7.1. Structures Related to Crustal Thinning: The Eita Shear Zone
 In the Valle Grosina, the contact between Campo and Grosina basements is characterized by the presence of the Eita shear zone. Previous studies [Meier, 2003] suggested that this shear zone was formed during late-Cretaceous thrusting. However, stretching lineations along this structure indicate a NNW sense of shear, which is oblique with respect to the generally NW-SE direction of transport along Alpine and Jurassic rift-related structures in the area [Froitzheim and Manatschal, 1996]. This observation indicates an oblique component for motion along the shear zone with respect to other rift structures, provided that the study area did not experience regional-scale rotation during the Alpine orogeny. As discussed below, the 40Ar/39Ar geochronological data presented in this study provides clear indication that a significant amount of crustal excision was accommodated along the Eita shear zone in the Late Triassic-Lower Jurassic. Local Alpine reactivation of this structure is possible and indeed likely. However, as discussed below, the regional cooling pattern indicates that its activity was largely restricted to the Late Triassic-Lower Jurassic.
 White mica and biotite separated from the Campo basement in the footwall of the Eita shear zone consistently yield ages ≤205 Ma (Figures 9 and 12). Biotite separated from samples 6 and 32 belonged to a high-T metamorphic fabric that was generated during emplacement of the Sondalo Gabbro at middle to lower crustal depth at ca. 270–300 Ma. These biotites yield ages in the 183–189 Ma range (Figures 9a, 9b, and 12). Sample 32 yielded the most concordant spectrum. This sample is characterized by a pervasive high-T metamorphic fabric with only one generation of biotite (Figure 7e), which yielded a relatively flat age spectrum in the 179–185 Ma range (Figure 9a). This estimate is more than 85 Myr younger that the inferred Permian age of the metamorphic fabric. The complete lack of chloritization of the biotites visible under the optical microscope allows to rule out this process as a possible cause of argon loss [Di Vincenzo et al., 2003]. Therefore, the Jurassic ages of the spectrum are interpreted as being related to cooling of the studied biotites below a temperature of ca. 300°C during the exhumation from middle/lower crustal depth (closure T is estimated from Harrison et al. , for the cooling rate of 10°C/Ma estimated by Braga et al.  for the plutonic rock of the Sondalo area). Sample 6 yields similar results, although with a slightly more discordant age spectrum, which may be related to the fact that the rock sample contained 3 generations of biotite with different grain sizes and, likely, slightly different Ar retention (Figure 9b).
Figure 12. Present-day geological map associated with a restored cross-section of the necking zone of the Adriatic rifted margin across the Bernina-Campo-Grosina units. The cross section summarizes the sediment architecture, fault geometry, 40Ar/39Ar data, and strain distribution observed in these units.
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 White mica from samples 17 and 233, which also belong to the Campo basement, yielded slightly older ages than biotite (Figures 9c and 9d). Both samples were taken from the upper part of the Campo basement, less than 10 m from the contact with the Grosina basement. Sample 17 yielded a relatively flat age spectrum, with the majority of steps, between steps 8–13 (68% of the total 39Ar released) in the 201–205 Ma range (Figure 9c). The age spectrum of sample 233 is characterized by a progressive age increase, with the majority of apparent ages (steps 7 to 17) falling in the 198–207 Ma range (Figure 9d). Although two generations of white mica are found in the studied samples, the rare fine grained aggregates (<100 μm) of white mica defining the mylonitic fabric of the Eita shear zone have largely been selected out by hand picking. Therefore, the apparent age spectrum of both samples is interpreted as resulting from the degassing of the argon reservoir related to the large porphyroclastic white mica crystals. The measured ages are likely to record cooling of the porphyroclastic white micas below temperatures at which a significant amount of radiogenic argon could be retained in the crystal lattice. This interpretation is supported by the 40Ar/39Ar age of 188 ± 4 Ma obtained by Meier  from white micas of the Campo basement. The Cretaceous ages obtained in the first 2 steps of the heating experiment from samples 208, 257, 233 may hint at local re-setting/recrystallization during the Alpine orogeny (Figure 9d).
 40Ar/39Ar ages of white micas from the Grosina basement are significantly older than those found in the Campo basement (Figure 9). As discussed above, the base of the Grosina basement is overprinted for a thickness of about 200 m by a pervasive greenschist facies flat-lying fabric related to the Eita shear zone (Figures 4 and 6e). Samples 208, 257 were located within this overprinted area, respectively at 100 m and in the vicinity (∼1 m) of the contact with Campo basement (Figures 4, 7a, 7b, and 9d). Both samples yielded staircase 40Ar/39Ar age spectra. Similarly to the Campo basement, samples 208 and 257 contained two generations of white micas (Figures 7a and 7b). Also in this case, the finest grained white mica II was selected out by hand picking. Therefore, the age spectra are interpreted to be derived largely from degassing of the coarse-grained porphyroclastic white mica I. Sample 208, which was located further away from the contact with the Campo basement and is less affected by the greenschist facies shear fabric yield Permian ages in the 250–275 Ma range over 86% of the radiogenic argon release (Figure 9d). Sample 257, instead, yielded slightly younger ages, with Triassic ages over 50% of the radiogenic argon release (Figure 9d).
 Shifting of the age spectrum toward younger ages from sample 208 to sample 257 is probably a result of the more pervasive overprint during shearing along the Eita Shear Zone (Figure 9d). Therefore, the 40Ar/39Ar geochronology performed on the Grosina basement indicates that the analyzed porphyroclastic white micas cooled in the Permian and were later partly reset during the activity of the Eita shear zone.
 The very fine-grained micas that define the mylonitic fabric of the Eita shear zone have not been dated in this study. Despite this, an Upper Triassic-Lower Jurassic age can be proposed for the activity of the Eita Shear Zone on the following grounds:
(1) The Eita shear zone juxtaposes rocks with Permian or older white mica cooling ages (>260 Ma) in hanging wall (Grosina basement) against rocks with younger cooling ages (∼205–183 Ma) in the footwall (Campo basement; Figure 12). This age gap brackets the time of activity of the interface between the Campo and Grosina units;
(2) Flat age spectra of biotites from the Campo basement (samples 6 and 32), which did not experience any fabric re-working after the Permian, allows us to exclude any argon loss during the Alpine orogeny. Such argon loss would have been inevitable if the Grosina basement had been thrust over the Campo basement in Alpine times at the conditions of 350–400°C estimated from the metamorphic and microstructures observed along the Eita shear zone.
(3) An Alpine activity is inconsistent with the observed structural style as well as the Alpine kinematics. Indeed, in the study area, Alpine thrust faults are constantly described as narrow shear zones (∼m) with synkinematic metamorphic conditions around 300°C [e.g., Froitzheim et al., 1994; Ferreiro Mählmann, 1996]. Furthermore the main thrusting event shows a dominance of WNW-shear sense. These characteristics are significantly different from those displayed by the Eita shear zone. Nevertheless, local Alpine reactivation of this structure as inferred by Meier  is considered to be likely, and is compatible with Late Cretaceous ages obtained in the earliest steps of the step-heating experiments in samples 208, 257, 233. However, such low-grade Alpine reactivation would have resulted in minor reworking of an already established large-scale crustal section (for details see Mohn et al. ).
 Furthermore, the substantial gap in 40Ar/39Ar ages between the Campo and Grosina units indicates that while the Grosina basement was already residing in an upper to mid-crustal position (T < 400°C) in the Permian, the Campo basement was still at temperatures sufficient to cause significant argon diffusion in white mica (∼405°C for a grain with a 100 μm radius cooling at 10°C/km and ca. 5 kbar [Harrison et al., 2009]). This conclusion is also supported by the intrusion of the Sondalo Gabbro at a mid- to lower crustal position (0.6 ± 0.2 GPa) [Tribuzio et al., 1999; Braga et al., 2001, 2003], which constrains the Permian depth of the Campo basement to ca. 15–25 km. Therefore, the Eita shear zone marks a significant crustal gap, indicating that it must have been active as an extensional shear zone (Figure 12). Based on the metamorphic conditions and microstructures it can be proposed that the Eita shear zone was active between 400 and 300°C. Assuming a thermal gradient of ca. 20–25°C/km as inferred for the Triassic time by Müntener et al. , this would indicate that this structure was active at 10 to 15 km depth separating pre-rift middle/lower crust (Campo basement) from pre-rift upper/middle crust (Grosina basement). Since this structure can be mapped along 20 km juxtaposing the same crustal levels and showing the same metamorphic conditions, we propose that this shear zone was active as a decollement horizon between a ductile upper/middle crust (Grosina) and rigid “brittle” middle/lower crust (Campo) (Figure 12). Several similar shear zones have been identified in the Campo and Grosina units. Although their exact role and age are not yet determined, we do not exclude that these shear zones may belong, together with the Eita shear zone, to a network of shear zones responsible for thinning of the ductile middle crust during rifting.
7.2. Structures Related to Exhumation: The Grosina Detachment
 The type locality of the Grosina detachment is located at Sasso Campana, where this low-angle intrabasement detachment fault is exposed at the top of the Grosina basement (Figures 4, 5b, 5c, and 8). We suggest that this structure can be correlated with the pre-Alpine brittle fault observed in the Sassalb area located more to the west, where it separates the exhumed mid- to lower crustal rocks of the Campo basement from pre-rift Triassic dolomites and upper crustal rocks of the Bernina unit (Figures 8a and 8b). Thus, in the Sassalb area, the Grosina detachment marks the contact between the Grosina-Campo units in the footwall and the Bernina unit in the hanging wall (Figures 8a and 8b). Crosscutting relationships allow the age of this structure to be constrained. At Sassalb, the Grosina detachment is folded and truncated by Alpine Campo and Grosina thrust faults, indicating that it pre-dates Late Cretaceous Alpine convergence (Figure 5). The observation that along this detachment Permian middle/lower crustal rocks (Campo basement) are juxtaposed against Triassic dolomites (Bernina unit) excludes a Permian or older age. This is also in line with the observation that the Grosina detachment cuts across Permian high-T fabrics in the Grosina and Campo units. Therefore a Jurassic age can be proposed for the Grosina detachment. From a map view, this interpretation seems to be confirmed by the fact that Grosina detachment crosscuts the Late-Triassic to Early Jurassic Eita shear zone (Figure 5). In contrast to the latter, we interpret the Grosina detachment as a downward concave exhumation fault along which rocks from deeper crustal levels, i.e., Grosina and Campo units, were exhumed and juxtaposed against upper crustal rocks and Mesozoic sediments, i.e., Bernina unit (Figure 12). Structural and microstructural observations show that the Grosina detachment fault was active in the brittle field below ∼300°C probably during Jurassic rifting. Significantly, within this brittle detachment, clasts of higher grade mylonitic rocks can be found, indicating brittle reworking of pre-existing ductile shear fabrics (Figures 8e and 8f). Crosscutting relationships between the brittle Grosina detachment and these quartz mylonites indicate an angular discordance of about 20°. Three main possibilities can be proposed to account for the mutual relationship between quartz mylonites and detachment: (1) the quartz mylonites formed in a deeper part of the Grosina detachment and were exhumed and overprinted by brittle deformation, or (2) the quartz mylonites belong to a structure similar to the Eita shear zone, i.e., they represent a decollement that was exhumed along the detachment fault (e.g., mylonitic front along the Wipple Mountains detachment [Lister and Davis, 1989]), or (3) the quartz mylonites are unrelated to Jurassic rifting and were formed as a result of Permian or Variscan tectonics. At the moment the exact significance of this quartz mylonite remains unclear.
7.3. A Low-Angle Top Basement Detachment Fault in the Distal Margin: The Bernina Detachment
 The Bernina detachment is interpreted as a low-angle top basement detachment fault accommodating the final extension in the distal margin (Figure 12). In the Val dal Fain and Piz Alv area (Figure 10), the first phase of rifting is characterized by high-angle normal faulting associated with the deposition of the Alv breccias. The high-angle normal faults of this earlier rift phase have been subsequently crosscut by younger low-angle detachment faults. This results in the formation of extensional allochthons that range in scale from several tens of kilometer wide blocks (e.g., Piz Alv), including basement, to small, extended fragments of pre-rift dolomites overlying exhumed basement (e.g., Val dal Fain; Figure 10b). Along the Bernina detachment, no variation of the metamorphic conditions can be observed and the structures related to the detachment are forming in the brittle field at temperatures below 300°C over more than 20 km in the direction of transport (Figures 4 and 11). This observation suggests that this fault was only active in the brittle field within the upper crust. Although it is difficult to determine the initial dip of the fault at depth, near the surface the fault was sub-horizontal and was exhumed at the seafloor, resulting in a low-angle top basement detachment fault. This is confirmed by the low-angle between the fault zone and the syn-rift sediments onlapping onto the fault (Figure 10b). The lower limit for the activity of the Bernina detachment can be inferred from truncation of the youngest pre-rift formation dated as Pliensbachian [Finger, 1978], while the last part of the Allgäu Fm., which was deposited in a post-tectonic setting until the Bathonian, provides the upper limit [Furrer et al., 1985, Eberli, 1988]. The Bernina detachment is interpreted as a similar structure to the Err detachment [Froitzheim and Eberli, 1990; Manatschal and Nievergelt, 1997; Manatschal, 1999, Masini et al., 2011], but located in a part of the distal margin closer to the continent (Figure 3). These structures accommodated extension in the distal continental margin, similar to the structures described from the present-day Iberia distal margin, which are responsible for mantle exhumation.
7.5. A Conceptual Model to Account for Extreme Crustal Thinning (Figure 14)
 Three major structures that were all active during early Jurassic rifting can be identified between the proximal and distal Adriatic rifted margin: the Eita shear zone, the Grosina detachment, and the Pogallo shear zone (Figure 14). Because the Grosina detachment was only active in the brittle crust and the Pogallo shear zone at deeper crustal levels [e.g., Handy, 1987], we assume that these structures soled out at in a decollement horizon in the ductile middle crust. Based on this evidence, we interpret that crustal thinning is accommodated by a system of conjugate crustal scale shear zones that are active in the upper brittle crust (e.g., Grosina detachment fault) and in the mafic lower crust and upper mantle (e.g., Pogallo type shear zone) and are decoupled along mid crustal decollements within the brittle-ductile transition (e.g., Eita shear zone) (Figures 13 and 14). We interpret the Margna shear zone [Müntener and Hermann, 2001] as an extraction shear zone/decollement from the necking zone to the distal margin, leading to the omission of the mid-crustal level and the direct juxtaposition of the uppermost and lowermost part of the continental crust in the distal margin (Figures 13 and 14) (e.g., extraction fault of Froitzheim et al. ).
Figure 14. Strain distribution and strain partitioning during lithospheric thinning: (a) exhumation phase, (b) thinning phase, and (c) initial stage. For a discussion see text.
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 The strain evolution, spatial relationships and structures that accommodate crustal thinning are shown in a conceptual way in Figure 14. We assume, based on the constrains presented above, a four layer rheology: (1) a strong felsic upper crust (from 0 to 15 km), (2) a weak felsic middle crust (15–25 km), (3) a strong mafic and felsic lower crust consisting of dry granulite and mafic rocks (25–30 km), and (4) a strong and cold subcontinental mantle. The conceptual model shown in Figure 14 is based on the fact that the mass of the crust needs to be conserved during rifting (problem of restoration of sections). The role of structural inheritance and the initial stretching phase (formation of rift basins during an initial stage of rifting) are neglected in our conceptual model shown in Figure 14.
 The onset of thinning occurs when deformation starts to localize within the future distal margin. In Figure 14c, this is shown by a conjugate set of shear zones that are in the upper felsic brittle crust. These structures are decoupled in a ductile layer in the felsic middle crust. Extension in the rigid mafic/felsic granulitic lower crust and upper mantle is accommodated along conjugate shear zones. This set of conjugate shear zone forms the limit between the future distal and proximal margins, and defines the necking zones of the two margins.
 During subsequent thinning, only the hatched areas (necking zones plus future distal margin) will deform. Crustal thinning is accommodated by conjugate shear zones shown in blue (e.g., Grosina and Pogallo) in the brittle upper and lower crusts. These crustal levels will deform by faulting and block rotation while ductile middle crust will act as a decoupling horizon between the two brittle layers. The importance of decoupling horizons during crustal thinning has been already emphasized by Brun and Beslier  from analog modeling. In the ductile middle crust, strain will increase toward the necking zone (see strain ellipsoid in Figure 14) as indicated by the formation of shear zones (e.g., Eita shear zone). On the scale of the crust, strain is accommodated, at this stage, by pure shear deformation. The ductile middle crust within the future distal margin will act as an extraction fault (e.g., Margna shear zone), along which the middle crust will be thinned between the rigid upper and lower crust together with the underlying rigid mantle. The higher strain of the ductile layer in the distal margin is compensated by more brittle deformation in the necking zones. A more complex process is supposed to occur in the mantle. Since extension in the brittle upper mantle results in omission and boudinage of the brittle layers, the space created by extension may be replaced by deeper, hotter impregnated mantle. This process may trigger thermal erosion and could be the cause of the strain localization and uplift of the future distal margin (e.g., key stone, Briançonnais [Lavier and Manatschal, 2006; Mohn et al., 2010]) during crustal/lithospheric thinning (for more discussion see Piccardo  and Müntener et al. [2010, and references therein]).
 The transition from symmetric thinning to asymmetric exhumation occurs when ductile mid-crustal layers cannot decouple anymore the deformation in the upper crust and mafic lower crust and subcontinental mantle. At this stage, faults can cut from the surface into the mantle and result in the exhumation of mantle rocks in the OCT. These faults (shown in red), are typically downward concave faults that sole out at shallow levels in the mantle and are expressed as low-angle top-basement detachment faults in the distal margin and adjacent OCT (e.g., Bernina and Err detachment faults). During this stage of deformation the former keystone Briançonnais block (or H-block in the terminology of Lavier and Manatschal, ) will be delaminated and will form units constituting the final distal margin.
7.6. Implications for Present-Day Rifted Margins (Figure 15)
 Interpretation of necking zones at present-day rifted margins remains difficult due to a lack of scientific drill hole data from the basement of the necking zone. In Figure 15, we show the refraction seismic line IAM5 across the Iberia rifted margin [Afilhado et al., 2008]. This line exemplifies the first order crustal structure of many rifted margins. In the IAM5 section (Figure 15a), the Moho as defined by velocities of ∼8 km/s is marked by surfaces that can dip up to 35°, corresponding to the necking zone where the crust is thinned from ±30 km to 10 km. Further continentward, in the necking zone, strong sub-horizontal reflections occur, which are clearly within the crust. These reflections are commonly interpreted as “Conrad” reflections, i.e., the contact between upper and lower crust. In the section, mid-crustal velocities are not observed in the most distal margin, while upper crustal velocities are found directly above a horizon characterized by velocities typical of the lower crust from the proximal margin. Therefore, Afilhado et al.  suggested that the middle crust is wedging out toward the necking zone. Restoration of the Adriatic rifted margin (Figure 13) shows many similarities with the refraction seismic data shown in the IAM5 section (Figure 15a) regarding the first-order crustal architecture of the necking zone. We therefore consider that the underlying processes and the strain accommodation within the two margins are comparable. This enables us to propose a structural interpretation of some of the reflections observed in the section in Figure 15b. The Margna-Pogallo shear zone could be linked to the contact between middle crust and mantle that dips continentward. Many authors have proposed that the seismic Moho at the base of the necking zone may correspond to a shear zone [e.g., Péron-Pinvidic and Manatschal, 2009]. The apparent dip of the Pogallo shear zone was estimated by Handy  between 10° to 34°, which is consistent with observations from present-day margins. The high velocity bodies that seem to be displaced by the Moho could in this case correspond to underplated gabbroic bodies, similar to those observed in the Margna and Ivrea units. The Eita shear zone and associated shear zones in the Campo unit could be equivalents of the reflections within the crust in the IAM5 section (Figure 15a). These reflections are at the top of the middle crust and point oceanward toward the wedge of the middle crust in the necking zone.
Figure 15. (a) Refraction seismic section across the West Iberia continental margin at 38°N [after Afilhado et al., 2008]. (b) Comparison of the crustal structure of the West Iberia continental margin (subdivision into upper, middle and lower crust) and the structures defined in this work (UCC, Upper Continental Crust; MCC, Middle Continental Crust; LCC, Lower Continental Crust).
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 The Grosina and Bernina detachments are top-basement detachment faults, locally overlain by extensional allochthons. These structures are therefore not easy to image and interpret in seismic sections [e.g., Hölker et al., 2003]. We suggest, however, that an equivalent structure to the Grosina detachment may occur west of the B1 high (see Figure 15a). This is supported by the observation that the middle crust wedges out in the footwall of this structure and that this structure also separates the distal and proximal margins showing different basin architectures.
 The similarity between the structures observed in the Bernina and Campo-Grosina units and those imaged in the IAM5 section (Figure 15a) show that the Alpine analogs represent a valuable natural laboratory to study present-day magma-poor rifted margins, with the significant advantage of allowing us to estimate the age and conditions under which the rift structures formed. Such information is necessary to constrain and test kinematic and dynamic models of rifted-margin formation.