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Queste bozze, corrette e accompagnate dall’al- legato preventivo firmato e dal buono d’ordine, SGI Bollettino Dgs10 143 debbono essere restituite immediatamente alla Segreteria della Società Geologica Italiana c/o Dipartimento di Scienze della Terra Piazzale Aldo Moro, 5 – 00185 ROMA Boll.Soc.Geol.It. (Ital.J.Geosci.), Vol. 126, No. 2 (2007), pp. 411-425, 11 figs.

Remarks on the Quaternary tectonics of the Insubria Region (, NW , and , SE )

GIANCANIO SILEO (*), FRANCESCA GIARDINA (*), FRANZ LIVIO (*), ALESSANDRO M. MICHETTI (*), KARL MUELLER (**) & EUTIZIO VITTORI (***)

ABSTRACT pino compreso tra i laghi Maggiore e Garda. Le evidenze di tettonica quaternaria nella zona di pianura a sudovest del Lago di Garda in- In the foothills of the Southern Alps between Lake Garda and cludono pieghe con circa 200 metri di rilievo strutturale in sedimenti , the relations between the recent tectonic evolution del Pleistocene inferiore-medio, affioranti nell’area epicentrale del and the accompanying seismic potential have been overlooked. terremoto di Brescia del 25 gennaio 1222 (Intensità epicentrale IX Published interpretations of structures consider shortening to cease MCS, Me 6.2). at the end of the Messinian; this is in contrast with geomorphic and Nonostante l’assenza di notizie di terremoti storici abbia fatto stratigraphic evidence for Plio-Pleistocene reverse faulting and sinora ritenere l’area ad ovest di Brescia, verso i laghi di e folding presented in this paper. Maggiore, sostanzialmente stabile ed asismica, il raccorciamento tet- Evidence for Quaternary shortening near Lake Garda includes tonico potrebbe essere tuttora attivo anche qui, sulla base della si- Quaternary folds with 200 m of structural relief recorded by Lower- smicità di bassa magnitudo che si allinea al piede dei rilievi prealpi- Middle Pleistocene deposits. These crop out on top of the Caste- ni fra Brescia e , dei dati geodetici, e della presenza di nedolo and Ciliverghe Hills, within the epicentral area of the histori- deformazioni quaternarie segnalate da vari autori. cal December 25, 1222 Brescia earthquake (epicentral intensity IX Questo ha spinto gli autori del presente lavoro a rivedere i dati MCS, Me 6.2; GUIDOBONI, 2002). esistenti e a raccoglierne ulteriori, al fine di comprendere meglio Shortening further west may also be occurring, but there is no l’evoluzione quaternaria del fronte alpino meridionale nella regione record of local damaging historical earthquakes along the foothills dell’Insubria (fra il Lago di Como e il Ticino). Qui vengono analizza- of the Alps. This motivated us to review available data and undertake te le due maggiori strutture nell’area: l’anticlinale di Albese con Cas- new reconnaissance field work, in order to better understand the sano e la dorsale del retroscorrimento della Gonfolite Lombarda. Quaternary tectonic evolution of the Southern Alps near Quest’ultima struttura, sinora datata al Miocene superiore, presenta in the Insubria region. tuttavia evidenze morfologiche consistenti con strutture quaternarie We focused our studies on two major structures located east and compressive. Nuovi dati di rilevamento indicano che questa struttu- west of Como, respectively, the Gonfolite backthrust and the Albese ra coinvolge depositi Pliocenici e probabilmente anche più recenti. con Cassano anticline. The Gonfolite backthrust, first recognized by A sua volta, l’anticlinale di Albese con Cassano è una struttura BERNOULLI et alii (1989) and reported as late Miocene in age, is che è risultata aver accumulato circa 200 metri di spostamento verti- accompanied by geomorphic features consistent with Quaternary cale post Pleistocene medio. Inoltre, il ritrovamento di paleoliquefa- compressional structures. Our new field mapping suggests that this zioni in depositi proglaciali medio-pleistocenici suggerisce che la thrust offsets deposits of Pliocene and possibly younger age. Neotec- crescita della struttura sia stata accompagnata da significativi terre- tonic evidence for the recent evolution of Albese con Cassano anticline moti locali. Nel complesso quindi, le evidenze di tettonica quaterna- was first presented by OROMBELLI (1976). Our investigations confirm ria nella regione dell’Insubria, una delle aree più vulnerabili dell’in- Orombelli’s observations, and suggest that the Albese con Cassano is tera Europa, sono consistenti con quanto osservato nell’area an actively growing anticline that accumulated ca. 200 m of post-Mid- bresciana. La modestissima pericolosità sismica attribuita all’area dle Pleistocene vertical displacement. In addition, paleoliquefaction Insubrica, essenzialmente sulla base del catalogo sismico, potrebbe features in Mid-Pleistocene proglacial deposits at this site suggest that quindi essere sottostimata. Ciò suggerisce la necessità di un riesame growth of this fold has been accompanied by strong local earthquakes. attento, attualmente in corso, degli indizi geologici e geomorfologici, Evidence of Quaternary tectonics in the Insubria region is con- di tettonica recente, anche attraverso l’analisi di possibili evidenze sistent with that observed in the well-known seismic area near Lake paleosismiche. Garda. Therefore, the seismic potential of this area should be care- fully re-evaluated based on the examination of available geological TERMINI CHIAVE: Tettonica Quaternaria, Alpi Centro-Meri- data and the results of new ad hoc paleoseismological analyses. Cur- dionali, pericolosità sismica, tettonica compressiva. rent assessment of seismic hazard, based only on the historical seis- mic catalog, supposedly heavily underestimates the earthquake potential in one of the most vulnerable areas of the whole Europe. INTRODUCTION KEY WORDS: Quaternary tectonics, Central-Southern Alps, seismic hazard, compressional tectonics. Active shortening and related seismicity in Northern Italy is the result of the growth of two thrust belts of RIASSUNTO opposite polarity, the Alps and the Apennines (e.g. SERVA, 1990; DOGLIONI, 1993; CASTELLARIN & CANTELLI, 2000; Considerazioni sulla tettonica Quaternaria della Regione BOCCALETTI & MARTELLI, 2004; fig. 1a). dell’Insubria (Lombardia, Italia Nord-Occidentale e Ticino, Sviz- Pleistocene shortening in the Southern Alps and zera sud-orientale). northern Plain has been documented on geological In questo studio vengono illustrate alcune potenziali evidenze di basis to decrease from east to west (e.g. SERVA, 1990; riattivazione tettonica tardo-quaternaria nel settore del fronte sudal- BIGI et alii, 1990; FANTONI et alii, 2004; GALADINI et alii, 2005; CASTELLARIN et alii, 2006). Historical seismicity fol- lows the same trend, and the frequency of strong seismic (*) University of Insubria, Dip. di Scienze Chimiche e Ambien- events decreases from Friuli toward Lake Garda, and then tali, via Valleggio, 11 - 22100 Como, [email protected]. (**) University of Colorado, Boulder, CO, USA. Lake Maggiore, where no historical seismicity with epi- (***) Italian Agency for Environment Protection and Technical central intensity greater than VI MCS is documented (e.g. Services (APAT), Rome, Italy. WORKING GROUP CPTI, 2004; fig. 1b). Furthermore, 20 143-Dgs10(411-426) 25-06-2007 14:57 Pagina 412

412 G. SILEO ET ALII

based on GPS data, the western Southern Alpine moun- Alpine structures (e.g. BERNOULLI et alii, 1989; DOGLIONI, tain front is currently characterized by noteworthy short- 1993; FANTONI et alii, 2004; CASTELLARIN et alii, 2006; ening rates of ca. 1.1 mmyr-1, even though half of the SCARDIA et alii, 2006). Others point out that during the value (2.2 mmyr-1) inferred for the seismically-active east- Late Alpine deformation the inner Western Alps are char- ern Southern Alps in Friuli (e.g. SERPELLONI et alii, 2005; acterized by Neogene to ongoing extension (MANCK- D’AGOSTINO et alii, 2006). TELOW, 1992; SUE & TRICART, 2003; GROSJEAN et alii, West of the Veneto-Friuli regions, the best evidence of 2004). PICOTTI et alii (1995) and BERTOTTI et alii (1997) active shortening is shown in the Lake Garda area. Desio have argued that even the Central-Southern Alps and the (1965) described the morphological and stratigraphic Northern Apennines are currently experiencing crustal expression of several Pleistocene anticlines outcropping extension, because of a post-Messinian state-of-stress southwest of Lake Garda, near Castenedolo and Ciliv- change related to the bulging of the Apennine Foreland. erghe (fig. 1a). These structures were subsequently stud- In the area between Lake Maggiore and Lake Como, ied by other workers, including BARONI & CREMASCHI this poor understanding of the Quaternary tectonic set- (1986), CASTALDINI & PANIZZA (1991), CURZI et alii ting has resulted in contrasting interpretations of the (1992), LOCATELLI & VERCESI (1998), CARCANO & PICCIN observed evidence of neotectonics and paleoseismicity (2002). At least 200 m of structural relief has been docu- (e.g. CASTALDINI & PANIZZA, 1991; ZANCHI et alii, 1997). mented in the Lower Pleistocene marine deposits affected For instance, opposing interpretations have been pre- by the Castenedolo anticline (CURZI et alii, 2002). A simi- sented to explain the Quaternary uplift of Mt. Morone, lar Middle Pleistocene uplift has been recently derived east of (e.g. FELBER, 1993; ZANCHI et alii, 1997; from magnetostratigraphic analysis along a borehole BINI et alii, 2001; figs. 2 and 3). BINI et alii (2001) suggest drilled southwest of Castenedolo (SCARDIA et alii, 2006). that the uplift could be interpreted as due either to Faulting and tilting in Middle Pleistocene glacial deposits reverse faulting along the Gonfolite backthrust or to late- has been observed in the Ciliverghe Hill, belonging to the Alpine extensional tectonics. same structure of Castenedolo (BARONI & CREMASCHI, This motivated us to review available data and under- 1986). The presence of Quaternary structures inferred take new reconnaissance field work, in order to better from geological data is in agreement with the historical understand the Quaternary tectonic evolution of the South- and instrumental seismicity data. Castenedolo and Ciliv- ern Alps near Lake Como in the Insubria region. In the first erghe are located within the epicentral area of the Decem- step of this project, we focused our studies on two major ber 25, 1222, Brescia earthquake (MAGRI & MOLIN, 1986; structures located east and west of Como, respectively, the GUIDOBONI, 1986; SERVA, 1990; GUIDOBONI, 2002), Gonfolite backthrust and the Albese con Cassano anticline, arguably the largest historical seismic event that has both displaying evidence of recent deformation according occurred in the Po Plain (WORKING GROUP CPTI, 2004): to literature data (e.g. OROMBELLI, 1976; FELBER, 1993; the intensity IX MCS attributed to this earthquake is ZANCHI et alii, 1997; BINI et alii, 2001). We seek to demon- equivalent to a macroseismically – derived magnitude of strate whether 1) these features are produced by active 6.2 (see GUIDOBONI, 2002). Besides, the seismicity of this shortening or extension, and 2) they can be compared to area and the mechanism of faulting has been more the Quaternary deformation observed in the seismically- recently documented by the November 24, 2004, Salò active portion of the western Southern Alps near Lake earthquake (e.g. MICHETTI et alii, 2004; fig. 1b), a moder- Garda. This paper illustrates initial results of new field ate event characterized by a reverse faulting focal mecha- mapping and geomorphic analyses along these two struc- nism (an overview of the historical seismicity near Salò is tures. In the next step of this research, we have conducted available in INGV, 2004). Likewise, a south-verging, blind a more regional study in collaboration with ENI Explo- reverse fault has been hypothesized as the causative tec- ration and Production, including the reinterpretation of tonic structure for the May 12, 1802, Soncino earthquake subsurface data in the whole Lombardia sector of the Po by BURRATO et alii (2003; fig. 1). Based on a detailed Plain and adjoining foothills; results from this further analysis of drainage network anomalies, these Authors research will be presented elsewhere in a companion suggest that the Southern Alps beneath the Po Plain in paper. The revision of the available literature based on new Lombardia are characterized by active shortening accom- field data collected so far and presented here allows to modated by thrust faulting and growing anticlines. This is draw some significant conclusions, even if preliminary, on in agreement with interpretations by ROEDER (1992), the Quaternary tectonics of the Insubria region, and to who, based on structural analysis of subsurface data delineate a rather new scenario for its seismicity potential. along the Po Plain piedmont belt in Lombardia, indicated «draping and/or warping of Pliocene sediments over deeper thrust architecture related to recent foothills GEOLOGICAL AND GEOMORPHOLOGICAL SETTING thrusting at low strain rate». OF THE INSUBRIA REGION Even if the studies described above consistently pro- pose a Quaternary tectonic setting due to late Alpine Insubria extends between Lake Maggiore and Lake shortening, the interpretation of the nature and features Como along the foothills of the western Southern Alps of Quaternary deformation along the Lombardia foothills (fig. 1a). The oldest strata exposed here include Late Tri- and piedmont belt is still controversial. Several Authors assic to Early Jurassic mainly carbonatic sediments suggest that shortening in the western Southern Alps vir- deposited in the Jura-Cretaceous Lombardian Basin fol- tually ceased in the late Messinian after the so-called lowing the opening of the Tethys Ocean (WINTERER & «Lombardic Tectonic Phase» (14 to 6 Myr B.P.; e.g. SCHU- BOSELLINI, 1981; GIANOTTI & PEROTTI, 1987; BERTOTTI et MACHER et alii, 1996), due to the northward propagation alii, 1993). The outset of the Alpine orogenesis then shed of the Northern Apennines lithospheric flexure, which sequences of flysch (Cretaceous to Eocene) and molasse tilted and deactivated the pre-existing south verging southward into the Po Plain Foredeep. The latter is repre- 20 143-Dgs10(411-426) 25-06-2007 14:57 Pagina 413

REMARKS ON THE QUATERNARY TECTONICS OF THE INSUBRIA REGION 413

Fig. 1 - a) Structural setting of Northern Italy (from CA- STELLARIN et alii, 2006, modi- fied); inset shows the study area, small triangles indicate the location of the Castene- dolo and Ciliverghe hills, Soncino and Salò; b) seismi- city map of Northern Italy showing events with MCS in- tensity higher than 5, modi- fied from the CPTI seismic catalogue (WORKING GROUP CPTI, 2004). – a) Assetto strutturale del Nord Italia (da CASTELLARIN et alii, 2006, modificato); l’inser- to indica l’area di studio, i pic- coli triangoli indicano l’ubica- zione dei rilievi di Castenedolo e Ciliverghe e delle località di Socino e Salò citate nel testo; b) carta della sismicità dell’Ita- lia Settentrionale che include tutti gli eventi con le intensità MCS maggiori di 5 (da CPTI, GRUPPO DI LAVORO, 2004, mo- dificato).

sented by the Oligo-Miocene sequence of the Lombardian were deposited in small troughs within the piedmont belt Gonfolite Group, which outcrops in Western (e.g. «Argille di Castel di Sotto»; BINI et alii, 2001; fig. 4). and is largely represented in the subsurface of the north- Locally, coarse alluvial deposits are found beneath the ern Po Plain (SCIUNNACH & TREMOLADA, 2004; FANTONI Pliocene clays, such as in the area west of Como («Ponte- et alii, 2004). The Gonfolite Group is made by up to 3000 m gana Conglomerates», tentatively dated at the late of resedimented conglomerates and sandstones, and can Messinian; see BINI et alii, 2001, and reference therein; be linked to a foredeep tectonic basin superimposed on fig. 4). This succession is covered by the Quaternary an active continental margin (e.g. GELATI et alii, 1988, glacial and fluvio-glacial sediments related to climatic- 1991). After deep erosion during the Messinian as a result driven fluctuations of the ice tongues of the Abduan glac- of a drop of the regional base level, Pliocene marine clays ier system (BINI et alii, 2001). 20 143-Dgs10(411-426) 25-06-2007 14:57 Pagina 414

414 G. SILEO ET ALII

Fig. 2 - Digital elevation model and location map of the study area: CO - Como; VA - Varese; LC - Lecco; MdN - Madonna della Neve; AcC - Albese con Cassano; Nov - Novazzano. – Modello digitale del rilievo e carta delle località dell’area di studio citate: CO - Como; VA - Varese; LC - Lecco; MdN - Madonna della Neve; AcC - Albese con Cassano; Nov - Novazzano.

Insubria underwent thin-skinned shortening during glacier (e.g. BINI et alii, 2001). During the Last Glacial the phase of the Alpine orogenesis that generated the Maximum (LGM, 22 to 20 kyr B.P.; e.g. IVY HOCHS et alii, Southern Alps (e.g. DOGLIONI, 1993; FANTONI et alii, 2004, and references therein) in the area between Como 2004), producing a system of mostly south-verging thrusts and Varese the landscape was mostly covered by ice, and and folds (e.g. SCHÖNBORN, 1992). Sediments deposited the whole drainage network was filled by glacial tongues in the foredeep of the Southern Alps during this period up to an elevation of 1100 m near Mt. Generoso and 800 and the external part of the belt are deeply buried m in the surrounding of Como town (fig. 2; e.g. BINI et beneath the Po Plain as shown in seismic reflection data alii, 1998, and references therein). During the LGM and (PIERI & GROPPI, 1981). These are represented by the previous glacial maxima, in this sector the glacial Upper Oligocene to Lower Miocene deepwater clastic processes were dominantly erosional, and most of the strata of the Gonfolite Lombarda Group (GELATI et alii, glacial to fluvio-glacial sediments have a post-LGM age. 1988), that unconformably overlie the mainly carbonatic The major accumulation of glacial deposits took place Meso-Cenozoic succession. further south, between Como and , where the huge From the geomorphic point of view, Insubria piedmont lobes and terminal moraines of the Abduan stretches east-west across the Lombardia and Ticino glacier were located during the LGM. The area south of foothills at the junction between the Alps and the Po the Varese and Como foothills is also characterized by a Plain, with elevations ranging from 198 m (the level of gently south-sloping fluvio-glacial and fluvial outwash Lake Como) and 1704 m a.s.l. (the elevation of Mt. Gen- plain (sandur-like environment), essentially built during eroso, the highest peak in the area). The digital elevation the LGM. This wide low-gradient depositional surface is a model shown in fig. 2 clearly illustrates the strong glacial typical feature of all the piedmont belts around the Po imprint on the landscape, described in detail by several Plain, and corresponds to the so-called «Livello fonda- authors, such as NANGERONI (1970), OROMBELLI (1976), mentale della pianura» of PETRUCCI & TAGLIAVINI (1969), BERNOULLI et alii (1989), MONTRASIO (1990), MOSCA- who first defined the basic features of this regional geo- RIELLO et alii (2000) and BINI et alii (2001). morphic unit (see CASTIGLIONI & PELLEGRINI, 2001; Since the Middle Pleistocene, several major glacial MARCHETTI, 2002; and references therein). expansions covered this area with large, up to 2 km Two major tectonic structures in the Insubria region thick, ice tongues (NANGERONI, 1970; BINI et alii, 1998; show geomorphic features clearly unrelated to glacial ero- MOSCARIELLO et alii, 2000; MUTTONI et alii, 2003). Suc- sion and indicative of a recent tectonic activity, the Gon- cessive advances and retreats of glacial flows coming folite backthrust and the Albese con Cassano anticline. from the Central Alps occupied the valleys and reached The evidence for Quaternary deformation and faulting the piedmont belt and the northern Po Plain. In particu- along these structures has been already described in the lar, during the Late Pleistocene the Insubria region was literature by several Authors (e.g. OROMBELLI, 1976; FEL- affected by several important glacial pulsations of the BER, 1993; ZANCHI et alii, 1997; BINI et alii, 2001). In the major ice tongue coming from Valtellina, i.e. the Abduan next sections we revise the published data and describe 20 143-Dgs10(411-426) 25-06-2007 14:57 Pagina 415

REMARKS ON THE QUATERNARY TECTONICS OF THE INSUBRIA REGION 415

Fig. 3 - Shaded relief 3D map (from 20 m resolution DEM courtesy of Regione Lombardia), of the Gonfolite Backthrust area, view from northwest; vertical exageration 3X. – Modello tridimensionale dell’area del retroscorrimento della Gonfolite, visto da nord-ovest (dal modello digitale del terreno, con risoluzione di 20 m, messo a disposizione dalla Regione Lombardia; esagerazione verticale 3X).

the results of new field mapping and geomorphic analy- between the Gonfolite Lombarda Group and the Pliocene ses. We show that the evidence of Quaternary shortening Formation of the Castel di Sotto Clays, outcropping along along these tectonic structures can be compared with the thalweg of a small valley. He interpreted this contact those previously described for the Southern Alps struc- as a possible normal fault, due to the normal drag of the tures southwest of Lake Garda. Pliocene bedding along the fault plane. ZANCHI et alii (1995, 1997) revised this outcrop giving the same inter- pretation of normal faulting. THE GONFOLITE BACKTHRUST The drainage along the Faloppia Valley, one of the originally south-sloping valleys carved in the Lombardia West of the city of Como, the Gonfolite backthrust is foothills during the Messinian drop of the Mediterranean located at the base of a 20 km long, steeply inclined, Sea level (see FINCKH, 1978; FELBER et alii, 1991, 1994), north-facing mountain front (figs. 2 and 3). During the was later inverted due to the raising of the Gonfolite construction of a railway tunnel in the Como area, a tec- relief. tonic contact between the Gonfolite Lombarda Group Around Monte Morone (figs. 2 and 3), ancient termi- and the carbonatic Mesozoic Succession was documented nal moraines have a single arc morphology in map view, and mapped for the first time (e.g. BERNOULLI et alii, while younger terminal moraines are arranged in two 1989). This structure is the surface expression of a major separate lobes. The uplift of Monte Morone influenced north-verging backthrust, reportedly dated to the Torton- the morphology of the local terminal moraines associated ian (see FANTONI et alii, 2004; SCIUNNACH & TREMOLADA, to the ice tongue. Mt. Morone elevation is 2004, for recent revisions of the relevant literature). 496 m a.s.l., more than 100 m higher than the surround- Despite many Authors regard the Gonfolite back- ing glacial outwash plain; this value can be taken as a thrust as a Miocene structure, evidence for more recent rough estimate of the minimum local vertical uplift. The activity has been pointed out (e.g. BINI et alii, 1992; FEL- lack of direct dating does not allow to more precisely con- BER, 1993; ZANCHI et alii, 1995, 1997; BINI et alii, 2001). strain the timing of this uplift. Since the onset of major ZANCHI et alii (1997) and BINI et alii (2001) were able to Quaternary glaciations in the Southern Alps is regarded show that the whole mountain front in the hangingwall of as having occurred at ca. 0.9 Myr B.P. (e.g. BINI et alii, the Gonfolite backthrust (figs. 3 and 4) displays evidence 1998; MUTTONI et alii, 2003; SCARDIA et alii, 2006), it is for a post-Miocene to Pleistocene uplift, and that Pliocene reasonable to hypothesize that the uplift of Monte to Pleistocene deposits in the surroundings are affected Morone occurred during the Middle Pleistocene. by systematic fracturing and faulting at the meso-scale The systematic offset of karstic conduits and collapse with centimetric to decimetric offsets. Near Novazzano of speleothems beneath the Mt. Campo dei Fiori at the (fig. 2), FELBER (1993) described a tectonic contact western margin of the Gonfolite backthrust near Varese 20 143-Dgs10(411-426) 25-06-2007 14:57 Pagina 416

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Fig. 4 - Geological sketch map of the Gonfolite backthrust area. – Carta geologica schematica dell’area del retroscorrimento della Gonfolite.

(fig. 2) are interpreted by BINI et alii (1992) as due to outcrop described by FELBER (1993) and ZANCHI et alii Quaternary shortening. Speleothems of the Mt. Campo (1995; 1997) in a ca. 15 m deep stream incision near dei Fiori cave system have been dated to ca. 350 kyr B.P. Novazzano, where a fault plane juxtaposes the Oligo- based on U-Th dating, and the observed collapses and Miocene conglomerates of the Lombardian Gonfolite deformations of the cave system is regarded as younger Group against the Pliocene Formation of the «Castel di than 350 kyr B.P. This is interpreted as evidence of paleo- Sotto Clays». The outcrop was better exposed than in seismicity due to the growth of a south-verging fold fac- 1993 and 1997 (as made clear by the comparison of the ing the north-verging Gonfolite backthrust during the present-day aspect of the outcrop shown in fig. 5 with fig. Middle Pleistocene. 9 in ZANCHI et alii, 1995), probably because of the valley It is important to remark that ZANCHI et alii (1997) pre- floor downcutting occurred during the November 2000 ferred interpretation is that the overall pattern of Plio-Plei- and November 2002 flooding events in the Lombardia and stocene tectonic activity in the area is characterized by Ticino Region. Our re-examination of this outcrop con- surface north-south extension due to «superficial accom- firms previous observations about the bedding of the Gon- modation of deformations related to south-verging blind folite Lombarda Group (strike N190, dip 70°-80°). Also, thrusting active at depth, locally producing differential the geometry of the upper part of the fault plane corre- uplifting and possibly reactivation of some of the major sponds to that described by other Authors; the fault plane Late Miocene thrust surfaces». However, these Authors is almost vertical and dips to the north, and the Castel di indicate that alternative active tectonic models are possible. Sotto Clays are dragged against the fault plane, while few In order to review these literature data and provide tens of meters away from the fault plane this formation better constraints for the age, nature and amount of post- takes again the original horizontal bedding. However, in Messinian to recent displacement along the Gonfolite the lower part of the fault plane, certainly newly exposed, backthrust we conducted new field mapping and geomor- the dip progressively shift to the south moving down phological analysis (figs. 3 and 4). We first mapped at along the exposed section, bringing the Gonfolite Group 1:10,000 scale the area along the thrust from Como to Mt. directly above the Castel di Sotto Clays. The Castel di Morello (fig. 2). During the field work, we re-visited the Sotto Clays in this lower part of the fault plane are over- 20 143-Dgs10(411-426) 25-06-2007 14:57 Pagina 417

REMARKS ON THE QUATERNARY TECTONICS OF THE INSUBRIA REGION 417

Fig. 5 - Outcrop of the Gonfolite backthrust at the Novazzano site (see location in fig. 2): a) the tectonic contact between Castel di Sotto Clays and Gonfolite Lombarda Group; b) our interpretation; c) particular of the overturned Castel di Sotto Clays in the footwall of the Gonfolite backthrust (photos taken by Giancanio Sileo on July 2004). – Il retroscorrimento della Gonfolite in località Novazzano (si veda l’ubicazione in fig. 2): a) il contatto tettonico tra le Argille di Castel di Sotto e il Gruppo della Gonfolite Lombarda; b) l’interpretazione degli autori; c) particolare delle Argille di Castel di Sotto uncinate nel footwall del retroscor- rimento (foto scattate da Giancanio Sileo nel mese di luglio 2004).

turned, with a planar bedding dipping south by 70°-80° largely affects the Castel di Sotto Clays, as shown in the (fig. 5). We interpreted this fault plane as an outcrop of geological section of fig. 6. Minimum amount of vertical the Gonfolite backthrust documented by BERNOULLI et component of Pliocene offset is 200 m. Therefore, in con- alii (1989). Field mapping around the Novazzano site trast to previous interpretations, the Gonfolite backthrust allowed us to confirm that the Gonfolite backthrust did not cease its activity in the Late Miocene.

Fig. 6 - Geological cross section across the Gonfolite backthrust. – Sezione geologica attraverso il retroscorrimento della Gonfolite. 20 143-Dgs10(411-426) 25-06-2007 14:57 Pagina 418

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Fig. 7 - Shaded relief map of the study area with the mountain front sinuosity analysis and corresponding indexes. We calculated the moun- tain front sinuosity index using a common GIS software and a 20 m Digital Elevation Model courtesy of the Regione Lombardia. The sinuo- sity index is a basic reconnaissance tool to identify areas with active tectonic deformations. Through this index, it is possible to evaluate the level of tectonic activity of a fault-generated mountain front, calculating the ratio between the length of the front and the length of the strai- ght line between beginning and end of the front. Values of the sinuosity index increase as the erosional processes dominate, producing more irregular mountain fronts (e.g. BULL & MCFADDEN, 1977; WELLS et alii, 1988; KELLER & PINTER, 1996; SILVA et alii, 2005). The sinuosity index of the Gonfolite Range front is ca. 1.2, indicating an active tectonic mountain front. The higher value along the Albese con Cassano hill is consistent with the presence of a blind thrust. – Mappa in scala di grigio dell’area di studio e indice di sinuosità del fronte montuoso. I valori dell’indice, consistenti nel rapporto tra la lunghezza del fronte e quella della linea retta congiungente i due estremi, sono stati calcolati mediante un comune software GIS ed il DEM a 20 metri messo a disposizione dalla Regione Lombardia. Questo indice è uno strumento di riconoscimento per identificare le aree con deformazioni tettoniche attive. I valori di sinuosità aumentano allorchè i processi erosivi sono dominanti producendo fronti molto irregolari (e.g., BULL & MCFADDEN, 1977; WELLS et alii, 1988; KELLER & PINTER, 1996; SILVA et alii, 2005). L’indice di sinuosità ottenuto dall’analisi del fronte montuoso del retro- scorrimento della Gonfolite ha un valore di circa 1.2 che indica un fronte tettonico attivo. Il valore di sinuosità più alto lungo il fronte della collina di Albese con Cassano è coerente con la presenza di un blind thrust.

Unfortunately, until now we were not able to observe continuing fault slip on deep-seated South-Alpine com- reverse displacement of Pleistocene deposits along the pressional structures. Our new data therefore support the fault zone. Direct observations of the fault plane can be interpretation mentioned above about the recent reactiva- done in fact only in a few spots, due to the dense vegeta- tion of some Late Miocene thrust. tion cover and diffuse man occupation of land, and in Also the geomorphic relations along the fault strongly these spots there are no exposures of well bedded Pleis- suggest a Quaternary activity for the Gonfolite back- tocene sediments across the fault zone. For instance, in thrust. The new field data allow to precisely constraint the Novazzano site only coarse glacial deposits outcrop in the trace of the backthrust, and therefore to better under- the upper part of the section of fig. 5, but they are badly stand the relationships between the fresh geomorphologic exposed and it is impossible to understand if they are expression of the Gonfolite reliefs and the presence of the faulted or not (see also ZANCHI et alii, 2005). Further fault. In the Chiasso Plain and near Novazzano the front work is in progress to stratigraphically define the of the Gonfolite relief is in contact with the Mesozoic car- youngest movements along the fault using geophysical bonatic sequence of the Lombardian Basin. Near Mt. prospecting and exploratory trenching. However, we were Olimpino we mapped outcrops of the Liassic formations able to confirm the presence of normal fault systems at of the «Moltrasio Limestone» and «Rosso Ammonitico» at the mesoscale affecting the Pliocene and Pleistocene a distance of a few tens of meters from the outcrop of the deposits in the nearby Mendrisiotto area, interpreted by Oligo-Miocene marls and conglomerates at the base of ZANCHI et alii (1997) as the accommodation at surface of the Gonfolite range front. All the Lias to Oligocene 20 143-Dgs10(411-426) 25-06-2007 14:58 Pagina 419

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sequence is missing. This contact is therefore clearly the surface expression of the Gonfolite backthrust (SILEO et alii, 2005), and is constantly located along the break in slope at the base of the Gonfolite range front. As already pointed out, in this area Quaternary glacial erosion was very strong and the pulsations of the Abduan ice tongue obliterated all preexistent structures and relief. In partic- ular, in the Chiasso plain several episodes of glacial advances and retreats are well documented (BINI et alii, 2001). Since the glacial flow was from the north, and the Gonfolite range front is facing north (figs. 2 and 3), the base of the range front should not be coincident with the fault trace. In other words, due to the glacial erosion the range front should have been retreated somewhere south of the trace of the backthrust. Glacial erosional processes wiped out the bedrock Mesozoic carbonatic formations in the Chiasso Plain but apparently they did not affect seri- ously the Gonfolite reliefs, which are made by much softer rocks. This is a serious geomorphic anomaly for a landscape carved by glacial erosion. Lack of retreat of the Gonfolite mountain front is a strong evidence that fault slip has been capable, during the Quaternary, to with- stand the glacial erosive activity. This is also suggested by the very regular curved trace of the range front in map view (fig. 2), for a length of about 20 km, with the concav- ity facing south. It seems very unlike that this regular morphology is only controlled by selective erosional processes. In fact, as shown in fig. 7 the sinuosity index of this front is ca. 1.2, indicating an active tectonic moun- tain front (KELLER & PINTER, 1996). Again, if the mor- phology of the Gonfolite relief was due to erosion, it is not clear why the harder carbonate bedrock in the foot- wall has been almost completely removed while the rela- Fig. 8 - 3D image of the Mt. Bolettone area derived from the 20 m tively softer conglomerates and marls in the hangingwall DEM of Regione Lombardia; view from south. White triangles indi- today form an outstanding range front. cate wind gaps on the Albese con Cassano Hill; note the path of the Summarizing, the Gonfolite backthrust did not cease Cosia Stream and its tributaries (highlighted in white) clearly diver- ted just north of the hill. to slip in the late Miocene, as commonly accepted in the – Immagine tridimensionale, con vista da sud, dell’area del Monte Bo- literature. Shortening was certainly reactivated during the lettone (ricavata dal DEM dalla Regione Lombardia con risoluzione di Pliocene, as observed near Novazzano, and produced sig- 20 metri) I triangoli bianchi indicano i wing gaps sul rilievo di Albese nificant uplift along the Gonfolite range front. Geomor- con Cassano. Si noti il tracciato del torrente Cosia ed i suoi affluenti (evidenziati in bianco) chiaramente deviati a nord del rilievo. phic evidence and mesostructural analysis (ZANCHI et alii, 1995, 1997; BINI et alii, 2001) along the whole range front strongly suggest that shortening and fault slip along the Gonfolite backthrust took place also during the Quater- Commonly, drainage pattern is strongly influenced by nary. Vertical uplift during Middle Pleistocene is poorly tectonic structures and can record their medium to long constrained, likely in the order of 100 m at Mt. Morone, term activity (BURBANK & ANDERSON, 2001). A stream located near the western termination of the Gonfolite which crosses an uplifting area can in fact maintain its relief. antecedence or be diverted, depending on the ratio between uplift and erosional rates. In the closeness of Albese con Cassano Hill, the Cosia Stream and two tribu- THE ALBESE CON CASSANO ANTICLINE taries change their trend from north-south to east-west, recording an abrupt deviation in the local drainage pat- Observations related to Quaternary deformation and tern (figs. 7 and 8); the drainage divide is marked by the displacement at this site have been already described by presence of three wind gaps, aligned with the deviated OROMBELLI (1976) and ZANCHI et alii (1997). The Albese streams. This diversion has been ascribed to the growing con Cassano Hill (fig. 8) is an east-west trending ridge, of the Albese con Cassano anticline, which has locally located at the foothills of Lake Como (fig 2), linked to the prevailed over the erosive strength of the Cosia Stream presence of an east-west trending smooth anticline (see CHUNGA et alii, 2006, and references therein). As (Albese con Cassano anticline; fig. 2), which locally already illustrated in the past (NANGERONI, 1970; affects the southward dipping Mesozoic mainly carbon- OROMBELLI, 1976), a sequence of glacial and fluvioglacial atic succession. Based on new detailed field mapping, we deposits (informally here called Albese con Cassano Con- interpret this structure as the western termination of the glomerates) outcrops on top of this relief. Facies analysis Mt. Barro thrust (BERSEZIO et alii, 1990; fig. 2) which, of the Albese con Cassano Conglomerates clearly relates due to a reduced cumulated displacement, is here blind them to a sandur-like depositional environment (well- and marked only by its associated fault related folding. bedded, well-rounded gravel and sand of open pro-glacial 20 143-Dgs10(411-426) 25-06-2007 14:58 Pagina 420

420 G. SILEO ET ALII

Fig. 9 - Geologic sketch map of Albese con Cassano area (from CHUNGA et alii, 2006, modified). The reverse fault just north of Albese con Cassano is illustrated in fig. 11a; the soft-sediment deformations in fig. 11b-d are located at the junction between the Cosia and the Valloni streams. – Carta geologica schematica dell’area di Albese con Cassano (modificato da CHUNGA et alii 2006). La faglia inversa a nord di Albese con Cassano è illustrata in fig. 11a; le deformazioni nei sedimenti lacustri illustrate in fig. 11b fino in fig. 11d, sono posizionati alla giunzione fra i torrenti Cosia e Valloni.

alluvial plain). Field mapping at 1:10,000 scale performed ently south-dipping moving south (see the geological map for the new Geological Map of Italy, Sheet n. 75, «Como» of fig. 14 in ZANCHI et alii, 1996; and our fig. 9); these (CARG Project, Geological Survey of Italy), allowed us to conglomerates can be therefore seen as growth strata, study in detail the spatial and vertical distribution of the recording the progressive deformation of the Albese con Albese con Cassano Conglomerates. They crop out in two Cassano anticline. Their limited distribution in space is sites, between Como ad Lecco (Albese con Cassano and due to the extensive erosional activity exerted by the Abd- Madonna della Neve sites; fig. 2) at an elevation ranging uan Glacier during the Quaternary. These conglomerates between 550 m and 650 m a.s.l., directly overlying the could therefore be preserved only in particular positions, Moltrasio Limestone Fm. At the Albese con Cassano site acting like a sort of geomorphic refugee, while they com- they are mainly horizontal on top of the hill and appar- pletely lack, at the same elevations, in other nearby locali- 20 143-Dgs10(411-426) 25-06-2007 14:58 Pagina 421

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Fig. 10 - Geological cross section of the Albese con Cassano anticline. The folded Mesozoic sequence has been interpreted as due to a Quater- nary fault propagation fold formed above the blind Mt. Barro thrust. – Sezione geologica attraverso l’anticlinale di Albese con Cassano. Il piegamento della sequenza Mesozoica è stata interpretato come l’effetto della propagazione di una faglia quaternaria al disopra del blind thrust del Monte Barro.

ties along the mountain front and in the surrounding placed by a secondary steep reverse fault (figs. 9 and 11a). hills. Unfortunately, there is no absolute dating for these The vertical offset of about 5 m affecting the conglomer- conglomerates but, based on regional stratigraphy, degree ates and underlying glacial till indicates recent tectonic of weathering and soil development, they have been shortening. This outcrop was newly exposed after a land- attributed to the Middle Pleistocene (OROMBELLI, 1976; slide in late 2003 and has been subsequently covered by NANGERONI, 1970; BINI, 1993). As already pointed out, reparation works. Also, the glaciolacustrine deposits this age is in agreement with the glacial chronology com- entrenched in the basal Middle Pleistocene units (fig. 9) monly accepted for the Southern Alps (e.g. BINI, 1993; and outcropping along the Cosia Stream valley floor show MUTTONI et alii, 2003; SCARDIA et alii, 2006), which refers evidence for coseismic liquefaction probably related to the beginning of the formation of the most important a local seismogenic source, as recently discussed by Lario Amphitheatre’s frontal moraines to the Middle CHUNGA et alii (2006; fig. 11b-c). Pleistocene. Summarizing into a consistent structural framework As already illustrated by OROMBELLI (1976), facies all the observed evidence for Quaternary uplift and defor- analysis of the Albese con Cassano Conglomerates has mation, we propose that the Albese con Cassano Hill is a demonstrated that they have been deposed as part of a growing anticline that has absorbed ca. 200 m of vertical wide glacial outwash plain, which is definitely incompat- displacement from Middle Pleistocene to present. This is ible with their present geomorphic setting and elevation. clearly indicated by the evident anomaly in the drainage In fact, they outcrop over 200 m higher than the facing pattern, the presence of a syntectonic sequence of plain (ca. 350 m a.s.l .), that represents the upper part of deformed conglomerates (growth strata), and reverse dis- the regional gentle sloping surface of the LGM outwash placement along a secondary fault. The age of the Albese plain described above («Livello Fondamentale della Pia- con Cassano Conglomerates is at the moment the main nura»). In agreement with OROMBELLI (1976), we also source of error. We regard this structure as an active consider the Albese con Cassano Conglomerates correla- fault-propagation fold formed above a blind thrust fault tive to similar cemented conglomerates preserved (fig 10), representing the western termination of Mt. beneath the adjacent fluvio-glacial plain, and encoun- Barro fault, a ca. 20 Km long south-verging thrust in the tered during the excavation of water wells (fig. 10). Mesozoic carbonatic succession of the Lombardy sedi- Additional field observations confirm the Quaternary mentary sequence. East of Mt. Barro this thrust is seg- activity of the Albese con Cassano anticline (figs. 9 and mented by a north-south major structural lineament 11). As already described by OROMBELLI (1976) and (Lecco Line; JADOUL & GAETANI, 1986; GIANOTTI & PE- ZANCHI et alii (1997), the oldest depositional units are dis- ROTTI, 1986; BERSEZIO et alii, 1990). 20 143-Dgs10(411-426) 04-07-2007 8:50 Pagina 422

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Fig. 11 - Field observations of Quaternary shortening and paleoseismicity in the Mid-Pleistocene glacial deposits outcropping on top of the Albese con Cassano hill (see locations in fig. 2): a) reverse fault (ca. 5 m of displacement) in the basal Mid Pleistocene glacial deposits, i.e. from top to the bottom, fluvioglacial polygenic conglomerates and glacial till; b) and c) soft- sediment deformation in younger Mid-Pleistocene proglacial lake deposits; d) paleoliquefaction in the same deposits. – Osservazioni di terreno nei depositi glaciali Medio-Pleistocenici affioranti alla sommità del rilievo di Albese con Cassano (vedi fig. 2):a)faglia inversa (circa 5 m di dislocazione) nei depositi glaciali basali Medio-Pleistocenici, i.e., dall’alto verso il basso, conglomerati poligenici e till glaciale; b) e c) deformazioni in sedimenti lacustri proglaciali Medio-Pleistocenici; d) paleoliquefa- zioni negli stessi depositi. 20 143-Dgs10(411-426) 25-06-2007 14:58 Pagina 423

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DISCUSSION AND CONCLUSIONS et alii, 2003). In agreement with this conclusion our knowledges about the two major Quaternary tectonic The revision of the literature data, new field mapping structures in the Insubria region, bring us to suppose that at 1:10,000 scale and detailed geomorphic observation they have controlled the recent growth of the Gonfolite allow us to confirm previous interpretations by ROEDER Range Front west of Como, and of the Albese con Cas- (1992) and ZANCHI et alii (1997) about the post-Messi- sano Hill east of Como. We assessed a fault length of ca. nian to Quaternary reactivation of Late Miocene thrusts 20 km for both structures, and a Middle Pleistocene in the Insubria region at low strain rates, and under a structural relief of ca. 200 m for the Albese con Cassano continued north-south shortening. A similar conclusion anticline. A vertical uplift of a few hundreds of meters has been drawn by BURRATO et alii (2003), based on the during the Middle Pleistocene along the Gonfolite back- study of the causative fault of the May 12, 1802, Soncino thrust hangingwall is also a reasonable estimate, but this earthquake (fig. 1), for the whole frontal sector of west- value is poorly supported by the available data. ern Southern Alps underneath the Po Plain. The resulting If our hypothesis will be confirmed, eventual data on scenario is therefore different from the reported deacti- earthquakes magnitude and recurrence times associated vation of the western Southern Alps after the main phase to the investigated structures should be taken into account of emplacement of the foredeep fold and thrust belt in future seismic hazard estimates. («Lombardic Phase» of SCHUMACHER et alii, 1996; 14 to At this preliminary point of our work, the review of 6 Myr B.P.). the literature data and our new mapping and investiga- Based on previous geological studies, GPS data, and tions suggest that Quaternary shortening should occur in new data presented in this paper, we can argue that the a continuous belt in the foothills of Southern Alps Insubria region is currently contracting at a very slow between Lake Garda and Lake Maggiore. North-south velocity (averaging 1 mm/yr). We reach this conclusion directed Quaternary shortening and its relations with the based on evidence from long term strain in the Quater- current seismicity level has been well documented in the nary deposits and in spite of a lack of large historical eastern part of the Southern Alps (e.g. SERVA, 1990; earthquakes. We also suggest that strain rates in the lake CASTALDINI & PANIZZA, 1991; BENEDETTI et alii, 2001; Maggiore and Lake Como are lower than in the Lake GALADINI et alii, 2005). New detailed geoological and Garda region. This is consistent with both long and short paleoseismological investigations are needed in order to term patterns of contraction in the Northern Apennines prove, but also to disprove, that similar relations are valid and Southern Alps, which can be related to rotational also in the western Southern Alps and in particular in the opening of the Tyrrhenian Sea (BURRATO et alii, 2003; Insubria region. GIARDINA et alii, 2004; SERPELLONI et alii, 2005; D’AGO- STINO et alii, 2005). However, evidence of Quaternary shortening in the Insubria region is not very different in ACKNOWLEDGEMENTS terms of both a) style of faulting and folding, and b) local The authors wish to thank Cipriano Carcano, Sergio Rogledi, fault parameters (as derived from observed uplift rates) Leonello Serva, Carlo Doglioni and Giorgio Pasquaré for the helpful from that known southwest of Lake Garda. discussions and suggestions. Revisions from Kurt Decker and an anonymous referee allowed us to considerably improve the paper. We suggest that the strong effect of glaciations in this Andrea Berlusconi greatly helped during the field mapping of the region, including excavation of deep valleys and deposi- Gonfolite backthrust. PRIN 2005 funding to A.M. Michetti, and tion of large terminal moraines at the active mountain APAT (Agency for Environment Protection and Technical Services- front, has obscured additional evidence of active shorten- Geological Survey of Italy, G. Sileo Ph.D. project grant) supported this research. ing. In a similar stratigraphic and geomorphic setting, and within a moderately active tectonic region, the identi- fication of reliable strain and chronological markers is not an easy task. Very detailed geomorphic and strati- REFERENCES graphic investigation is needed to detect the relatively small tectonic signal in the local landscape. Only specific BARONI C. & CREMASCHI M. (1986) - Geologia e pedostratigrafia della collina di Ciliverghe (Brescia). Natura Bresciana, Ann. Mus. Civ. careful geomorphological and geological analysis might Sci. Nat. Brescia, 23, 55-78. allow to evaluate more precisely the displacement history, BENEDETTI L., TAPPONNIER P., GAUDEMER Y., MANIGHETTI I. & VAN amount and nature of each Quaternary structure identi- DE WOERD J. (2003) - Geomorphic evidence for an emergent acti- fied in the study area. 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Received 22 July 2005; revised version accepted 20 Juanary 2006.