Journal of the Virtual Explorer

The region between the southern Western Alps and the Northern Apennine exhibits a complex pattern of exhumation in the uppermost crustal levels, nicely imaged by fission-track (FT) data on zircon and apatite (Fig. 9). These geochronological systems constrain burial and exhumation of rocks across the 240° and the 120°-60°C isothermal surfaces (Gallagher et al. 1998), corresponding to ca.7 km and 3-4 km depth for a 30°C/km paleogeothermal gradient (Malusà et al. 2006).

In the Western Alps, FTs on apatite are generally completely reset during the Alpine orogeny (Fügenschuh et al. 1999; Malusà et al 2005). A remarkable exception is represented by the Chenaillet unit, which always resided at shallow levels in the nappe stack (Carpena and Caby, 1984; Schwartz et al. 2007). In the northwestern Alps, apatite FT data define a regional pattern with decreasing ages moving from the axial sector of the belt towards the European foreland (Malusà et al. 2005).

Figure 9. Bedrock fission-track data on apatite and zircon.

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After Carpena and Caby 1984, Abbate et al., 1994 ; Balestrieri et al. 1996, Seward et al. 1999, Fügenschuh et al. 1999, Vance 1999, Ventura and Pini 1999, Bigot-Cormier et al. 2000, Bogdanoff et al. 2000, Sabil and Menard 2000, Tricart et al. 2001, Ventura et al. 2001, Carrapa 2002; Balestrieri et al. 2003; Foeken et al. 2003 ; Fügenschuh and Schmid 2003; Balestrieri et al. 2004, Malusà et al. 2005, Bigot-Cormier et al. 2006, Fellin et al. 2007, Schwartz et al. 2007, Tricart et al. 2007, Labaume et al. 2008.

To the south, apatite FT ages show an opposite trend and get younger eastward, at least west of the Dora-Maira unit (Tricart et al. 2007). Evident breaks across major faults (e.g. Tricart et al. 2001; Malusà and Vezzoli 2006) and across lower-order faults (e.g. Bigot-Cormier et al. 2006; Malusà et al. 2006; 2009) testify to active tectonics during and after exhumation. In general terms, the External Massifs and the westernmost units of the Briançonnais fan experienced higher exhumation rates than most of the axial Western Alps since the Miocene (Malusà et al. 2005; Vernon et al. 2008). Such a differential exhumation was probably accommodated by reverse motion along the W-dipping Internal Houiller Fault (Malusà et al. 2009), by normal reactivation of the E-dipping Brianconnais Fault and associated Longitudinal Faults (Barfety and Gidon, 1975; Fabre et al., 1982; Tricart et al.2001), and by forward propagation of the external thrusts located beneath the External Massifs (Gratier et al. 1989).

In the Western Alps, an along-strike gradient with increasing fission-track ages from north to south is also described (Fügenschuh and Schmid, 2003; Malusà et al., 2005). To the south, in the Maritime Alps, the zircon fission-track system is sometimes not reset, as observed in large areas of the External Massifs and in the Helmintoid Flysch nappes, where burial never exceeded 7 km (Seward et al. 1999; Vance 1999; Bernet et al. 2001; Foeken et al. 2003; Bigot-Cormier et al. 2006). Such along-strike gradient implies higher exhumation rates in the northern sector of the Western Alps with respect to the southern sector. This may be due to an increasing importance of crustal shortening that promoted erosion to the north (Malusà et al. 2005; Malusà and Vezzoli, 2006), coupled with a greater influence of Apenninic subsidence to the south (Doglioni, 1994; Garzanti and Malusà 2008).

In the Ligurian Alps, apatites are generally reset during the Alpine orogeny (Carrapa 2002), whereas zircons locally yield unreset Mesozoic ages (Vance 1999; Bernet et al. 2001). In these latter areas, burial was thus in the range of 3-4 km to less than 7 km, like in the Internal and External Ligurian units of the Northern Apennine where apatite FT ages are in the range of 20-6 Ma, and zircon FT ages exceed 150 Ma (Balestrieri et al. 1996). In the Cervarola, Macigno and Modino sandstone, apatite FT ages range between 3 and 10 Ma, and the extent of exhumation of the nappe pile ranges between 5 and 7 km (Ventura et al. 2001). Apatite FT ages from the Marnoso-arenacea Fm record instead post-depositional burial ranging between 5 km and less than 2.5 km. The missing section, eroded in the last 5-4 Ma, would consist of foredeep successions and overlying Ligurian units (Zattin et al. 2002). In an innermost position, geochronological data from the Apuane Alps indicate that the Apuane rocks were structurally buried to 15–30 km at about 20 Ma, to be exhumed across the 240°C isothermal surface at 10–13 Ma, and finally reach the 70°C isothermal surface by 5 Ma. The Macigno Fm in the Apuane region reached its maximum depth of 7 km at 20-15 Ma (Fellin et al. 2007).

The characters of the large-scale depositional units, define a long-term, major transgressive-regressive cycle from Late Eocene to Miocene (Rossi et al, 2009). The maximum transgression took place in the Late Burdigalian and coincides with the maximum rate of tectonic space creation. This is recorded by the deposition of a km-thick basinwide and highly efficient turbidite system. This system separates the older southwestward coastal onlap and aggradation from the younger outbuilding related to the Middle Miocene uplift recorded along both the southwestern and southeastern basin margins, sometimes punctuated by forced regression pulses (Rossi et al., 2009)

Then, since Middle-Upper Miocene major accumulation and subsiding areas were roughly in the present central TPB, i.e. Savigliano and Alessandria basins, bounded to the north by continuous and progressive uplift of northern TPB arc (Torino Hill and Monferrato)

The TPB and its underling and adjacent substrata experienced kilometers-scale vertical movements (subsidence and exhumation) following the Late Eocene-Early Oligocene stages of major contraction. Upward and downward movements were active in different places at the same time and the sites of maximum exhumation and subsidence migrated through time (Mosca 2006; Mosca et al, 2005; Bertotti et al., 2006). In detail, the southern segment of the Alpine metamorphic system were rapidly exhumed and eroded during (Late Eocene-) Lower Oligocene times; after the Lower Oligocene clastic sedimentation, the southern Tertiary Piedmont Basin (namely the southernmost basin domains) experienced major km-scale subsidence and underwent subsequent exhumation. By contrast, the internal side of the Western alpine arc was more stable, recording since the Late Oligocene a more generalized subsidence. This pattern of vertical movements for TPB and its substrata, characterized by variations in magnitude and even sign through time and space, and its correlation with shift of major TPB depocenters have been interpreted as related with crustal folding (Bertotti e Mosca, 2009). Alternative, they may have developed, at least in part, during progressive rotation of the southernmost part of the alpine axial belt (Ligurian Alps domain) to reach the present position.

A solution to the relationships between Western Alps/Northern Apennine junction area has been recently proposed by Vignaroli et al. (2008), Maffione et al. (2008) and Vignaroli et al. (2009). The model suggests that the eclogitic-bearing units of the Voltri Massif were exhumed in an extensional transfer domain which accomodated an opposite outward migration of the Alpine and Apennine thrust fronts since about 35/30 Ma. It is mainly based on a interpretation of the deep structure derived by 3-D tomography, new paleomagnetic data (Maffione et al., 2008) and a re-interpretation of surface geology in Central Liguria in particular around the Voltri Massif.

The extensional domain was controlled by a progressive development of a low-angle detachement fault system with a general top-to-west kinematics and progressive steepening of the eastern side of the footwall (eastern flank of Voltri Massif) by a rolling-hinge mechanism to produce the final asymmetric doming of the eclogitic units. A comment with detailed compared overall architecture and tectono-metamorphic history of the area has been provided by Capponi et al., (2009) with reply of Vignaroli et al., (2009).

On the basis of the geological data presented in this paper we discuss some points of the broad regional tectonic aspects of this model (included also in Maffione et al. 2008 and Vignaroli et al., 2009) inviting the reader to refer also to the comment by Capponi et al., (2009) with reply of Vignaroli et al., (2009) for a more local regional discussion.

A first point concerns the large scale tectonic frame of the supposed 35/30 Ma onward opposite retreating subductions. This setting, fitting part of the geological history (up to Pliocene) of the Apennines, is not supported by the data of the southwestward Alpine foreland which do not record any lithospheric flexure and related space regional accomodation postdating-34 Ma as the absence of significant foreland propagation testifies (Ford et al., 2006; see above).

Moreover, the model does not really solve the problem of the exhumation of the deep seated metamorphic rocks of the Voltri Massif which reached the surface in the Late Eocene-early Oligocene as testified by sedimentary record of TPB basin and its unconformably overlapping. Exhumation was therefore basically achieved before the time of the supposed opposite retreating (as claimed in Vignaroli et al., 2008).

Finally, all the pre-early Oligocene structures of the Ligurian Alps including the Voltri Massif and the Ligurian units on top of it cannot be considered related with the Apenninic subduction system (as claimed in Vignaroli et al., 2009) but instead formed as all authors recognized (see references in the paper) within an alpine east-southeastward dipping subduction. This is well testified by the overall architecture of the belt, the superficial continuites of the alpine nappe stack from Cottian-Maritime to Ligurian Alps, the tectono-metamorphic history and kinematics data available.