The Oligocene Central Swiss Molasse Basin As an Example Fritzschlunegg Erl and Teresa E

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TECTONICS, VOL. 16, NO. 5, PAGES 823-840, OCTOBER 1997

Controls of erosional denudation in the orogen on foreland basin evolution: The Oligocene central Swiss Molasse Basin as an example FritzSchlunegg erl and Teresa E. Jordan DepartmentofEarth Sciences, Cornell University, Ithaca, New York

EvaMafia Klaper GeologischesInstitut, Universitat Bern, Bern, Switzerland

Abstract. A high-resolution three-dimensionalreconstruc- 1. Introduction tionof the 25-m.y.-old central Swiss Molasse Basin reveals Forelandbasins have been extensivelystudied because they twosedimentary domains separated by a ~5-km-wide flood- chroniclein greatdetail the tectonicand denudationhistory of plain.The proximal domain of thebasin attained a width of 20 the boundingmountain belt [Jordan,1981]. Whereasit is now kin, and its basement is steeply flexed (6ø-7ø dip). widely understoodthat crustaland/or subcrustalloading con- Petrographicdata indicate that it wasfilled by sedimentfrom trols basin subsidence,relationships between erosional de- theRigi dispersalsystem derived from the central Alps of nudationand the geometricaland architecturalevolution of easternSwitzerland and by locally sourcedbajadas. In contrast, forelandbasins have been postulatedbut not yet adequately the distal sedimentary domain, located farther north, was investigated[Flemings and Jordan, 1989, 1990; Jordanand gentlydipping (<2 ø) and was filled by the meanderingLac Flemings, 1991; Paolaetal., 1992]. L6manand Honegg dispersalsystems. Chronological data re- vealthat sedimentationin the northern proximal part of the A successfulreconstruction of the relationships between basinstarted at ~27 Ma, when sediment supply to the basin crustalthickening and erosionaldenudation of the orogenand startedto increase.Deflection of the foreland plate at ~25 Ma the evolution of the adjacent foreland basin requires (1) is successfullysimulated by flexural modeling of the thrust knowledgeof the structuralgeometries and the deformation load and the sediment load. The model reveals that the Lac historyin the hinterland,(2) cooling ages to assessthe long- L•manand Honegg dispersal systems are locatedon a buried termrate and the spacialhistory of denudation,(3) a high-resolution reconstruction of the architecture of the foreland basin, flexuralbulge. Furthermore,it showsthat burial and suppres- sionof the flexural bulge at ~27 Ma as well as an increaseof (4) petrographicdata from the forelandbasin to locate the thebasin wavelength were controlledby the contemporaneous sourcearea, and (5) knowledgeof the mechanicalproperties of increasein the sedimentsupply rate of the Rigi system.The the forelandplate [DeCellesand Mitra, 1995]. The Oligo- modelpresented suggests that the tectonic subsidenceof the Miocene Alps/Molasse Basin, which is among the best MolasseBasin was mainly controlledby tectonicevents in the understoodorogen-basin systems in the world, meetsmost of northernpart of the orogen,within ~70 km distancefrom the theserequirements [Pfiffner, 1986; $chmidet al., 1996]. On tipof the orogenicwedge. Crustalthickening in this part of the basis of flexural modeling and high-resolution magne- theorogen is reflectedin the proximal Molasse by sedimen- tostratigraphy,Sinclair et al. [1991], Royden [1993], and tarycycles characterized by an increasein the sedimentaccu- $chluneggeret al. [199r/a,b] revealedthe stratigraphicaland mulationrates up section and by the presenceof locally geometricalreponse of the MolasseBasin to crustalloading in sourcedbajada fans at the top of each cycle. Althoughsouth theAlpine orogen. Using a forwarddiffusion model of moun- vergentback thrusting along the InsubricLine ~150 km south tain belt uplift and erosion, Sinclair and Allen [1992] pro- of the foreland basin contributed little to flexure, it resulted in posedthe conceptsof linking erosionaldenudation in the hinterland to basin architecture. anincrease of thesediment supply to the forelandbasin. This isreflected in the Molasseby coarseningand thickeningup- In thispaper, we reconstructthe three-dimensional architecture and the deflection of the 25-m.y.-old Molasse Basin of wardtrends, an increaseof the basin wavelength,basinward centralSwitzerland. The reconstructionis based on a compila- shiftsof the depocentersof the dispersalsystems, and uplift tionof publishedchronological and stratigraphicaldata and on anderosion of the proximalbasin borden newsedimentological and petrographicalstudies carried out for three outcrop sections. Further attention is focused on restoringthe Alpinethrust load and the sedimentload of the INowat Insitut fiir Geowissenschaften, Friedrich-Schiller-Universit•it Molasse Basin at ~25 Ma. This guessof the load distribution Jena,Jena, Germany. in the late ¸ligocene as well as determinationof the foreland platedeflection are used to estimatethe mechanicalproperties Copyright1997 by the American Geophysical Union. of theforeland plate by flexural modeling.The resultof these Papernumber 97TC01657. calculationsallows us to evaluate to what extent the sediment 0278-7407/97/TC_01657512.00 load controlled the basin subsidence. This evaluation will be

823 824 SCHLUNEGGERET AL.:MOLASSE BASIN AND ALPINE DENUDATION usedto discussthe significanceof sedimentsupply as a control [Berli,1985; Keller, 1989; Hurni, 1991; Schlunegger et on the geometricaland stratigraphicalevolution of the central 1993]. It endswith Serravalianfluvial elasticsof theOSM. Swiss Molasse Basin in the early Chattian (30-25 Ma) The Rupelianto early Chattiandeposits of central [Schluneggeret at., 1996]. Switzerlandcompose theLower Marine Molasse group (UMM• andthe lower Lower Freshwater Molasse group (USM I of 2. Geological Setting Schlanke[1974]). The UMM formsthe transitionfrom the earlyunderfilled "Flysch" stage of the northAlpine foreland The present-dayAlps are the resultof the continent-conti- basinto the overfilled "Molasse" stage [Sinclair et al.,1991; nentcollision between the Adriaticpromontory of Africa and Sinclairand Allen, 1992]. It consistsof deepmarine turbidites the Europeanplate that startedin the Late Cretaceous[Lihou at itsbase, overlain by a regressivesequence comprising off. and Allen, 1996]. The Alps form a doubly vetgent compres- shoremudstones, nearshore sandstone/mudstone alternations, sionalorogen that consistsof thin-skinnedand thick-skinned and shorefacesandstones [Diem, 1986]. At the Alpinethrust mostlyunmetamorphosed thrust sheets at their externalflanks front,contemporaneous fan deltas interfingered with the peri. andhigh-grade metamorphic rocks in their core [Laubscher, alpine sea(O. Kempf, personalcommunication, 1995). The 1990: Schi•'nborn,1992; Pfiffner, 1992; $chmideta!., 1996] USMI consistsof bajadasand alluvial megafans at the tip 0[ (Figurela). On the basisof the geologicalinterpretation of theorogenic wedge, which pass down current into conglomer. geophysicaldata, Laubscher[1990], Pfiffner [1992] and ate channelbelt, sandstonechannel belt and floodplainde. Schmidet al. [1996] could show that collisionbetween the two posits [Schluneggeret al., !997b]. platesoccurred by insertionof the Adriaticlower crust into the At the Alpineborder, the Oli goceneMolasse is presentin a interfacebetween the southdipping European lower crustand stackof southwarddipping thrust sheets (Subalpine Molasse), the Europeanupper crust (Figure 1b). which in turn are overlainby the Helveticthrust nappes In the north, the present-dayAlpine edifice consistsof the (Figure 2). The Plateau Molasse, which representsthe more Helvetic zone, which is subdivided into a lower Infrahelvetic distalpart of the basin,is mainlyfiat lying and gentlydips complex(Aar massifand its autochthonous- parautochthonous toward the Alpine orogen. However, synsedimentaryunder. cover)and an upperHelvetic (sensu stricto) complex (Helvetic plating of USM thrust sheets beneath the Plateau Molassere. thrustnappes), separated by the north vetgentbasal Helvetic sultedin backthrusting of the latterunit and in the develop- thrust (Figure lb). In the easternpart of the Swiss Alps, the ment of a classicaltriangle zone just north of the Subalpine Helvetic zone is overlain by a piggyback stack of Penninic zone [Vollmayrand Wendt, 1987; Staubleand Pfiffner,199I]. and Austroalpinesedimentary nappes (Figure la). The Central Schlunegger et al. [1997b] reconstructed the two-dimen. Alps comprise a stack of north vergent Penninie and sional architecture of the central Swiss Molasse at ~25 Ma, us- Austroalpine sedimentary and crystalline nappes. They are ing the thicknessesand the sedimentologicaldata from anout. separatedfrom the south verging SouthernAlps [Schtnborn, crop section(Rigi) and three boreholes (Entlebuch-1,Boswil- 1992] by the mainly E-W striking Insubric Line (Figure la). 1, and Htinenberg-1; Figure 2). These authors demonstrated This suturepartly accommodatedthe identationof the Adriatic that at ~25 Ma the Molasse Basin consisted of two parts promontory by steep south directed synmagmatic reverse (northernand southernsedimentary domains) that are separated faulting and right-lateral strike-slip movements [Pfiffner, by a---5-km-wide floodplain (Figure 3). The southernmore 1992; Schmidet al., 1996] (Figure lb). proximalpart of the basinattained a restoredwidth of --20km The early Chattian (30-25 Ma) and partly the Rupelian (<30 and dippedas much as 6ø-7ø southat the Alpine thrustfront, Ma) depositsof central Switzerland,which are the focus of this There the sedimentary fill consists of a ~3700-m-thick paper, are part of the Oligo/Miocene north Alpine foreland shallowing,coarsening, and thickening upward megasequence, basin,which evolved as a responseto the tectonic load of the comprisingthe marinedeposits of the UMM at its base(~100 evolving Alpine thrust wedge (Figure 2) [Homewood et al., m), overlainby the floodplain, conglomeratechannel belt, 1986; Sinclair et at., 1991; $chlunegger et al., 1997a]. The alluvial megafan,and bajadaalepositional systems (~3600 m) sedimentological development of the north Alpine foreland (Figure3a). In contrast,the northernmore distal sedimenta•• basincan be describedin termsof an early deepwaterstage and realm was--,30 km wide, and its basementdipped gentb a later shallowwater/continental stage which have been re- southward(< 2ø south).There the basinfill is representedby a ferredto as 'Flysch" and "Molasse"in the classicAlpine liter- <500-m-thickseries comprising the sandstonemeander belt ature (see discussionsof Sinclair et al. [199!] and Sinclair and and floodplainalepositional systems, and no coarsening Allen [1992]). The "Molasse" has been traditionallydivided upwardtrend is present.At the Oligocene/Mioceneboundary, into four lithostratigraphic units, for which the conventional upliftand erosion occurred in thesouthern proximal part of the Germanabbreviations are usedin this paper (Figure 2) [Matter basin,and the depocenters of the alepositional systems shifted et al., 1980; Keller, 1989]: Lower Marine Molasse (UMM), 12 km farthernorth into the more distal reaches[Schlunegger Lower Freshwater Molasse (USM), Upper Marine Molasse et al., 1997b].Note that south of the areaof Entlebuch-1,the (OMM), and Upper FreshwaterMolasse (OSM). They form two changeof dipof thebasement increased (Figures 3a and3b). shallowing upward megasequences.The first megasequence Determinationof the conglomerate clast population and the 'comprises the Rupelian UMM, which is followed by the heavymineral suite of sandstonesreveals the presenceof five Chattian and Aquitanian fluvial elasticsof the USM. The sec- dispersalsystems (Figures 3b and3c) [Fiichtbauer,1959, ond megasequence,starting with the Burdigalian transgres- !964; Gasser,1968; Miiller, 1971; Starm, 1973; Schlunegger sion, consistsof shallow marine sandstones(OMM), which etal., 1997b].The depocenter of thefirst system, referred toas interfingerwith major fan deltasadjacent to the thrust front LacLtman river, was located at thewestern end of theMolasse SCHLUNEGGERET AL.:MOLASSE BASIN AND ALPINE DENUDATION 825

/'Black

Penninic Gotthard

Penninic

F-22Map

frica•A e Southern Alps

Dispersalsystems: I: Rigi I1: Honegg II1: Lake Thun IV: Lac L6man

Sites of thermal data

_ (• fromthe dispersalsource areasystems of (Fig. 4) E u r o p e a n P I a t e Adriatic Plate N NorthernAlps CentralAlps •.•.i••, Insubrlc ' LineS A .....,,,,,,•,,•,,•,,,,. Austroalpine nappes ,,.- -"•• ..B,'-.-•"•-,, ,.-" .....qnut h_r• n Alpiner M H. P._-•-- -.'%,,-, •!111ll! _ • i• ' ,•; nappes• , 0 km

10 km

20 km

30 km . 30 km

40 km

50 km _ Man,.e B: Bergell Pluton f•,•7,• Gneiss& Granites Go: Gotthard 'massif' • Clastics H: Basal Helvetic thrust r ...5 Schists I•.• J P: Basal Penninic thrust A: Basal Austroalpine thrust [11111llli!11111Carbonates ."'""'t TertiaryPlutons M: Molasse Basin Figure1. (a)Geological mapof the Oligo-Miocene SwissMolasse Basin and the adjacent Alpine orogen. Themajor tectonic unitsare labeled. Themap also shows thelocation ofthe major north Alpine dispersal sys- temsthat fed the Molasse Basin of thestudy area. Th6 numbers 1-4represent locations of the thermal data showninFigure 4.Note that in the eastern Alps, the Penninic andAustroalpine nappesare part of the northern AlpsIn the analyzed cross section, however, theycrop out in the central Alps south ofthe Aar massif. (b) Syntheticcrosssection through thepresent-day Alpinehinterland. Theeroded Penninic andAustroalpine nappesareprojected intothe cross section toillustrate thearchitecture ofthe Alpine edifice prior to erosion (modifiedafter Schmid et al. [1'996]). 826 SCHLUNEGGERET AL.: MOLASSE BASIN AND ALPINE DENUDATION

% % % % % % % '% ,,, Ma N S I I œ•SM --Molasse .....

Zurich

HOne•b .,., ß

USM • 'Entlebuch-1

UMM'--

% % %

. NHF

%.•'. ,., uj •_[r-•'•Conglomerates • Siltstones Ii •"i i UpperFreshwater Molasse (OSM) [•-•ß Sandstones • Marls

['[ . 11 UpperMarine Molasse (OMM) Temporalcalibration of stratigraphic sections: • LowerFreshwater Molasse (USM) •I magnetostratigraphy (Schluneggeret al., 1977a) I ' :1 NorthLower MarineHelvetic MolasseFlysch (NHF) (UMM)& 0 petrographiccorrelation with i LowerOligocene ofRheintal Graben magnetostratigraphicsections (Schluneggeretal., 1997a, Schlunegger, 1995) "•• Crystallinemassifs seismostratigraphic correlation with • magnetostratigraphicsections ':•• HelveticTertiary (Schluneggeret al., 1997b)

Figure2. Geologicalmapof the Oligo-Miocene SwissMolasse Basin and the adjacent Alpine orogen with a generalstratigraphy ofthe foreland basin and the location of the studied transcot, theboreholes, andthe analyzed sections.

Basin~150 km southwestof the studyarea, from whereit mension,three additional representative sections (Rossberg, flowedeastward along the featheredgeof the basinto the Sattel,and Einsiedeln) were analyzed for sedimentaryfacies, analyzedcross section (Figure 3c). The Lac L6man system was palcoflowdirections, and petrofacies.These sectionsare 10- fedby thesecond system, the transverse Lake Thun dispersal cated in the SubalpineMolasse adjacent to the Alpinethrust systemthat originatedin the Central Alps of western front(Figure 2) andcomprise the depositsof thesouthern de- Switzerlandand that entered the basin80 km southwestof the positional domain. Each of these sections measures between studyarea (Figures la and3c). The third dispersal system, re- 2600and 3600 m in thicknessand comprises a ~600 -to 1200- ferredto as Honegg fiver, was derived from the Central Alps ca. m-thicksuccession of predominantlymudstones at thebase, 55km farther east and interfingered with the Lac L6man system overlainby alternatingconglomerates and mudstones [Miiller, inthe Zurich area (Figures la and3c). These dispersal systems 1971; $tiirm, 1973; $chlanke, 1974]. occupiedthe northernsedimentary domain of the Molassein the analyzedcross section (Figure 3b). The strata in the southerndepositional domain were deposited by the Rigi 3. Three-Dimensional Reconstruction dispersalsystem and by local systems (Figures 3b and3c). The of the Foreland Basin Architecture Rigi fiver wasderived from the CentralAlps of eastern and Tectonic Controls Switzerlandand deposited the conglomerates andmudstones in theproximal part of the analyzedtransect (Figure la). The local dispersalsystems, however, were sourcedfrom the 3.1. Methods frontalAlpine units and formed the bajadas bordering the Floodplain,conglomerate channel belt, and bajadadeposi- Alpinethrust front (Figures 3b and3c). tionalsystems were differentiated following $chlunegger etal. In orderto extend the initial two-dimensionalreconstruc- [1997b]. The thickness of each unit was calculated based0n tion of the 25-m.y.-oldbasin architecture into the third di- measureddip angles of thebeds, new mapping,and a eompila- SCHLUNEGGERET AL.: MOLASSE BASIN AND ALPINE DENUDATION 827

Foreland basin Alpinewedge

DepositionalSystems

Boswil-1 H0nenberg-1 Entlebuch-1 Rigi

_____2 ø

B: Bajada AM: Alluvial megalan CC: Conglomeratechannel belt SC: Sandstone channel belt o 5 km F: Floodplain

Dispersal Systems Locally sourced

• Schafisheim(•Zurich Boswil-l• • \ •,Hdnenberg-1

i ,, '0• __ __50,km50km • k,•,•••pihe,, dispears alsyst ems• (• Figure3. Palcogeographicdiagramforthe time interval between 30and 25 Ma, showing (a)the basin ge- ometryandarchitecture, (b)the location ofthe dispersal systems, and(c) the drainage pattern [$chlunegger et al., 1997b]. tionof thegeological maps of Buxtorfet al. [1916], Ottiger et on foursites is justifiedbecause (1) exceptfor a few caseswe al.[1990], Maller [1971], $tiirm [1973], $chlanke [1974], and usedmultiple temporal calibrations for each thermalstage $chluneggeret al. [ 1997a]. (Figure4), (2) weadded error bars that comprise uncertainties Palcocurrentswere determined from furrows, large-scale in thetemporal calibration of eachthermal stage and uncer- (>0.5m) trough cross beds, and imbricate clasts in the con- taintiesin the closuretemperatures of the differentradiometric glomerates.The tectonictilt in the SubalpineMolasse was systems,and (3) we used the thermal data only to qualitatively removedin order to determinethe palcoflowdirection at the estimatethe change of sedimentsupply to the basin. timeof deposition.Petrofacies were determined by giving specialattention to the firstappearance of distinctiveclasts. 3.2 Results Wesought to estimatethe relativeabundance of sediment supplyto the basin throughtime throughuse of ther- Mappingof lithofaciesand petrofaciesreveals that the de- m0chronologicaldatafor four siteslocated in thesource area positsof theRigi and locally sourced dispersal systems, found ofthe Alpine rivers [Hunziker et aL,.1992] (Figures la and4). in thesouthern sedimentary domain (Figure 3b), are presentin If thepre-20-Ma exhumation in this part of the orogenwas all theanalyzed sections adjacent to theAlpine border between mainlycontrolled by erosionaldenduation which we will as- Einsiedelnand Rigi (Figure5a). At Rigi, a well-developed sumetobe true [e.g. $teck and Hunziker, 1994], the thermal coarseningand thickening upward megssequence, comprising dataare an indirect measurement of the sedimentsupply to the the floodplain,conglomerate channel belt, and alluvial basin.Indeed, a careful estimate of thesediment supply to the megafan/bajadaalepositional systems, together with the basinwould require a compilationof coolingages that covers changefrom initially axial to transversepalcoflow directions, thewhole source area. However, our simplistic approach based indicatefan progradation(Figure 5a). Alongstrike from west 828 SCHLUNEGGER ET AL.: MOLASSE BASIN AND ALPINE DENUDATION

Source area of the Rigi system

Site 1 Site 2 Age (Ma) Age (Ma) 0 10 20 30 40 0 10 2O 30 40 0

200 200

400 •-400

600

Gotthard 'massif'

,•numberof measurements Penninicnappes, East /• numberofmeasurements

Sourcearea of the Honegg-Thunersee(sites 3 & 4) and the Lac L•man systems(site 4)

Site 3 Site 4 Age (Ma) Age (Ma) 0 10 20 3O 40 0 10 20 30 40 0 0 -' ' i ß 200 (• 200 •400 •c•.400 _. , __

b--600 .. _ I.- 600

..,

Aar massif, West Penninicnappes, West //•',,number ofmeasurements /• numberofmeasurements Figure 4. Thermalevolution of thecatchment area of theRigi and Lake Thun systems. The cooling ages for gneisssamples form the Gotthard "massif" were determined with the following methods: (method a) apatiteand (methodb) zirconfission track [Michalski and Soorn, 1990], (method c) Rb-Srand K-Ar on biotite[Soom, 1990;Dernpster, 1986], and (method d) formationages of hornblende[Steiger, 1964; Deutsch and Steiger, 1985].The thermal data of thePenninic nappes in theeast represent (method a) apatiteand (method b) zircon fissiontrack ages, (method c) K-Arand Rb-Sr ages measured on biotite,and (method d) formationages of muskovite[Jigger etal., 1967;Giger, 1991; Hurford, 1991; Hunziker et al., 1992].For the Aar massif,the fissiontrack ages of (methoda) apatiteand (method b) zirconand the formation ages of (methodc) biotiteare takenfrom Dernpster [1986] and Michalski and Soom [1990]. A fulldiscussion of the thermal evolution of the Penninic•appes in thewest was published byHurford [1986], Hurford et al. [1989]and Engi et al. [1995] ((methoda) apatite and (method b)zircon fission track data, (method c) Rb-Sr and K-Ar cooling ages measured onbiotite, and (method d) formationages of metamorphicfabrics).

to east, the basal floodplain depositionalsystem thickens Einsiedeln.Furthermore, maximum clasts of >50 cmare ex- from initally400 m at Rigi to approximately1200 m at posedat Rigi, from wherethey fine to <20 cm at Einsiedeln. Einsiedeln(Figure 5a). Eastwardthickening of thefloodplain Thislateral change of faciesindicates that the point of entryof faciesis offset by a thinningof the conglomeratefacies Rigi riverinto the basinmight have been located at Rigi,from (conglomeratechannel belt, alluvial megalan, and bajada de- whereit flowedeastward toward Einsiedeln as supportedb) positionalsystems) from 2800 m at Rigi to <1000 m at paleoflowdirections (Figure 5a). SCHLUNEGGERET AL.:MOLASSE BASIN AND ALPINEDENUDATION 829

w Thickness Rigi R . E. Ei.... deln oHtu•nenb• (m) ossoerg S. Sa,e• I t I •?..,E Bajada • . Ro: RossbergI L..... •.•,.•' deposystem • Balaaa R Rini I •'/;'7 TM'"•' .... I 320(

•E • 6 •• • •'• • • Balada Eins•edeln 240•

1601 • • " • .... •_27ua--o• ......

deo. system I Il • Conglo•rat•I kilometers • Mu•t•es > / Siltstones S Rigi H0nenberg-1Boswil-1ioaosw•.••-• Thickness •HQnenberg-1•

• c• • • (.•....•.• (m) ' - "'

I I • [ { • e • -3000-• ' ,ooI•• • • •= •oo-• .,oo.E

• • 0 kilometers Ro•plain • > depo.•stem

Figure 5. (a) Stratigraphyand sedimentology of the analyzed sections located in the southernsedimentary domain,revealing the along-strike sedimentological change of theRigi dispersal system. Solid lines indicate faciesboundaries. Dashed lines show chronologic correlation. (b) Stratigraphyand sedimentologyof three representativesections located in thesouthern (Rigi) and northern (Htlnenberg-1 and Boswil-1) sedimentary realms.

Thedeposits of the Lac L6man and Honeggdispersal sys- talline clasts derived from the Central Alps of eastern tems,found in the northernalepositional domain at Htinenberg- Switzerland(Figure 6a). Farthereast, between Rossberg and 1 andBoswil-1, are an alternationof sandstones,siltstones, Einsiedeln(Figure 6a), the samepetrographic change coin- andmudstonesthat reflect the presenceof a sandstonemeander cideswith initiation of coarseningand thickening upward beltalepositional system (Figure 5b) [Schluneggeret al., trends(Figure 5a), suggestingenhanced fan progradationat 1997b].In contrastto the moreproximal southern part of the that time. Furthermore,the magnetic-polarity-basedchronol- basin,however, no verticalsedimentary trend is detectedin the ogyestablished at Rigi indicates that at ~27 Ma, thesediment- Htinenberg-1and Boswil-1 wells (Figure5b). accumulationrate increasedfrom initially 700 m/m.y. between Reconstructionof the facies relationshipsin the detailed 30 and 27 Ma to more than 1000 m/m.y. between27 and 25 temporalframework provided by magnetostratigraphyat Rigi Ma [Schluneggeret al., 1997a]. revealsthat sedimentation of fluvial depositsstarted at ~30 Ma The three-dimensional architecture of the Oligocene at the proximal basin border (Rigi section,Figure 6a). Molasse Basin reveals that the Molasse deposits onlap pro- Correlationof the HQnenberg-1and Boswil-1 wells by magne- gressivelythe unconformity above the top of the Mesozoic tostratigraphyand seismostratigraphyto the Rigi section carbonates(Figures 6b and 7), resultingin an angularuncon- [Schluneggeret al., 1997a,b] showsthat at Htlnenberg-1and formitybetween the forelanddeposits and the underlying Boswil-1accumulation of sedimentbegan at ~27 Ma (Figure basement.However, between 30 and 28 Ma, sediments were 6b).Also at ~27 Ma, the alluvialmegafan alepositional system predominantlydeposited in the southern,more proximal part •tartedat Rigi,associated with the first appearanceof crys- of the basin(Figures 6b and 7). There the angleof unconfor- 830 SCHLUNEGGERET AL.:MOLASSE BASI]q AND ALPINEDENUDATION

Ma W E Ma 24-• Rigi Rossberg Sattel Einsiedeln•--24

• •• • .'•••-'-r•-L_,•'-."•- -."., ß ,, ,J-'---7_•_,.'_-."_,."..,."_-, . ,•_•[,_•?_• I ß' ß ß -26

First appearance of crystalline clasts

ßChannel conglomerates ...... 28 • 28 ...... - Floodplain •---

30 I UMM -30

5 Kilometers

Ua S N Ua Rigi HOhenberg-1 Boswil-1

Uplift and erosion

AlluVial'm•g•fa•n,."'.,," ' ' ' Channel sandstone I -•Flood Onlap Cha•nn;•l•o•gl;:::)n';er;tel i '• ,•- ...... •,.•Onla

10 kilometers

Figure 6. (a) Wheeler diagram of the proximal basin border in a section along the thrust front, revealing stronglyheterochronous facies relationships.This diagram is basedon the magnetostratigraphiccalibration of the Rigi section[Schlunegger et al., 1997a]. The transition from the Lower Marine Molasse (UMM) to the terrestrialfloodplain facies is isochronousaccording to Schluneggeret al. [1996, 1997a]. However, except for the Rigi sectionno chronologyis availableto temporallycalibrate the first appearanceof crystalline clasts. Nevertheless,this petrographicchange is likely to be isochronousbetween Rigi and Einsiedeln given the short distanceof ~15 km. (b) Wheeler diagramof the Oligocenecentral Swiss MolasseBasin, showing the temporalrelationships between the proximaland the distal depositionalsystems (modified after Schlunegger etal. [1997a and b]).

mity between the Molasse deposits and the Mesozoic base- tures[Frey et al., 1980a, b; Erdelbrock,1994; Rahnet al, ment measured <3.5 ø. At--.27 Ma, accumulation of sediment 1994, 1995] indicatethat forwardthrusting of the Helvetic startedat HQnenberg-1and Boswil-1 almost contemporane- unitsand the piggyback stack of Penninicand Austroalpine ously (Figure 6b). This implies that, at ,-,27 Ma, the angle of nappesoccurred along the basal Alpine thrust between --30 and onlap betweenthe forelanddeposits and the Mesozoic carbon- 25 Ma (Calandaphase of deformation[Milnes and Pfiffner, atesdecreased significantly to •1.5 ø (Figure 7). 1977,1980]). Furthermore, to explain the increase of thesediment-accumulationrate at Rigi at ~27 Ma, we interpretthat at thattime, the rate of crustalloading at the Alpinethrust front 3.3. Interpretation increased,resulting in enhancedbasement deflection and hence 'The continuoussubsidence at Rigi between30 and 25 Ma is an increaseof the sedimentaccumulation rates [e.g., Flemings interpretedto have been causedby crustal loading by forward andJordan, 1989, 1990].Enhanced crustal thickening at the thru• 'ng of the frontal Alpine nappes. Indeed, relationships thrustfront is likely to haveincreased the topographic between temporally calibrated metamorphicfabrics and struc- gradient,which in turnwould promote enhanced erosional SCHLUNEGGERETAL.: MOLASSE BASIN AND ALPINE DENUDATION 831

N S

E LacL6man system Honeggsystem Rigisystem

.. <. / /1 - ..... • "•M[•• ' ' ' • ' ' ' Boswil-l•( HOne•b•g-1 '••, R' .

Averageangle of • • .• . z,,{• - - ß unconformity • 3.5ø • 2eUa • • •• . Average angle o -- unconformity

3O Ma

Figure7. Three-dimensionalreconstruction of the architecture of the 25-Ma Molasse Basin Depositional systems:AM, alluvial megafan; B, bajada; and CC, conglomerate channel belt. Stratigraphic sections: R, Rigi section;Ro, Rossbergsection; S, Sattel;and E, Einsiedeln.

denudationof the frontalAlpine unitsand thusexplain the (Figure4). Thisimplies that (1) thetotal supply of sedimentto presenceof locally sourced bajada fans at thetop of the the studyarea increased between 30 and20 Ma and (2) an sectionsbordering the frontal Alpine nappes [e.g., Paola et enhancedincrease of the averagesupply rate of sedimentis at., 1992] (Figure 7). likelyto haveoccurred at ~27 Ma. Usingdiffusion models of mountainbelt uplift and erosion, FlemingsandJordan [1989, 1990], Sinclair et al. [1991],and Paolaetal.[1992] tested the consequence of enhanced crustal 4. PalinspasticRestoration of theOrogenic loadingin thefrontal part of an orogenin whichall other and SedimentaryLoad at 25 Ma variablesremain constant. They concludedthat this situation causesan increaseof the accommodationspace/sediment sup- 4.1. Methods plyratio in theforeland basin. As a result,the facies bound- ariesretreat with respect to thethrust front. The stratigraphic In orderto calculatethe curvature of the forelandplate at 25 dataof theMolasse Basin, however, suggest that the time pe- Maand thereby estimate the strength of thelithosphere during riodof enhanced crustal loading at thethrust front (27-25 Ma) formationof the forelandbasin, it is necessaryto restorethe isassociated with an increaseof the progradationrate of the orogenicload for that time. In this paper,we usedthe Rigifan, expansion of the width of thebasin, and a decreaseof tectonostratigraphicbalanced cross section of Schmidet al. theangle of unconformitybetween the Molasse deposits and [1996](Figure lb). Despiteits obliqueorientation with re- theMesozoic basement. This implies that the sediment supply spectto the Alpine strike and despite the rather large distance ofthe major contributors of sediment to thestudy area (Rigi tothe study area (~50 km), we justify the usage of thissection andLake Thun rivers [Schlunegger et al., 1997b])must also because(1) it partlycovers the source area of theRigi river haveincreased significantly at ,--27 Ma. Indeed, metamorphic (Figure la), (2) itstectonic architecture and structural evolu- andcooling ages suggest that in thesource terrain of theRigi tionare reasonably well known [Schmi Iet at., 1996],and (:3) river,maximum exhumation rates occurred between 32+5 Ma the mechanicalstructure for this partof the Alps hasrecently nd25+2 Ma (Figure4). In thehinterland of theLake Thun beenexplored [Ol•aya et at., 1996a]. ri•er,maximum erosion rates appear to haveoccurred between At 25 Ma, thePenninic, Austrolapine and Helvetic nappes 27• Maand 22+2 Ma atan average rate that was ~50% higher hadalready been emplaced [Milnes and Pfiffner, 1977, 1980; thanthe one determined for the sourcearea of the Rigi river Burkhard,1988], and the Insubric phase of backthrusting in 832 SCHLUNEGGER ET AL.: MOLASSE BASIN AND ALPINE DENUDATION southeasternSwitzerland with a vertical displacementof >10 fledbecause the contribution of thesestructures to the total km as well as the intrusion of the Bergell pluton were almost amountof shorteningis negligible[Schmid et el., 1996]. completed[Gulson, 1973; Koeppe!and Griinenfelder,1975; In additionto theserestored offsets (Figure 8a), the lead Hurford,1986; $chrnid et el., 1989]. FollowingSchrnid et el. theAl•)ine orogen added onto the foreland plate at ~25 [1996], we restored a total of ~85 km of relative movements (Figure8c) must include material lost by exhumation.This was betweenthe Europeanand Adriaticplates, which compose~35 determinedby calculatingthe overburdenof the tecton0strati. km of shorteningnorth of the InsubricLine, and ~50 km of graphicunits using (1) temporallyand thermally calibrated shorteningin the Southern Alps [SchOnborn,1992] that are, metamorphicmineral paregeneses and coolingages for the however,badly constraineddue to imprecisechronological Alps(see the next paragraph),(2) an averagedensity of 2700 data [Schmidet el., 1996, Figure4]. North of the Insubric kg/m3 forthe Alpine rocks [Okaya et el.,1996a], and (3) aver. Line, we restored (1) 6 km of shorteningfor the Jura agegeothermal gradients of 20øC/km,25øC/kin, and 30øC/Ira Mountainsof easternSwitzerland [Neelet el., 1985], (2) 24 that are consideredto take into accountthe major uncertain. km for the SubalpineMolasse/Aar massif system [Pfiffner et ties.Of these,large uncertaintiesexist for the thermochr0n0. el., 1996;$chlunegger et el., 1997a],O) 5 km of shortening logical data and the geothermalgradients. Seem [1990] for the Helveticthrust nappes (Ruehi phaseof deformation demonstratedone approachof evaluatinggeothermal gradi. [Milnesand Pfiffi•er, 1977, 1980; Erdelbrock,1994; Rahnet ents. He showed,using apatite fission track data, a linearrela- el., 1995; Wanget el., 1995]) that is interpretedas gravita- tionship between the elevation above sea level wherethe tional collapse[Menkveld, 1995], and (4) <1 km of shorten- sampleswere collected and the time whenthe samplespassed ing for the sinistral transpressionalmovements along the throughthe closuretemperature of apatite.Extrapolating aver. EngadineLine [Schmidand Froitzheirn,1994]. In this restora- agepresent-day uplift ratesto the upperMiocene and using the tion, east directed extensionalfeatures in the Penninic nappes elevation/agerelationships of the apatite fissiontrack data, (Forcolaphase of deformation[$chrnid et el., 1996]) as well as Seem [1990] determined geothermal gradients between small-scalecompressional structures in the Penninit thrust 25øC/km and 30øC/km. However, thermalmodeling of a c0ali- nappes(e.g., crenulationfabrics) are discarded.This is justi- fication profile in the Subalpine Molasse of central

r o p e a n P I a t e Adriatic Plate InsubdcLine S

- ß I ß ;•- •1

• 50krn •'••2"r-•••Manrio Mantle l½: Infrah0lveticcomplex

Clastics I• OphiolitosSchists Go:Be: GotthardBergell pluton 'massif' Carbonates ' ' H: BasalHelvetic thrust • Gneiss& Granites P: BasalPenninic thrust A: Basal Austroalpinethrust IL: Insubric Line

soo ---"'"' '""•',, (• 25ø/kin 4oo (•) 30ø/kin Orogenicloado . Distance1•) 20ø/kinfrom thrust Ifront 0 5i1 1(•0 1•0 200 (kin)

Figure 8. (a) Restoredsection across the 25-Me Alps.The thicknessof the Alpineedifice was calculatedus- ingaverage values of temporallyand thermally calibrated metamorphic mineral paregeneses and a geothermal gradientof 25øC/kin.The errorbars represent possible variations in the crustalloads due to uncertaintiesin the geothermalgradient (20ø-30øC/km) and in thetemporal and thermal calibration of themetamorphic fabrics and theclosure temperatures. See text for discussion.(b) Oregenielead determined for a geothermalgradient of 25øC/kmand for theaverage temperatures of the buried Alpine units shown on Figure8a. (c) Effectiveoregenie leadadded onto the platefor geothermalgradients of 20øC/kin,25øC/km, and 30øC/km.Note that we usedthe averagetemperatures of the buried Alpine units at 25 Ma (Figure8a) asreference. SCHLUNEGGERET AL.: MOLASSEBASIN AND ALPINEDENUDATION 833

Switzerlandsuggests that in thelate Oligocene the geothermal due to an appliedload is a function of (1) the continuity of the gradientmeasured between 20ø and 23.5øC/km [Schegg, 1994]. plate, (2) the rheologicalproperties of the plate (elastic vs. Alternatively,onthe basis of theresults of a thermokinematicviscoelastic), (3) the distributionand magnitudeof the load, modelofthe Alpine orogeny, Okaya et al. [1996b]concluded and (4) the mechanicalproperties of the plate, expressedby thatin activecollisional zones temperature gradients change the flexural rigidity [Hetdnyi, 1946; Turcotte and Schubert, dramaticallywith depthand laterally through the collision 1982; Flemingsand Jordan, 1989; Peper, 1993; Johnsonand zone.Their model suggests that immediatelynorth of the Beaumont,1995; Okayaet al., 1996b]. !nsubricLine the present-daygeothermal gradient decreases A first evaluationof the mechanicalproperties of the north from20øC/km in uppercrustal levels to <5øC/kmin thelower Alpine foreland plate was performed by Karner and Watts crust. [1983] using gravity anomalies. These authors estimated an Theaverage temperatures of the buriedAlpine nappes at .-,25 elastic thickness(Te value)between 20 and 50 km for the plate Maarepresented onFigure 8a. At that time, progressiveheat- underlyingthe Alps. A more derailedstudy was carriedout by ingto ,-,250øC +--50øC of the Infrahelvetic complex was com- Lyon-Caenand Molnar [ 1989]. These authorsmodeled the pre- pleted[Rahn etal., 1994;Wang et al., 1995].Also at --,25Ma, sent-dayBouger gravity anomaliesfor four sections across the Rb-Srand K-Ar agesdetermined on biotite suggestthat the Alps and the MolasseBasin. They revealedthat the measured centralAar massifreached temperatures of ~350øC __.50øCac- completeBouger gravity anomaliesover the core of the pre- cordingto Dempster[1986] and Soom [1990]. The thermal sent-dayAlps are similar to those calculated assuming local datafor theGotthard "assif" is taken from Steiger [1964] and isostaticequilibrium but are makedly out of isostatic equilib- Deutschand Steiger [1985]. These authorssuggest that at ~25 rium in the areas surrouhclingthe Alps, for example, the Masynmetamorphic growth of hornblendeoccurred at a tem- MolasseBasin. The Bouger gravity anomalies of this basin peratureof ~450øCñ50øC. The thermalevolution of the was most successfullysimulated by these authors for a weak Penninicnappes of the Central Alps and the Bergell pluton plate with an elastic thickness of ~15 km. However, such a (Figureslb and 8a) is presentedand discussedby Hurford weak elastic plate does not account for the excessof mass in [1986],Hurfordetal. [1989], and Giger [1991]. Theseauthors the Vosgesand the Black Forest (Figure la). On the basis of suggestthat at ~25 Ma the crystallinecore of the Penninic these findings, Lyon-Caen and Molnar [1989] concluded that nappescooled down to ~550øCñ100øC (Figure 8a), whereas gravity anomaliesdo not place a useful constraint on the flex- thetemperature of the Bergellpluton measured ,-,180øC. ural rigidity of the plate or on the forces that formed the Thesediment load of the Molasse Basin (Figure 9a) was de- Molasse Basin. terminedusing the measuredthicknesses of stratigraphicsec- A careful assessmentof the mechanical properties of the ti0ns(Figure 3a) and a densityof 2400 kg/m3 for sedimentarynorth Alpine foreland plate at 17 Ma was carried out by rocks[Turcotte and Schubert, 1982]. Sinclair et al. [1991]. These authors determined the mechanical properties of the plate by simulating the deflection of the 4.2. Results basement,using a constant taper of 28 ø for the Alpine orogenie wedge [Pfiffner, 1986] and a broken plate model. They Therestoration of the Alpine edifice in the late Chattian is changedthe elastic thicknessof the plate (Te value) and the presentedon Figure 8a. Despite the uncertaintiesthat arise width of the orogenicload iteratively until they reached a best fromthe poorly constrainedgeothermal gradient and from the fit simulationof the foreland plate deflection. They concluded ratherlarge error bars in the temporal and thermal calibration that, at 17 Ma, the elasticthickness of the foreland plate mea- of themetamorphic fabrics and the closure temperatures,our sured 10ñ5 km and that the deflection of the basement was restorationreveals that the A ustroalpinenappes, which formed controlledby a 100-km-wide thrust belt. BecauseSinclair et al. theorogenic lid, were almost completelyeroded at that time. [1991] iteratively changed the tectonic load and the elastic Furthermore,according to Figure8a the Helvetic thrustnappes thicknessof the plate as well, their determinationof the me- werenot exposed to erosion at 25 Ma. These findings are chanical properties of the foreland plate is nonunique. consistentwith petrographicdata collectedfrom the Molasse Nevertheless, these authors stated that it would be difficult to Basinof centralSwitzerland, which indicate that (1) the clasts usea Te value that was much greater than 15 km if parameters derivedfrom Austroalpine nappes disappear at ~24 Ma in the were kept within reasonablebounds. studyarea [Tanner, 1944; Speck, 1953; Schluneggeret al., Using gravity dam, the curvature of the basement between 1996a],and (2) erosionof the Helveticthrust nappes started at the featheredgeof the Molasse Basin and the thrust front, and a .-15Ma [Matter, 1964]. Furthermore,Figure 8b andFigure 8c broken plate model, Royden [1993] determined an elastic showthat the tectonic load in the northern and southern thicknessof ,-,50 km for the north Alpine foreland plate. or0genincreases from the proximalborder o• the northernand FollowingRoyden, the deflectionof the present-daybasement southernforelands to the Penniniccrystalline nappes, where beneaththe Alps and the MolasseBasin is controlledby the highestcrustal loads of ~1200MPa were present. orogenicload with contributionsof a vertical shearforce and a bendingmoment exerted at the end of the plate. The model of 5. Modelingof theForeland Basin Deflection Royden,however, fails to explain the steep curvatureof the plate beneaththe Molasse Basin and the limited width of this Regionalisostatic compensation is the prinicipaldriving basin [e.g., Sinclair et al., 1991]. forceof the subsidence in foreland basins, as the lithosphere The mostrecent estimate of the present-dayflexural rigidity accommodatestheloading by flexure[Watts and Ryan, 1976; of the Europeanplate was publishedby Okaya et al. [1996b]. Jordan,!981; Beaumont, 1981: Flemings and Jordan, 1989; These authors determined the mechanical structure of the litho- Sinclairetal., 1991;Jordan, 1995]. The deflection of a plate spherethrough the Alpine collision by calculatingstrength 834 SCHLUNEGGER ET AL.: MOLASSE BASIN AND ALPINE DENUDATION

BoswiI- 1 Hf• nen be rg- 1 Entlebuch- 1 Rigi

Load (MPa) Load of strata lOO within the basin

o T f I T • 5O 40 30 2O 10 o Distance from thrust front (kin)

Geothermalgradient = 30øC/km

Te=10.0km A Te=15.0km A Te=20.0km A Te=25.0km Distance(km) I Distance(km) j Distance(km) I Distance(km) I I 80 60 40 20 •! 80 60 40 20 • 80 60 40 20 • 80 60 40 20 • I 2000:,• L• 4000•--1 Modeled Subsidence Backstripped Subsidence

Geothermalgradient = 25øC/km

Te=5.0km A Te=10.0km A Te=15.0km A Te=20.0km A Distance(km) I Distance(km) Distance(km) I Distance(km) 806• 0 ,,• _-•__-.•.80 60 ,-4_40 20i • 8060 40 20 • -•80 •0 40 20 • /

• 4000 Modeled Subsidence BackstrippedSubsidence

Geothermalgradient = 20øC/km

Te=5.0km A Te=10.0km A Te=15.0km A Te=20.0km A Distance(km) 80 Distance60 40(km) 20 j 80 Distance60 40(kin) 20 • . 80 Distance60 40(km) 20 •I 2000• 4000• Modeled Subsidence BackstrippedSubsidence

Figure 9. (a) Crustalloads exerted by thesediments of the Molasse Basin. The magnitude of theseloads was determinedusing the thickness of strata (Figure 3a) anda density.of 2400 kg/m 3. (b) to (d) Modelresults, re- yealingthe profiles of thenorth Alpine foreland plate determined fordifferent Te values and geothermal gradi- entsof (b)30ø/km, (c) 25øC/km, and (d) 20øC/km. The tectonic subsidence was determined by backstripping. Seetext for furtherexplanation. SCHL••ER ET AL.:MOLASSE BASIN AND ALPINEDENUDATION 835 envelopesacross the orogen. This was performed byconsider- magnitude of the appliedload and the effective elastic thick- ingdepth profiles of lithosphericmaterial, rheology, strain nessof the plate. Low crustalloads that were obtained for a rate,and temperature that was gained from thermokinematic geothermal gradient of 30øC/km fail to explain the high slope modeling[Okaya et al., 1996a].Okaya et al. determinedTe of the basementadjacent to the thrust front (Figure 9b). Good valuesfor the north Alpine foreland plate that scatter between fits betweenthe back strippedsubsidence and the modeledsub- ~25and 40 kmdependent on whetherthey used a brokenor a sidenceare achievedfor (1) higher crustalloads that were de- continuousplate or whetherthey assumed that the crust and the terminedfor geothermalgradients of 20øC/km and 25øC/km mantleare mechanically coupled or decoupled. and (2)very low Te values between 10 and 15 km (Figures 9c Inthis paper, we used a modelfor a continuouselastic beam and 9d). This implies that the European foreland plate was overlyinga fluid substratum, the cross-sectional geometry of weak during deposition of the Lower Freshwater Molasse theMolasse Basin (Figure 3a), the distributedload of the Alps group. An elastic thicknessof ~10-15 km, which was deter- at.-25 Ma (Figure8c), andthe numericalsolutions given by minedby includingthe Alpine orogenicload and the sediment Pletgnyi[1946] and Jordan [1981] to simulatethe tectonic load of the basin, is consistentwith the low Te values of 10+5 subsidenceof the MolasseBasin and therebyassess the me- km determinedby Sinclair et al. [ 1991]. chanicalproperties of thenorth Alpine foreland plate. We jus- Despite the successfulsimulation of the deflection of the tifythe usage of a continuousrather than a brokenplate model 25-m.y.-old Molasse Basin, the presentedmodel has several becauseseismic, structural, geoehronological, and strati- drawbacks:(1)crustal heterogeneitiessuch as reactivatednor- graphicaldata indicate that insertion of theAdriatic lower crust mal faultsas well as thermal and densityanomalies [Rey et al., intothe interface between the south dipping Europeanlower 1990; Laubscher, 1990; Stiiuble and Pfiffner, 1991; Okaya e t crustand the European upper crust started in the Late Cretacous al., 1996a] were discarded in the model, which, however, [Schmidet al., 1996;Lihou and Allen, 1996], whichin turn might changethe elasticproperty of the crust [Washbuschand suggeststhat the Europeanand Adriaticplates behaved with Royden, 1992; Okaya et al., 1996b], (2) the assumptionthat continuousrather than broken plate conditions since late the mechanicalproperties of the Adriatic plate are the same as 01igoceneat the latest (Figure 8a). Indeed,any verticalmove- thoseof the Europeanplate might be incorrect[Royden, 1993; mentof the Europeanplate had to be accommodatedby deflec- Okaya et al., 1996a], (3) possible three-dimensionalhetero- t•onof the Adriatic plate or plastic flow of rocks in order to geneitiesin the orogenic load are discardedin our model, and keepa volumetricalbalance. The identationof the two plates (4) major uncertaintiesarise from underestimatesor overesti- impliesthat, although the palcogeographicallydistinct matesof the orogenic load (see section4). Nevertheless,our Europeanplate is discontinuousand brokenand although the model supportsthe findings of Lyon-Caen and Molnar [1989] Europeanand Adriatic plates are likely to have differentflexu- and Sinclair et al. [1991] that the strength of the European ralstrenghts [Royden, 1993; Okaya et al., 1996b], the me- lithosphereis possiblyvery low beneaththe Molasse Basin or chanicalas well as the rheological unit was continuous be- even zero beneath the roots of the Alps. If these conclusions neaththe Alps during the "Molasse" stage of the basin evolu- are correct, then differences in the flexural strength between tion. Furthermore, because sediment-accumulation rates and the Europeanand Adriatic plateshas an insignificant influence admixture and size of clasts derived from the frontal thrusts in- on the forces that caused the Molasse Basin to subside creasedsimultaneously (see above), we interpret that the fore- [Sinclair et al., 1991]. Furthermore, using a weak foreland landplate reacted instantaneously to crustal loading events. plate, we can explain (1) the high angle of deflection of the Thisimplies that elasticity representsa reasonableboundary plate beneath the Molasse Basin (Figure 3a), (2) the narrow conditionfor modeling the deflection of the Molasse Basin width of the basin, and (3) the small radiusof the plan view [seealso Turcotte and Schubert, 1982]. Finally, we justify the curvature of the Molasse Basin southwest of the study area usageof the cross-sectionalgeometry of the MolasseBasin as [Sinclair, 1996]. principalconstraints for our model because Sinclair et al. [1991]and Sinclair [1996] arguedsuccessfully that the cross- 6. Discussion and Conclusion sectionalas well as the plan view geometryof the Molasse Basinprovide the mostdetailed and the mostdecipherable in- The deflection of the central Swiss Molasse basin at 25 Ma formationabout possible controls on the forcesthat caused the is the net result of the deflection due to the orogenicload and platebeneath the Molasse Basin to flex. dueto the load of the basinfill (Figure 10). These two compo- To identifythe probableelastic thicknessat 25 Ma, we nents,however, are of significantly different wavelengths and comparethe resultsof forward and inverse (back stripping) amplitudes.Whereas the orogenic load causesa narrow, deep, modelsfor which the Te value is the unknown. The tectonic strongly curved foreland basin, the sedimentaryload of the subsidenceof the MolasseBasin was determinedby back basin tends to smooth the deflection of the foreland plate. strippingof the sediment load (Figures 9a to 9d).These results Furthermore,Figure 10 reveals that >60% of the total accom- forthe Molasse Basin were then compared with the forward modation spaceof the basin was formed by deflection due to modeleddeflection of theforeland plate (Figure 9b) thatwas the sediment load. calculatedusing the distributed orogenic load shown in Figure The calculatedmodel clearly reveals that the Lac Ltman and 8c.Both the back stripping and the forward modeling were it- Honeggdispersal systems are locatedon what would have been eratedthrough Te values. We divided the orogenic load (Figure a forebulgein a basin with no sedimentaccumulation (Figure 8½)and the sediment load (Figure 9a) into2.5-km-wide seg- 10). The hypothesisof the presenceof a tectonically driven mentsbefore performing the numericalcalculations. forebulgein this area as suggestedby the numerical model is Theresults of the flexural modeling reveal that a successfulsupported by the decreaseof the angle of unconformitysouth simulationof the deflection of the plate is dependenton the of Htinenberg-! (Figure 5) [see also Crampton and Allen, 836 SCHLUNEGGERET AL.: MOLASSE BASIN AND ALPINE DENUDATION

N S

Lac Ltman system Honegg system Rigi system

Sandstone meander belt 4000rn

Rigi Orn '25 Ma' I AM: Alluvial megafan B: Bajada

CC: Conglomerate channel belt 30 Ma

lO km

4000 m •. Horizontal line

'•'• Deflection due to the load of the • •'--- • basinfill

•'•TotalDeflection deflectiondue toorogenic load 0 m

Figure 10. Modelresults for Te=10km anda geothermalgradient of 25øC/km,revealing that 60% of the total deflection of the Molasse Basin is due to the sediment load. The model reveals that the Lac L•man and Honeggdispersal systems are located on a buriedflexural bulge.

1995]. The net subsidenceof this part of the basin is the sum Mesozoic basement suggeststhe presence of a forebulge of a negative and a positive component. Whereas the orogenic unconformity (underfilled stage), whereas burial of the load causedforebulge uplift, the sedimentaryfill of the basin teconicallydriven forebulgeby the 27 to 25 Ma strataindi- initiated subsidenceof the orogenically driven forebulge. cates overfilling of the basin. Because of the double control of subsidence as outlined above, The increase of the sediment accumulation rates from ini- enhanced crustal loading in the hinterland at a constant sedi- tally 700 m/m.y.to >1000 m/m.y. at Rigi, whichoccurred si- ment supply rate tends to promoteuplift in the northern alepo- multaneouslywith the 27 Ma geometry transition, suggests sitionaldomain and to createa forebulge unconformity, result- enhancedorogenic loading from frontal propagationand ing in undeffillingof the foreland basin [see Jordan, 1995]. If stacking[Schlunegger et al., 1997a].The coincidenceof over- instead sedimentsupply increaseswhile thrust rate is constant, filling with enhancedcrustal loading in the orogenimplies the result is suppression and burial of the forebulge and thatthe sedimentsupply rate to the forelandbasin increased overfilling of the basin [Flemings and Jordan, 1989, 1990; simultaneously.Indeed, cooling ages from the source area of Sinclair et al., 1991; Jordan, 1995; Crampton and Allen, the Rigi river indicate,as discussedabove, enhanced exhuma- 1995]. According to the model of Crampton and Allen [1995] tionof theCentral Alps of easternSwitzerland between ~30 the Oligocenedeposits of centralSwitzerland are likely to rep- and25 Ma (Figure4). It appearstherefore that enhanced ero- resentthe transitionfrom the underfilledto the oveffilled stage sionaldenudation in thispart of the Alps causedan augmenta- of the basin Northward onlap of 304o 28-Ma depositson the tionof thesediment supply by the Rigi river,which increased SCHLUNE•ER ET AL.: MOLASSEBASIN AND ALPINE DENUDATION 837 thesedimentary load in thebasin. As a result,subsidence in according to crosscutting relationships of temporally thenorthern domain was initiated, and the wavelength of the calibrated metamorphic fabrics [$chmid etal., 1996]. It basinincreased (Figure 10). appearstherefore that cessationof slip movement along the Whereasthe subsidencedriven by the tectonicload tendsto basalAlpine thrustand enhancedsediment sediment supply to concentratethe dispersal systems at the thrustfront, the sub- the distal axial drainage caused uplift and erosion of the sidencedue to the sedimentload favorsmigration of the proximal basin and a basinward shift of the axial depotenters. drainageaxis to thecentral paTt of the basin[Flemings and Given the flexural rigidity of the plate, one can critically Jordan,1989, 1990]. This impliesthat, if sedimentsupply to evaluatethe importanceof specific Alpine tectonic events theforeland basin increases at a constantthrust rate, the loca- (e.g., loading in SouthernAlps versusNorthern Alps) on the lionof maximumdeflection driven by the sedimentload shifts geometrical evolution of the north Alpine foreland basin awayfrom the thrust front. As a result,the subsidencerates in- (Figure11). For instance,load segment1, locatedin the area creasein the centerof the basin with respectto those at the of the Aar massif at ~60 km distance from the thrust front, thrustfront. This changein the basinsubsidence configuration causesflexural uplift at the thrust front (Rigi section)for a Te causesfan progradation and fanhead entrenchment. If, value of 5 km. The same crustal load, however, causes flexural however,thrust rates decrease and supply of sedimentto the downwarpat Rigi for a Te value >10 km. Load segment2, lo- basinincreases, erosion might occur in the proximal part of cated in the area of the Gotthard "massif" at I00 km distance theforeland basin. We interpret that the erosionalevent in the from the thrust front, has zero influence on the deflection at proximalMolasse Basin and the northwardshift of the de- Rigi for a Te value of 5 km. For an elastic thickness of 10 km, pocentersof the depositional systems at the the sameload causesuplift at Rigi, whereasit causesflexural 01igocene/Mioceneboundary (Figure 6b) are resultsof an in- downwarpat the same locality for Te values >20 km (Figure creasein the supply rate of sediment to the distal axial 11). Sinceat 25 Ma the elastic thicknessof the north Alpine drainageand a decreaseof crustalloading at the thrust front. forelandplate measured,-,10 km, the width of the orogenthat Indeed,whereas erosional denudation rates were constantin the causedflexural downwarp at the thrust front measured~70 km. CentralAlps of easternSwitzerland at the Oligocene/Miocene Orogenicloads beyond this area, that is, the Gotthard "massif" boundary(Figure 4), they increasedin the Central Alps of and the piggyback stack of Penninic nappes, either caused westernSwitzerland. This latter part of the orogen, however, flexural uplift of the proximal basin border or had zero influ- wasthe source terrain of the distal axial dispersal systems(see enceon the deflectionat the thrust front. This implies that, al- discussionin section 2). Furthermore, the rate of crustal thoughthe Central Alps north of the Insubric Line exerted the thickeningdue to slip movementalong the basalAlpine thrust greatestload onto the foreland plate at 25 Ma (Figure 8c), they decreasednearly to zero at the Oligocene/Miocene boundary, did not significantlyaffect the tectonic subsidencepattern and

N S Rigi,and Alpinethrust front Load Load insubric Line MolasseBasin segment segment

Orogenic load, determined fora geothermalgradient of 25øC/km

0 50 km

Distance from thrust [ront (krn} 0 50 100 150 200 0 ] Uplift• I I I 1 Sub-/ Te' $ km 1004 sidence• - 200Z"•Distance fromthrustfront (krn) O.r.. 50 100 150 200 0 I upon 1. • • l •,•,100•i;:•c• Te ,=10 km o 5o lOO 15o 2oo 0-1 I I •"•'•'•'•-•• !00•0 km

0 5o 1• 150 200

100

Figure !1. Model results,revealing the contributionof Alpinesegments to the deflectionat Rigi for differ- entflexural rigidities of the plate.See text for detaileddescription. 838 SCHLUNEGGERET AL.:MOLASSE BASIN AND ALPINEDENUDATION the geometricalevolution of the MolasseBasin (Figure 11). Acknowledgments.Thisproject was funded by the Swiss National However, given that denudationof the CentralAlps was con- ScienceFoundation andsupported byCornell University. We would like trolledby activityalong the InsubricLine, onecould conclude tothank N. Ussami, Sao Paulo, and Adrian Pfiffner, University ofBern, that there is an indirect tie betweenback thrustingalong the fora criticalreview of thepaper. Finally, F. Schluneggerthanks his Insubric Line and the geometricalevolution of the north colleagueswho acted as assistants andclimbing instructors. The science of thispaper improved significantly thanks to theconstructive reviews Alpine forelandbasin. Specifically, the crystallinecore of the by Philip Allen and by an anonymousreviewer. CentralAlps was a major sedimentsource area.

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