Solid Earth, 5, 611–629, 2014www.solid-earth.net/5/611/2014/doi:10.5194/se-5-611-2014© Author(s) 2014. CC Attribution 3.0 License.
Did Adria rotate relative to Africa?D. J. J. van Hinsbergen1, M. Mensink1, C. G. Langereis1, M. Maffione1, L. Spalluto2, M. Tropeano2, and L. Sabato2
1Department of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, the Netherlands2Dipartimento di Scienze della Terra e Geoambientali, UniversitaÌ degli Studi “Aldo Moro” di Bari, Via E. Orabona 4,70125, Bari, Italy
Correspondence to:D. J. J. van Hinsbergen ([email protected])
Received: 17 March 2014 – Published in Solid Earth Discuss.: 28 March 2014Revised: 28 May 2014 – Accepted: 2 June 2014 – Published: 2 July 2014
Abstract. The first and foremost boundary condition forkinematic reconstructions of the Mediterranean region isthe relative motion between Africa and Eurasia, constrainedthrough reconstructions of the Atlantic Ocean. The Adriacontinental block is in a downgoing plate position relativeto the strongly curved central Mediterranean subduction-related orogens, and forms the foreland of the Apennines,Alps, Dinarides, and Albanides–Hellenides. It is connectedto the African plate through the Ionian Basin, likely withLower Mesozoic oceanic lithosphere. If the relative motionof Adria versus Africa is known, its position relative to Eura-sia can be constrained through a plate circuit, thus allowingrobust boundary conditions for the reconstruction of the com-plex kinematic history of the Mediterranean region. Based onkinematic reconstructions for the Neogene motion of Adriaversus Africa, as interpreted from the Alps and from IonianBasin and its surrounding areas, it has been suggested thatAdria underwent counterclockwise (ccw) vertical axis rota-tions ranging from∼ 0 to 20◦. Here, we provide six new pa-leomagnetic poles from Adria, derived from the Lower Cre-taceous to Upper Miocene carbonatic units of the Apulianpeninsula (southern Italy). These, in combination with pub-lished poles from the Po Plain (Italy), the Istrian peninsula(Croatia), and the Gargano promontory (Italy), document apost-Eocene 9.8± 9.5◦ counterclockwise vertical axis rota-tion of Adria. Our results do not show evidence of signif-icant Africa–Adria rotation between the Early Cretaceousand Eocene. Models based on reconstructions of the Alps,invoking 17◦ ccw rotation, and based on the Ionian Basin,invoking 2◦ ccw rotation, are both permitted within the doc-umented rotation range, yet are mutually exclusive. This ap-parent enigma could possibly be solved only if one or moreof the following conditions are satisfied: (i) Neogene shorten-
ing in the western Alps has been significantly underestimated(by as much as 150 km); (ii) Neogene extension in the IonianBasin has been significantly underestimated (by as much as420 km); and/or (iii) a major sinistral strike-slip zone has de-coupled northern and southern Adria in Neogene time. Herewe present five alternative reconstructions of Adria at 20 Ma,highlighting the kinematic uncertainties, and satisfying theinferred rotation pattern from this study and/or from previ-ously proposed kinematic reconstructions.
1 Introduction
The complex geodynamic evolution of the central Mediter-ranean region has been dominated by convergent motion be-tween the African and European plates. Rather than beingaccommodated along a discrete plate boundary, the com-plex paleogeography of the region led to convergence be-ing accommodated along segmented subduction zones, andto distributed overriding plate shortening. In addition, sub-duction rollback since the late Eocene has formed a seriesof extensional back-arc basins and strongly curved subduc-tion zones and associated mountain belts (e.g., Dewey et al.,1989; Doglioni et al., 1997; Gueguen et al., 1998; Wortel andSpakman, 2000; Rosenbaum and Lister, 2004; Jolivet et al.,2009; Stampfli and Hochard, 2009). It is this complex evo-lution that has made the Mediterranean region instrumentalin the development of fundamental concepts that link sur-face deformation to deep mantle processes (Malinverno andRyan, 1986; Doglioni, 1991; Wortel and Spakman, 2000;Cavazza et al., 2004; Govers and Wortel, 2005; Jolivet et al.,2009; Faccenna and Becker, 2010; Carminati et al., 2012).
Published by Copernicus Publications on behalf of the European Geosciences Union.
612 D. J. J. van Hinsbergen et al.: Did Adria rotate relative to Africa?
Detailed kinematic reconstructions constitute a fundamen-tal tool for advancing our understanding of the complex geo-dynamics of the Mediterranean region. A common boundarycondition adopted by all reconstructions is represented by therelative motions summarized in the Eurasia–North America–Africa plate circuit based on marine magnetic anomalies ofthe Atlantic Ocean (e.g., Savostin et al., 1986; Dewey et al.,1989; Rosenbaum et al., 2002; Capitanio and Goes, 2006;Seton et al., 2012; Torsvik et al., 2012; Gaina et al., 2013;Vissers et al., 2013), which defined the area generated andconsumed between Africa and Europe since the breakup ofPangaea. A critical element in Mediterranean reconstruc-tions is the continental domain of Adria (Fig. 1). Adria isa fragment of continental crust intervening the European andAfrican plates composed of essentially undeformed platformcarbonates currently exposed on the Apulian peninsula andGargano promontory of southern Italy, the Istrian peninsulaof Croatia, and the Adige embayment of the southern Alps(Fig. 1). Adria is in a downgoing plate position relative toall surrounding mountain belts: it is overthrust by the Apen-nines in the west and the Dinarides–Albanides–Hellenidesin the east, and although it was originally in an overridingplate position in the Alps, it became overthrust by these sinceNeogene time. Tectonic slices of the Adriatic upper crustare currently exposed in all circum-Adriatic mountain ranges(Stampfli and Mosar, 1999; Faccenna et al., 2001; Vai andMartini, 2001; Schmid et al., 2008; Ustaszewski et al., 2008;Bernoulli and Jenkyns, 2009; Stampfli and Hochard, 2009;Handy et al., 2010; van Hinsbergen and Schmid, 2012; Gainaet al., 2013) (Fig. 1). To the south, Adria is separated from theNorth African passive continental margin by oceanic litho-sphere of the Ionian Basin (Catalano et al., 2001; Frizon deLamotte et al., 2011; Gallais et al., 2011; Speranza et al.,2012).
There is no zone of intense compression between Adriaand Africa, and Adria has been paleolatitudinally stable rel-ative to Africa within paleomagnetic error bars (of typi-cally several hundreds of kilometers) (e.g., Channell et al.,1979; Channell, 1996; Rosenbaum et al., 2004; Muttoni etal., 2013). Because the motion of Adria relative to Europewould be the best boundary condition to reconstruct the cen-tral Mediterranean kinematic history since the Mesozoic, itis crucial to reconstruct any past relative motions betweenAdria and Africa. Different approaches to this end, how-ever, led to contrasting results. The Ionian Basin’s sea flooris widely regarded as Mesozoic (e.g., Catalano et al., 2001;Frizon de Lamotte et al., 2011; Gallais et al., 2011; Schettinoand Turco, 2011; Speranza et al., 2012), implying a semi-rigid connection between Adria and Africa since that time.Eastward increasing Neogene shortening in the Alps (Schön-born, 1999; Schmid et al., 2013), however, has been usedto infer a Neogene∼ 20◦counterclockwise (ccw) rotation ofAdria relative to Eurasia (Ustaszewski et al., 2008), but only∼ 2◦ of which can be accounted for by Africa–Europe platemotion. Based on this kinematic model, therefore, Adria
Figure 1. Regional tectonic map of the Mediterranean region. AE –Adige embayment; AEs – Apulian escarpment; Ga – Gargano; HP– Hyblean Plateau; IAP – Ionian Abyssal Plain; II – Ionian Islands;Is – Istria; KFZ – Kefalonia Fault Zone; KP – Karaburun Peninsula;MAR – Mid-Adriatic Ridge; ME – Malta escarpment; TF – Tremitifault.
must have been decoupled from Africa during the Neogene.GPS measurements suggest that, at present, Adria moves ina northeastward motion relative to Africa (D’Agostino et al.,2008). These present-day kinematics are consistent with anortheastward motion of Adria versus Africa of 40 km overthe past 4 Myr inferred from kinematic reconstruction of theAegean region (van Hinsbergen and Schmid, 2012). Con-versely, Wortmann et al. (2001) argued for a Cenozoic 8◦
clockwise (cw) rotation of Adria versus Africa to avoid over-laps of Adria with Eurasia in pre-Cenozoic reconstructions,and Dercourt et al. (1986) postulated a 30◦ ccw rotation ofAdria relative to Africa between 130 and 80 Ma, assuming aCretaceous opening of the Ionian Basin.
Paleomagnetic data can provide useful quantitative con-straints on the vertical axis rotation history of Adria. How-ever, published results from Adria’s sedimentary coveryielded contrasting interpretations, involving (i) no rotation(Channell and Tarling, 1975; Channell, 1977), and assum-ing this, paleomagnetic data from Adria have been used to
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D. J. J. van Hinsbergen et al.: Did Adria rotate relative to Africa? 613
infer the African Apparent Polar Wander Path (APWP) forparticularly Early to Middle Mesozoic times (Muttoni et al.,2005, 2013; Channell et al., 2010) (ii) 20◦ cw rotation since30 Ma (Tozzi et al., 1988), (iii) 20◦ ccw rotation since theLate Cretaceous (Márton and Nardi, 1994), or (iv) more com-plex models where a 20◦ ccw Early–Late Cretaceous rotationwas followed by a Late Cretaceous–Eocene 20◦ cw rotationand a post-Eocene 30◦ ccw rotation (Márton et al., 2010).
In this paper, we present a new paleomagnetic study ofthe Lower Cretaceous to Upper Miocene stratigraphy of theApulian carbonate platform (southern Italy). We compare ourresults to and integrate these with published data sets, andevaluate the range of paleomagnetically permissible rotationsvalues in terms of their kinematic consequences for centralMediterranean region reconstructions.
2 Geological setting
Prior to the onset of Africa–Europe convergence in the Mid-Mesozoic, Adria was much larger continent stretching fromthe Italian Alps to Turkey (Vlahović et al., 2005). Gaina etal. (2013) introduced the term “Greater Adria” for the wholecontinental lithosphere including many Mesozoic intracon-tinental rift basins and platforms that are now incorporatedin the surrounding fold–thrust belts and that existed betweenthe Vardar ocean (or Neotethys) and the Ionian Basin.
Greater Adria was separated from Eurasia in the north-east by the Triassic Vardar, or Neotethys Ocean (Schmid etal., 2008; Stampfli and Hochard, 2009; Gaina et al., 2013),and in the north and west by the Jurassic Piemonte–Ligurianocean, or Alpine Tethys Ocean (e.g., Frisch, 1979; Favre andStampfli, 1992; Rosenbaum and Lister, 2005; Handy et al.,2010; Vissers et al., 2013). To the south the Ionian Basin sep-arated Adria from Africa (Fig. 1). Adria’s conjugate marginacross the Ionian Basin is likely the Hyblean Plateau of Sicilybounded to the east by the Malta escarpment (Catalano etal., 2001; Chamot-Rooke and Rangin, 2005; Speranza et al.,2012).
Before the Calabrian subduction zone retreated away fromSardinia in the late Miocene (Faccenna et al., 2001, 2004;Cifelli et al., 2007; Rosenbaum et al., 2008), the Ionian Basinextended farther to the northwest. This oceanic lithospherewas at least Jurassic in age, as evidenced by off-scrapedsediments now exposed in Calabria (Bonardi et al., 1988).The modern Ionian Basin is floored by a> 5 km thick se-quence of sediments, which in the west have been thrust inresponse to subduction below Calabria (the Calabrian ac-cretionary prism), and in the east in response to subduc-tion below the Aegean region (the “Mediterranean ridge”)(e.g., Finetti, 1985; Reston et al., 2002; Minelli and Fac-cenna, 2010; Gallais et al., 2011; Speranza et al., 2012). TheIonian Abyssal Plain is the only relatively undeformed por-tion that serves as the foreland of the central Mediterraneansubduction systems (Hieke et al., 2006; Gallais et al., 2011;
Speranza et al., 2012). Given the crustal thickness of 7–9 km(Chamot-Rooke and Rangin, 2005) and very low heat flow(Pasquale et al., 2005), this ocean floor is likely an old rem-nant of the Neotethys Ocean (e.g., Gallais et al., 2011; Sper-anza et al., 2012). The age of the Ionian Basin has been es-timated to range from late Paleozoic to Cretaceous (Sengöret al., 1984; Dercourt et al., 1986; Robertson et al., 1991;Stampfli and Borel, 2002; Golonka, 2004; Frizon de Lam-otte et al., 2011; Gallais et al., 2011; Schettino and Turco,2011), with the most recent suggestion giving a Late Triassicage (Speranza et al., 2012).
Despite the uncertainties on the opening age and direction(NE–SW according to Chamot-Rooke and Rangin, 2005, andSperanza et al., 2012, or NW–SE according to Frizon deLamotte et al., 2011, and Gallais et al., 2011), there is gen-eral agreement that the Ionian Abyssal Plain has not beenstrongly deformed since the Middle Mesozoic. Minor lateMiocene inversion was associated with only a few kilometersof shortening (Gallais et al., 2011). The Malta escarpmentgently dips towards the basin floor, and has not been reacti-vated since the Mesozoic except in the northwest. There, theescarpment was used since Pliocene times as a subductiontransform edge propagator (STEP) fault (Govers and Wortel,2005), accommodating Calabrian trench retreat (Argnani andBonazzi, 2005). Late Miocene and younger NE–SW exten-sion, however, has been documented within the African pas-sive margin, forming the∼ 140 km wide Sicily Channel riftzone (Argnani, 2009) between Sicily and the Tunisian coast(Fig. 1). This rift system is associated with up to∼ 40 %crustal thinning and contains active rift-related volcanoes(Civile et al., 2008). The dimension of the rifted zone andthe crustal attenuation may indicate some tens of kilome-ters of extension. This system connects southeastward to theSirte and Tripolitania basins of Libya (Capitanio et al., 2011)and was interpreted to result from renewed late Miocene andyounger NE–SW extension between Adria (and the IonianBasin) and Africa. This extension was likely caused by slab-pull forces of the subducting African plate (Argnani, 1990;Goes et al., 2004; Civile et al., 2010; Capitanio et al., 2011;Belguith et al., 2013).
Our study area, the Apulian carbonate platform, hereaftercalled “Apulia” (Fig. 2), is part of Adria and lies in thePlio-Pleistocene foreland of the Apennine fold–thrust beltto the west (D’Argenio et al., 1973). Recent NE–SW, low-magnitude extension evident from Apulia (Fig. 2) is inter-preted to result from flexural bending of the downgoing Adri-atic lithosphere into the Apennine subduction zone (Doglioniet al., 1994; Argnani et al., 2001). To the northeast, Apuliaborders the Adriatic Sea, which represents the late Miocene–Quaternary foredeep of the Dinarides–Albanides–Hellenidesbelt (de Alteriis, 1995; Argnani et al., 1996; Bertotti et al.,2001; Argnani, 2013). The southwestern margin of Apuliaappears to constitute a passive margin of the Ionian Basinin a narrow segment between the Calabrian prism and theMediterranean ridge along the Apulian escarpment (Finetti,
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614 D. J. J. van Hinsbergen et al.: Did Adria rotate relative to Africa?
Apenninic Front
Gargano
Murge
Salento
Apulia
PACR
CU
CN
PS
MA
TSOC; 4
MN; 2,3
1
5,6
8, 10, 11
79
Plio-Pleistocene clastics
Mesozoic-Cenozoic carbonatesSampling locality (this study, successful)
Sampling locality (published)
Figure 2
Otranto Channel
Adriatic Sea
Ionian Sea
Sampling locality (this study, unsuccessful)
50 km
Bari
Figure 2.A simplified geological map of the exposed Apulian fore-land (Apulia, southern Italy) indicating our new as well as previ-ously published paleomagnetic sampling sites (modified from Pieriet al., 1997). Numbers and codes correspond to sites listed in Ta-ble 1.
1985), where accumulation of sediment since the Mesozoichas compensated for the thermal subsidence of the oceaniclithosphere (Channell et al., 1979; Ricchetti et al., 1998).
The northern margin of the platform is exposed on theGargano promontory that was located close to the northeast-ern transition of Apulia toward the adjacent Adriatic Basin(Bosellini et al., 1999b; Graziano et al., 2013; Santantonio etal., 2013). The Adriatic Basin, from which the present-dayAdriatic Sea roughly inherited the location, was a Jurassicdeep-water continental rift basin that continued northwest-ward into the Umbria–Marche basin, now incorporated inthe Apennine fold–thrust belt, and southeastward into the Io-nian zone, which is now part of the Hellenides–Albanidesand should not be confused with the previously mentionedoceanic Ionian Basin, located on the opposite side of Apu-lia (Zappaterra, 1990, 1994; Flores et al., 1991; Mattavelliet al., 1991; Grandic et al., 2002; Picha, 2002; Fantoni andFranciosi, 2010). Basin-transition units of Apulia have inthe Pliocene and younger times become incorporated in thepre-Apulian zone of western Greece, exposed on the Io-nian Islands, which became separated from Apulia along theKefalonia Fault Zone (Underhill, 1989; van Hinsbergen etal., 2006; Royden and Papanikolaou, 2011; Kokkalas et al.,2012).
To the north of Apulia, in the central Adriatic Sea, thefronts of the external Dinarides and Apennines converge pro-ducing the Mid-Adriatic Ridge (Fig. 1). Along this structure,the Adriatic Sea floor is cut by Neogene NW–SE strikingthrusts, some of which invert Mesozoic extensional struc-tures (Grandic et al., 2002; Scrocca, 2006; Scisciani andCalamita, 2009; Fantoni and Franciosi, 2010; Kastelic et al.,2013). South of the Mid-Adriatic Ridge, several E–W to NE–
SW striking strike-slip structures have been suggested to dis-sect the Adriatic Sea floor. The exact location and kinemat-ics of these structures is controversial, but is primarily con-sidered dextral based on seismicity, low-resolution seismiclines, and GPS velocities (see below). As a result, three maindeformation zones, alternative to each other, were called intoconsideration to decouple northern and southern Adria. Thefirst one is the Pescara–Dubrovnik line (corresponding to theMid-Adriatic Ridge in Fig. 1), whose presence was hypoth-esized by Gambini and Tozzi (1996), and that roughly cor-responds to a segment of the boundary that, according toOldow et al. (2002), borders two fragments of Adria withdifferent GPS velocity. The second one is the Tremiti Line ofFinetti (1982) or the Tremiti Structure of Andre and Doul-cet (1991), whose presence is evident from both its seis-micity (Favali et al., 1990, 1993) and sea-floor structure(Argnani et al., 1993). According to Doglioni et al. (1994)and Scrocca (2006), this dextral lithospheric structure seg-ments Adria in order to accommodate a differential slab re-treat, and according to Festa et al. (2014), its subsurface ex-pressions were enhanced by salt tectonics; (iii) finally, alsothe Mid-Adriatic Ridge was interpreted to be a boundary be-tween two different sectors of Adria (Scisciani and Calamita,2009), assuming that some structural highs of the externalDinarides (i.e., the Palagruza High of Grandic et al. (2002)represent the southward prosecution of the same ridge in theeastern Adriatic Sea.
Apulia was considered an isolated carbonate platform thatdeveloped away from emerged continents (D’Argenio et al.,1973) until the discovery of dinosaur footprints that sug-gested the presence of some continental bridges betweenApulia and other coeval exposed regions in Late Jurassicto Early Cretaceous time (e.g., Bosellini, 2002). During theMesozoic, shallow-water carbonate deposition was able tocompensate for the regional subsidence, and led to the ac-cumulation of a stratigraphic succession up to 6000 m thick(Ricchetti et al., 1998). The succession, whose Cretaceousinterval is widely exposed, consists mainly of dolomiticand calcareous rocks (Ricchetti, 1975). In the Murge area(Fig. 2), where its age has been best constrained (Spallutoet al., 2005; Spalluto and Caffau, 2010; Spalluto, 2011),the succession forms a monocline dipping gently towardsthe SSW, thus exposing younger rocks from NNE to SSW(Ciaranfi et al., 1988) (Fig. 2). This monoclinal succes-sion is deformed by gentle undulations and steep normaland transtensional faults with an overall NW–SE orienta-tion (Festa, 2003). The southernmost part of the exposedApulia (i.e., the edge of the Salento peninsula facing theOtranto Channel, Fig. 2) represents the position of the Meso-zoic platform margin (Bosellini et al., 1999b). It probablysharply passed to a southern intraplatform pelagic basin, rec-ognized in the subsurface of the submerged Apulia (Del Benet al., 2010). Post-Cretaceous carbonate rocks cropping outalong this Salento margin show well-preserved tens of me-ters thick clinoforms, i.e slope deposits that formed along
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D. J. J. van Hinsbergen et al.: Did Adria rotate relative to Africa? 615
PA 6a
0.00
0.01
0 100 200 300 400 500 600 700
Tota
l Mag
netiz
atio
n (A
m2/
kg)
Temperature (°C)
PS 5.1
100
300
0 100 200 300 400 500 600 700
Tota
l Mag
netiz
atio
n (A
m2/
kg)
Temperature (°C)
TS46
0.00
0.01
0 100 200 300 400 500 600 700
Tota
l Mag
netiz
atio
n (A
m2/
kg)
Temperature (°C)
Figure 3Figure 3. Thermomagnetic curves measured on a Curie balance (Mullender et al., 1993) for representative samples. Arrows indicate heating(red) and cooling (blue).
and rework rocks of the old Apulia margin (Bosellini et al.,1999b). These slope deposits reach up to 25/30◦ of primarynon-tectonic dip (Tropeano et al., 2004; Bosellini, 2006).
3 Paleomagnetic sampling, analysis and results
3.1 Sampling and laboratory treatment
We collected 456 samples from nine localities covering theCretaceous and Cenozoic carbonate stratigraphy of Apulia.Cores samples were collected with a gasoline-powered mo-tor drill, and their orientation was measured with a magneticcompass.
The samples were measured at the Paleomagnetic Lab-oratory Fort Hoofddijk of Utrecht University, the Nether-lands. The nature of the magnetic carriers was investigatedfor representative samples using an in-house developed hor-izontal translation-type Curie balance, with a sensitivityof 5× 10−9 Am2 (Mullender et al., 1993). Approximately60 mg of powder obtained from each sample was subjectedto stepwise heating–cooling cycles up to 700◦C.
For each locality, 8 to 10 samples were selected as pilotsamples, and of each sample two specimens were retrievedfor both thermal (TH) and alternating field (AF) demagneti-zation. AF demagnetization and measurement of the rema-nence were carried out using an in-house developed robo-tized sample handler coupled to a horizontal pass-through 2GEnterprises DC SQUID cryogenic magnetometer (noise level1× 10−12 Am2) located in a magnetically shielded room(residual field< 200 nT). Samples were demagnetized bystepwise AF treatment (alternating field steps: 5, 8, 12, 15,20, 25, 30, 35, 40, 45, 50, 60, 70, 80 and 100 mT). Thermaldemagnetization was performed in a magnetically shieldedoven using variable temperature increments up to 500◦C.After each heating step the remanence was measured witha 2G Enterprises horizontal 2G DC SQUID cryogenic mag-netometer (noise level 3× 10−12 Am2).
Thermal demagnetization treatment demonstrated to bemore effective for the sampled rocks as it provided more sta-
ble demagnetization diagrams than the AF technique. Theremaining samples of each locality were therefore thermallydemagnetized.
Demagnetization diagrams were plotted on orthogonalvector diagrams (Zijderveld, 1967), and the characteristicremanent magnetization (ChRM) was isolated via princi-pal component analysis (Kirschvink, 1980). Samples witha maximum angular deviation (MAD) larger than 15◦ wererejected from further analysis. Because secular variation ofthe geomagnetic field induces scatter in paleomagnetic di-rections whose distribution gradually becomes more ellip-soidal towards equatorial latitudes (Creer et al., 1959; Tauxeand Kent, 2004), we calculated site mean directions usingFisher (1953) statistics on virtual geomagnetic poles (VGPs)following procedures described in Deenen et al. (2011). Ateach locality a 45◦ cutoff was applied to the VGPs (Johnsonet al., 2008). The results were then filtered by the paleomag-netic quality criteria of the N-dependent reliability envelopeof Deenen et al. (2011). Mean values and statistical parame-ters are listed in Table 1.
3.2 Results
Curie balance results are noisy because of the very low in-tensities of these carbonates, and do not reveal meaningfulinformation about the carriers of the remanence. Upon closeinspection it can be seen that some new magnetic mineral iscreated upon heating, just above 400◦C. This points to thepresence of minor amounts of pyrite converted to magnetite.The cooling curves are higher than the heating curves, con-firming that new magnetic minerals were created that werenot fully removed upon heating to 700◦C (Fig. 3).
As a result of very low Natural Remanent Magnetization(NRM) intensities, nearly 30 % of the demagnetized speci-mens (167) show an erratic demagnetization pattern yieldedno interpretable directions. Nevertheless, a total of 298 de-magnetized specimens show a weak but stable and measur-able remanence. In general, the lowest temperature steps (orAF steps) show a viscous or present-day overprint (Fig. 4).
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616 D. J. J. van Hinsbergen et al.: Did Adria rotate relative to Africa?
Figure 4. Orthogonal vector diagrams (Zijderveld, 1967), showing representative demagnetization diagrams for all sampled sites. Except forOC, TS and MN all sites are in tilt-corrected coordinates. Closed (open) circles indicate the projection on the horizontal (vertical) plane.
After removing this overprint, the characteristic remanentmagnetization (ChRM) directions can be interpreted. Mostspecimens show interpretable results up to temperatures ofapproximately 400–450◦C. Above this temperature intensi-ties become too low or spurious magnetization occurs thathampers any further interpretation (e.g., Fig. 4g). Of the moresuccessful demagnetization diagrams, we use 8 to 10 succes-
sive temperature steps for the ChRM directions determinedby principal component analysis.
3.2.1 Locality Petraro quarry (PA)
The Petraro quarry (PA) is located in NE Murge (Fig. 2).This section shows the oldest part of the Calcare di BariFormation cropping out in the Murge area and consists of awell-bedded, 55 m thick, shallow-water carbonate succession
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D. J. J. van Hinsbergen et al.: Did Adria rotate relative to Africa? 617
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935
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114.
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2.0
2.9
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12.
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68
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56.
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110
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54.
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211
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19.
563
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6
www.solid-earth.net/5/611/2014/ Solid Earth, 5, 611–629, 2014
618 D. J. J. van Hinsbergen et al.: Did Adria rotate relative to Africa?
in which few-decimeter-thick carbonate beds are irregularlyalternated with a few-meter-thick dolomitic beds (Luperto-Sinni and Masse, 1984). Carbonate lithofacies are made upof biopeloidal wackestones/packstones and microbial bind-stones with rare intercalations of biopeloidal and ooliticgrainstones interpreted as formed in inner shelf peritidal en-vironments. Dolomites consist of an anhedral or subhedralmosaic of dolomitic crystals, which totally or partly replacedthe carbonate precursor. Based on the study of the micro-fossiliferous assemblage of PA (mostly benthic foraminifersand calcareous algae), Luperto-Sinni and Masse (1984) as-sign this succession to the Valanginian (∼ 140–136 Ma; ac-cording to the geological timescale of Gradstein et al., 2012).We sampled a 10 m thick interval of this section and avoideddolomitic beds.
The NRM intensity of these samples is very low (30–300 µA m−1), and stable ChRMs were isolated for only 39specimens at temperature steps between 220 and 500◦C(Fig. 4a–c). The ChRMs show both normal and reversepolarities, and yield a positive reversal test (Johnson etal., 2008; McFadden and McElhinny, 1990) (classificationC; γ = 15.9< γc = 19.5). The distribution of the ChRMssatisfies the quality criteria of representing Paleo-SecularVariation (PSV) (i.e., A95min < A95< A95max; Deenen etal. (2011). The tilt-corrected mean ChRM direction for thislocality after a fixed 45◦ cutoff is D ± 1D = 130.8± 8.5◦,I ± 1I = −23.4± 14.6◦ (N = 29, K = 11.5, A95= 10.9◦)(Table 1 and Fig. 5).
3.2.2 Locality Casa Rossa quarry (CR)
The Casa Rossa quarry (CR) is a large limestone quarryin the NE Murge area (Fig. 2), located SW of Trani. Theoutcropping section consists of a well-bedded, more than40 m thick, shallow-water carbonate succession. Similarlyto the Petraro quarry, carbonate beds consist of biopeloidalwackestones/packstones and microbial bindstones showingevidence of desiccation features (mud cracks and birdseyes)suggesting inner shelf peritidal environments. Interbeddedwith the carbonate lithofacies, there are few-millimeter-thickgreen shale intercalations interpreted as paleosols. Basedon the study of the microfossiliferous assemblage of CR,Luperto-Sinni and Masse (1984) assign this succession tothe Barremian to lower Aptian (∼ 129–121 Ma). We sam-pled a stratigraphic thickness of 20 m in the lower part ofthe outcropping succession. The low intensity of these rocks(5–100 µA m−1) did not allow us to obtain high-quality re-manence components because of high MAD values. The dis-tribution of the isolated ChRMs is highly scattered, failing allthe adopted quality criteria (Fig. 5). The locality is thereforenot considered for further analyses.
3.2.3 Locality Cavallerizza quarry (CU)
The Cavallerizza quarry (CU) is located in the westernMurge area (Fig. 2), close to the town of Ruvo di Puglia.The outcropping section shows rudist biogenic beds, lateCenomanian in age (∼ 98–94 Ma), belonging to the upper-most part of the Calcare di Bari Fm (Iannone and La-viano, 1980). Rudist beds are topped by a horizon of greenclays, 1 m thick, interpreted as a paleosol, which marks aregional unconformity covering the whole Turonian (∼ 94–90 Ma). Peritidal limestones of the Calcare di Altamura Fm,Coniacian–Santonian in age (∼ 90–83.5 Ma), overlie greenshales and mark the recovery of carbonate marine sedimen-tation after the Turonian subaerial exposure. A total of 43samples were collected from the lower, 15 m thick, grey-brown rudist limestones of the Calcare di Bari Fm. Ac-cording to Laviano et al. (1998), upper Cenomanian rud-ist beds cropping out in the Ruvo area record the progra-dation of a rudist-inhabited margin into a shallow intraplat-form basin. Samples are characterized by generally low in-tensities (10–290 µA m−1), but show interpretable demagne-tization diagrams (Fig. 4d–e). The mean tilt-corrected di-rection after applying a 45◦cutoff to the ChRM distributionis D ± 1D = 333.2± 7.1◦, I ± 1I = 44.9± 8.0◦ (N = 32,K = 16.8, A95= 10.2◦) (Table 1 and Fig. 5). The VGP scat-ter for this site is consistent with that expected from PSV(A95min < A95< A95max).
3.2.4 Locality Caranna quarry (CN)
The Caranna quarry (CN) is located in SE Murge (Fig. 2),close to the town of Cisternino. The outcropping sectionconsists of an about 20 m thick succession of thin-beddedpelagic chalky limestones (microbioclastic mudstones towackestones) containing planktonic foraminifers and calci-spheres. According to Pieri and Laviano (1989) and Luperto-Sinni and Borgomano (1989), these deposits formed in rela-tively deep-water, distal slope environments in late Campa-nian to early Maastrichtian times (∼ 78–69 Ma). All 45 sam-ples were collected from the lower part of the outcroppingsuccession. Only 30 % of the analyzed specimens yielded in-terpretable demagnetization diagrams because of the low in-tensity of the NRM (8–34 µA m−1). Stable ChRMs were iso-lated at low temperatures commonly not exceeding 280◦C(Fig. 4f–g), and their distribution provided a mean valueof D ± 1D = 2.3± 11.8◦, I ± 1I = 51.7± 10.7◦ (N = 15,K = 15.7, A95= 9.9◦) (Table 1 and Fig. 5). Although thedistribution of the ChRMs reflects a PSV-induced scatter, theobtained mean direction is not statistically different from thepresent-day field direction (PDF; Fig. 5) and is inconsistentwith the expected Cretaceous inclinations. It is very likelythat a recent magnetic overprint affected this site, and the ob-tained results are not considered further.
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D. J. J. van Hinsbergen et al.: Did Adria rotate relative to Africa? 619
Locality PATilt corrected
a)
Rev. test: Cγ= 15.9 < γc=19.5
Locality CR
b)Tilt corrected
Locality CUTilt corrected
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Locality PS In situ
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Locality TSIn situ (bedding dip is primary)
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Locality OCIn situ (bedding dip is primary)
h)In situ (bedding dip is primary)
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Locality MN1 Locality MN2In situ (bedding dip is primary)k)
Locality MN3In situ (bedding dip is primary)
l)
Figure 5Figure 5. Equal area projections of the VGP (left) and ChRM directions (right) of all sites in both in situ and tilt-corrected coordinates. Open(closed) symbols correspond to the projection on the upper (lower) hemisphere. Large dots in the ChRM plots indicate the mean directionand relative cone of confidence (α95). Red (small) dots indicate the individual directions rejected after applying a 45◦ cutoff. Green asterisk(∗) indicates the present-day geocentric axial dipole (GAD) field direction at the sampled location. Sites TS, OC, and MN were sampled insediments with a primary bedding attitudes, and should be considered in in situ coordinates.
3.2.5 Locality Porto Selvaggio cove (PS)
The succession of the Porto Selvaggio cove (PS) crops outin western Salento. It mostly consists of upper Campa-nian chalky limestones (∼ 78–72 Ma), slightly dipping to theSE, overlying subhorizontal shallow marine limestones anddolomites (Reina and Luperto-Sinni, 1994a). According toMastrogiacomo et al. (2012) chalky limestones sampled inthis study formed in an intraplatform basin and record theevidence of a synsedimentary tectonic activity, as shown by
the occurrence of two horizons of soft-sediment deformationstructures (slumps). Out of the 52 demagnetized specimens,48 yielded interpretable diagrams for the calculation of theChRMs (Fig. 4h–i). The NRM of those samples is character-ized by relatively low intensities (10–2000 µA m−1) and bothnormal and reversed ChRMs that did not pass the reversal test(γ = 29> γc = 14.7) (McFadden and McElhinny, 1990).The mean normal polarity ChRM shows, after a fixed 45◦
cutoff, a D ± 1D = 357.7± 10.3◦, I ± 1I = 45.4± 11.4◦
(N = 23,K = 10.1, A95= 9.2◦) (Table 1, Fig. 5), very close
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620 D. J. J. van Hinsbergen et al.: Did Adria rotate relative to Africa?
to the present-day field, and likely the result of a recent over-print. The reverse polarity ChRMs yield a mean value thatis statistically different from the present-day field direction(D ± 1D = 165.0± 8.9◦, I ± 1I = −18.4± 16.2◦, N = 14,K = 21.6, A95= 8.8◦; see Table 1). The distribution of thereverse polarity ChRMs satisfies our criteria. Accordingly,only the reversed polarity ChRM is used for further analyses.
3.2.6 Locality Massafra (MA)
This locality was sampled from a road cut close to thetown of Massafra in the south of Murge (Fig. 2). We sam-pled a 15 m thick stratigraphic interval mostly compris-ing well-bedded white to light-brown shallow-water lime-stones with a Maastrichtian age (72–66 Ma) (Reina andLuperto-Sinni, 1994b). Sampled limestones mostly con-sist of peritidal, mud-supported, biopeloidal mudstonesand wackestones showing a benthic microfossiliferous as-semblage (mostly benthic foraminifers and ostracodes).The NRM intensity in those samples is relatively low(0.08–6 mA m−1), and only 18 samples yielded inter-pretable demagnetization diagrams (Fig. 4). The meandirection of the isolated ChRMs in tilt-corrected co-ordinates is D± 1D = 8.8± 10.7◦, I ± 1I = 46.6± 11.4◦
(N = 17, K = 15.2, A95= 9.4◦) (Table 1 and Fig. 5). Be-fore tilt correction, this direction is not statistically differ-ent from the present-day field (D ± 1D = 359.8± 12.6◦,I ± 1I = 52.2± 11.2◦, N = 17, K = 12.4, A95= 10.5◦)and is probably the effect of a recent overprint. Accordingly,this site is not considered for further analyses.
3.2.7 Locality Torre Specchialaguardia (TS)
An about 10 m thick succession of clinostratified brecciasand bioclastic deposits was sampled at the Torre Spec-chialaguardia locality (TS) in eastern Salento (Fig. 2).This succession belongs to the Upper Eocene (Priabonian,44–38 Ma) Torre Specchialaguardia Limestone Fm (Par-ente, 1994), which formed in a steep forereef slope on-lapping a rocky Cretaceous to Eocene paleocliff (Boselliniet al., 1999b). According to Parente (1994) and Boselliniet al. (1999b), this formation is the oldest non-deformedunit in eastern Salento, and its current dip of∼ 30◦ tothe ESE is a primary, non-tectonic orientation. A total of56 samples yielded NRM intensities ranging between 0.15and 3 mA m−1 and usually gave stable demagnetization di-agrams characterized by curie temperatures around 420◦C(Fig. 4l–n). The remanence displays both normal and re-verse polarities that pass the reversal test (McFadden andMcElhinny, 1990) (classification C,γ = 8.2 < γc = 11.7).After a fixed 45◦ cutoff, the mean in situ ChRM directionis D ± 1D = 356.0± 5.6◦, I ± 1I = 44.8± 6.3◦ (N = 47,K = 17.9, A95= 5.1◦) (Table 1, Fig. 5), and the ChRM dis-tribution satisfies our criteria.
3.2.8 Locality Castro (OC)
An about 10 m thick section was sampled close to the vil-lage of Castro (OC) in eastern Salento (Fig. 2). The out-cropping succession consists of Upper Oligocene (Chat-tian, 28–23 Ma) limestones belonging to the Castro Lime-stone Fm (Bosellini and Russo, 1992; Parente, 1994). Thisunit represents a fringing reef complex and shows a verywell-preserved lateral zonation of the reef subenvironments(Bosellini and Russo, 1992; Parente, 1994). The sampledsection shows clinostratified bioclastic deposits belongingto the reef slope subenvironment showing no evidenceof tectonic deformation (Bosellini and Russo, 1992; Po-mar et al., 2014). Very low NRM intensities character-ize these rocks (15–180 µA m−1), and stable ChRM com-ponents with maximum unblocking temperatures between220 and 500◦C were isolated from 31 specimens (Fig. 4o–p). The mean ChRM direction after a fixed 45◦cutoff isD ± 1D = 180.5± 3.2◦, I ± 1I = −44.2± 3.7◦ (N = 29,K = 85.8, A95= 2.9◦) (Table 1 and Fig. 5). The VGP dis-tribution does not entirely satisfy our criteria, since the A95value is lower than A95min, indicating that PSV is underrep-resented. The reverse polarity of the ChRMs and their lowinclinations excludes a present-day (or recent) overprint, andthe underrepresentation of PSV may be the result of someaveraging PSV within each limestone sample.
3.2.9 Locality Novaglie (MN1-3)
Three different sites belonging to the Lower Messinian suc-cession of the Novaglie Fm were sampled within 3 km ofeach other, close to the eastern Salento coast (Fig. 2). Theoutcropping successions consist of in situ coral reef biocon-structions, clinostratified breccias and associated bioclasticand lithoclastic prograding slope deposits and fine-grained,bioclastic base-of-slope calcarenites. Similarly to the pre-vious two localities, the bedding attitude in the sampledsites is most likely primary (Bosellini et al., 1999b, a, 2001;Vescogni, 2000). At each subsite 20 samples were collectedfrom a 10 m thick interval. NRM intensities range between9 and 5000 µA m−1. A total of 16, 13, and 6 ChRMs weresuccessfully isolated from subsite MN1, MN2, and MN3, re-spectively (Fig. 4q–s). Overall, the direction of the isolatedChRMs is substantially scattered, with both normal and re-verse polarities. The reversal test yielded a negative result(McFadden and McElhinny, 1990); therefore, separate meanvalues were calculated at each subsite.
After a fixed 45◦ cutoff, site MN1 yielded a meanpaleomagnetic direction of D ± 1D = 355.1± 12.5◦,I ± 1I = 61.1± 7.9◦ (N = 14, K = 19.5, A95= 9.2◦)(Table 1, Fig. 5). The VGP distribution passes our qualitycriteria. Only six specimens of MN2 yielded a poorly definedChRM, with a dispersion well beyond our quality criteria(Fig. 5). This subsite was discarded. The large scatter of theChRMs of subsite MN3 yields, after the 45◦cutoff, eight
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D. J. J. van Hinsbergen et al.: Did Adria rotate relative to Africa? 621
Locality OCNew data Tozzi et al., 1988
Tilt corrected Tilt correctedIn Situ
(bedding tilt is primary)In Situ
(bedding tilt is primary)
Figure 6Figure 6. Equal area projections of both in situ and tilt-corrected ChRMs from our site OC (left) and from the same locality of Tozzi etal. (1988) (right), illustrating the apparent clockwise rotation that would result from a tilt correction of the bedding at this locality. The stratahere have a primary dip (Fig. 7) and should be considered in in situ coordinates. Symbols are as in Fig. 5.
samples with a mean ChRM ofD ± 1D = 2 22.5± 13.7◦,I ± 1I = -33.2± 20.2◦ (N = 8, K = 19.0, A95= 13.0◦)(Table 1, Fig. 5). Despite the low number of specimens,the A95 envelope passes the Deenen et al. (2011) criteria(Table 1).
4 Discussion
4.1 Paleomagnetic constraints on the rotation of Adria
Reliable paleomagnetic poles were obtained from six locali-ties (out of nine) sampled throughout Apulia (Fig. 2). The re-sults from three localities were discarded because the distri-bution of the isolated ChRMs did not match the adopted qual-ity criteria or because of a present-day overprint. One moresite (MN3), although passing the quality criteria, yielded ananomalous declination (042.5± 13.7◦) indicating a strongclockwise rotation, not seen in the rest of the reliable sites.The anomalous direction at site MN3 may be explained con-sidering that the samples, collected in a forereef breccia,could represent a large fallen block within the Messinianslope deposits. Regardless of the cause of this local rotation,we consider this direction not meaningful for the analysis ofthe regional rotation of Adria.
The rotation of Adria and its relationship with the Africanplate has always been a moot point (Caporali et al., 2000;Márton et al., 2003, 2008). Our new data provide new con-straints for the rotation of Adria during the Cenozoic and,more importantly, can test the robustness and reliability ofthe available data set.
The results of the Oligocene site OC (Fig. 2) can be com-pared with those obtained by Tozzi et al. (1988) from thesame area. These authors interpreted the local∼ 30◦ east-southeastward bedding dip as a result of tectonic tilting, in-consistent with sedimentological studies (e.g., Tropeano etal., 2004; Bosellini, 2006), and calculated a post-Paleogene
∼ 25◦ cw rotation of Adria by restoring this bedding tothe horizontal. The paleomagnetic direction should be inter-preted in in situ coordinates, and our results as well as thoseof Tozzi et al. (1988) are coincident and indicate no or a mi-nor counterclockwise post-Oligocene rotation of Adria withrespect to Africa (Fig. 6).
To assess whether and when Adria rotated relative toAfrica, we combine our results with published data fromApulia, Gargano, Istria and the Adige embayment, and com-pare them to the expected directions for the European andAfrican plates calculated from the Global APWP of Torsviket al. (2012) using a reference location of 40.7◦ N, 17.2◦ E(Table 1, Fig. 7). Mean paleomagnetic directions and statisti-cal parameters from the existing database were recalculatedat each site by averaging VGPs obtained through parametricbootstrap sampling using the provided mean values and sta-tistical parameters (Table 1). This procedure overcomes theloss of information on the original data scatter that occurswhen only the mean paleomagnetic direction at a given lo-cality is computed by averaging site averages. In addition,sites with different numbers of samples should weigh differ-ently, since large data sets provide a better representation ofPSV than small data sets (see Deenen et al., 2011).
The updated paleomagnetic database is composed of 12poles from Apulia (6 from Tozzi et al., 1988; Scheepers,1992; Márton and Nardi, 1994, and the 6 successful polesfrom this study), 5 from the Gargano promontory (Channelland Tarling, 1975; Channell, 1977; Vandenberg, 1983; Sper-anza and Kissel, 1993), 12 poles from the Adige embayment(Márton et al., 2010, 2011) and 4 poles from the Veneto areain northern Italy (Agnini et al., 2006, 2011; Dallanave et al.,2009, 2012), and 8 poles from the Istrian peninsula of Croatia(Márton et al., 2003, 2008) (Table 1). At 6 out of 12 local-ities from the Adige embayment, PSV is underrepresented(A95< A95min; Table 1). We assume that this is a result ofwithin-sample averaging due to low sedimentation rates andhave included these sites in our analysis.
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622 D. J. J. van Hinsbergen et al.: Did Adria rotate relative to Africa?
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Figure 7. (A) Age (Ma, following the timescale of Gradstein et al.,2012) vs. declination plot for our new as well as published data fromAdria. The error envelopes for the African and Eurasian APWPs arefrom Torsvik et al. (2012). Vertical error bars correspond to the1Dcalculated at each site (Table 1); horizontal error bars correspondto age uncertainty. All data are recalculated to a reference location(45.0◦ N, 6.4◦ E) corresponding to the Neogene Apulia–Africa Eu-ler pole proposed by Ustaszewski et al. (2008). Numbers and siteabbreviations correspond to data entries in Table 1.(B) Same data,with a polynomial fourth-order trend line (purple dashed line) andthe declination component of a full-vector six-point moving averagewith error bars (1D) (red line).
Figure 7a shows all declinations vs. age, from all four sec-tors of Adria. Approximately 40 % of the poles are not statis-tically different from the expected African declinations. Theremaining poles, representing the majority of the data set,consistently show small counterclockwise deviations fromthe African APWP. The data provide no support for signif-icant rotations between the northern and southern sectors ofAdria.
To calculate the magnitude of rotation of Adria with re-spect to Africa, we combine the data sets from the differentregions. We used two approaches. One approach is to calcu-late a full-vector (six-point sliding window) moving averageat every data point, from which we determined theD valuesand a1D error envelope. The other approach is to calculatea (fourth-order) polynomial best fit based on declination val-
ues only (Fig. 7b). Both approaches show a remarkably coin-cident pattern that display a systematic ccw deviation of themean declination of Adria relative to Africa from the entireEarly Cretaceous to Late Cenozoic time interval. We interpo-lated the declination curve of the APWP of Africa (Torsviket al., 2012) to obtain the declination at the ages correspond-ing to our moving average, and determined the difference ateach data point. This yields an average deviation of all dataof 9.8± 9.5◦ ccw.
This obtained magnitude is accidentally comparable to thetotal rotation of Adria calculated from the Upper Cretaceousof the Adige embayment and Istria by Márton et al. (2010).These authors, however, interpreted their total rotation as theresult of two distinct phases of cw and ccw rotation. In par-ticular, an average of Eocene rocks was interpreted by Már-ton et al. (2010) to show 30◦ ccw rotation of Adria versusAfrica. They suggested a∼ 20◦ cw rotation of Adria betweenthe Cretaceous and Eocene, followed by a post-Eocene∼ 30◦
ccw rotation. These Eocene poles are included in our analy-sis, but taking all available data into account, we see no solidground for interpreting significant rotation phases betweenthe Early Cretaceous and the Late Cenozoic.
In summary, paleomagnetic data allow for a counterclock-wise rotation of Adria relative to Africa anywhere betweennegligible (1◦) and quite significant (18◦) values, but with avery consistent average of 9.5◦. The timing of this rotation isill constrained, but can be estimated from the average decli-nation shown in the graph of Fig. 7 to have occurred some-time in the second half of the Cenozoic, roughly 20± 10 Ma.
4.2 Regional kinematic implications
The rotation pattern of Adria as emerging from this studycan now be interpreted in the wider context of the centralMediterranean region. Our compilation of new and publishedpaleomagnetic data do not lend support to models that infereither large Cretaceous vertical axis rotations (Dercourt et al.,1986; Márton et al., 2010) or a small cw rotation (Wortmannet al., 2001). We observe that two major types of scenar-ios can be accommodated within the range of rotation docu-mented in this study (i.e., 1–18◦ ccw). One type of scenariois put forward from an Alpine point of view (post-20 Ma,∼ 20◦ ccw rotation of Adria relative to Europe around an Eu-ler pole in the western Alps, corresponding to a∼ 17◦ ccwrotation of Adria relative to Africa). The other type derivesfrom an Ionian Basin point of view (assuming near-rigiditybetween Africa and Adria and hence no differential rotation,according to Rosenbaum et al., 2004). The paleomagneti-cally permissible rotation range derived here can thereforenot discriminate the two end-member kinematic scenariosfor Adria. Accordingly, we will show the kinematic conse-quences of the permitted minimum and maximum rotationof Adria as a function of the location of its Euler pole.
An Euler pole for the relative motion between Adria andEurasia located at 45.0◦ N, 6.4◦ E, near the city of Turin was
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D. J. J. van Hinsbergen et al.: Did Adria rotate relative to Africa? 623
computed by Ustaszewski et al. (2008) based on westwarddecreasing Neogene shortening in the Alps, and northwardunderthrusting of Adria below the southern Alps. Their in-ferred 20◦ ccw rotation relative to Eurasia translates into apaleomagnetically permitted∼ 17◦ ccw rotation of Adria rel-ative to Africa. Assuming internal rigidity of Adria, a rotationaround this pole by 17◦ would require up to 420 km of ENE–WSW extension in the Ionian Basin measured at the mod-ern southeasternmost tip of stable Adria along the KefaloniaFault (Fig. 8a). This scenario would require that the entireIonian Basin is Miocene in age, inconsistent with any of theinferred ages that range from Permian to Cretaceous. Simi-larly, a 9.5◦ rotation of Adria (average rotation constrainedby our paleomagnetic analysis) would yield∼ 230 km ofENE–WSW extension, still much higher than what is geo-logically documented (Fig. 8b).
Adria could rotate ccw without extension in the IonianBasin if the Adria–Africa Euler pole is located in the farsoutheast of Adria (Fig. 8c). Assuming Adriatic rigidity, andapplying the 17◦ ccw rotation derived from reconstructionsof the Alps, this would, however, lead to a reconstructedoverlap of Adria and the Dinarides and Hellenides (whichwould suggest major Neogene extension in this area), andpredicts> 400 km of E–W convergence in the western Alps.In contrast to this scenario, geological data from the Dinar-ides and Hellenides show Neogene shortening, uplift and ex-humation (Bertotti et al., 2001; van Hinsbergen et al., 2006;Stojadinovíc et al., 2013), and the amount of Neogene short-ening in the western Alps is much smaller than required bythis scenario (e.g., Handy et al., 2010) (Fig. 8c).
Alternatively, we may explore the maximum amount ofrotation around the Euler pole constrained by Ustaszewskiet al. (2008) using constraints from the Ionian Sea and theAegean region. The∼ 40 % crustal attenuation in the 140 kmwide Sicily Channel (Civile et al., 2008) would suggesta (maximum) amount of NE–SW latest Miocene to Plio-Quaternary extension between Adria and Africa of∼ 40 km.A similar amount of Adria–Africa relative motion since theEarly Pliocene was inferred from a kinematic reconstructionof the Aegean region by van Hinsbergen and Schmid (2012)to avoid overlaps between Adria and the west Aegean fold–thrust belt. This corresponds to a 1.7◦ ccw rotation of Adria.This reconstruction is consistent with the geological recordof the circum-Ionian Basin, but it would require∼ 150 kmNW–SE-directed convergence between Adria and Europesince 20 Ma, to be accommodated in the western Alps, incontrast with widely accepted lower values of no more thansome tens of kilometers (Ustaszewski et al., 2008; Handy etal., 2010) (Fig. 8d).
Our discussion above identifies an inconsistency betweenthe kinematic interpretations from the geological record ofthe Alps and the Ionian Basin. Paleomagnetic data permitthe scenarios that fulfill the constraints from both regions, butthese scenarios cannot be reconciled with each other. More-
over, the average rotation suggested by the paleomagneticdata violates both end-member scenarios.
Since a key assumption in the above analysis is the rigid-ity of Adria, we explore a final scenario whereby we decou-ple northern and southern Adria, e.g., along the Mid-AdriaticRidge or along the Tremiti fault (Fig. 2), applying the recon-struction of Ustaszewski et al. (2008) for northern Adria (17◦
ccw rotation), and the reconstruction of van Hinsbergen andSchmid (2012) for southern Adria (1.7◦ ccw rotation). Thiswould require as much as 160 km of left-lateral strike-slipbetween northern and southern Adria, and none of the identi-fied structures appear likely candidates to accommodate suchmajor displacements (Fig. 8e).
The discussion above indicates that, although scenariosbased on kinematic interpretations from both the Alps andthe Ionian Basin infer Neogene Adria–Africa relative rota-tions that are within the range documented in this study, thesescenarios are mutually exclusive. Paleomagnetic data alonecannot solve this “Adriatic enigma”, but call for a reassess-ment of the kinematic evolution of three key areas, centeredaround the following questions: (i) since shortening recon-structions may underestimate the true amount of convergence– is the amount of Neogene shortening in the western Alpssignificantly underestimated? (ii) Is it possible to quantify thetiming and amount of displacement along strike-slip zonesthat may separate a northern and southern Adria block? (iii)Is it possible that there is a large amount of Neogene exten-sion along the Apulian escarpment, perhaps hidden below theadvancing Calabrian prism and the Mediterranean ridge?
5 Conclusions
We provide six new paleomagnetic poles from the LowerCretaceous to Upper Miocene of the Murge and Salentoareas of the Apulian Platform, southern Italy. These newdata, combined with recalculated published poles from theGargano promontory, the Istrian peninsula of Croatia, and theAdige embayment of the southern Alps, constrain a counter-clockwise rotation of Adria relative to Africa at 9.8± 9.5◦,occurring sometime after 20± 10 Ma. Our revised paleo-magnetic database for Adria discards significant rotations ofAdria versus Africa between the Early Cretaceous and theEocene, as invoked by several studies. The permissible rota-tion magnitude (1–18◦ counterclockwise) is consistent withtwo end-member models for the central Mediterranean re-gion requiring (i) a Neogene∼ 18◦ counterclockwise rota-tion of Adria relative to Africa (based on kinematic recon-struction of the Alps) or (ii) negligible rotation of Adriabased on kinematic reconstruction of the Ionian Basin. Al-though paleomagnetic data from Adria are not in disagree-ment with both models, we establish that these scenarios aremutually exclusive. We cannot solve this enigma, but call forkinematic studies that will address the following three keyquestions: (i) was Neogene shortening in the western Alps
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624 D. J. J. van Hinsbergen et al.: Did Adria rotate relative to Africa?
150 kmshorteningde�cit
ADR-EURADR-AFR
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Scenario 1: 20° ADR-EUR rotation (18° ADR-AFR)Pole constrained from the Alps (Ustaszewski et al., 2008)
420 km
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Albanides-Hellenides
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Scenario 2: 9.5° ADR-AFR rotationPole as in A; average rotation from paleomagnetism
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Albanides-Hellenides
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Scenario 4: 1.7° ADR-AFR rotationPole as in A; rotation based on maximum extension in Sicily Channel
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Albanides-Hellenides
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Scenario 3: 9.5° ADR-AFR rotation (average from paleomagnetism)Pole in SE of Adria (minimum ADR-AFR motion)
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AlpsCarpathians
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Albanides-Hellenides
Apennines
Africa
E
Scenario 5: Adria decoupled along Mid-Adriatic RidgeS Adria 1.7° ADR-AFR rot as in D; N Adria 18° ADR-AFR rot as in A
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40°N
50°N
10°E 20°E
40°N
10°E 20°E
40°N
50°N
Figure 8. Reconstructions at 20 Ma for rotation scenarios of Adria versus Africa that are permitted by paleomagnetic data, using(A) therotation pole of Adria versus Eurasia of Ustaszewski et al. (2008), and a 20◦ ccw rotation of Adria versus Europe, corresponding to 18◦ ccwrotation of Adria versus Africa, constrained from shortening reconstructions in the Alps by the same authors;(B) a 9.5◦ ccw rotation of Adriaversus Africa around the same pole corresponding to the paleomagnetically constrained mean rotation in this paper;(C) a 9.5◦ ccw rotationof Adria versus Africa corresponding to the paleomagnetically constrained mean rotation in this paper, around a pole located in the SE ofAdria and allowing minimum relative latitudinal motion between Africa and Adria;(D) a 1.7◦ ccw rotation of Adria versus Africa aroundthe pole of Ustaszewski et al. (2008), corresponding to the maximum amount of Neogene extension documented in the Sicily Channel;(E)decoupling northern and southern Adria along the Mid-Adriatic Ridge of Scisciani and Calamita (2009), with northern Adria following ascenario as in(A), and southern Adria following a scenario as in(D). Reconstructions of (i) the Adriatic front of the Alps, Carpathiansand Dinarides follow Ustaszewski et al. (2008), (ii) the western Mediterranean region follow Faccenna et al. (2004) and van Hinsbergen etal. (2014), and (iii) the Aegean–Albanian region follow van Hinsbergen and Schmid (2012). Reconstruction is given in a Europe-fixed frame,with the position of Africa determined using rotation poles of Gaina et al. (2002) and Müller et al. (1999) for the North Atlantic and centralAtlantic Ocean, respectively.
Solid Earth, 5, 611–629, 2014 www.solid-earth.net/5/611/2014/
D. J. J. van Hinsbergen et al.: Did Adria rotate relative to Africa? 625
significantly underestimated? (ii) Was Neogene extension inthe Ionian Basin significantly underestimated? (iii) Was anorthern Adria block decoupled from a southern Adria blockalong a large-offset sinistral strike-slip fault? Resolving thesequestions may lead to a solution of the conundrum associatedwith the kinematics of Adria.
Acknowledgements.Fieldwork was funded through the Nether-lands research school of Integrated Solid Earth Sciences (ISES).D. J. J. van Hinsbergen and M. Maffione were supported byERC Starting Grant 306810 (SINK) and an NWO VIDI grant toD. J. J. van Hinsbergen. L. Sabato, L. Spalluto and M. Tropeanowere supported by Bari University funds (PRIN/COFIN 2009)grant to M. Tropeano. We appreciated reviews of Gideon Rosen-baum and an anonymous reviewer.
Edited by: J. van Hunen
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