1993_argnanietal_anngeof

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ANNALI DI GE OFISICA, VOL. XXXVI, N, 2. May 1993

Foreland deformational patternin the Southern Adriatic Sea

Andrea Argnani(I), Paolo F a ~ a l i (~ ) ,rancesco F r ~ go n i ( ~ ) ,arco Ga sperhi('), Ma rco Ligi('),Michael M arani('), Giuseppina M attietti(2) and Giuliana M ele(2)( I ) Isrituto per la Geo logia Marina - CN R, Bologna, Italia

(') Istituro Naziona le dl G eofis ica, Ro ma , Italia

AbstractTwo major deformation belts occur in the portion of the Adriatic S ea offshore the Gargano Prom ontory. Althoughthese two belts display similar characters on seismic profiles, they are different in other respects. The NE-SW-trending Tremiti Deformation Belt, located north of the G argano Prom ontory, originated during the Plio-Quater-nary, while the E-W-trending South Garg ano Deformation Belt, located south of the Gargano Promon tory,formed in a time span that goes from Eocene to early Pliocene. On the ground of structural and stratigraficevidence these deformation belts are interpreted as originated by tectonic inversion of Mesozoic extensionalfaults. This inversion tectonics, of Tertiary age , can be related to the evolution of the fold-and-thrust belts thatsurround the Adriatic Se a.A moderate seismic activity, recorded around the Tremiti Island, and historical seismological data suggest thatthe whole of study area is, at present, seismically active. Therefore, this portion of the Adriatic block stillrepresents a preferental site of deformation.

1. IntroductionThis study aims at contributing to the investi-gation of an area in the Southern Adriatic Seawhere the presence of tectonic structures ofpossible regional significance has been reportedand where seismic activity occurred in the lastyears. The area is located offshore the GarganoPromo ntory and rests within the so-called hat is the foreland of theApennines and D inarides fold-and-thrust belts.Wh ether Adria belongs or not to the African plateis at present a matter of debate, and som e Authors(Favali et al., 1990; Westaway, 1990) favour,mainly on the ground of seismological data, aplate boundary running just across this part of theAdriatic Sea.Using a data set consisting of explorationwells and seismic profiles, mostly commercialbut som e collected on purpose, we attempt to definegeometries, trend and geologic evolution of theabove-mentioned structures and to relate them tothe observed seismicity. The results we obtain,

althoug h still preliminary , show a picture of com-plexity s o far unreported that we believe may berelevant to interpret the kinematics of th e region.

2. Geologic settingThe area we examine (fig. 1) lies within aportion of relatively undeformed continentalcrust (Adria) rimmed o n three sides by thrust andfolded units belonging to a previous passivemargin succession. They are: to the west theeast-verging Apennines, to the no rth the south-verging Southern Alp s, and, to the east the west-verging Dinarides (Channel1 e t al., 1979). Adriarepresents the foreland area of these fold-and-thrust belts an d, as such , it has been deflected inresponse to the load applied by the adjacentmountain belts (Moretti and Royden, 1988). Allalong the deformed belt surrounding Adria amarked seimicity is reported (McKenzie, 1972).This seismic activity defines the main plateboundary between Adria and Europe. The connec-


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[ I FRICAN FORELANDDEFORMED AFRICANCONTINENTAL MARGIN

, . ., .: FOR ED EEP. , .. . .. . . ..rn CEANIC REMNANTS

EUROPEAN FORELAND

DEFORMED EUROPEAN. . . . CONTINENTAL MARGIN

Fig. 1. Summary geological sketch of the central Mediterranean region. Bold outline represents the studyarea that surrounds the Ga rgano Promontory.230


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tion of Adria w ith the African plate is favouredby several Authors (Argand, 1924; McKenzie,1972; Channel1et al. , 1979; D'Argenio and Hor-vath, 1984; Dewey et al . , 1989), while Others(Vanderberg and Zijderveld, 1982; Morelli,1984; Jongsma e t al., 1987; Anderson and Jack-son, 1987; Favali et al. , 1990; Westaway, 1990)consider Adria, at present, as a separate microplate.Lithospheric thickness in this region is in theorder of (100+ 110) km (Calcagnile et al . , 1982;Mueller and Panza, 1984) while the crust is 30km thick on average (Morelli et al. , 1969; Geiss,1987), although some thickness variations dooccur. These geophysical data, therefore, indi-cate that we are dealing with a piece of trulycontinental lithosphere.Mo ho depth underneath the Gargano Promon-tory is estimated to be less than 25 km. This isconsiderably shallower than the adjacent regionwhere Moho depths are in the order of 35 krn(Geiss, 1987).Units belonging to the foreland sedimentarycover outcrop in the Gargano Prom ontory that islocated just onshore the study area (M artinis andPavan, 1967; Cremonini et al., 1971). They aremade up essentially by carbonate rocks rangingin age from Jurassic to middle Miocene with athickness of over 4000 m. E xploration wells havefound Triassic evaporites at depths of about 6000m. The G argano Promontory appears as a broadeast-west-elongated anticline affected by faultstrending NW-SE, E-W and, in minor extent, NE-SW . The nature of these fault systems and theircronology are not too well assessed. Apart fromthe Notes to the Ge ologic Map, only few papershave been devoted to the study of this Promon-tory and they differ significantly in their conclu-sions. Some Authors put more emphasis on si-nistral strike-slip tectonics acting along theE-W-trending Mattinata fault system (Funicielloet al . , 1988), while Others favour N-S and NE-SW compressional phases (Ortolani and Pa-gliuca, 1987). They only agree on the fact that thelast phase reactivated in extension many of theprevious faults and that some component ofstrike-slip motion occurred along the E-W-trending fault system.The T remiti Islands, located north of the Gar-gano Promontory, consist of four small islandswhere a Tertiary succession about 500 m thick

outcrops. Sediments range in age from Paleoceneto Upper Pleistocene and bedding planes gener-ally dip SE-ward defining a NE-SW-trendingmonocline. The tectonic evolution of these Is-lands, according to the Geologic Survey, is asfollows (Cremonini et al . , 1971). Minor folds,trending NE-SW and WNW-ESE, affected onlyPaleocene and Eocene sediments while faultswith little throw, mainly trending NW-SE butalso E-W , cut across sediments older than M iddlePliocene. Plio-Pleistocene sediments record aprogressive southeastward tilting of the mono-clin e of abo ut 5'-6'. A micro - and meso-structu-ral study carried out in these islands (Montoneand Funiciello, 1989) emphazises the role playedby E -W strike-slip faults. In their interpretationthe Tremiti Islands represent a pushed-up ridgewithin a E-W de xtral strike-slip system , with themain faults being, however, in an unknow n posi-tion offshore.As far as the geology offshore Gargano isconcerned, Finetti (19 84) in his synthesis of theAdriatic Sea, suggested the presence of twomajor dextral strike-slip faults, the NE-SW-trending Tremiti fault and the E-W-trendingMattinata fault, located north and south of theGargano Promontory, respectively. Unfortu-nately , the detail of these faults is not represented.The area south of Gargano has been further in-vestigated (De'Dominicis and Mazzoldi, 1987;Colantoni e t al. , 1990) and an E- W-trendingstructural high has been observed. The Authorsinterpret this structure either as a diapir anticlineoriginated during Mesozoic salt tectonics andlater reactivated by strike-slip faulting (De'Do-minicis and M azzoldi, 1987), or as a positiveflowe r structure originated during dextral strike-slip movements active until the base of Pliocene(Colantoni et al . , 1990).The E-W trend defined by the Gargano Pro-montory is also well expressed in the Bouguergravity anomaly contours (fig. 2). The positiveanomaly, that reaches 110 mGal over the Pro-mon tory, stretches offshore to a considerable ex-tent following the same trend (Finetti andMorelli, 1973). The sam e applies to the magneticbasement that underneath the Gargano Promon-tory and its offs hore is about 2 km higher than thesurrounding regions where it is deeper than 10km (Cassano et al . , 1986).


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Fig. 2. Bouguer gravimetric anomaly field and earthquake epicentres of the Gargano Promontory andsurrounding areas showing the + 100mGal anomaly located on the promontory and the focal mechanism of the5.3 magnitude earthq uake that occurred in 1988 midway between the Italian and Yugoslavian coasts. Seismicitydata from Console et al. (1992), gravity data from Finetti and M orelli (1973).

Paleomagnetic data collected in Upper Creta-ceous sediments of the Gargano Promontory,show a CCW rotation of about 23' with respectto the Africa pole (Chan nell, 1977; Vandenberg,1983). How ever, a rotation of 17' seems moreappropriate taking into account conservative dataevaluation (L owrie , 1986). Gargano Promontoryand Istria present the same amount of post LateCretaceous rotation suggesting that this part ofAdria behaved as a separate block with respect toAfrica. Recent data (Tozzi et al., 1989) show aCW rotation of 9' in Eocene limestones. Thiswould imply an alternate CC W and C W rotationof the Gargano area that, according to theAuthors, fits am od el of E-W-trending strike-slip

faults and block rotating about vertical axes.Howeve r, the limited amo unt of rotation and theslight worsening of the precision parametres ofthe Fisher statistics, after the tectonic correction,cast som e doubts as to the age of magnetizationthat in fact has the sam e direction of the presentEarth magnetic field before c orrection.

3. SeismicityThe Gargano Promontory and its neighbour-ing area are well known as a seismically activezone. Many earthquakes, such as the 1 627 ,164 6and 1731 events, reached destructive effects.


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Moreover, these earthquakes were often fol-lowed by many aftershocks and in some casesthe seismic sequences lasted over on e year (Ba-ratta, 19 01). Nevertheless, the seismicity wasoften mislocated as it was based only on ma-croseismic information. In fact, macroseismicinvestiga tion associates the epice ntral area of anearthqu ake to the mos t damaged localities, wherethe damage can be caused by local geologicalsituations. In many cases it is possible to inferthat the earthquakes took place offshore butcaused damages and site effects inland. Forexample the 1627 earthquake is classified as XIMCS. Such an intensity was reached only insome small centres of Gargano (San Severe,Apricena, etc.) (Postpischl, 1985a), but histori-cal chronicles about tsunami waves along thenorthern coast suggest an offshore hypocentralarea. A detailed analysis of the Italian seismiccatalogues (Po stpischl, 1985b; ING , 1990) pointsout the occurrence of seismic activity offshoreGargano. This activity is located both north andsouth the Promontory although, on the basis ofthese data, it is quite impossible to localize itcorrectly.On the other hand , the implementation of theItalian Telemetered Seismic Network, of whichthe Istituto Nazionale di Geofisica is in charge,has allowed a better localization of the lower-magnitude earthquakes occurring in Italy andsurroun dings. During the last years (1986- 1990)three interesting seismic sequences occurred inthe Southern Adriatic Sea and, when properlylocalized (Console et al., 1989; 1992) (fig. 2),they appe ar associated with the Trem iti fault zone(as defined in Finetti, 1982 and Finetti et al . ,1987) and demostrate that this structure is seis-mically active (Console et cd., 1992). Only themain shock of the 1988 sequence (mi, 5.3)allowed one to com pute a reliable focal mechan-ism. This mechanism was obtained using a(double couple^ model (using only teleseismicdata) and was also supported by the Centroid-Moment Tensor (CMT) method (Dziewonski etal., 1981). Both solutions show strike-slip com-ponent, more evident in the ,and thrust component, more evident in the CM T.Th a actual fault plane was interpreted as strikingENE -WSW with a sinistral strike-slip moveme nt(Console et al . , 1992).

4. Data setThis study is based on a data se t consisting of

several exploration wells and of a rather densegrid of seismic profiles. M ost of these profiles arecomm ercial but som e were collected on purposeduring a cruise carried out in 1991 (fig. 3). Abo ut1000 km of multichannel seismic reflectionprofiles were collected offshore the Garga no Pro-montory onboard of the R/V Bannoch of CNR.The details of acquisition and processing aregiven in tables I and 11, respectively. Not all ofthe profiles were processed to the final slip; forsome of them only the stacking was performed.Seismic profiles ha ve been interpreted defin-ing first the unfaulted panels and then, withinsuch panels, identifying reflection terminationsand geometric arrangements of reflection pack-ages.A set of about 15 exploration wells has beenused to work out the stratigraphy of the area and,whenever possible, well data have been tied toseism ic profiles in order to put stratigraphic con -straints to s eismic interpretation.Nine stratigraphic logs are shown in fig. 4 and9 where they are arranged in two transects cross-Table I. Acquisition parameters.Source Sodera G.I. GunGun mode True G.I.Capacity Generator 45 ci; Injector 105 ciPressure 2000 psiDepth 3.5 mShot interval 25 mStreamer TeledyneLength 600 mDepth 12 mGroups 24Hydrophones per group 20Group interval 25 mSeismograph Geometries 2420Sampling rate 1 msRecord length 5000 msAntialiasing filter 180 HzTape format SEG-D / 6250 bpiCoverage 1200%


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Fig. 3. Grid of seismic lines utilised in the present study (dotted: commercial; continuous: PG 91 cruise) andlocation of public domain exploration wells. Bold lines indicate seismic sections illustrated in fig. 5 an d 10.~ a t h ~ r n e t r ~ f r o r nBCM dataset.

ing the northern and sou thern parts, respectively,of study area.For the wells wh ere the stratigraph ic record ismore complete, water-loaded subsidence curveshave been computed using a modified version ofthe Program BA STA (Friedinger, 1988) that fol-lows Steckler and Watts's (1978) equations.Thickness of dated stratigraph ic intervals, poros-ity-depth relationships for different lithologiesand paleowater d epths are needed to calcu late thewater-loaded subsidence. The first piece of dataderives directly from well logs, while porosity-depth functions were taken from Sclater andChristie (1980 ) and Schrnoker and Halley (1982).

mine an d, for this reason, a depth range has beenused so as to take uncertainties into account.Paleowater depth estimates are taken from Woo d(1982). Airy isostasy has been assumed to de-scribe the lithospheric response to loading. Thisassum ption may be valid for the Mesozoic riftingsubsidence but is likely to be not so good anapproximation fo r the successive evolution of thearea where some flexural rigidity has been shownto exist (Royden, 1 988).Sea-level variations have not been taken intoaccount because the several sea-level curvespresented differ significantly both in time andmagnitude of fluctuatio ns, and no one has gainedPaleowater depths are more critical to deter- general acceptance.


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Table 11, Processing sequence. to this structure that is hereafter referred to asTrerniti Deformation Belt (TDB).Wells are unevenly distributed in this part (fig.True amplitude recovery 3) and none of them is located on top of the abo vementioned structure. A transect from coast tooffshore (fig. 4), constructed using three wells,illustrates the gross stratigraphy of this zone. Thedrowing of a carbonate platform system is wellPredictive deconvolution documented in the two offshore wells, while itOperator length: 255 ms never occurred in the near coa st one.The variation in thickness of the Mesozoicpelagic sediments likely reflects deposition in00-2000(*)ms2000(v)-5000 ms half-grabens as shown by the seismics (fig. 5 , ineB-427). It is noteworthy that above a depocentreTrace equalization of Mesozoic pelagics (well Famoso) the Paleo-Time variant filterCDP sorting gene section is incomp lete and mostly of shallowVelocity analysis water, while in an area of reduced MesozoicNMO correction pelagics (well Stella) the same section is com-Stack: 1200%Spike deconvolution BRANZINO STELLA FAMOSOTime variant filter-windows: 0(" )-0(** ) 12/65Hz km00 -15 00 (*' ) 10150Hz1500("")-50008/33 HZTrace equalizationFinite difference time migrationAGC-gate length: 500 ms("') Time from zero water bottom.

(") Time from base Plio-Quaternary.

5. InterpretationIn a describing and interpreting the da ta, thestudy area has been subdivided into two partsseparated by the Gargano Promontory. The rea-son for this is no t just purely geographic because,as will be shown below, these two parts appearto have unde rgone a different geologic evolution,at least from the Tertiary onward. After the tw o

trasted in a unified picture w ill be presented.

5.1. Zone North of Gargano Fig. 4. Summary stratigraphy from coast to off-shore north of the Gargano Pronlontory. Note theThe major feature in this Part is a NE-SW- expansion of the Mesozoic section and the reductiontrending structure that is well expressed in the of the Paleogene section that occurs passing frombathymetry (fig. 3). Th e Trem iti Islands be lon g Stella to Famoso. For further discussion, see text.235


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Fig. 5. Selection of seism ic lines north of the Gargano Prom ontory show ing the general stratigraphic featuresof the area (Line B-427 ) and the geom etry of the main structural feature of this zone, the TDB (Lines PG-16 andPG-5). For discussion, see text.plete, more expanded and made up by deeper- passive margins (Wooler et al., 1992). From 50water sediments. Following the Messinian evap- to 30 Ma minor pulses of uplift and subsidenceorites are the fine-grain ed clastic sediments of the are observed w hile at about 5Ma the onset of theApennine foredeep that thin northeastward. The Apennine foredeep is clearly marked.tectonic subside nce curves (fig. 6a)) show a typi- Three seismic lines have been selected to il-cal passive margin trend from 220 to 50 Ma. On lustrate the principal aspects of this zone (fig . 5).the contrary of what commonly reported in the The dating of horizons and seismic units haveliterature (Bern oulli and Jenkyns, 1974 ; Winterer been assessed by means of well ties wheneverand Bosellini, 1981) the main rifting episode is possible. The westernmost line (B-427) is farobserved in Late Triassic, before the carbonate away from the TDB and shows, at a depth ofplatform drowning occurred. A comparable about 3 s Two W ay Traveltime (TWT),apackageearly-to mid- Triassic stretching episode results of reflections onlapp ing onto a tilted horizon.from a study carried out along the Tethyan From well ties the onlapping package is at-

236


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STELLA - SOUTHERNADRIATIC SEA

Aue in , fa,

E T E R N O - S O U T H E R N A D R I A TI C S E A

FAMOSO 1 - SO U TH ERN AD RIATIC SE A

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ~ 1 1 1 1 ~200 is0 too 50 0

Ag e i n MaFig. 6a). Observed water-loaded tectonic subsidence curves based on well data from the area north of theGargano Promontory.

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GIULIANA - SOUTHE RN ADRIATIC SE AsCIGNO MARE - SOUTHERNADRIATIC S EA

GARGANOEM - SOUTHERNADRIATIC SEA

Age i n Ma

Fig. 6b). Observe d water-loaded tectonic subsidence curves based on well data from the area south of theGargano Promontory.tributed to Me sozoic pelagic sediments w hile the foredeep sediments display an onlapping unit, atunderlying horizon represents the top ofplatfor m the base, followed by a thick packet of north-carbonates tilted by extensional block faulting. eastward prograding clinoforms. Line PG-16 andAbove the southwestward tilted unconformity PG-5 cross each other on top of the TDB andMhat ma rks the base of the Plio-Quaternary, illustrate its general folded appe arance . Th e TDB


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formation Belt (SGDB). On th e contrary of whathappens fo r the TDB , the SGD B has no bathyme-tric expression but do es result as a relative highin the base Plio-Quaternary isochron map (fig.8).As discussed below, the SGDB displays no sub-stantial activity du ring the Plio-Quatern ary.Wells are located on either side of the struc-ture as well as on its top. A N-S transect, perpen-dicularly cro ssing the structu re is shown in fig. 9.As seen in the area north of Gargano, the Meso-zoic is characterised b y th e presence of Triassicand early Jurassic platform carbonates overlainby pelagic sediments. This transition from plat-form to basin sediments never occurred in thenear-coast well Jolly. Interestingly, the major

depocentre of Mesozoic pelagic sediments oc-curs in the well Gondola that is located right onthe top of the SGD B. These sediments have beenrelatively uplifted with respect to the adjacentcoeval sedim ents. Tertiary sediments are notice-ably absent over the SGDB while they occur intwo depocentres on either side of the structure.Paleocene and Eocene sediments have neverbeen encountered in any well of this area. Tec-tonic subsidence curves (fig. 6b)) show LateTriassic-Early Jurassic rifting and ensuingsmooth therm al subsidence interrupted , between35 and 15 Ma, by slight uplift followed by asubsidence pulse centred at about 20 Ma. An-other subsidence acceleration occurs around 5

Fig. 8. Base Pliocene isochron map of the sector south of the Garga no Promonto ry. The SGD B can befollowed from close to the Gargano Promontory, eastwards, where it changes direction and shows up moremarkedly. Deepening towards the north-east reflects the proximity of the foredeep related to the southernDinarides thrust belt.240


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JOLLY GIULIANA GRAZIA GONDOLA CIGNO MARE--

shaibw water cartsonates

Paleocene upper Trfassic

Fig. 9. Summary stratigraphy along a N-S transect south of the Gargano Promontory. Note that Gondola,located on the SGDB, shows the most expanded Mesozoic pelagic sequence, notwithstanding the probableerosion of further section since Plio-Quaternary elastics rest directly onto lower Cretaceous pelagic limestones(about 110Ma of missing section).M a and is related to the south Dinarid es foredeep . on the seismics and its relationship w ith the thickOf the three lines in fig. 10 two (D -436 and Mesozoic pelagic succession reported in the wellPG-2 2) cross the SGD B, while the third one (F-8) is not clear. How ever, on the southern flank ofis located on the easternmost continuation of this this broad asym metric fold a seismic unit withbelt where its trend changes substantially. Line parallel reflections onlapping at its base and tm n-D-436 crosses the structure where it is about to cated at its top by the Munconformity is welldie out westward. Th e SG DB is very narrow and displayed. Such a unit likely represents theto the south of it there is a wedge-shaped unit with Oligo-Miocene sequence and rests on the M eso-onlappin g reflections at its base. The low er part zoic pelagic sedime nts.of this unit represents Mesozoic pelagic sedi- Th e onlap of the Plio-Quaternary sedimentsments onlapping the tilted top of platform c arbo- onto the M eflection is remarkable and is innates, while the upper part, bounded on top by the agreement with what observed in the previous M reflector and onlapping at its base, is profile. Moving eastward, the SGD B changes itsthought to represent the Oligo-Miocene se- trend (fig. 8). Wo rk in this sec tor is still in pro-quenc e. In reality, as all of the wells are located gress, but there is good evidence of contractiononto relative structural highs, it is not given to as indicated by the fault-bend-fold geometry dis-know if the lack of Paleocene and Eocene is real played in Line F-8 where the folded ~ M orizonall over the area or if it represents a biasedpictu re. is onlapped by Plio-Quaternary sedime nts.In Line PG-2 2 the SG DB is wider and is better Also in this area, as in the north Garga no one,developed. The well Gondola is located afe w km Mesozoic rifting favoured the accumulation ofto the east of this line but, unfortunately, the great thicknesses of shallow-water carbonatesinternal geom etry of the structure is not resolved and led, eventua lly, to the breakdown of carbo-

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D-436 S. GARGANO DEFORMATION BE L T

F-8 D A U N O smt.-- ~ . - -. .. .- - -.- -~. : 3-.. - -- -. ~p--..- . - - .

Fig. 10. Seismic sections showing the structural characters of the SG DB passing from west (Line D-436) toeast (Lines PG-22 and F-8).For discussion, see text.nate platforms and to the deposition of pelagicsediments within half-grabens. Paleocene andEocene sedim ents are not reported in explorationwells although they m ay occ ur in undrilled struc-tural lows. The observed onlap of the Oligo-Miocene sediments onto the tilted Mesozoicunits indicates that some tectonic activity oc-curred sometime during Paleocene and Eocene.

This tectonic episode originated the SGDBwhich was later reactived in M essinian time, asdocumented by the tilted Oligo-Miocene sedi-ments and by the onlapping Plio-Quaternarystrata. Unlike the TD B, w here most of the defor-mation took place during the Plio-Quaternary,the SGD B apparently was not active in that timeinterval. The broadly folded reflectors of the


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Foreland deformational uattem in the Southern Adriatic Sea

SGDB, its position above a Mesozoic exten-sional fault (D-436) and the presence at its coreof a depocentre of Mesozoic pelagic sediments(well Gondola) su ggest that this structure mightbe related to tectonic inversion of a previousextensional fault.

5.3. SummaryTwo major deformation belts occur offshorethe Gargano Promontory with different trendsand different geologic evolution. The TDB , to thenorth, has a marked bathymetric expression anda NE-SW trend. The development of this struc-

ture occurred essentially during the Plio-Quater-nary and its activity lasted until very recent. TheE-W-trending SG DB , on the other hand, lacksbathymetric expression. The onset of this struc-ture is difficult to constrain, although it occurredsometime within the Paleocene-Eocene timespan. Assuming that this structure is due to inver-sion tectonics, an Eocene onset is more likelybecause it would be c oeval with a major tectonicpulse in the Din arides. A second activation of theSGDB occurred during the Messinian. Thisseems to be the main event responsible for thedeformatio n of the easternmo st part of the struc-ture whose trend swings from E-W to NE-SW .SUMMARY LITHOSTRATIGRAPHY AND

Although the Gargano Promontory and itssurroundines area are sites of seismicitv bothOnly a few papers have been devoted to this taskand the results are sometime controversial; for

1984; ~o la n t on i t al., 1990) and as strike-slipsinistral in the onsh ore (Funiciello et al. , 1988).Our study, that concerns only the offshore, allowsone to define the timing of activity of the twomajor deform ation belts present to the north andto the south of the Gargano Promontory. A sum-mary of the deformational events occurred inthese two zo nes, and described in greater lengthabove, is illustrated in fig. 11. Is is worth to pointout that the two deformation belts show a differ-ent time of activity. The main deformational epi -sode affected the TDB during Plio-Quaternarywhile in the SG DB it occurred about the Eocene.Given the position of the two deform ation belts,it appears reasonable to link these episodes to theevolution of the adjacent chains. In particular, theSGDB seems to have recorded the onset of theDinaric foredeep , while the deformation in TDBappears to be related to the evolution of the

FORELAND DEFORMATIONAL EVENTSNORTH GARGANO ZONE SOUTH GARGANO ZONE

I- - - - - -- - F FOUTER RAMPMARLS 1 0


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Plio-Quaternary A pennin ic foredeep. It is likelythat the Eocene episod e was recorded, to a minorextent, also in the northern zone where indica-tions of tectonic activity have been observed . Inour opinio n, this Eoce ne even t marks the transi-tion from an evolution controlled by extensionaltectonics to one where contraction is the domi-nant motif.As far as the structural style is concerned, thetwo belts show similar characters. They bothappear as broad and unfaulted folds when look-ing at the upper sedimentary units. As pointed outabove, stratigraphic and structural data suggestthat these folds may h ave originated by tectonicinversion of Mesozoic extensional faults. Al-though other Authors (Fin etti, 1984; Colanto ni etul., 1990) interpret the TDB and the SG DB es-sentially as strike-slip features, we believe thatthere is no need to call upon transcurrent motionto explain the structures observed. While thearguments used to support strike-slip tectonics(great length of the SG DB and one change in thesense of separation; Colantoni et al., 1990) arenot unambiguous, w e fail to recognize other char-acters considered typical of strike-slip tectonics.For example, en-echelon structures related to themain fault zone are absent, and, although theSGD B is rather curvilinear (fig. 8), no evidenceof structures connected to restraining and releas-ing bend has been observed. However, a strike-slip component of motion cannot be ruled outafter this, still prelim inary, interpretation.A moderate seismic activity offshore Garganohas been recorded in the last few years and mo stlyin connection with the TDB. S mall earthquakesoccurred also south of Gargan o, but their magni-tude is too small to allow areliable location. Dataconcerning historical earthquakes indicate thatseismic activity occurred both n orth and sou th ofGargano an d, therefore, both areas can be co n-sidered at present seismically active. The linkwith the structures observed on seismic profile isneverthelesses far from clear. In fact, althoughthe TDB has been actively deform ed until recenttime, the SGDB ended its activity by the earlyPliocene. It is worth to point out that the seis-micity of the Adriatic Promontory occurs in aportion of this continental block where a greatdeal of structural deform ation is also observed. Itseems therefore that the Gargano Promontory

and its offshore represent an area of forelandwhere d efo rm ati on preferentially concentratedsince the emplacement of the surrounding fold-and-thrust belts. The large gravimetric anomalypresent above the Gargano Promontory androughly elongated E-W (fig. 2) appears to berelated to a perturbing m ass located at a depth ofabout ( 3 5 ~ 4 0 ) rn , i.e. within the lithosphericmantle. The horizontal com ponen t of the gravityfield of a rotating Earth acting on this massanomaly can increase the local stress field (Ga-sperini et ul., in preparation) and this may alsocontribute to the recent seismicity.Looking at the whole of the Adriatic block(fig. 12), 2 ppears that the foredeep system isasymmetric and that the axis of the peripheralbulge presents a major kink. The Gargano Pro-montory is located within this zone of distorsionwhere the stress propagating from the surround-ing chains is likely to be amplified. This fact,couoled with the occurrence of inherited Meso-zoic weakness zones, may have favoured thetectonic inversion that we envisage as the maindeformational feature of the area.

7. ConclusionsThe area offshore the Gargano Promontoryhas been investigated using multichannel seis-mic reflection profiles and exploration wells.Two major deformation belts have been obser-ved north and south Gargano , the NE-SW-trend-ing TDB and the E-W-trending SG DB , respec-tively.Although these zones were known already inthe literature, our stud y points out that their evol-ution was more complex than previously men-tioned. The TDB is essentially of Plio-Quater-nary age, while the deformation of the SGDB

began during the Eocene and was completed bythe early Pliocene. Despite previous interpreta-tions regarding these structures as due to strike-slip faulting, we propose that tectonic inversionof Mesozoic extensional faults can equally ex-plain the observ ed geologic features and also fitsthe regional geologic setting of a foreland sur-rounded by fold- and-thru st belts. The stress pro-pagating from the adjacent mountain chains isbelieved to have caused the tectonic inversion:


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0FOREDEEP BASINS

> l k mISOBATHS (Base P liocene, Apennines;Base Oligocene, Dinarides)TZ,> 3 k m

FORELANDFj- ONSHORE OUTCROPS6lERTIARY THRUST-FRONTS(Early, Dinarides; Late, Apennines)

Fig. 12 . Sketch map of the Adriatic foreland with a n ou tline of the flexural bulge resulting from the d oubleload of the Apenn ines and Dinarides fold and thrust belts.

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the timings of deformation in the chains and inthe G arganic area are, in fact, almost coincidental.The area is at present seismically active sug-gesting that it still represents a preferential siteof deformation within the Adriatic continentalblock.

AcknowledgementsWe are indebted to the Captain and the Crewof R/V Bann ock (now gloriously retired!) for thesupport provided during the Cruise, and to thestudents and colleagues, from Urbino andNaples, that helped us on board. The expertiseand the endurance of G. Borto luzzi, that togetherwith L.Masini looked after the seismic acquisi-tion, have been an invaluable contribu tion to thesuccess of the survey. L. asoni carefully drawnmo st of the figures.

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