Episodic exhumation in the Western Alps
Barbara Carrapa* 
Jan Wijbrans*

Giovanni Bertotti* 
Faculteit der Aardwetenschappen, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV
Amsterdam, The Netherlands
ABSTRACT
Oligocene to Miocene clastic sediments of the Tertiary Piedmont Basin (northwest Italy)
were derived from erosion of Western Alps source rocks. Detrital white micas from different stratigraphic units and from sands of three present-day rivers draining the internal
Western Alps have been analyzed by 40Ar/39Ar geochronology. Our data suggest a widespread, fast cooling and exhumation event prior to ca. 38 Ma followed by a .30 m.y.
period of slower cooling and exhumation combined with erosion of crustal rocks with
uniform 40Ar/39Ar signatures. These processes have resulted in a pattern of regularly
increasing lag time up section.
Keywords: Western Alps, cooling, exhumation, steady state.
INTRODUCTION
The relationships between orogen dynamics, topography creation, erosion, and clastic
deposition in synorogenic basins have in recent years resulted in stimulating, if fairly theoretical, numerical and analogue models (e.g.,
Willett and Brandon, 2002, and references
therein). Through investigation of the detrital
mineral record derived from the Western Alps,
this study provides new constraints on the
cooling and exhumation evolution of the orogen since before 30 Ma.
OROGENS AND STEADY STATE
Orogens are considered to undergo a constructional phase during which material is accreted, a steady-state phase when the quantities of material accreted and eroded are in
balance, and a destructive phase during which
removal of material predominates (Jamieson
and Beaumont, 1989). The steady-state concept is of particular interest because it is representative of the orogen maturity (Willett and
Brandon, 2002). One way to investigate orogen dynamics is to look at the ages recorded
by different thermochronometers across it
(Willett and Brandon, 2002). However, this
approach investigates only a limited time segment because significant volumes of crust
have been eroded. The sedimentary record
represents the archive of past cooling and exhumation histories (e.g., Copeland and Harrison, 1990). Steady-state exhumation can be
recognized by dating clastic minerals recording isotopic ages related to upward rock motion through isotherms (Bernet et al., 2001;
Willett and Brandon, 2002).
One parameter used in these studies is lag
time (Brandon and Vance, 1992), defined as
the difference between the isotopic age of a
given detrital mineral and the stratigraphic age
of the sediment containing the mineral. A
*E-mail addresses: carb@geo.vu.nl; wijj@geo.
vu.nl; bert@geo.vu.nl.
short lag time corresponds to fast exhumation,
whereas a long lag time corresponds to slow
exhumation (Garver et al., 1999). Steady-state
exhumation is represented by constant lag
times (Bernet et al., 2001). However, episodic
rapid cooling and fast exhumation will tend to
produce rocks with isotopically indistinguishable ages (Brandon and Vance, 1992) that,
once eroded and deposited in progressively
younger sedimentary formations, will result in
a systematic increase in lag times.
Fission-track dating on zircons and 40Ar/
39Ar dating on white micas (von Eynatten et
al., 1999; Spiegel et al., 2000; Bernet et al.,
2001) in sedimentary rocks derived mainly
from the Central Alps suggest thermal overprinting during the Neogene. In contrast, the
lack of data on detrital minerals coming from
the Western Alps has, until now, limited the
interpretation of the Tertiary to present cooling and exhumation patterns of this large segment of the Alps.
SOURCE AND SINK AREAS IN THE
WESTERN ALPS
The Western Alps, formed following Eurasia and Africa plate collision (Platt et al.,
1989), display a dominant vergence toward
the European foreland (i.e., westward) and a
subordinate one toward the Adriatic foreland
(i.e., eastward). Intense exhumation is typically
associated with axial zones; high- to ultrahighpressure rocks (HP, UHP) are exposed in various localities along the internal part of the arc
(e.g., Voltri Group, Dora Maira Massif, Gran
Paradiso; Fig. 1A). The Western Alps, similar
to the Central Alps, underwent cooling following peak Alpine metamorphism between
40 and 32 Ma, as indicated by the youngest
ages recorded in the orogen (Fig. 1B).
The Tertiary Piedmont Basin (TPB, Fig.
1A) represents an ideal place for investigating
source rock cooling and exhumation patterns
as the basin received sediments eroded from
the internal Western Alps over a period of
.30 m.y. The basin hosts .4 km of lower
Oligocene to upper Miocene sedimentary
rocks (Gnaccolini et al., 1998) unconformably
overlying Alpine-Apennine tectonic structures
(Miletto and Polino, 1992). Rivers continue to
erode and drain the internal Western Alps
(Fig. 1A).
METHODS
The 40Ar/39Ar geochronology on detrital
white micas shed light on cooling and exhumation processes over time (e.g., Copeland
and Harrison, 1990; Renne et al., 1990; Najman et al., 1997). The 40Ar/39Ar geochronology (see Appendix)1 has been applied to single white micas (250–500 mm in size)
separated from eight different Oligocene–
Miocene Tertiary Piedmont Basin formations
and from three present-day river sands for a
total of .500 individual total-fusion experiments (Fig. 2; Table DR1; see footnote 1).
White micas have been chosen because of
their wide occurrence, their resistance to resetting by mild thermal disturbances owing to
their sufficiently high closure temperature
(;350–420 8C; McDougall and Harrison,
1999 and references therein), and their resistance to mechanical and chemical breakdown
(Heller et al., 1992).
Probability density diagrams (Sircombe,
2000) have been compiled for each formation
and present-day river sand sample in order to
isolate youngest peaks (Fig. 3; Table DR2; see
footnote 1). The youngest peaks are here considered as representative of regional signatures. In a few cases youngest ages are different from youngest peaks and are probably
related to local events (Table 1). The biostratigraphic age (in the framework of the time
scale of Berggren et al., 1995) of the selected
samples is based on a detailed study of planktic and benthic foraminifera and calcareous
nannofossils (Gnaccolini et al., 1998, and references therein; Table DR1 [see footnote 1]).
RESULTS
A southward marine transgression started in
the easternmost part of the Tertiary Piedmont
Basin in the early Oligocene and extended
into the southwest part of the basin during the
1GSA Data Repository item 2003086, Appendix
(description of laser analysis), Table DR1 (sample
descriptions), and Table DR2 (age data for Fig. 3),
is available from Documents Secretary, GSA, P.O.
Box 9140, Boulder, CO 80301-9140, editing@
geosociety.org, or at www.geosociety.org/pubs/
ft2003.htm.
q 2003 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
Geology; July 2003; v. 31; no. 7; p. 601–604; 4 figures; 1 table; Data Repository item 2003086.
601
Figure 1. A: Tectonic map of Western and Central Alps showing three major nappe complexes, modified after
Bigi et al. (1983). LA—Ligurian Alps; AGM—Argentera Massif; DM—Dora Maira; GP—Gran Paradiso; TPB—
Tertiary Piedmont Basin; VG—Voltri Group. Black-line rectangle—study area. B: Western Alpine basement rocks
and related youngest 40Ar/39Ar phengite ages: (1) Monié (1990); (2) Monié and Chopin (1991); (3) Hunziker et
al. (1992); (4) Scaillet et al. (1992); (5) Inger et al. (1996); and (6) Cortiana et al. (1998).
late Oligocene. Transgressive gravels to sands
(Molare Formation), mainly derived from the
south, were deposited (Gnaccolini et al.,
1998). Their isotopic age signatures cluster
mainly around 60–32 Ma and 330–270 Ma
(Fig. 2). The youngest age derived from sandstones of Rupelian age (zone P20 and P21 of
the early Oligocene) is 31.6 6 1.7 Ma, whereas the youngest peak is 37 Ma (Fig. 3; Table
1).
Transgressive sedimentary rocks are overlain by lower Oligocene–Aquitanian shallowwater conglomerates and sandstones belonging to the Rocchetta and Monesiglio
Formations (Gnaccolini et al., 1998). Paleocurrent data and sandstone petrography from
these sedimentary rocks indicate a southwestern or western provenance (Gnaccolini et al.,
1998, and references therein). The 40Ar/39Ar
ages for both formations are mainly between
160 and 37 Ma; there are a few Variscan ages
(Fig. 2). The youngest age in the Rocchetta
Formation is 22.7 6 0.4 Ma and is indistinguishable from the youngest peak (ca. 23 Ma)
(Fig. 3; Table DR2 [see footnote 1]). However,
this anomalous youngest peak is the result of
a duplicate analysis of a single sample of
Aquitanian age (ca. 23.8–20.5 Ma) derived
from the south. Therefore we reject the implied youngest peak ca. 23 Ma and consider
instead the second peak at 38 Ma as the more
relevant and regional youngest peak (Table 1).
Figure 2. Plot of detrital-mineral 40Ar/39Ar ages vs. minimum stratigraphic ages characteristic
of each formation and present-day river sands. Only analyses with errors of <10% are considered here. For more details see Table DR2 (see footnote 1 in text) (Carrapa, 2002).
602
Support for rejection comes from the sedimentologically similar Monesiglio Formation,
which yielded a youngest peak of 37 Ma and
a youngest age of 37.2 6 0.4 Ma (Fig. 3; Table 1).
Aquitanian to Tortonian sedimentation was
characterized by regular turbiditic bodies deposited in a deep-marine environment covering the whole basin and associated with an
increase in sedimentation rates (Gnaccolini et
al., 1998), basin accommodation space, and
possibly source-area reorganization (Carrapa
et al., 2003). Petrographic and paleocurrent
data indicate fairly uniform west to east transport (Gnaccolini et al., 1998, and references
therein). The Cortemilia and Paroldo Formations (Aquitanian–Langhian) show ages spanning mainly 38 Ma to 160 Ma, and few Variscan ages (Fig. 2). The youngest age of 37.0
6 0.9 Ma is indistinguishable from the youngest peak of 37 Ma (Fig. 3; Table 1).
The Murazzano and Cassinasco Formations
(Langhian–Serravallian) have comparable isotopic signatures and are characterized by ages
between 180 and 37 Ma and a few ages between 320 and 240 Ma (Fig. 2). In the Murazzano Formation, the youngest age observed
is 37.5 6 1.1 Ma and corresponds to the youngest peak (38 Ma; Fig. 3; Table 1). Similar
data were obtained from the Cassinasco Formation, which is characterized by a youngest
age of 37.5 6 0.8 Ma corresponding to the
youngest peak (39 Ma; Fig. 3; Table 1).
Two main isotopic signatures characterize
the Lequio Formation (Serravallian–Tortonian);
its detrital-mineral ages are mainly between
GEOLOGY, July 2003
TABLE 1. TERTIARY PIEDMONT BASIN SAMPLES
Samples
Tanaro (present-day sand)
Maira (present-day sand)
Lequio (11.2–7.12 Ma)
Cassinasco (16.4–11.2 Ma)
Murazzano (16.4–11.2 Ma)
Cortemilia 1 Paroldo (23.8–14.8 Ma)
Monesiglio (28.5–14.8 Ma)
Rocchetta (28.5–20.5 Ma)
Molare (33.7–23.8? Ma)
N
DA
(Ma)
21
21
58
87
32
58
38
95
109
0
0
11.2–7.12
14.8–11.2
14.8–11.2
20.5–14.8
20.5–14.8
23.8–20.5
33.7–28.5
YA
(Ma)
YP
(Ma)
Minimum cooling rates
(C8/m.y.)
6
6
6
6
6
6
6
6
6
38
9–11
38
39
38
37
37
38
37
14–11
15–13
16–13
18–15
18–15
24–20
49–41
35.7
39.7
38.2
37.5
37.5
37
37.2
22.7
31.6
1.5
1.3
0.5
0.8
1.0
0.9
0.4
0.4
1.7
Note: General depositional ages for each formation in brackets, N is the number of total analyses, DA is the
specific depositional age for the sample in which the youngest detrital age (YA) has been measured, YP is the
youngest peak calculated with probability density diagram (Fig. 3). Minimum cooling rates are calculated by using
the minimum depositional age (underlined values) and the YP.
çonnais rocks. The general spectrum is
characterized by ages between 320 and 35 Ma
(Fig. 2); the minimum detrital mineral age is
35.7 6 1.5 Ma (Table 1).
Figure 3. Probability density diagrams of
ages younger than 45 Ma and characteristic of each formation and present-day
river sands with indication of youngest
peaks. Probability curves are generated
by summing Gaussian distribution of
each individual measurement (defined by
age and its error).
130 and 38 Ma, and a minor fraction of the
detrital minerals contributed ages between 330
and 270 Ma (Fig. 2). The youngest age is 38.2
6 0.5 Ma, corresponding to the youngest peak
(38 Ma; Fig. 3; Table 1).
Present river sands were sampled from the
Maira, Stura, and Tanaro Rivers (Fig. 1). The
Maira drains the polymetamorphic HP-UHP
units of the southern part of the Dora Maira
Massif. Ages range mainly between 90 and 50
Ma; the minimum detrital mineral age is 39.7
6 1.3 Ma (Fig. 2; Table 1). The Stura River
primarily drains the Variscan Argentera Massif. The main age group ranges between 302
and 204 Ma (Fig. 2), and Alpine ages are absent. The Tanaro River comes from the Ligurian Alps and drains Piemontese and BrianGEOLOGY, July 2003
DISCUSSION
The youngest peak ages versus depositional
ages of Oligocene to Holocene sedimentary
units show a persistence of the youngest signature (ca. 38 Ma) in all samples, resulting in
increasing lag time up sequence (Fig. 4).
Cooling rates are calculated here by assuming
a closure temperature between 420 and 350 8C
and minimum depositional ages.
If we consider the youngest peak of 37 Ma
for the Molare Formation as the regional signature recorded by sedimentary rocks from
both the southern and western domains and
28.5 Ma as the minimum main depositional
age, a lag time of 8.5 m.y. is obtained. This
means that in 8.5 m.y., the sample cooled by
420–350 8C, yielding cooling rates of 49–41
8C/m.y. (Table 1). The youngest age of 32 Ma
of the Molare Formation suggests that local
fast cooling of ;100 8C/m.y. affected the Ligurian Alps during the Oligocene. This event
only partially reset the regional late Eocene
signature.
Cooling rates in the Central and Western
Alps averaged ,20 8C/m.y. and were locally
as fast as 85–100 8C/m.y. (e.g., Giger and
Hurford, 1989; Schlunegger and Willett, 1999,
and references therein). We interpret our data
as representative of cooling following the regional middle Tertiary HP-UHP metamorphism in the Western Alps and subsequent fast
exhumation (e.g., Hurford et al., 1991; Monié
and Chopin, 1991; Rubatto and Hermann,
2001). The presence of the 38 Ma signature
in both the present-day river sands and the
rock outcrops (Fig. 1B) suggests that the decrease in apparent cooling rates observed in
progressively younger sedimentary rocks (Table 1) is not simply the result of reworking
processes (e.g., Sherlock, 2001).
The general persistence of youngest peaks
and of youngest ages (ca. 38 Ma) indicates
that the Eocene (prior to ca. 38 Ma) fast cool-
Figure 4. Youngest peaks characteristic of each formation and
present-day river sands. Black diamond—Tanaro River; open
square—Stura River; open triangle—Maira River; open circle—
Lequio Formation (Fm.); filled circle—Cassinasco Fm.; black
triangle—Murazzano Fm.; cross—Cortemilia and Paroldo Fm.;
open diamond—Monesiglio Fm.; black triangle—Rocchetta Fm.;
filled triangle—Molare Fm.
603
ing and exhumation event was able to produce
sufficient crustal material with a uniform Ar
isotopic signature to supply .30 m.y. of erosive products to the Tertiary Piedmont Basin.
Thus, erosion from the late Oligocene to the
present was relatively slow, because it did not
exhume deeper units with presumably younger isotopic ages. The slowness of the late
Oligocene–Holocene erosion is in sharp contrast with the Eocene very fast cooling and
exhumation event (e.g., Rubatto and Hermann, 2001). Our data show that a fast and
short-lived tectonic exhumation event in the
Western Alps produced a constant isotopic age
signature in Tertiary Piedmont Basin sedimentary rocks for .30 m.y.
CONCLUSIONS
Our data set supports an episodic, fast,
cooling and exhumation event prior to ca. 38
Ma followed by relatively slow and continuous erosion of crustal material with a uniform
40Ar/39Ar signature. Thus, we propose a rapid
change in cooling and exhumation patterns
within the Western Alps in the Eocene. This
inferred abrupt change may correspond to the
cessation of the major deformation in the
Western Alpine domain.
Our data further show a substantially different thermal history for the Western Alps
when compared to the Central Alps, where
thermal overprinting over the past 20 m.y. is
widely recorded by different thermochronometers (e.g., von Eynatten et al., 1999; Spiegel
et al., 2000). For the Western Alps, we suggest
a pattern of episodic exhumation that cannot
be reconciled by a steady-state model such as
has been assumed to regulate the Alps (Bernet
et al., 2001). Therefore, our new results not
only shed light on regional differences in
cooling and exhumation patterns within the
Alpine orogen, but also highlight the need for
studies that use different thermochronological
systems in order to understand orogenic evolution in time and space. This study demonstrates that 40Ar/39Ar geochronology on detrital white micas can be applied with success to
exhumation studies of orogens, because it
yields detailed information on exhumation
processes in the past.
ACKNOWLEDGMENTS
We thank the reviewers Mark Brandon, Yani Najman, Hilmar von Eynatten, and two anonymous readers for their constructive advice and Glen Murrell for
the English review. Supported by the Netherlands Organization for Scientific Research (NWO). Netherlands Research School of Sedimentary Geology publication 2003.03.01.
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Revised manuscript received 7 March 2003
Manuscript accepted 19 March 2003
Printed in USA
GEOLOGY, July 2003