Tracing exhumation and orogenic wedge dynamics in the European
Alps with detrital thermochronology
Barbara Carrapa
Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA
ABSTRACT
Detrital cooling ages from the pro-foreland and retro-foreland basins of the European
Alps record distinctive exhumation trends that correlate with orogenic wedge states inferred
from thrust front propagation rates. Periods of rapid hinterland exhumation correlate with
relatively slow propagation of deformation toward the foreland and are interpreted to represent subcritical wedge conditions, whereas periods of slow hinterland exhumation correlate with rapid propagation of deformation toward the foreland and indicate supercritical
wedge conditions. Similar lag time trends recorded in both the pro-foreland and retro-foreland thus mimic orogenic wedge behavior and suggest that local tectonics and/or climate
events do not overprint the regional signal.
INTRODUCTION
The internal structure, kinematic history,
and surface topography of contractional mountain belts are products of complex interactions
between crustal shortening and/or thickening
and exhumation. Numerous studies have shown
that mountain belts can be successfully modeled as critically tapered orogenic wedges (e.g.,
Davis et al., 1983; Suppe and Medwedeff, 1990;
Willett et al., 1993; Koons, 1994; DeCelles and
Mitra, 1995). Critical taper models make predictions about the kinematic history of an orogenic
wedge in response to changes in mass distribution, which in turn is controlled by exhumation.
Therefore, in order to understand the relationships between exhumation and kinematic processes it is essential to determine the timing,
rates, and spatial distribution of exhumation.
The European Alps are a classic example of
a strongly asymmetrical continent-continent collisional orogen and one of the few cases of a doubly verging orogen with two foreland basin systems (e.g., Naylor and Sinclair, 2008). The North
and South Alpine foreland basins provide a natural laboratory in which to explore the distribution of exhumation along (east-west) and across
(north-south) the orogen over tens of millions of
years. This paper documents the relationships
between orogenic wedge taper and exhumation
in the Alps with detrital thermochronology.
WEDGE TECTONICS,
EXHUMATION, AND DETRITAL
THERMOCHRONOLOGY
The Alpine chain formed as a consequence of
convergence and subsequent collision between
the Eurasian and African continents from early
to middle Cenozoic time (e.g., Stampfli and
Marchant, 1997; Rosenbaum and Lister, 2005).
This was responsible for significant shortening
(as much as 195 km; Ford et al., 2006; Schmid
and Kissling, 2000; Pfiffner et al., 2000), crustal
thickening of the upper plate, and exhumation.
These processes, combined with later deformational and thermal events, produced the distribution of thermochronological ages observed
today (Fig. 1).
The Alps are formed by two oppositely verging tapered orogenic wedges (Figs. 1B and 2),
and loading of the upper plate by these wedges
produced foreland basins on both sides of the
orogen (Naylor and Sinclair, 2008). Critical
taper theory (e.g., Chapple, 1978; Davis et al.,
1983) predicts that the front of an orogenic
wedge develops taper and propagates toward
the foreland when the sum of the angles of
the basal and upper slopes (referred to as the
taper value) reaches a critical value. When the
taper value is less than the critical value (subcritical), the wedge will shorten internally by
out-of-sequence thrusting and/or duplexing to
build thickness and increase taper. Among other
things, subcritical conditions could be caused
by enhanced erosion due to wetter or more seasonal climate. If the taper value is greater than
the critical value (supercritical), the orogenic
wedge will broaden and reduce the overall
taper angle by forward thrust propagation and/
or internal extension in order to regain balance
between driving and resisting forces. Exhumation of material from the wedge may be viewed
as a response to changing taper states (Davis et
al., 1983; DeCelles and Mitra, 1995). Therefore,
if the Alps obey wedge theory, increasing hinterland exhumation reflects a subcritical wedge
state, whereas decreasing hinterland exhumation reflects a supercritical wedge state. Increasing exhumation will be recorded by detrital
minerals within foreland basin strata with an
upsection-decreasing lag time (e.g., Garver et
al., 1999) between cooling and depositional
ages, whereas decreasing exhumation will be
recorded by an increasing lag time upsection.
Episodic exhumation will be recorded by an
increasing lag time upsection, whereas steadystate exhumation will be recorded by a constant
lag time upsection (e.g., Bernet et al., 2001;
Carrapa et al., 2003a) (Fig. 2). In turn, the rate
of migration of the flexural wave into the foreland is expected to be slower during subcritical
conditions and faster during supercritical conditions. If those processes have influenced Alpine
exhumation along and across strike, we should
expect to find characteristic trends in the detrital record (von Eynatten et al., 1999; Spiegel et
al., 2001, 2004; Carrapa et al., 2003a, 2004a,
2004b; von Eynatten and Wijbrans, 2003).
ALPINE FORELAND BASIN RECORD
The area considered in this study covers the
entire orogenic system (Fig. 1) and the provenance of most of the samples is well constrained
by sandstone and conglomerate petrography and
paleocurrent data (e.g., Brügel, 1998; Carrapa
and DiGiulio, 2001; Evans and Elliott, 1999;
Spiegel et al., 2002; Dunkl et al., 2001, and
Carrapa et al., 2004a, and references therein).
Medium- to low-temperature detrital thermochronological data from the pro-foreland and
retro-foreland basins of the Western, Central,
and Eastern Alps (Fig. 3) show that different
thermochronometers record similar patterns of
exhumation. This suggests that differences in lag
time responses for different thermochronometers
are undetectable at this scale of observation and
that sediment reworking is not a problem.
In the pro-foreland of the Central and Eastern
Alps, the decreasing lag-time trend (Fig. 3A)
suggests increasing exhumation from ca. 30 to 10
Ma, suggesting subcritical taper conditions. The
pro-foreland of the Western Alps records increasing exhumation between 38 and 36 Ma (suggesting a subcritical state) and decreasing exhumation between ca. 16 and 8 Ma (supercritical state)
(Fig. 3B). Deep and rapid exhumation is also
indicated by ca. 34 Ma 40Ar/39Ar ages from pebbles in early Oligocene synorogenic conglomerates in the French Alps (Morag et al., 2008).
In the retro-foreland of the Western Alps
(Fig. 3C) the youngest thermochronological
signal (ca. 32–38 Ma) remains constant for >30
m.y. This represents rapid cooling and episodic
exhumation of the internal crystalline massifs
(e.g., Dora Maira) between ca. 38 and 32 Ma
(e.g., Carrapa et al., 2003a) and of the Periadriatic plutons (e.g., Bergell) in the Central Alps
(e.g., Garzanti and Malusá, 2008), suggesting
subcritical wedge conditions. This was followed by slower cooling (~10°/m.y.), indicating
a supercritical wedge state. It is interesting that
© 2009 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
GEOLOGY,
December
2009
Geology,
December
2009;
v. 37; no. 12; p. 1127–1130; doi: 10.1130/G30065A.1; 4 figures.
1127
48°N
A
ages from the Friuli-Venetian foreland (Eastern
Alps derived) indicate a change in lag time trend
ca. 12 Ma that may correspond to a change in
tectono-thermal regime, possibly related to
exhumation of the Lepontine Dome (Spiegel et
al., 2004; von Eynatten and Wijbrans, 2003).
(11)
Pro-foreland
sin
47°N
Sw
40
iss
ba
se(9)
s
a
ol A (10)(10)*
M
Aar
39
K-Ar and Ar/ Ar ages:
140–60 Ma
30–60 Ma
30–15 Ma
46°N
15–0 Ma
ZFT ages < 20 Ma
AFT ages < 15 Ma
AFT ages < 5 Ma
G
LD
DB
MB
G
SL
(1)(2)(5)
B
GP
Milano
(6)
Retro-foreland
Torino
Po Plain
45°N
TPB
(7)
Pro-foreland
Al
Arg
Digne
Valensole
basin
Bârreme
basin
5°E
6°E
Nice
7°E
8°E
9°E
10°E
11°E
Pro-foreland
A
PA
NCA
AU
VA
Aar
1
A
B 2
3
d
12°E
Retro-foreland
HE PF
MO
lan
4
B
5
AU MA
SU
TA
B
IL
SA
B
AD
GO
A
PA
10
4°E
Ligurian Sea
20
44°N
B
Ap (3)(4)
en
nin (4)*
ef
or
e
te
au
e H çe
s d en
pe rov
P
(12)(12*)
43°N
(8)
DM
km
Figure 1. Digital elevation model of Alps and flanking foreland basins with compilations of
40
Ar/ 39Ar ages younger than 140 Ma (after Hunziker et al., 1992, and references therein), zircon
fission-track (ZFT) ages younger than 20 Ma and apatite fission track (AFT) ages younger
than 20 Ma and younger than 5 Ma (after Vernon et al., 2008, and references therein). Boxes
(numbers in parentheses) indicate detrital thermochronological studies: (1, 2, 5) Gonfolite
studied by Giger and Hurford (1989), Spiegel et al. (2001), and Fellin et al. (2005); (3, 4,)
Macigno-Modino and Marnoso Arenacea Apennine units studied by Dunkl et al. (2001) and
Bernet et al. (2001); (4*) Bernet et al. (2009); (6) Stefani et al., 2007; (7) Tertiary Piedmont
Basin (TPB) studied by Carrapa et al. (2003a, 2004a) and Barbieri et al. (2003); (8) by Carrapa et al. (2004b); (9) Swiss Molasse Basin studied by von Eynatten and Wijbrans (2003);
(10) Spiegel et al. (2004); (10*) from Bernet et al. (2009); (11) area studied by Kuhlemann et
al. (2006). Note that area studied by Kuhlemann et al. extends until 14°E; (12) area studied by Morag et al. (2008) and (12*) Bernet et al. (2009). DM—Dora Maira Massif; GP—Gran
Paradiso; DB—Dent Blanche; MB—Monte Bianco; Arr—Aar Massif; G—Gottard Massif;
LD—Lepontine Dome; B—Bergell pluton; Arg—Argenetra Massif; G—Gonfolite; SL—SesiaLanzo. A–B schematic cross section through Central Alps (modified after Polino et al., 1990).
Bottom key: 1—Eastern Australpine system (AU) with Eoalpine very low grade (A) and greenshist-amphibolite facies (B) metamorphism; 2—Platta-Arosa (PA) and Malenco-Avers (MA)
Piedmont ophiolite units and Valais (VA) ophiolite and flysch units; 3— flysch décollement
units (mostly Cretaceous); 4—Cenozoic European molasse (A: MO) and Po Plain molasse
(B: Gonfolite); 5—Cenozoic Bergell (B) Periadriatic intrusion; PF—Penninic thrust front;
HE—Ultrahelvetic, Helvetic and Duphoinois; NCA—North Calcareous Alps; IL—Insubric line;
SA—Southern Alps; GO—Gotthard nappe; AD—Adula nappe; TA—Tambo nappe.
subcritical wedge conditions as inferred from
detrital thermochronology correlate with a
period of rapid plate convergence (Schmid et
al., 1996; Ford et al., 2006), whereas supercritical wedge conditions correspond to slow plate
convergence, suggesting a relationship between
plate convergence, shortening, and wedge state.
1128
The Central and Eastern Alps retro-foreland shows a more complex trend with overall
decreasing exhumation between ca. 32 and 12
Ma (Fig. 3D), suggesting supercritical conditions. Zircon fission track ages from Central
Alps–derived detritus, today preserved in the
Apennine foredeep, and apatite fission track
THRUST FRONT PROPAGATION,
SUBSIDENCE, AND EXHUMATION
From ca. 40 to 30 Ma the North Alpine proforeland basin (Swiss Molasse Basin) was an
~100-km-wide, deeply underfilled flexural
trough with <200 m of sedimentary fill. The
pro-wedge thrust front advanced northward at
high rates of 10–20 mm/a (Sinclair and Allen,
1992; Burkhard and Sommaruga, 1998). This
correlates to a supercritical wedge behavior.
Between 30 and 22 Ma, both the thrust front
and the distal foreland basin depositional pinchout migrated northwestward at a slower rate
of ~5 mm/a (Fig. 4). Increased tectonic subsidence (2.7 km) produced a basin that was totally
filled above sea level by sediment (Sinclair and
Allen, 1992; Burkhard and Sommaruga, 1998).
The thermochronological lag time decreased
during this time (30–12 Ma), indicating increasing exhumation (Fig. 3A) and subcritical taper
conditions. An increase in sediment flux into the
foreland basin, between 30 and 22 Ma (Kuhlemann et al., 2002), correlates with an increase
in source exhumation. From 22 to 12 Ma, the
foreland basin depositional pinchout continued
to migrate northwestward at a slower rate. The
basin had a width of ~100–140 km, and rapid
subsidence continued (Burkhard and Sommaruga, 1998). Lag times continued to decrease
upsection during this time interval, suggesting
continuing rapid exhumation and subcritical
taper conditions. After ca. 12 Ma the deformation front jumped ~100 km northward to the
external Jura, leading to exhumation of the
foreland basin. This suggests that the orogenic
wedge was in a supercritical state at the time
(Fig. 4).
In the Western Alps retro-foreland, deformation migrated at a nearly constant rate of ~6.3
mm/a between 30 Ma and 11 Ma (Fig. 4). This
rate was calculated using basin exhumation data
of Bertotti et al. (2006). Assuming that basin
exhumation was driven by deformation, the distance between two different samples exhumed
within the basin divided by the difference in timing of exhumation (constrained by [U-Th]/He
thermochronology) gives the velocity of propagation of deformation into the foreland. This
was coupled with significant subsidence in the
foreland basin. Detrital ages from the Western
Alps retro-foreland show slow erosion after a
period of fast exhumation (38–32 Ma), suggesting that the Western Alps retro-wedge was in a
subcritical state between 38 and 32 Ma and in a
supercritical state after 30 Ma (Figs. 3C and 4).
GEOLOGY, December 2009
ge
hu
la
ma
m
at
n
e
20
10
20
?
e2
0
0
30
supercritical
tim
e2
tim
subcritical
outlier
?
la g
la g
30
lag
e
tim
10
10
10
20
30
40
50
60
70
40
10
Thermochronological age (Ma)
20
30
50
40
(1) Gonfolite-Alpine foreland (ZFT), YP
(2) Gonfolite-Alpine foreland (AFT), YP
(11) Eastern Alps foreland (AFT: YP)
(3) Marnoso Arenacea-Apennines (ZFT), YP
(4) Macigno-Modino-Apennines (ZFT), YP
(4*) Macigno & Marnoso Arenacea (ZFT), YP
outlier (partially annealed after burial)
(5) Gonfolite-Alpine foreland (40Ar/39Ar), YA
General lag time trend
10
subcritical
(6) Venetian-Friulian Basin,
AFT; partially annealed.
10
change in tectono-thermal regime
20
20
supercritical
lag
0
e0
e2
tim
ti m
lag
30
30
supercritical?
10
20
30
40
50
60
Thermochronological age (Ma)
20
30
40
50
60
Thermochronological age (Ma)
Figure 3. Compilation of detrital thermochronological data from Alpine foreland basin deposits. A: Central and Eastern Alps pro-foreland. B: Western Alps pro-foreland. C: Western Alps
retro-foreland. D: Central and Eastern Alps retro-foreland. The 40Ar/ 39Ar data reported are
youngest ages (YA); apatite fission track (AFT) and zircon fission track (ZFT) data reported
are youngest populations (YP). 1—Spiegel et al. (2004); 2—Fellin et al. (2005); 3—Dunkl
et al. (2001); 4—Bernet et al. (2001); 4*—Bernet et al. (2009); 5—Giger and Hurford (1989)
(Bergell pluton–derived tonalite pebble); 6—Stefani et al. (2007); 7—Carrapa et al. (2003a);
8—Carrapa et al. (2004a); 9—von Eynatten and Wijbrans (2003); 10—Spiegel et al. (2004);
10*—Bernet et al. (2009); 11—Kuhlemann et al. (2006); (12*) after Bernet et al. (2009). Average
error in depositional (Dep.) ages is ~20% and average error in thermochronological ages is
<10% (in this case, error bars are undetectable on plots).
GEOLOGY, December 2009
tion
ma
xhu
10
15
20
25
30
35
40
Time (Ma)
Figure 4. Rates of migration of deformation
toward foreland. Data from Central Alps are
from Burkhard and Sommaruga (1998) and
data from Western Alps are after Bertotti et
al. (2006).
Data from the Central and Eastern Alps
retro-foreland (Eastern Po Plain) (Barbieri et
al., 2007) show continuous tectonic subsidence
between ca. 30 and 17 Ma (Fig. 3D). This time
span correlates with moderate rates (6.3 mm/a)
of propagation of deformation into the western
Alpine foreland. Overall, these data suggest
that the Central and Eastern Alps retro-wedge
was in a supercritical state between ca. 30 and
12 Ma and reached subcritical conditions after
ca. 12 Ma.
60
Central-Eastern Alps retro-foreland
0
Dep. age (Ma)
Dep. age (Ma)
D
(9) Central Alps foreland
(Honegg–Napf drainage), YA
40
39
( Ar/ Ar: youngest age; average error<5%)
(10) Central Alps foreland (ZFT: YP)
(10*) Central Alps foreland (ZFT: YP)
10
5
Thermochronological age (Ma)
A Central-Eastern Alps pro-foreland
0
subcritical
lag
0
e0
e
tim
lag
tim
e
tim
lag
40
0
(8) Eastern Tertiary Piedmont Basin
(40Ar/39Ar); YA
Increasing lag time up section=
episodic exhumation (UHP rocks) at ca. 32-38 Ma
followed by slow(er) erosion
supercritical
Dep. age (Ma)
Dep. age (Ma)
10
n
(7) Tertiary Piedmont Basin (40Ar/39Ar); YA
2
tio
Western Alps retro-foreland
0
(12*) SE France foreland
ZFT (YP)
ma
C
B Western Alps pro-foreland
0
hu
Figure 2. A: Schematic sketch of exhumation through a Coulomb wedge (modified after DeCelles and Mitra, 1995); α—surface slope, β—basal décollement angle, θ = α + β. B: Lag time
trends representing different orogenic wedge states (see text for explanation). 1—Subcritical
behavior; 2—supercritical behavior.
4
ex
Detrital cooling age
6
ing
tim
e.g., tA:cooling age at time tA < tB < tC
8
as
g
tion
tC
io
10
cre
nt
xhu
12
ge
1
sin
ex
sin
rea
te
rea
tC
ta
1
350 °C
2
tB
ta
14
Pro-foreland
Retro-foreland
Subcritical trend
Supercritical trend
inc
c
-s
inc
ns
tA
tA
tB
dy
Propagation of deformation (mm/yr)
120 °C
Depositional age
tB
ea
co
Foreland basin
stratigraphic section
tC
2
tA
st
16
de
Exhumation of a crustal column
Pro-wedge and/or retro-wedge behavior
decreasing exhumation
Forlandward migration of deformation
Source exhumation and
deposition of eroded material
in the adjacent foreland
CONCLUSIONS
This study demonstrates that both sides of
the orogen can be explained in the context of
critical taper theory. An inverse relationship
between source exhumation and propagation of
deformation toward the foreland is observed in
both the pro-foreland and retro-foreland (Figs. 3
and 4). However, the available detrital thermochronological data show that the source exhumation history of the pro-wedge and the retrowedge were quite different and out of phase.
Furthermore, the foreland basin detrital ages
record continuous and coherent signals suggesting either that short-term, local tectonic (or climatic) events cannot be deciphered using detrital thermochronology or, more likely, that such
changes are irrelevant with respect to orogenic
wedge behavior over tens of millions of years.
ACKNOWLEDGMENTS
This study greatly benefited from scientific discussions with Peter G. DeCelles and Sanjeev Gupta, and
from constructive comments from five anonymous
reviewers and Andrew Barth.
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Manuscript received 23 January 2009
Revised manuscript received 29 June 2009
Manuscript accepted 6 July 2009
Printed in USA
GEOLOGY, December 2009