383_406_1238_schlunegger 12.11.2007 10:06 Uhr Seite 383 1661-8726/07/030383-23 Swiss j. geosci. 100 (2007) 383–405 DOI 10.1007/s00015-007-1238-9 Birkhäuser Verlag, Basel, 2007 Possible environmental effects on the evolution of the Alps-Molasse Basin system FRITZ SCHLUNEGGER,DIRK RIEKE-ZAPP & KARL RAMSEYER Key words: Swiss Molasse, Alps, geodynamics, climate and tectonics ABSTRACT Molasse Basin. We propose that this decrease in sediment flux caused the Bur- We propose three partly unrelated stages in the geodynamic evolution of the digalian transgression of the OMM. We also speculate that this reduction of Alps and the sedimentary response of the Molasse Basin. The first stage com- surface erosion initiated the modification of Alpine deformation from vertical- prises the time interval between ca. 35 and 20 Ma and is characterized by a ly- to mainly horizontally directed extrusion (deformation of the Southern high ratio between rates of crustal accretion and surface erosion. The response Alps, and the Jura Mountains some Ma later). The third stage in the geody- of the Molasse Basin was a change from the stage of basin underfill (UMM) to namic development was initiated at the Miocene/Pliocene boundary. At that overfill (USM). Because the response time of erosional processes to crustal ac- time, palaeoclimate possibly became wetter, which, in turn, caused surface ero- cretion and surface uplift lasts several millions of years, the orogen first expe- sion to increase relative to crustal accretion. This change caused the Alps to rienced a net growth until the end of the Oligocene. As a result, the Molasse enter a destructive stage and the locus of active deformation to shift towards to basin subsided at high rates causing the topographic axis to shift to the proxi- the orogenic core. It also resulted in a net unloading of the orogen and thus in mal basin border and alluvial fans to establish at the thrust front. During the a flexural rebound of the foreland plate. Aquitanian, however, ongoing erosion and downcutting in the hinterland We conclude that the present chronological resolution is sufficient to pro- caused sediment discharge to the basin to increase and the ratio between the pose possible feedback mechanisms between environmental effects and lithos- rates of crustal accretion and surface erosion to decrease. The result was a pheric processes. Further progress will result from a down-scaling in research. progradation of the dispersal systems, and a shift of the topographic axis to- Specifically, we anticipate that climate-driven changes in sediment flux altered wards the distal basin border. The second stage started at ca. 20 Ma at a time the channel geometries of USM and OSM deposits, the pattern of sediment when palaeoclimate became more continental, and when the crystalline core transport and thus the stacking arrangement of architectural elements. This became exposed in the orogen. The effect was a decrease in the erosional effi- issue has not been sufficiently explored and awaits further detailed quantita- ciency of the Swiss Alps and hence a reduction of sediment discharge to the tive studies. Introduction plants (see Kuhlemann & Kempf, 2002, and Berger et al., High-resolution chronologies of foreland basin deposits are 2005a, b; for the most recent compilations). In addition, they prerequisites for relating geodynamic processes in orogens to provide indispensable information for relating sedimentary the stratigraphic response in adjacent sedimentary troughs. processes to Alpine tectonic events, climate variabilites, and This has been illustrated by detailed studies from different modifications in exposed bedrock in the Alps (e.g., Schluneg- basins in the world (e.g., Jordan et al., 2001; Horton & De- ger et al., 1998; Schlunegger, 1999; Kuhlemann & Kempf, Celles, 2001; Chen et al., 2002; von Rotz et al., 2005). This is 2002). Compilations of stratigraphic data resulted in the now also the case for the Oligo-Miocene evolution of the Swiss Mo- generally accepted knowledge that the width and the tilt direc- lasse Basin located north of the Central Alps (Fig. 1). For this tion of the basin, the pattern of subsidence rates and the loca- foreland basin, high-resolution magnetostratigraphy in combi- tion of the topographic axis are controlled by the relative im- nation with mammal biostratigraphy (Schlunegger et al., 1996; portance of lithospheric processes leading to crustal thickening Kempf et al., 1999; Strunck & Matter, 2002) resulted in the es- and loading in the Alps, and surface processes resulting in re- tablishment of a high-resolution chronological framework for distribution of mass from the Alps to the Molasse Basin the Molasse deposits (Fig. 2). These chronologies then build (Pfiffner et al., 2002). Recent applications of new theoretical the basis for assignments of ages to depositional systems, dis- concepts, however, resulted in the notion that variations in persion directions, petrofacies and the fossiliferous record of erosion rates modulated the crustal velocity field in the Alps Exogene Geology Group, Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, 3012 Bern, Switzerland, [email protected] Environmental effects on the evolution of the Alps-Molasse 383 383_406_1238_schlunegger 12.11.2007 10:06 Uhr Seite 384 Fig. 1. Tectonic map of the Alps and adjacent foreland basins and location of tectonic cross- section (Schlunegger & Simpson, 2002; Cederbom et al., 2004; Willett palaeoclimate. Finally, we combine these information into a et al., 2006), and the pattern and rates of Alpine exhumation synthesis thereby discussing possible effects of feedback mech- (Schlunegger & Willett, 1999). anisms between lithospheric and surface processes on the evo- This paper synthesizes our current understanding of how lution of the Central Alps and the Molasse. the development of the Swiss Molasse and the Central Alps We want to emphasize here that this synthesis is by far not chronicles possible feedback mechanisms between lithospheric complete. It is simply not possible to give insight into the thickening and sediment transfer. Three objectives related to wealth of available knowledge for the Swiss Molasse and the this topic will be presented here, and their effects on the Mo- Central Alps within one single paper. For further, more de- lasse stratigraphy and the Alpine geodynamics be discussed. tailed information the reader is referred to the many PhD and The first addresses the exploration of how sediment transfer diploma theses written at the universities of Fribourg, Bern, from the Alps to the Molasse has driven the stratigraphic de- Basel, Genève and the ETH Zürich. Also it is important to velopment of the foreland trough. The second and third objec- note here that this work has a strong bias as it explicitly ex- tives concern the interaction between sediment transfer and plores possible controls of surface erosion on the development the crustal velocity field in the Alps thereby recognizing that of the Alps and the Molasse. (i) lithospheric deformation exerts the first order control on topography and erosion, but that (ii) (re)distribution of topo- The development of the Molasse Basin graphic mass potentially results in a modulation of the lithos- pheric stress field (Willett et al., 1993). Specifically, it is possi- The Molasse deposits (Figs. 2, 3) comprise four lithostrati- ble that the Central Alps and the Molasse Basin evolved in re- graphic groups, for which the conventional German abbrevia- sponse to changes in the relative importance of crustal accre- tions are used in this paper (Matter et al., 1980). They form tion and surface erosion. This is the issue that we intend to two coarsening-, thickening and shallowing-upward megase- discuss in this paper. We will then conclude that the Oligocene quences, each of them recording transitions from basin under- history of the Central Alps and the Molasse was to a first order fill to overfill (Sinclair et al., 1991; Sinclair & Allen, 1992). The driven by tectonic processes, but that the Neogene evolution- first megasequence starts with the Rupelian Lower Marine ary stages could have been modulated to substantial extents Molasse unit (UMM; Untere Meeresmolasse) (Diem, 1986). by environmental effects (such as climate-driven variations in These deposits are overlain by the Chattian to Aquitanian flu- surface erosion rates). vio-lacustrine deposits of the Lower Freshwater Molasse According to the scope, we first introduce the stratigraphy (USM; Untere Süsswassermolasse) (Strunck & Matter, 2002). of the Swiss Molasse and summarize the development of facies The second megasequence, starting with the Burdigalian trans- and depositional systems. We then present the litho-tectonic gression on a presumably truncated surface (Sinclair et al., architecture of the Central Alps together with a general view 1991), comprises the shallow marine deposits of the Upper of the geodynamic evolution of this orogen. This will be fol- Marine Molasse (OMM; Obere Meeresmolasse) that interfin- lowed by a presentation of data about sediment discharge and ger with the Napf and Hörnli fan deltas at the thrust front 384 F. Schlunegger et al. 383_406_1238_schlunegger 12.11.2007 10:06 Uhr Seite 385 Fig. 2. Chronological framework for the lithostratigraphic groups of the Swiss Molasse Basin. It is based on the magnetostratigraphic calibration of mammal bio- zones. Arrows indicate mammal sites for which precise biostratigraphic and magnetostratigraphic ages are available.