Tracing Exhumation and Orogenic Wedge Dynamics in the European Alps with Detrital Thermochronology

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 lag time upsection (e.g., Bernet et al., 2001; Detrital cooling ages from the pro-foreland and retro-foreland basins of the European Carrapa et al., 2003a) (Fig. 2). In turn, the rate Alps record distinctive exhumation trends that correlate with orogenic wedge states inferred of migration of the fl exural wave into the fore- from thrust front propagation rates. Periods of rapid hinterland exhumation correlate with land is expected to be slower during subcritical relatively slow propagation of deformation toward the foreland and are interpreted to rep- conditions and faster during supercritical condi- resent subcritical wedge conditions, whereas periods of slow hinterland exhumation corre- tions. If those processes have infl uenced Alpine late with rapid propagation of deformation toward the foreland and indicate supercritical exhumation along and across strike, we should wedge conditions. Similar lag time trends recorded in both the pro-foreland and retro-fore- expect to fi nd characteristic trends in the detri- land thus mimic orogenic wedge behavior and suggest that local tectonics and/or climate tal record (von Eynatten et al., 1999; Spiegel et events do not overprint the regional signal. al., 2001, 2004; Carrapa et al., 2003a, 2004a, 2004b; von Eynatten and Wijbrans, 2003). INTRODUCTION These processes, combined with later deforma- The internal structure, kinematic history, tional and thermal events, produced the distri- ALPINE FORELAND BASIN RECORD and surface topography of contractional moun- bution of thermochronological ages observed The area considered in this study covers the tain belts are products of complex interactions today (Fig. 1). entire orogenic system (Fig. 1) and the prove- between crustal shortening and/or thickening The Alps are formed by two oppositely verg- nance of most of the samples is well constrained and exhumation. Numerous studies have shown ing tapered orogenic wedges (Figs. 1B and 2), by sandstone and conglomerate petrography and that mountain belts can be successfully mod- and loading of the upper plate by these wedges paleocurrent data (e.g., Brügel, 1998; Carrapa eled as critically tapered orogenic wedges (e.g., produced foreland basins on both sides of the and DiGiulio, 2001; Evans and Elliott, 1999; Davis et al., 1983; Suppe and Medwedeff, 1990; orogen (Naylor and Sinclair, 2008). Critical Spiegel et al., 2002; Dunkl et al., 2001, and Willett et al., 1993; Koons, 1994; DeCelles and taper theory (e.g., Chapple, 1978; Davis et al., Carrapa et al., 2004a, and references therein). Mitra, 1995). Critical taper models make predic- 1983) predicts that the front of an orogenic Medium- to low-temperature detrital thermo- tions about the kinematic history of an orogenic wedge develops taper and propagates toward chronological data from the pro-foreland and wedge in response to changes in mass distribu- the foreland when the sum of the angles of retro-foreland basins of the Western, Central, tion, which in turn is controlled by exhumation. the basal and upper slopes (referred to as the and Eastern Alps (Fig. 3) show that different Therefore, in order to understand the relation- taper value) reaches a critical value. When the thermochronometers record similar patterns of ships between exhumation and kinematic pro- taper value is less than the critical value (sub- exhumation. This suggests that differences in lag cesses it is essential to determine the timing, critical), the wedge will shorten internally by time responses for different thermo chrono meters rates, and spatial distribution of exhumation. out-of-sequence thrusting and/or duplexing to are undetectable at this scale of observation and The European Alps are a classic example of build thickness and increase taper. Among other that sediment reworking is not a problem. a strongly asymmetrical continent-continent col- things, subcritical conditions could be caused In the pro-foreland of the Central and Eastern lisional orogen and one of the few cases of a dou- by enhanced erosion due to wetter or more sea- Alps, the decreasing lag-time trend (Fig. 3A) bly verging orogen with two foreland basin sys- sonal climate. If the taper value is greater than suggests increasing exhumation from ca. 30 to 10 tems (e.g., Naylor and Sinclair, 2008). The North the critical value (supercritical), the orogenic Ma, suggesting subcritical taper conditions. The and South Alpine foreland basins provide a natu- wedge will broaden and reduce the overall pro-foreland of the Western Alps records increas- ral laboratory in which to explore the distribu- taper angle by forward thrust propagation and/ ing exhumation between 38 and 36 Ma (suggest- tion of exhumation along (east-west) and across or internal extension in order to regain balance ing a subcritical state) and decreasing exhuma- (north-south) the orogen over tens of millions of between driving and resisting forces. Exhuma- tion between ca. 16 and 8 Ma (supercritical state) years. This paper documents the relationships tion of material from the wedge may be viewed (Fig. 3B). Deep and rapid exhumation is also between orogenic wedge taper and exhumation as a response to changing taper states (Davis et indicated by ca. 34 Ma 40Ar/39Ar ages from peb- in the Alps with detrital thermochronology. al., 1983; DeCelles and Mitra, 1995). Therefore, bles in early Oligocene synorogenic conglomer- if the Alps obey wedge theory, increasing hin- ates in the French Alps (Morag et al., 2008). WEDGE TECTONICS, terland exhumation refl ects a subcritical wedge In the retro-foreland of the Western Alps EXHUMATION, AND DETRITAL state, whereas decreasing hinterland exhuma- (Fig. 3C) the youngest thermochronological THERMOCHRONOLOGY tion refl ects a supercritical wedge state. Increas- signal (ca. 32–38 Ma) remains constant for >30 The Alpine chain formed as a consequence of ing exhumation will be recorded by detrital m.y. This represents rapid cooling and episodic convergence and subsequent collision between minerals within foreland basin strata with an exhumation of the internal crystalline massifs the Eurasian and African continents from early upsection-decreasing lag time (e.g., Garver et (e.g., Dora Maira) between ca. 38 and 32 Ma to middle Cenozoic time (e.g., Stampfl i and al., 1999) between cooling and depositional (e.g., Carrapa et al., 2003a) and of the Periadri- Marchant, 1997; Rosenbaum and Lister, 2005). ages, whereas decreasing exhumation will be atic plutons (e.g., Bergell) in the Central Alps This was responsible for signifi cant shortening recorded by an increasing lag time upsection. (e.g., Garzanti and Malusá, 2008), suggesting (as much as 195 km; Ford et al., 2006; Schmid Episodic exhumation will be recorded by an subcritical wedge conditions. This was fol- and Kissling, 2000; Pfi ffner et al., 2000), crustal increasing lag time upsection, whereas steady- lowed by slower cooling (~10°/m.y.), indicating thickening of the upper plate, and exhumation. state exhumation will be recorded by a constant a supercritical wedge state. It is interesting that

© 2009 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, December December 2009; 2009 v. 37; no. 12; p. 1127–1130; doi: 10.1130/G30065A.1; 4 fi gures. 1127 48°N ages from the Friuli-Venetian foreland (Eastern A (11) Alps derived) indicate a change in lag time trend Pro-foreland ca. 12 Ma that may correspond to a change in tectono-thermal regime, possibly related to (9) exhumation of the Lepontine Dome (Spiegel et A (10)(10)* 47°N al., 2004; von Eynatten and Wijbrans, 2003). Swiss Molasse basin Aar G THRUST FRONT PROPAGATION, K-Ar and40 Ar/ 39 Ar ages: 140–60 Ma LD SUBSIDENCE, AND EXHUMATION 30–60 Ma From ca. 40 to 30 Ma the North Alpine pro- 30–15 Ma 46°N 15–0 Ma foreland basin (Swiss Molasse Basin) was an MB DB ZFT ages < 20 Ma (1)(2)(5) ~100-km-wide, deeply underfi lled fl exural AFT ages < 15 Ma G SL trough with <200 m of sedimentary fi ll. The AFT ages < 5 Ma B GP pro-wedge thrust front advanced northward at Milano (6) Retro-foreland high rates of 10–20 mm/a (Sinclair and Allen, Torino 45°N Po Plain 1992; Burkhard and Sommaruga, 1998). This TPB correlates to a supercritical wedge behavior. (7) (8) Between 30 and 22 Ma, both the thrust front Pro-foreland DM Alpes de and the distal foreland basin depositional pin- Arg Apennine(3)(4) foreland (12)(12*) Proven e (4)* chout migrated northwestward at a slower rate Digne Haute 44°N Valensole ç of ~5 mm/a (Fig. 4). Increased tectonic subsi- basin Bâ rreme dence (2.7 km) produced a basin that was totally basin Nice Ligurian Sea fi lled above sea level by sediment (Sinclair and Allen, 1992; Burkhard and Sommaruga, 1998). The thermochronological lag time decreased 43°N during this time (30–12 Ma), indicating increas- 4E° 5°E 6°E 7°E 8°E 9°E 10°E 11°E 12°E ing exhumation (Fig. 3A) and subcritical taper B Pro-foreland Retro-foreland conditions. An increase in sediment fl ux into the A AU MA MO HE PF NCA PA AU PA IL SA B foreland basin, between 30 and 22 Ma (Kuhle- mann et al., 2002), correlates with an increase VA SU B TA in source exhumation. From 22 to 12 Ma, the Aar 10 AD foreland basin depositional pinchout continued GO A A 20 1 34 5 to migrate northwestward at a slower rate. The B 2 B km basin had a width of ~100–140 km, and rapid Figure 1. Digital elevation model of Alps and fl anking foreland basins with compilations of subsidence continued (Burkhard and Somma- 40Ar/ 39Ar ages younger than 140 Ma (after Hunziker et al., 1992, and references therein), zircon ruga, 1998). Lag times continued to decrease fi ssion-track (ZFT) ages younger than 20 Ma and apatite fi ssion track (AFT) ages younger upsection during this time interval, suggesting than 20 Ma and younger than 5 Ma (after Vernon et al., 2008, and references therein). Boxes continuing rapid exhumation and subcritical (numbers in parentheses) indicate detrital thermochronological studies: (1, 2, 5) Gonfolite taper conditions. After ca. 12 Ma the deforma- 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 tion front jumped ~100 km northward to the Bernet et al. (2001); (4*) Bernet et al. (2009); (6) Stefani et al., 2007; (7) Tertiary Piedmont external Jura, leading to exhumation of the Basin (TPB) studied by Carrapa et al. (2003a, 2004a) and Barbieri et al. (2003); (8) by Car- foreland basin. This suggests that the orogenic rapa et al. (2004b); (9) Swiss Molasse Basin studied by von Eynatten and Wijbrans (2003); wedge was in a supercritical state at the time (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 stud- (Fig. 4). ied by Morag et al. (2008) and (12*) Bernet et al. (2009). DM—Dora Maira Massif; GP—Gran In the Western Alps retro-foreland, deforma- Paradiso; DB—Dent Blanche; MB—Monte Bianco; Arr—Aar Massif; G—Gottard Massif; tion migrated at a nearly constant rate of ~6.3 LD— Lepontine Dome; B—Bergell pluton; Arg—Argenetra Massif; G—Gonfolite; SL—Sesia- mm/a between 30 Ma and 11 Ma (Fig. 4). This Lanzo. A–B schematic cross section through Central Alps (modifi ed after Polino et al., 1990). rate was calculated using basin exhumation data Bottom key: 1—Eastern Australpine system (AU) with Eoalpine very low grade (A) and green- shist-amphibolite facies (B) metamorphism; 2—Platta-Arosa (PA) and Malenco-Avers (MA) of Bertotti et al. (2006). Assuming that basin Piedmont ophiolite units and Valais (VA) ophiolite and fl ysch units; 3— fl ysch décollement exhumation was driven by deformation, the dis- units (mostly Cretaceous); 4—Cenozoic European molasse (A: MO) and Po Plain molasse tance between two different samples exhumed (B: Gonfolite); 5—Cenozoic Bergell (B) Periadriatic intrusion; PF—Penninic thrust front; within the basin divided by the difference in tim- HE—Ultrahelvetic, Helvetic and Duphoinois; NCA—North Calcareous Alps; IL—Insubric line; SA—Southern Alps; GO—Gotthard nappe; AD—Adula nappe; TA—Tambo nappe. ing of exhumation (constrained by [U-Th]/He thermochronology) gives the velocity of propagation of deformation into the foreland. This subcritical wedge conditions as inferred from The Central and Eastern Alps retro-fore- was coupled with signifi cant subsidence in the detrital thermochronology correlate with a land shows a more complex trend with overall foreland basin. Detrital ages from the Western period of rapid plate convergence (Schmid et decreasing exhumation between ca. 32 and 12 Alps retro-foreland show slow erosion after a al., 1996; Ford et al., 2006), whereas supercriti- Ma (Fig. 3D), suggesting supercritical condi- period of fast exhumation (38–32 Ma), suggest- cal wedge conditions correspond to slow plate tions. Zircon fi ssion track ages from Central ing that the Western Alps retro-wedge was in a convergence, suggesting a relationship between Alps–derived detritus, today preserved in the subcritical state between 38 and 32 Ma and in a plate convergence, shortening, and wedge state. Apennine foredeep, and apatite fi ssion track supercritical state after 30 Ma (Figs. 3C and 4).

1128 GEOLOGY, December 2009 Forlandward migration of deformation Source exhumation and deposition of eroded material Pro-foreland in the adjacent foreland Retro-foreland

o 16 i Subcritical trend

m Pro-wedge and/or retro-wedge behavior Supercritical trend

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s t B n

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a 120° C tB increa a 10 p 1

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i n c 2

t e decreasing exhumation

e o tC 8

t 350° C 1 r

t a ation increasing exhumation

u 6

E e.g., tA:cooling age at time tA < tB < tC 4 Detrital cooling age Figure 2. A: Schematic sketch of exhumation through a Coulomb wedge (modiﬁ ed after De- 2 Celles and Mitra, 1995); α—surface slope, β—basal décollement angle, θ = α + β. B: Lag time 0

trends representing different orogenic wedge states (see text for explanation). 1—Subcritical Propagation of deformation (mm/yr) 5 10 15 20 25 30 35 40 behavior; 2—supercritical behavior. Time (Ma) Figure 4. Rates of migration of deformation toward foreland. Data from Central Alps are B Western Alps pro-foreland C Western Alps retro-foreland from Burkhard and Sommaruga (1998) and 0 (12*) SE France foreland 0 data from Western Alps are after Bertotti et ZFT (YP) (7) Tertiary Piedmont Basin (40Ar/ 39 Ar ); YA al. (2006). (8) Eastern Tertiary Piedmont Basin ();YA40Ar/ 39 Ar

) outlier

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10 3 a Data from the Central and Eastern Alps

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I 1 ea e 0 12 Ma and reached subcritical conditions after 1 0 40 40 ca. 12 Ma. 10 20 30 40 50 60 70 Thermochronological age (Ma) 10 20 30 40 50 60 Thermochronological age (Ma) CONCLUSIONS Central-Eastern Alps pro-foreland D Central-Eastern Alps retro-foreland A This study demonstrates that both sides of (9) Central Alps foreland (1) Gonfolite-Alpine foreland (ZFT), YP (Honegg–Napf drainage), YA the orogen can be explained in the context of (40 Ar/ 39 Ar: youngest age; average error<5%) (2) Gonfolite-Alpine foreland (AFT), YP 0 0 (10) Central Alps foreland (ZFT: YP) (3) Marnoso Arenacea-Apennines (ZFT), YP critical taper theory. An inverse relationship (10*) Central Alps foreland (ZFT: YP) (4) Macigno-Modino-Apennines (ZFT), YP between source exhumation and propagation of (11) Eastern Alps foreland (AFT: YP) (4*) Macigno & Marnoso Arenacea (ZFT), YP 40 39 deformation toward the foreland is observed in

outlier (partially annealed after burial) ) (5) Gonfolite-Alpine foreland (Ar/ Ar ), YA

)

a a General lag time trend (6) Venetian-Friulian Basin,

M M both the pro-foreland and retro-foreland (Figs. 3

( ( 10 10 AFT; partially annealed.

g g and 4). However, the available detrital thermo-

a a change in tectono-thermal regime

e chronological data show that the source exhu-

D D subcritical mation history of the pro-wedge and the retro- 20 20 wedge were quite different and out of phase. supercritical

la g Furthermore, the foreland basin detrital ages la g t im t im e e 2 record continuous and coherent signals suggest- 0 0 30 30 ing either that short-term, local tectonic (or cli- supercritical? matic) events cannot be deciphered using detri- 10 20 30 40 50 60 tal thermochronology or, more likely, that such Thermochronological age (Ma) changes are irrelevant with respect to orogenic 10 20 30 40 50 60 wedge behavior over tens of millions of years. Thermochronological age (Ma)

Figure 3. Compilation of detrital thermochronological data from Alpine foreland basin depos- ACKNOWLEDGMENTS its. A: Central and Eastern Alps pro-foreland. B: Western Alps pro-foreland. C: Western Alps This study greatly benefi ted from scientifi c discus- retro-foreland. D: Central and Eastern Alps retro-foreland. The 40Ar/ 39Ar data reported are sions with Peter G. DeCelles and Sanjeev Gupta, and youngest ages (YA); apatite fi ssion track (AFT) and zircon fi ssion track (ZFT) data reported from constructive comments from fi ve anonymous are youngest populations (YP). 1—Spiegel et al. (2004); 2—Fellin et al. (2005); 3—Dunkl reviewers and Andrew Barth. 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); REFERENCES CITED 8— Carrapa et al. (2004a); 9—von Eynatten and Wijbrans (2003); 10—Spiegel et al. (2004); Barbieri, C., Carrapa, B., DiGiulio, A., Wijbrans, J., 10*—Bernet et al. (2009); 11—Kuhlemann et al. (2006); (12*) after Bernet et al. (2009). Average and Murrell, G., 2003, Provenance of Oligocene error in depositional (Dep.) ages is ~20% and average error in thermochronological ages is synorogenic sediments of the Ligurian Alps (NW <10% (in this case, error bars are undetectable on plots). Italy): Inferences on belt age and cooling history:

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