D. Kondopoulou

Seismotectonics of the Aegean region

Abstract Fault plane solutions, neotectonic field observations, and in-situ stress measurements have been used to determine the stress field associated with the active deformation of the lithosphere in the Aegean and surrounding regions. A stress gradient and a zonal pattern of different tectonism away from the trench axis has been postulated. Pure thrust faulting occurs in the fore-arc side while the back-arc side is mainly dominated by almost N-S extensional tectonics. A narrow belt of strike-slip faults with predominantly thrust component separates these tectonic zones. This belt seems to represent a transition from the thrust-type faults to normal faults. Fault plane solutions of the whole region in question indicate that both the large and small magnitude shocks are caused by the same regional stress-field. A body of 33 focal mechanisms indicates that the intermediate depth shocks along the sinking Mediterranean slab are associated with thrust-type fracturing. Orientation of kinematic axes favours the suggestion that shear zones within the descending slab cause these shocks. The active crustal shortening along the Hellenic consuming boundary and the sinking of the Mediterranean lithosphere beneath the South Aegean area explain satisfactorily the seismotectonic features of the fore-arc and the southern back-arc sides. On the contrary, geophysical evidence concerning the Cenozoic geodynamic evolution of the Aegean is needed to interpret features of the north back-arc area.

Tectonic evolution of fault-bounded continental blocks : Comparison of paleomagnetic and GPS data in the Corinth and Megara basins (Greece)

[1] We report on new paleomagnetic and anisotropy of magnetic susceptibility (AMS) data from Plio-Pleistocene sedimentary units from Corinth and Megara basins (Peloponnesus, Greece). Paleomagnetic results show that Megara basin has undergone vertical axis CW rotation since the Pliocene, while Corinth has rotated CCW during the same period of time. These results indicate that the overall deformation in central Greece has been achieved by complex interactions of mostly rigid, rotating, fault bounded crustal blocks. The comparison of paleomagnetic results and existing GPS data shows that the boundaries of the rigid blocks in central Greece have changed over time, with faulting migrating into the hanging walls, sometimes changing in orientation. The Megara basin belonged to the Beotia-Locris block in the past but has now been incorporated into the Peloponnesus block, possibly because the faulting in the Gulf of Corinth has propagated both north and east. Paleomagnetic and GPS data from Megara and Corinth basins have significant implications for the deformation style of the continental lithosphere. In areas of distributed deformation the continental lithosphere behaves instantaneously like a small number of rigid blocks with well-defined boundaries. This means that these boundaries could be detected with only few years of observations with GPS. However, on a larger time interval the block boundaries change with time as the active fault moves. Paleomagnetic studies distinguishing differential rotational domains provide a useful tool to map how block boundaries change with time. INDEX TERMS: 1208 Geodesy and Gravity: Crustal movements—intraplate (8110); 1518 Geomagnetism and Paleomagnetism: Magnetic fabrics and anisotropy; 1525 Geomagnetism and Paleomagnetism: Paleomagnetism applied to tectonics (regional, global); 8107 Tectonophysics: Continental neotectonics; 9335 Information Related to Geographic Region: Europe;

Dextral rotations and tectonomagmatic evolution of the southern Rhodope and adjacent regions (Greece)

Abstract Palaeomagnetic measurements deduced from Eocene, Oligocene, Miocene and Pliocene formations, exposed in the Greek Rhodope and surrounding areas, reveal significant clockwise rotations, displaying an apparent systematic increase in magnitude from Thrace westwards and a decrease northwards. The Pelagonian zone in the west has suffered no rotation, at least since the Late Miocene. A transition from rotated to unrotated regions seems to exist close to the western boundary of the Vardar zone. No rotations are apparent earlier than the mid-Oligocene. Parts of the Rhodope may have been rotated clockwise by ∼12° during the Late Oligocene. Additional clockwise rotations occurred after the Early Miocene. It is proposed that at least the post-Early Miocene rotations were the result of plate tectonic motions in the north Aegean area, initiated by Middle Miocene times and continuing to the present day. Southward stretching and bending of the ductile part of the lithosphere was accomplished at brittle upper-crustal levels by the detachment, translation and rotation of the upper plate of the Rhodope core complex (the Serbomacedonian element), possibly followed by further fragmentation and rotation of individual blocks.

A selective procedure for absolute paleointensity in lava flows

[1]xa0We present a compilation of experiments of absolute paleointensity using double heating protocols on very recent lava flows from Hawaii, La Reunion, the Canary islands and Santorini. The existence of a sharp distribution of grain sizes carried by a single mineralogical phase always yielded successful determinations of paleointensity that deviate by less than 10% from the actual field value. Thus, a rapid decrease of at least 70% of the initial magnetization over a narrow range of temperatures prior to the Curie point combined with a unique mineralogical phase define an optimal situation for obtaining reliable estimates of absolute paleointensity. Consequently, we suggest that stepwise standard thermal demagnetization of companion specimens should be routinely performed prior to paleointensity experiments. Not only do these measurements provide important information about the characteristic magnetization, but they indicate which samples are appropriate for paleointensity experiments, which increases the success rate to almost 100%.

Palaeomagnetism in Greece: Cenozoic and Mesozoic components and their geodynamic implications

Abstract An important number of palaeomagnetic data have been obtained in Greece during the last 20xa0years. These data can be schematically divided into two groups according to their age. When plate movements are fairly well constrained, the Cenozoic provides a dense network of palaeomagnetic directions. The general pattern displays eastward declinations broadly distributed over a large distance — from the Ionian islands to the western Greek Rhodope — and westward declinations, less accurately constrained in space and time, in the Southern and Eastern Aegean and in Western Anatolia. Emphasis is given to the evaluation of this dataset by comparing it with new concepts of the regional tectonics. In the Mesozoic–Palaeozoic, more scarcely covered, reliable information is provided about latitudinal movements, resulting in a suggested palaeolatitude of 15–20°N for almost all studied areas. A model proposing prevailing counterclockwise rotations in the internal Hellenides and strong clockwise rotations for the external Hellenides appears plausible. Finally, issues for future research are outlined. For example, Mio-Pliocene formations from Central Greece, the Pindos ophiolites and Palaeocene–Upper Cretaceous data should receive further attention.

Complex rotational deformations in the Serbo-Macedonian massif (north Greece): structural and paleomagnetic evidence

Abstract Structural data concerning the neotectonic deformation of the Serbo-Macedonian massif suggest a possible counterclockwise rotation of the stress field which corresponds to a clockwise rotation of the geological structures. Such a rotation is confirmed by paleomagnetic data, providing a new, alternative interpretation of the geodynamic evolution of this area.

New paleomagnetic results from the Aegean extensional province

Various Oligocene formations from NE Greece (ignimbrites from the Medousa area, rhyolites from Zagradenia, granodiorites from Elatia) show discordant paleomagnetic signatures, in each case indicating small cw (clockwise) rotation and also inclination flattening. Marls from Pithion were partly remagnetized in a present-day field. Samples that contain ancient magnetization components also indicate small cw rotation and inclination flattening. However, the magnetization of andesites from Peplos reflects a considerably larger rotation, likely owing to local tectonics. In the context of previous work in the area, these results are used to propose a subdivision of NE Greece into four structural zones of distinctive rotational behaviour (from east to west): sites in zone 1, east of the Kavala-Xanthi-Komotini fault (KXK), show various cw and ccw (counterclockwise) rotation angles owing to complex kinematics resulting from the interaction of the KXK and the north-Anatolian fault zone. However, zone 2, between the KXK and the Strymon valley, is structurally homogeneous (∼ 10° cw rotation). The paleomagnetic signature of the Vertiskos massif (zone 3) implies a larger (> 30°) cw rotation, whereas sites in the Vardar basin (zone 4) contain a paleomagnetic signature similar to that of zone 2. This suggests a motion of the Vertiscos massif, a meta-ophiolitic nappe, relative to underlying strata. Indeed, zones 2 and 4 may be parts of the same structural unit which underlies this nappe.

An updated catalogue of Greek archaeomagnetic data for the last 4500 years and a directional secular variation curve

We present an updated compilation of Greek directional archaeomagnetic data for the last 4.5 millennia. The data set comprises 89 directions from archaeological artefacts and volcanic rocks. Most of the data come from the Late Bronze Age (1700-1400 BC) that is the flourishing period of the Minoan civilization in Crete, while parts of the classical (480-323 BC), Hellenistic (323-31 BC) and Roman (146 BC-330 AD) periods are also well covered. The dataset has been analysed using the Bayesian approach for curve building and a directional secular variation (SV) curve for Greece is proposed. Comparisons with regional and global model predictions show a general agreement even though some discrepancies are observed for some time intervals. The new curves together with the previously published intensity SV curve for Greece, also using the Bayesian approach, form a homogeneous set and enrich our knowledge of the full geomagnetic field vector variation in Greece during the last millennia.

First archaeomagnetic results and dating of Neolithic structures in northern Greece

Archaeomagnetism in Greece has continuously developed during the last decades. Numerous studies have provided high quality data and accurate secular variation curves for the direction and intensity of the geomagnetic field have been constructed. The Greek Secular Variation Curves (SVCs) cover the last 8 millennia for intensity and 6 millennia for direction. The coverage of the archaeological periods remains uneven, with several gaps, mostly in the directional dataset, with only two results for periods older than 2500 B.C. In the present contribution, the first archaeomagnetic results from Neolithic settlements in northern Greece are presented.For the present study, samples were collected from three different archaeological sites: burnt structures in Avgi (Kastoria) and Vasili (Farsala) and one oven from Sosandra (Aridaia). The natural remanent magnetization (NRM) grouping of all specimens indicated that the majority of the samples were burnt in situ, providing thus a reliable direction of the ancient field. Magnetic cleaning (both alternating-field and thermal) revealed the presence of one stable component of magnetisation. Rock magnetic experiments (acquisition of isothermal remanent magnetization (IRM), thermal demagnetisation of the IRM, thermomagnetic curves) have been performed on pilot samples indicating that low coercivity magnetic minerals such as magnetite or Timagnetite are prevailing. The mean directions (declination D, inclination I and parameters of the Fisherian statistics), which arose from the three sites are as follows: Sosandra: D = 343°, I = 55.6°, α95 = 4.8°; Avgi: D = 10.1°, I = 53.4°, α95 = 4.2° and Vasili: D = 357.5°, I = 43.1°, α95 = 4.1°. The obtained data are in a very good agreement with results from Neolithic Bulgaria. This study represents the beginning of an effort to fill the gaps of the Greek secular variation curves and their extension to the Neolithic period.

Rock magnetic property and paleointensity determination on historical Santorini lava flows

Eight historical dacitic lava flows from Santorini with ages between 46 A.D. and 1950 A.D. (four of them within the past century) have been subjected to detailed rock magnetic analyses and various experiments of absolute paleointensity. Thermomagnetic measurements and acquisition of isothermal magnetization have revealed the presence of two physically distinct magnetic phases with Curie temperatures of 280°C and 500°C. In most of the samples, the second phase does not play a prominent role for the characteristic remanent magnetization, which is dominated by titanomagnetite. Magnetostatic interaction is very limited and does not considerably change upon heating. Back-field curve spectra indicate a good thermochemical stability of these dacitic lava samples, which is also supported by the absence of noticeable changes in the remanent coercive force prior heating to 450°C. Hysteresis measurements show typical pseudo-single-domain behavior without noticeable superparamagnetism. Such characteristics were favorable to conduct and to test the most widely used experimental approaches for absolute paleointensity determination. Despite a success rate of 38%, the microwave technique has provided rather scattered within-flow determinations. The results obtained from approaches involving alternating field demagnetization were biased by considerable differences between the NRM and the TRM coercive force spectra. We have also noticed that most determinations obtained by microwave heating differ from the historical field value at the site for the most recent flows. Last, techniques involving double-heating protocols were successful due to a dominant low Curie temperature phase with a narrow grain size distribution. The results were characterized by low dispersion and were found in good agreement with the historical field.