The Early Stages of the Alpine Collision: an Image Derived from the Upper Eocene–Lower Oligocene Record in the Alps–Apennines Junction Area

Sedimentary Geology 171 (2004) 181–203 www.elsevier.com/locate/sedgeo

The early stages of the Alpine collision: an image derived from the upper Eocene–lower Oligocene record in the Alps–Apennines junction area

B. Carrapaa,*, A. Di Giuliob, J. Wijbransa

aDepartment of Isotope Geochemistry, Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, Amsterdam 1081 HV, The Netherlands bDipartimento di Scienze della Terra, Universita` di Pavia, Via Ferrata 1, Pavia 27100, Italy Received 15 October 2003; received in revised form 1 March 2004; accepted 12 May 2004

Abstract

The upper Eocene–lower Oligocene sediments deposited in the eastern part of the Tertiary Piedmont Basin in northern Italy provide a complete record of the unroofing of the Alpine orogenic prism during the early stages of exhumation in the Ligurian sector. From late Priabonian till late Rupelian time, the sediments in the study area were derived from two different sources, one characterised by white micas with Sib6.5 pfu and Permian 40Ar/39Ar ages (270 Ma), and the other characterised by white micas with SiN7 pfu and Eocene–Oligocene 40Ar/39Ar ages (32–50 Ma). The first source is considered to be indicative of low-pressure metamorphic rocks that covered the HP rocks of the Ligurian Alps, and were completely eroded by Chattian time. From this time on, the study area started to record the first input from western Alpine sources characterised by a larger span of ages with a more frequent Eoalpine signal. Thus, sediments deposited in the eastern part of the Tertiary Piedmont Basin contain the only available evidence of rocks belonging to high crustal levels in the Alpine orogenic prism that were not affected by the Alpine overprint. These data also provide time constraints to the poorly dated first conglomerates deposited in this area. 40Ar/39Ar geochronology reveals a minimum age of 33F1.4 Ma for the Pianfolco Conglomerates in the type locality, and of 31.4F3.5 Ma for the Borbera Conglomerates. D 2004 Elsevier B.V. All rights reserved.

Keywords: Provenance; Ligurian Alps; 40Ar/39Ar geochronology; Cooling/exhumation; Paleogeography

1. Introduction

q * Corresponding author. Present address: Institut f r Geo- Examining the provenance of clastic sediments wissenschaften, Universit7t Potsdam, Karl-Liebknecht-Str. 24/H25, 14476 Golm, Potsdam 14415, Germany. Tel.: +49 331 977 5078; derived from orogenic belts is a classical tool for fax: +49 331 977 5060. unravelling the evolution of collisional systems E-mail address: [email protected] (B. Carrapa). (Dickinson, 1974; Dickinson, 1985). Substantial

0037-0738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2004.05.015 182 B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203 advancements have been made in this field through The area has been extensively studied, mostly with the the application of mineral chemistry and geochronol- aim of unravelling the tectonic evolution of this ogy to clastic minerals, as these can provide informa- geologically complex region (Cavanna et al., 1989; Di tion on the cooling and exhumation paths of the Giulio, 1996; Di Giulio and Galbiati, 1995; Mutti et eroded rock units (Heller and Frost, 1988; Copeland al., 1995; Vanossi et al., 1994). and Harrison, 1990; Renne et al., 1990; Harrison et Recently, the provenance of clastic sediments in al., 1993; Najman et al., 1997; von Eynatten and the eastern TPB has been systematically investigated, Gaupp, 1999; Najman et al., 2001; Sherlock, 2001; in order to improve models of paleogeographic White et al., 2002; von Eynatten and Wijbrans, 2003). evolution of the orogenic system following collision Recently, this approach has been applied successfully (Cibin et al., 2001, 2003; Di Giulio and Galbiati, to clastic sediments deposited in the southern part of 1995; Gnaccolini, 1974; Gnaccolini and Rossi, 1994; the Piedmont Tertiary Basin (TPB) in northwestern Martelli et al., 1998). Sandstone petrography in the Italy (Fig. 1; Barbieri et al., 2003; Carrapa et al., 2003, study area suggests a possible low-pressure source for 2004). these sediments with south Alpine affinity, which The current study focuses on the eastern margin of were not affected by late Alpine metamorphism. the TPB, which is located on the tectonic junction Presumably, these sediments were once located on between the Ligurian Alps and the northern Apen- top of the Ligurian Alps and are presently completely nines (Fig. 1). Here, the clastic succession uncon- missing (Di Giulio, 1991). However, the lack of formably covers the Ligurian Alps to the south and thermochronological data has so far limited the contains the oldest sediments deposited in the TPB. validity of this proposition. If correct, this would

Fig. 1. Geological map of the Alps (modified from Polino et al., 1990). A: Adula nappe; Ad: Adamello; AU: eastern Austroalpine cover and basement nappes; B: Bergell; DI: Dinarides; EW/TW/RW: Engadina, Tauern, and Rechniz windows; HE: Ultrahelvetic, Helvetic, and Dauphinois units; LA: Ligurian Alps; LPN: lower Penninic nappes; MR/GP/DM/S: upper Penninic Monte Rosa, Gran Paradiso, Dora Maira and Suretta nappes; NCA: northern calcareous Alps; PF: Penninc front; SA: southern Alps; SB: Gran St. Bernard nappe; SC: Subalpine chains; SL/ DB: western Austroalpine Sesia Lanzo and Dent Blanche nappes; VG: Voltri Group. TPB: Tertiary Piedmont Basin; inset square: study area reported in Fig. 2. B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203 183 mean that the sediments deposited in the eastern part the eastern part of the TPB. Such a maximum of the TPB record the unique signal of rocks once estimate of the depositional age can be obtained exposed at the top of the Alpine edifice, which would (e.g. Najman et al., 2001) under the assumption have important paleogeographic implications. Also, that the depositional age of sediments cannot be the sediments preserved in the eastern part of the TPB greater than the 40Ar/39Ar ages of the detrital are the oldest sediments preserved in the study area, micas. This will be the case when no alteration meaning that their investigation would provide and/or resetting of the micas occurred after information on the paleogeography of the belt during deposition. the earliest, late Eocene steps of belt evolution after (2) To attempt a paleogeographic reconstruction of collision. Ultimately, continental conglomerates of the study area during the late Eocene–early uncertain ages deposited in the eastern TPB are here Oligocene. Provenance discrimination of the analysed with the aim of assessing a maximum investigated sediments is made in order to depositional age for these sediments (Najman et al., confirm the presence of sources with south 1997, 2001). Alpine affinity as previously proposed from Detrital mineral chemistry and 40Ar/39Ar thermo- sandstone petrography (Di Giulio, 1991). This chronology has been performed on continental to aim is pursued by looking at the white mica transitional and shallow marine sediments of the geochemical signal together with the 40Ar/39Ar Molare Formation which form the base of the south- detrital populations recorded by the studied ern part of the TPB (Barbieri et al., 2003). Detrital sediments. Differences in major element geo- 40Ar/39Ar ages in these sediments suggest two local chemistry and in 40Ar/39Ar age families reflect sources located in the Ligurian Alps. The first is the contribution in composition and ages present characterised mainly by high-pressure (HP) rocks and in the original source area surface at the time of Eocene–Oligocene 40Ar/39Ar ages (32–45 Ma) sediment deposition. recording the exhumation of deep crustal levels of the original orogenic prism. The second is charac- These objectives will be met through the integrated terised by low-pressure (LP) rocks and Carboniferous study of mineral chemistry and 40Ar/39Ar thermo- ages (Barbieri et al., 2003). In particular, the youngest chronology of clastic white micas, sampled in the 40Ar/39Ar detrital signal suggests a fast episodic lowermost part of the succession in the eastern part of cooling event occurring sometime in the Oligocene the TPB, where late Eocene sediments occur at the Ligurian belt (Barbieri et al., 2003; Carrapa et al., very base. 2003). However, due to a lack of paleontological markers in the mainly continental sediments of the Molare 2. Stratigraphic framework and sample strategy Formation, this formation has only a loosely defined early Oligocene age (Gnaccolini, 1974; Barbieri et al., The TPB is an episutural basin located in a complex 2003 and referenced therein) which consequently tectonic area that represents the boundary between the prevents a detailed provenance discrimination. On Alpine and the Apennine thrust belts (Fig. 2). The the other hand, the mainly marine sediments preserved stratigraphy of the area is complex, compounded by in the easternmost part of the TPB (e.g. Ranzano inconsistency in the published literature (Fig. 3). In Formation and Rigoroso Marls) are biostratigraphi- this study, we will use the stratigraphic scheme of Di cally well dated (Di Giulio et al., 2002; Mancin and Giulio (1991) integrated with other studies reported in Cobianchi, 2000; Mancin and Pirini, 2001; Martelli et Fig. 3. Biostratigraphic ages of the formations consid- al., 1998), allowing a more robust constraint on the ered in this study are given using works reported in time of cooling of the source area. Table 1 and the geological timescale of Haq and Van The aims of this study are: Eysinga (1998). In the western sector of the eastern TPB, (1) To better constrain the time of sedimentation of sedimentation was perceived to have started in the the poorly dated conglomerates outcropping in upper Eocene–early Oligocene, with continental to 184 B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203

Fig. 2. Sample locations with specification of the stratigraphic columns studied by Di Giulio (1991) reported in Fig. 3. transitional sediments (Costa Cravara Breccias and Contemporaneously, in the eastern sector, sed- Pianfolco Conglomerates; Charrier et al., 1964; imentation started with the Ranzano Formation, the Gnaccolini, 1978), unconformably overlying the lower part of which mainly comprises deep marine Ligurian Alps (Voltri Group in the study area; Fig. turbidite sandstones (Di Giulio and Galbiati, 1995; 2). The Pianfolco Conglomerates are mainly com- Martelli et al., 1998; Di Giulio et al., 2002). The posed of alternating continental conglomerates and Ranzano Formation as a whole has with a very sandstones supplied by local sources. These sedi- precise biostratigraphically determined age (Mancin ments have been tentatively dated as late Eocene– and Pirini, 2001; Martelli et al., 1998). Sandstone early Rupelian on the basis of a tropical flora petrography suggests a source mainly consistent with association and indirectly by means of the uncon- a Permo-Carboniferous cover, possibly related to formably overlying early Oligocene–Chattian Molare rocks once located at the top of the Penninic Formation to the west (Charrier et al., 1964; orogenic prism (Di Giulio, 1991). The Ranzano Gnaccolini, 1974, 1978; Fravega et al., 1994; Mutti Formation therefore records the first supply of the et al., 1995; Fig. 3). More recently, Mutti et al. unroofing products of the top part of the Alpine (1995) tentatively attributed a Rupelian age to the orogenic prism into the TPB. Samples from the Pianfolco Conglomerate according to their genetic Ranzano Formation have been collected in the same depositional relation to the Molare–Borbera unit. locality studied by Di Giulio (1991) (Figs. 1 and 3). Samples from the Pianfolco Conglomerates have The lowermost member of the Ranzano Formation been collected from the type locality (i.e. Pianfolco; (Pizzo d’Oca Member; Martelli et al., 1998)is Charrier et al., 1964) and the age of the sediments at referred to in the following as UNIT S1. this location is supposed to be late Eocene–early Sedimentation continued in the Rupelian with the Rupelian (Charrier et al., 1964). Nevertheless, at Borbera, the Savignone Conglomerates the upper part present, their age remains poorly defined. For this of the Ranzano Formation and the overlying Rigoroso reason, the Pianfolco Conglomerates will be treated Marls. Of these, the first two comprise fan delta separately in the following. deposits (Di Biase et al., 1997; Di Biase and Pandolfi, .Craae l eietr elg 7 20)181–203 (2004) 171 Geology Sedimentary / al. et Carrapa B.

Fig. 3. Correlation scheme for different stratigraphic units reported in literature for the study area. 185 186 B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203

Table 1 Synoptic depiction of samples analysed in this study Sample Total fusion Step heating Location Formation/lithology Depositional age Extra info code code code B2 0055+0073 Nivione Channel Rigoroso Marls Chattian (S4) Rigoroso V; Di Giulio and Galbiati (1995) B4 0074+0056 Fontana di Nivione Rigoroso Marls Chattian (S4) Rigoroso n; Di Giulio (Nivione Sst.) and Galbiati (1995) B6 0057 NW Dernice Ranzano Sst. (a) late Rupelian (S3) Ranzano a; Di Giulio and Galbiati (1995) B9 0058 NW Dernice Ranzano Sst. (s) late Rupelian (S3) Ranzano s; Di Giulio and Galbiati (1995) B11 0059 Pessola Ranzano Sst. (a) late Rupelian (S3) Ranzano a; Di Giulio and Galbiati (1995) B28 0069+0068 Fontana di Nivione Ranzano late Rupelian NP23 Ranzano C; Di Giulio (midium) (S3) and Galbiati (1995) B33 0071+0088 Dernice Val Borbera Cgl. early Rupelian (S2) Di Giulio and Galbiati (sandstone) (1995) B34 0053 C.na Lemmi Val Borbera Cgl. early Rupelian (S2) Di Biase et al. (1997) (cobble) B35 0054 C.na Lemmi Val Borbera Cgl. early Rupelian (S2) Di Biase et al. (1997) (cobble) B23 0050 Carrosio Savignone Cgl. early Rupelian (S2) Ghibaudo et al. (1985) (cobble) B24(B26) 0051 0079+0082+ Carrosio Savignone Cgl. early Rupelian (S2) Ghibaudo et al. (1985) 0091+0092 (cobble) B27 0052 Carrosio Savignone Cgl. early Rupelian (S2) Ghibaudo et al. (1985) (cobble) B20 0047 0083+0084+ Bosio-Voltaggio Pianfolco Cgl. late Eocene–early lithozone A; Gnaccolini 0093+0094 (cobble) Rupelian? (S1) (1978) B21 0048 0077+0089+ Bosio-Voltaggio Pianfolco Cgl. late Eocene–early lithozone A; Gnaccolini 0090 (cobble) Rupelian? (S1) (1978) B22 0049 Bosio-Voltaggio Pianfolco Cgl. late Eocene–early lithozone A; Gnaccolini (cobble) Rupelian? (S1) (1978) B15 0076+0061 C.na Pianfolco Pianfolco Cgl. late Eocene–early Charrier et al. (1964) (conglomeratic sst.) Rupelian? (S1) B17 0075+0062 C.na Pianfolco Pianfolco Cgl. late Eocene–early Charrier et al. (1964) (midium) Rupelian? (S1) B30 0070 Incisa Ranzano Fm. late Priabonian (S1) UNIT S1; Di Giulio (cross to Solarolo) (midium) and Galbiati (1995) B12 0060 P.zo d’Oca Ranzano Fm. (sst.) late Priabonian (S1) Ranzano a; Di Giulio (sez. Fontanelle) and Galbiati (1995) Sst.=sandstones; Cgl.=conglomerates; Fm.=formation.

1999). Locally, the Borbera and Savignone Conglom- The upper part of the Ranzano Formation (Di erates directly cover the Voltri Group of the Ligurian Giulio and Galbiati, 1995) together with the Rigoroso Alps (Di Biase et al., 1997) while towards the east Marls will be described in the following as UNIT S3 they partly interfinger with the Ranzano Formation, (middle–late Rupelian). The first formation (which passing laterally to its intermediate turbiditic member includes the S. Sebastiano and Curone members) is (Val Pessola Member). Samples from the Savignone characterised by siliciclastic turbidites while the Conglomerates come from the same sector studied by second is characterised by hemipelagic sediments Di Biase et al. (1997). These conglomerates will be (Di Giulio, 1991; Di Giulio and Galbiati, 1995; referred in the following as UNIT S2 (early Rupelian). Martelli et al., 1998). In Chattian time, a lenticular B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203 187 sandstone unit (Nivione sandstone; Cavanna et al., 40Ar/39Ar step heating experiments were performed 1989) was deposited within the upper part of the on selected single grains (250–1000 Am) from Rigoroso Marl. These sediments will be referred in the metamorphic cobbles, when the total fusion popula- following as UNIT S4 (Chattian). tion was unclear, to check on Ar homogeneities. Only experiments concordant within 95% confidence inter- vals, i.e. MSWDb2.5, have been used to derive 3. Techniques plateau ages. The 40Ar/39Ar experiments were carried out with Single grains of white mica were separated from 12 the VULKAAN laserprobe facility at the Isotope samples of the Eocene–Oligocene units of the eastern Geology Laboratory of the Vrije Universiteit in TPB clastic sequence (Fig. 2; Table 1) and analysed Amsterdam following laser extraction and mass by electron microprobe and 40Ar/39Ar analyses (single spectrometry methods for this facility described by fusion and step heating). Samples have been grouped Wijbrans et al. (1995). The irradiation facility used for in formations belonging to different sequences fol- this project was the cadmium-lined RODEO facility lowing the scheme of Fig. 3. of the HFR reactor of the ECN/JRC reactor facility in Petten, the Netherlands. Irradiation time was 7 h. 3.1. Mineral chemistry Correction factors for interferences of Ca and K isotopes were 0.000699 for 39Ar/37Ar, 0.000270 for Metamorphic pressure conditions and mineral para- 36Ar/37Ar and 0.00183 for 40Ar/39Ar, respectively. genesis influence the degree of substitution of Si+(Mg, These values were determined using zero age K- Fe) in phengite (Massonne and Schreyer, 1987; Velde, feldspar and anorthite glass. After irradiation, a J 1965, 1967). When the source rocks have experienced curve was derived for individual samples by inter- different metamorphic histories, phengites can be used polation between five single fusion experiments on to examine the provenance of clastic sediments. Ten every flux monitor. DRA sanidine (Steenbrink et al., grains (250–500 Am) from each sample were analysed 1999) was used as the flux monitor standard for this with electron microprobe analyses for a total of over project, with an age of 25.26F0.14 Ma. These values 200 analyses. Samples were disaggregated by mixing are compatible with the set of Renne et al. (1998), with 10% HNO3 and 10% Na-pyrophosphate and based on biotite GA1550 (at K/Ar age of 98.79F0.69 suspension in an ultrasound bath. After sieving, flat Ma). In the present study, system blanks were white micas were separated from the 0.25–0.5 and 0.5– determined after every five unknowns. The unknowns 1.0 mm fractions by using a Faul- (vibration) table and were corrected for the interpolated blank at the time of final handpicking. analysis of the unknown and the 2r error on the blank Chemical analyses of separated mineral phases were was further used for the error calculation of the performed on a JEOL JX-A8800M electron microp- unknown. 40Ar intensities for the analysed samples robe. Raw data corrections were done with JEOL on- were in the order of N100 times the blanks (see line ZAF-correction program (refer to Reed, 1993 for Wijbrans et al., 1995 for further details on mass more details) and atomic ratios have been calculated spectrometer sensitivity). The discrimination factor for 20 oxygens and 4 OH, F and Cl per formula unit. was on average equal to 1.059F0.04% (see Kuiper, The standards used are Na-jadeite, Mg, Si, Ca-diop- 2003 for further details on discrimination factor side, Al-syntheic Al2O3, K-orthoclase, Ti-ilmenite, Fe- calculation). Note that the 2r errors reported in Table olivine and Ba-barium-aluminate glass (Fig. 4). 2 do not include the uncertainties in J and uncertainties related to the age of the standards (the average of 3.2. 40Ar/ 39Ar geochronology J related errors is in the order of 0.3%). The exclusion of the J related errors in the analytical errors reported 40Ar/39Ar single fusion laser analyses on single in Table 2 enables a better comparison between grain white micas (250–500 Am) were performed on samples (Foland, 1983). For further details on the 10–20 grains from each sandstone sample and up to 5 calculation of the ages and related errors reported in grains from each cobble (UNITS 1 and 2; Table 2). Table 2, we refer to Koppers (2002). 188 B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203

Fig. 4. Microprobe results from phengites derived from the studied samples divided in sequences following the scheme of Di Giulio (1991) reported in Fig. 3. Note that microprobe analyses presented have a precision in the order of 1%. Current and count rate were set to optimum level in order to get the highest statistical resolution. B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203 189

Table 2 Total fusion 40Ar/39Ar data from clastic phengites of the study samples Sample 36Ar(a) 37Ar(ca) 38Ar(cl) 39Ar(k) 40Ar(r) Age 2r (Ma) 40Ar (%) 39Ar(k) (%) S1 (Ranzano Fm.; sst.) East (B12); 03M0060A fsn 0.00001 0.00000 0.00005 0.03714 3.03400 278.20F8.03 99.93 11.54 J=0.002041 03M0060B fsn 0.00007 0.00000 0.00000 0.01908 1.22461 222.06F14.60 98.42 5.93 03M0060C fsn 0.00003 0.00000 0.00003 0.03325 2.49720 257.27F8.68 99.65 10.33 03M0060D fsn 0.00001 0.00000 0.00001 0.02911 2.42997 283.85F10.31 99.85 9.05 03M0060E fsn 0.00011 0.00000 0.00000 0.02843 2.77676 327.89F12.08 98.81 8.84 03M0060G fsn 0.00001 0.00000 0.00013 0.05352 5.35631 335.35F6.32 99.95 16.63 03M0060H fsn 0.00002 0.00000 0.00000 0.02005 1.36525 234.81F13.75 99.57 6.23 03M0060I fsn 0.00003 0.00000 0.00000 0.02394 1.95348 277.94F12.40 99.54 7.44 03M0060J fsn 0.00003 0.00000 0.00000 0.02417 1.78793 253.70F11.83 99.51 7.51 03M0060K fsn 0.00004 0.00000 0.00016 0.05308 4.36623 280.00F5.71 99.74 16.50 East (B30); 03M0070A fsn 0.00003 0.00000 0.00000 0.05180 4.45394 282.81F5.54 99.79 15.31 J=0.001974 03M0070B fsn 0.00003 0.00000 0.00000 0.02828 2.44666 284.45F9.76 99.63 8.36 03M0070C fsn 0.00001 0.00000 0.00000 0.02113 1.82437 283.98F13.04 99.82 6.24 03M0070D fsn 0.00006 0.00000 0.00000 0.04651 3.71180 263.95F6.03 99.56 13.75 03M0070E fsn 0.00002 0.00000 0.00000 0.03810 3.26997 282.38F7.36 99.86 11.26 03M0070G fsn 0.00001 0.00000 0.00000 0.03503 2.80857 265.10F7.92 99.91 10.35 03M0070H fsn 0.00001 0.00000 0.00000 0.03353 2.99593 293.04F8.26 99.90 9.91 03M0070I fsn 0.00003 0.00000 0.00000 0.02715 2.35076 284.69F10.12 99.59 8.02 03M0070J fsn 0.00002 0.00000 0.00000 0.02362 2.02450 282.00F11.69 99.73 6.98 03M0070K fsn 0.00001 0.00000 0.00000 0.03318 2.98124 294.57F8.37 99.88 9.81

S1 (Pianfolco Cgl.; sst.) West (B17); 03M0075A fsn 0.00178 0.00000 0.00175 0.19678 2.55970 47.04F1.59 82.92 12.08 J=0.002031 03M0075B fsn 0.00276 0.00000 0.00039 0.05417 0.84780 56.45F6.67 50.94 3.33 03M0075D fsn 0.00071 0.00134 0.00287 0.34320 4.02750 42.49F0.96 95.03 21.07 03M0075G fsn 0.00097 0.00000 0.00263 0.30159 4.13745 49.58F0.95 93.50 18.52 03M0075J fsn 0.00166 0.00000 0.00306 0.35631 5.75315 58.21F0.79 92.12 21.88 03M0075K fsn 0.00089 0.00000 0.00327 0.37678 4.39114 42.21F0.90 94.31 23.13 03M0062A fsn 0.00100 0.00046 0.00237 0.27228 3.41735 45.41F1.34 92.00 27.88 03M0062B fsn 0.00041 0.00134 0.00167 0.17393 2.33581 48.55F2.04 95.07 17.81 03M0062D fsn 0.00025 0.00000 0.00056 0.06203 0.80085 46.69F5.40 91.60 6.35 03M0062E fsn 0.00038 0.00000 0.00167 0.20702 2.70084 47.18F1.47 95.98 21.20 03M0062G fsn 0.00099 0.00240 0.00273 0.26128 3.44049 47.61F1.44 92.18 26.76 West (B15); 03M0061A fsn 0.00167 0.00412 0.00154 0.15618 2.04131 47.33F1.53 80.51 19.26 J=0.002034 03M0061D fsn 0.00049 0.00124 0.00092 0.10736 1.56148 52.59F3.04 91.45 13.24 03M0061E fsn 0.00279 0.00046 0.00119 0.14204 1.91656 48.85F2.30 69.92 17.52 03M0061G fsn 0.00063 0.00034 0.00110 0.14023 1.80585 46.65F2.20 90.59 17.29 03M0061K fsn 0.00064 0.00148 0.00211 0.26507 3.07169 42.03F1.40 94.20 32.69 03M0076A fsn 0.00478 0.00173 0.00193 0.23308 3.28025 50.92F2.03 69.88 20.29 03M0076B fsn 0.00069 0.00061 0.00150 0.19852 2.09577 38.33F2.31 91.16 17.28 03M0076C fsn 0.00021 0.00000 0.00000 0.04887 0.67945 50.31F9.04 91.51 4.25 03M0076D fsn 0.00072 0.00038 0.00079 0.12582 1.67119 48.09F3.77 88.75 10.95 03M0076E fsn 0.00063 0.00135 0.00113 0.19541 2.42456 44.96F2.25 92.86 17.01 03M0076G fsn 0.00097 0.00106 0.00082 0.12796 1.50457 42.64F3.32 83.95 11.14 03M0076J fsn 0.00294 0.00327 0.00207 0.21674 3.03364 50.64F1.91 77.75 18.87

S1 (Pianfolco Cgl.; cbl.) Center (B22); 03M0049A fsn 0.00064 0.00193 0.00081 0.09168 0.92555 36.31F3.18 83.09 9.48 J=0.002014 03M0049B fsn 0.00059 0.00220 0.00140 0.16569 1.67484 36.36F1.60 90.59 17.14 03M0049C fsn 0.00037 0.00372 0.00248 0.25146 2.48844 35.60F1.37 95.72 26.01 03M0049D fsn 0.00092 0.00377 0.00225 0.24931 2.37759 34.32F1.03 89.74 25.78 03M0049E fsn 0.00079 0.00314 0.00194 0.20877 1.91100 32.96F1.43 89.04 21.59 (continued on next page) 190 B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203

Table 2 (continued) Sample 36Ar(a) 37Ar(ca) 38Ar(cl) 39Ar(k) 40Ar(r) Age 2r (Ma) 40Ar (%) 39Ar(k) (%) S1 (Pianfolco Cgl.; cbl.) Center (B21); 03M0048A fsn 0.00253 0.00341 0.00318 0.35714 9.40811 93.44F0.92 92.64 35.59 J=0.002018 03M0048B fsn 0.00257 0.00188 0.00200 0.23422 8.97329 134.35F1.20 92.18 23.34 03M0048C fsn 0.00048 0.00000 0.00027 0.05336 2.08339 136.83F3.88 93.61 5.32 03M0048D fsn 0.00322 0.00236 0.00196 0.22145 7.50164 119.29F1.39 88.73 22.07 03M0048E fsn 0.00145 0.00000 0.00133 0.13730 5.89451 149.89F2.51 93.22 13.68 Center (B20); 03M0047A fsn 0.00064 0.00000 0.00124 0.11900 1.36381 41.31F1.57 87.80 18.21 J=0.002021 03M0047B fsn 0.00022 0.00001 0.00140 0.13382 1.32208 35.67F1.60 95.21 20.48 03M0047C fsn 0.00135 0.00000 0.00137 0.18701 2.38221 45.86F1.21 85.63 28.62 03M0047D fsn 0.00045 0.00000 0.00075 0.08985 0.94290 37.86F3.25 87.60 13.75 03M0047E fsn 0.00043 0.00185 0.00084 0.12378 1.15260 33.63F1.51 90.10 18.94

S2 (Savignone Cgl.; cbl.) Center (B27); 03M0052A fsn 0.00045 0.00045 0.00163 0.15029 1.49744 35.46F1.72 91.86 23.46 J=0.001992 03M0052B fsn 0.00021 0.00032 0.00057 0.02927 0.32038 38.92F9.53 83.81 4.57 03M0052C fsn 0.00059 0.00019 0.00127 0.13930 1.52935 39.03F1.89 89.72 21.75 03M0052D fsn 0.00088 0.00014 0.00181 0.15671 1.81247 41.09F1.52 87.41 24.46 03M0052E fsn 0.00047 0.00200 0.00183 0.16502 1.94240 41.81F1.37 93.33 25.76 Center (B24); 03M0051A fsn 0.00126 0.00182 0.00114 0.15373 2.64021 61.08F2.50 87.66 21.38 J=0.002005 03M0051B fsn 0.00071 0.00122 0.00101 0.09923 2.39473 85.25F3.89 91.93 13.80 equivalent 03M0051C fsn 0.00079 0.00123 0.00177 0.21403 3.17958 52.95F1.75 93.17 29.76 to B26 03M0051D fsn 0.00069 0.00000 0.00141 0.15933 3.06206 68.21F2.39 93.77 22.16 03M0051E fsn 0.00057 0.00000 0.00098 0.09276 2.58411 98.05F3.84 93.87 12.90 Center (B23); 03M0050A fsn 0.00095 0.00252 0.00318 0.36746 4.90590 47.77F0.87 94.57 23.14 J=0.002010 03M0050B fsn 0.00085 0.00168 0.00257 0.29450 3.74685 45.55F1.00 93.70 18.54 03M0050C fsn 0.00072 0.00101 0.00361 0.36829 4.84758 47.11F0.81 95.78 23.19 03M0050D fsn 0.00111 0.00300 0.00255 0.28399 3.63875 45.87F1.13 91.71 17.88 03M0050E fsn 0.00078 0.00130 0.00298 0.27404 3.57013 46.63F1.21 93.95 17.25

S2 (Borbera Cgl.; cbl.) Center–east (B35); 03M0054A fsn 0.00161 0.00160 0.00059 0.04939 0.45798 32.25F5.06 48.99 12.70 J=0.001945 03M0054B fsn 0.00138 0.00163 0.00102 0.10281 1.00034 33.82F2.34 70.97 26.43 03M0054C fsn 0.00124 0.00008 0.00098 0.09234 0.88954 33.49F3.06 70.75 23.74 03M0054D fsn 0.00197 0.00097 0.00113 0.07778 0.70277 31.43F3.53 54.74 20.00 03M0054E fsn 0.00247 0.00140 0.00045 0.06662 0.66088 34.48F4.59 47.49 17.13 Center–east (B34); 03M0053A fsn 0.00308 0.00023 0.00072 0.06698 0.86390 44.88F3.61 48.71 21.96 J=0.001953 03M0053B fsn 0.00115 0.00034 0.00058 0.06153 0.83094 46.96F3.44 70.99 20.17 03M0053C fsn 0.00117 0.00024 0.00027 0.03984 0.56358 49.16F6.19 61.90 13.06 03M0053D fsn 0.01367 0.00196 0.00082 0.06544 0.69786 37.19F3.99 14.73 21.46 03M0053E fsn 0.00167 0.00093 0.00087 0.07122 0.88177 43.10F3.27 64.14 23.35

S2 (Borbera Cgl.; sst.) East (B33); 03M0071A fsn 0.00083 0.00002 0.00081 0.09124 6.29878 228.62F3.44 96.23 15.68 J=0.001957 03M0071B fsn 0.00006 0.00000 0.00009 0.03808 3.67479 312.09F7.12 99.51 6.54 03M0071C fsn 0.00004 0.00000 0.00000 0.02042 1.76263 281.56F13.13 99.30 3.51 03M0071D fsn 0.00005 0.00000 0.00002 0.03169 2.72097 280.25F8.53 99.43 5.44 03M0071E fsn 0.00006 0.00000 0.00000 0.02484 2.06326 271.77F10.86 99.09 4.27 03M0071G fsn 0.00003 0.00000 0.00000 0.01734 1.51895 285.39F15.48 99.39 2.98 03M0071H fsn 0.00083 0.00174 0.00149 0.15101 9.97363 219.29F2.61 97.59 25.95 03M0071I fsn 0.00006 0.00000 0.00013 0.04535 3.88482 279.63F6.18 99.57 7.79 03M0071J fsn 0.00003 0.00000 0.00000 0.02041 1.97022 312.16F13.21 99.58 3.51 03M0071K fsn 0.00057 0.00221 0.00176 0.14162 10.05960 234.83F2.47 98.35 24.33 03M0088B fsn 0.00016 0.00000 0.00000 0.02387 1.84995 254.75F9.92 97.57 7.10 B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203 191

Table 2 (continued) Sample 36Ar(a) 37Ar(ca) 38Ar(cl) 39Ar(k) 40Ar(r) Age 2r (Ma) 40Ar (%) 39Ar(k) (%) S2 (Borbera Cgl.; sst.) East (B33); 03M0088D fsn 0.00011 0.00310 0.00079 0.08718 7.13917 268.17F3.45 99.53 25.92 J=0.001957 03M0088E fsn 0.00002 0.00000 0.00024 0.02666 2.23459 274.00F9.01 99.77 7.93 03M0088F fsn 0.00042 0.00618 0.00125 0.08426 8.56981 327.49F3.70 98.57 25.05 03M0088H fsn 0.00006 0.00000 0.00011 0.02680 2.46767 298.93F8.70 99.28 7.97 03M0088I fsn 0.00002 0.00000 0.00002 0.01322 1.32388 322.97F16.88 99.55 3.93 03M0088J fsn 0.00012 0.00000 0.00028 0.04476 3.51278 257.77F5.73 98.96 13.31 03M0088K fsn 0.00002 0.00000 0.00003 0.01405 1.14056 266.04F16.09 99.60 4.18 03M0088L fsn 0.00004 0.00000 0.00008 0.01557 1.48684 309.02F14.22 99.29 4.63

S3 (Ranzano Sst.; sst.) East (B28); 03M0068A fsn 0.00012 0.00186 0.00019 0.01931 1.59762 274.37F14.74 97.84 10.26 J=0.001985 03M0068B fsn 0.00010 0.00072 0.00014 0.01561 1.22264 260.66F18.29 97.70 8.30 03M0068C fsn 0.00029 0.01105 0.00136 0.11417 9.59835 278.42F3.28 99.11 60.70 03M0068D fsn 0.00006 0.00000 0.00010 0.01962 1.64379 277.60F14.77 98.89 10.43 03M0068E fsn 0.00004 0.00000 0.00012 0.01940 1.65209 281.77F14.88 99.33 10.31 03M0069B fsn 0.00016 0.00016 0.00145 0.12389 10.53177 281.31F3.20 99.55 21.91 03M0069C fsn 0.00004 0.00130 0.00137 0.12052 9.96291 274.12F2.80 99.89 21.32 03M0069D fsn 0.00006 0.00000 0.00000 0.02149 1.79337 276.48F9.92 99.00 3.80 03M0069E fsn 0.00033 0.00098 0.00116 0.08783 7.36006 277.60F3.37 98.67 15.53 03M0069F fsn 0.00005 0.00000 0.00000 0.02211 1.74644 262.80F9.65 99.13 3.91 03M0069G fsn 0.00003 0.00000 0.00000 0.01692 1.44583 282.61F12.48 99.42 2.99 03M0069I fsn 0.00107 0.00770 0.00114 0.11989 9.74776 269.92F2.48 96.87 21.20 03M0069J fsn 0.00004 0.00000 0.00000 0.01513 1.29492 283.14F13.64 99.11 2.68 03M0069K fsn 0.00006 0.00000 0.00000 0.02131 1.84004 285.37F10.06 99.06 3.77 03M0069L fsn 0.00004 0.00000 0.00000 0.01632 1.46131 295.11F12.66 99.16 2.89 East (B11); 03M0059A fsn 0.00023 0.00000 0.00132 0.08926 7.26115 277.26F3.46 99.06 19.01 J=0.002042 03M0059B fsn 0.00005 0.00000 0.00000 0.01144 1.10641 325.05F14.37 98.80 2.44 03M0059C fsn 0.00002 0.00000 0.00000 0.01465 1.11572 260.75F10.19 99.34 3.12 03M0059D fsn 0.00004 0.00000 0.00000 0.01502 1.21112 274.98F10.06 99.15 3.20 03M0059E fsn 0.00004 0.00000 0.00000 0.01472 1.21300 280.58F10.42 99.11 3.14 03M0059G fsn 0.00032 0.00076 0.00088 0.09160 7.35693 273.98F3.36 98.72 19.51 03M0059H fsn 0.00050 0.00000 0.00109 0.11729 9.64263 279.98F2.93 98.50 24.98 03M0059I fsn 0.00001 0.00000 0.00000 0.01990 1.71088 291.82F7.70 99.80 4.24 03M0059J fsn 0.00035 0.00000 0.00114 0.08242 6.49275 269.09F3.54 98.43 17.56 03M0059K fsn 0.00001 0.00000 0.00000 0.01315 1.02952 267.60F15.00 99.84 2.80 East (B9); 03M0058A fsn 0.00101 0.00120 0.00344 0.34485 7.87230 82.27F1.24 96.35 22.16 J=0.002044 03M0058B fsn 0.00039 0.00086 0.00202 0.19012 3.01841 57.62F2.18 96.30 12.22 03M0058C fsn 0.00042 0.00114 0.00234 0.28784 2.93033 37.16F1.45 95.93 18.50 03M0058D fsn 0.00047 0.00125 0.00161 0.17460 2.55115 53.09F2.44 94.82 11.22 03M0058E fsn 0.00010 0.00000 0.00014 0.03030 1.40156 162.97F11.37 97.98 1.95 03M0058G fsn 0.00051 0.00000 0.00267 0.29152 3.47107 43.38F1.51 95.80 18.73 03M0058H fsn 0.00083 0.00033 0.00153 0.14998 4.16446 99.59F2.69 94.45 9.64 03M0058I fsn 0.00019 0.00069 0.00000 0.01844 0.24942 49.21F20.12 81.82 1.18 03M0058J fsn 0.00007 0.00000 0.00024 0.04110 1.36883 118.80F8.53 98.56 2.64 03M0058K fsn 0.00004 0.00000 0.00004 0.02737 1.80687 228.32F12.45 99.27 1.76 East (B6); 03M0057A fsn 0.00022 0.01004 0.00067 0.03715 0.45516 44.65F10.67 87.33 3.00 J=0.002045 03M0057B fsn 0.00027 0.00118 0.00029 0.02152 0.20147 34.21F17.63 71.81 1.74 03M0057C fsn 0.00067 0.00139 0.00198 0.15924 3.11400 70.74F2.41 94.05 12.87 03M0057D fsn 0.00060 0.00260 0.00288 0.29949 8.26834 99.08F1.39 97.90 24.21 03M0057E fsn 0.00071 0.00181 0.00076 0.03309 0.34924 38.53F12.04 62.33 2.67 03M0057G fsn 0.00042 0.00491 0.00016 0.01381 0.17452 46.02F21.96 58.47 1.12 03M0057H fsn 0.00046 0.00210 0.00029 0.04223 0.45422 39.25F8.10 77.15 3.41 (continued on next page) 192 B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203

Table 2 (continued) Sample 36Ar(a) 37Ar(ca) 38Ar(cl) 39Ar(k) 40Ar(r) Age 2r (Ma) 40Ar (%) 39Ar(k) (%) S3 (Ranzano Sst.; sst.) East (B6); 03M0057I fsn 0.00057 0.00179 0.00149 0.18201 2.37501 47.51F1.57 93.33 14.71 J=0.002045 03M0057J fsn 0.00024 0.00449 0.00271 0.29786 3.25029 39.82F1.05 97.84 24.08 03M0057K fsn 0.00059 0.00154 0.00117 0.15076 1.45921 35.36F2.26 89.34 12.19

S4 (Rigoroso Marls; sst.) East (B2); 03M0055A fsn 0.00084 0.00313 0.00204 0.23805 8.07743 120.75F1.18 97.02 14.05 J=0.002040 03M0055B fsn 0.00051 0.00208 0.00242 0.21978 4.28548 70.37F1.19 96.57 12.97 03M0055C fsn 0.00030 0.00105 0.00217 0.23797 5.24255 79.31F1.18 98.35 14.05 03M0055D fsn 0.00016 0.00000 0.00183 0.21054 2.53559 43.79F1.25 98.17 12.43 03M0055E fsn 0.00008 0.00130 0.00018 0.00685 0.12027 63.46F30.87 83.58 0.40 03M0055G fsn 0.00088 0.00101 0.00250 0.27293 5.50094 72.69F0.87 95.49 16.11 03M0055H fsn 0.00016 0.00992 0.00018 0.02283 0.30768 48.94F11.02 86.80 1.35 03M0055I fsn 0.00053 0.00078 0.00212 0.25406 6.22235 87.96F0.93 97.56 15.00 03M0055J fsn 0.00081 0.00069 0.00191 0.21812 6.32665 103.71F2.29 96.34 12.88 03M0055K fsn 0.00020 0.00259 0.00005 0.01290 0.20299 57.01F38.36 77.16 0.76 03M0073A fsn 0.00066 0.00339 0.00291 0.36332 8.82527 87.25F1.22 97.84 13.88 03M0073B fsn 0.00029 0.00412 0.00010 0.01399 0.13709 35.70F21.49 61.89 0.53 03M0073C fsn 0.00091 0.00472 0.00248 0.30231 9.32382 110.08F0.97 97.19 11.55 03M0073D fsn 0.00068 0.00344 0.00216 0.19051 4.22113 79.76F1.72 95.43 7.28 03M0073E fsn 0.00059 0.00276 0.00138 0.18354 8.07817 155.12F1.79 97.87 7.01 03M0073G fsn 0.00038 0.00161 0.00219 0.23607 4.63778 70.89F1.25 97.63 9.02 03M0073H fsn 0.00008 0.00000 0.00003 0.03017 1.33391 155.78F8.33 98.20 1.15 03M0073I fsn 0.00055 0.00295 0.00117 0.10776 4.45654 146.12F3.02 96.47 4.12 03M0073J fsn 0.00100 0.00376 0.00231 0.24870 6.21242 89.67F1.41 95.45 9.50 03M0073K fsn 0.00107 0.00113 0.00126 0.16060 4.98743 110.82F2.01 94.02 6.14 03M0073M fsn 0.00093 0.00208 0.00230 0.24854 5.71147 82.65F1.27 95.40 9.50 03M0073N fsn 0.00065 0.00000 0.00105 0.11597 3.86715 118.74F2.83 95.25 4.43 03M0073O fsn 0.00010 0.00000 0.00002 0.02813 0.42794 55.13F8.87 93.59 1.08 03M0073P fsn 0.00012 0.00000 0.00006 0.02329 1.14688 172.67F10.89 97.00 0.89 03M0073Q fsn 0.00078 0.00000 0.00366 0.36407 6.42049 63.76F0.92 96.52 13.91 East (B4); 03M0056A fsn 0.00093 0.00000 0.00282 0.29736 8.20864 98.98F0.89 96.76 15.39 J=0.002043 03M0056B fsn 0.00076 0.00000 0.00113 0.13792 4.96625 128.06F1.99 95.67 7.14 03M0056C fsn 0.00041 0.00000 0.00093 0.11926 1.23729 37.84F2.64 91.14 6.17 03M0056D fsn 0.00107 0.00000 0.00191 0.24622 9.72948 140.06F1.18 96.85 12.74 03M0056E fsn 0.00078 0.00000 0.00660 0.64402 10.29273 57.96F0.54 97.79 33.32 03M0056G fsn 0.00123 0.00028 0.00331 0.31884 10.06676 112.77F0.98 96.50 16.50 03M0056H fsn 0.00013 0.00000 0.00037 0.04954 1.36823 99.03F3.89 97.17 2.56 03M0056I fsn 0.00025 0.00000 0.00032 0.04830 1.84916 135.87F3.75 96.08 2.50 03M0056J fsn 0.00069 0.00075 0.00127 0.07132 1.01240 51.58F3.47 83.30 3.69 03M0074A fsn 0.00029 0.00000 0.00013 0.04444 2.14746 169.85F9.59 96.17 3.09 03M0074B fsn 0.00015 0.00000 0.00014 0.03940 2.08127 184.88F10.88 97.95 2.74 03M0074C fsn 0.00030 0.00013 0.00066 0.02955 0.33967 41.87F16.11 79.38 2.06 03M0074D fsn 0.00002 0.00000 0.00029 0.04132 1.14259 99.15F10.61 99.52 2.87 03M0074E fsn 0.00014 0.00000 0.00036 0.05001 1.56819 112.03F8.65 97.39 3.48 03M0074G fsn 0.00071 0.00000 0.00313 0.25615 9.89705 137.07F1.83 97.92 17.82 03M0074H fsn 0.00219 0.00000 0.00400 0.32353 9.17952 101.66F1.50 93.40 22.50 03M0074I fsn 0.00141 0.00000 0.00411 0.40582 5.92577 53.03F1.13 93.40 28.23 03M0074J fsn 0.00012 0.00000 0.00045 0.04729 1.81794 136.40F9.09 98.06 3.29 03M0074K fsn 0.00078 0.00000 0.00201 0.20022 4.68784 84.29F2.37 95.28 13.93 B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203 193

Probability distribution diagrams (Sircombe, 1999; (Barbieri et al., 2003; Cimmino et al., 1981). Sircombe, 2000) have been used to identify the main Chemical data for UNIT S1 reflect a low-pressure populations of detrital ages present in different units of source with Si content between 6 and 6.5 pfu, and Mg the studied sediments. The probability distribution content between 0.05 and 0.3 pfu. The Pianfolco curves are compiled by summing the Gaussian Conglomerates record high-pressure mica composi- distribution of each individual measurement, which is tion with Si content between 6.5 and 8 pfu, and Mg defined by the age and its error (e.g. Sircombe, 2000). content between 0.4 and 1.2 pfu. Chemical data from The ages obtained from the eastern TPB clastic UNIT S1 and from the Pianfolco Conglomerates phengites are interpreted to represent the time of samples directly reflect the composition of the source isotopic closure during cooling of the crystalline source outcropping at the time of deposition. through 350–420 8C (e.g. Hames and Bowring, 1994; UNITS S2 and S3 both contain low- and high- Kirschner et al., 1996; von Blanckenburg et al., 1989). pressure phengites. This composition could be due to Because the shape of minerals influences diffusion, the contribution of a primary source or it could be the grain size can have effect on cooling ages (e.g. result of reworking. In particular, sample B33 from McDougall and Harrison, 1999). 40Ar/39Ar ages from UNIT S2 shows a signal that is very similar to that of different grain sizes of a micaschist cobble derived by the underlying Ranzano Formation (B30, B12 of the Ligurian Alps show no dependency between grain UNIT S1; Pzo. D’Oca Member) and therefore could size and Oligocene 40Ar/39Ar ages. These results be the result of recycling of this older material. The indicate that for fast cooling rocks (i.e. Voltri Group) predominance of carbonate cobbles in the Borbera grain size does not significantly affect cooling ages. Conglomerates (UNIT S2), which form the framework of the rock where the sandstone matrix was sampled, suggests erosion of the underlying sedimen- 4. Mica chemistry tary units, which are mainly carbonatic in composition (Di Giulio, 1991). This reworking could be due to Electron microprobe data show two distinct groups erosion following a sea-level drop which occurred at of mica compositions for the analysed samples. One the Eocene–Oligocene boundary and which was group has a Si content ranging from 6.5 to 8 pfu and registered by an unconformity and locally by the Mg content ranging from 0.3 to 1.2 pfu; the other deposition of shallow marine sediments (Rio Trebbio group has a Si content between 6 and 6.5 pfu, and a Sst.; Cavanna et al., 1989; Di Giulio, 1991). Deposits Mg content between 0.05 and 0.3 pfu. The first from UNIT S3 in the study area unconformably cover composition (with general values of SiN7 pfu and the deposits of UNITS S1 and S2. Therefore the MgN0.5 pfu) is characteristic of high-pressure (HP) chemical composition of samples from UNIT S3 rocks of the Voltri Group and Montenotte Nappe could be partially due to recycling of the older (Ligure–Piemontese domain) where similar values sediments. have been recorded previously (e.g. Barbieri et al., Results from UNIT S4 show only high-pressure 2003). The second composition, with general values mica with Si content ranging from 6.5 to 7.5 pfu and of Sib6.5 pfu and of Mgb0.5 pfu is typical of low- Mg content between 0.3 and 1.2 pfu. This compo- pressure (LP) rocks of the Brianc¸onnais domain sition could be the result of either a primary source

Notes to Table 2: sst.=sandstones; cbl.=cobbles. 2r errors reported represent the analytical errors (errors in the regressions of the samples and blanks, in the mass discrimination factor and for correction of interfering nuclear reactions) excluding the uncertainties in J and age of the standards and uncertainties in the decay constant. Note that average of J related errors is in the order of 0.3%. The data listed for the 40Ar/39Ar experiments are: 36Ar(a): atmospheric component in 36Ar; 37Ar: calcium-derived 37Ar; 38Ar(cl): chlorine-derived component 38Ar; 39Ar(k): potassium-derived component in 39Ar; 40Ar(r): radiogenic 40Ar; age (Ma) with related 2r errors; 40Ar (%): percentage radiogenic component in Ar; 39Ar (%): increment size expressed as the percentage of 39Ar(k) compared to the total amount of 39Ar(k) released during the experiment. 194 B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203 or of reworking. In the case of reworking of the 45.5F1 and 47.8F1 Ma. Sample B24 is a metasedi- underlying sediments, HP micas would also be mentary rock with upper greenschist-facies metamor- expected in UNIT S4. Therefore it is more likely phic grade; five single fusion experiments gave ages that the mica composition of UNIT S4 reflects a ranging between 52.9F1.7 and 98F3.8 Ma. Sample primary source. B27 is a greenschist-facies metamorphic rock; five single fusion experiments gave ages between 35.5F 1.7 and 41.8F1.4 Ma. Sample B34 is a metasedi- 5. 40Ar/39Ar geochronology mentary rock with greenschist-facies metamorphism and five single fusion experiments give ages between 5.1. Single fusion ages 37.2F4 and 49.2F6.2 Ma. Single fusion experiments on sample B35, which is a metasedimentary rock Sediments from UNIT S1 produce only Permian– (calcschist) with greenschist-facies metamorphic Carboniferous micas with ages between 222.06F14.6 grade, yielded ages between 31.4F3.5 and 34.5F and 335.35F6.3 Ma. 4.6 Ma. The Pianfolco Conglomerates show two groups Samples from UNIT S3 recorded essentially the of ages, one Oligocene–Eocene with ages ranging same group of ages as samples from UNIT S2 but in between 33F1.4 and 58.2F0.8 Ma and the other different proportions. Four samples have been Cretaceous with ages ranging between 93.4F0.9 analysed from the uppermost member of the Ran- and 149.9F2.5 Ma. The Eocene–Oligocene group zano Formation (S. Sebastiano Curone Member), of ages is recorded in sandstones B15 and B17 and taken from the eastern sector of the study area (Fig. in cobbles B20 and B22 while cobble B22 records 3; see also Di Giulio and Galbiati, 1995). Samples Cretaceous ages. Sample B20 is a metasedimentary B28 and B11 show mainly Permo-Carboniferous rock with blueschist-facies metamorphism; single ages between 260.7F18.3 and 325F14.4 Ma (Fig. fusion experiments on five grains yielded ages 5). Samples B9 and B6 from the depocentre of the between 33.6F1.5 and 45.9F1.2 Ma. Sample B22 basin record ages ranging from 34.2F17.6 to is a metasedimentary rock with upper greenschist- 228.3F12.4 Ma with a greater proportion of Eocene facies metamorphic grade; five single fusion experi- ages. ments gave ages between 33F1.4 and 36.4F1.6 Samples from UNIT S4, from the Rigoroso Marls, Ma. Sample B21 is a metasedimentary rock with record ages ranging from 37.8F2.6 to 184.9F10.9 faint greenschist-facies metamorphic grade; five Ma. It is not possible to see a distinctive population. single fusion experiments gave mainly Cretaceous Ages between 100 and 160 Ma constitute a large part ages between 93.4F1 and 149.9F2.5 Ma. Samples of the total signal and these ages have not been found B30 and B12 from sandstones of the Ranzano in the underlying older units. Formation (Pzo. D’Oca member) record Permian– Carboniferous ages between 222.1F14.6 and 5.2. Step heating ages 335.3F6.3 Ma. Samples from UNIT S2 show two main age Eleven step heating experiments on metasedi- groups, one between 219.3F2.6 327.5F3.7 Ma and mentary cobbles from the Pianfolco (B20, B21; the other between 31.4F3.5 and 49.2F6.2 Ma but UNIT S1) and Savignone Conglomerates (B26) few ages around 90–100 Ma are also present. have been conducted. Four step heating experiments Sample B33, which is from a sandstone matrix of (0083, 0084, 0093, 0094) on sample B20 from the Borbera Conglomerates in the eastern sector (Fig. UNIT S1 (blueschist-facies metamorphism) all gave 5), recorded Permo-Carboniferous ages ranging from plateau ages between 40 and 46 Ma (Fig. 6a), 228.6F3.4 to 323F16.9 Ma. Cobbles from this unit, suggesting a homogeneous Eocene signal. Three however, recorded mainly Eocene–Oligocene ages step heating experiments (0077, 0089, 0090; Fig. (Fig. 5). Single fusion experiments on sample B23, 6b) on sample B21 (greenschist-facies metamor- which is a metasedimentary rock with blueschist phism) gave plateau or plateau-like ages between facies metamorphic grade, yielded ages between 124 and 129 Ma. Experiment 0077 yielded a B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203 195

Fig. 5. Cumulative probability curves of 40Ar/39Ar detrital ages from the selected samples divided in sequences following the scheme of Di Giulio (1991) reported in Fig. 3. 196 B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203

plateau age of 126.6F2.5 Ma, experiment 0089 a plateau age of 129.3F3.3 Ma, experiment 0090 a slightly disturbed age (MSWD=4.02) with a plateau-like age of 124.0F5.4 Ma. Sample B26 (same as B24) is a metasedimentary rock (calcschist) from the Savignone Conglomerates (UNIT S2) with upper greenschist-facies metamorphism. Four step heating experiments (0079, 0082,

Fig. 6. (a-c) Step heating experiments of the Pianfolco and Savignone Conglomerates. Fig. 6 (continued). B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203 197

0091, 0092) have been conducted (Fig. 6c). Experi- ments 0079, 0082 and 0091 gave plateau ages of 65.3F1.0, 70.8F3.08 and 137.8F1.3 Ma, respectively. Step heating experiment 0092 yielded a disturbed signal at 110.5F4 Ma with higher ages at lower T’s possibly due to alteration, excess or inherited Ar as discussed above.

6. Discussion

6.1. Implications for depositional ages

The youngest 40Ar/39Ar ages recorded in the Pianfolco Conglomerates allow establishment of a maximum age constraint to the stratigraphic ages of these poorly dated sediments as 40Ar/39Ar ages cannot be older than the sedimentation age. No post burial resetting is considered for these sediments since we know that TPB sediments never experienced temperatures higher than 1008C after deposition (Barbieri et al., 2003 and references therein). The Pianfolco Conglomerates were originally attributed to the late Eocene–early Rupelian (37–32 Ma; Charrier et al., 1964: Gnaccolini, 1978) while the youngest 40Ar/39Ar age recorded is 33.0F1.4 Ma (B22). A similar youngest age of 33.6F1.5 Ma is recorded by sample B20. Therefore these sediments can be attributed to the early Rupelian and can be considered as part of UNIT S2 (Molare–Borbera after Mutti et al., 1995).

6.2. Provenance discrimination

6.2.1. Late Priabonian (UNIT S1) Both the chemical and the 40Ar/39Ar data from this unit suggest a single source feeding the Pizzo d’Oca member of the Ranzano Formation. This source was mainly characterised by low-pressure (Sib6.5 pfu) rocks that recorded Permian ages around 270 Ma. The Permian signal may suggest the presence of low- pressure continental basement overlying the Penninic belt at some stage (Brianc¸onnais units; Fig. 7). This is also shown by petrographic data (Di Giulio, 1991), which suggest that rocks with south Alpine affinity, of which now only few relics exist, formed the top of the Voltri Group in the late Eocene (Di Giulio, 1991; Fig. 6 (continued). Polino et al., 1991). 198 B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203

Fig. 7. Paeogeographic maps for the late Priabonian, early Rupelian, late Rupelian, Chattian, respectively. DP=Dauphinoise-Provencal Units; DPF=Dauphinoise-Provencal Foredeep basin (Ventimiglia Flysch Basin); HF=Helminthoid Flysch Units of Ligurian Alps; LP=Pennidic units without high-pressure alpine metamorphism (Brianconnais Units of Ligurian Alps); HP=Pennidic units with high-pressure alpine metamorphism (Voltri Group and Montenotte Nappe); L=Ligurian Units of northern Apennines (mostly Helminthoid calcareous Flysch); SL=Subligurian Units; MF=Macigno Foredeep basin; PVC=Periadriatic volcanic centers; MF=Molare Formation (including Pianfolco Conglomerate at the very base); PzO=Pizzo d’Oca unit of Ranzano Formation; VP=Val Pessola Unit of Ranzano Formation; VM=Varano de´ Melegari Unit of Ranzano Formation; SSC=S. Sebastiano Unit of Ranzano Formation; SC=Savignone fan delta Conglomerates; BC=Borbera fan delta Conglomerates.

6.2.2. Early Rupelian (UNIT S2) by ages between 100 and 150 Ma, respectively, and a Both the chemical and the 40Ar/39Ar data suggest third low-pressure source characterised by Permo- three different sources for the sediments of UNIT S2: Carboniferous ages between 219 and 327 Ma. two main high-pressure (SiN7 pfu) sources charac- The main Mesoalpine population suggests a terised by 40Ar/39Ar ages between 31 and 56 Ma, and provenance from the Voltri and Montenotte Nappe B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203 199 affected by high-pressure metamorphism in the mid- meaningless. When excess Ar is incorporated in dle Eocene with retrograde metamorphic overprint in existing crystals by volume diffusion, one may the late Eocene–Oligocene (e.g. Barbieri et al., 2003; expect an age spectrum characterised by anom- Carrapa et al., 2004)(Fig. 7b). This interpretation is alously high apparent ages in the first steps, also supported by the chemical data which are very followed by a regular decreasing age pattern similar to those reported for the Voltri Group rocks where the final ages may be interpreted as a (Cimmino and Messiga, 1979). The same range of maximum estimate for a geological event (crys- ages recorded in sediments from UNIT S2 is also tallisation or cooling through the closure temper- recorded in Oligocene sediments of the southern TPB ature) (e.g. Pankhurst et al., 1973; Harrison and (Barbieri et al., 2003). The large span of total fusion McDougall, 1981). We refer to Carrapa and ages and step heating results within single cobbles of Wijbrans (2003) for an extended discussion on greenschist metamorphic grade (e.g. B21, B26) reflect excess versus inherited argon in detrital sediments the incomplete resetting of isotope systems in rocks from sediments of the TPB since this is beyond that have experienced only low to intermediate the scope of this paper. metamorphic temperatures (e.g. Wijbrans and McDou- gall, 1986; Scaillet et al., 1992; Leeps et al., 1999). The Ages from cobbles B21 and B24 (B26) strongly observed age ranges might be interpreted as evidence suggest the presence of an Eoalpine source (85–150 for several distinct scenarios: Ma) possibly related to the Sestri Voltaggio zone (e.g Schamel, 1974). The sedimentary facies of these – All total fusion ages are related to real cooling deposits suggests a very proximal source area (Di events and the discrepancy between them could be Biase and Pandolfi, 1999). This could imply that due to chemistry. For example Mg-rich phengite during the Eocene, rocks were present (on top of the could retain Cretaceous ages as observed by Voltri Group) in the Ligurian Alps, which exhibited a Scaillet et al. (1992). Cretaceous age signal (150–90 Ma). The same set of – Old ages are representative of real cooling while ages has been detected by Zircon Fission Track the young ages (e.g. ~90 Ma) are due to a partial Thermochronology in the Ligurian Alps and in opening of the system during later metamorphic western Corsica (Vance, 1999; Mailhe´ et al., 1986). events or to deformation-induced argon loss Also, the presence of Cretaceous ages that persist during denudation. Both of these processes will throughout Oligocene–Miocene sediments of the be referred to in the following as Ar loss. western-central TPB (Carrapa et al., 2004) suggests – Younger ages are representative of real cooling that this signal could be geodynamically significant events while older ages (150 Ma) are disturbed (refer to Carrapa and Wijbrans, 2003 for further ages due to alteration, inherited argon (refer to details). These ages suggest a complicated Eoalpine Dalrymple and Lanphere, 1969; Wijbrans and evolution of the Ligurian Alps as already suggested McDougall, 1986; Singer et al., 1998), or excess for sediments sourced by the Western Alps (Carrapa argon (refer to Dalrymple and Lanphere, 1969; and Wijbrans, 2003). If these signals are considered Reddy et al., 1996; Kelley, 2002). In the case of geologically meaningful, then older ages (~150 Ma) inherited argon, the relationship between radio- could be attributed to the Tethyan thermal anomaly active parent 40K and radiogenic daughter 40Ar is related to the spreading of the Liguro–Piemontese maintained and therefore these ages can still be Ocean (e.g. Vance, 1999) while younger ages (130– geologically meaningful as the compounded 93) could be due to the onset of the Ligure– effects of geological events preceding the main Piemontese intraoceanic subduction (Hurford and event of interest (e.g. Wijbrans and McDougall, Hunziker, 1989; Oberha¨nsli et al., 1985; Carrapa 1986; Villa, 1998; Forster and Lister, 2003). In and Wijbrans, 2003). A Cretaceous age of 100–80 Ma case of excess Ar, the relationship between the for the high-pressure (HP) metamorphism of the Voltri parent isotope 40K and its radiogenic daughter Group has been proposed by Hoogerduijn Strating et isotope 40Ar is disturbed and therefore calculated al. (1991) by analogy and comparison with rocks from ages for minerals affected by excess argon are the western Alps (e.g. Hunziker and Martinotti, 1984) 200 B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203 and Corsica (Cohen et al., 1981; Maluski, 1977). 7. Conclusions However, our data are the first indication of middle to late Cretaceous mica cooling ages in the Ligure– Our data shed new light on the unroofing history of Piemontese domain of the Western Alps. the Alps–Apennine junction area in the earliest stages The fact that the sandstone matrix (B33) of the of the Alpine orogeny. Chemical and geochronolog- Borbera Conglomerates records Permian ages while ical data combined with petrographical data of the the cobbles record mainly Mesoalpine ages could studied sediments indicate a source area located suggest that the cobbles are from a primary local mainly in the area of the Voltri Group. From late Mesoalpine source, whereas the sandstone has been Priabonian till late Rupelian time, the sediments recycled from older sediments (Ranzano Formation) deposited in the eastern part of the TPB record two as also suggested by the chemical data (Fig. 7b). different sources: one of LP rocks, characterised by Permian ages (270 Ma), and another of HP rocks, 6.2.3. Late Rupelian (UNIT S3) characterised by Mesoalpine ages (32–50 Ma). Eoal- Chemical and 40Ar/39Ar data of samples from the pine ages (~80–100 Ma) are also present and can be Ranzano Sst. indicate a dual source. One source is related to cooling following the onset of the Ligure– characterised by low-pressure micas and Permian ages Piemontese intra-oceanic subduction. These data are (280 Ma) that may represent low-pressure Permian here interpreted as recording the unroofing of the covers and/or partial recycling from older sediments tectonic nappe stack with first the erosion of LP (Fig. 7c). The second source is characterised by high- Penninic covers and later (post-Priabonian) of HP pressure micas and Mesoalpine–Eoalpine ages. Ages Piedmont units. Permian ages are no longer present in around 38 Ma can be attributed to the Voltri Group Chattian sediments, suggesting that LP rocks with and/or Montenotte Nappe (Fig. 7c). Permian ages once covering the Voltri Group were Ages between 60 and 100 Ma can still be attributed completely eroded by that time. The greater span of to the Sestri Voltaggio zone or (for ages ~100 Ma) to 40Ar/39Ar ages recorded in Chattian sediments also rocks with western Alpine and Corsican affinity (e.g. suggests a provenance from western Alpine sources. Hunziker and Martinotti, 1984; Cohen et al., 1981; In addition, the almost indistinguishable 40Ar/39Ar Maluski, 1977), outcropping in the Ligure–Piemon- detrital age of 31.4F3.5 from sample B35, and tese domain during the Eocene–early Oligocene. depositional age (early Rupelian, ~30–33.7 Ma) of sediments from the Borbera Conglomerates (UNIT 6.2.4. Chattian (UNIT S4) S2) suggests a rapidly exhuming source. Similar ages Chemical and 40Ar/39Ar data of the samples from have been recorded in the early Oligocene Molare the Rigoroso Marls in general show an even more sediments further to the west. They have been heterogeneous provenance than the late Rupelian interpreted as representative of a fast cooling and sediments. In particular, Eoalpine ages occur more exhumation episode affecting the Ligurian Alps frequently than in the previous sequence and this during the early Oligocene. suggests either a new source characterised by Eoal- Our data suggest a trend that provides an alternative pine ages (e.g. Western Alps) or a larger contribution view on the evolution of the Ligurian Alps during the of potential Eoalpine rocks belonging to the Voltri Eocene–Oligocene. Previous models (Vanossi et al., Group and Sestri Voltaggio zone. The main influx 1986) proposed that only Liassic–Triassic units were attributed to the western Alpine domain starts in on top of the Ligurian Alps. However, these models approximately late Oligocene–early Miocene time were based on geological observation and paleogeo- (Carrapa et al., 2004). This suggests that the Eoalpine graphic reconstruction but did not have any geo- signal from UNIT S4 most probably derives from chronological support. Also, the dataset presented western Alpine sources, which start to supply the TPB provides a new constraint on the stratigraphic age of and its eastern sectors already from Chattian time the Pianfolco Conglomerates with 40Ar/39Ar age data, onwards (Fig. 7d). Also, the disappearance of Permian suggesting that it can be no older than 33.0F1.4 Ma, ages suggests that the nappe on top of the Voltri which allows these sediments to be assigned to UNIT Group was eroded completely by Chattian time. S2, as proposed by Mutti et al., 1995. B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203 201

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