Tectonic Trends Research Articles

Seismicity and Seismotectonics of Jeddah-Makkah Region, West-Central Saudi Arabia

Jeddah-Makkah regionis have been suffering from earthquake crisis where some moderate to destructive earthquakes have been recorded. These earthquake activities are oriented along major faults or clustered in certain spots. Moreover, these earthquake events have annual recurrence periods, so the identification of these seismogenic source zones is of utmost importance for mapping the most hazardous localities which should be avoided in the future urban planning. Historical and instrumental earthquakes have been collected from national and international data centers and unified in catalogue. The existence of microearthquakes inland suggests that there is a significant level of tectonic activity at away from the axial trough of the Red Sea. Then, seismogenic source zones have been defined depending on the major tectonic trends; distribution of earthquake epicenters, seismicity rate (a & b-values) and fault plane solution of major earthquakes. It is concluded that Jeddah-Makkah region is affected by the outlined five seismogenic source zones; three of these zones aligned of the main Red Sea axial trough (southwestern Jeddah, western Jeddah, and northwestern Jeddah zones), while the other two zones are located in the land area of the region (Thewal-Rabegh and Jeddah-Makkah zones). These inland zones correlated well with the main trends of major tectonics which refleet the reactivation of tectonic movements along these fault trends. The Red Sea zones are in agreement with the main path of the axial trough. The range of b-value in these identified zones is 0.65 to 1.03 through these identified zones. The area characterized by higher b-values could be indicative of a relative low stress regime which was a result of resulting from the stress release by the earthquakes. Whereas, the areas of lower b-values can be considered as an evidence of a relatively higher stress regime associated with a dominantly extensional stresses. Based on aforementioned, the region is suffering from different stress level accumulations which, in turn, cause earthquakes with different magnitudes. Accordingly, deployment of local seismograph network through Jeddah-Makkah region is highly recommended. These results will support, to a great extent, seismic hazard assessment and risk mitigation of the region.

Integrated geological mapping approach and gold mineralization in Buhweju area, Uganda

Deeply dissected mountainous ridges, paucity of exposure, deep weathering, extensive regolith, dense vegetation cover and poor accessibility, are detrimental to conventional field geological mapping. Conditions such as these, characterize our current study area; the Buhweju area in Uganda. The latter is poorly explored and known to host alluvial gold. In this work, an integrated geological mapping approach aiming at updating the geology, building relationships between the lithology and structure and assessing its control on mineralization was tested using Landsat TM, shuttle radar topographic mission (SRTM) and potential filed datasets (radiometrics and magnetics) subsequently validated by field investigations. Several image processing and enhancement techniques were applied that improved the aesthetic quality of the various remotely sensed datasets for visualization and interpretation. Among the image enhancement techniques, image fusion and pan-sharpening of the radiometric composite image with multispectral data and SRTM digital elevation model (DEM) have resulted in better lithological differentiation. In general, the present work, found complementarity in lithology with the legacy geological map. On the basis of similar radiometric signature however, various basement rock units such as pelitic schist, quartzo-feldspathic gneiss and granite were delineated and mapped; some in areas which were not reported before. These were validated during field investigation. Similarly, the aeromagnetic data depicted worthwhile information regarding litho-magnetic domains and tectonics of the Buhweju study area.Three prominent structures trending NNW, ESE and NNE determine the tectonic grain of the area. These orientations are consistent with the two major and important regional tectonic trends known in the area; the NNE to the western Rift while the NNW attest to the regional Aswan shear zone. The control of the Buhweju gold field was also assessed in terms of the improved geological units and interpreted lineaments of the area. Accordingly, most gold occurrences are localized in preferred orientations and host rock. Density analysis carried out on surface and subsurface lineaments reveals the Buhweju group rocks and the undifferentiated schist and amphibolites are highly affected by surface lineaments whereas the basement rocks are affected mainly by subsurface lineaments. Two major clusters of gold occurrences were identified in the Buhweju gold field. The gold cluster found at Kitaka mine is characterised by a low magnetic anomaly and high lineament density, while the gold occurrences found at Katonga is hosted by mudstone and Lubare quartzite and underlain by a high magnetic anomaly. These gold clusters are spatially controlled by the NNW and NNE trending faults and their intersection determines the localization of most of the Buhweju gold occurrences. The two trends of structures in Buhweju could also be interpreted in terms of the Geita and Twangiza structural corridors (gold trends) known from the work of others. The NNW trending Geita structural corridors control gold mineralization in Tanzania while the NNE trending Twangiza corridor controls gold mineralization in the Democratic Republic of Congo (DRC).

Sector collapse events at volcanoes in the North Tanzanian divergence zone and their implications for regional tectonics

The North Tanzanian divergence zone along the East African Rift is characterized by active faults and several large volcanoes such as Meru, Ol Doinyo Lengai, and Kilimanjaro. Based on systematic morphostructural analysis of the Shuttle Radar Topographic Mission digital elevation model and targeted field work, 14 debris avalanche deposits were identified and characterized, some of them being—to our knowledge—previously unknown. Our field survey around Mount Meru allowed previous “lahar” deposits to be reinterpreted as debris avalanche deposits and three major collapse events to be distinguished, with the two older ones being associated with eruptions. We used topographic lineaments and faults across the North Tanzanian divergence zone to derive the main tectonic trends and their spatial variations and highlight their control on volcano collapse orientation. Based on previous analogue models, the tectonic regime is inferred from the orientation of the collapse scars and/or debris avalanche deposits. We infer two types of regime: extensional and transtensional/strike-slip. The strike-slip regime dominates along the rift escarpment, but an extensional regime is inferred to have operated for the recent sector collapses. The proposed interpretation of sector collapse scars and debris avalanche deposits therefore provides constraints on the tectonic regime in the region. It is possible that, in some cases, movement on regional faults triggered sector collapse.

Early middle Miocene tectonic uplift of the northwestern part of the Qinghai–Tibetan Plateau evidenced by geochemical and mineralogical records in the western Tarim Basin

The Tarim Basin in western China has been receiving continuous marine to lacustrine deposits during the Cenozoic as a foreland basin of the Qinghai–Tibetan Plateau (QTP). Clay mineralogy and geochemical proxy data from these sedimentary archives can shed light on climate and tectonic trends. Here we report on an abrupt mineralogical and weathering shift at 17 Ma ± 1 Myr in the Miocene Qimugan section in the northwestern part of the Qinghai–Tibetan Plateau. The rapid shift involves decreasing trends of chemical weathering indices, Rb/Sr and Ba/Sr ratios, and of minor and immobile elements with respect to upper crust composition as well as increasing trends of Na/Al and Na/Ti ratios, smectite, chlorite, and calcite contents. We ascribe these trends to changing source rocks due to uplift of the northern part of the QTP leading to exposures of younger intrusive bodies and older gneisses, schists, and carbonate-rich rocks. These uplifts potentially caused regional aridification reducing chemical weathering. The dating is indirect via magnetostratigraphically dated ostracod biostratigraphy and detrital zircon chronology and currently not good enough to compare the shift accurately in time with the onset of the global middle Miocene Climate Optimum (MMCO) at 16.5 Ma. Nevertheless, regional tectonics seem to have dominated over global climate as the warmer MMCO is expected to have increased weathering indices and decreased Na/Al and Na/Ti, rather than the observed reverse trends.

Estimation of crustal movements using the Global Positioning System (GPS) measurements along Nile Valley area, Egypt from 2007 to 2012

The Nile Valley in Egypt is located to the west of the Red Sea Rift and to the south of the Mediterranean Sea. Recently, some moderate earthquakes were occurred along the Nile Valley at the eastern and western side. Tectonically, the Nile Valley is controlled by NW–SE, NE–SW, E–W and N–S tectonic trends due to the exerted forces and stresses.A program of studying the recent crustal movements in Egypt has been started since 1984 to cover some areas which are characterized by the occurrence of felt Earthquakes. One of these areas is the Nile Valley. About 6 moderate earthquakes with magnitudes more than 4 were occurred on both sides of River Nile.The present study aimed to determine the recent crustal movement parameters along the Nile Valley using the Global Positioning System (GPS) measurements. To achieve this mission, a GPS network consisting of ten geodetic stations has been established on both sides along the Nile Valley area. GPS measurements have been collected from 2007 to 2012. The collected data were processed using Bernese 5.0 Software. The result of the data analysis indicates that the rate of local velocity is small ranging from 1 to 4mm/year. This rate is consistent with the low rate of occurrence of recent earthquakes activity along the Nile Valley area. But, the results obtained from the calculation of the regional velocity indicated that the velocity of the GPS stations including the African Plate motion is about 25mm/year in the northeast direction which is consistent with the African Plate motion direction.

Stress fields in and around metropolitan Osaka, Japan, deduced from microearthquake focal mechanisms

Present-day stress fields in and around metropolitan Osaka, Japan, have been investigated based on 238 microearthquake focal mechanism solutions from approximately the past 10years. The successful retrieval of the focal mechanisms in the noisy urban area owed much to the clean seismic data recorded at four borehole observatories deeper than 500m depth in Osaka prefecture, in which P-wave onset polarity of the microearthquake could be clearly identified. We found many microearthquakes with a pure reverse-faulting mechanism, which is consistent with the faulting style of major active faults within the area (42-km long east-dipping Uemachi and 38-km long east-dipping Ikoma fault zones); however, a considerable number of microearthquakes with strike-slip components were also found to occur. Most of the P-axes are oriented in the ESE–WNW to ENE–WSW directions sub-horizontally, conforming to the general tectonic trend of the area. The analysis of a stress tensor inversion delineated small but noticeable differences in stress regime, on a scale of at least 10km, where strike-slip faulting prevails throughout the region but the middle and southern sections of the Uemachi fault zone contain some reverse-faulting components. Based on the estimated stress fields, together with assumptions regarding the deep geometry of the Uemachi fault zone, we assessed the reactivation potential of the fault zone through slip-tendency analysis. The results indicate higher potential in the middle and southern sections of the fault zone, which is related to the locally increased reverse component detected there.

Deformation and Metamorphism of a Schistose Terrane Accreted with the Proterozoic Pakhal Sequence of Godavari Valley near Kothagudem, Andhra Pradesh

Abstract From the recent research by several workers it is becoming evident that the geology , particularly the structure and metamorphism of the Proterozoic sedimentary basins are not simple , which can at times be compared with fold-thrust belts. In the southern part of the Pranhita-Godavari (PG) basin in its close proximity with the Eastern Ghat Mobile belt (EGMB) in the east and Eastern Dharwar craton (EDC) in the west there is a supracrustal association exposed near Kothagudem in Andhra Pradesh, which was earlier referred in published literature as Mulug and Mallampalle Groups of lower Pakhal sequence. From the field and petrological studies we have attempted to enumerate here the deformation and metamorphic characteristics and the tectonic setup of this schistose unit. It is revealed from our studies that this lithotectonic unit represents a strongly deformed and metamorphosed deeper crustal supracrustal assemblage, which is in sharp contrast with the unmetamorphosed and weakly deformed true Pakhal sequence occurring in the western part of the area. In order to explain the spatial association of these two contrasting litho-tectonostratigraphic units we propose a westerly verging thrust model, supported by convincing structural and metamorphic evidences, whereby the schistose terrane is tectonically juxtaposed with the unmetamorphosed Pakhal sequence. Imprints of polyphase deformation and metamorphism could be recognized to document the complex history of the schistose terrane. At least three generation structures (D1, D2, D3) are identifiable in quartzite-metapelite-carbonate assemblages. D1 and D2 are quite pronounced and conspicuous by the development of respective planar and linear elements, while D3 is relatively mild in intensity. Development of different generations of folds (F1, F2, F3) and associated axial planar cleavage/ schistosity (S1, S2, S3) together with different types of fold interference patterns, demonstrated mainly in carbonate rocks, bear testimony to successive events of fold formation as a result of superposed deformations. The tectonic trend defined by N-S to NNESSW (approx.) elongation of the belt results from the combined effects of all the three deformations. The structural pattern indicate a possible combination of predominantly pure shear and subordinate simple shear transpressional type of tectonism accompanied by considerable E-W (approx.) tectonic shortening across the belt. The rocks show low to medium grade metamorphism with the characteric appearances of chlorite, biotite, andalusite, chloritoid, staurolite and garnet porphyroblasts in pelitic rocks. From a study on ‘Si-Se’ tectonites in different mineral assemblages we have elucidated the history of deformation and metamorphic mineral growth in this assemblage. A variety of relationship exists between ‘Si’ and ‘Se’ demonstrating multi-stage growth of porphyroblasts, particularly of andalusite and staurolite, which took place during at least two deformational events. The important mineral assemblages include: (i) garnet-bearing andalusite- biotite schist, (ii) garnet free andalusite-biotite schist, (iii) staurolite-andalusite-garnet-mica schist, (iv) andalusite-chloritoid-muscovite schist, and (v) garnet-chloritoid-chlorite-muscovite schist. The study establishes a prograde metamorphic evolution in each rock type followed by a retrograde history. But the relative timing of prograde and retrograde reactions with respect to successive phases of deformation varies widely.

A geophysical view of the Southeastern Brazilian margin at Santos Basin: Insights into rifting evolution

This study investigates the rifting structures of Santos Basin at the Southeastern Brazilian margin, based on an integrated geophysical approach. Our aim is to constrain the crustal basement topography of central and northern Santos basin, the presence of magmatism and the role of inherited structures in space and time through the rifting processes. We present a new high resolution aeromagnetic dataset, which in correlation with gravity anomalies enables us to interpret the tectonic trends and crustal basement structures. We calculated the magnetic basement depth for the central and northern Santos Basin using power spectrum analysis. The obtained depths range between 2 and 9 kms, and are comparable with results from previous works. From our integrated study, three margin domains could be identified, which display distinct rifting structures and are characterized by important lateral variation along the margin. The proximal domain displays trends and magnetic basement blocks NE–SW oriented, i.e., parallel to inherited onshore crustal basement with an inflexion to E–W oriented trends; the necking domain is characterized by oblique magnetic basement highs and lows (E–W and NW–SE) and a structural trend change. The trends and magnetic basement highs are bounded by NW–SE negative anomalies, interpreted as transfer zones. Oceanwards at the distal domain, the lineaments and transfer zones show a progressive structural inflexion to ENE and E–W, sub-parallel to adjacent South Atlantic Fracture Zones. The observed crustal basement architecture and segmentation suggest the reactivation of pre-rift structures at the proximal margin and the obliquity of rifting relative to them. From the proximal domain oceanwards the structural pattern may reflect the passage from a “continental type” domain, where lithospheric inheritance controls the deformation, to a distal margin where this influence diminishes and “new” structural trends are formed. We propose that northern Santos Basin show evidences of an intensely deformed zone, where rift evolved under oblique extension, similar to that observed at transform margin segments.

A two‐magma chamber model as a source of deformation at Grímsvötn Volcano, Iceland

AbstractGrímsvötn Volcano is the most active volcano in Iceland, and its last three eruptions were in 1998, 2004, and 2011. Here we analyze the displacement around Grímsvötn during these last three eruptive cycles using 10 GPS stations. The observed displacements in this region generally contain a linear component of tectonic and glacio‐isostatic origin, in agreement with the previously estimated values of plate motions and vertical rebound. Larger amplitude deformation observed close to Grímsvötn at the GFUM continuous GPS station clearly reflects a major volcanic contribution superimposed on a tectonic component. We estimate and subtract the tectonic trend at this station using regional observed displacement. The direction and pattern of the residual volcanic displacement (for coeruptive and intereruptive periods) are consistent for all three of these eruptive cycles. The posteruptive inflation is characterized by an exponential trend, followed by a linear trend. In this study, we explain this temporal behavior using a new analytic model that has two connected magma chambers surrounded by an elastic medium and fed by a constant basal magma inflow. During the early posteruptive phase, pressure readjustment occurs between the two reservoirs, with replenishment of the shallow chamber from the deep chamber. Afterward, due to the constant inflow of magma into the deep reservoir, the pressurization of the system produces linear uplift. A large deep reservoir favors magma storage rather than surface emission. Based on displacement measured at GFUM station, we estimate an upper limit for the radius of the deep reservoir of ∼10 km.

Distribution and geological control of mud volcanoes and other fluid/free gas seepage features in the Mediterranean Sea and nearby Gulf of Cadiz

Existing knowledge on the distribution of mud volcanoes (MVs) and other significant fluid/free gas-venting features (mud cones, mud pies, mud-brine pools, mud carbonate cones, gas chimneys and, in some cases, pockmark fields) discovered on the seafloor of the Mediterranean Sea and in the nearby Gulf of Cadiz has been compiled using regional geophysical information (including multibeam coverage of most deepwater areas). The resulting dataset comprises both features proven from geological sampling, or in situ observations, and many previously unrecognized MVs inferred from geophysical evidence. The synthesis reveals that MVs clearly have non-random distributions that correspond to two main geodynamic settings: (1) the vast majority occur along the various tectono-sedimentary accretionary wedges of the Africa-Eurasia subduction zone, particularly in the central and eastern Mediterranean basins (external Calabrian Arc, Mediterranean Ridge, Florence Rise) but also along its westernmost boundary in the Gulf of Cadiz; (2) other MVs characterize thick depocentres along parts of the Mesozoic passive continental margins that border Africa from eastern Tunisia to the Levantine coasts, particularly off Egypt and, locally, within some areas of the western Mediterranean back-arc basins. Meaningfully accounting for MV distribution necessitates evidence of overpressured fluids and mud-rich layers. In addition, cross-correlations between MVs and other GIS-based data, such as maps of the Messinian evaporite basins and/or active (or recently active) tectonic trends, stress the importance of assessing geological control in terms of the presence, or not, of thick seals and potential conduits. It is contended that new MV discoveries may be expected in the study region, particularly along the southern Ionian Sea continental margins.

Active Pacific North America Plate boundary tectonics as evidenced by seismicity in the oceanic lithosphere offshore Baja California, Mexico

Pacific Ocean crust west of southwest North America was formed by Cenozoic seafloor spreading between the large Pacific Plate and smaller microplates. The eastern limit of this seafloor, the continent–ocean boundary, is the fossil trench along which the microplates subducted and were mostly destroyed in Miocene time. The Pacific–North America Plate boundary motion today is concentrated on continental fault systems well to the east, and this region of oceanic crust is generally thought to be within the rigid Pacific Plate. Yet, the 2012 December 14 M_w 6.3 earthquake that occurred about 275 km west of Ensenada, Baja California, Mexico, is evidence for continued tectonism in this oceanic part of the Pacific Plate. The preferred main shock centroid depth of 20 km was located close to the bottom of the seismogenic thickness of the young oceanic lithosphere. The focal mechanism, derived from both teleseismic P-wave inversion and W-phase analysis of the main shock waveforms, and the 12 aftershocks of M ∼3–4 are consistent with normal faulting on northeast striking nodal planes, which align with surface mapped extensional tectonic trends such as volcanic features in the region. Previous Global Positioning System (GPS) measurements on offshore islands in the California Continental Borderland had detected some distributed Pacific and North America relative plate motion strain that could extend into the epicentral region. The release of this lithospheric strain along existing zones of weakness is a more likely cause of this seismicity than current thermal contraction of the oceanic lithosphere or volcanism. The main shock caused weak to moderate ground shaking in the coastal zones of southern California, USA, and Baja California, Mexico, but the tsunami was negligible.

Quality assurance of aeromagnetic data using lineament analysis

Lineament analysis is typically applied to geoscientific data to identify lithological contacts, faults, fractures and dyke swarms. We implement lineament analysis as a method for quantifying the adequacy of pre-processing of airborne magnetic datasets. This is accomplished through the identification of noise due to inappropriate levelling. Lineament analysis involves the extraction of linear features from a dataset using visual and/or automatic interpretation techniques and the statistical and directional analyses of these extracted lineaments. We apply lineament analysis to a levelled high resolution aeromagnetic dataset from the Northwest Territories, Canada, to assess the levelling quality. A priori knowledge will include geology defining regional tectonic trends such as fault systems and dyke swarms. Analysis of a lineament’s azimuth separates assumed geologic sources and noise associated with data acquisition or processing artefacts. The lineament azimuths are assessed as rose diagrams. This is an alternative method to standard computation of 2D radially averaged power spectrums in the frequency domain and sunshading orthogonal to flight path. The rose diagrams are compared with the 2D power spectrums which both provide quantitative directional information; however, the power spectrum provides spectral frequency content and rose diagrams provide frequency of occurrence. Calculation of the number of lineaments along a particular azimuth before and after pre-processing quantifies the degree to which flight-line variations have been suppressed and geological signal more apparent.

Subsurface tectonic pattern and basement topography as interpreted from aeromagnetic data to the south of El-Dakhla Oasis, western desert, Egypt

The study area lies to the south of El-Dakhla Oasis in the central part of the western desert, Egypt. It is limited by the latitudes 24–25° N and the longitudes 28–30° E. The main purpose of this work is the investigation of the subsurface structure and the delineation of the main structural elements at different subsurface levels. This study aims also to estimate the basement depth, the basement relief, and consequently, the thickness of the sedimentary cover. The study is based on acquired aeromagnetic data prepared by "La Compagnie General De Geophysique" for the Egyptian General Petroleum Company and Conoco (1977), geological information and results of previous studies in the region. The study involves the analysis for the aeromagnetic data and generating of reduced to pole magnetic map from which different magnetic maps are calculated. The calculated maps are first vertical derivative map and downward continuation map at depth level 400 m. Trend analysis technique is used to define the fault pattern affecting the studied area at different subsurface levels. It is applied to the reduced to pole magnetic map, the first vertical derivative map, and the downward continuation map at depth level 400 m of the study area. All results obtained from the interpretation process were combined together to draw the general view of the subsurface structures of the area. The NE–SW, E–W, and N–S trends are important surface and subsurface (basement) structural trends. This is attributed to the rejuvenation of movements on these old (basement) tectonic trends after the deposition of the sedimentary cover. Basement depth calculation from the aeromagnetic data is achieved using different techniques. The applied techniques included natural spectral analysis and Euler deconvolution. The depth values obtained vary from 400 to 1,700 m.

The Spatial and Genetic Relation between Seismicity and Tectonic Trends, the Bitter Lakes Area, North-East Egypt

The Spatial and Genetic Relation between Seismicity and Tectonic Trends, the Bitter Lakes Area, North-East Egypt The Bitter Lakes area was subjected to numerous seismic events which are genetically related to the well-known tectonic trends in NE Egypt. Based on the focal mechanism solutions and structural lineaments analysis, the spatial and genetic relationships of seismicity to these tectonic trends were clarified. The data set included eight focal mechanisms of recently recorded earthquakes that occurred during the period 1984–2003, and the structural lineaments extrtacted from the enhanced shaded relief image of the study area.

Chapter 9 Eruptive history and magmatic evolution of the island of Salina (central Aeolian archipelago)

Abstract Stratigraphic, structural, volcanological and geochemical data allow a detailed reconstruction of the geological history of the island of Salina (central Aeolian sector). Its subaerial volcanism ( c. 244 ka to 15.6 ka) developed through six successive Eruptive Epochs interrupted by major quiescence periods, volcano-tectonic collapses and recurrent episodes of marine terrace formation during MIS 7 and MIS 5. Several stratovolcanoes were constructed by strombolian and effusive (Pizzo Capo, Monte Rivi, Monte Fossa delle Felci, Monte dei Porri) to hydromagmatic and subplinian (Monte dei Porri, Pollara) activity, with a general east–west shift of active vents, controlled primarily by the dominant NNW–SSE and minor NE–SW regional tectonic trends, and a progressive chemical differentiation of the erupted products from calc-alkaline basalts to rhyolites. The magma compositions and variations through time are the result of contamination of primary magmas derived from a subduction-modified mantle source with the Calabro–Peloritano lower crust and subsequent differentiation dominated by polybaric fractional crystallization. Magma mixing and mingling processes occurred during individual eruptions. The early basalts were fed from deep reservoirs located near the crust–mantle boundary, whereas the later andesitic to dacitic and, ultimately, rhyolitic magmas originated through combined assimilation and fractional crystallization processes in magma reservoirs at mid- to upper-crustal levels. DVD: The 10 000 scale geological map of Salina is included on the DVD in the printed book and can also be accessed online at http://www.geolsoc.org.uk/Memoir37-electronic . Also included is a full geochemical dataset for Salina.

Chapter 8 Volcanism, magmatism, volcano-tectonics and sea-level fluctuations in the geological history of Filicudi (western Aeolian archipelago)

Abstract The geological history of Filicudi in the Middle–Late Pleistocene (246–29 ka) is described by four successive Epochs of activity separated by quiescence stages, erosional episodes during marine isotope stage (MIS) 7 and MIS 5 and the Vallechiesa sector collapse. The partially overlapping Casa Ficarisi, Fossa Felci and Chiumento stratocones and the Monte Guardia scoria cone are built up under control of the WNW–ESE regional tectonic trend, with a subsequent progressive south-eastwards shift and chemical differentiation towards the Monte Terrione and Capo Graziano domes. The Monte Montagnola dome (64 ka) and Case dello Zucco Grande pyroclastics (<56 ka) are the youngest products, together with the Canna neck (29 ka) of an independent volcanic centre offshore of Filicudi. Filicudi rocks are calc-alkaline basalts, basaltic andesites and high-K andesites with minor dacites. Incompatible elements increase from basalts to dacites, with much scatter. Sr, Nd and Pb isotopes show moderate variations. δ 18 O‰ of clinopyroxene phenocrysts ranges from +5.37 to +6.20, and shows the highest values in the mafic rocks. Geochemical data suggest a complex interplay of different evolution processes (fractional crystallization, crustal assimilation and mixing) for the various eruptive centres of Filicudi. Variable incompatible element ratios of mafic rocks are suggested to reflect source heterogeneity. DVD: The 10 000 scale geological map of Filicudi is included on the DVD in the printed book and can also be accessed online at http://www.geolsoc.org.uk/Memoir37-electronic . Also included are a full geochemical dataset and xenolith data for Filicudi.

Chapter 10 Stratigraphy and geological evolution of the Lipari volcanic complex (central Aeolian archipelago)

Abstract The Lipari volcanic complex, situated in the central Aeolian sector, was constructed between c. 267 ka and medieval ages by various lava flows, scoriaceous deposits, lava domes (coulees) and pyroclastic products related to hydromagmatic and strombolian activities. The eruptive history of Lipari is described by nine Epochs of activity interrupted by dormant periods, volcano-tectonic phases and episodes of terrace formation during the Last Interglacial. Several partially overlapping volcanic edifices were active through time, mostly under control of the NNW–SSE and north–south (minor east–west) regional tectonic trends. The latest eruptive events of M. Pilato and Rocche Rosse occurred from AD 776 to 1220. Lipari rocks range from calc-alkaline basaltic andesites to rhyolites, with silicic rocks dominating during the last 43 ka. There is a clear increase in K 2 O and incompatible elements with time, with distinct trends for mafic-intermediate and silicic rocks. Sr, Nd and Pb isotope ratios are highly variable. Petrographic and geochemical data suggest AFC (assimilation plus fractional crystallization) and mixing as the main magma evolution processes, with important effects of crustal anatexis, in the context of a polybaric feeding system. DVD: The 10 000 scale geological map of Lipari is included on the DVD in the printed book and can also be accessed online at http://www.geolsoc.org.uk/Memoir37-electronic . Also included is a full geochemical data set for Lipari.

Chapter 7 Volcanism, calderas and magmas of the Alicudi composite volcano (western Aeolian archipelago)

Abstract The Alicudi composite volcano (western Aeolian archipelago) was constructed between c. 106 and 28 ka by lava flows, domes and strombolian scoriae erupted during six Eruptive Epochs, interrupted by periods of dormancy and three caldera-type collapses in the summit area. Marine Isotope Stage (MIS) 5a (81 ka) terrace deposits and widespread Brown Tuffs of external origin are recognized and provide important marker beds for regional stratigraphic correlations. Volcanism was of central type, under control of the summit caldera collapses with negligible influence of regional tectonic trends. Alicudi rocks are basaltic to high-K andesitic and have the most primitive petrological compositions (high MgO, Ni, Cr contents), the lowest Sr–O and the highest Nd–He isotope ratios ( 87 Sr/ 86 Sr=0.70352 to 0.70410; 143 Nd/ 144 Nd=0.51289 to 0.51279; δ 18 O=+5.0 to 5.6; 3 He/ 4 He–R/Ra= c. 6.5 to 7.1) over the entire Aeolian archipelago. Their composition and variation through time are the result of polybaric crystal–liquid fractionation of parental calc-alkaline basalts to give basaltic andesitic and andesitic derivative melts. These underwent crustal assimilation during ascent, with basalts being contaminated more strongly than andesitic magmas. Sr–Nd–Pb isotopes suggest source metasomatic modification by fluids from an oceanic-type slab, with a minor role for subducted sediments. DVD: The 10 000 scale geological map of Alicudi is included on the DVD in the printed book and can also be accessed online at http://www.geolsoc.org.uk/Memoir37-electronic . Also included is a geochemical dataset for Alicudi.

The 3D magnetic structure beneath the continental margin of the northeastern South China Sea

Understanding the continental margin of the Northeastern South China Sea is critical to the study of deep structures, tectonic evolution, and dynamics of the region. One set of important data for this endeavor is the total-field magnetic data. Given the challenges associated with the magnetic data at low latitudes and with remanent magnetism in this area, we combine the equivalent-source technique and magnetic amplitude inversion to recover 3D subsurface magnetic structures. The inversion results show that this area is characterized by a north-south block division and east-west zonation. Magnetic regions strike in EW, NE and NW direction and are consistent with major tectonic trends in the region. The highly magnetic zone recovered from inversion in the continental margin differs visibly from that of the magnetically quiet zones to the south. The magnetic anomaly zone strikes in NE direction, covering an area of about 500 km × 60 km, and extending downward to a depth of 25 km or more. In combination with other geophysical data, we suggest that this strongly magnetic zone was produced by deep underplating of magma associated with plate subduction in Mesozoic period. The magnetically quiet zone in the south is an EW trending unit underlain by broad and gentle magnetic layers of lower crust. Its magnetic structure bears a clear resemblance to oceanic crust, assumed to be related to the presence of ancient oceanic crust there.

Magneto-seismic studies of the eastern Qattara Depression, Northwestern desert, Egypt

The potential field data are considered the main supporting factor in the geophysical exploration process for detecting and evaluating the subsurface structures. Therefore, a detailed land magnetic survey was performed in an area subjected to seismic investigations. The main target of this study was to detect the deeper subsurface structures and to investigate their possible relationships with earthquake activity.The RTP aeromagnetic map was used to detect the regional extension of the structures interpreted from the land magnetic survey. The RTP land and aeromagnetic maps were interpreted by the filtering technique, least-squares separations, tectonic trend analysis, spectral analysis, Werner method, Euler method, and 2D techniques. The results show that the main tectonic trends are 35° N–45° W, 45° N–65° E, E–W, and Aqaba.Moreover, two seismic lines, WQ85-31B and 127, were interpreted, and their location was matched with the deduced tectonic map. The results show great matching between the location of the faults deduced from both the geomagnetic and seismic data. They agree completely with the well logging data.Furthermore, these structures are correlated with the earthquake activities recorded by the Egyptian National Seismological Network (ENSN). The correlation implies that the studied area is more stable than other adjacent areas in the northern parts of Egypt close to the Mediterranean Sea and the Nile River Delta.