The Neogene evolution of the European Alps was characterized by the exhumation of crystalline basement, the so-called external crystalline massifs. Their exhumation presumably controlled the evolution of relief, distribution of drainage networks, and generation of sediment in the Central Alps. However, due to the absence of suitable proxies, the timing of their surficial exposure and thus the initiation of sediment supply from these areas are poorly constrained.
The northern Alpine foreland basin preserves the Oligocene to Miocene sedimentary record of tectonic and climatic adjustments in the hinterland. This contribution analyses the provenance of 25 to 14 Myr old alluvial fan deposits by means of detrital garnet chemistry. Unusually grossular- and spessartine-rich garnet is found (1) to be a unique proxy for identifying detritus from the external crystalline massifs and (2) to occur abundantly in ca. 14 Myr old deposits of the foreland basin. In contrast to previous assumptions, we therefore propose that the external massifs were already exposed to the surface ca. 14 Myr ago.
Tectonic processes drive the evolution of relief in mountain chains and
consequently control the development of the drainage network, sediment
supply, and deposition in the foreland basin. The Central European Alps and
their northern foreland basin, formed through the collision of the European
and the Adriatic continents since the Eocene (Schmid et al., 1996; Handy et al., 2010), are a classic example of such interactions (e.g. Schlunegger et
al., 1998; Pfiffner et al., 2002; Vernon et al., 2008, 2009; Baran et al.,
2014; Fox et al., 2015). The exhumation of large slices of mid-crustal rocks
from the European plate, the so-called external crystalline massifs,
occurred relatively late in the Alpine evolution, probably during the late
Miocene, although the exact timing is not well constrained. The external
crystalline massifs are today characterized by high relief, intense
glaciation, and some of the highest denudation rates in the Alps (up to 1.4 mm yr
Peak metamorphism of lower to upper greenschist facies conditions occurred
between 17 and 22 Ma in all northern external crystalline massifs (Mont
Blanc, Aar massifs, and the Gotthard nappe; Challandes et al., 2008; Rolland
et al., 2008; Cenki-Tok et al., 2014; Nibourel et al., 2018). Their
subsequent exhumation has been investigated using thermochronology (e.g.
Schaer et al., 1975; Wagner et al., 1977; Michalski and Soom, 1990; Vernon
et al., 2009; Glotzbach et al., 2010). Whereas some studies concluded that
exhumation was episodic (e.g. Vernon et al., 2009), others suggest
relatively constant exhumation rates of 0.5–0.7 km Myr
This study aims to constrain the timing of exposure and thus the beginning of sediment supply from the external crystalline massifs, by determining the provenance of the foreland basin deposits. Sedimentary rocks preserved in the northern peripheral foreland basin of the Central Alps, the Swiss part of the Molasse basin, are a well-studied archive recording tectonic and climatic adjustments in the central orogen between ca. 32 and 14 Myr ago (Schlunegger et al., 1993, 1996; Kempf et al., 1999; Spiegel et al., 2000; Kuhlemann and Kempf, 2002; von Eynatten, 2003; Schlunegger and Kissling, 2015). So far, the provenance of the Molasse deposits has been investigated using optical heavy mineral analysis, framework petrography, and both bulk and single-grain geochemical techniques, including epidote geochemistry and cooling ages derived from zircon fission track analysis and Ar–Ar dating of white mica (Spiegel et al., 2000, 2002; von Eynatten, 2003; von Eynatten and Wijbrans, 2003). No conclusive evidence for a contribution from the external crystalline massifs, however, has been found thus far, leading to the assumption that their exposure must post-date the youngest preserved (ca. 14 Myr old) Molasse deposits (von Eynatten, 2003).
In this study, we use major element geochemistry of detrital garnet in Miocene deposits from the central part of the Swiss foreland basin. The great compositional variability displayed by garnet from different source rocks means that it is a useful provenance tracer in a variety of settings (Spear, 1994; Mange and Morton, 2007). Furthermore, it is a common heavy mineral in orogenic sediments and sedimentary rocks (Garzanti and Andò, 2007) and is relatively stable during transport and diagenesis (Morton and Hallsworth, 2007). In the Central Alps, detrital garnet has recently been shown to be a valuable provenance indicator, especially for distinguishing detritus supplied from the external crystalline massifs (Stutenbecker et al., 2017). We aim (1) to explore if detrital garnet geochemistry can help identifying additional provenance changes in the Miocene Molasse deposits that have gone unnoticed so far and (2) to test whether detritus from the external massifs is present in the younger Molasse deposits in order to give independent constraints on the timing of crystalline basement exhumation.
The Central Alps evolved through convergence between the European continental margin in the north and the Adriatic plate in the south (Schmid et al., 1996). The convergence started during the Late Cretaceous with the subduction of the Alpine Tethys Ocean below the Adriatic microplate (Froitzheim et al., 1996) and ceased during the Paleogene after the European continental lithosphere entered the subduction zone. These Cretaceous to early Neogene orogenic processes are reflected by the syn-orogenic deposition of deep-marine flysch units preserved throughout the Alps (e.g. Wildi, 1985; Winkler, 1996). Around 32 Myr ago, the sedimentation style in the northern foreland basin changed from marine, flysch-like deposition to shallow marine and terrestrial sedimentation (Allen et al., 1991; Sinclair, 1997). This is thought to represent the transition to Molasse-type sedimentation in an overfilled basin and is discussed to be potentially related to a break-off of the European slab around the time of the Eocene–Oligocene boundary (e.g. Sinclair et al., 1991; Sinclair, 1997; Schlunegger and Kissling, 2015). Since this time, the northern foreland basin has become a major sink of orogenic detritus and an important sedimentary archive.
The sedimentary rocks in the Swiss part of the northern foreland basin are divided into four lithostratigraphic units that represent two shallowing- and coarsening-up megacycles (Schlunegger et al., 1998). The first cycle consists of the Rupelian Lower Marine Molasse and the Chattian and Aquitanian Lower Freshwater Molasse. The second megacycle comprises a transgressive facies of Burdigalian age (the Upper Marine Molasse) overlain by Langhian to Serravallian deposits of the Upper Freshwater Molasse. The depositional ages of these units were constrained using mammal biostratigraphy and magnetostratigraphy (Engesser, 1990; Schlunegger et al., 1996). Throughout the Oligocene and the Miocene, the proximal Molasse deposits are thought to have been formed through a series of large alluvial fans (Fig. 1) aligned along the Alpine thrust front (Schlunegger et al., 1993; Kuhlemann and Kempf, 2002). The more distal parts of the basin, on the other hand, were characterized by axial drainage directed towards the Paratethys in the east–northeast (31–20 Ma) and the western Mediterranean Sea in the southwest (after 20 Ma) (Kuhlemann and Kempf, 2002). Whereas the more distal deposits could be significantly influenced by long-distance transport from the northeast or southwest, the alluvial fans are thought to carry a local provenance signal from the rocks exposed immediately south of each fan system due to their proximal nature.
Simplified tectonic map of the Central Alps after Schmid et al. (2004) highlighting the location of alluvial fan deposits within the
northern Alpine foreland basin as well as the most important source rock
units in the hinterland. The Honegg–Napf fan, marked by the black rectangle,
is located in the central part of the Swiss foreland basin (SFB). For cross
section X–X
The hinterland of the central Swiss foreland basin comprises, from north to
south, potential source rocks derived from the following tectonic units
(Figs. 1, 2).
The Romandes Prealps; a stack of non-metamorphic and weakly metamorphosed
sedimentary cover nappes (Mesozoic carbonate and Cretaceous–Eocene flysch),
interpreted as the accretionary wedge of the Alpine Tethys, detached from
its basement and thrust northwards onto the European units. The Helvetic nappes; the non- or very low-grade metamorphic sedimentary
cover sequence of the European continental margin (mostly Mesozoic
carbonate). The external crystalline massifs; lentoid-shaped autochthonous bodies of
European continental crust that consist of a pre-Variscan polycyclic gneiss
basement intruded by upper Carboniferous to Permian granitoid rocks and an
overlying metasedimentary cover. They were buried within the Alpine nappe
stack during the Oligocene (Cenki-Tok et al., 2014), reaching greenschist
facies peak-metamorphic conditions between 17 and 22 Myr ago (Fig. 2a) and
were exhumed during the Miocene. The Gotthard nappe, although not a
“massif” sensu stricto because of its allochthonous nature, will be included in the
term “external crystalline massifs” from here on because the timing and
the rates of exhumation are comparable (Fig. 2b, Glotzbach et al., 2010). The Lepontine dome; an allochthonous nappe stack of European Palaeozoic
gneiss basement and its Mesozoic metasedimentary cover (Berger et al.,
2005). Amphibolite facies peak metamorphism (Frey and Ferreiro Mählmann,
1999; Fig. 2a) in the Lepontine occurred diachronously at around 30–27 Myr
ago in the south (Gebauer, 1999) and possibly as late as 19 Myr ago in the
north (Janots et al., 2009). Although the onset of exhumation of the
Lepontine dome might have been equally diachronous, it is generally assumed
to have occurred before 23 Myr ago (Hurford, 1986). The Penninic nappes, containing ophiolite of the Alpine Tethys as well as
the continental crust of Briançonnais, a microcontinent located within
the Alpine Tethys between the southern Piedmont–Ligurian ocean and the
northern Valais trough (Schmid et al., 2004). The Austroalpine nappes, containing the basement and sedimentary cover of
the Adriatic plate with a Cretaceous (“Eoalpine”, ca. 90–110 Ma)
metamorphic peak of greenschist facies conditions (Schmid et al., 2004). The
Austroalpine nappes were probably part of the nappe stack in the Central
Alps prior to their erosion during the Oligocene and Miocene, although they
are found exclusively in the Eastern Alps to the east of the Lepontine dome
today. The Sesia–Dent Blanche nappe, probably representing rifted segments of the
basement and sedimentary cover of a distal part of the Adriatic plate
(Froitzheim et al., 1996). In contrast to the Austroalpine nappes, the
Sesia–Dent Blanche nappe was subducted and exposed to blueschist facies
(Fig. 2a; Bousquet et al., 2012) and to eclogite facies metamorphism (e.g.
Oberhänsli et al., 2004).
The Central Alps are generally regarded as the major sediment source of
all proximal Molasse basin deposits, and compositional changes in the
foreland are thought to directly reflect tectonic and erosional processes in
the immediate Alpine hinterland (Matter, 1964; Schlunegger et al., 1993,
1998). The compositional evolution in the basin is diachronous and
non-uniform between the different fan systems (e.g. Schlunegger et al.,
1998; Spiegel et al., 2000; von Eynatten, 2003). In this study, we will
focus on the Honegg–Napf fan, located in the central part of the basin. It
most likely preserves a provenance signal related to external massif
exhumation due to its proximity to the large crystalline basement slices of
the Aar massif and the Gotthard nappe (Fig. 1). In the Honegg–Napf fan,
three major compositional trends have been previously identified (Fig. 3).
Compilation of published compositional data in the Honegg–Napf fan. Heavy mineral and rock fragment data from the sand grain size after von Eynatten (2003), pebble petrography after Schlunegger et al. (1998), epidote isotope ratios after Spiegel et al. (2002) and zircon fission track (FT) data after Spiegel et al. (2000). The two pink lines represent the dominant provenance changes as discussed in the text. Abbreviations used: LMM – Lower Marine Molasse; LFM – Lower Freshwater Molasse; UMM – Upper Marine Molasse; UFM – Upper Freshwater Molasse; ZTR – zircon–tourmaline–rutile index; sil. – siliceous.
The external crystalline massifs have not been regarded as a possible sediment source. The exact time of their surficial exposure is unknown, but it is believed to post-date the youngest preserved Molasse deposits. This interpretation is based on the lack of granitic pebbles attributable to the external massifs in the Molasse (Trümpy, 1980) and on structural reconstructions (e.g. Pfiffner, 1986) in combination with thermochronological data (e.g. Michalski and Soom, 1990).
In order to characterize the detrital garnets in the foreland, three samples were taken from 25, 19, and 14 Myr old fine- to medium-grained fluvial sandstones within the Honegg–Napf fan deposits located ca. 40 km to the east and southeast of Bern in the central part of the Swiss Molasse basin. The exact sampling sites were chosen based on the availability of published petrographical, chemical, and mineralogical data (von Eynatten, 2003) as well as magnetostratigraphic calibration (Schlunegger et al., 1996).
It is possible to compare potential source compositions to the detrital ones because the potential source rocks were already narrowed down to particular regions based on other provenance proxies and because many of these rocks are still preserved in the Alpine chain today. For comparison we used detrital data from Stutenbecker et al. (2017) as well as published source rock data from different units across the Central Alps (Steck and Burri, 1971; Chinner and Dixon, 1973; Ernst and Dal Piaz, 1978; Hunziker and Zingg, 1980; Oberhänsli, 1980; Sartori, 1990; Thélin et al., 1990; Reinecke, 1998; von Raumer et al., 1999; Cartwright and Barnicoat, 2002; Bucher and Bousquet, 2007; Angiboust et al., 2009; Bucher and Grapes, 2009; Weber and Bucher, 2015).
In addition, three river sand samples were collected from small
monolithological catchments (3–30 km
Sample locations and characteristics of the Molasse sandstones from the Honegg–Napf fan. Abbreviations used: UFM – Upper Freshwater Molasse; UMM – Upper Marine Molasse; LFM – Lower Freshwater Molasse.
Sample locations and characteristics of potential sources (tributary sampling approach).
The sandstone samples were carefully disintegrated using a jaw breaker and a
pestle and mortar. The disintegrated sandstones and the source rock
tributary sands were sieved into four grain size classes of
The grains were subsequently arranged in lines on sticky tape, embedded in
epoxy resin, ground with SiC abrasive paper (grits 400, 800, 1200, 2500,
4000), polished using 3, 1, and
Molecular proportions were calculated from the measured main oxide
compositions on the basis of 12 anhydrous oxygen atoms. The
Garnet classification scheme of Mange and Morton (2007).
Most of the detrital garnets are dominated by Fe-rich almandine with varying
amounts of grossular, pyrope, spessartine, and andradite (Fig. 4). Other
endmembers (e.g. uvarovite) are negligible. Average endmember contents are
summarized in Table 3; for the full dataset we refer to Stutenbecker (2019).
Garnet compositions do not differ significantly between the two analysed
grain size fractions of the same sample, although slight variations are
visible (Fig. 4): in sample LS2016-18 (25 Ma; Fig. 4a) garnet of the 125–250
Average contents (including standard deviation in brackets) of the five common garnet endmembers in the Molasse sandstones, the fluvial samples from the Lepontine gneisses and the Gurnigel flysch (this study), and three potential source rocks from the literature: external crystalline massif granites (Stutenbecker et al., 2017), eclogite facies rocks (Chinner and Dixon, 1973; Ernst and Dal Piaz, 1978; Oberhänsli, 1980; Sartori, 1990; Thélin et al., 1990; Reinecke, 1998; Cartwright and Barnicoat, 2002; Angiboust et al., 2009; Bucher and Grapes, 2009; Weber and Bucher, 2015), and granulite facies rocks (Hunziker and Zingg, 1980). For the full dataset we refer to Stutenbecker (2019).
Although some individual garnet grains show distinct internal compositional zoning from core to rim, the intra-grain chemical variability is generally negligible (see Stutenbecker, 2019).
The major part of garnet in all three samples (
Results from classification following Mange and Morton (2007) and
Tolosana-Delgado et al. (2018). Using the linear discriminant method of Tolosana-Delgado
et al. (2018), garnet was attributed to one single class if the probability for that
class was
Distinct compositional changes between the 25, 19, and 14 Myr old Molasse sandstones are mostly related to the ratio of almandine and grossular contents (Table 3, Fig. 5). At 25 Ma, the garnets are dominantly almandine-rich (average 70 %) and grossular-poor (average 9 %). At 19 Ma, both grossular-poor and grossular-richer garnets occur (average 16 %). Garnets in the 14 Myr old sandstone are generally almandine-poorer (average 50 %) and grossular-rich (average 32 %).
Garnets from the Lepontine gneisses (Table 3, Fig. 4d) are generally almandine-rich, but those in the paragneiss tend to be grossular-richer (22 %) compared to the ones in the orthogneiss (11 %). The Gurnigel flysch garnets (Fig. 4e) are almandine-rich with elevated pyrope contents (14 %).
Relative frequency of the four most common endmembers almandine, grossular, spessartine, and pyrope in the three detrital samples from the Molasse basin.
Although detrital garnet chemistry suggests the presence of only one relatively uniform, amphibolite facies source rock in the hinterland of the Honegg–Napf fan during the late Oligocene, the identification of the exact nature of this source is difficult. This is mostly due to the large compositional overlap of garnet sourced by diverse amphibolite facies metamorphic rocks (e.g. metasedimentary versus meta-igneous; Krippner et al., 2014; Tolosana-Delgado et al., 2018).
Amphibolite facies conditions of Alpine age were only reached in the Lepontine dome (Fig. 2a; Bousquet et al., 2012). However, many gneisses in the Central Alps preserve a prealpine amphibolite facies metamorphic signature as well (Frey et al., 1999), for example in the Austroalpine Bernina nappe (Spillmann, 1993; Spillmann and Büchi, 1993), the middle Penninic Briançonnais basement (Sartori et al., 2006), or the polycyclic basement of the external massifs (von Raumer et al., 1999). In fact, the Gurnigel flysch, a Late Cretaceous to Eocene flysch nappe in the Romandes Prealps that did not undergo Alpine metamorphism (Fig. 2a), contains abundant almandine-rich B-type garnets (Fig. 4e).
Zircon fission track ages from sandstones of the same age are mostly
Paleogeographic reconstruction of the Central Alps and in
particular of the hinterland of the Honegg–Napf fan. Situation during
The drainage divide was probably located close to the Insubric line (e.g. Schlunegger et al., 1998) but north of the Bergell pluton (Fig. 6a), whose detritus is exclusively found in the retro-foreland to the south (Gonfolite Lombarda; Giger and Hurford, 1989; Carrapa and Di Giulio, 2001).
The larger spread of garnet compositions in the early Miocene
(
The B-type garnet compositions match the range of garnets found in the
Lepontine nappes (Fig. 4b, d), which is supported by the occurrence of
predominantly young (
Granulite facies metamorphic conditions in the Central Alps were only reached in the Gruf complex located close to the Insubric line between the Lepontine dome and the Bergell intrusion (Fig. 2a). Furthermore, there is evidence for pre-Mesozoic granulite facies metamorphism in some rocks in the southern Alpine Ivrea zone south of the Insubric line (Hunziker and Zingg, 1980), in the Sesia Zone (Fig. 1; Engi et al., 2018; Giuntoli et al., 2018), and in the Dent Blanche nappe (Fig. 1; Angiboust et al., 2009). It is unlikely that erosion reached that far to the south during the Miocene because the Penninic and probably also the exhuming Lepontine nappe stack would have acted as a topographic barrier to the fluvial drainage network (Fig. 6b). However, it has been proposed that the flysch deposits preserved in the Romandes Prealps were partially fed by these units during the Late Cretaceous and the Eocene (Wildi, 1985; Ragusa et al., 2017). This interpretation is supported by the Gurnigel flysch sample (Fig. 4e), which contains garnets of the granulite facies type that are similar to those found in the Ivrea zone (Table 3, Fig. 4h). A recycled flysch origin is supported further by the abundance of flysch sandstone pebbles in Molasse strata of the same age (Schlunegger et al., 1998).
A potential, but minor, contribution from ophiolites, as suggested by Spiegel et al. (2002), could be supported by the two eclogite facies garnet grains found in the 19 Myr old sample (Fig. 4b) that match eclogite facies garnets from Alpine ophiolites (Table 3, Fig. 4g). Eclogite facies garnets occur both in metamorphic rocks of the Penninic Alpine ophiolites (e.g. Bucher and Grapes, 2009; Weber and Bucher, 2015; Fig. 2a) and in Palaeozoic (?) gneisses of the middle Penninic Briançonnais basement (Sartori, 1990; Thélin et al., 1990). Both sources are not distinguishable (Fig. 4g) but would have probably been located in relative close geographic proximity, either in the Penninic hanging wall south of the Simplon fault (Zermatt area) or in the Penninic nappes located between the eastern rim of the Lepontine and the adjacent Austroalpine nappes (Arosa zone; Fig. 6b).
Previous provenance studies have identified metasedimentary detritus in the
middle Miocene Molasse and located its source in the unroofing sedimentary
cover of the Lepontine dome (e.g. von Eynatten, 2003). This was strongly
supported by the young detrital zircon fission track ages (youngest peak at
However, garnet compositions in the youngest Molasse sandstones are not
comparable to Lepontine garnets sampled in this study nor to any detrital
garnet found in the main rivers draining the Lepontine dome today (Andò
et al., 2014). Instead, the detrital garnet signature of the 14 Myr old
sample mirrors almost exactly the compositional range of garnets from the
external crystalline massifs (Table 3, Fig. 4c, f). In the external
crystalline massifs, these garnets grew in Permo-Carboniferous plutons under
Alpine greenschist facies metamorphic conditions (Steck and Burri, 1971,
Fig. 2a). They are restricted to the granitoid basement of the external
massifs and do not occur anywhere else in the Central Alps, which makes them
an excellent provenance proxy (Stutenbecker et al., 2017). A further
distinction among garnets supplied by the different plutons (e.g. the
Central Aar granite from the Aar massif, the Rotondo granite from the
Gotthard nappe and the Mont Blanc granite from the Mont Blanc massif) is not
possible based on major element garnet geochemistry alone (Stutenbecker et
al., 2017). Until now, the surficial exposure of the external massifs in the
Central Alps was thought to post-date Molasse deposition. This
interpretation relies principally on the absence of pebbles of external
massif origin (e.g. Aare granite) in the foreland basin (Trümpy, 1980).
However, many Alpine granite bodies closely resemble each other
mineralogically and texturally, especially if present as altered pebbles in
the Molasse deposits, and hence it is difficult to discount a specific
source only on this basis. Further support of late surficial exposure of the
external massifs comes from structural reconstructions (e.g. Pfiffner, 1986, 2017) that have located the top of the crystalline basement at an
elevation that is similar to the modern topography, based on a relatively
flat-lying contact between the crystalline basement and the overlying
Mesozoic sedimentary cover (Fig. 7a). According to this model and the
published exhumation rates of 0.5–0.7 km Myr
Cross sections from X to X
However, Nibourel et al. (2018) recently proposed a revised geometry of the contact between crystalline basement and overlying cover, which allows ca. 8 km of additional crystalline basement on top of the present-day topography (Fig. 7b). The presence of external massif-sourced garnets in the youngest Molasse deposits provides independent evidence that parts of the crystalline crust contained in the external massifs were already at the surface at ca. 14 Ma (Fig. 6c). Assuming the aforementioned average exhumation rates, 7–10 km of crystalline basement would have already been exhumed and subsequently eroded during the past 14 Myr, which is in good agreement with the geometric reconstructions by Nibourel et al. (2018).
We suggest that the drainage divide was shifted northwards due to the exhumation of the Gotthard nappe and/or the Aar massif and that it was essentially located at its current position (Fig. 6c, d), but this warrants corroboration from other deposits in the foreland and the retro-foreland.
Garnet geochemistry is a useful tool to further constrain the provenance of sandstones in orogens such as the Central Alps. We have demonstrated that it is possible to distinguish detrital garnets using a combination of garnet classification schemes (Mange and Morton, 2007; Tolosana-Delgado et al., 2018) and case-specific comparison with available Alpine source rock compositions (Stutenbecker et al., 2017). For the Miocene deposits of the Swiss Molasse basin, we were able to (1) confirm the provenance shift possibly related to the exhumation of the Lepontine dome between 25 and 19 Myr ago as suggested previously (e.g. von Eynatten, 2003) and (2) to identify an additional provenance shift between ca. 19 and 14 Myr ago that had not been noticed before. This shift is related to the erosion of granites from the external crystalline massifs, which provides a minimum age for their surficial exposure and corroborates their recently revised structural geometry. We conclude that the exposure of the crystalline basement happened already ca. 14 Myr ago, which is several million years earlier than previously assumed.
The data (chemical composition of garnets from Molasse sandstones and source
samples) can be found online:
LS designed the project. AM helped during field work and sample collection. PMET and PL gave advice for sample preparation and supported the microprobe measurements and data acquisition at the University of Bern. LS prepared the paper with contributions by all co-authors.
The authors declare that they have no conflict of interest.
We would like to thank Fritz Schlunegger for guidance in the field and Alfons Berger and Lukas Nibourel for stimulating discussions. We thank reviewers Carita Augustsson and Lorenzo Gemignani for their constructive comments.
This research has been supported by the International Association of Sedimentologists (post-doctoral research grant, spring session 2018 grant).
This paper was edited by Kei Ogata and reviewed by Carita Augustsson and Lorenzo Gemignani.