Seismic image of the compressional and extensional structures in the Gubbio area (Umbrian-Pre Apennines)

ABSTRACT

This paper is about the results obtained from the analysis offour commercial seismic profiles located in a zone of the UmbrianPre-Apennines comprised between the structure of Gubbio and theVal Tiberina basin. The aim of the research has been to reconstrustthe deep setting of the main compressive and extensional structuresin the area.

The good resolution of the seismic profiles allowed the easylocalization of the seismic markers and isochronous and isobathmaps of the main seismic markers have been produced.

The Carbonate sequence appears to be interested by two maincompressive structures: the Gubbio anticline and the Mt. Subasioanticline whose northern continuity can be traced in sub-surfaceuntil a depth of about 3-4 km. This data processing have shown thatthe Tops of Bisciaro, Marne a Fucoidi, Triassic evaporites are invol-ved in these folds, whereas the Top of basement shows a dishar-monic pattern: this character underlines the presence of a detach-ment within the evaporites.

The geometry of the Alto Tiberina fault (ATF), an extensionalshear zone put in evidence by the CROP03 profile, and of the ATFantithetic, WSW dipping, normal Gubbio fault (GuF) has been char-acterized. The GuF cuts the internal limb of the Gubbio anticline,with a strike of N120, a dip direction toward SW. The seismic datashow that the deeper part of fault plane joints the frontal thrust thatgenerates the anticline, representing phenomena of inversion tectonics.

A good relationship between the geometry of the GuF obtainedfrom this work and the seismicity, has been found.

KEY WORD: Extensional and compressional tectonics,Umbrian pre-Apennines, Seismic reflection profiles.

RIASSUNTO

Immagine sismica delle strutture compressive e distensivenellarea di Gubbio (Pre-Appennino Umbro).

Lintegrazione delle informazioni provenienti dalle indaginisismiche con i dati geologici di superficie e con quelli provenientidai sondaggi profondi, ha notevolmente migliorato la definizionedelle strutture geologiche presenti nellAppennino, come ha dimo-strato il progetto CROP03 (PIALLI et alii, 1998).

In questo lavoro si sono utilizzate le informazioni provenientida quattro sezioni di sismica a riflessione, messe a disposizione daEni/Agip Division, che interessano una porzione, di circa 30 km2, delPre Appennino Umbro in una zona compresa tra la struttura diGubbio ed il Bacino Tiberino. La buona qualit delle linee sismicheutilizzate, ha permesso la facile individuazione dei riflettori markerpi significativi che, insieme ai dati geologici di superficie e ai datiprovenienti da perforazioni effettuate in questa zona, ha consentitola descrizione efficace delle principali strutture compressive e disten-sive presenti nel sottosuolo.

Per ricostruire landamento in profondit delle strutture, evi-denziate nei profili sismici, si proceduto alla costruzione delle

carte delle isocrone dei vari riflettori marker e delle carte delle iso-bate relative al riflettore della faglia di Gubbio.

I principali risultati ottenuti da tale elaborazione di dati, si pos-sono raggruppare in quattro punti principali:

In tutte le sezioni, la successione torbiditica della MarnosoArenacea appare interessata da strutture compressive a piccola lun-ghezza donda che non coinvolgono la sottostante successione car-bonatica.

Le strutture compressive principali rilevate che coinvolgono lasuccessione umbra sono lAnticlinale di Gubbio e lAnticlinale delMonte Subasio.

I risultati ottenuti in questo lavoro hanno messo in evidenzacome entrambe le strutture si propaghino in profondit. Le cartedelle isocrone delle varie formazioni evidenziano per i riflettori delBisciaro, delle Marne a Fucoidi e del Top evaporiti andamenti ana-loghi, mentre le isocrone del Top basamento sono completamentediverse. Questo risultato suggerisce la presenza di un livello di scol-lamento posizionato allinterno delle evaporiti Triassiche.

Le strutture estensionali principali esaminate sono: la fagliaAlto Tiberina e la faglia di Gubbio.

La faglia Alto Tiberina, che borda ad ovest il bacino dellaltaValle del Tevere, pu essere tracciata sulle linee sismiche con conti-nuit fino ad una profondit di circa 8 km. Il piano di faglia pre-senta una direzione circa 140 N, unimmersione verso ENE eduninclinazione variabile. Lanalisi delle linee a disposizione ha evi-denziato inoltre una traiettoria a gradini del piano di faglia. Que-sta faglia, di significato crostale, presenta un rigetto complessivo di5 km.

Le linee sismiche consentono anche di precisare la geometriadei depositi che riempiono il Bacino dellAlta Val Tiberina. I depo-siti mostrano un andamento asimmetrico con depocentro spostatoverso Ovest, con una profondit massima di circa 1700 m.

La faglia di Gubbio taglia il fianco interno dellAnticlinale diGubbio, immerge verso SW ed ha un rigetto complessivo di circa1 km. La faglia ha una direzione 120 N e uninclinazione che variatra 50 e 70. Dalle sezioni sismiche studiate si evince come la fagliain profondit riattivi un preesistente piano di sovrascorrimento,invertendone il movimento.

Di notevole interesse stato inoltre analizzare il rapporto esi-stente fra la struttura distensiva di Gubbio e la sismicit. Esiste,infatti, unevidente correlazione tra la distribuzione degli ipocentridella crisi sismica di Gubbio del 1984 e le geometrie della faglia diGubbio ricavata. In particolare la sismicit appare in relazione alsegmento meridionale della faglia di Gubbio. Gli ipocentri sonolocalizzati tra 3.5 e 7 km di profondit, in una fascia che corrispondea quella compresa tra lemersione della faglia in superficie e la suaintersezione con la faglia Alto Tiberina.

TERMINI CHIAVE: Tettonica compressiva ed estensionale,pre-Appenino Umbro, Sismica a riflessione.

INTRODUCTION

The integration between information coming fromseismic soundings, surface geology data and from deepwells has remarkably improved the definition of the geo-logical structures in the Northern Apennines, as welldemonstrated by the CROP03 project (PIALLI et alii,

Boll. Soc. Geol. It., Volume speciale n. 1 (2002), 263-272, 7 ff.

Seismic image of the compressional and extensional structuresin the Gubbio area (Umbrian-Pre Apennines)

CRISTINA PAUSELLI (*), ROSITA MARCHESI (*) & MASSIMILIANO R. BARCHI (*)

(*) Dipartimento di Scienze della Terra, Piazza dellUniversit,06100 Perugia. E-mail [email protected]

1998). In particular, in the structural province of theUmbrian pre-Apennines, (sensu DEIANA & PIALLI, 1994),lying between the Val Tiberina basin and the main ridgeof the Umbria-Marche Apennines (fig. 1), the availablenetwork of seismic reflection profiles, NNW-SSE andWSW-ENE trending, has allowed a good characterizationof the geometry of the main compressional and exten-sional geological structures in the area (BARCHI et alii,1998; BARCHI et alii, 1999). In this paper we improve thedefinition of these structures, analyzing a set of transver-sal (WSW-ENE) commercial seismic profiles across theupper Val Tiberina basin, in a region comprised betweenCitt di Castello and Perugia, and the adjacent Umbria-Marche Apennines. The trace of the seismic profiles ana-lyzed is reported in fig. 1. This research has been carriedout within a project of the Structural Geology group ofPerugia aimed at the definition of the potentially seismo-genic structures of the Umbria region.

The Umbrian Pre-Apennines, from a lithological pointof view, are characterized by the outcropping of the Mio-cene turbidites of the Marnoso Arenacea Fm., which rep-resents the uppermost unit of a litho-structural sequence,also including, from top to bottom, the Meso-Cenozoiccarbonatic multilayer, the Triassic evaporites and a Per-mian-Triassic phyllitic basement. The stratigraphy of theregion is schematically reported in fig. 2: for more detai-led description see CENTAMORE et alii, 1986; CIARAPICA etalii, 1987; CRESTA et alii, 1989. This area, like the entireNorthern Apennines, has been interested by two principaltectonic phases migrating from west to east: a compres-sional event followed by an extensional one (ELTER et alii,1975; LAVECCHIA et alii, 1987; LAVECCHIA et alii, 1994).

The compressional phase, acting since the MiddleMiocene, gave rise to the Umbria-Marche Apennines foldand thrust belt, which has been interpreted as a typicalthin-skinned structure (BALDACCI et alii, 1967; BALLY etalii, 1986). Others authors suggested the involvement ofthe basement in the main thrust sheets, following a thick-skinned model of deformation (LAVECCHIA, 1985; LAVEC-CHIA et alii, 1987). This latter interpretation has beendeveloped by BARCHI et alii, 1998, describing the structu-ral style of the Umbria-Marche belt as determined by thepresence of multiple detachments located at differentstructural levels that gave rise to a complex pattern ofcontractional structures. The main detachments are loca-ted at the top of carbonates, within the evaporites andwithin the Phyllitic basement: each of these levels gaverise to the formation of different structures, having differ-ent wavelengths: the shallower the detachment the shor-ter the wavelength of the structures. The hierarchicarrangement of the compressional structures results,thus, in, from top to bottom (BARCHI et alii, 1998): 1)Shallow Embricates mainly involving the turbidites,achieving a complex pattern of short wavelength, asym-metric folds, embricate fans, duplexes and backthrusts(MENICHETTI & PIALLI, 1986; DE FEYTER, 1989; RIDOLFIet alii, 1995); 2) Umbria-Marche Folds (the two majorcompressional structures present in the area, i.e., theGubbio-Mt. Subasio anticlines along whose culminationthe carbonates crop out) involving carbonates and eva-porites; 3) Basement Wedge involving evaporites and thebasement rocks.

Intense extensional deformation, following the com-pressional phase, affected the Umbrian pre-Apenninesduring the Pliocene-Quaternary giving rise to continental

basins with a NNW-SSE trending (N150 20) borderedby normal faults. The most important among them is theVal Tiberina basin, related to the presence of an impor-tant east-dipping normal fault called the Alto Tiberinafault (hereinafter ATF). It crops out at the western side ofthe Val Tiberina basin and dips toward east until a depthof about -5s TWT (~12 km) beneath the western ridge ofthe Umbria-Marche Apennines. The extensional characterand the geometry of this fault has been recently derivedby an integration of geological, geophysical and seismo-logical data (BONCIO et alii, 1998; BARCHI et alii, 1999;BONCIO et alii, 2000; COLLETTINI et alii, 2000) underliningits seismo-tectonic role in the region.

Other intermountain basins are present to the east ofthe Val Tiberina basin disrupting the preexisting com-pressional structures. The most important among themare the Gubbio, Gualdo Tadino, Colfiorito, Norcia, Casciaand Castelluccio basins, bordered by SW dipping normalfaults interpreted as active faults, based on seismologicaland geomorphologic data (BARCHI et alii, 2000).

The Gubbio area is particularly interesting because itoffers a catalogue of the different tectonic environmentsthat have interested the Umbrian Pre-Apennines. Itsstructural setting and stratigraphy has been studied indetail by many Authors (MENICHETTI & MINELLI, 1991and references therein). It consists of a NE verging root-less anticline whose axis strikes N130 and which over-thrusts a syncline of Upper Miocene Marnoso Arenacea,separating the Gubbio anticline from the Umbria MarcheApennines. The local stratigraphy has been obtained bythe outcrops located at the core of the Gubbio anticline(Bottaccione Gorge and the Contessa Valley, LUTER-BACHER & PREMOLI SILVA, 1962; ALVAREZ et alii, 1977)and by a borehole (Gubbio 1) in the NW part of the struc-ture (fig. 1). Many compressional structures interest theexternal limb of the anticline: the age of these featuresspans from Late Tortonian to Messinian and, probably, toEarly Pliocene (DE FEYTER et alii, 1990). The anticline isalso affected in the SW limb by the normal fault that orig-inated the Gubbio basin during the Plio-Pleistocene time(MENICHETTI & MINELLI, 1991).

In order to define the compressional and extensionalstructures in the Umbrian-pre Apennines, we interpreted aset of transversal (WSW-ENE) commercial seismic profiles.

DESCRIPTION OF THE SEISMIC REFLECTION PROFILES

The profiles analyzed are located in the sector of theUmbrian-pre Apennines, comprised between the Val Tib-erina basin and the western side of the Umbria-MarcheApennines (fig. 1). The good general resolution of theseismic reflections has easily allowed the individuation ofa relatively homogeneous sequence of seismic markers.These markers, whose geological interpretation has beencalibrated on surface geology (existing maps and fieldcontrol) and on the data from a deep borehole drilledbetween Umbertide and Gubbio (Monte Civitello 1), havebeen related, from top to bottom, to:

the bottom of the Miocene turbidites (Bisciaro Fm.) a marly Fm. embedded in the carbonate sequence

(Marne a Fucoidi Fm.) the top of the evaporites the top of the phyllitic basement

264 C. PAUSELLI ET ALII

SEISMIC IMAGE OF THE COMPRESSIONAL AND EXTENSIONAL STRUCTURES IN THE GUBBIO AREA 265

Fig. 1 - Schematic map of the study area. Localizzazione dellarea di studio.

The bottom of the continental deposits can be alsorecognized in the seismic profiles, infilling the Val Tibe-rina and Gubbio basins.

In order to define the correct geometry of the maincompressional and extensional structures, the interpretedseismic profiles have been depth converted, using thedata of interval velocities available in the literature (fig. 2),and Geosec software.

In this paper we present two profiles (A and D in fig. 1)representative of the structural setting of the study area.

PROFILE A (fig. 3a-b)

The northernmost profile extends from SW to NE,from the town of Umbertide to Pian di Serra, north of thetown of Gubbio, crossing the Val Tiberina basin and thenorthern termination of the Gubbio anticline (fig. 1).

The seismic profile clearly shows the geometry of theGubbio anticline (fig. 3a-b). Its backlimb is bordered by anormal fault (the Gubbio fault, hereinafter GuF), dippingwestwards, that reaches the ATF at 2 s (TWT), about 5 kmin depth.

The forelimb is bordered by a thrust fault that gener-ates the anticline: the GuF joins the thrust at about 1.5 s(TWT), about 3 km, representing a probable phenomenonof inversion tectonics. Along the GuF fault plane the seis-

mic markers point out an inversion in the throw, fromextensional at the surface (top carbonates and Marne aFucoidi reflectors) to compressional at depth, as evi-denced by the Top of the evaporites.

The profile also puts in evidence that the MarnosoArenacea Fm. is interested by short wavelength folds,whereas the underlying carbonate sequence is not invol-ved in this shallow deformation.

Along the profile, the ATF is underlined by some ali-gned oblique reflections and by sharp interruption of theseismic markers in the hanging wall. Its trajectory, dip-ping toward ENE,can be traced with continuity on theseismic line until to depth of about 7 km. The good qual-ity of the profile has also shown some significant detailsof the internal geometry of the continental deposits infill-ing the Val Tiberina basin. The shape of the basin isasymmetric, with the progressive eastward onlap of thestrata, and the thickening of the sequence from east towest: the thickness of the sediment reaches 1.4 s (TWT),about 1500 m.

PROFILE D (fig. 4a-b)

The seismic profile D starts about 7 km north of Peru-gia, crosses the southernmost part of the Gubbio basinand reaches the westernmost part of the Umbria MarcheApennines to the north of Gualdo Tadino (Fossato diVico) (fig. 1).

Immediately to the east of the Val Tiberina basin ananticline is clearly recognizable that represents the north-ern ending of the Mt. Subasio anticline, involving the car-bonate sequence (fig. 4). Its outern limb is bordered by athrust fault that joins the ATF at depth of about 1.8 s (4.5km). The crest of the anticline is buried under the turbi-dites at a depth of approximately 600 ms (TWT), corre-spondent to approximately 1000 m, and its southern limbdips toward east until a depth of about 3-4 km (fig. 4b).

The Gubbio anticline is crossed at its southern end-ing, and appears much less pronounced compared to theother section, maintaining however, its fundamentalstructural characteristics. The disharmonic pattern ofdeformation between the Marnoso Arenacea Fm. and theunderlying carbonatic formations is also shown.

Here, the ATF has an emphasized staircase trajectory,with a rather abrupt connection between the section witha high angle attitude, delimiting the Val Tiberina basin,and the section to low angle, dipping towards the struc-ture of Gubbio. After an initial steep attitude, down to adepth of about 1.6 s (about 4 km), it evolves into a flatshear zone and finally into gently east-dipping segment inthe lowermost part.

The upward concavity of the seismic reflectors, under-lines the internal geometry of the continental depositsinfilling the Val Tiberina basin: the thickness is about 900ms (TWT), corresponding to approximately 1000 m. Asplay of the fault border the Val Tiberina basin on the west.

DATA PROCESSING AND DISCUSSION

Isochronous maps of the Top Fucoidi, Top evaporitesand Top basement (fig. 5) have been produced in order toverify the consistency of the proposed interpretation ofthe seismic profiles and to obtain a map view of the sub-surface structures at different structural levels. We choice


Fig. 2 - Schematic stratigraphy of the Umbria-Marche Apennines.The mean values of the seismic interval velocities that have beenused for depth conversions of the seismic profiles are also reported(after BARCHI et alii, 1998). Stratigrafia dellAppennino Umbro-Marchigiano. Sono riportati ivalori medi delle velocit sismiche di intervallo usate per la conver-sione in profondit (da BARCHI et alii, 1998).

these markers because their lateral continuity, the unifor-mity of the facies and thickness and the possibility of cal-ibration offered by several wells, makes them key reflec-tors representative of the geometry of the geologicalstructures in the area. The maps were drawn utilizing thedata coming from two other seismic profiles, E and F(PAOLACCI, 1998), in order to improve the areal distribu-tion of the data (see the trace of the profiles in fig. 1). Themaps (fig. 5) were constructed picking in regular spatialintervals the depth in double times (TWT) of the markers(the crosses reported in fig. 5). The intersections betweenfaults and reflectors are also sampled. The data obtainedwere interpolated with common contouring techniques.

In the making of these maps, the markers were con-sidered continuous, and, for this reason, although themaps are a good tool for improving the regional trend ofthe main geological structures of the area, they do notindicate the thrust of the Gubbio anticline. For this rea-son, in order to reconstruct the geometry of the thrustfault, an isochronous map was drawn of the Top of

Fucoidi, that represents the best key-reflector for thestructural interpretation of the carbonate units (fig. 6).This map was obtained by separating the data on the seis-mic marker found in the hanging wall of the thrust fromthe data on the marker in the foot wall (fig. 6). In fig. 6the intersection between the fault plane and the Top ofFucoidi of the hangingwall is also shown.

The isobath map of the GuF was also constructed: infig. 7, the intersections of the fault plane with both thetopography and the ATF are shown.

This data processing, together with the analysis of theseismic profiles, have allowed the characterization of themain geological structures in the area and the resultsobtained will be discussed in following.

COMPRESSIONAL STRUCTURES

A common characteristic in all the seismic profiles, isthe different attitude between the shallow seismic reflec-tors (above the Bisciaro Fm.) and the reflectors in the


Fig. 3 - a) Line drawing of the seismic profile A; b) depth converted profile. The location of the profile is reported in fig. 1. a) Schema dei riflettori del profilo sismico A; b) profilo convertito in profondit. La localizzazione del profilo riportata in fig. 1.

lower part of the profiles. In fact, whereas the former arecharacterized by steep and bent reflectors and compres-sive deformations with short wavelengths, the latter aresmooth and continuous. This different attitude underlinesthe disharmonic deformation of the turbidites comparedto the underlying carbonates and evaporites: thus, theMarnoso Arenacea Fm. is interested by short wavelengthfolds, whereas the underlying carbonate sequence is notinvolved in this shallow deformation. This structural dis-harmony and the overall flat attitude of the carbonatemultilayer is clearly shown in fig. 3.

The main compressive structures involving the carbo-nate sequence are the Gubbio and the Mt. Subasio anti-clines, whose geometries are clearly visible in respectivelyfig. 3 and fig. 4. They consist of NE-verging anticlinesstriking about N130, as shown by isochronous maps (fig.5). The back limb of the Gubbio anticline is bordered bya normal fault (GuF), which is an antithetic of the ATF,and inverts a previously formed thrust fault: in fact, theGuF joins the thrust fault, which generates the anticline,at about 3 km in depth.

The figure 6 shows that the axis strike of the anticlineshows a slight eastward convexity, following the samebending of the thrust north of Gubbio. The westwardshift of the axis strike of the anticline in correspondencewith the Gubbio town, is probably apparent and con-nected to the lack of data in this sector. The recently anal-ysis of an available set of seismic profiles in the sectorsouth of the Gubbio town (MIRABELLA et alii, 2000) willimprove the definition of this structure. The thrust gener-ating the anticline shows a strike of approximately N130and a SW dip direction (fig. 6).

The Mt. Subasio anticline is shown at its northern ter-mination: for this reason its crest is buried under the tur-bidities to a depth of approximately 1000 m, as shown infig. 4. The thrust fault bordering its forelimb, joins theATF at depth of about 1.8 s (4.5 km).

It is interesting to note that the isochronous maps(fig. 5) clearly show that the whole carbonatic sequenceand at least the upper part of the evaporites are involvedin these structures, whereas the Top of the basementshows a different disharmonic pattern (compare the fig.


Fig. 4 - a) Line drawing of the seismic profile D; b) depth converted profile. The location of the profile is reported in fig. 1. a) Schema dei riflettori del profilo sismico D; b) profilo convertito in profondit. La localizzazione del profilo riportata in fig. 1.


Fig. 5 - Isochronous maps (seconds TWT) of the: a) Top Fucoidi;b) Top evaporites; c) Top basement. The time depth of the reflectorhas been picked along the seismic profiles. Crosses indicate themeasure points used for contouring. Dashed lines indicate the axisof the antliclines. Mappe delle isocrone (secondi TWT) del: a) Top Fucoidi; b) Topevaporiti; c) Top basamento. La profondit in tempi dei riflettori stata ricavata dai profili sismici. Le croci indicano i punti di misurautilizzati per il countouring. Le linee tratteggiate indicano gli assi delleanticlinali.

5a-5b with the fig. 5c). This characteristic points out thepresence of a detachment within the evaporites, whichare traditionally considered as the main decollement ofthe Umbria-Marche fold and thrust belt (BALDACCI et alii,1967; BALLY et alii, 1986). Considering the four profiles,going from north to south, a gradual transfer of the com-pressive deformations was found. In fact, the shorteningof the Gubbio anticline is gradually transferred to Mt.Subasio, in an inner position, depicting an n echelonpattern (fig. 5).

EXTENSIONAL STRUCTURES

The profiles analyzed show a good definition of themain extensional structures in the area, i.e. the Alto Tib-erina fault (ATF) and the Gubbio fault (GuF).

The seismic expression of the ATF is marked by astrong alignment of reflections and is confirmed by thesharp interruption, at its hanging wall, of the seismicsequence so far described. It outcrops at the western bor-

der of the Val Tiberina basin and dips toward ENE, witha strike of N150, and it can be traced with continuity onthe seismic profiles to a depth of 8 km. The seismic pro-files underline the staircase trajectory of the fault, whosegeometry has been described in detail (BONCIO et alii,1998; BARCHI et alii, 1999; BONCIO et alii, 2000; COLLET-TINI et alii, 2000): after an initial steep attitude, to a depthof about 1.6 s (TWT), it evolves into a flat shear zone ata depth of 2 s (TWT) and, finally, into a gently east-dip-ping segment in the lower part.

The geometry of the Val Tiberina basin is also char-acterized by the good quality of the seismic profiles. Thebasin shows an asymmetrical shape infilled by continen-tal, fluvial and lacustrine deposits of Upper Pliocene (?)to Quaternary age (AMBROSETTI et alii, 1978; AMBROSETTIet alii, 1995; CATTUTO et alii, 1995) with the progressiveeastward onlap of the strata and the thickening of thesequence from east to west. Considering the four profiles,a complex variation of the thickness of the Val Tiberinabasin has been observed going from north to south: it isabout 1500 ms (TWT), corresponding to approximately1500 m, in the northernmost profile (A); the thicknessremains more or less constant in the two central profiles(B and C), reaching a depth of about 50 ms (500 m); andit then reaches 900 ms (TWT) corresponding to 1000 m,in the southernmost profile (D).

The GuF cuts the internal limb of the Gubbio Anti-cline, with a strike of N120 and a dip direction towardSW. The inclination of the fault is variable from 50 to70. The seismic data show that the deeper part of thefault plane joins the frontal thrust that generates the anti-cline, representing a phenomenon of inversion tectonics.The total displacement of the normal fault, as detectableby the position of the Marne a Fucoidi reflector, can beestimated as being more than 1500 m.

The intersection between the ATF and GuF south ofGubbio is located at a depth of about 6 km in the south-ernmost sector, reaching 4.5 km toward the north, andthen reaching a depth of 5 km near the town of Umber-tide (fig. 7). (The relationships between the ATF, GuF andthe seismicity have also been described by BONCIO &LAVECCHIA, 2000).


Fig. 6 - Isochronous map (seconds TWT) of the Top Fucoidi of thehangingwall and footwall of the thrust fault generating the Gubbioanticline. In this map the measure points of the thrust hangingwallhave been contoured separately from those of the footwall. The thickblack line indicates the intersections of the thrust plane with the TopFucoidi of the hangingwall. Crosses indicate the measure pointsused for contouring. Mappa delle isocrone (secondi TWT) del Top Fucoidi, campionato altetto e al letto del thrust generante lanticlinale di Gubbio. In questamappa i punti di misura del tetto del thrust sono stati campionatiseparatamente da quelli del letto. La linea nera spessa indica linterse-zione tra il piano di faglia del thrust e il Top Fucoidi campionato altetto. Le croci indicano i punti di misura utilizzati per il countouring.

Fig. 7 - Isobath map (meter) of the Gubbio normal fault. The distri-bution of the hypocenters of the 1984 Gubbio earthquake (HAESSLERet alii, 1988) is also reported. The thick black line indicates the traceat surface of the Gubbio normal fault. Dashed line indicates theintersection of the Gubbio normal fault with the Alto Tiberina Fault.Crosses indicate the measure points used for contouring. Mappa delle isobate della faglia diretta di Gubbio. con riportata ladistribuzione della sismicit legata alla crisi sismica del 1984 (HAESS-LER et alii, 1988). La linea nera spessa indica la traccia in superficiedella faglia normale di Gubbio. La linea tratteggiata indica linterse-zione tra la faglia normale di Gubbio e la Faglia Altotiberina. Le crociindicano i punti di misura utilizzati per il countouring.

The isobath map of the GuF satisfactorily describesthe regional attitude of the fault, showing a steep trajec-tory a shallow depth (about 40), emphasized by thethickening of the curves, evolving into a flat shear zone(less than 20) up to its intersection with the Alto Tibe-rina fault (fig. 7). The trace of the GuF drawn by the offi-cial maps (Foglio Gubbio), has been given on the map,showing a good correspondence with the locating of thefault at the surface indicated by the seismic profile (0 iso-bath). In the southernmost sector of the area studied,where the fault displaces the Marnoso-Arenacea turbi-dites, a slight discrepancy (about 1 km, eastward) isobtained (fig. 7). The poor correlation found south ofGubbio between the tip line and the intersection with thetopography of the GuF obtained from this data process-ing could be explained by two different hypotheses. Thefirst is the lack of data in this sector, as above underlined;the second is the unclear location of the tip line in thefield, being an intraformational fault.

The locating of the GuF at the surface shows a leftbending in correspondence with the Gubbio town: thesegment NW of the town of Gubbio appears shiftedtoward the west. The isobath map (fig. 7) shows that thisgeometry is maintained even in the deepest part of thefault and, thus, it is probable that the GuF is character-ized by two different segments. Showing the distributionof the hypocenters of the 1984 Gubbio earthquake(HAESSLER et alii, 1988) on the isobath map (fig. 7), agood correlation can be observed between the geometryobtained and the seismicity. The hypocenters are in factlocated from 3.5 to 7 km in depth, involving the regioncomprised between the Gubbio fault plane and its inter-section with the ATF. The distribution of the hypocentersalso suggests that only the southern sector of the faultwas activated during the 1984 earthquake.

The seismic profiles do not allow an easy reconstruc-tion of the geometry of the Gubbio basin: the thickness ofthe continental deposits is about 500 m, a value in accor-dance with that hypothesized by MENICHETTI, 1992.

CONCLUSIONS

1) The good quality of the seismic profiles made iteasy to localize the main seismic markers, whose inter-pretation has defined a good tool in constraining thegeometry of the main compressional and extensionalstructures in the area.

2) The interpretation of the seismic profiles demon-strates the presence of at least three major detachmentslevels, confirming the hierarchic arrangement of the com-pressional structures observed by BARCHI et alii, 1998.

From the top, the first detachment is located at anintra-turbidites level or at the top of the carbonates. Infact, the seismic profiles have shown that the compressivedeformations involving the Marnoso-Arenacea Fm. do notpropagate to the underlying carbonate sequence. The dif-ferent behavior of the deformation could be due to thedifferent mechanical properties of the litho-structuralunits forming the Umbria-Marche sequence. In fact, thesequence of the turbidites exhibits ductile behavior undercompressive deformation, with the nucleation of shortwavelength folds, whereas the carbonates, and at least theupper part of the evaporites, respond to the compressivedeformation generating anticlines. These anticlines, in

the area studied, are the structures of Gubbio and of Mt.Subasio, having wavelengths of about 5-10 km. Lookingat the isochronous maps, the basement is not involved inthese structures, suggesting the presence of a secondimportant decollement level within the evaporites.

The top of the Phyllites, traditionally regarded as thebasement, is involved in thrust sheets. This data confirmsthe only direct geological control on the presence of thesestructures of the S. Donato 1 well, where a repetition ofthe evaporites and Phyllites on the border between West-ern Umbria and the Umbria pre-Apennines was drilled.

3) The geometry of the Gubbio normal fault and itsrelation with the ATF have been defined. A good correla-tion was found between the GuF geometry obtained andthe seismicity. In fact the distribution of the hypocentersof the 1984 Gubbio earthquake are located from 3.5 to 7km in depth, involving a region comprised between theGubbio fault plane and its intersection with the ATF. Apossible segmentation of the GuF in two different seg-ments has been supposed. The 1984 earthquakes appearsto be in relation with the southernmost segment of thefault, whose continuity is about 15 km. This dimension iscomparable with the magnitude of the recent and histor-ical seismicity of the region.

ACKNOWLEDGEMENTS

The authors are deeply grateful to ENI-AGIP Division for theirpermission to study the seismic profiles, without which this studywouldnt have been possible. The authors are also grateful to Prof.Giusy Lavecchia and Dr. Sergio Rogredi for improving the manu-script with valuable suggestions and to Dr. Saverio Merlini for thediscussion during the seismic interpretation.

This study was supported by the CNR-GNDT 1998 (resp. M. Bar-chi) and Ateneo 1998 (resp. C. Federico) grants.

REFERENCES

ALVAREZ W., ARTHUR M.A., FISCHER A.G., LOWRIE W., NAPOLEONEG., PREMOLI SILVA I. & ROGGENTHEN W.M. (1977) - Upper Cre-taceous-Paleocene magnetic stratigraphy at Gubbio, Italy. Typesection for the late Cretaceous-Paleocene magnetic reversal timescale, Geol. Soc. Am. Bull., 88, 383-389.

AMBROSETTI P., CARBONI M.G., CONTI M.A., COSTANTINI A., ESU D.,GADIN A., GIROTTI O., LAZZAROTTI A., MAZZANTI R., NICOSIA U.,PARISI G. & SANDRELLI F. (1978) - Evoluzione paleogeograficadei bacini tosco-umbro-laziali nel Pliocene e nel Pleistocene infe-riore. Mem. Soc. Geol. It., 19, 573-580.

AMBROSETTI P., BASILICI G., CIANGHEROTTI A.D., CODIPIETRO G.,CORONA E., ESU D., GIROTTI O., LO MONACO A., MENEGHINIM., PAGANELLI A. & ROMAGNOLI M. (1995) - La foresta fossile diDunarobba (Terni, Umbria Centrale): Contesto litostratografico,sedimentologico, palinologico, dendrocronologico e paleomalaco-logico. Il Quaternario, 8(2), 465-508.

BALDACCI F., ELTER P., GIANNINI E., GIGLIA G., LAZZAROTTO A.,NARDI R. & TONGIORGI M. (1967) - Nuove osservazioni sul pro-blema della falda toscana sulle interpretazioni dei flisch arenaceidi tipo Macigno dellAppennino Settentrionale. Mem. Soc. Geol.It., 6, 218-244.

BALLY A.W., BURBI L., COOPER C. & GHELARDONI R. (1986) - Bal-anced sections and sismic reflection profiles across the CentralApennines. Men. Soc. Geol. It., 35, 257-310.

BARCHI M.R., DE FEYTER A., MAGNANI M.B., MINELLI G., PIALLI G.& SOTERA B.M. (1998) - The structural style of the Umbria-Mar-che fold and thrust belt. Mem. Soc. Geol. It., 52, 557-578.

BARCHI M., PAOLACCI S., PAUSELLI C., PIALLI G. & MERLINI S.(1999) - Geometria delle deformazioni estensionali recenti nelbacino dellAlta Val Tiberina fra S. Giustino Umbro e Perugia: evi-denze geofisiche e considerazioni geologiche. Boll. Soc. Geol. It.,118, 617-625.


BARCHI M., GALADINI F., LAVECCHIA G., MESSINA P., MICHETTIA.M., PERUZZA L., PIZZI A., TONDI E. & VITTORI E. (2000) - Sin-tesi delle conoscenze sulle faglie attive in Italia Centrale: parame-trizzazione ai fini della caratterizzazione della pericolosit sismica.GNDT, Gruppo Nazionale per la Difesa dai Terremoti, Roma,spec. publ.: 62 pp.

BONCIO P., BROZZETTI F., PONZIANI F., BARCHI M., LAVECCHIA G. &PIALLI G. (1998) - Seismicity and extensional tectonics in theNorthern Umbria Marches Apennines. Mem. Soc. Geol. It., 52,539-555.

BONCIO P. & LAVECCHIA G. (2000) - A geological model for the Colfi-orito earthquakes (September-October 1997, Central Italy). Journ.of Seism., 4(4), 345-356.

BONCIO P., BROZZETTI F. & LAVECCHIA G. (2000) - Architecture andseismotectonics of a regional Low-Angle Normal Fault Zone inCentral Italy. Tectonics, 19, 1038-1055.

CATTUTO C., CENCETTI C., FISAULI M. & GREGORI L. (1995) - I baciniPleistocenici di Anghiari e Sansepolcro nellalta valle del Tevere. IlQuaternario, 8, 119-128.

CENTAMORE E., DEIANA G., MICARELLI A. & POTETTI M. (1986) - IlTrias-Paleogene delle Marche. In: Centamore E. & Deiana G. (Ed.)La geologia delle Marche. Studi Geologici Camerti, vol. spec.1986, 9-27.

CIARAPICA G., CIRILLI S., PASSERI L., TRINCIANTI E. & ZANINETTI L.(1987) - Anidriti di Burano et formation du Monte Cetona(Nouvelle Formation), biostratigraphie de deux series-types duTrias Superieur dans lApennin Septentrional. Revue de Palo-biologie, 6, 341-409.

COLLETTINI C., BARCHI M., PAUSELLI C., FEDERICO C. & PIALLI G.(2000) - Seismic expression of active extensional faults in North-ern Umbria (Central Italy). Journal of Geodynamics, 29, 309-321.

CRESTA S., MONECHI S. & PARISI G. (1989) - Stratigrafia del Meso-zoico e Cenozoico nellarea umbro-marchigiana. Itinerari geologicisullAppennino umbro-marchigiano (Italia). Mem. Descr. CartaGeol. dIt., 39, 1-182.

DE FEYTER A.J. (1989) - Gravity tectonics and sedimentation of theMontefeltro, Italy. Geologica Ultraiectina, 35, 168 pp.

DE FEYTER A.J., MOOLENAR N., PIALLI G., MENICHETTI M. &VENERI F. (1990) - Paleotectonic significance of gravity displace-ment structures in the Miocene turbidite series of the M. PolloSyncline (Umbro-Marchean Apennines, Italy). Geol. Mijnbouw,69, 69-86.

DEIANA G. & PIALLI G. (1994) - The structural provinces of theUmbro-Marchean Apennines. Mem. Soc. Geol. It., 48, 473-484.

ELTER P., GIGLIA G., TONGIORGI M. & TREVISAN L. (1975) - Tensionaland compressional areas in the recent (Tortonian to Present) evo-lution of north Apennines. Boll. Geof. Teor. Appl., 17, 3-18.

HAESSLER H., GAULON R., RIVERA L., CONSOLE R., FROGNEUX M.,GASPARINI G., MARTEL L., PATAU G., SICILIANO M. & CISTERNASA. (1988) - The Perugia (Italy) earthquake of 29 April 1984: amicroearthquake survey. Bull. Seism. Soc. Am., 78(6), 1948-1964.

LAVECCHIA G. (1985) - Il sovrascorrimento dei Monti Sibillini: analisicinematica e strutturale. Boll. Soc. Geol. It., 104, 161-194.

LAVECCHIA G., MINELLI G. & PIALLI G. (1987) - Contractional andextensional tectonics along the cross-section Lake Trasimeno-Pesaro (Central Italy). In: BORIANI A., BONAFEDE M., PICCARDOG.B. & VAI G.B. (Eds.), The lithosphere in Italy: advances inEarth Sciences Research. Accad. Naz. Lincei, Roma, 177-194.

LAVECCHIA G., BROZZETTI F., BARCHI M., KELLER J. & MENICHETTIM. (1994) - Seismotectonic zoning in east-central Italy deducedfrom the analysis of the Neogene to present deformations and rela-ted stress fields. Geological Society of America Bulletin, 106,1107-1120.

LUTERBACHER H.P. &. PREMOLI SILVA I. (1962) - Note preliminairesur une revision du profil de Gubbio, Italie. Riv. Ital. Paleont.Stratig., 68, 253-288.

MENICHETTI M. (1992) - Evoluzione tettonico-sedimentaria della Valledi Gubbio. Studi Geologici Camerti, Vol. Spec. 1992/1, 155-163.

MENICHETTI M. & PIALLI G. (1986) - Geologia strutturale del Preap-pennino umbro tra i Monti di Gubbio e la catena del M. Petrano-M. Cucco (Appennino Umbro-Marchigiano). Mem. Soc. Geol. It.,35, 371-388.

MENICHETTI M. & MINELLI G. (1991) - Extensional tectonics andseismogenesis in Umbria (Central Italy): the Gubbio Area. Boll.Soc. Geol. It., 110, 857-880.

MIRABELLA F., CIACCIO M.G. & PAUSELLI C. (2000) - Geometria delledeformazioni dalla Valle del Tevere alla Ruga interna dellAppen-nino Umbro-Marchigiano: il punto di vista della sismica a rifles-sione. 19 Convegno Nazionale GNGTS, Roma, 7-9 novembre2000, Volume dei Riassunti, 20-21.

PIALLI G., BARCHI M.R. & MINELLI G. EDS. (1998) - Results of theCrop03 deep seismic reflection profile. Mem. Soc. Geol. It., 52.

PAOLACCI S. (1998) - Interpretazione di linee sismiche a riflessioneattraverso le strutture compressive e distensive del pre-AppenninoUmbro. Tesi di laurea, Perugia, 79 pp.

RIDOLFI A., LUCHETTI L., MENICHETTI M., NEGRI A. & PIALLI G.(1995) - Use of key beds in stratigraphic and structural analysisof the Marnoso-arenacea fm of the Assino valley area (NorthernUmbrian Apennine). Giornale di Geologia, 57, 113-129.


Manoscritto pervenuto il 20 dicembre 2000; testo approvato per la stampa il 6 Agosto 2001; ultime bozze restituite il 14 Gennaio 2002.

Documents

Seismic image of the compressional and extensional structures in the Gubbio area (Umbrian-Pre Apennines)