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 24Ð22 Ma and 21.5Ð15 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. 225Ð241; 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-southÐoriented 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 series of conglomerates, sand- coarsening-, thickening-, and shallowing-upward mayr and Wendt, 1987). The Subalpine Molasse stones, and marls ≈500 m thick (unit C). megasequences. The first megasequence begins consists of Lower Marine Molasse (UMM) and A partly contemporaneous distal sequence with Lutetian to Priabonian NHF and with the Lower Freshwater Molasse (USM), which build comprising a 750-m-thick alternation of fluvial

Figure 1. Geological map of the OligoceneÐMiocene Swiss Molasse basin and the adjacent Alpine oro- gen, with location of the study areas and general stratigraphy of the fore- land basin (modified after Schlun- egger et al., 1996).

226 Geological Society of America Bulletin, February 1997 MOLASSE BASIN AND ALPINE OROGENY

Figure 2. Synthetic chronostrati- graphic (Wheeler) diagram of the proximal Molasse of central Switz- erland. The analyzed sections and their approximate time span are marked by solid lines. The Med- iterranean mammalian zonation is taken from Engesser (1990), with modifications by Mödden and Gad (1992) and Mödden (1990).

channel sandstones and overbank fines is repre- 1968). The USM rests directly on upper Jurassic For magnetostratigraphy, brown to yellowish sented by the Fischenbach section in the upturned limestones and is composed of carbonate-rich laminated or massive mudstones and siltstones Plateau Molasse (Gasser, 1966; Schlunegger et sandstones and siltstones (USM I) overlain by were preferentially sampled (for sample location al., 1996). The Fischenbach and Höhronen sec- alternating arkoses and mudstones (USM II). maps, refer to Schlunegger, 1995). An individual tions constitute the USM II of Schlanke (1974), sample strategy had to be chosen for each section which is characterized by the low carbonate and METHODOLOGY dependent on availability of mudstones and silt- high feldspar content of its sandstones as well as stones, biostratigraphically defined time span, and by the presence of the key heavy minerals apatite Each stratigraphic section was measured by expected number of reversals. At the top of the and zircon in the lower part (USM IIa) and epi- using a Jacob’s staff and Abney level. The sec- Rigi section, fine-grained rock types are sparse, dote in the upper part (USM IIb, Gasser, 1966; tions were divided into large-scale facies associ- and samples were collected every 50Ð100 m. Four Müller, 1971; Schlanke, 1974). ations that were mapped at a scale of 1:25 000, to five oriented samples were taken for each site, The Napf section is located 40 km west of the making use of previously published geological and bedding orientation was measured for tilt cor- study area in the Napf alluvial fan. It consists at maps. rection. Detailed thermal and alternating field de- the base of 100 m of OMM overlain by a succes- Paleocurrents were determined from furrows, magnetization analysis of specimens from differ- sion of OSM more than 1400 m thick (Matter, large-scale (>0.5 m) trough cross-beds, and im- ent parts of the basin and different depositional 1964), ranging in age from Burdigalian to Lan- bricate clasts in the conglomerates. The tectonic systems revealed the presence of three to four ghian (Keller, 1989; Hurni, 1991), or approxi- tilt in the Subalpine Molasse was removed in or- magnetic components (Burbank et al., 1992; mately 19 to 15 Ma, according to recent magne- der to determine the paleoflow direction at the Schlunegger et al., 1996): (1) a low-temperature tostratigraphic calibration (Schlunegger et al., time of deposition. Conglomerate compositions magnetic signal, removed at 250 ¡C, which is 1996). Here, the OSM is partly contemporane- were determined every 100 m on 200 clasts with probably carried by titanomagnetite minerals, ous with the OMM (Hurni, 1991). The OSM a long diameter >2 cm collected from 1 m2 of (2) a magnetic signal in the 250Ð550 ¡C tempera- consists of fluvial fining-upward cycles domi- outcrop. Moreover, special attention was given to ture window related to magnetite group minerals, nated by conglomerates. the first appearance of distinctive clasts. In addi- (3) a magnetization remaining at high tempera- The well Hünenberg-1 explored an undis- tion, using the method described by Schlunegger tures, most probably carried by hematite crystals, turbed sequence of the entire Plateau Molasse et al. (1993), 37 heavy mineral analyses were car- and (4) occasionally sulfide minerals. These pilot down to the Mesozoic basement (Lemcke et al., ried out on cuttings from the Hünenberg-1 well. studies showed that specimens containing titano-

Geological Society of America Bulletin, February 1997 227 A

B

C

Figure 3. The central Swiss Molasse basin with (A) a compilation of geologic maps (Kopp et al., 1955; Gasser, 1966, 1968; Matter, 1964; Stürm, 1973; Schlanke, 1974; Ottiger et al., 1990), (B) cross section, and (C) palinspastic restoration.

228 Geological Society of America Bulletin, February 1997 MOLASSE BASIN AND ALPINE OROGENY magnetite, magnetite, and hematite minerals re- land. The detailed magnetostratigraphies chron- OMM reflections seen in seismic data (Schlun- veal reliable characteristic remanent magnetiza- icle the changes of the depositional environments egger et al., in press) and southward-convergent tions (CRMs) within the 250Ð550 ¡C temperature and basin subsidence. These record four thrusting OSM conglomerate horizons with increasingly window, and that magnetic directions in the lower events at the Alpine front and indicate incorpora- fine-grained pebbles and cobbles toward the Tri- and upper temperature windows are unstable and tion of Lower Freshwater Molasse (USM) into angle Zone. of opposite and mixed polarities. However, the orogenic wedge as early as 23 Ma. organic-rich rock types, which are prone to con- Chronostratigraphy tain sulfide minerals, displayed stable demagneti- Structure and Palinspastic Restoration zation vectors within a 250Ð350 ¡C temperature The high sample density in the studied sec- window. On the basis of these pilot studies, The structural style of the Subalpine Molasse tions defines rather unambiguous magnetic re- Schlunegger et al. (1996) concluded that (1) de- in the Lake Lucerne area is characterized by a lat- versal patterns. The magnetic polarity stratigra- spite the possible postdepositional growth of ti- eral change from a thrust belt to a fold-and-thrust phy (MPS) of the Fischenbach and Napf sections tanomagnetite, sulfide, and hematite crystals, the belt. In addition, in this area the strike of the basal (Fig. 4, A and B) were correlated with the mag- intensities of these secondary mineral phases are Alpine thrust is discordant to the bedding of the netic polarity time scale (MPTS) of Cande and too weak to have a significant influence on the underlying Rigi thrust sheet (Fig. 3A). Kent (1992, 1995) by Schlunegger et al. (1996) CRM of the samples; and (2) reliable CRMs are The north-south tectonostratigraphic section on the basis of (1) the Ries event at 14.6 ± detected in the 250Ð350 ¡C window for all fine- (Fig. 3B) is based on (1) the geological and struc- 0.6 Ma, postdating the top of the Napf section, grained rock types of the Molasse basin. There- tural data shown in Figure 3A and (2) three deep (2) chemostratigraphic data of Keller (1989), and fore, on the basis of this conclusion, all specimens wells that constrain the depth of the Mesozoic (3) comparison with magnetostratigraphies of were demagnetized at 250, 300, and 350 ¡C in this basement, the location of thrust planes, and—to- neighboring sections. These authors calibrated study. The coherence of the magnetic directions gether with the measured sections—the thick- the Fischenbach and Napf sections with the time of each site was tested by using Fisher (1953) sta- nesses of the individual lithostratigraphic units. span from chrons 6Cn.1 to 5C—i.e., Aquitanian tistics. Sites were classified as class I if three sam- These data reveal a dip angle of 4¡ for the top of to Langhian (Fig. 4C). Furthermore, they also ples were grouped with k ≥ 10. They were classi- the Mesozoic basement and suggest that the main identified and estimated the duration of uncon- fied as class II if k < 10 for three samples and decollement horizon lies within the basal Mo- formities within the USM and the OSM as well k ≥ 10 for two samples. The mean magnetic vec- lasse sequence. Moreover, the wells and field as at the base of the OMM. tor of each site was used to calculate the virtual data indicate decreasing thicknesses of UMM, The base of the Rigi MPS is correlated with geomagnetic pole (VGP; Fisher, 1953). The lati- USM I, and USM II (inclusive of the Höhronen the MPTS on the basis of (1) mammalian index tude of the VGP formed the basis for the local conglomerate) away from the Alpine front. The fossils indicating MP25 and (2) recognition of a magnetopolarity stratigraphy (MPS). Addition- USM I shows the most significant change in distinctive reversal pattern (Fig. 5A). According α ally, an 95 error envelope was calculated for each thickness from at least 3600 m at Rigi Mountain to Schlunegger et al. (1996), MP25 lasted from VGP latitude. The chronology of Schlunegger et to ≈450 m in the Hünenberg-1 and Entlebuch-1 approximately 29 Ma to 27.5 Ma. However, the al. (1996), which calibrates the Molasse groups of wells (Lemcke et al., 1968; Vollmayr and Wendt, correlation of the long interval of normal polar- central Switzerland and the Mediterranean mam- 1987), indicating, as discussed herein, that the ity in the upper part of the section is poorly con- malian assemblage zonation of Engesser (1990), dip angle of the Mesozoic basement had to strained. Because of the low sample density in Mödden and Gad (1992), and Mödden (1993) is change through space and time as a response to this part of the section, reversed polarities could used as reference for correlating the MPS with the ongoing thrust wedge advance (see also Fig. 3). have been missed either at the base or the top of global magnetic polarity time scale (MPTS) of The anticline in the Triangle Zone (labeled as “a” this long interval of normal polarity (solutions 2 Cande and Kent (1992, 1995). The MPSs of the in Fig. 3, A and B) is interpreted as a fault-propa- and 3, Fig. 5A). Solution 3 suggests that chron analyzed sections were correlated to the MPTS gation fold with a blind thrust in its core which 8n is represented in the Rigi section by a rela- through (1) consideration of index fossils (mam- reaches the surface west of Lake Lucerne. The tively short interval of normal polarity (Fig. 5C), malian fauna), (2) recognition of distinctive rever- amount of shortening in the Triangle Zone is on which implies a hiatus at the base of unit B, sal patterns, and (3) consideration of regional the order of ≈7 km, which is 9 km less than pos- given the regular pattern of reversals in the un- lithostratigraphy. tulated by Burkhard (1990). Because the thick- derlying unit A. However, new mapping and A tectonostratigraphic balanced cross-section ness of USM II was previously underestimated careful examinations of large exposures in three of the proximal Molasse basin was constructed (Burkhard, 1990), greater thrust displacements of dimensions (Buxtorf et al., 1916) do not reveal by using a compiled geologic map and subsur- USM I were previously required to balance the any unconformities or onlap relationships, face data from deep wells. A palinspastic resto- section within the triangle zone. A minimum of which are key indicators for the presence of hia- ration of this section was made in order to estab- 12 km is necessary to restore the Rigi thrust tuses. We therefore reject solution 3. Solution 2 lish the amount of shortening and to determine sheet. However, the total amount of shortening implies that the top of the Rigi section correlates the basin geometry and the original facies pat- for the Molasse basinÐJura Mountains systems is either with chron 7An or 8n, which suggests that terns through time. on the order of 30 km, on the basis of seismic it may be as young as 25.5 Ma. data (Pfiffner et al., 1996a), which includes 6 km Correlation of the magnetostratigraphy of the RESULTS of displacement for the Jura Mountains, accord- Höhronen section (Fig. 5B) is guided by mam- ing to Matter et al. (1988a). These data suggest malian index fossils at the base and the top, Five sections on a palinspastically restored that ≈16 km of shortening must be considered to which indicate MN1. Because the lower bound- cross section reported here give insights into the restore the Rigi thrust sheet (Fig. 3C). ary of MN1 is precisely dated with chron 6Cn.1n evolution of the shape of the basin, the facies A progressive unconformity appears to be (Schlunegger et al., 1996), the long reversed- architectures, and the dispersal systems as a re- present in the backthrust part of the Plateau Mo- polarity interval at the base of the Höhronen sec- sponse to orogenic events in the Alpine hinter- lasse as indicated by southward onlap of OSM on tion most probably correlates with chron 6Br

Geological Society of America Bulletin, February 1997 229 SCHLUNEGGER ET AL.

Figure 4. Magnetostratigraphy and sedimentology of (A) Fischenbach and (B) Napf sections, and (C) correlation with the magnetic polarity time scale (modified after Schlunegger et al., 1996).

(Fig. 5C). The calibration of the uppermost four Höhronen section either with chron 6AAr.2 or the base of USM II 50 km farther northeast (Matter reversals of the Höhronen section, however, is 6AAr.1 (Fig. 5C). et al., 1988b) and the magnetostratigraphic calibra- poorly constrained because no chronology is No magnetostratigraphy could be established on tion of the Höhronen section indicate an age of ca. available to date exactly the upper boundary of the Hünenberg-1 well because only cuttings were 24 Ma (Fig. 6). The upper part of USM II is corre- MN1 (Schlunegger et al., 1996). Nevertheless, available. Nonetheless, a general calibration of the lated with the dated Fischenbach section (Fig. 6), the top of the Höhronen MPS can be calibrated sequence can be achieved through (1) lithostrati- which represents the same facies belt (Schlunegger either with chrons 6AAr.2, 6AAr.1 or 6Ar. How- graphic correlation with the other well-dated sec- 1995), assuming similar accumulation rates. The ever, unit B of the Höhronen section contains the tions shown in Figure 6, (2) seismic stratigraphy proposed correlation results in a hiatus of ≈1.5 m.y. key heavy minerals apatite and zircon (USM IIa; (Schlunegger, 1995), and (3) magnetostratigraphic for the unconformity that is seen on seismic lines Schlanke, 1974), which are replaced in the Mo- dating of the USM-OMM and OMM-OSM between USM I and USM II (Schlunegger, 1995). lasse by epidote at the base of USM IIb at boundaries within the study area (Schlunegger et 21.5 Ma, at the latest (upper boundary of chron al., 1996). This calibration suggests that USM I at Lithofacies and Accumulation Rates 6Ar) (Gasser, 1966; Schlanke, 1974; Schluneg- Hünenberg-1 (Fig. 6) represents a distal facies ger et al., 1996). Furthermore, at Höhronen, the equivalent of the conglomeratic succession crop- Three coarsening- and thickening-upward overlying 800-m-thick alternation of conglomer- ping out at Rigi Mountain and spans approximately megasequences are preserved in the proximal ates and siltstones (unit B) also contains the key 28.5Ð25.5 Ma. The lower age limit of USM II is Molasse of central Switzerland (Fig. 7). The heavy minerals apatite and zircon (Schlanke, difficult to assess because no chronologic data are lower megasequence, comprising Rupelian to 1974). Therefore, it appears most reasonable to available within the study area. However, a mam- early Chattian UMM and USM I, is best devel- calibrate the uppermost reversed polarity of the malian fauna detected approximately 150 m above oped in the Rigi section. Here, a 2000-m-thick

230 Geological Society of America Bulletin, February 1997 MOLASSE BASIN AND ALPINE OROGENY

Figure 5. Magnetostratigraphy and sedimentology of (A) Rigi and (B) Höhronen sections, and (C) correlation with the magnetic polarity time scale. The magnetostratigraphic calibration of the mammal assemblage zonation is taken from Schlunegger et al. (1996). The magnetopolarity stratigraphies labeled as 1, 2, and 3 in A represent alternative solutions (refer to text for explanation). sequence of alternating conglomerates and mud- ates. This unit appears to have been laid down by best developed in the Höhronen section, where stones (unit A) is overlain by a 1300-m-thick suc- high concentration to hyperconcentration flows. ≈300 m of alternating sandstones and mudstones cession of massive conglomerates (unit B), Furthermore, mapping reveals that unit C en- (unit A) are overlain by a succession of con- which are topped by mudstones with intercalated croaches laterally onto the alluvial fan. This geo- glomerates and mudstones more than 1100 m ribbon conglomerates (unit C), measuring 300 m metrical relationship suggests that the conglom- thick (unit B) and an alternating series of con- (Fig. 6). The conglomerate beds of unit A are be- erates and mudstones of unit C were deposited on glomerates, sandstones and mudstones ≈500 m tween 1.5 and 5 m thick and display a strong con- local (bajada) fans (Schlunegger et al., 1993). thick (unit C). Given the possible calibrations of cave shaped erosive base with deep scours. The Given the possible calibrations of the Rigi section the Höhronen section as discussed previously interbedded fine-grained sediments reveal plane- as discussed previously herein (Fig. 5C), the fa- herein, the intermediate coarsening- and thick- lamination, ripple cross-bedding, mottling, root cies evolution from the base to the top of the Rigi ening-upward trend is accompanied by an in- traces, and bioturbation. The geometry and tex- section is associated with a steady increase in ac- crease in accumulation rates from less than ture of the lithofacies found in unit A suggest the cumulation rates, from 0.7 km/m.y. in unit A to 0.2 km/m.y. in unit A to >1 km/m.y. in unit B and presence of an alluvial plain with channelized more than 1.0 km/m.y. toward the end of fan con- probably unit C (Fig. 8). Farther north in the Hü- water flow. Unit B consists of massive and hori- struction. In contrast, the distal part of the basin is nenberg-1 section (Figs. 6, 7), contemporaneous zontally bedded well-sorted conglomerates with characterized by a low accumulation rate of successions are predominantly sandstone. In imbricated clasts. This unit is interpreted to have <0.3 km/m.y. and by deposition of a sandstone- contrast to the Höhronen section, the accumula- been deposited by sheetflood dispersion on an al- dominated lithofacies (Figs. 7, 8). tion rate at Hünenberg-1 decreases up-section luvial fan. Unit C, however, contains mottled The intermediate coarsening- and thickening- from >0.5 to 0.3 km/m.y. (Fig. 8). Mapping at mudstones, matrix-supported conglomerates, upward megasequence postdates a hiatus of Höhronen reveals highly variable thicknesses and 3-m-thick, deeply incised ribbon conglomer- ≈1.5 m.y. and begins at ca. 24 Ma (Fig. 7). It is and compositions of conglomerate beds in

Geological Society of America Bulletin, February 1997 231 Figure 6. Correlation of the magnetopolarity stratigraphy of the studied sections with the magnetic polarity time scale of Cande and Kent (1992, 1995). The magnetostratigraphic calibration of the Molasse groups is taken from Schlunegger et al. (1996).

232 Geological Society of America Bulletin, February 1997 MOLASSE BASIN AND ALPINE OROGENY

Figure 7. Chronologic (Wheeler) diagram of a palinspastically restored section across the central Swiss Molasse basin, supporting a strong het- erochroneity of facies.

unit C, as well as matrix-supported conglomer- shallow marine sandstones. Approaching the study area have been interpreted as the product ates and sandstones. These are interpreted to rep- Napf fan, these sandstone strata are progres- of a broad spectrum of depositional processes in- resent deposition by rivers and hyperconcen- sively replaced by terrestrial deposits of the cluding turbidity currents, tidal and wave activ- trated flows on locally derived bajada fans OSM up to 1400 m thick. The magnetostratigra- ity (Diem, 1986; Keller, 1989), and streamflows (Schlunegger et al., 1993). phy established on the Napf section suggests the and hyperconcentrated flows on alluvial sys- The upper coarsening- and thickening-up- presence of another unconformity within the tems. The petrofacies and lithofacies data shown ward megasequence measured in the Napf and OSM. This unconformity is interpreted to result in Figure 9 reveal three major (Rigi, Höhronen, Fischenbach sections and in the Hünenberg-1 from base-level lowering during a Burdigalian Napf) and two local dispersal systems (Fig. 10). well follows a hiatus at approximately 21.5 Ma regression described by Keller (1989) in the The depocenter of the Rigi alluvial fan system (Fig. 7). The section comprises the whole Mo- OMM and observed in seismic lines (Schluneg- is located at Rigi Mountain east of Lake Lucerne, lasse succession from USM IIb to OSM and in- ger et al., in press). In the time interval between where stacked massive conglomerates form pre- cludes the OMM. Measuring 350 m thick, the 21 and 15.5 Ma, the accumulation rates continu- cipitous rock cliffs. Magnetostratigraphic chron- USM IIb is made up of fining-upward fluvial ously decreased in megasequence III, from 0.5 ologies reveal that it was active from ca. 30 to ca. sandstone-mudstone cycles. The sandstone is to 0.35 km/m.y. (Fig. 8). However, a marked in- 25.5 Ma (Fig. 10). The encroachment of the Rigi typically massive or trough cross-bedded, crease of the accumulation rate up to a maxi- fan into this area is interpreted from abrupt up- whereas the upper part of the mudstones com- mum value of 1.7 km/m.y. occurs in the upper- ward coarsening, acceleration in the sediment- monly has mature paleosols. This facies associa- most part of the section (15.5Ð15 Ma). accumulation rate, and a switch from axial to tion is interpreted to represent a sinuous river transverse paleocurrents (Figs. 5A, 6, and 8). The belt depositional system. An unconformity span- Dispersal Systems and Provenance Rigi system is characterized by the presence of ning about 0.5 m.y. in the study area lies at the the key heavy minerals spinel, zircon, and apatite base of a 950-m-thick Burdigalian succession of The various lithofacies of the Molasse in the and by a high content of carbonate rock frag-

Geological Society of America Bulletin, February 1997 233 SCHLUNEGGER ET AL.

Figure 8. Palinspastically restored cross section of the central Swiss Molasse basin showing variations of decompacted sediment accumulation rates.

ments in the sandstones as well as by red granite lence of flysch clasts suggest increased relief and muddy limestones also occur in the younger clasts and a predominance of carbonate and erosion of the Alpine frontal units. In the USM I Höhronen conglomerates. Paleocurrent direc- flysch sandstone clasts in the conglomerates of the Hünenberg-1 section, however, traces of tions document a northeastward paleoflow, indi- (Fig. 9). The relative frequencies of these con- spinel and abundant apatite and zircon, as well as cating axial drainage of the Höhronen fan stituents show clear trends. The high percentage a high carbonate content, are interpreted to result (Fig. 5B). The fan apex is not exposed, but it must of sedimentary clasts in the lower part of the sec- from deposition by early Chattian axial systems be located directly west of Rigi Mountain where tion and key clasts (Müller, 1971; Stürm, 1973) derived from farther west (Gasser, 1968; Schlun- the Höhronen fan disappears beneath the thrust suggest derivation from the sedimentary cover of egger, 1995). plane of the Subalpine Molasse. the Penninic and Austroalpine nappes of eastern The second major alluvial system, referred to No systematic clast counts were carried out Switzerland. The appearance of red granite clasts as Höhronen, was active from ca. 24 to ca. 22 Ma for the bajada fans at the top of the Höhronen and apatite in the upper part of the section, how- and is characterized by the key heavy minerals section (unit C). However, new mapping reveals ever, indicates unroofing of these nappes down to apatite and zircon (Füchtbauer, 1959; Gasser, that unit C contains clast types identical to those their crystalline cores. Moreover, the presence of 1966; Müller, 1971; Schlanke, 1974), (Figs. 9, found in conglomerates in units A and B of the two types of flysch sandstones (Stürm, 1973) 10). The conglomerate population consists of Rigi section (see also Müller, 1971). However, a with different heavy mineral suites characterized abundant crystalline clasts, mainly granites, and careful look at the conglomerate and sandstone by spinel and apatite record erosion of South admixtures of sedimentary components derived composition revealed that the evolution of the Penninic and North Penninic to Ultrahelvetic from the crystalline cores and the sedimentary petrographic composition of the bajada fan con- Flysch nappes, respectively (Gasser, 1967). cover of the Penninic and Austroalpine nappes of glomerates and sandstones reveals a trend oppo- Clasts of flysch derived from these nappes pre- eastern Switzerland (Renz, 1937; Kleiber, 1937; site to that found in the Rigi section, suggesting dominate in the local bajada-fan conglomerates Speck, 1953; Müller, 1971). However, rare key normal unroofing of Rigi Mountain (see also at the top of the Rigi section. An increase in clast clasts of the Rigi dispersal system such as peb- Colombo, 1994). This is supported by transverse size in the upper part of the section and the preva- bles of red granites and highly bioturbated paleoflow directions measured in unit C (Fig. 6),

234 Geological Society of America Bulletin, February 1997 Figure 9. Schematic lithologic logs and sedimentary petrography of the studied sections, suggesting heterochroneity of petrofacies.

Geological Society of America Bulletin, February 1997 235 SCHLUNEGGER ET AL.

Figure 10. Chronologic (Wheeler) dia- gram, revealing the temporal and spatial distribution of dispersal systems. This dia- gram is based on the palinspastic restoration shown in Figure 3B and the calibration of petrographic units shown in Figure 9.

indicating provenance from Rigi Mountain. tion and duration of each dispersal system are alpine nappes of eastern Switzerland. As a result, Representing the third major drainage in the discussed. Subsequently, the evolution of the submarine boulder conglomerates were de- study area, the Napf system was active from geometric shape and facies architecture of the posited in the Gonfolite Lombarda south of the about 21.5 to 15 Ma (Fig. 10). In contrast to the basin is reconstructed in relation to thrust-wedge Alps (Gunzenhauser, 1985), and construction of other sections, the Napf system is distinguished evolution and processes, such as in-sequence the Rigi fan began. However, the distance from by a totally different petrofacies with a predom- and out-of-sequence thrusting, backthrusting, the source area to the basin margin was much inance of epidote in the heavy mineral spectrum and underplating. larger for the Rigi river than for the system feed- and a change from a crystalline-dominated to a ing the Gonfolite Lombarda. This conclusion is sediment-dominated conglomerate population Relationships Between Alpine Evolution based on studies on conglomerate provenance (Fig. 9). Furthermore, in the upper part of the and Dispersal System Activity (Speck, 1953; Müller, 1971; Giger, 1991), which section the flysch clasts, mainly of Ultrahelvetic suggest a water divide between both catchment to North Penninic origin, given their heavy min- The present-day Alpine hinterland of the areas immediately north of the Bergell intrusion. eral composition, increase in abundance and study area consists of the following structural After restoration to their prethrusting positions, size. These results and previous studies on the units: the Helvetic zone, which is subdivided in the resulting distance between the source area origin of clast types and heavy minerals (Fücht- a lower Infrahelvetic complex (Aar massif and and receiving basins is on the order of 40Ð50 km bauer, 1964; Matter, 1964) indicate that the de- its autochthonous-parautochthonous cover) and for the Gonfolite Lombarda and 70Ð80 km for tritus was derived from the upper Penninic and an upper Helvetic (sensu stricto) complex (Hel- the Rigi dispersal system. There are, however, in- Austroalpine nappes of southwestern Switzer- vetic thrust nappes), separated by the basal Al- dications that backthrusting of Austroalpine and land. Mapping of maximum clast size distribu- pine thrust. The Helvetic zone, in turn, is over- Penninic units had already commenced by about tion in relation to paleocurrent directions sug- lain by a piggyback stack of Penninic and 35Ð30 Ma (Mancktelow, 1990). This is reflected gests that the fan apex was located 80 km Austroalpine nappes (Fig. 11). in the presence of Rupelian conglomerates in the southwest of Lucerne and indicates a change Chronostratigraphic and petrographic data Gonfolite Lombarda (Gunzenhauser, 1985). Dur- from transverse to axial drainage toward the from the Molasse strata reveal erosion of the Aus- ing that same time, turbidites gradually filled the study area (Schlunegger, 1995). However, the in- troalpine and Penninic nappes of eastern Switzer- submarine trough of the North Alpine foreland, creasing admixture and size of flysch clasts to- land during the entire Chattian and early Aqui- forming the regressive sequence of the UMM ward the top of the section, together with the tanian—i.e., from 30 to 22 Ma (Rigi and (Diem, 1986). change from axial to transverse paleoflows, sug- Höhronen systems). First, conglomerates rich in The Aar massif also underwent major uplift in gest more pronounced erosion of the frontal carbonate clasts were deposited on the Rigi allu- Miocene and Pliocene time (Kammer, 1989; North Penninic and Ultrahelvetic nappes (Mat- vial fan between about 30 and 25.5 Ma. This dep- Burkhard, 1990; Lihou et al., 1995; Pfiffner et ter, 1964). ositional episode coincides with the Insubric al., 1996b). The massif was buried by stacking of phase of backthrusting that has been interpreted Alpine nappes in the course of thrust wedge ad- DISCUSSION to result in >10 km of vertical displacement (Hur- vance (Pfiffner et al., 1996b). Stacking of nappes ford, 1986; Schmid et al., 1989). During the same resulted in an increase in temperature and pres- The structure, stratigraphy, and petrography time span (30Ð25 Ma), the intrusion of the Ber- sure, reaching greenschist facies conditions of of the Molasse strata presented previously herein gell (ca. 30 Ma) and Novate (ca. 26 Ma) plutons about 350 ¡C in the buried Aar massif (Niggli, record the development of the Alpine thrust occurred (Gulson, 1973; Koeppel and Grünen- 1970; Frey et al., 1980a, 1980b; Erdelbrock, wedgeÐMolasse basin system through time. In felder, 1975). Synmagmatic backthrusting along 1994) by 25 Ma at the latest (Hunziker et al., the following sections, the causal relationships the Insubric Line is likely to have increased the 1986). Such metamorphic conditions correspond between the major orogenic events on the initia- denudation rates of the Penninic and Austro- to a burial depth of at least 12 km, if we use an

236 Geological Society of America Bulletin, February 1997 MOLASSE BASIN AND ALPINE OROGENY

Figure 11. Tectonic map of the central Alps. Arrows represent the major dispersal systems in the central Swiss Molasse basin. Cir- cled dates indicate cessation of drainage systems.

average geothermal gradient of 30 ¡C/km as pos- Miocene Höhronen river compared to that of the (Frey et al., 1973, 1980a; Groshong et al., 1984; tulated for the central Alps (Pfiffner, 1986; Hur- late Oligocene Rigi river. The drainage reconfig- Hunziker et al., 1992). Thrusting of the Helvetic furd et al., 1991; Erdelbrock, 1994; Pfiffner et uration resulting from deformation of the Aar thrust nappes above the Infrahelvetic complex al., 1996b). Subsequent exhumation caused massif might thus explain the deactivation of the (Aar massif and its autochthonous to parau- cooling through the “annealing temperature win- Höhronen dispersal system at approximately tochthonous cover) along the basal Alpine thrust dows” of the apatite and zircon fission track sys- 22 Ma (Figs. 10 and 11). This interpretation is resulted in low-grade metamorphism in the latter tems. Zircon fission tracks chronicle cooling supported by the time of deactivation of the ad- units between 20 and 25 Ma (Niggli, 1970; Frey through the 200Ð250 ¡C temperature window, jacent major dispersal systems (Fig. 11): The et al., 1980b; Erdelbrock, 1994; Rahn et al., which corresponds to a burial depth of ≈8 km, Napf and Hörnli drainage systems to the west 1994, 1995). Thrusting of the Helvetic thrust whereas apatite fission tracks record cooling and east of the Aar massif were active until about nappes in eastern Switzerland (Calanda phase; through the 80Ð120 ¡C temperature window, 15 and 13 Ma, respectively, whereas the Höhro- Milnes and Pfiffner, 1977, 1980) seems to have which is equivalent to a burial depth of about nen and Beichlen rivers, which had to cross the been contemporaneous with the Insubric phase 4 km (Hurford et al., 1991). Cooling histories Aar massif, were deactivated much earlier of backthrusting (Burkard, 1988), which started based on zircon and apatite fission-track data (Schlunegger, 1995). One might expect that in southeastern Switzerland prior to 30 Ma and from the external massifs (Hurford, 1986; Soom, bedrock uplift of the Aar massif would lead to reached maximum uplift rates between 30 and 1990; Michalski and Soom, 1990) suggest that the establishment of new large dispersal systems 25 Ma (Hurford, 1986; Schmid et al., 1989). A the eastern Aar massif reached shallow crustal in the foreland. This was not the case in the prox- subsequent forward thrusting event along the levels of ≈8 km at about 20 Ma. Thus, at this imal Molasse, because deformation of the Aar basal Alpine thrust was identified by a disconti- time, the piggyback stack of Penninic and Aus- massif was coupled with thrusting in the Mo- nuity in the metamorphic gradient between the troalpine nappes overlying the eastern Aar mas- lasse basin (Pfiffner et al., 1996a), which be- Helvetic thrust nappes and the Infrahelvetic sif had apparently been denuded by at least 4 km came, as discussed later herein, subject to ero- complex (Frey et al., 1980b; Frey, 1988), which due to uplift of the eastern Aar massif (Pfiffner et sion and redeposition on bajada fans at this time. resulted in an amount of post 20Ð25 Ma offset on al., 1996b). The erosional products are now the order of 5Ð10 km (Erdelbrock, 1994; Rahn et found in the Rigi and Höhronen fans. The fis- Tectonic Processes in the Alpine Orogen, al., 1995; Wang et al., 1995). This tectonic event sion-track data suggest that, in early Miocene Basin Geometry, and Facies Architecture has been termed the “Ruchi phase of deforma- time, the zone of maximum denudation and tion” (Milnes and Pfiffner, 1977, 1980). bedrock uplift shifted from the Insubric Line to In early Chattian time, the Penninic and Aus- Another Alpine unit that underwent intense de- the Aar massif, which was at least 40 km to the troalpine nappes had already been emplaced formation during deposition of the Molasse strata north (see also Sinclair and Allen, 1992). This (Burkhard, 1988). These units overlay the Hel- is the Infrahelvetic complex. The exhumation his- northward shift would have significantly re- vetic thrust nappes, which underwent low-grade tory of this unit was reconstructed by using the duced the size of the catchment area of the early metamorphism sometime between 30 and 35 Ma temporal calibration of the “annealing closing

Geological Society of America Bulletin, February 1997 237 SCHLUNEGGER ET AL. temperatures” of zircon and apatite fission-track mayr and Wendt, 1987). In this part of the basin, cross section shows a flexural angle of 5¡ for the systems (Michalski and Soom, 1990; Soom, predominantly mudstones and sandstones were underlying basement and a wedge-shaped basin 1990) and by using relationships between meta- deposited by an axial system derived from farther filled by conglomerates and mudstones south of morphic fabrics and structures (Pfiffner, 1982; west at a rate of <0.3 km/m.y. The different petro- the Fischenbach section, which were deposited Groshong et al., 1984). These data suggest initial facies and sedimentary trends at Rigi, when com- by the Höhronen system and local-source rivers. bedrock uplift and denudation of the eastern Aar pared to those at Hünenberg-1 and Entlebuch-1, In the more distal part of the foreland, however, massif and its cover in the early Miocene (ca. suggest that progradation of the Rigi fan was re- no conglomerates were deposited, and a monoto- 23Ð20 Ma), which appears to represent the Ca- stricted. This phase of basin evolution coincides nous series of mudstones and sandstones pre- landa phase of deformation (Milnes and Pfiffner, with the Insubric phase of backthrusting along the dominates. The latter unit was deposited by the 1977, 1980). This in turn is coupled with thrusting Insubric Line approximately 100 km south of the axial Napf dispersal system (Figs. 9, 10). As with in the Subalpine Molasse (Pfiffner et al., 1996b). Rigi fan and with a phase of forward thrusting the Rigi fan, the northward progradation of the Final uplift of the Aar massif occurred in late along the basal Alpine thrust (Calanda phase). Höhronen was probably controlled by the rela- MioceneÐPliocene time and is associated with Subsidence driven by crustal loading in the central tively rapid subsidence of the proximal basin in folding and thrusting of the Jura Mountains (Li- orogen appears to have restricted progradation of comparison to the sediment supply. During this hou et al., 1995; Pfiffner et al., 1996a). the Rigi fan, whereas the loading, in combination interval, proximal parts of the USM I were incor- The structural evolution in the orogen, as dis- with forward thrusting, seems to have caused in- porated into the orogenic wedge by in-sequence cussed above, yields a relative sequence of de- creased erosion and redeposition of frontal North thrusting, as evidenced by redeposited Rigi fan formation events with fairly well constrained nu- Penninic and Ultrahelvetic Flysch units on bajada conglomerates on bajada fans (unit C of the merical ages for each phase. In this section, we fans and to have created a wedge-shaped basin. Höhronen section). The recycling of the Rigi discuss the geometrical, sedimentological, pet- This first phase of basin formation is suc- conglomerates into the Höhronen fan contrasts rographic, and structural evolution of the proxi- ceeded by a basinwide unconformity at about with previous interpretations of the origin of this mal Molasse of central Switzerland in three time 25 Ma. No major loading and forward-thrusting unit, which represented it either as a transition slices in relation to tectonic events in the orogen. events are recorded in the Alpine orogen for that from Rigi to Höhronen dispersal system The tectonostratigraphic configuration of the time, according to cooling ages and metamorphic (Schlanke, 1974) or as a mixture of both Alpine study area at 25.5 Ma (Fig. 12A) is based on mea- and radiometric studies (Hurford, 1986; Frey, rivers (Müller, 1971). The end of this phase is sured thicknesses of USM I in the Hünenberg-1, 1988; Schmid et al., 1989). It appears that this marked by an unconformity within USM II (see Entlebuch-1, and Rigi sections, on determination phase of tectonic quiescence was responsible for Fig. 7), which spans the studied part of the basin. of the petrofacies (Figs. 9, 10), and on the uplift and erosion across the whole basin at The dip of the basement under the proximal palinspastic restoration of the Subalpine Molasse 25 Ma (see also Flemings and Jordan, 1990). foreland appears to be slightly shallower shown in Figure 3C. This configuration shows that The second phase of basin fill history spans (Fig. 12B) than in the previous reconstruction the Alpine front was formed mainly by North Pen- the time interval between ca. 24 and 23 Ma. Re- (25.5 Ma; Fig. 12B). Thrusting and incorporation ninic and Ultrahelvetic Flysch nappes, which were construction of the basin geometry for that time of USM I into the orogenic wedge coincides with shedding their detritus northward to the bajada interval is difficult because we do not see the the Calanda phase of deformation in the Infrahel- fans. The Helvetic thrust nappes, which form the base of USM II at Höhronen (Schlanke, 1974) vetic complex between ca. 23 and 20 Ma. Ac- present-day Alpine border, had to be buried under and because time control in the Hünenberg-1 cording to Pfiffner et al. (1996b) this phase of de- the stack of the tectonically higher Penninic section is rather speculative. Nonetheless it formation caused initial bedrock uplift of the Aar nappes at that time (Matter, 1964; Stürm, 1973). At seems that during an initial stage the more distal massif by about 4 km and forward propagation of the thrust front, the Molasse basin formed a sites located at Hünenberg-1 subsided more rap- the orogenic wedge. Loading in the northern part wedge-shaped trough >37 km wide that was filled idly than the more proximal ones, represented by of the central orogen and forward thrusting seem in its proximal part with a 4-km-thick succession the Höhronen section (Fig. 8). Furthermore, no to be the major controls on the basin geometry of predominantly conglomerates deposited by the 25Ð23-m.y.-old sediments are present in the and architecture during this time period. How- prograding Rigi system (Rigi fan) and by local- most proximal basin at Rigi, suggesting bypass- ever, crustal loading of the Infrahelvetic complex source rivers on bajada fans. In order to determine ing or erosion. This phase of basin evolution is is probably small compared to the effect of the dip angle for the basement at the toe of the interpreted as the flexural response of the basin >10 km of vertical displacement, which occurred wedge, surface slopes and stratigraphic tapers to renewed initial loading in the orogen after a in southeastern Switzerland during the Insubric must be considered. Although surface slopes of al- phase of tectonic quiescence. This bypassing phase of backthrusting. These differences might luvial fans in foreland basins are poorly known, may result from initiation of motion on the Rigi explain the slight reduction of the flexural angle Jordan et al. (1988) argued that a total of thrust and initial uplift and erosion of the most of the underlying basement at the end of this 300Ð400 m stratal thickness is attributable to ele- proximal part of the foreland. phase. Part of the decreased dip might result also vation change in prograding alluvial fan systems in The cross section representing the foreland from rebound that occurred during the preceding a foreland basin. Using an additional 200 m for the basin-thrust-wedge configuration at 21.5 Ma quiescence. In addition, by 21 Ma, the proximal UMM beneath the Rigi section (Greber et al., (Fig. 12B) is based on lithofacies, thicknesses, USM I was incorporated into the orogenic 1994) results in a dip angle of 6¡Ð7¡ for the base- and magnetostratigraphies of measured sections, wedge. This represents an event of accretion at ment at the toe of the wedge (Fig. 12A). An en- the palinspastic restoration of the Subalpine Mo- the toe of the wedge, as opposed to stable sliding hanced flexure hypothesis at the proximal basin lasse shown in Figure 3C, and the strong diminu- of the entire wedge. Accretion simply redefined margin is supported by an increase in sediment ac- tion of North Penninic and Ultrahelvetic Flysch the leading edge of the wedge as being farther on cumulation rate from 0.7 to >1.0 km/m.y. in the clasts in comparison with USM I. As for the re- to the flexed plate. Given the likelihood of rapid Rigi section. Between Entlebuch-1 and Hünen- construction of the 25 Ma basin architecture, a to- erosion of the uplifted USM I (see also Burbank berg-1, USM I is less than 500 m thick and is even tal of 300Ð400 m of stratigraphic thickness is at- and Beck, 1991), no significant new loads would slightly repeated by a thrust in Entlebuch-1 (Voll- tributed to elevation (Jordan et al., 1988). The have been added above the flexed plate by this

238 Geological Society of America Bulletin, February 1997 MOLASSE BASIN AND ALPINE OROGENY

Figure 12. (A) Schematic diagram showing the tectonostratigraphic con- figuration of the Molasse basin at 25.5 Ma. A wedge-shaped basin was created as a result of crustal loading by the Insubric phase of backthrusting along the Insubric Line and by forward thrusting of North Penninic and Ultra- helvetic Flysch nappes onto the Alpine foreland. Forward thrusting at the tip of the orogenic wedge resulted in uplift, erosion, and redeposition of the frontal Alpine units on bajada fans. (B) Sketch illustrating the stratigraphy and struc- ture of the basin margin at 21.5 Ma. It shows incorporation of proximal USM I into the thrust wedge and rede- position of conglomerates derived from the Rigi fan on bajada fans. Uplift of the Rigi thrust sheet and loading by un- derplating of the Aar massif resulted in renewed flexural subsidence of the Mo- lasse. (C) Reconstruction of the tecton- ostratigraphic configuration of the proximal basin margin at 15.5 Ma, re- vealing that underplating of Molasse deposits resulted in backthrusting at the southern margin of the Plateau Mo- lasse and in formation of a south-ver- gent progressive unconformity.

accretion. Thus, the shape of the flexural depres- mation is a likely cause of the unconformity at and Triangle Zone. This interpretation is sup- sion would not change, but the proximal edge of 21.5 Ma (Fig. 7). ported by (1) a gradually decreasing accumula- the depositional foreland would be shifted north- The reconstruction of the tectonostratigraphic tion rate from 0.5 to 0.35 km/m.y. adjacent to the ward above more gently flexed crust. configuration at 15.5 Ma (Fig. 12C) depicts the backthrust, (2) southward onlap of OSM on Farther northward, propagation of the thrust penultimate major deformation and related depo- OMM, and (3) the south-vergent progressive un- system into the Höhronen (Fig. 12B, dashed sition in the study area. It shows in-sequence in- conformity in the OSM north of the backthrust. lines) would have caused rock uplift in the hang- corporation of USM II and OMM into the oro- The origin of the basal Burdigalian unconformity ing walls. Denudation in response to this defor- genic wedge and development of the backthrust at 20 Ma might be caused by forward propaga-

Geological Society of America Bulletin, February 1997 239 SCHLUNEGGER ET AL.

tion of the thrust front into the more distal part of Determination of petrofacies in the detailed is marked by a basinwide unconformity, inter- the basin. The discordance at 18 Ma, however, is temporal framework allows reconstruction of the preted to result from crustal rebound after initial related to a sea-level drop proposed by Keller denudation history of the Alpine hinterland in re- loading. A subsequent increase in accumulation (1989) in the upper part of the OMM and also lation to its exhumation. The Insubric phase of rates to >1 km/m.y. between 23 and 21.5 Ma co- represented on seismic lines as a major sequence backthrusting, which occurred in southeastern incides with initial uplift of the eastern Aar mas- boundary (Schlunegger et al., in press). Finally, Switzerland between 30 and 25 Ma, resulted in sif and with incorporation of early Chattian con- the decreasing accumulation rates during progra- increased erosion of the Austroalpine and Pen- glomerates into the orogenic wedge. The third dation of the Napf fan, as recorded by the coars- ninic nappes of eastern Switzerland and in acti- advance of the Alpine wedge between 21 and ening- and thickening-upward megasequence in vation of the Rigi dispersal system. Downcutting 15.5 Ma caused underplating of Molasse depos- the Napf section, suggest that fan progradation of these nappes to their crystalline cores is its, resulting in synsedimentary backthrusting of was caused by a relative drop in subsidence. recorded in the Molasse strata by an increase in previously deposited Molasse sequences and in The final stage of basin evolution, from 15.5 to crystalline clasts in the conglomerates. Subse- the development of a progressive unconformity. 15 Ma, is characterized by thrusting of the Hel- quent initial uplift of the Aar massif 40 km far- A third increase in accumulation rates to vetic nappes and the piggyback stacking of North ther north delineated the time span during which >1 km/m.y. between 15.5 and 15 Ma marks the Penninic and Ultrahelvetic nappes along the dispersal systems were active. The Höhronen final loading event in the wedge, an event coeval basal Alpine thrust. This interpretation is based dispersal system, which crossed the Aar massif with out-of-sequence thrusting of the Helvetic on (1) increasing accumulation rates up-section on its eastern flank, was deactivated at 22 Ma, border chain and of the piggyback stack of North in the Napf fan from 0.35 to >1.0 km/m.y., sug- whereas the Napf and the Hörnli drainage sys- Penninic and Ultrahelvetic Flysch nappes along gesting additional loading; (2) an angular uncon- tems to the west and east of the Aar massif were the basal Alpine thrust. formity between the conglomerates of the Rigi active until about 15 and 13 Ma, respectively, thrust sheet and the basal Alpine thrust, indicat- which indicates that they were not affected by ACKNOWLEDGMENTS ing that thrusting of the Helvetic border chain the initial uplift. postdates uplift of the Rigi thrust sheet (out-of- The detritus shed from the evolving Alpine This project was carried out as part of Schlun- sequence thrusting); (3) an increasing admixture orogen was deposited in a foreland basin whose egger’s Ph.D. project, with funding provided by and size of North Penninic and Ultrahelvetic Fly- geometry changed according to thrusting events the Swiss National Science Foundation (grant sch clasts associated with a change from predom- in the orogen. During phases of uplift in the cen- 21-36219.91). We thank B. Engesser and C. inantly axial to transverse drainage, implying in- tral orogen and forward thrusting at the tip of the Mödden, who identified the mammal fragments; creased erosion of the frontal Alpine nappes; and orogenic wedge, a wedge-shaped basin formed, S. Lund, who advised us in interpreting the de- (4) first appearance of Helvetic clasts in the OSM characterized by a high lateral gradient, the accu- magnetization data; H. Haas, for the time- farther east (Leupold et al., 1942; Tanner, 1944). mulation rate ranging from more than 1 km/m.y. consuming heavy-mineral separation; M. Mange, Out-of-sequence forward thrusting in the Hel- adjacent to the tip of the orogenic wedge to only for determination of heavy minerals; H. Bärtschi, vetic nappes along the basal Alpine thrust at ca. 0.3 km/m.y. in the more distal settings. Con- for technical instructions; S. Henyey, for process- 15 Ma is interpreted to represent the Ruchi phase glomerates deposited in river channels, on allu- ing numerous magnetic samples; S. Burns, for of deformation (Milnes and Pfiffner, 1978, vial fans, and on bajada fans are the characteris- help with English; A. Pfiffner, G. Schreurs, O. 1980), which caused a horizontal displacement tic lithofacies in proximal reaches of this basin Kempf, and P. Strunck for fruitful discussions; of ≈5Ð10 km (Erdelbrock, 1994; Rahn et al., type, whereas channel sandstone bodies embed- and Schlunegger’s colleagues, who acted as as- 1995; Wang et al., 1995). This interpretation, ded in overbank fines are found in axial river sys- sistants and climbing instructors. Constructive re- however, is in disagreement with Schmid et al. tems in more distal positions. A change in the views by C. Paola and D. McNeill greatly im- (1996), who suggested that this phase of defor- load in the Alpine orogen by tectonic quiescence proved the quality of the manuscript. mation occurred in early Miocene time. It is not after a major loading event or by activation of a yet certain whether increased accumulation of new thrust in the proximal foreland caused major REFERENCES CITED

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