COMPILATION OF A UNIFIED AND HOMOGENEOUS AEROMAGNETIC MAP OF THE GREEK MAINLAND

We present a unified and homogeneous digital aeromagnetic map of the Hellenic mainland, based on the 1:50,000 map series of IGME. These maps cover the areas A1, A2, B, C1, C2, C3, D1 compiled by Hunting Geology and Geophysics Ltd. and measured at nominal ground clearances (flight altitudes) 150m AGL, 150m AGL, 300m AGL, and 2300m AMSL respectively (part of C2 with 3000m AMSL). We also include the entire area of Northern Greece, measured by ABEM AB with nominal ground clearance 275±75m AGL. The original map sheets were digitally imaged, georeferenced, digitized along contour lines and interpolated onto regular 250´250m grids. The unified aeromagnetic map was constructed by collating the mosaic of the resulting gridded data. Using upward/ downward continuation techniques various homogeneous versions of the map, were compiled by referencing of the observed mosaic total magnetic field to a unique constant ground clearance or to a unique constant elevation above mean sea level. This is the first time there is a complete and unified image of the magnetic signature of the isopic zones and rock formations comprising the Hellenic mainland, with particular reference to the ophiolite suites, which provides additional insight into the Alpine and post-alpine tectonics of the area

An investigation of the active tectonics in central-eastern mainland Greece with imaging and decomposition of topographic and aeromagnetic data

We report the results of a joint analysis of aeromagnetic, topographic and tectonic data in central-eastern mainland Greece. The emphasis of the analysis is placed on the detection of coherent lineations (discontinuities), collocated and correlated with faulting structures detected by geological field observation. To this effect, edge detection and image enhancement were applied to digital aeromagnetic anomaly maps and digital elevation models, comprising bidirectional differentiation, wavelet transformation (imaging) and spatial decomposition/reconstruction in the wavenumber domain. The analysis facilitated the detection of significant topographic lineaments with NNE-SSW, ENE-WSW and ESE-WNW orientations. Respectively, the aeromagnetic data exhibit two families of significant NE-SW, and one family of ESE-WNW lineaments. The major aeromagnetic and topographic lineaments coincide and have comparable width scales of the order of 2-3 km, indicating that they are produced by significant discontinuities in the upper crust. The kinematics of the NE-SW faults varies between oblique-slip and strike-slip. These faults affect Neogene to Late Quaternary deposits and have been responsible for the formation of transverse depressions and horsts. This is also corroborated by focal plane solutions from small earthquakes recorded by local networks. The nature of these structures is not yet clear. However, they have been detected by diverse methodologies, they have considerable extent and are apparently active. These attributes suggest that they may possibly be related to the propagation and diffusion of the North Anatolian and North Aegean fault systems into the Greek mainland. © 2009 Elsevier Ltd. All rights reserved

Newly compiled gravity and topographic data banks of Greece

A brief description of the gravity and topographic data banks of Greece is given. About 22 000 gravity stations are included in the newly compiled gravity data bank from an initially available data set of 33 000 stations, after the application of specific rejection criteria, and classified in squares of 100 km × 100 km. The data cover an area between 18° and 27°E, and 33° and 42°N (Fig. 1). A matrix of 2 km × 2 km mean elevation values was also estimated for the same area. These data constitute the topographic data bank. The preparation and structure of both data banks are also briefly discussed here

Evidence of recent plutonic magmatism beneath Northeast Peloponnesus (Greece) and its relationship to regional tectonics

This work reports evidence of recent tectonically controlled plutonic magmatism related to Neogene volcanism in a broad area of Northeast Peloponnesus (Greece) that is straddled by the Hellenic Volcanic Arc and comprises the Argolid, the Argolic and Saronic gulfs and eastern Corinthia including the province of Crommyonia at the western half of Megaris peninsula (western Attica).We assess the contemporary stress field based on formal inversion of well-constrained crustal earthquake focal mechanisms and determine that it is principally extensional and NE-SW oriented, with σ1 strike and plunge being N64° and 77°, respectively and σ3 strikes and plunge N210° and 10°. This generates WNW-ESE and NW-SE faults, the former being dominant in the Saronic Gulf and the latter in the Argolic. In addition, the analysis predicts E-W and N330° faults with non-trivial right- and left-lateral heave, respectively, which are consistent with the R and R' directions of Riedel shear theory and explain a number of observed earthquake focal mechanisms and earthquake epicentre alignments.We also present a semi-quantitative analysis of observed aeromagnetic anomalies by performing numerical modelling of the radially averaged power spectrumwith an efficient anomaly separation scheme based on a new type of 2-D Fourier domain filter introduced herein, the Radial Extended Meyer Window. This analysis identifies an extensive complex of magnetized rock formations buried at depths greater than 3 km which, given the geology and geotectonic setting of the area, can hardly be explained with anything other than calc-alkaline intrusions (plutons). At northeastern Corinthia andCrommyonia, this type of intrusive activity is unexceptional, mainly concentrated in theGulf ofMegara-Sousaki areas and consistent with the low-intensity, small-scale Pliocene dacitic volcanism observed therein. Conversely, large-scale elongate anomalies of E-W and N330° orientation have been identified in the Argolid, generally collocated with and delimited by extensional tectonic structures (grabens and major faults) of analogous orientation. These are interpreted to comprise calc-alkaline plutons whose placement has been controlled by the regional tectonic activity (syn-rift magmatism); their nature and origin is demonstrated with convergent evidence from deep magnetotelluric, seismological, seismic tomography and other investigations. A large number of shallow and superficial (less than 2 km) magnetic sources have also been identified; these are generated by a complex of distributed nearsurface formations consisting of subvertically developing buried or extrusive volcanics and outcropping or shallow-buried ophiolitic formations (thin nappes of tectonic mélange and dismembered ophiolitic complexes). The joint analysis of the data facilitates the formulation of a tentative geotectonic model for Argolis peninsula, according towhich the strain differential caused by the disparate extensional trends of the Argolic and Saronic gulfs is accommodated by right-lateral block motion associated with igneous intrusive activity at major block boundaries. © The Author(s) 2017. Published by Oxford University Press on behalf of The Royal Astronomical Society

Tectonic deformation in the Santorini volcanic complex (Greece) as inferred by joint analysis of gravity, magnetotelluric and DGPS observations

Tectonic activity is very difficult to study in the Santorini volcanic complex (SVC) as it comprises a cluster of small/awkwardly shaped islands covered by pyroclastic deposits from which tell-tale markers are swiftly erased, while seismicity is generally absent. We address the problem by combining geophysical exploration methods to evaluate the long-term effects of tectonic deformation and time-lapse differential GPS to directly evaluate the magnitude and kinematics of present-day deformation. The former comprise 3-D gravity modelling to investigate the footprint of tectonics on the pre-volcanic Alpine basement and natural-field EM induction to map conductivity anomalies epiphenomenal to fluid circulation in faults. Our analysis identified the following principal tectonic elements: The Trans-Santorin Divide (TSD), a segmented NNW-SSE dextral strike-slip fault splitting the SVC sideways of the line joining Cape Exomytis, the Kammeni Islets and the Oia-Therassia Strait. It is collocated with a major vertical conductive zone and forms a series of dents and depressions in the basement. The Columbo Fault Zone (CFZ) is a pair of parallel NE-SW subvertical normal-sinistral faults straddling the northern SVC and terminating against the TSD; it may be associated with fluid injection into the shallow crust but appears to have limited effect on crustal conductivity (compared to TSD). The Anhydros Fault Zone (AFZ) is detected by its footprint on the basement, as a set of parallel northerly dipping NE-SW faults between the Athinios-Monolithos line and Fira. If it has any heave, it is left-lateral. It does not have distinguishable electrical signature and does not contribute to present-day horizontal deformation. The CFZ and AFZ are antithetic and form a graben containing the volcanic centre of Kammeni Islets. E-W extension was identified lengthwise of a zone stretching from Cape Exomytis to Athinios and along the east flank of the caldera to Imerovigli. N-S normal faulting confirmed therein, may have contributed to the localization of the east caldera wall. NNE-SSW compression was observed at SW Thera; this may have produced E-W failure and contributed to the localization of the south caldera wall. The footprint of the caldera on the basement is a parallelogram with N-S long and WNW-ESE short dimensions: if the east and south flanks collapsed along N-S normal and E-W inverse failures, then the west and north flanks may have formed analogously. Present-day deformation is localized on the TSD and CFZ: this can only be explained if the former is the synthetic (dextral) Riedel-R shear and the latter the antithetic (sinistral) Riedel-R′ shear, generated by N-S σ1 and E-W σ3 principal stress axes. Accordingly, NW-SE right-lateral shearing of the broader area is expected and indicated by several lines of indirect evidence. The geographic extent of this shearing and its role in the regional tectonics of the south Aegean remains to be confirmed and appraised by future research. Contemporary volcanic centres develop at the interface of the TSD with the CFZ/AFZ graben; volcanism appears to be controlled by tectonics and the SVC to be shaped by tectonic rather than volcanic activity. © 2019 The Author(s). Published by Oxford University Press on behalf of The Royal Astronomical Society

The Methana Volcano – Geothermal Resource, Greece, and its relationship to regional tectonics

Geophysical methods of analysis were applied, in order to investigate the deep structure and the geothermal potential of the Methana Volcano (NE Peloponnesus, Greece). The study is based on a re-evaluation and reinterpretation of legacy magnetotelluric (MT) data with modern analysis methods, as well as 3-D inversion of aeromagnetic data constrained by in situ measurements of magnetic susceptibility. Magmatic systems are located in regions of active tectonic processes that often play a controlling role. The MT method is effective in delineating low resistivity functional elements of volcanic systems, such as magma chambers, vents, thermal fluid reservoirs and thermal fluid circulation conduits, the latter two of which are typically associated with active faults. The aeromagnetic data can assist in mapping the configuration, hence emplacement modes of volcanic rocks at depth. Accordingly, the joint interpretation of these lines of evidence, together with structural and geochemical information, is expected to allow insight into the influence of contemporary tectonics on the inception and evolution of the volcano. The contemporary stress field is mainly extensional, NNE-SSW oriented and overall homogeneous; in the area of Methana it allows for the formation of WNW-ESE north-easterly dipping normal faults, W-E faults consistent with the synthetic (dextral) R-shear direction of Riedel's shear theory and NW-SE faults consistent with the antithetic (sinistral) R′-shear direction; all such features have been mapped on Methana Peninsula. The magnetotelluric data imaged a significant geothermal reservoir developing around an intersection of the three active fault zones (normal, R and R′) at depths of 1–1.5 km below the centre of the peninsula, as well as elongate epiphenomenal conductivity anomalies associated with the circulation of thermal fluids along all three fault zones. The 3-D magnetic susceptibility model strongly suggests that the intrusion and emplacement of magmas were guided by the same active fault zones, with particular reference to the R and R′ shears whose influence is imprinted on the configuration of volcanic rocks at depth. The joint interpretation of all lines of evidence indicates that magmatism and volcanism at Methana are almost completely controlled by tectonic activity in a manner analogous to the situation of the large Santorini Volcanic Complex. It also indicates that the reservoir is replenished through the weak permeable zone created by the intersection of the R and R′ shears, which is very probably collocated with the main vent of intrusive magmatic activity and may connect with a shallow magma chamber at depths greater than 4.5 km. The apparently common origin and similarities/differences in the circulation paths of thermal fluids may amply explain both the individual characteristics and similarities/differences in the chemical composition of thermal spring discharges, which have been reported by hitherto geochemical investigations. © 2020 Elsevier B.V

Simulation of macroseismic field in Central Greece

The distribution of seismic intensity is generally influenced by major geological and tectonic features and, on a smaller scale, by local geological conditions, such as the type of surface soil, the surface-to-bedrock soil structure in sedimentary basins and the depth of the saturated zone, The present paper attempted to determine the distribution of macroseismic intensities based on published attenuation laws in the area of Central Greece, using the epicentral intensity, magnitude, length and direction of fault and a considerable number of observation sites, for which the above mentioned information is available, The expected intensity values were then compared to those observed in the same sites, from four earthquakes in Volos, Central Greece, for which the fault plane solutions are also known. The deviations of the observed values from the theoretical model were then related to the local geological conditions and the corresponding correction factor determined for each site

Simulation of macroseismic field in Central Greece

The distribution of seismic intensity is generally influenced by major geological and tectonic features and, on a smaller scale, by local geological conditions, such as the type of surface soil, the surface-to-bedrock soil structure in sedimentary basins and the depth of the saturated zone, The present paper attempted to determine the distribution of macroseismic intensities based on published attenuation laws in the area of Central Greece, using the epicentral intensity, magnitude, length and direction of fault and a considerable number of observation sites, for which the above mentioned information is available, The expected intensity values were then compared to those observed in the same sites, from four earthquakes in Volos, Central Greece, for which the fault plane solutions are also known. The deviations of the observed values from the theoretical model were then related to the local geological conditions and the corresponding correction factor determined for each site