View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Bern Open Repository and Information System (BORIS) TECTONICS, VOL. 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 becausethey 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-res- olution 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 hin- terland to basin architecture. anincrease of thesediment supply to the forelandbasin. This isreflected in the Molasseby coarseningand thickeningup- In thispaper, we reconstructthe three-dimensional architec- ture 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