Paleomagnetic Implications on the Stability of the Moesian Platform and the Bulgarian Rhodope Since the Paleogene: Surviving in Between Two Major Rotating Systems
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Paleomagnetic implications on the stability of the Moesian Platform and the Bulgarian Rhodope since the Paleogene: surviving in between two major rotating systems Karen Oud MSc Geology student, Faculty of Geosciences, Utrecht University July 2008 Abstract To investigate whether or not the Moesian Platform and the Bulgarian Rhodope accommodated for the major clockwise rotation of both the Carpathian and Aegean systems since 13 Ma, paleomagnetic analysis was done on Paleogene sediments and volcanics from the Moesian Platform and volcanics from the Bulgarian Rhodope. The mean paleomagnetic direction for the overall Bulgarian region shows a declination of 11 o ± 10 o N at 30 Ma. Hence, with respect to the overall rotation of the Eurasian continent, the Bulgarian region is a stable block with a negligible rotation of 4.3 o ± 10.9 o counterclockwise since 30 Ma. 1. Introduction systems. Schmid et al (1998) state that during the Late Cretaceous, the western The Moesian Platform in northern part of the Rhodope moved northwards, Bulgaria lies in between two domains past the Moesian Platform. They also that have shown a clear rotation since state that the stable Moesian Platform the Middle Miocene, which is proven in served as a corner, around which several studies. The southern Carpathian oroclinal bending (arc formation) of the foreland in Romania, north of the Southern Carpathians occurred during Moesian Platform, rotated from 13 to 8 the Eocene. This bending would cause Ma by 30 o clockwise (CW) (Dupont-Nivet major dextral wrenching and clockwise et al, 2005). During the same period, the rotations in the Southern Carpathian Greek Rhodope massif in the northeast units relative to the Moesian Platform. of the Aegean region underwent a However, because the Carpathian rotation of 20 o to 30 o CW (Van and Aegean domains underwent such a Hinsbergen et al, 2005). Both mountain major rotation, it is very well possible belts formed during the Alpine orogeny and it may seem logical that the Moesian in the Tertiary. They are joined together platform accommodated for this south (with the Balkanides, see figure 1) movement with a similar rotation itself. and west of the Moesian platform The goal of this study is to (Schmid et al, 1998). investigate whether or not the Moesian Not much is known yet about the platform and the Bulgarian Rhodope rotational evolution of the Moesian indeed underwent such a rotation since Platform and the Bulgarian Rhodope at least 13 Ma, and if so, how much this during the Alpine orogeny. Moreover, rotation was exactly. For this, extensive previous studies that have dealt with paleomagnetic sampling was done on the these areas do not give a decisive Moesian Platform and in the Bulgarian outcome of the results. Jordanova et al Rhodope, from both late Cretaceous to (2001) cite some contrasting results Miocene sediments and Oligocene from different studies concerning volcanic plugs; the latter mostly in the northern Bulgaria: the Moesian platform Rhodope volcanic massif. Also, some would have rotated counterclockwise rock magnetic experiments were carried (CCW), or, it underwent hardly any out to determine the dominant rotation. Also, it is long believed that the ferromagnetic minerals in the different Moesian platform is a stable domain in localities. between the two tectonically active 1 Figure 1 . Map of the main tectonic zones in Bulgaria with sampling localities. Moesian Platform: stable part, mainly Neogene-Quaternary sedimentary cover; Balkanides: low degree of Alpine deformation, mainly Cretaceous-Paleogene cover; Srednogorie: volcanic (island arc) basement, folded and thrusted northward during Late Cretaceous; Rhodope unit: Alpine metamorphic complex formed from Paleozoic-Mesozoic crustal and mantle fragments with Paleogene sedimentary and volcanic cover. Sedimentary localities: BO – Bojouritsa; PV – Pleven; LU – Lukovit; MZ – Mezdra; VA – Varna; KA – Kavarna. Volcanic localities: SU – Suhindol; BR – Bratsigovo; YA – Yabalkovo; ZV – Zvesdel; DO – Dospat; BA – Banichan. After Georgiev et al (2000). 2 2. Geological setting and sampling extrusives varying from rhyolitic lavas to ignimbrites (Bratsigovo, Dospat and 2.1. Sites Banichan). Sampling was done at twelve localities from a large area covering the 2.2. Sampling Moesian Platform and the Bulgarian Sampling was done with a portable Rhodope (see figure 1). At each locality, drill powered with gasoline. Drill bits with around 7 to 12 sites (with exceptions of diamond coating were cooled with water Mexdra and Banichan, having 20 and 4 from a pump. Cores from such a drill are sites, respectively) were drilled, which about 2.5 cm in diameter and they must were spread out over several outcrops to have a minimum length of about 6 cm to average possible local rotations and be useful for getting multiple samples include as much time as possible. Each from each core. The orientation of the site consisted generally of 8 samples, sample was measured with a magnetic resulting in a total number of 960 compass. A correction of 4 o for local samples. declination in Bulgaria was taken into On the Moesian Platform, upper account for all measurements. Cretaceous and younger sediments were Orientation and identification marks were sampled, as well as a volcanic complex put on the sample before it was wrapped with basaltic plugs intruding the in aluminium foil for optimal sediments. This was done to enhance preservation. At every site, GPS-points chances of results, because the were taken for precise location. In case sediments were mainly marine of a visible bedding (in sampled carbonates (limestones and mudstones), sediments, or surrounding a sampled which do not always have a high chance volcanic horizon), the strike and dip were of preserving the paleomagnetic signal. measured. For the same reason, the volcanic sites Some remarks can be made on the in the Rhodope were sampled. sampling methods above. It is important Moesian sediments were sampled at that samples are taken from seemingly six localities, with ages ranging from fresh, non-weathered rocks. Weathering upper Cretaceous-Paleocene (in Mezdra can alter the paleomagnetic signal in a and Bojouritsa localities) to Eocene (in way that it is weakened and is Pleven and Varna localities), and even overprinted by the present-day magnetic younger sediments having Miocene age signal through oxidation of primary (in Lukovit and Kavarna localities). All magnetite or hematite. Thus, it was sediments are not, or only, slightly tilted. necessary to be careful on choosing Bedding dips range from a maximum of fresh sites, like road cuts, or creating 17 o for some samples from the fresh outcrop by digging into sediments Paleocene, to <5-7o dip for samples from and removing the outer layer. the Miocene. Paleomagnetic directions Furthermore, at outcrops with loose resulting from the demagnetizations blocks much care must be taken on were corrected for this bedding-tilt. drilling samples in a fixed block, because A series of three basaltic plugs was otherwise, orientations could have been sampled on a locality near Suhindol, in false. the Moesian Platform. These basalts For sediments, another measure was have been radiochronologically dated to also taken into account. Sedimentation be of early Miocene age, ranging from 20 occurs relatively slowly compared to to 25 Ma. cooling of lava flows and igneous In the Rhodope of southern Bulgaria, intrusions and therefore, sediment layers volcanics were sampled at five localities. can cover a large time range. It is They have been radiochronologically important to try and take samples from dated at 25 to 35 Ma. From the same one site as much as possible in the same age range, localities have been sampled sedimentary layer to diminish effect of (1) in the eastern Rhodope including the observed variation in the lavas near Yabalkovo and basaltic to paleomagnetic field through time. andesitic lavas near Zvezdel, and (2) in the western Rhodope. Those are all felsic 3 3. Paleomagnetic analysis grains that have a blocking temperature below the demagnetization temperature. 3.1. Demagnetization Most of the samples were at least The magnetic signal preserved in heated to 180 oC or 210 oC, however, rocks, before treatment, is the natural when the result at that stage did not remanent magnetization (NRM). This make any sense anyhow, the sample NRM often consists of multiple would be rejected from further analysis. components; a primary component that Other samples were rejected at later originated during rock formation, and a stages because those also gave useless, secondary component that was acquired non-interpretable results and/or because at later times by other processes, either most samples from that site were gradual or short-term. The latter can already rejected earlier. From sediment alter the first component and adds up to samples with a reasonable or good the total NRM. However, because in most signal, the rest of the site was prepared studies the primary is needed, the and measured in the DC-SQUID by the secondary NRM must be disposed off same procedure. from the sample. Fortunately, this is, in All of the volcanics were most cases, the less stable component. demagnetized by applying an alternating Partial demagnetization can remove it field (AF). The AF demagnetizer is from the sample and will isolate the situated in a magnetically shielded room more stable component. The latter is and is aided by a robot-navigated device then called the characteristic NRM which handles the samples. In the AF (ChRM), because it is not fully certain demagnetizer, the samples are exposed whether this is only the primary NRM. to an alternating field which decreases All the samples were treated at the with time and destroys the secondary paleomagnetic lab of Fort Hoofddijk in NRM with less coercivity than the original Utrecht. The NRM of all samples was applied pick field. The demagnetization measured in a 2G Enterprises horizontal was done with pick field steps of 5 mT, DC-SQUID cryogenic magnetometer, starting from 0 mT and with steps of 10 which is able to handle samples that are mT, from 30 mT on, to a maximum pick only weakly magnetized.