Dimitrios Papanikolaou

Crustal investigation of the Hellenic subduction zone using wide aperture seismic data

Abstract We present the results of a wide-aperture seismic onshore–offshore study (Crete Seismic Experiment) in the Cretan region as part of the Hellenic arc compressional system. Three seismic lines were carried out on and around the island of Crete in order to investigate the crustal structure of the region. Up to 119 three-component recording stations were deployed on each profile that observed seismic energy generated by a 48-l airgun array and eight 20-kg landshots. A total of 6208 shots were fired. Upon completing the fieldwork, the vertical components of all stations were evaluated; 300 Common-Receiver-Gather (CRG) sections of the ocean bottom seismographs (OBS) and land stations as well as 100 Common-Source-Gather (CSG) sections of the land shots and selected airgun shots were compiled and modeled in order to generate a 2D P-wave velocity–depth model for each profile. The accuracy of the model depends on the depth and position along the profiles and does not exceed 5% for both depth and P-wave velocity. We identified strong lateral variations in crustal and sedimentary thickness mainly in a north–south direction but also along strike (east–west). The crust is continental and has a maximum thickness of 32.5 km below northern central Crete. Its subdivision in an upper ( v p =5.8–6.3 km/s, locally up to 6.5 km/s) and a lower ( v p =6.4–6.9 km/s) part is justified by a first-order discontinuity with v p -velocity a contrast of up to 0.6 km/s. The eastern part of Crete shows a significantly thinner crust of 24 to 26 km. To the North, the crustal thickness decreases to 15 km below the central Cretan Sea. The prominent decrease of the Moho depth north of central Crete is interpreted to represent the northern end of a microcontinent that was subducted in Oligocene times and later surfaced by ‘buoyant escape’ (Stockhert et al., 1999; Thomson et al., 1999). The P – T – t – D history of the high-pressure rocks of Crete, Greece: denudation by buoyant escape. In: Exhumation Processes: Normal Faulting, Ductile Flow and Erosion. Ring, U., Lister, G., Willet, S., Brandon, M. (Eds.), Spec. Publ. of the Geol. Soc. of London, p. 154]. To the south and southwest of the island, the continental crust gradually thins to a minimum of 17 km and at approximately 100 km off the southern coast of Crete, it is in contact with oceanic crust below the Mediterranean Ridge. Upper mantle velocities were determined to be 7.7 km/s below the Cretan Sea and 8.0 km/s south of Crete. Below the continental Cretan crust, a 6- to 7-km-thick layer with v p -velocities between 6.6 and 7.1 km/s was identified on each line and could be followed by reflections to a depth of 42 km. It is decoupled from the overlying continental crust at central Crete and is interpreted as oceanic crust presently under subduction towards the NNE below the Aegean Sea.

Slab segmentation and late Cenozoic disruption of the Hellenic arc

The Hellenic subduction zone displays well-defined temporal and spatial variations in subduction rate and offers an excellent natural laboratory for studying the interaction among slab buoyancy, subduction rate, and tectonic deformation. In space, the active Hellenic subduction front is dextrally offset by 100–120 km across the Kephalonia Transform Zone, coinciding with the junction of a slowly subducting Adriatic continental lithosphere in the north (5–10 mm/yr) and a rapidly subducting Ionian oceanic lithosphere in the south (∼35 mm/yr). Subduction rates can be shown to have decreased from late Eocene time onward, reaching 5–12 mm/yr by late Miocene time, before increasing again along the southern portion of the subduction system. Geodynamic modeling demonstrates that the differing rates of subduction and the resultant trench offset arise naturally from subduction of oceanic (Pindos) lithosphere until late Eocene time, followed by subduction of a broad tract of continental or transitional lithosphere (Hellenic external carbonate platform) and then by Miocene entry of high-density oceanic (Ionian) lithosphere into the southern Hellenic trench. Model results yield an initiation age for the Kephalonia Transform of 6–8 Ma, in good agreement with observations. Consistency between geodynamic model results and geologic observations suggest that the middle Miocene and younger deformation of the Hellenic upper plate, including formation of the Central Hellenic Shear Zone, can be quantitatively understood as the result of spatial variations in the buoyancy of the subducting slab. Using this assumption, we make late Eocene, middle Miocene, and Pliocene reconstructions of the Hellenic system that include quantitative constraints from subduction modeling and geologic constraints on the timing and mode of upper plate deformation.

A geologic reconnaissance of the Cycladic blueschist belt, Greece

The Cycladic blueschist belt consists of two distinctive segments separated by a broad zone of superposed granitic and high-temperature metamorphic rocks. The northern segment contains early metamorphic fold axes and parallel glaucophane lineations that trend ∼060° with a progressive increase in metamorphism toward the southeast. The southern segment contains similar fold axes and glaucophane lineations that trend ∼010° with an apparent increase in metamorphism toward the northwest. Radiometric dating of metamorphic minerals from both segments give apparent ages of about 40 to 80 m.y. These data suggest the existence of late Mesozoic or early Cenozoic subduction zones in the Aegean region that subsequently collided.

Evolution and dynamics of the Cenozoic tectonics of the South Balkan extensional system

The South Balkan extensional system consists of normal faults and associated sedimentary basins within southern Bulgaria, Macedonia, eastern Albania, northern Greece, and northwestern Turkey. Extensional tectonism began during the final convergence across the Vardar, Intra-Pontide, and Izmir-Ankara suture zones, where oceanic regions closed between continental Europe and continental fragments that make up the Pelagonian, Sakar, and western Anatolian tectonic units. Earliest extension of latest Cretaceous-middle Eocene age appears to have occurred within a regional convergent tectonic setting and may be related to an increase in gravitation potential energy within a thickening continental lithosphere. Following diachron-ous closure across the suture zone, from the middle Eocene to late Oligocene, the transition from a regionally convergent to a regionally extensional tectonic setting occurred and was associated with abundant magmatism and formation of sedimentary basins. Extension was associated with lithospheric thinning probably related to changes in geometry of the subducted slab, dynamics of the mantle wedge, and beginning of slab rollback along the Hellenic subduction zone. A short period of local and diachronous (?) shortening (during latest Oligocene-early Miocene time) occurred in the Thrace basin of northwestern Turkey and in some basins in western Bulgaria and eastern Macedonia. Regional extension began in middle Miocene time and was related to the regional extensional tectonic setting that has dominated the Aegean extensional region to the present. Trench rollback was the dominant dynamic process, but during late Miocene time it was modi-fled by the formation of the western part of the North Anatolian fault zone that partially decoupled the South Balkan extensional system from the Aegean extensional region. During late Cenozoic time, east-west-striking normal faults and associated sedimentary basins in the eastern part of the South Balkan extensional system propagated westward in tandem with westward migration of north-south-striking normal faults and sedimentary basins from western Bulgaria into eastern Albania. This migration was caused by evolution of the Hellenic subduction zone as it increased its curvature during trench rollback and clockwise and counterclockwise rotation of crustal fragments in the west and east, respectively. After formation of the western part of the North Anatolian fault zone, extension within the eastern part of the South Balkan extensional system was related to southward movement of its lithosphere at a slower rate than the extension within the Aegean extensional region. Active extension and basin formation show two provinces of extension that are nearly at right angles to one another and their overlap in the central South Balkan extensional system: east-west extension in central Albania to eastern Macedonia and north-south extension from northwestern Greece and eastern Macedonia to eastern Bulgaria and northwestern Turkey.

Tectonic Evolution of the Cycladic Blueschist Belt (Aegean Sea, Greece)

The Cycladic blueschist belt comprises a number of tectonic units showing transitional paleogeographic affinities of a paleomargin from the carbonate platform of the external Hellenides to the Pindos-Cyclades oceanic basin. The blueschists were emplaced between the underlying Ios-Menderes crystalline basement plus its Mesozoic cover and the overlying non-metamorphic internal Hellenides (carbonate platform with the Axios-Vardar ophiolite nappe emplaced during Late Jurassic — Early Cretaceous) during Late Eocene — Early Miocene. Several successive stages of the Cycladic blueschists can be detected, on the basis of metamorphic, structural and magmatic events, throughout their evolution from the Eocene subduction zone to the present situation behind the active volcanic arc. The internal structure of the blueschists is characterized by a radial distribution of fold hinges and co-parallel stretching lineations , probably representing a-structures, forming an amphi-theatre with an overall tectonic transport from NE to SW, similar to the present plate-kinematics of the Hellenic arc and trench system.

Morphotectonic structure of the western part of the North Aegean Basin based on swath bathymetry

Abstract The morphotectonic structure of the North Aegean Basin is studied on the basis of a new detailed swath bathymetric survey. The resulting bathymetric map is presented in reduction with 20-m isobaths. The slope analysis gives an accurate scheme of the geometry of the basin and distinction of several sub-basins (approximately 20). The overall basin geometry is a rectangular tetrahedron shaped by the major slope discontinuity separating the continental platform from the continental slope. The area distribution with depth shows a maximum at depths between 300 and 450 m along the sub-horizontal edge of the continental platform and at depths between 1000 and 1200 m at the basinal areas of the sub-basins. The separation of the western part of the North Aegean Basin from the eastern part (Saros Bay) is very clear in the area between Limnos and Thasos, with a maximum depth of 490 m. The 3.2% of the basin area is characterized by slope values >20%, which correspond to active fault zones. Their trend is NE–SW (N46°) and NW–SE (N136°). Some secondary E–W faults are also present within the basin with morphological expression only on the orientation of slopes

The three metamorphic belts of the Hellenides: a review and a kinematic interpretation

Summary New results obtained in the three metamorphic belts of the Hellenides suggest that each belt has a different structure, geotectonic setting and evolution. The external metamorphic belt of Peloponnesus-Crete has probably resulted from nappe movement in the vicinity of an evolving island arc system. The blueschists occurring within this external belt are allochthonous and have been transported with other nappes over the external carbonate platform of the Pre-Apulian-Plattenkalk-Ionian-Tripolitza-Almyropotamos Units. This implies that there was only one true blueschist domain, initiated in the Cycladic area during ?Early Eocene. The kinematic interpretation is based on the distinction of a- and b-structures and on the nature of the shear zones between the different domains, especially that between the metamorphic Hellenides below and the non-metamorphic Hellenides above. The existence of the probably Lower Palaeozoic meta-sedimentary sequence of Kastoria in the northern Pelagonian zone indicates that the overlying, ophiolite-bearing Almopias unit has been derived from within the Hercynian domain. The Kastoria sedimentary sequence, the ophiolites of Vertiskos and the carbonate platform of Pangeon are considered to be most likely elements of a probable Hercynian orogeny.

Predictive value of procalcitonin for bowel ischemia and necrosis in bowel obstruction.

BACKGROUND To our knowledge, the predictive value of procalcitonin for bowel strangulation has been evaluated in only 2 experimental studies that had conflicting results. The objective of this study was to evaluate the value of procalcitonin for early diagnosis of intestinal ischemia and necrosis in acute bowel obstruction. METHODS We performed a prospective study of 242 patients with small- or large-bowel obstructions in 2005. A total of 100 patients who underwent operation were divided into groups according to the presence of ischemia (reversible and irreversible) and necrosis, respectively, as follows: ischemia (n = 35) and nonischemia groups (n = 65) and necrosis (n = 22) and nonnecrosis groups (n = 78). Data analyzed included age, sex, vital signs, symptoms, clinical findings, white blood cell count, base deficit, metabolic acidosis, procalcitonin levels on presentation, the time between symptom onset and arrival at the emergency department and the time between arrival and operation, and the cause of the obstruction. RESULTS Procalcitonin levels were greater in the ischemia than the nonischemia group (9.62 vs 0.30 ng/mL; P = .0001) and in the necrosis than the non-necrosis group (14.53 vs 0.32 ng/mL; P = .0001). Multivariate analysis identified procalcitonin as an independent predictor of ischemia (P = .009; odds ratio, 2.252; 95% confidence interval, 1.225-4.140) and necrosis (P = .005; odds ratio, 2.762; 95% confidence interval, 1.356-5.627). Using receiver operating characteristic (ROC) curve analysis, the area under the curve (AUC) of procalcitonin for ischemia and necrosis was 0.77 and 0.87, respectively. A high negative predictive value for ischemia and necrosis of procalcitonin levels <0.25 ng/mL (83% and 95%, respectively) and a positive predictive value of procalcitonin >1 ng/mL were identified (95% and 90%, respectively). CONCLUSION Procalcitonin on presentation is very useful for the diagnosis or exclusion of intestinal ischemia and necrosis in acute bowel obstruction and could serve as an additional diagnostic tool to improve clinical decision-making.

Thinned continental crust below northern Evoikos Gulf, central Greece, detected from deep seismic soundings

Abstract In March 1996, a wide aperture reflection/refraction profiling (WARRP) seismic survey was carried out across the Maliakos–Sporades and northern Evoikos basins, central Greece. Two onshore/offshore seismic lines were recorded, using 14 ocean bottom seismographs (OBS) and 10 land stations. As an energy source, we used one sleevegun of 60 l volume, operating at 120-bar pressure. The results acquired by kinematic, two-point raytracing modelling of the time sections provided evidence of a thinned, stretched continental crust, of only 20 km thickness, below the central part of the northern Evoikos Basin. This basin was developed by the separation of the island of Evia—with continental crust of approximately 30 km thickness—from the Greek mainland, by transtension and stretching of the crust, forming the north Evia and the Sperchios valleys. Along the Maliakos–north Sporades Basin, the crust thins from 34 km at the mainland to 22 km below the Sporades Basin. The crust at the northern part of the island of Evia and the Trikeri Straits is 30–32 km thick and of normal continental structure; it is separated by an intercrustal discontinuity between an upper and lower crust. The sediments thicken significantly towards the Sporades Basin, where they obtain maximum thicknesses of the order of 8–10 km. The igneous and metamorphic crust thins significantly in the basin maintaining however its continental character along the entire section. The transition from the Maliakos–Trikeri Straits to the Sporades Basin is controlled by a steep listric fault that downthrows the basin crust by more than 6–7 km. The crustal transition of the thinned Sporades Basin to the 30-km thick crust of the northern Evia and the Maliakos Straits is poorly understood. Simple or pure shear stretching cannot have developed this margin, and we favour the assumption that it denotes the limit of two different crustal domains; these were merged together during a compressive tectonic episode, prior to the present-day extension and stretching of the Aegean Sea that was activated 5 Ma ago.

Unintentional parathyroidectomy during total thyroidectomy

Unintentional parathyroidectomy during thyroidectomy has been evaluated in a few studies. Moreover, the impact of the surgeons experience and operative technique has not been evaluated. Our aim was to identify the incidence of unintentional parathyroidectomy during total thyroidectomy, its clinical consequences, and factors affecting its occurrence.