Magnetostratigraphic constraints on relationships between evolution of the central Swiss Molasse basin and Alpine orogenic events F. Schlunegger* Geologisches Institut, Universität Bern, Baltzerstrasse 1, CH-3012 Bern, Switzerland A. Matter } D. W. Burbank Department of Earth Sciences, University of Southern California, Los Angeles, California 90089-0740 E. M. Klaper Geologisches Institut, Universität Bern, Baltzerstrasse 1, CH-3012 Bern, Switzerland ABSTRACT thrusting along the eastern Insubric Line, sedimentary basins than in the adjacent fold- where >10 km of vertical displacement is inter- and-thrust belt, abundant stratigraphic research Magnetostratigraphic chronologies, to- preted. During the same time span, the Alpine has been done in foreland basins to assess the gether with lithostratigraphic, sedimentologi- wedge propagated forward along the basal evolutionary processes of the orogenic thrust cal, and petrological data enable detailed re- Alpine thrust, as indicated by the coarsening- wedge (Jordan et al., 1988; Burbank et al., 1986; construction of the Oligocene to Miocene and thickening-upward megasequence and by Burbank et al., 1992; Colombo and Vergés, history of the North Alpine foreland basin in occurrence of bajada fans derived from the 1992). Despite a more complete and better dated relation to specific orogenic events and ex- Alpine border. The end of this tectonic event is record within a foreland, the correlation of sedi- humation of the Alps. The Molasse of the study marked by a basinwide unconformity, inter- mentary events recorded in the foreland with tec- area was deposited by three major dispersal preted to have resulted from crustal rebound tonic events in the adjacent hinterland is com- systems (Rigi, Höhronen, Napf). Distinguished after initial loading. A subsequent increase in monly speculative, because direct physical by characteristic heavy mineral suites, con- accumulation rates to >1 km/m.y. between 23 stratigraphic and structural ties between these glomerate clast populations, and the presence and 21.5 Ma coincides with initial uplift of the areas have been removed by subsequent erosion. of key clasts, these systems record three major eastern Aar massif by at least 4 km. This phase This is certainly the case for past studies of the phases of denudation of the Alpine edifice. The of high accumulation rates is associated with Oligocene to Miocene Swiss Molasse basin, on Rigi system eroded the Austroalpine and Pen- incorporation of early Chattian conglomerates the northern side of the Alpine orogenic wedge, ninic nappes of eastern Switzerland from 30 to into the orogenic wedge. The third advance of where there have been several attempts to relate 25.5 Ma as a result of backthrusting and uplift the Alpine wedge between 21 and 15.5 Ma foreland basin stratigraphy to orogenic evolution of these units along the Insubric Line. Subse- caused underplating of Molasse deposits, re- of the adjacent hinterland (Pfiffner, 1986; Home- quent uplift of the Aar massif some 40 km to sulting in synsedimentary backthrusting of wood et al., 1986; Sinclair et al., 1991; Sinclair the north appears to have controlled the dura- previously deposited Molasse sequences and in and Allen, 1992). Construction of coarse- tion of the Höhronen and Napf dispersal sys- the development of a progressive unconfor- grained alluvial fans, for instance, has been at- tems, spanning 24–22 Ma and 21.5–15 Ma, re- mity. A rapid increase in accumulation rates tributed to hinterland uplift, whereas deposition spectively. They record downcutting into the from 0.35 to >1 km/m.y. between 15.5 and of finer grained material has been considered to crystalline cores of the Penninic and Aus- 15 Ma marks the final loading event in the reflect tectonic quiescence. This is, however, ex- troalpine nappes of eastern (Höhronen) and wedge, which may be caused by further major actly opposite to the signature that is predicted western (Napf) Switzerland. High-resolution displacement and loading of the Aar massif. for the medial (and more commonly preserved) reconstruction of the structural and geometri- This deformation is coeval with out-of- parts of foreland basins. Because lithofacies cal evolution of the proximal Molasse reveals sequence thrusting of the Helvetic border variation is a function of subsidence, source-area in-sequence and out-of-sequence thrusting chain and of the piggyback stack of North Pen- lithology, and position within the foreland, re- events at the Alpine front and incorporation of ninic and Ultrahelvetic Flysch nappes along construction of external controls based on sedi- the Molasse into the orogenic wedge by in-se- the basal Alpine thrust. mentary facies alone is misleading. quence thrusting and underplating. Further- The first reconstruction of the structural and more, it reveals close relationships between pe- INTRODUCTION stratigraphic evolution of the northern Alpine riods of rapid denudation in the central Alps foreland was made on a section in eastern Switz- and phases of increased sediment accumula- Foreland-basin sequences potentially contain erland (Pfiffner, 1986; Sinclair et al., 1991). Poor tion rates at the proximal basin border. An ini- a decipherable record of the tectonic history of time control, however, has hindered the recon- tial increase in Molasse accumulation rates to the bounding mountain belt, because the stratig- struction of the causal relationships among basin >1 km/m.y. occurred between 30 and 25.5 Ma raphy within the basin is controlled mainly by subsidence, facies evolution, and unroofing his- and coincides with the Insubric phase of back- the development of the thrust wedge system tory of the Alpine wedge. through time (Beaumont, 1981; Jordan, 1981). We used high-resolution magnetostratigraphy *E-mail address: [email protected] Because chronologies are better established in to calibrate the sedimentary, structural and geo- GSA Bulletin; February 1997; v. 109; no. 2; p. 225–241; 12 figures. 225 SCHLUNEGGER ET AL. metrical evolution of the proximal Molasse Rupelian UMM, which are followed by the Chat- the thick Rigi thrust sheet adjacent to the Alpine basin in a section across central Switzerland. On tian and Aquitanian fluvial clastic rocks of the border. The Subalpine Molasse itself is overlain the basis of detailed temporal control, we discuss USM. The second megasequence, starting with by the Helvetic nappes, and structural unconfor- here the subsidence history and the relationships the Burdigalian transgression, consists of shallow mities separate the Molasse thrust sheets from the between Alpine orogeny and foreland-basin evo- marine sandstones (OMM), which interfinger Helvetic border chain (Haus, 1937; Scherer, lution. We assess timing of individual thrusting with major fan deltas adjacent to the thrust front 1966; Haldemann et al., 1980). events at the tip of the orogenic wedge using the (Berli, 1985; Keller, 1989; Hurni, 1991). This Four representative sections (Rigi, Höhronen, detailed chronologic record of the Molasse megasequence ends with Serravalian fluvial clas- Fischenbach, Napf) were measured for analysis strata. We also focus on determining the con- tic rocks of the OSM. of facies, paleoflow directions, petrofacies, and glomerate populations and the heavy-mineral The study area, located in central Switzerland magnetostratigraphy (Fig. 3A). They enable re- composition of sandstones in order to identify (Fig. 1), is a classic region of Molasse strata re- construction of a synthetic north-south–oriented dispersal systems within the detailed temporal search. Good exposures and three deep boreholes cross section encompassing the time range from framework. Finally, we attempt to identify the provide detailed knowledge on lithostratigraphy Rupelian to Langhian (Fig. 2). catchment areas and their orographic evolution and structure (Kopp et al., 1955; Matter, 1964; The ≈4-km-thick Rigi section, located in the as a function of orogeny. Gasser, 1966, 1968; Lemcke et al., 1968; Stürm, Subalpine Molasse, comprises three units con- 1973; Schlanke, 1974; Vollmayr and Wendt, sisting of early Chattian proximal, alluvial fan GEOLOGIC SETTING 1987; Ottiger et al., 1990; Greber et al., 1994). conglomerates with few fine-grained facies The Molasse strata of central Switzerland in- (Stürm, 1973). This succession, which is domi- In the North Alpine foreland basin five litho- cludes both southernmost Plateau Molasse and nated by carbonate rock fragments in both con- stratigraphic units are distinguished, for which Subalpine Molasse (Fig. 1). The sedimentary se- glomerates and sandstones, is referred to as the conventional German abbreviations are used quence of the Plateau Molasse in this region is USM I (Schlanke, 1974). in this paper (Matter et al., 1980; Keller, 1989; made up of the USM, OMM and OSM, and The Höhronen section consists of a 300-m- Sinclair et al., 1991; Fig. 1): North Helvetic ranges in age from Chattian to Langhian (Figs. 2, thick Aquitanian alternation of sandstones and Flysch (NHF), Lower Marine Molasse (UMM), 3A). The generally flat lying Plateau Molasse has mudstones in the lower part (unit A), followed Lower Freshwater Molasse (USM), Upper Mar- been affected by a south-vergent backthrust ow- by a predominantly conglomeratic succession ine Molasse (OMM), and Upper Freshwater Mo- ing to underthrusting of a stack of imbricate 1100 m thick (unit B, Höhronen conglomerate) lasse (OSM). The Molasse deposits form two USM thrust slices (Triangle Zone, Fig. 3B; Voll- and an alternating