Tectonic Trends Research Articles

Subsurface Structures and Hydrogeologic Aquifers at the Western Side of Lake Nasser, Southwestern Desert, Egypt

This work deals with the detection of water aquifers and the delineation of subsurface structures predominant in the basement rocks, and their relationship with these aquifers, at west Lake Nasser, southwestern desert, Egypt. An integrated study using geoelectric, geomagnetic, and well logging methods was conducted. Geoelectric surveys were initially performed to obtain vertical electrical sounding (VES) profiles to identify the subsurface geoelectric layers predominant in the area. This was followed by land magnetic surveys using a proton precession magnetometer in a mesh-like network covering the area of investigation. These surveys allowed us to determine the depth to basement rock, and establish the subsurface structures and their relation with the detected aquifers. To confirm the results, more than 20 boreholes were drilled and logged within the study area to determine the distribution of the subsurface water reservoirs and geological sequences in the area. The results indicate that the depth to the main water aquifer, represented by saturated Nubian sandstone, ranges from [Formula: see text]. The thickness of this reservoir varies from [Formula: see text]. The Nubian sandstone formation consists of three members (layers) that unconformably overlie the basement rocks. The depth to the basement rocks ranges from [Formula: see text]. The structure trend analyses indicate that the dominant tectonic trends are NW, NE, and Aqaba trends. These structures play an important role in the water supply from Lake Nasser and recharging the main water aquifer.

Aeromagnetic constraints on the subsurface structure of Stromboli Volcano, Aeolian Islands, Italy

Two helicopter-borne magnetic surveys were conducted over Stromboli Volcano and its surrounding areas on the Aeolian Islands, southern Italy in 2002 and 2004 to better understand the subsurface structure of the area. Observed data from those surveys were merged and aeromagnetic anomalies for Stromboli Island and its vicinity were reduced onto a smoothed surface, assuming equivalent anomalies below the observed surface. The magnetic terrain effects were calculated for the magnetic anomalies of the study area, assuming the magnetic structure comprised of an ensemble of prisms extending from the ground surface to a depth of 3000 m below sea level: the average magnetization intensity was calculated to be 2.2 A/m for the edifice of Stromboli shallower than 1200 m below sea level by comparing the observed and synthetic data. Next, apparent magnetization intensity mapping was applied to the observed anomalies using a uniform magnetization of 2.2 A/m as the initial value. The apparent magnetization intensity map indicates magnetic heterogeneities among volcanic rocks which constitute the edifice of the volcano. The most remarkable feature of the magnetization intensity map is a magnetization low which occupies the center of the island where the summit craters reside, suggesting demagnetization caused by the heat of conduits and/or hydrothermal activity in addition to the thick accumulation of less magnetic pyroclastic rocks. By comparing topographic and geologic maps, it can be seen that magnetization highs are distributed on the exposures of basaltic-andesite to andesite lavas (Paleostromboli I), shoshonitic lavas with an eccentric vent and a shield volcano (Neostromboli), on the south, north and west coasts of the volcano, respectively. These magnetization highs further extend offshore, implying the seaward continuation of these volcanic rocks. 3-D magnetic imaging was preliminarily applied to the same magnetic anomalies as well as for the magnetization intensity mapping. The result implies that the bottom depths of these magnetic structures are relatively shallow (< 1500 m below sea level at the maximum). A distinctive regional magnetization high is distributed on a saddle area between Stromboli and Basiluzzo islands and extends to the two submarine eruptive centers on the southwestern submarine edifice of Stromboli. A careful examination on the results of the 2-D and 3-D magnetic imaging implies that the saddle area is occupied by volcanic rocks from these eruptive centers and is also underlain by partially and/or completely concealed volcanic structures formed along a NW–SE direction conjugate to the main axis of regional tectonic trend in this area.

Aeromagnetic Data Interpretation of Wadi Hawashiya Area for Identifying Surface and Subsurface Structures, North Eastern Desert, Egypt

The studied area is located within the North Eastern Desert of Egypt. The lithology of this area comprises rocks related to Precambrian, Cambrian, Carboniferous, Albian to Cenomanian, Late Cenomanian to Early Turonian and Quaternary. Aeromagnetic data are fully analyzed and interpreted where the analysis and interpretation are mainly devoted towards outlining significant surface and subsurface tectonic trends and their relationship with T.C. radiometric anomalous zones. According to the qualitative interpretation of the geological and aeromagnetic, and its shaded relief maps, the lineament structural map is constructed through the signatures of the contours and shaded relief of the total aeromagnetic data. Werner de-convolution (dyke model) was applied to the aeromagnetic anomaly map. The depth estimates calculated for these major magnetic anomalies range between 0.5 and 3.9 km. lineament structural map and magnetic depth map values have been utilized to construct the interpretation of the main subsurface structures affecting the studied area, which correlate with the previous studies of aeroradiomtric anomalous zones allover the study area. The obtained results showed the most significant structural trends affecting the distribution of these radiometric anomalies in the present study area. Four main tectonic trends are outlined from the total aeromagnetic and shading relief maps. These trends were defined as N60E, N45W and E-W.

Coherence between teleseismic tomography and long-wavelength features of the gravity and magnetic fields of southeastern Australia

During 2007 the NSW Department of Primary Industries contracted the Australian National Research Facility for Earth Sounding (ANSIR) to conduct the SEAL2 (South East Australia Linkage experiment, part 2) teleseismic program in southwestern New South Wales, to supplement earlier teleseismic acquisition in Victoria and southeastern South Australia (Figure 1). Interpretation of the results of the earlier SEAL1 program (Rawlinson et al., 2006), acquired with a seismic array in the southern Murray basin, appeared to conflict with recent interpretations of upper-crustal tectonic trends inferred primarily from aeromagnetic surveys in southwestern NSW (Hallett et al., 2005), which suggested that the Stawell Zone of western Victoria continued northwards into NSW. Instead, the SEAL1 tomographic model suggested a break in upper mantle velocities near the state border; the model implied that the Stawell Zone in Victoria is underlain by high vp upper mantle, consistent with lithospheric basement of Proterozoic age, while the putative extension of the Stawell Zone into NSW is underlain by low vp mantle, corresponding to Palaeozoic basement. Analysis of the SEAL2 dataset included a synthesis of all the southeast Australian teleseismic data into a single tomographic model of the upper mantle (Rawlinson, 2008). This model complicated the interpretation of the Stawell Zone further, by indicating a boundary between high vp ?Proterozoic? and low vp ?Phanerozoic? upper mantle cutting obliquely across the boundary between the Stawell and Bendigo zones in Victoria. In addition, two lobes of high vp upper mantle east of 145°E lie below Phanerozoic upper crust of the Lachlan Orogen, and lack any apparent relationship to internal tectonic divisions within the Lachlan Orogen. This paper investigates, and seeks to resolve, the apparent discrepancy between the tomographic upper mantle models on the one hand, and the combination of mappable surface geology and interpreted upper-crustal geophysics on the other, by comparing both with geophysical and geological evidence which arises from sources located in the middle to lower crust. Potential field anomalies with wavelengths greater than about 20 km should reveal middle crustal features, although the ability to discriminate them is compromised by the presence of broad anomalous sources (granitoids) in the upper crust, and, for the magnetic field, by suppression of long wavelengths during grid merging.

A tectonic model of the Sinai Peninsula based on magnetic data

The present work deals with the tectonic position of the Sinai Peninsula based on the interpretation of magnetic data. A high-resolution magnetic survey was undertaken, combined with aeromagnetic data, GPS and seismic stress-tension analysis. The magnetic data was interpreted using the analytical signal, horizontal gradient, Euler and Werner deconvolutions and 2.5 dimensional magnetic modelling methods. The seismological focal mechanism and stress-tension forces have also been evaluated. We conclude that the structural trend N35??N45? west reflects the most predominant tectonic force in the southern part of the Sinai Peninsula, with a secondary N45??N65? east tectonic trend. Interestingly, this situation is reversed in the northern part of the peninsula. The depth of the basement rocks ranges from zero at the southern end to more than 4 km below sea level at the northern end of the studied area. Furthermore, we found that the depth of the basement rocks increases from east to west, which is consistent with available well log data. This reflects a source of stress situated in the south that is pushing in the NE direction. Correlating the seismological and GPS analysis has enabled the construction of a tectonic model for the Sinai Peninsula.

Formation and evolution of Lakshmi Planum, Venus: Assessment of models using observations from geological mapping

Detailed geological analysis of the Lakshmi Planum region of western Ishtar Terra results in the establishment of the sequence of major events during the formation and evolution of western Ishtar Terra, an important and somewhat unique area on Venus characterized by a raised volcanic plateau surrounded by distinctive folded mountain belts, such as Maxwell Montes. These mapping results and the stratigraphic and structural relationships provide a basis for addressing the complicated problem of Lakshmi Planum formation and for testing the suite of models previously proposed to explain this structure. We review and classify previous models of formation for western Ishtar Terra into “downwelling” models (generally involving convergence and underthrusting) and “upwelling” models (generally involving plume-like upwelling and divergence). The interpreted nature of units and the sequence of events derived from geological mapping are in contrast to the predictions of the divergent models. The major contradictions are as follows: (1) The very likely presence of an ancient (craton-like) tessera massif in the core of Lakshmi, which is inconsistent with the model of formation of Lakshmi due to rise and collapse of a mantle diapir; (2) The absence of rift zones in the interior of Lakshmi that are predicted by the divergent models; (3) The apparent migration of volcanic activity toward the center of Lakshmi, whereas divergent models predict the opposite trend; (4) The abrupt cessation of ridges of the mountain ranges at the edge of Lakshmi Planum and propagation of these ridges over hundreds of kilometers outside Lakshmi; the divergent models predict the opposite progression in the development of major contractional features. In contrast, convergent models of formation and evolution of Lakshmi Planum appear to be more consistent with the observations and explain this structure by collision and underthrusting/subduction of lower-lying plains with the elevated and rigid block of tessera. These models are capable of explaining formation of the major features of western Ishtar (for example, the mountain belts), the sequences of events, and principal volcanic and tectonic trends during the evolution of Lakshmi. To explain the pronounced north–south asymmetry of Lakshmi these models need to consider the likelihood that the major focal points of collision are at the north and north-west margins of the plateau. We note that pure downwelling models, however, face three important difficulties: (1) The possibly unrealistically long time span that appears to be required to produce the major features of Lakshmi; (2) The strong north–south asymmetry of the Planum; the pure downwelling models predict the formation of a more symmetrical structure; and (3) The absence of radial contractional structures (arches and ridges) in the interior of Lakshmi that would represent the predictions of the downwelling models.

Radiospectrometric and magnetic signatures of a gold mine in Egypt

The purpose of this research is to study the signatures of a gold mine (Um Salim), located in the Eastern Desert of Egypt, using aeroradiospectrometric and aeromagnetic data. To achieve this goal, reduction-to-pole (RTP), high-pass filtering and analytical signal techniques were applied to the aeromagnetic data. Besides, interpretation of the total-count (TC) radiometric map, the three radioelements (K, eU and eTh) maps and the ternary radioelement composite image were carried out. Moreover, a two-dimensional trend analysis for the structural lineaments as traced from the geologic, total-count radiometric and reduced-to-pole magnetic maps was conducted to define the relationship which might exist between location of the gold mine, lithologies and major structures of the area under consideration. The study revealed that the known gold mine is associated with a high positive magnetic anomaly that trends in the NE direction and a high-amplitude analytic signal. The Um Salim gold mine is associated with very low aeroradiospectrometric levels on the four maps (TC, K, eU and eTh). The location of the gold mine is controlled by the intersection of both NW and NE trends. There are other particular radiospectrometric ratio signatures for gold deposits (eU/eTh, eU/K and eTh/K). Ternary radioelement maps can discriminate also alteration zones in various degrees that correlate with the gold mine. These zones can be used as a diagnostic exploration guide for various types of non-radioactive mineral deposits (e.g., copper and gold). Four major tectonic trends were identified that have the following directions: NE, NW, N–S and WNW.

Geophysical investigations of the plumbing system of Stromboli volcano (Aeolian Islands, Italy)

In this work, we report the results of an integrated approach using both seismological and geodetic data provided by the INGV-CT (Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania) Stromboli volcano monitoring systems, in order to improve the knowledge of its plumbing system. In particular, we investigated the relationships between the June 1999 seismic swarm, occurring in the area of Stromboli, and the possible activation of the NE–SW oriented volcano-tectonic structure. We analyzed this seismic swarm proposing new locations and a morphological analysis of the waveforms. This approach allowed us to demonstrate that there are relationships between the tectonic activity near Stromboli and the rising of magma. This evidence supports the hypothesis that during the 1999 swarm an intrusive process started from a crustal level where earthquakes were located (about 10–15 km b.s.l.). As already testified by other similar episodes (deformation anomaly recorded between December 1994 and March 1995 after the November 1994 swarm), months after the 1999 seismic activity, the tiltmeters and the GPS permanent stations deployed in Stromboli, showed slow variations over three months (May–July 2000). We performed an analytical inversion of these geodetic observations and found that the modelled sources are characterized by NE–SW trend, compatible with the regional faults cutting the volcano. The modelled sources could represent the rising pathway connecting the “deep” plumbing system (about 10–15 km b.s.l.) to the “shallow” one located in the body of the Stromboli volcano (about 500 m a.s.l.). These new evidences support the hypothesis of the existence of a simple plumbing system with different levels of magma storage, where batches of magma are periodically pushed up along the main NE–SW tectonic trend.

The role of geomagnetic methods in the detection of mineral deposits at El‐Hody area, south‐eastern Aswan, Egypt

ABSTRACTIn this study we delineate the subsurface structures within an area of about 840 km2 in south‐western Egypt, using magnetic methods. The main target is to establish the tectonic framework of the studied area and to understand the role of tectonics in the mineralization process. To do so, a land magnetic survey was conducted, complemented with available aeromagnetic data. This data set is processed using a series of techniques, namely, trend analysis, Werner deconvolution and 2.5 direct modelling. Results indicate that the most predominant tectonic trends are NW–SE, NE–SW and E–W directions. The depth to the basement rocks ranges from 0 to 1.3 km. These results agree with the drilled well data. We show a good correlation between the structures deduced from the magnetic analysis and the known mineral distributions. We conclude that mineralization is structurally controlled by faulting, probably as the result of hydrothermal circulation on fault planes.

Faulting-assisted lateral collapses and influence on shallow magma feeding system at Ollagüe volcano (Central Volcanic Zone, Chile-Bolivia Andes)

The role played by the regional tectonics and relationships between volcanic structures and their shallow magmatic feeding system are of paramount importance in understanding multiple lateral collapse events and, thus, for hazard assessment in active volcanoes. Ollagüe (21°18′S, 68°11′W, Andean Central Volcanic Zone, Chile-Bolivia border) is an active, andesite-dacite, composite volcano developed on a regional NW-striking extensional fault system that bisects the volcanic edifice. Geological and stratigraphic studies on the summit part of the cone, together with new 40Ar/ 39Ar dating and petrographic and geochemical (major-trace elements) analyses, have allowed us to delineate the volcanic history of Ollagüe and recognize multiple deformation and lateral collapse events. The evolution of Ollagüe appears to be characterized by four stages of volcano building, separated by three main events of deformation of the cone. These deformation events developed progressively by downthrow of the SW sector of the volcano (∼ 800 ka), along the NW-striking normal fault propagating from the substrate, to a final catastrophic failure (∼ 300 ka) of the SW sector and debris avalanche emplacement. Both NW-trending regional tectonics and weakness of the edifice caused by the successive deformation and collapse events are responsible for both the geometric variations of the shallow magma feeding system and the coexistence of summit and parasitic dacitic vents. Central and lateral vents shifted in a SW-trending direction, i.e. perpendicular to the tectonic trend and along the axis of the collapse movement, and developed along a NW-trending alignment, i.e. parallel to the main tectonic trend and collapse scarps.

MINERALOGY AND PETROLOGY OF ASSOCIATED Mg-RICH ULTRAPOTASSIC, SHOSHONITIC, AND CALC-ALKALINE ROCKS: THE MIDDLE LATIN VALLEY MONOGENETIC VOLCANOS, ROMAN MAGMATIC PROVINCE, SOUTHERN ITALY

In the Middle Latin Valley, in the Roman Magmatic Province, southern Italy, volcanism developed during the Late Pleistocene, with eruption of high-Mg magmas characterized by a low content of phenocrysts. Small volumes of magma, transported to the surface rapidly, favored by an extensional post-collision regime, led to small monogenetic centers aligned along two main tectonic trends. The absence of a large magmatic reservoir allowed the occurrence of a fairly large amount of mafic, strongly primitive volcanic rocks with a fairly large enrichment in potassium. They range in composition from plagioclase-free ultrapotassic melilite-bearing (kamafugites) to ultrapotassic leucitites and plagioclase leucitites, to trachybasalts (shoshonites), and calc-alkaline basaltic rocks (subalkaline). The composition of minerals from the four different groups of rock is reported. The Fe–Mg distribution between olivine crystals and whole rocks indicates equilibrium crystallization. The clinopyroxene shows the typical trend of alkaline potassic and ultrapotassic rocks, ranging in composition from diopside to hedenbergite. Melilite-bearing ultrapotassic rocks are found beside feldspar-bearing ultrapotassic and potassic primitive rocks. Magnesiochromite is the main spinel; it has been found enclosed in olivine cores. Olivine–spinel pairs indicate that the magmas from which they crystallized were in equilibrium with a strongly depleted mantle source. An origin from a common mantle source, in terms of a peridotitic component, is inferred for the different varieties of ultrapotassic to subalkaline primitive rocks. This source was characterized by different degrees of metasomatic enrichment arranged in a network of veins, possibly within the lithospheric upper mantle.

Evaluating the subsurface structures due to the Syrian Arc Tectonics at El-Hasana area, Sinai Peninsula, Egypt

Three major domal structures are trending northeast-southwest directions prevailed at El-Hasana area, northern part of Sinai Peninsula. These structures and their altered rock constituents attracted us to study these formations from geophysical view. The main target of this work is to establish the relation between these structures and Syrian Arc Tectonic System (SATS) prevailed at the studied area. The investigations were carried out using land magnetic survey, aeromagnetic, gravity and laboratory measurements.An intensive analysis using tectonic trend analysis and, 3D Euler deconvolution with different coefficients were applied to the potential field data. Also, cross-sections Euler deconvolution, Werner deconvolution and radial power spectrum have been applied. The results illustrate that most prevailed tectonics that could forming these anticlines are trending at N 35°–65° E direction which related to the Syrian tectonics (SATS) direction. The depth to the basement rocks ranges between 2 km at the southern parts to 2.3 km at the northern parts of the studied area.The magnetic susceptibility map was constructed using laboratory measurements for more than 120 collected rock samples along the surveyed area. The average values range between 0.7 × 10 −3 to 1 × 10 −3 SI for limestone samples, whereas the sandstone varies between 1 × 10 −3 to 1.5 × 10 −3 SI unit. The results show that the magnetic values are higher for the rock samples picked along the anticlinal structures.

Regional-scale aeromagnetic survey of the south-west of Algeria: A tool for area selection for diamond exploration

The main objective of the most regional-scale aeromagnetic surveys is to assist in mineral development through improved geologic mapping. The aeromagnetic survey of the south-west of Algeria is used for diamond potential evaluation and target-area selection. This region of Algeria, forming part of the west African Craton, has until very recently been under-explored for diamonds. This paper is a contribution to area selection for primary diamond deposits based on a conceptual model for kimberlitic occurrences. According to the proposed exploration model, the emplacement of diamondiferous kimberlites is controlled by three principal criteria: (1) existence of a sufficiently thick lithosphere, favourable to diamond genesis and preservation, (2) presence of major tectonic trends and lineaments that could act as pathways for kimberlitic magma, the principal diamond-bearing lithology, and (3) the recognition of magnetic anomalies related to mafic–ultramafic intrusives, signs of deeply-rooted magma. Interpretation of the aeromagnetic data, using energy spectral analysis and different data-enhancement filters, has permitted elucidation of all three criteria. Their combination provides a final assessment for three possible areas as targets for primary diamond deposits emplacement.

Unveiling the Sources of the Catastrophic 1456 Multiple Earthquake: Hints to an Unexplored Tectonic Mechanism in Southern Italy

We revisited data related to the 1456 seismic crisis, the largest earthquake to have ever occurred in peninsular Italy, in search of its causative source(s). Data about this earthquake consist solely of historical reports and their intensity assessment. Because of the age of this multiple earthquake, the scarcity and sparseness of the data, and the unusually large damage area, no previous studies have attempted to attribute the 1456 events to specific faults. Existing analytical methods to identify a likely source from intensity data also proved inappropriate for such a sparse dataset, since historical evidence suggests that the cumulative damage pattern contains at least three widely separated events. We subdivided the 1456 damage pattern into three independent mesoseismal areas; each of these areas falls onto east–west tectonic trends previously identified and marked by deep (>10 km) right-lateral slip earthquakes. Based on this evidence we propose (1) that the 1456 events were generated by individual segments of regional east–west structures and are evidence of a seismogenic style that involves oblique dextral reactivation of east–west lower crustal faults; (2) that each event may have triggered subsequent but relatively distant events in a cascade fashion, as suggested by historical accounts; hence (3) that the 1456 sequence reveals a fundamental but unexplored mechanism of tectonic deformation and seismic release in southern Italy. This style dominates the region that lies between the northwest–southeast system of large extensional faults straddling the crest of the southern Apennines and the buried outer front of the chain. Although the quality of the available information concerning the 1456 earthquake is naturally limited, we show that the overlap of the damage distribution, the orientation and characteristics of regional tectonic structures, the seismicity patterns, and the focal mechanisms all concur with our interpretations and would be difficult to justify otherwise.

Seismic structure of Kuwait

SUMMARY We have used data from the Kuwait National Seismic Network (KNSN) to estimate the seismic structure of Kuwait using a limited amount of seismic data. First, we made surface wave dispersion measurements and calculated receiver functions from the relatively small amount of data available from the broad-band station, KBD. Models were derived from the joint inversion of teleseismic receiver functions and Rayleigh and Love fundamental mode surface wave group velocity dispersion. While both surface waves and receiver functions by themselves can be used to estimate lithospheric structure, we have successfully combined the two to reduce non-uniqueness in estimates based on the individual data sets. The resulting KUW1 model features a thick (8 km) sedimentary cover and crustal thickness of 45 km. Crustal velocities below the sedimentary cover are consistent with global averages for stable platforms. We infer upper-mantle velocities (7.84 km s ‐1 P-wave velocity; 4.40 km s ‐1 S-wave velocity) that are slightly lower than expected for a stable platform. In comparison with other crustal structure estimates for the Arabian platform to the west, the crust is thicker and the mantle is slower in Kuwait. This is consistent with the overall tectonic trends of the region that find increasing crustal thickness between the divergent plate boundary at the Red Sea and the convergent plate boundary at the Zagros Mts, as well as slow mantle velocities beneath this nearby orogenic zone. The resulting model fits the traveltimes of regional phases (Pn, Pg, Sn and Lg). Independent inversion of local earthquake traveltimes recorded by KNSN (allowing for event hypocentre relocation) results in a remarkably similar velocity structure, providing confidence that the joint inversion of receiver functions and surface wave group velocities can impose accurate constraints on crustal structure for local event location and network operations. Relocation of events in Kuwait improves the clustering of events and results in shallower hypocentres.

Astronomical tuning as the basis for high resolution chronostratigraphy: the intricate history of the Messinian Salinity Crisis

The Messinian Salinity Crisis (MSC) in the Mediterranean resulted from a complex interplay between tectonic gateway closure and climate evolution. The climate factor, in turn, can be separated into two components, one associated with dominantly precession controlled regional climate change and the other with dominantly obliquity related glacial cyclicity. The influence of these climate changes occurred superimposed on a long(er)-term tectonic trend. Discrimination into the various forcing factors only recently came within reach due to the development of an integrated high-resolution stratigraphy and astronomically tuned age models, both for the Messinian (pre-)evaporite successions in the Mediterranean and for benthic oxygen isotope records from the open ocean. The application of these time scales in combination with a high-resolution integrated stratigraphic approach showed that 1) the onset of the MSC proper at 5.96 Ma is not related to glacio-eustatic sealevel lowering but its timing can best be attributed to the influence of the 400-kyr eccentricity cycle on regional climate superimposed on a tectonic trend, 2) the main desiccation phase between the Lower and Upper Evaporites coincides with the twin peak glacials TG12-14 suggesting a glacio-eustatic control, 3) the beginning of the Upper Evaporites and Lago Mare phase coincides with the onset of the major deglaciation following peak glacial TG 12, indicating that glacio-eustatic sealevel rise played a role, 4) the Pliocene flooding of the Mediterranean is not related to a glacioeustatic sealevel rise, and 5) the evaporite cycles are controlled by precession induced regional climate changes rather than by obliquity forced glacio-eustatic sealevel change. Our astronomical age model for the Mediterranean Messinian is consistent with 40Ar/39Ar ages of ash layers intercalated in Messinian successions in the Mediterranean. However, the intercalibration between astronomical and 40Ar/39Ar dating has to be taken into account for a direct comparison of these ages.

Tectonic trends delineated from drainage lineament analysis and azimuthal resistivity survey: a case study of S.E. Nigeria gully erosion belt

Tectonic trends delineated from drainage lineament analysis and azimuthal resistivity survey: a case study of S.E. Nigeria gully erosion belt

Crustal architecture of the Transantarctic Mountains between the Scott and Reedy Glacier region and South Pole from aerogeophysical data

Aerogeophysical data collected in transects between the South Pole and West Antarctica, crossing the Transantarctic Mountains at the 150°W meridian, are used to constrain the sub-ice topography, the sub-ice geology and the inland structure of the Transantarctic Mountains. Forward modeling of gravity data suggests slight crustal thickening of 5 km beneath the mountain front indicating partial isostatic compensation by thickened crust. New magnetic data help characterize the sub-ice geology inland of the Transantarctic Mountains with the observed magnetic anomaly field dominated by Granite Harbour Intrusives, similar to the magnetic field in Victoria Land. However, the typical pattern of anomalies caused by Jurassic tholeiitic magmatism elsewhere along the Transantarctic Mountains is not observed, nor is the mesa topography that is often associated with the Ferrar Dolerite. Together, these observations rule out the widespread presence of Ferrar Dolerite sills within the survey area. A pronounced magnetic lineament, herein named the South Pole Lineament, parallel to the 0°/180° longitudinal meridian, beneath the South Pole defines a previously unknown tectonic trend of the East Antarctic craton. The lineament suggests the presence of a lithospheric-scale structure beneath South Pole, projecting into a fault mapped from ice-penetrating radar data and extending to Shackleton Glacier, the site of a major geological boundary across the Transantarctic Mountains. Potentially, the lineament is the expression of the edge of the undeformed craton, an inherited structure created during assembly or breakup of Rodinia and Gondwana supercontinents; or an intracontinental transform.

A morphometric analysis of the submarine volcanic ridge south-east of Pico Island, Azores

A region of crustal extension, the Azores Plateau contains excellent examples of submarine volcanic edifices constructed over a wide range of ocean depths along the Pico Ridge. Using bathymetric data and Towed Ocean Bottom Instrument (TOBI) side-scan sonar imagery, we measured the dimensions (diameter, height, slopes), shape, and texture of these volcanic edifices to further understanding of the geometric development of a submarine ridge. Our analysis and interpretation of the measurement and texture data suggest the following: (1) the various edifice types do not correlate with depth ranges, suggesting that ambient water pressure is not a controlling factor in configuration of the edifice formed; (2) the cones have a mean diameter of 948 m, height of 152 m, and slope of 15.6°, and are peaked rather than displaying flat-topped summits; and (3) while hummocky-textured cones also occur, smooth-textured cones predominate. The cones seem to develop preferentially outwards, then upwards, with only a weak correlation between diameter and height, suggesting that the cone population does not evolve self-similarly. Although smooth and hummocky cone populations are not statistically different in mean slope angle and eruption depth, the hummocky cones have a significantly greater mean diameter than the smooth cones. We suggest that hummocky-textured cones (probably involving eruption of pillow lavas) are formed after voluminous smooth textured flows. Nearest-neighbour analysis suggests that submarine cones are distributed randomly whereas subaerial cones are not. We interpret this finding to suggest subaerial cones being masked by over-covering flows, which tend to flow further than submarine lava flows. Furthermore, the spatial distribution of cones away from the ridge centre is not easily explained by a magma supply fed via lava tubes from central eruptions, because of a lack of consistent pathway down gradient. Fissure vents seem to be important in determining construction of the ridge and result in linear arrangements of edifices and cone elongation, consistent with the regional tectonic trend of this part of the Azores Plateau. Cones form as each feeding dyke intrusion cools and the eruption becomes localized along point-source vents along the fissure. The ridge seems to be predominantly formed from fissure eruptions along the ridge axis, with subordinate transport of lava down its flanks.

Local seismic array observations at north Evoikos, central Greece, delineate crustal deformation between the North Aegean Trough and Corinthiakos Rift

In order to better constrain and define the microseismic activity at the north Evoikos Gulf and its surrounding area we deployed an onshore/offshore seismic array consisting of 31 three-component seismic digital stations. The array was active from 30 June to 24 October 2003, and covered an area of 2500 km 2. We located more than 2000 seismic events ranging from 0.7 to 4.5 M L by using six stations as a minimum in order to define the foci parameters. Recorded seismicity delineated three major zones of deformation: from south to north, the Eretria–Parnis–eastern Corinthiakos zone, the Psachna–Viotia zone, and the Northern Sporades–North Evia–Bralos zone. Alignments of the recorded seismicity follow the tectonic trends and their orientation in the above zones. The whole area accommodates the stress field between the North Aegean Trough and the Corinthiakos Gulf. Rate of deformation intensifies from north to south, as revealed also by historical and instrumental seismicity. The successive change of orientation between the two stress fields fragments the crust in relatively small units and the fault systems developed do not permit the generation of major earthquakes in the north Evoikos area and its immediate vicinity. This is also supported by the instrumental seismicity of the last century. Larger events reported in historical times are probably overestimated. Most seismic activity is crustal. Subcrustal events were recorded mainly below the Lichades area and are interpreted as the consequence of the subduction of the Ionian oceanic lithosphere below the Hellenides. The Lichades volcano is the most northern end of the Hellenic volcanic arc. At present the highest seismic activity is associated with the Psachna region of north Evia that has been continuously active since 2001. Considering, however, the development of the seismic activity during the last decade, there has been a sequence of large events, i.e., Parnis in 1999, Skyros in 2001 and Psachna in 2001–2003. This demonstrates the fact that the tectonic deformation in all this area is intense and important for the accommodation of the stress field of the North Aegean Trough to that of the Corinthiakos Rift.