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.
Introduction
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-1), which all contribute to their relevance as a sediment source
(Kühni and Pfiffner, 2001; Wittmann et al., 2007; Stutenbecker et al.,
2018). The exhumation is discussed as being related to crustal delamination in
response to lithospheric mantle rollback (Herwegh et al., 2017), slab
detachment (Fox et al., 2015), or erosional unloading (Champagnac et al.,
2009), possibly due to increased precipitation rates in the Pliocene
(Cederbom et al., 2004) or enhanced glacial erosion in the Pleistocene (Fox
et al., 2015; Herman et al., 2013).
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-1 since 14 Ma (Michalski
and Soom, 1990; Glotzbach et al., 2010). A focus in this debate concerns the
late Neogene cooling and the onset of glaciation in the Pleistocene and
their possible effect on the exhumation, erosion, and sediment accumulation
rates (e.g. Kuhlemann et al., 2002; Herman et al., 2013; Schildgen et al.,
2018). In contrast, the early Neogene exhumation history received comparably
little attention. In particular, the timing of the first surficial exposure
of the external massifs has never been constrained because estimates of
their total thickness have not been established yet. In most geometric
reconstructions (e.g. Pfiffner, 1986, 2017; Schmid et al., 2004),
the contact between the crystalline basement and the overlying Mesozoic
cover is assumed to be relatively flat, and the top of the crystalline
basement is hypothesized to have been less than 1 km above the
modern topography. Conversely, a new reconstruction of this tectonic contact
allows for a substantially greater amount (∼8 km) of (now
eroded) crystalline rock on top of the present-day topography (Nibourel et
al., 2018).
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.
Geological setting
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′′ see Fig. 7. Abbreviations used: AR – Aiguilles Rouges
massif; BD – Belledonne massif; DB – Dent Blanche nappe; HN – Honegg–Napf fan; MB – Mont Blanc massif; GN – Gotthard nappe; PE – Pèlerin fan; PF – Pfänder fan; RH – Rigi–Höhronen fan; SKH – Speer–Kronberg–Hörnli fan; SZ – Sesia zone.
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).
(a) Metamorphic map of the Central Alps (Bousquet et al., 2012)
showing the distribution and grade of Alpine metamorphism. Note the increase
from north to south from lower greenschist to eclogite facies conditions.
(b) In situ bedrock zircon fission track ages according to a compilation of
Bernet et al. (2009). Note the predominantly young (<30 Ma) cooling
ages in the area around the Lepontine dome and the external massifs in
contrast to the predominantly old (>50 Ma) cooling ages in the
Austroalpine nappes to the east. The river network (blue) and the thick
black outlines of selected geological units (external massifs, Romandes Prealps, and Dent Blanche nappe; cf. Fig. 1) are used to facilitate the
orientation and the comparison with Fig. 1. Abbreviations used: PE – Pèlerin fan; HN – Honegg–Napf fan; RH – Rigi–Höhronen fan; SKH – Speer–Kronberg–Hörnli fan; PF – Pfänder fan.
Compositional trends in the Honegg–Napf fan
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).
Phase 1. Between ∼31 and ∼25 Myr ago, the
heavy minerals are dominated by the zircon–tourmaline–rutile assemblage and
garnet (von Eynatten, 2003). Rock fragments are dominantly of sedimentary
origin and zircon fission track ages are Palaeozoic to late Mesozoic (Spiegel
et al., 2000). This phase is consistently interpreted to reflect the erosion
of Austroalpine flysch-like sedimentary cover nappes, which are structurally
the top of the central Alpine nappe stack and probably extended further west
during this time (Schlunegger et al., 1998; Spiegel et al., 2000; von
Eynatten, 2003).
Phase 2: 25–21 Myr ago. Around 25 Myr ago, the occurrence of epidote as well
as an increase in granitoid rock fragments mark a major compositional change
in the foreland. The presence of characteristic colourful granite pebbles
suggests an origin from the Austroalpine Bernina nappe (Matter, 1964).
Sediments of this phase clearly reflect the cutting down into crystalline
basement and are consistent with a continuation of a normal unroofing
sequence. Additionally, Schlunegger et al. (1998) report the occurrence of
quartzite pebbles, possibly sourced from the middle Penninic
Siviez–Mischabel nappe and argue that parts of the epidote could originate
from Penninic ophiolites as well, thus suggesting that erosion might have
reached down into the Penninic nappes already by then. Spiegel et al. (2002)
argued against this Penninic contribution based on the 87Sr/86Sr
and 143Nd/144Nd isotopic signatures of the epidote.
Phase 3: 21–14 Myr ago. At ∼21 Ma, metamorphic rock fragments
occur in the sediments, whereas the heavy mineral assemblages remain
epidote-dominated and overall similar to the second phase. Zircon fission
track ages are exclusively Cenozoic (age peaks between ∼32
and ∼19 Ma). In contrast to the first two phases, the
sediment composition allows several, partially contradictory
interpretations. Whilst petrographic and mineralogic data might suggest
recycling and sediment mixing (von Eynatten, 2003), young
40Ar/39Ar cooling ages in white mica (von Eynatten, 2003; von
Eynatten and Wijbrans, 2003) and a population of zircons with a fission
track central age of 19.5±0.9 Ma (Spiegel et al., 2000) point to an
additional, newly exhumed source identified as the Lepontine dome (Fig. 2b;
von Eynatten, 2003; Spiegel et al., 2000). Based on the abundance of flysch
pebbles after ∼21 Ma, Schlunegger et al. (1998) favour an
alternative scenario, in which the erosional front shifted northwards into
the flysch nappes of the Romandes Prealps. A mixture of both sources seems
possible. Furthermore, the isotopic signature of detrital epidotes suggests
a contribution of mantle source rocks between ca. 21 and 19 Myr ago, which
could point to a contribution by Penninic ophiolites (Spiegel et al., 2002).
However, this is not reflected in the heavy mineral spectra (von Eynatten,
2003) that do not contain typical ophiolite minerals such as Cr spinel.
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).
Sampling strategy and methodology
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 km2) draining potentially
garnet-bearing source rocks that were previously not, or only partially,
considered in the literature. We prefer this “tributary sampling approach”
(first-order sampling scale according to Ingersoll, 1990) over in situ sampling of
specific source rocks because small monolithological catchments are more
likely to comprise all garnet varieties of the targeted source rock and to
average out spatial variations of the source rock properties, e.g. mineral
size or fertility (Malusà et al., 2016). The targeted plausible source
areas are located in the Gurnigel flysch (Romandes Prealps), the Antigorio
nappe orthogneisses of the Lepontine dome, and the Lebendun nappe
paragneisses of the Lepontine dome (Fig. 1). Sample characteristics are
summarized in Tables 1 and 2. For detailed lithological descriptions of
the sampling sites in the Honegg–Napf area, see Schlunegger et al. (1993)
and von Eynatten (2003).
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 nameSampling locationLithostratigraphy (Matter, 1964; Schlunegger et al., 1996)Magnetostratigraphic section (Schlunegger et al., 1996)Magnetostratigraphic age (Schlunegger et al., 1996)LS2017-347.005667.971325UFM, Napf bedsFontannen sectionca. 14 MaLS2018-546.939137.950800UMM, Luzern formationSchwändigraben sectionca. 19 MaLS2016-1846.774637.732383LFM, Thun formationPrässerebach sectionca. 25 Ma
Sample locations and characteristics of potential sources (tributary sampling approach).
Sample nameSampling locationRiver catchmentMetamorphic gradeLithological unitLS2018-1246.72026 7.24548Ärgera, ca. 30 km2Not metamorphicGurnigel flysch (detrital garnets)LS2018-4046.39026 8.54124Valle di Foiòi, ca. 3 km2Alpine amphibolite faciesOrthogneiss, Antigorio nappe, Lepontine domeLS2016-4346.43955 8.50115Valletta di Fiorina, ca. 8 km2Alpine amphibolite faciesParagneiss, Lebendun nappe, Lepontine dome
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 <63, 63–125, 125–250, and >250µm. The
fractions of 63–125 and 125–250 µm were further processed in
sodium polytungstate heavy liquid at 2.85 g cm-3 to concentrate heavy
minerals. The heavy mineral concentrates were dried and, depending on the
obtained amounts, split into two to four parts using a microsplitter. All analysed
garnet grains were hand-picked from the concentrate of one split part per
fraction under a binocular microscope.
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 1/4µm diamond suspensions,
and graphite-coated. Major element oxides were analysed using a JEOL
JXA-8200 electron probe micro-analyser at the Institute of Geological
Science at University of Bern, Switzerland, under standard operating
conditions for garnet (see Giuntoli et al., 2018): accelerating voltage of
15 keV; electron beam current of 15 nA; beam diameter of 1 µm; 20 s
peak acquisition time for Si, Ti, Al, Fe, Mn, Mg, and Ca and 10 s for both
backgrounds. Natural and synthetic standard olivine (SiO2, MgO, FeO),
anorthite (Al2O3, CaO), ilmenite (TiO2), and tephroite (MnO)
were used for calibration by applying a CITIZAF correction (Armstrong,
1984). Garnet compositions were measured as close as possible to the
geometric centres of the grains, unless the area was heavily fractured or
showed inclusions of other minerals. In some randomly selected grains, core
and rim compositions were measured to identify intra-grain chemical
variability; these core–rim pairs are reported separately in Stutenbecker
(2019).
Molecular proportions were calculated from the measured main oxide
compositions on the basis of 12 anhydrous oxygen atoms. The
Fe2+/Fe3+ ratio was determined based on charge balance (Locock,
2008) because ferric and ferrous iron were not measured separately (FeO – Fetotal). Garnet endmember compositions were subsequently calculated
using the Excel spreadsheet by Locock (2008). The relative proportions of
the endmember components almandine (Fe3Al2Si3O12),
grossular (Ca3Al2Si3O12), pyrope
(Mg3Al2Si3O12), spessartine
(Mn3Al2Si3O12), and andradite
(Ca3Fe2Si3O12) depend on bulk rock composition and
intensive parameters (such as temperature and pressure), which can vary
substantially depending on the metamorphic or magmatic history of the
protolith (Deer et al., 1992; Spear, 1994). The data were plotted and
classified using the ternary diagram of Mange and Morton (2007) as well as
the linear discriminant function method of Tolosana-Delgado et al. (2018)
based on a global data compilation on garnet compositions from different
source rocks (Krippner et al., 2014).
Garnet classification scheme of Mange and Morton (2007). (a–c)
Detrital garnet compositions in the 25, 19, and 14 Myr old Molasse deposits
(this study). Source rock data from (d) Lepontine gneisses (this study), (e)
the Gurnigel flysch (this study), (f) external massif granites (Stutenbecker
et al., 2017), (g) 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 (h) granulite
facies rocks from the Ivrea zone in the southern Alps (Hunziker and Zingg,
1980).
Results
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 µm fraction
is more enriched in pyrope than garnet of the 63–125 µm fraction. In sample LS2018-5 (19 Ma; Fig. 4b) 4 “outliers” that are
very pyrope- and grossular-rich (n=2) or grossular- and andradite-rich
(n=2) occur only in the 63–125 µm grain size fraction. Furthermore,
garnet grains of the 63–125 µm fraction are more frequently
grossular-rich compared to the 125–250 µm fraction. In sample LS2017-3
(14 Ma; Fig. 4c), the 63–125 µm fraction contains some garnet grains
(n=8) of high almandine and low grossular content that are absent in the
125–250 µm fraction.
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 (>80 %) belong
to the B-type garnet of Mange and Morton (2007) and thus point to a dominant
contribution by amphibolite facies source rocks (Table 4). Minor amounts are
classified as C-type (high-grade metabasic), A-type (granulite facies), and
D-type (metasomatic) garnet. The 25 Myr old sandstone contains almost
exclusively B-type garnet (92 %; Table 4). The 19 Myr old sandstone shows a
larger spread with some A-, C-, and D-type garnet (Fig. 4b; Table 4). The 14 Myr old sandstone contains B-, C-, and D-type garnet (Fig. 4c; Table 4).
Classification through linear discriminant analysis (Tolosana-Delgado et
al., 2018) yields a similar trend with generally high proportions of
amphibolite facies source rocks (class-B garnets, >70 %;
Table 4). Some grains (5 %, 3 %, and 12 % in the 25, 19, and
14 Myr old deposits, respectively) were classified as igneous garnet (Table 4).
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 ≥50 %. Several grains were assigned mixed probabilities with <50 % per class; these are listed separately below (italic numbers).
Mange and Morton (2007) Tolosana-Delgado et al. (2018) Types after Mange and Morton (2007)25 Ma19 Ma14 MaClasses after Tolosana-Delgado et al. (2018)25 Ma19 Ma14 MaCi type (high-grademetabasic)5 %15 %Eclogites (Class A)1 %B type (amphibolite facies)96 %84 %80 %Amphibolites (Class B)92 %81 %78 %A type (granulite facies)3 %8 %Granulites (Class C)9 %5.5 %D type (metasomatic)1 %3 %5 %Igneous (Class E)7 %3 %12 %Mixed probabilitiesClasses B-C1 %1 %Mixed probabilitiesClasses A-B-C5 %4.5 %
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
>100 Myr old with a smaller and younger age peak of 41±9 Ma (Fig. 3; Spiegel et al., 2000). This would favour an input from the
Austroalpine nappes and/or the Romandes Prealps (Fig. 6a), which yield
related cooling ages >50 Ma (Fig. 2b; e.g. Bernet et al., 2009),
rather than from the Lepontine dome, which is characterized by zircon
fission track ages <30 Ma (Fig. 2b; e.g. Hurford, 1986). The
presence of granite pebbles attributable to the Austroalpine Bernina nappe
(Matter, 1964; Schlunegger et al., 1998) would further support an
Austroalpine rather than a Lepontine provenance.
Paleogeographic reconstruction of the Central Alps and in
particular of the hinterland of the Honegg–Napf fan. Situation during (a) the
late Oligocene (∼25 Ma), (b) the early Miocene
(∼19 Ma), (c) the middle Miocene (∼14 Ma), and
(d) today (after Schmid et al., 2004). The colour coding in (a–c) corresponds
essentially to the colour coding in (d) (see Fig. 1 for detailed legend).
However, we have summarized the lower, middle, and upper Penninic nappes and
the Dent Blanche nappe (pink colour) as well as the carbonate and flysch
nappes of the Helvetic nappes and the Romandes Prealps (blue colour).
Abbreviations used: AA – Aar massif; BP – Bergell pluton; GN – Gotthard nappe; HN – Honegg–Napf fan; IL – Insubric line; RH – Rigi–Höhronen fan; RSL – Rhône–Simplon lineament; SKH – Speer–Kronberg–Hörnli fan.
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).
Early Miocene (<inline-formula><mml:math id="M70" display="inline"><mml:mrow><mml:mo>∼</mml:mo><mml:mn mathvariant="normal">19</mml:mn></mml:mrow></mml:math></inline-formula> Myr ago)
The larger spread of garnet compositions in the early Miocene
(∼19 Ma) sample indicates the presence of several or mixed
sources with different metamorphic grades, including amphibolite-,
eclogite-, and granulite facies rocks.
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 (<30 Ma) zircon fission track ages (Fig. 3) in
agreement with the young cooling ages of the Lepontine dome (Fig. 2b; Bernet
et al., 2009). Due to the overlap of amphibolite facies garnets, it cannot
be excluded that at least some of the garnets were contributed by
Austroalpine sources or were recycled from older strata. The Lepontine dome
was probably drained both towards the north and the south (Fig. 6b) because
old basement detritus with young cooling ages (∼30 Ma;
derived from K-Ar on white mica) was found in the Gonfolite Lombarda group
in the southern retro-foreland (Giger and Hurford, 1989).
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
19.5±0.9 Ma, Fig. 3; Spiegel et al., 2000) that match the zircon
fission track ages of the Lepontine dome (Fig. 2b; e.g. Hurford, 1986;
Bernet et al., 2009).
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-1 (Michalski and Soom, 1990;
Glotzbach et al., 2010), the top of the basement was buried 7–10 km below
the surface 14 Myr ago.
Cross sections from X to X′′ in Fig. 1 through the Aar massif
simplified after Pfiffner (2017) and Nibourel et al. (2018). (a) The
reconstructed top of the crystalline basement in the Aar massif is located
ca. 1–2 km higher than the present-day topography according to Pfiffner
(2017). (b) In a revised version by Nibourel et al. (2018) the contact
between the basement and the overlying Helvetic cover nappes is
reconstructed to be steeper, resulting in ca. 8 km of (now eroded)
crystalline crust on top of the present-day topography.
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.
Conclusions
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.
Data availability
The data (chemical composition of garnets from Molasse sandstones and source
samples) can be found online:
10.6084/m9.figshare.8269742.v1 (Stutenbecker, 2019).
Author contributions
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.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
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.
Financial support
This research has been supported by the International Association of Sedimentologists (post-doctoral research grant, spring session 2018 grant).
Review statement
This paper was edited by Kei Ogata and reviewed by Carita Augustsson and Lorenzo Gemignani.
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