New Cretaceous paleomagnetic results from the foreland of the Southern Alps and the refined apparent polar wander path for stable Adria

Tectonophysics 480 (2010) 57–72

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Tectonophysics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t e c t o

New Cretaceous paleomagnetic results from the foreland of the Southern Alps and the refined apparent polar wander path for stable Adria Emő Márton a,⁎, Dario Zampieri b, Paolo Grandesso b, Vlasta Ćosović c, Alan Moro c a b c

Eötvös Loránd Geophysical Institute of Hungary, Palaeomagnetic Laboratory, Columbus u. 17-23, H-1145 Budapest, Hungary University of Padova, Department of Geosciences, Via Giotto 1, 35137 Padova, Italy University of Zagreb, Department of Geology and Palaeontology, Horvatovac 102a, 10000 Zagreb, Croatia

a r t i c l e

i n f o

Article history: Received 22 May 2009 Received in revised form 8 September 2009 Accepted 10 September 2009 Available online 20 September 2009 Keywords: Stable Adria Tithonian–Maastrichtian APW Rotations

a b s t r a c t The central-western and the eastern Southern Alps are separated by the triangular shaped Adige embayment, which belongs to stable Adria and was the site of pelagic sedimentation from the Tithonian through Maastrichtian. The first part of this study presents paleomagnetic results from the Tithonian– Cenomanian Biancone and Turonian–Maastrichtian Scaglia Rossa formations sampled at 33 geographically distributed and biostratigraphically dated localities. The new and high quality paleomagnetic results from the Adige embayment are then combined with coeval paleomagnetic directions from autochthonous Istria (Márton et al., 2008), which also belongs to stable Adria. The combined data set (which for the Late Albian–Maastrichtian time period is constructed similarly to the synthetic African curve by Besse and Courtillot, 2002, 2003) reveals an important tectonic event (Late Aptian–Early Albian) characterized by 20° CCW rotation and sedimentary hiatus. Comparison between paleomagnetic declinations/inclinations expected in an African framework (i.e. with the assumption that Adria is still an African promontory) leads to the following conclusions. The timedistributed Tithonian and Berriasian (150–135 Ma) paleomagnetic directions exhibit the “African hairpin” with an inclination minimum and a sudden change from CW to CCW rotation at 145 Ma. Concerning the younger ages, the declinations for Adria continue to follow the African trend of CCW rotation till the end of Cretaceous. However, the Tithonian–Maastrichtian declination curve for stable Adria is displaced by 10° from the “African” curve as a result of two rotations. The first, an about 20° CW rotation of Adria with respect to Africa took place between the Maastrichtian and the mid-Eocene. During this time the orientation of Adria remained the same, while Africa continued its CCW rotation. The younger rotation (30°CCW) changed the orientation of Adria relative to Africa as well as to the present North. © 2009 Elsevier B.V. All rights reserved.

1. Introduction It is well established today that the central-western and the eastern Southern Alps and their related foreland basins are separated by the triangular shaped region comprising the Lessini and Berici Mountains and the Euganei Hills (Bigi et al., 1990; Laubscher, 1996; Castellarin and Cantelli, 2000; Castellarin et al., 2006; Massironi et al., 2006) (Fig. 1). This region, bordered by NW- and NNE-trending transfer zones (Fig. 1) and unaffected by Alpine thrusting, with the unique exception of the Marana thrust at the north-eastern border (Fig. 2) is called “Adige embayment” by Laubscher (1996). In contrast, the central-western (Lombardian) and the eastern (Venetian) Southern Alps experienced an important post-collisional shortening starting with the Neogene, which was accommodated by thrusting of the upper crust of the Adria margin. Based on different structural models, the amount of shortening is estimated as <30 km (e.g. Viola ⁎ Corresponding author. Fax: +36 1 2480378. E-mail address: [email protected] (E. Márton). 0040-1951/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2009.09.003

et al., 2001; Castellarin et al., 2006) or >50 km (e.g. Laubscher, 1985; Schönborn, 1992, 1999). The shortenings for the Lombardian and the Venetian sectors are conceived as different, e.g. by Schönborn (1992, 1999) but others propose similar values for the two sectors (e.g. Castellarin et al. 2006). The Adige embayment is a unique place in autochthonous Adria, being the only emerged area of reasonably large size where typical basinal facies are accessible for paleomagnetic sampling. It is surprising, therefore, that no systematic and tectonically-oriented paleomagnetic study was reported from this region, although both the Tithonian–Cenomanian Biancone formation and the Turonian–Maastrichtian Scaglia Rossa formation, which are widespread in the Adige embayment, have been known as good targets e. g. in the Apennines for tectonically-oriented paleomagnetism as well as for magnetostratigraphy studies (e.g. Channell and Tarling, 1975, VandenBerg and Wonders, 1976, Lowrie and Channell, 1984, Cirilli et al., 1984). The efforts to obtain paleomagnetic constraints from direct measurements led to controversial interpretation for Gargano (compare Channell (1977) and VandenBerg (1983)) or produced a mixture of statistically well and poorly defined paleomagnetic directions

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Fig. 1. Simplified structure of the Southern Alps. Geometrical relationship with the southern foreland zones (Po plain and N Apennines), after Castellarin et al. (2006). The central inset shows the study area of Fig. 2, i.e. the Adige embayment, while the left corner inset shows the main Mesozoic palaeogeographic elements, still well preserved in the South Alpine domain.

(Márton and Veljović, 1983), due to the extreme weakness of the paleomagnetic signal in platform carbonates. The targets of renewed efforts in the 1990s were the platform carbonates from Murge and provided positive results only for the Cenomanian–Turonian (Márton and Nardi, 1994). More recently, the platform facies from stable Istria were systematically investigated and yielded good paleomagnetic directions for the Tithonian–Coniacian interval (Márton et al., 2008) and for the Eocene (Márton et al., 2003). In order to obtain an even more detailed APW for stable Adria the Tithonian through Maastrichtian rocks of the Adige embayment were also subjected to a systematic paleomagnetic study. In this paper we are reporting the results of this work. Nevertheless, the final goal is to combine the two data sets so that a robust and as detailed as possible APW is defined from direct paleomagnetic observations for this tectonically important stable block of the Mediterraneum. 2. Geological background The Alpine orogenic belt originated from the Late Cretaceous–Present convergence between the Adriatic upper plate and the subducting European lower plate (e.g., Dewey et al., 1989; Kurz et al., 1998; Dal Piaz et al., 2003). It is composed of a Europe-vergent collisional wedge (Alpine domain s.s.) and a south-propagating fold-and-thrust belt (South Alpine domain) separated by a major fault system, the Periadriatic Lineament (Fig. 1). During the first stages of the Alpine orogeny (Late Cretaceous– Early Paleocene), the central and western Southern Alps constituted the slightly deformed hinterland of the Europe-vergent Austroalpine– Penninic collisional wedge. In the central-western area (Pre-Adamello belt, Fig. 1) there are south-vergent structures of pre-Middle Eocene

age (Brack, 1981). In contrast, the easternmost sector was deformed by Dinaric SW-vergent thrusts during Eocene–Early Oligocene (Doglioni and Bosellini, 1987). From the Miocene onward (Neoalpine phase), the whole of the Southern Alps was shortened as a southvergent fold-and-thrust belt, which developed as a retro-wedge (Doglioni and Bosellini, 1987; Castellarin and Cantelli, 2000). The Southern Alps are subdivided into two main sectors (Lombardian and Venetian) by the NNE–SSW-trending Giudicarie belt (Fig. 1). The western sector exposes a complete crustal section. The eastern sector exposes low-grade metamorphic basement and Permo-Mesozoic cover sequences, well represented in the classic Dolomites. In between the two sectors, the triangular shaped block of the Adige embayment comprising the Lessini and Berici Mountains and the Euganei Hills represents the “undeformed” foreland of the Southern Alps (Bigi et al., 1990; Castellarin et al., 2006) and thus the autochthonous core of the Adria plate. This block is bounded to the west by the Giudicarie belt, which being oblique to the N–S shortening of the Lombardian Alps, has accommodated sinistral transpression (Doglioni and Bosellini, 1987). The eastern boundary of the block is the NW–SE-trending Schio–Vicenza Line, which is a poliphase structure presently accommodating the NNW–SSE convergence between Adria and Europe by sinistral strike-slip kinematics (e.g. Castellarin et al., 2006; Massironi et al., 2006). Despite the Alpine shortening, the Southern Alps preserve the different paleogeographic units of the Mesozoic Adriatic passive margin. From east to west they are the Julian basin, the Friuli platform (part of the Adriatic Carbonate Platform, see Cati et al., 1989), the Belluno basin, the Trento platform and the Lombardian basin (e.g. Bertotti et al., 1993). The Adige embayment was part of the Trento platform, which however was drowned during Middle Jurassic, when the lacunose and condensed succession started to deposit on top of a seamount (Trento Plateau)

E. Márton et al. / Tectonophysics 480 (2010) 57–72

59

Fig. 2. Simplified geological map of the study area with the paleomagnetic sampling localities numbered. The numbers are used throughout the paper.

bounded by normal faults. Since the Tithonian, deep water sedimentation took place even on the Trento Plateau. The main Mesozoic pelagic units of the Adige embayment are the Rosso Ammonitico (Baiocian–Early Tithonian), the Biancone (Late Tithonian–Cenomanian) and the Scaglia Rossa (Turonian–Maastrichtian). During Tertiary the Adige embayment had an elevated structural position (“Lessini shelf” of Bosellini, 1989) where shallow water sedimentation alternated with periods of emersion. However, SE of the Riviera dei Berici fault (Fig. 2) basinal conditions occurred in the Eocene and afterwards (Massari et al., 1977). From stratigraphic and structural points of view, the Adige embayment shows prominent features of the Paleogene extension and magmatism (De Vecchi et al., 1976). The main extensional structure is the NNW-trending AlponeAgno graben located in the eastern Lessini Mountains and bounded to the west by the Castelvero master normal fault (Fig. 2). The extensional deformation produced a widespread network of normal faults either planar (domino style) or listric with low to moderate tilting of blocks (Zampieri, 1995). In the eastern Lessini Mountains mafic and ultramafic rocks erupted during the Late Paleocene–Late Eocene. The volcanic activity continued in the Early Oligocene east of Schio, in the Berici Mountains and in the Euganei Hills (De Vecchi et al., 1976). 3. Sampling and laboratory procedures In Trento plateau the Cretaceous interval is represented by the Biancone and Scaglia Rossa formations (Fig. 3). The first unit overlies the typical reddish nodular facies of the Rosso Ammonitico Superiore (RAS) of Tithonian–Kimmeridgian age. The Biancone formation is characterized, at the bottom, by a rapid decrease in nodularity, a change in colour from pink to white, and by the occurrence of red chert nodules. The typical Biancone facies is represented by white thin

bedded micritic limestone (locally pseudonodular or flaser bedding), with light brown to grey chert nodules and bands. The uppermost part of the formation contains grey or dark grey marly levels between beds of bioturbated grey-white limestones, with black chert lenses. A few decimetre of black shale is present at the top (Bonarelli level of latest Cenomanian age). A generalized stratigraphic gap exists, with variable width, in the Late Aptian–Early Albian. The thickness of the Biancone ranges between 80 and 150 m. The Scaglia Rossa consists of well-bedded whitish-pink to pink and red micritic and/or marly limestones. In the lower part of the unit red cherty nodules and lenses are common, while red-brown shaley interbeds are frequent in the upper part, which is characterized by marly limestones with flaser bedding and sometimes by hard grounds marked by brown ferruginous-phosphatic crusts. In fact, the Campanian to Paleocene successions of the Trento plateau are characterized by a reduced thickness, wide stratigraphic gaps and condensation (Channel and Medizza, 1981). In the Trento plateau the thickness of the Scaglia Rossa ranges between 40 and 120 m. Several intervals of Biancone and Scaglia Rossa units show frequent stylolitic seams, which together with flaser structures (Fig. 3) suggest important compaction and volume loss of the sediment. In both units, the microfossils content is abundant and mostly consists of calpionellids, radiolarians, calcareous nannoplancton and planktonic foraminifera. Macrofossils are rare and mainly represented by echinoids, inocerams and less frequent ammonoids. Bioturbation is common throughout the units. In this work the biostratigraphic subdivision of the Cretaceous pelagic succession is based on calpionellids and on planktonic foraminifera. The species have been identified in thin sections of the same beds sampled for paleomagnetism. The adopted zonal scheme is drawn from Gradstein et al. (2004). It is a combination of biozones

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Fig. 3. Bio-Magneto- and Chronostratigraphic scheme (after Gradstein et al., 2004) correlated with the lithostratigraphic studied succession. Numbers refer to the sampling localities of Fig. 2 and Tables 1a, 1b. The variable amplitude of the stratigraphic hiatuses was established also from data of other authors (e.g. Massari and Medizza, 1973; Massari et al., 1977; Channel and Medizza, 1981).

proposed by several authors (e.g. Caron, 1985; Remane, 1985; Premoli Silva and Sliter, 1995) and it is calibrated with the paleomagnetic reversal sequence and correlated to the chronostratigraphic scale (Fig. 3). The zonal assignment of Hauterivian–Barremian samples is not well defined, because poorly significant foraminiferal assemblages have been recognized. Scaglia Rossa was sampled at a large number of localities, mostly abandoned quarries, which represent the Turonian–Maastrichtian time interval (Fig. 3). A number of quarries were visited, but some of them were not sampled because they looked chaotic due to syn-sedimentary tectonics or the strata were disintegrated along joints. At the eventually

sampled localities (Fig. 2, and Table 1b, localities 14–33) the beds were regular and gently dipping. In one quarry, however, close to the Schio– Vicenza fault sub-horizontal strata (locality 18, Fig. 2 and Table 1b) as well as steeply dipping beds of somewhat younger age were drilled. The latter represent part of a footwall drag fold of the steep poliphase Schio–Vicenza fault (locality 19, Fig. 2 and Table 1b). It was an interesting question if the beds in the footwall drag would show some evidence of relative rotation with respect to the undisturbed part and if so, in what sense. Another quarry (locality 23, Fig. 2 and Table 1b) was of special importance from the viewpoint of possible re-magnetization during Paleogene volcanism (here sub-horizontal limestone beds were intruded by a volcanic body),

E. Márton et al. / Tectonophysics 480 (2010) 57–72

Fig. 4. Susceptibility versus NRM intensity (both are locality mean values) plot for the studied localities from Adige embayment and from stable Istria. Note that the susceptibility is always negative for the latter and the NRM intensities are extremely weak.

61

since some of the Scaglia Rossa sampling localities, namely 15, 16, 22, 23 and 29 were relatively close to igneous bodies. From the Biancone, natural outcrops or road cuts (exception is locality 13, which is an abandoned quarry) were drilled, far away from known volcanic bodies. The samples are distributed over the time interval of Late Tithonian–Cenomanian (Fig. 3). From each bed drilled for paleomagnetic study, hand samples were also taken for micropaleontological investigation. The species were identified from thin sections and the ages assigned according to the zonal scheme proposed by Premoli Silva and Sliter (1995) and calibrated with the paleomagnetic reversal sequence and correlated to the chronostratigraphic scale (Fig. 3). The drill cores were oriented in situ with a magnetic compass. The cores were cut into standard-size specimens. The natural remanent magnetization (NRM) and the magnetic susceptibility of each specimen were measured in the Paleomagnetic Laboratory of ELGI, using JR-4 and JR-5A spinner magnetometers and a KLY-2 susceptibility bridge. Sister specimens from each locality were selected for

Fig. 5. Typical demagnetization curves for the Scaglia Rossa and for the Biancone. AF demagnetizations: SA734A, SA839 and SA1021A (Zijderveld diagrams accompanied by intensity (circles) versus demagnetizing field diagrams); thermal demagnetizations: SA763A and SA1070 (Zijderveld diagrams accompanied by NRM intensity (circles)/susceptibility (dots) versus temperature diagrams). Key to Zijderveld diagrams: full dots: projection of the NRM vector onto the horizontal, circles: into the vertical. Geographical system.

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E. Márton et al. / Tectonophysics 480 (2010) 57–72

detailed stepwise demagnetization. One of the sisters was subjected to alternating field (AF), the other to thermal demagnetization. Based on the behaviour of the selected samples, the remaining samples from each locality were demagnetized in several steps with one of the above methods. In general, white and grey samples responded well to AF, while the red or pink samples required thermal demagnetization. During thermal demagnetization, possible mineralogical changes were monitored by re-measuring susceptibility after each heating step. Although it was possible to gain information for the carrier of the NRM from the thermal demagnetization curves, special magnetic mineralogy experiments were also carried out, like isothermal remanent magnetization (IRM) acquisition experiments followed by

the stepwise thermal demagnetization of the three-component IRM (method by Lowrie, 1990) accompanied by susceptibility monitoring. Susceptibility was mostly in the range of 10− 5 SI. Higher values were only found for the red coloured varieties, where measurements of the magnetic anisotropy (AMS) revealed extremely weak anisotropy, invariably less than one percent. 4. Paleomagnetic results The NRM intensities of both the Biancone and the Scaglia samples are fairly high in the category of sediments (Fig. 4). Both NRM intensity and susceptibility is the highest for the red (pink) Scaglia, low positive values characterize the whitish variety of the Scaglia and

Fig. 6. AF and thermal demagnetization curves for sister specimens for a Scaglia Rossa sample documenting that the hard component (showing no CCW rotation)-point of magnetite both in direction and in polarity, from CCW rotated normal, to slightly CW rotated reversed polarity remanence.

E. Márton et al. / Tectonophysics 480 (2010) 57–72

63

zation of the 3-component IRM (Lowrie, 1990) combined with the monitored susceptibility suggest that the soft mineral is magnetite and the hard magnetic phase is hematite (Fig. 7) The hematite is a secondary mineral, for during the thermal demagnetization the NRM either decays by the Curie-point of the magnetite or the direction changes (e.g. Fig. 6, SA 772) from CCW rotated to a “non-rotated” direction (secondary remanence of invariably reversed polarity).

the Biancone samples. In comparison, the earlier studied platform carbonates from Istria (Márton et al., 2008) are diamagnetic and have extremely low NRM intensities (Fig. 4). Both the Biancone and Scaglia samples exhibit excellent demagnetization behaviour. The white and grey samples respond very well to AF demagnetization, while the red and pink ones usually need thermal treatment. Sometimes the NRM is of single component (e.g. Fig. 5 specimen SA 734A), but the demagnetization curves more often reveal the presence of two components. Typical examples of the different types of the latter are shown in Figs. 5 and 6. There are cases when the component decaying towards the origin is overprinted by a small normal polarity component (Fig. 5 SA 839, SA 1021A, SA 1070) and the overprint is easily removed. Reversed polarity overprint is more often observed in the Scaglia samples. It is usually removed early during demagnetization (Fig. 5 SA 763A), but occasionally survives the decay of the remanence interpreted as the ancient one (Fig. 6). It is important to emphasize that even in the latter case, the Kirschvink (1980) analysis of the demagnetization curves permits to separate the more ancient component (showing CCW rotation to the present North) and the overprint to such a degree that the overall mean paleomagnetic directions based on the former have excellent statistical parameters (Tables 1a and 1b). At some localities, the overprint component of reversed polarity is present in most samples and also well-clustered (Table 2). Their significance will be discussed later, in the context of similar overprints observed elsewhere in the Adriatic region (Satolli et al., 2007; Márton et al., 2008). The isothermal remanence acquisition (IRM) experiments show the presence of a soft magnetic mineral either alone or in the company of a hard magnetic phase (Fig. 7). The stepwise thermal demagneti-

5. Discussion of the paleomagnetic results from the Adige embayment The locality mean paleomagnetic directions obtained (Tables 1a and 1b) are based on independently oriented samples, which were fully demagnetized, subjected to component analysis and are characterized by excellent statistical parameters. The abundance of data allows the definition of overall mean paleomagnetic directions for several age groups of the Late Cretaceous, namely Cenomanian, Turonian–Early Coniacian, Coniacian, Late Coniacian–Santonian, and Campanian–Maastrichtian. For all these groups, the clustering of the directions improves on tilt corrections (Table 3). This also applies to localities 18 and 19, although the latter represents a large block from within the Schio–Vicenza drag fault. For the Early Cretaceous, tilt corrections slightly worsen the statistical parameters, probably because of the very shallow tilts, which may unfavourably influence the outcome of the test. Nevertheless, the possibility of re-magnetization must be also considered. This could have taken place during the Aptian– Albian which is a hiatus in the sedimentary record (Fig. 3). It is, however, unlikely, for Aptian–Albian belongs to the Cretaceous normal quiet interval, while the majority of the Early Cretaceus localities from the Adige

Table 1a Summary of locality mean palaeomagnetic directions for Biancone, based on the results of principal component analysis (Kirschvink 1980). Locality 1

2 3

4 5 6 7

8 9 10 11 12 13

D°

I°

α95°

k

α95°

Lat.N, Lon.E

n/no

Tithonian Arzere SA 1080–087

45° 35' 11" 11° 00' 38"

0/8

Berriasian Campofontana SA 1068–079 Corbellari SA 1060–067

45° 11° 45° 11°

38' 32" 09' 02" 37' 44" 69' 37"

10/12

116

− 38

91

5

121

− 44

91

5

76/8

7/8

124

− 38

463

3

124

− 38

463

3

Horizontal

Valanginian–Hauterivian M. Padella SA 1049–059 Novale SA 909–917 Laghi SA 1011–022 Fontana fredda SA 1088–095

45° 11° ⁎45° ⁎11° 45° 11° 46° 11°

37' 22" 09' 51" 40' 17" 17' 50" 32' 21" 07' 56" 12' 33" 39' 27"

10/11

124

− 42

147

4

116

− 38

147

4

235/11

9/9

128

− 37

82

6

128

− 38

82

6

194/2

12/12

114

− 35

60

6

118

− 49

60

6

100/14

8/8

315

+ 34

25

11

321

+ 44

31

10

95/16 95/12

Cenomanian M. Spinazzola SA 857–866 Chiampo SA 956–966 Canova di Cazzano SA 1023–036 S. Felice SA 1037–048 Miotti SA 1108–116 Cava Bomba SA 1096–107

⁎45° ⁎11° 45° 11° 45° 11° 45° 11° 45° 11° 45° 11°

22' 45" 40' 13" 33' 30" 16' 27" 27' 48" 12' 58" 28' 34" 11' 46" 39' 56" 18' 09" 16' 26" 39' 24"

0/10

Too weak

DC°

IC°

k

Large scatter

Dip 260/8

Horizontal

10/11

339

+ 35

200

3

344

+ 38

200

3

95/6

11/12

317

+ 35

94

5

316

+ 37

97

5

9/12

321

+ 31

233

3

324

+ 42

207

4

5/9

331

+ 25

419

4

340

+ 34

419

4

Horizontal 150/5 115/13 140/9 97/17

11/12

334

+8

248

3

329

+ 37

372

2

174/33 174/26

Localities are numbered according to Fig. 2. Key: Lat.N, Lon.E: Geographic coordinates (WGS84) measured by GPS except for those marked with ⁎, which were calculated on IGM 1:25 000 topographic sheets, n/no: number of used/collected samples (the samples are independently oriented cores); D, I (Dc, Ic): declination, inclination before (after) tilt correction; k and α95: statistical parameters (Fisher 1953).

64

E. Márton et al. / Tectonophysics 480 (2010) 57–72

Table 1b Summary of locality mean palaeomagnetic directions for Scaglia Rossa, based on the results of principal component analysis (Kirschvink 1980). Locality Turonian – Early Coniacian 14 Monticello Euganei SA 732–751 16 Lovertino SA 771–778 18 S. Vito di Leguzzano, undisturbed, SA 895–902 20 M. Bellocca SA 928–936

α95°

DC°

IC°

333

2

327

+ 33

+ 13

356

3

327

324

+ 36

212

4

9/9

343

+ 29

145

12/12

312

+ 32

14/17

316

α95°

IC2°

Dip

334

2

+ 39

+ 28

356

3

+ 33

Variable sub-horizontal 135/15

320

+ 35

212

4

+ 41

240/6

4

341

+ 36

145

6

+ 42

182/7

280

3

321

+ 37

280

3

+ 43

65/14

+ 23

42

6

338

+ 38

82

4

+ 44

Lat.N, Lon.E

n/no

D°

I°

⁎45° ⁎11° ⁎45° ⁎11° 45° 11° ⁎45° ⁎11°

23' 49" 35' 23" 20' 51" 37' 19" 40' 23" 22' 49" 32' 01" 12' 14"

20/20

330

+ 36

8/8

326

7/8

45° 11° 45° 11°

35' 28" 15' 37" 40' 23" 22' 49"

k

k

Coniacian 17 S. Pietro Mussolino SA 978–989 19 S. Vito di Leguzzano displaced by Schio–Vicenza fault, SA 903–908, 967–977 21 S. Pancrazio SA 752–759

⁎45° 24' 23" ⁎11° 33' 52"

8/8

330

+ 43

220

4

339

+ 34

220

4

+ 40

96/26 57/46 89/52 332/9

Late Coniacian – Santonian 22 Rovolon SA 845–856 23 Albettone SA 589–603 24 Pozzetto SA 825–836 25 Fornasetta SA 837–844 26 Cazzano di Tramigna SA 918–927

⁎45° ⁎11° 45° 11° ⁎45° ⁎11° ⁎45° ⁎11° ⁎45° ⁎11°

22' 34" 40' 11" 21' 48" 35' 53" 23' 33" 40' 02" 23' 37" 40' 22" 29' 09" 13' 11"

6/12

326

+ 28

177

5

329

+ 28

177

5

+ 33

100/2

6/6

344

+ 30

836

2

344

+ 30

836

2

+ 35

Horizontal

11/12

340

+ 36

368

2

336

+ 45

368

2

+ 52

190/11

8/8

316

+ 42

146

5

325

+ 36

75

6

+ 42

9/10

338

+ 38

127

5

340

+ 38

127

5

+ 44

340/11 43/14 74/2

Campanian 15 Monselice SA 453–457 27 Villaga SA 760–766 28 Casette, North SA 867–876 31 Ca' Barbaro SA 886–894 32 Teolo SA 1187–192 33 Teolo, hard ground SA 1165–172

⁎45° ⁎11° ⁎45° ⁎11° ⁎45° ⁎11° ⁎45° ⁎11° 45° 11° 45° 11°

15' 49" 44' 11" 23' 42" 32' 26" 14' 31" 42' 27" 13' 40" 42' 16" 20' 54" 40' 47" 20' 54" 40' 47"

4/5

339

+ 32

670

4

339

+ 40

670

4

+ 47

158/8

5/7

336

+ 32

85

8

333

+ 34

85

8

+ 40

230/6

10/10

340

+ 37

275

3

338

+ 34

275

3

+ 40

310/3

9/9

317

+ 47

48

7

312

+ 56

48

7

–

160/10

5/5

164

− 50

1218

2

157

− 39

1218

2

− 46

310/13

5/8

347

+ 57

1024

2

338

+ 46

1024

2

–

310/13 Mixed polarities

Maastrichtian 29 Monselice, red SA 448–452 30 Casette, South SA 877–885

⁎45° ⁎11° ⁎45° ⁎11°

15' 49" 44' 11" 14' 05" 42' 19"

4/5

339

+ 27

140

8

339

+ 34

140

8

+ 40

152/7

9/9

334

+ 36

374

3

336

+ 32

374

3

+ 37

4/4

Localities are numbered according to Fig. 2. Key: as for Table 1a. IC2°: corrected for inclination shallowing due to compaction (see text). Locality 31 left out from further evaluation, because of suspected slump.

embayment have reversed polarities (Table 1a). Re-magnetization during Paleogene volcanism is not likely either. First of all, there is no evidence of any intrusion or lava flow near the Early Cretaceous localities. But even if we assume that hidden volcanic bodies are not far away, the chances are very small that they could re-magnetize limestones, except very close to an intrusion. This follows from a test carried out in a quarry (locality 23) where a volcanic intrusion, the thermally altered limestone surrounding it and the limestone 100 m away from the volcanic body was sampled. The experiment clearly demonstrated that the limestone, at a distance of 100 m from the volcanic neck was not re-magnetized (Fig. 8). 6. Combination of coeval paleomagnetic results from Adige embayment and stable Istria Stable Istria is a low-lying remnant of the rigid Adriatic microplate situated in the North-eastern Adriatic region. During the Cretaceous stable Istria was the loci of shallow water sedimentation (Adriatic Carbonate Platform) while the region to the west was a basin where

pelagic sedimentation took place. The platform margin can be traced as a continuous and sinuous belt roughly trending north–south from Friuli towards the Adriatic Sea (Fig. 1). A small Neogene sinistral strike-slip movement along the NW-trending Schio–Vicenza fault can be assumed (e.g. Castellarin et al., 2006; Massironi et al., 2006), but the north–south component of a possible shift of Istria with respect to the Adige embayment is negligible. The above situation justifies, from geological point of view, the combination of the paleomagnetic directions from the two areas (Figs. 9 and 10). There are time-intervals, like the Tithonian–Berriasian, the Valanginian–Barremian, the Cenomanian and the Turonian–Early Coniacian when such combination is possible. However, Albian paleomagnetic results represent only stable Istria, while Coniacian, Late Coniacian–Santonian and Campanian–Maastrichtian only the Adige embayment. Coeval paleomagnetic directions from the two stable areas can be regarded as one population till the Turonian (Fig. 9), which is the onset of the Scaglia sedimentation. Agreement between the declinations from

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65

Table 2 Statistically well-defined reversed polarity overprint components observed on Late Jurassic–Cretaceous carbonates from stable Adria and overprint components of similar directions from the Umbria–Marche area. α95°

DC°

IC°

k

63 401 111

8 6 4

190 242 179

− 68 − 65 − 60

63 401 111

− 67 − 63

270 270

3 4

177 210

− 85 − 64

190

− 70

67

9

190

204 211 182 199

− 29 − 52 − 51 − 52

4.3 9.9 14.1 25.3

22 10 10 7

208 213 109 29

Locality

n/no

D°

I°

k

Adige embayment Villaga, SA 760–766 Lovertino, SA 771–778 Rovolon, SA 845–858

6/7 3/8 12/12

198 209 175

− 63 − 65 − 58

10/25 6/6

185 174

5/5

14 25 16 20

Stable Istria Kamenjak, Njive HR 506–517 Kamenjak, Grakalovac HR 488–493 Lanterna, HR 628–643 Umbria–Marche Gargo a Cerbara Contessa Arcevia Bosso

α95°

Dip

Reference

8 6 4

230/6 135/15 100/2

Present study Present study Present study

270 270

3 7

85/10 106/17

Márton et al. 2008 Márton et al. 2008

− 67

67

9

15/3

Márton et al. 2008

+ 12 − 32 − 65 − 78

4.0 8.7 14.5 14.4

23 10 10 9

Satolli Satolli Satolli Satolli

et al. et al. et al. et al.

2007 2007 2007 2007

Key: as for Table 1a.

the two areas is also observed for the Turonian–Early Coniacian but the inclinations are clearly separated. The shallower inclinations for the Scaglia (Fig. 10) can be either due to unremoved overprint or the consequence of compaction. As it was mentioned earlier, reversed polarity overprint was isolated in several samples at a number of localities from the Adige embayment as well as from Istria (Table 2). The overprint component postdates the deformation (Fig. 11) and is similar in direction and polarity to what was observed earlier by Márton and Nardi (1994) in Apulia and by Satolli et al. (2007) in the Umbria–Marche area (Table 2). Such overprint, even if not in a pure form, appears at several of the presently studied localities, thus it must be widespread in the Adriatic region. In order to see if overprint can explain shallower than expected inclinations in stable Adria, we treated separately the Scaglia localities, where the reversed

polarity overprint was detectable (seven localities, mostly red and pink varieties) and where the NRM was single component (11 localities, whitish varieties). The difference in inclinations between the two groups is two degrees (while the declinations are exactly the same), the former being shallower. However, this value is well within the limit of error, since α95 for the two groups are 5° and 4°, respectively. It is not likely, therefore, that overprinting can explain the lower than “African” inclinations for Adria. Compaction, on the other hand, is a likely mechanism, since the porosity of the Scaglia Rossa is reduced from 80% to 60% at the depth of 200 m (Tucker and Wright, 1990) while it is absent in the platform carbonates of stable Istria, due to early cementation. Correction for inclination shallowing due to compaction in the Scaglia Rossa was obtained with two methods. For the white varieties, a magnetic

Fig. 7. Examples of IRM acquisition, the behaviour of the three components of IRM (Lowrie, 1990) on stepwise thermal demagnetization and the susceptibility monitored during heating. The components of the IRM were acquired in fields of 1T (squares), 0.36T (dots) and 0.12T (circles), respectively.

66

E. Márton et al. / Tectonophysics 480 (2010) 57–72

Table 3 Summary of the overall mean paleomagnetic directions for the Adige embayment before and after tilt corrections.

Berriasian – Hauterivian Cenomanian Turonian – Early Coniacian Coniacian Late Coniacian – Santonian Campanian – Maastrichtian

N

D°

I°

k

α95°

DC°

IC°

6

304

+ 38

140

6

305

+ 42

101

7

5 5

329 326

+ 27 + 30

35 41

13 12

331 327

+ 38 + 35

72 115

9 7

+ 41

3 5

329 333

+ 32 + 35

27 55

24 10

339 335

+ 36 + 36

1191 76

4 9

+ 42 + 41

6

339

+ 40

47

10

337

+ 37

218

4

+ 41

Key as for Table 1a. and N is number of localities.

α95°

k

IC2°

approach was possible. Corrections were estimated from the anisotropy of the anhysteric remanence (AARM) using the method by Jackson et al. (1991). Although F test for AARM measurements was negative (values varied between 0.7 and 0.9), the foliations were near-horizontal in the tectonic system (Fig. 12), which is a condition set to the applicability of the method. The correcting factors calculated for the Scaglia Rossa localities are between 1.11 and 1.26, and the average is 1.175. The specimens studied from Istria, in order to see if the inclinations are really unaffected by compaction, had even lower F values (around 0.3), AARM foliation planes were never horizontal and repeated measurements for the same specimen yielded differently oriented ellipsoids. These are in line with the lack of compaction due to early diagenesis in platform carbonates. In the red and pink varieties of the Scaglia Rossa, where the abundance of secondary hematite prevented the application of the magnetic method, the correction was estimated by comparing the inclinations for a 0.5 m thick hard ground and for the compacted Scaglia (about 5 m below the

Fig. 8. Albettone, locality 23. Demagnetization behaviour documented by Zijderveld diagrams and accompanying decay diagrams (for Key, see Fig. 5) and comparison of the paleomagnetic mean directions for the limestone, 100 m away from the intrusion (normal polarity) and for the volcanic intrusion and the contact limestone (both are of reversed polarities).

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67

Fig. 9. Locality mean paleomagnetic directions with α95 (circles) for Tithonian–Berriasian, for Valanginian–Barremian, for Albian and for Cenomanian age groups from the Adige embayment (squares) and from stable Istria (diamonds) before and after tilt corrections. Stereographic plots.

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Fig. 10. Locality mean paleomagnetic directions with α95 for Turonian– Early Coniacian, for Coniacian, for Late Coniacian–Santonian and for Campanian–Maastrichtian age groups. Key as for Fig. 9. Inclinations for the Adige embayment are without correction for inclination shallowing. Note that in some cases α95 are so small that the squares with encircling α95 look like dots.

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69

Fig. 11. Tilt test showing that the reversed polarity overprints (Table 2) from the Adige embayment and from stable Istria were acquired after deformation, since the best K statistics is observed at zero unfolding.

hard ground) from the same section. This method yielded the same factor (1.17) as the magnetic method. By applying the factor of 1.17, the paleomagnetic inclinations for Turonian–Early Coniacian Scaglia move closer to the ones obtained for the platform carbonates and naturally, the younger Cretaceous inclinations for the Scaglia Rossa from the Adige embayment also become steeper (Table 1b column IC2).

7. Discussion and conclusions The paleomagnetic directions from geographically distributed localities in basin (Adige embayment) and in platform (stable Istria) carbonates are first combined according to their stratigraphic ages (Table 4). For such groups the statistical parameters are better after (tectonic system) than before (geographic system) tilt corrections. When the so defined overall mean paleomagnetic directions are plotted on a stereonet (Fig. 13), a sudden change in declination is seen suggesting an about 20° CCW rotation of stable Adria, which took place during the time of emersion and sub-aerial erosion in Istria (Márton et al., 2008) and submarine erosion in the foreland of the Southern Alps (Fig. 3). The coincidence of the large and relatively fast CCW rotation (Fig. 13) with stratigraphic hiatus implies that we are dealing with the most important Cretaceous tectonic event in the Adriatic region. In the future, it will be an interesting task to trace this event in the mobile areas surrounding stable Adria (Apennines, Southern Alps. Dinarides) and in tectonic units which were originally connected to Adria, but later drifted away from it (e.g. Northern Calcareous Alps, Transdanubian Range). Within the Alpine–Mediterranean region the paleomagnetic directions (in the tectonic system) of Table 4 are convenient as

Table 4 Summary of the overall mean paleomagnetic directions for the stable Adria (Adige embayment + stable Istria) before and after tilt corrections.

Fig. 12. Principal directions of the AARM ellipsoid for white coloured Scaglia Rossa samples from the Adige embayment. Tectonic system, stereographic plot. Key: maximum-square, intermediate-triangle, minimum-dot. 1 — locality 14, sample SA 741A, 2 — locality 28, sample SA 868A, 3 — locality 30, sample SA 877, 4 — locality 17, sample SA 984A.

Tithonian – Berriasian Valanginian – Barremian Late Albian Cenomanian Turonian – Early Coniacian Coniacian Late Coniacian – Santonian Campanian – Maastrichtian

N

D°

I°

k

α95°

DC°

IC°

k

α95°

4 7 6 8 13 3 5 7

307 307 334 325 328 319 333 339

+ 41 + 42 + 41 + 33 + 39 + 33 + 35 + 39

70 62 53 27 38 41 55 52

11 8 9 11 7 19 10 8

306 308 330 331 332 333 335 337

+ 45 + 43 + 42 + 40 + 48 + 43 + 41 + 42

153 67 168 82 50 110 71 364

7 7 5 6 6 12 9 3

Key as for Table 1a. but IC° is corrected for compaction effect for Turonian and younger localities from the Adige embayment. N is number of localities.

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E. Márton et al. / Tectonophysics 480 (2010) 57–72

Fig. 13. Paleomagnetic overall mean directions combined from data from the Adige embayment and stable Istria (data from Table 4) for different age groups.

reference directions when tectonic movements with respect to Adria during the late Jurassic–Cretaceous are explored, especially when the ages of the source rocks are stratigraphically controlled. The situation is different when possible displacements with respect to Africa are discussed. In this case the reference is the widely used synthetic APW for Africa (Besse and Courtillot, 2002, 2003) which is tied to a million year time scale (and not to geological stages), either with a 10 Ma or of 20 Ma sliding window. In the first case, the paleomagnetic poles are calculated for every 5 Ma, in the second case, for every 10 Ma. In our data set, the Late Albian–Maastrichtian is so well covered that it is possible to define a high-resolution APW curve for stable Adria from direct observations for every 5 Ma with a 10 Ma sliding window (Table 5). The Tithonian–Early Albian segment is less constrained, due to the Late Aptian–Early Albian sedimentary hiatus (Aptian–Early Albian hiatus in the paleomagnetic record) and to the relatively few Tithonian–Barremian paleomagnetic results from stable Adria, (Table 3), basically due to the scarcity of suitable outcrops. Thus, the manner of comparison with expected declination/inclinations in an African framework is different. For the 150–135 Ma time interval precisely dated individual locality mean paleomagnetic declinations/inclinations (localities 2, 3 and 7 from the Adige

embayment and localities 17 and 12b from stable Istria) are plotted against time (Fig. 14). These exhibit a pattern which follows the African trend suggesting fast rotations in opposite senses, accompanied by inclination variation. This trend was also recognized elsewhere in the Adriatic area, namely in the Umbrian Apennines (Satolli et al., 2007). It has to be noted, however, that the declinations for stable Adria are more westerly than the African ones. The same is true for the declinations defined for 133 Ma. (mean declination based on five localities). Concerning the Albian–Maastrichtian time interval, which is densely covered now with direct paleomagnetic data from stable Adria, inclinations are systematically shallower and declinations are more westernly than those expected in an African framework (Fig. 14). Shallower than expected inclinations characterize times when the source rocks of the direct paleomagnetic results are solely (105 and 100 Ma) or dominantly (95 Ma) platform carbonates as well as the younger ages where the inclinations for the Scaglia Rossa localities were corrected by a factor of 1.17 for the effect of compaction. Earlier Lowrie (1986) reviewed paleomagnetic results from the “Adriatic promontory” and found that the Late Cretaceous overall mean inclinations for the Southern Alps, for Gargano and for Umbria were 42°, 38° and 42°, respectively. These values were close to the inclination expected in the time current African framework (40°) so he regarded the steeper inclination for stable Istria (Márton and Veljović, 1983) as an outlier. Today, the situation is the opposite. The generally accepted synthetic APW for Africa (Besse and Courtillot, 2002, 2003) would require higher inclination for Adria than the directly measured ones. Thus, we must conclude that inclinations are not the best guides when the degree of co-ordination is discussed between Africa and Adria (and Adria derived tectonic units), not only because the APW for Africa did and may change with time, but the likely inclination shallowing (in this paper corrected for) in basin facies of the Adriatic realm. The declination trend for stable Adria during the late Albian– Maastrichtian corresponds to the African trend (Fig. 14), although the latter seems to be more complicated than the former. The westernly displacement of the declinations for the former with respect to the latter is systematic and suggests an about 10° CCW net rotation of Adria with respect to Africa. However, the 10° is not the consequence of a single rotational event, but the resultant of two rotations in opposite senses. This follows from the situation expressed by Fig. 14., where the Maastrichtian and Late Eocene paleomagnetic declinations for stable Adria are practically the same i.e. stable Adria does not exhibit rotation with respect to the present North between 70 and 40 Ma while declinations calculated from the synthetic African APW for Adria shift from 340° to zero, i.e. Africa continued rotating in the CCW sense. The implication is an about 20° CW rotation of stable Adria, with respect to Africa (incidentally accompanied by sedimentary hiatus both in the Adige embayment and in Istria), which is large enough to signify decoupling. Comparison between declinations for 40 Ma reveals that stable Adria rotated with respect to

Table 5 Summary of the overall mean paleomagnetic directions and paleomagnetic poles for stable Adria (Adige embayment + stable Istria) before and after tilt corrections calculated for each 5 Ma with a 10 Ma sliding window.

70 ± 5 75 ± 5 80 ± 5 85 ± 5 90 ± 5 95 ± 5 100 ± 5 105 ± 5

N

D°

I°

k

α95°

DC°

IC°

k

α95°

Lat.

Lon.

k

α95°

5 4 6 17 24 14 11 6

339.2 340.3 336.2 329.3 328.4 325.7 328.2 333.8

+ 42.5 + 44.2 + 45.1 + 42.8 + 38.3 + 35.4 + 40.9 + 41.1

74 78 61 41 35 34 44 53

8.9 10.5 8.7 5.6 5.1 7.0 7.0 9.3

338.0 336.9 334.9 334.2 332.8 330.2 330.2 329.5

+ 42.0 + 43.3 + 45.3 + 46.6 + 45.0 + 41.4 + 41.7 + 42.1

339 360 198 64 58 67 105 168

4.2 4.8 4.8 4.5 3.9 4.9 4.5 5.2

62.4 62.6 62.7 63.5 61.8 58.0 58.1 57.9

239.0 241.8 246.9 246.9 251.1 251.0 251.9 253.8

550 449 181 55 50 56 88 151

3.3 4.3 5.0 4.9 4.2 5.4 4.9 5.5

Key as for Table 1. but IC° is corrected for compaction effect for localities younger than 94 Ma from the Adige embayment. Lat. and Lon. are the coordinates of the paleomagnetic pole. N is number of localities.

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Fig. 14. Comparison of declinations/inclinations (with error bars) for stable Adria (Adige embayment and autochthonous Istria combined) with those calculated from the synthetic APW for Africa (Besse and Courtillot, 2002, 2003). In the 150–135 Ma interval, the values for Adria represent single localities (hollow diamonds), the ones plotted at 133 Ma and at 41 Ma (full diamonds with error bars also for ages) are in the first case overall mean declination/inclination based on 5 paleomagnetic locality mean directions of relevant ages from both autochthonous study areas, the second is based on 5 published (Márton et al., 2003) and one unpublished directions from autochthonous Istria. In the 105–70 Ma interval, the values are calculated for every 5 Ma with a 10 Ma sliding window (full diamonds, data from Table 5).

Africa after the Eocene, since the Late Eocene declination for stable Istria is CCW rotated by about 30° with respect to Africa as well as the present North.

Acknowledgements The authors thank Francesco Massari for calling attention to the existence of hard grounds within the Scaglia Rossa and Gábor Imre for assistance in the field and in the laboratory. Helpful suggestions from the referees are gratefully acknowledged. This work was supported by Italian–Hungarian Intergovernmental Scientific and Technological project no. I-12/2003, the staff mobility exchange agreement between the Eötvös Loránd University and the University of Padova (2004–2007) and Croatian–Hungarian Intergovernmental Scientific and Technological project no. Cro-0t6/2006. Additional support was provided by Hungarian Scientific Research Fund OTKA project no. K049616.

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