Western Alps/Northern Apennine junction area: structures.
Relevant shallow and deep structures across the Western Alps/Northern Apennine junction area are illustrated through geological cross-sections of Figs. 6 and 9.
Cross-section 6.1 is representative of the Cottian Maritime Alps the southernmost segment of the Western Alps, and to the north/northeast includes the present day Po Plain domain and its subsurface features. The cross-section is mainly based on Ford et al. (2006) and Mosca et al. (2009) and includes some geological data derived from the “Geo-France 3-D”in Lardeaux et al. (2006), Bigot Cormier et al. (2006), Larroque et al. (2009).
Figure 6. Regional cross-sections across the Western Alps and Northern Apennine junction area.
Traces of the cross-sections in Figure 2. 6.1) from Digne to the Po plain. Mainly based on Ford et al. (2006), Mosca et al. (2009), Lardeaux et al. (2006), Bigot Cormier et al. (2006), Larroque et al. (2009); 6.2) from the Ligurian Sea to the Western Po Plain (modified after Mosca 2006). Structural data for the western side mainly came from Bigot Cormier et al., 2002, 2006; Piana et al., 2009, whereas for the N/NE side mainly from Piana et al., 2009; Rossi et al., 2009. Depth of Moho is from Waldahauser et al., 1998; Cassinis et al., 2002; cross-section across the Bobbio window after Molli (2008) and based on subsurface data in Biella et al. (1987); Cassinis et al. (1990); Laubscher et al. (1992); Cassano et al., (1986); Toscani et al. (2006) and geological data contained in Elter et al. (1992); Labaume (1992); Bernini et al. (1997); Cerrina et al. (2002); Cross-section 6.4 (after Molli, 2008) is traced across the Alpi Apuane window. It includes data of Cassano et al. (1986); Labaume (1992); Castellarin (1992, 2001) for the external part and surface geology of Carmignani et al. (1978); Molli et al. (2000, 2002) and Molli and Vaselli (2006). Colors as in Fig.2.
In the SW part of the section below the Argentera Massif is drawn as a crustal blind thrust (Bigot Cormier et al., 2006; Larroque et al., 2009 and references) splaying southwest within the decollement level of the Triassic evaporites of the Castellane and Nice area. Deep-seated thrusting below the Argentera has been invoked to explain the denudation rate since 10 Ma with increase at 3.5 Ma in association with reverse and strike-slip faulting (see below) along the Argentera-Bersezio and Frama Morte fault zones Bigot-Cormier et al. 2006; Larrouque et al., 2009). Deep-seated thrusting of the Ivrea-body is drawn according to Roure et al., 1990; Ford et al., 2006; Béthoux et al., 2006; Larroque et al., 2009). In the cross-section the Ivrea body extends below the Dora Maira which directly overlays its roof at c. 10 Km of depth. The Ivrea body played the special role of buttress during early collision of the European and Adriatic plates and those of indenter during later evolution (Laubscher, 1988; Roure et al., 1990; Schmid and Kissling, 2000; Lardeaux et al., 2006).
In the northeastern part of the section, complex interference between thrusts with opposite vergence produced the pop-up structure and the antiformal shape of the Torino Hill where well recognizable south-verging thrusts acting until Burdigalian time were followed by north-verging thrusts controlling the present northward translation of TPB over the Po Plain foredeep. Along the internal side of the metamorphic Alpine axial sector, the section shows the westernmost occurence of the non-metamorphic Ligurian units, buried by TPB succession. The Ligurian units progressively thin out from their eastern outcrop exposure toward the south between metamorphic units and the southwestern extension of the Adriatic units.
Cross-section 6.2 is representative of the “LA/NA” domain, which is characterized at surface levels by Alpine tectonic structures developed above a “Ligurian” Moho. The cross-section depicts the double-vergent transpressive system of the Ligurian Alps, that on the NE-side caused the superposition of slices of the metamorphic Alpine belt onto the Adria crust, with involvement of Ligurian non-metamorphic units (Helminthoid Flysches) and synorogenic Oligocene-Early Miocene terrigenous sediments of the Tertiary Piemonte Basin (see below), while on the SW side it induced, since Early Oligocene times, the stacking of the Ligurian Brianconnais thrust sheets, detached above the European-Ligurian crust, and the SW-vergent thrusting of Ligurian Brianconnais onto the Dauphinoise domain, with coheval superposition of Helminthoides Flysch at the top of the Ligurian Alps stack.
Cross-section 6.3 is traced across the Bobbio window, one of the most significant structures of the north-west Apennines. In the Bobbio window the Tuscan foreedeep deposits of the Bobbio Fm. are exposed below a composite system of Ligurian and subLigurian units. The Bobbio Fm. (upper Chattian-Burdigalian; Catanzariti et al., 2002) can be subdivided into a lower member (Brugnello Shale) which is made of mudstones with intercalations of thin-bedded and fine grained sandstones, and an upper member (San Salvatore Sandstone) consisting of thick alternations of thick-bedded and coarse grained sandstones and sequences showing lithologies similar to those of the Brugnello Shale. The pre-foredeep deposits are represented by the Marsaglia Complex (Labaume, 1992) formed by debris flow breccias and olistoliths reworking Subligurian wedge-derived elements. The Bobbio Fm., together with the underlying Marsaglia Complex, is structured according to a kilometer scale syn-sedimentary NE-verging syncline developed at the front of the Early Miocene submarine thrust front of the Ligurian and SubLigurian wedge (Labaume, 1992). Stratigraphy, age and structural features of the Bobbio Formation allowed a direct correlation with the Early Miocene Cervarola foredeep system of which the sandstones outcropping in the Bobbio window would represent the northwesternmost outcropping extension (Reutter and Schluter, 1968; Plesi, 1975; Labaume, 1992). The cross-section is mainly based on available geological data contained in Elter et al. (1992); Labaume (1992); Bernini et al. (1997); Cerrina et al. (2002) and references; refraction seismic interpretations of Biella et al. (1987); Cassinis et al. (1990); Laubscher et al. (1992); and borehole-controlled reflection profiles for the external area (Cassano et al. 1986; Toscani et al. 2006). The southwestern part of the section is characterized by two crustal scale thrusts. The westernmost thrust which brings a 6-6.1 Km/sec layer to a depth of 5 Km is connected at the surface with the subLigurian overthrust surface (Molli, 2008). The layer, overlain by Ligurian nappes (Antola, Internal and External Ligurian units) can be interpreted as related to sub-Ligurian and External Ligurian basement. The first activation of this thrust which shows out-of-sequence relationships post-dating the synsedimentary emplacement of the Sub-Ligurian unit on top of the Bobbio Fm. can be constrained as post-Early Burdigalian (Labaume 1992; Elter et al. 1992). The second thrust displaces the top of Mesozoic Adria carbonates (reflector with velocity of 6 Km/sec) and bounds in subsurface the Bobbio composite structure. The development of the Bobbio crustal antiform produced the final emplacement (Tortonian in age) with eastward sliding away from the crest of the antiform of the Ligurian units on top of middle Miocene (Serravallian) sandstone reached in the Ponte dell’Olio deep hole (Elter et al. 1992 and references, Toscani et al. 2006). Along the profile the Moho gently dips west reaching a depth of c.40 Km southwest of Bobbio where it rises abruptly to a shallower position in the Ligurian-Tyrrhenian basin (Laubscher et al. 1992; Castellarin 1992, 2001).
Cross-section 6.4 (modified after Molli, 2008) is traced across the Alpi Apuane window. It includes data of Cassano et al. (1986); Labaume (1992); Castellarin (1992, 2001) for the external part and surface geology of Carmignani et al. (1978); Molli et al. (2000, 2002) and Molli and Vaselli (2006). In the cross-section the top of basement dips eastward from the Alpi Apuane metamorphic complex and reaches c.10 Km of depth east of the Val di Lima fold (Carmignani and Kligfield 1990; Argnani et al. 2003). The basement below is subdivided into two parts: the upper one is considered part of the Tuscan metamorphic units exposed in the Alpi Apuane, whereas the lower is interpreted as an external slice analogue of the basement reached in an Agip hole (Pontremoli hole) north of the Alpi Apuane (Anelli et al., 1994; Montanini and Molli 1999; Molli, 2008). This second basement slice is floored by a thrust (t2) whose activity could be Tortonian to Messinian in age and overlies the more external basement and cover thrust sheets. The two internal crustal thrusts (t1 and t2) which correspond to the “Apuan Alps” and “Abetone” thrusts of Boccaletti and Sani (1998) are here considered as having a component of motions out of section, as structural data in their surface splays seem to indicate (e.g. Plesi et al. 1998; Cerrina et al., 2002). Along the profile, the Moho is gently west dipping reaching a depth of c.50 Km and jumps to a shallower position in the Tyrrhenian Moho (references in Pialli et al. 1998; Castellarin 2001; Scafidi et al., 2010).
Boundary Faults
Figure 7 presents the boundary fault zones playing a major role during the interaction between the evolving Alps and Apennines orogenic belts. In the scheme are also included some fault zones showing an ongoing seismic activity related with present-day tectonics.
Moving from the south/western to north-north/eastern zones and from younger to older structures the different fault systems are indicated with kinematics (where well-defined) also reported.
South-western Fault Zones: The boundary between the CMA and LA
Since the early ’70s, many authors invoked a Oligocene-onward, E-W sinistral strike slip fault zone placed at the southern boundary of the arc of the Western Alps, in the Maritime Alps (Laubscher, 1971; Guillame, 1980). Evidences of this major strike slip zone were documented by Ricou (1981) and Lefebvre (1983) along the so-called “Couloir de la Stura”, where sinistral strike-slip displacement along a N110 fault zone (Stura Fault, SF and associated Preit and Frama Morte lines) affected the Mesozoic-Lower Oligocene succession that rests on the NE boundary of the Argentera Massif. The successions involved in the km-thick fault zone are steeply dipping and partially overturned to NE, while the net displacement of the fault zone, that locally merges with the Penninic Front, could be of the order of tens kilometers. The role of the Stura Fault have been underlined later by Ricou and Siddans (1986), Giglia et al. (1996) and Bigot-Cormier et al. (2006) who strongly remarked the role of strike-slip tectonics in the formation of the Alpine orogenic belt. Other evidences of transpressive, NE-vergent deformations along the Stura couloir were reported by Perello (1997) that documented the local overthrusting of Argentera massif onto the Stura valley Brianconnais succession and the lower Oligocene Annot sandstones.
More recently, Piana et al. (2009) described another km-scale, ESE-WNW transpressive fault zone (Limone-Viozene deformation zone) placed some kilometers to the south east of the Stura Fault eastern termination, that runs for many kilometers roughly along the boundary between the Ligurian Brianconnais and Dauphinois-Provencal domains. This seems to be a major sinistral transpressive zone active since the first Alpine tectonic stage of the external Ligurian Alps, although important later dextral reactivations occurred along several individual minor faults of the zone.
E-W strike-slip faults and shear zones are present along the boundary between the External and Internal Brianconnais units (i.e. the Verzera Fault zone, Piana et al., 2009) and within the Internal Brianconnais, suggesting a possibile prolongation of the fault system in more eastern sectors and maybe in the Celle-Sanda fault zone that marks the boundary between the continental crust slices of the “Savona Crystalline units and the internal Ligurian Alps of the Voltri Unit meta-ophiolites (as inferred by Mosca et al., 2009).
In this work, the Oligocene-Neogene kinematic boundary between the Western Alps arc and the Ligurian Alps is thus individuated in a wide corridor bounded to the north by the Stura Fault-Penninic Front, and that comprehends the E-W strike slip faults of Ligurian Brianconnais, the Limone-Viozene sinistral transpressive zone, the boundary faults of the NE side of the Argentera Massif (Bersezio Fault, Tricart, 2004; Bigot-Cornier et al., 2006, the Bagni-Vinadio Fault, Perello et al., 2000; Baietto et al., 2009) and other minor faults such as the Saorge-Taggia fault that allowed since Early Oligocene an indipendent kinematics of Ligurian Alps with respect to the Maritime-Cottian Alps, with sinistral regional main transfer in the Oligocene followed by dextral movements in Late Miocene-Pliocene up to now.
Figure 7. Boundary faults and subsurface structures across the Western Alps/Northern Apennine junction area.
The figure reports the alpine foreland European-derived units; the inner Alpine nappe stack with metamorphic and unmetamorphic upper units (the same colors are used for the Alps s.s. and for their southern prolongation e.g. Ligurian Alps and Northern Apennine p.p.); Alpine retrowedge and the Apenninic units. All unit-types are reported in outcrop and subsurface occurences. Argentera Bersezio Fault System; CS, Celle Sanda; FBF, Frontal Briançonnais Front; LVVFS, Limone, Verzera, Viozene Fault System; OL, Ottone Levanto Line; PTF, Padane Frontal Thrust; RFDZ, Rio Freddo Deformation zone; SF, Stura Fault; STF; Saorge Taggia Fault; VVL Villalvernia Varzi L; Trace of fault zones, thrust and oblique thrust are also reported altogether with the stretching lineations on then main exhumation related foliation (Mid Eocene-early Oligocene in age) (after Menardi Noguera, 1988; Crispini and Capponi, 2001; Carminati, 2004; Seno et al., 2006). Rotation value (after Collombet et al., 2002; Maffione et al., 2008 and references) are reported with the poles of rotation for Corsica-Sardinia and BTP basin according to different authors: 1) Elter and Pertusati, 1973; 2) Vanossi et al., 1994; 3) Laubscher, 1988, 1991; 4) Rehault et al., 1984; 5) Hogerdujin Strating et al., 1994.
North-eastern Fault Zones: the boundary between the LA and NA
The AX fault (AXF)
A first order unexposed fault zone has been recently illustrated in subsurface below the Western Po Plain by Mosca et al., 2009; Rossi et al. 2009. These authors indentified this boundary as the front of the Alpine axial sector (AXF). In this paper this medium to low angle fault zone marks the type of the present substratum of the Alps-Apennine junction (Figs. 6,7). It separates buried elements of the axial Alpine belt, here representing the northward prolongation of LA, made up of HP/LT metamorphic rocks and low grade to unmetamorphic units, (both parts of the same pre-Late–Eocene Alpine-orogenic wedge) by the Helminthoid Flysch units that constitute the bedrock of the Torino and Monferrato Late Eocene-Miocene sedimentary successions. These Helminthoid Flysch units are in physical continuity with External Ligurian units of the NA, which also overlie Adria-derived continental units.
Figure 8. Main tectono-metamorphic units.
a) Sketch showing the main tectono-metamorphic units in the area encompassing the Voltri Massif, the Sestri-Voltaggio Fault Zone and the Flysch Units of central Liguria (Ligurian Alps, see text for details). Trace of cross-section in Fig. 8d is shown; in red the main faults; b) General crustal scale cross section through SVZ and the sourronding domains redrawn and modified after Biella et al., 1988; c) Symplified cross-section of the area close to the Sestri-Voltaggio Fault Zone. This can be considered as a high strain zone (Crispini et al., 2009) characterized by intense shearing and fracturing. The "old Sestri-Voltaggio Line" is interpreted as an early nappe contact, later reworked at shallow crustal levels. A regional backthrusting (top to E-NE) phase involves both Oligocene TPB successions and the metamorphic basement; 8d) WNW to ESE cross-section (modified from Capponi and Crispini, 2008 - Foglio GENOVA 1:50.000) showing the simplified structural architecture at very shallow level of the area across the innermost units of the Ligurian Alps (Voltri Massif, Sestri-Voltaggio Zone and Flysch units).
The Padane Thrust Front (PTF)
The PTF is a South-dipping reflector sealed by Pleistocene sediments that marks the northern boundary of the Helminthoid Flysch units and their overlying Torino Hill and Monferrato successions, and separates them from the Tertiary and Mesozoic sediments resting on the subsurface Adria basement. The PTF is mostly a blind thrust whose vertical projection at surface roughly corresponds to the geomophologic southern boundary of the Po plain.
Between the AXF and PTF, several transpressive km-scale fault zones developed during Oligocene and Miocene times, controlling the physiographic features of the evolving sedimentary basins, as well as sedimentation rates and provenance and deposenter migration; among these structures, at least two (the Rio Freddo Deformation Zone and the Villalvernia-Varzi Line) are to be described hereafter in some detail.
The Rio Freddo Deformation Zone (RFDZ)
The RFDZ (Piana and Polino, 1994; 1995; Piana, 2000) marks the boundary between Torino Hill and Monferrato areas. The RFDZ and its minor associated structures strongly controlled the sedimentary evolution of the Oligocene syn-orogenic basins of the north-TPB at least until the Early Burdigalian, in a prevalent strike slip tectonic regime, transtensional during the Early Oligocene and transpressive in late Oligocene-Early Miocene (Dela Pierre et al., 2003; Festa et al., 2005).
The Villalvernia-Varzi Line (VVL)
The VVL line is a E-W trending high angle fault zone which, along the Staffora Valley (Oltrepò Pavese area), separates the clastic successions of the eastern TPB unconformably overlying the Antola Unit, to the south, from the Epiligurian Succession and the underlying Ligurian stacks at the top of Tuscan tectonic units of the Bobbio window, to the north (Figs.6,7). It is considered a fault with main sinistral strike slip movement developed during a synsedimentary activity recorded in the TPB and Epiligurian basins. Biella et al. (1988) recognized its sub-surface features characterized by a steep attitude bounding at southwest the Tuscan foredeep units of the Bobbio tectonic window. Mosca et al. (2009) and Rossi et al. (2009) describe the VVL as a high-angle fault system characterized by an original extensional behavior accommodating the flexural tilting close to the southern edge of Adria, and since the Oligocene reactivated as contractional fault during the N-NE verging thrusting of Apenninic Ligurian units
The main activity of the VVL is considered to be Late Oligocene-Early Miocene in age by Laubscher et al. (1992) (see also Schumacher and Laubscher, 1996) who interpreted it as an outstanding sinistral transfer fault zone at the southern margin of the Adriatic indenter (Laubscher, 1988; Laubscher, 1991), parallel to the Insubric Line and with an opposite kinematics. In this framework the VVL was dissected by the post-Messinian thrusting, as the Adriatic indenter appears to have been inactivated in post-Miocene times (Schumacher and Laubscher, 1996).
According to Di Giulio and Galbiati (1995) the transpressive left-lateral activity of the VVL can be constrained using the sedimentary records since the middle-late Rupelian with two main stages of activity. The first stage occured during late Rupelian, whereas the second at Chattian/Aquitanian boundary. Recent tectonic activity along the Villalvernia-Varzi Line has been argued as well as the influence of this structure on the morphology and fluvial dynamics of the area (Meisina and Piccio, 2003). The kinematic evolution of the VVL is not yet completely well defined being missing a systematic structural study. This explains why the VVL line has been also interpreted as a high angle dextral strike slip fault zone (Cerrina Feroni et al., 2002).
The Ottone-Levanto Line (OL)
The Ottone-Levanto was originally defined by Elter and Pertusati (1973) as the southern prolongation of the VVL. It has been considered as the frontal thrust separating the pre-Oligocene alpine structural nappe stack of the LA and TPB by the early Miocene NA (Laubscher, 1988). After the early original proposition no detailed structural studies have been devoted to the recognition and analyses of the surface expression of such line that according to the seismic data in Biella et al. (1988) is here considered as a post-Aquitanian deformation zone formed by three major splays rooting in a WNW dipping thrust (Fig.8b).
The Sestri Voltaggio Zone (SVZ)
The Sestri Voltaggio Zone is a km-wide north-south oriented structural domain that includes tectono-metamorphic units of the Alpine belt and it is limited to the west by the Sestri-Voltaggio Line and to the east is contact with very low grade and unmetamorphic Ligurian units (Fig.2,8). Actually the Sestri Voltaggio Zone can be considered as a high strain zone or fault zone and it can be better referred to as Sestri-Voltaggio Fault Zone. In the present-day map view it marks the contact among units of different paleogeographic derivation and reequilibrated at different P-T metamorphic conditions (Fig.8).
The WNW to ESE cross-section of Fig.8b,c (modified from Capponi and Crispini, 2008) shows the simplified structural architecture at very shallow level of the area across the innermost units of the Ligurian Alps (Voltri Massif, Sestri-Voltaggio Zone and Flysch units). The cross-section is representative of the geometric stacking of the units, of the relationships among the units and their internal structural arrangement. The poorly exposed contacts among the tectonic units are generally steeply dipping to the east so that the HP-LT units are the lowermost units and the very low grade Flysch units the uppermost units of the tectonic pile; the Antola Unit is in the top structural position, with a low-dipping tectonic contact. At the outcrop scale, the boundaries among the metamorphic units are folded shear zones commonly reactivated by strike-slip faults with the same longitudinal trend. Moreover the area close to the Sestri-Voltaggio Line (Fig.8) can be considered as a high strain zone (Crispini et al., 2009) and is characterized by intense shearing and fracturing.
Figure 8d shows a schematic insight into the tectonic features close to the Sestri-Voltaggio Fault Zone. A regional top to E-NE backthrusting phase that involves Oligocene TPB successions and the metamorphic basement is described in the LA and testifies to a major tectonic activity lasting up to the Oligocene (d’Atri et al., 1997; Capponi et al., 2001; Piana et al., 2006. ; Spagnolo et al., 2007; Capponi et al., 2009). In the same way the tectonic activity of the Sestri-Voltaggio Fault Zone possibly lasted up to Late Oligocene-early Miocene as the related subsidiary structures involve the Aquitanian–Burdigalian TPB deposits (Capponi et al., 2009 and references therein). The SVFZ and the backthrusts can be inserted in the same tectonic framework where SVFZ acted as a dextral tear fault in the general migration of the LA towards NE-N.