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
a
Department of Isotope Geochemistry, Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085,
Amsterdam 1081 HV, The Netherlands
b
Dipartimento di Scienze della Terra, Università 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;
40
Ar/39Ar geochronology; Cooling/exhumation; Paleogeography
1. Introduction
* Corresponding author. Present address: Institut fqr Geowissenschaften, Universit7t Potsdam, Karl-Liebknecht-Str. 24/H25,
14476 Golm, Potsdam 14415, Germany. Tel.: +49 331 977 5078;
fax: +49 331 977 5060.
E-mail address: carrapa@geo.uni-potsdam.de (B. Carrapa).
0037-0738/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.sedgeo.2004.05.015
Examining the provenance of clastic sediments
derived from orogenic belts is a classical tool for
unravelling the evolution of collisional systems
(Dickinson, 1974; Dickinson, 1985). Substantial
182
B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203
advancements have been made in this field through
the application of mineral chemistry and geochronology to clastic minerals, as these can provide information on the cooling and exhumation paths of the
eroded rock units (Heller and Frost, 1988; Copeland
and Harrison, 1990; Renne et al., 1990; Harrison et
al., 1993; Najman et al., 1997; von Eynatten and
Gaupp, 1999; Najman et al., 2001; Sherlock, 2001;
White et al., 2002; von Eynatten and Wijbrans, 2003).
Recently, this approach has been applied successfully
to clastic sediments deposited in the southern part of
the Piedmont Tertiary Basin (TPB) in northwestern
Italy (Fig. 1; Barbieri et al., 2003; Carrapa et al., 2003,
2004).
The current study focuses on the eastern margin of
the TPB, which is located on the tectonic junction
between the Ligurian Alps and the northern Apennines (Fig. 1). Here, the clastic succession unconformably covers the Ligurian Alps to the south and
contains the oldest sediments deposited in the TPB.
The area has been extensively studied, mostly with the
aim of unravelling the tectonic evolution of this
geologically complex region (Cavanna et al., 1989; Di
Giulio, 1996; Di Giulio and Galbiati, 1995; Mutti et
al., 1995; Vanossi et al., 1994).
Recently, the provenance of clastic sediments in
the eastern TPB has been systematically investigated,
in order to improve models of paleogeographic
evolution of the orogenic system following collision
(Cibin et al., 2001, 2003; Di Giulio and Galbiati,
1995; Gnaccolini, 1974; Gnaccolini and Rossi, 1994;
Martelli et al., 1998). Sandstone petrography in the
study area suggests a possible low-pressure source for
these sediments with south Alpine affinity, which
were not affected by late Alpine metamorphism.
Presumably, these sediments were once located on
top of the Ligurian Alps and are presently completely
missing (Di Giulio, 1991). However, the lack of
thermochronological data has so far limited the
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
mean that the sediments deposited in the eastern part
of the TPB record the unique signal of rocks once
exposed at the top of the Alpine edifice, which would
have important paleogeographic implications. Also,
the sediments preserved in the eastern part of the TPB
are the oldest sediments preserved in the study area,
meaning that their investigation would provide
information on the paleogeography of the belt during
the earliest, late Eocene steps of belt evolution after
collision. Ultimately, continental conglomerates of
uncertain ages deposited in the eastern TPB are here
analysed with the aim of assessing a maximum
depositional age for these sediments (Najman et al.,
1997, 2001).
Detrital mineral chemistry and 40Ar/39Ar thermochronology has been performed on continental to
transitional and shallow marine sediments of the
Molare Formation which form the base of the southern part of the TPB (Barbieri et al., 2003). Detrital
40
Ar/39Ar ages in these sediments suggest two local
sources located in the Ligurian Alps. The first is
characterised mainly by high-pressure (HP) rocks and
Eocene–Oligocene 40 Ar/ 39 Ar ages (32–45 Ma)
recording the exhumation of deep crustal levels of
the original orogenic prism. The second is characterised by low-pressure (LP) rocks and Carboniferous
ages (Barbieri et al., 2003). In particular, the youngest
40
Ar/39Ar detrital signal suggests a fast episodic
cooling event occurring sometime in the Oligocene
Ligurian belt (Barbieri et al., 2003; Carrapa et al.,
2003).
However, due to a lack of paleontological markers
in the mainly continental sediments of the Molare
Formation, this formation has only a loosely defined
early Oligocene age (Gnaccolini, 1974; Barbieri et al.,
2003 and referenced therein) which consequently
prevents a detailed provenance discrimination. On
the other hand, the mainly marine sediments preserved
in the easternmost part of the TPB (e.g. Ranzano
Formation and Rigoroso Marls) are biostratigraphically well dated (Di Giulio et al., 2002; Mancin and
Cobianchi, 2000; Mancin and Pirini, 2001; Martelli et
al., 1998), allowing a more robust constraint on the
time of cooling of the source area.
The aims of this study are:
(1)
To better constrain the time of sedimentation of
the poorly dated conglomerates outcropping in
(2)
183
the eastern part of the TPB. Such a maximum
estimate of the depositional age can be obtained
(e.g. Najman et al., 2001) under the assumption
that the depositional age of sediments cannot be
greater than the 40Ar/39Ar ages of the detrital
micas. This will be the case when no alteration
and/or resetting of the micas occurred after
deposition.
To attempt a paleogeographic reconstruction of
the study area during the late Eocene–early
Oligocene. Provenance discrimination of the
investigated sediments is made in order to
confirm the presence of sources with south
Alpine affinity as previously proposed from
sandstone petrography (Di Giulio, 1991). This
aim is pursued by looking at the white mica
geochemical signal together with the 40Ar/39Ar
detrital populations recorded by the studied
sediments. Differences in major element geochemistry and in 40Ar/39Ar age families reflect
the contribution in composition and ages present
in the original source area surface at the time of
sediment deposition.
These objectives will be met through the integrated
study of mineral chemistry and 40Ar/39Ar thermochronology of clastic white micas, sampled in the
lowermost part of the succession in the eastern part of
the TPB, where late Eocene sediments occur at the
very base.
2. Stratigraphic framework and sample strategy
The TPB is an episutural basin located in a complex
tectonic area that represents the boundary between the
Alpine and the Apennine thrust belts (Fig. 2). The
stratigraphy of the area is complex, compounded by
inconsistency in the published literature (Fig. 3). In
this study, we will use the stratigraphic scheme of Di
Giulio (1991) integrated with other studies reported in
Fig. 3. Biostratigraphic ages of the formations considered in this study are given using works reported in
Table 1 and the geological timescale of Haq and Van
Eysinga (1998).
In the western sector of the eastern TPB,
sedimentation was perceived to have started in the
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
Pianfolco Conglomerates; Charrier et al., 1964;
Gnaccolini, 1978), unconformably overlying the
Ligurian Alps (Voltri Group in the study area; Fig.
2). The Pianfolco Conglomerates are mainly composed of alternating continental conglomerates and
sandstones supplied by local sources. These sediments have been tentatively dated as late Eocene–
early Rupelian on the basis of a tropical flora
association and indirectly by means of the unconformably overlying early Oligocene–Chattian Molare
Formation to the west (Charrier et al., 1964;
Gnaccolini, 1974, 1978; Fravega et al., 1994; Mutti
et al., 1995; Fig. 3). More recently, Mutti et al.
(1995) tentatively attributed a Rupelian age to the
Pianfolco Conglomerate according to their genetic
depositional relation to the Molare–Borbera unit.
Samples from the Pianfolco Conglomerates have
been collected from the type locality (i.e. Pianfolco;
Charrier et al., 1964) and the age of the sediments at
this location is supposed to be late Eocene–early
Rupelian (Charrier et al., 1964). Nevertheless, at
present, their age remains poorly defined. For this
reason, the Pianfolco Conglomerates will be treated
separately in the following.
Contemporaneously, in the eastern sector, sedimentation started with the Ranzano Formation, the
lower part of which mainly comprises deep marine
turbidite sandstones (Di Giulio and Galbiati, 1995;
Martelli et al., 1998; Di Giulio et al., 2002). The
Ranzano Formation as a whole has with a very
precise biostratigraphically determined age (Mancin
and Pirini, 2001; Martelli et al., 1998). Sandstone
petrography suggests a source mainly consistent with
a Permo-Carboniferous cover, possibly related to
rocks once located at the top of the Penninic
orogenic prism (Di Giulio, 1991). The Ranzano
Formation therefore records the first supply of the
unroofing products of the top part of the Alpine
orogenic prism into the TPB. Samples from the
Ranzano Formation have been collected in the same
locality studied by Di Giulio (1991) (Figs. 1 and 3).
The lowermost member of the Ranzano Formation
(Pizzo d’Oca Member; Martelli et al., 1998) is
referred to in the following as UNIT S1.
Sedimentation continued in the Rupelian with the
Borbera, the Savignone Conglomerates the upper part
of the Ranzano Formation and the overlying Rigoroso
Marls. Of these, the first two comprise fan delta
deposits (Di Biase et al., 1997; Di Biase and Pandolfi,
B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203
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
code
Total fusion
code
B2
Step heating
code
Location
Formation/lithology
Depositional age
Extra info
0055+0073
Nivione Channel
Rigoroso Marls
Chattian (S4)
B4
0074+0056
Fontana di Nivione
Chattian (S4)
B6
0057
NW Dernice
Rigoroso Marls
(Nivione Sst.)
Ranzano Sst. (a)
late Rupelian (S3)
B9
0058
NW Dernice
Ranzano Sst. (s)
late Rupelian (S3)
B11
0059
Pessola
Ranzano Sst. (a)
late Rupelian (S3)
B28
0069+0068
Fontana di Nivione
B33
0071+0088
Dernice
late Rupelian NP23
(S3)
early Rupelian (S2)
B34
0053
C.na Lemmi
early Rupelian (S2)
B35
0054
C.na Lemmi
early Rupelian (S2)
Di Biase et al. (1997)
B23
0050
Carrosio
early Rupelian (S2)
Ghibaudo et al. (1985)
B24(B26)
0051
early Rupelian (S2)
Ghibaudo et al. (1985)
B27
0052
early Rupelian (S2)
Ghibaudo et al. (1985)
B20
0047
B21
0048
B22
0049
Bosio-Voltaggio
B15
0076+0061
C.na Pianfolco
0075+0062
C.na Pianfolco
B30
0070
B12
0060
Incisa
(cross to Solarolo)
P.zo d’Oca
(sez. Fontanelle)
late Eocene–early
Rupelian? (S1)
late Eocene–early
Rupelian? (S1)
late Eocene–early
Rupelian? (S1)
late Eocene–early
Rupelian? (S1)
late Eocene–early
Rupelian? (S1)
late Priabonian (S1)
lithozone A; Gnaccolini
(1978)
lithozone A; Gnaccolini
(1978)
lithozone A; Gnaccolini
(1978)
Charrier et al. (1964)
B17
Ranzano
(midium)
Val Borbera Cgl.
(sandstone)
Val Borbera Cgl.
(cobble)
Val Borbera Cgl.
(cobble)
Savignone Cgl.
(cobble)
Savignone Cgl.
(cobble)
Savignone Cgl.
(cobble)
Pianfolco Cgl.
(cobble)
Pianfolco Cgl.
(cobble)
Pianfolco Cgl.
(cobble)
Pianfolco Cgl.
(conglomeratic sst.)
Pianfolco Cgl.
(midium)
Ranzano Fm.
(midium)
Ranzano Fm. (sst.)
Rigoroso V; Di Giulio
and Galbiati (1995)
Rigoroso n; Di Giulio
and Galbiati (1995)
Ranzano a; Di Giulio
and Galbiati (1995)
Ranzano s; Di Giulio
and Galbiati (1995)
Ranzano a; Di Giulio
and Galbiati (1995)
Ranzano C; Di Giulio
and Galbiati (1995)
Di Giulio and Galbiati
(1995)
Di Biase et al. (1997)
0079+0082+
0091+0092
Carrosio
Carrosio
0083+0084+
0093+0094
0077+0089+
0090
Bosio-Voltaggio
Bosio-Voltaggio
late Priabonian (S1)
Charrier et al. (1964)
UNIT S1; Di Giulio
and Galbiati (1995)
Ranzano a; Di Giulio
and Galbiati (1995)
Sst.=sandstones; Cgl.=conglomerates; Fm.=formation.
1999). Locally, the Borbera and Savignone Conglomerates directly cover the Voltri Group of the Ligurian
Alps (Di Biase et al., 1997) while towards the east
they partly interfinger with the Ranzano Formation,
passing laterally to its intermediate turbiditic member
(Val Pessola Member). Samples from the Savignone
Conglomerates come from the same sector studied by
Di Biase et al. (1997). These conglomerates will be
referred in the following as UNIT S2 (early Rupelian).
The upper part of the Ranzano Formation (Di
Giulio and Galbiati, 1995) together with the Rigoroso
Marls will be described in the following as UNIT S3
(middle–late Rupelian). The first formation (which
includes the S. Sebastiano and Curone members) is
characterised by siliciclastic turbidites while the
second is characterised by hemipelagic sediments
(Di Giulio, 1991; Di Giulio and Galbiati, 1995;
Martelli et al., 1998). In Chattian time, a lenticular
B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203
sandstone unit (Nivione sandstone; Cavanna et al.,
1989) was deposited within the upper part of the
Rigoroso Marl. These sediments will be referred in the
following as UNIT S4 (Chattian).
3. Techniques
Single grains of white mica were separated from 12
samples of the Eocene–Oligocene units of the eastern
TPB clastic sequence (Fig. 2; Table 1) and analysed
by electron microprobe and 40Ar/39Ar analyses (single
fusion and step heating). Samples have been grouped
in formations belonging to different sequences following the scheme of Fig. 3.
3.1. Mineral chemistry
Metamorphic pressure conditions and mineral paragenesis influence the degree of substitution of Si+(Mg,
Fe) in phengite (Massonne and Schreyer, 1987; Velde,
1965, 1967). When the source rocks have experienced
different metamorphic histories, phengites can be used
to examine the provenance of clastic sediments. Ten
grains (250–500 Am) from each sample were analysed
with electron microprobe analyses for a total of over
200 analyses. Samples were disaggregated by mixing
with 10% HNO3 and 10% Na-pyrophosphate and
suspension in an ultrasound bath. After sieving, flat
white micas were separated from the 0.25–0.5 and 0.5–
1.0 mm fractions by using a Faul- (vibration) table and
final handpicking.
Chemical analyses of separated mineral phases were
performed on a JEOL JX-A8800M electron microprobe. Raw data corrections were done with JEOL online ZAF-correction program (refer to Reed, 1993 for
more details) and atomic ratios have been calculated
for 20 oxygens and 4 OH, F and Cl per formula unit.
The standards used are Na-jadeite, Mg, Si, Ca-diopside, Al-syntheic Al2O3, K-orthoclase, Ti-ilmenite, Feolivine and Ba-barium-aluminate glass (Fig. 4).
3.2.
40
40
Ar/ 39Ar geochronology
Ar/39Ar single fusion laser analyses on single
grain white micas (250–500 Am) were performed on
10–20 grains from each sandstone sample and up to 5
grains from each cobble (UNITS 1 and 2; Table 2).
40
187
Ar/39Ar step heating experiments were performed
on selected single grains (250–1000 Am) from
metamorphic cobbles, when the total fusion population was unclear, to check on Ar homogeneities. Only
experiments concordant within 95% confidence intervals, i.e. MSWDb2.5, have been used to derive
plateau ages.
The 40Ar/39Ar experiments were carried out with
the VULKAAN laserprobe facility at the Isotope
Geology Laboratory of the Vrije Universiteit in
Amsterdam following laser extraction and mass
spectrometry methods for this facility described by
Wijbrans et al. (1995). The irradiation facility used for
this project was the cadmium-lined RODEO facility
of the HFR reactor of the ECN/JRC reactor facility in
Petten, the Netherlands. Irradiation time was 7 h.
Correction factors for interferences of Ca and K
isotopes were 0.000699 for 39Ar/37Ar, 0.000270 for
36
Ar/37Ar and 0.00183 for 40Ar/39Ar, respectively.
These values were determined using zero age Kfeldspar and anorthite glass. After irradiation, a J
curve was derived for individual samples by interpolation between five single fusion experiments on
every flux monitor. DRA sanidine (Steenbrink et al.,
1999) was used as the flux monitor standard for this
project, with an age of 25.26F0.14 Ma. These values
are compatible with the set of Renne et al. (1998),
based on biotite GA1550 (at K/Ar age of 98.79F0.69
Ma). In the present study, system blanks were
determined after every five unknowns. The unknowns
were corrected for the interpolated blank at the time of
analysis of the unknown and the 2r error on the blank
was further used for the error calculation of the
unknown. 40Ar intensities for the analysed samples
were in the order of N100 times the blanks (see
Wijbrans et al., 1995 for further details on mass
spectrometer sensitivity). The discrimination factor
was on average equal to 1.059F0.04% (see Kuiper,
2003 for further details on discrimination factor
calculation). Note that the 2r errors reported in Table
2 do not include the uncertainties in J and uncertainties related to the age of the standards (the average of
J related errors is in the order of 0.3%). The exclusion
of the J related errors in the analytical errors reported
in Table 2 enables a better comparison between
samples (Foland, 1983). For further details on the
calculation of the ages and related errors reported in
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
Table 2
Total fusion
189
40
Ar/39Ar data from clastic phengites of the study samples
Sample
36
37
Ar(a)
Ar(ca)
38
Ar(cl)
39
Ar(k)
40
Ar(r)
Age 2r (Ma)
40
Ar(k)
(%)
Ar (%)
39
S1 (Ranzano Fm.; sst.)
East (B12);
03M0060A
J=0.002041
03M0060B
03M0060C
03M0060D
03M0060E
03M0060G
03M0060H
03M0060I
03M0060J
03M0060K
East (B30);
03M0070A
J=0.001974
03M0070B
03M0070C
03M0070D
03M0070E
03M0070G
03M0070H
03M0070I
03M0070J
03M0070K
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
0.00001
0.00007
0.00003
0.00001
0.00011
0.00001
0.00002
0.00003
0.00003
0.00004
0.00003
0.00003
0.00001
0.00006
0.00002
0.00001
0.00001
0.00003
0.00002
0.00001
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00005
0.00000
0.00003
0.00001
0.00000
0.00013
0.00000
0.00000
0.00000
0.00016
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.03714
0.01908
0.03325
0.02911
0.02843
0.05352
0.02005
0.02394
0.02417
0.05308
0.05180
0.02828
0.02113
0.04651
0.03810
0.03503
0.03353
0.02715
0.02362
0.03318
3.03400
1.22461
2.49720
2.42997
2.77676
5.35631
1.36525
1.95348
1.78793
4.36623
4.45394
2.44666
1.82437
3.71180
3.26997
2.80857
2.99593
2.35076
2.02450
2.98124
278.20F8.03
222.06F14.60
257.27F8.68
283.85F10.31
327.89F12.08
335.35F6.32
234.81F13.75
277.94F12.40
253.70F11.83
280.00F5.71
282.81F5.54
284.45F9.76
283.98F13.04
263.95F6.03
282.38F7.36
265.10F7.92
293.04F8.26
284.69F10.12
282.00F11.69
294.57F8.37
99.93
98.42
99.65
99.85
98.81
99.95
99.57
99.54
99.51
99.74
99.79
99.63
99.82
99.56
99.86
99.91
99.90
99.59
99.73
99.88
11.54
5.93
10.33
9.05
8.84
16.63
6.23
7.44
7.51
16.50
15.31
8.36
6.24
13.75
11.26
10.35
9.91
8.02
6.98
9.81
S1 (Pianfolco Cgl.; sst.)
West (B17);
03M0075A
J=0.002031
03M0075B
03M0075D
03M0075G
03M0075J
03M0075K
03M0062A
03M0062B
03M0062D
03M0062E
03M0062G
West (B15);
03M0061A
J=0.002034
03M0061D
03M0061E
03M0061G
03M0061K
03M0076A
03M0076B
03M0076C
03M0076D
03M0076E
03M0076G
03M0076J
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
0.00178
0.00276
0.00071
0.00097
0.00166
0.00089
0.00100
0.00041
0.00025
0.00038
0.00099
0.00167
0.00049
0.00279
0.00063
0.00064
0.00478
0.00069
0.00021
0.00072
0.00063
0.00097
0.00294
0.00000
0.00000
0.00134
0.00000
0.00000
0.00000
0.00046
0.00134
0.00000
0.00000
0.00240
0.00412
0.00124
0.00046
0.00034
0.00148
0.00173
0.00061
0.00000
0.00038
0.00135
0.00106
0.00327
0.00175
0.00039
0.00287
0.00263
0.00306
0.00327
0.00237
0.00167
0.00056
0.00167
0.00273
0.00154
0.00092
0.00119
0.00110
0.00211
0.00193
0.00150
0.00000
0.00079
0.00113
0.00082
0.00207
0.19678
0.05417
0.34320
0.30159
0.35631
0.37678
0.27228
0.17393
0.06203
0.20702
0.26128
0.15618
0.10736
0.14204
0.14023
0.26507
0.23308
0.19852
0.04887
0.12582
0.19541
0.12796
0.21674
2.55970
0.84780
4.02750
4.13745
5.75315
4.39114
3.41735
2.33581
0.80085
2.70084
3.44049
2.04131
1.56148
1.91656
1.80585
3.07169
3.28025
2.09577
0.67945
1.67119
2.42456
1.50457
3.03364
47.04F1.59
56.45F6.67
42.49F0.96
49.58F0.95
58.21F0.79
42.21F0.90
45.41F1.34
48.55F2.04
46.69F5.40
47.18F1.47
47.61F1.44
47.33F1.53
52.59F3.04
48.85F2.30
46.65F2.20
42.03F1.40
50.92F2.03
38.33F2.31
50.31F9.04
48.09F3.77
44.96F2.25
42.64F3.32
50.64F1.91
82.92
50.94
95.03
93.50
92.12
94.31
92.00
95.07
91.60
95.98
92.18
80.51
91.45
69.92
90.59
94.20
69.88
91.16
91.51
88.75
92.86
83.95
77.75
12.08
3.33
21.07
18.52
21.88
23.13
27.88
17.81
6.35
21.20
26.76
19.26
13.24
17.52
17.29
32.69
20.29
17.28
4.25
10.95
17.01
11.14
18.87
S1 (Pianfolco Cgl.; cbl.)
Center (B22);
03M0049A
J=0.002014
03M0049B
03M0049C
03M0049D
03M0049E
fsn
fsn
fsn
fsn
fsn
0.00064
0.00059
0.00037
0.00092
0.00079
0.00193
0.00220
0.00372
0.00377
0.00314
0.00081
0.00140
0.00248
0.00225
0.00194
0.09168
0.16569
0.25146
0.24931
0.20877
0.92555
1.67484
2.48844
2.37759
1.91100
36.31F3.18
36.36F1.60
35.60F1.37
34.32F1.03
32.96F1.43
83.09
90.59
95.72
89.74
89.04
9.48
17.14
26.01
25.78
21.59
(continued on next page)
190
B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203
Table 2 (continued)
Sample
36
37
38
39
Ar(a)
Ar(ca)
Ar(cl)
Ar(k)
40
Ar(r)
Age 2r (Ma)
40
39
Ar(k)
(%)
Ar (%)
S1 (Pianfolco Cgl.; cbl.)
Center (B21);
03M0048A
J=0.002018
03M0048B
03M0048C
03M0048D
03M0048E
Center (B20);
03M0047A
J=0.002021
03M0047B
03M0047C
03M0047D
03M0047E
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
0.00253
0.00257
0.00048
0.00322
0.00145
0.00064
0.00022
0.00135
0.00045
0.00043
0.00341
0.00188
0.00000
0.00236
0.00000
0.00000
0.00001
0.00000
0.00000
0.00185
0.00318
0.00200
0.00027
0.00196
0.00133
0.00124
0.00140
0.00137
0.00075
0.00084
0.35714
0.23422
0.05336
0.22145
0.13730
0.11900
0.13382
0.18701
0.08985
0.12378
9.40811
8.97329
2.08339
7.50164
5.89451
1.36381
1.32208
2.38221
0.94290
1.15260
93.44F0.92
134.35F1.20
136.83F3.88
119.29F1.39
149.89F2.51
41.31F1.57
35.67F1.60
45.86F1.21
37.86F3.25
33.63F1.51
92.64
92.18
93.61
88.73
93.22
87.80
95.21
85.63
87.60
90.10
35.59
23.34
5.32
22.07
13.68
18.21
20.48
28.62
13.75
18.94
S2 (Savignone Cgl.; cbl.)
Center (B27);
03M0052A
J=0.001992
03M0052B
03M0052C
03M0052D
03M0052E
Center (B24);
03M0051A
J=0.002005
03M0051B
equivalent
03M0051C
to B26
03M0051D
03M0051E
Center (B23);
03M0050A
J=0.002010
03M0050B
03M0050C
03M0050D
03M0050E
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
0.00045
0.00021
0.00059
0.00088
0.00047
0.00126
0.00071
0.00079
0.00069
0.00057
0.00095
0.00085
0.00072
0.00111
0.00078
0.00045
0.00032
0.00019
0.00014
0.00200
0.00182
0.00122
0.00123
0.00000
0.00000
0.00252
0.00168
0.00101
0.00300
0.00130
0.00163
0.00057
0.00127
0.00181
0.00183
0.00114
0.00101
0.00177
0.00141
0.00098
0.00318
0.00257
0.00361
0.00255
0.00298
0.15029
0.02927
0.13930
0.15671
0.16502
0.15373
0.09923
0.21403
0.15933
0.09276
0.36746
0.29450
0.36829
0.28399
0.27404
1.49744
0.32038
1.52935
1.81247
1.94240
2.64021
2.39473
3.17958
3.06206
2.58411
4.90590
3.74685
4.84758
3.63875
3.57013
35.46F1.72
38.92F9.53
39.03F1.89
41.09F1.52
41.81F1.37
61.08F2.50
85.25F3.89
52.95F1.75
68.21F2.39
98.05F3.84
47.77F0.87
45.55F1.00
47.11F0.81
45.87F1.13
46.63F1.21
91.86
83.81
89.72
87.41
93.33
87.66
91.93
93.17
93.77
93.87
94.57
93.70
95.78
91.71
93.95
23.46
4.57
21.75
24.46
25.76
21.38
13.80
29.76
22.16
12.90
23.14
18.54
23.19
17.88
17.25
S2 (Borbera Cgl.; cbl.)
Center–east (B35); 03M0054A
J=0.001945
03M0054B
03M0054C
03M0054D
03M0054E
Center–east (B34); 03M0053A
J=0.001953
03M0053B
03M0053C
03M0053D
03M0053E
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
0.00161
0.00138
0.00124
0.00197
0.00247
0.00308
0.00115
0.00117
0.01367
0.00167
0.00160
0.00163
0.00008
0.00097
0.00140
0.00023
0.00034
0.00024
0.00196
0.00093
0.00059
0.00102
0.00098
0.00113
0.00045
0.00072
0.00058
0.00027
0.00082
0.00087
0.04939
0.10281
0.09234
0.07778
0.06662
0.06698
0.06153
0.03984
0.06544
0.07122
0.45798
1.00034
0.88954
0.70277
0.66088
0.86390
0.83094
0.56358
0.69786
0.88177
32.25F5.06
33.82F2.34
33.49F3.06
31.43F3.53
34.48F4.59
44.88F3.61
46.96F3.44
49.16F6.19
37.19F3.99
43.10F3.27
48.99
70.97
70.75
54.74
47.49
48.71
70.99
61.90
14.73
64.14
12.70
26.43
23.74
20.00
17.13
21.96
20.17
13.06
21.46
23.35
S2 (Borbera Cgl.; sst.)
East (B33);
03M0071A
J=0.001957
03M0071B
03M0071C
03M0071D
03M0071E
03M0071G
03M0071H
03M0071I
03M0071J
03M0071K
03M0088B
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
0.00083
0.00006
0.00004
0.00005
0.00006
0.00003
0.00083
0.00006
0.00003
0.00057
0.00016
0.00002
0.00000
0.00000
0.00000
0.00000
0.00000
0.00174
0.00000
0.00000
0.00221
0.00000
0.00081
0.00009
0.00000
0.00002
0.00000
0.00000
0.00149
0.00013
0.00000
0.00176
0.00000
0.09124
0.03808
0.02042
0.03169
0.02484
0.01734
0.15101
0.04535
0.02041
0.14162
0.02387
6.29878
3.67479
1.76263
2.72097
2.06326
1.51895
9.97363
3.88482
1.97022
10.05960
1.84995
228.62F3.44
312.09F7.12
281.56F13.13
280.25F8.53
271.77F10.86
285.39F15.48
219.29F2.61
279.63F6.18
312.16F13.21
234.83F2.47
254.75F9.92
96.23
99.51
99.30
99.43
99.09
99.39
97.59
99.57
99.58
98.35
97.57
15.68
6.54
3.51
5.44
4.27
2.98
25.95
7.79
3.51
24.33
7.10
B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203
191
Table 2 (continued)
Sample
36
37
Ar(a)
Ar(ca)
38
Ar(cl)
39
Ar(k)
40
Ar(r)
Age 2r (Ma)
40
39
Ar(k)
(%)
Ar (%)
S2 (Borbera Cgl.; sst.)
East (B33);
03M0088D
J=0.001957
03M0088E
03M0088F
03M0088H
03M0088I
03M0088J
03M0088K
03M0088L
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
0.00011
0.00002
0.00042
0.00006
0.00002
0.00012
0.00002
0.00004
0.00310
0.00000
0.00618
0.00000
0.00000
0.00000
0.00000
0.00000
0.00079
0.00024
0.00125
0.00011
0.00002
0.00028
0.00003
0.00008
0.08718
0.02666
0.08426
0.02680
0.01322
0.04476
0.01405
0.01557
7.13917
2.23459
8.56981
2.46767
1.32388
3.51278
1.14056
1.48684
268.17F3.45
274.00F9.01
327.49F3.70
298.93F8.70
322.97F16.88
257.77F5.73
266.04F16.09
309.02F14.22
99.53
99.77
98.57
99.28
99.55
98.96
99.60
99.29
25.92
7.93
25.05
7.97
3.93
13.31
4.18
4.63
S3 (Ranzano Sst.; sst.)
East (B28);
03M0068A
J=0.001985
03M0068B
03M0068C
03M0068D
03M0068E
03M0069B
03M0069C
03M0069D
03M0069E
03M0069F
03M0069G
03M0069I
03M0069J
03M0069K
03M0069L
East (B11);
03M0059A
J=0.002042
03M0059B
03M0059C
03M0059D
03M0059E
03M0059G
03M0059H
03M0059I
03M0059J
03M0059K
East (B9);
03M0058A
J=0.002044
03M0058B
03M0058C
03M0058D
03M0058E
03M0058G
03M0058H
03M0058I
03M0058J
03M0058K
East (B6);
03M0057A
J=0.002045
03M0057B
03M0057C
03M0057D
03M0057E
03M0057G
03M0057H
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
0.00012
0.00010
0.00029
0.00006
0.00004
0.00016
0.00004
0.00006
0.00033
0.00005
0.00003
0.00107
0.00004
0.00006
0.00004
0.00023
0.00005
0.00002
0.00004
0.00004
0.00032
0.00050
0.00001
0.00035
0.00001
0.00101
0.00039
0.00042
0.00047
0.00010
0.00051
0.00083
0.00019
0.00007
0.00004
0.00022
0.00027
0.00067
0.00060
0.00071
0.00042
0.00046
0.00186
0.00072
0.01105
0.00000
0.00000
0.00016
0.00130
0.00000
0.00098
0.00000
0.00000
0.00770
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00076
0.00000
0.00000
0.00000
0.00000
0.00120
0.00086
0.00114
0.00125
0.00000
0.00000
0.00033
0.00069
0.00000
0.00000
0.01004
0.00118
0.00139
0.00260
0.00181
0.00491
0.00210
0.00019
0.00014
0.00136
0.00010
0.00012
0.00145
0.00137
0.00000
0.00116
0.00000
0.00000
0.00114
0.00000
0.00000
0.00000
0.00132
0.00000
0.00000
0.00000
0.00000
0.00088
0.00109
0.00000
0.00114
0.00000
0.00344
0.00202
0.00234
0.00161
0.00014
0.00267
0.00153
0.00000
0.00024
0.00004
0.00067
0.00029
0.00198
0.00288
0.00076
0.00016
0.00029
0.01931
0.01561
0.11417
0.01962
0.01940
0.12389
0.12052
0.02149
0.08783
0.02211
0.01692
0.11989
0.01513
0.02131
0.01632
0.08926
0.01144
0.01465
0.01502
0.01472
0.09160
0.11729
0.01990
0.08242
0.01315
0.34485
0.19012
0.28784
0.17460
0.03030
0.29152
0.14998
0.01844
0.04110
0.02737
0.03715
0.02152
0.15924
0.29949
0.03309
0.01381
0.04223
1.59762
1.22264
9.59835
1.64379
1.65209
10.53177
9.96291
1.79337
7.36006
1.74644
1.44583
9.74776
1.29492
1.84004
1.46131
7.26115
1.10641
1.11572
1.21112
1.21300
7.35693
9.64263
1.71088
6.49275
1.02952
7.87230
3.01841
2.93033
2.55115
1.40156
3.47107
4.16446
0.24942
1.36883
1.80687
0.45516
0.20147
3.11400
8.26834
0.34924
0.17452
0.45422
274.37F14.74
260.66F18.29
278.42F3.28
277.60F14.77
281.77F14.88
281.31F3.20
274.12F2.80
276.48F9.92
277.60F3.37
262.80F9.65
282.61F12.48
269.92F2.48
283.14F13.64
285.37F10.06
295.11F12.66
277.26F3.46
325.05F14.37
260.75F10.19
274.98F10.06
280.58F10.42
273.98F3.36
279.98F2.93
291.82F7.70
269.09F3.54
267.60F15.00
82.27F1.24
57.62F2.18
37.16F1.45
53.09F2.44
162.97F11.37
43.38F1.51
99.59F2.69
49.21F20.12
118.80F8.53
228.32F12.45
44.65F10.67
34.21F17.63
70.74F2.41
99.08F1.39
38.53F12.04
46.02F21.96
39.25F8.10
97.84
97.70
99.11
98.89
99.33
99.55
99.89
99.00
98.67
99.13
99.42
96.87
99.11
99.06
99.16
99.06
98.80
99.34
99.15
99.11
98.72
98.50
99.80
98.43
99.84
96.35
96.30
95.93
94.82
97.98
95.80
94.45
81.82
98.56
99.27
87.33
71.81
94.05
97.90
62.33
58.47
77.15
10.26
8.30
60.70
10.43
10.31
21.91
21.32
3.80
15.53
3.91
2.99
21.20
2.68
3.77
2.89
19.01
2.44
3.12
3.20
3.14
19.51
24.98
4.24
17.56
2.80
22.16
12.22
18.50
11.22
1.95
18.73
9.64
1.18
2.64
1.76
3.00
1.74
12.87
24.21
2.67
1.12
3.41
(continued on next page)
192
B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203
Table 2 (continued)
Sample
36
37
38
39
Ar(a)
Ar(ca)
Ar(cl)
Ar(k)
40
Ar(r)
Age 2r (Ma)
40
39
Ar(k)
(%)
Ar (%)
S3 (Ranzano Sst.; sst.)
East (B6);
03M0057I
J=0.002045
03M0057J
03M0057K
fsn
fsn
fsn
0.00057
0.00024
0.00059
0.00179
0.00449
0.00154
0.00149
0.00271
0.00117
0.18201
0.29786
0.15076
2.37501
3.25029
1.45921
47.51F1.57
39.82F1.05
35.36F2.26
93.33
97.84
89.34
14.71
24.08
12.19
S4 (Rigoroso Marls; sst.)
East (B2);
03M0055A
J=0.002040
03M0055B
03M0055C
03M0055D
03M0055E
03M0055G
03M0055H
03M0055I
03M0055J
03M0055K
03M0073A
03M0073B
03M0073C
03M0073D
03M0073E
03M0073G
03M0073H
03M0073I
03M0073J
03M0073K
03M0073M
03M0073N
03M0073O
03M0073P
03M0073Q
East (B4);
03M0056A
J=0.002043
03M0056B
03M0056C
03M0056D
03M0056E
03M0056G
03M0056H
03M0056I
03M0056J
03M0074A
03M0074B
03M0074C
03M0074D
03M0074E
03M0074G
03M0074H
03M0074I
03M0074J
03M0074K
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
fsn
0.00084
0.00051
0.00030
0.00016
0.00008
0.00088
0.00016
0.00053
0.00081
0.00020
0.00066
0.00029
0.00091
0.00068
0.00059
0.00038
0.00008
0.00055
0.00100
0.00107
0.00093
0.00065
0.00010
0.00012
0.00078
0.00093
0.00076
0.00041
0.00107
0.00078
0.00123
0.00013
0.00025
0.00069
0.00029
0.00015
0.00030
0.00002
0.00014
0.00071
0.00219
0.00141
0.00012
0.00078
0.00313
0.00208
0.00105
0.00000
0.00130
0.00101
0.00992
0.00078
0.00069
0.00259
0.00339
0.00412
0.00472
0.00344
0.00276
0.00161
0.00000
0.00295
0.00376
0.00113
0.00208
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00028
0.00000
0.00000
0.00075
0.00000
0.00000
0.00013
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.00204
0.00242
0.00217
0.00183
0.00018
0.00250
0.00018
0.00212
0.00191
0.00005
0.00291
0.00010
0.00248
0.00216
0.00138
0.00219
0.00003
0.00117
0.00231
0.00126
0.00230
0.00105
0.00002
0.00006
0.00366
0.00282
0.00113
0.00093
0.00191
0.00660
0.00331
0.00037
0.00032
0.00127
0.00013
0.00014
0.00066
0.00029
0.00036
0.00313
0.00400
0.00411
0.00045
0.00201
0.23805
0.21978
0.23797
0.21054
0.00685
0.27293
0.02283
0.25406
0.21812
0.01290
0.36332
0.01399
0.30231
0.19051
0.18354
0.23607
0.03017
0.10776
0.24870
0.16060
0.24854
0.11597
0.02813
0.02329
0.36407
0.29736
0.13792
0.11926
0.24622
0.64402
0.31884
0.04954
0.04830
0.07132
0.04444
0.03940
0.02955
0.04132
0.05001
0.25615
0.32353
0.40582
0.04729
0.20022
8.07743
4.28548
5.24255
2.53559
0.12027
5.50094
0.30768
6.22235
6.32665
0.20299
8.82527
0.13709
9.32382
4.22113
8.07817
4.63778
1.33391
4.45654
6.21242
4.98743
5.71147
3.86715
0.42794
1.14688
6.42049
8.20864
4.96625
1.23729
9.72948
10.29273
10.06676
1.36823
1.84916
1.01240
2.14746
2.08127
0.33967
1.14259
1.56819
9.89705
9.17952
5.92577
1.81794
4.68784
120.75F1.18
70.37F1.19
79.31F1.18
43.79F1.25
63.46F30.87
72.69F0.87
48.94F11.02
87.96F0.93
103.71F2.29
57.01F38.36
87.25F1.22
35.70F21.49
110.08F0.97
79.76F1.72
155.12F1.79
70.89F1.25
155.78F8.33
146.12F3.02
89.67F1.41
110.82F2.01
82.65F1.27
118.74F2.83
55.13F8.87
172.67F10.89
63.76F0.92
98.98F0.89
128.06F1.99
37.84F2.64
140.06F1.18
57.96F0.54
112.77F0.98
99.03F3.89
135.87F3.75
51.58F3.47
169.85F9.59
184.88F10.88
41.87F16.11
99.15F10.61
112.03F8.65
137.07F1.83
101.66F1.50
53.03F1.13
136.40F9.09
84.29F2.37
97.02
96.57
98.35
98.17
83.58
95.49
86.80
97.56
96.34
77.16
97.84
61.89
97.19
95.43
97.87
97.63
98.20
96.47
95.45
94.02
95.40
95.25
93.59
97.00
96.52
96.76
95.67
91.14
96.85
97.79
96.50
97.17
96.08
83.30
96.17
97.95
79.38
99.52
97.39
97.92
93.40
93.40
98.06
95.28
14.05
12.97
14.05
12.43
0.40
16.11
1.35
15.00
12.88
0.76
13.88
0.53
11.55
7.28
7.01
9.02
1.15
4.12
9.50
6.14
9.50
4.43
1.08
0.89
13.91
15.39
7.14
6.17
12.74
33.32
16.50
2.56
2.50
3.69
3.09
2.74
2.06
2.87
3.48
17.82
22.50
28.23
3.29
13.93
B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203
Probability distribution diagrams (Sircombe, 1999;
Sircombe, 2000) have been used to identify the main
populations of detrital ages present in different units of
the studied sediments. The probability distribution
curves are compiled by summing the Gaussian
distribution of each individual measurement, which is
defined by the age and its error (e.g. Sircombe, 2000).
The ages obtained from the eastern TPB clastic
phengites are interpreted to represent the time of
isotopic closure during cooling of the crystalline source
through 350–420 8C (e.g. Hames and Bowring, 1994;
Kirschner et al., 1996; von Blanckenburg et al., 1989).
Because the shape of minerals influences diffusion,
grain size can have effect on cooling ages (e.g.
McDougall and Harrison, 1999). 40Ar/39Ar ages from
different grain sizes of a micaschist cobble derived by
the Ligurian Alps show no dependency between grain
size and Oligocene 40Ar/39Ar ages. These results
indicate that for fast cooling rocks (i.e. Voltri Group)
grain size does not significantly affect cooling ages.
4. Mica chemistry
Electron microprobe data show two distinct groups
of mica compositions for the analysed samples. One
group has a Si content ranging from 6.5 to 8 pfu and
Mg content ranging from 0.3 to 1.2 pfu; the other
group has a Si content between 6 and 6.5 pfu, and a
Mg content between 0.05 and 0.3 pfu. The first
composition (with general values of SiN7 pfu and
MgN0.5 pfu) is characteristic of high-pressure (HP)
rocks of the Voltri Group and Montenotte Nappe
(Ligure–Piemontese domain) where similar values
have been recorded previously (e.g. Barbieri et al.,
2003). The second composition, with general values
of Sib6.5 pfu and of Mgb0.5 pfu is typical of lowpressure (LP) rocks of the Briançonnais domain
193
(Barbieri et al., 2003; Cimmino et al., 1981).
Chemical data for UNIT S1 reflect a low-pressure
source with Si content between 6 and 6.5 pfu, and Mg
content between 0.05 and 0.3 pfu. The Pianfolco
Conglomerates record high-pressure mica composition with Si content between 6.5 and 8 pfu, and Mg
content between 0.4 and 1.2 pfu. Chemical data from
UNIT S1 and from the Pianfolco Conglomerates
samples directly reflect the composition of the source
outcropping at the time of deposition.
UNITS S2 and S3 both contain low- and highpressure phengites. This composition could be due to
the contribution of a primary source or it could be the
result of reworking. In particular, sample B33 from
UNIT S2 shows a signal that is very similar to that of
the underlying Ranzano Formation (B30, B12 of
UNIT S1; Pzo. D’Oca Member) and therefore could
be the result of recycling of this older material. The
predominance of carbonate cobbles in the Borbera
Conglomerates (UNIT S2), which form the framework of the rock where the sandstone matrix was
sampled, suggests erosion of the underlying sedimentary units, which are mainly carbonatic in composition
(Di Giulio, 1991). This reworking could be due to
erosion following a sea-level drop which occurred at
the Eocene–Oligocene boundary and which was
registered by an unconformity and locally by the
deposition of shallow marine sediments (Rio Trebbio
Sst.; Cavanna et al., 1989; Di Giulio, 1991). Deposits
from UNIT S3 in the study area unconformably cover
the deposits of UNITS S1 and S2. Therefore the
chemical composition of samples from UNIT S3
could be partially due to recycling of the older
sediments.
Results from UNIT S4 show only high-pressure
mica with Si content ranging from 6.5 to 7.5 pfu and
Mg content between 0.3 and 1.2 pfu. This composition 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
36
Ar; 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
underlying sediments, HP micas would also be
expected in UNIT S4. Therefore it is more likely
that the mica composition of UNIT S4 reflects a
primary source.
5.
40
Ar/39Ar geochronology
5.1. Single fusion ages
Sediments from UNIT S1 produce only Permian–
Carboniferous micas with ages between 222.06F14.6
and 335.35F6.3 Ma.
The Pianfolco Conglomerates show two groups
of ages, one Oligocene–Eocene with ages ranging
between 33F1.4 and 58.2F0.8 Ma and the other
Cretaceous with ages ranging between 93.4F0.9
and 149.9F2.5 Ma. The Eocene–Oligocene group
of ages is recorded in sandstones B15 and B17 and
in cobbles B20 and B22 while cobble B22 records
Cretaceous ages. Sample B20 is a metasedimentary
rock with blueschist-facies metamorphism; single
fusion experiments on five grains yielded ages
between 33.6F1.5 and 45.9F1.2 Ma. Sample B22
is a metasedimentary rock with upper greenschistfacies metamorphic grade; five single fusion experiments gave ages between 33F1.4 and 36.4F1.6
Ma. Sample B21 is a metasedimentary rock with
faint greenschist-facies metamorphic grade; five
single fusion experiments gave mainly Cretaceous
ages between 93.4F1 and 149.9F2.5 Ma. Samples
B30 and B12 from sandstones of the Ranzano
Formation (Pzo. D’Oca member) record Permian–
Carboniferous ages between 222.1F14.6 and
335.3F6.3 Ma.
Samples from UNIT S2 show two main age
groups, one between 219.3F2.6 327.5F3.7 Ma and
the other between 31.4F3.5 and 49.2F6.2 Ma but
few ages around 90–100 Ma are also present.
Sample B33, which is from a sandstone matrix of
the Borbera Conglomerates in the eastern sector (Fig.
5), recorded Permo-Carboniferous ages ranging from
228.6F3.4 to 323F16.9 Ma. Cobbles from this unit,
however, recorded mainly Eocene–Oligocene ages
(Fig. 5). Single fusion experiments on sample B23,
which is a metasedimentary rock with blueschist
facies metamorphic grade, yielded ages between
45.5F1 and 47.8F1 Ma. Sample B24 is a metasedimentary rock with upper greenschist-facies metamorphic grade; five single fusion experiments gave ages
ranging between 52.9F1.7 and 98F3.8 Ma. Sample
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 metasedimentary rock with greenschist-facies metamorphism
and five single fusion experiments give ages between
37.2F4 and 49.2F6.2 Ma. Single fusion experiments
on sample B35, which is a metasedimentary rock
(calcschist) with greenschist-facies metamorphic
grade, yielded ages between 31.4F3.5 and 34.5F
4.6 Ma.
Samples from UNIT S3 recorded essentially the
same group of ages as samples from UNIT S2 but in
different proportions. Four samples have been
analysed from the uppermost member of the Ranzano Formation (S. Sebastiano Curone Member),
taken from the eastern sector of the study area (Fig.
3; see also Di Giulio and Galbiati, 1995). Samples
B28 and B11 show mainly Permo-Carboniferous
ages between 260.7F18.3 and 325F14.4 Ma (Fig.
5). Samples B9 and B6 from the depocentre of the
basin record ages ranging from 34.2F17.6 to
228.3F12.4 Ma with a greater proportion of Eocene
ages.
Samples from UNIT S4, from the Rigoroso Marls,
record ages ranging from 37.8F2.6 to 184.9F10.9
Ma. It is not possible to see a distinctive population.
Ages between 100 and 160 Ma constitute a large part
of the total signal and these ages have not been found
in the underlying older units.
5.2. Step heating ages
Eleven step heating experiments on metasedimentary cobbles from the Pianfolco (B20, B21;
UNIT S1) and Savignone Conglomerates (B26)
have been conducted. Four step heating experiments
(0083, 0084, 0093, 0094) on sample B20 from
UNIT S1 (blueschist-facies metamorphism) all gave
plateau ages between 40 and 46 Ma (Fig. 6a),
suggesting a homogeneous Eocene signal. Three
step heating experiments (0077, 0089, 0090; Fig.
6b) on sample B21 (greenschist-facies metamorphism) gave plateau or plateau-like ages between
124 and 129 Ma. Experiment 0077 yielded a
B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203
Fig. 5. Cumulative probability curves of
Giulio (1991) reported in Fig. 3.
40
195
Ar/39Ar detrital ages from the selected samples divided in sequences following the scheme of Di
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). Experiments 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
Fig. 6 (continued).
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 lowpressure continental basement overlying the Penninic
belt at some stage (Briançonnais 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;
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 dé 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)
Both the chemical and the 40Ar/39Ar data suggest
three different sources for the sediments of UNIT S2:
two main high-pressure (SiN7 pfu) sources characterised by 40Ar/39Ar ages between 31 and 56 Ma, and
by ages between 100 and 150 Ma, respectively, and a
third low-pressure source characterised by PermoCarboniferous ages between 219 and 327 Ma.
The main Mesoalpine population suggests a
provenance from the Voltri and Montenotte Nappe
B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203
affected by high-pressure metamorphism in the middle Eocene with retrograde metamorphic overprint in
the late Eocene–Oligocene (e.g. Barbieri et al., 2003;
Carrapa et al., 2004) (Fig. 7b). This interpretation is
also supported by the chemical data which are very
similar to those reported for the Voltri Group rocks
(Cimmino and Messiga, 1979). The same range of
ages recorded in sediments from UNIT S2 is also
recorded in Oligocene sediments of the southern TPB
(Barbieri et al., 2003). The large span of total fusion
ages and step heating results within single cobbles of
greenschist metamorphic grade (e.g. B21, B26) reflect
the incomplete resetting of isotope systems in rocks
that have experienced only low to intermediate
metamorphic temperatures (e.g. Wijbrans and McDougall, 1986; Scaillet et al., 1992; Leeps et al., 1999). The
observed age ranges might be interpreted as evidence
for several distinct scenarios:
–
–
–
All total fusion ages are related to real cooling
events and the discrepancy between them could be
due to chemistry. For example Mg-rich phengite
could retain Cretaceous ages as observed by
Scaillet et al. (1992).
Old ages are representative of real cooling while
the young ages (e.g. ~90 Ma) are due to a partial
opening of the system during later metamorphic
events or to deformation-induced argon loss
during denudation. Both of these processes will
be referred to in the following as Ar loss.
Younger ages are representative of real cooling
events while older ages (150 Ma) are disturbed
ages due to alteration, inherited argon (refer to
Dalrymple and Lanphere, 1969; Wijbrans and
McDougall, 1986; Singer et al., 1998), or excess
argon (refer to Dalrymple and Lanphere, 1969;
Reddy et al., 1996; Kelley, 2002). In the case of
inherited argon, the relationship between radioactive parent 40K and radiogenic daughter 40Ar is
maintained and therefore these ages can still be
geologically meaningful as the compounded
effects of geological events preceding the main
event of interest (e.g. Wijbrans and McDougall,
1986; Villa, 1998; Forster and Lister, 2003). In
case of excess Ar, the relationship between the
parent isotope 40K and its radiogenic daughter
isotope 40Ar is disturbed and therefore calculated
ages for minerals affected by excess argon are
199
meaningless. When excess Ar is incorporated in
existing crystals by volume diffusion, one may
expect an age spectrum characterised by anomalously high apparent ages in the first steps,
followed by a regular decreasing age pattern
where the final ages may be interpreted as a
maximum estimate for a geological event (crystallisation or cooling through the closure temperature) (e.g. Pankhurst et al., 1973; Harrison and
McDougall, 1981). We refer to Carrapa and
Wijbrans (2003) for an extended discussion on
excess versus inherited argon in detrital sediments
from sediments of the TPB since this is beyond
the scope of this paper.
Ages from cobbles B21 and B24 (B26) strongly
suggest the presence of an Eoalpine source (85–150
Ma) possibly related to the Sestri Voltaggio zone (e.g
Schamel, 1974). The sedimentary facies of these
deposits suggests a very proximal source area (Di
Biase and Pandolfi, 1999). This could imply that
during the Eocene, rocks were present (on top of the
Voltri Group) in the Ligurian Alps, which exhibited a
Cretaceous age signal (150–90 Ma). The same set of
ages has been detected by Zircon Fission Track
Thermochronology in the Ligurian Alps and in
western Corsica (Vance, 1999; Mailhé et al., 1986).
Also, the presence of Cretaceous ages that persist
throughout Oligocene–Miocene sediments of the
western-central TPB (Carrapa et al., 2004) suggests
that this signal could be geodynamically significant
(refer to Carrapa and Wijbrans, 2003 for further
details). These ages suggest a complicated Eoalpine
evolution of the Ligurian Alps as already suggested
for sediments sourced by the Western Alps (Carrapa
and Wijbrans, 2003). If these signals are considered
geologically meaningful, then older ages (~150 Ma)
could be attributed to the Tethyan thermal anomaly
related to the spreading of the Liguro–Piemontese
Ocean (e.g. Vance, 1999) while younger ages (130–
93) could be due to the onset of the Ligure–
Piemontese intraoceanic subduction (Hurford and
Hunziker, 1989; Oberhänsli et al., 1985; Carrapa
and Wijbrans, 2003). A Cretaceous age of 100–80 Ma
for the high-pressure (HP) metamorphism of the Voltri
Group has been proposed by Hoogerduijn Strating et
al. (1991) by analogy and comparison with rocks from
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).
However, our data are the first indication of middle to
late Cretaceous mica cooling ages in the Ligure–
Piemontese domain of the Western Alps.
The fact that the sandstone matrix (B33) of the
Borbera Conglomerates records Permian ages while
the cobbles record mainly Mesoalpine ages could
suggest that the cobbles are from a primary local
Mesoalpine source, whereas the sandstone has been
recycled from older sediments (Ranzano Formation)
as also suggested by the chemical data (Fig. 7b).
6.2.3. Late Rupelian (UNIT S3)
Chemical and 40Ar/39Ar data of samples from the
Ranzano Sst. indicate a dual source. One source is
characterised by low-pressure micas and Permian ages
(280 Ma) that may represent low-pressure Permian
covers and/or partial recycling from older sediments
(Fig. 7c). The second source is characterised by highpressure micas and Mesoalpine–Eoalpine ages. Ages
around 38 Ma can be attributed to the Voltri Group
and/or Montenotte Nappe (Fig. 7c).
Ages between 60 and 100 Ma can still be attributed
to the Sestri Voltaggio zone or (for ages ~100 Ma) to
rocks with western Alpine and Corsican affinity (e.g.
Hunziker and Martinotti, 1984; Cohen et al., 1981;
Maluski, 1977), outcropping in the Ligure–Piemontese domain during the Eocene–early Oligocene.
6.2.4. Chattian (UNIT S4)
Chemical and 40Ar/39Ar data of the samples from
the Rigoroso Marls in general show an even more
heterogeneous provenance than the late Rupelian
sediments. In particular, Eoalpine ages occur more
frequently than in the previous sequence and this
suggests either a new source characterised by Eoalpine ages (e.g. Western Alps) or a larger contribution
of potential Eoalpine rocks belonging to the Voltri
Group and Sestri Voltaggio zone. The main influx
attributed to the western Alpine domain starts in
approximately late Oligocene–early Miocene time
(Carrapa et al., 2004). This suggests that the Eoalpine
signal from UNIT S4 most probably derives from
western Alpine sources, which start to supply the TPB
and its eastern sectors already from Chattian time
onwards (Fig. 7d). Also, the disappearance of Permian
ages suggests that the nappe on top of the Voltri
Group was eroded completely by Chattian time.
7. Conclusions
Our data shed new light on the unroofing history of
the Alps–Apennine junction area in the earliest stages
of the Alpine orogeny. Chemical and geochronological data combined with petrographical data of the
studied sediments indicate a source area located
mainly in the area of the Voltri Group. From late
Priabonian till late Rupelian time, the sediments
deposited in the eastern part of the TPB record two
different sources: one of LP rocks, characterised by
Permian ages (270 Ma), and another of HP rocks,
characterised by Mesoalpine ages (32–50 Ma). Eoalpine ages (~80–100 Ma) are also present and can be
related to cooling following the onset of the Ligure–
Piemontese intra-oceanic subduction. These data are
here interpreted as recording the unroofing of the
tectonic nappe stack with first the erosion of LP
Penninic covers and later (post-Priabonian) of HP
Piedmont units. Permian ages are no longer present in
Chattian sediments, suggesting that LP rocks with
Permian ages once covering the Voltri Group were
completely eroded by that time. The greater span of
40
Ar/39Ar ages recorded in Chattian sediments also
suggests a provenance from western Alpine sources.
In addition, the almost indistinguishable 40Ar/39Ar
detrital age of 31.4F3.5 from sample B35, and
depositional age (early Rupelian, ~30–33.7 Ma) of
sediments from the Borbera Conglomerates (UNIT
S2) suggests a rapidly exhuming source. Similar ages
have been recorded in the early Oligocene Molare
sediments further to the west. They have been
interpreted as representative of a fast cooling and
exhumation episode affecting the Ligurian Alps
during the early Oligocene.
Our data suggest a trend that provides an alternative
view on the evolution of the Ligurian Alps during the
Eocene–Oligocene. Previous models (Vanossi et al.,
1986) proposed that only Liassic–Triassic units were
on top of the Ligurian Alps. However, these models
were based on geological observation and paleogeographic reconstruction but did not have any geochronological support. Also, the dataset presented
provides a new constraint on the stratigraphic age of
the Pianfolco Conglomerates with 40Ar/39Ar age data,
suggesting that it can be no older than 33.0F1.4 Ma,
which allows these sediments to be assigned to UNIT
S2, as proposed by Mutti et al., 1995.
B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203
Acknowledgments
J. Kuhlemann and S. Sherlock are greatly acknowledged for their constructive advice and criticism.
P.A.M. Andriessen is kindly acknowledged for his
help in trying to get usable Zircon Fission Track
results out of early separated samples and for his
fundamental support of this project. Wim J. Lustenhower is greatly thanked for help in microprobe
analyses. We have greatly appreciated the constructive
advice of Glen R. Murrell.
References
Barbieri, C., Carrapa, B., Di Giulio, A., Wijbrans, J., Murrell, G.,
2003. Provenance of Oligocene syn-orogenic sediments of the
Ligurian Alps (NW Italy): inferences on belt age and cooling
history. International Journal of Earth Sciences 90, 534 – 548.
Carrapa, B., Wijbrans, J., 2003. Cretaceous 40Ar/39Ar detrital
mica ages in Tertiary sediments shed a new light on EoAlpine evolution. Virtual Explorer. Geochronology and Structural Geology (13) (http://www.virtualexplorer.com.au/2003/
forsterwijbrans/).
Carrapa, B., Wijbrans, J., Bertotti, G., 2003. Episodic exhumation in
the Western Alps. Geology 31 (7), 601 – 604.
Carrapa, B., Wijbrans, J., Bertotti, 2004. Detecting variations in
exhumation/erosion patterns within the western Alpine Arc
through 40Ar/39Ar thermochronology on detrital minerals
(Tertiary Piedmont Basin, NW Italy). In: Bernet, M., Spiegel,
C. (Eds.), Detrital Thermochronology, Special Paper - GSA,
vol. 378, chap. 5.
Cavanna, F., Di Giulio, A., Galbiati, B., Mosna, S., Perotti, S., Pieri,
S., 1989. Carta Geologica del settore orientale del bacino
Ligure–Piemontese. Atti Ticinesi di Scienze della Terra 32
(Table 1).
Charrier, G., Fernandez, D., Malaroda, R., 1964. La Formazione di
Pianfolco (Bacino Oligocenico Ligure–Piemontese). Atti Accademia Nazionale Lincei. Memorie della Classe di Scienze
Fisiche, Matematiche e Naturali. S 7 (2), 25 – 81.
Cibin, U., Spadafora, E., Zuffa, G.G., Castellarin, A., 2001.
Continental collision history from arenites of episutural basins
in the Northern Apennines, Italy. Geological Society of America
Bulletin 113 (1), 4 – 19.
Cibin, U., Di Giulio, A., Martelli, L., 2003. Oligocene–Early
Miocene tectonic evolution of the northern Apennines (northwestern Italy) traced through provenance of piggy-back basin
fill successions. In: McCann, T.S. (Ed.), Tracing Tectonic
Deformation Using the Sedimentary Record, Special Publication - Geological Society of London, vol. 208, pp. 269 – 287.
Cimmino, F., Messiga, B., 1979. Calcschists of the Voltri Massif
(western Liguria); compositional variations of the white micas
related to the Alpine tectonic–metamorphic evolution. Ofioliti 4
(3), 269 – 294.
201
Cimmino, F., Messiga, B., Piccardo, G.B., 1981. Le caratteristiche paragenetiche dell’evento eo-Alpino di alta pressione
nei diversi sistemi (pelitici, femici, ultrafemici) delle ofioliti
metamorfiche del Gruppo di Voltri (Liguria occidentale).
Rendiconti della Societa Italiana di Mineralogia e Petrologia 37
(1), 419 – 446.
Cohen, C.R., Schweickert, R.A., Leroy, O.A., 1981. Age of
emplacement of the schistes lustrés nappes, Alpine Corsica.
Tectonophysics 73, 267 – 283.
Copeland, P., Harrison, M.T., 1990. Episodic rapid uplift in the
Himalaya revealed by 40Ar/39Ar analysis of detrital K-feldspar
and muscovite, Bengal fan. Geology 18, 354 – 357.
Dalrymple, G.B., Lanphere, M.A., 1969. Potassium–argon Dating.
Freeman, San Francisco, pp. 258.
Di Biase, D., Pandolfi, L., 1999. I Conglomearti della Val
Borbera (Bacino Terziario Piemontese) e relative implicazioni per l’esumazione di unita’ alpine a metamorfismo di
alta pressione/bassa temperatura, Riunione del Gruppo
Informale di Sedimentologia, Rimini. Giornale di Geologia
61, 178 – 181.
Di Biase, D., Marroni, M., Pandolfi, L., 1997. Age of the
deformation phases in the Internal Liguride Units: evidences
from Lower Oligocene Val Borbera Conglomerate of Tertiary
Piedmint Basin (Northern Italy). Ofioliti 22 (2), 231 – 238.
Di Giulio, A., 1991. Detritismo della parte orientale del Bacino
Terziario Piemontese durante l’Eocene–Oligocene: composizione delle Arenarie ed evoluzione tettonico-stratigrafica. Atti
Ticinesi di Scienze della Terra 34, 21 – 41.
Di Giulio, A., 1996. Eo-Alpine geodynamics; an integrated approach.
Bolletino della Societa Geologica Italiana 115 (3), 649 – 671.
Di Giulio, A., Galbiati, B., 1995. Interaction between tectonics
and deposition into an episutural basin in the Alps–
Appennine knot. In: Polino, R.S., R. (Eds.), Rapporti AlpiAppennino, Scritti e Documenti dell’ Acc. Naz. delle Scienze
14, pp. 113 – 128. Roma.
Di Giulio, A., Mancin, N., Martelli, L., 2002. Geohistory of the
Ligurian orogenic wedge: first inferences from paleobathyimetric data of the Epiligurian sediments. Bolletino della Societa
Geologica Italiana 1, 375 – 384.
Dickinson, W.R., 1974. Plate tectonics and sedimentation. In:
Dickinson, W.R. (Ed.), Tectonics and Sedimenation, Spec.
Publ. - Soc. Econ. Paleontol. Mineral., pp. 1 – 27. Tulsa, OK.
Dickinson, W.R., 1985. Interpreting provenance relations from
detrital modes of sandstones. In: Zuffa, G.G. (Ed.), Provenance
of Arenites, NATO ASI Ser., vol. 148, pp. 333 – 361.
Foland, K.A., 1983. 40Ar/39Ar incremental heating plateaus for
biotites with excess argon. Isotopic Geoscience 1, 3 – 21.
Forster, M.A., Lister, G.S., 2003. The interpretation of apparent age
spectra produced in the Argon Partial Retention Zone: application of the method of asymptotes and limits. Journal of
Structural Geology. In press.
Fravega, P., Piazza, M., Stockar, R., Vannucci, G., 1994. Oligocene
coral and algal reef and related facies of Valzamola (Savona,
NW Italy). Rivista Italiana di Paleontologia e Stratigrafia 423–
456 (3), 423 – 456.
Ghibaudo, G., Clari, P., Perello, M., 1985. Litostratigrafia,
sedimentologia ed evoluzione tettonico-sedimentaria dei depo-
202
B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203
siti miocenici del margine sud-orientale del Bacino Terziario
Ligure–Piemontese. Rivista Italiana di Paleontologia e Stratigrafia 104, 349 – 397.
Gnaccolini, M., 1974. Osservazioni sedimentologiche sui conglomerati oligocenici del settore meridionale del Bacino Terziario
Ligure–Piemontese. Rivista Italiana di Paleontologia e Stratigrafia 80 (1), 85 – 100.
Gnaccolini, M., 1978. Depositi Oligocenici di ’Fan-delta’ nella
regione compresa tra Bosio e la Cresta di Cravara. Rivista
Italiana di Paleontologia e Stratigrafia 84 (3), 673 – 699.
Gnaccolini, M., Rossi, P.M., 1994. Sequenze deposizionali e
composizione delle arenarie nel Bacino Terziario Ligure–
Piemontese: osservazioni preliminari. Atti Ticinesi di Scienze
della Terra 37, 3 – 15.
Hames, W.E., Bowring, S.A., 1994. An empirical evaluation of the
argon diffusion geometry in muscovite. Earth and Planetary
Science Letters 124, 161 – 167.
Haq, B.U., Van Eysinga, F.W.B., 1998. Geological Time Scale.
Elsevier, Amsterdam.
Harrison, T.M., McDougall, I., 1981. Excess 40Ar in metamorphic
rocks from Broken Hill, New South Wales: Implications for
40
Ar/39Ar age spectra and the thermal history of the region.
Earth and Planetary Science Letters 55, 123 – 149.
Harrison, T.M., Copeland, P., Hall, S.A., Quade, J., Burner, S., Ojha,
T.P., Kidd, W.S.F., 1993. Isotopic preservation of Himalayan/
Tibetan Uplift, Denudation, and Climatic Histories of two
molasse deposits. Journal of Geology 101, 157 – 175.
Heller, P.L., Frost, C.D., 1988. Isotopic provenance of clastic
deposits; application of geochemistry to sedimentary provenance studies. In: Kleinspehn, K., Paola, C. (Eds.), New
Perspectives in Basin Analysis. Springer-Verlag, New York, pp.
27 – 42.
Hoogerduijn Strating, E.H., van Wamel, W.A., Vissers, R.L.M., 1991.
Some constraints on the kinematics of the Tertiary Piemonte
Basin (northwestern Italy). Tectonophysics 198, 47 – 51.
Hunziker, J.C., Martinotti, G., 1984. Geochronology and evolution
of the western Alps: a review. Memorie della Societa Geologica
Italiana 29, 43 – 56.
Hurford, A.J., Hunziker, J.C., 1989. A revised thermal history for
the Gran Paradiso massif. Schweizerische Mineralogische und
Petrographische Mitteilungen 69, 319 – 329.
Kelley, S., 2002. Excess argon in K–Ar and Ar–Ar geochronology.
Chemical Geology 188, 1 – 22.
Kirschner, D.L., Cosca, M.A., Masson, H., Hunziker, J.C., 1996.
Staircase 40Ar/39Ar spectra of fine-grained white mica: timing
and duration of deformational and empirical constraints on
argon diffusion. Geology 24, 747 – 750.
Koppers, A.P., 2002. ArArCALC—software for 40Ar/39Ar age
calculations. Computers & Geosciences 28, 605 – 619.
Kuiper, K., 2003. Direct intercalibration of radio-isotopic and
astronomical time in the Mediterranean Neogene. Geologica
Ultraiectina 235, 224. ISSN 90-5744-094-6.
Leeps, A.L.W., White, S.H., Wijbrans, J., 1999. 40Ar/39Ar laserprobe direct dating of discrete deformational events: continuous
record of Early Alpine tectonics in the Pelagonian Zone, NW
Aegean area, Greece. In: Robertson, A.H.F. (Ed.), Tectonophysics Special Issue, vol. 298 (1–3), pp. 133 – 153.
Mailhé, D., Lucazea, F., Vasseur, G., 1986. Uplift history of thrust
belts: an approach based on fission track data and thermal
modelization. Tectonophysics 124, 177 – 191.
Maluski, H., 1977. Application de la méthode 40Ar/39Ar aux
minéraux de roches cristallines perturbées par des événements
thermiques et tectoniques en Corse. Bulletin de la Societe
Geologique de France 7 (4), 849 – 855.
Mancin, N., Cobianchi, M., 2000. Le Marne di Montepiano della
Val di Nizza (Appennino Settentrionale): biostratigrafia integrata e considerazini paleoambientali. Atti Ticinesi di Scienze
della Terra 41, 145 – 162.
Mancin, N., Pirini, C., 2001. Middle Eocene to Early Miocene
foramminiferal biostratigraphy in the Epiligurian succession
(Northern Apennines, Italy). Rivista Italiana di Paleontologia e
Stratigrafia 107 (3), 371 – 393.
Martelli, L., Cibin, U., Di Giulio, A., Catanzariti, R., 1998.
Litostratigrafia della Formazione di Ranzano (Priaboniano–
Rupeliano, Appennino Settentrionale e Bacino Terziario
Piemontese). Bolletino della Societa Geologica Italiana 117,
151 – 185.
Massonne, H.J., Schreyer, W., 1987. Phengite geobarometry based on
the limiting assemblage with K-feldspar, phlogopite, and quartz.
Contributions to Mineralogy and Petrology 96, 212 – 224.
McDougall, I., Harrison, T.M., 1999. Geochronology and Thermochronology by the 40Ar/39Ar Method. Oxford University Press,
Oxford. pp. 269.
Mutti, E., Papani, L., Di Biase, D., Davoli, G., Mora, S., Segadelli, S.,
Tinterri, R., 1995. Il Bacino Terziario Epimesoalpino e le sue
implicazioni sui rapporti tra Alpi ed Appennino. Memorie di
Scienze Geologiche di Padova 47, 217 – 244.
Najman, Y.M.R., Pringle, M.S., Johson, M.R.W., Robertson,
A.H.F., Wijbrans, J.R., 1997. Laser 40Ar/39Ar dating of single
detrital muscovite grains from early foreland-basin sedimenatry
deposits in India: implications for early Himalayan evolution.
Geology 25 (6), 535 – 538.
Najman, Y., Pringle, M., Godin, L., Grahame, O., 2001. Dating of
the oldest continental sediments from the Himalayan foreland
basin. Nature 410, 194 – 197.
Oberh7nsli, R., Hunziker, J.C., Martinotti, G., Stern, W.B., 1985.
Geochemistry, geochronology and petrology of Monte Mucrone:
an example of Eoalpine eclogitization of Permian granitoids in
the Sesia-Lanzo Zone, Western Alps, Italy. Isotope Geoscience
52, 165 – 184.
Pankhurst, R.J., Moorbath, S., Rex, D.C., Turner, G., 1973. Mineral
age patterns in ca. 3700 my old rocks from West Greenland.
Earth and Planetary Science Letters 20, 157 – 170.
Polino, R., Dal Piaz, V., Gosso, G., 1990. Tectonic erosion at the
Adria margin and accretionary processes for the Cretaceous
orogeny of the Alps. In: Roure, F., Heitzmann, P. and Polino, R.
(Eds.), Deep structure of the Alps. Société Geologique de
France, Société Geologique Suisse, Società Geologica Italiana,
345-367.
Polino, R., Ruffini, R., Ricci, B., 1991. Le molasse terziarie della
Collina di Torino: relazioni con la cinematica alpina. Atti
Ticinesi di Scienze della Terra 34, 85 – 95.
Reddy, S.M., Kelley, S.P., Wheeler, J., 1996. A 40Ar/39Ar laser probe
study of micas from the Sesia zone, Italian Alps: implications for
B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203
metamorphic and deformation histories. Journal of Metamorphic
Geology 17, 493 – 508.
Reed, S.B.J., 1993. Electron Microprobe Analysis (2nd edition).
Cambridge Univ. Press. 326 pp.
Renne, P.R., Becker, T.A., Swapp, S.M., 1990. 40Ar/39Ar laserprobe dating of detrital micas from the Montgomery Creek
Formation: clues to provenance, tectonics, and weathering
processes. Geology 18, 563 – 566.
Renne, P.R., et al., 1998. Intercalibration of standards, absolute ages
and uncertainties in 40Ar/39Ar dating. Chemical Geology 145,
117 – 152.
Scaillet, S., Féraud, G., Ballèvre, M., Amouric, M., 1992. Mg/Fe
and [(Mg,Fe)Si–Al2] compositional control on Ar behaviour in
high-pressure white micas: a continuous 40Ar/39Ar laser-probe
study from the Dora-Maira nappe of the Western Alps, Italy.
Geochimica et Cosmochimica Acta 56, 2851 – 2872.
Schamel, S., 1974. Eocene Subduction in Central Liguria. Yale
University, Italy, pp. 167.
Sherlock, S.C., 2001. Two-stage erosion and deposition in a
continental margin setting: an 40Ar/39Ar laserprobe study of
offshore detrital white micas in the Norwegian Sea. Journal of
the Geological Society (London) 158, 793 – 799.
Singer, B.S., Wijbrans, J.R., Nelson, S.T., Pringle, M.S., Feeley,
T.C., Dungan, M.A., 1998. Inherited argon in a Pleistocene
andesite lava: 40Ar/39Ar incremental-heating and laser-fusion
analyses of plagioclase. Geology 26, 427 – 430.
Sircombe, K.N., 1999. Quantitative comparison of large sets of
geochronological data using multivariate analysis: a provenance
study example from Australia. Geochimica et Cosmochimica
Acta 64 (9), 1593 – 1616.
Sircombe, K.N., 2000. The utility and limitations of binned
frequency histograms and probability density distributions for
displaying absolute age data. Current Research - Geological
Survey of Canada F2, 11.
Steenbrink, J., van Vugt, N., Hilgen, F.J., Wijbrans, J.R.,
Meulenkamp, J.E., 1999. Sedimentary cycles and volcanic ash
beds in the lower Pliocene lacustrine succession of Ptolemais
(NW Greece); discrepancy between 40Ar/39Ar and astronomical
ages. Palaeogeography, Palaeoclimatology, Palaeoecology 152
(3–4), 283 – 303.
Vance, J.A., 1999. Zircon Fission track evidence for a Jurassic
(Tethyan) thermal event in the Western Alps. Memorie di
Scienze Geologiche di Padova 51 (2), 473 – 476.
203
Vanossi, M., Cortesogno, L., Galbiati, B., Messiga, B., Piccardo,
G., Vannucci, R., 1986. Geologia della Alpi Liguri: dati,
problemi, ipotesi. Memorie della Società Geologica Italiana 28,
5 – 75.
Vanossi, M., Perotti, C.R., Seno, S., 1994. The Maritime Alps arc
in the Ligurian and Tyrrhenian systems. Tectonophysics 230,
75 – 89.
Velde, B., 1965. Phengitic micas: synthesis, stability and natural
occurrence. American Journal of Science 263, 886 – 913.
Velde, B., 1967. Si+4 content of natural phengites. Contributions to
Mineralogy and Petrology 14, 250 – 258.
Villa, I., 1998. Isotopic closure. Terra Nova 10, 42 – 47.
von Blanckenburg, F., Villa, I.M., Baur, H., Morteani, G.,
Steiger, R.H., 1989. Time calibration of a PT-path from the
Western Tauern Window, Eastern Alps: the problem of
closure temperatures. Contributions to Mineralogy and Petrology 101, 1 – 11.
von Eynatten, H., Gaupp, R., 1999. Provenance of Cretaceous
synorogenic sandstones in the Eastern Alps; constraints from
framework petrography, heavy mineral analysis and mineral
chemistry. Sedimentary Geology 124 (1–4), 81 – 111.
von Eynatten, H., Wijbrans, J., 2003. Precise tracing of exhumation
and provenance using 40Ar/39Ar geochronology of detrital white
mica: the example of the Central Alps. In: McCann, T.S. (Ed.),
Tracing tectonic deformation using the sedimentary record,
Special Publication - Geological Society of London, vol. 208,
pp. 289 – 305.
White, M.W., Pringle, M., Garzanti, E., Bickle, M., Najman, Y.,
Chapman, H., Friend, P., 2002. Constraints on the exhumation
and erosion of the High Himalayan Slab, NW India, from
foreland basin deposits. Earth and Planetary Science Letters 195
(1–2), 29 – 44.
Wijbrans, J.R., McDougall, I., 1986. 40Ar/39Ar dating of white
micas from an Alpine high-pressure metamorphic belt on Naxos
(Greece). The resetting of the argon isotopic system. Contributions to Mineralogy and Petrology 93, 187 – 194.
Wijbrans, J., Pringle, M.S., Koppers, A.A.P., Scheveers, R., 1995.
Argon geochronology of small samples using the Vulkaan argon
laserprobe. Proceedings of the Koninklijke Nederlandse Akademie van Wetenschappen 98 (2), 185 – 218.