Subduction of continental crust in the Western Alps

1T:CTONOPHYSlCS IIIIIIIII II

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Tectonophysics 269 (1997) 217-235

Subduction of continental crust in the Western Alps R.H. Marchant *, G.M. Stampfli lnstitut de G~ologie et Pal~ontologie, BFSH-2, Universit~ de Lausanne, CH-1015 Dorigny, Switzerland Received 30 December 1994; accepted 16 July 1996

Abstract As a result of recent deep reflection and refraction seismology the crustal structure of the Western Alps is now quite well-defined. However, this raises the question of what is present below the Moho, such as a crustal eclogitic root. This study attempts to estimate the volume of this eclogitic root on the basis of palinspastic reconstructions. Even with a minimum estimate of the crustal material involved in the subduction processes which took place during the Alpine orogeny, a significant eclogitized crustal root must be present down to depths of around 100 km below the Po plain. A maximum estimate suggests that a large part of this root could now be recycled in the asthenosphere. Keywords: Alpine orogeny; subduction; continental crust; deep seismic sounding; lithosphere; tomography; mass balance

1. Introduction Due to a dense network of deep seismic reflection lines (Fig. 1) and several thousand kilometres of refraction profiles, the crustal structure of the Western Alps is now quite well defined. Furthermore, the improved resolution of tomographic models allows a better understanding of the lithospheric structure of this mountain belt. From all this geophysical evidence combined with an impressive compilation of geological information, a general consensus has emerged about the deep structures of the Western Alps: the European plate is subducted southward, below the Adriatic plate, down to a depth of around 175 km below the Po plain (e.g., Spakman et al., 1993; Marchant, 1993). However, this subduction of the European continental lithosphere raises the question of how much crustal material has been sub-

* Corresponding author.

ducted. By subducted crustal material we mean all the continental crust now found below the Adriatic lithospheric mantle: this material can be considered as overthrusted, underplated, subducted a n d / o r detached. Unfortunately, the geophysical information which would allow estimation of the mass of subducted crustal material is scarce. Therefore, the approach used in this study is based mainly on geological information: the crustal volume of the various plates and terranes involved in the Alpine orogeny is estimated on the basis of actualistic models of passive margins. Even though the precision of such a method is rather low, we arrive at the conclusion that a significant amount of continental crust has been subducted during the Alpine orogeny and that most of it is now situated in the upper mantle, if not recycled in the asthenosphere. Through his understanding of the structural geology of the Internal Alps and by embracing the theory of continental drift, Argand (1916, 1924) was certainly the first scientist who proposed a significant

0040-1951/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PH S 0 0 4 0 - 1951 ( 9 6 ) 0 0 1 7 0 - 9

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subduction of continental crust in the Alpine orogeny. His lithospheric scale cross-sections (Argand, 1924) show the European plate subducting far south below the African promontory (i.e. the Adriatic plate), thus defining a fundamentally asymmetric crustal root. After this pioneering work, Laubscher addressed this problem of the subduction of continental crust in the Alps in quite a number of papers (e.g., Laubscher, 1990b,d,e). This author arrives at the conclusion that a large amount of crustal material has been transported to great depth, but he views this in the context of a symmetrical lithospheric subduction zone (Laubscher's "Verschluckung" concept, 1974). However, other authors (Platt, 1986; M~nard et al., 1991) come to the conclusion that subduction of continental crust beneath Adria is not required by mass-balancing. Nevertheless, they do not discount this possibility because of the imponderables concerning the required quantities. In the first part of this contribution, we summarise the crustal and lithospheric structures of the Western Alps which we believe to have an asymmetrical root. The second part focuses on palinspastic reconstructions in order to define the amount of crustal material involved in the Alpine orogeny and the third part discusses the problem of subduction of continental crust.

2. Crustal and lithospheric structures of the Western Alps

Numerous papers have already presented detailed interpretations of the deep seismic profiles shot in the Westem Alps (see e.g., Roure et al., 1990; Pfiffner et al., 1996). Comments on the interpretations will essentially be limited to the problem of continental subduction; thus, the reader is referred to the various papers cited in the text for additional information on the interpretation of these profiles. Fig. 2 presents a lithospheric scale cross-section (for location, see Fig. 1) which follows the NFP-20 (the Swiss deep-seismic programme) Western tra-

219

verse. This traverse is a composite of five deep seismic profiles (W1-W5, Figs. 3 and 4 4; for location, see Fig. 1) which have surveyed the Alps of western Switzerland from the Prealps to the Internal Alps. The structures south of the Insubric line have been surveyed by the eastern part of the ECORSCROP Alp deep seismic line (Figs. 1 and 5). On the basis of the individual interpretation of these six profiles, it is possible to construct by means of structural projections a synthetic profile across the entire mountain belt (Marchant, 1993). Together with tomographic images (Fig. 5), the crustal structure can be extrapolated at a lithospheric scale (Fig. 2). The W5 deep seismic section (Fig. 3; see also Valasek, 1992; Levato et al., 1993; Steck et al., 1996), clearly shows a major detachment between the layered lower crust and the upper crust. Whereas the lower crust is smoothly subducted below the Alpine edifice, the entire upper crust is strongly affected by intense deformation. The reflectivity of the layered lower crust and of the Moho disappears progressively below the Internal Alps. However, as shown by the wide-angle data shot in the vicinity the ECORS-CROP Alp profile (ECORS-CROP Deep Seismic Sounding Group, 1989a,b; Thouvenot et al., 1990; S6n6chal, 1991), the European Moho can be traced at least 20 km further to the southeast (Fig. 4). Thus, the disappearance of the lower crust and the Moho can be related to the data acquisition (the instruments used were unable to penetrate to such depths. Moreover, the complex geology of the Internal Alps would tend to diffuse and scatter the energy. Furthermore, progressive granulitization and eclogitization of the subducting crustal material diminish the acoustic contrast with the underlying mantle (e.g., Valasek, 1992; Marchant, 1993). The extention to greater depth of the lower crust and of the Alpine crustal root (Fig. 2) can be inferred by tomographic models (e,g., Spakman, 1990; Spakman et al., 1993). As shown in Fig. 5, the positive anomaly due to the European lithosphere can be followed about 100 km south of the Insubric line, down to a depth of around 150-170 km below the

Fig. 1. Tectonic map modified after Berthelsen et al., 1992 with location of the deep seismic profiles (black lines) shot in the Western Alps and location of the two lithospheric cross-sections (white lines) of Fig. 2Fig. 7. Ao = Adamello intrusions; DB = Dent Blanche nappe; Go = Gotthard massif; GP = Grand Paradis massif; LE = Lower Engadine window; MB = Mont Blanc massif; MR = Monte Rosa nappe; Pr = Provence basin; TW = Tauern window; VVL = V i l l a l v e r n i a - V a r z i - L e v a n t o line.

220

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line (after Steck et al., 1996). This section clearly shows the European layered lower crust and Moho smoothly subducting below the intensely deformed Alpine crust. Their reflectivity disappears below 50 km depth along this profile. Po plain. Between 200 and 350 km depth below the Po plain, another positive P-wave anomaly appears which can be related to the subducted and detached Valais ocean (Stampfli and Marchant, 1995, 1996; Marchant and Stampfli, 1996). Thus, the European continental lithosphere at depth extends at least 100 km further than the surface expression of the Alpine belt. This implies subduction of crustal material, the amount of which is discussed below. Fig. 6 is a lithospheric cross-section which follows the European GeoTraverse (EGT: see Blundell et al., 1992) and coincides with the NFP-20 E1 (e.g., Butler, 1990; Laubscher, 1990a,b,c,d; Pfiffner et al., 1990, 1991; Valasek et al., 1991; Pfiffner, 1992; Valasek, 1992; Marchant, 1993) and the CROP Alpi Centrali (Montrasio and Sciesa, 1994) sub-vertical seismic reflection traverses (Figs. 7 and 8; for location see Fig. 1). Along the NFP-20 E1 profile (Fig. 7), the reflectivity of the European layered lower crust and Moho can be followed up to the Internal Alps, where it disappears. However it reappears again on the CROP Alpi Centrali profile (Fig. 8) which extends the NFP-20 E1 profile through the Southern Alps up to the Po plain. On this section the reflectivity of the European layered lower crust and Moho can be observed to a depth of more than 60 km and more than 30 km south of the Periadriatic line. Thus, the lack of reflectivity observed on the

221

NFP-20 E1 is most probably due to acquisition parameters and to energy scattering because of the complex structures of the internal Alps. Here again, refraction seismology (Buness, 1992; Ye, 1992; see Figs. 7 and 8) enables the identification of the European Moho beyond the range of reflection seismology. Although the structure and composition of the Adriatic indenter is here (Fig. 6) different from the Western traverse (Fig. 2; for a discussion see Marchant, 1993; Marchant and Stampfli, 1996; Stampfli and Marchant, 1996), the lithospheric structure, as shown by tomographic models (Spakman, 1990; Spakman et al., 1993), is very similar. Also the general crustal structures and the upper mantle show a pronounced asymmetry (see also e.g., Frank, 1983; Frei et al., 1989; Laubscher, 1990c; Guyoton, 1991; Valasek et al., 1991; Kissling, 1993) in contrast with the "Verschluckung" model proposed by Laubscher (1974 and subsequent publications).

3. Palinspastic reconstructions We have seen above that the Alpine belt is the result of a significant subduction of the European plate below the Adriatic microcontinent. One way of assessing the amount of crustal material dragged down in this subduction is by reconstructing the initial amount of material involved, and this can be done on the basis of actualistic models of passive margins. This task has been carded out over the past years by Stampfli and Marthaler (1990), Favre and Stampfli (1991), Stampfli (1993), Froitzheim and Manatschal (1996) or Stampfli and Marchant (1996). In the Western Alps such reconstructions are complicated by the fact that, not only the European plate and the Adriatic microplate were involved, but also the Brian~onnais (Stampfli, 1993) and Austroalpine (Stampfli and Marchant, 1996) terranes. Furthermore, significant lateral movements between these elements occurred, which implies that mass-balancing of a 2D cross-section must be considered in a 3D context. Therefore, we have positioned the lithospheric scale cross section (Fig. 2) on a palinspastic map from the early Cretaceous (around 125 Ma, Fig. 9). The cross-section had to be segmented in four parts corresponding to the four plates involved in the

222

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Alpine orogeny: a European, a Brian~onnais, an Austroalpine and an Adriatic (or Apulian) segment. Due to significant lateral movements and rotations which occurred later on, these four segments do not form a straight and continuous line during the early Cretaceous: the Adriatic segment was even at right angles to the European segment. This palinspastic map of early Cretaceous times (Fig. 9) corresponds to the initiation of oceanisation (just following the rifting phase) in the Valais trough (Stampfli, 1993; Stampfli and Marchant, 1996). Due to the opening of this basin which is related to the rotation of the Iberic plate, the Piemont ocean has entered a compressional regime, with probably two SW-oriented subduction zones (Hoogenduijn Strating, 1991; Stampfli, 1993): one at the mid-oceanic ridge, the other at the Austroalpine margin. Part of the Austroalpine terrane is being subducted together with the southern Liguro-Piemont ocean as illustrated by cross-sections in Fig. 10. Fig. 10 shows reconstructions based on the palinspastic map of Fig. 9. For the sake of clarity, the four plate segments have been put together and

223

aligned. These reconstructions are based on actualistic asymmetrical rifting models (Stampfli et al., 1991; Froitzheim and Manatschal, 1996; Stampfli and Marchant, 1996), i.e. Red Sea or Central Atlantic type. For instance, the two margins of the Brian~onnais terrane are considered to be upper-plate (or flexural) margins and therefore only little upper mantle and lower crust is found in this plate. However, the exact length of the European, Brian~onnais and Austroalpine continental crust is subject to debate and the reconstruction shown in Fig. 10a is a maximum estimate and Fig. 10b a minimum estimate. These two reconstructions represent two extremes. The true size of the plates involved lies somewhere in between. We will now discuss in more detail how we arrived at these two extremes (using a drawing computer program, CANVAS TM,to measure the area of the various elements involved; values are shown in Table 1). One of the most important features into reconstructing the geometry of a continental margin is the rift shoulder. Inland of the rift shoulder, a rim basin is usually developed and characterised by weak extension ([3 < 1.2). Offshore of this shoulder, one finds the margin with strong extension ([3 > 2.0). Present-day continental margins reveal a width of 100-250 km between the rift shoulder and the transition to oceanic crust; transtensional margins usually have a narrower margin than purely extensional margins. The width of the rift shoulder can vary from a few tens to a few hundred kilometres and can have prominent relief (up to 3000 m for the Red Sea). Thus, a rift shoulder will be characterised by strong erosion while the rim basin and the margin will be subject to continuous sedimentation. This makes the rift shoulder an excellent geodynamic marker easily recognisable in the stratigraphic record. In previous papers (e.g., Stampfli et al., 1991; Favre and Stampfli, 1992; Stampfli, 1993; Stampfli and Marchant, 1996), we have analysed the Alpine stratigraphic record in the light of such geodynamic markers (rim basins, rift shoulder, upper- or lower-plate margins, etc.). For the European margin, the rift shoulder (shown in Fig. 9) can be traced from east to west: between the Simano and Adula nappes, between the Antigorio and Verampio nappes and to the southeast of the Argentera massif (Stampfli, 1993; Stampfli and Marchant, 1996). Thus, the Dauphinr-Helvetic do-

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Fig. 7. Depth-migrated section of the NFP-20 E 1 profile (after Valasek, 1992, fig. B9) with an overlay of the migration of the Vibroseis data in the Penninic domain (after Marchant, 1993, Fig. 6-19b). Continuous thick black lines correspond to the projection of the European and Adriatic Moho and the dotted lines to the "Conrad" discontinuity as determined by the seismic refraction modelling performed by Ye, 1992. On this section, the reflectivity of the European lower crust and Moho appear clearly on the first 50 km of the profile. Further south they become rather indistinct but can be followed about 60 km further south by refraction siesmology.

main corresponds to the rim basin and the Ultrahelvetic domain to part of the margin. Extension in the rim basin (Jura, Swiss Plateau and Helvetic) was very weak, as shown by the crustal stretching factors varying between 1.03 and 1.20 as determined by Loup (1992) on the basis of subsidence curves. Thus, the one model (Fig. 10a) has a rim basin with a Moho ranging between 26 and 30 km depth and the other model (Fig. 10b) between 23 and 27 km (the initial crustal thickness of the European continent was in the order of 27-30 km, e.g., Blundell et al., 1992). The width and the crustal stretching factor for the rift shoulder and the margin are more difficult to assess due to an incomplete record as only a few corresponding units are exposed in outcrop; most of them are to be found at depth below the Alps or have been subducted. Therefore, our reconstructions of the South Helvetic and Ultrahelvetic domains are based on comparisons with present-day continental margins: our maximum estimate (Fig. 10a) shows the European continent with a length of 570 km and our minimum estimate (Fig. 10b) a length of 380 km; the

main difference resides in the width of the South Helvetic domain. The corresponding cross-sectional area estimates vary between 13,411 and 7575 km 2. For the Brian~onnais terrane, palinspastic reconstructions of the Prralpes M~dianes nappe along the considered transect indicate a minimum length of 52 km (Baud and Septfontaine, 1980). However, this nappe does not represent the total width of the Brian~onnais terrane: the Valais and Piemont (Pre Piemontais) margins have to be added as well as part of the Piemont rift shoulder. According to our reconstructions this terrane has a width ranging from 120-230 km (Fig. 10). Concerning the crustal thickness, we have calculated crustal stretching factors on the basis of subsidence curves published by Borel (1995) for the Prralpes Mrdianes nappe, using the method described by Favre (1995) which is based on the equations of Hellinger and Sclater (1983). For these calculations which assumed an initial crustal thickness of 30 km, we considered the curve from late Triassic to mid-Cretaceous when flexing of the Brian~onnais terrane started. For the Subbrian~on-

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nais rim basin we obtained crustal stretching factors of 1.27-1.30 and for the Brian~onnais plaform factors of 1.13-1.14. These values could be slightly higher as we did not take the Permo-Triassic rifting phase into account (Stampfli and Marchant, 1996; Stampfli, 1996). Therefore, we attributed a maximum crustal thickness of 20 km to our minimum estimate and 22 km to our maximum estimate (see Table 1 and Fig. 10). The initial volume of the Austroalpine terrane is much more difficult to assess. The Austroalpine units along the NFP-20 Western traverse are strongly deformed and metamorphosed and the stratigraphic record is very poor (Venturini, 1995). Therefore, we based our minimum estimate of the Austroalpine crust on the area shown on our present-day crosssection (Fig. 2; 1027 km 2) to which we added an estimate of the eroded Austroalpine material (1609 km2). For our maximum estimate, we increased the

PL =

Periadriatic (or Insubric) line.

length of this terrane by 60% and its thickness by 10%, reaching an area of 2636 km 2. Finally, our minimum estimate of crustal material is 11,862 km 2 and our maximum 21,265 km 2 (Table 1). The large difference between these two extreme values reflects the imponderables in paleogeographic reconstructions. To this we can add that our minimum estimate is certainly close to the lowermost possible boundary and that our maximum estimate is close to a realistic value, but we cannot rule out that even more crustal material was involved.

4. Crustal budget The idea behind these two extreme reconstructions was to compare their volume (or rather crosssectional areas) of continental crust with the present

227

R.H. Marchant, . G.M. Stampfli / Tectonophysics 269 (1997) 217-235 location of restored cross-sections

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R.H. Marchant, .G.M. Stampfli / Tectonophysics 269 (1997)217-235

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R.H. Marchant, .G.M. Stampfli / Tectonophysics 269 (1997) 217-235

229

Table 1 Cross-sectional areas (in km 2) of the elements involved in the crustal thickening of the Alpine orogeny. Only the elements situated north of the Adriatic plate are considered. The first two columns are based on two extreme palinspastic reconstructions: a maximum estimate (Fig. 10a) and a minimum estimate (Fig. 10b). The third column corresponds to the areas from the present-day cross-section in Fig. 2, without the eclogitic root (which has an area of 3682 km2). The last three columns are the estimates with their corresponding margins of error from M~nard et al. (1991). Their values are not directly comparable to ours as their mass-balancing was done along a different cross-section with an area of 6525 km 2. See text for explanations Areas in km 2

Fig. 10a

Fig. 10b

Fig. 2

Mrnard et al., 1991 Margins of error

European crust Brian~onnais crust Austroalpine crust Eroded material Total Eclogitic root

13,411 3307 4547 21,265 12,255

7675 1551 2636 11,862 2852

(e.g., Pfiffner, 1992); we have, therefore, not considered the Adriatic crust in our crustal budget. The estimation of the present-day crustal material involved in the Alpine orogeny (in the cross-section of Fig. 2) is rather complex due to the imbricated structures: we calculated the areas by adding the different bits and pieces belonging to the same plate (Table 1). The amount of eroded material was estimated on the basis of the mesoalpine regional metamorphism, which peaked during the mid-Eocene (around 38 Ma, Steck and Hunziker, 1994). Since then, most of the nascent Alpine belt was gradually uplifted above sea-level and thus the mid-Eocene is probably the best reference for estimating the amount of erosion. The envelope of eroded material in Fig. 2 was constructed mainly by means of pressure measurements of the mesoalpine metamorphism in the Helvetic and Internal Alps (Frank, 1983; Hammerschmidt and Frank, 1991) and on the basis of illite crystallinity in the Prealps (Mosar, 1988). Results of Table 1 show that even for a minimum estimate of the crustal material involved in the Alpine orogeny, an eclogitic root (with an area of at least 2852 km 2) must be present. The eclogitic root in Fig. 2 was drawn prior to this work and has an area of 3642 kin2; thus, quite close to the minimum estimate. Considering the maximum estimate, the area of the eclogitic roots would be 12,255 km 2. Such a model would imply that together with the subducted and detached Valais ocean, a large part of the eclogitic root was dragged down into the astheno-

5648 499 1027 1836 9010 3682

4356 2500 0 (650) 6856 341

+ 1404 + 600 0 ( + 650) + 2004 2335

- 1268 - 600 0 ( - 650) - 1868 - 1537

sphere. This would not be unrealistic, Cloos (1993) has demonstrated that the lithosphere bulk density of thinned continental crust ( < 15-20 km) is larger than the density of the asthenosphere.

5. C o m p a r i s o n w i t h other estimates

Several papers have addressed the problem of subduction of continental crust in the Alpine belt sometimes reaching opposite conclusions. For instance, Mrnard et al. (1991) have performed a mass-balancing exercise very similar to ours, but along a cross-section corresponding to the ECORSCROP ALP deep seismic traverse (for location, see Fig. 1). They reach the conclusion that little crustal material was subducted during the Alpine orogeny. However, because of the imponderables, they do not rule out a substantial underthrusting of continental crust. Why such a discrepancy compared to our results? A close comparison reveals the following: - Their area estimate for the European crust is not directly comparable to ours as they start from the southern side of the Molasse basin, whereas we started from the northem side of the Jura. If we remove the Jura and Molasse basin from our estimates (about 2500 km2), we obtain a minimum area for the European crust of 5175 km 2 and a maximum estimate of 10,911 km 2. These values can now be compared with the estimate of Mrnard et al. (1991) as shown in Table 1:4356 km 2 ( + 1404; 1708). Our

230

R.H. Marchant, .G.M. Stampfli / Tectonophysics 269 (1997) 217-235

minimum value is thus in the range of the M6nard et al. (1991) estimate, but our maximum estimate is much larger. This is due to the fact that M6nard et al. (1991) consider the Valais trough as an aborted rift while we have evidence (Stampfli, 1993) for oceanisation. For this reason, we place a rift shoulder and a margin to the southeast of the Dauphin6-Helvetic basin which lengthens our European plate substantially (100-300 km), whereas M~nard et al. (1991) consider the European plate to extend 100 km beyond the Dauphin6-Helvetic basin. What we call the Brian~onnais terrane corresponds to what M6nard et al. (1991) call the Valais, Subbrian~onnais, Brian~onnais and Piemontais realms. Their total estimate is 2500 km2: ( + 6 0 0 ) which is in the same order of magnitude as our estimates (1551-3307 km2). M6nard et al. (1991) have not considered the Austroalpine domain in their crustal budget, even though their cross-section covers the Sesia-Lanzo zone. Our estimates for the Austroalpine terrane vary between 2636 and 4547 km 2. - M6nard et al. (1991) give a value of 650 km 2 ( _+650) for the eroded material along their cross-section. Based on the Tertiary regional metamorphism, we arrived at a value of 1836 km 2 along our transect. This value is very close to the estimate (2100 km 2) which can be deduced from the study of England (1981) based on the sediment thicknesses in basins surrounding the Alps. Even though the transect considered by M6nard et al. (1991) was slightly less eroded than ours, we believe their value to be underestimated (in particular their minimum estimate of 0 km 2 is unrealistic). Thus, the main differences between our crustal budget and the one from M6nard et al. (1991) reside in the fact that they did not take into account the Austroalpine domain and that they considered the Valais trough as an aborted rift; also they probably underestimated the volume of eroded material. In a similar mass-balancing exercise along a profile close to the one of MEnard et al. (1991), Butler (1989) presents a lithospheric cross-section which shows an eclogitic root below the Po plain with an area of about 5500 km 2, which is in the same order as our estimates (between 2852 and 12,255 km2). For a cross-section through central Switzerland, Laubscher (1990c) arrives at the conclusion that

8000 km 2 of crustal material are present in an eclogitic root. The same author (Laubscher, 1990b) supposes that approx. 4500 km 2 of crustal material disappeared in the mantle during the Neoalpine orogeny along a traverse corresponding to the NFP-20 Eastern traverse. If the Austroalpine terrane is subtracted from our estimates in Table 1 (the crustal material involved in Eoalpine orogeny), a crustal root with an area of between 2852 and 10,346 km 2 is obtained for Neoalpine orogeny. The values of Laubscher (1990b,e) fall well within the range of our estimates. On the basis of the reconstructions from Dercourt et al. (1985, 1986) and Savostin et al (1986), Le Pichon et al. (1988) calculated that a volume of at least 10 × 106 km 3 must have disappeared in the mantle below the Alps. Considering a length of 1000 km for the Alps, this implies an area of about 10,000 km 2, which is close to our maximum estimate.

6.

Discussion

Our mass-balancing based on palinspastic reconstructions requires that a considerable amount of continental crustal material has been subducted in the Western Alps and that this material should now be found in the upper mantle and perhaps also in the asthenosphere. Subduction processes in the Western Alps took place over a long period of time. They had already started, in the early Cretaceous (Eoalpine metamorphic phase) for the Austroalpine terrane (e.g., Venturini et al., 1994) and the Piemont ocean. A small part of this Eoalpine subducted material has been exhumed. A little outcrop in the Dora Maira massif testifies, by the presence of coesite, a subduction to about 100 km (Chopin, 1984). A similar depth of the ecolgitic root is shown in Fig. 2. Our results are based on a single cross-section passing through western Switzerland: based on the map (Fig. 9) we propose the following for the adjacent areas: - For the French-Italian Alps, the volume of the European crust and Austroalpine terrane was certainly smaller and the Brian~onnais terrane about the same. Here a smaller crustal root can be expected than in western Switzerland. - For a cross-section through eastern Switzerland (i.e. the NFP-20 Eastern traverse, Fig. 6), the nar-

R.H. Marchant, .G.M. Stampfli / Tectonophysics 269 (1997) 217-235

rowing of the Brian~onnais terrane is compensated by the widening of the Austroalpine terrane (see Fig. 9); the European margin stays about the same size. Thus, the eclogitic root should be of a similar size here as along the NFP-20 Western traverse. - For a cross-section through the Eastern Alps, the Brianqonnais terrane disappears but the Austroalpine terrane becomes substantially larger; the European margin becomes narrower (Fig. 9; see also Stampfli and Marchant, 1996); furthermore, erosion has been less important than in the Central Alps. Here a smaller size for the eclogitic root can be expected than along the NFP-20 Western traverse. This would support the suggestion of Helwig (1976) that no significant subduction of continental crust occurred in the Eastern Alps. Therefore, the isostatic rebound was smaller, resulting in less erosion of the upper plate. Isostatic rebound and topography are also dependent on the ratio of subducted versus underplated crustal material. This ratio is likely to change with place and time, particularly in relation to the age of the subducting oceanic lithosphere. A young, thus buoyant, oceanic lithosphere will find it difficult to drag a continental margin down the subduction zone, resulting in substantial underplating and therefore producing a strong relief. Whereas an old and cold oceanic lithosphere will be sufficiently more dense than the asthenosphere to create roll-back and will favour subduction of a continental margin as long as its crust does not exceed a thickness of around 15-20 km (Cloos, 1993). Other arguments than palinspastic reconstructions also favour a significant subduction of continental crust in the Western Alps. One of these arguments can be deduced from finite-element modelling of compressional orogens, such as carried out by Beaumont and Quinlan (1994) or Beaumont et al. (1994a,b). These authors show that for a model implying a total subduction of the crust (Beaumont et al., 1994b, Fig. 3) no retro-deformation (backthrusting or backfolding) occurs, but as soon as part of the crust is not allowed to subduct (most models in Beaumont and Quinlan, 1994 and in Beaumont et al., 1994a,b) important retro-deformation appears. In the Western Alps, a small amount of backthrusting started probably in the Late Cretaceous (Schtinborn, 1992), but significant retro-deformation only started

231

around 35-30 Ma (SchSnborn, 1992; Steck and Hunziker, 1994). This implies that up to 35-30 Ma most of the continental crustal material (European, Brian~onnais and Austroalpine) was rather easily subducted. The change to a dominantly double-vergent orogen at around 35-30 Ma is probably due to the thickness of the subducting European plate. As can be deduced from the work of Cloos (1993), the thinned European margin as well as the thin Brian~onnais terrane could easily be subducted, but as soon as the crustal thickness exceeded 15-20 km, the bulk density of the European plate had positive buoyancy relative to the asthenosphere, thus resisting subduction and initiating significant retrodeformation (Mosar et al., 1996). Furthermore, analogical models of orogens (Chemenda et al., 1995) have produced subduction of continental crust down to depths of 200-300 km before the crust detaches from the lithospheric mantle and tends to be uplifted by the force of buoyancy. Geophysical evidence for a deep subduction of continental crust in the Western Alps is rather poor (Kissling, 1993), but for the reflections observed down to 60 km along the CROP Alpi Centrali profile (Fig. 8). As mentioned above this is most probably due to the low resolution of refraction and reflection seismology at such depths. However, alternative geophysical methods have been able to trace eclogitic roots at greater depths in other orogens. For instance in the Pyrenees, Pous et al. (1994) have carried out magnetotelluric soundings along the ECORSPyrenees deep seismic reflection profile. This survey has revealed a high resistivity zone following the subducted Iberic plate from a depth of 30-80 km, whereas the reflectivity of the Iberic crust stops short at a depth of 60 kin. These authors interpret this high resistivity zone as being related to melts originating from the crustal subducted material. For the HinduKush range, Roecker (1982) on the basis of earthquakes, estimates that continental crust has been subducted to depths of at least 150 km. From geological and seismological evidence, Burtman and Molnar (1993) can follow continental crust down to depths of 150 km, if not 200 km below the Pamir range. The length of the subducted continental lithosphere must be at least 250 to 300 km there; thus, nearly double our estimates for the Western Alps. Unfortunately, none of these magnetotelluric or seis-

232

R.H. Marchant, .G.M. Stampfli / Tectonophysics 269 (1997) 217-235

mologic methods have as yet been carried out at such depths in the Western Alps. Using a very different approach, i.e. mineralogical data and comparative planetology, Anderson (1989) comes to the conclusion that much of the Earth's crust could be buried in the mantle. On the strength of geochemical mass-balancing arguments, Amstrong (1991) concludes that the importance of recycling of crustal material is such that since about 3 Ga the mass of continents has stayed roughly constant. Finally, a number of occurrences of ultra-high pressure rocks of continental origin, such as the West Gneiss Region (Jamtweit, 1987; Andersen et al., 1991), the Bergen Arcs (Austrheim, 1991) both in Norway, the Shadong province in east China (Enami and Zang, 1990) or the Dora Maira massif (Chopin, 1984), testify that continental crust can be dragged down to depths of at least 100 km and that eventually under certain circumstances, they may be brought to the surface.

7. Conclusions Although geophysical evidence is scarce in the Western Alps, there are sufficient arguments to suggest the existence of a significant eclogitized crustal root below the Western Alps. As deduced from crustal scale mass-balancing, the volume of the ecologitic root (or the Alpine crustal root) shown here in Figs. 2 and 7 is close to a minimum estimate. A maximum estimate would suggest that this volume is at least three times larger and that part of it is recycled in the asthenosphere. Thus, subduction of continental crust is certainly a major geodynamic, process, usually ignored in most plate-tectonic models, and an increasing body of evidence (from the Western Alps as well as from other orogens) suggests that this phenomenon could be more widespread than usually thought.

Acknowledgements The authors are grateful to Philippe Favre for his contribution in determining the crustal stretching factors. The manuscript benefited from the reviews by J. Ansorge, J. Platt and G. SchSnborn.

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Note: This paper was initially intended to be published as part of the Special Issue "Seismic Reflection Probing of the Continents and their Margins", edited by D.J. White, J. Ansorge, T.J. Bodoky and Z. Hajnal. Tectonophysics, 264 (1996), 392 pp.