Tectonic Significance of Cenozoic Exhumation and Foreland Basin

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RESEARCH ARTICLE Tectonic signiﬁcance of Cenozoic exhumation and foreland 10.1002/2016TC004132 basin evolution in the Western Alps Key Points: Barbara Carrapa1, Andrea Di Giulio2, Nicoletta Mancin2, Daniel Stockli3, Roberto Fantoni4, • Topographic growth in the Alps is the 1 5 result of asymmetric and episodic Amanda Hughes , and Sanjeev Gupta

Oligo-Miocene exhumation 1 2 • Miocene exhumation and Department of Geosciences, University of Arizona, Tucson, Arizona, USA, Department of Earth and Environmental 3 retroforeland deformation indicate a Sciences, University of Pavia, Pavia, Italy, Department of Geological Sciences, University of Texas at Austin, Austin, Texas, phase of orogenic construction USA, 4ENI E. & P. Division, San Donato Milanese, Italy, 5Faculty of Engineering, Department of Earth Science and • Miocene orogenic growth is controlled Engineering, Imperial College, London, UK by an increase in accretionary ﬂux

Supporting Information: Abstract The Alps are the archetypical collisional orogenic system on Earth, and yet our understanding of • Supporting Information S1 processes controlling topographic growth in the Cenozoic remains incomplete. Whereas ideas and models • Table S1 on the Alps are abundant, data from the foreland basin record able to constrain the timing of erosion and • Table S2 • Table S3 sedimentation, mechanisms of basin accommodation and basin deformation are sparse. We combine 40 39 • Table S4 seismic stratigraphy, micropaleontology, white mica Ar/ Ar, detrital zircon (U-Th)/He and apatite • Table S5 fission track thermochronology to Oligocene-Pliocene samples from the retrowedge foreland basin (Saluzzo • Table S6 • Table S7 Basin in Italy) and to Oligocene-Miocene sedimentary rocks from the prowedge foreland basin (Bârreme Basin in France) of the Western Alps. Our new data show that exhumation in the Oligocene-Miocene was Correspondence to: nonuniform across the Western Alps. Topographic growth was underway since the Oligocene and B. Carrapa, exhumation was concentrated on the proside of the orogenic system. Rapid and episodic early Miocene [email protected] exhumation of the Western Alps was concentrated instead on the retroside of the orogen and correlates with a major unconformity in the proximal retroforeland basin. A phase of orogenic construction is recorded by Citation: exhumation of the proximal proforeland in both the Central and Western Alps at circa 16 Ma. This is Carrapa, B., A. Di Giulio, N. Mancin, D. Stockli, R. Fantoni, A. Hughes, and associated with high sedimentation rates, and by inference erosion rates, and suggests that an increase in S. Gupta (2016), Tectonic significance of accretionary flux associated with the dynamics of subduction of Europe under Adria controlled orogenic Cenozoic exhumation and foreland expansion in the Miocene. basin evolution in the Western Alps, Tectonics, 35, doi:10.1002/ 2016TC004132. 1. Introduction Received 18 JAN 2016 The Alps resulted from the collision between Eurasia and Adria in the early Cenozoic following the closure of Accepted 27 JUN 2016 the Tethyan ocean [e.g., Handy et al., 2010] and are characterized by a doubly vergent wedge, high relief Accepted article online 30 JUN 2016 (>3 km), and relatively high elevations. The Western Alps include the highest mountains within the Alpine orogen (Mont Blanc, 4810 m; Figure 1a) and are defined by an asymmetric topography with a steeper retroside (eastern) and a less inclined proside (western) (Figure 1b) and by Eocene-Oligocene high pressure (HP) to ultrahigh pressure (UHP) metamorphism followed by Oligocene-Neogene exhumation [e.g., Dûchene et al., 1997; Carrapa et al., 2003a; Jourdan et al., 2013; Malusà et al., 2011; Rubatto and Hermann, 2001]. The tectonic evolution of the Alps has been intensely studied and form the basis of some of the fundamental models on collisional orogens [e.g., Boyer and Elliott, 1982; Handy et al., 2010; Pfiffner, 1992; Schmid and Kissling, 2000]; however, the timing and mechanisms underlying orogenic growth as recorded by exhumation and basin subsidence in the prowedge and retrowedge basins of the Western Alps are in places still unresolved or debated [e.g., Bernet et al., 2001; Carrapa et al., 2003a; Bernet et al., 2009; Carrapa, 2009; Fauquette et al., 2015; Malusà et al., 2011; Schlunegger and Willett, 1999; Schlunegger and Simpson, 2002]. Episodes of rapid exhumation in the Alps have been associated, for example, to several processes including vertical extrusion, tectonic exhumation, and dynamic processes following slab tear (partly detached) or break-off or climate- enhanced erosion [e.g., Baran et al., 2013; Champagnac et al., 2007; Fox et al., 2015; Glotzbach et al., 2011; von Eynatten et al., 1999; Wolff et al., 2012]. Subsidence in the prowedge and retrowedge basins of the Alps has been attributed to flexure of the lithosphere [e.g., Carrapa and Garcia-Castellanos, 2005; Sinclair, 1997a] and has been related to hinterland loading, exhumation, and wedge dynamics [e.g., Carrapa, 2009; Schlunegger and Willett, 1999; Willett and Schlunegger,

©2016. American Geophysical Union. 2010]. The overall mechanisms of basin subsidence have been largely associated with a contractional regime All Rights Reserved. and ﬂexural processes [Carrapa et al., 2003b; Bertotti and Mosca, 2009]. Alternatively, the retrowedge basin of

CARRAPA ET AL. TECTONIC EVOLUTION OF THE WESTERN ALPS 1 Tectonics 10.1002/2016TC004132

the Western Alps (Tertiary Piedmont Basin; Figure 1) has been interpreted as the result of a combination of extension and back-arc thinning [Maino et al., 2013] and dynamic processes associated with northwestward subduction and subsequent southeastward rollback of the Adriatic plate under the European plate [Maffione et al., 2008]. Therefore, despite being one of the most studied orogenic systems on Earth, ongoing discussions exist on the mechanisms, timing, spatial distribution, and pattern of exhumation and erosion across the Western Alps, as well as on the cause of subsidence in its adjacent sedimentary basins. Understanding the timing and mechanisms of exhumation in the hinterland and basin subsidence in the foreland is essential for resolving the tectonics of the Western Alps and for contributing to testing current tectonic and geodynamic models. The most critical issue in interpreting processes controlling the evolution of the Alps to date is the lack of a model combining observations between the sedimentary record and the hinterland from both sides of the same orogenic system and reconciling similarities and differences along strike between the Western and Central Alps. Although along-strike correlations are difficult because of the noncylindrical geometry of the orogen, we will explore similarities and differences in the timing of erosion, basin evolution, and deformation along strike in an attempt to understand regional-scale tectonic and geodynamic processes. In this paper we investigate the record preserved in the proforeland and retroforeland basins of the Western Alps in order to understand the mechanisms driving erosion and deposition. Comparison between the erosional and deformational history of the Western and Central Alps will be used to interpret processes controlling orogenic growth. This study contributes to resolving the following scientific issues: (1) whether exhumation can be considered to respond in a steady way to changes in tectonics over the last 30 Ma [e.g., Bernet et al., 2001, 2009] or if changes in exhumation can be resolved and directly tied to orogenic wedge growth [e.g., Carrapa, 2009]; (2) how exhumation patterns, as recorded by eroded material deposited in the proforeland and retroforeland basins, may constrain the age of development of asymmetric topography observed today; and (3) how changes in erosion rates, basin deformation, and subsidence in the Oligocene- Pliocene reflect tectonic processes. This multifacetted data set, when combined with existing data, resolves the distribution and timing of exhumation and deformation across the Western Alps during the Cenozoic.

2. Geological Setting The European Alps are the result of Cretaceous to Cenozoic convergence and collision between the European and the African-Adriatic plates [e.g., Frisch, 1979; Handy et al., 2010; Stampﬂi and Marchant, 1997]. The Western Alps contain basement rocks (lower Penninic) characterized by high pressure (HP) and ultrahigh pressure (UHP) metamorphism forming the inner (eastern) side of the belt. The Briançonnais nappes, now exposed in the axial part of the belt, together with the lower Penninic, represent the European continental margin partially subducted and then exhumed during collision. The External Crystalline Massifs represent slices of the European continental crust accreted in front of the orogenic wedge during convergence and now form the outer (western) side of the belt (Figure 1). The inner side of the belt experienced HP metamorphism between circa 66 and 33 Ma with rapid exhumation continuing as late as circa 30 Ma [e.g., Agard et al., 2002; Cliff et al., 1998; Dûchene et al., 1997; Monié and Philippot, 1989; Monié and Chopin, 1991; Rubatto and Hermann, 2001; Schwartz et al., 2000]; rapid exhumation of high pressure-low temperature (HP-LT) rocks during the early Oligocene is also widespread in the internal Western Alps including the Ligurian Alps [Barbieri et al., 2003]. After Oligocene time average exhumation rate seems to have decreased [Beucher et al., 2012; Carrapa et al., 2003a; Jourdan et al., 2013; Malusà et al., 2005]; however, the signiﬁcance of changes in post-Oligocene exhumation recorded in both the retroforeland and proforeland basins remain largely unresolved. The Oligocene phase of deformation in the Western Alps corresponds to approximately 35 km of shortening and is interpreted as the response of Adria northward movement (indentation) with respect to Europe; this was followed by approximately 60 km of shortening during Miocene time related to European wedging [Schmid and Kissling, 2000]. Alternatively, orogen-parallel extension has been suggested to have affected

CARRAPA ET AL. TECTONIC EVOLUTION OF THE WESTERN ALPS 2 Tectonics 10.1002/2016TC004132

Figure 1. (a) Digital elevation model of the Alps with location of the Bârreme and Saluzzo Basins and of the studied wells (white circles); pink square defines the area in Figure 2a. (b) Topography across Western Alps (A-A′ profile); (c) simplified geological map of the Alps, modified after Handy et al. [2010]; and (d) structural profile across the Western Alps (A-A′ profile), modified after Ford et al. [2006].

the Alps regionally in the Miocene as a result of orogenic collapse or extrusion [Bistacchi et al., 2001; Champagnac et al., 2006; Wolff et al., 2012]. The connection between hinterland deformation and exhumation has been described by coupled erosion-deformation models and by a synthesis of thermochronological data from the Central Alps [Schlunegger and Willett, 1999]. Outer growth of the Central Alps after circa 20 Ma has been explained as either a result of a decrease in erosion rates or an increase in frontal accretion [Schlunegger and Simpson, 2002]. In the case of the Western Alps, flexural loading by the growing Alpine orogenic wedge was responsible for foreland basin development on both sides of the orogenic wedge since the Eocene-Oligocene [Carrapa and Garcia-Castellanos, 2005; Ford et al., 2004]. On the proside (western-northern) of the belt, starting at ~35 Ma, the Swiss Molasse basin migrated forelandward (northward) [Allen et al., 1991; Burkhard and Sommaruga, 1998; Ford and Lickorish, 2004] in response to hinterland deformation and exhumation, which incorporated parts of the proximal foreland basin into the orogenic wedge. Similarly, along strike to the west, a foreland basin developed between circa 55 Ma and 34 Ma as a response to northward convergence, crustal thickening, and loading by the Adriatic plate. This was followed by northwestward migration of the orogenic front and thrusting of the Umbrunais-Ubaye Nappes leading to the formation of wedge top basins such as the Bârreme basin starting at circa 35 [Ford et al., 2006; Joseph et al., 2012] and of the Valensole basin starting at circa 26 Ma until Pliocene time [Couëffé and Maridet, 2003; Ford et al., 1999]. On the retroside (western-southern) of the Western Alps the Tertiary Piedmont Basin (Figures 1a and 1c) started to form in the late Eocene to early Oligocene, and its subsidence increased in the Miocene [Carrapa et al., 2003b] largely in response to far-field contractional deformation superimposed on flexural

CARRAPA ET AL. TECTONIC EVOLUTION OF THE WESTERN ALPS 3 Tectonics 10.1002/2016TC004132

Figure 2. (a) Structural map and isobath map of the Messinian unconformity for the study area. The main velocity we use to create Figure 2a and convert travel time to depth is 2000 m/s. (b) Seismic stratigraphy along B-B′ proﬁle and (c) along A-A′ proﬁle (for location refer to Figure 1 and for coordinate of the wells see Supporting Information).

loading [Carrapa and Garcia-Castellanos, 2005]. Subsidence in the Saluzzo (Savigliano) and Alessandria Basins (Figure 1a) during the Miocene and Pliocene has been interpreted as a result of contractional deformation and crustal folding [e.g., Bertotti and Mosca, 2009] or alternatively as a response to upper plate extension related to rotation of the Corsica-Sardinia block associated with the rollback of the Apennine slab and con- comitant northwestward rollback of the European plate [Mafﬁone et al., 2008].

3. Methods Newly processed seismic lines through the Saluzzo area (courtesy of Ente Nazionale Idrocarburi (ENI)) retrowedge foreland basin of the Western Alps were used to interpret and correlate Moretta 1, Saluzzo 1, Saluzzo 2, and Sommariva del Bosco 1 wells (Figures 2 and 3). These wells were tied to the seismic lines using check shot survey. The correlation between Saluzzo 1 and Saluzzo 2 strata (for which new biostratigraphic data are available) provides a robust-integrated biochronostratigraphic framework (Figure 3). Seismic lines also provide information on the deformation history of the region and, together with well stratigraphy, allow for the timing of activity along structures down to approximately 2.5 km depth to be constrained (Figure 2). Although previous seismic lines were presented for the same general region [Mosca et al., 2005; Rossi et al., 2009;

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Figure 3. Biostratigraphic correlation between Saluzzo 1, Saluzzo 2, and Sommariva del Bosco 1 wells. Thermochronological data are also indicated.

Mosca and Bertotti, 2010; Mosca et al., 2010], our lines cover a larger area and better constrain the timing and location of Alpine verging structures (east verging reverse faults cutting basement), and we provide a different interpretation of Apennine-related structures (west verging). Micropaleontological analyses of foraminifera were performed on selected samples from the Saluzzo 2 and Sommariva del Bosco 1 wells (courtesy of ENI), from well cores and cuttings (60 samples from Saluzzo 2 and 31 from Sommariva del Bosco 1, respectively), to determine the depositional age of the sampled units (Figure 3) and to constrain the paleowater depth of the basin (Table S1 and Figures S1 and S2 in the supporting information). No published micropaleontological analyses were available before for these units. These new data were used as input parameters in the geohistory analysis (using the BasinMod 1-D software by Platte River Associates, http://www.platter.com) for the Saluzzo 2 and Sommariva del Bosco 1 wells presented in Figure 4 and which provides information on the subsidence/uplift history of the retrowedge foreland basin. This data set is summarized in the supporting information (Table S3). The methodologies used to reconstruct the paleobathymetric and geohistory curves are described in Di Giulio et al. [2013]. We refer to supporting information for further details about the geohistory analysis. We collected 13 samples from Oligocene-Pliocene strata from wells within the retrowedge Saluzzo Basin (seven pebbles from Saluzzo 1 and six sandstones from Saluzzo 2) and ﬁve sandstone samples from the Oligocene to lower Miocene prowedge Bârreme Basin for 40Ar/39Ar, zircon (U-Th)/He (ZHe) and apatite ﬁssion track (AFT) thermochronology (Table 1). Combined 40Ar/39Ar, ZHe, and AFT thermochronology constrain the provenance and the approximately 350–120°C time-temperature history of the sample sediment sources and sedimentary basin [Carrapa, 2010]. Unfortunately, no samples were available from the basement at the

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Figure 4. Total subsidence (decompaced) curves calculated for the Saluzzo 2 and Sommariva del Bosco 1 wells. The average sedimentary rate curves are also indicated (in red). On the right we show the simpliﬁed chronostratigraphy and the paleobathymetric evolution along the studied wells as recorded by fossil foraminiferal assemblages. The complete micropaleontological data set is available in the supporting information (Figures S1 and S2 and Tables S1–S3).

CARRAPA ET AL. TECTONIC EVOLUTION OF THE WESTERN ALPS 6 ARP TA.TCOI VLTO FTEWSENAP 7 ALPS WESTERN THE OF EVOLUTION TECTONIC AL. ET CARRAPA

Table 1. Summary of Thermochronological Data Depo Weighted 40 39 40 39 Sample Name Age Mean W. M. AFT Ar/ Ar Age Ar/ Ar Age a (Depth) Formation Lithology (Ma) Error Extra Information ZHe Age Error Age Error Age/YP Error YA Error PA/YP Saluzzo 1 S1-461-1 molare Fm. matemorphic 20 1 approximate age based on 20.6 1.7 24.7 6.8 17.6 2.4 46.5 pebble correlation with Saluzzo 2

S1-461-2 21.4 1.7 Tectonics S1-461-3 32.6 2.6 S-1-461-4 41.1 3.3 S1-552-1 molare Fm. matemorphic 20.5 1 approximate age based on 22.4 1.8 20.3 1.7 18.4 2 pebble correlation with Saluzzo 2 S1-552-2 18.1 1.4 S1-552-3 21.9 1.8 S1-552-4 20.1 1.6 S1-700-1 molare Fm. matemorphic 21 1 approximate age based on 20.2 1.6 20.5 0.4 37.3 pebble correlation with Saluzzo 2 S1-700-2 20.1 1.6 S1-700-3 21.1 1.7 S1-1184-1 molare Fm. matemorphic 22 1 approximate age based on 25.7 2.1 22.6 1.9 32.08 pebble correlation with Saluzzo 2 S1-1184-2 22.3 1.8 S1-1184-3 20.8 1.7 S1-1461 molare Fm. matemorphic 23 1 approximate age based on 46.5 pebble correlation with Saluzzo 2 S1-1523-1 molare Fm. matemorphic 23 1 approximate age based on 25.0 2.0 19.1 5.7 pebble correlation with Saluzzo 2 S1-1523-2 27.7 2.2 S1-1523-3 23.0 1.8 S1-1523-4 13.8 1.1 S1-1528 molare Fm. matemorphic 24 1 approximate age based on 25.5 3.1 pebble correlation with Saluzzo 2 Saluzzo 2 S2-903 lower asti group sandstone 4 1 age based on bio 35.04 0.14 61 S2-1101 lower Asti group sandstone 4.4 0.9 age based on bio 33.5 2.74 57 S2-1391 Sartirana sandstone sandstone 6.6 0.6 age based on bio 17.7 1.4 S2-1850 Gallare group sandstone 15.3 0.5 age based on bio 28.8 2.3 44 S2-2055 Gallare Group sandstone 15.8 0.6 age based on bio 30.6 2 42 S2-2494 molare Fm. sandstone 25 1.5 age based on 16.0 2.4 26.7 0.8 33 bio/thermochron. Bârreme Basin GUI-046 Grés Verts sandstone 23.8–22 181 SG_049 (n = 30) Série Grise sandstone 25–23.8 from literature 251

MR2_052 (n = 24) molasse rouge sandstone 28.5–26 from literature 51 10.1002/2016TC004132 Slb1_UPT1-32 Conglomérat sandstone 30–28.5 from literature 28.2 de St. Lions CL1-048 Conglomérat sandstone 30–28.5 from literature 55 de Clumanc (La Poste member) aThe age and errors are calculated by interpolating between the youngest and oldest availabe biostratigraphic ages (Table S3) or thermochronological ages for the same sample or for samples above and below; when the thermochronological age is used, the error is the analytical error; age range for the Bârreme Basin is from the literature (see text and Figure 8 for explanation); ZHe ages in italics: interpreted as detrital and not considered for the mean age; YA: youngest detrital age; excluding ages with small errors and or small grain sizes; YP: youngest detrital peak; PA: plateau age; error for AFT is 1 s and for 40Ar/39Ar and Zhe is 2 s; and in bold are the ages used in Figure 7. Tectonics 10.1002/2016TC004132

bottom of the Saluzzo wells; however, the lithologies of the pebbles provide information on the provenance of these deposits (Table 2). Four of the seven pebbles from Saluzzo 1 were analyzed for 40Ar/39Ar step heating analysis, five for ZHe, and three for AFT analyses. Five sandstone samples from the Saluzzo 2 well were analyzed for detrital 40Ar/39Ar thermochronology; samples S2_2492 and S2_1391 were also analyzed for detrital AFT thermochronology (Figures 3, 5, and 6). The number of analyses was dictated by the availability of material (sample size) and apatite yield, which was generally low such that 25 and 55 grains could be analyzed for samples S2-2492 and S2-1391, respectively. For detrital 40Ar/39Ar analyses, 100 white mica grains were targeted for single grain total fusion experiments (Table S4 in the supporting information). In addition, five crystalline basement pebbles from the nearby Saluzzo 1 well were collected from the basal conglomerate for 40Ar/39Ar step-heating analyses on mica, zircon (U-Th)/He (ZHe), and AFT analyses (Figure 3) (Tables S4, S5, and S6). 40Ar/39Ar analyses were performed at Arizona State University and at the University of Potsdam, (U-Th)/He at University of Texas at Austin, and AFT at the University of Arizona; for details on analytical procedures we refer to the supporting information. Detrital data are represented using probability density functions, and populations are calculated using DensityPlotter for 40Ar/39Ar and fission track data [Vermeesch, 2012]; the program was run in automated mode. We refer to supporting information for more details. Four samples from the Bârreme Basin from the Conglomérat de Clumanc, Molasse Rouge, Série Grise, and Grés Verte were analyzed for detrital white mica 40Ar/39Ar thermochronology (Table S4 and Figure S3 in supporting information) (Figure 6); one sample from the Conglomérat de St. Lions was analyzed for detrital apatite fission track (AFT) thermochronology (Table S5 in the supporting information).

4. Results 4.1. Seismic Interpretation Seismic surveys were performed in the studied area from 1977 until 1982 for a total amount of about 300 km of acquired lines. Seismic calibration is provided by three hydrocarbon wells drilled from 1957 (Saluzzo 1 and 2 wells) until 1991–1992 (Sommariva del Bosco 1 and Moretta 1 wells; Figure 2). The wells are located in different structural positions and reach different depth and stratigraphic horizons; Sommariva del Bosco 1, Saluzzo 1, and Saluzzo 2 wells drill the Oligocene-Miocene clastic sequence at a depth ranging between about 1500 m (Saluzzo 1) and 3800 m (Sommariva del Bosco 1), whereas the Moretta well, located on a structural high, reaches about 3100 m depth and drills the basement top at around 2900 m (Figure 2). From a structural point of view, the studied area is quite complex as imaged by seismic (Figure 2) and mostly characterized by clear compressional structures (reverse faults, thrusts, and related anticlines) with two main senses of motion an east (Alpine) and north-northwest sense of motion (Appenine) [Bertotti and Mosca, 2009; Pieri and Groppi, 1981, Figure 2]. These structures moved along décollement surfaces that divide the sedimen- tarycoverfromthecrystallinebasementandonlytheoutermost(northernmost)oneseemtoinvolvetheupper- most part of the basement. The east verging reverse faults involving basement are interpreted on the basis of different amplitude and frequency of the reﬂectors on the different sides of the structure. This interpretation is consistent with similar structures reported by the CROP proﬁle for the Western Alps [Bernabini et al., 2003]. The main north-northwest verging thrust in Figure 2c (1) is interpreted as a fault-propagation fold juxtaposed onafaultbendfoldalongthetopofthebasement.Thedashedlineisinterpretedasasynclinalaxialsurfaceasso- ciatedwiththetipofthethrust. Structuralgrowth intheMessinianisevidencedbythenarrowingoftheforelimb width and rotation of backlimb dip in Figure 2c (2), as well as by the onlap onto the top of the Tortonian. Late reactivation in the late Pliocene/Pleistocene is shown by narrowing of the forelimb in Figure 2c (3). Calibration of seismic lines through wells data documents that these structures grew during the Tortonian- Messinian and late Pliocene-Pleistocene time. These north vergent late Miocene to Pleistocene Apennines- related structures cut an older east vergent structure responsible for the tectonic-related elevation of the top of the crystalline basement in the Saluzzo 1 well with respect to the Saluzzo 2 (Figure 2a).

4.2. Biostratigraphy Foraminiferal assemblages from the Saluzzo 2 and Sommariva del Bosco 1 wells vary from rare, in the coarse- grained strata, to quite abundant in the ﬁne-grained marly intervals (Figure 3 and Table S1 in the supporting

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Table 2. Petrological Description of Pebbles From Saluzzo 1 Sample Mineral Assemblage Petrological Interpretation

S1-947 Sillimanite, Muscovite, Clinopyroxene (diopside), Kyanite, Schist, Pelitic rock- Amphibolite facies Quartz, Orthopyroxene, Amphibole (horneblende) S1-1184 Muscovite, Quartz, Epidote, Feldspars Orthogneiss, Greenshist facies S1-700 Quartz, Garnet, Epidote, Biotite, Muscovite, Zircon Paragneiss, Epidote Amphibolite facies S1-461 Quartz, Garnet, Epidote, Biotite, Muscovite, Zircon, K-Feldspar Paragneiss, Epidote Amphibolite facies S1-1528 Quartz, Garnet, Epidote, Biotite, Muscovite, Zircon, K-Feldspar, Clinozoisite Paragneiss, Epidote Amphibolite facies S1-552 Quartz, Muscovite, Biotite, Clinozoisite, Epidote, Zoesite, Cordierite Paragneiss, Epidote Amphibolite facies

information). Microfossils are usually well preserved, diversified, and characterized by a fairly continuous occurrence of both planktonic and benthic taxa. This allows for the biostratigraphic dating of the sampled intervals to be constrained and provides detailed chronostratighaphic ages of the samples collected for thermochronological analyses (Figure 3). Furthermore, they provide the age inputs for paleobathymetric and geohystory analyses (Figures 4, S1, and S2 in the supporting information). Unfortunately, the coarse-grained samples from the lower portion of the Saluzzo 1 well are barren in foraminifera; therefore, the stratigraphic age derives from seis- mostratigraphic correlation with the adjacent Saluzzo 2 and Sommariva 1 wells (Figures 2 and 3). The turbidite deposits from the lower portion of the Saluzzo 2 well, from the bottom to the first erosional unconformity at a depth of 2250 m, mostly contain reworked foraminiferal species (e.g., Paragloborotalia opima opima, Globoquadrina sellii, and Zeaglobigerina ampliapertura, Table S1 in the supporting information); only in the three uppermost samples (from 2290 m to 225 m depth) sporadic specimens of the younger species Paragloborotalia kugleri are found (Figure 3 and Table S1 in the supporting information). The first appear- ance of Paragloborotalia kugleri is dated at circa 23.7 Ma [Wade et al., 2011] and is also found in Aquitanian samples [Mancin et al., 2007]. This combined with the youngest 40Ar/39Ar age 26.7 ± 0.8 Ma (Figure 5) from sample S2-2494 constrains the stratigraphic age of the deepest sample to circa 27–23.7 Ma and the stratigraphic age of strata below the unconformity to circa 23.7–20.5 Ma (Figure 3). The first erosional unconformity (at 2250 m depth, Figure 3) is covered by a fine-grained succession, made in the lower to middle part (up to 1500 m depth) by marly deposits, Langhian to Tortonian in age (Praeorbulina spp.—Globorotalia menardii/Neogloboquadrina acostaensis zones, from circa 16.5 to circa 7.25 Ma). This unconformity, being bounded by Chattian-Aquitanian sedimentary rocks (below) and Langhian units (above), is here interpreted to be Burdigalian in age, and the stratigraphic gap it contains can be estimated to be at least 5 Ma long. Above Tortonian sedimentary units, the upper portion of the Miocene sequence (from 1500 m up to 1390 m depth) is made by evaporitic deposits containing typical oligotype foraminiferal assemblages with abundant Bulimina and Bolivina taxa and common gypsum crystals (Table S1 in the supporting information) both indicative of an early Messinian age (circa 7.25–6.1 Ma). These data are supported by the results collected from the adjacent Sommariva del Bosco 1 well (Figure 3 and Table S1 in the supporting information), which is from a more depocentral position within the basin (Figure 2), thus recording a thicker and more continuous succession. The Langhian-Messinian sequence is topped by a second unconformity (at 1309 m depth in Saluzzo 2, Figure 3) formed during the upper Messinian sea level drop dated at circa 5.5 Ma [Roveri et al., 2014]. The upper and youngest portion of the drilled succession consists of marly to sandy deposits containing abundant marine macrofossils and lower Pliocene foraminiferal taxa (MPl 2–MPl 3 to NPD1 zones in the scheme of Mancin et al. [2007]; circa 5.3–3.3 Ma); these deposits trend upward, through an erosional unconformity that omits the upper Pliocene strata, toward continental sandstone and conglomerates which are barren in foraminifera, and are supposedly Pleistocene to Holocene in age (Figure 3). 4.3. Paleobathymetry ThepaleobathymetricdatacollectedfromtheSaluzzo2andtheSommarivadelBosco1wellsarereportedinthe supportinginformation(FiguresS1,S2,andTableS1)andsynthesizedinFigure4.Thereconstructedpaleobathy- metry is indicated as depth ranges (minimum, average, and maximum) following the bathymetric subdivision proposed by Van Morkhoven et al. [1986]. Displaced benthic index species are rare and limitedly distributed in certain stratigraphic intervals (e.g., the Serravallian-lower Tortonian sediments in the Saluzzo 2 well; Figure S1 in the supporting information); thus, foraminiferal assemblages provide reliable paleobathymetric information.

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Figure 5. Thermochronological data from Saluzzo 2; probability density functions and populations (in pink) are calculated using DensityPlotter [Vermeesch, 2012].

In the Saluzzo 2 well, the lower Miocene turbiditic deposits (up to the Burdigalian unconformity) contain poorly preserved foraminiferal assemblages characterized by very rare planktonics (P% < 5) in assemblage with sporadic benthic indexes, such as Gyroidinoides girardanus, Oridorsalis umbonatus, Vulvulina pennatula, and the genus Recurvoides. Since these species occur within the same sample but indicate different water depths, from upper bathyal to abyssal bathymetries (Table S2 in the supporting information), the proposed paleobathymetric range of 600–1000 m must be taken with caution because the species assemblage could

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indicate reworking; at the same time, the proposed paleobathymetry (600–1000 m), which is based on the upper depth limit of the deepest indexes, can be assumed as the minimum possible depositional depth. Unfortunately, in the adjacent Sommariva del Bosco 1 well, coeval strata were not drilled as the well stops before reaching the Burdigalian unconformity (Figure 3). The overlying Langhian-Serravallian deposits (up to 2074 m depth, Figure S1 in the supporting information) contain quite abundant foraminiferal assemblages, characterized by frequent planktonics (P% = 50) in assemblage with benthic indexes indicative of upper bathyal bathymetries, varying from 200 to 600 m (Anomalinoides helicinus, Cibicidoides pseudoungerianus, and Lenticulina spp., Heterolepa dertonensis and Recurvoides spp., and frequent Uvigerina spp.). In lower Tortonian deposits, the deepest middle to lower bathyal indexes (e.g., Cibicidoides wuellerstorﬁ, Oridorsalis umbonatus, and Uvigerina auberiana) ﬁrst appear or increase in abundance, whereas planktonic foraminifera furtherly increase, reaching abundances of 80% to 95%. This faunistic change can be interpreted as due to a progressive increase in the depositional depth, passing from approximately 400 m to more than 600 m. Toward the top of the Miocene sequence, in upper Tortonian to lower Messinian strata, foraminiferal assemblages change further, indicating another paleowater depth shift. Planktonic species rapidly decrease (P% value changes from 95% to 45% then reaching 0% in samples from the evaporitic layers), whereas neritic species (the genera Bolivina and Brizalina, and Cibicidoides ungerianus) increase. In the uppermost Messinian strata, foraminifera become very rare to absent and characterized by sporadic specimens belonging to the genus Ammonia indicative of a littoral environment. The reconstructed paleobathymetry indicates an upper neritic paleodepth (approximately 0–50 m), transitioning upsection to transitional/continental environments at the top. A similar Miocene paleobathymetric trend can be reconstructed for the Sommariva del Bosco 1 well (Figure S2 in the supporting information) although most of the analyzed samples contain very rare foraminifera (Table 1 in the supporting information). The lower Pliocene samples document the reestablishment of open marine conditions in the basin after the Messinian Salinity Crisis. In both wells, fossil assemblages are characterized by rare planktonic foraminifera (P % < 1%) in assemblage with benthic species indicative of upper bathyal environments (200–600 m), such as C. pseudoungerianus, Heterolepa bellincionii, Lenticulina spp., Planulina arimininens, Siphonina reticulata, and Uvigerina rutila. In the samples from the upper Zanclean strata, the shallower neritic indexes (e.g., the genera Elphidium and Ammonia, miliolids, and A. helicinus) markedly increase in abundance, documenting a progressive decrease in water depth from approximately 0–100 m to approximately 0–25 m. In the Pleistocene portion of the well, foraminifera rapidly disappear indicating the transition to more proximal subaerial environments as supported by deposition of sandstones and conglomerates.

4.4. Geohistory Analysis Geohistory analysis was performed on both the Saluzzo 2 and Sommariva del Bosco 1 wells on the basis of input data reported in the supporting information (Table S3). The resulting total subsidence/uplift curves calculated for the average paleowater depths and decompacted sedimentation rates are shown in Figure 4. During the latest Oligocene to early Miocene (circa 26–21 Ma), the Saluzzo Basin experienced a weak subsidence phase that is fully balanced by sedimentation (100 m/My) as indicated by the constant bathymetry in the Saluzzo 2 well; this is followed in the Burdigalian/earliest Langhian (circa 21–16 Ma) by a probable uplift event that caused a sharp bathymetric shallowing and the consequent subaereal erosion of part of the basin. During the early Langhian to the late Messinian (circa 15.8–5.5 My), the basin records a continuous subsidence trend, initially fast and then slower, 1 order of magnitude higher in the depocenter (Sommariva well) and lower toward the basin margin (Saluzzo2 well); this is counterbalanced by basin-wide high sedimentation rates (ranging from 150 m/My (Saluzzo well) to over 600 m/My (Sommariva del Bosco well) responsible for the progressive decrease of the paleobathymetry. This subsidence trend is interrupted at circa 6 Ma by the abrupt sea level drop due to the late Messinian salinity crisis that caused extensive erosion of the basin [Ghielmi et al., 2013; Manzi et al., 2005; Roveri et al., 2003; Roveri et al., 2014]. The Pliocene to Quaternary history of the Saluzzo Basin (since circa 5 Ma) records an ongoing subsidence trend with high sedimentation rates reaching values of circa 800 m/My during the Zanclean (5.3–3.6 Ma). This was followed by uplift during late Pliocene-Pleistocene, which rapidly caused the basin to ﬁll leading to the development of the modern alluvial plain.

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4.5. Thermochronology and Geochronology Overall the Miocene-Pliocene samples from the Saluzzo 2 well (Figure 5) show significant Cenozoic detrital 40Ar/39Ar components between circa 30 and 55 Ma and Cretaceous and Jurassic signals (Figure 5). The deepest and oldest sample, S2_2492, shows a youngest 40Ar/39Ar cooling age of 26.7 ± 0.8 Ma (Figures 3 and 5). Considering that the stratigraphic age for this sample is Chattian (circa 27–23.7 Ma), this results in a relatively short lag time (i.e., difference between detrital cooling age and depositional age) [Brandon and Vance, 1992] of no longer than circa 3 My. The youngest detrital 40Ar/39Ar ages and populations for Saluzzo 2 sandstone samples seem to increase upsection suggesting a provenance signal (Figure 7). The two sand samples (S2- 2494 and S2-1391; Table 1) analyzed for detrital AFT thermochronology give youngest populations at 16 ± 2.4 Ma and 17.7 ± 1.4 Ma, respectively (Figure 6). We note that the youngest population for sample S2- 2494 is younger than the Chattian (circa 27–23.7 Ma) depositional age determined by biostratigraphy and thus indicates partial annealing of fission tracks in apatites. Basement pebbles from Saluzzo 1 well, sourced from medium to high grade metamorphic rocks (Table 2), show Miocene ZHe and AFT ages, which are all older than the depositional ages of the hosting strata and range between 24.7–19.1 and 25.5–17.6 (Table 1; Figure 3). Four of the same clasts were also analyzed by 40Ar/39Ar step-heating analyses and produced Eocene-Oligocene plateau ages (Figure S3). Selected samples from the proside foreland basin (Bârreme Basin) were also analyzed for detrital 40Ar/39Ar and AFT thermochronology. Sample CL1-048 from the Conglomérat de Clumanc (Late Rupelian) [Carbonnel et al., 1972; Joseph et al., 2012], and references therein] displays a range of 40Ar/39Ar ages from ~40 to ~320 Ma and a significant population at circa 56 Ma (Figure 8). Detrital AFT data from sample Slb1_UPT1-32, from the time equivalent Conglomérat de St. Lions, show two populations at circa 28 and 42 Ma (Figure 8) (26 and 45 Ma Binomftit populations; Table S5). Sample MR2-052, from the Molasse Rouge (early Chattian), shows mostly Carboniferous-mid Triassic 40Ar/39Ar ages and only a few Cenozoic (~50 Ma) ages. Sample SG-049 from the Série Grise (late Chattian) exclusively displays Carboniferous to middle Triassic 40Ar/39Ar ages and sample GUI-046, from the Grés Verts (early Aquitanian), contains Permian- Carboniferous ages with a few early Jurassic ages.

5. Interpretation 5.1. Dynamics of the Retrowedge Foreland Basin Margin Revealed by Combined Seismic Biostratigraphy and Geohistory Analyses The integrated record of seismic stratigraphy and biostratigraphy indicates that since the latest Oligocene to early Miocene to the present day the retroforeland basin margin was characterized by three main steps of deposition, separated by two prominent unconformities. The ﬁrst unconformity is described by westward onlap of Langhian-Tortonian beds (drilled by Saluzzo 2 well) on top of lower Miocene conglomerates (drilled by Saluzzo 1 well) (Figures 2 and 3). This unconformity records a transient period of erosion of the basin margin that followed the climax of the coarse progradational sedimentary episode responsible for the fast deposition of approximately 1 km of fan delta conglomerates along the basin margin (Saluzzo 1 well). According to the geohistory analysis (Figure 4), this erosional event seems to be linked to a short-lived phase of uplift of the retrowedge foreland basin margin during approximately Burdigalian time (circa 20–16 Ma). This history is supported by the east verging structure monitored by seismic in the Saluzzo 2 well and interpreted as an Alpine verging basement involved reverse fault overlain by onlap termination of the Langhian- Tortonian clastic wedge (Figure 2). Evidence of retrowedge deformation is also present in the Central Alps where the Monte Olimpino back-thrust in the Como area was active in the early Miocene and was responsible for uplift of the Gonfolite Group foreland basin deposits [Bernoulli et al., 1989]. Above the Burdigalian unconformity the sedimentary sequence records an overall shallowing upward trend within a fairly regularly subsiding basin represented by the Langhian-Messinian deposits truncated by the upper Messinian erosional unconformity linked to the Messinian Salinity Crisis. Above the upper Messinian unconformity, the youngest basin history is characterized by a well-developed transgressive-regressive Pliocene-Pleistocene cycle; the lower transgressive part of the cycle records the early Pliocene reestablishment of open marine conditions after the end of the Messinian salinity crisis, followed by a progradational trend that seems to be promoted by increase of sediment supply possibly due to climate-driven enhanced

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Figure 6. Kernel distributions and radial plots of detrital AFT data from two Saluzzo subsurface samples (S2_2492 and S2_1391); for stratigraphic information refer to Figure 3.

erosion [Kuhlemann et al., 2002; Mancin et al., 2009; Valla et al., 2011; Vernon et al., 2008]. Pliocene-Pleistocene structures revealed by seismic data are interpreted to have developed during growth of the Monferrato hills related to the Northern Apennines northward accretion (Figure 2). Overall the retrowedge, including the Tertiary Piedmont and Saluzzo depocenters, is characterized by a relatively deep, gently deformed basin. Oligocene-Miocene subsidence in the retrowedge foreland can be explained by ﬂexure due to loading and growth of the Western Alps [Carrapa et al., 2003b]; later overprinted by north verging Apennines deformation mostly in the late Miocene-Pliocene.

5.2. Exhumation of the Western Alps Revealed by Thermochronology In the retroside foreland basin, Cenozoic 40Ar/39Ar ages recorded in Saluzzo 1 and 2 samples are typical of HP- LT sources within the axial Western Alps. In particular, Oligocene thermochronological ages are typical of the Dora Maira crystalline Massif [Rubatto and Hermann, 2001; Di Vincenzo et al., 2006; Beucher et al., 2012], Grand Paradiso, Monte Rosa [Rubatto and Gebauer, 1999; Gabudianu Radulescu et al., 2009], and the Ivrea-Verbano zone [Wolff et al., 2012] suggesting widespread exhumation of the internal Western Alps at this time. This is in agreement with the metamorphic assemblages of the analyzed pebbles (Table 2) typical of the Dora Maira massif [Chopin et al., 1991]. On the contrary, Carboniferous-Permian 40Ar/39Ar ages are typical of Briançonnais sources [e.g., Barbieri et al., 2003; Hunziker et al., 1992] including the Briançonnais-derived covers of the Dora Maira Massif. The dominant signal of Cenozoic ages in Miocene-Pliocene deposits suggests a source from deeply exhumed rocks within the axis of the Alps at this time. The fact that the lithologies, metamorphic grade (Table 2) and AFT ages (25.5–17.6 Ma; Table 1) of pebbles in Saluzzo 1 are consistent with a provenance from the Dora Maira Massif [Beucher et al., 2012; Chopin et al., 1991] combined with the similarity between ZHe and AFT ages of the pebbles indicates rapid exhumation of the inner side of the Western Alps in the late Oligocene-Miocene followed by slower erosion starting at circa 16 Ma (Figure 7). The longer lag times recorded by the youngest detrital 40Ar/39Ar age components for the Saluzzo samples suggest that basement covers were eroded together with a rapidly exhumed basement and incorporated into the synorogenic deposits.

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Figure 7. Lag time plot calculated from the new Saluzzo well samples and samples from the Torino Hill and from the French proside basins [Jourdan et al., 2013; this study].

In the proside foreland basin Cenozoic detrital 40Ar/39Ar ages from the Conglomérat de Clumanc (late Rupelian) from the Bârreme Basin are consistent with Cenozoic plateau ages from HP-LT cobbles from the same conglomeratic units [Morag et al., 2008] and with ZFT ages reported for the same deposits [Jourdan et al., 2013] (Figure 8). Apatite U-Pb geochronology of the Conglomérat de Clumanc shows a peak at circa 85 Ma [Mark et al., 2016] supporting the interpretation that mica 40Ar/39Ar ages represent exhumation ages. Apatite U-Pb ages from the Molasse Rouge instead show peaks at circa 36 and 49 Ma [Mark et al., 2016]; whereas the 49 Ma peak is within error from the youngest white mica 40Ar/39Ar population, suggesting cooling and exhumation following metamorphism, the youngest apatite U-Pb peak at circa 36 Ma indicates a different source for apatite than for mica in this sample highlighting the importance of multidating for resolving provenance and tectonic-exhumation histories. Overall these data indicate erosion of two different sources, one characterized by Cenozoic cooling ages and the other by Carboniferous to middle Triassic cooling ages. The Cenozoic ages are recorded by HP-LT blushiest facies metamorphic pebbles indicating unequivocal provenance from inner Alpine units [Morag et al., 2008]; this is consistent with the age of HP-LT rocks found in the core of the Western Alps [e.g., Ford et al., 2006, and references therein], whereas the Carboniferous to middle Triassic ages are typical of LT external Briançonnais units and external crystalline massifs [e.g., Barbieri et al., 2003; Corsini et al., 2004]. The fact that Cenozoic ages are only present in the upper Rupelian Conglomérat de Clumanc and lower Chattian Molasse Rouge (Figure 8), whereas they disappear up-section, suggests that a paleodrainage reorganization occurred in the Western Alps at circa 26 Ma; this is agreement with a change

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Figure 8. White mica 40Ar/39Ar and AFT (Slb1_UPT1-32) thermochronological data from the Bârreme Basin; for location refer to Figure 1; simpliﬁed stratigraphic scheme modiﬁed after Jourdan et al. [2013].

in provenance at circa 26 Ma based on apatite U-Pb data discussed by Mark et al. [2016] and interpreted to be related to paleodrainage reorganization. The occurrence of widespread Cenozoic cooling ages in Miocene- Pliocene detrital minerals in the retrowedge basin and their lack in the proside basin suggest asymmetric drainage and higher magnitude of erosion of the internal Western Alps with respect to the external Western Alps during this time interval.

6. Discussion The new thermochronological, stratigraphic, seismic, and subsidence history results presented in this paper, combined with existing information from the literature, provide the basis for evaluating the morphotectonic evolution of the Western Alps during the Oligocene-Pliocene. In the early Oligocene the prowedge foreland basin (Bârreme Basin) recorded a phase of rapid exhumation involving Alpine metamorphic units in the axial part of the belt as supported by the Oligocene synorogenic conglomerates with Alpine Cenozoic 40Ar/39Ar signature (Conglomérat de Clumanc) and very short lag times (Conglomérat de St. Lions) (Figures 7 and 8). This exhumation phase is consistent with exhumation recorded by retrowedge proximal foreland basin deposits derived from the Ligurian Alps [e.g., Barbieri et al., 2003; Jourdan et al., 2013] and seems to represent a regional-scale exhumation event affecting the axial part of the belt characterized by Alpine-age

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metamorphism. Material from this exhumation event was able to feed foreland basins on both sides of the orogenic system. Oligocene hinterland exhumation is coupled with significant shortening in the Western Alps [Ford et al., 2006; Schmid and Kissling, 2000] and with a general increase of Alpine-sourced sediment yield in the surrounding foreland basin [Kuhlemann and Kempf, 2002; Kuhlemann et al., 2001, 2002, 2006]. This suggests Oligocene topographic growth of the Alpine belt in agreement with Fauquette et al. [2015]. Topographic growth in the Oligocene may have been related to slab break-off, which has been suggested to be responsible for changes in foreland basin subsidence, uplift, and erosion in the mid-Oligocene [Sinclair, 1997b]. During the latest Chattian-Aquitanian the prowedge side (Bârreme Basin) records the disappearance of white micas with Cenozoic (Alpine) 40Ar/39Ar ages, and the predominance of material derived from sources characterized by Carboniferous-Triassic 40Ar/39Ar ages suggesting drainage reorganization. We interpret this to be the result of a shift of the orogenic front and drainage divide toward the proside of the belt (westward), consistent with upper Oligocene forward propagation of deformation progressively involving more external units [Ford et al., 2006]. A plausible scenario is that part of the drainage divide is atop the Argentera massif between circa 28 and 20 Ma [Mark et al., 2016]. In the retrowedge side of the belt, the predominance of a Cenozoic 40Ar/39Ar signature in Miocene-Pliocene detritus records a provenance from HP units of the axial and internal part of the belt. This supports a shift of the drainage divide, which starting in the Miocene seems to coincide with the tectonic boundary between the Briançonnais complex, delivering material to the west and HP units delivering material to the east. Very short to zero lag times recorded by ZHe and AFT ages in lower Miocene synorogenic conglomerates from the retroside of the belt (Saluzzo Basin) support a phase of rapid exhumation of the inner part of the orogenic wedge between circa 25 and 16 Ma (Figures 3 and 7). This rapid exhumation phase correlates with an important phase of conglomeratic deposition within the retroside foreland basin (Saluzzo 1 well), which culminates into an erosional unconformity involving the basin margin at circa 17–16 Ma (Figures 2 and 3). The resulting basin unconformity coupled with the rapid exhumation of the basin margin suggests a phase of lateral growth of the retrowedge during the early Miocene (Figure 4). Evidence of synsedimentary structures associated with NE-SW contraction between circa 16 and 11 Ma are present in the Tertiary Piedmont Basin [Carrapa et al., 2003b]. This coupled with the newly observed Alpine east verging basement structure active in the early Miocene at the Saluzzo Basin margin indicates that the retrowedge basin was deforming internally on top of the wedge at this time. Deformation on the proside of the orogenic system propagated westward incorporating the Bârreme Basin into a wedge-top position starting in the mid-Oligocene [Evans and Elliott, 1999]. The Western Alps were thus expanding on both sides in the early to middle Miocene. Sediment budget analyses [Kuhlemann and Kempf, 2002; Kuhlemann et al., 2001, 2002, 2006], although we recognize they bear large uncertainties, show two peaks in bulk denudation rates for the Western Alps at circa 22 and 18 Ma followed by a decrease in sediment budget after circa 18 Ma. Our study when combined with existing data on deformation of the retrowedge and prowedge during the Miocene suggests that the orogen was in a constructive phase between circa 22 and 16 Ma. Orogenic construction can be explained by either an increase in accretionary flux and or a decrease in erosion [Beaumont et al., 1996; Schlunegger and Willett, 1999; Schlunegger and Simpson, 2002]. Our data, and results from sediment budget calculations, support an increase in erosion between circa 22 and 16 Ma and therefore suggest that orogenic construction of the Alps at this time is related to an increase in accretionary flux rather than decrease in erosion. Furthermore, the eastern portion of the Tertiary Piedmont Basin records an increase in sediment supply consistent with an increase in erosion in the late Oligocene-Aquitanian [Di Giulio, 1999]; this combined with sediment budget data suggests that an increase in accretionary flux, rather than a decrease in erosion, is the most probable cause for orogenic construction at least until Aquitanian time. Other processes such as lithospheric removal could also perturb the state of the orogenic wedge and control the locus of deformation and erosion [e.g., DeCelles et al., 2009]. Lag times from the retroforeland basin increase after circa 16 Ma, and bulk denudation rates for the Western Alps [Kuhlemann et al., 2006] also seem to have decreased between circa 16 and 12 Ma (Figure 9e) suggesting that orogenic construction may have been partially a result of a decrease in erosion after circa 16 Ma. A similar orogenic behavior is observed in the Central Alps where the Gonfolite (proximal retrowedge) was exhumed starting at circa 16 Ma [Bernoulli et al., 1989; Malusà et al., 2011] and the Southern Alps started to grow after circa 20 Ma [Schlunegger and Willett, 1999]. This suggests that both the Western and Central Alps grew

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Figure 9. Schematic cross sections of the Western Alps from Oligocene to Miocene (a–d) time modiﬁed after Fauquette et al. [2015] and Ford et al. [2006]; dashed line: position of the drainage divide. (e) Bulk denudations rates for the Alps after Kuhlemann et al. [2006].

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laterally in the Miocene suggesting an orogen-scale controlling mechanism. The drainage divide migrated back eastward acquiring a position similar to the modern one at this time [Mark et al., 2016]. Interestingly rollback of the European lithosphere has been suggested to explain subsidence of the Swiss Molasse Basin and widening of the Alps in the Miocene [Schlunegger and Kissling, 2015]. In this view stacking of the crustal root could have driven extrusion and outward growth of the orogenic system after circa 20 Ma. Our data show an increase in erosion coupled with orogenic growth in the early Miocene, which we interpret to correlate with an increase in accretionary flux between circa 20 and 16 Ma. Northwestward slab rollback of Europe at this time may have contributed to accretion of crustal material from the subducting plate by driving the wedge to grow outward. Although direct evidence of slab rollback is hard to find, slab rollback provides a plausible mechanism for creation of space for Adria to fill and a possible suction force [e.g., Sternai et al., 2014] driving Adria to continue its convergence against Europe. Mid to late Miocene to Pleistocene west and north verging structures are most likely associated with Apennines-related deformation indicating that by this time two separate subduction regimes were in place.

7. Conclusions Orogenic growth in the Western Alps was characterized by rapid early Oligocene exhumation of HP rocks recorded in both the proside and retroside of the orogenic wedge. During the latest Chattian-Aquitanian deformation migrated westward in the prowedge, side leading to a shift of the drainage divide. In the Miocene (circa 20–16 Ma) the drainage divide migrated eastward and the retrowedge hinterland and proximal foreland were actively deforming and exhuming. This tectonic phase correlates with regional Miocene shortening across the Western and Central Alps. We interpret early Miocene (circa 20–16 Ma) exhumation and deformation of the Western Alps and of its proximal retroforeland to represent a phase of regional orogenic construction in the Alps. We favor a scenario where changes in accretionary ﬂux associated with subduction dynamics (e.g., rollback) between Europe and Adria exerted a primary control on accretionary ﬂux, exhumation, and topographic growth.

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