the erosion of the alps: nd isotopic and geochemical constraints on the sources of the peri-alpine...

Earth and Planetary Science Letters 146 (1997) 627-644

EPSL

The erosion of the Alps: Nd isotopic and geochemical constraints on the sources of the peri-Alpine molasse sediments

Philippe Henry a, * , Etienne Deloule a, Annie Michard b ’ CRPG-UPR 9046 CNRS, 15 rue Norre Dame des Pauvres, BP 20, 54501 Vandoeuvre les Nancy, France

b LGE-CEREGE, Universirt? Aix-Marseille III, CNRS FU 017. BP SO, 13345 Aix en Provence Cedex 04, France

Received 20 September 1995; revised 2 October 1996; accepted 22 October 1996

Abstract

Sm-Nd data from molasse sedimentary deposits from eastern France and Switzerland are used to quantify the erosion of

the Alps during the Oligocene and Miocene. The average present day l Nd value of the continental sedimentary deposits increases from - 11 for Cretaceous and Eocene substratum to - 9 for the first Oligocene molasse sediments. This increase

requires the erosion of Mesozoic marine sediments to explain the average Ed,, value ( - 9.1) of the Rupelian and Lower Chattian sediments. Then the average l Nd value (-9.7) and the chemical compositions of the Upper Chattian and Aquitanian sediments are consistent with the erosion of granitic rocks of the Variscan crust. This change in source allows us

to define a second cycle in the per&Alpine molasse which began at 24.5 Ma with the deposition of the “Calcaires et Dolomies”. This basin-wide unit represents the sedimentary record of a halt in the input of Alpine detritus in the western

part of the molasse basin, and we suggest that the limit between Lower and Upper Chattian sediments, at 24.5 Ma, corresponds to a major tectonic event in the Alps. A further increase of 1 eNd unit recorded by the Burdigalian marine sandstones (average of - 8) defines a third molasse cycle which resulted from the erosion of late Variscan alkaline granites havmg high eNd values between - 1.8 and -5.2.

This study concludes that the erosion of the Alps increased the +, values of the sedimentary mass in two stages: (1) during the Rupelian and Lower Chattian, by recycling of marine chemical sediments having Q, values similar to that of

Tethys seawater (eNd = -8), and which represent 90% of the eroded materials, and (2) during the Burdigalian, by the erosion of a Variscan cmst representing at least 20% of the eroded material. Half of this Variscan material was composed of alkaline granites with high l Nd values, suggesting the addition of mantle-derived material to the crust during late Variscan

events.

Keywords: geochemistry; Nd-144/Nd-143; Sm/Nd; Alps; erosion; sedimentation

1. Introduction mentary rocks (see reviews [1,2]). In particular, the

Sm-Nd isotope studies have been shown to be a

powerful tool for investigating the sources of sedi-

* Corresponding author’s present address: Giosciences, Univer- sit6 de Franche Comtt, 16 route de Gray, 25030 Besancon cedex, France. E-mail: [email protected]

eNd values of shales have been used to determine the average isotopic composition of the eroded continental crust and, subsequently, to determine rates of continental growth [3-lo]. In all of these studies, the Nd isotopic composition of the detrital sediments is assumed to result from mechanical mixing between old eroded crust and more recent detrital input. This

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628 P. Henn, et al. /Earth and Planetary Science Letters 146 (1997) 627-644

latter input is derived, more or less directly, from a juvenile mantle-derived component, and its introduc-

tion into the continental mass contributes to the growth of the continental crust.

Furthermore, the Nd isotopic compositions of the

different superficial reservoirs of the Earth’s surface

have been investigated. Many of these studies focus

on sedimentary reservoirs (e.g., [7-131) and a compi-

lation of Sm-Nd data [14] shows the influence of the

geodynamic locations (active or passive margins) on the isotopic compositions of the sediments. Other

studies use oceanic chemical sediments to determine

the isotopic compositions of the oceans [15] and their

secular evolution (reviews by [16,17]). The modelling of crustal growth by Nd depleted

mantle ages CT,,, or Nd crust residence times) of sedimentary rocks, assumes that the Nd evolves with

a constant crustal ‘47Sm/ 144Nd ratio (about 0.12

13-71) after its extraction from the depleted mantle. However, higher Sm/Nd ratios are observed in

chemical sediments, resulting in model ages older

than those of contemporaneous detrital sediments [5,6]. Furthermore, in the case of detrital sediments,

two studies [ 18,191 propose that secondary processes,

and especially diagenetic reactions, can modify both ‘47 Sm/ 144 Nd and 143Nd/ 144Nd ratios. Such modifi-

cations are important variables to be considered for

the interpretation of Sm-Nd data of old sediments

for which we need to calculate the initial eNd val-

ues. The Alpine orogen results from a continental

collision between the African and European plates

after the subduction of the Tethys Ocean. The three

major tectonic phases are the eo-, meso- and neo-Al- pine phases, associated respectively with metamorphic ages of 140-80, 40-30 and less than 30 Ma

[20,21]. This succession corresponds also to a displacement of the active tectonic zones, with the structuring of the internal Alps, followed by tectonic

motions in the external Alpine zones. A previous study of the sandstones of Taveyannaz

and the volcanism of Saint Antonin [22] determined eNd values between + 1 and +4, which define the peak of the juvenile contribution to the Alpine orogen, by comparison with the Variscan, Caledonian and Cadomian Nd isotopic peaks 131. The positive l Nd values found in these syn-tectonic Alpine sediments provide evidence for juvenile mantle-like ad-

ditions during the Alpine orogenesis. In a previous

paper [23], we presented Rb-Sr data obtained on peri-Alpine molasse sediments showing an evolution

of the metamorphic ages of the eroded rocks through time. The goal of the present study was to use Nd

isotopes, with major and trace elements, to verify the

evolution of the sedimentary record during the time

of the molasse deposition. Three major points will be

discussed with the aim of: (1) testing the importance of mantle-like additions during Alpine orogenesis;

(2) using Nd isotopes to identify and quantify the

erosion of the Alps; and (3) trying to correlate sedimentary changes with major tectonic events of

the Alpine collision.

1 0 Drillholes

Fig. 1. Samples location map (modified after [34]) with graphic units [27]. The ages refer to the correlative chart

European Oligocene and Miocene [30].

strati- of the

P. Henry et al. /Earth and Planetary Science Letters 146 (1997) 627-644 629

2. Geological setting and sample descriptions

The Alpine orogenesis began with Mesozoic sedimentation, followed by the deposition of the Upper Cretaceous and Tertiary flysch, and ended with the Oligocene and Miocene peri-Alpine molasse sedimentation [2 1,241. The molasse Helveto-Savoyard basin (Fig. 1) is a foreland basin [25], also called flexural basin [26], which resulted from the overload-

200 300 400 500 400 700

100 200 300 400 500 600 700 800 m

1000 1100 lzo0 1300 14cm 1500 1600 1700 1800 1900 2000 2100

300 400 500 600 700

E 1000 1100 1200 1300 1400 1500

ing created when the internal zones over-thrust the external European platform. Reviews of previous work already exist both for the Alpine chain [2 1,241 and the molasse sediments [25].

The molasse basin is usually divided into two parts, defined by their tectonic locations. The autochthonous molasse (“Molasse du Plateau”, Fig. 1) covers a large part of Switzerland and continues into France in the synclines of the external sub-Alpine

column A (wt S) column B (WI %) column C @pm)

0 20 40 60 80 loo 0 5 IO 15 20 25 0 400 800 1200 1600

Tiefenbrunnen

Savoy - 101

Tuggen

qz ill CO:- MgO* K20 P205 Ba Cr Ni Zr Rb Sr q q [ID q omB&?jmo HrJ

Fe203 NaaO TiOa L.O.I.

Helvetian

Burdigalian

Burdigalian

allochthonous Upper Chattian

fault

Aquitanian

Upper Chattian

“Calcaires et Dolmlies”

Lower Chattian

“Calcairea inf&iwrs” Rupelian, Eocene Cretaceous

Lower Chattian to Upper Rupelian

Fig. 2. Major and trace element data in depth distribution. Column A shows the estimated proportion in wt% of quartz (qz), illite (ill) and

carbonate (CO:- ). Column B reports the contents in wt% of Fe,O,, MgO * , Na,O, K,O, TiO,, P,O, and L.O.I. (loss by ignition), where

MgO * represents an estimate of MgO contained in the silicates (see text). Column C shows Ba, Cr, Ni, Zr, Rb, and Sr concentrations in

ppm. Samples at 566 m and at 1371 m from Sv-101 have Sr contents of 2262 and 1389 ppm, respectively.

630 P. Henry et al./Earth and Planetary Science Letters 146 (1997) 627-644

chains. The Jura mountains constitute its northwest- em geological limit and the main Alpine thrust its

southeastern boundary. The allochthonous molasse (“Sub-Alpine Molasse”, Fig. 1) over-thrusts the au-

tochthonous molasse with a possible displacement of

50-100 km from the south and the east [25]. Since

Studer in 1854 [27], four stratigraphic units have

been recognized in the molasse, from bottom to top:

the Lower Marine Molasse (UMM = Untere Meeres-

molasse), the Lower Freshwater Molasse (USM = Untere Stisswassermolasse), the Upper Marine Mo-

lasse (OMM = Obere Meeresmolasse) and the Upper Freshwater Molasse (OSM = Obere Siisswassermo-

lasse).

The molasse sediments analyzed were sampled from three drillholes (Fig. 1):

The Tiefenbrunnen drilEhole (T.f.b.), located near

Zurich, Switzerland, cross-cuts, from top to bottom,

the Burdigalian OMM, divided into “Helvetian”

marls (190-310 m> and Burdigalian green sand-

stones (310-720 m>, and reaches the top of the Aquitanian USM (720-736 m).

The Sauoy 101 drillhole (Sv-1011, near Annecy,

France, begins in the Burdigalian green sandstones (loo-245 m), cross-cuts an allochthonous Upper Chattian unit (245-700 m, [28]) and the whole USM

series [27], including the Aquitanian “Molasse Grise

de Lausanne” (Grey Molasse of Lausanne, 700- 1100 ml, the Upper Chattian “Gres et Marries Gris ?I

Gypse” (Grey Sandstones and Marls with gypsum, 1100-1400 m), including between 1300 and 1400 m

the “Calcaires et Dolomies” (Freshwater Lime-

stones and Dolomites), dated at 24.5 Ma on the basis

of micro-paleontologic data [29,30], and, finally, the Lower Chattian “Mames bariolees inferieures”

(Motley Marls, 1400-1946 ml, including the

“Calcaires inferieurs” (Lower Limestones, 1870- 1915 m), which are dated between 26 and 34 Ma as

a function of their proximal or distal locations in the basin [29]. A thin unit of carbonated sandstones (1946-1961 m) attributed to the Rupelian UMM [13,23] is found below the Lower Chattian USM. The drillhole reaches the substratum formed by Eocene laterites (196 l-2002 m) and Cretaceous

oolitic limestones (2002-2064 ml. The Tuggen drillhole (south of Lake Zurich,

Switzerland) cross-cuts a thick unit of alternating marls and dolomitic sandstones. By their location in

the allochthonous molasse and by their lithologic features, these formations are considered to represent

the UMM (113,231, and this study) with an age between the Upper Rupelian and the Lower Chattian

[29]. Thus, the top of Tuggen drillhole is presumed to be contemporaneous with the thin Rupelian unit at

the bottom of Sv- 10 1.

Major and trace element analyses were used to

further characterize the stratigraphy of the drillholes.

This procedure is reviewed in Section 4 (Fig. 2),

however more detailed petrological investigations are given in [13].

In addition, six other sediments from different sites (Fig. 11, donated by J.P. Berger, were analyzed

for comparison with the drillholes ones: Pra is a

Burdigalian carbonated sandstone (“shelly sand-

stone”) located near the limit between the molasse basin and the Jura Mountains; Zlettes an al-

lochthonous Lower Chattian red marl from the Val

d’Illiez Window; Ogoz is a Lower Chattian sandstone from the conglomerates of the Mont Pelerin

Formation; Ver is a sample of the Lower Chattian “Calcaires inferieurs”; Tor is an Eocene ferruginous

sandstone from the small Tertiary basin of Moutier; and Bir is a sample of Rupelian UMM from the

Tertiary basin of Delemont, located between the molasse basin and the Rhine Graben.

3. Analytical techniques

For the chemical and isotopic analyses, rocks

were crushed in an agate mortar. Major and trace elements were analyzed by ICP at CNRS-CRPG

(Nancy, France). The Sm-Nd sample preparation began by the dissolution of CaCO, in

1 N HCl to prevent the formation of fluorides when concentrated hydrofluoric acid was added. The samples were spiked with a 147Sm-150Nd tracer, then

dissolved with mixtures of distilled concentrated acids (HF, HNO, and HClO,) in Teflon” vessels. Complete homogenization of the solutions was as- sured by a mixture of concentrated HNO, and HCl. The chemical separations followed the techniques described by Alibert et al. [31] and Michard et al. [32]. The Nd isotopic ratios were measured with a Finnigan MAT 262 mass spectrometer. The Sm concentrations were determined with a Cameca TSN

P. Henry et al./ Earth and Planetary Science Letters 146 (1997) 627-644 631

206 SA mass spectrometer. During the period of this study, the total blank procedure was 50 and 20 pg for Nd-rich (200 ng> or Nd-poor (< 100 ng) solutions, respectively, and is negligible. The concentrations and the Sm/Nd ratios are given with an accuracy of 2% and the reproducibility of 143Nd/ 144Nd ratios is less than 25 . 1O-6 (2~). These values were determined both by 35 measurements of La Jolla and J&M standards with 143Nd/ ‘44Nd ratios of 0.511858 (22) and 0.5 11124 (24), respectively, and by some duplicates that were melted before dissolution, which confirm that the samples were completely dissolved in the Teflon’ vessels [13].

4. Major and trace element data

The major and trace element concentrations are summarized as chemical stratigraphic columns in Fig. 2 (the data base for this figure and following ones is available as an EPSL Online Background dataset ’ ). Column A gives the proportions of quartz, illite and carbonate, the three major constituents of the sedimentary rocks. The proportion of carbonate is calculated by the combination of CaO and CO, wt%, with an estimation of the proportion of dolomite by the addition of MgO to balance (CaO + MgO) = CO,. The proportion of illite is calculated by assum- ing that all Al,O, is contained in illite (SiO, = 48.6 and Al,O, = 24.0, in wt% [33]). The proportion of this theoretical illite represents an estimation of the alumino-silicate content (clays, feldspars, glauconites and muscovites) and is in good agreement with the mineralogical observations. Finally, the SiO, wt% in excess is expressed as quartz. Column B shows the wt% of the other major elements, where MgO* is the fraction of MgO in the silicates (MgO* =

Mg”,O,,r - Mg”dolomite ). Column C illustrates the variations in Ba, Cr, Ni, Zr, Rb and Sr concentrations.

The chemical compositions of the molasse sediments provide another means of monitoring stratigraphic changes in each of the drillholes. From the

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chemical changes illustrated in Fig. 2 the following observations can be made:

(1) The Helvetian marls (drillhole T.f.b.) show high carbonate contents ( = 60%) and low quartz and illite clay contents (= 20% each).

(2) The Burdigalian feldspathic and glauconitic sandstones (T.f.b. and Sv-101) have high quartz ( = 40%), and relatively low carbonate ( = 30%) contents. The calculated illite clay content ( = 30%), higher than in the Helvetian marls, has been shown by mineralogical observations to represent feldspars and glauconites. The Burdigalian sandstones are characterized by low contents of clay minerals, essentially present in unconsolidated clay pebbles or in small beds of marl (T.f.b., 457 m and Sv-101, 192 m).

(3) The Aquitanian sandy marls (“Molasse Grise de Lausanne’ ’ , Sv-101) have about 20% quartz and 40% of both carbonates (rock fragments and cements) and theoretical illite, which represents a mixture of clay minerals, feldspars and micas.

(4) The allochthonous and autochthonous Upper Chattian “G&s et Marnes Gris avec Gypse” units from Sv-101 have similar compositions, with quartz, carbonate and illite clay contents of 20-50%, 30- 50% and 20-40%, respectively. The sandstones at 1 lOO- 1200 m ( = 50% quartz), which define the limit between the Aquitanian and Upper Chattian units, and the “Calcaires et Dolomies” at 1400 m (= 50% carbonate and high Sr concentrations), which define the limit between the Upper and Lower Chat- tian series, are easily identifiable by their chemical compositions (Fig. 2). The high Sr concentrations found at 566 and 682 m (2262 and 833 ppm, respectively) are caused by one or both of the following: either the faults [28] have favoured Sr remobilisa- tions, or they are characteristic of the “Calcaires et Dolomies”, as is the case for the autochthonous Upper Chattian at 1323 and 137 1 m (647 and 1389 ppm, respectively).

(5) The Lower Chattian sediments of Sv-101 (“Marnes bariolees inferieures”), contain about 30% quartz, 20% carbonates and 50% illite. The major differences with the other molasse sediments are the higher MgO * , Cr and Ni concentrations caused both by the presence of Mg-rich chlorites (average: MgO = 25.5, Cr,O, = 0.4 and NiO = 0.2 wt%; electron- probe analyses [ 131) and rock fragments having the

632 P. Henry et al./ Earth and Planetary Science Letters 146 (1997) 627-644

chemical composition of hydrated serpentinites (average: MgO = 25.5, Cr,O, = 0.04 and NiO = 0.1 wt% [13]). On the basis on MgO * , Cr and Ni contents (Fig. 2), the sandstones at 1921 and 1922 m, located below the “Calcaires infkrieurs’ ‘, belong to the same Lower Chattian unit. The Na,O concentrations, higher than those of the Aquitanian and the Upper Chattian sediments, can be explained by the presence of sodic chert fragments having Na,O between 3.7 and 6.5 wt% (electron-probe analyses in [13]). Moreover, Zr and Rb concentrations are lower than in the other molasse sediments, suggesting that the average continental crust was not the major erosional source during the Lower Chattian.

(6) The Rupelian sediments of Tuggen have quartz and carbonate contents of about 90 wt%. These sediments are essentially composed of detrital quartz, siliceous and carbonate rocks fragments, and carbonate cements. The presence of dolomitic fragments seems to be the major characteristic by comparison with the other molasse sediments. For drillhole Sv- 101, the sample at 1952 m, which represents the first molasse sediments deposited directly on the Eocene

WI (wm) 14’Srn /**Nd 0 10 20 30 cl.!0 0.1.2 0.1.4

substratum, has low MgO * , Cr and Ni concentrations. This chemical composition, very different from that of the Lower Chattian sandstones, argues for the presence of a thin Rupelian unit at the base of drillhole Sv- 101 ([ 131 and the original description of the drillhole Sv-101).

5. Sm-Nd data

5.1. ENd values of the molasse sediments

The Sm-Nd data are plotted in Fig. 3 as a function of depth for each drillhole and the averages for each stratigraphic unit are listed in Table 1. For all molasse sediments, the l Nd values range from - 12 to - 6. The important observations are:

(1) The average eNd value of the pre-molasse Cretaceous and Eocene substratum of Sv-101 is - 11 .O f 0.2 (weighted average eNd of i samples = X:( eNdi. [Ndli)/XINdIi, where [Nd] is the Nd concentration). This value agrees well with both eNd values from Phanerozoic shales (between - 10 and

TDM @a) %d

1.2 1.4 1.6 1.8 -13 -11 -9 -7

A Helvetian (H) •I Aquitanian (A) m Lower Chattian (IC) - Eocene @I A Burdigalian(B) q Upper Chattian (UC) 0 Rupelian W X Cretaceous (0)

(auto- and allochthonous)

Fig. 3. Sm-Nd data vs. depth for each drillhole. In comparison with the Eocene and Cretaceous substratum (eNd = - 11.01, the RuFlian

and Lower Chattian molasse sediments (eNd = - 9.1) record an increase of 2 l Nd units. The average l Nd values of each stratigraphic level

divide the molasse sedimentation into three parts: Rupelian and Lower Chattian ( eNd = - 9.1), Upper Chattian and Aquitanian (eNd = - 9.71,

Burdigalian and Helvetian (eNd = - 8.2). The Nd concentrations (ppm), 14’Sm/ 144Nd ratios and T oM ages (depleted mantle model ages

[37]) are also plotted as function of the depth for each drillhole.

P. Henry et al. / Earth and Planetary Science Letters 146 (1997) 627-644 633

- 13, e.g., [3]) and from pre-Variscan formations [35], and can be interpreted as the signature of the erosion products of a crust older than the Variscan crust.

(2) The Rupelian and the Lower Chattian sediments (Tuggen and Sv-101) have similar average eNd values of - 9.1 + 0.4. Therefore, the erosion of the Alps increased the l Nd value of the sedimentary mass by about 2 eNd units. This implies that at least a part of the eroded Alpine rocks have l Nd values higher than the average value recorded by these molasse sediments. A Lower Chattian carbonate sandstone @v-l01 at 1861 m) has an l Nd value of - 12.0 + 0.4, different from the other Lower Chat- tian sediments but similar to the pre-molasse substratum. This sandstone was deposited after the

“Calcaires inferieurs”, which are classically interpreted as evidence for a hiatus in the Alpine detrital input in the molasse basin [27]. The sample Ver (“Calcaires inferieurs”) has an eNd value of - 11.2 f 0.2, similar to those of Cretaceous limestones, in agreement with an eroded source dominated by the same material. Finally, the l Nd values of the Lower Chattian sediments Ilettes (- 9.3) and Ogoz (- 8.9) are consistent with those of the Lower Chattian from Savoy.

(3) The Upper Chattian and the Aquitanian sediments of Sv-101 have the same average l Nd value of -9.7 + 0.5, slightly lower than the value of the Lower Chattian sediments. The Lower to Upper Chattian transition corresponds to major changes in MgO * , Cr and Ni contents, which can be related to a

Table 1 Sm-Nd data for the molasse sediments (average + 1 D )

Sediment Depth n

Cm)

Nd

@em)

tNd

Tiefenbrunnen OMM Helvetian

OMM Burdigalian

USM Aquitanian

Savoy-101

OMM Burdigalian

USM alloch. Upper Chattian

USM Aquitanian

USM autoch. Upper Chattian

USM Lower Chattian

except carbonate sandstone

except “Calcaires Inferieurs”

UMM Rupelian

Eocene

Cretaceous

Tuggen UMM Rupelian

Samples from different sites OMM Burdigalian

USM. Lower Chattian

USM “Calcaires Infkieurs”

USM Lower Chattian

UMM Rupelian

Eocene

190-310

3 lo-720

717 m

736 m

4

6

1

1

4

7

4

7

10

1

3

1

1 3

14

12.3 f 3.6

12.3 + 3.5

13.0

6.67

0.124 + 0.003

0.124 f 0.003

0.1240

0.1311

100-245

245-700

700-l 100

1100-1400

1400-1946

1861 m

1870-1915

1952 m

1988 m

2002-2064

12.9 k 5.6

21.7 f 4.7

16.5 + 2.3

15.3 * 3.5

15.1 + 4.6

22.9

0.6 f 0.5

16.5

6.56

1.3 f 1.3

0.128 + 0.003

0.121 f 0.003

0.126 + 0.008

0.122 + 0.004

0.128 + 0.003

0.1048

0.121 + 0.004

0.1181

0.0882

0.142 f 0.019

300-1600 7.0 + 4.3 0.121 + 0.003

Pra 6.35 0.1197

Ilettes 20.4 0.1332

Ver 0.902 0.1238

ogoz 15.3 0.1200 Bir 9.32 0.1197

Tor 13.4 0.1074

- 8.4 + 0.5

- 7.9 + 0.6

- 8.2 * 0.2

- 6.8 f 0.2

1.42 f 0.02

1.38 + 0.06

1.41

1.40

-8.6 f 1.0

- 9.8 * 0.4

-9.6 + 0.1

- 9.7 + 0.4

-9.1 + 0.4

- 12.0 * 0.2

- 9.6 f 0.2

- 8.9 f 0.2

- 10.7 * 0.2

-11.0*0.2

1.52 k 0.06

1.49 f 0.05

1.57 f 0.16

1.51 * 0.06

1.56 + 0.04

1.43

1.49 + 0.07

1.38

1.16

2.20 + 0.60

-9.1 * 0.4 1.44 f 0.03

- 7.8 * 0.2 1.31

- 9.3 + 0.2 1.68

-11.2*0.2 1.67

- 8.9 + 0.2 1.41

- 8.5 + 0.2 1.37

- 9.4 & 0.2 1.28

The Nd concentrations (ppm) and the 14’Sm/ ‘44Nd ratios are reported with an accuracy better than 2%. The cNd values are calculated for the present using the CHUR isotopic values of 0.1967 and 0.512638 for 14’Sm/ ‘44Nd and ‘43Nd/ ‘44Nd ratios, respectively. T,, is the

depleted mantle model age [37]. This table reports the averages ( f 1 (T 1 which are representative of each sedimentary unit. The complete database is available as an EPSL Online Background Dataset.

634 P. Henry et al./Earth and Planetary Science Letters 146 (1997) 627-644

major change in the nature and/or proportions of the eroded Alpine materials. Moreover, this limit also corresponds to the deposition of the “Calcaires et Dolomies’ ’ , which are a synchronous lithostrati- graphic level for a large part of the western molasse basin, dated at 24.5 Ma [29,30]. However, two Aqui- tanian samples from Central Switzerland (T.f.b. at 717 and 736 m) have different l Nd values (- 8.2 and - 6.8, respectively), in better agreement with the Burdigalian eNd values. This suggests that the flu- vial inputs are different for the western and the eastern parts of the molasse basin, at least for the end of the Aquitanian.

(4) The Burdigalian and Helvetian sediments have an average eNd value of - 8.2 f 0.8, and the glauconitic sandstones have the highest l Nd values (average of - 7.9 k 0.6). The Burdigalian “shelly sandstone” Pra has a similar eNd value (-7.8). These values show that the stratigraphic limit between the Aquitanian USM and the Burdigalian OMM corresponds to a change in the detrital Alpine inputs. The uniformity of the eNd values recorded by the Burdigalian sediments from the drillholes Sv-101 and T.f.b. and from Pra suggests that the entire molasse basin received the same detrital material at this time. This point does not support the presence of two cells of sedimentation, which were suggested from heavy mineral contents [36].

The main conclusion from the above observations is that both the chemical compositions and the average l Nd values allow us to divide the molasse sedimentation into three cycles: (1) Rupelian and Lower Chattian (average Q,, = - 9.11; (2) Upper Chattian and Aquitanian (average l Nd = - 9.7); and (3) Burdigalian and Helvetian (average eNd = - 8.2). The limit between Lower and Upper Chattian is marked by a small change in the l Nd values, but is also supported both by a major change in the overall chemistry of the sediments, with the Lower Chattian sediments having high Mg, Cr and Ni concentrations, and by the deposition of the “Calcaires et Dolomies’ ’ . The Burdigalian marine transgression is marked by a change of more than 1 l Nd unit.

5.2. Nd concentrations, ‘47Sm/‘44Nd ratios, and

TDM ages

Fig. 3 shows the variations in Nd concentrations, ‘47Sm/ L44Nd ratios, eNd values and T,, ages (de-

pleted mantle model ages after [37]) with depth. Because quartz and carbonate are abundant in these molasse sediments and are Nd-poor, the Nd concentrations are always less than 30 ppm, which is the characteristic average Nd concentration for the shales [S]. Most of the molasse sediments have 147Sm/ ‘44Nd ratios higher than 0.115, which is the average value usually reported for the detrital sediments and corresponds to the average value for the upper continental crust (e.g., [3-lo]). This suggests either that the molasse sedimentary processes favour higher Sm/Nd ratios or that at least one end-member had a Sm/Nd ratio higher than the typical continental crust. The To, ages vary between 1.3 and 1.8 Ga, which are typical of Variscan European crust [35] despite the higher Sm/Nd ratios. The significance of these depleted mantle ages will be discussed below.

5.3. Sm-Nd data for pebbles from the Burdigalian conglomerates

In order to determine the eNd values of the eroded rocks directly, some pebbles extracted from Burdigalian conglomerates (drillhole T.f.b.1 were analyzed (Table 2). The sedimentary pebbles include three talc-arenites (Ga 2, Ga 4 and Ga 51, and one dolomitic limestone (Ga 6). Their l Nd values range from - 10.1 to - 8.1 and overlap with values for Tethysian carbonates and cherts [15]. The granitic pebbles give more surprising results: two have l Nd values of - 9.3 (G 4) and - 8.5 (G 8), in agreement with the average composition of European Variscan crust [35], but the other eight have cNd values between - 5.2 and - 1.8, an unusual range of values for the European upper continental crust. These eight alkaline granites are composed essentially of quartz (SiO, = 74-79 wt%), orthoclase or microcline and albite (Al,O, = 11-14 wt%; K,O = 3.8-5.1 wt% and Na,O = 3.4-4.8 wt%) with rare Fe-Mg-silicates. The Rb-Sr data [13,23] suggest late Variscan ages, between 300 and 250 Ma. Therefore, the initial l Nd values, between 0 and -6, are outside of the range defined for the average European Variscan crust [35]. The erosion of these granites would explain the higher present day eNd values measured for the Burdigalian sandstones of the OMM both in Switzerland and Savoy.

P. Henry et al. / Earth and Planetary Science Letters 146 (1997) 627-644 635

Table 2

Sm-Nd data for mineral separates and for pebbles extracted from the Burdigalian conglomerates (T.f.b.)

Sample Sm Nd 14’Sm/ 144Nd ‘43Nd/ ‘44Nd

(2a) ‘Nd T DM

Muscovite-s

Sv-101 at 151 m 2.15

Sv-101 at 520 m 2.99

Sv-101 at 1921 m 0.806

T.f.b. at 518 m 0.815

Chloritized biotites

So-101 at 1473 m bi-chl 1 4.51

bi-chl 2 3.84

T$b. at 518 m bi-chl 1 3.34

bi-chl 2 6.98

bi-chl 3 11.16

Pebbles from T.f.b. at 342 m

Pebbles of sedimentav rocks a

Ga2 2.34

Ga4 0.479

Ga 5 0.257

Ga6 0.294

Pebbles of alkaline granites Ga 1 3.37

Ga3 4.28

Pebbles from T.f.b. at 349 m

Pebbles ofplagioclase-rich granites G4 2.09

G8 1.08

Pebbles of alkaline granites Gl 4.81

G2 4.06

G3 4.35

G7 0.987

G 13 1.51

G 14 1.18

10.8 0.1206 0.512059 (15) - 11.3 f 0.3 1.62

16.7 0.1081 0.512044 (15) -11.6*0.3 1.45

4.72 0.1032 0.512048 (12) - 11.5 f 0.3 1.38

4.55 0.1084 0.512064 (14) - 11.2 + 0.3 1.42

20.9 0.1304 0.512158 (7)

18.4 0.1260 0.512117 (18)

1.63

1.62

19.2 0.1053 0.512150(11)

33.6 0.1256 0.512220 (10)

52.4 0.1289 0.512235 (8)

-9.4 * 0.1

- 10.2 + 0.4

- 9.5 + 0.2

- 8.2 f 0.2

- 7.9 + 0.2

1.26

1.43

1.46

11.4 0.1247 0.512224 (13) -8.1 +O.l 1.41

2.52 0.1147 0.512120 (14) - 10.1 f 0.1 1.43

1.47 0.1060 0.512206 (15) -8.4 f 0.1 1.19

1.35 0.1319 0.512167 (12) -9.2 + 0.1 1.65

21.6 0.0942 0.512397 (67) -4.7 + 1.3 0.83

14.8 0.1754 0.5 12548 (9) - I .8 + 0.0 2.04

12.2 0.1042 0.512162 (10) - 9.3 * 0.2 1.23

7.44 0.0877 0.512202 (25) - 8.5 + 0.5 1.02

27.0 0.1079 0.512448 (23) - 3.7 + 0.4

22.3 0.1100 0.512452 (15) - 3.6 + 0.3

22.7 0.1159 0.512444 (13) - 3.8 + 0.3

2.81 0.2121 0.512371 (19) - 5.2 + 0.4

7.34 0.1244 0.512450 (12) -3.7 f 0.1

4.37 0.1629 0.512433 (6) -4.0+0.1

0.87

0.88

0.94 _

1.02

1.87

a Ga 2 = carbonate feldspathic sandstone: Ga 4 = carbonate sandstone; Ga 5 = sandy limestone; Ga 6 = dolomitic limestone. Notes as in Table 1.

5.4. Sm-Nd data of mineral separates

Muscovites and chloritized biotites were separated

from some samples and analyzed (Table 2). The eNd values of the muscovites range from - 11.2 to - 11 A, similar to the l Nd values of both the Creta-

ceous and Eocene substratum of Sv-101 and the pre-Variscan continental crust [3,12,35]. The chloritized biotites give eNd values between - 10.2 and

-7.9, in the range of the European Variscan crust

13.51. However, for these chloritized biotites, the To,

ages are, on average, 1.6 Ga for Lower Chattian (Sv- 101, 1472 m) and 1.4 Ga for Burdigalian (T.f.b.,

5 18 m). This confirms that the continental crust eroded during Burdigalian times was, on average, younger that the typical European Variscan crust, in agreement with the eNd values of granitic pebbles described above.


6. Discussion

The plot of eNd values as a function of 147Sm/ 144Nd ratios (Fig. 4), Al,O,/Nd and K,O/Nd ratios (Fig. 5) shows the general trend due to sedimentary mixing. These diagrams suggest that at least three end-members are needed to explain the dispersion of the data. The analyses of the pebbles and mineral separates allow the determination of three end-members defined by the analyses of the muscovites (A), the biotites and two granitic pebbles (0, and by the alkaline granites (D). These three end-members have typical upper crustal 147Sm/ 144Nd ratios (from 0.10 to 0.12), and l Nd values of about - 12, - 8 and - 4, respectively (Fig. 4). In Fig. 5, l Nd values between - 12 and - 8 are associated with crustal Al,O,/Nd (between 0.4 and 1.0) and K,O/Nd (between 0.1 and 0.2) ratios. The eNd value of - 12, characteristic of the end-member A, argues for the presence of pre-Variscan formations [3,35]. This end-member (A) also takes into account the lower l Nd value of the sample Sv-101 at

1861 m, deposited after the ‘ ‘Calcaires inferieurs’ ‘, which provides a good evidence for a short halt in the detrital Alpine inputs. This sandstone could record the erosion of local sources before the deposition of the homogeneous detrital Lower Chattian unit, and argues that the pre-Variscan formations do not have an exclusively Alpine origin. The l Nd value of about - 8 associated with crustal chemical compositions (Figs. 4 and 5) argues for the definition of an end-member C, representing the erosion of the typical European Variscan crust, for which an average l Nd value of - 8.3 + 0.5 has been previously proposed [35]. The end-member D is defined by the alkaline granites found as pebbles in Burdigalian conglomerates having an average eNd value of - 3.7 f 0.5. This value explains the l Nd values of the Burdigalian sandstones, which is higher by 1 l Nd unit than those of the other molasse units. Moreover, this end-member is characterized by crustal ‘47Sm/ ‘44Nd (average = 0.12, Fig. 4) and Al,O,/Nd (0.8, Fig. 5a) and high K,O/Nd (0.3, Fig. 5b), due both to the high alkali and low Nd

- l-- 120 molassic sediments

- 8--

‘Nd - 9--

-IO-- n Lower Chattian

-ll--

-12-- A . I-

0.10 0.11 0.12 0.13 0.14

-1 1 ,

I

- molassic sediients + Pebbles -2 --

-3 --I 20 -4 -- -5 -- +

‘Nd 1; -- -8 -- -9-- m

-10 -- -11 -- -12 --

+ + alkaline granites m granites with plagioclases X sedimentary rocks

Mineral separates 0 muscovitcs e chloritized biotitcs

Presumed eroded rocks (averages)

I A: prc-Variscao formations

0.08 0.09 0.10 0.11 0.12 0.13 0.14 0.15 0.16 0.17 0.18

‘47Sm/144Nd D: alkaline granites with high E+J~

Fig. 4. eNd vs. 14’Sm/ 144Nd This diagram illustrates the mixture of various end-members, with the comparison between Sm-Nd data . obtained for the molasse sediments and those obtained for the pebbles extracted from the Burdigalian conglomerates and for mineral

separates.


A Helvetian m Upper Chattian

A Burdigalian n Lower Chattian

q Aquitanian 0 Rupelian

a -4

-5 &Nd 0

-6 -1 0

A A

-8

-9

i 0 B @o n

-10 I

-II f

-12

-13 I Al203 /Nd

I

0.0 0.2 0.4 0.6 0.8 1.0

b -4

-5 &Nd

-6t -1 0

A

-8 *

-9

-10

-11

-12 K20 /Nd -13 I 4

0.00 0.05 0.10 0.15 0.20 0.25 0.30

Fig. 5. l Nd vs. (a) Al,O, /Nd and (b) K,O/Nd ratios (Al,O,

and K,O in wt% and Nd in ppm). These mixing diagrams permit

discrimination between the end-members. A = the pre-Variscan

formations; B = the presumed Mesozoic sediments; C = the aver-

age Variscan European crust; and D = the alkaline granite pebbles

from the Burdigalian conglomerates.

contents (Nd average = 15 + 9 ppm) in these gran-

ites. To account for the total dispersion in Fig. 4,

especially that due to the high Sm/Nd ratios, a fourth end-member (B), having a high ‘47Sm/ 144Nd

ratio (at least 0.141, is needed. An alternative explanation could be the existence of modifications of the

Sm/Nd ratios due to sedimentary processes. How- ever, in agreement with the rock fragments found in our study, previous studies concerning the pet-i-Al- pine molasse emphasise the importance of the erosion of flyschs, limestones and cherts during the deposition of the Rupelian and Lower Chattian mo-

lasses [38]. The interpretation of Figs. 4 and 5 in

term of mixing diagrams suggests the presence of an end-member (B) characterized by high Sm/Nd, low

Al,O,/Nd and low K,O/Nd ratios, which are asso-

ciated with Q,, values between - 8 and - 10. This

is in agreement with the idea that Mesozoic marine

sediments may constitute an important source of

molasse sedimentation. Therefore, an end-member

(B) having eNd between - 8 and - 10 and representing the erosion of Mesozoic chemical sediments

is proposed. This additional end-member can account for the total dispersion observed in Figs. 4 and 5. Its

range of l Nd values, from - 8 to - 10, can be

explained by the fact that these marine sediments are

a mixture of pure chemical components and detrital

particles, for which eNd values have been estimated at - 8 and - 12, respectively [ 151. Finally, the ex-

planation of the high Sm/Nd ratios of the molasse

sediments by the erosion of older chemical sediments is not really in disagreement with the altema-

tive proposition that Sm/Nd ratios could be modified by sedimentary processes. In our model, the

Sm/Nd modifications would have occurred during a previous sedimentary cycle, with the deposition dur-

ing Mesozoic times of chemical marine sediments

known to fractionate the Sm/Nd ratios [5].

At this stage, three end-members (A, B and C)

have been proposed to explain the sedimentary mixing during the Rupelian-Aquitanian. During the

Burdigalian, the data argue for the erosion of a

fourth component CD). In the following section, we will test this interpretation, which is possibly not unique.

A depleted mantle model age (To, 1 vs. Na,O/Nd ratio (Fig. 6a) diagram is used to investigate the

presence of sodic cherts, essentiaily in the Lower

Chattian sediments. The interest of the To, age is

that it depends on both the l Nd value and the ‘47Sm/ 144Nd ratio. Fig. 6a shows that some Lower Chattian and most of the Burdigalian sediments have

high Na,O/Nd ratios ( > 0.10) but their To, ages are different. In the case of the Burdigalian sand-

stones, the young To, ages (average = 1.42 Ga) are

easily explained by the erosion of alkaline granites having younger T,, ages (= 1 Ga) than those of typical Variscan granites ( = 1.6 Ga [35]). In contrast, the Lower Chattian sediments have older To, ages (average = 1.56 Ga) than Upper Chattian and

638 P. Henry et al. /Earth and Planetary Science Letters 146 (1997) 627-644

A Helvetian 81 Upper Chattian * Burdigalian n Lower Chattian q Aquitanian 0 Rupelian

0

A A

Naz 0 /Nd

MgOINd

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Fig. 6. (a) T,, vs. Na,O/Nd. This figure shows high Na,O/Nd

ratios associated with young T nM ages for the Burdigalian sedi-

ments and associated with old T,, ages for some Lower Chattian

sediments. This suggests different Na-rich eroded rocks: alkaline

granites having high l Nd values and young To, ages, and

chemical marine sediments, such as sodic cherts, having high

14’Sm/ ‘44Nd ratios, resulting in older T,, ages. (b) To, vs.

MgO/Nd. The lack of clear correlation between MgO and To,

ages emphasizes that the Mg-rich end-member, which could be

Tethys oceanic basalts with positive l Nd values, does not control

the Nd isotopic compositions of whole-rock sediments.

Aquitanian sediments (both with average T,, = 1.5 1 Ga). Their high Na,O/Nd ratios are explained by the presence of sodic chert fragments [13]. These chemical sediments are known to have high Sm/Nd ratios and average Nd concentrations up to 17 ppm [39] and can therefore contribute significantly to the Sm and Nd budgets. In the case of Lower Chattian sediments, Fig. 6a suggests that To, ages record an increase in the Sm/Nd ratios during a previous sedimentary cycle, possibly during the Mesozoic. Finally, the comparison between Na,O/Nd and Nd isotopic compositions argues for the conservation of

both geochemical and isotopic characteristics of the eroded sources. Similarly, Fig. 6b compares To, ages with MgO/Nd ratios. The presence of Mg-rich chlorites and serpentinitic rock fragments in the Lower Chattian sediments suggests the erosion of mafic to ultramafic rocks, which could explain their slightly higher eNd values ( - 9.1) than Upper Chat- tian sediments ones ( - 9.7). An addition of 4 wt% of basalts (Nd = 15 ppm, Ed,, = + 8 and 14’Sm/ 144Nd = 0.19) to the Upper Chattian average composition (Nd = 19 ppm, l Nd = -9.7 and 14’Sm/ 144Nd = 0.121) is needed to reach the average l Nd value of Lower Chattian sediments. However, such an addition would decrease the TDM age by 0.02 Ga, when the Lower Chattian unit has an older To, age. Therefore, a minor introduction of a mafic component remains possible but cannot explain the To, ages of the Lower Chattian sediments.

In the following section we will try to discrimi- nate between the possibilities of the erosion of chemical sediments or modifications of Sm/Nd ratios by sedimentary processes for explaining the high Sm/Nd ratios (Fig. 4). In Fig. 7 we report Sm-Nd data vs. Sr/Nd ratio. The variations shown by Sr/Nd ratios can correspond either to a mixture between two end-members having different Sr/Nd ratios or to processes such as weathering, sorting or diagenesis in the case of sedimentary rocks. The silicate end-members (A, C and D) have 14’Sm/ ‘44Nd ratios between 0.10 and 0.12, associated with Sr/Nd of about 10 [40]. The Sr/Nd ratios of molasse sediments range from 10 to 100, when this ratio is higher than 50, sedimentary processes, especially carbonate cementation, leading to strong Sr enrichment, provide a good explanation. For these samples there is no evidence for Sm/Nd modifications (Fig. 7). Con- versely, the maximum dispersion shown both by 14’Sm/ ‘44Nd and l Nd values corresponds to Sr/Nd ratios between 10 and 50. This is further evidence that the erosion of Sr-rich rocks contributes to the Nd budget of the molasse sediments. The erosion of chemical sediments, clearly demonstrated by the abundance of such rocks fragments in all molasse sediments, therefore provides a better explanation of the data.

A further argument is provided by Rb-Sr data [13,23]. The 87Sr/86Sr ratios of the molasse sediments result from mixing between silicate minerals

P. Henyv et al./ Earth and Planetay Science Letters 146 (1997) 627-644 639

A Helvetian q Aquitanian = Lower Chattian

A Burdigalian q Upper Chattian 0 Rupelian

0 10 20 30 40 50 60 70 80 90 100 0.15 I

0 10 20 30 40 50 60 70 80 90 100

Fig. 7. (a) lq7Srn/ Id4Nd and (b) eNd vs. Sr/Nd. These diagrams

point out that, for samples with Sr/Nd values lower than 50, the

mixture of silicate (A. C and D) and S-rich (B) end-members is

prevailing over weathering (see text for discussion).

and limestones [ 13,231. Henry et al. [23] showed that

by using a 87Sr/ 86Sr vs. 87Rb/ “Sr diagram, the

intercept 87Sr/ 86Sr value (when Rb = 0) provides a

good estimation of the 87Sr/ 86 Sr ratios of the eroded Mesozoic limestones. This estimation gives a

*‘Sr/ 86Sr ratio of about 0.707 for the limestones eroded during the Lower Chattian molasse sedimentation, in agreement with the erosion of Upper Juras-

sic marine sediments 1411. Therefore, for the Ru- pelian and Lower Chattian, we propose an end value

similar to that of Jurassic Tethys seawater (eNd = - 8.1 [ 151) for the eroded chemical sediments. Con-

versely, for the Upper Chattian and Aquitanian sediments, the estimated 87Sr/86Sr ratio for the eroded limestones (0.708) is in better agreement with Creta- ceous limestones deposited on a continental platform [23]. reflecting a larger contribution of detrital inputs. Therefore, at these times, the end value of - 8.1 characterizing the eroded Mesozoic sediments could be a maximum value. For the two Aquitanian samples of T.f.b., as well as Burdigalian and Helve-

tian molasse sediments, the l Nd value (- 8.1) of the

end-member B is in agreement with the average end value (- 8.3) deduced from the pebbles of sedimentary rocks extracted from the Burdigalian conglomer-

ates. Consequently, we suggest the presence of a sedi-

mentary component B having, on average, a Sr/Nd ratio about of 50 and for which we propose

14’Sm/ 144Nd of 0.145 and end of -8.1, similar to

the Tethys Ocean value [ 151 to account for the dispersion observed in Fig. 7.

Having characterized the different end-members,

we can now quantify the erosion of the Alps by

calculating the relative amounts of each end-member

for the different stratigraphic levels. For our calculations, we used the following mixing equations [2]:

X, = ~(fi ’ [Nd]i . Xi)/~(fi . [Nd],)

where X, is either end value, Sm/Nd or X/Nd ratio; and fi = M,/( 2 M,), is the weight percentage

of each end-member i, M, being its mass. To a first approximation, this equation assumes

that the Sm-Nd compositions in the molasse sediments result from mechanical mixing between the

end-members A, B, C and D. The abundance of rock

fragments and coarse detrital minerals supports this

model. Because the sum of fi is equal to 1, we can

determine the percentages of three end-members A, B and C by using the ‘47Sm/ 144Nd ratios and l Nd

values. In the case of the Burdigalian sediments, with four end-members, we add the Rb/Nd ratios to

solve the system. High Rb contents (133-283 ppm) characterize the end-member D, as shown in Fig. 8, where high l Nd values of Burdigalian sediments are

-3 -I- -4 --

-5 -- -6-- -I-- -8 --

-9 -- -lO--

-ll--

-12 -- -13 l-

0 2 4 6 8 10 12 14 16

Fig. 8. eNd vs. Rb/Nd ratio for Helvetian (T.f.b.), Burdigalian

(T.f.b. and Sv-101) and Aquitanian (T.f.b.) sediments and the

end-members A. B. C and D.

640

Table 3

P. Heq et al./Earth and Planetary Science Letters 146 (1997) 627-644

End-member compositions for mixing calculations

End-member [Ndl

(ppm)

‘Nd [Rbl/[Ndl

A: pre-Variscan formations 25

B: Mesozoic chemical sediments 5”

c: Variscan crust 30

D: alkaline granites 15

- 12.0 0.105

-8.1 0.145

-8.3 0.115

-3.1 0.120

3.5 (Rb = 88 ppm)

2 (Rb = 10 ppm)

3 (Rb = 90 ppm)

14 (Rb = 210 ppm)

a [Nd] = 2 ppm in the case of the Tuggen drillhole to take into account the low Nd concentrations of these sediments (see text for

discussion).

explained by the presence of these alkaline granitic pebbles.

The Nd contents, 147Sm/ 144Nd ratios and eNd values for each mixing end-member are summarized in Table 3. The definition of the 147 Sm/ 144 Nd ratios and eNd values has been discussed earlier in the text. The Nd concentrations are determined as follows: [Nd], = 25 ppm, as measured on Sv-101 at 1861 m; [Nd], = 5 ppm, the average value for limestones [S] (except for the formation of Nd-poor Tuggen sediments, for which [Nd], = 2 ppm will be used because of the great abundance of pure limestone rock fragments); [Nd], = 30 ppm, the average value for the Variscan crust [35]; and [Nd], = 15 ppm, the average calculated for the alkaline granitic pebbles.

Results of the calculation are reported in Table 4, and Fig. 9 shows the relative abundances of each source calculated for each molasse sample. These results reproduce the variations observed in the average l Nd values and allow the molasse series to be divided into 3 cycles: cycle 1 = UMM (Rupelian) -t Lower USM (Lower Chattian); cycle 2 = Upper USM (Upper Chattian and Aquitanian) and cycle 3 = top of eastern USM (Aquitanian of T.f.b.) + OMM (Burdigalian and Helvetian). Furthermore, the following conclusions may be drawn for the quantification of the erosion of the Alps during the Oligocene and Miocene:

(1) On average, the relative proportion of the pre-Variscan formations (A in Fig. 9) is 8 wt% for cycles 1 and 3, and 16 wt% for cycle 2. The higher percentages of A are calculated for the Lower Chat- tian “Calcaires inferieurs” and the Upper Chattian “Calcaires et Dolomies”, which correspond to a decrease in the detrital Alpine inputs in the molasse

basin. Therefore, the pre-Variscan inputs should have, in this case, an origin other than the erosion of the Alps. The erosion of surrounding mountain chains, such as the Massif Central, the Vosges, the Black Forest, and the Bohemian massif, could be sus- pected. Furthermore, the constancy of the A proportion calculated from Rupelian to Lower Chattian and Burdigalian sediments, suggests that the erosion of A is independent of the Alpine tectonic events and, therefore, could have a non-Alpine origin.

In our quantification, the end-member B is defined by the l nd value of the Tethys Ocean ( - 8.11, but the en,, value of Mesozoic sediments eroded during the cycle 2 could be lower, between - 8.1

0 10 20 30 40 M 60 70 80 90 IOOB

2% - top of the eastern USM

-21.5 Ms-

a - Upper USM

-24.5 Ms-

Fig. 9. Relative proportions of end-members A, B, C and D vs.

depth for each drillhole. Table 4 gives the average proportion

calculated for each stratigraphic unit. The major changes, discussed in the text, are used to divide the pe&Alpine molasse

sedimentation into three cycles.


Table 4

Results of mixing calculations between sedimentary sequences and end-members of Table 3 (averages, minima and maxima values)

Sedimentary sequence Number of A average B average C average D average

samples (min-max) (min-max) (min-max) (min-max)

OMM, Helvetian

OMM, Burdigalian

USM, Aquitanian

From drillhole T.f.b.

From drillhole Sv-101

USM, Upper Chattian

Allochthonous

Autochthonous

USM, Lower Chattian

UMM, Rupelian

4 9 (l-18)

11 8 (O-17)

2 7 (5 and 9) 73 (70-77)

4 14 G-18) 78 (65-96)

7 19 (2-28)

7 16 (11-23)

12 9 (2-14)

16 6 (l-14)

71 (65-77)

71 (37-83)

70 (50-82)

74 (62-88)

86 (79-93)

83 (76-93)

9 (l-16) 10 (5-18)

7 (O-497) 14 (3-22)

4 (O-8) 16 (13-21)

8 (O-17)

11 (12-24)

11 (2-17)

5 (O-12)

11(4-19)

and - 12, as discussed above. In setting the l Nd

value of B at - 10, the new mass-balance would be 10 wt% for A and 20 wt% for C, without significant

effect on B, due to its lower Nd contents. Therefore,

depending on the l Nd value chosen for B during the Upper Chattian and Aquitanian, two alternative inter-

pretations can be proposed. On the one hand (end(a) = -8.1), the molasse sediments deposited during

cycle 2 could record a more important erosion of

pre-Variscan formations. On the other hand (~~~(a, = - lo), cycle 2 could record the erosion of both a

continental crust (A + C) being, on average, older

than the Variscan crust, and Mesozoic sediments (B), which would be different from those eroded during

cycles 1 and 3. An average Ed,, value of - 10, which seems more likely on the basis of mixing

relationships discussed above, could correspond to Mesozoic sediments deposited on a continental plat-

form, in agreement with the estimation of *‘Sr/ 86Sr

ratios [23], as discussed above.

(2) The Mesozoic sediments represent the major

source for the molasse sedimentation. The proportion of B is, on average, 85 wt% during cycle 1, then 70 wt% during cycles 2 and 3. During cycle 1 our

model proposes that the eroded rocks are dominated by limestones and cherts, with eNd values similar to that of Tethys seawater (eNd = - 8.1, [15]), and are interpreted as true oceanic sediments deposited on

the oceanic Tethys crust. This agrees with previous source studies for the molasse basin ([38] and after U. Gasser in [24]). Our data show that 60-90 wt% of these chemical sediments can explain both the chemical compositions of the molasse sediments and the

increase of 2 eNd units between the Eocene (- 11)

and the Rupelian (-9) with the beginning of the

erosion of the Alps. As the isotopic composition of Tethys seawater is explained by the introduction of

about 20% of material derived from the depleted mantle [15], we can estimate that about 10 wt% of

juvenile Nd is incorporated in the molasse sedimen-

tation. Therefore, the erosion of chemical sediments,

such as cherts [39,42,43], could contribute to modify

the ‘Nd value of the sedimentary mass. The use of

T nM ages of sediments for calculating the crustal

growth could be slightly modified. Hence, in our case, the erosion of chemical sediments produces an

increase m the average end value but also an increase in T,, ages, because Sm/Nd ratios have

been modified in an older sedimentary cycle. Adding our data to that of Tri.impy and Bersier [38], we

propose a long and complex tectono-sedimentary

cycle as follows: (1) Mesozoic sedimentation on the

Tethys oceanic crust; (2) deposition of flyschs during

the first Alpine tectonic events (Late Cretaceous and

Middle Eocene); and (3) tectonic exhumation of both the Mesozoic sediments and the flyschs. This view is supported by the identification of the Simme nappe as the main source for the pebbles in the Chattian

conglomerates [38] and by the lack of metamorphism in the Prealps, good evidence for their early tectonic exhumation [2 1,241. Finally, a latter stage (4), during

the Early to Middle Oligocene, corresponds to the erosion of this “Oligocene Simme nappe” [38] composed of oceanic sediments and flyschs. Moreover, during this period, our calculations emphasize that crustal igneous rocks represent only = 10 wt% of


the eroded rocks. This low percentage suggests that the eroded regions were protected by an allochthonous sedimentary cover during the Rupelian and Lower Chattian.

(3) For the Upper Chattian and Miocene, our results and calculations show two major variations. On the one hand, we suggest that the average end value of the eroded Mesozoic sediments changes, as discussed above. On the other hand, the erosion of igneous rocks has become more important since the Upper Chattian. These crustal materials (A + Cl could represent about 30 wt% of the eroded products from the Upper Chattian to Burdigalian (including “Helvetian” facies). This change in the nature of the rocks eroded is recorded by the molasse sedimentation just after the deposition of the “Calcaires et Dolomies” unit, which is interpreted as evidence of a dramatic decrease in the detrital Alpine inputs in the molasse basin. Furthermore, Rb-Sr data [13,23] on the same samples point to a variation in the source at the boundary between the Chattian and the Aquitanian sequences, with the introduction of Alpine and metamorphic minerals in the Aquitanian Mo- lasse. The Upper Chattian seems to be a transition period associated with important tectonic events, marked by the deposition of the “Calcaires et Dolomies” at 24.5 Ma in the molasse basin [29,30]. Events such as a rapid variation in the uplift in the Alps [44] have been recently proposed, associated with syn-sedimentary faulting in the molasse basin of Savoy [28].

Furthermore, the l Nd values of the Burdigalian and Helvetian sediments show that about of 50 wt% of the eroded Variscan continental crust is composed of alkaline granites with higher l Nd values than the average European Variscan continental crust [35]. The sedimentary currents [21,36] and the heavy mineral contents [36] suggest that the source of these granites could be found in the Alpine Aar massif, or in the Variscan Bohemian massif, or in both.

Finally, a comparison can be made between the isotopic compositions recorded for similar stratigraphic levels representing various lithologies deposited at different locations. At the end of the Aquitanian Freshwater Molasse (USM), the sediments from Savoy and from Tiefenbrunnen have different eNd values (- 9.7 and - 8.2, respectively). This implies that the two localities received different

detrital material from different rivers. In contrast, during the deposition of the Burdigalian marine Mo- lasse (OMM), the end values of the molasse sediments were similar in Savoy (Sv-lOl), in Western Switzerland (Pra) and in Central Switzerland (T.f.b.). This suggests the same source for all, which dis- agrees with the proposition of two cells of sedimentation for the Burdigalian sea defined by heavy minerals [36]. The uniform l Nd value of the Burdi- galian molasse suggests sedimentary currents strong enough to carry the detrital minerals from eastern Switzerland as far as Savoy. The explanation of this disagreement may be that the Nd isotopic record is carried largely by light minerals homogeneously dis- tributed at the scale of the whole basin, while heavy minerals may be deposited along the sedimentary path. Therefore, for distant molasse sediments, the heavy minerals do not seem to be an efficient tool for tracing the nature of the eroded rocks.

7. Conclusions

The Sm-Nd data from molasse sedimentation shows that the erosion of the Alps increased the l nd values of the continental sediments by about 2 eNd units, through the erosion of oceanic sediments con- taining a mantle-like component. A mass balance calculation suggests that the addition of about 10 wt% of this juvenile component can explain the l Nd values of the Oligocene molasse sediments. The recycling of this juvenile component may have occurred by a long and complex cycle which included: Jurassic and Cretaceous sedimentation on the Tethys oceanic crust; tectonic uplift leading to the deposition of Upper Cretaceous flyschs; and tectonic thrust- ing of the large sedimentary nappes, followed by their erosion.

The formation of the peri-Alpine molasse can be divided in three cycles. During the first one, during the Rupelian and Lower Chattian, the beginning of the erosion of the flyschs is marked by the increase ofthe en,, units in the sediments, but the addition of a mantle-like component is not observed in the T nM ages, due to the high Sm/Nd ratios of the eroded chemical sedimentary rocks. After 24.5 Ma, the Upper Chattian and the Aquitanian sediments record the beginning of the erosion of the Variscan crust. During this period, the l nd values and the

P. Henry et al./Earth and Planetary Science Letters 146 (1997) 627-644 643

chemical compositions of the sediments allow the

definition of a second cycle in the molasse sedimentation. The age of 24.5 Ma, which corresponds to the deposition of the “Calcaires et Dolomies”, is inter-

preted as a time of a major change in the molasse sedimentation caused by major tectonic changes in

the Alps. Following the Burdigalian transgression, the eNd values of the molasse sandstones increase to

about -8. This second increase in the eNd values

recorded by the sedimentary mass, which define a

third cycle in the molasse sedimentation, is due to

the erosion of alkaline granites having high l Nd

values and young To, g a es, which could result from

the recycling of the Variscan Nd isotopic peak [3].

Here, the decrease in the To, ages is due to the incorporation of a mantle-like component in the continental crust during a previous and older orogeny,

and its evolution after incorporation with a low rh7Sm/ ‘44Nd ratio (about 0.12).

Acknowledgements

The authors thank F. Albarede for his help in initiating this work. The progress in this study was

greatly aided by help in the laboratory from D. Dole,

and discussions with L. Deny, C. France-Lanord, A. Galy and L. Reisberg. The manuscript benefitted

from critical reviews by D. Ben Othman, J.P. Berger, S.M. McLennan and an anonymous reviewer, and

from English improvement by L. Reisberg and R.K. Stevenson. [CL1

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the erosion of the alps: nd isotopic and geochemical constraints on the sources of the peri-alpine...

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