Vassil Thesis

Magnetostratigraphy and palaeomagnetism of the area around the Momchilgrad Palaeogene depression, the East Rhodope massif, Bulgaria

Vassil V. Karloukovski

Previous palaeomagnetic studies of the Eastern Rhodope Tertiary volcanics in southern Bulgaria have yielded mostly high, European inclinations, contrary to the results from the neighbouring Greek part of the Rhodopes. Volcanics from 14 sites and a 425 metre high profile in sediments and pyroclastics in the Momchilgrad-Krumovgrad area (41.55^oN, 25.45^oE; Fig. 1) were studied by palaeomagnetic and rock magnetic techniques with the aim of investigating this apparent contradiction and establishing the magnetic polarity sequence in the sediments.

The main magnetic carriers of remanence in the volcanics are multidomain to pseudo-single-domain medium titanomagnetites and magnetites of low oxidation classes (2 to 3). Pyrrhotite has been identified in basalt andesites; pyrrhotite and hemoilmenite - in the sediments of one section. Widespread weak maghaemitisation is suspected in the volcanics. The NRM of the (partially zeolitised) pyroclastic flows to the east was a PTRM and emplacement temperatures between 150 and 400^oC have been obtained.

Reliable polarities from multiple sections were obtained in the sediments. With the help of Ar⁴⁰/Ar³⁹ dates from the lowermost and uppermost pyroclastics ranging between 32.28±0.07 and 31.82±0.07 Ma, most of the sedimentary profile has been assigned to the 12r chron (fig. 2). Among the volcanics, the dacites have probably been extruded during a R®N polarity transition, whilst all the rest are of reverse polarity and most likely also belong to the 12r chron.

The mean palaeomagnetic direction from the sediments (D=184.5^o, I=-54.1^o, a₉₅=7.0^o, N=7) and the more reliable one from the volcanics (D=182.9^o, I=-51.4^o, a₉₅=5.2^o, N=12) are in accordance with the data from the wider Aegean region in the south and their anomalously low inclinations (figs 3 & 4). With a poleward displacement, relative to Europe, of 5.6^o from the volcanics (Fig. 5), the data from the Momchilgrad depression support the apparent trend in the Aegean sites of a progressive increase in the values of northward displacement towards the west.

	Fig. 1. Map of the Aegean sea region (Beck and Schermer, 1994) with the locations of the palaeomagnetic studies (filled circles). The filled triangle denotes the location of the present study in the Momchilgrad region.
	Fig. 2. Correlation of the combined magnetic polarity profile of Dambaluk to the GPTS (Cande and Kent, 1995) and the three Ar/Ar dated pyroclastics. The hatched zones in the leftmost column denote the pyroclastic flows, and the asterisk indicates the K/Ar date of Mushinski (1980).
	Fig. 3.Stereoplot of the site-mean directions in the Momchilgrad (f=41.55^oN, l=25.45^o) volcanics and the sediments with the corresponding cones of confidence. The mean negative (D=182.8^o, I=-51.8^o, a95=4.2^o, N=15) and positive (D=6.5^o, I=54.8^o, a95=14.4^o, N=4) directions pass the reversal test (class Ra).
	Fig. 4. Palaeolatitude versus age for the Aegean palaeomagnetic data (after Beck and Schermer 1994) with the new data from the Momchilgrad volcanics (empty square) and sediments and pyroclastics (filled square) added. The empty and filled diamonds denote the APWP reference curves of Africa and Europe, correspondingly (Besse and Cuortillot, 1991).
	Fig. 5. Poleward displacement of the Aegean...

Mushinski, A., 1980, Petrology and geology of the acid Tertiary volcanics of the Lozen mountain (Vissoka Elha) and the Eastern Rhodopes (Hissar), PhD thesis (in Bulgarian).

Beck, M., Schermer, E., 1994, Aegean paleomagnetic inclination anomalies.
Is there a tectonic explanation?, Tectonophysics, 231, 281-292.

Besse, J., Courtillot, V., 1991, Revised and synthetic apparent polar wander paths of the African, Eurasian, North American and Indian plates, and true polar wander since 200 Ma, J. Geophys. Res., 96, 4,029-4,050.