6-8 kbar by about 35 M a Figures8c and 8e (indicatedby the relative movement (Figure 8d), the onsetof temperature-dominatedmeta- of pointslabeled "a" and "c ", seeTable I and Figure8) morphism(so-called "Lepontine" metamorphism) [Frey is a minimum estimate because it does not account for et al., 1980] in this particular area (Figure 4). This an unknown amount of retroshearing associatedwith temperature-dominated metamorphism resulted from the initial phasesof back thrusting and vertical extru- decompressionalong a continuousPT loop [LSw,1987]. sion in the vicinity of the Insubric line. Becausethe Bergell intrusion is located in direct con- The time span correspondingto the collisionalevent tact abovethe Adula nappe (Plate 1) this early phase (50 to 35 Ma) was characterizedby penetrativede- of exhumation of the Adula nappe must have predated formation in the Tambo, Suretta, and Schams nappes the intrusion. The Bergell tonalite reached the solidus (Ferrera phaseof Figure 4). The Ferrera phasede- at a depth of merely some 20 km at its base dur- formation within the North Penninic Biindnerschiefer ing the early Oligocene according to pressureestimates falls entirely within this time interval. Intense imbrica- [Reusser,1987; Davidsonet al., 1996]. Early exhumation of Mesozoic sediments, continental basement, and tion of the Adula nappe appears to have been extremely mafic rocks of possiblyoceanic origin within the future rapid, having occurredover a time span of only 5 m.y. Adula nappe(Sorreda phase) [LSw, 1987], occurred un- (betweenthe stagesrepresented in Figures8c and 8d). 1056 SCHMID ET AL.: GEOPHYSICAL-GEOLOGICAL TRANSECT OF THE ALPS

Such rapid exhumation by corner flow, extension,or vide excellent time constraintsfor dating deformation buoyancyforces (see discussion by [Platt, 1993])iscon- along the Insubric line and the adjacent Penninic units. sidered unlikely for this early phase of exhumation re- Figure 8e depicts the situation immediately after final lated to collision. Forcedextrusion (recently proposed emplacement of this intrusion at a depth constrained as a viable alternative model for the exhumation of the by hornblendebarometry [Reusser,1987; Davidsonet Dora Maira eclogites)[Michard et al., 1993]parallel to al., 1996]. The southernsteep belt and the Insubricline the subductionshear zone (arrow in Figure8c) seemsto must have existed prior to the ascent and final emplace- be the most likely mechanism for differential exhuma- ment of the Bergellintrusion [Rosenberg et al., 1995]. tion of the Adula nappe in respect to both higher and Therefore this steep zone is inferred to have formed ear- lower tectonic units which did not suffer eclogitefacies lier (Figures4 and 8d). As alsosuggested by Triimpy metamorphism. [1992]the ascentof the Bergellpluton is facilitatedby movementsalong the Insubric fault which lead to up- Slab Detachment, Bergell Intrusion, and lift of the entire southern Penninic zone relative to the Refolding of the Nappe Stack (35-30 Ma) Southern Alps between the stagesin Figures 8d and 8e. During the convergenceand collisionstages discussed This differential uplift of the southern Penninic zone above, substantial parts of the more internal Penninic was probably caused by upward directed flow in the units (oceaniccrust, Briangonnais, and distalEuropean southern steep belt that was deflected into north di- continentalcrust) were subducted.The entire volume rected horizontal movement of the Tambo-Suretta pair of upper crust in the present-day Subpenninic nappes, (Niemet-Beverinphase of Figure 4, seediscussions by representing the more proximal parts of the European Merle and Guillier [19891,Schmid et al. [1990, 1996], crust, however, was accreted to the orogenicwedge af- and Schreurs[1993, 1995]). This in turn resultedin the ter Eocene collision. This led to excessivethickening of spectacularrefolding of the Schamsnappes and parts of the orogenicwedge after the Eocene, effectively"clog- the North Penninic Biindnerschieferaround the hinge of ging" the subduction system, such that only the de- the Niemet-Beverinfold (axial trace indicatedin Plate tached lower crust of the European foreland continued 1 and Eigures8d and 8e). to be subducted from now on. Movements during this Niemet-Beverin phase were Figure 8e depicts the result of a first stage of postcol- contemporaneouswith the closing stages of the Zap- lisional shorteningdescribed in this section. In this Fig- port phase in the Adula nappe, the Calanda phase in ure the present-dayportion of the SouthernAlps along the Helveticnappes [Pfiffner, 1986], and the initiationof the •ransect(Plate 1, Figures8f and 8g) is replacedby thrustingalong the Glarusthrust (Eigure 4). Movement a profile acrossthe Ivrea zone [after Zingget al., 1990] of the trailing edge of the Helvetic nappesat this time, in order to account for about 50 km of dextral strike- depicted in Eigure 8e, ensured that these nappes were slip motion along the Insubric line after early Oligocene not affectedby any significantmetamorphism (these times [Fumasoli,1974; Schmid et al., 1987, 1989].Also rocksnow exhibit anchizonal conditions). Hence the ini- depicted in Figure 8e is slab break-offof the subducting tial stagesof movementsin the vicinity of the Insubric lowerparts of the Europeanlithosphere [yon Blancken- line were contemporaneouswith north directed trans- burgand Davies, 1995;Dal Piaz and Gosso,1994]. The port in the northern foreland. Eigure 4 documentsthat subduction of light continental lithosphereduring col- this contemporaneity of "proshears" and "retroshears" lision created extensional forces within the slab, due in the senseof Beaumont et al. [1994] is maintained to opposing buoyancy forces between the deeper sub- during the later stages of post-collisionaldeformation ducted, relatively dense lithosphere, and the shallower all through the Neogene. continentallithosphere [Davies and yon Blanckenburg, Intrusion of the Bergell granodiorite at 30 Ma also 1995]. As a result, the slab brokeoff. This mechanism provides a useful time mark for the end of this earliest led to the heating and melting of the overridinglitho- stage of postcollisionalshortening, the Niemet-Beverin spheric mantle by the upwelling asthenosphere.Melt- phase(Eigure 4) [Schmidet al., 1990].This phasewas ing probably resultedin mixing of basalticwith assimi- associatedwith E-W extension[Baudin and Marquer, lated crustal material. Such mixing is indicated by the 1993],affecting the AversBfindnerschiefer, which were geochemicaland isotopic signaturesof the Periadriatic cut by an east directed normal fault at the base of the intrusionsIron Blanckenburgand Davies,1995], in par- orogenic lid: the Turba normal fault depicted in plate ticular the Bergell intrusion. Ascent and final emplace- 1 [Nievergeltet al., 1996]. This normal fault is trun- ment of the Bergell intrusion are related to postcolli- cated by the Bergell granodiorite, indicating that the sionalshortening [Rosenberg et al., 1994, 1995;Berger Niemet-Beverin phase ended before 30 Ma. Orogen- and Gier4, 1995]and are depictedin Figures8d and 8e. parallel extension resulted in substantial area reduction The 30 and 32 Ma radiometric ages of the Bergell of the Avers Bfindnerschieferbetween the stagesof Eig- tonalite and granodioriteIron Blanckenburg,1992] pro- ures 8d and 8e. SCHMID ET AL.' GEOPHYSICAL-GEOLOGICAL TRANSECT OF THE ALPS 1057

Back Thrusting Along the Insubric Line and 1977; Milnes and Pfiffner, 1977]. Effectsof contempo- Foreland Propagation of the Helvetic Nappes raneous back folding south of the external massifsand (32 to 19 Ma) in the northern part of the transect (Carassinophase and formationof the Chiera synformin Figure 4) [LSw, This secondstep during postcollisionalshortening is 1987]were relatively minor alongour transect,but their characterizedby back thrusting of the Central Alps over importance rapidly increasesalong strike further to the the Southern Alps along a mylonite belt associatedwith west. There very intense back folding of substantially the Insubricline [Schmidet al., 1989].This backthrust- youngerage [Steckand Hunziker, 1994] is observedat ing, in combinationwith erosion,led to amazingly rapid exhumationof the Bergellarea at a rate of 5 mm yr-• the southernmargin of the westernAar massif(see Fig- ure 10) [Escheret al., 1988].The initiationof thrusting [Giger and Hutford, 1989]. Depositionof bouldersof in the Molassebasin associatedwith shorteningwithin Bergell rocksin the Tertiary coverof the Sourthem Alps the Aar massif (Grindelwaldphase in Figure 4) also (mainlyin the Comoformation, Figure 4) occurredonly occurred during this secondstage of postcollisionalde- a few million years after intrusion. Exhumation of the formation. entire Bergell area to a shallowcrustal depth must have been completedby about 20 Ma accordingto the cool- Lower Crustal Wedging and Foreland ing agesfrom the southern part of the Tambo nappe in Propagation in the Southern Alps the vicinity of this intrusion(cooling below 300øC be- (Post 19 Ma) tween21 and 25 Ma, seeFigure 4) (data from dSgeret al. [1967],Purdy and Jgiger[1976], and Wagneret al. Deformation in the southern part of the profile out- [19771). weighted the one in the northern part during this third Effects of the Domleschgand Leis phases[Schmid stage of postcollisionalshortening: 46 km out of a total et al., 1996] which were contemporaneouswith back of 61 km shortening(see Table 1) took placewithin the thrusting along the Insubricmylonite belt (Figure 4) Southern Alps after the early Miocene. It is this stage were relatively weak within the central Penninicregion. of postcollisionalshortening that profoundly influenced The large-scalestructure of this region was not sub- the deep structure of the Alps as revealedby geophysi- stantially altered. However, important contemporane- cal information but which only led to uplift and erosion ousmovements affect the northernforeland (Figure 4). in the central part of the Alps along our transect. There the main activity involvedmovements along the Accordingto Table 1 the greatestpart of post-Adamel- Glarus thrust [Schmid,1975] and the formation of a lo shortening in the Southern Alps occurredduring this penetrativecleavage above and belowthis thrust (Ca- last stage of orogeny. It led to the impressive fore- landa phase)[Pfiffner, 1986]. The leadingedge of the land thrust wedge of the Southern Alps, sealed by the Helvetic nappes emerged at the erosionalfront of the Messinian unconformity that formed at around 7 Ma early Alps in mid-Miocenetimes (Figure 8f) [Pfiffner, [Pieri and Groppi,1981]. This post-Adamelloshorten- 1985, 1986],as witnessedby Helveticpebbles in the Up- ing was mainly achievedby thrusting along the Milan per FreshwaterMolasse (OSM). The basalthrust of the thrust and the later out-of-sequenceLecco thrust (Plate Helveticnappes (Glarus thrust in Plate 1) migratedto- 1). Accordingto Schdnborn[1992] this N-S-shortening ward the foreland. Outward migration was alsotrue for was contemporaneousand related to the displacement the nappe-internaldeformation (folds, thrusts, cleav- of the Periadriatic line by youngermovements along the age),which is attributedto the Calandaphase (Figure Giudicarie line. Retrodeformation of the post-Adamello 4). This phaseis of early Oligoceneage in the Hel- shorteningleads to a perfect alignmentbetween the In- vetic nappes[Hunziker et al., 1986] and of mid to late subricand Pustertallines (Figure 9) [SchSnborn,1992]. Oligoceneage in the internal Aar massif. This providesfurther evidencefor postulating a direct Movements at a time near the Oligocene-Miocene relationship between shortening in the Southern Alps boundary led to the situation depicted in Figure 8f. and indentation of the Adriatic lower crustal wedge These movementsinclude (1) dextral strike slip under to the north. Indentation of the Adriatic lower crust brittle conditions along the Insubric line, without as- into the interface between the south dipping European sociatedback thrusting[Schmid et al., 1989],(2) dif- lowercrust and the Europeanupper crust (Subpenninic ferential uplift of the Bergell intrusion with respect to nappes)postdated movements along the Insubricline, the Penninic units, causedby block rotation along the as alreadysuggested by Laubscher[1990]. sinistralEngadine line [Schmidand Froitzheim,1993], A large part of the shorteningwithin the external Aar (3) E-W orogen-parallelextension at the easternmargin massif(Grindelwald phase of Figure4) and associated of the Lepontinedome (Forcola phase of Figure4), and thrusting in the Molasse basin were contemporaneous (4) further movementalong the Glarus thrust, leading with indentation of the Adriatic lower crustal wedge to a second,crenulation cleavage (Ruchi phasein Fig- [Michalskiand $oom, 1990]. Outward and downward ure 4) in the area of the internal Aar massif[Pfiffner, (in-sequence)propagation of thrustingalso affected the 1058 SCHMID ET AL.: GEOPHYSICAL-GEOLOGICAL TRANSECT OF THE ALPS

Molassebasin. The depot centerof this foredeep(in- the various time intervals were determined using an av- cluding the OligoceneNorth Helvetic Flysch deposited eragerotation pole (-15ø/310) determinedfrom the data on the internalAar massif)migrated outward at a rate of Dewey et al. [1989].These velocities were then con- of 0.3 cm yr-1 in the Oligocene,slowing down to 0.2 cm vertedto localvelocities (in centimentersper year) and yr-1 in Miocenetimes [Pfiffner, 1986]. The Grindel- are listed in Table 1. The averagevelocity over the en- wald phase postdates and in Central Switzerland ac- tire time span(65 Me[to present)is 0.72cm yr-•, and tually deforms the basal thrust of the Helvetic nappes. very closeto the estimateof 0.79 cm yr-x determined Within the transect, exhumation of the Aar massif from from our retrodeformation. The total convergencebe- a paleodepth of approximately 7 km to about 4 km be- tweenEurope (Rorschach) and Africa (northernLibya) neath the paleoland surface occurred in the Miocene, is 481 km as estimated from the rotation parameters. as indicatedby fissiontrack data [Michalskiand Soom, Velocities of plate motion suggestmuch slower con- 1990]. The thrust indicatedin Figure 8g at the baseof vergencerates (0.22-0.4cm yr-1) in Paleocenetimes the Aar massif delimits the boundary between deformed than the estimatesbased on our reconstruction(1.33 and undeformed European foreland. Updoming of the cm yr-x) . The reasonsfor this discrepancyare not Aar massif may be viewed as a crustal-scale ramp fold clear but might be due to the independent motions related to a detachment at the interface between lower of microcontinents such as the Brianconnais-Iberia ter- and upper European crust. This detachment is kine- rane [Stampill, 1993]. Plate tectonicconvergence rates matically linked to the Adriatic lower crustal wedge. duringEocene-Oligocene times (0.94 to 1.2 cm yr-X), The intersection point between proshears and retro- however, are in excellent agreement with our kinematic shears[Beaumont et al., 1994]is nowsituated further to reconstruction. Surprisingly good agreement is also the north (i.e., at the nortl•rn tip of the lowercrustal reachedregarding Miocene to recenttimes (Table 1). wedge)and at the interfacebetween lower and upper European crust. Discussion The describedAlpine sectionis similar in many ways Plate Tectonic Constraints on Tertiary to some of the numerical models of crustal-scale de- Convergence formation providedby Beaumontet al. [1994]. The Amounts and rates of Tertiary convergencededuced driving force in these models is provided by under- from the kinematic reconstructionsof Figure 8 and sum- thrustingof the underlyingmantle lithosphere(in our marized in Table 1 may be compared to estimates de- case European lithosphere),coupled with asymmetric termined by plate reconstructions. The reconstructions detachment emerging from a velocity discontinuity at in Figure 8 were determined independentlyof plate tec- a point (point "S") where two inclinedstep-up shear tonic constraints with only one exception: the ill-dated zones(proshear and retroshear)meet. The gentlydip- stage represented in Figure 8a was placed in the lower ping proshearmay easily be comparedto the south dip- Paleocenein order to avoid convergencerates in excess ping thrust faults in the Helvetic and northern Penninic of 1.5 cm yr-X. zone. More steeply inclined retroshears such as the In- Tertiary convergenceis ultimately linked to relative subric line developabove the subductionzone, causing motions between the European and African plates. Al- relative uplift of the southern Penninic zone. though overall convergencerates between these two The analogy between model 5 of Beaumont et al. platescan be estimated(see below), many details of the [1994],characterized by subductionof one third of the smaller-scalekinematics are influencedby the motion of crustal column, and the postcollisionalstages depicted smaller blocks such as Iberia, Apulia, Corsica-Sardinia, in Figures 8e-8g is particularly striking. Intracrustal Mallorca,and Menorca[see Dewey ½t al., 1989].In addi- detachmentallows for simultaneousforeland migration tion, the overall convergenceis divided into shortening to the north and south. As illustrated by model M4 accommodated in the Mediterranean area and short- [Beaumontet al., 1994],denudation amplifies the move- ening in the Alpine transect studied here. The com- ment along the retroshear. Erosion of a thicknessof parisons made below are thus concerningthe order of several kilometers of material did in fact occur in late magnitudes and not the detail. Oligocenetimes north of the Insubric line [Giger and Europe-Africa plate convergencerates have been ana- Hutford, 1989]. This resultedin rapid exhumationof lyzed basedon the rotation parametersgiven by Dewey high- grade metamorphic rocks north of the Insubric ½t al. [1989]. The plate convergencerates were cal- line (Figures8e and 8f). culated between a point fixed on the northern end of In the course of Miocene and Pliocene times the sin- the easterntransect (Rorschach at -t-9.50longitude and gularity point "S" [Beaumontet al., 1994]between the 47.50latitude) and a movingpoint on Africa (presently proshearand retroshearmigrated northward (Figures located in northern Libya) at the samedistance from 8f and 8g). This correspondsto the tip of the Adri- the rotation pole as Rorschach. Angular velocities for atic wedge encroachingalong the top of the European SCHMID ET AL.: GEOPHYSICAL-GEOLOGICAL TRANSECT OF THE ALPS 1059

lower crust, finally exhuming the Alpine nappe stack. [1992]),but no generalizationsshould be made. Weak Between19 Ma (early Miocene,Figure 8f) and 8 Ma lowercrust is expectedunder an elevatedgeotherm, for (mid Miocene: end of sedimentationin the Molasse exampleduring postorogenicextension leading to the basin) the tip of the Adriatic wedgemigrated over a elimination of mountain roots, as was the case at the distanceof about 38 km (related to shorteningin the end of Variscanorogeny [e.g., Eisbacher et al., 1989]. Subalpinemolasse and Aar massif),i.e., at a rate of 0.4- It is this late or post-Variscanevent whichis generally 0.5 cmyr-•. Miocenethrust loading created the accom- held responsiblefor producing the high reflectivity of modationspace [Sinclair and Allen, 1992]in the fore- the European crust, preserved underneath the Alpine deep, which was then filled with Molassesediments, i.e., orogen. During Alpine orogeny,however, •his prestruc- Lower FreshwaterMolasse (USM), Upper Marine Mo- tured lower crust appears to have been subductedwith- lasse(OMM), and Upper FreshwaterMolasse (OSM). out interal deformation due to its high s•rength to be Migration of the singularitypoint (tip of the Adriatic expectedunder conditionsof a low geotherm(see dis- wedge) during the Miocene results in exhumationof cussionby Ord and HobOs[1989]). that part of the orogenic wedge which is located be- Wedging indicates that the Adriatic lower crust also tween the proshears and retroshears. During this lat- has a relatively high strength in repect to the upper est stage the step-up of the proshearincludes the basal crust, although thickening of this lower crust within thrust of the Aar massif. The retroshear comprisesthe this wedgeindicates limited deformation by brittle fail- thrust fault at the top of the north moving wedge of ure and/or crystal plasticflow. Indentationis kinemat- Adriatic lower crust, which links to the thrust faults in ically related to the contemporaneousformation of a the SouthernAlps (but not the Insubricline, which is fold and thrust belt confined to upper crustal levels in inactiveby this time). the Southern Alps. This again demands detachment The model of tectonic evolution depicted in Figure near the upper interface of the lower crust and, to some 8 has implications for the theological properties of the extent, also at the lowerinterface (Moho). It is note- lithosphere, lending strong support to the concept of worthy that model M9 of Beaumo•t et al. [1994],as- theologicalstratification [e.g., Ord and Hobbs,1989]. suminga two-layertheology for the crust (wet quartz Shallow-dippingupper crustal detachments,typically at and wet feldspartheologies) [Jaoul et al., 1984;$helto• depths of around 4-8 km below the top of the basement, a•d Tullis• 1981], did indeedproduce a lowercrustal are characteristic for nappe formation in the Penninic wedgevery similar to that shownin Figure 8g. zone (Figure 8). The depth of theserelatively shallow After having strongly focusedon one singlecross sec- detachment levels possiblycoincides with the depth cor- tion through the eastern Central Alps, we needto briefly respondingto the temperaturesrequired for the onsetof discusshow representative this section really is for the crystal plasticity in quartz [e.g. Ord and Hobbs,1989]. entire Alps. Figure 9 addressessome of the three- As a consequence,substantial parts of European and dimensional problems in the Alps in the vicinity of the Briangonnais continental crust were subducted during transect of Plate 1 but does not include the results of the stagesof early convergence,subduction, and colli- a profile provided by the French Etude Continentale sion accordingto Figures 8a-8d, in contrast t•othe find- et Oc•anique par R•fiexion et R•fraction Sismique and ingsof M4nard et al. [1991]. the Italian ConsiglioNazionale delle Pdcerche[Nicolas During the postcollisionalstage the thicknessof the et al., 1990]located outside the areacovered by this Fig- European crust increased significantly, as more proxi- ure (ECORS-CROP profile). The contourmap of Fig- real parts of the Europeanmargin (underlyingthe Hel- ure 9 indicates that the Adriatic lower crustal wedge vetic realm) enteredthe subductionzone. This led to depicted in Plate 1 underthrusts the Insubric line by the detachment of the entire upper crust near the in- some45 km in the east (measuredbetween the inter- terface with the lower crust, apparently a characteristic section point with the north dipping Insubric line and of postcollisionalshortening (Figures 8e-8g). Complete the tip of the wedge). This displacementdrops to zero detachment near the top of the highly reflective Eu- at a point northwest of the Ivrea zone. Hence the Ivrea ropean lower crust indicates a strength minimum situ- geophysicalbody, which is located to the southeastand ated immediately abovethe lower crust. The unchanged underneath the NW dipping Insubric line, representsa thicknessof European lower crust all the way from the separate and shallower wedge that reachesthe surface northern foreland beneath the Adriatic wedge argues in the Ivrea zone. The contours of the Insubric line for a relatively high strength of this lower crust. This in Figure 9 have been determined from the profile of goesagainst a widespreadbelieve in an inherently weak Plate 1 and migrated sectionsof seismiclines S1 and lowercrust [e.g., Meissner,1989]. Lowercrustal rocks C2 [after Valasek,1992], from 3-D gravity modelingof whosetheology is largely controlledby that of feldspar the Ivrea body along profilesG1, G2, and G3 [Kissling, and/or mafic mineralsmay indeedbe weak under cer- 1980, 1982], and from field data [$chmidet al., 1987, tain circumstances(see discussion by Rutter and Brodie 1989]. 1060 SCHMID ET AL.: GEOPHYSICAL-GEOLOGICAL TRANSECT OF THE ALPS

Westward decreasingamount of wedgingof the Adri- 3-D gravity modelingby Ii'issling[1980, 1982]point to atic lower crust indicates that the amount of post early the existence of a velocity and density inversion un- MioceneN-S shorteningwithin the SouthernAlps (46 derneath the Ivrea body and a second Adriatic Moho km accordingto Table 1) also decreasestoward the (labeled"M" in Figure 10). This second(present-day) west. This points to the existenceof a left-lateral trans- Moho may be continuouswith the north dipping Adri- pressivetransfer zone running along the westernmargin atic Moho (Plate 1) eastof Figure 10. The causeof the of the SouthernAlps [Laubscher,1991] and possiblyto density inversion below the Ivrea zone is controversial. some counterclockwise rotation of the Southern Alps. Schmidet al. [1987]speculated that it may be due to Another left-lateral transpressivezone (the Giudicarie Cretaceous-agesubduction of parts of the Sesia zone belt) [Laubscher,1991], extending along the Giudicarie below the Ivrea body. Figure 10 showsthat the lower line (Figure 9), delimitsthe likely easterntermination crustalwedge of our transect(Plate 1) is not identi- of the Adriatic wedge. On geometrical grounds this cal with the Ivrea geophysicalbody and that the Ivrea wedgewhich underthruststhe Central Alps is replaced zone has been underthrust by this wedgewhich occupies by the indenter of the eastern Southern Alps which en- a deeper structural level. However, the exact geometry compassesthe entire crustal section situated south of of the underthrust material and associated overthrust the Tauern window [Ratschbacheret al., 1991]. Note blockswithin this part of the Southern Alps is unknown that the Giudicarie line does not appear to offset the and only drawn schematicallyin Figure 10. contourlines of the Europeanand Adriatic Moho (Fig- The southern extensionof the Ivrea geophysicalbody ure 9). This meansthat the Adriatic Moho underly- was encounteredalong the ECORS-CROP profileof the ing the lower crustal wedgein the profile of Plate 1 is WesternAlps [Nicolaset al., 1990]whose trace is situ- continuouswith the Moho underlying the indenter of ated southwestof the area coveredby Figure 9. From the eastern Southern Alps. On both sidesof the Giu- a purely geometrical point of view this profile, also dicarie line the Adriatic Moho overrides the uniformly characterizedby wedging, exhibits similarities with the southdipping EuropeanMoho. This confirmsthe kine- transect of the Swiss-Italian Alps. In the case of the matic link betweenpost lower Mioceneshortening in the ECORS- CROP profile, however,wedging involves large Southern Alps, movementsalong the Giudicarie line, slabs of mantle material accordingto gravity modeling and north directed indentation of the Adriatic lower [Rey et al., 1990]. Also, as previouslydiscussed with crustalwedge underneath the Penniniczone [Laubscher, the help of Figure 9, the lower crustal wedgeof Plate 1990; SchSnborn,1992]. i does find its western termination near the Canavese The 3-D problemsassociated with the westwardter- line and cannot be directly connectedwith the mantle mination of the Adriatic lower crustal wedge are far wedgeencountered along the ECORS-CROP profile. As from solved due to incomplete seismiccoverage in this discussedby Nicolaset al. [1990],it is not clearif these area. The profile of Figure 10 represents an attempt slabs formed by rupture of the entire European litho- to discussa possible relationship between the Ivrea sphere,or alternatively,by wedgingof an Adriatic man- body and the Adriatic lower crustal wedge. Exhuma- tle slice which representsthe western extensionof the tion of the Ivrea zone [$chmid,1993] was largely re- Ivrea geophysicalbody at a depthof 25-30km [Routeet lated to Mesozoicpassive continental margin formation al., 1990].According to Routeet al. [1990]this wedging near the continent-oceantransition (immediatelyad- must have occurredprior to the depositionof Neogene jacent to the Sesiaextensional allochthon indicated in depositsin the westernPo plain. Hencewedging in the Figure 5). Alpine orogenysteepened the entire crustal ECORS-CROP profile is contemporaneouswith back sectioninto its present-daysubvertical orientation (Fig- thrustingalong the Insubricline (Oligocene)and pre- ure 10) [Zingget al., 1990],thereby probably exposing dates wedgingin the transect acrossthe Swiss-Italian a segmentof the ancient Moho of the Adriatic crust Alps during the Miocene. at the surface. This led someauthors [e.g., Gieseand Reconciliation of our estimate of some 500 km of N-S Buness,1992] to postulate a dramatic changein the shortening along the profile of Plate i since the Pa- present-daytopography of the Adriatic Moho near the leocene with the smaller amounts of E-W shortening western termination of the Ivrea zone; the northward recordedin the WesternAlps [e.g.,Nicolas et al., 1990] dip of the Moho along the profile of Plate 1 is postu- remains a major problem, providing a challengefor fu- lated to changeinto a southeasterlydip, awayfrom the ture investigations.Laubscher [1991] pointed out that Ivrea zone. large amountsof strike-slipfaulting and independent This direct correlation of the paleo-Moho exposedin motionsof large blockssuch as the Adriatic block are the Ivrea zone with the present-dayMoho beneath the inevitable. The large amount of N-S convergencepos- Southern Alps is at odds with seismicrefraction data tulated alongthe easternSwiss traverse (Plate 1) indi- obtained along a refraction profile parallel to the Ivrea cates that much of the E-W shorteningin the Western zone[Ansorge, 1968]. Theseseismic data, togetherwith Alps must be due to an independentwestward motion SCHMID ET AL.: GEOPHYSICAL-GEOLOGICAL TRANSECT OF THE ALPS 1061 of the Adriatic blockduring the Neogeneand decoupled Acknowledgments. We would like to thank all the nu- from the Central Alps by dextral movementsalong the merous colleaguesand students who helped in making such Insubric line and its precursors.Deflection of a unique a compilation possible, both by their valuable research and through numerous discussions. Albert Uhr is thanked for west-northwest directed plate movement vector of the drawing the final version of Plate 1 with great skill. Rudolf Adriatic block (part of the Apulian microplate)into Triimpy and Alan Green helped to improve an earlier ver- north and west directed components of tectonic trans- sion of the manuscript. Reviews by Giorgio Dal Piaz and port (in the Westernand Central Alps, respectively) an anonymous reviewer are gratefully acknowledged. Much of the work presented in this compilation has been funded due to gravitationalforces [Platt et al., 1989]can hardly by various grants of the SchweizerischeNationalfonds, in lead to the very substantialamount of N-S convergence particular by the National Research Program on the Deep deduced in this study. Structureof Switzerland(NFP 20).

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