From accretion to exhumation in a fossil accretionary wedge: a case history from Gottero unit (Northern Apennines, Italy)

© 2004 Lavoisier SAS. All rights reserved.

Geodinamica Acta 17 (2004) 41–53

From accretion to exhumation in a fossil accretionary wedge:a case history from Gottero unit

(Northern Apennines, Italy)

Michele Marroni

a, b,

*, Francesca Meneghini

a

, Luca Pandolfi

a, b

a

Dipartimento di Scienze della Terra, Universita' di Pisa, Pisa, 56126, ITALY

b

C.N.R., Istituto di Geoscienze e Georisorse, Pisa, 56126, ITALY.

Abstract

The Gottero unit of the Northern Apennines, Italy, is representative of the sedimentary cover of the Ligure-Piemontese oceanic litho-sphere. This unit consists of a thick sedimentary sequence that includes Valanginian-Santonian pelagic deposits and Campanian-earlyPaleocene turbiditic deposits. The latter are overlain by early Paleocene trench deposits related to frontal tectonic erosion of the accre-tionary wedge slope. This sequence is interpreted as recording trenchward motion of the oceanic lithosphere.

The Gottero unit records a pre-Late Oligocene, complex deformation history related to subduction and accretion events. This defor-mation history has developed through underthrusting (D1a), underplating (D1b and D1c) and later exhumation (D2a and D2b) episodes.The folding phase related to the main underplating sub-phase (D1b) is predated by a sub-phase (D1a) connected to rapid fluid escape andfollowed by a sub-phase dominated by the development of shear zones (D1c). The D1b sub-phase is characterized by similar folds anda slaty cleavage developed under P/T conditions of 0.4GPa/210°-270 °C. The D1c sub-phase, characterized by west-verging thrusts, isparticularly signficative in understanding the dynamics of the Ligure-Piemontese accretionary wedge because it testifies active shorten-ing of the Gottero unit also after its transfer to the prism. In addition, sub-phase D1c represents the transition from the sub-phasesconnected to accretion and the tectonics dominated by extension, characterized by parallel folds and low-to high-angle normal faults.The gravity driven extension is represented by the D2a and D2b sub-phases and can be interpreted as the result of the thicknening of theLigure-Piemontese accretionary wedge, produced by continuous underplating at its base but also by shortening of the previously under-plated units. These final tectonic events resulted in the exhumation of the Gottero unit to the surface during the Early Oligocene, whenthis Unit became one of the source areas of the conglomerates deposited in the Tertiary Piedmont basin.

This deformation history suggests the occurrence of a complex sequence of deformations during the transition from accretion to exhu-mation, even in the intermediate levels of the accretionary wedge.

© 2004 Lavoisier SAS. All rights reserved.

Keywords:

Northern Apennines; Internal Liguride units; Ophiolites; Underthrusting; Underplating; Exhumation

* Corresponding author.PROF. MICHELE MARRONIDIPARTIMENTO DI SCIENZE DELLA TERRAUNIVERSITA’ DEGLI STUDI DI PISAVIA S. MARIA, 53-56126 PISATel.: 050 2215732Fax: 050 2215800

E-mail:

[email protected]

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M. Marroni et al. / Geodinamica Acta 17 (2004) 41–53

1. Introduction

The accretionary wedge represents one of the most activegeodynamic settings where deformation and metamorphismof oceanic crust occur. The main process affecting the accre-tionary wedges is the transfer of large volumes of materialfrom the downgoing plate to the overriding plate. The mate-rial from the downgoing plate can be accreted to thedeformation front of the accretionary wedge by processesknown as “frontal accretion” or “offscraping” or, alterna-tively, it can be transferred at depth to the underside of thewedge by “underplating”. In addition, most of the accretedmaterials are subjected to a wide range of post-accretiondeformations, mainly connected with the exhumation toshallow structural levels.

In the last decades, investigations on the internal structureof modern accretionary wedges have been performed bymarine geophysical surveys and deep-sea drilling projects.These investigations have revealed the complexities of thetectonic processes that occur in the shallow levels at the frontof the accretionary wedges. In contrast, the mechanism bywhich the oceanic crust and its sedimentary cover areaccreted at medium to deep levels of the accretionarywedges, and the subsequent deformations during exhuma-tion, are poorly known, because they cannot be evaluated bymarine geophysical techniques. However, this deformationhistory can be fully reconstructed in the exhumed examplesof fossil accretionary wedges, where the structural history ofaccreted sequences can be directly observed.

In the Northern Apennines of Italy, the Internal Ligurideunits are regarded as an example of a fossil accretionarycomplex (e.g. [1, 2, 3]), where the accretion processes andthe subsequent deformations can be fully reconstructed. Inthis paper the pre-collisional deformation history of the sed-imentary sequence from the Gottero unit, one of the InternalLiguride units, is described and discussed in order to outlinethe deformation history experienced from accretion to exhu-mation in a fossil accretionary wedge.

2. Geological setting

In the Alpine-Apennine belt, from the Eastern Alps to theNorthern Apennines, Corsica and Calabria, Jurassic ophiolitesequences represent the remnants of oceanic lithosphere of theLigure-Piemontese oceanic basin and its transition to conti-nental margins. The Ligure-Piemontese oceanic basin wasformed between the European and the Adria plates during Tri-assic-Jurassic rifting and drifting, and was subsequentlyinvolved in Late Cretaceous-Eocene subduction-relateddeformations before the onset of the continental collision.

Fragments of the oceanic lithosphere, today preserved inthe Alpine-Apennine belt, are thus interpreted as slices ofoceanic lithosphere accreted to the accretionary wedge thatdeveloped during the Late Cretaceous-Eocene intra-oceanicsubduction. They show a high pressure metamorphism asso-

ciated to a complex, pre-collisional structural evolution,which can be regarded as a record of the deformationsachieved in the accretionary wedge.

In the Northern Apennines, the ophiolites and associatedsedimentary sequences are grouped into the Liguride units,which outcrop at the top of the nappe pile in the westernmostareas of the belt (Fig. 1). Whereas the External Ligurideunits are regarded as representative of the ocean-continenttransition [4], the Internal Liguride Units represent the mostcomplete and best preserved section of the oceanic lithos-phere formed in the Ligure-Piemontese oceanic basin. In theInternal Liguride units, the Jurassic ophiolite rocks arecapped by a Middle Jurassic/Late Cretaceous sedimentarycover consisting of Radiolarite Formation, CalpionellaLimestone and Palombini Shale. At the top, the Internal Lig-uride sequence is completed by Late Cretaceous/EarlyPaleocene turbidites and mass-gravity deposits.

Although only slightly metamorphosed, the Internal Lig-uride units are affected by a complex deformation sequenceresulting from the interference between two tectonic events,each characterized by a different structural style and kine-matic significance. The first event is represented by pre-LateOligocene deformations acquired during the subduction ofoceanic crust of the Ligure-Piemontese basin, whereas thelatter event, characterized by more gentle deformation, is theresult of Oligo-Miocene continental collision between theAdria and Europe continental margins.

The Internal Liguride units are arranged in a stack of tec-tonic units, generally deformed under very low-grademetamorphic conditions [5 and quoted references]. The unitsof the Bracco-Lavagna Valley sector (Gottero, Bracco-ValGraveglia and Colli-Tavarone units), as well as those of theTrebbia-Scrivia valleys sector (Portello, Vermallo and DuePonti units), show very-low grade metamorphism at orimmediately below the Anchizone-Epizone transition [6].The Internal Liguride units stack includes also the ophioliteunits of the Sestri/Voltaggio area, where the lowermost ones,known as Cravasco/Voltaggio and Mt. Figogna units, showlow-grade blueschist mineral assemblages [7 and quoted ref-erences]. Along the Sestri-Voltaggio line, these units arejuxtaposed against the Voltri Group, consisting of Jurassicophiolite sequences and related sedimentary cover, meta-morphosed to eclogite facies (Fig. 1). According toHoogerduijn Strating [8], the Sestri-Voltaggio line repre-sents an east-dipping, low-angle normal fault of LatePaleocene to Early Eocene age. Along this fault a pressuregap of about 8 Kbars can be detected between Voltri Groupand the Internal Liguride units. In this picture, the InternalLiguride units can be regarded as representative of the inter-mediate to shallow levels of the Ligure-Piemonteseaccretionary wedge.

The Internal Liguride units, as well as the Voltri Group,are unconformably overlain by the post-orogenic successionof the Tertiary Piedmont Basin whose oldest deposits aremainly represented by the Early Oligocene, continental ValBorbera Conglomerates [9].

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3. Stratigraphical setting

Despite the polyphase deformations, the ophiolitic unitsfrom Northern Apennines display sequences with well-pre-served sedimentary features. Whereas some of the InternalLiguride Units (Bracco-Val Graveglia and Colli-Tavarone)display a well-preserved Jurassic ophiolitic sequence, others(Gottero, Due Ponti, Vermallo and Portello) consist only ofEarly Cretaceous-Early Paleocene sedimentary successionsdetached from their oceanic basement.

According to Abbate

et al.

[10], the stratigraphy of theInternal Liguride ophiolites is characterized by a basementof mantle lherzolite associated with a gabbroic complex. Themantle ultramafics and the gabbroic complex are covered bya volcano-sedimentary complex, up to 300/400 m thick, thatconsists of pillow-lavas and massive basaltic flows interfin-gering with ophiolite breccias and cherts. This sequence istopped by a thick sedimentary cover [11 and quoted refer-ences] whose stratigraphic log can be fully reconstructedonly by integration of data available from the different tec-tonic units. The ophiolites are capped by hemipelagicdeposits represented by Radiolarite Formation (Callovian-Tithonian), Calpionella Limestone (Berriasian-Valanginian)and Palombini Shale (Valanginian-Santonian).

The Palombini Shale represents the oldest formation rec-ognizable in the Gottero succession (Fig. 2), grading upwardto siliciclastic turbidites, ranging from Campanian to EarlyPaleocene age (the Manganesiferi Shale, the Monte VerziMarl, the Zonati Shale and the Monte Gottero Sandstone),interpreted as a complex turbidite fan system [12].

The Manganesiferi Shale formation (Early Campanian) iscomposed of silicilastic, coarsening-upward basin plain tur-bidites, and grades up to mixed siliciclastic-carbonaticturbidites known as Monte Verzi Marl (Early to Late Cam-panian). This formation is represented by siliciclastic basinplain turbidites interbedded with carbonatic megaturbidites.The upper part of the turbidite system is an overall siliciclas-tic sequence showing a thickening and coarsening upwardtrend, represented by the Zonati Shale (Late Campanian-Early Maastrichtian) and the Monte Gottero Sandstone(Early Maastrichtian-Early Paleocene). The Zonati Shaleformation is made by thin-bedded turbidites interpreted asbasin plain deposits. This formation grades upward to theMonte Gottero Sandstone that is composed of coarse-grained siliciclastic turbidites interpreted as the proximalportion of the deep water fan. The youngest formation of theGottero unit is represented by the Early Paleocene BoccoShale, coarse-grained deposits characterized by the occur-

Fig. 1 Tectonic sketch map of the Western Alps and Northern Apennines. Post-orogenic sedimentary sequences of the Tertiary Piedmont (TPB) andEpiligurian (EB) Basins. Alpine and Apennines Ophiolitic units: Internal Liguride (IL), Sestri-Voltaggio zone (SV), Voltri group (VG), Piemonteseunits (OPH). Helminthoid flysch and associated sedimentary complexes: Prealpine units (PR), External Liguride units (EL), Antola unit (AN), Autapieand S. Remo-Mt. Saccarello (HF) units. Canetolo and Umbrian-Tuscan units (TU). Austroalpine and Southern Alps cover units (AU). Briançonnais(BR) and Lower Penninic (LP) units. Dauphinois-Helvetic units (DH). Austroalpine and Southern Alps basement units: Sesia-Lanzo zone (SL), Ivreazone (IV) and Dent Blanche Nappe (DB). Internal crystalline massifs of the Western Alps: Dora Maira (DM), Gran Paradiso (GP) and Monte Rosa(MR). External crystalline massifs of the Western Alps: Argentera (AM), Belledonne (BM), Mont Blanc (MB) and Pelvoux (PV). Rio Freddo line(RFL). Sestri-Voltaggio line (SVL). Villalvernia-Varzi line (VVL). The location of the study area is indicated.

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rence of debris flow and slide deposits. This formation hasbeen interpreted by Marroni and Pandolfi [13] as trenchdeposits related to frontal tectonic erosion of the accretion-ary wedge slope. The transition from hemipelagic depositsto turbidites up to debris flows and slide deposits can beinterpreted as reflecting the trenchward motion of a portionof the Ligure-Piemontese oceanic lithosphere.

4. Deformation history of the Gottero Unit

The Gottero unit displays a complex, pre-upper Oli-gocene deformation history [14, 15, 7, 8, 3] that will bedescribed here in detail. In addition, the geological map andthe related section of a selected area, where large—andmeso—scale structures are well exposed, is also provided.The area investigated corresponds to the Mt. Ramaceto andLavagna valley (Figs. 3 and 4), where detailed geological

mapping associated to structural studies have been per-formed. According to Marroni and Pandolfi [3], the pre-LateOligocene deformation history reconstructed in this areaincludes two main deformation phases, referred hereafter asD1 and D2 phases, each subdivided in different sub-phasesof veining, folding and thrusting. The sequence of these sub-phases includes:

4.1. D1a sub-phase: pre-folding vein development

As suggested by micro—and mesoscale evidence, coarse-grained, quartz—and calcite-filled veins are generally devel-oped before any folding phase. These structures,characterized by mosaic texture, are mainly observed in thefine-grained sediments, i.e. the Palombini Shale, Man-ganesiferi Shale and Zonati Shale, whereas in the coarse-grained sediments, as the Monte Gottero Sandstone, they arerare. The veins are generally bedding-parallel, and located at

Fig. 2 Stratigraphic log of the Gottero unit. The apparent thickness of the studied formations is indicated. (Time scale from Elter

et al.

, [53]).

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the boundaries of different lithologies (Fig. 5), althoughveins at high-angle to bedding are also found. Generallythese veins display good lateral persistence. Their features,such as a “dirty” appearance, due to the occurrence of scat-tered shaly inclusions, and the presence of irregularities atthe grain scale in the wall cracks, suggest a formation inpoorly lithified sediments [3].

4.2. D1b sub-phase: west-verging folding

This phase is characterized by strongly non-cylindrical,isoclinal folds with an approximately similar geometry (F1,

Fig. 6a). The folds display thickened hinge zones, while thelimbs are generally affected by boudinage and necking.After retrodeformation from subsequent phases, the distribu-tion of A1 axes remains scattered because of the stronglynon-cylindrical geometry of F1 folds, as observed in the out-crops (Fig. 6b). However, a cluster ranging from W/E toNW/SE can be observed (Fig. 7). The facing of the F1 mes-oscale folds indicates a westward vergence [14, 2, 16]. TheS1 axial-plane foliation, generally parallel to the beddingsurfaces, is seen in the shales as a continuous and penetrativesurface. The LS0-S1 intersection lineations are mainly rep-resented by foliation-bedding intersections and mullion

Fig. 3 Structural sketch map of the Internal Liguride units cropping out in the Lavagna Valley, Mt. Panigaro and Mt. Ramaceto areas. The A-B cross-section of Fig. 4 is indicated.

Fig. 4 Geological section across the Gottero unit cropping out along the Lavagna Valley. The location of the geological section is shown in Fig. 3.

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structures. Extension veins, ranging in thickness from 2-3 to40-50 mm, are well developed along the limbs of the F1folds throughout all the lithologies. These veins, generallyparallel to the strike of the A1 folds, show infillings ofquartz, calcite and, more rarely, of phyllosilicate fibers per-pendicular to vein walls (Fig. 6c). In thin section, theantitaxial quartz and/or calcite fibers are characterized by thepresence of a widespread median line and by inclusion bandsof wall-rock fragments related to crack-seal deformation[17]. The twins in calcite fibers correspond to type II of Bur-khard [18], that develops in the temperature range from 180°to 300 °C. The L1 mineral lineations occur as fibers of cal-cite and/or quartz infilling pressure shadows around pyritecrystals on S1 foliation. In the shales, the continuous axial-plane foliation S1 can be classified as slaty cleavage(Fig. 6d) and is characterized by elongate quartz-albite-micaaggregates surrounded by fine-grained, aligned phyllosili-cates. A metamorphic mineral assemblage of quartz+ calcite + albite + chlorite + white mica (illite and/or para-gonite) + Fe-oxides has been detected in the shales. Pressureshadows around detrital minerals are well developed, show-ing infillings of fibrous minerals such as quartz,phyllosilicates and calcite (Fig. 6e).

The amount and type of finite strain has been measured inquartz-rich arenites, occurring in the Palombini Shale, usingthe techniques discussed by Fry [19] and Hanna and Fry[20]. The volume reduction due to porosity loss is taken as20%, following Baldwin and Butler [21]. The data show onthe logarithmic Flinn diagram the coexistence of oblate andprolate ellipsoids with K values ranging from 0.566 to 2.461,but oblate-type ellipsoids belonging to a constrictional stressfield are prevalent [3].

According to illite and chlorite crystallinity and to illite b

0

parameters [6], the peak of metamorphism developed underP/T conditions of 0.4GPa/210°-270 °C. The calculated P/Tranges are in agreement with vitrinite reflectance data [22]and with the type of twins in calcite fibers. In addition, atemperature lower than 300 °C is confirmed also by theoccurrence of zircon grains in the Monte Gottero Sandstonethat display fission tracks pointing to Paleozoic ages [23].This implies that the annealing temperature for zircon hasnever been reached during the whole tectono-metamorphicevolution of the Gottero unit.

4.3. D1C sub-phase: west-directed thrusting

Shear zones marked by foliated cataclasites (Fig. 6f)crosscut all the structures related to the D1a and D1b sub-phases. These shear zones, showing smooth NE/SW strikingtrajectories, can be found both parallel and at high angle tothe S1 foliation. The foliated cataclasites display a welldeveloped foliation (S foliation), represented by a composi-tional layering defined by alternating, mm-size mica-richand calcareous layers. This foliation is associated with shearsurfaces with an orientation corresponding to the R surfaceof the Riedel shear model. In thin section, the foliated cata-clasites are characterized by a S foliation defined by thinlayers of very fine-grained, mica-rich material, whichenclose ribbons of elongated grains displaying often relictsof S1 foliation. Minor recrystallization of quartz, calcite,albite, chlorite and white mica also occurs. Most of the elon-gated calcareous grains are represented by fragments ofveins with calcite fibers. Movement criteria at the micro-scale (book-shelf sliding, asymmetric clasts, etc…) furthersuggest a top-to-NW sense of shear.

4.4. D2a sub-phase: East-verging folding and low-angle, normal faulting

This sub-phase is characterized by recumbent, asym-metric folds with NW-SE subhorizontal axes and flat-lyingaxial planes (Fig. 8a). These folds show an approximatelyparallel geometry with sub-rounded to rounded hinges.Hinge collapse is widespread. On the stereonet the A2 axescluster from N/S to WSW/ENE (Fig. 9). No stretching lin-eations related to D2a folds were developed. The D2a foldsare everywhere superimposed on the D1 foliation or shearzones; the deformation of D1b folds by the D2a ones pro-duces a type 3 interference pattern [24]. The D2a foldsgenerally show an eastward vergence, opposite to that rec-ognized for the D1b folds. Consequently, an origin of thesefolds through progressive D1b folding is unlikely becausethey generally show opposite facing. In addition, uprightD2a folds have been also recognized. These folds, found inoutcrops where the D2a folds with subhorizontal axialplains are lacking, show a crenulation cleavage as axialplane foliation. The D2a folds are systematically associ-ated with N/NW-striking brittle shear zones, which appear

Fig. 5 D1a pre-folding vein (D1aV) from Zonati Shale, Forcella Pass.The bedding (S0) is indicated.

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as low-angle surfaces after restoration from later deforma-tion phases (Fig. 8a). Normal motion along these brittleshear zones is indicated by displacement of markers as wellas by shear sense criteria (slickenfibers, extensionfractures, etc.). During the D2a sub-phase antitaxial veinswith calcite infillings developed, generally along the limbsof the folds. In these veins the calcite shows micro-twinsand straight narrow lamellae corresponding to type I ofBurkhard [18], that develop at temperatures lower than200 °C. The S2 axial-plane foliation developed in the

shales can be recognized as parallel crenulation cleavage(Fig. 8b), ranging from discrete to zonal types. No meta-morphic recrystallization associated to S2 foliations hasbeen observed.

4.5. D2b sub-phase: high-angle normal faulting

The last sub-phase is represented by high-angle, normalfaults that cut all the pre-existing structures. These faultsshow a strike ranging from N160 to N20.

Fig. 6 D1 related structures in the Gottero unit. a) D1b isoclinal fold (F1b) from Zonati Shale, N of Favale di Malvaro village. Hammer for scale. b)non-cylindrical fold (F1b) from Palombini Shale, SW of Gattorna village. The A1b axis is indicated. Coin diameter is 26 mm. c) D1b-related antitaxialextension vein with calcite fibers from Zonati Shale, Forcella Pass. d) S1b slaty cleavage from Zonati Shale, SW of Mt. Pagliaro. e) well developedcalcite fibers (Cc) in pressure shadows developed around framboidal pyrite (pyr) in siltstone from Palombini Shale, Mt Verzi. The S1b slaty cleavage isindicated. f) D1b-related cataclasite in siltstone from Zonati Shale, Brugnoni village, N of Forcella Pass.

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5. Map-scale structures

The deformation path described above can be easily rec-ognized not only at the microscopic and mesoscopic scalebut also at the map scale. In the geological map and section(Figs. 3 and 4) all the structural elements of the selected areaare shown. The area comprises the Lavagna valley to Mt.Panigaro, and Mt. Ramaceto (Fig. 4). In the Lavagna valleythe lowest part of the succession belonging to the Gotterounit is exposed. Here the well preserved NW-SE trendingD2a folds occur as overturned synclines and anticlines thatinvolve the Palombini Shale, the Manganesiferi Shale andthe Monte Verzi Marl. All these D2a large-scale structuresare characterized by sub-horizontal axial planes and east-ward vergence. These map-scale folds are considered asminor regional structures associated with the larger anti-

form, with Palombini Shale at the core, that outcrops alongthe left side of the Lavagna valley, with a strike parallel tothe Lavagna River [16]. The best preserved structure is theeast-vergent syncline, with Monte Verzi Marl at the core,outcropping in the Mt. Panigaro area. All the elements of thefold (limbs and hinges) are easily recognizable along theroad cut around Mt. Panigaro, and the same structure is alsowell visible in the Mt. Verzi area, as shown in the geologicalmap by tracing of the D2a axial plane. This fold is associatedwith a NW-striking low-angle D2a shear zone showing anormal sense of shear. This structure is an example of thewidespread intimate association between the D2a folds andthe low-angle extensional shear zones.

In the Mt. Ramaceto area all the structures described atthe mesoscale can be observed also at the map scale. Asshown in the geological map (Fig. 3), a large-scale D1b syn-

Fig. 7 Equal-area, lower hemisphere stereographic representation of D1b structural data from the Gottero unit in the Lavagna Valley area. A1b = foldaxes; S0 = bedding; S1b = foliations.

Fig. 8 D2-related structures from the Gottero unit. a) D2a asymmetric fold from Manganesiferi Shale, N of Cicagna village. The D2a east-verging foldsare associated to a low-angle shear zones (2aSZ). The AP2a axial plane is also indicated. View looking N. Hammer for scale. b) S2a crenulationcleavage superimposed on S1b slaty cleavage from Zonati Shale, SW of Mt. Pagliaro.

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cline with Bocco Shale at the core occurs. The fold affects asuccession consisting of Zonati Shale, Monte Gottero Sand-stone and Bocco Shale. The structural setting is complicatedin this area, because of the occurrence of a N-S/NE-SWtrending strike-slip fault system that causes block rotationevidenced by rotation of the A1 axes on a horizontal plane[16]. However, the outcropping F1 fold displays a roughlyNW/SE trend and it is characterized by a sub-vertical axialplane. As shown in the geological section (Fig. 4), the defor-mation of this structure by NE-verging D2a meso-folds issuggested by the average sub-vertical orientation of strataand of the D1b axial plane, that can be therefore explainedby interpreting this area as a large-scale D2a hinge zoneinvolving a pre-existing D1b structure. Also in this area wehave observed the association between the D2a folds withlow-angle surfaces showing normal sense of shear that trun-cate and offset all the previously formed structures (Fig. 4).

6. Dating of the deformations

In the Gottero unit, the calculated time span for the defor-mation path from the D1a to D2a sub-phases corresponds toabout 25 ma, from Early Paleocene, i.e. the age of the young-est deposits involved in the deformations, to EarlyOligocene, i.e. the age of the Val Borbera Conglomerateswhere pebbles of lithologies from the Internal Liguridesequence have been recognized [9]. In these conglomerates,that show an undeformed matrix, pebbles derived from thesuccession of the Gottero unit that display a slaty cleavageoverprinted by a crenulation cleavage have been found [9].This implies that all the deformation sub-phases from D1a toD2a are pre-Oligocene in age. In addition, Mutti

et al.

[25]

described high-angle normal or transtensive faults activeduring the sedimentation of the Val Borbera Conglomerates.These faults can tentatively be correlated with the deforma-tions related to the D2b sub-phase.

7. Discussion

According to van Zupthen

et al.

, [14], Marroni [15],Hoogerduijn Strating [7, 8] and Marroni and Pandolfi [3],the pre-Late Oligocene structures recognized in the Gotterounit can be related to a complex and long-lived deformationpath achieved in the Ligure-Piemontese accretionary wedge.As recognized in many examples of both fossil and activeaccretionary complexes [26, 27, 28, 29], also in the Gotterounit this deformation path includes different sub-phases,each representing a different step during the accretion his-tory and the subsequent exhumation towards the surface(Fig. 10).

The veins that developed during the D1a sub-phase dis-play features, such as the mosaic texture, the occurrenceonly in fine-grained lithologies and their developmentbefore any folding or thrusting phase that suggest they mightbe be interpreted as small-scale fractures representing con-duits for fluid migration in partially lithified sediments,probably during stress-controlled compaction. According toMarroni and Pandolfi [3], the development of veins duringthe D1a sub-phase can be related to fluid pressure higherthan hydrostatic pressure caused by tectonic loading duringthe underthrusting of the sedimentary cover detatched fromthe oceanic lithosphere. This sub-phase probably developedimmediately before or in the first deformation stages relatedto accretion of the sedimentary successions, as observed in

Fig. 9 Equal-area, lower hemisphere stereographic representation of D2a structural data from the Gottero unit in the Lavagna Valley area. A2a = foldaxes; S2a = foliations.

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many other examples of underplated sequences [30, 31, 26,32, 33, 34, 35, 36, 27, 28].

The D1b sub-phase (Fig. 10) developed during accretionto the Ligure-Piemontese accretionary wedge of the Gotterounit as a single, huge, tectonic nappe made up of a sedimen-tary succession detached from its oceanic substratum [14,15, 7, 8, 3]. The D1b deformation is represented by isoclinalfolds and the associated foliation that deformed pervasivelyall the lithologies belonging to the sedimentary succession ofthe Gottero unit. According to Marroni and Pandolfi [3], thedevelopment of the D1b folds at the peak metamorphic con-ditions, the development of a S1 continuous foliation, theoccurrence of metamorphic mineral assemblages that sug-gest deformation at moderately deep levels ranging from 10to 12 kilometers and the absence of soft-sediment deforma-tions suggest that underplating at depth rather than frontalaccretion is the most probable accretion mechanism. This isconfirmed also by the absence of deposits covering the suc-cession of the Gottero unit immediately after the D1bdeformation, as postulated for offscraped units (e.g. [8]). Inaddition, the presence of well preserved sedimentarysequences deformed during the accretion only by foldingand the absence of tectonic melanges, suggest that theGottero unit was underplated coherently. On the whole, theD1b folds can be interpreted as generated in response to non-coaxial deformation along the decollement zone at the baseof the Ligure-Piemontese accretionary wedge during coher-ent underplating.

The top-to-west sense of shear, inferred for the whole D1phase by the facing of structures, allows to interpret thisunderplating phase as developed into an accretionary wedgeconnected with an east-dipping subduction zone (Fig. 10),

according to geodynamic reconstructions proposed by Elterand Pertusati, [38], Hoogerduijn Strating, [7]; Marroni andPandolfi, [3].

The transition from the D1b to the D2a sub-phase is rep-resented by shear zones developed during the D1c sub-phase(Fig. 10). This sub-phase indicates that the sedimentary suc-cession of the Gottero unit was internally imbricated after itsfolding during the coherent underplating. This intra-wedgeshortening produced a stack of tectonic slices probably asso-ciated with steepening of all the previous structures. Thechange from pervasive deformation (folding + foliation) tolocalized deformation (shear zones) can be probably relatedto a rapid loss of porosity of the sedimentary rocks derivedfrom compaction during the D1a and D1b sub-phases, aspostulated in theoretical models (e.g. [39, 40]). Similardeformation histories related to underplating have beenobserved in other examples of fossil accretionary wedges, asfor instance in the Kodiak Islands, Alaska [31, 36, 26, 27], inthe Shimanto belt, Japan [41, 42, 43], in the Torlesse Ter-rane, New Zealand [44] or in the Diego Ramirez Islands,Southern Chile [45].

However, imbrication in the accretionary wedge aftercoherent underplating can be also regarded as the result ofperturbations in the steady state subduction system. Pertur-bations in the steady state in the Ligure-Piemonteseaccretionary wedge have been postulated by Marroni andPandolfi [13] to explain the occurrence of sedimentarydeposits originated from multiple events of frontal tectonicerosion. As suggested by Gutscher

et al.

[46, 47, 48], pertur-bations in the steady state subduction system caused by highbasal friction of the accretionary wedge can produce alter-nate cycles of frontal accretion and underplating. This

Fig. 10 2D sketch illustrating the geodynamic setting of the Gottero unit during the different evolution stages described in the text.

M. Marroni et al. / Geodinamica Acta 17 (2004) 41–53

51

cyclical behaviour is accompanied by the advancing andretreat of the deformation front producing compressiveevents in the underside of the accretionary wedge. Anincrease in the basal friction of the Ligure-Piemontese accre-tionary wedge can be originated, as suggested byHoogerduijn Strating [7], by a sharp reduction of the conver-gence rate between the Adria and Europe plates or,alternatively, can be explained by the underthrusting of oce-anic lithosphere with positive topographic reliefs.Irrespective of its origin, the occurrence of the D1c sub-phase implies that the thickening of the Ligure-Piemonteseaccretionary wedge was achieved not only by continuousunderplating of oceanic lithosphere slices but also by imbri-cation of the previously accreted sequences.

The following D2a and D2b sub-phases represent theconnection between the deformation sub-phases related tocoherent underplating and that related to exhumation of theGottero unit to shallow structural levels, and then its expo-sure at the surface, as suggested by the decrease of the T and,probably, P values from the D1b to the D2a sub-phase. TheD2a and D2b sub-phases can be likely related to deformationdominated by gravity-driven extension experienced by theLigure-Piemontese accretionary wedge to accomodate thethickening caused by continuous underplating, as predictedby Platt’s model [49]. This is in agreement with the featuresshown by the D2a folds and the associated low-angle normalfaults that can be interpreted as originated during verticalshortening, as required by extensional tectonics. In this pic-ture the D2a folds with sub-horizontal axial planes can beinterpreted as developing in a slip transfer zone with pureshear deformation between two detachment levels [50].

Both the flat-lying axial plane of the D2a folds and thenormal motion along the D2a shear zones can be regardedas originated during this process. In addition, the D2afolds are mostly confined along meter—to kilometre-widelevels bounded by D2a low-angle normal faults, asobserved in the geological section. Normal motion alongthe D2a low-angle normal faults is also demonstrated bythe regional setting of the Gottero unit that is overlain inthe northern areas by the less metamorphosed Portello unit[5]. This was also observed by Ellero

et al.

[51] in theInternal Liguride units of the Sestri/Voltaggio zone, whereP and T gaps have been recognized in correspondence ofthe shear zones interpreted as low-angle normal faults.These normal faults are also described by HoogerduijnStrating and Van Wamel [52] in the Bracco unit, even ifthey have been differently placed in the chronology of thefolding sub-phases. In the Gottero unit, the D2a folds andthe associated low-angle normal faults indicate an overalltop-to-the-east displacement. This implies that during theextensional tectonics the Gottero unit was located in theeast flank of a large-scale dome. In the geological settingof the northern Apennines, this dome can be representedby the Voltri group, where eclogite-facies ophiolitesequences, are directly overlain by the less metamorphicsequences of the Internal Liguride units.

The pre-Late Oligocene history reconstructed in thispaper is followed by Late Oligocene-Miocene weak defor-mation events related to the collision between the Adria andthe Europe plates. During these events the Liguride Units,already deformed and unconformably covered by the Terti-ary Piedmont Basin, have been involved in large-scalethrusting onto the Adria paleo-margin, i.e. the Sub-Ligurianand Tuscan domains (e.g. [38]).

8. Conclusions

The structural history reconstructed here for the Gotterounit suggests that deformation related to coherent underplat-ing is more complex than previously proposed (Fig. 10). Thefolding phase related to the main sub-phase of underplating(D1b) is predated by a sub-phase (D1a) connected to rapidfluid escape and followed by a shear zone sub-phase (D1c).This last sub-phase is particularly significative forunderstanding the dynamics of the Ligure-Piemontese accre-tionary wedge because it testifies active shortening of theGottero unit after its accretion. In addition, the last sub-phase represents the transition from the sub-phases con-nected to accretion to tectonics dominated by gravity-drivenextension, characterized by folds and low—to high-anglenormal faults. The local extension represented by the D2aand D2b sub-phases can be interpreted as the result of thethicknening of the Ligure-Piemontese accretionary wedgeproduced by continuous underplating at its base but also byshortening of the previously underplated units. Finally, thegravity-driven extension resulted in the exhumation of theGottero unit up to the surface during the Early Oligocene,when its sedimentary succession represented a source area ofthe conglomerates deposited in the Tertiary Piedmont basin.

On the whole, the data collected on the deformation his-tory of the Gottero unit suggest the occurrence of a complexsequence of deformations during the transition from accre-tion to exhumation also in the intermediate levels of theaccretionary wedge.

Acknowledgements

This research was supported by the Italian NationalResearch Council (C.N.R., Istituto di Geoscienze e Geori-sorse) and by the Italian Ministry for University and Scientific Research (M.I.U.R. – COFIN Project). We arealso obliged to M. Gattiglio, B. Lombardo and J. Malavieillefor their careful reviews.

References

[1] Treves B., Orogenic belts as accretionary prisms: the example of thenorthern Apennines, Ofioliti 9 (1984) 577-618.

52

M. Marroni et al. / Geodinamica Acta 17 (2004) 41–53

[2] van Wamel W.A., On the tectonics of the Ligurian Apennines(northern Italy), Tectonophysics142 (1987) 87-98.

[3] Marroni M. Pandolfi L., The deformation history of an accretedophiolite sequence: the Internal Liguride units (Northern Appen-nines, Italy), Geodin. Acta 9 (1996) 13-29.

[4] Marroni M., Molli G., Ottria G. Pandolfi L., Tectono-sedimentaryevolution of the External Liguride units (Nothern Appennines,Italy): insights in the pre-collisional history of a fossil ocean-conti-nent transition zone, Geodin. Acta 14 (2001) 307-320.

[5] Ducci M., Lazzaroni F., Marroni M., Pandolfi L., Taini A., Tectonicframework of the northern Ligurian Appennine, Italy, C.R. Acad.Sci. Paris 324 (série IIa) (1997) 317-324.

[6] Leoni L., Marroni M., Tamponi M., Sartori F., The grade of meta-morphism in the metapelites of the Internal Liguride units (NorthernApennines, Italy), Eur. J. Min. 8 (1996) 35-50.

[7] Hoogerduijn Strating E.H., The evolution of the Piemonte-Ligurianocean, A structural study of the ophiolite complexes in Liguria (NWItaly). PhD Dissertation, Utrecht University (1991) 127 pp.

[8] Hoogerduijn Strating E.H., Extensional faulting in an intraoceanicsubduction complex—working hypothesis for the Paleogene of theAlps-Apennine system, Tectonophysics 238 (1994) 255-273.

[9] di Biase D., Marroni M., Pandolfi L., Age of the deformation phasesin the Internal Liguride units: evidences from lower Oligocene ValBorbera Conglomerates of Tertiary Piedmont Basin (northern Italy),Ofioliti 22(2) (1997) 231-238.

[10] Abbate E., Bortolotti V., Principi G., Apennine ophiolites: a peculiaroceanic crust. In G. Rocci (Ed): Ofioliti, Special Issue on Tethyanophiolites,

vol. I, Western area (1980) 59-96.[11] Marroni M., Monechi S., Perilli N., Principi G., Treves B., Late Cre-

taceous flysch deposits of the northern Apennines, Italy: age ofinception of orogenesis-controlled sedimentation, Cretaceous Res.13 (1992) 487-504.

[12] Nilsen T.H., Abbate E., Submarine fan facies associations of theUpper Cretaceous and Paleocene Gottero Sandstone, LigurianAppennine, Italy, Geomarine Letters 3 (1983-84) 193-197.

[13] Marroni M., Pandolfi L., Debris flow and slide deposits at the top ofthe Internal Liguride ophiolite sequence, Northern Appennines,Italy: A record of frontal tectonic erosion in a fossil accretionarywedge, The Island Arc 10 (2001) 9-21.

[14] van Zupthen A.C.A., van Wamel W.A., Bons A.J., The structure ofthe Val Lavagna Nappe in the region of the Monte Ramaceto andVal Graveglia (Ligurian Apennines, Italy). Geologie en Mijnbouw64 (1985) 373-384.

[15] Marroni M., Deformation history of the Mt. Gottero Unit (northernApennines, Italy), Boll. Soc. Geol. It. 110 (1991) 727-736.

[16] Marroni M., Della Croce G., Meccheri M., Structural evolution ofthe Mt. Gottero Unit in the Mt. Zatta—Mt. Ghiffi sector (Ligurian-Emilian Apennines), Ofioliti 13 (1988) 29-42.

[17] Ramsay J.G., The crack-seal mechanism of rock deformation,Nature 284 (1980) 135-139.

[18] Burkhard, M., Calcite twins, their geometry, appearance and signif-icance as stress-strain markers and indicators of tectonic regime: areview, J. Struct. Geol. 15 (1993) 351-368.

[19] Fry N., Random point distribution and strain measurement in rocks,Tectonophysics 60 (1979) 89-105.

[20] Hanna S.S., Fry N., A comparison of methods of strain determina-tion in rocks from southwest Dyfed (Pembrokeshire) and adjacentareas, J. Struct. Geol. 1 (1979) 155-162.

[21] Baldwin B., Butler C.O., Compaction curves, Bull. Am. Ass. Petro-leum Geol. 69 (1985) 622-626.

[22] Bonazzi A., Cortesogno L., Galbiati B., Reinhardt M., Salvioli Mar-iani E., Vernia L., Nuovi dati sul metamorfismo di basso grado nelleunità Liguridi interne e loro possibile significato nell’evoluzionestrutturale dell’Appennino Settentrionale, Acta Naturalia del’Ateneo Parmense 23 (1987) 17-47.

[23] Balestrieri M.L., Abbate E., Bigazzi G., Insights on the thermal evo-lution of the Ligurian Apennines (Italy) through fission-trackanalyses, J. Geol. Soc. London 153 (1996) 419-425.

[24] Ramsay J.G., Folding and fracturing of rocks, Mc Graw-Hill (1967)568 pp.

[25] Mutti E., Papani L., Di Biase D., Davoli G., Mora S., Segadelli S.,Tinterri R., Il Bacino Terziario Epimesoalpino e le sue implicazionisui rapporti tra Alpi ed Appennino, Mem. Soc. Geol. It. 47 (1995)217-244.

[26] Fisher D., Byrne T., Structural evolution of underthrusted sedi-ments, Kodiak Islands, Alaska, Tectonics 6 (1987) 775-793.

[27] Kusky T.M., Bradley D.C., Haeussler P.J., Karl S., Control on accre-tion of flysch mélange belts at convergent margins: Evidence fromthe Chugach Bay thrust and Iceworm melange, Chugach accretion-ary wedge, Alaska, Tectonics 16 (1997) 855-878.

[28] Hashimoto Y., Kimura G., Underplating process from melange for-mation to duplexing: Example from the Cretaceous Shimanto Belt,Kii Peninsula, southwest Japan, Tectonics 18 (1999) 92-107.

[29] Lu C. Y., Chang K. J., Malavieille J., Chan Y. C., Chang C. P., LeeJ. C., Structural evolution of the southeastern Central Range, Tai-wan, West. Pacific Earth Sci., 1 (2001) 213-226.

[30] Sample J.C., Fisher D.M., Duplexes and underplating in an ancientaccretionary complex, Kodiak islands, Alaska, Geology 14 (1986)160-173.

[31] Sample J.C., Moore J.C., Structural style and kinematics of anunderplated slate belt, Kodiak Islands, Alaska, Geol. Soc. Am. Bull.99 (1987) 7-20.

[32] Fisher D., Byrne T., The character and distribution of mineralizedfractures in the Kodiak Formation, Alaska: implications for fluidflow in an underthrust sequence, J. Geophys. Res. 95 (1990) 9069-9080.

[33] Vrolijk P., Tectonically driven fluid flow in the Kodiak accretionarycomplex, Alaska, Geology 15 (1987) 466-469.

[34] Paterson S.R., Sample J.C., The development of folds and cleavagein slaty belts by underplating in accretionary complexes: a compari-son of the Kodiak Formation, Alaska and the Calaveras Complex,California, Tectonics 7 (1988) 859-874.

[35] Fisher D.M., Brantley S.L., Everett M., Dzvonik J., Cyclic fluid flowthrough a regionally extensive fracture network within the Kodiakaccretionary prism, J Geophys. Res. 100 (B7) (1995) 12.881-12.894.

[36] Fisher D.M., Fabrics and veins in the forearc: a record of cyclic fluidflow at depths of < 15 km. In Bebout G.E., Scholl D.W., Kirby S.H.and Platt J.P. (Eds.): Subduction: top to bottom. Geophys. Mono-graph, American Geophysical Union, (1996) 75-89.

[37] Moore J.C., Tectonics and hydrogeology of accretionary prisms:role of the decollement zone, J. Struct. Geol. 11 (1989) 95-106.

[38] Elter P., Pertusati P.C. Considerazioni sul limite Alpi-Appennino esulle relazioni con l’arco delle Alpi occidentali. Mem. Soc. Geol. It.12 (1973) 359-375.

[39] Hill P.R., Masters J.C., Control on physical properties of Peru con-tinental margin sediments and their relationships to deformationstyle, In Suess E., von Huene R.

et al.

(Eds.): Proceedings of ODP112 (1990) 623-632.

[40] Davis D.M., Accretionary mechanism with properties that vary inspace and time. In Bebout, Scholl, Kirby and Platt (Eds.): Subduc-tion: top to bottom, Geophys. Monograph, American GeophysicalUnion (1996) 39-48.

[41] Mackenzie J.S., Needham D.T., Agar S.M., Progressive deformationin an accretionary complex: an example from the Shimanto belt ofeastern Kyushu, southwest Japan, Geology 15 (1987) 353-356.

[42] Agar S.M., The interaction of fluid processes and progressive defor-mation during shallow level accretion: examples from the ShimantoBelt of SW Japan, J. Geophys. Res. 95 (B6) (1990) 9133-9147.

[43] Di Tullio L, Byrne T., Deformation paths in the shallow levels of anaccretionary prism: The Eocene Shimanto belt of SW Japan, Geol.Soc. Amer. Bull. 102 (1990) 142.

M. Marroni et al. / Geodinamica Acta 17 (2004) 41–53

53

[44] George A.D., Deformation processes in an accretionary prism: astudy from the Torlesse terrane of New Zealand, J. Geophys. Res. 12(1990) 747-759.

[45] Wilson T.j., Hanson R.E., Grunow A.M., Multistage melange for-mation within an accretionary complex, Diego Ramirez Islands,southern Chile, Geology 17 (1989) 11-14.

[46] Gutscher M.A., Kukowski N., Malavieille J., Lallemand S., Cyclicalbehavior of thrust wedges: insights from high basal friction sandboxexperiments, Geology 24 (1996) 135-138.

[47] Gutscher M.A., Kukowski N., Malavieille J., Lallemand S., SergeE., Episodic imbricate thrusting and underthrusting; analog experi-ments and mechanical analysis applied to the Alaskan accretionarywedge, J. Geophys. Res. 103 (1998a) 10.161-10.176.

[48] Gutscher M.A., Kukowski N., Malavieille J., Lallemand S., SergeE., Material transfer in accretionary wedges from analysis of a sys-tematic series of analog experiments, J. Struct. Geol. 20 (1998b)407-416.

[49] Platt J. P, Dynamics of orogenic wedges and uplift of high-pressuremetamorphic rocks, Geol. Soc. of Am. Bull. 97 (1986) 1037-1053.

[50] Rykkelid E., Fossen H., Composite fabrics in mid-crustal gneisses:observation from Øygarden Complex, West Norway Caledonides, J.Struct. Geol. 14 (1992) 1-9.

[51] Ellero A., Leoni L., Marroni M., Sartori F., Internal Liguride Unitsfrom Central Liguria, Italy: new constraints to the tectonic settingfrom white mica and chlorite studies, Schweiz. Miner. Petr. Mitt. 81(2001) 39-53.

[52] Hoogerduijn Strating E.H., Van Wamel W.A., The structure of theBracco complex (Ligurian Apennines, Italy): a change from Alpineto Apennine polarity, J. Geol. Soc. London 146 (1989) 933-944.

[53] Elter, P., Ghiselli, F., Marroni, M., Ottria, G. 1997—Note illustra-tive della Carta geologica d’Italia, scala 1: 50.000, Foglio 197-Bobbio.