- Email: [email protected]
The tectonic development of the Northern Adriatic region constrained by Jurassic and Cretaceous paleomagnetic results
Tectonophysics 490 (2010) 93–102
Contents lists available at ScienceDirect
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
The tectonic development of the Northern Adriatic region constrained by Jurassic and Cretaceous paleomagnetic results Emő Márton a,⁎, Vlasta Ćosović b, Damir Bucković b, Alan Moro b a b
Eötvös Loránd Geophysical Institute of Hungary, Palaeomagnetic Laboratory, Columbus u. 17-23, H-1145 Budapest, Hungary Department of Geology, Faculty of Science, University of Zagreb, Horvatovac 102a, 10000 Zagreb, Croatia
a r t i c l e
i n f o
Article history: Received 17 February 2010 Received in revised form 12 April 2010 Accepted 19 April 2010 Available online 25 April 2010 Keywords: External Dinarides Jurassic Albian–Santonian Paleomagnetism Tectonic implications
a b s t r a c t In the Northern Adriatic region, there is a clear boundary between the weakly deformed stable core of the Adriatic microplate and its tectonically complicated NE margin, the External Dinarides which are further subdivided on stratigraphic and structural grounds. Although most authors distinguish an Adriatic and a Dinaridic realm within the External Dinarides, the relationship and the present boundary between the realms are matters of discussion. The aim of this study was to obtain paleomagnetic directions from different parts of the Northern Adriatic segment of the External Dinarides and discuss their bearing on the different types of the tectonic models. For this purpose we carried out standard paleomagnetic measurements on biostratigraphically well-controlled samples from 28 localities from the Adriatic mainland and the Northern Adriatic islands and from one Middle Jurassic locality representing the stable core of the Adriatic microplate. In addition, six localities from the Northern Adriatic islands, earlier studied paleomagnetically, were biostratigraphically updated and thus included in the data set. For the Northern Adriatic islands an Albian (D/ I = 333°/48°, k = 44, α95 = 9°) and a Cenomanian–Santonian (D/I = 334°/46°, k = 188, α95 = 5°) paleomagnetic direction was defined. They are in perfect agreement with coeval paleomagnetic directions from stable Adria. Thus, the paleomagnetic data clearly support the models which conceive the area as the imbricated margin of the Adriatic microplate. The Early (D/I = 338°/49°, k = 118, α95 = 11°), the Middle (D/I = 342°/54°, k = 112, α95 = 7°) and the Late (D/I = 336°/42°, k = 62, α95 = 16°) Jurassic paleomagnetic directions, all representing the tectonic units of the Adriatic mainland, suggest an about 30° CW rotation of this belt of the External Dinarides with respect to stable Adria. The difference can be interpreted as inherited from the differencial rotations of two independent carbonate platforms, an Adriatic and a Dinaric. Alternatively, thrusting of the more internal belt of the External Dinarides above the Adriaticum may be responsible for the difference. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The most problematic geological issue in the NE Adriatic region is the relation between structures exposed along the eastern Adriatic coast (Istria, Adriatic islands and coastline and those situated in the mainland (e. g. Gorski kotar, Velebit, Lika). The complexity of their relationship is discussed in a dozen of papers published since the 1970s (e.g. D'Argenio et al., 1971; Grandić, 1974; Pamić et al., 1998; Tari, 2002; Korbar, 2009). These publications based their models on stratigraphic and structural observations, except Korbar (2009) who discussed also some implications of the paleomagnetic studies published in recent years. The pioneer paleomagnetic studies on the Northern Adriatic region date back to the 1980s (Márton and Veljović, 1983, 1987; Márton et al., 1990). The subjects of the mentioned studies were mostly weakly
⁎ Corresponding author. Fax: +36 1 2480378. E-mail address: [email protected] (E. Márton). 0040-1951/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2010.04.032
magnetic platform carbonates from the autochthonous part of Istria and from the more tectonized thrust belt of the Dalmatian–Herzegovinian zone (Dimitrijević, 1982, practically corresponding to the zone called imbricated Adria by Tari, 2002) of the External Dinarides. Since that time, there have been important developments in carbonate sedimentology, in biostratigraphic age determination (which quite often lead to age revision) and in the paleomagnetic methodology. That is why a new wave of paleomagnetic investigations started in the Adriatic region during the last 10 years. The recent paleomagnetic investigations already lead to the definition of the Late Jurassic–Eocene segment of the apparent polar wander path for stable Adria (Márton et al., 2003, 2008, 2010) based on the modern paleomagnetic study of biostratigraphically wellcontrolled platform carbonates (Márton et al., 2003, 2008) and also pelagic sediments from the foreland of the Southern Alps (Márton et al., 2010). Thus, a reliable reference framework is now available to which the paleomagnetic directions from the tectonically more complicated fold and thrust belts can be compared. We started such investigations in 2004 and more or less the same time Lewandowski
94
E. Márton et al. / Tectonophysics 490 (2010) 93–102
et al. took 18 hand samples from the post-Carboniferous deposits of the Velebit Mt. for rock magnetic and paleomagnetic investigations. The results they published are interesting from the rock magnetic point of view, but the large scatter of the paleomagnetic vectors prevented the authors to define paleomagnetic directions for localities or age groups (Lewandowski et al., 2009). In this paper we are presenting new and also biostratigraphically updated Late Cretaceous paleomagnetic results from the Northern Adriatic islands, new Early Jurassic–Earliest Cretaceous paleomagnetic observations from the Adriatic mainland in the NW part of Croatia, including the Velebit Mt. and results from Middle Jurassic platform carbonates in autochthonous Istria which conveniently contribute to the already published Late Jurassic–Eocene APW for stable Adria. 2. Geological background The studied localities (except one, which belongs to stable Istria) are situated in the External Dinarides, which is the NE deformed margin of
the Adriatic microplate (Fig. 1). The whole area is composed of thick succession of sediments (up to 10 km, Vlahović et al., 2005) ranging in age from the Carboniferous to the Eocene. Igneous rocks overlain by siliciclastics occur only locally. The sediments were deposited in shallow water environment, first in an epiric sea (Carboniferous to the mid-Triassic) and after a long emergent phase, in a shallow water carbonate platform environment (latest Triassic to the middle Eocene), from time to time interrupted by episodes of emersion or drowning. It is a matter of discussion if during the second time span, a single platform existed (Adriatic–Dinaridic carbonate platform, Grandić, 1974; Gušić and Jelaska, 1993; Pamić et al., 1998; Vlahović et al., 2005) or there had been two carbonate platforms. In the two platform models (D'Argenio et al., 1971; Chorovitz, 1975) a deep-water interplatformal belt separates the Adriaticum and the Dinaricum, which is called Epiadriticum (Herak, 1986), or Budva–Cukali basin (Tari, 2002). In a recently published paper, Korbar (2009) also distinguishes an Adriatic and a Dinaridic segment of the Adriatic–Dinaridic carbonate platform domain.
Fig. 1. Tectonic map of the Adriatic region (redrawn and simplified after Korbar, 2009) showing the distribution of the major thrusts and faults zones. a = NE Adriatic fault zone; b = thrust front of the External Dinarides; c = supposed thrust front of the External Dinarides; d = Dinaridic SW unit thrusts; e = Adriatic NE unit thrusts; f = Dinaridic NE unit thrust, g = other major faults; h = sampling localities. From Ist and surrounding islands and from Cres island only the localities with positive results are shown.
E. Márton et al. / Tectonophysics 490 (2010) 93–102
95
Table 1a Summary of sedimentological structures, fossil content, age description and locality mean palaeomagnetic directions for samples collected on the Adriatic mainland, based on the results of principal component analysis (Kirschvink, 1980), except otherwise indicated ep + gc: combination of stable end points and re-magnetization circles (McFadden and McElhinny, 1988). Localities are numbered according to Fig. 1. Key: Lat.N, Lon.E: Geographic coordinates (WGS84) measured by GPS; 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). The following localities have precise age determination: 4 — Aalenian–Bajocian, 5 — Bathonian and 10 — Oxfordian. The applied limestone classification considers the dominating groundmass types and depositional fabric (Dunham, 1962).
1
2 3
4
5
Locality
Lat.N, Lon.E
Sedimentol. structure
Early Jurassic Velebit, Prezid, HR 1267-277
44° 15′ 57″ 15° 48′ 42″
Mudstones & dolomitized mudstones Ooidal grainstones
Velebit, Mali Alan, HR 1043-050 Melnice HR 1022-027 Mid Jurassic Gornje Jelenje HR 1051-062
44° 15° 44° 15°
17′ 13″ 38′ 55″ 57′ 45″ 02′ 13″
45° 22′ 26″ 14° 34′ 28″
Velebit, Mali Alan, 44° 16′ 37″ HR 1035-042 15° 38′ 30″
6
Velebit, Sušanj HR 1222-233 7 Velebit, Prezid, HR 1257-266 8 Breze HR 1005-015 9 Banska Vrata HR 1016-021 Late Jurassic 10 Velebit, Mali Alan HR 1028-034
44° 15° 44° 15° 45° 14° 45° 14°
31′ 50″ 07′ 45″ 15′ 31″ 48′ 44″ 11′ 50″ 53′ 41″ 09′ 48″ 57′ 11″
Fossil content
Glomospira sp.
Mudstones
Mudstones
Mesoendothyra croatica Gušić
Mudstones to packstones
Palaeopfenderina salernitana (Sartoni and Crescenti), Verneulina sp., Thaumatoporella parvovesiculifera (Raineri) Mudstones to peloidal Favreina sp., wackestones Mesoendothyra sp. Mudstones to peloidal Siphovalvulina sp. wackestones Mudstones Mesoendothyra sp. Mudstones
Pelecypod casts
44° 16′ 26″ 15° 38′ 33″
Mudstones to wackestones
11 Velebit, Prezid, HR 1251-256
44° 15′ 16″ 15° 48′ 39″
Peloidal wackestones to packstones
Kurnubia jurassica (Henson), Verneulina sp. Unidentified benthic foraminifera
12 Velebit, Sušanj HR 1234-239 13 Velebit, Otočac HR1240-250
44° 15° 44° 15°
31′ 40″ 07′ 37″ 51′ 44″ 10′ 18″
Peloidal wackestones Peloidal wackestones to laminated packstones
14 Platak HR 1063-067 15 Žuta Lokva HR 689-698 Early Cretaceous 16 Ogulin HR 699-712
45° 14° 44° 15°
23′ 50″ 32′ 56″ 58′ 04″ 04′ 30″
Ooidal grainstones
45° 16′ 08″ 15° 16′ 09″
Ostracods, gastropods, unidentified benthic foraminifera Cyanophyta balls
Mudstones
Mudstones to grainstones
Moncharmontia apenninica (de Castro), Cuneolina sp., Nummuloculina sp., ostracods
α95° Dip
Remark
50 + 62 18 332 + 49 452
19 2
171/27
Softer Harder
8
332 + 52 65
8
200/42
332 + 34 39 4 + 23 84
15 8
309 + 33 39 349 + 45 84
15 8
222/30
4/4 4/4 4/4 7/8
42 22 7 8
11 14 8 8
41 9 317 357
11 14 8 8
223/33
3/12
354 −6
n/no
D°
Early Jurassic
10/11
24 + 42 18 337 + 23 452
19 2
Early Jurassic Early Jurassic
7/8
351 + 18 65
4/6 5/6
Aalenian– Bajocian Bathonian
I°
+ 42 + 25 + 49 + 14
k
α95° DC°
Age
74 75 122 71
+ 75 + 54 + 67 + 58
k
74 75 122 71
5/10
337 + 29 28
10/11 11/11 6/6 6/6
334 25 328 16
Oxfordian
4/7
352 0
Late Jurassic
0/6
too weak
Late Jurassic Late Jurassic
5/6
7
+ 47 91
8
221 + 59 91
8
202/71
11/11
329 + 35 60
6
338 + 49 60
6
118/17
Late Jurassic Late Jurassic
0/5
too weak
10/10
338 + 41 49
7
335 + 46 49
7
170/15
Early 14/14 Cretaceous 10/14
278 + 51 39 245 −44 97
6 5
285 + 42 39 234 + 49 97
6 5
340/11
The fold and thrust belt of the External Dinarides show a complex geotectonic structure, which was the object of numerous studies for a long period of time (e. g. Aubouin, 1973; Dimitrijević, 1982; Herak, 1986). More recently two geotectonic models of the region were published suggesting different subdivisions for the NE part of Adria. Tari (2002) defines three main units, which are separated by two NW– SE oriented major thrust fronts. These are Stable Adria, Imbricated Adria and the Dinaric Nappe System. The boundary which separates Imbricated Adria from the Dinaric Nappe System is also recognized by Korbar (2009) as an important tectonic feature, but with a different role in the tectonic evolution of the area. In his interpretation this structure
340 + 45 1035 4
204/61
15
330 + 54 28
15
172/27
48 68 107 123
7 6 7 6
299 339 309 344
48 68 107 123
7 6 7 6
340/11
111
9
335 + 30 111
9
228/62
+ 35 + 57 + 38 + 52
Softer Harder
Site1 Site2 Site3 200/46 ep + gc
Middle Jurassic Middle Jurassic Middle Jurassic Middle Jurassic
+ 54 + 44 + 53 + 51
1035 4
IC°
Endpoints
Softer Harder 267/24 Softer Harder
141/38
230/42
Softer Harder
is a long-lived (from Triassic to recent) right lateral transcurrent fault (NE Adriatic fault) which separates Adriatic and Dinaridic domains (Fig. 1). However, he suggests that on the surface there are Dinaridic elements even SW of the NE Adriatic fault, which were transported as nappes on top of the Adriatic domain. Within the Dinaridic domain, Korbar (2009) distinguishes several thrust units. The most important boundary between these seems to be what separates the Inner Karst from the High Karst (Fig. 1, thrust boundary f). Both authors suggest further subdivisions. According to Tari (2002) Imbricated Adria is subdivided by transversal faults into three segments which are, from north to the south, Imbricated Adria
96
E. Márton et al. / Tectonophysics 490 (2010) 93–102
Table 1b Summary of sedimentological structures, fossil content, age description and locality mean palaeomagnetic directions for samples collected from Stable Istria and from Cres island, based on the results of principal component analysis (Kirschvink, 1980). Localities are numbered according to Fig. 1. Key as for Table 1a. Results for localities 18, 19, 21–24 were published earlier (Márton et al., 1990) but the stratigraphic ages are revised. According to the new age assignment all localities from Cres are of Albian age, except locality 21, which is lower Cenomanian. The applied limestone classification considers the dominating groundmass types and depositional fabric (Dunham, 1962). Locality Stable Istria Bathonian 17 Rovinj, Monsena HR 1121-136 Cres Island Late Cretaceous 18 Punta Križa YM 814-826
k
α95° Dip
Fossil content
Age
n/no
D°
45° 06′ 30″
Mudstones to wackestones
Palaeopfenderina salernitana (Sartoni and Crescenti)
Bathonian
11/16
314 + 39 29
9
309 + 43 29
9
188/7
Albian (earlier: Barremian–Aptian)
11/13
322 + 35 29
9
324 + 47 56
6
85/15 129/12 179/20
Albian (earlier: Barremian– Aptian)
9/9
324 + 63 135 4
348 + 50 135 4
28/20
Albian
6/9
322 + 25 112 6
341 + 39 112 6
85/33
Lower Cenomanian 12/12 (earlier: Cenomanian– Turonian)
320 + 53 52
7
346 + 41 52
7
44/26
Albian (earlier: Cenomanian– Turonian)
7/7
293 + 31 30
11
312 + 58 30
11
86/32
Albian (earlier: Cenomanian– Turonian)
12/12
311 + 51 23
9
311 + 49 24
9
96/6 102/10 236/10 314/16
Albian (earlier: Cenomanian– Turonian)
7/7
292 + 45 31
11
341 + 46 31
11
54/47
44° 38′ 22″ 14° 30′ 04″
19 Krčina YM 827-835
44° 55′ 25″ 14° 25′ 16″
20 Hrasta HR 1278-286
44° 48′ 42″ 14° 24′ 38″
21 Osor YM 836-847
44° 42′ 24″ 14° 24′ 25″
22 Valun YM 860-866
44° 53′ 38″ 14° 22′ 55″
23 Valun
44° 53′ 38″
YM 867-878
14° 22′ 55″
24 Predoščica YM 885-891
45° 02′ 19″ 14° 22′ 31″
Mudstones to wackestones/ Pseudonummuloculina Packstones to grainstones heimi (Bonet), Istriloculina sp., ostracods Mudstones to packstones – Pseudonummuloculina grainstones heimi (Bonet), Rumanoloculina sp., Istriloculina sp. Mudstones to wackestones/ Pseudonommuloculina Packstones to grainstones heimi (Bonet), Axiopolina sp., Istriloculina sp. Wackestones to packstones Aeolisaccus kotori (Radoičić), Nezzazata sp., ostracods, unidentified miliolids Mudstones to wackestones/ Pseudonummuloculina Packstones to grainstones heimi (Bonet), Rumanoloculina sp., Istriloculina sp., Axiopolina sp. Mudstones to wackestones/ Pseudonummuloculina Packstones to grainstones heimi (Bonet), Rumanoloculina sp., Istriloculina sp., Axiopolina sp. Grainstones to laminated Pseudonummuloculina mudstones to grainstones heimi (Bonet), Rumanoloculina sp.
north of Istria, Imbricated Adria of Kvarner islands and Imbricated Adria of Central Dalmatia. She does not, however, continue the faults into the Dinaric Nappe System. Korbar's model (2009) contains more units, since he recognizes several transversal faults which cut not only the Adriatic but also the Dinaridic domains. Eventually, he defines two Adriatic and four Dinaridic domains between the Kvarner and the Jabuka–Vis faults (Fig. 1). He considers these as distinct tectonostratigraphic entities although their stratigraphic evolution does not always show profound differences. 3. Paleomagnetic and biostratigraphic samplings In order to obtain biostratigraphically well-constrained paleomagnetic directions from different units of the NW segment of the External Dinarides we visited a large number of localities (e.g. fresh outcrops along the newly constructed motor way, working and abandoned quarries and natural outcrops) in the years of 2004–2009 on the Adriatic mainland, and on the northern Adriatic islands. Eventually, 260 paleomagnetic samples were drilled from 28 localities. We also inspected Middle Jurassic outcrops in the Rovinj area (the oldest rocks on the surface in stable Istria) and collected 13 samples in order to obtain a Middle Jurassic reference direction for direct comparison with results of similar age from the External Dinarides, i.e. from the Adriatic mainland (Tables 1a, 1b and 1c).
k
IC°
Sedimentol. structure
13° 36′ 44″
I°
α95° DC°
Lat.N, Lon.E
The beds for drilling were carefully selected. They were preferably the mud supported type, non-fossiliferous limestones, devoid of visible late diagenetic changes and secondary alterations. The paleomagnetic cores were oriented in the field with a magnetic compass. For biostratigraphic interpretation hand samples were collected either from the beds drilled for paleomagnetic measurements, or from a bed that represents lateral extension of the sampled one. In the case when pinch-out relationship did not exist we took hand samples from neighbouring beds. The sampling localities are geographically distributed (Fig. 1) and represent a fairly long time span (Tables 1a, 1b and 1c). Nevertheless, there is a concentration of the sampling localities in the Velebit Mt., where the rocks were collected along three sections crossing the mountains at different longitudes. The most westerly is exposed along the Otočac–Karlobag road (Fig. 1, localities 6, 12, and 13,), the most easterly is the Prezid section (Fig. 1, localities 1, 7 and 11) and the middle one is the Mali Alan section (Fig. 1, localities 2, 5 and 10).
4. Age and lithology of the sampled rocks The oldest sediments sampled are of Early Jurassic age (Table 1a and Fig. 1, localities 1–3). They are dark grey mud supported limestones (wackestones) which alternate with grain-supported varieties (ooidal
E. Márton et al. / Tectonophysics 490 (2010) 93–102
97
Table 1c Summary of sedimentological structures, fossil content, age description and locality mean palaeomagnetic directions for samples collected on Ist and surrounding islands, based on the results of principal component analysis (Kirschvink, 1980). Localities are numbered according to Fig. 1. Key as for Table 1a. Paleomagnetic results were published earlier (Márton and Moro, 2009), but the precise age assignments for the localities is published for the first time. Locality
25 26
27 28
Lat.N, Lon.E Sedimentol. structure
Ist and surrounding islands Late Cretaceous Ist, port 44° 16′ 45″ HR 953-961 14° 45′ 26″ Ist, abandoned 44° 15′ 23″ quarry HR 987-995 14° 45′ 55″ Tramerka 44° 13′ 22″ HR 996-1004 14° 46′ 08″ Funestrala 44° 15′ 08″ HR 1155-166 14° 44′ 39″
29 Silba, Nozdre bay HR 962-971 30 Premuda, Široka bay HR 1137-154
44° 21′ 20″ 14° 43′ 24″ 44° 19′ 05″
Wackestones to packstones/grainstones Wackestones to packstones/grainstones
k
α95° Dip
n/no D°
Nummuloculina sp.
Cenomanian
7/9
340 + 52 21
Miliolids
Cenomanian
9/9
336 + 49 108 5
338 + 48 108 5
314/10 252/25 42/2
Cenomanian
6/9
337 + 51 230 4
325 + 45 230 4
271/12
Cenomanian
8/12
330 + 44 98
6
332 + 48 98
6
125/5
Turonian–Santonian
6/10
328 + 40 55
9
334 + 48 55
9
110/10
Santonian–Campanian
5/18
3
−20
9
347 + 45 73
9
219/82
Aeolisaccus kotori (Radoičić) Mudstones-wackestones Broeckina to packstones/grainstones (Pastrikella) balcanica Cherci, Radoičić & Schroeder Wackestones-packstones Calcispheres, pithonellas Mudstones –wackestones Calcispheres, globotruncanids
k
IC°
Age
Wackestones-packstones
I°
α95° DC°
Fossil content
73
13
329 + 42 42
9
14° 37′ 52″
to peloidal grainstones) deposited in peritidal conditions. These beds contain rare fossils. Middle Jurassic massive, thick bedded dark grey mudstones were sampled at localities 4–9 in the External Dinarides and well-bedded cream coloured mudstones to wackestones at locality 17 from stable Istria. Benthonic foraminifera allowed age determination with high precision for locality 5 (Bathonian), for locality 4 (Aalenian–Bajocian) and for locality 17 (Bathonian) (Tables 1a and 1b). The Late Jurassic limestones are typically coarse grained ooidal grainstones that pass into peloidal wackestones to mudstones with rare foraminifera. We collected the thin bedded mud supported variety (localities 10–15). Sporadic occurrences of benthic foraminifera allowed precise age determination for locality 10, pointing to Oxfordian age. Although the Cretaceous sediments in the Adriatic mainland are widespread and we inspected several working or abandoned quarries, in addition to the fresh road cuts along the Zagreb–Split motorway, we
found only one locality in the Dinaridic domain (locality 16, fresh road cut on the new motorway) suitable for sampling. There we collected dark grey mudstones for paleomagnetic measurements, and the grainsupported variety for biostratigraphic analysis. At the other places, the limestones were karstified, or massive without visible bedding plane, or coarse grained with considerable biogenetic components. On the Northern Adriatic islands the oldest outcrops are of Cretaceous age. They are characteristically shallow water peritidal limestones and subordinately pelagic limestones, light grey or white in colour. Earlier, a number of localities were paleomagnetically studied from the islands of this area (Márton et al., 1990). Since that time the ages of several rock units have been revised, therefore we returned to Cres island, which yielded the best paleomagnetic results from the Kvarner islands, collected hand samples for biostratigraphic age determination and took the GPS coordinates from the localities which provided good quality paleomagnetic directions and added a new
Fig. 2. Typical demagnetization curves for the Late Cretaceous platform carbonates from Cres island (HR1279B) and for the Doggerian from stable Istria (HR1127 and 1131). accompanied by NRM intensity versus demagnetizing field diagrams. AF demagnetizations. Key to Zijderveld diagrams: full dots: projection of the NRM vector onto the horizontal, circles: into the vertical. Geographical system.
98
E. Márton et al. / Tectonophysics 490 (2010) 93–102
Fig. 3. Typical demagnetization curves for Early (HR1025, HR1273A), Middle (HR1016, HR1222, HR1036) and Late (HR697, HR1236, HR1241B) Jurassic platform carbonates from the External Dinarides. Zijderveld diagrams are accompanied by intensity (circles) versus demagnetizing field diagrams, when the method of demagnetization is AF and by NRM intensity (circles)/susceptibility (dots) versus temperature diagrams, when the method of demagnetization is thermal.
locality (locality 20) to the previous collection. It is important to note that indeed the stratigraphic ages of most of the earlier published paleomagnetic directions from Cres island had to be seriously revised (Table 1b). Further to the south, paleomagnetic and biostratigraphic samples were collected from Ist and surrounding islands. From this region the paleomagnetic results were published by Márton and Moro (2009), but the precise biostratigraphic age assignment for the sampled localities is published the first time in the present paper (Table 1c).
5. Laboratory processing of the paleomagnetic samples and results The drill cores were cut in the laboratory into standard-size specimens which were subjected first to measurements of the natural remanent magnetization (NRM) and the magnetic susceptibility in the natural state. The instruments used were JR-4 and JR-5A magnetometers and a KLY-2 Kappabridge. Sister specimens of
E. Márton et al. / Tectonophysics 490 (2010) 93–102
99
Fig. 4. Typical demagnetization curve for the Early Cretaceous locality 16. Zijderveld diagram is accompanied by NRM intensity versus demagnetizing field diagram and by the stereographic plot showing the directional change of the NRM on demagnetization. Key as for Fig. 3.
representative samples were then stepwise demagnetized, one of them with alternating field (AF), the other with thermal (TH) method, till the NRM signal was lost. Based on the experience with the pilot specimens the rest of the samples from the respective localities were demagnetized in several increments with the method which provided better defined demagnetization curves. In case of extremely weak initial NRM intensities (below 5 × 10− 5 A/m), only AF demagnetization was used, as years of experience with platform carbonates showed that the thermal method in such cases was not practicable (Márton et al., 2003, 2008). In the samples collected from stable Istria (Middle Jurassic) and from the Northern Adriatic islands the NRM signal was extremely weak (usually 2–9 × 10− 5 A/m, but for localities 17, 20 and 28 values up to 65× 10− 5 A/m were measured). The magnetic susceptibilities were negative (12–15× 10− 6 SI), except for locality 17, where low positive values were measured (below 10 × 10− 6 SI). Nevertheless, six of the Late Cretaceous localities representing Ist and the surrounding islands, as documented in Márton and Moro (2009) and the samples from Cres island yielded well-defined demagnetization curves (Fig. 2, HR1279B). The initial NRM intensities and the susceptibilities for the samples from the Adriatic mainland (mostly of Jurassic age) are extremely variable, the former is in the range of 1– 2800 × 10− 5 SI, the latter from minus 13 to plus 732 × 10− 6 SI. Except for localities 11 and 14, the NRM was sufficiently strong to lend itself to demagnetization in increments and the resulting demagnetization curves are good (Figs. 3 and 4). Concerning locality Rovinj (Monsena) from stable Istria, the specimens with higher NRM intensities exhibited a large overprint component which was successfully removed (Fig. 2, HR1127), the weaker specimens yielded rather noisy demagnetization curves (Fig. 2, HR1131) or their intensities dropped to the noise level of the measuring instruments at the first step of demagnetization (3 mT, these samples were omitted from further evaluation). As the magnetic mineralogy experiments suggest, the magnetic minerals in all the studied localities are dominantly or exclusively (Ist and surrounding islands) soft (Fig. 5). The mineral which appears as soft on the curves of the acquisition of the isothermal remanence (IRM) is magnetite, although in some cases the presence of maghemite is evidenced by decreasing susceptibility on heating above 250° (e.g. HR1025, Fig. 3). The harder component of the IRM
must be connected to hematite, which does not seem to contribute to the NRM (fast reduction of the NRM intensity on AF demagnetization and the elimination of the NRM on thermal demagnetization before the Curie point of the magnetite). Following the demagnetization experiments, the curves were subjected to component analysis (Kirschvink, 1980) and the directions of the characteristic remanences (and in some cases those of the overprint magnetizations, denoted as soft, indicating also lowblocking components when the method of demagnetization was thermal) determined. The results before and after tilt corrections are shown in Tables 1a, 1b and 1c.
Fig. 5. The acquisition behaviour of the isothermal remanent magnetization (IRM). The acquisition curves indicate solely (HR1131) or dominantly soft magnetic mineral (most probably magnetite, sometimes accompanied by maghemite, as deduced from the decrease of the magnetic susceptibility on heating, e.g. HR1025 and HR1273 in Fig. 3. Note that maghemite decomposes before the component interpreted in terms of tectonics appears. In several samples minor amount of hematite is evidenced by the moderate increase of the IRM intensity from 0.2 T onward (HR1270B, HR1241A, HR1279B, and HR1241B), but this mineral is not likely to contribute to the NRM, for the NRM always unblocks below the Curie point of the magnetite (Fig. 3, HR1025, HR1273A, HR1222, and HR1236, HR1241B).
100
E. Márton et al. / Tectonophysics 490 (2010) 93–102
Table 2 Summary of the overall-mean paleomagnetic directions and paleomagnetic poles for the Northern Adriatic islands, the Adriatic mainland and for a single Middle Jurassic locality from stable Istria before and after tilt corrections. Key as for Table 1a and Lat. and Lon. are the coordinates of the paleomagnetic pole. N is number of localities, and (n) number of samples.
Cres island, Albian (Early Cenomanian) Ist and surrounding islands, Cenomanian (Turonian–Santonian) Adriatic mainland Early Jurassic Middle Jurassic Late Jurassic Stable Istria, Rovinj, Bathonian
N
D°
I°
k
α95°
DC°
IC°
k
α95°
Lat.
Lon.
k
α95°
7 6 3 5 3 (11)
311.3 340.1 350.7 2.4 340.6 313.8
+ 44.2 + 38.1 + 21.7 + 27.5 + 25.9 + 38.9
23 8 40 8 11 29
12.7 25.4 19.7 27.9 39.0 8.7
333.0 334.1 338.1 341.7 335.9 308.8
+ 48.0 + 46.2 + 49.0 + 53.5 + 41.7 + 42.7
44 188 118 112 62 29
9.1 4.9 11.4 7.2 15.8 8.7
63.3 63.4 67.2 72.6 61.9 44.3
258.5 254.5 253.3 254.8 247.2 275.0
30 129 74 45 130 29
11.1 5.9 14.4 8.9 10.9 8.7
6. Discussion and conclusions The sampling localities from the Northern Adriatic islands are conveniently divided into two groups. The first is an Albian group of localities (18–24, the group includes one Early Cenomanian locality) from Cres, the second is a Cenomanian–Campanian one from Ist and surrounding islands (localities 25–30). The overall-mean paleomagnetic directions calculated for the first (Table 2 and Fig. 6) have better statistical parameters after than before applying tilt corrections for the respective localities, the Enkin (2003) test is positive and the best statistics is achieved at 74% untilting, indicating that the characteristic remanence is basically of pre-tilting origin. For the second group the tilt test (based on k statistics) is positive and yields the best result close to 100% untilting, which points to the purely pre-tilting age of the magnetizations (Márton and Moro, 2009). The overall-mean directions for the above two groups can be compared directly with coeval paleomagnetic directions from stable Adria, which are biostratigaphically and also paleomagnetically wellconstrained (Márton et al., 2010). The plot of the paleomagnetic overall directions with their respective confidence circles from stable Adria and the Northern Adriatic islands are statistically identical (Fig. 7A), i.e. there is no indication for relative movements between the two realms after the Campanian (after about 72 Ma). On the Adriatic mainland all the sampled localities belong to the External Dinarides s.s., except locality 16 which is situated in the Inner Karst, i.e. in a higher tectonic position (Korbar, 2009) than e.g. the localities in the Lika area or in the Velebit Mt. (Fig. 1). The results from the External Dinarides can be subdivided into Early, Middle and Late Jurassic groups (Table 1a). In the following discussion the harder/ higher blocking (or the only) components of the NRM will be evaluated from the viewpoint of the time of the acquisition of the remanence in relation to the deformation in the area (most probably taking place during Late Eocene, e.g. Korbar, 2009). The Early Jurassic localities have extremely shallow inclinations before tilt corrections, which becomes 49° after corrections while the scatter of the directions is at minimum close to 100% untilting (Table 2 and Fig. 6), suggesting that the magnetization was acquired before deformation. The highly scattered Doggerian locality mean paleomagnetic directions (in the geographic coordinate system) become tightly clustered after tilt corrections (Fig. 6), the best statistics is achieved at 110% untilting and the test (based on k statistics) is positive (Table 2). From the calculations locality 4 is omitted since the different sites sampled from this locality do not yield consistent paleomagnetic directions (Table 1a). The overall-mean paleomagnetic direction for the Late Jurassic has the best statistical parameters close to 100% untilting, the scatter of the individual directions is considerably reduced (Table 2 and Fig. 6) suggesting that the magnetizations were acquired prior to deformation. From the calculation locality 12 is omitted, because it is suspected of full remagnetization quite recently (the paleomagnetic direction is close to the direction of the present Earth magnetic field in the sampling area).
The above groups of overall-mean paleomagnetic directions are characterized by moderate CCW rotation of the declinations with respect to the present North. The single locality from the Inner Karst exhibits two NRM components (Fig. 4), which are significantly different in direction from that of the present Earth magnetic field. The magnetically softer component shows large CCW, the harder component exhibits large CW rotation with respect to the present North. The tectonic interpretation of the two paleomagnetic vectors from this locality is problematic, since it is an isolated observation, so far the only representative of the Inner Karst. From a study by Márton et al. (2010) a paleomagnetic reference direction for the Tithonian–Berriasian of stable Adria (from direct measurements) is available for comparison with the Late Jurassic group of localities from the External Dinarides. In addition, the present study provides a reference direction for the Doggerian, based on locality 17 from Istria. Compared to the Late and Middle Jurassic paleomagnetic directions for stable Adria, the Late and Middle Jurassic paleomagnetic directions from the External Dinarides exhibit an about 30° CW rotation while the respective inclinations are close (Fig. 7B). The above discussed relations between the paleomagnetic directions from stable Adria, on one hand and from the Adriatic mainland and the Northern Adriatic islands, on the other hand have important tectonic implications. The close agreement between coeval Late Cretaceous paleomagnetic directions from stable Adria and from the North Adriatic islands supports the model of Tari (2002) in which the mentioned islands belong to the deformed margin of stable Adria (Imbricated Adria). In the model by Korbar (2009) the Northern Adriatic islands are Dinaridic elements in nappe position (thin skinned tectonics), which is unlikely since the studied segment of the External Dinarides on the mainland shows distinctly less rotation in the CCW sense than the coeval paleomagnetic declinations for stable Adria (Middle and Late Jurassic). The tectonic implication of the 30° difference between Jurassic declinations for the Northern segment of the External Dinarides s.s. and stable Adria is less straightforward for lack of younger than Jurassic data from the former. However, relying on the general opinion that the External Dinarides and Adria became docked during Late Eocene deformation processes, which means that the External Dinarides must have participated in the post-Eocene CCW rotation of about 30° of Adria, we are obliged to conclude that the External Dinarides rotated in the CW sense with respect to Adria before the Late Eocene. In such case, there is a possibility that the Adriatic and Dinaric (Dinaridic domain in Korbar, 2009) platforms were indeed independent after the Middle Triassic (but could not be too far from each other, for the coeval inclinations are practically identical, see Table 2) and the difference in declinations is due to the displacements which brought the two platforms (domains) into contact. Another possibility is that the External Dinarides rotated in CW sense during trusting over the Adriaticum. In this case, we do not have to calculate with two independent platforms. Finally, it has to be mentioned that
E. Márton et al. / Tectonophysics 490 (2010) 93–102
101
Fig. 6. Locality mean paleomagnetic directions (dots) with α95 (circles) for the Albian (Early Cenomanian) platform carbonates from Cres island, for the Early, Middle and Late Jurassic platform carbonates from the Adriatic mainland before and after tilt corrections. The overall-mean paleomagnetic directions (diamonds) with confidence circles are also shown. Stereographic plots.
the paleomagnetic observations so far available do not exclude the possibility that the Dinaricum and the Adriaticum moved independently even after the Eocene and the observed CCW rotations for Adria reflect African origin and decoupling from Africa with a net CCW rotation after the Eocene. At the same time the External Dinarides were not belonging to the African plate and the CCW rotations observed on the Jurassic rocks are the consequence of the thrusting of the Dinaricum over the Adriaticum, after the final CCW rotation of Adria. In order to exclude the last possibility, systematic paleomag-
netic studies are needed in the future on the sediments of the Neogene intramontaine basins of the External Dinarides. Acknowledgement The authors thank Tvrtko Korbar for the fruitful discussion about the tectonics of the study area, Gábor Imre for assistance in the field and in the laboratory, Robert Košćal for technical assistance, and Slavijan Deroko for boat transportation. Constructive suggestions to
102
E. Márton et al. / Tectonophysics 490 (2010) 93–102
Fig. 7. Paleomagnetic overall-mean directions A: for Cres island and for Ist and surrounding islands (Table 2) compared with coeval reference directions from stable Adria (Márton et al., 2010) and B: for the Adriatic mainland (Early, Middle and Late Jurassic) compared with Tithonian–Berriasian (Márton et al., 2010) and Bathonian (present study) paleomagnetic directions from stable Adria.
improve the manuscript by the referees are gratefully acknowledged. This work was supported by the Croatian–Hungarian Intergovernmental Scientific and Technological projects nos. Hr-18/2004 and Cro06/2006. Additional support was provided by the Hungarian Scientific Research Fund OTKA project no. K049616, and the Croatian Ministry of Science, Education and Sports project nos. 119-1191152-1167 and 119-1191152-1171.
References Aubouin, J., 1973. Des tectoniques superposees et de leur signification par rapport aux modeles geophysiques, lexemple des Dinarides, paleotectonique, tectonique, tarditectonique, neotectonique. Bulletin de la Societe Geologique de France 15 (5–6), 426–460. Chorovitz, J., 1975. Le devenir de la zone du Budva vers le Nord-Ouest de la Yougoslavie. Bulletin de la Societe Geologique de France 7 (17), 699–709. D'Argenio, B., Radoičić, R., Sgrosso, I., 1971. A paleogeographic section through the Italodinaric external zone during Jurassic and Cretaceous times. Nafta 22 (4–5), 195–207. Dimitrijević, M.D., 1982. Dinarides, an outline of the tectonics. Earth Evolution Sciences 2 (1), 4–23. Dunham, R.J., 1962. Classification of carbonate rocks according to depositional texture. In: Ham, W.E. (Ed.), Classification of Carbonate Rocks. A Symposium: Amer. Ass. Petrol. Geol. Mem., I, pp. 108–171. Enkin, R., 2003. The direction-correction tilt test: an all-purpose tilt/fold test for paleomagnetic studies. Earth and Planetary Science Letters 212, 151–166. Fisher, R.A., 1953. Dispersion on a sphere. Proceedings. Royal Society of London 217, 295–305. Grandić, S., 1974. Some regional petroleum geological characteristics of the External Dinarides deposits. Nafta 25 (3), 111–120. Gušić, J., Jelaska, V., 1993. Upper Cenomanian–Lower Turonian sea-level rise and its consequences on the Adriatic–Dinaric carbonate platform. Geologische Rundschau 82, 676–686. Herak, M., 1986. A new concept of geotectonics of Dinarides. Acta Geologica 16, 1–42.
Kirschvink, J.L., 1980. The least-squares line and plane and the analysis of paleomagnetic data. Geophysical Journal of the Royal Astronomical Society 62, 699–718. Korbar, T., 2009. Orogenic evolution of the External Dinarides in the NE Adriatic region a model constrained by tectonostratigraphy of Upper Cretaceous to Paleogene carbonates. Earth Science Reviews 96 (4), 296–312. Lewandowski, M., Velić, I., Sidorczuk, M., Vlahović, I., Velić, J., 2009. First rock magnetic and paleomagnetic analyses of the pre-Cenozoic rocks of the Velebit Mt. (Croatia): prospects for applications in paleogeographic and geotectonic studies. Geologia Croatica 62 (1), 45–61. Márton, E., Moro, A., 2009. New paleomagnetic results from imbricated Adria, Ist island and related areas. Geologia Croatica 62 (2), 107–114. Márton, E., Veljović, D., 1983. Paleomagnetism of the Istria peninsula, Yugoslavia. Tectonophysics 91, 73–87. Márton, E., Veljović, D., 1987. Paleomagnetism of Cretaceous carbonates from the northwestern part of the Dinaric fold belt. Tectonophysics 134, 331–338. Márton, E., Milicević, V., Veljović, D., 1990. Paleomagnetism of Kvarner Islands, Yugoslavia. Physics of the Earth and Planetary Interiors 62, 70–81. Márton, E., Drobne, K., Ćosović, V., Moro, A., 2003. Palaeomagnetic evidence for Tertiary counterclockwise rotation of Adria. Tectonophysics 377, 143–156. Márton, E., Ćosović, V., Moro, A., Zvocak, S., 2008. The motion of Adria during the Late Jurassic and Cretaceous: New paleomagnetic results from stable Istria. Tectonophysics 454, 44–53. Márton, E., Zampieri, D., Ćosović, V., Moro, A., 2010. New Cretaceous paleomagnetic results from the foreland of the Southern Alps and the refined apparent polar wander path for stable Adria. Tectonophysics 480, 57–72. McFadden, P.L., McElhinny, M.W., 1988. The combined analysis of remagnetization circles and direct observations in paleomagnetism. Earth and Planetary Science Letters 87, 161–172. Pamić, J., Gušić, I., Jelaska, V., 1998. Geodynamic evolution of the Central Dinarides. Tectonophysics 297 (4), 251–268. Vlahović, I., Tišljar, J., Velić, I., Matičec, D., 2005. Evolution of the Adriatic Carbonate Platform paleogeography, main events and depositional dynamics. Palaeogeography, Palaeoclimatology, Palaeoecology 220 (3–4), 333–360. Tari, V., 2002. Evolution of the Northern and Western Dinarides, a tectonostratigraphic approach. European Geosciences Union: Stephan Mueller Special Publication, 1, pp. 223–236.
Contents lists available at ScienceDirect
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
The tectonic development of the Northern Adriatic region constrained by Jurassic and Cretaceous paleomagnetic results Emő Márton a,⁎, Vlasta Ćosović b, Damir Bucković b, Alan Moro b a b
Eötvös Loránd Geophysical Institute of Hungary, Palaeomagnetic Laboratory, Columbus u. 17-23, H-1145 Budapest, Hungary Department of Geology, Faculty of Science, University of Zagreb, Horvatovac 102a, 10000 Zagreb, Croatia
a r t i c l e
i n f o
Article history: Received 17 February 2010 Received in revised form 12 April 2010 Accepted 19 April 2010 Available online 25 April 2010 Keywords: External Dinarides Jurassic Albian–Santonian Paleomagnetism Tectonic implications
a b s t r a c t In the Northern Adriatic region, there is a clear boundary between the weakly deformed stable core of the Adriatic microplate and its tectonically complicated NE margin, the External Dinarides which are further subdivided on stratigraphic and structural grounds. Although most authors distinguish an Adriatic and a Dinaridic realm within the External Dinarides, the relationship and the present boundary between the realms are matters of discussion. The aim of this study was to obtain paleomagnetic directions from different parts of the Northern Adriatic segment of the External Dinarides and discuss their bearing on the different types of the tectonic models. For this purpose we carried out standard paleomagnetic measurements on biostratigraphically well-controlled samples from 28 localities from the Adriatic mainland and the Northern Adriatic islands and from one Middle Jurassic locality representing the stable core of the Adriatic microplate. In addition, six localities from the Northern Adriatic islands, earlier studied paleomagnetically, were biostratigraphically updated and thus included in the data set. For the Northern Adriatic islands an Albian (D/ I = 333°/48°, k = 44, α95 = 9°) and a Cenomanian–Santonian (D/I = 334°/46°, k = 188, α95 = 5°) paleomagnetic direction was defined. They are in perfect agreement with coeval paleomagnetic directions from stable Adria. Thus, the paleomagnetic data clearly support the models which conceive the area as the imbricated margin of the Adriatic microplate. The Early (D/I = 338°/49°, k = 118, α95 = 11°), the Middle (D/I = 342°/54°, k = 112, α95 = 7°) and the Late (D/I = 336°/42°, k = 62, α95 = 16°) Jurassic paleomagnetic directions, all representing the tectonic units of the Adriatic mainland, suggest an about 30° CW rotation of this belt of the External Dinarides with respect to stable Adria. The difference can be interpreted as inherited from the differencial rotations of two independent carbonate platforms, an Adriatic and a Dinaric. Alternatively, thrusting of the more internal belt of the External Dinarides above the Adriaticum may be responsible for the difference. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The most problematic geological issue in the NE Adriatic region is the relation between structures exposed along the eastern Adriatic coast (Istria, Adriatic islands and coastline and those situated in the mainland (e. g. Gorski kotar, Velebit, Lika). The complexity of their relationship is discussed in a dozen of papers published since the 1970s (e.g. D'Argenio et al., 1971; Grandić, 1974; Pamić et al., 1998; Tari, 2002; Korbar, 2009). These publications based their models on stratigraphic and structural observations, except Korbar (2009) who discussed also some implications of the paleomagnetic studies published in recent years. The pioneer paleomagnetic studies on the Northern Adriatic region date back to the 1980s (Márton and Veljović, 1983, 1987; Márton et al., 1990). The subjects of the mentioned studies were mostly weakly
⁎ Corresponding author. Fax: +36 1 2480378. E-mail address: [email protected] (E. Márton). 0040-1951/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2010.04.032
magnetic platform carbonates from the autochthonous part of Istria and from the more tectonized thrust belt of the Dalmatian–Herzegovinian zone (Dimitrijević, 1982, practically corresponding to the zone called imbricated Adria by Tari, 2002) of the External Dinarides. Since that time, there have been important developments in carbonate sedimentology, in biostratigraphic age determination (which quite often lead to age revision) and in the paleomagnetic methodology. That is why a new wave of paleomagnetic investigations started in the Adriatic region during the last 10 years. The recent paleomagnetic investigations already lead to the definition of the Late Jurassic–Eocene segment of the apparent polar wander path for stable Adria (Márton et al., 2003, 2008, 2010) based on the modern paleomagnetic study of biostratigraphically wellcontrolled platform carbonates (Márton et al., 2003, 2008) and also pelagic sediments from the foreland of the Southern Alps (Márton et al., 2010). Thus, a reliable reference framework is now available to which the paleomagnetic directions from the tectonically more complicated fold and thrust belts can be compared. We started such investigations in 2004 and more or less the same time Lewandowski
94
E. Márton et al. / Tectonophysics 490 (2010) 93–102
et al. took 18 hand samples from the post-Carboniferous deposits of the Velebit Mt. for rock magnetic and paleomagnetic investigations. The results they published are interesting from the rock magnetic point of view, but the large scatter of the paleomagnetic vectors prevented the authors to define paleomagnetic directions for localities or age groups (Lewandowski et al., 2009). In this paper we are presenting new and also biostratigraphically updated Late Cretaceous paleomagnetic results from the Northern Adriatic islands, new Early Jurassic–Earliest Cretaceous paleomagnetic observations from the Adriatic mainland in the NW part of Croatia, including the Velebit Mt. and results from Middle Jurassic platform carbonates in autochthonous Istria which conveniently contribute to the already published Late Jurassic–Eocene APW for stable Adria. 2. Geological background The studied localities (except one, which belongs to stable Istria) are situated in the External Dinarides, which is the NE deformed margin of
the Adriatic microplate (Fig. 1). The whole area is composed of thick succession of sediments (up to 10 km, Vlahović et al., 2005) ranging in age from the Carboniferous to the Eocene. Igneous rocks overlain by siliciclastics occur only locally. The sediments were deposited in shallow water environment, first in an epiric sea (Carboniferous to the mid-Triassic) and after a long emergent phase, in a shallow water carbonate platform environment (latest Triassic to the middle Eocene), from time to time interrupted by episodes of emersion or drowning. It is a matter of discussion if during the second time span, a single platform existed (Adriatic–Dinaridic carbonate platform, Grandić, 1974; Gušić and Jelaska, 1993; Pamić et al., 1998; Vlahović et al., 2005) or there had been two carbonate platforms. In the two platform models (D'Argenio et al., 1971; Chorovitz, 1975) a deep-water interplatformal belt separates the Adriaticum and the Dinaricum, which is called Epiadriticum (Herak, 1986), or Budva–Cukali basin (Tari, 2002). In a recently published paper, Korbar (2009) also distinguishes an Adriatic and a Dinaridic segment of the Adriatic–Dinaridic carbonate platform domain.
Fig. 1. Tectonic map of the Adriatic region (redrawn and simplified after Korbar, 2009) showing the distribution of the major thrusts and faults zones. a = NE Adriatic fault zone; b = thrust front of the External Dinarides; c = supposed thrust front of the External Dinarides; d = Dinaridic SW unit thrusts; e = Adriatic NE unit thrusts; f = Dinaridic NE unit thrust, g = other major faults; h = sampling localities. From Ist and surrounding islands and from Cres island only the localities with positive results are shown.
E. Márton et al. / Tectonophysics 490 (2010) 93–102
95
Table 1a Summary of sedimentological structures, fossil content, age description and locality mean palaeomagnetic directions for samples collected on the Adriatic mainland, based on the results of principal component analysis (Kirschvink, 1980), except otherwise indicated ep + gc: combination of stable end points and re-magnetization circles (McFadden and McElhinny, 1988). Localities are numbered according to Fig. 1. Key: Lat.N, Lon.E: Geographic coordinates (WGS84) measured by GPS; 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). The following localities have precise age determination: 4 — Aalenian–Bajocian, 5 — Bathonian and 10 — Oxfordian. The applied limestone classification considers the dominating groundmass types and depositional fabric (Dunham, 1962).
1
2 3
4
5
Locality
Lat.N, Lon.E
Sedimentol. structure
Early Jurassic Velebit, Prezid, HR 1267-277
44° 15′ 57″ 15° 48′ 42″
Mudstones & dolomitized mudstones Ooidal grainstones
Velebit, Mali Alan, HR 1043-050 Melnice HR 1022-027 Mid Jurassic Gornje Jelenje HR 1051-062
44° 15° 44° 15°
17′ 13″ 38′ 55″ 57′ 45″ 02′ 13″
45° 22′ 26″ 14° 34′ 28″
Velebit, Mali Alan, 44° 16′ 37″ HR 1035-042 15° 38′ 30″
6
Velebit, Sušanj HR 1222-233 7 Velebit, Prezid, HR 1257-266 8 Breze HR 1005-015 9 Banska Vrata HR 1016-021 Late Jurassic 10 Velebit, Mali Alan HR 1028-034
44° 15° 44° 15° 45° 14° 45° 14°
31′ 50″ 07′ 45″ 15′ 31″ 48′ 44″ 11′ 50″ 53′ 41″ 09′ 48″ 57′ 11″
Fossil content
Glomospira sp.
Mudstones
Mudstones
Mesoendothyra croatica Gušić
Mudstones to packstones
Palaeopfenderina salernitana (Sartoni and Crescenti), Verneulina sp., Thaumatoporella parvovesiculifera (Raineri) Mudstones to peloidal Favreina sp., wackestones Mesoendothyra sp. Mudstones to peloidal Siphovalvulina sp. wackestones Mudstones Mesoendothyra sp. Mudstones
Pelecypod casts
44° 16′ 26″ 15° 38′ 33″
Mudstones to wackestones
11 Velebit, Prezid, HR 1251-256
44° 15′ 16″ 15° 48′ 39″
Peloidal wackestones to packstones
Kurnubia jurassica (Henson), Verneulina sp. Unidentified benthic foraminifera
12 Velebit, Sušanj HR 1234-239 13 Velebit, Otočac HR1240-250
44° 15° 44° 15°
31′ 40″ 07′ 37″ 51′ 44″ 10′ 18″
Peloidal wackestones Peloidal wackestones to laminated packstones
14 Platak HR 1063-067 15 Žuta Lokva HR 689-698 Early Cretaceous 16 Ogulin HR 699-712
45° 14° 44° 15°
23′ 50″ 32′ 56″ 58′ 04″ 04′ 30″
Ooidal grainstones
45° 16′ 08″ 15° 16′ 09″
Ostracods, gastropods, unidentified benthic foraminifera Cyanophyta balls
Mudstones
Mudstones to grainstones
Moncharmontia apenninica (de Castro), Cuneolina sp., Nummuloculina sp., ostracods
α95° Dip
Remark
50 + 62 18 332 + 49 452
19 2
171/27
Softer Harder
8
332 + 52 65
8
200/42
332 + 34 39 4 + 23 84
15 8
309 + 33 39 349 + 45 84
15 8
222/30
4/4 4/4 4/4 7/8
42 22 7 8
11 14 8 8
41 9 317 357
11 14 8 8
223/33
3/12
354 −6
n/no
D°
Early Jurassic
10/11
24 + 42 18 337 + 23 452
19 2
Early Jurassic Early Jurassic
7/8
351 + 18 65
4/6 5/6
Aalenian– Bajocian Bathonian
I°
+ 42 + 25 + 49 + 14
k
α95° DC°
Age
74 75 122 71
+ 75 + 54 + 67 + 58
k
74 75 122 71
5/10
337 + 29 28
10/11 11/11 6/6 6/6
334 25 328 16
Oxfordian
4/7
352 0
Late Jurassic
0/6
too weak
Late Jurassic Late Jurassic
5/6
7
+ 47 91
8
221 + 59 91
8
202/71
11/11
329 + 35 60
6
338 + 49 60
6
118/17
Late Jurassic Late Jurassic
0/5
too weak
10/10
338 + 41 49
7
335 + 46 49
7
170/15
Early 14/14 Cretaceous 10/14
278 + 51 39 245 −44 97
6 5
285 + 42 39 234 + 49 97
6 5
340/11
The fold and thrust belt of the External Dinarides show a complex geotectonic structure, which was the object of numerous studies for a long period of time (e. g. Aubouin, 1973; Dimitrijević, 1982; Herak, 1986). More recently two geotectonic models of the region were published suggesting different subdivisions for the NE part of Adria. Tari (2002) defines three main units, which are separated by two NW– SE oriented major thrust fronts. These are Stable Adria, Imbricated Adria and the Dinaric Nappe System. The boundary which separates Imbricated Adria from the Dinaric Nappe System is also recognized by Korbar (2009) as an important tectonic feature, but with a different role in the tectonic evolution of the area. In his interpretation this structure
340 + 45 1035 4
204/61
15
330 + 54 28
15
172/27
48 68 107 123
7 6 7 6
299 339 309 344
48 68 107 123
7 6 7 6
340/11
111
9
335 + 30 111
9
228/62
+ 35 + 57 + 38 + 52
Softer Harder
Site1 Site2 Site3 200/46 ep + gc
Middle Jurassic Middle Jurassic Middle Jurassic Middle Jurassic
+ 54 + 44 + 53 + 51
1035 4
IC°
Endpoints
Softer Harder 267/24 Softer Harder
141/38
230/42
Softer Harder
is a long-lived (from Triassic to recent) right lateral transcurrent fault (NE Adriatic fault) which separates Adriatic and Dinaridic domains (Fig. 1). However, he suggests that on the surface there are Dinaridic elements even SW of the NE Adriatic fault, which were transported as nappes on top of the Adriatic domain. Within the Dinaridic domain, Korbar (2009) distinguishes several thrust units. The most important boundary between these seems to be what separates the Inner Karst from the High Karst (Fig. 1, thrust boundary f). Both authors suggest further subdivisions. According to Tari (2002) Imbricated Adria is subdivided by transversal faults into three segments which are, from north to the south, Imbricated Adria
96
E. Márton et al. / Tectonophysics 490 (2010) 93–102
Table 1b Summary of sedimentological structures, fossil content, age description and locality mean palaeomagnetic directions for samples collected from Stable Istria and from Cres island, based on the results of principal component analysis (Kirschvink, 1980). Localities are numbered according to Fig. 1. Key as for Table 1a. Results for localities 18, 19, 21–24 were published earlier (Márton et al., 1990) but the stratigraphic ages are revised. According to the new age assignment all localities from Cres are of Albian age, except locality 21, which is lower Cenomanian. The applied limestone classification considers the dominating groundmass types and depositional fabric (Dunham, 1962). Locality Stable Istria Bathonian 17 Rovinj, Monsena HR 1121-136 Cres Island Late Cretaceous 18 Punta Križa YM 814-826
k
α95° Dip
Fossil content
Age
n/no
D°
45° 06′ 30″
Mudstones to wackestones
Palaeopfenderina salernitana (Sartoni and Crescenti)
Bathonian
11/16
314 + 39 29
9
309 + 43 29
9
188/7
Albian (earlier: Barremian–Aptian)
11/13
322 + 35 29
9
324 + 47 56
6
85/15 129/12 179/20
Albian (earlier: Barremian– Aptian)
9/9
324 + 63 135 4
348 + 50 135 4
28/20
Albian
6/9
322 + 25 112 6
341 + 39 112 6
85/33
Lower Cenomanian 12/12 (earlier: Cenomanian– Turonian)
320 + 53 52
7
346 + 41 52
7
44/26
Albian (earlier: Cenomanian– Turonian)
7/7
293 + 31 30
11
312 + 58 30
11
86/32
Albian (earlier: Cenomanian– Turonian)
12/12
311 + 51 23
9
311 + 49 24
9
96/6 102/10 236/10 314/16
Albian (earlier: Cenomanian– Turonian)
7/7
292 + 45 31
11
341 + 46 31
11
54/47
44° 38′ 22″ 14° 30′ 04″
19 Krčina YM 827-835
44° 55′ 25″ 14° 25′ 16″
20 Hrasta HR 1278-286
44° 48′ 42″ 14° 24′ 38″
21 Osor YM 836-847
44° 42′ 24″ 14° 24′ 25″
22 Valun YM 860-866
44° 53′ 38″ 14° 22′ 55″
23 Valun
44° 53′ 38″
YM 867-878
14° 22′ 55″
24 Predoščica YM 885-891
45° 02′ 19″ 14° 22′ 31″
Mudstones to wackestones/ Pseudonummuloculina Packstones to grainstones heimi (Bonet), Istriloculina sp., ostracods Mudstones to packstones – Pseudonummuloculina grainstones heimi (Bonet), Rumanoloculina sp., Istriloculina sp. Mudstones to wackestones/ Pseudonommuloculina Packstones to grainstones heimi (Bonet), Axiopolina sp., Istriloculina sp. Wackestones to packstones Aeolisaccus kotori (Radoičić), Nezzazata sp., ostracods, unidentified miliolids Mudstones to wackestones/ Pseudonummuloculina Packstones to grainstones heimi (Bonet), Rumanoloculina sp., Istriloculina sp., Axiopolina sp. Mudstones to wackestones/ Pseudonummuloculina Packstones to grainstones heimi (Bonet), Rumanoloculina sp., Istriloculina sp., Axiopolina sp. Grainstones to laminated Pseudonummuloculina mudstones to grainstones heimi (Bonet), Rumanoloculina sp.
north of Istria, Imbricated Adria of Kvarner islands and Imbricated Adria of Central Dalmatia. She does not, however, continue the faults into the Dinaric Nappe System. Korbar's model (2009) contains more units, since he recognizes several transversal faults which cut not only the Adriatic but also the Dinaridic domains. Eventually, he defines two Adriatic and four Dinaridic domains between the Kvarner and the Jabuka–Vis faults (Fig. 1). He considers these as distinct tectonostratigraphic entities although their stratigraphic evolution does not always show profound differences. 3. Paleomagnetic and biostratigraphic samplings In order to obtain biostratigraphically well-constrained paleomagnetic directions from different units of the NW segment of the External Dinarides we visited a large number of localities (e.g. fresh outcrops along the newly constructed motor way, working and abandoned quarries and natural outcrops) in the years of 2004–2009 on the Adriatic mainland, and on the northern Adriatic islands. Eventually, 260 paleomagnetic samples were drilled from 28 localities. We also inspected Middle Jurassic outcrops in the Rovinj area (the oldest rocks on the surface in stable Istria) and collected 13 samples in order to obtain a Middle Jurassic reference direction for direct comparison with results of similar age from the External Dinarides, i.e. from the Adriatic mainland (Tables 1a, 1b and 1c).
k
IC°
Sedimentol. structure
13° 36′ 44″
I°
α95° DC°
Lat.N, Lon.E
The beds for drilling were carefully selected. They were preferably the mud supported type, non-fossiliferous limestones, devoid of visible late diagenetic changes and secondary alterations. The paleomagnetic cores were oriented in the field with a magnetic compass. For biostratigraphic interpretation hand samples were collected either from the beds drilled for paleomagnetic measurements, or from a bed that represents lateral extension of the sampled one. In the case when pinch-out relationship did not exist we took hand samples from neighbouring beds. The sampling localities are geographically distributed (Fig. 1) and represent a fairly long time span (Tables 1a, 1b and 1c). Nevertheless, there is a concentration of the sampling localities in the Velebit Mt., where the rocks were collected along three sections crossing the mountains at different longitudes. The most westerly is exposed along the Otočac–Karlobag road (Fig. 1, localities 6, 12, and 13,), the most easterly is the Prezid section (Fig. 1, localities 1, 7 and 11) and the middle one is the Mali Alan section (Fig. 1, localities 2, 5 and 10).
4. Age and lithology of the sampled rocks The oldest sediments sampled are of Early Jurassic age (Table 1a and Fig. 1, localities 1–3). They are dark grey mud supported limestones (wackestones) which alternate with grain-supported varieties (ooidal
E. Márton et al. / Tectonophysics 490 (2010) 93–102
97
Table 1c Summary of sedimentological structures, fossil content, age description and locality mean palaeomagnetic directions for samples collected on Ist and surrounding islands, based on the results of principal component analysis (Kirschvink, 1980). Localities are numbered according to Fig. 1. Key as for Table 1a. Paleomagnetic results were published earlier (Márton and Moro, 2009), but the precise age assignments for the localities is published for the first time. Locality
25 26
27 28
Lat.N, Lon.E Sedimentol. structure
Ist and surrounding islands Late Cretaceous Ist, port 44° 16′ 45″ HR 953-961 14° 45′ 26″ Ist, abandoned 44° 15′ 23″ quarry HR 987-995 14° 45′ 55″ Tramerka 44° 13′ 22″ HR 996-1004 14° 46′ 08″ Funestrala 44° 15′ 08″ HR 1155-166 14° 44′ 39″
29 Silba, Nozdre bay HR 962-971 30 Premuda, Široka bay HR 1137-154
44° 21′ 20″ 14° 43′ 24″ 44° 19′ 05″
Wackestones to packstones/grainstones Wackestones to packstones/grainstones
k
α95° Dip
n/no D°
Nummuloculina sp.
Cenomanian
7/9
340 + 52 21
Miliolids
Cenomanian
9/9
336 + 49 108 5
338 + 48 108 5
314/10 252/25 42/2
Cenomanian
6/9
337 + 51 230 4
325 + 45 230 4
271/12
Cenomanian
8/12
330 + 44 98
6
332 + 48 98
6
125/5
Turonian–Santonian
6/10
328 + 40 55
9
334 + 48 55
9
110/10
Santonian–Campanian
5/18
3
−20
9
347 + 45 73
9
219/82
Aeolisaccus kotori (Radoičić) Mudstones-wackestones Broeckina to packstones/grainstones (Pastrikella) balcanica Cherci, Radoičić & Schroeder Wackestones-packstones Calcispheres, pithonellas Mudstones –wackestones Calcispheres, globotruncanids
k
IC°
Age
Wackestones-packstones
I°
α95° DC°
Fossil content
73
13
329 + 42 42
9
14° 37′ 52″
to peloidal grainstones) deposited in peritidal conditions. These beds contain rare fossils. Middle Jurassic massive, thick bedded dark grey mudstones were sampled at localities 4–9 in the External Dinarides and well-bedded cream coloured mudstones to wackestones at locality 17 from stable Istria. Benthonic foraminifera allowed age determination with high precision for locality 5 (Bathonian), for locality 4 (Aalenian–Bajocian) and for locality 17 (Bathonian) (Tables 1a and 1b). The Late Jurassic limestones are typically coarse grained ooidal grainstones that pass into peloidal wackestones to mudstones with rare foraminifera. We collected the thin bedded mud supported variety (localities 10–15). Sporadic occurrences of benthic foraminifera allowed precise age determination for locality 10, pointing to Oxfordian age. Although the Cretaceous sediments in the Adriatic mainland are widespread and we inspected several working or abandoned quarries, in addition to the fresh road cuts along the Zagreb–Split motorway, we
found only one locality in the Dinaridic domain (locality 16, fresh road cut on the new motorway) suitable for sampling. There we collected dark grey mudstones for paleomagnetic measurements, and the grainsupported variety for biostratigraphic analysis. At the other places, the limestones were karstified, or massive without visible bedding plane, or coarse grained with considerable biogenetic components. On the Northern Adriatic islands the oldest outcrops are of Cretaceous age. They are characteristically shallow water peritidal limestones and subordinately pelagic limestones, light grey or white in colour. Earlier, a number of localities were paleomagnetically studied from the islands of this area (Márton et al., 1990). Since that time the ages of several rock units have been revised, therefore we returned to Cres island, which yielded the best paleomagnetic results from the Kvarner islands, collected hand samples for biostratigraphic age determination and took the GPS coordinates from the localities which provided good quality paleomagnetic directions and added a new
Fig. 2. Typical demagnetization curves for the Late Cretaceous platform carbonates from Cres island (HR1279B) and for the Doggerian from stable Istria (HR1127 and 1131). accompanied by NRM intensity versus demagnetizing field diagrams. AF demagnetizations. Key to Zijderveld diagrams: full dots: projection of the NRM vector onto the horizontal, circles: into the vertical. Geographical system.
98
E. Márton et al. / Tectonophysics 490 (2010) 93–102
Fig. 3. Typical demagnetization curves for Early (HR1025, HR1273A), Middle (HR1016, HR1222, HR1036) and Late (HR697, HR1236, HR1241B) Jurassic platform carbonates from the External Dinarides. Zijderveld diagrams are accompanied by intensity (circles) versus demagnetizing field diagrams, when the method of demagnetization is AF and by NRM intensity (circles)/susceptibility (dots) versus temperature diagrams, when the method of demagnetization is thermal.
locality (locality 20) to the previous collection. It is important to note that indeed the stratigraphic ages of most of the earlier published paleomagnetic directions from Cres island had to be seriously revised (Table 1b). Further to the south, paleomagnetic and biostratigraphic samples were collected from Ist and surrounding islands. From this region the paleomagnetic results were published by Márton and Moro (2009), but the precise biostratigraphic age assignment for the sampled localities is published the first time in the present paper (Table 1c).
5. Laboratory processing of the paleomagnetic samples and results The drill cores were cut in the laboratory into standard-size specimens which were subjected first to measurements of the natural remanent magnetization (NRM) and the magnetic susceptibility in the natural state. The instruments used were JR-4 and JR-5A magnetometers and a KLY-2 Kappabridge. Sister specimens of
E. Márton et al. / Tectonophysics 490 (2010) 93–102
99
Fig. 4. Typical demagnetization curve for the Early Cretaceous locality 16. Zijderveld diagram is accompanied by NRM intensity versus demagnetizing field diagram and by the stereographic plot showing the directional change of the NRM on demagnetization. Key as for Fig. 3.
representative samples were then stepwise demagnetized, one of them with alternating field (AF), the other with thermal (TH) method, till the NRM signal was lost. Based on the experience with the pilot specimens the rest of the samples from the respective localities were demagnetized in several increments with the method which provided better defined demagnetization curves. In case of extremely weak initial NRM intensities (below 5 × 10− 5 A/m), only AF demagnetization was used, as years of experience with platform carbonates showed that the thermal method in such cases was not practicable (Márton et al., 2003, 2008). In the samples collected from stable Istria (Middle Jurassic) and from the Northern Adriatic islands the NRM signal was extremely weak (usually 2–9 × 10− 5 A/m, but for localities 17, 20 and 28 values up to 65× 10− 5 A/m were measured). The magnetic susceptibilities were negative (12–15× 10− 6 SI), except for locality 17, where low positive values were measured (below 10 × 10− 6 SI). Nevertheless, six of the Late Cretaceous localities representing Ist and the surrounding islands, as documented in Márton and Moro (2009) and the samples from Cres island yielded well-defined demagnetization curves (Fig. 2, HR1279B). The initial NRM intensities and the susceptibilities for the samples from the Adriatic mainland (mostly of Jurassic age) are extremely variable, the former is in the range of 1– 2800 × 10− 5 SI, the latter from minus 13 to plus 732 × 10− 6 SI. Except for localities 11 and 14, the NRM was sufficiently strong to lend itself to demagnetization in increments and the resulting demagnetization curves are good (Figs. 3 and 4). Concerning locality Rovinj (Monsena) from stable Istria, the specimens with higher NRM intensities exhibited a large overprint component which was successfully removed (Fig. 2, HR1127), the weaker specimens yielded rather noisy demagnetization curves (Fig. 2, HR1131) or their intensities dropped to the noise level of the measuring instruments at the first step of demagnetization (3 mT, these samples were omitted from further evaluation). As the magnetic mineralogy experiments suggest, the magnetic minerals in all the studied localities are dominantly or exclusively (Ist and surrounding islands) soft (Fig. 5). The mineral which appears as soft on the curves of the acquisition of the isothermal remanence (IRM) is magnetite, although in some cases the presence of maghemite is evidenced by decreasing susceptibility on heating above 250° (e.g. HR1025, Fig. 3). The harder component of the IRM
must be connected to hematite, which does not seem to contribute to the NRM (fast reduction of the NRM intensity on AF demagnetization and the elimination of the NRM on thermal demagnetization before the Curie point of the magnetite). Following the demagnetization experiments, the curves were subjected to component analysis (Kirschvink, 1980) and the directions of the characteristic remanences (and in some cases those of the overprint magnetizations, denoted as soft, indicating also lowblocking components when the method of demagnetization was thermal) determined. The results before and after tilt corrections are shown in Tables 1a, 1b and 1c.
Fig. 5. The acquisition behaviour of the isothermal remanent magnetization (IRM). The acquisition curves indicate solely (HR1131) or dominantly soft magnetic mineral (most probably magnetite, sometimes accompanied by maghemite, as deduced from the decrease of the magnetic susceptibility on heating, e.g. HR1025 and HR1273 in Fig. 3. Note that maghemite decomposes before the component interpreted in terms of tectonics appears. In several samples minor amount of hematite is evidenced by the moderate increase of the IRM intensity from 0.2 T onward (HR1270B, HR1241A, HR1279B, and HR1241B), but this mineral is not likely to contribute to the NRM, for the NRM always unblocks below the Curie point of the magnetite (Fig. 3, HR1025, HR1273A, HR1222, and HR1236, HR1241B).
100
E. Márton et al. / Tectonophysics 490 (2010) 93–102
Table 2 Summary of the overall-mean paleomagnetic directions and paleomagnetic poles for the Northern Adriatic islands, the Adriatic mainland and for a single Middle Jurassic locality from stable Istria before and after tilt corrections. Key as for Table 1a and Lat. and Lon. are the coordinates of the paleomagnetic pole. N is number of localities, and (n) number of samples.
Cres island, Albian (Early Cenomanian) Ist and surrounding islands, Cenomanian (Turonian–Santonian) Adriatic mainland Early Jurassic Middle Jurassic Late Jurassic Stable Istria, Rovinj, Bathonian
N
D°
I°
k
α95°
DC°
IC°
k
α95°
Lat.
Lon.
k
α95°
7 6 3 5 3 (11)
311.3 340.1 350.7 2.4 340.6 313.8
+ 44.2 + 38.1 + 21.7 + 27.5 + 25.9 + 38.9
23 8 40 8 11 29
12.7 25.4 19.7 27.9 39.0 8.7
333.0 334.1 338.1 341.7 335.9 308.8
+ 48.0 + 46.2 + 49.0 + 53.5 + 41.7 + 42.7
44 188 118 112 62 29
9.1 4.9 11.4 7.2 15.8 8.7
63.3 63.4 67.2 72.6 61.9 44.3
258.5 254.5 253.3 254.8 247.2 275.0
30 129 74 45 130 29
11.1 5.9 14.4 8.9 10.9 8.7
6. Discussion and conclusions The sampling localities from the Northern Adriatic islands are conveniently divided into two groups. The first is an Albian group of localities (18–24, the group includes one Early Cenomanian locality) from Cres, the second is a Cenomanian–Campanian one from Ist and surrounding islands (localities 25–30). The overall-mean paleomagnetic directions calculated for the first (Table 2 and Fig. 6) have better statistical parameters after than before applying tilt corrections for the respective localities, the Enkin (2003) test is positive and the best statistics is achieved at 74% untilting, indicating that the characteristic remanence is basically of pre-tilting origin. For the second group the tilt test (based on k statistics) is positive and yields the best result close to 100% untilting, which points to the purely pre-tilting age of the magnetizations (Márton and Moro, 2009). The overall-mean directions for the above two groups can be compared directly with coeval paleomagnetic directions from stable Adria, which are biostratigaphically and also paleomagnetically wellconstrained (Márton et al., 2010). The plot of the paleomagnetic overall directions with their respective confidence circles from stable Adria and the Northern Adriatic islands are statistically identical (Fig. 7A), i.e. there is no indication for relative movements between the two realms after the Campanian (after about 72 Ma). On the Adriatic mainland all the sampled localities belong to the External Dinarides s.s., except locality 16 which is situated in the Inner Karst, i.e. in a higher tectonic position (Korbar, 2009) than e.g. the localities in the Lika area or in the Velebit Mt. (Fig. 1). The results from the External Dinarides can be subdivided into Early, Middle and Late Jurassic groups (Table 1a). In the following discussion the harder/ higher blocking (or the only) components of the NRM will be evaluated from the viewpoint of the time of the acquisition of the remanence in relation to the deformation in the area (most probably taking place during Late Eocene, e.g. Korbar, 2009). The Early Jurassic localities have extremely shallow inclinations before tilt corrections, which becomes 49° after corrections while the scatter of the directions is at minimum close to 100% untilting (Table 2 and Fig. 6), suggesting that the magnetization was acquired before deformation. The highly scattered Doggerian locality mean paleomagnetic directions (in the geographic coordinate system) become tightly clustered after tilt corrections (Fig. 6), the best statistics is achieved at 110% untilting and the test (based on k statistics) is positive (Table 2). From the calculations locality 4 is omitted since the different sites sampled from this locality do not yield consistent paleomagnetic directions (Table 1a). The overall-mean paleomagnetic direction for the Late Jurassic has the best statistical parameters close to 100% untilting, the scatter of the individual directions is considerably reduced (Table 2 and Fig. 6) suggesting that the magnetizations were acquired prior to deformation. From the calculation locality 12 is omitted, because it is suspected of full remagnetization quite recently (the paleomagnetic direction is close to the direction of the present Earth magnetic field in the sampling area).
The above groups of overall-mean paleomagnetic directions are characterized by moderate CCW rotation of the declinations with respect to the present North. The single locality from the Inner Karst exhibits two NRM components (Fig. 4), which are significantly different in direction from that of the present Earth magnetic field. The magnetically softer component shows large CCW, the harder component exhibits large CW rotation with respect to the present North. The tectonic interpretation of the two paleomagnetic vectors from this locality is problematic, since it is an isolated observation, so far the only representative of the Inner Karst. From a study by Márton et al. (2010) a paleomagnetic reference direction for the Tithonian–Berriasian of stable Adria (from direct measurements) is available for comparison with the Late Jurassic group of localities from the External Dinarides. In addition, the present study provides a reference direction for the Doggerian, based on locality 17 from Istria. Compared to the Late and Middle Jurassic paleomagnetic directions for stable Adria, the Late and Middle Jurassic paleomagnetic directions from the External Dinarides exhibit an about 30° CW rotation while the respective inclinations are close (Fig. 7B). The above discussed relations between the paleomagnetic directions from stable Adria, on one hand and from the Adriatic mainland and the Northern Adriatic islands, on the other hand have important tectonic implications. The close agreement between coeval Late Cretaceous paleomagnetic directions from stable Adria and from the North Adriatic islands supports the model of Tari (2002) in which the mentioned islands belong to the deformed margin of stable Adria (Imbricated Adria). In the model by Korbar (2009) the Northern Adriatic islands are Dinaridic elements in nappe position (thin skinned tectonics), which is unlikely since the studied segment of the External Dinarides on the mainland shows distinctly less rotation in the CCW sense than the coeval paleomagnetic declinations for stable Adria (Middle and Late Jurassic). The tectonic implication of the 30° difference between Jurassic declinations for the Northern segment of the External Dinarides s.s. and stable Adria is less straightforward for lack of younger than Jurassic data from the former. However, relying on the general opinion that the External Dinarides and Adria became docked during Late Eocene deformation processes, which means that the External Dinarides must have participated in the post-Eocene CCW rotation of about 30° of Adria, we are obliged to conclude that the External Dinarides rotated in the CW sense with respect to Adria before the Late Eocene. In such case, there is a possibility that the Adriatic and Dinaric (Dinaridic domain in Korbar, 2009) platforms were indeed independent after the Middle Triassic (but could not be too far from each other, for the coeval inclinations are practically identical, see Table 2) and the difference in declinations is due to the displacements which brought the two platforms (domains) into contact. Another possibility is that the External Dinarides rotated in CW sense during trusting over the Adriaticum. In this case, we do not have to calculate with two independent platforms. Finally, it has to be mentioned that
E. Márton et al. / Tectonophysics 490 (2010) 93–102
101
Fig. 6. Locality mean paleomagnetic directions (dots) with α95 (circles) for the Albian (Early Cenomanian) platform carbonates from Cres island, for the Early, Middle and Late Jurassic platform carbonates from the Adriatic mainland before and after tilt corrections. The overall-mean paleomagnetic directions (diamonds) with confidence circles are also shown. Stereographic plots.
the paleomagnetic observations so far available do not exclude the possibility that the Dinaricum and the Adriaticum moved independently even after the Eocene and the observed CCW rotations for Adria reflect African origin and decoupling from Africa with a net CCW rotation after the Eocene. At the same time the External Dinarides were not belonging to the African plate and the CCW rotations observed on the Jurassic rocks are the consequence of the thrusting of the Dinaricum over the Adriaticum, after the final CCW rotation of Adria. In order to exclude the last possibility, systematic paleomag-
netic studies are needed in the future on the sediments of the Neogene intramontaine basins of the External Dinarides. Acknowledgement The authors thank Tvrtko Korbar for the fruitful discussion about the tectonics of the study area, Gábor Imre for assistance in the field and in the laboratory, Robert Košćal for technical assistance, and Slavijan Deroko for boat transportation. Constructive suggestions to
102
E. Márton et al. / Tectonophysics 490 (2010) 93–102
Fig. 7. Paleomagnetic overall-mean directions A: for Cres island and for Ist and surrounding islands (Table 2) compared with coeval reference directions from stable Adria (Márton et al., 2010) and B: for the Adriatic mainland (Early, Middle and Late Jurassic) compared with Tithonian–Berriasian (Márton et al., 2010) and Bathonian (present study) paleomagnetic directions from stable Adria.
improve the manuscript by the referees are gratefully acknowledged. This work was supported by the Croatian–Hungarian Intergovernmental Scientific and Technological projects nos. Hr-18/2004 and Cro06/2006. Additional support was provided by the Hungarian Scientific Research Fund OTKA project no. K049616, and the Croatian Ministry of Science, Education and Sports project nos. 119-1191152-1167 and 119-1191152-1171.
References Aubouin, J., 1973. Des tectoniques superposees et de leur signification par rapport aux modeles geophysiques, lexemple des Dinarides, paleotectonique, tectonique, tarditectonique, neotectonique. Bulletin de la Societe Geologique de France 15 (5–6), 426–460. Chorovitz, J., 1975. Le devenir de la zone du Budva vers le Nord-Ouest de la Yougoslavie. Bulletin de la Societe Geologique de France 7 (17), 699–709. D'Argenio, B., Radoičić, R., Sgrosso, I., 1971. A paleogeographic section through the Italodinaric external zone during Jurassic and Cretaceous times. Nafta 22 (4–5), 195–207. Dimitrijević, M.D., 1982. Dinarides, an outline of the tectonics. Earth Evolution Sciences 2 (1), 4–23. Dunham, R.J., 1962. Classification of carbonate rocks according to depositional texture. In: Ham, W.E. (Ed.), Classification of Carbonate Rocks. A Symposium: Amer. Ass. Petrol. Geol. Mem., I, pp. 108–171. Enkin, R., 2003. The direction-correction tilt test: an all-purpose tilt/fold test for paleomagnetic studies. Earth and Planetary Science Letters 212, 151–166. Fisher, R.A., 1953. Dispersion on a sphere. Proceedings. Royal Society of London 217, 295–305. Grandić, S., 1974. Some regional petroleum geological characteristics of the External Dinarides deposits. Nafta 25 (3), 111–120. Gušić, J., Jelaska, V., 1993. Upper Cenomanian–Lower Turonian sea-level rise and its consequences on the Adriatic–Dinaric carbonate platform. Geologische Rundschau 82, 676–686. Herak, M., 1986. A new concept of geotectonics of Dinarides. Acta Geologica 16, 1–42.
Kirschvink, J.L., 1980. The least-squares line and plane and the analysis of paleomagnetic data. Geophysical Journal of the Royal Astronomical Society 62, 699–718. Korbar, T., 2009. Orogenic evolution of the External Dinarides in the NE Adriatic region a model constrained by tectonostratigraphy of Upper Cretaceous to Paleogene carbonates. Earth Science Reviews 96 (4), 296–312. Lewandowski, M., Velić, I., Sidorczuk, M., Vlahović, I., Velić, J., 2009. First rock magnetic and paleomagnetic analyses of the pre-Cenozoic rocks of the Velebit Mt. (Croatia): prospects for applications in paleogeographic and geotectonic studies. Geologia Croatica 62 (1), 45–61. Márton, E., Moro, A., 2009. New paleomagnetic results from imbricated Adria, Ist island and related areas. Geologia Croatica 62 (2), 107–114. Márton, E., Veljović, D., 1983. Paleomagnetism of the Istria peninsula, Yugoslavia. Tectonophysics 91, 73–87. Márton, E., Veljović, D., 1987. Paleomagnetism of Cretaceous carbonates from the northwestern part of the Dinaric fold belt. Tectonophysics 134, 331–338. Márton, E., Milicević, V., Veljović, D., 1990. Paleomagnetism of Kvarner Islands, Yugoslavia. Physics of the Earth and Planetary Interiors 62, 70–81. Márton, E., Drobne, K., Ćosović, V., Moro, A., 2003. Palaeomagnetic evidence for Tertiary counterclockwise rotation of Adria. Tectonophysics 377, 143–156. Márton, E., Ćosović, V., Moro, A., Zvocak, S., 2008. The motion of Adria during the Late Jurassic and Cretaceous: New paleomagnetic results from stable Istria. Tectonophysics 454, 44–53. Márton, E., Zampieri, D., Ćosović, V., Moro, A., 2010. New Cretaceous paleomagnetic results from the foreland of the Southern Alps and the refined apparent polar wander path for stable Adria. Tectonophysics 480, 57–72. McFadden, P.L., McElhinny, M.W., 1988. The combined analysis of remagnetization circles and direct observations in paleomagnetism. Earth and Planetary Science Letters 87, 161–172. Pamić, J., Gušić, I., Jelaska, V., 1998. Geodynamic evolution of the Central Dinarides. Tectonophysics 297 (4), 251–268. Vlahović, I., Tišljar, J., Velić, I., Matičec, D., 2005. Evolution of the Adriatic Carbonate Platform paleogeography, main events and depositional dynamics. Palaeogeography, Palaeoclimatology, Palaeoecology 220 (3–4), 333–360. Tari, V., 2002. Evolution of the Northern and Western Dinarides, a tectonostratigraphic approach. European Geosciences Union: Stephan Mueller Special Publication, 1, pp. 223–236.